This example shows how to measure the packet error rate (PER) of an IEEE® 802.11p™ link using an end-to-end simulation with a Vehicle-to-Vehicle (V2V) fading channel and additive white Gaussian noise. The PER performance of a receiver with and without channel tracking is compared. In a vehicular environment (high Doppler), a receiver with channel tracking performs better.
IEEE 802.11p [ 1 ] is an approved amendment to the IEEE 802.11™ standard to enable support for wireless access in vehicular environments (WAVE). Using the half-clocked mode with a 10 MHz channel bandwidth, it operates in 5.85-5.925 GHz bands to support applications for Intelligent Transportation Systems (ITS) [ 2 ].
In this example, an end-to-end simulation is used to determine the packet error rate for an 802.11p [ 1 ] link with a fading channel at a selection of SNR points with and without channel tracking. For each SNR point, multiple packets are transmitted through a V2V channel, demodulated and the PSDUs are recovered. The PSDUs are compared to those transmitted to determine the number of packet errors. For each packet, packet detection, timing synchronization, carrier frequency offset correction and phase tracking are performed at the receiver. For channel tracking, decision directed channel estimation [ 3 ] is used to compensate for the high Doppler spread. The figure below shows the processing chain with channel tracking.
An 802.11p non-HT format transmission is simulated in this example. A non-HT format configuration object contains the format specific configuration of the transmission. This object is created using the
wlanNonHTConfig function. In this example, the object is configured for a 10 MHz channel bandwidth and QPSK rate 1/2 (MCS 2) operation.
% Link parameters mcs = 2; % QPSK rate 1/2 psduLen = 500; % PSDU length in bytes % Create a format configuration object for an 802.11p transmission cfgNHT = wlanNonHTConfig; cfgNHT.ChannelBandwidth = 'CBW10'; cfgNHT.PSDULength = psduLen; cfgNHT.MCS = mcs;
The V2V radio channel model defines five scenarios to represent fading conditions within a vehicular environment. In this example, 'Urban NLOS' [ 4 ] scenario is used. This corresponds to a scenario with two vehicles crossing each other at an urban blind intersection with building and fences present on the corners.
% Create and configure the channel fs = wlanSampleRate(cfgNHT); % Baseband sampling rate for 10 MHz chan = V2VChannel; chan.SampleRate = fs; chan.DelayProfile = 'Urban NLOS';
For each SNR (dB) point in the vector
snr a number of packets are generated, passed through a channel and demodulated to determine the packet error rate.
snr = 15:5:30;
The number of packets tested at each SNR point is controlled by two parameters:
maxNumErrors is the maximum number of packet errors simulated at each SNR point. When the number of packet errors reaches this limit, the simulation at this SNR point is complete.
maxNumPackets is the maximum number of packets simulated at each SNR point. It limits the length of the simulation if the packet error limit is not reached.
The numbers chosen in this example lead to a short simulation. For statistical meaningful results these numbers should be increased.
maxNumErrors = 20; % The maximum number of packet errors at an SNR point maxNumPackets = 200; % The maximum number of packets at an SNR point % Set random stream for repeatability of results s = rng(98);
For each SNR point, a number of packets are tested and the packet error rate is calculated. For each packet the following processing steps occur:
A PSDU is created and encoded to create a single packet waveform.
The waveform is passed through the channel. Different channel realizations are used for each transmitted packet.
AWGN is added to the received waveform to create the desired average SNR per active subcarrier after OFDM demodulation.
The per-packet processing includes packet detection, coarse carrier frequency offset estimation and correction, symbol timing and fine carrier frequency offset estimation and correction.
The L-LTF is extracted from the synchronized received waveform. The L-LTF is OFDM demodulated and initial channel estimates are obtained.
Channel tracking can be enabled using the switch
enableChanTracking. If enabled, the channel estimates obtained from L-LTF are updated per symbol using decision directed channel tracking as presented in J. A. Fernandez et al in [ 3 ]. If disabled, the initial channel estimates from L-LTF are used for the entire packet duration.
The non-HT Data field is extracted from the synchronized received waveform. The PSDU is recovered using the extracted data field and the channel estimates and noise power estimate.
% Set up a figure for visualizing PER results h = figure; grid on; hold on; ax = gca; ax.YScale = 'log'; xlim([snr(1), snr(end)]); ylim([1e-3 1]); xlabel('SNR (dB)'); ylabel('PER'); h.NumberTitle = 'off'; h.Name = '802.11p '; title(['MCS ' num2str(mcs) ', V2V channel - ' chan.DelayProfile ' profile']); % Simulation loop for 802.11p link S = numel(snr); per_LS = zeros(S,1); per_STA = per_LS; for i = 1:S enableChanTracking = true; % 802.11p link with channel tracking per_STA(i) = v2vPERSimulator(cfgNHT, chan, snr(i), ... maxNumErrors, maxNumPackets, enableChanTracking); enableChanTracking = false; % 802.11p link without channel tracking per_LS(i) = v2vPERSimulator(cfgNHT, chan, snr(i), ... maxNumErrors, maxNumPackets, enableChanTracking); semilogy(snr, per_STA, 'bd-'); semilogy(snr, per_LS, 'ro--'); legend('with Channel Tracking','without Channel Tracking') drawnow; end axis([10 35 1e-3 1]) hold off; % Restore default stream rng(s);
SNR 15 dB with channel tracking completed after 51 packets, PER: 0.41176 SNR 15 dB without channel tracking completed after 59 packets, PER: 0.35593 SNR 20 dB with channel tracking completed after 201 packets, PER: 0.069652 SNR 20 dB without channel tracking completed after 109 packets, PER: 0.19266 SNR 25 dB with channel tracking completed after 201 packets, PER: 0.0199 SNR 25 dB without channel tracking completed after 182 packets, PER: 0.11538 SNR 30 dB with channel tracking completed after 201 packets, PER: 0.0099502 SNR 30 dB without channel tracking completed after 201 packets, PER: 0.094527
For meaningful results
maxNumPackets should be increased. The below plot provides results for
maxNumErrors: 1000 and
Try changing the channel delay profile, the length of the packet or the data rate (
mcs values ) and observe the performance of channel tracking. For some configurations channel tracking provides little performance improvement. For a small number of OFDM symbols (small PSDU length or high MCS), temporal averaging performed during decision directed channel tracking may not be effective. The characteristics of the channel may also limit the performance for higher order modulation schemes (
mcs > 5 ).
This example uses the following helper functions and objects:
IEEE Std 802.11p-2010: IEEE Standard for Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Amendment 6: Wireless Access in Vehicular Environments, IEEE, New York, NY, USA, 2010.
J. A. Fernandez, D. D. Stancil and F. Bai, "Dynamic channel equalization for IEEE 802.11p waveforms in the vehicle-to-vehicle channel," 2010 48th Annual Allerton Conference on Communication, Control, and Computing (Allerton), Allerton, IL, 2010, pp. 542-551. doi: 10.1109/ALLERTON.2010.5706954
P. Alexander, D. Haley and A. Grant, "Cooperative Intelligent Transport Systems: 5.9-GHz Field Trials," in Proceedings of the IEEE, vol. 99, no. 7, pp. 1213-1235, July 2011.