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Matlab官方代码OFDM Synchronization 注释.docx
OFDMSynchronizationExample.m
generateOFDMSignal.m
OFDMScope
processOFDMScopes.m
applyOFDMChannel
OFDMSynchronizationExample.m
Copyright 2013-2014 The MathWorks, Inc.
%% OFDM Synchronization
% This example shows a method for digital communication with OFDM
% synchronization based upon the IEEE 802.11a standard. System objects
% from the Communication System Toolbox are utilized to provide OFDM
% modulation and demodulation and help synchronization functionality. In
% particular, this example illustrates methods to address real-world
% wireless communication issues like carrier frequency recovery, timing
% recovery, and frequency domain equalization.
%
%% Implementations
% This example describes a MATLAB implementation of OFDM synchronization,
% based upon the IEEE 802.11a standard [ <#9 3> ].
%% Introduction
% The IEEE 802.11a standard describes the transmission of an OFDM modulated
% signal for information exchange between systems in local and metropolitan
% area networks. This example utilizes the physical layer outlined by that
% standard, specifically the preamble symbols and the OFDM grid structure.
%
% The purpose of this example is:
%
% * To model a general OFDM wireless communication system that is able to
% successfully recover a message, which was corrupted by various simulated
% channel impairments.
%
% * To illustrate the use of key Communications System Toolbox(TM) tools
% for OFDM system design and OFDM symbol synchronization
%
% * To illustrate the performance benefits of MATLAB Coder(TM)
%% Initialization
% Adjustable transmitter parameters including the payload message in each
% frame that consists of several OFDM symbols and the number of transmitted
% frames.
message = 'Live long and prosper, from the Communications System Toolbox Team at MathWorks!';
numFrames = 1e2; %100
%%需要发送的信息message 是一串字母
% Adjustable channel parameters
% Channel noise level (dB)
EbN0dB = 12;
frequencyOffset = 1e4; % Frequency offset (Hz)
phaseOffset = 15;
% Phase offset (Degrees)
delay = 80;
% Initial sample offset for entire data stream (samples)
% Display recovered messages
displayRecoveredMsg = false;
% Enable scope visualizations
useScopes = true;
% Check for MATLAB Coder license
生成一次就有100帧
useCodegen = checkCodegenLicenseOFDM;
% By default the transmitter and receiver functions will be recompiled
% between every run, which is not always necessary. To disable receiver
% compilation, change "compileIt" to false.
compileIt = useCodegen;
%% Code Architecture for the System
% This example models a digital communication system based upon the IEEE
% 802.11a standard [ <#9 3> ]. The system is broken down into four
% functions: generateOFDMSignal, applyOFDMChannel, receiveOFDMSignal, and
% calculateOFDMBER.
%
% 1) generateOFDMSignal: set up and step an OFDMTransmitter System object.
% The object converts the payload message into a bit stream which is first
% PSK modulated, then OFDM modulated, and finally prepended by preamble
% OFDM symbols to form an individual frame. The transmitter repeats this
% frame |numFrames| times.
%
% 2) applyOFDMChannel: models the channel with carrier offset, timing
% offset, and additive white Gaussian noise (AWGN).
%
% 3) receiveOFDMSignal: set up and step an OFDMReceiver System object. The
% object models a series of components at the receiver, including timing
% recovery, carrier frequency recovery, channel equalization, and
% demodulation. The object can also be configured to show multiple scopes
% to visualize the receiver processing. The output of the OFDMReceiver
% object's step method is the decoded bit stream from those detected
% frames.
%
% 4) calculateOFDMBER: calculate the system frame error rate (FER) and bit
% error rate (BER) based on the original payload message in each frame and
% the bit output from the OFDMReceiver System object.
%% Description of the Individual Components and Algorithms
% *Transmitter*
%
% The OFDMTransmitter System object generates an OFDM signal based upon the
% IEEE 802.11a standard with a supplied ASCII payload. Each transmission
% frame is made up of several OFDM symbols, including preamble and data
% symbols. Identical frames are repeated by the transmitter based on the
% value supplied. Frames are padded to fill the OFDM grid when necessary.
