Re: Large baseband spikes from gr-digital OFDM transmitter

[Marcus Müller]

Hi Manav,

the phenomenon you're referring to is called PAPR, 
peak-to-average-power-ratio, and it's really *the* standard problem for 
OFDM. There's a wealth of information out there on PAPR reduction. Must 
be thousands of papers on IEEE Xplore alone[1]!

As you can imagine, that's because the problem depends on what kind of 
channel you do which kind of transmission with which kind of 
restrictions using which kind of error correction accepting which kind 
of power backoff...

So, pick the most cited paper there [2], it gives a bit of an overview. 
Cheers!

Best regards,
Marcus

[1] 
https://ieeexplore.ieee.org/search/searchresult.jsp?queryText=ofdm%20papr&highlight=true&returnFacets=ALL&returnType=SEARCH&matchPubs=true&sortType=paper-citations
[2] Seung Hee Han and Jae Hong Lee, "An overview of peak-to-average 
power ratio reduction techniques for multicarrier transmission," in IEEE 
Wireless Communications, vol. 12, no. 2, pp. 56-65, April 2005, doi: 
10.1109/MWC.2005.1421929.

On 23/06/2020 16.34, Manav Kohli 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.
>
> Thank you very much,
> Manav

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