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Mitigating Interference in LTE Networks
With Sequans AIR™ - Active Interference Rejection




sequans.com
Mitigating Interference in LTE Networks                                                                                                                             sequans.com
    With Sequans AIR™ – Active Interference Rejection




Contents
Executive summary......................................................................................................................................... 3
Introduction....................................................................................................................................................... 4
	      LTE market.................................................................................................................................................. 4
	      Inter-cell interference in LTE networks........................................................................................... 4
	      Impact of small cells................................................................................................................................. 4
	      Network-based interference management.................................................................................... 5
	      Terminal-based interference management.................................................................................... 5
Receiver design................................................................................................................................................. 5
	      Interference mitigation in LTE............................................................................................................. 5
	      Interference mitigation techniques................................................................................................... 6
	      Introducing Sequans AIR....................................................................................................................... 6
	      Support on Sequans products............................................................................................................. 6
Performance results....................................................................................................................................... 7
	      Link level performance........................................................................................................................... 7
	      System level performance.................................................................................................................... 9
Conclusion........................................................................................................................................................11
Acknowledgements......................................................................................................................................11
	ArrayComm..............................................................................................................................................11
	Siradel.........................................................................................................................................................11
Acronyms..........................................................................................................................................................12
References........................................................................................................................................................12




© 2012 Sequans Communications, www.sequans.com                                                                                                                                        2
Mitigating Interference in LTE Networks                                          sequans.com
  With Sequans AIR™ – Active Interference Rejection




Executive summary
As LTE (long term evolution) networks proliferate and network traffic
increases, interference is becoming an issue for LTE operators. Because LTE
spectrum is limited, most operators are deploying single frequency networks
to maximize capacity; however, while single frequency networks increase
spectral efficiency, they also increase the potential for interference. Network-
based interference mitigation solutions are specified in future versions of
the LTE standard, but these are not yet available to address the interference
problems of today’s LTE networks, and will not remove all interference.
Terminal-based interference solutions, however, are available today and they
offer operators a powerful weapon to combat interference. LTE chipmaker
Sequans Communications has introduced a terminal-based interference
solution called Sequans Active Interference Rejection – or Sequans AIR™.
Sequans AIR provides key benefits to both end-users and operators: end
users will experience higher throughput and better service continuity, and LTE
operators will improve coverage and increase the capacity of their networks.


Key benefits
 	 Up to 3.5 x throughput increase at cell-edge
 	 Up to 2 x network capacity increase*
 	 Improved user experience in dense deployments
*Assumes all user terminals equipped with Sequans AIR




© 2012 Sequans Communications, www.sequans.com                                                   3
Mitigating Interference in LTE Networks                                                                                                       sequans.com
  With Sequans AIR™ – Active Interference Rejection




1. Introduction                                                                 Figure 1 – Inter-cell interference

1.1. LTE market
LTE is a 4G wireless technology standardized by the 3GPP (3G                                                      Serving BS
Partnership Project) that is being deployed today by leading operators                                            (Useful signal)
around the world to provide high-speed data and multimedia services.                                                                             Neighbor BS
The LTE market is growing rapidly. According to telecom research                                                                                 (Interference)
firm IDATE, there were more than 10 million LTE subscribers and 50
deployed LTE networks at the end of 2011, growing to an estimated 118
million subscribers and 200 networks in 2013. All leading operators are
moving to LTE.

The LTE standard is an evolving standard with several planned releases.
Most operators began deployment with Release 8, and are now in the
                                                                                This is a well-known issue that has been the topic of many publications,
process of moving to Release 9. LTE-Advanced networks based on
                                                                                such as Managing Interference in LTE Networks by Senza Fili Consulting
Release 10 are expected to begin deploying near the end of 2013.
                                                                                [SENZA-FILI].

Table 1 – LTE standard versions                                                 1.3. Impact of small cells
                                                                                Data is replacing voice as the predominant application, and therefore
  Version        Date             Main features
                                                                                overall capacity and continuity of service are becoming key concerns for
  Rel 8          2008 Q4          First LTE release.                            operators as data hungry devices flood cellular networks.
                                  All-IP network (SAE).
                                                                                As a means of increasing capacity, operators have been trending towards
  Rel 9          2009 Q4          SAE enhancements, WiMAX and LTE/UMTS
                                                                                deploying small cells in a layered configuration, whereby a macro cell is
                                  interoperability.                             deployed for coverage and several smaller overlapping pico or femto
                                                                                cells are deployed for capacity. The small cells can be turned off/on
  Rel 10         2011 Q1          LTE Advanced fulfilling IMT Advanced 4G re-   dynamically, depending on traffic, in order to save energy. See Figure 2.
                                  quirements. Backwards compatible with
                                  release 8 (LTE).
                                                                                Figure 2 - LTE small cell deployment
  Rel 11         2012 Q3          Advanced IP interconnection of services.
                                                                                                                                    Pico cell covers hotzone

                                                                                                                  Macro cell (3 sectors)
1.2. Inter-cell interference in LTE networks
Due to limited spectrum resources, most operators are deploying their                                                                      Hotzone
LTE networks in a frequency reuse =1 configuration, which means                        Directory connected to
that a single carrier frequency is reused in all cells of the network. This            centralized unit by e.g.
                                                                                       optical fiber
deployment scheme is also referred to as a single-frequency network,
and it is different from schemes used in predecessor cellular networks,
where predefined planning ensured limited inter-cell interference. Single
                                                                                                                                       Pico cell (omni)
frequency networks are the most efficient in terms of overall spectral
efficiency, but by nature they are limited by inter-cell interference. See
Figure 1.
                                                                                In these small cell deployment scenarios, interference can become
                                                                                greatly exacerbated.
  Single frequency LTE networks
  are by nature limited by inter-cell
  interference.




© 2012 Sequans Communications, www.sequans.com                                                                                                                    4
Mitigating Interference in LTE Networks                                                                                                sequans.com
  With Sequans AIR™ – Active Interference Rejection



