【正文】 rent. The （論文） 15 rectified current thus produced varies in accordance with the signal originally modulated on the wave radiated at the transmitter and so reproduces the desired signal. Thus, when the modulated wave is rectified, the resulting current is seen to have an average value that varies in accordance with the amplitude of the original signal. Receiver circuit are made up a of a number of stages. A stage is a single transistor connected to ponents which provide operating voltages and currents and also signal voltages and currents. Each stage has its input circuit from which the signal es in and it output circuit from which the signal, usually amplified, goes out. When one stage follows another, the output circuit of the first feeds the signal to the circuit of the second. And so the signal s amplified, stage by stage, until it strong enough to operate the loudspeaker. Mobile Communication Cordless Telephone Systems Cordless telephone systems are full duplex munication systems that use radio to connect a portable handset to a dedicated base station, which is then connected to a dedicated telephone line with a specific telephone number on the public switched telephone work (PSTN). In first generation cordless telephone systems (manufactured in the 1980’s) , the portable unit municates only to the dedicated base unit and only over distances of a few tens of meters. Early cordless telephones operate solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for inhome use. Second generation cordless telephones have recently been introduced which allow （論文） 16 subscribers to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes bined with paging receivers so that a subscriber may first be paged and then respond to the page using the cordless telephone. Cordless telephone systems provide the user with limited range and mobility, as it is usually not possible to maintain a call if the user travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters. Cellular Telephone Systems A cellular telephone system provides a wireless connection to the PSTN for any user location within the radio range of the system. Cellular systems acmodate a large number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often parable to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another. A basic cellular system consists of mobile station, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO), since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile munication via radio with one of the base stations and may be handedoff to any number of base stations throughout the （論文） 17 duration of a call. The mobile station contains a transceiver, an antenna, and control circuitry, and may be mounted in a vehicle or used as a portable handheld unit. The base stations consist of several transmitters and receivers which simultaneously handle full duplex munications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC. The MSC coordinates the activities of all of the base stations and connects the entire cellular system to the PSTN. A typical MSC handless 100 000 cellular subscribers and 5000 simultaneous conversations at a time, and acmodates all billing and system maintenance functions, as well. In large cities, several MSCs are used by a single carrier. Why GPS and How It Works? What’s GPS? Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome. Over the years all kinds of technologies have tried to simplify the task, but every one has had some disadvantage. Finally, the . Department of Defense (DoD) decided that the military had to have a super precise form of worldwide positioning. And fortunately they had the kind of money. It took to build something really good. The result is the Global Positioning System, a system that’s changed navigation forever. The Global Positioning System (GPS) is a worldwide radionavigation system formed from a constellation of 24 satellites and their ground station. GPS uses these （論文） 18 ―manmade stars‖ as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter. In a sense it’s like giving every square meter on the pla a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are being very economical. And that makes the technology accessible to virtually everyone. These days GPS is finding us way into cars, boats, planes, construction equipment, movie making gear, farm machinery, even laptop puters. Soon GPS will bee almost as basic as the telephone. Indeed it just may bee a universal utility. Working in Five Logical Steps In principle, GPS operates in five logical steps: Step 1: Triangulating from satellites Improbable as it may seem, the whole idea behind