adapteqpt1.m

利用matlab软件对各种频率选择性信道进行仿真

%% Adaptive Equalizer Simulation (Part I)
% This script simulates a communication link with PSK modulation,
% raised-cosine pulse shaping, multipath fading, and adaptive equalization.
%
% It is the first of two parts:  Part I (adapteqpt1.m) sets simulation
% parameters and creates channel and equalizer objects.  Part II
% (adapteqpt2.m) performs a link simulation based on these settings, which
% are stored in the MATLAB workspace.  For information on the Part II
% script, enter demo('toolbox','comm') at the MATLAB prompt, and select
% "Adaptive Equalizer Simulation (Part II)".
%
% Part I sets up three equalization scenarios, and calls Part II multiple
% times for each of these.  Each call corresponds to a transmission block.
% The pulse shaping and multipath fading channel retain state information
% from one block to the next.  For visualizing the impact of channel fading
% on adaptive equalizer convergence, the simulation resets the equalizer
% state every block.  
% 
% To experiment with different simulation settings, you can edit the Part I
% script.  For instance, you can set the ResetBeforeFiltering property of
% the equalizer object to 0, which will cause the equalizer to retain state
% from one block to the next.

%   Copyright 1996-2006 The MathWorks, Inc.
%   $Revision: 1.1.4.3 $  $Date: 2006/06/23 19:24:52 $

%% Modulation and Transmission Block
% Set parameters related to PSK modulation and the transmission block.  The
% block comprises three contiguous parts: training sequence, payload, and
% tail sequence.  All use the same PSK scheme. The training and tail
% sequences are used for equalization.
Tsym = 1e-6; % Symbol period (s)
bitsPerSymbol = 2;  % Number of bits per PSK symbol
M = 2.^bitsPerSymbol;  % PSK alphabet size (number of modulation levels)
nPayload = 400;  % Number of payload symbols
nTrain = 100;  % Number of training symbols
nTail = 20; % Number of tail symbols
pskmodObj = modem.pskmod(M); % modulator object
xTrain = modulate(pskmodObj, randint(1, nTrain, M));  % Training sequence
xTail = modulate(pskmodObj, randint(1, nTail, M));  % Tail sequence

%% Transmit/Receive Filters
% Create structures containing information about the transmit and receive
% filters (txFilt and rxFilt).  Each filter has a square-root raised cosine
% frequency response, implemented with an FIR structure. 
%
% The transmit and receive filters incorporate upsampling and downsampling,
% respectively, and both use an efficient polyphase scheme (see Part II
% script for more information).  These multirate filters retain state from
% one transmission block to the next, like the channel object (see
% "Simulation 1: Frequency-flat fading channel" below).
%
% The peak value of the impulse response of the filter cascade is 1.  The
% transmit filter uses a scale factor to ensure unit transmitted power. 
%
% To construct the pulse filter structures, this script uses an auxiliary
% function adapteq_buildfilter.m.  An improved approach would be to use
% multirate filter objects from the Filter Design Toolbox.

% Filter parameters
nSymFilt = 8;  % Number of symbol periods spanned by each filter
osfFilt = 4;  % Oversampling factor for filter (samples per symbol)
rolloff = 0.25;  % Rolloff factor
Tsamp = Tsym/osfFilt; % TX signal sample period (s)
cutoffFreq = 1/(2*Tsym);  % Cutoff frequency (half Nyquist bandwidth)
orderFilt = nSymFilt*osfFilt;  % Filter order (number of taps - 1)

% Filter responses and structures
sqrtrcCoeff = firrcos(orderFilt, cutoffFreq, rolloff, 1/Tsamp, ...
              'rolloff', 'sqrt');
txFilt = adapteq_buildfilter(osfFilt*sqrtrcCoeff, osfFilt, 1);
rxFilt = adapteq_buildfilter(sqrtrcCoeff, 1, osfFilt);

%% Additive White Gaussian Noise
% Set signal-to-noise ratio parameter for additive white Gaussian noise.
EsNodB = 20;  % Ratio of symbol energy to noise power spectral density (dB)
snrdB = EsNodB - 10*log10(osfFilt); % Signal-to-noise ratio per sample (dB)

