In the previous two chapters, we studied specific techniques for communication over wireless channels. In particular, Chapter 3 is centered on the point-to-point communication scenario and there the focus is on diversity as a way to mitigate the adverse effect of fading. Chapter 4 looks at cellular wireless networks as a whole and introduces several multiple access and interference management techniques.

The present chapter takes a more fundamental look at the problem of communication over wireless fading channels. We ask: what is the optimal performance achievable on a given channel and what are the techniques to achieve such optimal performance? We focus on the point-to-point scenario in this chapter and defer the multiuser case until Chapter 6. The material covered in this chapter lays down the theoretical basis of the modern development in wireless communication to be covered in the rest of the book.

The framework for studying performance limits in communication is information theory. The basic measure of performance is the capacity of a channel: the maximum rate of communication for which arbitrarily small error probability can be achieved. Section 5.1 starts with the important example of the AWGN (additive white Gaussian noise) channel and introduces the notion of capacity through a heuristic argument. The AWGN channel is then used as a building block to study the capacity of wireless fading channels. Unlike the AWGN channel, there is no single definition of capacity for fading channels that is applicable in all scenarios.

Book objective

Wireless communication is one of the most vibrant areas in the communication field today. While it has been a topic of study since the 1960s, the past decade has seen a surge of research activities in the area. This is due to a confluence of several factors. First, there has been an explosive increase in demand for tetherless connectivity, driven so far mainly by cellular telephony but expected to be soon eclipsed by wireless data applications. Second, the dramatic progress in VLSI technology has enabled small-area and low-power implementation of sophisticated signal processing algorithms and coding techniques. Third, the success of second-generation (2G) digital wireless standards, in particular, the IS-95 Code Division Multiple Access (CDMA) standard, provides a concrete demonstration that good ideas from communication theory can have a significant impact in practice. The research thrust in the past decade has led to a much richer set of perspectives and tools on how to communicate over wireless channels, and the picture is still very much evolving.

There are two fundamental aspects of wireless communication that make the problem challenging and interesting. These aspects are by and large not as significant in wireline communication. First is the phenomenon of fading: the time variation of the channel strengths due to the small-scale effect of multipath fading, as well as larger-scale effects such as path loss via distance attenuation and shadowing by obstacles.

Why we wrote this book

The writing of this book was prompted by two main developments in wireless communication in the past decade. First is the huge surge of research activities in physical-layer wireless communication theory. While this has been a subject of study since the sixties, recent developments such as opportunistic and multiple input multiple output (MIMO) communication techniques have brought completely new perspectives on how to communicate over wireless channels. Second is the rapid evolution of wireless systems, particularly cellular networks, which embody communication concepts of increasing sophistication. This evolution started with second-generation digital standards, particularly the IS-95 Code Division Multiple Access standard, continuing to more recent third-generation systems focusing on data applications. This book aims to present modern wireless communication concepts in a coherent and unified manner and to illustrate the concepts in the broader context of the wireless systems on which they have been applied.

Structure of the book

This book is a web of interlocking concepts. The concepts can be structured roughly into three levels:

1. channel characteristics and modeling;

2. communication concepts and techniques;

3. application of these concepts in a system context.

A wireless communication engineer should have an understanding of the concepts at all three levels as well as the tight interplay between the levels. We emphasize this interplay in the book by interlacing the chapters across these levels rather than presenting the topics sequentially from one level to the next.

In this chapter we look at various basic issues that arise in communication over fading channels. We start by analyzing uncoded transmission in a narrowband fading channel. We study both coherent and non-coherent detection. In both cases the error probability is much higher than in a non-faded AWGN channel. The reason is that there is a significant probability that the channel is in a deep fade. This motivates us to investigate various diversity techniques that improve the performance. The diversity techniques operate over time, frequency or space, but the basic idea is the same. By sending signals that carry the same information through different paths, multiple independently faded replicas of data symbols are obtained at the receiver end and more reliable detection can be achieved. The simplest diversity schemes use repetition coding. More sophisticated schemes exploit channel diversity and, at the same time, efficiently use the degrees of freedom in the channel. Compared to repetition coding, they provide coding gains in addition to diversity gains. In space diversity, we look at both transmit and receive diversity schemes. In frequency diversity, we look at three approaches:

• single-carrier with inter-symbol interference equalization,

• direct-sequence spread-spectrum,

• orthogonal frequency division multiplexing.

