i collected data from a spectrum analyzer and it saved as a .mat file. I confirmed that the signals I collected are baseband signals. Do I still need to create a .bb format file from this data? How is the .mat file different from the .bb file?

I collected data of a bluetooth device using a spectrum analyzer. I saved the data as a .mat file to be used in MATLAB. I am trying to use the Communication library to analyze the data. Using the library, I have run across .bb files and most of the communications examples I have seen have used .bb files instead of just the .mat file. What is the difference? My data is already in baseband, do I need to still convert my .mat files to .bb files?

Also, in using the bluetooth toolbox, I have seen in the example that (https://www.mathworks.com/help/comm/examples/bluetooth-low-energy-receiver.html) OQPSK was the modulation type used. Bluetooth utilizes GFSK. Why is OQPSK used in the analysis?

Answer 1

[Ajay Pattassery]

It is not mandatory to save base band signal in the .bb format.

comm.BasebandFileWriter object store the base band signals in .bb format.

This object associate properties like Sample rate, Center Frequency, meta data which can be any numeric, logical or character data type and with any number of dimensions.

This additional information that can be associated with the base band signal helps during further data processing whereas with the .mat file with just the base band data, the associated properties needs to stored and tracked separately.

Refer here for more information regarding usage of comm.BasebandFileWriter.

In the Bluetooth Low Energy example link provided, GMSK demodulation is done to retrieve the data bits.

% Extract message information
    [cfgLLAdv,pktCnt,crcCnt,remStartIdx] = helperBLEPhyBitRecover(rcvFilt,...
                                prbIdx,pktCnt,crcCnt,bleParam);
 

In the above part, the helperBLEPhyBitRecover function uses GMSK demodulator for demodulation.

[Akpabio Ekpewoh]

Thanks for the response, I truly appreciate it. I now understand about the .bb file. However, I still have problems with my other question. I will post the code used for carrier frequency compensation and gain control:

% Initialize System objects for receiver processing

agc = comm.AGC('MaxPowerGain',20,'DesiredOutputPower',2);

freqCompensator = comm.CoarseFrequencyCompensator('Modulation', 'OQPSK',...

'SampleRate',bbSampleRate,...

'SamplesPerSymbol',2*bleParam.SamplesPerSymbol,...

'FrequencyResolution',100);

The compensator uses the modulation type 'OQPSK'. Does that not affect the compensation done and the coarse frequency offset determined?

[Ajay Pattassery]

OQPSK and GMSK are almost similar, with the difference being the baseband rectangular pulses in OQPSK are replaced with half-sinusoidal pulses in GMSK, the coarse frequency compensator works well.

[Akpabio Ekpewoh]

Thank you very much. I would do some research on 'OQPSK' to get a better understanding. Thanks for the time and the great answers. It was a great help