Multipath comparison of IEEE802.11g High Rate Proposals May 2001 Multipath comparison of IEEE802.11g High Rate Proposals Sean Coffey, Anuj Batra, Srikanth Gummadi, Chris Heegard, Matthew Shoemake Texas Instruments 141 Stony Circle, Suite 130 Santa Rosa California 95401 (707) 521-3060, coffey@ti.com Coffey et al, Texas Instruments

Contents CCK-OFDM is not 802.11a Receiver structures May 2001 Contents CCK-OFDM is not 802.11a Receiver structures Multipath performance comparisons Conclusions Coffey et al, Texas Instruments

What is wrong with CCK-OFDM that is right with pure 11a OFDM? May 2001 What is wrong with CCK-OFDM that is right with pure 11a OFDM? Coffey et al, Texas Instruments

Overhead and Data Payloads May 2001 Overhead and Data Payloads PBCC 11a CCK-OFDM No acks, 500 byte packets Coffey et al, Texas Instruments

May 2001 Relative throughputs Coffey et al, Texas Instruments

May 2001 The CCK-OFDM Dilemma Any receiver requires overhead for channel estimation, tracking, etc: 802.11a is “pay as you go” - ultra-short preamble, 16 musecs 802.11b and PBCC-22 are “pay all up front” - 11b “short” preamble, 96 musecs CCK-OFDM is “pay up front and again as you go” - 11b “short” preamble, plus (non-standard) OFDM preamble, 110 musecs “Double the pain” Coffey et al, Texas Instruments

Packet size & system performance May 2001 Packet size & system performance Compare performances – results are critically dependent on packet size Short packets (e.g., MPEG-4 packets of 188 bytes) strongly favor “pay-as-you-go” approach  802.11a/Hiperlan 2 Long packets increasingly favor “pay-all-up-front” approach”  PBCC-22 aimed at this application Short or long, it won’t work well if it’s CCK-OFDM Coffey et al, Texas Instruments

May 2001 PBCC-22 features Excellent performance in full range of multipath conditions - much better than CCK-OFDM at comparable rates This is the central technical point in dispute Range advantage of PBCC 22 Mbps over CCK-OFDM 24 Mbps in multipath conditions is 30-40%. These claims documented later; standard IEEE models were used. Coffey et al, Texas Instruments

Multipath comparison of the proposals May 2001 Multipath comparison of the proposals First, for each proposal, assume same ground rules: floating point implementation full channel knowledge standard IEEE multipath model off-the-shelf algorithms assume each uses receiver structure presented by proposers Coffey et al, Texas Instruments

May 2001 PBCC-22 Receiver: treat multipath and code as forming a composite state machine, or “super code” decode the “super trellis” using any standard reduced state algorithm Simulation results here assume whitened matched filter plus M-algorithm; standard material, very well understood: whitened matched filter - Forney, 1972. M-algorithm - Anderson, 1969. Coffey et al, Texas Instruments

M-algorithm decoder background: May 2001 M-algorithm decoder background: M-algorithm operates like regular trellis decoder, but retains only best “M” paths at each depth No restrictions on choice of M straightforward way of trading performance versus complexity natural receiver upgrade path Main results use M = 64 we also present M = 8, M = 16, M = 32, M = 128. Coffey et al, Texas Instruments

May 2001 M-algorithm decoder: Assume the “state” consists of input data bits at last 8 time units Compare last 4 time units to represent pure code state Choice of 8 is arbitrary, other values possible Assume each “state” remembers the full impact of the past on the future Curve shown in Doc. 01/140 assumes instead that multipath is regenerated from last 8 time unit inputs Coffey et al, Texas Instruments

Baseline comparisons, IEEE multipath model, 100 ns May 2001 Baseline comparisons, IEEE multipath model, 100 ns From Doc. 00/392r1 5 dB Ideal channel knowledge, floating point implementations Coffey et al, Texas Instruments

Baseline comparisons, 100 ns, contd. May 2001 Baseline comparisons, 100 ns, contd. From Doc. 00/392r1 Coffey et al, Texas Instruments

Implications of 5 dB advantage: May 2001 Implications of 5 dB advantage: 5 dB translates to a factor of 3.1 For similar throughput and range, PBCC requires 3 times less received power than CCK-OFDM 24 Mbps – translates to greater battery life For similar throughput and received power, PBCC has 40% more range than CCK-OFDM 24 Mbps Assuming the “power of 3.3” model for path loss – this is the standard model used in 802.15.2 (Doc. 802.15/138r0) Coffey et al, Texas Instruments

100ns: 40% PBCC range advantage May 2001 100ns: 40% PBCC range advantage PBCC 22 Mbps CCK-OFDM 24 Mbps Double the coverage Coffey et al, Texas Instruments

Relative throughputs: May 2001 Relative throughputs: Coffey et al, Texas Instruments

Actual PBCC receiver algorithms, 100 ns May 2001 Actual PBCC receiver algorithms, 100 ns 2.9 dB Ideal channel knowledge, floating point implementation for CCK-OFDM Coffey et al, Texas Instruments

Baseline comparisons, contd: 250 ns May 2001 Baseline comparisons, contd: 250 ns 4.5 dB Ideal channel knowledge, floating point implementations Coffey et al, Texas Instruments

Baseline comparisons, 250 ns, contd. May 2001 Baseline comparisons, 250 ns, contd. Coffey et al, Texas Instruments

May 2001 Conclusions PBCC-22 has a natural superiority over CCK-OFDM in multipath. Established IEEE multipath model used. CCK-OFDM tries to juggle two incompatible things – makes an underperforming system out of a merger of two otherwise good components A better way of doing things – PBCC-22 + .11a! Coffey et al, Texas Instruments