Multicarrier OpenBTS on UmTRX

Alexander Chemeris alexander.chemeris at gmail.com
Tue Oct 30 11:50:03 UTC 2012


Yes, I suppose that by default. Thanks for clarification anyway.

On Tue, Oct 30, 2012 at 3:48 PM, Srdjan Milenkovic
<s.milenkovic at limemicro.com> wrote:
> Forgot to mention, < -60dBc image suppression has been achieved by
> correcting both phase and gain imbalance.
>
>
> On 30/10/2012 10:10, Alexander Chemeris wrote:
>>
>> 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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