gusucode.com > wlan工具箱matlab源码程序 > wlan/wlan/wlanVHTLTFChannelEstimate.m
function est = wlanVHTLTFChannelEstimate(rxSym,cfgVHT,varargin)
% wlanVHTLTFChannelEstimate Channel estimation using the VHT-LTF
% EST = wlanVHTLTFChannelEstimate(RXSYM,CFGVHT) returns the estimated
% channel between all space-time streams and receive antennas using the
% Very High Throughput Long Training Field (VHT-LTF). The channel
% estimate includes the effect of the applied spatial mapping matrix and
% cyclic shifts at the transmitter.
%
% EST is a complex Nst-by-Nsts-by-Nr array characterizing the estimated
% channel for the data and pilot subcarriers, where Nst is the number of
% occupied subcarriers, Nsts is the total number of space-time streams,
% and Nr is the number of receive antennas.
%
% RXSYM is a complex Nst-by-Nsym-by-Nr array containing demodulated
% VHT-LTF OFDM symbols. Nsym is the number of demodulated VHT-LTF
% symbols.
%
% CFGVHT is a packet format configuration object of type <a href="matlab:help('wlanVHTConfig')">wlanVHTConfig</a>.
%
% EST = wlanVHTLTFChannelEstimate(RXSYM,CHANBW,NUMSTS) returns the
% estimated channel for the specified channel bandwidth, CHANBW, and the
% number of space-time streams, NUMSTS. Both CHANBW and NUMSTS have the
% same attributes as the corresponding ChannelBandwidth and
% NumSpaceTimeStreams properties of the wlanVHTConfig format
% configuration object.
%
% EST = wlanVHTLTFChannelEstimate(...,SPAN) performs frequency smoothing
% by using a moving average filter across adjacent subcarriers to reduce
% the noise on the channel estimate. The span of the filter in
% subcarriers, SPAN, must be odd. If adjacent subcarriers are highly
% correlated frequency smoothing will result in significant noise
% reduction, however in a highly frequency selective channel smoothing
% may degrade the quality of the channel estimate.
%
% Examples:
%
% Example 1:
% % Generate a time domain waveform txWaveform for an 802.11ac VHT
% % packet and combine all transmit antennas onto one receive antenna.
% % Extract and demodulate the VHT-LTF and perform channel estimation
% % with a frequency smoothing span of 3.
%
% cfgVHT = wlanVHTConfig; % Create packet configuration
% cfgVHT.NumTransmitAntennas = 2; % 2 transmit antennas
% cfgVHT.NumSpaceTimeStreams = 2; % 2 spatial streams
% txWaveform = wlanWaveformGenerator([1;0;0;1],cfgVHT);
%
% rxWaveform = sum(txWaveform,2); % Combine all transmit antennas
% idnVHTLTF = wlanFieldIndices(cfgVHT,'VHT-LTF');
% sym = wlanVHTLTFDemodulate( ...
% rxWaveform(idnVHTLTF(1):idnVHTLTF(2),:), cfgVHT);
% smoothingSpan = 3;
% est = wlanVHTLTFChannelEstimate(sym,cfgVHT,smoothingSpan);
%
% Example 2:
% % Generate a time domain waveform for an 802.11ac VHT packet, pass it
% % through a TGac fading channel and perform VHT-LTF channel
% % estimation.
%
% cfgVHT = wlanVHTConfig; % Create packet configuration
% txWaveform = wlanWaveformGenerator([1;0;0;1],cfgVHT);
%
% % Configure channel
% tgacChannel = wlanTGacChannel;
% tgacChannel.SampleRate = 80e6;
% tgacChannel.ChannelBandwidth = 'CBW80';
%
% % Pass through channel (with zeros to allow for channel delay)
% rxWaveform = tgacChannel([txWaveform; zeros(15,1)]);
% rxWaveform = rxWaveform(5:end,:); % Synchronize for channel delay
%
% % Extract VHT-LTF and perform channel estimation
% indVHTLTF = wlanFieldIndices(cfgVHT,'VHT-LTF');
% sym = wlanVHTLTFDemodulate(rxWaveform( ...
% indVHTLTF(1):indVHTLTF(2),:),cfgVHT);
% est = wlanVHTLTFChannelEstimate(sym,cfgVHT);
%
% See also wlanVHTConfig, wlanVHTLTFDemodulate, wlanVHTDataRecover,
% wlanLLTFChannelEstimate, wlanHTLTFChannelEstimate.
% Copyright 2015-2016 The MathWorks, Inc.
