[srslte-users] Frequency offsets in srsUE

[Philip Shaw]

Xavier,

We are adding some analogue hardware that has a lower frequency cut off.  It does not work to DC.

The idea is to move the Baseband signal away from zero Hz.

I hope this helps.

Regards

Philip

From: Xavier Arteaga <xavier at srs.io>
Sent: 24 August 2021 13:06
To: Philip Shaw <philip at surewavetechnology.co.uk>
Cc: srslte-users at lists.softwareradiosystems.com
Subject: Re: [srslte-users] Frequency offsets in srsUE

Hi Philip,
I guess you are asking how to avoid carrier leakage<https://en.wikipedia.org/wiki/Carrier_wave#Carrier_leakage>. If so, this email will answer your question.

There are a few advanced no documented `device_args` parameters that can do this function.

You can use the parameter `lo_freq_offset_hz` implemented here<https://github.com/srsran/srsRAN/blob/master/lib/src/phy/rf/rf_uhd_generic.h#L175>. To set the local oscillator frequency out of the band of interest and avoid carrier leakage.

When tuning the LO with X300, there is a tune request<https://github.com/srsran/srsRAN/blob/master/lib/src/phy/rf/rf_uhd_generic.h#L368>. This provides a target RF frequency and other two frequencies: LO frequency and DDC/DUC intermediate frequency.

By setting `lo_freq_offset_hz` the LO frequency is moved `lo_freq_offset_hz` Hz from the target frequency while the DSP frequency is set to an auto policy.

I recommend setting `lo_freq_offset_hz` to equal the sampling rate (i.e. 23.04e6) to make sure it is far enough from the band of interest. If you are working with FDD, make sure TX LO frequency is not in the RX band.

Regards,
Xavier



On Tue, 24 Aug 2021 at 13:33, Philip Shaw <philip at surewavetechnology.co.uk<mailto:philip at surewavetechnology.co.uk>> wrote:
Hi,

Neel said:
Hello Philip:

Are you basically asking how to do offset tuning with srsRAN/srsLTE/srsUE?
I'm not sure whether this is currently supported.  You should be able to
have srsUE tune to a non-3GPP frequency band (which is basically the effect
of using offset tuning), but I'm not sure that the srsUE PHY would be able
to accommodate for that in its processing.

--Neel Pandeya
Hi Neel / Paul,
I have a X300 with a UBX160.  I need to move the analogue BB signal away from DC zero Hz before it enters the X300 for ADC etc.  I can set an offset using the property in the config file.  I can see that the search for a cell is now at this offset frequency (at the analogue signal point on the interface between the UBX160 and X300).  I don’t have a cell to connect to (for testing), so the question is will srsUE connect to a cell using a large offset value?

A offset of 1Mhz would be perfect for what we are doing.  Will the PHY work?  So, the system knows that the received signal is offset from zero Hz and is able to recover it?

Thank you.

Philip
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