Topics

Testing resources

ve7dhm@...
 

Looking for testing resources / testing methods for installed UDRs at remote sites.

- can the UDR be put into a loop back mode at the RF or modem stage remotely?

- constellation display application of RX data

- SNR / BER data for determining RF path quality

- anyone writing python code in support of UDR testing ?

In the PNW there will probably some long RF paths across water and it would certainly

help to have testing resources available to troubleshoot connection / data throughput

issues at initial UDR install and throughout the year as weather conditions change the

RF path.


Thanks for any feedback


Paul VE7DHM


ross@...
 

Hi Paul,

Get the transmitter to send a known pseudo-random bit sequence that has good autocorrelation properties (eg a maximal length sequence from a linear feed back shift register: https://en.wikipedia.org/wiki/Linear_feedback_shift_register).

This is not encryption because the LFSR tap settings will be published and known to all (just like the "scrambler" used in K9NG/G3RUH 9k6 FSK modems).

Use a correlator at the receiver to demodulate this signal. The output of the correlator will tell you if you have any multipath echos (and what magnitude & delay they are, so that you can implement adaptive channel equalisation...). After channel equalisation, you can also use the correlator to determine S/N by measuring the difference between the correlation level and the noise level.
This technology has been around a long time eg every 2G GSM repeater sends repeated 64bit pseudorandom "training sequences" so that your mobile phone can sync to the repeater and adaptively equalise multipath and STANAG 4285 uses 80 symbols taken from a LSFR to achieve the same goal on HF. Adaptive channel equalisation is also absolutely vital for old fashioned dial up modems to operate 9k6 ~ 56k kb/sec in a 3 kHz voice channel. I ain't no patent lawyer, but I believe that there is plenty of published prior art from long enough ago for the relevant patents to have expired by now.


I have attached a spreadsheet (libre/open office .ods) and a few old screenshots. "channel equaliser.PNG" shows the TX bits, the raw RX bits, and the equalised RX bits. "impulse response1.PNG" shows the correlation function of the TX bits, and what comes out of the correlator at the simulated receiver. "impulse response2.PNG" shows the simulated receive correlator and what comes out of a correlator after the channel equaliser.

Basically, it sends the STANAG 4285 sequence over a bad channel (SNR is adjustable and the FIR is to simulate multipath), then it uses the autocorrelation properties of the received '4285 sequence to attempt to correct the channel impulse response (multipath). As you decrease the S/N, you will see that the channel equalisation is degraded.



73
Ross Whenmouth ZL2WRW

bhhoyer@...
 

Hi Paul,

When you say remote location, I assume you have no network access, only RF.

Looping 25W back into our receiver would be a bad idea. :) Looping the modem would just tell you the processor is running.

Do you mean more of a digipeater mode where received packets are stored then re-transmitted? We could disable checksum to guarantee a response for analysis locally.

As part of the Hi-Speed effort, we are building a channel sounder for testing purposes. This is a PN generator designed for analysis.

The easiest thing to do would be to put the remote UDR in sounder mode and use your local UDR to analyze the received data which includes RSSI and BER. Constellation is an internal design tool at this time, but we will provide the hooks to some enterprising WebGL folks for display.

The results could be logged locally. We could also run sounder as a chron job (telemetry or beacon) then you'd get results even if you couldn't successfully talk to the remote unit. A few second burst hourly would be interesting.

Great topic. What if you had several internet connected UDRs receiving results for comparison?

I'd like more input, I think this topic has a lot of merit.

Bryan - K7UDR

Steve <yahoo-udr@...>
 

Can someone please translate these question and response into a bit less tech-speak for the rest of us?

---------- Original Message ----------
[ Sent by bhhoyer@... [UniversalDigitalRadio] at 05/11/2014 08:07 AM ]

Hi Paul,

When you say remote location, I assume you have no network access, only RF.


Looping 25W back into our receiver would be a bad idea. :) Looping the modem would just tell you the processor is running.


Do you mean more of a digipeater mode where received packets are stored then re-transmitted? We could disable checksum to guarantee a response for analysis locally.


As part of the Hi-Speed effort, we are building a channel sounder for testing purposes. This is a PN generator designed for analysis.



The easiest thing to do would be to put the remote UDR in sounder mode and use your local UDR to analyze the received data which includes RSSI and BER. Constellation is an internal design tool at this time, but we will provide the hooks to some enterprising WebGL folks for display.


The results could be logged locally. We could also run sounder as a chron job (telemetry or beacon) then you'd get results even if you couldn't successfully talk to the remote unit. A few second burst hourly would be interesting.


Great topic. What if you had several internet connected UDRs receiving results for comparison?


I'd like more input, I think this topic has a lot of merit.


Bryan - K7UDR