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Below is an example of a digital modulation system using 16-QAM. I have copied this example from MATLAB communications toolbox, 

   M = 16;                     % Size of signal constellation
   k = log2(M);                % Number of bits per symbol
   n = 30000;                  % Number of bits to process
   numSamplesPerSymbol = 1;    % Oversampling factor
   dataIn = randi([0 1],n,1);
   dataInMatrix = reshape(dataIn, length(dataIn)/4, 4); % Reshape data into binary 4-tuples
   dataSymbolsIn = bi2de(dataInMatrix);
   dataMod = qammod(dataSymbolsIn, M);

I am wondering if

1) there is a need to reshape the bits into tuples of 4 bits

2) there is a need to do the mapping from binary to decimal and then do the QAM modulation, couldn't I just have done

dataMod = qammod(dataIn, M);

Thanks.

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First of all, I recommend getting familiar with Matlab's doc command. doc qammod has the answers to your second question: qammod requires a vector of integers between 0 and M-1.

Regarding your first question: the reason to reshape your bits into groups of four is that 16-QAM transmits $\log_2(16)=4$ bits per symbol. So, your bi2de command requires binary numbers of four bits to get decimal integers betwen 0 and 15.

If you want to save lines of code, what you could do is generate integers between 0 and 15 directly, with dataSymbolsIn = randi([0 15], n/4, 1). However, doing this has one disadvantage: calculating the bit error rate at the receiver is more complicated if all you have are the integers. Let's say that you transmit a 15 and receive a 13 (obtained with qamdemod), how many bit errors there were? This is why it's common practice to start with actual bits in the transmitter and compare them one-to-one with the received bits.

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  • $\begingroup$ thank you, I understand the reasoning, however I thought in a real digital communication system, we don't have such a mapping from binary to decimal, am I right? i.e we directly go from bits (0 and 1) to symbols? $\endgroup$
    – Tyrone
    Commented Apr 14, 2015 at 22:46
  • $\begingroup$ by the way I noticed that you answer alot of digital communications questions, I am relatively new in this site. Was wondering is it OK to tag you in my questions or comments so you can help me further? $\endgroup$
    – Tyrone
    Commented Apr 14, 2015 at 22:59
  • $\begingroup$ @Tyrone You are correct that one always maps bits to symbols. The requirement of having a vector of integers as argument is particular to qammod. Personally, in my simulations I prefer to map groups of bits to the actual analog amplitude that the transmitted signal will have, because this makes it easier to calculate the noise power that results in a specific SNR. $\endgroup$
    – MBaz
    Commented Apr 14, 2015 at 22:59
  • $\begingroup$ @Tyrone, if you tag your questions as 'digital-communications', I'll see them, and I'll be happy to answer them if I have the time and know the answer :) Other site regulars know a lot about this subject, too. $\endgroup$
    – MBaz
    Commented Apr 14, 2015 at 23:01
  • $\begingroup$ I see, I have I noticed in MATLAB that they send the QAM symbols as is through the AWGN channel, for example following the notation of the code I presented above, they add, receivedSignal = awgn(dataMod, snr, 'measured'); Isn't this wrong I mean why arent we converting the symbols to an analog signal and then transmitting it over the AWGN channel? $\endgroup$
    – Tyrone
    Commented Apr 14, 2015 at 23:02

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