mafi_sch_new.m

小区初搜为GSM系统中的一个关键过程

function [Y, Rhh] = mafi_sch_new(r,Lh,T_SEQ,OSR)

%

% MAFI:     This function performes the tasks of channel impulse

%           respons estimation, bit syncronization, matched 

%           filtering and signal sample rate downconversion.

%

% SYNTAX:   [y, Rhh] = mafi(r,Lh,T_SEQ,OSR)

%

% INPUT:    r     Complex baseband representation of the received

%                 GMSK modulated signal

%       Lh    The desired length of the matched filter impulse

%                 response measured in bit time durations

%       T_SEQ A MSK-modulated representation of the 26 bits long

%                 training sequence used in the transmitted burst, 

%                 i.e. the training sequence used in the generation of r

%           OSR   Oversampling ratio, defined as f_s/r_b.

%

% OUTPUT:   Y     Complex baseband representation of the matched 

%                 filtered and down converted received signal

%

%           Rhh   Autocorrelation of the estimated channel impulse 

%                 response. The format is a Lh+1 unit long column vector 

%                 starting with Rhh[0], and ending with Rhh[Lh]. 

%                 Complex valued.

%

% SUB_FUNC: None

%

% WARNINGS: The channel estimation is based on the 64 most central 

%           bits of the training sequence only

%

% TEST(S):  Tested manually through test script mf_ill.m

%

% AUTOR:    Jan H. Mikkelsen / Arne Norre Ekstr?m

% EMAIL:    [e-mail] / [e-mail]

%

% $Id: mafi.m,v 1.1 1998/10/01 10:20:21 hmi Exp $

 

DEBUG=0;

 

% PICK CENTRAL 16 BITS [ B | C | A ] AS COMPROMISE

% AND PERFORM COMPLEX CONJUGATION

%

T16 = conj(T_SEQ(6:59));%conj(T_SEQ(6:21));
T16 = [0 0 0 0 0 T16 0 0 0 0 0];
 

% EXTRACT RELEVANT PART OF THE RECEIVED SIGNAL. USING

% GUARD TIMES AS GUIDELINES IMPLIES EXTRACTING THE PART 

% STARTING APPROXIMATELY AT 10 Tb's BEFORE THE 16 MOST 

% CENTRAL TRAINING SEQUENCE BITS AND ENDING APPROXIMATELY

% 10 Tb's AFTER. ASSUME THAT BURSTS TEND TO BE CENTERED IN

% A SAMPLE STREAM.

%

GUARD = 10;

center_r=round(length(r)/2);        %未抽样位置准确的一个SB的数据的中点

start_sub=center_r-(GUARD+32)*OSR;  %取训练序列的开始位置，向前保护10个bit

end_sub=center_r+(GUARD+32)*OSR;    %取训练序列的结束位置，向后保护10个bit

 

% YOU MAY WANT TO ENABLE THIS FOR SPECIAL DEBUGGNIG

%

%start_sub=1;

%end_sub=length(r);

 

r_sub = r(start_sub:end_sub);        %取了保护的训练序列

 

if DEBUG,

  % DEBUGGING, VERIFIES THAT WE PICK THE RIGHT PART OUT

  % 

  count=1:length(r);

  figure

  plot(count,real(r));

  plug=start_sub:end_sub;

  hold on;

  plot(plug,real(r_sub),'r')

  hold off;

  title('Real part of r and r_sub (red)');

  %pause;

end

 

% PREPARE VECTOR FOR DATA PROCESSING

%

chan_est = zeros(1,length(r_sub)-OSR*64);

 

% ESTIMATE CHANNEL IMPULSE RESPONSE USING ONLY EVERY

% OSR'th SAMPLE IN THE RECEIVED SIGNAL

%

for n = 1:length(chan_est),

  chan_est(n)=r_sub(n:OSR:n+63*OSR)*T16.';   %做相关

end



if DEBUG,

  % DEBUGGING, PROVIDES A PLOT OF THE ESTIMATED IMPULSE

  % RESPONSE FOR THE USER TO GAZE AT

  figure;

  plot(abs(chan_est));

  title('The absoulte value of the correlation');

