4G Broadband
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Overview of 3GPP LTE technology
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- 1. 21-Jan-11 4G Broadband what you need to know about LTE Fanny Mlinarsky President octoScope, Inc. www.octoscope.com 7-Oct-11
- 2. 2 Peak Data Rate (Mbps) G The G’s Downlink Uplink 1 Analog 19.2 kbps 2 Digital – TDMA, CDMA 14.4 kbps Improved CDMA variants (WCDMA, CDMA2000) 144 kbps (1xRTT); 3 384 kbps (UMTS); 2.4 Mbps (EVDO) 3.5 HSPA (today) 14 Mbps 2 Mbps HSPA (Release 7) DL 64QAM or 2x2 MIMO; UL 16QAM 28 Mbps 11.5 Mbps 3.75 HSPA (Release 8) DL 64QAM and 2x2 MIMO 42 Mbps 11.5 Mbps WiMAX Release 1.0 TDD (2:1 UL/DL ratio), 10 MHz channel 40 Mbps 10 Mbps 3.9 LTE, FDD 5 MHz UL/DL, 2 Layers DL 43.2 Mbps 21.6 Mbps LTE CAT-3 100 Mbps 50 Mbps 4 LTE-Advanced 1000 Mbps 500 Mbps www.octoscope.com
- 3. 3 AT&T Test • AT&T launched its LTE network in 5 cities on 9/18/11 • PC Magazine article: AT&T vs. Verizon: LTE, Head-to-Head http://www.pcmag.com/article2/0,2817,2393182,00.asp#fbid =fD0LlOUpHzx Unable to roam between AT&T and Verizon LTE networks AT&T has put coverage maps on its site advocating merger with T-Mobile Dallas-Fort Worth San Antonio Houston Atlanta Chicago www.octoscope.com
- 4. 4 octoScope’s LTE Throughput Measurements in MA DL/UL, Mbps Samsung Galaxy 4G Tablet www.octoscope.com
- 5. 5 Measurements Performed Here www.octoscope.com
- 6. 6 Output Captured by speedtest.com kbps msec Date ConnType Lat Lon Download Upload Latency ServerName Internal IP External IP 10/2/2011 11:10 Lte 42.41827 -71.6034 19518 4920 98 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 11:10 Lte 42.41827 -71.6034 19518 3983 106 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 11:09 Lte 42.41827 -71.6034 17300 2772 96 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 11:05 Ehrpd 42.28415 -71.6087 1917 1000 194 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 11:00 Ehrpd 42.28415 -71.6087 742 1000 148 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:57 Ehrpd 42.28415 -71.6087 1373 842 150 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:56 Ehrpd 42.28415 -71.6087 1910 901 180 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:55 Lte 42.28415 -71.6087 11467 309 98 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:55 Lte 42.28415 -71.6087 35694 6542 96 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:54 Lte 42.28415 -71.6087 31827 7324 97 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:53 Lte 42.28415 -71.6087 21281 7423 90 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:53 Lte 42.28415 -71.6087 9455 6937 90 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:52 Lte 42.28415 -71.6087 18291 4633 94 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:39 Ehrpd 42.28415 -71.6087 2341 954 179 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:37 Lte 42.28415 -71.6087 14298 989 94 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:36 Lte 42.28415 -71.6087 41880 7882 92 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:36 Lte 42.28415 -71.6087 34324 7346 92 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:36 Lte 42.28415 -71.6087 42962 8904 90 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:35 Lte 42.28415 -71.6087 44814 7583 94 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:35 Lte 42.28415 -71.6087 22561 9205 100 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:35 Lte 42.28415 -71.6087 14173 3284 104 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:32 Ehrpd 42.28415 -71.6087 1593 830 192 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:29 Lte 42.28415 -71.6087 8507 262 92 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:29 Lte 42.28415 -71.6087 12333 1002 97 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:28 Lte 42.28415 -71.6087 34996 10387 88 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:28 Lte 42.28415 -71.6087 49833 14801 85 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:25 Lte 42.28415 -71.6087 29931 8027 90 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:25 Lte 42.28415 -71.6087 20394 8460 100 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 10/2/2011 10:25 Lte 42.28415 -71.6087 17250 5815 99 Boston, MA 10.133.86.195, 10.165.70.146 166.248.1.123 www.octoscope.com Geolocation recorded by speedtest.com is incorrect