1.4. Network-based interference management                                  Table 2 - 3GPP reference receivers
The 3GPP is currently evaluating interference solutions. Future versions
                                                                              3GPP Name                     Reference receiver
of the LTE standard (releases 10 and 11) will incorporate network-based
interference management techniques. These techniques will help to             Type 0           =            RAKE
manage interference, but will not be available before LTE-Advanced
                                                                              Type 1           =            Diversity receiver (RAKE)
networks are deployed. Furthermore, these will only limit interference,
not suppress it. Examples of such techniques are eICIC (enhanced              Type 2           =            Equalizer
inter-cell interference coordination) and CoMP (coordinated multipoint
transmission and reception). These are well described in publications         Type 2i          =            Equalizer with interference awareness
such as [SENZA-FILI].
                                                                              Type 3           =            Diversity equalizer
1.5. Terminal-based Interference management
Due to the lack of availability and limitations of network-based              Type 3i          =            Diversity equalizer with
                                                                                                            interference awareness
interference solutions, terminal-based solutions that can be deployed
now are gaining interest. These solutions must be able to operate on          Type M           =            Multiple input multiple output (MIMO)
current LTE networks (Release 8 and 9), and be ready to operate on
future LTE-Advanced networks (Release 10 and 11).                             Similarly, the 3GPP has recently begun to investigate interference-
Terminals with embedded interference mitigation technology benefit            aware receivers for Release 11, as described in [TR36.829].
users by providing:                                                           While the theoretical aspects and algorithm principles of interference
 	 Overall higher throughput (especially at the cell edge)                    cancelation are well understood, numerous challenges lie in the
 	 More stable performance across various locations in the cell               implementation of these techniques in an LTE terminal:
 	 Better continuity of service when moving across a network                    	 First, the LTE waveform is based on OFDMA modulation, which is
                                                                                 not the same as 2G/3G modulation. New techniques have had to
Terminal-based interference solutions benefit operators by providing:            be developed because channel estimation and receiver design for
 	 Higher total capacity                                                         the multi-carrier OFDMA modulation of LTE is very different from
                                                                                 that used for the single-carrier and WCDMA modulation of 2G/3G.
 	 Improved coverage
                                                                                 Practical implementation of interference mitigation theory in LTE
These benefits will be described in detail in section 3.                         requires intimate knowledge of OFDMA architecture.
                                                                                	 Second, LTE throughput is significantly higher than 2G/3G throughput,
2. Receiver design                                                               but the overall budget for power consumption is constrained in order
2.1. Interference mitigation in LTE                                              to meet the requirements of battery-powered devices. Therefore,
There are numerous publications on the topic of interference                     the implementation of interference mitigation techniques must be
cancelation techniques. For instance, [IRC-GSM] deals with interference          designed to require minimal hardware resources and consume minimal
mitigation for 2G systems, and the 3GPP has defined and standardized             power. This goal can be achieved only by accounting for interference
UE receiver classes related to interference cancelation capability for 3G        mitigation in the modem architecture from the initial design.
systems (WCDMA) [TR25.963] for several receiver types as defined in             	 Finally, the LTE standard has defined several transmission modes (see
table 2.                                                                          Table 3), from which specific interference mitigation techniques must
                                                                                  be derived. Specific terminal feedback information transmitted to the
                                                                                  eNodeB (for dynamic throughput optimization) must be taken into
                                                                                  account in the design of interference mitigation techniques.




© 2012 Sequans Communications, www.sequans.com                                                                                                         5
Mitigating Interference in LTE Networks                                                                                                                 sequans.com
  With Sequans AIR™ – Active Interference Rejection



Table 3 – LTE transmit modes                                                        Figure 4 - Receive beamforming

  TM             Description                                                                                                         Active Beam          Targeted User
                                                                                                      Antennas                       Antenna                                Interfering
                                                                                                                                     Array                                  User
  1              Single transmit antenna
                                                                                                                                            Interfering
                                                                                                                                            User
  2              Transmit diversity                                                                                                                                       Antenna
                                                                                                                                                                          Array

  3              Open loop spatial multiplexing with cyclic delay diversity (CDD)
                                                                                       Conventional                  Switched                               Adaptive
                                                                                       Beamforming Array             Antenna Array                          Antenna Array
  4              Closed loop spatial multiplexing

  5              Multi-user MIMO
                                                                                    Note that for both of these approaches, channel estimation, whereby
  6              Closed loop spatial multiplexing using a single transmission       the channel and interference are accurately estimated, is a key step in
                 layer                                                              the process. Another key step is to detect the presence or absence of
                                                                                    interference. This eases overall processing in the UE, considering that
  7              Beamforming
                                                                                    interference is highly unpredictable and dependent on variable factors
  8              Dual-layer beamforming                                             such as channel conditions, traffic from other terminals, and scheduling
                                                                                    from the eNodeB.

                                                                                    2.3. Introducing Sequans AIR
2.2. Interference mitigation techniques
                                                                                    Based on the specific requirements of interference mitigation in
There are two possible approaches to implementing interference
                                                                                    LTE, and considering the rapidly changing interference conditions
mitigation in a terminal receiver:
                                                                                    in packet-switched networks, Sequans has designed Sequans AIR
 	 Nonlinear: In this approach, the interfering signal is estimated and
                                                                                    (active interference rejection), a compact LTE receiver that includes
  then subtracted from the received signal, possibly in an iterative
                                                                                    interference mitigation capability, suited to the various transmission
  manner. This requires explicit modeling of the interfering signal. Such
                                                                                    modes of LTE. Sequans AIR adopts the linear approach to interference
  an approach provides excellent performance, but is very sensitive to
                                                                                    mitigation. It has been co-developed with technology partner
  errors in the estimation of the interfering signal.
                                                                                    ArrayComm, a pioneer in antenna processing and interference
Figure 3 – Nonlinear interference cancelation                                       management techniques. Sequans has leveraged its own expertise with
                                                                                    OFDMA and MIMO receivers, and the combined efforts of Sequans and
                                                                                    ArrayComm have resulted in an innovative and powerful interference
        FRONT-END               EQUALIZER                      DECODER              mitigation algorithm and an optimized implementation on silicon.



                                                INTERFERENCE
                                                 ESTIMATION                           Sequans AIR mitigates interference not
                                                                                      only from data channels, but also from
 	 Linear: In this approach, the receiver uses multiple antennas to                   control channels.
  perform spatial suppression of the interfering signal. Specifically,
  the receiver forms a receive antenna beam, with a spatial null in the
  direction of the interferer. This works best with a large number of               Sequans AIR has been designed to mitigate the interference not only
  receive antennas, but provided with proper spatial properties, this               from data channels but also from control channels. Even though control
  technique can handle a number of interferers. Such receivers are                  channels are designed to be more robust than data channels, they may
  usually called IRC (interference rejection combiner) receivers.                   also suffer from strong interference. If they do, the terminal may not be
                                                                                    able to demodulate the control channel and may lose its connection to
                                                                                    the network.

                                                                                    2.4. Support on Sequans products
                                                                                    Sequans AIR is designed for use on Sequans’ latest LTE platforms:
                                                                                     	 Andromeda (based on SQN3110 baseband IC) for handsets and
                                                                                      tablets
                                                                                     	 Mont-Blanc (based on SQN3120 baseband IC) for dongles, mobile
                                                                                      hotspots, M2M applications, and other data-centric devices.



