Re: Large baseband spikes from gr-digital OFDM transmitter

[Ron Economos]

There is a PAPR reduction block in GNU Radio, but it's specific for DVB-T2. It uses a tone reservation algorithm where excess power is dumped into unused/reserved carriers.

https://github.com/gnuradio/gnuradio/blob/master/gr-dtv/lib/dvbt2/dvbt2_paprtr_cc_impl.cc

Ron

On 6/23/20 08:36, Derek Kozel wrote:

To add on to what Marcus and Brian have said, one of the ways of slightly reducing this problem is Crest Factor Reduction. It would be very useful to have some of the standard CFR algorithms added to GNU Radio. Peak Cancellation is one that has looked promising to me. Peak Windowing and Noise Shaping are two others.

If you do implement any of these methods please consider contributing them back to GNU Radio so that we can all benefit.

Regards,
Derek

On 6/23/2020 3:49 PM, Brian Padalino wrote:

On Tue, Jun 23, 2020 at 10:38 AM Manav Kohli <mpk2138@columbia.edu> wrote:

Hello,

This problem is visualized here: https://www.dropbox.com/s/w7kdmf9dewwdomx/20M_2974_20_15_nocal_packet_time_tx.png?dl=0

This is an OFDM packet consisting of 6 symbols: the default sync word 1&2, SIG field and three data symbols. The data symbols are QPSK modulated and the sync words are BPSK.

Is there any way that I could reduce or eliminate these large spikes that even with a moderate baseband scaling still go considerably above a magnitude of 1? I have tried to use different data, and a different number of packet data symbols, but to no avail. I am definitely able to reduce the overall baseband scaling and get those spikes within range, but this is not desirable as I am trying to maximize transmit power.

The usage of the USRP-2974 and sampling rate is immaterial; this happens using a variety of different radios (should not matter as this is a GNU radio "issue") and sampling rates.

If anyone has seen this before or may have any advice, please let me know.

It's the nature of OFDM to have a high Peak-to-Average-Power Radio (PAPR).  You can look at PAPR reduction techniques, but otherwise you're in for around 10dB PAPR.

Good luck.

Brian