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-Shift Keying and fill in the incomplete portions. Note that the code is not complete and will not execute properly as written. How does the spectrum of the transmitted signal change with Tsymb? 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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