CONCEPTS IN TRANSMISSION TRANSPORT
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.
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
Suppose we are using a 48-Mbps bit stream to input to our transmitter, which was using
16-QAM modulation. Its baud rate, which measures transitions per second, would be 48/4
megabauds/sec. If we allowed 1 baud/Hz, then a 12-MHz bandwidth would be required. If
we used a roll-off factor of 1.5, then the practical bandwidth required would be 18 MHz.
Carry this one step further to 64-QAM. Here the theoretical bit packing is 6 bits/Hz, and
for the 48-Mbps bit stream a 12-MHz bandwidth would be required (practical).
There are no free lunches. As
M increases (e.g., M = 64), for a given error rate,
increases. Figure 9.13 illustrates a family of
curves for various
modulation schemes plotted against BER.
In summary, to meet these bit rate/bandwidth requirements, digital LOS microwave
commonly uses some form of QAM; as a minimum, it uses 64-QAM, 128-QAM, 256-
QAM, or 512-QAM. The theoretical bit packing capabilities of these QAM waveforms