- 7. 7 What’s eHRPD? • eHRPD is Verizon’s 3G; upgrade path to LTE CDMA based; enhanced HRPD (EVDO ) Maintains the same private IP when handset moves from tower to tower Reduces dropped sessions and decreases the handover latency • eHRPD will be used by Verizon for VOIP calls until 2020 eHRPD = enhanced high rate packet data EVDO = Evolution-Data Optimized www.octoscope.com
- 8. 8 3G Network Latency • HSPA+ is aimed at extending operators’ investment in HSPA 2x2 MIMO, 64 QAM in the downlink, 16 QAM in the uplink Data rates up to 42 MB in the downlink and 11.5 MB in the uplink. Traditional HSPA One tunnel HSPA One tunnel HSPA+ GGSN GGSN Gateway GGSN One-tunnel architecture GPRS flattens the network by Control Support enabling a direct Data Serving Node transport path for user SGSN GPRS SGSN SGSN data between RNC and Support the GGSN, thus Node Radio minimizing delays and RNC RNC Network Controller set-up time User RNC Data Node B Node B Node B www.octoscope.com
- 9. 9 LTE EPC (Evolved Packet Core) HSS SGSN GPRS Core Trusted MME EPS Access Gateway PCRF Serving gateway PDN gateway IP Services SGSN (Serving GPRS (IMS) Support Node) PCRF (policy and charging Trusted rules function) Non- HSS (Home Subscriber Wi-Fi 3GPP Server) MME (Mobility Management eNode-B Non- Trusted non-3GPP IP Access (CDMA, TD- Entity) Trusted SCDMA, WiMAX) PDN (Public Data Network) Flat, low-latency architecture www.octoscope.com
- 10. 10 OFDM and MIMO • OFDM transforms a frequency- and time- variable fading channel into parallel correlated flat-fading channels, enabling wide bandwidth operation … … Channel Quality Frequency Frequency-variable channel appears flat over the narrow band of an OFDM subcarrier. OFDM = orthogonal frequency division multiplexing MIMO = multiple input multiple output www.octoscope.com
- 11. 11 OFDMA OFDM is a modulation scheme Time Time OFDMA is a LTE modulation and access scheme Frequency Multiple Access Frequency allocation per Frequency per user is user is continuous vs. time dynamically allocated vs. time slots User 1 User 2 User 3 User 4 User 5 OFDM = orthogonal frequency division multiplexing OFDMA = orthogonal frequency division multiple access www.octoscope.com
- 12. 12 OFDMA vs. SC-FDMA (LTE Uplink) • Multi-carrier OFDM signal exhibits high PAPR (Peak to Average Power Ratio) due to in-phase addition of subcarriers. • Power Amplifiers (PAs) must accommodate occasional peaks and this results low efficiency of PAs, typically only 15-20% efficient. Low PA efficiency significantly shortens battery life. In-phase addition of sub-carriers • To minimize PAPR, LTE has adapted SC- creates peaks in FDMA (single carrier OFDM) in the the OFDM signal uplink. SC-FDMA exhibits 3-6 dB less PAPR than OFDMA. www.octoscope.com
- 13. 13 Multiple Antenna Techniques • SISO (Single Input Single Output) Traditional radio • MISO (Multiple Input Single Output) Transmit diversity (STBC, SFBC, CDD) • SIMO (Single Input Multiple Output) Receive diversity, MRC • MIMO (Multiple Input Multiple Output) SM to transmit multiple streams simultaneously; can be used in conjunction with CDD; works best in high SNR environments and channels de-correlated by multipath TX and RX diversity, used independently or together; used to enhance throughput in the presence of adverse channel conditions • Beamforming SM = spatial multiplexing SFBC = space frequency block coding STBC = space time block coding CDD = cyclic delay diversity MRC = maximal ratio combining SM = Spatial Multiplexing SNR = signal to noise ratio www.octoscope.com
- 14. 14 MIMO Based RX and TX Diversity • When 2 receivers are available in a MIMO radio MRC can be used to combine signals from two or more antennas, improving Peak SNR • MIMO also enables transmit diversity techniques, including CDD, STBC, SFBC • TX diversity spreads the signal creating artificial multipath to decorrelate signals Null from different transmitters so as to optimize signal reception MIMO = multiple input multiple output SIMO = single input multiple outputs SM = spatial multiplexing SFBC = space frequency block coding Delay is inside the TX STBC = space time block coding CDD = cyclic delay diversity MRC = maximal ratio combining SM = Spatial Multiplexing SNR = signal to noise ratio www.octoscope.com
- 15. 15 Distributed-Input-Distributed-Output (DIDO) Distributed Antenna System + Beamforming ? Recent white paper from Rearden Companies + Beamforming Distributed Antenna System www.octoscope.com
- 16. 16 LTE Scalable Channel Bandwidth Channel bandwidth in MHz Transmission bandwidth in RBs Center subcarrier (DC) not transmitted in DL Channel bw 1.4 3 5 10 15 20 MHz Transmission bw 1.08 2.7 4.5 9 13.5 18 # RBs per slot 6 15 25 50 75 100 www.octoscope.com
- 17. 17 FDD vs. TDD • FDD (frequency division duplex) Paired channels • TDD (time division duplex) TD-LTE Single frequency channel for uplink an downlink Is more flexible than FDD in its proportioning of uplink vs. downlink bandwidth utilization Can ease spectrum allocation issues DL UL DL UL www.octoscope.com
- 18. LTE Frequency Bands - FDD Source: 3GPP TS 36.104; V10.1.0 (2010-12) Band Uplink (UL) Downlink (DL) Regions 1 1920 -1980 MHz 2110 - 2170 MHz Europe, Asia 2 1850 -1910 MHz 1930 - 1990 MHz Americas, Asia 3 1710 -1785 MHz 1805 -1880 MHz Europe, Asia, Americas 4 1710 -1755 MHz 2110 - 2155 MHz Americas 5 824-849 MHz 869 - 894 MHz Americas 6 830 - 840 MHz 875 - 885 MHz Japan 7 2500 - 2570 MHz 2620 - 2690 MHz Europe, Asia 8 880 - 915 MHz 925 - 960 MHz Europe, Asia 9 1749.9 - 1784.9 MHz 1844.9 - 1879.9 MHz Japan 10 1710 -1770 MHz 2110 - 2170 MHz Americas 11 1427.9 - 1452.9 MHz 1475.9 - 1500.9 MHz Japan 12 698 - 716 MHz 728 - 746 MHz Americas 13 777 - 787 MHz 746 - 756 MHz Americas (Verizon) 14 788 - 798 MHz 758 - 768 MHz Americas (D-Block, public safety) 17 704 - 716 MHz 734 - 746 MHz Americas (AT&T) 18 815 – 830 MHz 860 – 875 MHz 19 830 – 845 MHz 875 – 890 MHz 20 832 – 862 MHz 791 – 821 MHz 21 1447.9 – 1462.9 MHz 1495.9 – 1510.9 MHz www.octoscope.com
- 19. 19 UHF Spectrum, Including CH 52-59, 692-746 MHz A B C D E A B C White Space Bands Band17 Band17 US (FCC) White Spaces Band12 Band12 54-72, 76-88, 174-216, 470-692 MHz Low 700 MHz band European (ECC) White Spaces (470-790 MHz) 0 100 200 300 400 500 600 700 800 900 MHz High 700 MHz band A B A B CH 60-69, 746-806 MHz www.octoscope.com ECC = Electronic Communications Committee
- 20. 20 High 700 MHz Band D-Block MHz 758 763 775 788 793 805 Band 13 Band 13 Band 14 Band 14 Guard band Guard band Public Safety Broadband (763-768, 793-798 MHz) Public Safety Narrowband (769-775, 799-805 MHz), local LMR LMR = land mobile radio www.octoscope.com
- 21. 21 TV Channels and White Space Allocation US – FCC Channel # Frequency Band *Channel 37 (608-614 MHz) is reserved for radio astronomy 2-4 54-72 MHz **Shared with public safety Fixed 5-6 76-88 MHz VHF TVBDs only 7-13 174-216 MHz Transition from NTSC to ATSC (analog to digital TV) in June 12, 14-20 470-512 MHz** 2009 freed up channels 52-69 (above 692 MHz) UHF White Spaces 21-51* 512-692 MHz http://www.fcc.gov/mb/engineering/usallochrt.pdf Europe – ECC Channel # Frequency Band 5-12 174-230 MHz VHF White Spaces 21-60 470-790 MHz UHF 61-69 790-862 MHz www.octoscope.com
