Symbol Sync parameters

Question

I'm trying to implement a flowgraph in gnuradio that is a basic OFDM transceiver to communicate between two USRP devices (Ettus N210). I'm using an Octoclock of National Instruments to have the same clock source for both devices. My problem is that I don´t want to use this octclock to synchronize the devices, instead of using this octoclock device I would like to use the Symbol Sync block to achieve synchronism between devices, but I'm not getting any good results from that. Can anyone help me with the parameters or does anyone have ever worked with this block?

Answer 1

There's no Symbol Sync Parameters that can solve this!

The largest part of your flow graph makes no sense:

• Your USRP takes baseband signals and shifts them to the frequency you want. You shouldn't mix this yourself.
  • Your receive-side mixer (don't use that at all) is totally broken; you're missing the low-pass filter. Again, don't mix at all. That's your USRP's job.
  • If you want to only occupy an upper 1/4 of the bandwidth, simply select these carriers with the carrier map parameter. No need for resampling. Really no reason to use the arbitrary resampler for a rate of 4.
  • Your receive side first throws away half of your signal, don't do that, if you want an analytical signal.
  • These are digital complex signals, if you really wanted to do the mixing (don't!), you could simply multiply with a single complex sine, or use the "rotator" block. Again, don't mix.
• Your low-pass filter is totally wrong; your signal of interest has more than 240 kHz of bandwidth.
• Your sync block hurts you – your OFDM receiver has built-in synchronization.
• You're completely missing channel coding, which won't turn out in your favor even at really good SNR. Which is strange - you even have the definition of a decoder in your flow graph, you just don't use it.

Essentially:

• everything after the OFDM Transmitter should be removed.
• everything before the OFDM Receiver should be removed.
• Add channel coding.

I'm a bit confused: This really looks like you took the working ofdm_loopback.grc example that comes with GNU Radio and reworked it until it didn't work anymore. Also, GNU Radio 3.8 has been out for a year – might be worth to update your system.

Answer 2

Thanks for the answer. The purpose of those blocks of sine and cosine, both on the transmitter and the receiver sides, is to put the signal to an intermediate frequency, before apply the carrier of USRP sink/source. Basically, I need the signal on intermediate frequency (on both Tx/Rx sides) to do some tasks. In the attached file I try to give you a better explanation of what I want to do. The variable O it's the oversampling factor (O=4).

Tx_Rx_scheme

Best Regards