%
% *Channel*
%
% This component simulates the effects of over-the-air transmission. It
% degrades the transmitted signal with both phase and frequency offset, a
% delay to mimic channel delay between transmitter and receiver, and AWGN.
% The noise level of the AWGN is given in dB.
%
% *Receiver*
%
% This OFDMReceiver System object recovers the original transmitted payload
% message. It is divided into four primary operations in this order:
%
% 1) Timing Recovery: This component is responsible for determining the
% sample location of the start of a given frame. More specifically, it
% utilizes a known preamble sequence in the received frame found through a
% cross-correlation. The cross-correlated data will contain a specific
% peak arrangement/spacing which allows for identification. The preamble
% itself is designed to produce this specific shape in the time domain.
% This identification method is based upon [ <#9 1> ]. The locatePreamble
% method of the object, which is responsible for this operation, uses a
% normalized minimum peak height, and a minimum number of required peaks to
% provide a possible preamble match.
%
% 2) Carrier Frequency Recovery: Frequency estimation is accomplished by
% calculating the phase difference in the time domain between halves of the
% long portion of the 802.11a preamble. This phase difference Phi is then
% converted to a frequency offset. This is a common technique originally
% published by Schmidl and Cox [ <#9 2> ]. This implementation of the
% phase measurement assumes that the true offset is within pi, or one
% frequency bin of the FFT. In the case of 802.11a a bin is 312.5kHz wide.
%
% 3) Frequency Domain Equalization: Since the frequency estimate can be
% inaccurate, additional phase rotation will exist at the subcarrier level
% of the OFDM symbol. As well as phase rotations, channel fading will also
% affect the received signal. Both of these impairments are corrected by a
% frequency domain equalizer. The equalizer has two stages, utilizing both
% preamble and pilot data. First, the received payload is equalized
% through the use of taps generated from the received long preamble
% samples. Then the pilot subcarriers are extracted, and interpolated in
% frequency to provide a full channel estimate. The payload is next
% equalized using these pilot estimates.
%
% 4) Data Decoder: Finally the OFDM subcarriers are demodulated and then,
% PSK demodulated into bits, from which the original payload message can be
% recovered.
%
% *BER Calculation*
%
% This component calculates the system FER and BER based on the original
% payload message and the decoded bit stream from the detected frames at
% the receiver. The undetected frames are not counted in the calculation.
%
% *Display of Recovered Message*
%
% The recovered message at the receiver is displayed for each detected
% frame. Since the original message length is not sent to the receiver, the
% padded bits in each frame are also recovered into characters and
% displayed. So you may see up to 7 meaningless characters at the end of
% each recovered message.
%
% *Scopes*
%
% * constellation diagrams showing the received signal before and after
% frequency domain equalization
%
% * vector plot of the equalizer taps used for a given frame
%
% * a spectrum analyzer displaying detected frames of data
%
% * a time plot displaying the start of detected frames
%
% * a time plot displaying the frequency estimate of the transmitter's
% carrier offset for detected frames
%% OFDM Synchronization Test Overview
% A large data vector is regenerated for a given EbN0 value by the
% generateOFDMSignal function. This data is then passed through the
% applyOFDMChannel function which introduces several common channel
% impairments. Finally the data is passed to the receiver for recovery.
% The receiveOFDMSignal function operates by processing data on a
% frame-by-frame basis. This processing mechanism is self-contained for
% performance benefits when using code generation and for code simplicity.
% This script by default generates code for the transmitter and receiver
% functions; this is accomplished by using the *codegen* command provided
% by the MATLAB Coder(TM) product. The *codegen* command translates
% MATLAB(R) functions to a C++ static or dynamic library, executable, or to
% a MEX file, producing a code for accelerated execution. The generated C
% code runs several times faster than the original MATLAB code.
