-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi Jawad,
I think you might want to post follow-ups to this discussion to [email protected], since so far this has not much to do with GNU Radio. On 23.07.2014 14:11, Jawad Seddar wrote: > 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. 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/ iQEcBAEBAgAGBQJTz6w1AAoJEBQ6EdjyzlHtChEH/AxmZSpRFyGI04q76dRSqJEb Nii9WhIlKDxxza5lU6qOiVfO9ovSnGwfNZraWSOVsEz7GRNv6v9JPkmq0rlEXzaC 00fT1E8fGTKy6Eaq5QAHqb1gfhMtuhi0S4lVDtckQXaeGykoTmWNiIwKPmFoUkrH knJg8d8jyzE8R1WWdY/sAOBmhR0/C3EEz9gvuYOeY2rOx9znYHXXZK5wR10oiPos onoAZIl+gnZIaRGxf1yVJQhbhlO8lthtXt6tBZKi/0wwAKvZ3JzpNLEnyddzLmHM NBNw4pjLoar7vA1kZg2ClzpAoXhVPBFDSiWNV5WwG3C5y0wkWvsCfYO6coLOgB0= =Stii -----END PGP SIGNATURE----- _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
