saloha.m

系统分别对无线通信中的几种重要的多址接入协议进行了仿真

% Program 6-7
% saloha.m
% 
% Slotted ALOHA System
%
% Input argument
% now_time :now time but,now_time<0 initializes the access terminals
%
% Output argument
% next_time :next state change time
%

function [next_time]=saloha(now_time)
global STANDBY TRANSMIT COLLISION      % definition of the global variable
global Srate Plen
global Mnum Mplen Mstate
global Tint Rint
global Spnum Splen Tplen Wtime

persistent mgtime mtime slot          % definition of the static variable
if now_time <0                        % initialize access terminal
    rand('state',sum(100*clock));     % resetting of the random table
    slot=Plen/Srate;                  % slot length
    mgtime=-Tint*log(1-rand(1,Mnum)); % packet generation time
    mtime=(fix(mgtime/slot)+1)*slot;  % packet transmitting time 
    Mstate=zeros(1,Mnum);
    Mplen(1:Mnum)=Plen;               % packet length
    next_time=min(mtime);
    return
end

idx=find(mtime==now_time & Mstate==TRANSMIT);
                                      % finding of the terminal which
                                      % transmission succeeded
if length(idx)>0
    Spnum      =Spnum+1;
    Splen      =Splen+Mplen(idx);
    Wtime      =Wtime+now_time-mgtime(idx);
    Mstate(idx)=STANDBY;
    mgtime(idx)=now_time-Tint*log(1-rand);     % next packet generation time
    mtime(idx) =(fix(mgtime(idx)/slot)+1)*slot;% next packet transmitting time
end

idx=find(mtime==now_time & Mstate==COLLISION);
                                      % finding of the terminal which
                                      % transmission failed
if length(idx)>0
    Mstate(idx)=STANDBY;
    mtime(idx)=now_time-Rint*log(1-rand(1,length(idx)));% waiting time
    mtime(idx) =(fix(mtime(idx)/slot)+1)*slot;          % resending time
end

idx=find(mtime==now_time);
                                      % finding of the terminal which
                                      % transmission start
if length(idx)>0
    Mstate(idx)=TRANSMIT;
    mtime(idx)=now_time+Mplen(idx)/Srate; % end time of transmitting
    mtime(idx) =round(mtime(idx)/slot)*slot;
    Tplen=Tplen+sum(Mplen(idx));
end

next_time=min(mtime);                 % next state change time