Orthogonality (as the O in OFDM) guarantees a fixed phase relationship for every symbol unless a pattern is introduced in an effort to reduce peak to average power ratio (which I do not believe is happening here). PAPR is bane of OFDM and much research has gone in to reduce this problem which requires high linearity in the amplifiers in cell towers. Linearity is typically accompanied by poor efficiency (lots of heat goes along with watts).
In addition to this , there is indeed a random process involved in the synchronization. Schmidl-Cox is imperfect and "fine tuning" of the sampling phase also has some randomness. On the air you will have phase offsets due to frequency dependent group delay through the analog components of the RF systems, group delay due to multipath, and on and on. I didn't open your grc file but I assume this was the perfect "wire" channel with additive noise in which case ignore all of the other analog stuff I added. Bob On Fri, Jan 12, 2018 at 2:22 PM, Jeff Long <willco...@gmail.com> wrote: > Ah, normalization was the secret. The phase offset is there because the > subcarriers frequencies each look like a phasor that keeps moving (at a > rate relative to its offset). You want to predict what the phase will be at > the next symbol. > > > On 01/12/2018 01:59 PM, edwin wrote: > >> Hi Jeff, >> >> I just found out that if I normalize these taps by the FFT number(64 in >> this case), they have magnitude of 1! Now my questions are: >> >> Why are there phase offset? The phase offset for each subchannel seems >> different. Is it because of imperfect synchronization? >> >> >> Regards, >> >> Edwin >> >> > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > -- Bob McGwier Founder, Federated Wireless, Inc Founder and Technical Advisor, HawkEye 360, Inc Research Professor Virginia Tech Chief Scientist: The Ted and Karyn Hume Center for National Security and Technology Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY Faculty Advisor Virginia Tech Amateur Radio Assn, Trustee K4KDJ Director of AMSAT Member of PVRC (Roanoke-Blacksburg), TAPR, life member of ARRL and AMSAT, NRVR.ORG (Rocketry)
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