28-05-2013, 03:26 PM
Wireless Communication and Network
History ofWireless Communications
The first wireless networks were developed in the Pre-industrial age. These systems transmitted information over line-of-sight distances (later extended by telescopes) using smoke signals, torch signaling, flashing mirrors, signalflares, or semaphore flags. An elaborate set of signal combinations was developed to convey complex messages with these rudimentary signals. Observation stations were built on hilltops and along roads to relay these messagesover large distances. These early communication networks were replaced first by the telegraph network
by Samuel Morse in 1838) and later by the telephone. In 1895, a few decades after the telephone was invented,Marconi demonstrated the first radio transmission from the Isle of Wight to a tugboat 18 miles away, and radiocommunications was born. Radio technology advanced rapidly to enable
better quality, less power, and smaller, cheaper devices, thereby enabling public and private radio communications,elevision, and wireless networking. Early radio systems transmitted analog signals. Today most radio systems transmit digital signals composedof binary bits, where the bits are obtained directly from a data signal or by digitizing an analog signal. A digitalradio can transmit a continuous bit stream or it can group the bits into packets. The latter type of radio is called a packet radio and is characterized by bursty transmissions: the radio is idle except when it transmits a packet.The first network based on packet radio, ALOHANET, was developed at the University of Hawaii in 1971. This network enabled computer sites at seven campuses spread out over four islands to communicate with a centralcomputer on Oahu via radio transmission. The network architecture used a star topology with the central computer at its hub. Any two computers could establish a bi-directional communications link between them by going throughthe central hub. ALOHANET incorporated the first set of protocols for channel access and routing in packet radio systems, and many of the underlying principles in these protocols are still in use today. The U.S. military was extremelyinterested in the combination of packet data and broadcast radio inherent to ALOHANET. Throughout the1970’s and early 1980’s the Defense Advanced Research Projects Agency (DARPA) invested significant resources to develop networks using packet radios for tactical communications in the battlefield.
Wireless Vision
The vision of wireless communications supporting information exchange between people or devices is the communications frontier of the next few decades, and much of it already exists in some form. This vision will allow multimedia communication from anywhere in the world using a small handheld device or laptop. Wireless networkswill connect palmtop, laptop, and desktop computers anywhere within an office building or campus, as wellas from the corner cafe. In the home these networks will enable a new class of intelligent electronic devices that
can interact with each other and with the Internet in addition to providing connectivity between computers, phones,and security/monitoring systems. Such smart homes can also help the elderly and disabled with assisted living,patient monitoring, and emergency response. Wireless entertainment will permeate the home and any place thatpeople congregate. Video teleconferencing will take place between buildings that are blocks or continents apart,
and these conferences can include travelers as well, from the salesperson who missed his plane connection to theCEO off sailing in the Caribbean. Wireless video will enable remote classrooms, remote training facilities, and remote hospitals anywhere in the world. Wireless sensors have an enormous range of both commercial and military
applications.
Technical Issues
Many technical challenges must be addressed to enable the wireless applications of the future. These challenges extend across all aspects of the system design. As wireless terminals add more features, these small devices must incorporate multiple modes of operation to support the different applications and media. Computers process voice,
image, text, and video data, but breakthroughs in circuit design are required to implement the same multimode operation in a cheap, lightweight, handheld device. Since consumers don’t want large batteries that frequently
need recharging, transmission and signal processing in the portable terminal must consume minimal power. The ignal processing required to support multimedia applications and networking functions can be power-intensive.
Thus, wireless infrastructure-based networks, such as wireless LANs and cellular systems, place as much of the processing burden as possible on fixed sites with large power resources.
Cellular Telephone Systems
Cellular telephone systems are extremely popular and lucrative worldwide: these are the systems that ignited the wireless revolution. Cellular systems provide two-way voice and data communication with regional, national, or international coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof. Today these systems have evolved to support lightweight handheld mobile terminals operating inside and outside buildings at both pedestrian and vehicle speeds. The basic premise behind cellular system design is frequency reuse, which exploits the fact that signal power falls off with distance to reuse the same frequency spectrum at spatially-separated locations. Specifically, the coverage area of a cellular system is divided into nonoverlapping cells where some set of channels is assigned
to each cell. This same channel set is used in another cell some distance away, as shown in Figure
Ci denotes the channel set used in a particular cell. Operation within a cell is controlled by a centralized base station, as described in more detail below. The interference caused by users in different cells operating on the same channel set is called intercell interference. The spatial separation of cells that reuse the same channel set, the reuse distance, should be as small as possible so that frequencies are reused as often as possible, thereby maximizing
spectral efficiency. However, as the reuse distance decreases, intercell interference increases, due to the smaller propagation distance between interfering cells. Since intercell interference must remain below a given threshold for acceptable system performance, reuse distance cannot be reduced below some minimum value.