 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, Eb/No increases. Figure 9.13 illustrates a family of Eb/No curves for various M-QAM 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 RADIO 8-phase 16-QAM 32-QAM 64-QAM types. (9.16) 9.2.3.6 CONCEPTS IN TRANSMISSION TRANSPORT usually display a 55% efficiency or somewhat better. With such an efficiency, gain (G, in decibels) is then G db 20 l0g10D+20 l0g10F+1 (9.17a) where F is the frequency in gigahertz and D is the diameter of the parabolic reflector in feet. In metric units we have (9.17b) where D is in meters and F is in gigahertz. 9.2.3.7 Running a Path/Site Survey. This exercise can turn out to be the most important step in the design of an LOS microwave link (or hop). We have found through experience that mountains move (i.e., map error), buildings grow, grain elevators appear where none were before, east of Madrid a whole high-rise community goes up, and SO forth. Another point from experience: If someone says "line-of-sight" conditions exist on a certain path, don't believe it Line of sight must be precisely defined. We reiterate that for each obstacle in the LOS microwave path, earth curvature with proper K-factor must be added to obstacle height, 0.6 of the first Fresnel zone must be added on top of that, and then 50 ft for trees and 10 ft more for growth must be added if in a vegetated area (to 