信息科学与电子工程专业英语 教学课件 Unit 6 Mobile Communications.ppt

Technical English For Information Science and Electronic Engineering Unit 6 Mobile Communications Part I Mobile Communications New Words New Words 1 A mobile system is one in which users can physically move while communicating with one another. Examples include pagers, cellular phones, and cordless phones. It is the mobility that has made RF communications powerful and popular. The transceiver carried by the user is called the “mobile unit” (or simply the “mobile”), the “terminal,” or the “hand-held unit.” 1 The complexity of the wireless infrastructure often demands that the mobiles communicate only through a fixed, relatively expensive unit called the “base station.” Each mobile receives and transmits information from and to the base station via two RF channels called the “forward channel” or “downlink” and the “reverse channel” or “uplink,” respectively. 1 Most of our treatment relates to the mobile unit because, compared to the base station, hand-held units constitute a much larger portion of the market and their design is much more similar to other types of RF systems. 2 Cellular system With the limited available spectrum (e.g., 25 MHz around 900 MHz), how do hundreds of thousands of people communicate in a crowded metropolitan area? To answer this question, we first consider a simpler case: thousands of FM radio broadcasting stations may operate in a country in the 88-108 MHz band. 2 This is possible because stations that are physically far enough from each other can use the same carrier frequency (frequency reuse) with negligible mutual interference, except at some point in the middle where the stations are received with comparable signal levels.1 The minimum distance between two stations that can employ equal carrier frequencies depends on the signal power produced by each. 3 In mobile communications, the concept of frequency reuse is implemented in a “cellular” structure, where each cell is configured as a hexagon and surrounded by six other cells a