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Advances in Electrical and Electronic Engineering                                                                             200


      WCDMA MOBILE RADIO NETWORK SIMULATOR WITH HYBRID LINK
                           ADAPTATION

                                                       Vladimír Wieser
                                Katedra telekomunikácií, Elektrotechnická fakulta ŽU v Žiline
                           Ve ký diel, 010 26 Žilina, tel.: +421 41 513 2260, mail: wieser@fel.utc.sk

                                                       Vladimír Pšenák
                                         Siemens Program and System Engineering s.r.o,
                      Byt ická 2, 010 01 Žilina,, tel.: +421 907 898 215, mail: vladimir.psenak@siemens.sk



Summary The main aim of this article is the description of the mobile radio network model, which is used for simulation of
authentic conditions in mobile radio network and supports several link adaptation algorithms. Algorithms were designed to
increase efficiency of data transmission between user equipment and base station (uplink). The most important property of
the model is its ability to simulate several radio cells (base stations) and their mutual interactions. The model is created on the
basic principles of UMTS network and takes into account parameters of real mobile radio networks.

1. INTRODUCTION                                                            We are looking for better performance of 3G
                                                                      mobile radio networks in our project. The goal of
    The evolution of mobile radio networks is
                                                                      this paper is the description of the tool, which was
affected by tendency to achieve as the highest data
                                                                      developed during the project VEGA - 1/0140/03
rates as it is possible. The customer wants to own
                                                                      (Effective radio resources management methods in
the latest technology, what can be translated to
                                                                      next generations of mobile communication
common used expression “high data rate”. The
                                                                      networks). The whole project is based on 3G
condition to reach the highest data rate supports
                                                                      Partnership Project (3GPP) specifications.
development of the new technology, which will by
used in next generation of mobile networks (e.g.
                                                                      2. MODEL STRUCTURE
OFDM) [1, 9].
    The most common method for delivering                                The model was created in Matlab environment
maximum data rate to user is the link adaptation.                     with block structure. This design allows the simple
We are dealing only with system parameters                            addition of new blocks or algorithms. Also
adaptation (power and modulation). Other                              description of functionality is very simple. As it is
adaptation methods suited for CDMA systems can                        shown on figure 1, the model contains five main
be found in [11].                                                     blocks (steps):


           1st step                    2nd step                   3rd step                   4th step                   5th step
         Simulation                   Network                      Radio                    Simulation                  Results
         parameters                      model                channels and                open loop                   calculating
        number of BS                type of cells             environment                 closed loop                graphs
        MS in cells                 BS positions              Doppler effect              outer loop                 average data
        radio channel               MS positions              coherent                    link adaptation            speed
        environment                 etc.                      channel time                process                    outage
        etc.                                                  initial seed for            etc.                       probability
                                                              Clark’s model                                          etc.
                                                              etc.




                                                       Fig. 1. Model structure.
201                                                   WCDMA mobile radio network simulator with hybrid link adaptation


      Input parameters processing (1st step).                          type of cells is the same as microcells, but the
      Creating model (2nd step).                                       highest output power is lower (the main difference
      Test environment parameters and radio                            is in the used test environment) [3].
      channels processing (3rd step).                                      The next important parameter of mobile radio
      Simulation of uplink connections between                         network simulation is the test environment.
      base station (BS) and mobile station (MS),                       According to European Telecommunications
      intracell and intercell interference (4th step).                 Standards Institute (ETSI) specification, simulator
      Results calculating (5th step).                                  supports three types of them [3]. The main
                                                                       difference lies in supported MS velocity and also in
    The 1st step. Simulation model supports two                        a different path loss model, which is used.
main types of radio cells - microcells and                                 Types of supported test environments:
macrocells [2]. In macrocells there are used three
                                                                            Low Range Outdoor (Non-line of sight).
sectored antennas with 120° coverage. Microcells                            Suburban Outdoor (Non-line of sight and line
are created by one antenna in the middle of the cell
                                                                            of sight).
with full space coverage (360°). User can set up                            Rural Outdoor (Indoor office) (Line of sight).
almost all of the base stations important parameters                      The most important parameters of test
(e.g. the highest and the lowest output power,                         environments are in table 1.
receiver sensitivity, cable, connector and combiner
losses, transmitter antenna gain, receiver antenna                        Radio channel model is represented by main
gain, cell radius, etc.) The example of graphical                      blocks (the 3 rd step):
user interface (GUI) is on figure 2. The maximum                            Long-term fading (Path loss and Shadow
allowed number of cell is 81 for microcells and 57                          fading)
for macrocells.                                                             Short-term fading
    In 3GPP specification it is also described the
third type of cells: picocells [2]. Structure of these

