Evaluating the Performance of IEEE802.11 HRb Proposals in the ...
September 2000 doc.: IEEE 802.11-00/282 Evaluating the Performance of HRb Proposals in the Presence of Multipath Steve Halford, Ph.D., Karen Halford, Ph.D. and Mark Webster Intersil Corporation September, 2000 Submissio n Slide 1 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Goals Multipath is recognized as major WLAN impairment To select best waveform, must include multipath performance Multipath Model was left as TBD by teleconference Want a model close to 802.11b model Want a model well-defined Compare proposal against the same measure Cross-validate multipath performance numbers Want a model that is fair to all proposals Want a model that reflects real radio conditions as much as possible Submissio n Slide 2 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282
Overview Multipath Models for WLAN Motivation Exponential Channel Model (IEEE 802.11b model) Truncation to FIR model Sample Rate Normalization Rayleigh Fading Model AWGN with multipath Use of Channel Model : Suggested Test Bed Summary of Proposal Sample Code Submissio n Slide 3 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Multipath in WLAN reflector receiver transmitter transmitted signal Channel Model reflector received signal Submissio n Slide 4
time S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Exponential Model Used by Task Group b for 802.11 (see docs 97/96, 97/125, 97/157r1) Average Power Profile decays exponentially 1 2 1 2 hk N (0, k ) j N (0, k ) 2 2 for k 0,1,..., k max 1 2 kTs / rms o 2 k e where o2 1 e Ts / rms kmax 10 rms /Ts Truncate to represent with FIR Average gain of channel is 0 dB Submissio n Slide 5 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282
Exponential Channel Model Average Power Profile Sample Realization 0.7 0.4 0.6 0.35 0.5 0.3 0.4 0.25 0.2 0.3 0.15 0.2 0.1 0.1 0 0.05 Ts 2Ts 3Ts 4Ts 5Ts 6Ts 7Ts 8Ts 9Ts 10Ts 11Ts time Submissio n Slide 6 0 Ts 2Ts 3Ts 4Ts 5Ts 6Ts 7Ts 8Ts 9Ts 10Ts 11Ts time S. Halford, K. Halford, and M. Webster
September 2000 doc.: IEEE 802.11-00/282 Tap Truncation Truncate to represent with an FIR model. Value of last tap in truncated exponential channel: e kmax Ts / rms e 10 4.5 10 5 Exponential channel is monotonically decreasing Therefore, remaining unmodeled taps e 10 Unmodeled taps are insignificant. Submissio n Slide 7 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Sample Rate and Exp Model Sample rate determines resolution of taps 0.7 1 11 MHz Example 0.9
0 0 50 100 150 200 nsec 250 300 350 50 45 Only a problem at low sample rate and low multipath delay. Actual Delay Spread 40 35 Theoretical Delay Spread 30 Sample Rate = 11 MHz 25 Sample Rate = 22 MHz 20 Sample Rate = 44 MHz 15 Sample Rate = 88 MHz 10 5
0 0 5 10 15 20 25 30 35 40 45 50 Theoretical Delay Spread Submissio n Slide 8 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Normalization Channel model is normalized in an expected value sense 1 1 Recall: hk N 0, k2 j N 0, k2 2 2 1 2 E hk k2 02 e kTs / RMS 02 1 e Ts / RMS k
k k Select 02 1 e Ts / RMS to make average power = 1 This is not same as normalizing each realization Power Gain of Channel (sum of taps squared) 2.4 Power varies on a per trial basis Average gain is one 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 Submissio n Slide 9 0 500 1000 1500 2000 2500 3000 Trial Number 3500 4000 4500 5000 S. Halford, K. Halford, and M. Webster
September 2000 doc.: IEEE 802.11-00/282 Normalization Discussion Does HRb want to normalize per realization? e.g., Force each channel realization K max 2 hk 1 k 0 Problem: Not realistic --- multipath causes relative power loss/gain Problem: May not get consistent results across channel sample rates Normalization would apply to entire bandwidthnot signal bandwidth 3 Example 2.5 Sample Rate = 88 MHz Normalized power of each realization Power Gain for 22 MHz signal Shows the power variation Penalizes samples rates >> bandwidth 2 1.5 1 0.5 0 Submissio n Slide 10 0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Rayleigh Fading Classic model for multipath components with delay much less than sample rate Amplitude has a Rayleigh Distribution with uniform random phase Memoryless -- affects all signal frequencies the same (flat fade) For convenience, can consider to be a limiting case of exponential channel Single tap channel with 0 RMS delay spread fix kmax equal to one Single tap will scale and rotate the received signal affect all frequencies in the same way since it is a multiplication not a convolution hRayleigh 1 2 1 2 h0 N (0, 0 ) j N (0, 0 ) 2 2
lim RMS 0 02 lim RMS 0 1 e Ts / RMS 1 02 1 Submissio n Slide 11 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Multipath with AWGN Q: Does HRb include additive noise in multipath comparisons? Additive noise can have a major impact on multipath performance Example: Zero-forcing & MMSE equalizer have same performance w/o noise Performance can be vastly different in presence of noise Realistic to include both impairments simultaneously Sweep Packet Error Rates across a range of noise & multipath values A: Yes. Submissio n Slide 12 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Use of Channel Model: Suggested Test Bed Multipath results should include Eb/N0 Setting Desired Eb Channel Sample rate Delay spread Packet Length (1000 bytes) Packet Error Rate
N0 Calculate Noise Power (N0) Generate Noise Measure energy per bit Measure Packet Error Rate Transmitter Model Exponential Channel Model Packet Length Data Rate Sample Rate Delay Spread Submissio n Receiver Model Slide 13 Packet Error Rate S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Summary of Proposal Propose using the current exponential channel model Identical to IEEE802.11b
Truncate using k 10 /T max rms s Sample rate should be given (not specified by CFP) No additional normalization Rayleigh fading included as special case of exp model Showed suggested test bed Recommend using PER with 1000 byte packets Include noise with multipath Vary levels of both noise and multipath Cross-Verification: Include description of equalizer type (not required to give design details) Submissio n Slide 14 S. Halford, K. Halford, and M. Webster September 2000 doc.: IEEE 802.11-00/282 Matlab Code for Exponential Channel %**************************************************************** % function [taps] = ExpChanTaps(sampRateMHz, delaySprdNsec) % % Return the FIR channel taps for the exponential channel model % for indoor multipath. % % INPUT PARAMETERS: % sampRateMHz = sampling rate in MHz % (reciprocal of tap spacing in usec) % delaySprdNsec = delay spread in nsec (0 generates Rayleigh % % OUTPUT VALUE: % taps = complex channel taps for the exponential % channel model % ***************************************************************
% % % % % % % % % % % % % % % function [taps] = ExpChanTaps(sampRateMHz, delaySprdNsec) sampTimeNsec = 1000 / sampRateMHz; if delaySprdNsec == 0 Kmax = 0; vark = 1; else Kmax = ceil( 10 * delaySprdNsec/sampTimeNsec ); var0 = 1 - exp( - sampTimeNsec/delaySprdNsec ); k = (0:Kmax)'; vark = var0 * exp( - k*sampTimeNsec/delaySprdNsec ); end stdDevReOrIm = sqrt(vark/2); taps = stdDevReOrIm .* (randn(Kmax+1,1) + j*randn(Kmax+1,1)); Submissio n Slide 15 S. Halford, K. Halford, and M. Webster
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