[SOLVED] CS代考计算机代写 %% wifitransmitter: Creates a Wi-Fi packet

%% wifitransmitter: Creates a Wi-Fi packet
% output = WiFi packet
% Inputs: message = text message, snr = signal to noise ratio,
% level = number of stages of encoding
function output = wifitransmitter(message, level, snr)

%% Default values
if(nargin < 2)level = 4;endif(nargin < 3)snr = Inf;end %% Sanity checks% check if message length is reasonableif(length(message) > 10000)
fprintf(2, ‘Error: Message too long
’);
output=[];
return;
end

% check if level is between 1 and 4
if(level > 4 || level < 1)fprintf(2, ‘Error: Invalid level, must be 1-4
‘);output=[];return;end%% Some constants% We will split the data into a cluster of 2*nfft bits% mulitplicative factor 2 is due to 4 QAM modulation used laternfft = 64;% This is the Encoder/decoder trellis used by WiFi’s turbo encoderTrellis = poly2trellis(3,[7,5]);% Every WiFi packet will start with this exact preamblepreamble = [1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1,1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1];preamble_symbols = qammod(preamble.’,4,’InputType’,’bit’); % Every 128 bits are mixed up like below:Interleave = reshape(reshape([1:2*nfft], 4, []).’, [], 1);%% Lets learn about the message% Lengthlen = length(message);%% Level #1: First lets do interleaving, which permutes the bits if (level >= 1)
% This basically converts the message into a sequence of bits
bits = reshape(dec2bin(double(message).’, 8).’, 1, [])-‘0’;

% We append as many bits as necessary to make this a multiple of
% 2*nfft
bits = [bits, zeros(1, mod(-length(bits), 2*nfft))];

nsym = length(bits)/(2*nfft);

output = [];
% Number of symbols in the message
for ii = 1:nsym
% Collect the iith symbol
symbol = bits((ii-1)*2*nfft + 1: ii*2*nfft);
% Interleave the symbol and append to output
output = [output, symbol(Interleave)];
end
% Finaally, let’s pre-pend the length to the message
output = [dec2bin(len,2*nfft)-‘0′,output];

end

%% Level #2: Next, lets do convolutional encoding, which adds redundancy to the bits
% and modulate the bits to symbols
if (level >= 2)

% Next, we apply the convolutional encoder to the message part
coded_message = convenc(output(2*nfft + 1:end),Trellis);

% Pre-pend the length to the coded message
output = [output(1:2*nfft),coded_message];

% Next, lets do modulation, which maps the bits to a modulation (QAM)
% Do 4-QAM modulation
output = qammod(output.’,4,’InputType’,’bit’);
% Prepend a preamble
output = [preamble_symbols; output];

end

%% Level #3: Next, lets create an OFDM packet
if (level >= 3)
% Number of symbols in message
nsym = length(output)/nfft;
for ii = 1:nsym
% Collect the iith symbol
symbol = output((ii-1)*nfft+1:ii*nfft);
% Run an FFT on the symbol
output((ii-1)*nfft+1:ii*nfft) = fft(symbol);
end
end

%% Level #4: Finally, lets add some random padding and noise
if (level >= 4)
% Lets add some (random) empty space to the beginning and end
noise_pad_begin = zeros(1, round(rand*1000)).’;
noise_pad_end = zeros(1, round(rand*1000)).’;
length(noise_pad_begin)
output = [noise_pad_begin; output; noise_pad_end];

% Let’s add additive white gaussian noise
output = awgn(output, snr);
end

end