© 2012 Sequans Communications, www.sequans.com                                                                                                                                            6
Mitigating Interference in LTE Networks                                                                                                sequans.com
  With Sequans AIR™ – Active Interference Rejection



Sequans AIR is designed to work in any LTE network, regardless of         With respect to the interference profile, we assume that the
eNodeB vendor, carrier frequency, channel bandwidth, or duplexing         interference consists of data transmitted in either the same transmit
scheme (TDD/FDD).                                                         mode as the useful data from the serving eNodeB, or using a different
                                                                          transmit mode. In all cases, the downlink sub-frames are fully allocated,
Sequans will further enhance Sequans AIR on future generations of its
                                                                          from both the serving and interfering sides. In the case of the PDCCH
products to support LTE Releases 10 and 11 and to cope with potential
                                                                          (physical downlink control channel), we assume for the sake of
new interference scenarios.
                                                                          simulation that the serving and interfering eNodeBs consider the same
                                                                          aggregation level.
3. Performance results
In this section, we present performance results of the Sequans AIR        The next figures illustrate the performance of the AIR receiver
receiver, based on simulations at the link level and system level:        compared to a reference MRC receiver. In Figure 5, we consider a single
                                                                          interferer, having a constant C/I (carrier to interferer) ratio of –3dB.
 	 The link performance data provides good information about the
                                                                          This means that the power level of the interferer is twice the power
  receiver performance in a range of interference conditions (from
                                                                          level of the useful signal. The useful signal is using TM2 (the most robust
  noise-limited to interference-limited). These results do not directly
                                                                          way to transmit information within the various transmission modes),
  translate to show the benefits of Sequans AIR in a real system.
                                                                          while the interferer is using TM1.
 	 The system level results were obtained through partnership with
  SIRADEL, a leading provider of advanced RF tools, using realistic        Figure 5 - Single-interferer link-level PDSCH performance
  geographical data. These results clearly demonstrate the benefits of
                                                                                                 40
  Sequans AIR in real operational deployment conditions.
                                                                                                 35

3.1. Link level performance
In order to evaluate the benefit of Sequans AIR in the receiver, the                             30

Sequans AIR algorithm was implemented in Sequans’ LTE simulator,
                                                                                                 25
which is bit accurate and can represent the true performance of
                                                                             Throughput (Mbps)




the chip. Only downlink is considered in these simulations. The first
                                                                                                 20
two simulations assume an ideal link adaptation where the best
MCS (modulation and coding scheme) per SNR (signal to noise ratio)
                                                                                                 15
point is selected independently for a Sequans AIR receiver and a
reference implementation for an industry-standard MRC (maximum
                                                                                                 10
ratio combining) receiver. The channel considered is the extended-
vehicular-A channel in low mobility as defined by the 3GPP. The
                                                                                                  5
carrier frequency is 2.6 GHz and we assume cell planning such that                                                                          Reference MRC
                                                                                                                                            Sequans AIR
the interfering cell(s) and the serving cell reference signals are non-                           0
                                                                                                      −10   −5   0   5         10   15      20         25
overlapping. We assume up to three interfering cells in the link layer                                                SNR (dB)
simulation.

                                                                          In this scenario, the MRC receiver has a throughput floor of about 10
  The Sequans AIR receiver yields 3.5                                     Mb/s while the AIR receiver yields much higher throughput up to 35

  times more throughput than the                                          Mb/s or about 350 percent of the reference MRC. In this scenario, even
                                                                          with a good SNR, the performance is interference-limited for the MRC
  default MRC receiver.                                                   and the AIR receiver therefore provides much higher throughput.

                                                                          Figure 6 presents a very challenging scenario with three interferers.
                                                                          The first interferer has the same power as the serving cell while the
                                                                          second has power 3dB below it and the third, 6dB below. Even in this
                                                                          challenging scenario that requires rejecting interference from three
                                                                          interferers with only two UE antennas, the AIR receiver provides about
                                                                          35 percent higher throughput than the reference MRC receiver.




© 2012 Sequans Communications, www.sequans.com                                                                                                              7
Mitigating Interference in LTE Networks                                                                                                                            sequans.com
     With Sequans AIR™ – Active Interference Rejection



Figure 6 - 3-interferer link-level PDSCH performance
                         14                                                                                     For a given performance, the Sequans
                                                                                                                AIR receiver can handle nearly twice as
                         12
                                                                                                                much interference.
                         10
                                                                                                            The final performance curve in Figure 8 shows the effect of using the
                                                                                                            AIR receiver on the PDCCH control channel with an aggregation level
     Throughput (Mbps)




                          8
                                                                                                            of two, and a single interferer at a C/I = -3dB. In this scenario, the
                                                                                                            reference MRC fails to maintain the link since it cannot decode the
                          6
                                                                                                            PDCCH 35 percent of the time. The AIR receiver, on the other hand,
                                                                                                            operates well below a 1 percent error level, already at a low SNR,
                          4
                                                                                                            which means that it rejects the interference and receives the control
                                                                                                            information correctly.
                          2
                                                                                                            This document presents only a few representative scenarios of
                                                                                       Reference MRC
                                                                                       Sequans AIR          the benefits of Sequans AIR, whereby the Sequans AIR receiver
                          0
                                −10        −5     0        5         10       15       20         25        demonstrates a clear gain over the MRC receiver, even for one of the
                                                            SNR (dB)
                                                                                                            most challenging cases where the desired signal is interfered by many
The two previous figures assume a constant level of interferer(s) power                                     interferers and there are only two antennas at the UE. For both the
versus the serving cell(s) power, with a varying SNR. Another way to                                        traffic channel, PDSCH (physical downlink shared channel), and the
illustrate the link layer performance is to present the performance                                         control channel, PDCCH, large gains over a reference MRC receiver
assuming a given SNR (depending typically on the distance from the                                          were observed, showing that a Sequans AIR mobile device would be
receiver to the base station and its environment), and a varying C/I.                                       able to decode the control channel and maintain connectivity, while the
                                                                                                            reference MRC mobile device would be disconnected.
In Figure 7, there are two interferers both using TM2 (transmit
diversity), effectively making four interfering eNB antenna ports that                                      Figure 8 - Single-interferer link-level PDSCH performance
must be rejected with only two UE antennas. Both interferers have                                                               0
                                                                                                                               10

the same power, but their power levels vary while each maintains
the desired signal 15dB above the noise floor. The Sequans AIR
receiver shows a gain of 2-3 dB above the reference MRC receiver,
demonstrating that for a given performance the AIR receiver can handle
nearly twice as much interference as the reference MRC receiver.
                                                                                                                                −1
                                                                                                                               10
                                                                                                             PDCCH fail rate




Figure 7 - Dual-interferer link-level PDSCH performance

                         15




                     12.5

                                                                                                                                −2
                                                                                                                               10

                         10
                                                                                                                                                                          Reference MRC
                                                                                                                                                                                     TM2 PDCCH
 Throughput (Mbps)




                                                                                                                                                                          Sequans AIR

                                                                                                                                     −5   0   5              10    15          20
                                                                                                                                                  SNR (dB)
                         7.5




                         5




                         2.5


                                                                                       Reference MRC
                                                                                       Sequans AIR
                         0
                          −10         −8    −6   −4   −2       0          2   4    6         8         10
                                                            CIR (dB)
                                                               1




© 2012 Sequans Communications, www.sequans.com                                                                                                                                                   8
Mitigating Interference in LTE Networks                                                                                             sequans.com
  With Sequans AIR™ – Active Interference Rejection



                                                                         Table 4 – System-level parameters

  The Sequans AIR receiver                                                 Modeling       Real environment (high-resolution geo map data.)

  demonstrates a clear gain                                                               Propagation model: Ray-based Volcano model.

  over the MRC receiver, even in the                                       Study area     0.87 km² corresponding to 12 cells.

  most challenging situations.                                             System         LTE FDD - 2 x 10 MHz.
                                                                                          Central frequency: 2.6 GHz.
Although link level performance can help to characterize the                              Transmission mode 3 (SU-MIMO) with 2 layers.
performance of a given receiver, it does not highlight the promise
of having a network deployed with such receivers. The next section         Macro-cell     Three sectors per site.
                                                                           layout         Hexagonal site deployment: three rings around the central site,
addresses this by looking at system level performance.
                                                                                          i.e. 37 sites corresponding to 114 cells.
3.2. System level performance                                                             Average inter-site distance (ISD): 500 m.
A typical urban LTE deployment has been simulated, using central Paris                    Average antenna height: 32 m above ground.
as an example. The zone of interest covers a 1 km2 zone, with macro                       Maximum transmit power per antenna: 46 dBm.
base stations deployed. Base stations outside the zone of interest are
                                                                                          Antenna: directional, 14 dBi gain.
included to generate an accurate interference pattern inside the zone.
                                                                                          Antenna electric down-tilt: 6°.
Figure 9 - Paris VI simulation zone                                                       Number of antennas per sector: 2.

