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[Discuss-gnuradio] TIMEOUT from "digital_ofdm_sampler.cc"

From: Sahoo, Anirudha
Subject: [Discuss-gnuradio] TIMEOUT from "digital_ofdm_sampler.cc"
Date: Wed, 20 Mar 2013 15:06:27 -0400


   I am trying to implement an algorithm over the ofdm implementation (modifying

benchmark_xx.py) so that the txmitter and rcvr can communicate changing

the subcarriers dynamically.

   The scheme is roughly as follows:

(1)     Inially the txmitter and the rcvr listen on a well-known set of subcarriers.

(2)    then the txmitter sends a carrier map (set of subcarriers)

over which the subsequent transmissions will happen.

(3)    the rcvr acks that it has received the new carrier map (and tunes to those subcarriers for reception)

(4)    On receiving ACK, the txmitter also sets its carrier map to new carrier map.

(5)    Now both the ends are synced to this new carrier map

(6)    some data packets are sent over this new carrier map

(7)    then the txmitter decides to change the carrier map and goes to step (2).


  Here is the problem that I encounter:

  The rcvr gets few packets (usually first two times the carrier map reset works and the rcvr

   is able to get the packets).


    But then the txmitter sends the new carrier map. The rcvr receives it, sends an ACK, sets its

   carrier map appropriately. The txmitter receives the ACK, sets its carrier map and then starts

  sending data packets.

    But now the rcvr does not get any packets (even though it is tuned to the same subcarriers as

   the txmitter is transmitting on).

    Then I see the message “TIMEOUT” on the rcvr (coming from general_work() of “digital_ofdm_sampler.cc”).


      I have even put some delays between resetting carrier map and sending data (so that the receiver is

    ready with the new carrier map), but that did not help.


    Any idea why the receiver stops getting packets after few times the carrier map is changed?


   I am running gnuradio version 3.6.2 on USRP N210. The txmitter and the rcvr are connected over

  a channel emulator (using RF cable), which emulates a perfect lossless channel.


thanks and regards


--Anirudh Sahoo

Advanced Network Technology Div.

National Institute of Standards and Technology (NIST)

100 Bureau Drive,

Gaithersburg, MD - 20878

Room – B230, bldg.- 222

Phone- 301-975-4439


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