 is single sideband modulation. 9.2 RADIO SYSTEMS Octal-PSK BPSK and QPSK E,/N. in dB Figure 9.10 Bit error probability (BER) versus Eb/No performance for BPSK/QPSK and 8-ary PSK (octal PSK). Eight-ary PSK modulation is not uncommon. In this case the phase circle is broken up into 45° phase segments. Now for every transition, 3 bits at a time are transmitted. The bit packing in this case is 3 bits/Hz theoretical. Now add two amplitude levels to this, making a hybrid waveform covering both amplitude modulation as well as phase modulation. This family of waveforms is called quadrature-amplitude modulation (QAM). For example, 16-QAM has 16 different state possibilities; eight are derived for 8-ary PSK and two are derived from the two amplitude levels. We'd call this 16-QAM, where for each state transition 4 bits are transmitted at once. The bit packing in this case is theoretically 4 bits/Hz. Certain digital LOS microwave system use 256-QAM and 512-QAM, theoretically achieving 8 and 9 bits/Hz of bit pack- ing. The difference between theoretical bit packing and the practical deals with filter design. For QAM-type waveforms, depending on design, practical bit packing may vary CONCEPTS IN TRANSMISSION TRANSPORT Modulated Stream Carrier Carrier phase positions Figure 9.11 Conceptual block diagram of a QPSK modulator. Note that these are two BPSK modulators working jointly, and one modulator is 90° out of phase with the other. The first bit in the bit stream is directed to the upper modulator, the second to the lower, the third to the upper, and so on. Figure 9.12 A 16-QAM state diagram. I stands for in-phase, Q stands for quadrature. the baud-rate bandwidth from 1.25 to 1.5. The extra bandwidth required provides a filter with spectral space to roll-off. In other words, a filter's skirts are not perfectly vertical. Figure 9.12 is a space diagram for 16-QAM. The binary values for each of the 16 states are illustrated. Suppose we are using a 48-Mbps bit stream to input to our transmitter, which was using 16-QAM modulation.