%
% During operation, the receiver will display a series of plots
% illustrating certain synchronization results and effects on the signal.
codegen generateOFDMSignal -args {coder.Constant(message), coder.Constant(numFrames)}
% Compile transmitter with MATLAB Coder用matlab coder来编译发射器
if compileIt
end
% Generate transmission signal
if useCodegen
else
[txSig, frameLen] = generateOFDMSignal_mex(message, numFrames);
[txSig, frameLen] = generateOFDMSignal(message, numFrames); %%有这句就能生成
[txSig, frameLen]是函数返回的两个值，message和numFrames是函数的两个形参
End
%%生成的txsig是待发送的数据，有numFrames=100帧，每帧长度为framelen=1280
%%那simulink模型生成的是有多少帧，每帧长为多少的数据？把这个和simulink模型进行对比
codegen receiveOFDMSignal -args {rxSig, coder.Constant(frameLen),
% Pass signal through channel 经过信道，可以不管
rxSig = applyOFDMChannel(txSig, EbN0dB, delay, frequencyOffset, phaseOffset);
%可以看到信道有频偏和相位偏移，还有delay
% Compile receiver with MATLAB Coder用matlab coder来编译接收器
if compileIt
coder.Constant(displayRecoveredMsg), coder.Constant(useScopes)}
end
% Recover signal
if useCodegen
displayRecoveredMsg, useScopes);
else
displayRecoveredMsg, useScopes);
end
% Calculate average BER
[FER, BER] = calculateOFDMBER(message, decMsgInBits, numFramesDetected);
fprintf('\nAt EbNo = %5.2fdB, %d frames detected among the %d transmitted frames with FER = %f
and BER = %f\n', ...
[decMsgInBits, numFramesDetected] = receiveOFDMSignal(rxSig, frameLen,
[decMsgInBits, numFramesDetected] = receiveOFDMSignal_mex(rxSig, frameLen,
%%有这句就能接收
EbN0dB, numFramesDetected, numFrames, FER, BER);
%% Summary
% This example utilizes several MATLAB System objects to simulate digital
% communication with OFDM over an AWGN channel. It shows how to model
% several parts of the OFDM system such as modulation, frequency
% estimation, timing recovery, and equalization. The simulation also
% displays information about the operation of the synchronization
% algorithms through a series of plots. This example also utilizes code
% generation, allowing the simulation to run several times faster than the
% original MATLAB code.
%% Appendix
% The following System objects are used in this example:
%
% *
% *
% *
%
% The following helper functions are used in this example:
%
% *
% *
% *
% *
% *
% *
% *
%% References
% # Minn, H.; Zeng, M.; Bhargava, V.K., "On timing offset estimation for
% OFDM systems," Communications Letters, IEEE , vol.4, no.7, pp.242,244,
% July 2000
% # Schmidl, T.M.; Cox, D.C., "Robust frequency and timing synchronization
% for OFDM," Communications, IEEE Transactions on , vol.45, no.12,
% pp.1613,1621, Dec 1997
% # IEEE Std 802.11a, "Part 11: Wireless LAN Medium Access Control (MAC)
% and Physical Layer (PHY) Specifications," 1999.
displayEndOfDemoMessage(mfilename)
OFDMTransmitter.m
%% classdel是定义类名，后面的<是表示继承，Matlab允许多重继承，继承自matlab.system类，matlab.system
类定义了很多关于object的处理函数，例如addListener以及notify还有delete等对对象的函数，是一个抽象类，不能
实例化。Matlab中搜matlab.System
这里创建了一个简单的 system object OFDMTransmitter 继承自 matlab.system
classdef OFDMTransmitter < matlab.System
% OFDMTransmitter Generate OFDM signal repeatedly for a payload message
% based on the 802.11a standard.
% Copyright 2014 The MathWorks, Inc.
properties (Nontunable)
%SampleRate Sample rate
SampleRate = 20e6 %采样速率20Mhz，这是初始化，后面还能改
%PayloadMessage Payload message
PayloadMessage = 'Live long and prosper, from the Communications System Toolbox Team at
%NumFrames Number of frames to transmit
NumFrames = 1 %要传输的帧的个数？初始化是1，运行后是100
MathWorks!'