Finally, we study the impact of channel uncertainty on the performance of diversity combining schemes. We will see that, in some cases, having too many diversity paths can have an adverse effect due to channel uncertainty.

In Chapters 8 and 9, we have studied the role of multiple transmit and receive antennas in the context of point-to-point channels. In this chapter, we shift the focus to multiuser channels and study the role of multiple antennas in both the uplink (many-to-one) and the downlink (one-to-many). In addition to allowing spatial multiplexing and providing diversity to each user, multiple antennas allow the base-station to simultaneously transmit or receive data from multiple users. Again, this is a consequence of the increase in degrees of freedom from having multiple antennas.

We have considered several MIMO transceiver architectures for the point-to-point channel in Chapter 8. In some of these, such as linear receivers with or without successive cancellation, the complexity is mainly at the receiver. Independent data streams are sent at the different transmit antennas, and no cooperation across transmit antennas is needed. Equating the transmit antennas with users, these receiver structures can be directly used in the uplink where the users have a single transmit antenna each but the base-station has multiple receive antennas; this is a common configuration in cellular wireless systems.

It is less apparent how to come up with good strategies for the downlink, where the receive antennas are at the different users; thus the receiver structure has to be separate, one for each user. However, as will see, there is an interesting duality between the uplink and the downlink, and by exploiting this duality, one can map each receive architecture for the uplink to a corresponding transmit architecture for the downlink.

A good understanding of the wireless channel, its key physical parameters and the modeling issues, lays the foundation for the rest of the book. This is the goal of this chapter.

A defining characteristic of the mobile wireless channel is the variations of the channel strength over time and over frequency. The variations can be roughly divided into two types (Figure 2.1):

• Large-scale fading, due to path loss of signal as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the mobile moves through a distance of the order of the cell size, and is typically frequency independent.

• Small-scale fading, due to the constructive and destructive interference of the multiple signal paths between the transmitter and receiver. This occurs at the spatial scale of the order of the carrier wavelength, and is frequency dependent.

We will talk about both types of fading in this chapter, but with more emphasis on the latter. Large-scale fading is more relevant to issues such as cell-site planning. Small-scale multipath fading is more relevant to the design of reliable and efficient communication systems – the focus of this book.

We start with the physical modeling of the wireless channel in terms of electromagnetic waves. We then derive an input/output linear time-varying model for the channel, and define some important physical parameters. Finally, we introduce a few statistical models of the channel variation over time and over frequency.

In this chapter we consider the actions of the UE when it does not have an active radio connection (RRC idle mode state) and also when it is in certain RRC connected mode states (CELL_PCH, URA_PCH and CELL_FACH). For historical reasons, these actions are generally referred to as the idle mode procedures; here we follow the same terminology but add comments where required for the applications of the procedures to the other states.

In the idle mode there are two closely related topics that we need to consider together and which interrelate with one another. These topics are the idle mode substate machine and the idle mode procedures. The idle mode substate machine defines the different states that the UE may be in, the potential transitions between these states and the reasons for being in the states and making the transitions between states. The idle mode procedures define the set of actions that are undertaken by the UE, the outcome of which drives the transitions between the different states in the state machine.

Now, the idle mode procedures and the idle mode substates span the NAS and the AS. Aspects of the procedures and substates apply to both the AS and NAS and so we will consider these in turn. The NAS procedures and substates relate specifically to the activities of the MM sublayer, which is considered in Chapter 13. These NAS issues are further separated into the CS domain activities (MM entity) and the PS domain activities (GMM entity).

In this chapter we consider the RF aspects for the WCDMA transceiver. We focus on the FDD mode, starting with a review of the basic transmitter specifications for the UE and the Node B. Following this, we introduce some terminology and parameters that define the receiver characteristics. Then, we examine the receiver specifications themselves with some comments on the likely design targets for the receiver. In the final section, we review elements of an example design, taking the design issues for a UE transceiver as the reference.

