www.pudn.com > ofdm_matlab.rar > receive.m
function [Datarx, DiffPhRx] = receive(TimeSignal,ifftsize,carriers,...
wordsize,guardtype,guardtime,DataAvg,NoData,OutWordSize)
%RECEIVE Decodes a COFDM time waveform back into its data.
% It decode the data from one data frame. This function requires
% that the TimeSignal has the correct starting position.
% function [Datarx, DataOut, DiffPh] = receive(TimeSignal,ifftsize,carriers,...
% wordsize,guardtype,guardtime,DataAvg,NoData)
%
% INPUTS:
% ========
% TimeSignal : This is the input time signal for the COFDM waveform. The format
% of the output is a row vector.
% ifftsize : Size of ifft to use for generating the waveform
% carriers : Which carriers to use for the transmission
% wordsize : Number of bits to transmit on each carrier eg. 2 => QPSK
% 1 => BPSK, 4 => 16PSK, 8 => 256PSK.
% Must be one of: 1,2,4 or 8
% guardtype : What type of guard period to use
% Options:
% 0 = No Guard period
% 1 = zero level guard period
% 2 = cyclic extension of end of symbols
% 3 = same as 2 but with the first half of the guard period = zero
% guardtime : Number of sample to use for the total guard time
% DataAvg : Data Averaging. Number of repeats sent of the same
% data word, so that it can be averaged at the receiver
% to help reduce the phase error.
% NoData: is used for removing padding of the data. The padding is added
% by the transmitter on the last symbol if the number of data
% words doesn't fit into an integer number of symbols. NoData
% is the number of data words transmitted in the frame being
% decoded, e.g. if 10 bytes of data was sent and the DataOut
% format is byte format then NoData = 10.
% If padding hasn't been used set NoData to 0.
% OutWordSize: This is the wordsize of the output data (DataOut). This will
% normally be set to 8, if the original data is in a byte format.
% However OutWordSize may be set to higher values (e.g. 16) if
% analog signal are being transmitted on the COFDM instead of
% digital data. The value of OutWordSize must match InWordSize
% on the transmitted. Note : OutWordSize is an optional
% parameter with a default of 8 bits.
%
% OUTPUTS:
% ========
% Datarx : This is the output data that has been decoded from the 'TimeSignal'
% Its format is in words the same size as 'wordsize'. The output
% format of the data is a row vector.
%
% DiffPhTx : This is the actual phase difference between each symbol for the data
% This includes any noise or aborations due to the channel. It can be
% used for generating a histogram of the phase error. It has been adjusted
% so that the phase is centered around the phase locations of the data
% For Example for QPSK, the output phase is from -45 deg to 315 deg. This
% is for IQ locations at, 0, 90, 180, 270 deg.
%
% Copyright (c) Eric Lawrey 1997
%
% See: TRANSMIT, WRFILE, CHANNEL
% Modifications:
% 16/6/97 Started working on the function, it isn't finished yet.
% 17/6/97 Continued work on the receive function, it is now mostly finished
% and has been partly tested.
% 18/6/97 Changed the way thay the DiffPhTx is generated, so that the phase out is based
% on the phase locations. This was done so the phase error can be easily
% calculated. Negative phase errors centered around the 0deg phasor are no
% longer 359.6 deg for example but -0.4 deg. Fixed a bug due to having
% guardtype = 0
% 3/8/97 Added data averaging allowing duplicate data to be sent to
% reduce the error rate.
% 12/8/97 Updated to data averaging to the same method used by the transmitter
% and added the ability to trim the data back to compensate for padding.
% The padding doesn't work yet
% 13/8/97 The padding problem was fixed by rotating the transmitted data
% which was incorrect, and clipping the DiffPhRx array to the
% correct length.
%
% Required External Functions
% subsamp.m
if nargin < 9,
OutWordSize = 8; %Set the default
end
%=========================================================================
%Strip back the number of samples to make it a multiple of the symbol size
%=========================================================================
if guardtype == 0,
guardtime = 0;
end
SymbLen = length(TimeSignal)+guardtime; %Find total number of samples
%per symbol, including guard.
TimeSignal = TimeSignal(1:(SymbLen-rem(SymbLen,ifftsize+guardtime)));
%Find the number of symbols in the input time waveform
numsymb = length(TimeSignal)/(ifftsize+guardtime);
%==========================================================================
%Reshape the linear time waveform into fft segments and remove guard period
%==========================================================================
if guardtype ~= 0,
symbwaves = reshape(TimeSignal,ifftsize+guardtime,numsymb);
symbwaves = symbwaves(guardtime+1:ifftsize+guardtime,:); %Strip off the guard period
else
symbwaves = reshape(TimeSignal,ifftsize,numsymb);
end
fftspect = fft(symbwaves)'; %Find the spectrum of the symbols
DataCarriers = fftspect(:,carriers); %Extract the used carriers from the symbol spectrum.
clear fftspect; %Save some memory
CarrPh = angle(DataCarriers); %Find the phase angle of the data
NegCarrPh = find(CarrPh<0); %Map the phase angle to 0 - 2pi radians
CarrPh(NegCarrPh) = rem(CarrPh(NegCarrPh)+2*pi,2*pi);
clear NegCarrPh;
%================================
%Apply DQPSK on the received data
%================================
DiffPh = diff(CarrPh); %Compare phase of current to previous symbol
DiffPh = DiffPh*360/(2*pi); %convert from radians to degrees
NegPh=find(DiffPh<0); %Make all phases between 0 - 360 degrees
DiffPh(NegPh)=DiffPh(NegPh)+360;
DiffPh = reshape(DiffPh',1,size(DiffPh,1)*size(DiffPh,2)); %Convert back to a serial stream
DiffPh = subsamp(DiffPh,DataAvg,1,0); %Average the phase for duplicate words
NegPh=find(DiffPh<0); %Make all phases between 0 - 360 degrees
DiffPh(NegPh)=DiffPh(NegPh)+360;
%DiffPh = reshape(DiffPh,size(DiffPh,1)*DataAvg,size(DiffPh,2)/DataAvg);
Datarx = zeros(size(DiffPh));
PhInc = 360/(2^wordsize); %Find the increment between the phase locations
DiffPhRx = rem(DiffPh/(PhInc)+0.5,(2^wordsize))*(PhInc)-(PhInc/2);
Datarx = floor(rem(DiffPh/(360/(2^wordsize))+0.5,(2^wordsize)));
if NoData == 0,
%Stip back the length of Datarx so it is a multiple of the
%OutWordSize/wordsize.
Datarx = Datarx(1:floor(length(Datarx)/(OutWordSize/wordsize))*(OutWordSize/wordsize));
else
if (NoData*(OutWordSize/wordsize) > length(Datarx)),
disp('WARNING : Received Data Shorter than expected');
disp(['Expected = ', num2str(NoData*(OutWordSize/wordsize))]);
disp(['Received = ', num2str(length(Datarx))]);
else
Datarx = Datarx(1:NoData*(OutWordSize/wordsize));
DiffPhRx = DiffPhRx(1:NoData*(OutWordSize/wordsize));
end
end