Re: Large baseband spikes from gr-digital OFDM transmitter

[Brian Padalino]

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