Extending the LTE range

[shao]

We have successfully used LimeSDR to build a LTE basestation.

When we use a 2W PA/LNA@800MHz, the range is 300 meters. If we use a 10W PA/LNA@1.8GHz, the range is 500 meters.

We want to use LNA with higher gain to increase the range to around 1km or 2km. But when the UE came nearby, it would cause the receiving channel of LimeSDR to saturate. We know the AGC function should be able to help with that. But we don’t know how to setup with proper parameters. We found when AGC is enabled, some noise start to arouse.

Could you give us some advice? Thanks.

[9a4db]

Just curious:
Are you using some BPF on Lime RX side?
With 10W PA range must be much better…

[shao]

We are currently using duplexer as our filter. Downlink channel is easy to extend range, we can simply use larger PA, but for uplink it’s difficult. That’s why we are trying to use AGC.

[9a4db]

My advice is to measure and readjust that duplexer first…
Than try to add LNA4ALL in RX path after duplexer.

[shao]

How much is the gain of this LNA4ALL?
We are designing a new duplexer with an isolation of 100dBc.

[9a4db]

Pls see here -> https://lna4all.blogspot.com/

[Zack]

You can not have an AGC at BB side at all. Let us assume that UE#1 is near to the station while UE#2 is not. In this case UE#2 will lost coverage while BB will try to lower power not to saturate UE#1. This must be done from UE side i.e. UE must adjust its PA accordingly not to saturate BB LNA using technical information from BB.

[shao]

Thanks Zack. Noted.

We think it’s necessary for some extra filtering on the limesdr board, for higher out of band rejection. How to configure the analog and digital filter inside LMS7002 for the tx and rx path? Is there additional filter implemented by the FPGA that we can use?

[Zack]

Hi @shao,

Check LMS API documentation functions LMS_SetGFIRLPF and LMS_SetLPFBW for more information.

[shao]

Hi @Zack,

Do you have some documents focusing on how to configure those filters in LimeSuiteGUI? We want to configure them in LimeSuiteGUI first, to intuitively see the filtering effects before dive into the code. Because our lte software is using soapy instead of directly calling LMS API. It’s not very easy for us to find the right place to call LMS_SetGFIRLRF and LMS_SetLPFBW.

If we setup the register of filters in LimeSuiteGUI and then start up the lte software, will it flush out our configuration when initiating?

[Karolis]

Hello shao,

There is no specific document for the GFIR configuration - it is covered in the datasheet section “TRANSCEIVER SIGNAL PROCESSOR” in “General Purpose FIR Filters” sub-section. Please refer to it, on how to manually configure the GFIR filters.

Digital filter behavior will depend on the LTE software behavior when it starts up. If it overwrites all registers at some point (or at least registers related to GFIR banks, RXTSP/TXTSP and digital interface configuration), the GFIR filter setup done prior with LimeSuite will be discarded or function not as intended.

[shao]

Thanks Karolis.

These days we modified the code to call the functions of digital filter. But when we see on the uplink channel the received signal spectrum distorted. It seems that the passband isn’t linear? Is this normal or we should configure it for a larger passband?

[Karolis]

Hello shao,

could you explain in more detail what do you mean by saying distorted. Maybe you have a photo of the spectrum for evaluation? What is the pass band of your wanted signal and what are the setting for the LMS7002M (analig and digital filter settings, TSP speed and interface configuration)? Can you actually do a setting dump in the form of .ini file?

[shao]

Hello Karolis,

I am still reading the datasheet of LMS7002. We are now calling the function related to digital filter through Soapy. We will look for a similar way to do the register dump through Soapy.

At the beginning, we found our signal distorted like the photo below, the Rx spectrum has a shape of half ellipse shape, like a mountain. It is actually similar as Tx in amarisoft.

After a few times of running, now we don’t have that problem anymore. Both the rx and tx spectrum looks normal in shape. But they are attenuated too much in the passband.

This is the tx signal. I am seeing it on another spectrum analyzer. The pink one below is when the digital filter is on and the yellow one is when it’s off. Yellow means high signal while pink means low.

And then I tested the Rx filter. You can see when the rx filter is on, the received signal strength on spectrum is around -20dB.

But when I turned it off. It’s around 20dB

There is a 40dB difference. I noticed on some documents that the digital filter of LMS7002 has an adjacent channel rejection of 40dB. Is it just a coincidence or I have wrongly setup the passband? But there is only a parameter called bandwidth for me to configure. My intended bandwidth is 11.5MHz. And I configured the filter to be 12MHz. There is no place to setup the center frequency of this filter.

I am wondering is there a special sequence to set the filter? Like setting the LO for tx and rx path first, then the filters? Should I setup analog filter before the digital ones or the other way around?

By the way I don’t know yet whether the analog filter has been set or not, because I haven’t seen the message printed in terminal regarding analog filters. I will check also the TSP speed and interface configuration in the documents first and come back to you with the data.

Thanks

[Karolis]

Hello shao,

is there a possibility for you to send me the configuration file of the entire chip? Seems your configuration is off.

Q: I am wondering is there a special sequence to set the filter? Like setting the LO for tx and rx path first, then the filters? Should I setup analog filter before the digital ones or the other way around?
A: Analog-digital or digital-analog filter setup does not interfere with one another. For digital filter, interface must be setup prior, because it will affect the TSP clock. For analog filters, the tuning procedures are self-contained and no specific requirements are needed to setup prior. Of course external power down command via RXEN or TXEN would interfere with analog filter tuning, but this is common sense kind of requirement (have a disabled chip and try to do something with it …).

[Securipro38]

Hi
Can you show your 2w power amplifier picture ?
Where did you buy from?
Did you mesure 2 w with professionnal power meter?
Be carefull about seller who claims large band power amplifer
Thanks

[shao]

Hello Karolis,

Today, I am having issue that I don’t see any downlink signal from my basestation. Then I disassembled it, and took my LimeSDR out. At the beginning there is no signal coming out from LimeSDR, but after a while of plugging and unplugging the ipex cable, it works again. I am still guessing the cause. Tomorrow I will try to make it work as it used to be. Then try to export the configuration file.

[shao]

The amplifier is custom made by my friend. I’m not a professional of PA designer, so I haven’t measured it with power meter. I am focusing on the LimeSDR side, trying to play with its built-in filters and agc functions to maximize its performance.

[shao]

Hello Karolis,

Sorry for late response. Could you please give me some instruction to get the configuration file of LMS7002M? Can I do that in LimeSuiteGUI? Should I do that while the enb is running or once the enb program is closed?

Today I substituted my 2W PA/LNA to an 20W PA/LNA and its uplink gain is 25db. When I set up the tx_gain and rx_gain to 30 in the LTE software, it works normally. Then I increased the tx_gain into 35. LimeSDR suddenly stopped to work, I got messages about USB error. Once I reset my computer and decrease tx_gain to 30, then it works again. What problem could that be? Is it because my transmitted signal too strong and my USB cable is interfered?

Regards,
Shao

[9a4db]

Yes, can be RF in USB cable.
Try to put some click on ferrite cores on the both ends.