%% Simulation 1: Frequency-Flat Fading Channel
% Begin with single-path, frequency-flat fading.  For this channel, the
% receiver uses a simple 1-tap LMS (least mean square) equalizer, which
% implements automatic gain and phase control.  
%
% The Part II script (adapteqpt2.m) runs multiple times.  Each run
% corresponds to a transmission block.  The equalizer resets its state and
% weight every transmission block.  (To retain state from one block to the
% next, you can set the ResetBeforeFiltering property of the equalizer
% object to 0.)
%
% Before the first run, the Part II script displays the initial properties
% of the channel and equalizer objects.  For each run, a MATLAB figure
% shows signal processing visualizations.  The red circles in the signal
% constellation plots correspond to symbol errors.  In the "Weights" plot,
% blue and magenta lines correspond to real and imaginary parts,
% respectively. (The HTML version of this demo shows the last state of the
% visualizations.)
simName = 'Frequency-flat fading';  % Used to label figure window.

% Multipath channel
fd = 30;  % Maximum Doppler shift (Hz)
chan = rayleighchan(Tsamp, fd);  % Create channel object.
chan.ResetBeforeFiltering = 0;  % Allow state retention across blocks.

% Adaptive equalizer
nWeights = 1;  % Single weight
stepSize = 0.1;  % Step size for LMS algorithm
alg = lms(stepSize);  % Adaptive algorithm object
eqObj = lineareq(nWeights, alg, pskmodObj.Constellation);  % Equalizer object

% Link simulation
nBlocks = 50;  % Number of transmission blocks in simulation
for block = 1:nBlocks, adapteqpt2; end  % Run Part II script in loop.  

%% Simulation 2: Frequency-Selective Fading Channel and Linear Equalizer
% Simulate a three-path fading channel (frequency-selective fading).  The
% receiver uses an 8-tap linear RLS (recursive least squares) equalizer
% with symbol-spaced taps.  The simulation uses the channel object from
% Simulation 1, but with modified properties.
simName = 'Linear equalization of frequency-selective fading channel';

% Multipath channel
chan.PathDelays = [0 0.9 1.5]*Tsym; % Path delay vector (s)
chan.AvgPathGaindB = [0 -3 -6]; % Average path gain vector (dB)
reset(chan, 0);  % Reset channel to known state.

% Adaptive equalizer
nWeights = 8;
forgetFactor = 0.99;  % RLS algorithm forgetting factor
alg = rls(forgetFactor);  % RLS algorithm object
eqObj = lineareq(nWeights, alg, pskmodObj.Constellation);  % Equalizer object
eqObj.RefTap = 3;  % Reference tap

% Link simulation.  Store BER values.
for block = 1:nBlocks, adapteqpt2; BERvect(block)=BER; end
avgBER2 = mean(BERvect)  % Average BER over transmission blocks


%% Simulation 3: Decision Feedback Equalizer (DFE)
% The receiver uses a DFE with a six-tap fractionally spaced forward filter
% (two samples per symbol) and two feedback weights.  The DFE uses the same
% RLS algorithm as in Simulation 2.  The receive filter structure is
% reconstructed to account for the increased number of samples per symbol.
% This simulation uses the same channel object as in Simulation 2.  
simName = 'Decision feedback equalizer (DFE)';

% Reset channel to known state.
reset(chan, 0);

% Receive filter
nSamp = 2;  % Two samples per symbol at equalizer input
rxFilt = adapteq_buildfilter(sqrtrcCoeff, 1, osfFilt/nSamp);

% Adaptive equalizer
nFwdWeights = 6;  % Number of feedforward equalizer weights
nFbkWeights = 2;  % Number of feedback filter weights
eqObj = dfe(nFwdWeights, nFbkWeights, alg, pskmodObj.Constellation, nSamp);
eqObj.RefTap = 3;  % Reference tap

for block = 1:nBlocks, adapteqpt2; BERvect(block)=BER; end
avgBER3 = mean(BERvect)

block = 1;  % Reset variable (in case Part II is run independently).


displayEndOfDemoMessage(mfilename)