Hi Take, On HF channels OFDM is a neat solution to multipath problems like delay spread and frequency selective fading. Single carrier modems are possible but require a different design, in particular an equaliser to handle the channel.
As Hams tend to have SSB radios, they are already designed for waveforms (like analog SSB) that already have significant PAPR. So OFDM is reasonable fit to available hardware, despite the PAPR issues. Cheers, David On 18/6/20 10:20 am, Tsutsumi Takehiko wrote: > Hi Glen, > > > > What is your position of Single-carrier FDMA or LP-OFDMA technology > compare to PARP reduction scheme which you have rich experience in your > memo? > > > > I wonder why radio amateur society leaves PARP issue untouched when they > adopt OFDM technology. > > > > Does it comes from SSB experience which has about 5dB PARP? > > > > Regards, > > > > take > > > > de JA5AEA > > > > Windows 10 版のメール <https://go.microsoft.com/fwlink/?LinkId=550986>か > ら送信 > > > > *差出人: *Glen English <mailto:[email protected]> > *送信日時: *Wednesday, June 17, 2020 8:09 AM > *宛先: *[email protected] > <mailto:[email protected]> > *件名: *Re: [Freetel-codec2] FreeDV Tx power > > > > Hi David > > I've been across OFDM PAPR reduction development for many years. > > Most successful simple schemes involve some sort of selective mapping or > bit flipping. > > -IE the PAPR is computed and an alternative trajectory is generated . > > -These can be either tweaking the source bitstream slightly (many methods) > > -and also providing overmapping of the constellation (say 64 QAM points > might only map to 48 unique symbols, giving the system a choice of some > other point that can convey the same data. > > -and also deliberate FEC violation (modify the most encoded code and > remove/reduce the offenders) > > -and also selective bit flipping - easy and fast. > > I would encourage some form of PAPR reduction for any ham OFDM development. > > Without it, OFDM is often and (and ignorantly) overdriven creating > spectrum headaches for adjacent channel users > > > > -glen > > On 17/06/2020 07:33, David Rowe wrote: > > Hello List, > > > > I've been working with Peter, VK3RV on the surprisingly tricky problem > > of measuring FreeDV Tx power. > > > > Over the weekend I built an inline "sampler" and tried a few averaging > > options to measure the power of a 700D waveform on my spec-an. I am > > using my IC7200 with the drive adjusted so that the ALC just moves. I > > am sampling the power between the IC7200 and my ATU, which is connected > > to a dipole on 40M. > > > > > > _______________________________________________ > Freetel-codec2 mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > _______________________________________________ Freetel-codec2 mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/freetel-codec2
