Bandwidth
Today the three transmission modes are copper cable lines, wireless, and optical fiber lines. Telephone lines, as they are currently configured, limit digital data transmission to 56 kb/s. Digital subscriber lines are available that give 1 to 10 Mb/s data rate, but the expense and availability of these still preclude their widespread use. Fiber lines can provide the required bandwidth for most users, but their cost also precludes widespread use. The use of the wireless in the unlicensed wireless bands of 2 to 5 GHz and 20 to 40 GHz can provide data rates in the 5 Mb/s and 10 to 1000 Mb/s, respectively. The economic solution appears to bring fiber lines to a base station, and provide wireless for the last 3 to 5 km individual users. This may ease deployment and lower infrastructure cost. These systems are not significantly affected by multipath because of the high placement of antennas and their narrow beam width. The disadvantages to wireless broadband systems are its restriction to line-of-sight signal transmission resulting in less than 100% coverage, weather effects on radio signals, new technical challenges for commercial application of millimeter wave electronics, and a lack of standards [6].
In 1998 the FCC auctioned the frequency spectrum between 27.5 and 31.3 GHz for local distribution of video, voice, and data for what is called local multipoint distribution service (LMDS). This would provide two-way wireless communications at a high data rate. There are multipoint systems in the 24 to 26 GHz frequency bands as well. Actual frequency plans and bandwidth allocations vary: in the United States (24.25每24.45 GHz and 25.05每25.25 GHz), in Korea (24.25每24.75 GHz and 25.5每26.0 GHz), Germany (24.55每25.05 GHz and 25.56每26.06 GHz), and so on. Bandwidths in the 39 GHz band (38.6每40 GHz) and in the 60 GHz band (59每64 GHz) have also been reserved for future communication systems. Wider bandwidths are naturally available at higherfrequency bands. The need for wider bandwidth is quite clear, since users will demand more and more high-quality multimedia and data applications.