Bulletins from the Pacific Packet Radio Society - page 160

If the link performance may get very poor temporarily, e.g.,in space communication or with Amateur Radio, one or two bytes for synchronization are not adequate. In this environment it makes sense to require that sync (the beginning of a block) is declared only if the block start is positively identified. If the receiver is just getting noise, the modem will output random data, and the probability of this random data declaring "sync" should be reasonably small to prevent this false sync masking the real sync vector possibly coming a little later. (This assumes that no squelch can be utilized in the receiver -- the are no practical ways to design an effective squelch with the better communication modes requiring low signal levels.)

The time between declaration of false sync on random data is t = 2~n/data rate, n being the length of the sync vector in bits. With n = 8 every 256 bits (about once every 1/4 second at 1000 bit/s) a false sync is declared. Even with a sync vector of 1 6 bits long ( 2 bytes) about once per minute a false sync will be "recognized." Because of this problem AMSAT adopted a 4-byte sync vector for the Phase III satellites. NASA also uses 4-byte vectors in their telemetry transmission. With the 4-byte vector the probability of false sync is so low that only every 50 days or so a false sync will occur. Since at the end of the block a CRC normally will be used to identify errors, this performance is quite satisfactory.

Modulation Formats

Most high-performance data transmission systems use some form of phase-shift keying (psk) because these modulation systems offer excellent Eb/No performance for vhf and uhf data transmission. The receiver for these formats must reconstruct carrier and bit-rate phase and frequency for proper operation. This reference information then is used by the circuitry to demodulate the received data.

It should be noted that the quality (absence of phase jitter) of the reconstructed carrier determines if the theoretical predictions as to the Eb/No performance can be obtained. Without going into the mathematics it can be stated here that for 2-phase modulation formats the effective two-sided bandwidth of the carrier-recovery PLL should not exceed the data rate/4. With 4phase techniques (e.g., qpsk or msk) this PLL bandwidth must be reduced to the data rate/100. (With 2-phase systems the signal degradation is proportional to the cosine of the phase error; with 4-phase systems the degradation results from quadrature-channel crosstalk proportional to the sine of the phase error.)

The loop bandwidth determines the time required for the receiver PLL to become locked after the transmitter is keyed on. A very careful PLL design is necessary in order to make this time to lock short enough so that the transmitter on time is not governed by receiver acquisition requirements rather than the data content of the packets. In the design of the Phase III

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