The best mode to use depends on how much data you want to transmit in a given bandwidth. Moving lots of data in a small bandwidth requires sending one or more bits per subcarrier. Otherwise, you can spread one bit out over multiple subcarriers. For any given user data rate, increasing the bandwidth decreases the required signal to noise ratio.
MFSK uses multiple subcarriers but activates only one at a time, encoding 3 bits onto 8 subcarriers, 4 bits onto 16 subcarriers, etc. This is just N-ary RTTY. Multiple subcarriers can be used simultaneously with error-correcting codes like the Walsh function (a Hadamard matrix) which maps 8-bits into 128 subcarriers, 7 bits into 64 subcarriers, 6 bits into 64 subcarriers, etc. This is actually OFDM, but with rate 1/16, 7/64 or 3/32 error-correcting codes. With low SNRs, the hard part is achieving time and frequency synchronization. It seems to me that the ideal HF modem would use OFDM that occupyies a constant bandwidth using a subcarrier spacing that suits the Doppler spread and multipath spread for the current path and changes the code rate to suit the available SNR. 73, John KD6OZH ----- Original Message ----- From: Rud Merriam To: digitalradio@yahoogroups.com Sent: Thursday, October 25, 2007 18:10 UTC Subject: [digitalradio] QEX Article on HF Digital Propagation There is a great article in the QEX I just received (Nov/Dec 2007). The author is Daniel Crausaz HB9TPL in Switzerland. He reports on modeling and testing PSK, RTTY, Olivia, MFSK, DominoEx and Feld-Hell under various propagation conditions. I need to digest his work with respect to the OFDM proposal since the results indicate PSK may not be an optimal choice. Olivia works better under all the conditions tested. MFSK seems to be second. At first glance I would say this is because the transmission rate is so slow for Olivia at 2.5 character per second. I would find that painfully slow for even a chat mode. Interestingly, RTTY performs about the same under all the conditions tested. PSK either works well or just fails. It has problems in flutter conditions which seem to me be the conditions prevalent a lot of the time. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net