  %pause;

end

 

% chan_est = chan_est./64;

plot(abs(chan_est)); 

% EXTRACTING ESTIMATED IMPULS RESPONS BY SEARCHING FOR MAXIMUM

% POWER USING A WINDOW OF LENGTH OSR*(L+1)

%

% WL = OSR*(L+1);

WL = OSR*(Lh+1);        %确定窗长

 

search = abs(chan_est).^2;    %得到功率

for n = 1:(length(search)-(WL-1)),   %滑动窗找最大

  power_est(n) = sum(search(n:n+WL-1));

end

 

if DEBUG,

  % DEBUGGING, SHOWS THE POWER ESTIMATE

  figure;

  plot(power_est);

  title('The window powers');

  %pause;

end

 

% SEARCHING FOR MAXIMUM VALUE POWER WINDOW AND SELECTING THE

% CORRESPONDING ESTIMATED MATCHED FILTER TAP COEFFICIENS. ALSO,

% THE SYNCRONIZATION SAMPLE CORRESPONDING TO THE FIRST SAMPLE

% IN THE T16 TRAINING SEQUENCE IS ESTIMATED

%

[peak, sync_w] = max(power_est);

h_est = chan_est(sync_w:sync_w+WL-1);

 

[peak, sync_h] = max(abs(h_est));

sync_T16 = sync_w + sync_h - 1;

 

if DEBUG,

  % DEBUGGING, SHOWS THE POWER ESTIMATE

  figure;

  plot(abs(h_est));

  title('Absolute value of extracted impulse response');

  %pause;

end

 

% WE WANT TO USE THE FIRST SAMPLE OF THE IMPULSERESPONSE, AND THE

% CORRESPONDING SAMPLES OF THE RECEIVED SIGNAL.

% THE VARIABLE sync_w SHOULD CONTAIN THE BEGINNING OF THE USED PART OF

% TRAINING SEQUENCE, WHICH IS 3+57+1+6=67 BITS INTO THE BURST. THAT IS

% WE HAVE THAT sync_T16 EQUALS FIRST SAMPLE IN BIT NUMBER 67.

%

burst_start = ( start_sub + sync_T16 - 1 ) - ( OSR * 42 + 1 ) + 1;

 

% COMPENSATING FOR THE 2 Tb DELAY INTRODUCED IN THE GMSK MODULATOR.

% EACH BIT IS STRECHED OVER A PERIOD OF 3 Tb WITH ITS MAXIMUM VALUE

% IN THE LAST BIT PERIOD. HENCE, burst_start IS 2 * OSR MISPLACED. 

%

burst_start = burst_start - 2*OSR + 1;

 

% CALCULATE AUTOCORRELATION OF CHANNEL IMPULSE

% RESPONS. DOWN CONVERSION IS CARRIED OUT AT THE SAME

% TIME

%

R_temp = xcorr(h_est);

 

pos = (length(R_temp)+1)/2;

 

Rhh=R_temp(pos:OSR:pos+Lh*OSR);

 

% PERFORM THE ACTUAL MATCHED FILTERING

%

 

m = length(h_est)-1;

 

% A SINGLE ZERO IS INSERTED IN FRONT OF r SINCE THERE IS AN EQUAL 

% NUMBER OF SAMPLES IN r_sub WE CANNOT BE TOTALLY CERTAIN WHICH 

% SIDE OF THE MIDDLE THAT IS CHOSEN THUS AN EXTRA SAMPLE IS 

% NEEDED TO AVOID CROSSING ARRAY BOUNDS.

%

GUARDmf = (GUARD+1)*OSR;

r_extended = [ zeros(1,GUARDmf) r zeros(1,m) zeros(1,GUARDmf)];

 

% RECALL THAT THE ' OPERATOR IN MATLAB DOES CONJUGATION

%

 

for n=1:148,

  aa=GUARDmf+burst_start+(n-1)*OSR;

  bb=GUARDmf+burst_start+(n-1)*OSR+m;

  Y(n) = r_extended(aa:bb)*h_est'; 

end