%#codegen
narginchk(2,4);
if ischar(cfgVHT) % chanBW input
narginchk(3,4);
wlan.internal.validateParam('CHANBW', cfgVHT, mfilename);
chanBW = cfgVHT;
numSTSVec = varargin{1};
wlan.internal.validateParam('NUMSTS', numSTSVec, mfilename);
if nargin == 4
span = varargin{2};
enableFreqSmoothing = true;
else
enableFreqSmoothing = false;
end
else % VHT configuration object input
narginchk(2,3);
% cfgVHT validation
validateattributes(cfgVHT, {'wlanVHTConfig'}, {'scalar'}, ...
mfilename, 'VHT format configuration object');
% Dependent validation not needed for necessary fields (CHANBW, numSTS)
chanBW = cfgVHT.ChannelBandwidth;
numSTSVec = cfgVHT.NumSpaceTimeStreams;
if nargin == 3
enableFreqSmoothing = true;
span = varargin{1};
else
enableFreqSmoothing = false;
end
end
% Validate symbol type
validateattributes(rxSym,{'single','double'},{'3d'}, mfilename, ...
'VHT-LTF OFDM symbol(s)');
numSC = size(rxSym,1);
numRxAnts = size(rxSym,3);
numSTSTotal = sum(numSTSVec);
% Return an empty if empty symbols
if isempty(rxSym)
est = zeros(numSC,numSTSTotal,numRxAnts);
return;
end
% Verify number of subcarriers to estimate
% Get OFDM configuration
[cfgOFDM,dataInd] = wlan.internal.wlanGetOFDMConfig(chanBW, 'Long', ...
'VHT',numSTSTotal);
FFTLen = cfgOFDM.FFTLength;
[ind,sortIdx] = sort([cfgOFDM.DataIndices; cfgOFDM.PilotIndices]);
coder.internal.errorIf(numSC~=numel(ind), ...
'wlan:wlanChannelEstimate:IncorrectNumSC',numel(ind),numSC);
% Get cyclic shifts applied at transmitter
csh = wlan.internal.getCyclicShiftVal('VHT',numSTSTotal,FFTLen/64*20);
if numSTSTotal==1
% Perform channel estimation for all subcarriers
est = wlan.internal.vhtltfEstimate(rxSym,chanBW,numSTSTotal,ind);
else
% Perform channel estimation for data carrying subcarriers as we
% must interpolate the pilots
estData = wlan.internal.vhtltfEstimate(rxSym(dataInd,:,:), ...
chanBW,numSTSTotal,cfgOFDM.DataIndices);
% Undo cyclic shift for each STS before averaging and interpolation
estData = wlan.internal.wlanCyclicShift(estData,csh,FFTLen,'Rx', ...
cfgOFDM.DataIndices);
% Estimate pilot subcarriers
estPilots = pilotInterpolation(estData,FFTLen,cfgOFDM.DataIndices, ...
cfgOFDM.PilotIndices);
% Combine data and pilots into one container
allSubcarriers = [estData; estPilots];
est = allSubcarriers(sortIdx,:,:);
end
% Perform frequency smoothing
if enableFreqSmoothing
% Smooth segments between DC gaps
switch chanBW
case 'CBW20'
numGroups = 1;
case 'CBW40'
numGroups = 2;
case 'CBW80'
numGroups = 2;
otherwise % 'CBW160'
numGroups = 4;
end
groupSize = size(est,1)/numGroups;
for i = 1:numGroups
idx = (1:groupSize)+(i-1)*groupSize;
est(idx,:,:) = wlan.internal.frequencySmoothing(est(idx,:,:),span);
end
end
% Re-apply cyclic shift after averaging and interpolation
if numSTSTotal>1
est = wlan.internal.wlanCyclicShift(est,csh,FFTLen,'Tx',ind);
end
end
%--------------------------------------------------------------------------
function estPilots = pilotInterpolation(estData,Nfft,dataIndices,pilotIndices)
% Interpolate over the pilot locations
numSTS = size(estData,2);
numRxAnts = size(estData,3);
% Construct full FFT size to allow us to interpolate over DC nulls
est = complex(ones(Nfft,numSTS,numRxAnts),ones(Nfft,numSTS,numRxAnts));
est(dataIndices,:,:) = estData;
% Interpolate over missing parts of the waveform in magnitude and
% phase (as opposed to real and imaginary)
magPart = interp1(dataIndices,abs(est(dataIndices,:,:)),1:Nfft);
phasePart = interp1(dataIndices,unwrap(angle(est(dataIndices,:,:))),1:Nfft);
[realPart,imagPart] = pol2cart(phasePart,magPart);
estInterp = complex(realPart,imagPart);
if isrow(estInterp)
est = estInterp(:,:,1).';
else
est = estInterp;
end
% Extract pilots
estPilots = est(pilotIndices,:,:);
end