                                      Maximum data         Maximum MS
         Environment                                                  Model of test channel                      Coverage
                                       rate (QPSK)           velocity

         Rural Outdoor                   144 kbit/s             500 km/h       Vehicular (A & B)                 Macrocells
                                                                               Outdoor to Indoor and
                                                                                                                 Microcells
                                                                               Pedestrian (A & B)
         Suburban Outdoor                384 kbit/s             120 km/h
                                                                               Vehicular (A)                     Macrocells
                                                                               Outdoor to Indoor and
         Low Range Outdoor              2048 kbit/s             10 km/h                                          Microcells
                                                                               Pedestrian (A)
         Indoor Office                  2048 kbit/s             10 km/h        Indoor (A & B)                    Picocells

                                              Tab. 1. Parameters of test environments


   The path loss LPATH-LOSS for Low range outdoor                                                         λ
environment is defined [3]:                                               LPATH − LOSS = −20 . log 10              [dB],      (3)
                                                                                                        4.π .r
LPATH−LOSS = 40.log10(r ) + 30. log10( f ) + 49 [dB], (1)
                                                                       where λ [m] is the wavelength.
                                                                           Shadow fading is similar for all environments:
where r [km] is the distance between transmitter                       Log-normal shadow fading with a standard
and receiver and f [MHz] is frequency.                                 deviation of 10 dB is appropriate for outdoor
         LPATH-L OSS  for      Suburban outdoor                        environments (urban and suburban areas) and 12
environment is defined [3]:                                            dB for indoor one. The building penetration loss
                                                                       with mean value of 12 dB with a standard deviation
L PATH   − LOSS   = 128 ,1 + 37 , 6 . log 10 (r ) [dB].   (2)          of 8 dB was chosen [3].
                                                                           Short-term fading is represented by Clark’s
       This formula is valid for a carrier frequency                   model for Rayleigh fading [4]. Rayleigh fading
of 2000 MHz and a base station antenna height of                       rates are generally set in the simulator by means of
15 meters.                                                             walk or vehicle speed (depending on used
       LPATH-L OSS for Rural outdoor environment is                    environment).
defined [3]:
Advances in Electrical and Electronic Engineering                                                           202


    The detailed description of radio channel model          Open and Closed loops are termed together as
is possible to find in another article in this issue.    Inner loop. In our simulation program each of these
                                                         power control loops can be modified in various
                                                         ways.
                                                             The main purpose of the open loop power
                                                         control is to set up the initial output power for the
                                                         MS transmitter to a specific value. This loop is used
                                                         when MS is accessing the network [1]. There is a
                                                         Dedicated Physical Control Channel DPCCH [1],
                                                         which serves for this purpose. The MS measures
                                                         the received power of DPCCH. The output power
                                                         of transmitter has to be sufficient to overcome the
                                                         various signal loss in uplink. We suppose that path-
                                                         loss and shadow fading in downlink is the same as
                                                         in uplink.
                                                             The initial transmitter output power PMS_out is
                                                         given:
                                                                          PMS_out ≥ PMS_in (4)
               Fig. 2. The first page of GUI
                                                         where PMS_in is measured received power of
          nd                                             DCCH, and is given:
    The 2 step. The position of base stations is
calculated by appropriate algorithm (one is for
                                                                    PMS_in = PBS_out – Lsun     (5)
microcell model and the other for macrocell one).
Positions of the traced mobile stations (MST) are
                                                         where Lsum is the sum of all losses in the radio
random, but required distance from BS is kept.
                                                         channel (downlink). The MS and the BS antenna
Traced MST are highlighted on the figure 3. All
                                                         gains, other losses and power reserve have to be
traced MST use OVSF codes (Orthogonal Variable
                                                         also respected.
Spreading Factor) from the same code tree branch
                                                             Closed loop power control adjusts actual
[5]. Other MSs use different OVFS codes (they are
                                                         transmit power of MS (PMS_out) according to the
grey on the figure 3) and represent the intracell
                                                         selected applicable mathematic formulas. Basic
interference [6].
                                                         algorithm of closed loop power control compares
    In the 2nd step there are calculated all distances
                                                         actual signal to interference ratio SIR (SIRA ) and
among active network elements. These values are
                                                         required SIR (SIRR):
important later for the simulation, when
interference level is calculated [6].
                                                               if SIRA > SIR R      PMS_out - ∆P      (6)
                                                               if SIRA < SIR R      PMS_out + ∆P      (7)