                                                                           User           UE antenna heights: 1.5 m for outdoor UEs;
                                                                                          1.5 m, 13.5 m and 25.5 m above ground for indoor UEs.
                                                                                          UE antenna: omni-directional, 0 dBi.
                                                                                          Number of antennas: 2.
                                                                                          UE noise figure: 9 dB.

                                                                           Traffic        Downlink traffic load:
                                                                                          -0% (no interference);
                                                                                          -50%;
                                                                                          -100%.


                                                                         Three scenarios were simulated:

                                                                          	 One scenario without interference (meaning the neighboring cells have
                                                                           no traffic on downlink data channel).
                                                                          	 Two scenarios with, respectively, 50 percent and 100 percent downlink
                                                                           traffic loads.
                                                                           This traffic load represents the average portion of signal resources
                                                                           allocated to the cell users. The MAC layer abstraction does not
                                                                           consider any network-based interference mitigation technique, thus
                                                                           the set of resources allocated by each cell is viewed as random and
                                                                           independent.
                                                                         We assume that the network is deployed at 2.6 GHz, using 10 MHz
                                                                         of bandwidth, and operating in TM3. In this simulation, we made
                                                                         the conservative assumption that TM3 was restricted to 1-layer
                                                                         transmission. Other simulation parameters used are given in Table 4.
For the simulation, a typical hexagonal configuration was used, as
described in Table 4.




© 2012 Sequans Communications, www.sequans.com                                                                                                              9
Mitigating Interference in LTE Networks                                                                                                sequans.com
  With Sequans AIR™ – Active Interference Rejection



To derive the throughput values, both outdoors and indoors, the system       The performance for indoor users is quite different because of
simulation [SIR] takes into account the SNR, CQI (channel quality            penetration losses. Figure 11 illustrates the performance of indoor users
indicator) and throughput values based on the properties of the receiver     at ground floor. In this case, the maximum data rate is not achieved, as
– the default MRC receiver or the Sequans AIR receiver (reusing link-        there is no use of small indoor cells.
level simulation results). The peak user data rate maps represent the
net data rate experienced by a single user in the cell benefiting from the   Figure 11 - Indoor coverage without interference at ground floor

entire bandwidth (50 resource blocks).

For indoor performance evaluation, a specific outdoor-to-indoor
channel model was considered using high-resolution 3D map data.

Figure 10 presents the interference-free case for the outdoor users. In
this case, both the Sequans AIR and the MRC receivers display optimal
performance. It is quite interesting to see that with this typical macro
deployment, usually dimensioned to offer capacity, the network is not
noise-limited outdoors (the maximum throughput is obtained almost
everywhere).

Figure 10 - Outdoor coverage without interference




                                                                             The two previous figures depict scenarios with no interference, and
                                                                             thus represent the performance boundary of a perfect interference
                                                                             cancelation receiver.

                                                                             Now let us consider real-life scenarios with interference. Figure 12
                                                                             shows the coverage map for the default MRC receiver and the Sequans
                                                                             AIR receiver.

                                                                             Figure 12 - Outdoor coverage with 100% interference




                                                                                  (a) - Default MRC receiver        (b) - Sequans AIR receiver




© 2012 Sequans Communications, www.sequans.com                                                                                                     10
Mitigating Interference in LTE Networks                                                                                           sequans.com
  With Sequans AIR™ – Active Interference Rejection



When comparing the results of the full interference case with the ideal
interference-free case, the following observations can be made:
                                                                             4. Conclusion
                                                                             Interference is a key issue in LTE networks. Solutions implemented
 	 The network is interference-limited. Even at good SNR levels, the
                                                                             at the terminal side provide key benefits, both for end users and LTE
  throughput drops considerably with the default receiver as compared
  to the Sequans AIR receiver.                                               operators. Sequans AIR is a solution that has been designed to fit on
                                                                             Sequans’ chipset architecture, with proper hardware accelerators to
 	 A standard receiver may not be able to connect even in a deployment
                                                                             enable full line-rate performance. Sequans AIR works in both TDD
  that was designed for capacity (i.e. over-dimensioned with respect to
                                                                             and FDD modes for all of the transmission modes defined in the LTE
  coverage).
                                                                             standard. The benefits of Sequans AIR have been proven, both at link-
 	 A receiver able to mitigate interference can recover most of the          level and system-level.
  degradation in a realistic deployment.
A similar result may be seen for indoor users as illustrated in Figure 13,   5. Acknowledgements
although the number of areas with no service is even larger (reflecting      Sequans wishes to thank its key technology partners, ArrayComm and
the cumulative effects of interference and the loss of signal strength).     Siradel.
However, in the deep indoor areas where the SNR is much affected by
the indoor penetration losses, the performance of the default receiver       5.1. ArrayComm
and the Sequans AIR receiver become close. In this case communication        ArrayComm is a provider of physical layer solutions for wireless
is no longer interference-limited, but noise-limited.                        infrastructure and client device applications. ArrayComm is a world
                                                                             leader in multi-antenna signal processing, delivering commercial
Figure 13 - Indoor coverage with 100% interference at ground floor           A-MAS™ software now that combines MIMO, beamforming, and
                                                                             interference cancelation to improve end user experience and radio
                                                                             network economics through gains in coverage, client data rates, and
                                                                             system capacity. The company’s comprehensive and flexible PHY
                                                                             solutions include optimized DSP software and hardware accelerators
                                                                             that save development costs and time-to-market.
                                                                             www.arraycomm.com

                                                                             5.2. Siradel
                                                                             SIRADEL (www.siradel.com) is a high-tech company (small-medium
                                                                             enterprise) created in 1994 and based in France, China (Hong-Kong) and
     (a) - Default MRC receiver        (b) - Sequans AIR receiver            Canada (Toronto). Siradel provides products and services for the ICT
                                                                             Industry in particular wireless telecommunications.
Finally, in a scenario with a lower level of interference (neighboring       The portfolio of the company is composed of:
cells have a traffic load of 50 percent), the relative gain of the Sequans
                                                                              	 3D GIS data and RF measurements
AIR receiver compared to the default MRC receiver is lower than in
                                                                              	 Advanced RF tools (Volcano, VolcanoLab)
the full interference scenario, as illustrated in Figure 14. Nonetheless,
the Sequans AIR receiver is able to recover the interference-free             	 Management and technology consulting.
performance, except in the few areas with a very low C/I.                    More than 50 people work at Siradel, serving more than 250 customers
                                                                             in about 50 countries. Siradel’s solution brings more reliable and realistic
Figure 14 - Outdoor coverage with 50% interference
                                                                             assessments of wireless network and wireless equipment performance.
                                                                             The profile of its customers is diverse and includes wireless carriers,
                                                                             radio access equipment companies, manufacturers, regulatory bodies,
                                                                             utilities, and consultants.




