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Re: Byte Boundary alignment

From: Artur Nogueira
Subject: Re: Byte Boundary alignment
Date: Wed, 22 Jul 2020 13:13:27 -0300

Hello Lannan,

As far as I could understand, you want to do the following operations:

(i) Generate bytes -> (ii) Modulate -> (iii) Transmit -> (iv) Receive -> (v) Demodulate -> (vi) Recover bytes

and then you want to align the byte sequences from steps (i) and (vi).
Am I correct?
What hardware (model) are you using to transmit/receive?

I've been trying to solve a similar issue lately and I am convinced that my signals are not aligned because of 3 main reasons: wave propagation time, GNU Radio processing time (latency) and carrier frequency offset (CFO).
What I've been doing is: the two first issues are minimized by applying a cross-correlation operator (not necessarily in GNU Radio; you can post-process using another software if you want) to the transmitted and received  base-band signals. The peak value allows you to find the lag between them.
The third problem (CFO) comes from an unalignment between the transmitting and receiving units (I use HackRF One) and can be solved with a mixer at the receiver level tuned with the frequency offset value.

Best regards,

Em qua., 22 de jul. de 2020 às 12:56, lannan jiang <jln925@live.com> escreveu:
Hi everyone, 
   (It's me again.)
   I am working on an audio channel using QPSK modulation. I currently am transmitting through a signal source that outputs bytes. I am looking for a method to align the byte boundaries so I am able to hear a clean audio at the receiver. 
   Here is an idea of what I want to do: send packets that have, for example, 7 bytes of data and 1 byte of known pattern, and so I can sync with the receive block. However, I do not know how to implement this in GRC (I see a block named packet header generator, is this what I want to use?). Could someone please advise me on how to approach this?
   Thanks in advance.
   Lannan Jiang 

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