Multicarrier OpenBTS on UmTRX

Alexander Chemeris alexander.chemeris at gmail.com
Mon Oct 29 13:26:13 UTC 2012


Hi Srdjan,

If I understand correctly, I/Q imbalance calibration precision depends
on the sampling rate. What sampling rate do you use to achieve <-60dBc
image level?

On Mon, Oct 29, 2012 at 5:19 PM, Srdjan Milenkovic
<s.milenkovic at limemicro.com> wrote:
> Hi Alexander,
>
> It is IQ imbalance indeed. It has to be corrected in BB. I believe you have
> access to the document:
>
> "LMS6002D Improving transceiver performance using digital techniques
> -1.0r1.pdf"
>
> It may be useful. Also, we recommend to implement RX DC auto cancellation as
> explained in section 3.3 of the same document. This should solve RX LO
> leakage issue.
>
> We usually measure < -60dBc unwanted image level when IQ imbalance is
> corrected.
>
> Regards, Srdjan
>
>
> On 29/10/2012 12:33, Alexander Chemeris wrote:
>>
>> 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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