Re: [Discuss-gnuradio] Half-Duplex Relay

[David Halls]

Hi Martin and all,

I am looking at some AF stuff for now, where the delay is much better.

I am using something similar to HPD at the relay to implement some gain to the payload samples only. I call

                    // Copy header
                    copy_n_symbols(in, out, 0, samples_per_header);
                    // Copy payload
                    copy_n_symbols(in, out, 0, samples_per_payload);

where the function is:

    void packet_detector_impl::copy_n_symbols(const unsigned char *in, unsigned char *out, int port, int n_symbols)
    {
        // Copy samples
        memcpy((void *) out, (void *) in, n_symbols*sizeof(gr_complex));
        // Copy tags
        std::vector<tag_t> tags;
        get_tags_in_range(tags, 0, nitems_read(0), nitems_read(0) + n_symbols);
        for (unsigned t = 0; t < tags.size(); t++)
        {
            int new_offset = tags[t].offset - nitems_read(0);
            add_item_tag(port, nitems_written(port) + new_offset, tags[t].key, tags[t].value);
        }
    }

My questions is probably stupid, but I want to multiply the payload symbols by a certain gain, say 'G', but I am not clear how to do it.  I am not clear how to access the items individually to scale them.

As a first step, I tried first replacing the memcpy with

                  for(int i = 0; i < samples_per_packet; i++) out[i] = in[i];

but that of course doesn't successfully replace the functionality?

Regards,

David

________________________________________
From: Martin Braun address@hidden
Sent: 30 January 2014 15:56
To: David Halls; address@hidden
Subject: Re: [Discuss-gnuradio] Half-Duplex Relay

On 30.01.2014 16:49, David Halls wrote:
> Thanks Martin,
>
> So your recommendation would be not to spend too much time poking around
> with USRP latency stuff:
>
> http://code.ettus.com/redmine/ettus/projects/uhd/wiki/latency

This is an excellent guide, and a good read. In this case, I doubt UHD
is the limiting factor.

> Is there an obvious way to benchmark the latency in GNU Radio? I have
> basically stuck your decoder and encoders back to back in the relay
> code, so it is pretty intensive. I suppose that implementing some of it
> in the FPGA may help? I have no experience in that field at all though...

It would certainly help, but you'd be looking at an absurd amount of
work. I can't think of a good pure GNU Radio way to measure latency, though.

> The current implementation is enough to show proof of concept but is
> very inefficient.

That's true, but given that it's clicked together, 10 ms latency is not
all that bad.

> Is there an obvious way to increase the payload length without getting
> buffer issues in the code - have you ever tried increasing it
> significantly in your implementation?

See Aditya's question. Tagged stream blocks operate on one packet at a
time, so it's limited.

MB

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