                                                         where ∆P is the power control step. The power
                                                         control step size can take several predefined values
                                                         (e.g.: 0.5, 1, 1.5 or 2 dB) [1]. The value of SIR R
                                                         varies according to type of modulation (Table 2).

                                                                Modulation       SIR R (BER = 10-3)
                                                                  BPSK                   9 dB
                                                                  QPSK                  11 dB
                                                                 16QAM                  21dB
                                                                 64QAM                  24dB

                                                                Tab. 2. Required SIRR of modulations [3]

         Fig. 3. Model of mobile radio network               The outer loop power control adjusts value of
                                                         SIR R for used modulation. It is used to maintain the
3. SIMULATIONS                                           quality of communication at the level of bearer
The 4th step. The process of simulation is based on      service quality requirement, while using as low
algorithm used in UMTS system. This algorithm            power as possible [1]. At the start of the simulation
includes three power adaptation loops [1]:               the SIRR is equal to the appropriate value from table
      Open loop power control.                           2, but the algorithm of outer power adaptation loop
      Closed loop power control.                         compares BERA and BER R and sets the value
      Outer loop power control.                          respectively:
203                                            WCDMA mobile radio network simulator with hybrid link adaptation



      if BERA > BERR             SIR R + ∆SIR (8)                    On the figure 4 there are shown two basic
      if BERA < BERR             SIR R - ∆SIR (9)                 adaptation algorithms, where hybrid adaptation of
                                                                  modulation and power is used:
where ∆SIR is SIR control step of predefined size
(e.g. 1dB). The BERR is set to 10 -3 for speech                        •    Modulation is assigned in Outer loop
services and 10-6 for data based services in our                            power control (figure 4a).
model.                                                                 •    Modulation is assigned in Closed loop
    The primary advantage of our model is its                               power control (figure 4b).
possibility to change modulation together with
power adaptation. The UMTS system uses QPSK                           We defined radio frame duration according to
modulation, which has very good performance in                    3GPP specifications (3G UMTS). One radio frame
all environments specified by 3GPP. We expected                   is 10 ms long and contains of 15 time slots [1]. The
that also other higher-order modulations might be                 MS output power is adjusted every slot (0.667 ms)
used successfully in WCDMA mobile radio                           and modulation scheme can be changed every 10ms
networks, but with several limitations (for example               in the first algorithm:
the most important is SIRA ). The benefit of high-                                          set appropriate
order modulation using is the higher data rate and                  if SIRA > SIR R                              (10)
                                                                                          modulation scheme
better OVSF code utilization (Table 3) [7].                           The second algorithm offers changes of
                                                                  modulation scheme and the MS output power
           Theoretical maximum data rate [kbit/s]                 adjustment in each slot (in 0.667 ms steps). The
      SF                                                          assignment of modulation in the Inner loop power
             4        8     16      32    64   128 256
Mod.                                                              control can cause an oscillation of switching
 BPSK      1024      512    256     128    64 32 16               between modulations, when boundary conditions
 QPSK      2048     1024    512     256   128 64 32               are reached. For restriction of this behavior, the
16QAM      4096     2048   1024     512   256 128 64
                                                                  second algorithm has to include particular
64QAM      6144     4096   2048    1024   512 256 128
                                                                  mechanism.
                 Tab. 3. Maximum data rate




              Fig. 4. Two basic types of used algorithm: a) modulation is assigned in outer loop power control
                                                         b) modulation is assigned in closed loop power control


    The block “Uplink data transmission” (Fig. 4)                 channel) were created in Simulink environment.
covers simplified radio transmission chain (inspired              Simulink environment makes possible to observe
by 3GPP UMTS specifications). All elements of                     dynamic changes of transmitted signal. List of
transmission chain (also the model of radio                       block elements:
Advances in Electrical and Electronic Engineering                                                                              204


