As far as I know, most OFDM implementations use differential keying either in time or frequency. HamDRM uses multiple pilot carriers as phase references. I think that there is one pilot carrier for every 4-5 data carriers. I assume that 8PSK is used in the federal standards because of the characteristics that I cited. TCM was originally used in a series of modems developed and standardized for voice-grade telephone lines and has been used for many purposes, including U.S. digital TV. OFDM was implemented in some voice-grade modems, but lost out in the standardization process as the characteristics of a landline are fairly stable over time.
The single tone modems operate similarly to U.S. DTV. There is an equalizer that estimates the amplitude, time and phase of all rays arriving at the receiver and forces all components to add in phase. The only disadvantage is that the equalizer takes time to make estimates and can lag behind the actual signal. OFDM doesn't have this problem as it ignores the time period over which the leading edge of the various rays arrive. However it has a high crest factor (like SSB voice) and about 1 dB of the signal is lost during this guard interval. OFDM has a big advantage when the receiver or transmitter are moving so it is being used in the U.S. for mobile TV. European countries use OFDM for all digital TV. The single-tone modems have an advantage when both stations are fixed. The FCC cited this when they evaluated the U.S. and European DTV systems and stated that no one would ever watch TV in a car so OFDM wasn't useful. I suspect that the real reason was that U.S. broadcasters were behind schedule in implementing DTV and a switch to OFDM would have created another delay. The FCC wants the analog TV channels for more mobile telephone and data services and the NAB had agreed to a switchover to DTV by 2006, freeing up of about 20 channels. Since hams like to push the limit and operate over barely usable HF paths, OFDM is probably better for amateur use. 73, John KD6OZH ----- Original Message ----- From: Rick To: digitalradio@yahoogroups.com Sent: Thursday, October 25, 2007 02:23 UTC Subject: Re: [digitalradio] OFDM Proposal: Details John, Are there any cases where any of the current amateur radio digital modes do not use differential keying? Based on your comments on 8PSK, is this why it is the base waveform used in the MIL-STD/FED-STD/STANAG modems? What is your view on single tone modems as used in those standards vs. the OFDM that is proposed by Rud and is used in Pactor 3? 73, Rick, KV9U John B. Stephensen wrote: > Differential PSK should be more reliable in the presence of frequency > drift and Doppler spread. There are two ways to do this: 1) compare > the phase with the previous phase of the same subcarrier or 2) compare > the phase with the phase of the next higher or lower subcarrier. In > the first case, the first symbol transmitted is always all zeroes. In > the second case, there would have to be at least one pilot subcarrier > that is unmodulated. If you want pilot subcarriers, it should be > possible to put them 62.5 Hz above and below the outermost data > subcarriers as they take no extra space if they are not modulated. > > A good way to do FEC is to use trellis-coded modulation (TCM). One bit > is added to the data stream for each subcarrier. This, 1 data bit is > sent using QPSK and 2 data bits are sent using 8PSK. The advantage > of sending the data and ECC bits on one subcarrier is that the > error-correcting code can be designed so that no extra bandwith is > needed and that the addition of the extra bit actually decreases the > required SNR rather than increasing it as you would first expect by > increasing the number of points in the constellation. Ungerboek came > up with a set of codes that can decrease the required SNR by 3-6 dB > (with no fading) when going from QPSK with no ECC to 8PSK with ECC. > The improvement is larger when fading occurs. > > The amount of improvement provided by TCM depends on the complexity of > the state machine used to generate the ECC bit. However, > a simple algorithm with 4 states provides a 3 dB improvement. A > Viterbi decoder is used to calculate the most probable set of state > transitions that the incoming signal has taken from symbol to symbol > and then backtracks to determine the most likely combination over an > entire data frame. It can also make decisions based on the actual > value of the incoming signal rather than on 3 already decoded > buts. This adds another 2 dB of improvement. > > Its probably useful to place the audio subcarrier frequencies in the > 500-1000 Hz range or higher so that harmonics of low frequency > subcarriers don't interfere with higher-frequency subcarriers. > > 73, > > John > KD6OZH