end
properties(Nontunable, SetAccess = private, Dependent)
%FrameLength Frame length
FrameLength
end
properties(Nontunable, Access = private) %nontunable是不可调的意思
pPayloadBits
pNumPadBits
pNumOFDMSymbols
pPreamble
pPilots
pBPSKMod
pDataOFDMMod
% Number of bits from payload message
% Number of random bits padded in each frame
% Number of OFDM symbols per frame
% Preamble for each frame
% Pilots for each frame
% BPSK Modulator System object
% OFDM Modulator System object
end
properties(Constant, Access = private) % OFDM Modulator configuration constant是常量
= 64
FFTLength
NumGuardBandCarriers = [6; 5] %数据载波前后的空子载波有6个和5个
NumDataCarriers
CyclicPrefixLength
PilotCarrierIndices = [12;26;40;54] %slx中是6;20;34;48,所以每个数是加了6应该是Guardband
%这个pilot的下标是加了guardbandleft=6，guardbandright=5，中间加了DC后pilot的下标，进行
%comm.OFDMModulator的时候，会把DC=0位变到第一位，然后26~39也是0，39是数据开始，然后进入ifft
NumOFDMSymInPreamble = 5 %有五个preamble
NumBitsPerCharacter = 7
%使用的数据子载波数有48个
= 48
= 16
end
%类的方法是放在methods ...end里面。%用OFDMTransmitter创建一个实例，前面定义了OFDMTransmitter类
OFDMTransmitter是类的方法
methods
function obj = OFDMTransmitter(varargin) %function函数定义关键字varargin函数中输入的可选参数
setProperties(obj, nargin, varargin{:});
%这是matlab.system里面的函数，nargin 函数中参数输入个数。setProperties（THIS，PROPS）设置THAP对象的
结构PROPS中定义的属性。 这个方法将在THIS构建之后被调用。
end
%设置samplerate的函数
function set.SampleRate(obj, Rs)
propName = 'SampleRate';
validateattributes(Rs, {'double'}, ...
{'real','scalar','positive','finite'}, ...
[class(obj) '.' propName], propName);
obj.SampleRate = Rs;
end
%设置payloadmessage的函数，PayloadMessage已经定义为一句话了，这里是把payloadmessage赋值给msg吧
function set.PayloadMessage(obj, msg)
propName = 'PayloadMessage';
validateattributes(msg, {'char'}, {'row'}, ...
[class(obj) '.' propName], propName);
obj.PayloadMessage = msg;
end
%设置NumFrames Number of frames to transmit %NumFrames=1 numFrm是形参
function set.NumFrames(obj, numFrm)
propName = 'NumFrames';
validateattributes(numFrm, {'double'}, ...
{'real','scalar','positive','integer','finite'}, ...
[class(obj) '.' propName], propName);
obj.NumFrames = numFrm;
end
%计算framelen的大小，framelen大小=msg*msg每个字母的bits数再除传数据的载波数，每次一个载波传一个bit（得
到的是传数据的帧的个数）然后乘（FFTlength+循环前缀长度)+preamble的ofdmsymbol的数量*fftlength
即msg帧个数*（fftlength+cyclength）+preamble个数+fftlength。 Preamble没有cyc？
function frameLen = get.FrameLength(obj)
frameLen = ceil(length(obj.PayloadMessage)*obj.NumBitsPerCharacter/obj.NumDataCarriers)
end
(obj.FFTLength+obj.CyclicPrefixLength) +
* ...
obj.NumOFDMSymInPreamble*obj.FFTLength;
end
%这里有coder.关键字，没有用matlab的coder会不会对结果有影响？
methods(Access = protected)
function setupImpl(obj)
coder.extrinsic('dec2bin','getOFDMPreambleAndPilot');
% Convert message to bits 转换msg到bit用到了dec2bin，得到的是一个7*80的矩阵，7是7个bit，80是80
个字符，但是7个都是显示49或48，不是二进制，因为用了double显示所以1变成了49，所以后面减去了48，所以
pPayloadBits是一个560*1的矩阵（因为msgInBits(:)铺开了）7个bit的0和1代表每个字符的二进制
msgInBits = coder.const(double(dec2bin(obj.PayloadMessage, obj.NumBitsPerCharacter).'));
obj.pPayloadBits = msgInBits(:) - 48;
的数据，所以总的bit除每个symbol传的bit就是总的symbol数，这里求得是560/48=11.6=12个symbol
% Calculate number of OFDM symbols per frame，每一次ofdm调制（即每一个OFDMsymbol）只能传48bit
如果ofdm前用的是bpsk就是这么算，如果用的是qpsk那就不是这么算
obj.pNumOFDMSymbols = ceil(length(obj.pPayloadBits)/obj.NumDataCarriers);
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