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Re: [Discuss-gnuradio] feedback loop in gnu radio

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] feedback loop in gnu radio
Date: Wed, 22 Feb 2017 13:28:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

Hi Sagnik,

right now, you can't build loops out of stream connections. That has causality reasons – if you had such a loop, and the "merging" block at the start of the loop would need input on both channels, GNU Radio couldn't guarantee your flow graph could ever start¹.

If you can live with a loosely–coupled feedback loop, message passing would be an appropriate method: You can pass messages from any block to any block without any restrictions - but also, without backpressure. See chapter four or five of http://tutorials.gnuradio.org . (If you haven't already, read the tutorial from the start – I think it's rather nice!)

Best regards,


¹ In fact, that restriction has historical reasons, and we know we could remove it in some cases, but we didn't, so far. I used to have an experimental branch where we didn't have that restriction, but then a lot of our not-directly-stream-connection-related stuff breaks; basically, a lot of algorithm in GNU Radio's internal data handling depend on the Flow Graph being cycle-free.

On 22.02.2017 12:51, Sagnik Basu wrote:
I am using a GRC flowgraph for testing the energy detection algorithm for cognitive radio.I want to update my threshold for probability detection by estimating the SNR of the channel.

However, for this, I need a feedback loop, which I want to know how to implement.

Sagnik Basu
B. Tech | 2013-2017
Dept. of Electronics and Communication
NIT Rourkela
M: 8337942928

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