police and military systems. Repeater spacing with microwave varies depending on the frequency of trans- mission. Remember from Chapter 1 that lower frequencies can travel farther than higher frequencies before they attenuate. Higher frequencies lose power more rap- idly. In microwave systems that operate in the 2GHz, 4GHz, and 6GHz bands, tow- ers can be separated by 45 miles (72 km). In the higher-frequency allocations, such as 18GHz, 23GHz, and 45GHz, the spacing needs to be much shorter, in the range of 1 to 5 miles (1.6 to 8 km). This is an important issue in network design and, depending on the scope over which you want to deploy these facilities, it can have a significant impact on the investment required. Another important design criterion is that microwave requires line of sight and is a highly directional beam. Microwave requires a clear, unobstructed view, and it can't move through any obstacles, even things you wouldn't think would be obsta- cles, such as leaves on a tree. Technologies that depend on line of sight may work brilliantly in areas that have the appropriate terrain and climate, and they may not perform very well where there are many obstacles or where there is a lot of precipi- tation. Furthermore, line of sight is restricted by the curvature of the earth, which interrupts the line of sight at about 90 miles (144 km). However, new spectrum utilization techniques such as Orthogonal Frequency Division Multiplexing (OFDM) permit non-line-of-sight operation, greatly expanding the use of micro- wave. (Chapter 15 discusses OFDM in detail.) The impact of precipitation on microwave can be great. Microwave beams are small, and as you go up into the higher bands, the waveforms get smaller and smaller. Pretty soon, they're smaller than a raindrop, and they can be absorbed by a raindrop and then scattered in a million directions. Therefore, in wet atmospheric conditions, there is a great potential for problems with microwave. As a result, practicing network diversity-using both terrestrial and nonterr