- 22. 22 LTE Frequency Bands - TDD TD-LTE Band UL and DL Regions 33 1900 - 1920 MHz Europe, Asia (not Japan) 34 2010 - 2025 MHz Europe, Asia 35 1850 - 1910 MHz 36 1930 - 1990 MHz 37 1910 - 1930 MHz 38 2570 - 2620 MHz Europe 39 1880 - 1920 MHz China 40 2300 – 2400 MHz Europe, Asia 41 2496 – 2690 MHz Americas (Clearwire LTE) 42 3400 – 3600 MHz 43 3600 – 3800 MHz Source: 3GPP TS 36.104; V10.1.0 (2010-12) www.octoscope.com
- 23. 23 WiMAX Frequency Bands - TDD Band (GHz) Bandwidth Certification Group Code Class BW (MHZ) (BCG) 1 2.3-2.4 8.75 1.A 5 AND 10 1.B WiMAX Forum 2 2.305-2.320, 2.345-2.360 Mobile 3.5 2.A (Obsolete, replaced by 2.D) Certification Profile 5 2.B (Obsolete, replaced by 2.D) v1.1.0 10 2.C (Obsolete, replaced by 2.D) A universal 3.5 AND 5 AND 10 2.D frequency step 3 2.496-2.69 size of 250 KHz is 5 AND 10 3.A recommended for 4 3.3-3.4 all band classes, 5 4.A while 200 KHz 7 4.B step size is also 10 4.C recommended for 5 3.4-3.8 band class 3 in 5 5.A Europe. 7 5.B 10 5.C 7 0.698-0.862 5 AND 7 AND 10 7.A 8 MHz 7.F www.octoscope.com
- 24. 24 WiMAX Frequency Bands - FDD Band (GHz)BW (MHZ) Duplexing Mode Duplexing Mode MS Transmit Band (MHz) BS Transmit Band Bandwidth Class BS MS (MHz) Certification Group Code 2 2.305-2.320, 2.345-2.360 2x3.5 AND 2x5 AND 2x10 FDD HFDD 2345-2360 2305-2320 2.E** 5 UL, 10 DL FDD HFDD 2345-2360 2305-2320 2.F** 3 2.496-2.690 2x5 AND 2x10 FDD HFDD 2496-2572 2614-2690 3.B 5 3.4-3.8 2x5 AND 2x7 AND 2x10 FDD HFDD 3400-3500 3500-3600 5.D 6 1.710-2.170 FDD 2x5 AND 2x10 FDD HFDD 1710-1770 2110-2170 6.A 2x5 AND 2x10 AND FDD HFDD 1920-1980 2110-2170 6.B Optional 2x20 MHz 2x5 AND 2x10 MHz FDD HFDD 1710-1785 1805-1880 6.C 7 0.698-0.960 2x5 AND 2x10 FDD HFDD 776-787 746-757 7.B 2x5 FDD HFDD 788-793 AND 793-798 758-763 AND 763-768 7.C 2x10 FDD HFDD 788-798 758-768 7.D 5 AND 7 AND 10 (TDD), TDD or FDD Dual Mode TDD/H- 698-862 698-862 7.E* 2x5 AND 2x7 AND 2x10 (H-FDD) FDD 2x5 AND 2x10 MHz FDD HFDD 880-915 925-960 7.G 8 1.710-2.170 TDD 5 AND 10 TDD TDD 1785-1805, 1880-1920, 1785-1805, 1880-1920, 8.A 1910-1930, 2010-2025 1910-1930, 2010-2025 WiMAX Forum Mobile Certification Profile R1 5 v1.3.0 www.octoscope.com
- 25. 25 Summary • LTE is here Verizon and ATT • Beyond commercial markets LTE is also being embraced by Military and Public Safety markets Intelligent Transportation Systems Possibly Smart Grid • Carrier to carrier roaming remains to be seen www.octoscope.com
- 26. 26 For More Information • White papers, presentations, articles and test reports on a variety of wireless topics www.octoscope.com www.octoscope.com
- 27. 27 LTE Resource Allocation 180 kHz, 12 subcarriers with normal CP User 2 User 3 User 2 User 1 0.5 ms User 2 User 3 User 2 User 1 7 symbols with normal CP User 2 User 3 User 3 User 2 Time User 2 User 1 User 3 User 2 User 1 User 1 User 3 User 1 Resource Block (RB) Frequency • Resources are allocated per user in time and frequency. RB is the basic unit of allocation. • RB is 180 kHz by 0.5 ms; typically 12 subcarriers by 7 OFDM symbols, but the number of subcarriers and symbols can vary based on CP CP = cyclic prefix, explained ahead www.octoscope.com
- 28. 28 Resource Block A resource block (RB) is a basic unit of access allocation. RB bandwidth per slot (0.5 ms) is 12 subcarriers times 15 kHz/subcarrier equal to 180 kHz. 1 slot, 0.5 ms … Resource block 12 … subcarriers Subcarrier (frequency) … Resource Element 1 subcarrier 1 subcarrier QPSK: 2 bits 16 QAM: 4 bits v 64 QAM: 6 bits … Time www.octoscope.com
- 29. 29 SC-FDMA vs. OFDMA 15 kHz subcarrier Downlink – lower symbol rate Uplink – higher symbol rate, lower PAPR S1 S2 S3 S4 S5 S6 S7 S8 … 60 kHz Sequence of symbols Time Frequency www.octoscope.com
- 30. Intelligent Transportation Systems (ITS) • Emerging market • Embracing 802.11p and LTE with 802.11p sophisticated LTE software stacks on top ITS www.octoscope.com
