If radiated power is not limited, data rate is directly proportional to bandwidth, but the maximum data rate per kHz depends on the amount of time (multipath) spreading and amount of frequency (Doppler) spreading. NVIS has a multipath spread of 6-12 ms and there needs to be a gap between symbols to accomodate this. Long paths have much lower multipath. Near the equator, there is little frequency spread (< 4 Hz), but it is larger in near-polar paths and can be very large (up to 40 Hz) under disturbed conditions. The maximum symbol rate is limited to much less than 1/multipath spread and this decreases with the number of symbol states so QPSK can tolerate only half the Doppler spread of BPSK, etc. The acheivable data rate for any given bandwith depends on ionospheric conditions and where the two stations are located. A good guess for maximum acheivable data rates with 1.5 kW PEP might be 3 kbps per kHz to 1/4 kbps per kHz depending on location and ionospheric conditions. When the doppler spread and multipath spread both become large there is no chance for digital communication.
As far as maximum bandwidth is concerned, most countries have no limit, but 8 kHz is in the regulations of some countries in Europe so that may be a good upper limit. 73, John KD6OZH ----- Original Message ----- From: DuBose Walt Civ AETC CONS/LGCA To: [email protected] Mark you said..."The question I have been struggling with is how much is enough/too much. I guess what I am looking for is a curve showing bandwidth vs. throughput for parallel tone modems, or maybe more precisely where is the point of diminishing returns? " Yes...perhaps you are right about seeing curve...but I'm not sure that there is enough data (on/off the air or simulator) available to make such a curve. Rick...you said..."The maximum accepted bandwidth for most modes is the width of an SSB transmitter since you can not go wider than that and communicate with the typical rigs of the day." But is this any reason to not use wider modes?
