Channels, Propagation and Antennas for Mobile Communications
This exceptional book introduces the reader to the principles, theory and applications of physical layer wireless/mobile communications. applicators and millimetric antennas. The book emphasises the basic principles needed to establish an understanding of this technology, whilst treating the tools required - such as the mathematics and statistics - in the manner of a practical handbook, thus avoiding detailed derivations.
Inspec keywords: mobile antennas; signal processing; mobile communication; radiowave propagation; multipath channels
Other keywords: negative group delay; signal processing; antenna design; mobile communication; nonline-of-sight path; short-term multipath channel behaviour; propagation mechanism; scattering mechanism
Subjects: Mobile radio systems; Antennas; Signal processing and detection; Radiowave propagation
This introductory chapter gives a coverage of the evolution and basics of mobile and personal communications. Details of the digital communications systems issues will not be pursued in the following chapters, except where necessary for a particular topic. The roles of the propagation and antenna engineering are seen respectively as limiting and mitigating factors in a mobile link. The multipath propagation and the path loss limit communications capacity, and the classical gain and diversity gain from signal processing undertaken by the antennas serve to mitigate the capacity limitations. Some of the subsequent chapters discuss the mechanisms of multipath propagation and the antenna signal-processing action and antenna configurations. It is these processes that govern the performance of mobile links.
When a receiver is moving in a complicated, multipath environment, it is continuously receiving new contributions with changes in delay, amplitude and polarisation. The problem of exactly analysing the multipath problem is a formidable task, but is usually one without interest. What is important is to get a statistical description of the propagation parameters combined with a knowledge of their relevance, and to get an engineering understanding of the variety of multipath phenomena by reducing the problem as much as possible. In this section, the various propagation parameters are introduced for more detailed treatment in later sections.
The simplest case of multipath propagation comprises one reflected path in addition to a direct path. This section discusses the fields resulting from a reflection from a smooth planar surface, which has direct relevance to propagation along streets and across rural areas.
A variety of methods have been applied to the problem of path loss in real environments, natural or man-made. A number of methods have been parametric in the sense that a few parameters describe the path loss based on experimental results, thus yielding fast results for network planning. Others have been more based on simple modelling, like a few knife-edges, and then treated by approximate methods. Recently, there has been some interest in a more accurate, deterministic modelling of the environment, especially for indoor and microcell situations. First, the outdoor above-rooftop case will be treated in the urban environment, followed by the rural case and the indoor environment. Additionally, the time-domain response as well as the angular response are treated since they are important for wideband systems and for the use of adaptive antennas.
The waves incident on a mobile receiving antenna sometimes arrive via a line-of-sight from the transmitter, but in a general case they emanate mostly from scatterers. The scatterers can be viewed as electromagnetic sources. The scenario comprises these sources and their geometric relation to the receiving antenna. Scenario and propagation channel models are developed. The deterministic two-path model is used extensively as a model since it offers a simple tutorial and can produce representations of most of the behaviour from real-world channels. The deterministic model is developed into stochastic forms, and finally into averaged forms. During the development, the distortion of a signal in the channel is addressed and the antenna interfacing principles are also discussed.The waves incident on a mobile receiving antenna sometimes arrive via a line-of-sight from the transmitter, but in a general case they emanate mostly from scatterers. The scatterers can be viewed as electromagnetic sources. The scenario comprises these sources and their geometric relation to the receiving antenna.
The extension from the two-path model to the N-paths is conceptually straightforward from the point of view that the channel can be described by simple summations. This chapter looks at the short-term channel behaviour arising from the many-path model, where no single path is dominant, and discusses the scenarios of sources used in channel simulations.
