Re: [Discuss-gnuradio] OFDM test with USRP

[Martin Braun]

On 19.03.2015 14:11, Henry Jin wrote:
> Hi Marcus,
>
> I later added a throttle and time scope to the tx end to see how large
> the amplitude can be. Indeed, when the multiplier is 60m, the amplitude
> increases beyond 1, which will be clipped by the USRP driver. Setting it
> to be 0.45 seems like a safe choice.

You might be able to push it a bit more. But definitely stay clear from 1.

> For small amplitude cases such as when the multiplier is 20m, the SNR is
> also low. The reason the OFDM receiver fails to receive many packets is
> actually because the threshold used in digital.ofdm_sync_sc_cfb() for
> the plateau detector. Currently, the code leaves the threshold value to
> be default, which is 0.9. In low SNR cases, in fact, we need to set it
> smaller to tolerate noise. Originally, let's say the synchronization
> rate is only 1% or even lower, after this adjustment, it can be over
> 90%. I suggest adding a parameter in ofdm_txrx.py to adjust this parameter.

Hm. S&C works fine for low SNRs, and changing the threshold is usually 
not required, due to normalization. I'd be hesitant to add that 
parameter, there might be another issue here. When you say low SNR, 
which regime are you talking about? Because the slicers (QPSK demapping) 
will usually fail before the detection.

> Back to the high SNR cases, one issue puzzling me now is that when the
> multiplier is 60m, although indeed there are samples exceeding the
> amplitude limitation of 1, many other samples (in fact most of them)
> have a small amplitude less than 0.5. This greatly limits the overall
> SNR we can get. I wonder if there can be any improvements over this?

This is a well-known OFDM issue. You can choose an encoding that will 
limit PAPR, use pilot tones that decrease PAPR... and many other 
PAPR-reducing techniques.
This also depends on how many subcarriers you use. For random data, your 
PAPR usually stays within 2*ln(N), which is usually not that bad.

Cheers,
M