On Mon, Jun 21, 2010 at 12:00 PM,
<address@hidden> wrote:
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 :) 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
On 19 June 2010 17:10, Rahman Doost <address@hidden> wrote:
> 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
>
>
> On Thu, Jun 17, 2010 at 5:29 AM, Mike Jameson <address@hidden> wrote:
>
>> missed off the filename: ofdm_receiver.py
>>
>> :)
>>
>>
>>
>> On 17 June 2010 10:27, Mike Jameson <address@hidden> wrote:
>>
>>> 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
>>>
>>> On 16 June 2010 23:13, Rahman Doost <address@hidden> wrote:
>>>
>>>> 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
>>>>
>>>> On Tue, Jun 15, 2010 at 5:53 AM, Mike Jameson <address@hidden>wrote:
>>>>
>>>>> 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
>>>>> 1.
>>>>> - 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
>>>>>
>>>>> On 15 June 2010 10:20, Chris_Spain <address@hidden>wrote:
>>>>>
>>>>>>
>>>>>> 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:
>>>>>> >
>>>>>> > 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:
>>>>>> >>
>>>>>> >> 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?
>>>>>> >>
>>>>>> >>
>>>>>> >
>>>>>> >
>>>>>>
>>>>>> --
>>>>>> 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.
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Discuss-gnuradio mailing list
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>>>>>>
>>>>>
>>>>>
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