1. Introduction

In 1888, Heinrich Hertz  successfully demonstrated the first transmission and reception of electromagnetic waves, and this lead to the birth of wireless communication, which has been used in radio and TV broadcasting, microwave links, satellite communications, mobile communications and network access.

The major advantage of wireless compared to wired communication is mobility.  The most popular mobile communication device at the moment is the cellular phone.  Like mains water and electricity, mobile communications has improved our lives considerably.  At a smaller scale, mobile communication is found within gadgets and appliances for example TV infrared remote control and Bluetooth enabled devices that make it possible for these devices to form a network among themselves.  Wireless communication is also widely used in fixed communications.  Fixed communications include microwave links, wireless local loop and broadband fixed wireless access (BFWA) networks .  The advantages of using fixed wireless link compared to wired link are:

·        faster speed of deployment

·        accessibility of difficult to reach areas

·        low marginal cost and effort in adding or removing a subscriber compared to the sunk cost required to install cables for wired access.

Wireless communication also has its disadvantages.  For example, a wireless signal can be received anywhere within the coverage area, which compromises security since it is easier to gain unauthorized network access.  It is more difficult to transmit at a high data rate in a wireless channel than a wired channel since the wireless channel is more hostile.

1.1.          Wireless Channel

In the 19^th century, James Clerk Maxwell  proposed that electromagnetic waves propagate through space via the ether  in a similar manner to the propagation of sound (pressure) waves due to the movement of molecules in the medium.  Using the concept of the ether, Maxwell’s equations were developed to describe the propagation of electromagnetic waves.  He predicted the speed of electromagnetic waves to be similar to that of light and that light belongs to the family of electromagnetic waves.  Although the concept of ether has been abandoned, Maxwell’s equations are still being used to model electromagnetic propagations.

As with sound waves, electromagnetic waves can be reflected, diffracted and attenuated depending upon the medium and the size of the obstacles the wave encounters.  Reflection occurs when a wave hits a surface of an object whose dimension is much larger than its wavelength and the reflected wave follows Snell’s Law.  Diffraction occurs when a wave hits the edge of an object or an object whose dimension is smaller than its wavelength.  The wave will be scattered at the diffraction point.

An electromagnetic wave generated from a point source spreads out as it propagates further from its source.  For an input power P_IN (Watts), the power density P_D (Watts/m²) of this wave at a distance d is:

If a receiver at distance d captures an area A_RX experiencing a certain power density, the receive power will be a fraction of the input power P_IN.  The loss of power at the receiver is termed spatial attenuation .  The electromagnetic wave may also face further attenuation due to absorption depending upon the medium it is traversing.  For example, microwaves may be absorbed by water or foliage and hence experience large attenuation.  The received wave may also lose power when it is diffracted or when reflection is not perfect.

A transmitted radio signal is an electromagnetic wave.  Since the transmitted signal can be reflected and diffracted, it may go through many different paths before arriving at a receive antenna.  The various signals arriving at the receive antenna may have different phases and powers (due to different attenuation).  The received signal is said to have experienced multi-path propagation  and the overall received signal is the sum (which may be destructive or constructive) of all these individual multi-path signals.  Multi-path propagation causes different areas to have a non-uniform spatial distribution of signal power and this phenomenon gives rise to so called multi-path fading , where the physical spacing between the fading depends upon the signal’s transmitted wavelength.

1.2.         Path Loss

In free space , the received signal power P_RX (in Watts) at a distance d from a source transmitted with power P_TX is [1]:

Where, z is the path loss exponent  (which is equal to 2 in free space propagation), l is the wavelength of the transmitted signal, and G_RX and G_TX are the receiver and transmitter antenna gains respectively. 

Free space propagation does not occur in most environments because of the presence of the ground, which acts as a reflective surface and also owing to physical variations (e.g. buildings or hills) between the transmitter and receiver.  These variations cause shadowing , giving variations in the received signal strength.  An empirical path loss model taking into account the effects of shadowing and fast fading, can be expressed as:

where F_S(d) is the shadowing margin and F_M(d) is the multi-path fading margin.  For radio signals where the wavelength is less than a metre, the received signal strength variation over a short distance owes more to multi-path fading than it does to shadowing.  Multi-path fading and shadowing are also known as short-term fading  and long-term fading respectively .  In a typical radio environment, the path loss exponent is usually between 2 and 5 and long-term fading follows a log-normal distribution [1] .  The short term fading or fast-fading follows a Rayleigh distribution where there is no line of sight path between transmit and receive antennas and with a line of sight path it follows a Rician distribution.

1.3.         Cellular Concept

A wireless operator is usually given a fixed block of frequency spectrum.  In order to satisfy the traffic demand in a large geographical area, the spectrum needs to be reused.  Mac Donald [2] introduced the cellular concept  where the radio coverage area of a base station is represented by a cell.  A regular hexagon is chosen to represent a cell because it covers a larger area with the same centre-to-vertex distance (or radius) compared to a square or an equilateral triangle.  Consequently, fewer hexagonal cells are required to cover a given geographical area.   These cells are placed in a cellular structure covering a geographical area as shown in Figure 1.1.   

Figure 1.1: Cellular layout and frequency reuse

The cells in a geographical area are grouped into clusters .  The entire block of frequencies is completely allocated to each cluster and the cells in each cluster use different frequencies.  In this way, the limited block of frequency spectrum is reused.  The concept of frequency reuse  is shown in Figure 1.1, where the cluster size is 7 cells and a set of co-channel cells – i.e. cells using the same frequencies – are shown shaded.  The co-channel reuse ratio  is defined as:

where R_u is the distance between the two closest co-channel cells, R_b is the cell radius and N_c is a positive integer representing the number of cells per cluster.

