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[Discuss-gnuradio] Problem using GFSK modulation and demodulation blocks

From: Giancarlo Murillo
Subject: [Discuss-gnuradio] Problem using GFSK modulation and demodulation blocks
Date: Fri, 5 Jul 2019 20:30:18 -0500

Hello Everyone

I was dealing with a problem for the past 2 years with the GNURadio, i made a very simple loopback flowgraph which consists into something like this

Wav File Source --> Rational resampler --> multiply const --> float to char --> GFSK mod  --> GFSK demod --> Char to float  --> multiply const ---> Audio sink

I made the following assumptions to desing my digital communication system
Audio_rate: 32 KHz
Audio_bit_depth: 8 bits
Bits_per_symbol: 1 bit.
Samples_per_symbol: 8 sps

And made the following calculations:

Bit_rate(Audio): Audio_rate * Audio_bit_depth = 8 * 32e3 = 256 Kbps
Baud_rate(Symbols): Bit_rate * Bits_per_symbol = 256e3 * 1 = 256 KSyps
Sample_rate (modulated samples): Baud_Rate * Sample_per_symbol = 256e3 * 8 = 2.048 MSps

I did this calculations thanks to the Wikipedia and GNURADIO wiki
Frequency_deviation = Bit_rate/2 = 256e3/2 =128 KHz
Modulation_index =  Frequency_deviation /(baud_rate/2) = 128e3 / (256e3 / 2) = 1
Sensitivity = (math.pi* Modulation_index )/( Samples_per_symbol ) = (3.1416..* 1)/8 = 392.699e-3

In the transmitter side:
Wav File source: An audio file sampled at 44100 KHz
Rational resampler: Interpolation 320 Decimation 441 con resample to 32 KHz
Multiply Const: 0.5x to avoid clipping
Float to char: scale of 256 levels

In the Receiver side:
Char to float: scale of 256 levels
Multiply Const: 2x to restore level
Audio sink: to play the wav file at 32000 KHz

I tested it without the GFSK blocks, and works awesome without errors, the main problem is at conecting the GFSK blocks which produce bit errors, i made the followind configurations.
GFSK mod:
Samples/Symbol: 8
Sensitivity: 392.699e-3
BT: 350e-3
GFSK demod:
Samples/Symbol: 8
Sensitivity: 392.699e-3
BT: 350e-3
Gain Mu: 175e-3
Mu: 500e-3
Omega relative frequency: 5e-3
Freq error: 0

But looks like im doing something wrong in the GFSK blocks, can you help me experts to debug it
im attaching my flowgraph.

Con la mejor de las intenciones
Giancarlo Freddy Murillo Arenas
Bachiller de Ingeniería Electrónica y de telecomunicaciones
Universidad Católica San Pablo

Attachment: fsk_nohier.grc
Description: Binary data

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