Simple baseband TRx design concept in matlab

Greeting everyone.

I am trying some experiments on how to process the data through wireless communication with the simple communication link below.

Source (bit stream) > Tx modulator > Tx filter > AWGN channel > Rx filter > Rx demodulator > Receiver bit (bit stream)

So, my expectation is that the receiver site should demod and receive the correct data from the source.

Example: Input data "111100001111"; the receiver should demod the data as "111100001111".

Anyway, what I got when I checked the output demoded message was that the data was not the same as the input.

Example: Input "111100001111"; demod data : "111000011111" which has an error on bit no 4 and bit no 8

Below are the matlab code and input parameters that I set on my experiments.

load("Data.mat") % Input data

numBits = length(B4mod_bit);

modOrder = 2; % for BPSK

bitsPerSymbol = log2(modOrder) % modOrder = 2^bitsPerSymbol

txFilt = comm.RaisedCosineTransmitFilter; % create Tx Raised Cosine Object

rxFilt = comm.RaisedCosineReceiveFilter; % create Rx Raised Cosine Object

dbpskmod = comm.DBPSKModulator(pi); % create dbpsk modulation object

dpbpskdemod = comm .DBPSKDemodulator(pi,"OutputDataType","Full precision"); % create dbpsk demodulation object

modOut = dbpskmod(B4mod_bit); %modulate input bits

txFiltOut = txFilt(modOut); % add tx filter after modulate

SNR = 35; % dB

chanOut = awgn(txFiltOut,SNR,"measured"); % add awgn channel

rxFiltOut = rxFilt(chanOut); % filter the receive signal

demodOut = dpbpskdemod(rxFiltOut) % demod the signal

numofBitErrors=nnz(B4mod_bit ~= demodOut) % compare Input and demod output here

% calculate the bit error rate follow from Wireless communication On ramp excersise

delayInSymbols=txFilt.FilterSpanInSymbols/2+rxFilt.FilterSpanInSymbols/2

delayInBits=delayInSymbols * bitsPerSymbol

srcAligned=B4mod_bit(1:(end-delayInBits))

demodAligned=demodOut((delayInBits+1):end)

numBitErrors=nnz(srcAligned ~= demodAligned)

BER=numBitErrors/length(srcAligned)

Source bit / Bit per frame : 208 bits

Modulation type : DBPSK with Pi phase rotation

bit rate / Symbol rate : 1 bps

Tx filter :

Shape: 'Square root'

RolloffFactor: 0.2000

FilterSpanInSymbols: 10

OutputSamplesPerSymbol: 8

Gain: 1

Rx filter :

Shape: 'Square root'

RolloffFactor: 0.2000

FilterSpanInSymbols: 10

InputSamplesPerSymbol: 8

DecimationFactor: 8

DecimationOffset: 0

Gain: 1

Can anyone please give me some idea what is happening?

Any feedback is appreciated.

Best Regards,

Fumihiko Sato

[Santosh Fatale]

Hi Fumihiko,

Could you please share "Data.mat" file?

[Fumihiko Sato]

Dear Santosh,

Please find the attached for the "Data.mat" file.

The data is the column vector of data frame that included the 19 bits header preamble 0b1010101010101010101.

The rest are data.

Best Regards,

Fumihiko

Answer 1

[Santosh Fatale]

Hi Fumihiko,

It is my understanding that you are talking about the mismatch happening for the variable "numofBitErrors". Note that each of the Tx and Rx filters introduces a delay of (Filter Span in Symbol/2) * (Number of Bits/Symbol). You should not compare modulated and demodulated bits without taking the respective delay into account.

The same delay factor is taken into consideration in the second part of the code, where you followed the bit error calculation as per the Wireless Communication Onramp exercise.

You can also check out the following MATLAB Answer for more information on end-to-end implementation of wireless communication systems.

https://www.mathworks.com/matlabcentral/answers/519233-how-to-generate-bpsk-signal