Bulletins from the Pacific Packet Radio Society - page 162

- An even longer sync vector is needed to allow a probabilistic recognition of this vector. AMSAT-DL uses an 8-byte vector - if 60 of the 64 bits are correct, symbol sync is declared.

- Differential coding (which normally is used with psk to eliminate the sign ambiguity of the carrier) cannot be used. For this reason the sync-recognition program also looks for the inverse sync vector and sets a sign flag accordingly.

- A CRC is not needed at the end of the block. Either the decoder can decode the received block - there is a negligible probability of error - or the decoder will declare the block as undecodable in the time allocated for decoding.

Conclusions

Some of the properties of the advanced synchronous communication techniques giving the best-known weak-signal performance were discussed. In order not to preclude the use of these techniques in the emerging amateur-packet-radio systems it is recommended that standards with the following properties be adopted:

- Sufficiently long sync vectors (4 bytes for uncoded, 8 bytes for coded transmission) should be adopted. The AMSAT vectors allow both simple hardware and software recognition.

- A fixed block length should be adopted to make the block end uniquely identifiable even in the case of bad error rates. This also allows making the blocks entirely transparent, i.e., arbitrary bytes (like binary data) may be included in the data of the block.

- Synchronous techniques should be recommended using efficient modulation techniques. Four-phase techniques (like qpsk or msk) should be used for data rates in excess of 10 kbit/s only because otherwise the carrier-recovery loop bandwidth becomes too narrow and the time to lock becomes excessive.

The author hopes that these hints based on the experience gathered during the development of the digital communication system of the AMSAT Phase III satellites will help to implement state-of-the-art communication technology with amateur packet radio. The price for this is only the understanding of the issues - realizing these ideas for the most part requires only appropriate software and is well with the reach of amateurs due to the widespread availability of low-cost microcomputers.

Reference

\1} Magnuski, "Agreement on Packet Radio Standards," QEX 12 October 1982 and ASR.

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