Calculating sample delay between transmit and receive (some help required)

Projects Lime Suite
1

Hi everyone,

I am back doing some LimeMini programming and I need a bit of help.
My application transmits a signal and then monitors what is being sent to the
transmitter and compares it to what is actually being received at the antenna.

To do this I have to keep a delayed copy of the transmitted signal so I can
compare the monitored signal to the ‘perfect transmitted signal’

I am assuming that if I use a single channel to do this then the sample rate
and frequency will be exactly the same as they use a common reference.

Easy part
What I want to know is what is the best approach to make sure that
when I send a block of samples I can read an identical sized block of
received samples with no loss of alignment (i.e dropped samples).

Hard Part
I need to know the exact delay between a transmitted sample and
the receive sample so I can delay my ‘golden reference’ by that amount.
What is the best way to do this using LimeSuite?

I do realise there will still be a small extra delay due to the length of cables
and delays through the external Power Amplifier but I can use a correlator
to fine tune that.

At the moment I have no idea how best to work out the loopback delay (via RF)
of the system. I have searched the message board but I have not found an
exact answer to my current problem.

  • Charles
2

What I have done to achieve the hard part is to read the HW timer, add some time
to it then send a block of samples specifying it to delay tx until the know time, then
every time I send a block of samples I increment a uint64_t time variable in
my code, using that and the HW timer values returned in the received metadata
I hopefully have a reasonable idea of the lag between tx and receive.

Anyone got any better ideas?

  • Charles
3

Well that didn’t work as it appears the hardware timer on the Rx and TX side of a channel have different values.

4

OK if I stop and then start the tx and rx stream on a channel it resets the hardware timer on the receive and transmit channel. Whether that helps with my delay issue I don’t know. I can query the Tx and Rx fifo size but I am not
sure how much help that is going to be either.

5

Can you explain what you mean by they have different values?

Because what I figured out from the FPGA code is that both the RX and TX paths use the RX PLL for their timestamp values.

6

Well I called LMS_GetStreamStatus for the receive stream and the transmit stream
on the same port one immediately after the other and they returned hugely different
timestamp values, the TX side 0 and the receive side 5062260. It could of course be
that the TX side value is always zero (just thought of that).

If I do an LMS_StopStream followed by an LMS_StartStream on the rx and tx stream
then do a GetStreamStatus on the two streams I then get them returning the same
value. It is as if they are simply counting the number of samples. I am using a Lime Mini
the classic Lime maybe totally different.

  • C
7

Just checked the TX count does change but the values appear totally wrong.
The receive side increments by the mount I would expect but the tx side seems to return silly values.

8

Hey,

Thanks for the explanation. Can you provide me your stream config, sampling rate and whether if you are writing any metadata timestamp using your application. I can maybe reproduce the issue with those info.

Best regards,
Saptarshi

9

I am only using channel 0 and the stream config is basically what was in the example.
The sample rate is 2664000 (333KS/s * 8) The sample block sizes are 4096
I am not using the metadata to time the transmit times. I have checked and I am not dropping any samples
either on transmit or receive.

What I am actually doing is trying to write a program to do digital pre-distortion of a Power Amplifier.
I have all the maths written to solve Ax=b and hence calculate the set of coefficients required
to do the pre-distortion but I need to have before and after samples from the PA that are perfectly
aligned so I can do the Ax=b (or in my case (U^HU)a=bU^H U is built from the received samples and b from the
delayed copy of the transmitted samples. For further information have a look at

https://www.keysight.com/upload/cmc_upload/All/DigitalPreDistortion_MicroApps.pdf

    m_rx_streams[i].channel = i; //channel number
   	m_rx_streams[i].fifoSize = 1024 * 128; //fifo size in samples
   	m_rx_streams[i].throughputVsLatency = 1.0; //optimize for maximum throughput
   	m_rx_streams[i].isTx = false; //RX channel
   	m_rx_streams[i].dataFmt = lms_stream_t::LMS_FMT_I16; //16-bit integers
   	if (LMS_SetupStream(m_lms_device, &m_rx_streams[i]) != 0) error();

   	m_tx_streams[i].channel = i; //channel number
   	m_tx_streams[i].fifoSize = 1024 * 128; //fifo size in samples
   	m_tx_streams[i].throughputVsLatency = 1.0; //optimize for  throughput
   	m_tx_streams[i].isTx = true; //TX channel
   	m_tx_streams[i].dataFmt = lms_stream_t::LMS_FMT_I16; //16-bit integers
   	if (LMS_SetupStream(m_lms_device, &m_tx_streams[i]) != 0) error();

    m_rx_metadata[i].flushPartialPacket = false; //Do not discard data remainder when read size differs from packet size
    m_rx_metadata[i].waitForTimestamp = false; //Do not wait for specific timestamps
	m_tx_metadata[i].flushPartialPacket = false; //Do not discard data remainder when read size differs from packet size
	m_tx_metadata[i].waitForTimestamp = false; //Do not wait for specific timestamps