Introduction

The chapters so far have looked at the main functionality of a cognitive radio through an exploration of the ‘observe, decide, act’ cycle. We now step back from this and look at security issues specifically related to cognitive radio. This chapter is the shortest chapter of the book. This is not indicative of the level of importance of the topic of security and cognitive radio but of the fact that cognitive radio security has to date received much less attention than other topics.

All communication systems need to be made secure to operate. Typically any users of a system have to authenticate themselves on the network. Authentication is the process of determining whether someone or something is, in fact, who or what it is declared to be. Some authentication processes may involve the simple use of a password but others are more complex. The use of digital certificates, issued and verified by what is known as a Certificate Authority (CA) as part of a public key infrastructure, is an example of a more stringent process. Following authentication, authorisation processes, to ensure data and services are accessible only to those who have the correct entitlements, are needed. During all communication privacy may need to be guaranteed. Encryption is typically used to achieve this either using a public key infrastructure, a symmetric cryptographic approach or hash algorithms. Eavesdropping by man-in-the-middle attacks must be avoided.

Wireless mesh networking is a hot and growing topic, still in its infancy in some ways, whilst already shown to be capable in others. From a military beginning, mesh networks moved to civilian use and are now being deployed worldwide as both local area networks (LANs) and metropolitan area networks (MANs). However, these deployments are still ‘leading edge’ and it is not yet clear what the most enduring applications of mesh will be – particularly as the market moves from early adopters towards widespread take up.

Some of the claims for what a mesh network may deliver have been very ambitious to say the least. In this book we investigate such claims versus the real qualities of mesh networks and identify the key time scales and drivers for the challenges involved with making meshes. Throughout the book we attempt to keep mathematics to a minimum. Where an equation is shown, it remains practical to follow the flow of the book without needing to understand the maths fully.

The book takes a very pragmatic but balanced approach to the issues. We are particularly interested in meshes with an external access capability, for example to the Internet. We supply a technical assessment of mesh and multi-hop networking, highlight the attractions, identify the pitfalls, provide clear and concise hints and tips for success – summarised inside the back cover – and finally evaluate some real-world examples of good mesh applications. These include wireless cities, community networking and vehicular ad hoc networks (VANETs). Wireless sensor networks (WSNs) are another important application of mesh techniques with their own unique challenges, and these receive their own chapter.

Here we examine two of the most common real-world mesh deployments: firstly wireless cities and secondly community Internet. We show how their reasons for success align with the content presented in earlier chapters in this book. Interestingly, wireless city deployments are targeted at urban areas which already have wired Internet connectivity but where the addition of mobility is valued, whilst in contrast community Internet is targeted at those places where the wired Internet is sparse and connectivity can be added most easily by using wireless to serve fixed locations.

Thirdly, we also show a rising application of mesh networking – vehicular ad hoc networks (VANETs). These systems are targeted at improving road safety and have had spectrum allocated in many countries, and enjoyed success in industrial trials. We expect VANETs to experience particularly strong future growth.

Wireless cities

Several wireless cities are now up and running which provide easy Internet access on the move. In the UK, London and Bristol were early examples, whilst in the USA there is New York, Portland, OR and a rapidly growing number of others. The aim in each case is to enable easy mobile connection to the Internet. This can serve the general public, business users and the city authorities, who may use it for operational purposes, including for public services such as law enforcement.

The wireless nodes are deployed at street level and each includes a normal WiFi access point, so that users may connect with their existing WiFi enabled devices, such as laptops and a growing number of converged cellular-WiFi mobile handsets and PDAs.

To summarise once more, at this point in the book it has been shown that practical mobile meshes are not chosen primarily for spectral efficiency nor for any notion of self-generation of capacity. Meshes should be chosen because they have other benefits. Section 2.2 provided an introduction to how meshes offer coverage benefits, which is possibly their major attribute. In this chapter we revisit our six most likely applications which we have been considering throughout the book. These are

• cellular multi-hopping or WiFi hotspot extension,

• community networking,

• home and office indoor networking,

• micro base station backhaul,

• vehicle ad hoc networks (VANETs), and

• wireless sensor networks (WSNs).

The first five applications are considered in detail in this chapter, whilst wireless sensor networks receive their own treatment in Chapter 10, since they have some unique features. In this chapter, we also look at the barriers to mesh adoption and the time scales likely for them to be overcome.

For the following discussion we find it useful to group the applications into those which form a mesh on the user side and those which form a mesh on the network side, in other words those where the users' nodes themselves mesh together, versus those where only the backhaul forms a mesh. There is one case where the mesh can be for both users and network backhaul; this occurs in VANETs.

