Multicarrier OpenBTS on UmTRX

Alexander Chemeris alexander.chemeris at gmail.com
Tue Oct 30 10:10:09 UTC 2012


Thank you. We will check how well we could compensate I/Q imbalance on UmTRX.

On Mon, Oct 29, 2012 at 5:40 PM, Srdjan Milenkovic
<s.milenkovic at limemicro.com> wrote:
> I am not sure calibration precision is affected by sample rate.
> Theoretically, we are rotating I vector by alpha/2 and Q vector by -alpha/2
> and that is done by multiplication. Hence, the resolution of quadrature
> phase error (alpha) which we can correct is defined by the with of
> multipliers.
>
> Any way, we clocked ADCs at 15.36MHz (30.72MS/s digital IQ interface) in the
> experiment I mentioned.
>
>
> On 29/10/2012 13:26, Alexander Chemeris wrote:
>>
>> 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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