Both direct sequence and frequency hop operation can be used. Directional antennas at the higher frequencies provide considerably longer range than radiated light systems, up to several miles or more. No FCC license is required. A prin- cipal user of these higher-frequency bands is microwave ovens with their interference potential. CSMA and CSMA/CD (IEEE 802.3) protocols are often employed. There is also a standard microwave WLAN (nonspread spectrum) that operates in the band 18-19 GHz. FCC licensing is required. Building wall penetration loss is high. The basic application is for office open spaces. 18.11 MOBILE SATELLITE COMMUNICATIONS 18.11.1 Background and Scope This section contains a brief review of low earth orbit (LEO) satellite systems serving the PCS/cellular market. Most LEO satellite systems discussed here utilize frequency reuse and are based on a cellular concept. 18.11.2 Advantages and Disadvantages of LEO Systems Delay One-way delay to a GEO satellite is budgeted at 125 msec; one-way up and down is double this value, or 250 msec. Round-trip delay is about 0.5 sec. Delay to a typical LEO satellite is 2.67 msec and round-trip delay is 4 X 2.67 msec or about 10.66 msec. Calls to/from mobile users of such systems may be relayed still again by conventional satellite services. Data services do not have to be SO restricted on the use of "handshakes" and stop-and-wait ARQ as with similar services via a GEO system. Higher Elevation Angles and "Full Earth Coverage." The GEO satellite provides no coverage above about 80° latitude and gives low-angle coverage of many of the world's great population centers because of their comparatively high latitude. Typ- ically, cities in Europe and Canada face this dilemma. LEO satellites, depending on orbital plane spacing, can all provide elevation angles >40°. This is particularly attractive in urban areas with tall buildings. Coverage with GEO systems would only be available on the south side of such buildings in the Northern Hemisphere with a clear shot to the horizon