Re: [Discuss-gnuradio] Designing a good receiver

[Marcus D. Leech]

On 06/02/2011 07:10 AM, Mike Clark wrote:
> This comes from a discussion over on the USRP-users mailing list, but
> I felt it would be more appropriate to post here. Over on that list,
> someone was asking about problems they were having consistently
> receiving data on their USRP. My background is in CS, not RF comms, so
> please forgive me as I'm sure the answer will be fairly standard :)
>
> Anyways, the question I have is, is there a general procedure one can
> follow to design a decent receiver in gnuradio? For example, I have a
> project that I'm using for experimentation where my receiver looks
> like this: USRP Source -> GMSK Demod -> Packet Decoder -> File Sink.
> This works well when I have the USRPs cabled together and even when I
> have antennas with line of sight (I haven't checked max distance).
> When I don't have line of sight, however, I stop receiving packets.
> Are there any other gnuradio blocks I can add in to my setup which
> will help get better performance when there is no line of sight? I
> haven't tested it, but my guess is that when there is no line of
> sight, there must be a frequency shift or something of that nature
> happening which ruins the connection.
>
> Mike
>
>   
There are a few things that could be a problem:

    o multipath
    o received signal strength

What type of daughtercards are you using, and when you cable them
together, I hope that you're
  using an attenuator?  Otherwise, you risk damaging the LNA on the
receive side, which will cause
  it to be either partially or completely deaf.

Frequency offset *is* a problem, but is usually not sensitive to path
loss.  In most *real* radios, you
  need to determine the frequency offset, as seen by the receiver and
compensate.  This is more, or less,
  of a problem depending on the signal bandwidth.  A wideband signal is
less sensitive to frequency
  offsets, because the 'error' is a smaller fraction of the total
bandwidth.  OFDM is the exception, since
  in effect there are dozens of individual carriers, of relatively
narrow bandwidth.




-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org