Re: [Discuss-gnuradio] Improve FSK4 decodes in OP25

[Luke Berndt]

Thanks Marcus! I will try putting in a low pass in first after the Quad demod. That BPF / Polyphase Channelizer approach sounds pretty cool too!

I have attached a GRC file that I use for debugging and a link the raw dump from a File Sink. The sample rate at the sink is 96k/s, the system channel rate is 48k/s, with 10 samples per symbol. The FSK Level has to be set to about 0.66.

I am sure that some of the problem is that I don’t have a great signal coming in, but if there is any additional processing I could do to improve the reception that would be great.

debug.grc
Description: Binary data

Wav file

https://www.dropbox.com/s/ncpz6wt8tubm2v3/32784-1484187802_4.90762e%2B08.wav?dl=0

Raw capture from a file sink 

https://www.dropbox.com/s/810zj7pudc66yo3/32784-1484187802_4.90762e%2B08.raw?dl=0

On Jan 11, 2017, at 12:12 PM, Marcus Müller <address@hidden> wrote:

Hi Luke,

you don't happen to have a waterfall plot or a raw IQ recording?

Quadrature demodulation isn't the most resilient to noise. So, a low-pass filter that makes sure that events that are sufficiently shorter than a symbol after the Quadrature demodulator would sound wise. Then again, I don't know the OP25 infrastructure overly well, so chances are the FSK4 demod already contains one.

An alternative to your Quad FM demod would be a four-bandpasses-based decider. An easy way to build one is to either use four individual BPFs, or use one set of filter taps in the Polyphase Channelizer (which divides your f_sample_in  stream into four f_sample_in/4 -rate streams with channel centers). Just use a filter that would select the "zeroth" carrier. The magic of polyphase math will add three f_sample_in/4-shifted versions (you might need to use Rotator or similar to put the zeroth subcarrier at f=0). I have a "playing with channelizers" flow graph [1]. Advantage of the polyphase channelizer is that it uses very little more ressources than a single bandpass filter would – basically, you get the three additional filters for the cost of a short FFT. Yay!

Cheers,

Marcus

[1] https://gist.github.com/babffaa88f54a7fba9b52fc09bda0462

On 01/11/2017 04:44 PM, Luke Berndt wrote:

I am using the C++ OP25 libs to capture and record the P25 transmissions from a trunked radio system ( https://github.com/robotastic/trunk-recorder ).

Transmission can either use 4-Level FSK or CPSK. I am getting great decodes from a system using CPSK but I am having a lot of trouble with a system using 4FSK.

At a high level, here is my 4FSK flow graph (https://github.com/robotastic/trunk-recorder/blob/master/trunk-recorder/recorders/p25_recorder.cc):

Xlating + LPF to roughly capture the channel

ARB Resampler to get the system sample rate

Quad FM Demod

Multiply Const - to get the right levels, -3 to 3

FSK4 Demod - from OP25

FSK4 Slicer - from OP25

OP25 Decode

it then converts it to what is needed to record it as a Wav file

Are there some obvious blocks I should add in to help condition the input better? I think the incoming levels may not be constant. There also seems to be occasional interference. Is there a block that will discard values above a certain range? It would be great to throw-away samples above 3.5 or below -3.5.

Attached is a picture with what I think is an example of the interference.

Here is an example of a very clean recording: https://beta.openmhz.com/wmata?call-id=587652495833790100186198

Here is an example of a bad decode: https://beta.openmhz.com/wmata?call-id=58765250583379010018619c

Any tips would be greatly appreciated!!

- Luke

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