function varargout = channel(sig,varargin)%CHANNEL MIMO channel model.%   Y = CHANNEL(X) corrupts input signal X by desired type of channel%   fading. 通過衰落信道使輸入信號X惡化的輸出%%   [Y,ALPHA] = CHANNEL(X) same as above but returns a matrix of complex%   path fading. This may be useful when performing data detection. %%   [Y,...] = CHANNEL(X,'PropertyName',PropertyValue,...)%%   Channel Property List%%   Arrangement    [m n]%   Fading         {'none'} | 'awgn' | 'rayleigh'%   SignalPower    value%   SNR            value [db]%   SymbolTime     value [s]%   SamplePeriod   value [s]%   Echo           'on' | {'off'}%%   NOTE: When the 'none' option is chosen than there are no noise in%         the channel, however the direct signals will be%         combined at the receiver input.%%   See also DETECT, EST.%   Copyright 2001-2003 Kamil Anis, anisk@feld.cvut.cz%   Dept. of Radioelectronics, %   Faculty of Electrical Engineering%   Czech Technical University in Prague%   $Revision: 2.1 $  $Date: 2003/1/16 17:33:28 $%   --%   <additional stuff goes here>global ECHO CH_CONF SIG_PWR SYMB_TIME SMPLPERname = 'CHANNEL';[idt,tag] = iecho(name);nopts = length(varargin) / 2;opts = reshape(varargin,[2 nopts])';ord1 = strmatch('Echo',opts(:,1));ord2 = strmatch('Arrangement',opts(:,1));ord3 = strmatch('Fading',opts(:,1));ord4 = strmatch('SignalPower',opts(:,1));ord5 = strmatch('SNR',opts(:,1));ord6 = strmatch('SymbolTime',opts(:,1));ord7 = strmatch('SamplePeriod',opts(:,1));% Echoif ~isempty(ord1) % first check whether local option exists	value1 = opts{ord1,2};	switch value1	case 'on'		ech = 1;	case 'off'		ech = 0;	otherwise		disp(' ');		disp([tag,'Invalid option for Echo property.']);		disp([idt,'Possible values are ''on''|{''off''}.']);		disp(' ');		ech = 0;	endelse	if ~isempty(ECHO) % than check whether global option exists		switch ECHO		case 'on'			ech = 1;		case 'off'			ech = 0;		otherwise			disp(' ');			disp([tag,'Invalid option for Echo property.']);			disp([idt,'Possible values are ''on''|{''off''}.']);			disp(' ');			ech = 0;		end	else % if there are no settings use the defaults		ech = 0; % default value	endend	% Arrangementif ~isempty(ord2) % first check whether local option exists	ch_conf = opts{ord2,2};else	if ~isempty(CH_CONF) % than check whether global option exists		ch_conf = CH_CONF;	else % if there are no settings use the defaults		ch_conf = 0; % default value	endend% Fadingif isempty(ord3) % there's no such option	fading = 'none'; % default valueelse % there's relevant option	fading = opts{ord3,2};end% SignalPowerif ~isempty(ord4) % first check whether local option exists	sig_pwr = opts{ord4,2};else	if ~isempty(SIG_PWR) % than check whether global option exists		sig_pwr = SIG_PWR;	else % if there are no settings use the defaults		sig_pwr = 1; % default value	endend% SNRif isempty(ord5) % there's no such option	snr = 5; % default valueelse % there's relevant option	snr = opts{ord5,2};end% SymbolTimeif ~isempty(ord6) % first check whether local option exists	symb_time = opts{ord6,2};else	if ~isempty(SYMB_TIME) % than check whether global option exists		symb_time = SYMB_TIME;	else % if there are no settings use the defaults		symb_time = 0.0001; % default value	endend% SamplePeriodif ~isempty(ord7) % first check whether local option exists	smplper = opts{ord7,2};else	if ~isempty(SMPLPER) % than check whether global option exists		smplper = SMPLPER;	else % if there are no settings use the defaults		smplper = 1; % default value	endend%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% BODY BEGIN %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%[sig_length,space_dim,frames] = size(sig);sigma_ch = 1;esr = symb_time * sigma_ch ^ 2 * ch_conf(1) * sig_pwr;n0 = esr / (10 ^ (0.1 * snr));sigma = sqrt(2 * n0 / smplper);const = sqrt(2);switch fadingcase 'rayleigh'  ch_coefs = (randn(ch_conf(1),ch_conf(2),frames) +...    i * randn(ch_conf(1),ch_conf(2),frames)) / const;  ch_noise = (randn(sig_length,space_dim,frames) +...    i * randn(sig_length,space_dim,frames)) * sigma / const;case 'awgn'  ch_coefs = ones(ch_conf(1),ch_conf(2),frames);  ch_noise = (randn(sig_length,space_dim,frames) +...    i * randn(sig_length,space_dim,frames)) * sigma / const;case 'none'  ch_coefs = ones(ch_conf(1),ch_conf(2),frames);  ch_noise = zeros(sig_length,space_dim,frames);end% signal mixturefor k = 1:frames % no 3D arrays multiplication is available    sig_add(:,:,k) = sig(:,:,k) * ch_coefs(:,:,k);endsig_corr = sig_add + ch_noise;varargout = {sig_corr,ch_coefs};%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% BODY END %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%if ech	switch fading	case 'none'      str2 = [''];	otherwise      str1 = num2str(snr);      str2 = [' ,SNR -> ',str1,' [dB]'];	end		disp(' ');	disp([tag,'Channel fading -> ',fading,str2,'.']);	disp(' ');end