In generating signals similar to those encountered in mobile communications, it is practical to compute signal samples from the scenario model by moving the receiving antenna amongst discrete sources. In simulations, calculations of any metric over a finite sample size will provide only an estimate of the ensemble average value. In this section, basic considerations for computer simulations of the short-term fading channel are discussed. Two aspects of simulation are: the generation of discrete channel samples, that may be mutually independent and have the correct ensemble averaged statistics; and the generation of time (i.e. space-) series which are samples of continuous channel behaviour, and these may comprise closely spaced, correlated samples, which should also have the correct ensemble-averaged statistics.
This section introduces the fundamental antenna parameters for the context of mobile communications. The basic antenna elements - the dipole and loop - are discussed both to fix ideas and to demonstrate design considerations for compact antennas. Antenna elements are often categorised from the basis of the antenna's construction: hence the terms wire antennas, slot antennas, patch antennas, aperture (e.g. horn) antennas, etc. Other categorisations reflect a physics approach: electric source antennas, magnetic source antennas, etc., or some engineering parameter type; for example: low-, medium- or high-gain antennas. We are mostly interested in compact and relatively simple elements, although the mounting, often on a complicated platform such as a handheld terminal, can inadvertently make the antenna appear complicated.
In the previous chapters receiver diversity related to antenna arrays has been discussed to some extent, but the additional flexibility derived from having more than one antenna element leads to many more uses. The interdependence between the antennas and the propagation environment is even more clear in this new connection. The signal correlation between elements is very important for the potential applications, and this correlation depends on the angular spread as seen from each array antenna. In general there will be M transmitting antennas in a transmit array and N receiving antennas in a receive array. Although there will be reciprocity between any two antennas, it will be necessary to distinguish between the transmit and receive situation, since the knowledge of the channel parameters may be different in the two situations. Also in the most general case the angular spread of the same environment will be different seen from the two sides. Another important distinction will be between one-port and multi-port arrays. In the first case the different elements may have different complex weights, but combined into one port, whereas in the second case each element may be connected to a network with several ports.
In general, the path loss, as a deterministic parameter, is defined in the following way. A transmitter transfers Pj (W) into the port of an antenna with gain Gx, and the perfectly co-polarised receiving antenna with gain Gr delivers power Pr (W) into a perfectly matched load at its port. The path loss L is defined from the Friis transmission equation.
This appendix reviews some basic statistics which are commonly used in mobile communications and signal processing, and is associated with Chapter 2. The results are well known and are drawn from several sources such as the classical text by Papoulis (1965) and more recent texts such as Kay (1993). The probability of the real random variable z being less than or equal to a value Z, is written P(z<Z) = J pz(x)dx, (B.l) J - oo where Pχ is the cumulative density function (cdf), also called the cumulative prob ability function (cpf), and pz{z) is the probability density function (pdf); k random variables are statistically independent if their joint pdf, pz(x), factors into the product of the individual random variable pdfs: Pziχ) = Pzι (xi)Pz2(χ2) ....Pzkiχk) (B-2) where z and x are the set of zι and x,-, respectively.
This appendix reviews the basic results for many of the distributions which are useful in mobile communications and is associated mainly with Chapter 2. The distributions are motivated by the need for descriptions for the electrical and field signals and the signal coverage and communications outage, etc. Gaussian statistics form the basis of most of the distributions. No physical modelling is included here so that the statistics are confined to envelope and power, and some phase distributions. The distributions of phase derivatives, for example, from physical models, are treated in the text and make use of the results here. The overbar and the angle brackets are used interchangeably with the usual expectation operator for simplicity here. Similarly, whereas in classical statistical notation, upper case letters are normally used for a random variable, this is not adhered to in this text.
In the context of Huygens' principle and physical optics, it makes sense to consider a volume over which the phase of contributing waves is fairly constant, centred around the line-of-sight where the phase is stationary. Consider the situation in Figure D.l where the distance between the two antennas is d and the total distance to a scattering point is R1 + R2.
This appendix shows the equivalence between the time and frequency domain definitions for delay. The time domain expression for the mean excess delay is the energy-weighted delay. This is shown to be the same as the frequency domain energy-weighted group delay.