     (a) - Default MRC receiver       (b) - Sequans AIR receiver




© 2012 Sequans Communications, www.sequans.com                                                                                                       11
Mitigating Interference in LTE Networks                                                                                                  sequans.com
  With Sequans AIR™ – Active Interference Rejection




6. Acronyms                                                                             7. References
                                                                                        [IRC-GSM] J. Karlsson, J. Heinegkd, “Interference rejection combining
 3GPP             3rd Generation Partnership Project                                    for GSM“, Proc. of 5th IEEE International Conference Universal
                                                                                        Personal Communications, pp.433-437, Sep.1996
 CoMP             Coordinated multi-point transmission and reception
                                                                                        [SENZA-FILI] Senza Fili Consulting, “Managing Interference in LTE”,
 C/I              Carrier to interference ratio
                                                                                        April 2012
 CDD              Cyclic delay diversity
                                                                                        [TR25.963] 3GPP TR 25.963: Feasibility study on interference
 CQI              Channel quality indicator                                             cancellation for UTRA FDD User Equipment (UE)

 DL               Downlink
                                                                                        [TR36.829] 3GPP TR 36.829: Enhanced performance requirement for
                                                                                        LTE User Equipment (UE) (Release 11).
 eICIC            Enhanced inter-cell interference coordination
                                                                                        [SIR] F. Letourneux, Y. Corre, E. Suteau, and Y. Lostanien, 3D
 LTE              Long term evolution                                                   performance analysis of a heterogeneous LTE network with urban
                                                                                        femto-cells, COST IC1004 + iPLAN Joint Workshop on Small Cell
 MCS              Modulation and coding scheme
                                                                                        Cooperative Communications, May 2012, Lyon, France.
 MRC              Maximum ratio combining

 OFDMA            Orthogonal frequency division multiple access

 PDCCH            Physical downlink control channel

 PDSCH            Physical downlink shared channel

 SNR              Signal to noise ratio

 UE               User equipment (terminal)

 WCDMA            Wideband code division multiple access




Sequans and Sequans AIR are trademarks or registered trademarks of Sequans
Communications. All rights reserved.

Sequans Communications S.A. (NYSE: SQNS) is a 4G chipmaker, supplying LTE and
WiMAX chips to original equipment manufacturers and original design manufacturers
worldwide. Sequans is based in Paris, France with additional offices throughout the
world, including United States, United Kingdom, Israel, Hong Kong, Singapore, Taiwan,
South Korea, and China.

