Main Ways to Enhance Throughput

Transcription

1 802.11n Sanna Puha

2 Contents 1. Introduction 2. Ways to Enhance Throughput 3. MIMO operation 4. Structure of Operating Channel 5. MIMO Transmission Modes 6. Modulation Rates 7. Physical Transmission, PLCP: WWiSE 8. Physical Transmission, PLCP: TGnSync 9. Physical Transmission, PMD 10. Conclusion 11. Sources

3 Introduction IEEE Task Group N (TGn) begun work on January 2004 and the work should be completed by the end of 2007 TGn s goal to achieve 100 Mbps net throughput Two competing proposals for standard: WWiSE (World-Wide Spectrum Efficiency) and TGnSync

4 Main Ways to Enhance Throughput The enhanced throughput is achieved by 3 methods Using Multiple-Input/Multiple-Output (MIMO) operation Taking a wider channel into use a uses 20 MHz and doubling bandwidth to 40 MHz theoretically doubles the throughput. MAC efficiency methods frame bursting frame aggregation MAC header compression

5 MIMO Operation Most of the devices have used only one antenna so far MIMO operation means that there are multiple antennas and each of these antennas have one RF chain attached to it. Each RF chain can receive and transmit at the same time. Thus better throughout is achieved Each RF chain and antenna corresponding to it are responsible for transmitting a one spatial stream It is possible to multiplex a single frame across multiple spatial streams by breaking it up and reassembling it again at the receiver. Thus it is possible to have more antennas than spatial streams.

6 Structure of Operating Channel WWiSE Channel divided into MHz subcarriers 20 MHZ divided into 56 and 40 MHz into 112 subcarriers (4 pilot carriers) 54 data subcarriers in 20 MHz mode and 108 in 40 MHZ 40 MHZ supported only in 5 GHz band TGnSync Channel divided into MHz subcarriers 20 MHz identical to a channel structure: 52 subcarriers with 4 pilot carries 40 MHZ channel divided into 128 subcarriers, 6 pilots. This makes 40 MHz mode 2.25 faster than 20 MHz Figure 1. Channel Structure of WWiSE (Lörincz, Josip & Begušić, Dinko, 2006). Figure 2. Channel Structure of TGn Sync (Lörincz, Josip & Begušić, Dinko, 2006).

7 MIMO Transmission Modes WWiSE 14 transmission modes depending on Number of transmit antennas (from 1 to 4) Frame used either in greenfield or mixed mode Channel bandwidth (20 or 40 MHz) 20 MHx Channels 40 MHx Channels GreenField 2TX20GF 1TX40GF 3TX20GF 4TX20GF 2TX40GF 3TX40GF 4TX40GF Mixed Mode 2TX20MM 1TX40MM 3TX20MM 2TX40MM TGnSync 3 modes Basic MIMO mode: number of spatial streams is equal to number of antennas (mandatory) Basic MIMO with beamforming: every channel coded in the same way, based on information from sounding exchange the power and coding for each spatial stream is selected (optional) Advanced Beamforming MIMO: works in the same way as basic beamforming, but different transmission power can be used on each transmit stream and each spatial stream can be coded and modulated in different way (optional) 4TX20MM 3TX40MM Table 1. Transmission modes of WWiSE. (Gast, 2005) 4TX40MM

8 Modulation Rates WWiSE Data rate = x channel bandwidth x number of spatial streams x coded bits per subcarrier x code rate Channel Bandwidth: 20 or 40 MHz Number of spatial streams: 1-4 Coded bits by subcarrier: 6 (64-QAM) or 1 (16-QAM) Code rate: ½ or ¾ with 16-QAM, 2/3, ¾ or 5/6 with 64-QAM Maximum throughput 540 Mbps TGnSync Date rate = 12 x channel bandwidth factor x number of spatial streams x coded bits per subcarrier x code rate x guard interval factor Channel bandwidth: 1 for 20 MHz or 2.25 for 40 MHz Number of spatial streams: 1-4 Coded bits by subcarrier: 6 for 16-QAM, 4 for 64-QAM, 2 for QPSK and 1 for BPSK Code rate: ½ with BPSK; ½ or ¾ with QPSK and 16-QAM; 2/3, ¾, 7/8 with 64- QAM Guard interval factor: basic interval 800ns assigned factor 1, 400ns assigned factor 1.11 Maximum throughput 630 Mbps

