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You will simulate a quadrature phase-shift keying (QPSK) digital transmitter in MATLAB.

Matlab simulation

MATLAB is commonly used to design filters and determine frequency responses of systems, but it is also very useful as a simulation tool.

Use the following MATLAB code skeleton to simulate the QPSK transmitter from Digital Transmitter: Introduction to Quadrature Phase-ShiftKeying and fill in the incomplete portions. Note that the code is not complete and will not execute properly aswritten. How does the spectrum of the transmitted signal change with T symb ? How do you interpret the figure created by plot(rI,rQ) ?

1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 2 % MATLAB Code Skeleton for QPSK Digital Transmitter 3 4 % Generate random bits 5 bits_per_symbol=2; 6 num_symbols=128; 7 numbits=bits_per_symbol*num_symbols; 8 bits=rand(1,numbits)>0.5; 9 10 Tsymb = 16; % symbol length 11 omega = pi/2; % carrier frequency 12 13 %%%%%%%%%%%%%%%%%%%%%%%% 14 % Transmitter section 15 % initialize transmit sequence 16 t = zeros(1,num_symbols*Tsymb); 17 i = 1; % initialize bit index 18 n = 1; % initialize time index 19 20 while (n <= num_symbols*Tsymb) 21 if ( bits(i:i+1) == [ 0 0]) 22 Igain = 1/sqrt(2); 23 Qgain = 1/sqrt(2); 24 % ------>Insert code here<------- 25 26 end; 27 i = i+2; % next 2 bits 28 29 % Generate symbol to be transmitted 30 t(n:n+Tsymb-1) = %------>Insert code here<------- 31 32 n = n+Tsymb; % next symbol 33 end; 34 35 % Show the transmitted signal and its spectrum 36 % ------>Insert code here<------- 37 38 % Show the transmitted signal constellation 39 rI = t.*cos(omega*[1:num_symbols*Tsymb]); 40 rQ = t.*sin(omega*[1:num_symbols*Tsymb]); 41 42 % Filter out the double-frequency term 43 low_pass=fir1(512,0.5); 44 rI=conv(rI,low_pass); 45 rQ=conv(rQ,low_pass); 46 figure; 47 plot(rI,rQ);

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Source:  OpenStax, Dsp laboratory with ti tms320c54x. OpenStax CNX. Jan 22, 2004 Download for free at http://cnx.org/content/col10078/1.2
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