    •    Data source (Bernoulli Random Binary                                 4. RESULTS PROCESSING
         Generator).
                                                                              The 5th step. Graphs of several important variables
    •    PN code and OVFS code generator [8].                                 can be shown during and at the end of the
    •    Convolutional encoder and decoder.                                   simulation: SIR R, SIRA , PBS_rec, BER, type of
    •    Spreading, scrambling and modulation                                 modulation and data rate. The bounds of channel
         blocks [5].                                                          coherence time Tcoh and the end of transmission of
    •    Rake receiver (four fingers) [5].                                    UMTS frames are highlighted (Fig. 5). Graphs are
    •    Error rate calculation and measurement                               generated for each traced mobile station
         block.                                                               independently or the time dependency of the same
    •    Model of radio channel (6 signal paths).                             variables can be shown in one graph.




                                          Fig. 5. Example of an output graph for one traced mobile station


    At the end of the simulation (tsim elapsed) BER,                              Time interval t0,042_i is set by satisfied user
FER, average data rate Rt , average data rate with                            requirement [3], by which the user is satisfied if he
regard to satisfied user Rmod , probability of outage                         is not interrupted. The session is interrupted if BER
Pout and probability of outage with regard to                                 > BER req during time longer than tdropp
satisfied user Pout_su is calculated for each traced                                                        10
mobile station (Fig. 4).                                                           t dropp = max 5,                  [sec] (14)
    Average data rate Rt :                                                                          Rt ⋅ BER req
                               Nb                                                 We supposed (for real time services) mean
                    Rt =           ,          (11)                            session duration 120 sec [3], so tdropp is 5/120 =
                           t sim                                              0.042 %.
where Nb is the number of correct transferred bits                                Probability of outage Pout :
and tsim is duration of simulation. Average data rate                                             t BER > BERreq
with regard to satisfied user (modified data rate)                                      Pout =                     ,        (14)
Rmod :                                                                                                t sim
                                 N b _ mod                                    where tBER>BERr is time interval where BER is higher
                    Rmod =                 ,  (12)                            than required BERreq.
                                 t sim                                            Finally, probability of outage with regard to
where Nb_mod is the number of correct transferred                             satisfied user Pout_su:
bits except time interval terr:                                                                                t
                   M                                                                              Pout _ su = err .         (15)
         t err =          t0, 042 _ i ,                     (13)                                               tsim
                   i =1
where M is number of the time intervals t0,042_i .
Duration of the time interval t0,042_i is 0,042tsim and
BER in it is higher than required BERreq.
205                                        WCDMA mobile radio network simulator with hybrid link adaptation




                                  Fig. 6. Example of table with simulation results


5. CONCLUSION                                                 [5] 3GPP TS 25.213 V6.2.0 (2005-03):
    Simulation model of the mobile radio network is                "Spreading and modulation (FDD)".
used for proposal of better algorithms for link               [6] Cátedra, F.M.- Pérez-Arriaga, J.: Cell
adaptation process than is available in WCDMA                      Planning for Wireless Communications.
networks now. Fast power adaptation process is                     Artech House Publishers, Boston, USA, 1999.
essential for correct functionality of this kind of           [7] Hrdina, Z. – Vejražka, F.: Digitální rádiová
mobile networks. Optimization of the link                          komunikace, Vydavatelství VUT, 1995.
adaptation algorithms together with possibility to
                                                              [8] 3GPP TS 25.212 V6.4.0 (2005-03):
change modulation (BPSK, QPSK, 16QAM and
                                                                   "Multiplexing and channel coding (FDD)".
64QAM) brings more efficient data transmission
and saves system resources [10]. Algorithms can be            [9] Doboš, .- ižmár, A.-Palitefka, R.: Next
designed to reach as high data rate as possible in                 Generation Mobile Communication system.
simulated conditions or data rate is input parameter               Proceedings     Renewable      Sources    and
for each user and algorithm is designed to reach                   Environmental            Electro-technologies,
maximum users in each cell (combination OVSF                       RSEE´98, Oradea, May 27-29, 1998, pp.78-
code and modulation).                                              83. ISSN-1223-2106.
    Making changes in algorithms is easy and                  [10] Doboš, .-Gori , J.: Call Admission Control in
graphs of various variables make possible to                       Mobile Wireless. Radioengineering. December
simulate and compare behaviors of each traced                      2002 Volume 11, No.4, pp. 17-23. ISSN 1210-
element of simulated mobile networks in different                  2512.
conditions (state of radio channel, speed of mobile           [11] Marchevský, S.- Ben o, S.: Adaptive detectors
user etc.).                                                        for CDMA Signals. Zborník prednášok zo
                                                                   6.medzinárodnej      vedeckej      konferencie
Acknowledgement                                                    TELEKOMUNIKÁCIE 2000, 2.-3.mája 2000,
                                                                   Bratislava, pp.281-283.
      This paper was supported by the grant
VEGA - 1/0140/03 and state program No. 2003 SP
51/028 09 00/028 09 10.

