Multicarrier OpenBTS on UmTRX

Alexander Chemeris alexander.chemeris at gmail.com
Mon Oct 29 12:33:17 UTC 2012


Hi 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




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