# Beamforming with GNU Radio and USRPs **URL:** \ **Category:** GNU Radio\ **Created:** [July 23, 2014, 2:12pm UTC](https://www.ruby-forum.com/t/beamforming-with-gnu-radio-and-usrps/231140 "2014-07-23T14:12:52Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [July 23, 2014, 2:12pm UTC](https://www.ruby-forum.com/t/beamforming-with-gnu-radio-and-usrps/231140/1 "2014-07-23T14:12:52Z")
Hi everyone, I am currently trying to get 2 N210 to transmit coherently to a third N210. All devices have GPSDO kits and the “gps\_locked” status is on “locked”, there is no MIMO cable in the setup. I am currently doing some preliminary tests where I feed both sinks with the same signal i.e a complex sine wave using the signal source from GNU Radio and I observe the signal received by the third USRP. I made sure that both transmitted signals are received with the same amplitude when taken independently. My understanding is that if both transmitters are locked to the GPS there should be no frequency offset between them and there should only be a constant phase offset between them which would give me a somewhat constant amplitude signal at the receiver. On the receiver, I observe the signal with both a time sink and a frequency sink. My problem is that the received signal seems to be varying a lot in amplitude, it looks like a slow AM modulation which is something I would get if both transmitters weren’t perfectly aligned in frequency. Changing the frequency of one of the transmitters makes the problem worse. Does anyone have prior experience with this kind of setup and experienced the same issues? Thanks in advance, Jawad ---
**Author:** ![Jawad\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/d07c76/32.png) [@Jawad\_S](https://www.ruby-forum.com/u/Jawad_S)\ **Post date:** [July 23, 2014, 2:22pm UTC](https://www.ruby-forum.com/t/beamforming-with-gnu-radio-and-usrps/231140/2 "2014-07-23T14:22:18Z")
I am not aware of the details of the GPSDO implementation but your sources must be phase locked, not only synchronized through the GPS signal. The GPS control loop on the LO only has a finite bandwidth above which each oscillator is free to drift at will. Under such conditions, the interference pattern will be fluctuating because the PLL bandwidth is necessarily much lower than the carrier frequency. JM > Changing the frequency of one of the transmitters makes the problem worse. > > Does anyone have prior experience with this kind of setup and experienced > the same issues? > > Thanks in advance, > Jawad – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France ---
**Author:** ![Jawad\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/d07c76/32.png) [@Jawad\_S](https://www.ruby-forum.com/u/Jawad_S)\ **Post date:** [July 23, 2014, 2:37pm UTC](https://www.ruby-forum.com/t/beamforming-with-gnu-radio-and-usrps/231140/3 "2014-07-23T14:37:43Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Jawad, I think you might want to post follow-ups to this discussion to [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), since so far this has not much to do with GNU Radio. On 23.07.2014 14:11, Jawad S. wrote: > source from GNU Radio and I observe the signal received by the > third USRP. I made sure that both transmitted signals are received > with the same amplitude when taken independently. > So you put your N210s in an anechoic chamber and calibrated the > system, and where able to exclude any changing fading effects? That > sounds like a lot of work, so kudos! A few parameters would be nice to know, though: What’s the tuning frequency, and did you actually generate sines in software (so your over-the-air signal would have $f\_carrier = f\_tune + f\_sine$) or did you just send a constant value at a specific frequency ($f\_carrier=f\_tune$)? Range, antenna and amplification used would be nice to know, too, and daughterboard model of course. > My understanding is that if both transmitters are locked to the GPS > there should be no frequency offset between them and there should > only be a constant phase offset between them which would give me a > somewhat constant amplitude signal at the receiver. > Clever measurement setup! > > On the receiver, I observe the signal with both a time sink and a > frequency sink. My problem is that the received signal seems to be > varying a lot in amplitude, it looks like a slow AM modulation > which is something I would get if both transmitters weren’t > perfectly aligned in frequency. > “Slow AM modulation” does sound _a lot_ like fading. Also, your > receivers are not “perfectly” synced, they’re synced within the > specification of your GPSDO, which is quite good, but still \>2ppb (if > I remember correctly). Since you add up sines, you get a resulting > sine with a phase that is the result of adding up all the multipath > signals of your two transmitters. The slightest variation in > environment might shift a lot of not-direct signals, which then might > be cancelling or constructively overlay with other paths, thus > shifting the overall phase and amplitude quite a bit. Also notice that heat will change the the characteristics of the transceiver’s analog chain, so over time the signal of both TX USRPs might differ due to amplifiers etc reacting differently at different temperature. A few numbers would be nice here, too: What’s the received amplitude in the single-TX cases, what’s the range for a two-RX case? How fast is “slow AM modulation”? Do you tune to the same frequency or do you avoid DC-component problems by intentionally receive at a slightly offset frequency? > Changing the frequency of one of the transmitters makes the problem > worse. > > Does anyone have prior experience with this kind of setup and > experienced the same issues? > Usually, the frequency stability of the USRPs with GPSDOs is quite > good, and I really can’t tell if the slow amplitude variation you’re > seeing (but not quantifying) isn’t even covered by the spec of the > GPSDO. I think you should actually measure the phase and phase derivative of your received signal. > Thanks in advance, Jawad Greetings, Marcus -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 Comment: Using GnuPG with Thunderbird - [http://www.enigmail.net/](http://www.enigmail.net/) iQEcBAEBAgAGBQJTz6w1AAoJEBQ6EdjyzlHtChEH/AxmZSpRFyGI04q76dRSqJEb Nii9WhIlKDxxza5lU6qOiVfO9ovSnGwfNZraWSOVsEz7GRNv6v9JPkmq0rlEXzaC 00fT1E8fGTKy6Eaq5QAHqb1gfhMtuhi0S4lVDtckQXaeGykoTmWNiIwKPmFoUkrH knJg8d8jyzE8R1WWdY/sAOBmhR0/C3EEz9gvuYOeY2rOx9znYHXXZK5wR10oiPos onoAZIl+gnZIaRGxf1yVJQhbhlO8lthtXt6tBZKi/0wwAKvZ3JzpNLEnyddzLmHM NBNw4pjLoar7vA1kZg2ClzpAoXhVPBFDSiWNV5WwG3C5y0wkWvsCfYO6coLOgB0= =Stii -----END PGP SIGNATURE----- ---
**Author:** ![Jawad\_S](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/d07c76/32.png) [@Jawad\_S](https://www.ruby-forum.com/u/Jawad_S)\ **Post date:** [July 23, 2014, 2:50pm UTC](https://www.ruby-forum.com/t/beamforming-with-gnu-radio-and-usrps/231140/4 "2014-07-23T14:50:10Z")
On 07/23/2014 08:11 AM, Jawad S. wrote: > from GNU Radio and I observe the signal received by the third USRP. I > My problem is that the received signal seems to be varying a lot in > amplitude, it looks like a slow AM modulation which is something I > would get if both transmitters weren’t perfectly aligned in frequency. > > Changing the frequency of one of the transmitters makes the problem worse. > > Does anyone have prior experience with this kind of setup and > experienced the same issues? > > Thanks in advance, > Jawad > To clarify, your transmitters are both locked to _different_ GPSDO > units? If so, they won’t be at “exactly the same frequency”. – Marcus L. Principal Investigator Shirleys Bay Radio Astronomy Consortium > **[Live Draw HK Tercepat | Result HK | HK Pools | Live HK | HK Prize Hari Ini](https://www.sbrac.org)** > > Temukan live draw hk tercepat melalui halaman hk pools yang resmi dan dapatkan result hk prize melalui live hk secara langsung di situs kami.