Digital Design of Communication Systems - Digital Design of Signal Processing Systems: A Practical Approach - page 572

% Equalization
Estimated_Signal = [];
for Symbol_Number = 1:1:ofdm_burst_size-1
Present_Symbol = Received_Signal_WOZP(:,Symbol_Number);
Estimated_Symbol = Present_Symbol.*W;
Estimated_Signal = [Estimated_Signal Estimated_Symbol];
end
Received_Signal_WZP = reshape(Estimated_Signal, 1, N *
(ofdm_burst_size-1)); % Parallel to serial
% Scatterplot(Received_Signal_WZP)
% Title('Equalizer Output')
Recovered_Signal = Received_Signal_WZP;
% Received_Signal_WZP = reshape(Estimated_Signal, 1,
N*(ofdm_burst_size-1)); % Parallel to serial
% Slicer
Bit_Counter = 1;
Bits = zeros(1,length(Recovered_Signal)*bps);
for Symbol_Counter = 1:length(Recovered_Signal)
% Slicer for decision making
[Dist Decimal] = min(abs(const_map -
Recovered_Signal(Symbol_Counter)));
% Checking for mimimum Euclidean distance
est = const_map(Decimal);
Bits(1,Bit_Counter:Bit_Counter+bps-1) = de2bi(Decimal-1,bps,
'left-msb');
% Constellation Point to Bits
Bit_Counter = Bit_Counter + bps;
end
scatterplot(Recovered_Signal)
title('Received Constellation')
grid
Data = bit_stream(1:N*(ofdm_burst_size-1)*bps);
ErrorCount = sum((Data =Bits))
scatterplot(Received_Signal_WZP); title('Received Constellation');
grid
store = ErrorCount/length(Data)
Figure 13.44 shows the constellation of QPSK-based OFDM receiver of this section for different
channel conditions. For deep fades in the channel frequency resource, an OFDM-based system is
augmented with error-correction codes and interleaving. The interleaving helps in spreading the
consecutive bits in the input sequence to different locations in the transmitted signal.
It is quite evident from the MATLAB code that simple FFT and IFFT cores can be used for
mapping the OFDM Tx/Rx in hardware. The channel estimation should preferably be mapped on a
programmable DSP. The timing error and frequency errors can also be calculated in hardware. The
system needs to be thought of as a cascade of PEs implementing different parts of the algorithm.
Depending on the data rates, the HW mapping may be fully parallel or time-shared.
13.3.7 Digital Up-conversion and Mixing
The modulated signal is to be digitallymixedwith an intermediate frequency (IF) carrier. In a digital
receiver one of the two frequencies, 70 MHz or 21.4 MHz, is commonly used. The digital carrier is