© 2012 Sequans Communications, www.sequans.com

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Sequans air-wp-final-050212

  • 1. Mitigating Interference in LTE Networks With Sequans AIR™ - Active Interference Rejection sequans.com
  • 2. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Contents Executive summary......................................................................................................................................... 3 Introduction....................................................................................................................................................... 4 LTE market.................................................................................................................................................. 4 Inter-cell interference in LTE networks........................................................................................... 4 Impact of small cells................................................................................................................................. 4 Network-based interference management.................................................................................... 5 Terminal-based interference management.................................................................................... 5 Receiver design................................................................................................................................................. 5 Interference mitigation in LTE............................................................................................................. 5 Interference mitigation techniques................................................................................................... 6 Introducing Sequans AIR....................................................................................................................... 6 Support on Sequans products............................................................................................................. 6 Performance results....................................................................................................................................... 7 Link level performance........................................................................................................................... 7 System level performance.................................................................................................................... 9 Conclusion........................................................................................................................................................11 Acknowledgements......................................................................................................................................11 ArrayComm..............................................................................................................................................11 Siradel.........................................................................................................................................................11 Acronyms..........................................................................................................................................................12 References........................................................................................................................................................12 © 2012 Sequans Communications, www.sequans.com 2
  • 3. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Executive summary As LTE (long term evolution) networks proliferate and network traffic increases, interference is becoming an issue for LTE operators. Because LTE spectrum is limited, most operators are deploying single frequency networks to maximize capacity; however, while single frequency networks increase spectral efficiency, they also increase the potential for interference. Network- based interference mitigation solutions are specified in future versions of the LTE standard, but these are not yet available to address the interference problems of today’s LTE networks, and will not remove all interference. Terminal-based interference solutions, however, are available today and they offer operators a powerful weapon to combat interference. LTE chipmaker Sequans Communications has introduced a terminal-based interference solution called Sequans Active Interference Rejection – or Sequans AIR™. Sequans AIR provides key benefits to both end-users and operators: end users will experience higher throughput and better service continuity, and LTE operators will improve coverage and increase the capacity of their networks. Key benefits Up to 3.5 x throughput increase at cell-edge Up to 2 x network capacity increase* Improved user experience in dense deployments *Assumes all user terminals equipped with Sequans AIR © 2012 Sequans Communications, www.sequans.com 3
  • 4. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection 1. Introduction Figure 1 – Inter-cell interference 1.1. LTE market LTE is a 4G wireless technology standardized by the 3GPP (3G Serving BS Partnership Project) that is being deployed today by leading operators (Useful signal) around the world to provide high-speed data and multimedia services. Neighbor BS The LTE market is growing rapidly. According to telecom research (Interference) firm IDATE, there were more than 10 million LTE subscribers and 50 deployed LTE networks at the end of 2011, growing to an estimated 118 million subscribers and 200 networks in 2013. All leading operators are moving to LTE. The LTE standard is an evolving standard with several planned releases. Most operators began deployment with Release 8, and are now in the This is a well-known issue that has been the topic of many publications, process of moving to Release 9. LTE-Advanced networks based on such as Managing Interference in LTE Networks by Senza Fili Consulting Release 10 are expected to begin deploying near the end of 2013. [SENZA-FILI]. Table 1 – LTE standard versions 1.3. Impact of small cells Data is replacing voice as the predominant application, and therefore Version Date Main features overall capacity and continuity of service are becoming key concerns for Rel 8 2008 Q4 First LTE release. operators as data hungry devices flood cellular networks. All-IP network (SAE). As a means of increasing capacity, operators have been trending towards Rel 9 2009 Q4 SAE enhancements, WiMAX and LTE/UMTS deploying small cells in a layered configuration, whereby a macro cell is interoperability. deployed for coverage and several smaller overlapping pico or femto cells are deployed for capacity. The small cells can be turned off/on Rel 10 2011 Q1 LTE Advanced fulfilling IMT Advanced 4G re- dynamically, depending on traffic, in order to save energy. See Figure 2. quirements. Backwards compatible with release 8 (LTE). Figure 2 - LTE small cell deployment Rel 11 2012 Q3 Advanced IP interconnection of services. Pico cell covers hotzone Macro cell (3 sectors) 1.2. Inter-cell interference in LTE networks Due to limited spectrum resources, most operators are deploying their Hotzone LTE networks in a frequency reuse =1 configuration, which means Directory connected to that a single carrier frequency is reused in all cells of the network. This centralized unit by e.g. optical fiber deployment scheme is also referred to as a single-frequency network, and it is different from schemes used in predecessor cellular networks, where predefined planning ensured limited inter-cell interference. Single Pico cell (omni) frequency networks are the most efficient in terms of overall spectral efficiency, but by nature they are limited by inter-cell interference. See Figure 1. In these small cell deployment scenarios, interference can become greatly exacerbated. Single frequency LTE networks are by nature limited by inter-cell interference. © 2012 Sequans Communications, www.sequans.com 4
  • 5. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection 1.4. Network-based interference management Table 2 - 3GPP reference receivers The 3GPP is currently evaluating interference solutions. Future versions 3GPP Name Reference receiver of the LTE standard (releases 10 and 11) will incorporate network-based interference management techniques. These techniques will help to Type 0 = RAKE manage interference, but will not be available before LTE-Advanced Type 1 = Diversity receiver (RAKE) networks are deployed. Furthermore, these will only limit interference, not suppress it. Examples of such techniques are eICIC (enhanced Type 2 = Equalizer inter-cell interference coordination) and CoMP (coordinated multipoint transmission and reception). These are well described in publications Type 2i = Equalizer with interference awareness such as [SENZA-FILI]. Type 3 = Diversity equalizer 1.5. Terminal-based Interference management Due to the lack of availability and limitations of network-based Type 3i = Diversity equalizer with interference awareness interference solutions, terminal-based solutions that can be deployed now are gaining interest. These solutions must be able to operate on Type M = Multiple input multiple output (MIMO) current LTE networks (Release 8 and 9), and be ready to operate on future LTE-Advanced networks (Release 10 and 11). Similarly, the 3GPP has recently begun to investigate interference- Terminals with embedded interference mitigation technology benefit aware receivers for Release 11, as described in [TR36.829]. users by providing: While the theoretical aspects and algorithm principles of interference Overall higher throughput (especially at the cell edge) cancelation are well understood, numerous challenges lie in the More stable performance across various locations in the cell implementation of these techniques in an LTE terminal: Better continuity of service when moving across a network First, the LTE waveform is based on OFDMA modulation, which is not the same as 2G/3G modulation. New techniques have had to Terminal-based interference solutions benefit operators by providing: be developed because channel estimation and receiver design for Higher total capacity the multi-carrier OFDMA modulation of LTE is very different from that used for the single-carrier and WCDMA modulation of 2G/3G. Improved coverage Practical implementation of interference mitigation theory in LTE These benefits will be described in detail in section 3. requires intimate knowledge of OFDMA architecture. Second, LTE throughput is significantly higher than 2G/3G throughput, 2. Receiver design but the overall budget for power consumption is constrained in order 2.1. Interference mitigation in LTE to meet the requirements of battery-powered devices. Therefore, There are numerous publications on the topic of interference the implementation of interference mitigation techniques must be cancelation techniques. For instance, [IRC-GSM] deals with interference designed to require minimal hardware resources and consume minimal mitigation for 2G systems, and the 3GPP has defined and standardized power. This goal can be achieved only by accounting for interference UE receiver classes related to interference cancelation capability for 3G mitigation in the modem architecture from the initial design. systems (WCDMA) [TR25.963] for several receiver types as defined in Finally, the LTE standard has defined several transmission modes (see table 2. Table 3), from which specific interference mitigation techniques must be derived. Specific terminal feedback information transmitted to the eNodeB (for dynamic throughput optimization) must be taken into account in the design of interference mitigation techniques. © 2012 Sequans Communications, www.sequans.com 5