REFERENCES

[1] Castro, P. J.: The UMTS Network and Radio
    Access Technology - Air interface Techniques
    for Mobile Systems. Willey, USA, 2001.
[2] Laiho J., Wacker, A., Novosad, T.: Radio
    Network Planning and Optimization for
    UMTS. Wiley, USA, 2002.
[3] ETSI TR 101 112 V3.2.0. (1998-04):
    "Selection procedures for the choice of radio
    transmission technologies of the UTMS".
[4] Rappaport, T.S.: Wireless Communications.
    Principles and Practice. Prentice Hall, New
    Jersey, USA, 1996.

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200 205 wieser

  • 1. Advances in Electrical and Electronic Engineering 200 WCDMA MOBILE RADIO NETWORK SIMULATOR WITH HYBRID LINK ADAPTATION Vladimír Wieser Katedra telekomunikácií, Elektrotechnická fakulta ŽU v Žiline Ve ký diel, 010 26 Žilina, tel.: +421 41 513 2260, mail: wieser@fel.utc.sk Vladimír Pšenák Siemens Program and System Engineering s.r.o, Byt ická 2, 010 01 Žilina,, tel.: +421 907 898 215, mail: vladimir.psenak@siemens.sk Summary The main aim of this article is the description of the mobile radio network model, which is used for simulation of authentic conditions in mobile radio network and supports several link adaptation algorithms. Algorithms were designed to increase efficiency of data transmission between user equipment and base station (uplink). The most important property of the model is its ability to simulate several radio cells (base stations) and their mutual interactions. The model is created on the basic principles of UMTS network and takes into account parameters of real mobile radio networks. 1. INTRODUCTION We are looking for better performance of 3G mobile radio networks in our project. The goal of The evolution of mobile radio networks is this paper is the description of the tool, which was affected by tendency to achieve as the highest data developed during the project VEGA - 1/0140/03 rates as it is possible. The customer wants to own (Effective radio resources management methods in the latest technology, what can be translated to next generations of mobile communication common used expression “high data rate”. The networks). The whole project is based on 3G condition to reach the highest data rate supports Partnership Project (3GPP) specifications. development of the new technology, which will by used in next generation of mobile networks (e.g. 2. MODEL STRUCTURE OFDM) [1, 9]. The most common method for delivering The model was created in Matlab environment maximum data rate to user is the link adaptation. with block structure. This design allows the simple We are dealing only with system parameters addition of new blocks or algorithms. Also adaptation (power and modulation). Other description of functionality is very simple. As it is adaptation methods suited for CDMA systems can shown on figure 1, the model contains five main be found in [11]. blocks (steps): 1st step 2nd step 3rd step 4th step 5th step Simulation Network Radio Simulation Results parameters model channels and open loop calculating number of BS type of cells environment closed loop graphs MS in cells BS positions Doppler effect outer loop average data radio channel MS positions coherent link adaptation speed environment etc. channel time process outage etc. initial seed for etc. probability Clark’s model etc. etc. Fig. 1. Model structure.
  • 2. 201 WCDMA mobile radio network simulator with hybrid link adaptation Input parameters processing (1st step). type of cells is the same as microcells, but the Creating model (2nd step). highest output power is lower (the main difference Test environment parameters and radio is in the used test environment) [3]. channels processing (3rd step). The next important parameter of mobile radio Simulation of uplink connections between network simulation is the test environment. base station (BS) and mobile station (MS), According to European Telecommunications intracell and intercell interference (4th step). Standards Institute (ETSI) specification, simulator Results calculating (5th step). supports three types of them [3]. The main difference lies in supported MS velocity and also in The 1st step. Simulation model supports two a different path loss model, which is used. main types of radio cells - microcells and Types of supported test environments: macrocells [2]. In