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14.7.3 Carrier recovery
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A factor, which must be taken into account with TDMA is that the various bursts in a frame lack coherence so that carrier recovery must be repeated for each burst. This applies to the traffic as well as the reference bursts. Where the carrier recovery circuit employs a phase-locked loop such as shown in Fig. 10.20, a problem known as hangup can occur. This arises when the loop moves to an unstable region of its operating characteristic. The loop operation is such that it eventually returns to a stable operating point, but the time required to do this may be unacceptably long for burst-type signals. One alternative method utilizes a narrowband tuned circuit filter to recover the carrier. An example of such a circuit for quadrature phaseshift keying (QPSK), taken from Miya (1981), is shown in Fig. 14.15. The QPSK signal, which has been downconverted to a standard IF of 140 MHz, is quadrupled in frequency to remove the modulation, as described in Sec. 10.7. The input frequency must be maintained at the resonant frequency of the tuned circuit, which requires some form of automatic frequency control. Because of the difficulties inherent in working with high frequencies, the output frequency of the quadrupler is downconverted from 560 to 40 MHz, and the AFC is applied to the voltage controlled oscillator (VCO) used to make the frequency conversion. The AFC circuit is a form of phase-locked loop (PLL), in which the phase difference between input and output of the single-tuned circuit is held at zero, which ensures that the 40-MHz input remains at the center of the tuned circuit response curve. Any deviation of the phase difference from zero generates a control voltage which is applied to the VCO in such a way as to bring the frequency back to the required value.
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Figure 14.15 An example of carrier recovery circuit with a single-tuned circuit and AFC. (Courtesy of Miya, 1981.)
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Interburst interference may be a problem with the tuned-circuit method because of the energy stored in the tuned circuit for any given burst. Avoidance of interburst interference requires careful design of the tuned circuit (Miya, 1981) and possibly the use of a postamble, as mentioned in the Sec. 14.7.2. Other methods of carrier recovery are discussed in Gagliardi (1991).
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14.7.4 Network synchronization
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Network synchronization is required to ensure that all bursts arrive at the satellite in their correct time slots. As mentioned previously, timing markers are provided by the reference bursts, which are tied to a highly stable clock at the reference station and transmitted through the satellite link to the traffic stations. At any given traffic station, detection of the unique word (or burst code word) in the reference burst signals the start of receiving frame (SORF), the marker coinciding with the last bit in the unique word. It would be desirable to have the highly stable clock located aboard the satellite because this would eliminate the variations in propagation delay arising from the uplink for the reference station, but this is not practical because of weight and space limitations. However, the reference bursts retransmitted from the satellite can be treated, for timing purposes, as if they originated from the satellite (Spilker, 1977). The network operates what is termed a burst time plan, a copy of which is stored at each earth station. The burst time plan shows each earth station where the receive bursts intended for it are relative to the SORF marker. This is illustrated in Fig. 14.16. At earth station A the SORF marker is received after some propagation delay tA, and
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Start of receive frame (SORF) marker in a time burst plan.
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the burst time plan tells station A that a burst intended for it follows at time TA after the SORF marker received by it. Likewise, for station B, the propagation delay is tB, and the received bursts start at TB after the SORF markers received at station B. The propagation delays for each station will differ, but typically they are in the region of 120 ms each. The burst time plan also shows a station when it must transmit its bursts in order to reach the satellite in the correct time slots. A major advantage of the TDMA mode of operation is that the burst time plan is essentially under software control so that changes in traffic patterns can be accommodated much more readily than is the case with FDMA, where modifications to hardware are required. Against this, implementation of the synchronization is a complicated process. Corrections must be included for changes in propagation delay which result from the slowly varying position of the satellite (see Sec. 7.4). In general, the procedure for transmit timing control has two stages. First, there is the need for a station just entering, or reentering after a long delay, to acquire its correct slot position, this being referred to as burst position acquisition. Once the time slot has been acquired, the traffic station must maintain the correct position, this being known as burst position synchronization.
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