- 31. 31 Voice over LTE Solutions • CSFB (3GPP 23.272) whereby voice calls are switched to 2G/3G CS networks • VoLGA whereby voice calls are encapsulated in data packets traversing LTE networks • Over-the-Top (OTT) voice, for example Skype operating over LTE networks • GSMA’s selected Voice over LTE (VoLTE) based on IMS CSFB = circuit switched fallback CS = circuit switch VoLGA = voice over LTE with Generic Access OTT = over-the-top VoLTE = voice over LTE IMS = IP multimedia subsystem www.octoscope.com
Notes de l'éditeur
- 3GPP has defined EPS in Release 8 as a framework for an evolution or migration of the3GPP system to a higher-data-rate, lower-latency packet-optimized system that supports multiple radio-access technologies. The focus of this work is on the packet switcheddomain, with the assumption that the system will support all services—including voice—in this domain. (EPS was previously called System ArchitectureEvolution.)Although it will most likely be deployed in conjunction with LTE, EPS could also be deployed for use with HSPA+, where it could provide a stepping-stone to LTE. EPS willbe optimized for all services to be delivered via IP in a manner that is as efficient as possible—through minimization of latency within the system, for example. It will supportservice continuity across heterogeneous networks, which will be important for LTE operators that must simultaneously support GSM/GPRS/EDGE/UMTS/HSPA customers.One important performance aspect of EPS is a flatter architecture. For packet flow, EPS includes two network elements, called Evolved Node B (eNodeB) and the AccessGateway (AGW). The eNodeB (base station) integrates the functions traditionally performed by the radio-network controller, which previously was a separate nodecontrolling multiple Node Bs. Meanwhile, the AGW integrates the functions traditionally performed by the SGSN. The AGW has both control functions, handled through theMobile Management Entity (MME), and user plane (data communications) functions. The user plane functions consist of two elements: a serving gateway that addresses 3GPPmobility and terminates eNodeB connections, and a Packet Data Network (PDN) gateway that addresses service requirements and also terminates access by non-3GPP networks.The MME, serving gateway, and PDN gateways can be collocated in the same physicalnode or distributed, based on vendor implementations and deployment scenarios.The EPS architecture is similar to the HSPA One-Tunnel Architecture, discussed in the“HSPA+” section, which allows for easy integration of HSPA networks to the EPS. EPSalso allows integration of non-3GPP networks such as WiMAX.EPS will use IMS as a component. It will also manage QoS across the whole system,which will be essential for enabling a rich set of multimedia-based services.The MME, serving gateway, and PDN gateways can be collocated in the same physicalnode or distributed, based on vendor implementations and deployment scenarios.The EPS architecture is similar to the HSPA One-Tunnel Architecture, discussed in the“HSPA+” section, which allows for easy integration of HSPA networks to the EPS. EPSalso allows integration of non-3GPP networks such as WiMAX.EPS will use IMS as a component. It will also manage QoS across the whole system,which will be essential for enabling a rich set of multimedia-based services.Elements of the EPS architecture include:- Support for legacy GERAN and UTRAN networks connected via SGSN.- Support for new radio-access networks such as LTE.- The Serving Gateway that terminates the interface toward the 3GPP radio-accessnetworks.- The PDN gateway that controls IP data services, does routing, allocates IPaddresses, enforces policy, and provides access for non-3GPP access networks.- The MME that supports user equipment context and identity as well asauthenticates and authorizes users.- The Policy Control and Charging Rules Function (PCRF) that manages QoSaspects.
