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Large baseband spikes from gr-digital OFDM transmitter


From: Manav Kohli
Subject: Large baseband spikes from gr-digital OFDM transmitter
Date: Tue, 23 Jun 2020 15:34:44 +0100

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.

Thank you very much,
Manav

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