Re: [Discuss-gnuradio] DBPSK and USRPs

Tx and Rx are on their own USRP1s, the daughter board is the RFX2400. They are roughl 3ft apart from each other on a desk. The Rx side is using the RX2 port.

I will definitely try upping the gain when I get back into the office tomorrow. I will check the SMA ports, I didn't configure these myself so I should probably double check them anyways.

Logan Washbourne

Electrical Engineering Graduate Student

(Electromagnetics)

On Mon, Oct 26, 2015 at 2:46 PM, Marcus Müller <address@hidden> wrote:

Hi!

* looking at your constellation, relief comes setting in: It looks pretty circular to me; notice how the axes are scaled differently. So no catastrophic IQ imbalance.
* the fact that it's circular and not a line is probably the result of the constellation sink not being able to achieve timing synchronization, i.e. it doesn't "know" when a symbol starts
* You're right, RX SNR is terrible. However, RX power is very little indeed
* Rule of thumb: if(not clipping && in doubt) increase gain;
* What exactly is your "RF channel": antennas, direct cable with attenuator?
* The mistake I do every few months: Did you connect the antenna/cable to the wrong SMA port?

Best regards,
Marcus
On 26.10.2015 20:33, Washbourne, Logan wrote:
I looked at the spectrums of both sides and the RX side looks to be inundated with a lot of noise. I'm not sure if its my environment or my USRPs, but I really expected to see some resemblance of a signal. I am posting links to screencaps of the TX and RX sides.

http://imgur.com/GBTHw8T- TX
http://imgur.com/DBy8gqs - RX
Logan Washbourne
Electrical Engineering Graduate Student
(Electromagnetics)
On Mon, Oct 26, 2015 at 1:31 PM, Marcus Müller <address@hidden> wrote:
Can you compare the TX spectrum with the RX spectrum? Maybe it'd worthwhile to first look at at e.g. 500kHz of RX spectrum to see whether, due to frequency offset, some of the wanted signal simply gets cut off; admittedly, with 2S/sym that's not too likely...

Best regards,
Marcus
On 26.10.2015 19:16, Washbourne, Logan wrote:
I am using 250k!
Logan Washbourne
Electrical Engineering Graduate Student
(Electromagnetics)
On Mon, Oct 26, 2015 at 1:05 PM, Marcus Müller <address@hidden> wrote:
Hello!
what's the sampling rate you're using?

Best regards,
Marcus

On 26.10.2015 18:38, Washbourne, Logan wrote:
Hello all,

I'm trying to take a step back from my previous postings and try a simple approach to try and understand working with over the air communications with the USRPs. Let me know if it would be better to post this in the USRPs-Users list.

I'm trying to have a simple TX and simple RX side so I can eliminate and unnecessary complications.

The TX side has the following flowgraph: Vector Source(preamble+data)->DPSK Mod(DBPSK,2samples/symbol)-> Multiply Const(.707) -> UHD:USRP SINK(2.448GHz center freq, 10dB default gain)

RX Side: UHD:USRP Source(2.448GHz center freq, 10dB gain default) -> DPSK DEMOD(DBPSK, 2 samples/ symbol, rest default values) - > file sink

I'm checking the progress of the communication by looking at a constellation plot that is connected directly to the USRP SOURCE block on the receiver side. I'm getting a very elliptical shape, more spread out in the horizontal direction but not by much. I'm also looking at the bits from the receiver side in matlab and my preamble is not showing up.

These were the same problems I had when trying to use a correlated system for OTA communications. I feel like I'm missing something really simple.

Does anyone know of a simple TX, RX grc setup that I can use to test my USRPs?

Again, I really appreciate the help you guys give.

Logan Washbourne
Electrical Engineering Graduate Student
(Electromagnetics)
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From:	mleech
Subject:	Re: [Discuss-gnuradio] DBPSK and USRPs
Date:	Mon, 26 Oct 2015 16:04:16 -0400
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