CLOCK DEMOD SYMBOL FILTER DETECTOR CONTROLLE FIGURE 102.9 DSP demodulator functional diagra to phase noise and frequency offset, thus allowing the use of lower-cost LNBs in the VSAt terminals. However as compared to BPSK, convolutionally encoded DPSK requires about 2 dB greater Eb/No at a BER of 10-. In addition, if operation is required below 10 dB, some form of low-level interleaving may be required. In lieu of performing the VSaT demodulation function via the traditional analog circuit techniques, an all digital implementation using digital signal processing(DSP)techniques may be considered. The merits of DSP include the development of a more testable, producible, maintainable, configurable, and cost-effective demod- ulator Figure 102.9 presents an illustration of the dSP demodulator functions to be implemented using the DSP processor(s). The functions of the major blocks are as follows: phase locked loop(PLL) for carrier acquisition, narrowband Costas loop for data detection, external automatic gain control(AGC), dynamically advance/retard sampling to achieve optimum data sampling, and A/D converters for signal analog-to-digital conversion. A VSAT system must employ frequency agility in the remote terminal to use an assigned block of frequencies within a transponder. Within the assigned frequency band, one or more outbound carriers and a number of inbound carriers are precisely located. On the VSAT receive or outbound side, the LNB output can be demod ulated directly using a synthesizer-controlled local oscillator, or further downconversion can be used under synthesizer control to obtain the demodulator input signal at a standard IF frequency such as 70 or 140 MHz. In the inbound direction, channel selection can be accomplished by two methods. First, the carrier frequency of the modulator can be shifted to select the appropriate channel and a fixed upconverter may be used to obtain the RF signal. Second, the synthesizer output frequency may be multiplied up to RF to obtain the carr may then be modulated directly with the data as described in Cannistraro and McCarter [1990] Satellite Access protocols The multiple satellite access protocol is one of the most critical elements to the performance of a VSAT network. VSAT systems tend to be used in applications where message delay is critical and this protocol is the controlling element to the delay-throughput performance of the system. During the past 15 years, there have been numerou multiple-access protocols developed and simulated in the context of satellite packet communications [Ray- haudhuri and Joseph, 1988. Table 102.4 provides a comparison of throughput vs. delay for various satellite access protocols In the outbound or hub-to-VSAT direction, a TDM channel is employed. This channel may be regarded as point-to-multipoint or broadcast channel with node selectivity being achieved by the use of addressing e 2000 by CRC Press LLC© 2000 by CRC Press LLC to phase noise and frequency offset, thus allowing the use of lower-cost LNBs in the VSAT terminals. However, as compared to BPSK, convolutionally encoded DPSK requires about 2 dB greater Eb/No at a BER of 10–5. In addition, if operation is required below 10 dB, some form of low-level interleaving may be required. In lieu of performing the VSAT demodulation function via the traditional analog circuit techniques, an all￾digital implementation using digital signal processing (DSP) techniques may be considered. The merits of DSP include the development of a more testable, producible, maintainable, configurable, and cost-effective demod￾ulator. Figure 102.9 presents an illustration of the DSP demodulator functions to be implemented using the DSP processor(s). The functions of the major blocks are as follows: phase locked loop (PLL) for carrier acquisition, narrowband Costas loop for data detection, external automatic gain control (AGC), dynamically advance/retard sampling to achieve optimum data sampling, and A/D converters for signal analog-to-digital conversion. A VSAT system must employ frequency agility in the remote terminal to use an assigned block of frequencies within a transponder. Within the assigned frequency band, one or more outbound carriers and a number of inbound carriers are precisely located. On the VSAT receive or outbound side, the LNB output can be demod￾ulated directly using a synthesizer-controlled local oscillator, or further downconversion can be used under synthesizer control to obtain the demodulator input signal at a standard IF frequency such as 70 or 140 MHz. In the inbound direction, channel selection can be accomplished by two methods. First, the carrier frequency of the modulator can be shifted to select the appropriate channel and a fixed upconverter may be used to obtain the RF signal. Second, the synthesizer output frequency may be multiplied up to RF to obtain the carrier, which may then be modulated directly with the data as described in Cannistraro and McCarter [1990]. Satellite Access Protocols The multiple satellite access protocol is one of the most critical elements to the performance of a VSAT network. VSAT systems tend to be used in applications where message delay is critical and this protocol is the controlling element to the delay-throughput performance of the system. During the past 15 years, there have been numerous multiple-access protocols developed and simulated in the context of satellite packet communications [Ray￾chaudhuri and Joseph, 1988]. Table 102.4 provides a comparison of throughput vs. delay for various satellite access protocols. In the outbound or hub-to-VSAT direction, a TDM channel is employed. This channel may be regarded as a point-to-multipoint or broadcast channel with node selectivity being achieved by the use of addressing FIGURE 102.9 DSP demodulator functional diagram