OFMD TX and RX

GNU Radio
1

Hello everybody,

I’ am trying to test and understand the OFDM Modulation implemented in
GNU
Radio. I’m using the deafult values, fft_lenth=512, occupied_tones =200,
cp_length= 128 and bpsk modulation, besides, I’m using a RFX900
Daughterboards on USPR and I’m connecting the TX and RX with a coaxial
cable.

I try to send a package of 1024 bytes but I’m not receiving all packets,
always lost the first packet and then is a little bit random, sometimes
I
receive and I demodulated the 2º,3º,4º package and lost 5º, or lost 3º
and
recive ok 2 ºand 4º…
I don’t know what are happening, because I don’t know how the OFDM
implementation map the 1024 bytes onto 200 carriers of data.

If I’m understanding the OFDM theory, with a bpsk modulation each
carrier
are modulated for two bits and my OFDM symbol length are 512+128
(fft_length+cp_length).So, ¿with 2 bits per data carrier (200), each
symbol
transport 400 bits, 50 bytes?

That can´t be true because I receive some packages with no errors.
Somebody
can explain me, How works OFDM Modulation implemented in GNU Radio?

–
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2

Hi,

I’m not sure why you can’t receive all the packet correctly, but I can
briefly tell you how ofdm implement in GNU Radio.

First, fft_length=512 means it will take 512 subcarriers to do ifft/fft,
and
occupied_tones=200 means it only take 200 subcarriers to put your
modulation
symbol. Besides, the 200 subcarriers are set at the center of 512
subcarriers. Then you will see there are the remaining 312(512-200)
subcarriers which is zero outside the occupied_tones.

likes this:

xxxxxOOOOOOxxOOOOOOxxxxx
--------------±---------------> f

O => occupied_tones
x => remaining subcarriers
the two x at the center is to avoid DC offset.
After ifft, the CP is added.

OFDM is sensitive to the bitrate(interpolate/decimate), so set the
bitrate
carefully.

Hope this will help you little.

Fisheep.

Chris_Spain wrote:

I try to send a package of 1024 bytes but I’m not receiving all packets,

That can´t be true because I receive some packages with no errors.
Somebody can explain me, How works OFDM Modulation implemented in GNU
Radio?

–
View this message in context:
http://old.nabble.com/OFMD-TX-and-RX-tp28871027p28888368.html
Sent from the GnuRadio mailing list archive at Nabble.com.

3

Thank you Fisheep, you confirmed what I was thinking but, I don’t Know
why,
the packets are been received with no errors, I lost packets but the
packets
that I receive are good.

I don’t Know how GNU Radio implementation maps 1024 bytes over 200
carriers
with a bpsk modulation, and then the transmission it’s OK.

Cheers

Fisheep wrote:

subcarriers which is zero outside the occupied_tones.

Chris_Spain wrote:

I try to send a package of 1024 bytes but I’m not receiving all packets,

That can´t be true because I receive some packages with no errors.
Somebody can explain me, How works OFDM Modulation implemented in GNU
Radio?

–
View this message in context:
http://old.nabble.com/OFMD-TX-and-RX-tp28871027p28888978.html
Sent from the GnuRadio mailing list archive at Nabble.com.

4

Hi Chris,

The default settings do drop alot of packets as you are seeing.

These are some of the things I’ve adjusted which happen to have brought
my packet reception rate up to 99% using 1 USRP with a Basic RX and
TX joined with a 10db attenuator:

  • For benchmark_tx.py and benchmark_rx.py add ‘-m qpsk’ to the command
    line
    for qpsk instead of bpsk.
  • For benchmark_tx.py add ‘-s 4’ to the command line to reduce the
    packet
    size sent.
  • For benchmark_tx.py add ‘-d 1’ to the command line to reduce the rate
    and
    change the benchmark_tx.py script to sleep for 0.01 seconds instead of
  • For benchmark_tx.py add ‘–tx-amplitude=3000’ to the command line to
    boost
    the default output power.
  • In transmit_path.py pad_for_usrp=true and msgq_limit=1

Cheers,

Mike

5

Hi Rahman,

Try broadening the bandwidth of the receiver to allow for the frequency
offset between your USRP2s:

In /usr/lib/python2.6/dist-packages/gnuradio/blks2impl

on line 67 look for “tb = bw*0.08”

change this to a higher value such as “tb = bw*1.5”

This works for me using usrps.

Mike

6

Guys,

I’m working on the same thing, I’m trying to receive something with OFDM
example, USRP2 and XCVR boards. But it doesn’t work. The transmitter is
sending and the rx signal is crystal clear with spectrum analyzer but no
packet is caught.
Could anyone of you send me the command you’re using at both transmitter
and
receiver with benchmarks? I really appreciate it.

Thanks
Rahman

7

Thanks Mike for the comment. I did change the bandwidth, but some error
poped up at runtime for 1.5. Lower values like 0.1 or 0.2 did not change
the
behavior of the receiver. Still no correct packet. I receive the packets
though but they fail the CRC check I guess, since I get this “ok:false”
status on each of them. Any idea?

Thanks,
Rahman

8

Hi Rahman,

Just like you, my USRPs have suddenly started failing the CRC for every
packet.

By changing the frequency to 1MHZ I’m back to 99% correctly received :slight_smile:
From
searching the mailing list it looks like it’s related to the increased
frequency offset due to aliasing at the higher frequencies.

Also, there is a setting in ofdm_receiver.py for SYNC. For me, SYNC=“ml”
works the best.

Mike

9

missed off the filename: ofdm_receiver.py

:slight_smile: