Bulletins from the Pacific Packet Radio Society - page 161

modem (the so-called AFDEM) a 3-level search strategy was necessary to accomplish a time to lock of 5 seconds (with a 5kHz search range and two-sided loop bandwidth of 100 Hz).

With coded transmission the acquisition time becomes even more constraining. This situation clearly favors 2-phase PSK (which also is simpler to implement) because the total amount of data transmitted in a given time may be larger even if the data rate is cut in half because the receiver lock-up time is so much shorter.

Formats for Coded Transmission

Although not known very much to amateurs, there are simple ways to considerably improve the weak-signal performance over the capabilities offered by straight transmission of data. These methods make use of the pre diction of the communication theory of Shannon that the performance may be improved by redundancy, i.e., transmitting more information than just the desired data and using this information to remove errors.

Typically the data stream transmitted is doubled (or also sometimes tripled) by producing two symbols from each data bit using a simple shift register and exclusive-OR gate arrangement (a so-called convolutional coder). This operation also can be performed entirely by software.

Recovering the data from the symbols is the more complex part of the operation. There are two different methods in widespread use. The so-called Viterbi decoding requires special and fairly complex hardware and is used mostly with continuous data. For amateur use this technique is probably too complex. For blocked data there is another technique called sequential or "Fano" decoding. This decoder can be implemented by software and requires little or no change in the receiving hardware.

AMSAT-DL implemented Fano decoders on the Radio Shack TRS-80 (tm) Model I and the Color Computer as a backup for the Phase III satellite -- should something make the links poorer than calculated. These decoders are fast enough to decode a few hundred bits/s in real time.

The performance of these decoders is such that they can recover a block of data if the total number of symbol errors does not exceed 3to 3.5%. This means that an Eb/No of 2 to 3 dB at the symbol rate is sufficient, while normally 10 to 11 dB Eb/No is required for acceptable performance. Because the data rate is only 1/2, there is a net gain of about 5 dB - and all that is required is a short program in the computer which is needed anyway.

In view of this situation it would be a good idea not to preclude the use of coded transmission for packet radio. These considerations should be taken into account:

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