Examples of wireless communication systems
- 1. By, Dr. D. Veera Vanitha, Asso. Prof. /ECE, School of Engineering, Avinashilingam Institute for Home Science and Higher Education for Women, Coimbatore
- 2. Paging systems are communication systems that send brief messages to a subscriber Depending on the type of service, the message may be either a numeric message , an alphanumeric message or a voice message Paging systems are typically used to notify a subscriber of the need to call a particular telephone number or travel to a known location to receive further instruction In modern paging system, news headlines, stock quotation and faxes may be sent A message is sent a paging subscriber via the paging system access number (usually a toll-free telephone number) with a telephone keypad or modem. The issued message is called a page
- 3. The paging system then transmits the page throughout the service area using base station which broadcast the page on a radio carrier Paging system vary widely in their complexity and coverage area While simple paging systems may cover a limited range of 2km to 5km,or may even be confined to within individual buildings, wide area paging systems can provide worldwide coverage Though paging receivers are simple and inexpensive, the transmission system required is quite sophisticated. Wide area paging systems consists of a network of telephone lines, many base station transmitters and large radio towers that simultaneously broadcast a page from each base station this is called simulcasting.
- 4. Simulcast transmitters may be located within the same service area or in different cities or countries. Paging systems are designed to provide reliable communications to subscribers wherever the are; whether inside a building , driving in a highway, or flying in an airplane. This necessitates large transmitter power (on the order of kilowatts) and low data rates (a couple of thousand bits per second) for maximum coverage from each base station.
- 5. Cordless telephone systems are full duplex communication systems that use radio to connect a portable handset to a dedicated base station, which is the connected to a dedicated telephone line with a specific telephone number on the public switched telephone network (PSTN) In first generation cordless telephone system (manufactured in 1980s), the portable unit communicates only to the dedicated base unit and only over distance 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 in home use Second generation cordless telephones have recently been introduced which allow subscribers to use their handsets at many outdoor locations within urban centers
- 6. Modern cordless telephone are sometimes combined 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 station provide coverage ranges up to a few hundred meters
- 7. A cellular telephone system provides a wireless connections to the PSTN for any user location within the radio range of the system Cellular systems accommodate 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 comparable 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
- 8. The diagram shows a basic cellular system which consists of mobile stations, base station 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
- 9. Each mobile communications via radio with one of the base station and may be handed-off to any number of base station throughout the 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 hand-held unit The base stations consists of several transmitters and receivers which simultaneously handle full duplex communication and generally have towers which supports 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
- 10. A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous conversations at a time, and accommodates all billing and system maintenance functions, as well In large cities, several MSCs are used by a single carrier Communication between the base station and the mobiles is defined by a standard common air interface(CAI) that specifies four different channels The channels used for voice transmission from the base station to the mobiles are called forward voice channel(FVC) and the channels used for voice transmissions from mobiles to the base station are called reverse voice channel(RVC)
- 11. The two channels responsible for initiating mobile calls are the forward control channels(FCC) and reverse control channels (RCC) Control channels are often called setup channels because they are involved in setting up a call and moving it to an unused voice channel Control channels transmit and receive data messages that carry call initiations and services request, and are monitored by mobiles when they do not have a call in progress Forward control channels also serve as beacons which continually broadcast all of the traffic requests for all mobiles in the system The towers represents base stations which provide radio access between mobile users and the mobile switching center (MSC) (Ref.: T.S. Rappaport, “Wireless Communications Principles and Practice”, 2nd Edition, )