cdma_cd.m

802.11无线网络通信中基本CSMA/CA基本方式和RTS/CTS的仿真实现比较。

%%---------------------------------------------%%
%%---802.11 MAC CSMA/CD------%%


close all;
clear all;

% M=3                         %numbers of stations
for M = 2:4
ChannelBusy = 0
Start = 0
Collition = 0
DeferenceTime = 0           %Station's deference time，发送期
Throughput = 0
Result = 1:20              %simulation result, in seconds
ArrivalTime = 1:M           %Packets arrival time
PacketLength = 1:M          %Packets length
HasPacket = zeros(1,M)
CW = 1:M
BackoffTimer = 1:M

PacketBuff = zeros(M,1501);  %stations Packets buffer，1501的意思?
CollitionStations = zeros(1,M+1);    %Collition station sequence

PhyRate = 2*10^6            %物理层数据率, physical rate，2Mbit/s
SlotTime = 20*10^(-6)       %时隙间隔, slot time,20us
TotalTime = 200/SlotTime    %total simulation time，in slots, 200s,10M slot
SIFS = 0.5        %SIFS=0.5*slot,10us
DIFS = 2.5        %DIFS=2.5*slot,50us 
ACK = 14*8/(PhyRate*SlotTime) %ACK 14bytes换算成slot
AverageArrivalTime = 110    %slots
AveragePacketLength = 50    %slots
CurBufferSize = zeros(1,M)  %slots，indicate the current buffer length,当前帧长为
Buffer_Threshold = 8*10^6/(PhyRate*SlotTime)%the max buffer length is 1M byte

%初始化
for i = 1:M
    ArrivalTime(i) = ExpDis(AverageArrivalTime);  %initial packet initial time，初始化到达时间  
    PacketLength(i) = ExpDis(AveragePacketLength);%initial packet length，初始化分组长度
    CW(i) = 32; %竞争窗口，CW = 32 slots
    BackoffTimer(i) = 1000; %BackoffTimer initial value,退避时间 1000 slots
end


for t = 1:TotalTime
    for i = 1:M
    %renew process，every station has almost infinite buffer(l-100)
        if t == ArrivalTime(i)
            %目前不能发送，push分组进PackeBuff，修改退避计数器
            if CurBufferSize(i) < Buffer_Threshold - PacketLength(i)
                PacketBuff = Push(PacketBuff,i,PacketLength(i));
            %Push the new packet
                CurBufferSize(i) = CurBufferSize(i) + PacketLength(i);
                HasPacket(i) = 1;

                if BackoffTimer(i) == 1000
                    BackoffTimer(i) = RandSel(CW(i));%退避计数器达到最大1000时，重置退避计数器
                end
            end
            ArrivalTime(i) = ExpDis(AverageArrivalTime) + PacketLength(i) + t;
            %update packet arrival time and length
            PacketLength(i) = ExpDis(AveragePacketLength);
        end
    end

    for i = 1:M
        if HasPacket(i) == 1 & ChannelBusy == 0 %PackeBuff中有数据包发送并且信道空闲
            if BackoffTimer(i) == 0 %退避时间=0 -> 发送
                CollitionStations = Add(CollitionStations,i);%加入冲突站点序列中
                Start = 1;
            else
                BackoffTimer(i) = BackoffTimer(i) - 1;%退避时间=!0 -> 退避时间-1
            end
        end
    end

    if Start == 1%信道改为忙碌
        ChannelBusy = 1;
        n = CollitionStations(1);
        
        if n == 1%信道中只有一个站点发送数据则为正常发送情况
            DeferenceTime = floor(t + SIFS + DIFS + ACK + PacketBuff(CollitionStations(2),2));
            %PacketBuff(CollitionStations(2),2)数据长度
            %成功发送时间
            Collition=0;%没有碰撞
        else
            DeferenceTime = floor(t + DIFS + MaxLength(PacketBuff,CollitionStations));
            %不能发送是的虚拟计数器时间?
            Collition=1;%发生碰撞
        end
        Start=0;        
    end
    
    if t == DeferenceTime & ChannelBusy == 1%信道忙的时候达到站点的等待时间
        if Collition == 0
            n = CollitionStations(2);
            CurBufferSize(n) = CurBufferSize(n) - PacketBuff(n,2);
            Throughput = Throughput + PacketBuff(n,2) * SlotTime * PhyRate;
            PacketBuff = Pop(PacketBuff,n);
            
            CW(n) = 32;
            k = PacketBuff(n,1);
            if k ==0%如果没有数据等待发送，将HasPacke置0，BackoffTimer置Max
                HasPacket(n) = 0;
                BackoffTimer(n) = 1000;
            else%还有数据分发送，修改碰撞计数器
                BackoffTimer(n) = RandSel(CW(n));
            end    
        else
            n = CollitionStations(1);
            for i = 1:n
                j = CollitionStations(i+1);
                CW(j) = Increase(CW,j);
                BackoffTimer(j) = RandSel(CW(j));
            end    
        end
        CollitionStations = zeros(1,M+1);
        DeferenceTime = 0;
        ChannelBusy = 0;
        Collition = 0;
    end
    
    if mod(t,10/SlotTime) == 0
        s = t * SlotTime/10;
        s = round(s);
        Result(s) = Throughput;
        Throughput = 0;
    end
end
    
% x = 1:200;      %%%750
% plot(x,Result);
% title('csma')
% axis([0,20,0,1.8*10^6]);
savefile = sprintf('cdma_ca_M=(%d).txt',M);
fid = fopen(savefile,'a+');
fprintf(fid,'%1.6f     ',Result);
fclose(fid);
end


% function y = ExpDis(a)
% y=0;
% while y == O
%     A = rand(1);
%     y = floor((-a) * log(1-A));
% end
% 
% function y = Increase(CW,n)
% i = log2(CW(n));
% if  i < 8
%     i = i + 1;
% end
% y = 2^i;
% 
% function y = MaxLength(PacketBuff,CollitionStations)
% max=0;
% for i = l:CollitionStations(1)
%     if PacketBuff(Collitionstations(i+l),2) > max
%         max = PacketBuff(Collitionstations(i+l),2);
%     end
% end
% y=max;
% 
% function y = Pop(PacketBuff,n)
% PacketBuff(n,:) = [PacketBuff(n,1),PacketBuff(n,3:1501),0];
% %here，accomadate with packet buffer length
% PacketBuff(n,l) = PacketBuff(n,1) - 1;
% y = PacketBuff;
% 
% function y = Push(PacketBuff,index,PacketLength)
% i = PacketBuff(index,l);
% if i < 1500     %length(PacketBuff(index,:))-l
%     PacketBuff(index,i+2) = PacketLength;
%     PacketBuff(index,1) = PacketBuff(index,1) + l;
% end
% y = PacketBuff;
% 
% function y = RandSel(CW)
% y = 0
% while y == 0
%     y = floor(rand * CW);
% end
% 
% function y = Add(CollitionStations,index)
% CollitionStations(1) = CollitionStations(1) + 1;
% i = CollitionStations(1);
% CollitionStations(i+1) = index;
% y = CollitionStations;