macrocells there are used three Low Range Outdoor (Non-line of sight). sectored antennas with 120° coverage. Microcells Suburban Outdoor (Non-line of sight and line are created by one antenna in the middle of the cell of sight). with full space coverage (360°). User can set up Rural Outdoor (Indoor office) (Line of sight). almost all of the base stations important parameters The most important parameters of test (e.g. the highest and the lowest output power, environments are in table 1. receiver sensitivity, cable, connector and combiner losses, transmitter antenna gain, receiver antenna Radio channel model is represented by main gain, cell radius, etc.) The example of graphical blocks (the 3 rd step): user interface (GUI) is on figure 2. The maximum Long-term fading (Path loss and Shadow allowed number of cell is 81 for microcells and 57 fading) for macrocells. Short-term fading In 3GPP specification it is also described the third type of cells: picocells [2]. Structure of these Maximum data Maximum MS Environment Model of test channel Coverage rate (QPSK) velocity Rural Outdoor 144 kbit/s 500 km/h Vehicular (A & B) Macrocells Outdoor to Indoor and Microcells Pedestrian (A & B) Suburban Outdoor 384 kbit/s 120 km/h Vehicular (A) Macrocells Outdoor to Indoor and Low Range Outdoor 2048 kbit/s 10 km/h Microcells Pedestrian (A) Indoor Office 2048 kbit/s 10 km/h Indoor (A & B) Picocells Tab. 1. Parameters of test environments The path loss LPATH-LOSS for Low range outdoor λ environment is defined [3]: LPATH − LOSS = −20 . log 10 [dB], (3) 4.π .r LPATH−LOSS = 40.log10(r ) + 30. log10( f ) + 49 [dB], (1) where λ [m] is the wavelength. Shadow fading is similar for all environments: where r [km] is the distance between transmitter Log-normal shadow fading with a standard and receiver and f [MHz] is frequency. deviation of 10 dB is appropriate for outdoor LPATH-L OSS for Suburban outdoor environments (urban and suburban areas) and 12 environment is defined [3]: dB for indoor one. The building penetration loss with mean value of 12 dB with a standard deviation L PATH − LOSS = 128 ,1 + 37 , 6 . log 10 (r ) [dB]. (2) of 8 dB was chosen [3]. Short-term fading is represented by Clark’s This formula is valid for a carrier frequency model for Rayleigh fading [4]. Rayleigh fading of 2000 MHz and a base station antenna height of rates are generally set in the simulator by means of 15 meters. walk or vehicle speed (depending on used LPATH-L OSS for Rural outdoor environment is environment). defined [3]:
  • 3. Advances in Electrical and Electronic Engineering 202 The detailed description of radio channel model Open and Closed loops are termed together as is possible to find in another article in this issue. Inner loop. In our simulation program each of these power control loops can be modified in various ways. The main purpose of the open loop power control is to set up the initial output power for the MS transmitter to a specific value. This loop is used when MS is accessing the network [1]. There is a Dedicated Physical Control Channel DPCCH [1], which serves for this purpose. The MS measures the received power of DPCCH. The output power of transmitter has to be sufficient to overcome the various signal loss in uplink. We suppose that path- loss and shadow fading in downlink is the same as in uplink. The initial transmitter output power PMS_out is given: PMS_out ≥ PMS_in (4) Fig. 2. The first page of GUI where PMS_in is measured received power of nd DCCH, and is given: The 2 step. The position of base stations is calculated by appropriate algorithm (one is for PMS_in = PBS_out – Lsun (5) microcell model and the other for macrocell one). Positions of the traced mobile stations (MST) are where Lsum is the sum of all losses in the radio random, but required distance from BS is kept. channel (downlink). The MS and the BS antenna Traced MST are highlighted on the figure 3. All gains, other losses and power reserve have to be traced MST use OVSF codes (Orthogonal Variable also respected. Spreading Factor) from the same code tree branch Closed loop power control adjusts actual [5]. Other MSs use different OVFS codes (they are transmit power of MS (PMS_out) according to the grey on the figure 3) and represent the intracell selected applicable mathematic formulas. Basic interference [6]. algorithm of closed loop power control compares In the 2nd step there are calculated all distances actual signal to interference ratio SIR (SIRA ) and among active network elements. These values are required SIR (SIRR): important later for the simulation, when interference level is calculated [6]. if SIRA > SIR R PMS_out - ∆P (6) if SIRA < SIR R PMS_out + ∆P (7) where ∆P is the power control step. The power control step size can take several predefined values (e.g.: 0.5, 1, 1.5 or 2 dB) [1]. The value of SIR R varies according to type of modulation (Table 2). Modulation SIR R (BER = 10-3) BPSK 9 dB QPSK 11 dB 16QAM 21dB 64QAM 24dB Tab. 2. Required SIRR of modulations [3] Fig. 3. Model of mobile radio network The outer loop power control adjusts value of SIR R for used modulation. It is used to maintain the 3. SIMULATIONS quality of communication at the level of bearer The 4th step. The process of simulation is based on service quality requirement, while using as low algorithm used in UMTS system. This algorithm power as possible [1]. At the start of the simulation includes three power adaptation loops [1]: the SIRR is equal to the appropriate value from table Open loop power control. 2, but the algorithm of outer power adaptation loop Closed loop power control. compares BERA and BER R and sets the value Outer loop power control. respectively:
  • 4. 203 WCDMA mobile radio network simulator with hybrid link adaptation if BERA > BERR SIR R + ∆SIR (8) On the figure 4 there are shown two basic if BERA < BERR SIR R - ∆SIR (9) adaptation algorithms, where hybrid adaptation of modulation and power is used: where ∆SIR is SIR control step of predefined size (e.g. 1dB). The BERR is set to 10 -3 for speech • Modulation is assigned in Outer loop services and 10-6 for data based services in our power control (figure 4a). model. • Modulation is assigned in Closed loop The primary advantage of our model is its power control (figure 4b). possibility to change modulation together with power adaptation. The UMTS system uses QPSK We defined radio frame duration according to modulation, which has very good performance in 3GPP specifications (3G UMTS). One radio frame all environments specified by 3GPP. We expected is 10 ms long and contains of 15 time slots [1]. The that also other higher-order modulations might be MS output power is adjusted every slot (0.667 ms) used successfully in WCDMA mobile radio and modulation scheme can be changed every 10ms networks, but with several limitations (for example in the first algorithm: the most important is SIRA ). The benefit of high- set appropriate order modulation using is the higher data rate and if SIRA > SIR R (10) modulation scheme better OVSF code utilization (Table 3) [7]. The second algorithm offers changes of modulation scheme and the MS output power Theoretical maximum data rate [kbit/s] adjustment in each slot (in 0.667 ms steps). The SF assignment of modulation in the Inner loop power 4 8 16 32 64 128 256 Mod. control can cause an oscillation of switching BPSK 1024 512 256 128 64 32 16 between modulations, when boundary conditions QPSK 2048 1024 512 256 128 64 32 are reached. For restriction of this behavior, the 16QAM 4096 2048 1024 512 256 128 64 second algorithm has to include particular 64QAM 6144 4096 2048 1024 512 256 128 mechanism. Tab. 3. Maximum data rate Fig. 4. Two basic types of used algorithm: a) modulation is assigned in outer loop power control b) modulation is assigned in closed loop power control The block “Uplink data transmission” (Fig. 4) channel) were created in Simulink environment. covers simplified radio transmission chain (inspired Simulink environment makes possible to observe by 3GPP UMTS specifications). All elements of dynamic changes of transmitted signal. List of transmission chain (also the model of radio block elements:
