[Discuss-gnuradio] OQPSK Demodulation and convolutional decoding

Hello all,

I have an s-band radio that passes HDLC frames through a reed-solomon encoder (255,223) , then an NRZ-M PCM encoder, then a convolutional coder (octal 171, 133 connection vectors), and finally through an OQPSK modulator.

I'm trying to recover the ASM flag from the reed-solomon encoder, which after being passed through the NRZ-M PCM encoder is 0xEC8AA816. In order to do this I am sending complex data from a file source (previously received idle data from the radio) through the gr-digital MPSK receiver and then through the "Decode CCSDS 27" block. I was hoping to see that NRZ-M encoded ASM flag here, but I'm initially getting all 0xFF, and then some gibberish data comes through after a few repeats of the file source (not sure what's causing that). I attached a screenshot of my flowgraph.

The constellation I'm seeing after the MPSK receiver doesn't seem right to me. I was expecting four nodes at pi/4, 3pi/4, 5pi/4, and 7pi/4 with horizontal and vertical symbol transitions, but instead I'm seeing them on the axes with transitions close to the origin. I attached a screenshot of this constellation plot.

Not sure where I'm going wrong here. Has anyone dealt with a similar issue or demodulated OQPSK a different way?

Any help is greatly appreciated!

Justin

From:	Justin Shetty
Subject:	[Discuss-gnuradio] OQPSK Demodulation and convolutional decoding
Date:	Tue, 10 Jul 2018 15:01:24 -0400