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Alexander Chemeris alexander.chemeris at gmail.comHi Srdjan, Have you seen a question at the end of this e-mail, regarding the LMS calibration? To me it looks like a result of the I/Q imbalance which we should compensate at the baseband level. Is that correct? On Thu, Oct 25, 2012 at 10:16 PM, Thomas Tsou <ttsou at vt.edu> wrote: > Hi all, > > I spent some time yesterday testing UmTRX with MCBTS. Aside from some > trivial fixes (inverted sample rate), the code ran fine with no > issues. The default bandwidth of the LPF was insufficient, so that was > widened. I tested with 9 handsets between 3 and 7 carriers on a Core2 > Duo (P7570) laptop. > > git://github.com/ttsou/openbts-multi-arfcn.git umtrx > > Configuration is through sqlite database, which is unchanged from > mainline; just set the number of ARFCN (7 max) and C1 channels. > Because of the bandwidth involved, be sure to increase the maximum > buffer sizes for UHD. The uhd_usrp_probe utility will warn if the > target size cannot be set. > > http://files.ettus.com/uhd_docs/manual/html/transport.html#linux-specific-notes > > Design > ===== > There are three resampling stages. > > 1. Outer resampler converts from the device rate to a multiple of the > 400 kHz channel spacing. > 2. Channelizer demultiplexes input stream to M channels > 3. Inner resampler converts from 400 kHz to a multiple of the GSM symbol rate. > > UmTRX rates: > > Channelizer Rate Device Rate Decimation > > 1.6 Msps 1.625 Msps 8 > 3.2 Msps 3.250 Msps 4 > > Options > ====== > > To broadcast dummy bursts on all carriers for spectrum testing, > uncomment the following preprocessor declaration in radioParams.h. > > #define ENABLE_ALL_CHANS > > With dummy bursts on all carriers: > > http://filebox.vt.edu/users/ttsou/http/7carrier_umtrx.PNG > > Calibration > ======== > > Calibration remains a concern. With single carrier, the carrier > leakage and and IQ imbalance appear as in-band distortion. For MCBTS, > OpenBTS offsets C0 on to the lowest carrier. This makes the carrier > and image quite visible, as shown in the following uncalibrated > capture. I checked with a swept tone to verify that this issue was not > a result of baseband DSP. The USRP also shows similar effects. > > http://filebox.vt.edu/users/ttsou/http/carrier_image.PNG > > Any idea how much carrier and image suppression we should expect with > calibration on UmTRXv2? > > Thomas -- Regards, Alexander Chemeris. CEO, Fairwaves LLC / ООО УмРадио http://fairwaves.ru