9 Physical Transmission: PLCP, WWiSE In Greenfield mode backwards compatible physical headers are omitted PLCP Preamble SIGNAL-N DATA Reserved CONFIG LENGHT CRC LPI SIG-N TAIL FIELD MIMO- OFDM PLCP Preamble SIGNAL-N CONTENT Bit sequences to lock the receiver on to the signal. Same preamble transmitted on all the antenna but with small time shift Information to decode the data stream Config: Number of spatial streams Channel bandwidth modulation coding CRC Length: number of bytes in payload of physical frame LPI (Last PSDU Indicator): when multiple frames sent in bursts this indicates burst coming to an end Service Identical to a Data Sequence of 4 ms of symbols that carry data

10 Physical Transmission: PLCP, TGnSync L-STF L-LTF L- HT-Signal HT-STF Signal HT-LTF #1 HT-LTF #N DATA HT Length MCS Adv. Coding Snd. # frm LTF Sh. GI Agg. Scram. Init. 20/ 40 CRC Tail FIELD L-STF, L-LTF, L-Signal HT- Signal High Throughput Training Fields Data and Tail CONTENT Legacy Short Training Field, Legacy Long Training Field and Legacy Signal respectively. Identical to a Used to detect whether frame is carrying TGnSync encoded data or is it just a legacy data frame HT-length: number of bytes in PLCP payload frame MCS, Modulation and Coding Set: selects modulation, coding scheme and number of spatial streams Advanced Coding: whether optional coding is used or not Sounding Packet: when set, every antenna is transmitting own spatial stream. If not set, the frame is used to measure channel information Number of HT-LTFs: number of long training fields following the HT-Signal field Short Guard interval: When set, short guard interval is used Aggregation: If set, the PLCP frame carries several MAC frames Scrambler Initialization: used to seed the scrambler 20/40 BW: if set to 1, 40 MHZ channel used, 0 indicates 20 MHz channel CRC: calculated over legacy signal field and HT-signal field Tail: six bits needed to ramp down the convolutional coder Long and short training fields. A single training field over whole operating channel. One long training field used for each spatial stream Data bits encoded according the modulation and coding methods defined by HT-header

11 Physical Transmission: PMD WWiSE Transmitter is essentially the same as a but it has multiple transmit chains. Interleaver divides coded bits among different transmit chains and spatial streams TGnSync Incoming scrambled frame is taken to forward-error correction coder Spatial parser divides the unified bit stream into subsidiary stream for transmission Each spatial stream punctured up to desired rate and then mapped to ODFM by interleaver In advanced MIMO mode spatial steering matrix assigns symbols to any transmit chain Figure 5. WWiSE PMD (Source: Gast, 2005). Figure 4. TGnSync PMD (Source: Gast, 2005).

12 Conclusions Final standard seems to go well beyond the target of 100 Mbps Spectral usage seems to be a point of contention with both proposals although WWiSE is more concerned with this aspect and relies more on improving MAC efficiency than date rates Both of the proposals make use of MIMO and OFDM technology in several modes and configurations Both proposals are backwards compatible with legacy systems in the same frequency bands Both require support for 2x2 mode, so this is supposed to be mandatory for the standard TGnSync s peak rates are higher but they use more aggressive coding

13 Sources Gast, Matthew S.; Wireless Networks, The Definitive Guide; 2 nd Ed; O Reilly, 2005 Lörincz, Josip & Begušić, Dinko; Physical Layer Analysis of Emerging IEEE n WLAN Standard; Advanced Communication Technology 2006 (ICACT 2006), The 8 th International Conference; Volume 1, pp 20-22; February 2006