  • 6. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Table 3 – LTE transmit modes Figure 4 - Receive beamforming TM Description Active Beam Targeted User Antennas Antenna Interfering Array User 1 Single transmit antenna Interfering User 2 Transmit diversity Antenna Array 3 Open loop spatial multiplexing with cyclic delay diversity (CDD) Conventional Switched Adaptive Beamforming Array Antenna Array Antenna Array 4 Closed loop spatial multiplexing 5 Multi-user MIMO Note that for both of these approaches, channel estimation, whereby 6 Closed loop spatial multiplexing using a single transmission the channel and interference are accurately estimated, is a key step in layer the process. Another key step is to detect the presence or absence of interference. This eases overall processing in the UE, considering that 7 Beamforming interference is highly unpredictable and dependent on variable factors 8 Dual-layer beamforming such as channel conditions, traffic from other terminals, and scheduling from the eNodeB. 2.3. Introducing Sequans AIR 2.2. Interference mitigation techniques Based on the specific requirements of interference mitigation in There are two possible approaches to implementing interference LTE, and considering the rapidly changing interference conditions mitigation in a terminal receiver: in packet-switched networks, Sequans has designed Sequans AIR Nonlinear: In this approach, the interfering signal is estimated and (active interference rejection), a compact LTE receiver that includes then subtracted from the received signal, possibly in an iterative interference mitigation capability, suited to the various transmission manner. This requires explicit modeling of the interfering signal. Such modes of LTE. Sequans AIR adopts the linear approach to interference an approach provides excellent performance, but is very sensitive to mitigation. It has been co-developed with technology partner errors in the estimation of the interfering signal. ArrayComm, a pioneer in antenna processing and interference Figure 3 – Nonlinear interference cancelation management techniques. Sequans has leveraged its own expertise with OFDMA and MIMO receivers, and the combined efforts of Sequans and ArrayComm have resulted in an innovative and powerful interference FRONT-END EQUALIZER DECODER mitigation algorithm and an optimized implementation on silicon. INTERFERENCE ESTIMATION Sequans AIR mitigates interference not only from data channels, but also from Linear: In this approach, the receiver uses multiple antennas to control channels. perform spatial suppression of the interfering signal. Specifically, the receiver forms a receive antenna beam, with a spatial null in the direction of the interferer. This works best with a large number of Sequans AIR has been designed to mitigate the interference not only receive antennas, but provided with proper spatial properties, this from data channels but also from control channels. Even though control technique can handle a number of interferers. Such receivers are channels are designed to be more robust than data channels, they may usually called IRC (interference rejection combiner) receivers. also suffer from strong interference. If they do, the terminal may not be able to demodulate the control channel and may lose its connection to the network. 2.4. Support on Sequans products Sequans AIR is designed for use on Sequans’ latest LTE platforms: Andromeda (based on SQN3110 baseband IC) for handsets and tablets Mont-Blanc (based on SQN3120 baseband IC) for dongles, mobile hotspots, M2M applications, and other data-centric devices. © 2012 Sequans Communications, www.sequans.com 6
  • 7. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Sequans AIR is designed to work in any LTE network, regardless of With respect to the interference profile, we assume that the eNodeB vendor, carrier frequency, channel bandwidth, or duplexing interference consists of data transmitted in either the same transmit scheme (TDD/FDD). mode as the useful data from the serving eNodeB, or using a different transmit mode. In all cases, the downlink sub-frames are fully allocated, Sequans will further enhance Sequans AIR on future generations of its from both the serving and interfering sides. In the case of the PDCCH products to support LTE Releases 10 and 11 and to cope with potential (physical downlink control channel), we assume for the sake of new interference scenarios. simulation that the serving and interfering eNodeBs consider the same aggregation level. 3. Performance results In this section, we present performance results of the Sequans AIR The next figures illustrate the performance of the AIR receiver receiver, based on simulations at the link level and system level: compared to a reference MRC receiver. In Figure 5, we consider a single interferer, having a constant C/I (carrier to interferer) ratio of –3dB. The link performance data provides good information about the This means that the power level of the interferer is twice the power receiver performance in a range of interference conditions (from level of the useful signal. The useful signal is using TM2 (the most robust noise-limited to interference-limited). These results do not directly way to transmit information within the various transmission modes), translate to show the benefits of Sequans AIR in a real system. while the interferer is using TM1. The system level results were obtained through partnership with SIRADEL, a leading provider of advanced RF tools, using realistic Figure 5 - Single-interferer link-level PDSCH performance geographical data. These results clearly demonstrate the benefits of 40 Sequans AIR in real operational deployment conditions. 35 3.1. Link level performance In order to evaluate the benefit of Sequans AIR in the receiver, the 30 Sequans AIR algorithm was implemented in Sequans’ LTE simulator, 25 which is bit accurate and can represent the true performance of Throughput (Mbps) the chip. Only downlink is considered in these simulations. The first 20 two simulations assume an ideal link adaptation where the best MCS (modulation and coding scheme) per SNR (signal to noise ratio) 15 point is selected independently for a Sequans AIR receiver and a reference implementation for an industry-standard MRC (maximum 10 ratio combining) receiver. The channel considered is the extended- vehicular-A channel in low mobility as defined by the 3GPP. The 5 carrier frequency is 2.6 GHz and we assume cell planning such that Reference MRC Sequans AIR the interfering cell(s) and the serving cell reference signals are non- 0 −10 −5 0 5 10 15 20 25 overlapping. We assume up to three interfering cells in the link layer SNR (dB) simulation. In this scenario, the MRC receiver has a throughput floor of about 10 The Sequans AIR receiver yields 3.5 Mb/s while the AIR receiver yields much higher throughput up to 35 times more throughput than the Mb/s or about 350 percent of the reference MRC. In this scenario, even with a good SNR, the performance is interference-limited for the MRC default MRC receiver. and the AIR receiver therefore provides much higher throughput. Figure 6 presents a very challenging scenario with three interferers. The first interferer has the same power as the serving cell while the second has power 3dB below it and the third, 6dB below. Even in this challenging scenario that requires rejecting interference from three interferers with only two UE antennas, the AIR receiver provides about 35 percent higher throughput than the reference MRC receiver. © 2012 Sequans Communications, www.sequans.com 7
  • 8. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Figure 6 - 3-interferer link-level PDSCH performance 14 For a given performance, the Sequans AIR receiver can handle nearly twice as 12 much interference. 10 The final performance curve in Figure 8 shows the effect of using the AIR receiver on the PDCCH control channel with an aggregation level Throughput (Mbps) 8 of two, and a single interferer at a C/I = -3dB. In this scenario, the reference MRC fails to maintain the link since it cannot decode the 6 PDCCH 35 percent of the time. The AIR receiver, on the other hand, operates well below a 1 percent error level, already at a low SNR, 4 which means that it rejects the interference and receives the control information correctly. 2 This document presents only a few representative scenarios of Reference MRC Sequans AIR the benefits of Sequans AIR, whereby the Sequans AIR receiver 0 −10 −5 0 5 10 15 20 25 demonstrates a clear gain over the MRC receiver, even for one of the SNR (dB) most challenging cases where the desired signal is interfered by many The two previous figures assume a constant level of interferer(s) power interferers and there are only two antennas at the UE. For both the versus the serving cell(s) power, with a varying SNR. Another way to traffic channel, PDSCH (physical downlink shared channel), and the illustrate the link layer performance is to present the performance control channel, PDCCH, large gains over a reference MRC receiver assuming a given SNR (depending typically on the distance from the were observed, showing that a Sequans AIR mobile device would be receiver to the base station and its environment), and a varying C/I. able to decode the control channel and maintain connectivity, while the reference MRC mobile device would be disconnected. In Figure 7, there are two interferers both using TM2 (transmit diversity), effectively making four interfering eNB antenna ports that Figure 8 - Single-interferer link-level PDSCH performance must be rejected with only two UE antennas. Both interferers have 0 10 the same power, but their power levels vary while each maintains the desired signal 15dB above the noise floor. The Sequans AIR receiver shows a gain of 2-3 dB above the reference MRC receiver, demonstrating that for a given performance the AIR receiver can handle nearly twice as much interference as the reference MRC receiver. −1 10 PDCCH fail rate Figure 7 - Dual-interferer link-level PDSCH performance 15 12.5 −2 10 10 Reference MRC TM2 PDCCH Throughput (Mbps) Sequans AIR −5 0 5 10 15 20 SNR (dB) 7.5 5 2.5 Reference MRC Sequans AIR 0 −10 −8 −6 −4 −2 0 2 4 6 8 10 CIR (dB) 1 © 2012 Sequans Communications, www.sequans.com 8