- OFDM has proven to make the best use of the challenging wireless channel. The figure at the lower left shows that the quality of the wireless channel varies as a function of frequency and as a function of time. Even if I stand with my wireless device in one place, the signal at its receiver will fluctuate. The nulls in the signal are due to multipath and doppler fading. The wider the channel, the more difficult it is to equalize the received signal. OFDM takes a divide and conquer approach.OFDM transforms the frequency- and time-variable fading channel into multiple parallel correlated flat-fading channels. The narrow channels of each OFDM subcarrier exhibit small variations, making equalization simple. Thus, the OFDM channel can be arbitrarily wide. When OFDM is combined with multiple antenna techniques that we will discuss later, we can very effectively combat the time and frequency variability of the channel.
- LTE uses a variety of multiple antenna techniques. Sometimes we loosely refer to these as MIMO (Multiple Input Multiple Output). MIMO enables spatial multiplexing whereby multiple streams of data (called layers in LTE) are transmitted in the same channel simultaneously. Spatial Multiplexing is only possible in a decorrelated channel and with multiple transmitters and receivers.In addition to Spatial Multiplexing, Multiple antenna techniques include transmit and receive diversity in MISO, SIMO and MIMO configurations. Spatial Multiplexing typically requires high signal to noise ratio (SNR) conditions. In the presence of low SNR or excessive doppler, multiple transmitters can be used for transmit diversity such as Cyclic Delay Diversity CDD and multiple receivers can be used for receive diversity techniques such ash MRC maximal ratio combining. Both transmit and receive diversity can be used simultaneously, further improving the robustness of the channel. While spatial multiplexing of 2 layers has the potential of doubling the data rate, diversity techniques use multiple radios for redundant transmission of a single stream and hence have lower theoretical throughout. LTE MIMO radios can dynamically select Spatial Multiplexing in channel conditions that are suitable for this and then switch to transmit and receive diversity when channel conditions deteriorate.
- A MIMO device with multiple radios can implement transmit diversity in addition to receive diversity. Receive diversity on a MIMO device can also more sophisticated than on a single-radio device because the complete packet and not just preamble can be received by multiple receivers and then the receive source can be selected based on signal quality or by combining multiple received signals. This technique is known as maximal ratio combining (MRC).One can think of receive diversity as analogous to having two ears and transmit diversity as analogous to having two mouths. Transmit diversity techniques aim to produce multiple versions of the same signal and they are specifically designed to carefully control the relationship of these multiple versions of the signal so as to optimize signal reception.Transmit and receive diversity techniques can be used independently or together.When channel conditions allow, MIMO radios can also use spatial multiplexing whereby multiple radios are used to transmit more than one simultaneous data stream thereby multiplying the capacity of the airlink.
- By having control over which subcarriers are assigned in which sectors, LTE can control frequency reuse. By using all the subcarriers in each sector, the system wouldoperate at a frequency reuse of 1; but by using a different one third of the subcarriers in each sector, the system achieves a looser frequency reuse of 1/3. The looser frequency reduces overall spectral efficiency but delivers high peak rates to users.
- Most WCDMA and HSDPA deployments are based on FDD, where the operator uses different radio bands for transmit and receive. An alternate approach is TDD, in whichboth transmit and receive functions alternate in time on the same radio channel.Many data applications are asymmetric, with the downlink consuming more bandwidth than the uplink, especially for applications like Web browsing or multimedia downloads. A TDD radio interface can dynamically adjust the downlink-to-uplink ratio accordingly, hence balancing both forward-link and reverse-link capacity.TDD systems require network synchronization and careful coordination between operators or guard bands.
- This table shows the FDD bands that are allocated in different regions of the world. The regions are shown in right column. FDD spectrum is paired spectrum, so for each channel we have the uplink band and the downlink band. The FDD frequency range spans from around 700 MHz to just under 2700 MHz.
- The TDD bands are generally higher in frequency than the FDD channels. One reason for this is that TDD bands are more recent allocations. FDD bands have also been allocated for use by 3G. The TDD frequency range is from 1850 to 2620 MHz.
- The multiple-access aspect of OFDMA comes from being able to assign different usersdifferent subcarriers over time. A minimum resource block that the system can assign toa user transmission consists of 12 subcarriers over 14 symbols (approx 1.0 msec.)