# Periodically varying channel gain mesurements

**URL:** <https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685>\
**Category:** GNU Radio\
**Created:** [February 25, 2011, 1:56am UTC](https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685 "2011-02-25T01:56:04Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\
**Post date:** [February 25, 2011, 1:56am UTC](https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685/1 "2011-02-25T01:56:04Z")

</div>

Hi,

I’m using Ubuntu 10.04 with a recent gnuradio-next branch and the UHD  
driver on USRP2/WBX boards.  
In an outdoor setup I have two receiving nodes about 40m away from  
each other and from the transmitter. The transmitter is a PC with two  
USRP2s connected with a MIMO cable. It is sending two different PN  
sequences every 120ms on both interfaces at the same time. I correlate  
the known sequences at the receivers to measure the channel gain. The  
results can be seen here (x-axis is the packet number, y is the  
absolute gain):

[http://cs.ucsb.edu/~veljko/downloads/channel\_gains\_outdoor.jpg](http://cs.ucsb.edu/~veljko/downloads/channel_gains_outdoor.jpg)

The gain seems to be varying with around 2 Hz frequency. Since the  
setup is static and the tx/rx gain is not modified during the course  
of the experiment I’m almost certain that the artefact comes from the  
system rather than from the environment. Does anyone have a good  
explanation for this? Perhaps there is some sort of a gain control in  
the USRP2 that I’m not aware of? I got similar behavior when I tried  
the same experiment indoors.

Thanks,

Veljko

---

<div class="post-metadata">

**Author:** ![Veljko\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a3d4f5/32.png) [@Veljko\_P](https://www.ruby-forum.com/u/Veljko_P)\
**Post date:** [February 25, 2011, 2:03am UTC](https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685/2 "2011-02-25T02:03:34Z")

</div>

> The gain seems to be varying with around 2 Hz frequency.

Quite possible, the WBX transmit has an analog power control loop  
formed by the ADL5386. The “gain” setting on WBX transmit is really  
more of a setpoint for the power control loop than gain

Jason

---

<div class="post-metadata">

**Author:** ![Veljko\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a3d4f5/32.png) [@Veljko\_P](https://www.ruby-forum.com/u/Veljko_P)\
**Post date:** [February 25, 2011, 2:19am UTC](https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685/3 "2011-02-25T02:19:53Z")

</div>

On Thu, Feb 24, 2011 at 16:54, Veljko P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> [http://cs.ucsb.edu/~veljko/downloads/channel\_gains\_outdoor.jpg](http://cs.ucsb.edu/~veljko/downloads/channel_gains_outdoor.jpg)
> 
> The gain seems to be varying with around 2 Hz frequency. Since the  
> setup is static and the tx/rx gain is not modified during the course  
> of the experiment I’m almost certain that the artefact comes from the  
> system rather than from the environment. Does anyone have a good  
> explanation for this? Perhaps there is some sort of a gain control in  
> the USRP2 that I’m not aware of? I got similar behavior when I tried  
> the same experiment indoors.

This looks like the effect of timing skew between the transmitters and  
receivers. Your correlation magnitude will vary in time with a period  
related to how long it takes for the receiver sample times to “slide”  
through one PN chip length. One way to test whether this is the case is  
to  
(temporarily) lock both the transmitter and receiver to the same  
external 10  
MHz reference; your correlation magnitudes should then be the same (with  
some small noise variance.)

Johnathan

---

<div class="post-metadata">

**Author:** ![Veljko\_P](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/a3d4f5/32.png) [@Veljko\_P](https://www.ruby-forum.com/u/Veljko_P)\
**Post date:** [February 25, 2011, 6:27pm UTC](https://www.ruby-forum.com/t/periodically-varying-channel-gain-mesurements/203685/4 "2011-02-25T18:27:19Z")

</div>

Thanks for the suggestions. I’ll see if the same 10MHz clock changes  
anything. However, my initial idea was to have devices far apart.  
Clocking the with the same ref clock in that setup seems impossible.

Cheers,

Veljko

On Thu, Feb 24, 2011 at 5:16 PM, Johnathan C.