Introduction

In this chapter a distributed dynamic channel assignment (DCA) algorithm applicable for the TDD mode of the UMTS terrestrial radio access (UTRA) is presented. It is closely related to the DCA used in the DECT (digitally enhanced cordless telecommunications) system (Punt et al., 1998). Once again, the discovery made in Chapter 3 is exploited; that is, that for certain scenarios opposed synchronisation of TSs between neighbouring cells is advantageous. The new distributed DCA algorithm is supported by the results of the investigation in section 4.2. In this section it was demonstrated that synchronous transmission and reception between neighbouring cells may not yield the greatest capacity that is attainable in a single cell. It was found, however, that when the centralised DCA algorithm developed in section 4.3 was applied to multiple cells it was not feasible to fully exploit the potential gains revealed by the capacity analysis of a single cell.

In this chapter it is demonstrated that by applying the novel distributed DCA algorithm, which utilises the TS-opposing idea, greater capacity can result than would be obtained by synchronous transmissions. Most importantly, this is shown to be valid for a TDMA-CDMA/TDD (TD-CDMA/TDD) network, which accounts for full spatial coverage. Channel asymmetry is assumed to be arranged by code pooling rather than TS pooling (3GPP, TSG, RAN, 2000c).

Atmospheric effects add significant complicating factors to the job of the radio-system designer. Initially, the way in which multipath propagation can be established due to the atmospheric structure is explained together with the effect this can have on the received signal. Another phenomenon, ducting, whose existence depends upon the structure of the atmosphere, is then discussed. It is seen that ducting can lead to levels of long-distance interference rising. The way in which diversity techniques can reduce the effect of multipath fading is explained. Another form of fading (‘diffraction fading’) is then described. Diffraction fading occurs when the atmosphere causes the path of the radio wave to bend upwards as it travels, leading to no line of sight existing between points for which a clear line of sight would be expected. Next, it is shown that, in severe cases of multipath propagation, the delay between two paths can be significant. This leads to the received spectrum in large-bandwidth links becoming distorted. The amount of fading caused by rain is explained as another factor that must be considered when designing a microwave radio link. Further, the fact that, even if no fading occurs, the ever-present molecules present in the atmosphere will cause attenuation is described, together with an indication of the frequency dependence of this attenuation. Finally, the way in which atmospheric losses affect the noise performance of radio systems, particularly Earth–space systems, is analysed.

When a radio wave can reach a receiver by more than one route, we say that the receiver is in a multipath environment. The way in which a standing wave pattern is established when the received signal is the combination of both a direct and a reflected signal is explained. The characteristics of the standing wave are shown to depend upon the nature of the reflection as determined by its reflection coefficient. Further examples of propagation paths involving reflection include propagation over a flat plane and propagation over water, the latter having the additional complication of tidal variation often causing the position of the reflecting surface to change. The more complex situation that arises when there are many different routes from transmitter to receiver is analysed. It is seen that the nature of the standing wave depends on whether one of the contributing paths is dominant (the ‘Rician’ environment) or whether the strengths of all of the signals on all paths are about equal (the ‘Rayleigh’ environment). It is further shown that the reflected signal depends on whether the reflecting surface is smooth or rough and the difference in the nature of the reflected signal is analysed. A further possible propagation mechanism is that of penetration of materials. The amount of penetration is seen to be dependent upon the electrical characteristics of the material and the frequency of the electromagnetic wave.

Introduction

In practical situations, radio waves will reflect off walls and off the ground.

Predicting the strength of a radio signal in the shadow of an obstacle is a vital function for propagation engineers. The mechanism by which a wave enters into the shadow of an obstacle is known as diffraction. Even the simplest of practical obstacles pose severe mathematical challenges. More easily solved approximations are adopted in order to estimate the strength of diffracted signals. The starting point for diffraction problems is the case where a receiver is in the shadow of a perfectly absorbing ‘knife-edge’ obstacle. This is then extended to encompass the situation where there are several such obstacles on the path. Many approximate multiple-knife-edge prediction methods exist and the most commonly used are analysed and compared. More accurate ‘near-exact’ methods are discussed. Although these methods usually make better predictions of the signal strength in the shadow of obstacles, they require significantly more computing time as well as being significantly more complicated to implement. Once an understanding of the properties of a diffracted signal has been obtained, it is possible to derive clearance requirements for a point-to-point path so that diffraction effects may be safely ignored. The insights gained by investigating the mechanism of diffraction into the shadow of an obstacle can be used to analyse two related phenomena: reflection from a finite surface and the formation of the radiation pattern of an aperture antenna.

Knife-edge diffraction

Diffraction is the name given to the mechanism by which waves enter into the shadow of an obstacle.

Point-to-area transmission is the generic name given to the way in which broadcasting transmitters or base stations for mobile communications provide coverage to a given area. A general overview is provided in order to deliver the most significant information as quickly as possible. Following that, the concept of electric field strength as an alternative to power density, prediction methods and the effect of frequency are explained in more detail. Digital mobile radio is selected for further study as a specific example of point-to-area communication. Path-loss-prediction methods specific to digital mobile radio are examined and various types of antenna that are used for the base stations of these systems are described. Finally, the effect that interference can have on the coverage range of a base station is explained.

Overview

The simplest form of antenna used is an ‘omni-directional’ antenna. These radiate equally in all directions in the horizontal plane. They often have a narrower beam (perhaps less than 20 degrees) in the vertical plane. Such antennas typically have a gain of 10 dBi. Two collinear wire elements that are fed at their junction with a signal form what is known as a dipole antenna. The basic omni-directional antenna is a dipole that is half a wavelength in height (a ‘half-wave dipole’ with each of the wire elements being a quarter of a wavelength in length). This has a wide vertical beam and has a gain of 2.1 dBi.

A further benefit from using the decibel scale becomes apparent when dealing with what is known as the link budget. Using decibels, a power budget on a radio link becomes as straightforward as a simple financial budget. Transmit power and gains can be thought of as equivalent to income, with losses and required margins being equivalent to expenditure.

For example, suppose that we transmit with a power of 30 dBm (1 watt). There are feeder losses, antenna gains, free-space loss, absorption loss, fading margin etc. The link budget is really a method of organising these parameters so as to make the calculation of the received signal level (under conditions of maximum fade, if the fade margin is considered) as straightforward as possible. The received signal level should be sufficient to deliver an acceptably low bit error ratio. Table A4.1 gives an example of such a link budget.

An attempt has been made to provide an insight into the way in which signal strength can be predicted for a variety of situations. Information has been presented with the intention of stimulating an intuitive understanding of radio wave propagation together with essential formulas that will allow rapid estimates of signal strength to be made. It is the sort of information that experienced radio-propagation engineers will carry around in their heads (with the exception of the more complicated equations). Further, detailed information will be gained from consulting more detailed books such as those recommended as further reading and the ITU recommendations (also listed). Further, a radio-propagation engineer will often have software modules available that implement the ITU recommendations and other methods for propagation prediction such as the Okumura–Hata method.

Although radio wave propagation is really a single subject, all the diverse factors that affect the strength of a received signal make a comprehensive calculation of signal strength almost impossible. As a result, radio-propagation engineers concentrate on the factors that have the most significant effect for the circumstances in hand. It is seen that the task of predicting the signal received when propagation is in free space is relatively straightforward and depends upon antenna gains, path length and frequency. The concept of antennas possessing gain, although they are passive devices, is explained: the ‘gain’ is associated with the ability of an antenna to direct the transmitted energy in the required direction and prevent the energy spreading as it travels.