Thanks for your email. Have been reading through the link you sent me, just trying to relate the direct UHD stuff with the equivalent in gr-uhd code.
I am planning to implement some LDPC code that I wrote for another application. Have you tried implementing something similar?
Regards,
David
________________________________________
From: Johannes Demel address@hidden
Sent: 29 January 2014 18:53
To: David Halls
Cc: Martin Braun; address@hidden
Subject: Re: [Discuss-gnuradio] Half-Duplex Relay
Hi David,
you could consider to tweak the latency [1] of your system between host and USRP. This way your relay is less dependent on that and you can reduce the gaps between packets if the host side signal processing can keep up.
If you increase the packet size of your OFDM system you might also consider to use channel coding, e.g. a convolutional code.
On Wed, Jan 29, 2014 at 10:11 AM, David Halls <address@hidden<mailto:address@hidden>> wrote:
Hi Martin, and all,
I am making good progress with the relay.
At the source, I transmit packets interspersed with 0's to create a silent period. This is achieved using vector_insert. Perhaps there are better ways, but it works well. Currently the gap is 20ms (2e4 samples at 1e6Ms/s) between tx'd packets from source.
At the relay I take the rx_time UHD tag, then in HPD I work out the actual time that each trigger is received (i.e. beginning of each packet) using nitems_read(0) with rx_time and sample_rate, I then decode, re-encode, and then add sob, eob, and a tx_time created by adding 10ms to the rx_time, so that it is transmitted half-way between the packets from the source.
This works but gives large gaps - each burst is (3+3+16)*80 = 1760 samples, a lot of time is wasted.
The decoding/encoding delay in the relay seems to vary between around 3000 and 10000 samples, so I can't reduce the timeslot length without some packets at the relay arriving at the USRP late 'L'.
I am not sure how to proceed. I can increase the packet length at the transmitter to fill the gap, but not sure if this will lengthen the relay decoding delay or not. Also with the OFDM_tx code, I get buffer errors with a payload longer than 96Bx2. i.e. 32 symbol payload.
Any thoughts?
David
________________________________________
From: address@hidden<mailto:address@hidden> address@hidden<mailto:address@hidden>] on behalf of Martin Braun address@hidden<mailto:address@hidden>]
Sent: 14 January 2014 16:56
To: address@hidden<mailto:address@hidden>
Subject: Re: [Discuss-gnuradio] Half-Duplex Relay
On Tue, Jan 14, 2014 at 11:29:51AM +0000, David Halls wrote:
> Thanks Martin,
>
> Yes, I am using USRP N210. I aim to have separate code on S, R and D as you suggest. I have built a 2 x 1 MISO system developing from your OFDM GRC code - thanks :)
>
> I have added a number of new elements including 802.16e randomiser, random vector source, SNR estimate, BER estimate and orthogonal headers... anyway I digress.
>
> I am already using some tagging. I hope I can use the rx_time timestamp and number of samples received to work out the time I want to begin and end transmission at the relay. This, as Marcus comments, will be much more complicated as the network topology grows. Currently I will assume just two time slots and allow a generous amount of time for propagataion and decoding/reencoding delay.
>
> I will then try to use 'set_command_time' to control when the relay txs.
>
> Do you feel this makes sense? I will let you know how I progress.
Sounds good. Tell us how you're coming along, and do ask for advice
if necessary.
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