  • 5. Advances in Electrical and Electronic Engineering 204 • Data source (Bernoulli Random Binary 4. RESULTS PROCESSING Generator). The 5th step. Graphs of several important variables • PN code and OVFS code generator [8]. can be shown during and at the end of the • Convolutional encoder and decoder. simulation: SIR R, SIRA , PBS_rec, BER, type of • Spreading, scrambling and modulation modulation and data rate. The bounds of channel blocks [5]. coherence time Tcoh and the end of transmission of • Rake receiver (four fingers) [5]. UMTS frames are highlighted (Fig. 5). Graphs are • Error rate calculation and measurement generated for each traced mobile station block. independently or the time dependency of the same • Model of radio channel (6 signal paths). variables can be shown in one graph. Fig. 5. Example of an output graph for one traced mobile station At the end of the simulation (tsim elapsed) BER, Time interval t0,042_i is set by satisfied user FER, average data rate Rt , average data rate with requirement [3], by which the user is satisfied if he regard to satisfied user Rmod , probability of outage is not interrupted. The session is interrupted if BER Pout and probability of outage with regard to > BER req during time longer than tdropp satisfied user Pout_su is calculated for each traced 10 mobile station (Fig. 4). t dropp = max 5, [sec] (14) Average data rate Rt : Rt ⋅ BER req Nb We supposed (for real time services) mean Rt = , (11) session duration 120 sec [3], so tdropp is 5/120 = t sim 0.042 %. where Nb is the number of correct transferred bits Probability of outage Pout : and tsim is duration of simulation. Average data rate t BER > BERreq with regard to satisfied user (modified data rate) Pout = , (14) Rmod : t sim N b _ mod where tBER>BERr is time interval where BER is higher Rmod = , (12) than required BERreq. t sim Finally, probability of outage with regard to where Nb_mod is the number of correct transferred satisfied user Pout_su: bits except time interval terr: t M Pout _ su = err . (15) t err = t0, 042 _ i , (13) tsim i =1 where M is number of the time intervals t0,042_i . Duration of the time interval t0,042_i is 0,042tsim and BER in it is higher than required BERreq.
  • 6. 205 WCDMA mobile radio network simulator with hybrid link adaptation Fig. 6. Example of table with simulation results 5. CONCLUSION [5] 3GPP TS 25.213 V6.2.0 (2005-03): Simulation model of the mobile radio network is "Spreading and modulation (FDD)". used for proposal of better algorithms for link [6] Cátedra, F.M.- Pérez-Arriaga, J.: Cell adaptation process than is available in WCDMA Planning for Wireless Communications. networks now. Fast power adaptation process is Artech House Publishers, Boston, USA, 1999. essential for correct functionality of this kind of [7] Hrdina, Z. – Vejražka, F.: Digitální rádiová mobile networks. Optimization of the link komunikace, Vydavatelství VUT, 1995. adaptation algorithms together with possibility to [8] 3GPP TS 25.212 V6.4.0 (2005-03): change modulation (BPSK, QPSK, 16QAM and "Multiplexing and channel coding (FDD)". 64QAM) brings more efficient data transmission and saves system resources [10]. Algorithms can be [9] Doboš, .- ižmár, A.-Palitefka, R.: Next designed to reach as high data rate as possible in Generation Mobile Communication system. simulated conditions or data rate is input parameter Proceedings Renewable Sources and for each user and algorithm is designed to reach Environmental Electro-technologies, maximum users in each cell (combination OVSF RSEE´98, Oradea, May 27-29, 1998, pp.78- code and modulation). 83. ISSN-1223-2106. Making changes in algorithms is easy and [10] Doboš, .-Gori , J.: Call Admission Control in graphs of various variables make possible to Mobile Wireless. Radioengineering. December simulate and compare behaviors of each traced 2002 Volume 11, No.4, pp. 17-23. ISSN 1210- element of simulated mobile networks in different 2512. conditions (state of radio channel, speed of mobile [11] Marchevský, S.- Ben o, S.: Adaptive detectors user etc.). for CDMA Signals. Zborník prednášok zo 6.medzinárodnej vedeckej konferencie Acknowledgement TELEKOMUNIKÁCIE 2000, 2.-3.mája 2000, Bratislava, pp.281-283. This paper was supported by the grant VEGA - 1/0140/03 and state program No. 2003 SP 51/028 09 00/028 09 10. REFERENCES [1] Castro, P. J.: The UMTS Network and Radio Access Technology - Air interface Techniques for Mobile Systems. Willey, USA, 2001. [2] Laiho J., Wacker, A., Novosad, T.: Radio Network Planning and Optimization for UMTS. Wiley, USA, 2002. [3] ETSI TR 101 112 V3.2.0. (1998-04): "Selection procedures for the choice of radio transmission technologies of the UTMS". [4] Rappaport, T.S.: Wireless Communications. Principles and Practice. Prentice Hall, New Jersey, USA, 1996.