1.3.1.       Interference

Co-channel interference  is the interference caused by users and base stations in co-channel cells.  Since the received power is inversely proportional to the distance from the source, the co-channel interference power is dependent upon the distance of the co-channel cells.  Therefore, from (1.4) , the cluster size N_c is governed by the level of co-channel interference that can be tolerated.

Apart from co-channel interference, adjacent channel interference can also occur when signals from adjacent channels enter the receiver since the receiver is not perfect .  A sufficiently wide guard band is required to reduce adjacent channel interference.

1.3.2.      Duplex Communication

In most wireless systems, communication is usually two-way, that is the subscriber must be able to communicate with the base station and vice versa.  Duplexing  is used to permit two-way communication and it can be achieved using Frequency Division Duplex (FDD), Time Division Duplex (TDD) or the recently proposed Code Division Duplex (CDD).  The choice of duplexing affects the amount of interference in a network.

Two separate frequencies are used in FDD , one for uplink communication (subscriber to base station) and the other for downlink communication (base station to subscriber).  In FDD, interference can occurs in two ways: adjacent subscribers (subscribers from another co-channel or adjacent channel cell) to base station being interfered with (i.e. home base station) and adjacent base station to home subscriber.  A system employing FDD requires two separate bands of frequencies and for asymmetric traffic (e.g. if downlink traffic is heavier than uplink traffic) these bands are not fully utilized.

TDD  uses the same frequency for uplink and downlink communications.  The subscriber and base station take turns to transmit and hence they must agree upon the timeslot for uplink and downlink transmission.  In TDD four kinds of interference can occur: adjacent subscriber to home subscriber, adjacent base station to home base station, adjacent subscriber to home base station and adjacent base station to home subscriber.  Hence, TDD has a worse interference performance than FDD.  However, for asymmetric traffic TDD better utilizes the frequency spectrum compared to FDD as downlink and uplink transmission can occur consecutively on the same frequency band.  TDD also eases frequency allocation issues for regulatory bodies since only a single block of frequencies is required as compared with FDD, which requires a pair of frequency bands [3].  Note that the SNR performance of a TDD system can be improved by having synchronization among the base stations.  It is shown in [4] that the signal to interference power ratio (SIR) performance of a fully synchronous TDD system approaches that in a FDD system.

A CDD  system – similar to a TDD system – uses the same frequency band for the uplink and downlink transmission.  However, instead of separating the uplink and downlink transmission in time, CDD separates them using different smart codes [5].  A set of smart codes is able to maintain the required orthogonality property among codes even with a time shift in the received signal.  This requires the smart codes to have a low value of auto-correlation and cross-correlation for the received signals that are time shifted less than the time delay spread.  Theoretically, multipath effect will not affect a code with an auto-correlation of zero, while a cross-correlation of zero will eliminate multi-user interference.  Hence, a system using CDD will not experience any interference and the SNR is only affected by receiver noise.  For asymmetric traffic, CDD better utilizes the frequency spectrum compared to FDD.

1.4.         Frequency Spectrum

With the growth in communications, there has been a huge increase in demand for frequency spectrum and consequently this has made it very valuable.  For example, the British government enjoyed a gain of £22.5 billion on 27 April 2000 while the German government pocketed £30.8 billion on 17 August 2000 by auctioning the Third Generation  (3G) Mobile Telephone Licenses to telecom operators [6].  Billions of pounds were gained by apparently selling air.  The winners of the auction will incur billions more pounds in debt in order to build the infrastructure required to provide the 3G Mobile Telephony services.  Clearly, the high cost of obtaining a frequency spectrum creates pressure to use it effectively and efficiently.

Unlicensed frequency spectrum such as the ISM2 (2 GHz) or ISM5 (5 GHz) bands can be used free of charge.  The unlicensed frequency spectrum (subject to regulatory constraints) allows anyone to act as an Internet Service Provider (ISP) and this has helped fuel the growth of Wireless Fidelity  (Wi-Fi) networks.  Wi-Fi uses the IEEE 802.11 standards and can be cheaply and easily set up to provide wireless data access.  The Wi-Fi service can be free and it is becoming increasingly available in highly populated areas (Hot Spots ) e.g. airports, restaurants, shopping malls and hotels.  The estimated number of Wi-Fi users in 2002 is 2 million worldwide and this is expected to exceed 5.4 million by 2003 [7].  However, since anyone can use unlicensed spectrum, Wi-Fi base stations will experience high levels of interference from each other.  Apart from interference from like equipment, Wi-Fi will also experience interference from other systems e.g. Bluetooth.  Hence, effective methods are required to reduce interference.

Whether the frequency spectrum is licensed or unlicensed, it is limited and the operator needs to reuse it.  Frequency reuse causes interference  and this lowers the received Signal to Noise ratio (SNR), where noise includes both the receiver noise and the interference.  For a system with a fixed capacity (bits per second), a poor SNR causes a higher bit error rate and lowers the overall effective data throughput.  A poor SNR also decreases the capacity of a system with variable capacity (e.g. one using adaptive modulation).  Therefore, reducing the interference will lead to a higher capacity and data throughput for a wireless system.

Since co channel and adjacent channel interferences are caused by equipment in other cells using the same frequency, good channel allocation among the cells will reduce the overall level of interference.  Hence, effective channel allocation methods will improve the data throughput of a packet switched network and decreases the blocking and dropping probabilities of a circuit switched network.