Multicarrier OpenBTS on UmTRX

Srdjan Milenkovic s.milenkovic at limemicro.com
Tue Oct 30 11:48:06 UTC 2012


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
>>>>>
>>>>>
>>>
>
>





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