Record and Playback PHY Abstraction for 802.11n MAC Simulations March 15 2004 Record and Playback PHY Abstraction for 802.11n MAC Simulations Hemanth Sampath Erik Lindskog Ravi Narasimhan Atul Salhotra hsampath@marvell.com eriklindskog@yahoo.com ravin@marvell.com atuls@marvell.com H. Sampath,E. Lindskog, R. Narasimhan, and A. Salhotra, Marvell

Record & Playback PHY Abstraction Scheme March 15 2004 Record & Playback PHY Abstraction Scheme PHY Record: Generate 802.11n channel sequence of N samples, with sampling time dT Pass channel sequence through PHY simulator including rate adaptation [Black Box Methodology IEEE 802.11-04/01 72r0] Generate a PHY record with sequence of chosen rates and corresponding PASS/FAIL decisions. MAC Playback: The MAC replays PHY record for each user H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Simulation Diagram March 15 2004 H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Record & Playback Features March 15 2004 Record & Playback Features PHY simulations do not scale with number of users Include rate adaptation (& power control) as part of PHY [IEEE 802.11-04/01 72r0]. Good modeling of 11n channel characteristics & variations. Accurate modeling of PHY proposals with all impairments. Easy interface to merge different PHY and MAC proposals ! No mapping approximations between PER, BER, rate, capacity etc. H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Differences with capacity based PHY abstraction March 15 2004 Differences with capacity based PHY abstraction Issue#1: The capacity to PER/rate mapping not unique. Example #1: Flat-fading and frequency selective fading channels can have same average capacity but different rates and PERs! Example #2: Problem exacerbated for MIMO channels. A full rank channel with low SNR can have same capacity as low rank channel with high SNR, but different rates and PERs! Simulations: Initial SISO simulations show high variability in capacity  PER mapping. Same capacity can yield different PERs for different channel realizations. [see Appendix] Issue #2: Capacity mapping does not allow MAC based rate adaptation schemes. Assumes only PHY based rate adaptation. Record & Replay method circumvents above issues H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Channel Sequence Simulations Results: March 15 2004 Channel Sequence Simulations Results: 25 channel coherence times is sufficient to capture richness of channel. Possible values of N and dT for ~ 25 coherence times (assuming ~ 160msec coherence time from 11n channel models at 5 GHz) ~ 4msec sampling with 1000 channels is sufficient for correctly predicting PHY performance with (PHY based rate adaptation.) Smaller N desirable for smaller PHY records N dT (msec) 250 16 500 8 1000 4 4000 1 H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Performance Validation with N=1000 channel realizations March 15 2004 Performance Validation with N=1000 channel realizations PHY performance accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Capacity Calculation with N=1000 channels March 15 2004 Capacity Calculation with N=1000 channels Capacity distribution accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Example PHY Record (1000 byte pkt) March 15 2004 Example PHY Record (1000 byte pkt) Avg SNR = 3 dB Avg SNR = 27 dB Avg SNR = 30 dB time R1 P/F T 6 t+dT 12 t+2dT 1 t+(P1)dt R1 P/F 54 1 48 R1 P/F 72 1 54 ……. Avg SNR can vary with path-loss and shadowing. [Example: 0:3:30] dB Records are computed for different packet sizes in usage model. H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

MAC Simulations t = Inter-packet spacing March 15 2004 MAC Simulations For each user, playback sequence of recommended rates and associated packet pass or fail events t = Inter-packet spacing For time < dT, MAC packets are transmitted with identical rate and pass/fail decision (as specified by the PHY record) H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Potential Issue & Improvements March 15 2004 Potential Issue & Improvements Interpolation: Multiple packets are sent using the same pass/fail and rate, leading to increased throughput variability Define: Inter-packet spacing = t (< dT) Number of packets (M) transmitted per dT time = dT / t Worse case scenario: For dT ~ 4 msec and worse case t ~ 200 micro sec, M = 20 packets may be sent with identical rate & pass-fail decisions. Issue addressed: Decrease (dT) sampling time for worse case scenarios. Modify PHY record to include more than one rate for each channel realization. H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

MAC Throughput Validation for dT = 4msec, t = 0.2msec March 15 2004 MAC Throughput Validation for dT = 4msec, t = 0.2msec MAC throughput for (dT = 4msec, t = 0.2 msec) is similar to ideal MAC simulation with dT = t = 0.2 msec (if PHY rate adaptation is accurate). H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Potential Issue & Improvements March 15 2004 Potential Issue & Improvements Issue: What about MAC based rate adaptation ? Issue addressed: The PHY record will have pass/fail decisions for recommended rate and alternative rates, for each channel realization. Enables MAC based rate adaptation. Algorithm for simulating alternate rates: If recommended rate FAILS, simulate lower rates until packet passes for the current channel realization. If recommended rate PASSES, simulate higher rates until packet fails for the current channel realization. H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Example PHY Record with Alternate Rates March 15 2004 Example PHY Record with Alternate Rates Avg SNR = 30 dB R1 P/F R2 R3 … 54 1 48 36 72 .. …. Only a few rates need to be simulated around the recommended rate regardless of total number of rates. (Record size does not increase drastically!) MAC based rate adaptation algorithms and feedback delays can be modeled MAC rate adaptation will further reduce throughput variation (jitter). H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Example MAC Simulation March 15 2004 Example MAC Simulation Example MAC adaptation: Increase rate if 2 consecutive packets pass. Decrease rate if 2 consecutive packets fail. H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Estimated Simulation Time for Generating Records March 15 2004 Estimated Simulation Time for Generating Records In Marvell MATLAB PHY simulator: Simulating a 1000 byte packet transmission on a 2 GHz processor takes 2.5 seconds on average. Record with N=1000 entries and 10 Avg. SNR indices (0, 3, 6,..,30 dB) with 1 rate per time instant would take 1000 entries x 2.5 seconds/entry x 10 SNR ~= 7 hours. Comparison: This time is similar to a typical PHY simulation that generates one PER vs SNR plot (assuming 1000 channels per SNR and 10 SNR points). Only 1 PHY record generated per channel model & packet-size. Simulation time does not scale with the number of users H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Conclusions Record-Playback methodology has several advantages: March 15 2004 Conclusions Record-Playback methodology has several advantages: Includes full 802.11n channel models. Complete modeling of PHY with impairments. Includes rate adaptation in PHY and MAC. Easy to merge different PHY and MAC proposals! No mapping approximations between BER, PER, capacity and rate! H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

March 15 2004 References 11-03/0863 Packet Error Probability Prediction 802.11 MAC Simulation (Intel) 11-04/0064 Time Correlated Packet Errors in MAC Simulations (STm) 11-04/0120 PHY Abstraction to be Used in MAC Simulation (Mitsubishi) 11-04/0172 Black Box PHY Abstraction Methodology (Atheros / Mitsubishi) 11-04/0182 Record and Playback PHY Abstraction 802.11n MAC Simulations Using Soft PER Estimates (Marvell) 11-04/0183r1 Record and Playback PHY Abstraction 802.11n MAC Simulations using Binary PER Estimates (Marvell) 11-04/0184 Proposal PHY Abstraction In MAC Simulators (STm) H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

March 15 2004 Appendix H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Capacity Calculation with N=1000 channels March 15 2004 Capacity Calculation with N=1000 channels Capacity distribution accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Capacity Calculation with N=1000 channels March 15 2004 Capacity Calculation with N=1000 channels Capacity distribution accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Capacity Calculation with N=1000 channels March 15 2004 Capacity Calculation with N=1000 channels Capacity distribution accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Performance Validation with N=1000 channel realizations March 15 2004 Performance Validation with N=1000 channel realizations PHY performance accurately modeled with N=1000 & dT = 4msec H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell

Variability in capacity  PER mapping March 15 2004 Variability in capacity  PER mapping Note: Plot generated by ST-Microelectronics H. Sampath, E. Lindskog, R.Narasimhan, A. Salhotra, Marvell