  • 9. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection Table 4 – System-level parameters The Sequans AIR receiver Modeling Real environment (high-resolution geo map data.) demonstrates a clear gain Propagation model: Ray-based Volcano model. over the MRC receiver, even in the Study area 0.87 km² corresponding to 12 cells. most challenging situations. System LTE FDD - 2 x 10 MHz. Central frequency: 2.6 GHz. Although link level performance can help to characterize the Transmission mode 3 (SU-MIMO) with 2 layers. performance of a given receiver, it does not highlight the promise of having a network deployed with such receivers. The next section Macro-cell Three sectors per site. layout Hexagonal site deployment: three rings around the central site, addresses this by looking at system level performance. i.e. 37 sites corresponding to 114 cells. 3.2. System level performance Average inter-site distance (ISD): 500 m. A typical urban LTE deployment has been simulated, using central Paris Average antenna height: 32 m above ground. as an example. The zone of interest covers a 1 km2 zone, with macro Maximum transmit power per antenna: 46 dBm. base stations deployed. Base stations outside the zone of interest are Antenna: directional, 14 dBi gain. included to generate an accurate interference pattern inside the zone. Antenna electric down-tilt: 6°. Figure 9 - Paris VI simulation zone Number of antennas per sector: 2. User UE antenna heights: 1.5 m for outdoor UEs; 1.5 m, 13.5 m and 25.5 m above ground for indoor UEs. UE antenna: omni-directional, 0 dBi. Number of antennas: 2. UE noise figure: 9 dB. Traffic Downlink traffic load: -0% (no interference); -50%; -100%. Three scenarios were simulated: One scenario without interference (meaning the neighboring cells have no traffic on downlink data channel). Two scenarios with, respectively, 50 percent and 100 percent downlink traffic loads. This traffic load represents the average portion of signal resources allocated to the cell users. The MAC layer abstraction does not consider any network-based interference mitigation technique, thus the set of resources allocated by each cell is viewed as random and independent. We assume that the network is deployed at 2.6 GHz, using 10 MHz of bandwidth, and operating in TM3. In this simulation, we made the conservative assumption that TM3 was restricted to 1-layer transmission. Other simulation parameters used are given in Table 4. For the simulation, a typical hexagonal configuration was used, as described in Table 4. © 2012 Sequans Communications, www.sequans.com 9
  • 10. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection To derive the throughput values, both outdoors and indoors, the system The performance for indoor users is quite different because of simulation [SIR] takes into account the SNR, CQI (channel quality penetration losses. Figure 11 illustrates the performance of indoor users indicator) and throughput values based on the properties of the receiver at ground floor. In this case, the maximum data rate is not achieved, as – the default MRC receiver or the Sequans AIR receiver (reusing link- there is no use of small indoor cells. level simulation results). The peak user data rate maps represent the net data rate experienced by a single user in the cell benefiting from the Figure 11 - Indoor coverage without interference at ground floor entire bandwidth (50 resource blocks). For indoor performance evaluation, a specific outdoor-to-indoor channel model was considered using high-resolution 3D map data. Figure 10 presents the interference-free case for the outdoor users. In this case, both the Sequans AIR and the MRC receivers display optimal performance. It is quite interesting to see that with this typical macro deployment, usually dimensioned to offer capacity, the network is not noise-limited outdoors (the maximum throughput is obtained almost everywhere). Figure 10 - Outdoor coverage without interference The two previous figures depict scenarios with no interference, and thus represent the performance boundary of a perfect interference cancelation receiver. Now let us consider real-life scenarios with interference. Figure 12 shows the coverage map for the default MRC receiver and the Sequans AIR receiver. Figure 12 - Outdoor coverage with 100% interference (a) - Default MRC receiver (b) - Sequans AIR receiver © 2012 Sequans Communications, www.sequans.com 10
  • 11. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection When comparing the results of the full interference case with the ideal interference-free case, the following observations can be made: 4. Conclusion Interference is a key issue in LTE networks. Solutions implemented The network is interference-limited. Even at good SNR levels, the at the terminal side provide key benefits, both for end users and LTE throughput drops considerably with the default receiver as compared to the Sequans AIR receiver. operators. Sequans AIR is a solution that has been designed to fit on Sequans’ chipset architecture, with proper hardware accelerators to A standard receiver may not be able to connect even in a deployment enable full line-rate performance. Sequans AIR works in both TDD that was designed for capacity (i.e. over-dimensioned with respect to and FDD modes for all of the transmission modes defined in the LTE coverage). standard. The benefits of Sequans AIR have been proven, both at link- A receiver able to mitigate interference can recover most of the level and system-level. degradation in a realistic deployment. A similar result may be seen for indoor users as illustrated in Figure 13, 5. Acknowledgements although the number of areas with no service is even larger (reflecting Sequans wishes to thank its key technology partners, ArrayComm and the cumulative effects of interference and the loss of signal strength). Siradel. However, in the deep indoor areas where the SNR is much affected by the indoor penetration losses, the performance of the default receiver 5.1. ArrayComm and the Sequans AIR receiver become close. In this case communication ArrayComm is a provider of physical layer solutions for wireless is no longer interference-limited, but noise-limited. infrastructure and client device applications. ArrayComm is a world leader in multi-antenna signal processing, delivering commercial Figure 13 - Indoor coverage with 100% interference at ground floor A-MAS™ software now that combines MIMO, beamforming, and interference cancelation to improve end user experience and radio network economics through gains in coverage, client data rates, and system capacity. The company’s comprehensive and flexible PHY solutions include optimized DSP software and hardware accelerators that save development costs and time-to-market. www.arraycomm.com 5.2. Siradel SIRADEL (www.siradel.com) is a high-tech company (small-medium enterprise) created in 1994 and based in France, China (Hong-Kong) and (a) - Default MRC receiver (b) - Sequans AIR receiver Canada (Toronto). Siradel provides products and services for the ICT Industry in particular wireless telecommunications. Finally, in a scenario with a lower level of interference (neighboring The portfolio of the company is composed of: cells have a traffic load of 50 percent), the relative gain of the Sequans 3D GIS data and RF measurements AIR receiver compared to the default MRC receiver is lower than in Advanced RF tools (Volcano, VolcanoLab) the full interference scenario, as illustrated in Figure 14. Nonetheless, the Sequans AIR receiver is able to recover the interference-free Management and technology consulting. performance, except in the few areas with a very low C/I. More than 50 people work at Siradel, serving more than 250 customers in about 50 countries. Siradel’s solution brings more reliable and realistic Figure 14 - Outdoor coverage with 50% interference assessments of wireless network and wireless equipment performance. The profile of its customers is diverse and includes wireless carriers, radio access equipment companies, manufacturers, regulatory bodies, utilities, and consultants. (a) - Default MRC receiver (b) - Sequans AIR receiver © 2012 Sequans Communications, www.sequans.com 11
  • 12. Mitigating Interference in LTE Networks sequans.com With Sequans AIR™ – Active Interference Rejection 6. Acronyms 7. References [IRC-GSM] J. Karlsson, J. Heinegkd, “Interference rejection combining 3GPP 3rd Generation Partnership Project for GSM“, Proc. of 5th IEEE International Conference Universal Personal Communications, pp.433-437, Sep.1996 CoMP Coordinated multi-point transmission and reception [SENZA-FILI] Senza Fili Consulting, “Managing Interference in LTE”, C/I Carrier to interference ratio April 2012 CDD Cyclic delay diversity [TR25.963] 3GPP TR 25.963: Feasibility study on interference CQI Channel quality indicator cancellation for UTRA FDD User Equipment (UE) DL Downlink [TR36.829] 3GPP TR 36.829: Enhanced performance requirement for LTE User Equipment (UE) (Release 11). eICIC Enhanced inter-cell interference coordination [SIR] F. Letourneux, Y. Corre, E. Suteau, and Y. Lostanien, 3D LTE Long term evolution performance analysis of a heterogeneous LTE network with urban femto-cells, COST IC1004 + iPLAN Joint Workshop on Small Cell MCS Modulation and coding scheme Cooperative Communications, May 2012, Lyon, France. MRC Maximum ratio combining OFDMA Orthogonal frequency division multiple access PDCCH Physical downlink control channel PDSCH Physical downlink shared channel SNR Signal to noise ratio UE User equipment (terminal) WCDMA Wideband code division multiple access Sequans and Sequans AIR are trademarks or registered trademarks of Sequans Communications. All rights reserved. Sequans Communications S.A. (NYSE: SQNS) is a 4G chipmaker, supplying LTE and WiMAX chips to original equipment manufacturers and original design manufacturers worldwide. Sequans is based in Paris, France with additional offices throughout the world, including United States, United Kingdom, Israel, Hong Kong, Singapore, Taiwan, South Korea, and China. © 2012 Sequans Communications, www.sequans.com