For your amusement and consternation here are my latest thoughts on doing an
OFDM protocol.
Symbol rate: 62.5 Hz (128 samples @ 8000 Hz)
Guard interval: 2, 4, 8 ms adaptive to conditions
Subchannels: 8 (62.5 125 187.5 250 312.5 375 437.5 500)
Bandwidth: 437.5 Hz
Raw BPS: 1778, 1600, 1333 adaptive (guard band change)
Base frequency: undetermined
MODULATION (somewhat firm)
Waveform: DQPSK with constellation at 45, 135, 225, 315 degrees
Generation: 8 separate generators providing continuous waves through the
guard bands
Phase change: start of symbol period
Shaping: post generation raised cosine over symbol and guard period
DEMODULATION (somewhat speculative)
FT: 128 bin every 32 samples for locating subchannels
Synchronization: square of subchannels identified by FT
to locate bottom subchannel by 125 Hz signal
Frequency drift: subchannel selection based on output of synchronization
Phase detection: phase averaged over symbol period,
differential with last symbol
A main goal is to keep the bandwidth within 500 Hz.
The symbol rate is as suggested by John KD6OZH. First testing will probably
be with his 8 ms guard band but I would like to make it adaptive to short
that period if multipath conditions allow.
DQPSK to get more throughput and because getting the absolute phase is a
challenge. Any suggestion to use absolute phase would be appreciated since
that gains a couple dB.
The Fourier transform is mainly to identify the potential subchannel
locations to allow adjusting for frequency drift. Once high energy bins are
determined the signal is filtered at various of those frequencies and the
square used to detect the doubled lowest frequency (125 Hz). That also
locates the symbol period for synchronization. Actually, the possible
frequency includes the guard band so it may be one of three values. By
determining that value the guard band period is also determined and the
actual guard band removed.
Rud Merriam K5RUD
ARES AEC Montgomery County, TX
http://TheHamNetwork.net
