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Re: [Discuss-gnuradio] Real-time fading simulation?

From: Brian Padalino
Subject: Re: [Discuss-gnuradio] Real-time fading simulation?
Date: Wed, 30 May 2012 08:02:56 -0400

On Wed, May 30, 2012 at 4:23 AM, J Mc <address@hidden> wrote:
>> Date: Tue, 29 May 2012 22:15:43 -0400
>> Subject: Re: [Discuss-gnuradio] Real-time fading simulation?
>> From: address@hidden
>> To: address@hidden
>> CC: address@hidden
>> On Tue, May 29, 2012 at 9:47 PM, J Mc <address@hidden>
>> wrote:
>> > I have been considering using GnuRadio with the USRPN210 as a realtime
>> > fading simulator for radio hardware testing, however any approaches I've
>> > considered in doing this seem to fall down fundamentally if I limit to
>> > using
>> > a single USRP. I'm still relatively sure it could be done, was wondering
>> > if
>> > anyone had any advice/input.
>> >
>> > The main issue I've had is trying to understand how to do this with
>> > single
>> > antennas systems, if I take something like 2 cheap WiFi nodes both
>> > attached
>> > to a common Tx and Rx port is there any way to prevent the transmitting
>> > node's signal feedback when it hits the receiving node's antenna. If
>> > anyone
>> > has looked at this question, opinions would be appreciated...
>> I think you can do it with an one USRP1, or two USRPN210s using some
>> circulators and a special FPGA load.
>> Circulators move in a clockwise motion:
>> [WiFi] <-> [ Circulator ] <-> [USRP Rx/Tx]
>> ^
>> |
>> v
>> [ Circulator ] <-> [WiFi]
>> ^
>> |
>> v
>> [USRP Rx/Tx]
>> I think that diagram shows the WiFi card transmitting to the USRP
>> Rx/Tx port, the Tx from the USRP goes to the other circulator, and
>> into WiFi card.
>> The second WiFi card transmits into the circulator then into the USRP
>> Rx/Tx port, and the Tx from the USRP goes to the original circulator,
>> and into the original WiFi card.
>> FPGA load would essentially be programmable with your noise/fading
>> profile, and with little host intervention create noise on the
>> baseband then retransmit.
>> Does that work?
>> Brian
> I may be missing something however anyway I try construct this design on
> paper (which is variations on what I've been considering) the following
> issues seem to occur;
> -Many Daughterboards only have 1 Tx channel, which both the nodes must
> connect to, how at transmissions "addressed" i.e. the circulator won't be
> able to avoid this
> -Because WiFi and other standards that use TDD share a single antenna, they
> must near-simultaneously transmit and receiver. If a USRP Transmission is
> heard by the original node, it will create a feedback loop

That is why you need either 2 USRPN210's or 1 USRP1.  The USRP1 can
have 2 daughterboards which allows you to have 2 independent RX/TX
ports.  The circulators handle the rest for you.

You're correct that using a single TX on a single daughterboard isn't
good enough.

> For the fading I was thinking of just using gnuradio without any FPGA
> alterations, (theres a fading model/noise sources/notch filters), my only
> uncertainty is the ability to receiver and transmit to the same USRP on
> different antenna ports.

I believe there are latency requirements in 802.11 that may be
difficult/impossible to achieve if you're shipping samples back and
forth.  If latency isn't a concern, I think that the above setup
should be valid.

Do you agree?


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