1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: FPP-SUN Bad Urban GFSK vs OFDM
Date Submitted: July 30, 2009
Source: Rishi Mohindra, MAXIM Integrated Products
Contact: Rishi Mohindra, MAXIM Integrated Products
Voice: ,
Re: TG4g Call for proposals
Abstract: PHY proposal towards TG4g
Purpose: PHY proposal for the TG4g PHY amendment
Notice: This document has been prepared to assist the IEEE P It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P
Slide 1
Rishi Mohindra, MAXIM

2. FPP-SUN Bad Urban GFSK vs OFDM
Rishi Mohindra

3. Contents Bad Urban Channel Model GFSK simulation set up
OFDM simulation details
PER simulation results
Channel Statistics
PER with FHSS for GFSK
Conclusions
Appendix
Channel models in Matlab and ADS

4. Bad Urban Channel Model
Channel Type
Tap Number
Tap Relative Delay (us)
Average relative power (dB)
Bad Urban 1 2 5 -3
Obstacle (Building etc) Tx Rx
Transmitted signal
Received signal
Slide 4

5. GFSK Simulation set up Modulation etc Data Rates Channels:
100 kb/s
500 kb/s
Modulation etc
GFSK, h = BT = 0.5
Receiver filter Noise Bandwidth = 1.5x Data Rate = approximate signal bandwidth
Channels:
AWGN
Bad Urban
Payload Packet Length for PER measurement
100 bytes (appended after 24-bit preamble)
1000 bytes (appended after 24-bit preamble)
Up to 8000 packets simulated, each with a random channel snap shot to keep PER variance small
Synchronization for Integrate & Dump & data sampling
Cross correlation with 24-bit preamble pattern for maximum peak, for every channel snap shot. Used 8 samples per bit after post-demodulator down sampling.
Demodulation & Data estimation
Non-coherent FM demodulation followed by down sampled synchronized Integrate & Dump over each bit period.
Slide 5

6. Data Rates used for Bad Urban OFDM simulations = 188 kb/s and 750 kb/s
OFDM Simulation set up
OFDM Option 1
OFDM Option 2
OFDM Option 3
OFDM Option 4
OFDM Option 5
Unit
FFT size
128 64 32 16 8
Active Tones
108 52 26 14 6
# Pilots tones 4 2
# Data Tones
100 48 22 12
Approximate Signal BW
1064
518
264
146 68
kHz
BPSK 1/2 rate coded and 4x repetition 98
kbps
BPSK 1/2 rate coded and 2x repetition
195 94 43
BPSK 1/2 rate coded
391
188 86 47
BPSK 3/4 rate coded
586
281
129 70
QPSK 1/2 rate coded
781
375
172
QPSK 3/4 rate coded
562
258
141
16-QAM 1/2 rate coded
750
344
187 62
16-QAM 3/4 rate coded
516
Data Rates used for Bad Urban OFDM simulations = 188 kb/s and 750 kb/s
Slide 6

7. PER Simulation Results
GFSK
OFDM
GFSK
GFSK in
AWGN
OFDM
GFSK simulations performed by Rishi Mohindra, MAXIM
OFDM simulations performed by Partha Murali, Redpine Signals
Slide 7

8. PER Simulation Results
OFDM
GFSK
GFSK in
AWGN
GFSK in
AWGN
GFSK
OFDM
GFSK simulations performed by Rishi Mohindra, MAXIM
OFDM simulations performed by Partha Murali, Redpine Signals
Slide 8

9. Bad Urban Channel Statistics for 100 kb/s GFSK
Slide 9

10. Bad Urban Channel Statistics for 500 kb/s GFSK
Slide 10

11. PER with FHSS for GFSK 500 kb/s
Looked at channel snap shots (seed values) that produced bit errors in a packet.
Then swept the carrier frequency for the above seed values of the channel model, in order to check if frequency diversity (or FHSS) will help for GFSK
It was observed that frequency diversity (or FHSS) did not help i.e. PER = 100% over frequency once a bad multipath channel occurs even for very high SNR values of 45 dB
This is because in the simple 2-ray model with Rayleigh fading, the channel frequency response is repetitive, producing frequency selective fading for 500 kb/s GFSK signal
A more elaborate channel model has to be used for depicting “Frequency Correlation Function” over the operating band, and for determining the improvement with frequency diversity (i.e. FHSS)
However, even with FHSS, the expected PER will still be worse than 20% for each channel for the Bad Urban multipath case, and this will require multiple re-transmissions leading to more battery drain compared to OFDM.
Slide 11

12. Conclusions for Bad Urban channels
188 kb/s OFDM has at least 20 to 30 dB SNR advantage over 100 kb/s GFSK in Bad Urban channel conditions for PERs from 10% down to 1 %. The lower the required PER, more the advantage of OFDM over GFSK.
188 kb/s OFDM can operate at 10 dB lower power than 100 kb/s GFSK considering actual receiver power levels
Even 750 kb/s OFDM has 12 dB SNR advantage over 100 kb/s GFSK at 1% PER
750 kb/s OFDM can operate at 6 dB lower power than 100 kb/s GFSK considering actual receiver power levels
500 kb/s GFSK will not work efficiently in Bad Urban channels for Gas and Water meters due to PER in excess of 20% that would lead to higher battery drain due to multiple re-transmissions at different hop channels
Receiver power level where OFDM operates at 1-10% PER, the GFSK systems will operate at % PER for similar data rates.
GFSK systems will require a very high density of repeaters to cover the range in a mesh network compared to OFDM
Slide 12

13. Appendix Bad Urban Channel models in Matlab (used for OFDM)
ADS (used for GFSK)
Slide 13

14. Bad Urban Channel Model in Matlab for OFDM simulations
function [y]=two_ray_multipath_channel(x,amp1,amp2,n_FsDelay)
% x is the input to the Multipath channel
% y is the output of the Multipath channel
%amp1 = Average amplitude of 1'st path in dB
%amp2 = Average amplitude of 2'nd path in dB
%n_FsDelay = Delay in Number of Samples at the sampling rate of x.
% If x is sampled at 3125KHz (5x) and n_FsDelay=16 then sample delay of second tap is 5us.
RV1=(randn(1,1)+j*randn(1,1))/sqrt(2)*10^(amp1/20);
RV2=(randn(1,1)+j*randn(1,1))/sqrt(2)*10^(amp2/20);
y=(x*RV1 + [zeros(1,n_FsDelay) x(1:(length(x)-n_FsDelay))]*RV2)/sqrt(10^(amp1/20) + 10^(amp2/20));
OFDM simulations performed by Partha Murali, Redpine Signals (refer to document IEEE g, FPP SUN Simulation Results – Redpine Signals)
Slide 14

15. Bad Urban Channel Model in ADS for GFSK simulations
Above ADS channel model replicates the Matlab code in the previous slide.
Power levels and SNR are set in the top-level simulation after obtaining the ensemble average power
Slide 15