Frequency issues

UMTS frequency bands

Table 5.1 illustrates the ‘current’ proposed bands for the deployment of the UMTS system. Band I is the ‘IMT-2000’ band, Band II is the US personal communication system (PCS) band, and Band III is the digital cellular network at 1800 MHz (DCS1800) band.

North American PCS bands

To understand the frequency allocations in the US PCS bands, it is useful to examine the band allocations for the US bands. Table 5.2 presents the US PCS bands. The ‘C block’ licences have been reauctioned to create a number of subbands as shown in the table. Figure 5.1 illustrates the band structure for the PCS bands, including the different band allocations for the ‘C block’.

MAC introduction

The MAC layer is the lower part of WCDMA layer 2 protocol architecture. At the input to the MAC there are logical channels and at the output there are transport channels. The logical channels define ‘what’ the information is that is being transported, whilst the transport channels define ‘how’ the information is transported. One of the prime functions of the MAC, therefore, is to map a specific logical channel onto the appropriate transport channel. We will see later that this mapping can change dynamically as the characteristics of the network or the user vary (for instance due to a rise or fall of the loading in the network).

This chapter starts by considering the logical channels that enter the MAC, their structure and uses, and also transport channels that exit the MAC, their structure and uses. When we consider the architecture of the MAC, we see that it comprises a number of component parts to reflect its distributed nature within the UTRAN. The functions and services provided by the MAC include items such as random access procedure and transport format selection control, and the mapping and switching between logical and transport channels. To fully understand the function of the MAC we then explore in greater detail some of the key operations it provides, including the random access procedure, the control of CPCH and the TFC selection in the uplink within the UE. We start this first section with an introduction to the logical channels that the MAC provides for the transportation of higher layer data, and review the transport channels considered in earlier chapters.

WCDMA symbol rate transmission path

In this chapter we review the basic principles of what we call the symbol rate processing functions, but are also often referred to as bit rate processing functions. These functions apply to both transmit and receive paths of the WCDMA system. Additionally, we explore functions such as error protection coding, rate matching and the topic of transport channel combination. We start with a review of the uplink/downlink symbol rate transmission path (this is the same for the downlink/uplink receive path, but traversed in the reverse direction). Then we review some of the basic principles of convolutional error correcting codes, and finally we finish with an exploration of turbo codes, the turbo decoder and in particular the maximum a-posteriori probability (MAP) algorithm.

Coding introduction

Figure 7.1 presents the basic structure of the lower layers of the transmission link between the UE and the UTRAN. The diagram is a simplification of the processing stages that are considered in greater detail shortly. The diagram illustrates the basic principles in the operation of the symbol rate processing stages within the WCDMA system. Data are received via transport channels from the MAC. The data blocks may have cyclic redundancy check (CRC) bits appended for error detection purposes in the receiver. The data are then encoded using either a convolutional encoder or a turbo encoder. Next, interleaving is performed to ‘shuffle’ the data prior to transmission to help combat fading and interference across the radio link.

Introduction

In this chapter we examine the structure and the operation of the RRC protocol. The RRC protocol is the main AS control protocol. It is responsible for the configuration and control of all of the different layers that create the radio connection between the UE and the UTRAN. It is a large and complex protocol and consequently, in this chapter, we consider only some key aspects of its operation, leaving the interested reader to consult the relevant specification [24] for a more thorough description.

We start this chapter with a review of the RRC protocol architecture before considering specific key elements of its operation.

Architecture and messages

The RRC protocol architecture is illustrated (from the perspective of the UE) in Figure 11.1. The key functions of the architecture are the dedicated control functional entity (DCFE), the paging and notification functional entity (PNFE) and the broadcast control functional entity (BCFE).

The RRC messages are passed between the UE and the UTRAN. They are used to configure and control the RRC connection between the UE and the UTRAN. The RRC messages can be loosely grouped into four categories: RRC connection management messages; RB control messages; RRC connection mobility messages and RRC measurement messages.