1.3 Wireless communications  (Page 7/5)

Introduction

We first learned about the propagation of electromagnetic waves through the works of Maxwell and Hertz. Later, Tesla demonstrated the transmission of information using these waves, and in 1898, Marconi first demonstrated wireless communication from a boat to the Isle of Wight in the English Channel. In 1948, Claude Shannon’s work established the possibility of error-free communication under restriction for data rate (R) and signal-to-noise ratio (SNR) in a digital communication system. Thus began the era of active research in information theory and channel coding, with the goal to achieve data rates at channel capacity (C) in a digital communication system.

Digital vs. analog transmission

Digital signals are easy to regenerate, as they operate in binary state. This is not true for analog signals, which have infinite states. A pulse in a digital system is affected by distortion because of frequency characteristics and noise present in the channel. Before it can degrade to an ambiguous shape, amplifiers in the transmission path restore the pulse shape to its original form and retransmit. Digital circuits are also more reliable and less costly to design. Digital hardware is flexible and reconfigurable via software and can accommodate the operation of different communication techniques on the same hardware. Digital techniques lend themselves easily to signal-processing functions that protect against interference and jamming and allow for encryption and information security.

Requirements for wireless systems

In wireless systems, different applications have different requirements in terms of range, data rate and mobility energy consumption, for example. A few of these are listed below.

Data rate

Sensor networks that monitor temperature, humidity, speed and acceleration usually require data rates from a few bits per second to about 1 kbps. The central node in a sensor network might require data rates as high as 10 Mbps, since its tasks are coordination and data gathering. Speech communication requires between 5 kbps and 64 kbps, depending on the fidelity and amount of compression. Cellular networks with higher spectral efficiency operate at 10 kbps, while high-speed data services like WLAN and 3G/4G can go as high as 100 Mbps or more by utilizing space and time diversity techniques.

Coverage and number of subscribers

The task of a communication system is to convey information at a distance (d) with minimum probability of error (Pb) using a minimum amount of transmit power. In a mesh network, the coverage area of a system can be made independent of the range by adding multiple base stations. In sensor networks, nodes can be converted to routers that can communicate with the coordinator nodes over multiple hops.

Fixed or mobile installation

Wireless systems are designed to operate in mobile environments; this incurs costs in terms of system performance, as channel effects such as multipath (fading) and speed of mobility (Doppler shifts) can degrade bit-error-rate performance and reduce channel capacity.