Introduction

Cognitive radio is a topic of great interest and holds much promise as a technology that will play a strong role in communication systems of the future. This book focuses on the essential elements of cognitive radio technology and regulation. This is a challenging task in that cognitive radio is still very much an emerging technology. There is much debate over its exact definition, its potential role in communication systems, whether cognitive radios should in fact be permitted in the first place and if yes, what the regulatory policies should be. However, while acknowledging the flux in this field, the book aims to identify the core concepts that will remain central to the field irrespective of how precisely it develops. The aim of this first chapter is to briefly define cognitive radio and to then focus on the all important question of why cognitive radios are needed. This chapter therefore motivates all that is to come in the book.

Brief history and definition

The term cognitive radio was coined by Mitola in an article he wrote with Maguire in 1999 [1]. In that article, Mitola and Maguire describe a cognitive radio as a radio that understands the context in which it finds itself and as a result can tailor the communication process in line with that understanding.

Introduction

To study observations, we return yet again to the definition of the cognitive radio laid out in Chapter 1 and note once more that ‘A cognitive radio is a device which has four broad inputs, namely, an understanding of the environment in which it operates, an understanding of the communication requirements of the user(s), an understanding of the regulatory policies which apply to it and an understanding of its own capabilities.’ Getting these four inputs is what we mean by the phrase ‘observing the outside world’.

We can further detail some of the observations that are needed if we go through the various action categories outlined in the last chapter. To take action from a frequency perspective the cognitive radio must observe which signals are currently being transmitted, which channels are free, the bandwidth of those channels and perhaps whether the available channels are likely to be short lived or more durable. To take action from a spatial perspective, the cognitive radio needs to make observations about the spatial distribution of systems that must be avoided, or the spatial distribution of interferers and of the target radios. The cognitive radio needs to be able to monitor its power output and the power output of other systems. To take action to make a signal more robust or to maximise the throughput of the transmitted signal, the cognitive radio needs to make observations about the signal-to-noise ratio (SNR) at the target receivers, about the bit error rates and about the propagation conditions experienced by the transmitted signal (e.g. delay spread, doppler spread).

Introduction

To discuss regulation and standardisation in the context of cognitive radio is a challenge. Currently there are almost no regulations or standards in place for cognitive radio, as cognitive radios are still very much a thing of the future. Hence this chapter is more about classifying the general types of regulations that may be needed and the standards that are emerging than discussing what is already in place. In reality there is a wealth of regulatory issues that relate directly, indirectly or just ‘kind of relate’ to cognitive radio. Chapter 1 explored the role of cognitive radios in delivering new ways of managing the spectrum and looked at applications in the military, public safety and commercial domains. The new spectrum management regimes and the various potential applications may each give rise to the need for new regulations, some of which are specifically related to cognitive radios and some of which are related to creating the kind of environment in which cognitive radio applications can thrive. The purpose of this chapter, therefore, is to give a broad sense of what those issues might be, as well as to describe the current status of the standardisation efforts.

Regulatory issues and new spectrum management regimes

Much of the discussion about ‘regulations for cognitive radio’ is about ‘regulations for new spectrum management regimes in which cognitive radios can operate’.

Introduction

The first chapter of this book focused on the application areas that will drive cognitive radio technology. This chapter acts as a bridge to the remainder of the book. It seeks to provide the reader with a broad sense of all that is involved in cognitive radio technology. In order to do this we go to the heart of the cognitive radio but not at first using technology as an example. Instead we step back and take a look at how decisions are made in a more abstract manner before returning to the radio world. The final part of the chapter provides a roadmap for the rest of the book.

Setting the scene for understanding cognitive radio

The first question to think about is: how do we make decisions? How do we reason and come to conclusions? We begin this discussion by looking at a simple example.

The lone radio

Scenario 1: I am about to go out and must decide whether I should take an umbrella with me or not. The umbrella is heavy and cumbersome and, while I don't want to get wet, I don't want to take the umbrella with me if it is not necessary.

In this example two actions are possible, namely take umbrella or don't take umbrella. I need to determine how likely it is to rain in order to decide whether to take the umbrella or not.

Introduction

We now reach the ‘decide’ part of the ‘observe, decide and act’ cycle. In very simple terms the decision-making process is about selecting the actions the cognitive radio should take. Using the vocabulary introduced in Chapter 2, it is about choosing which ‘knobs’ to change and choosing what the new settings of those ‘knobs’ should be. Decision-making goes very much to the heart of a cognitive radio.

The decision-making process: part 1

In Table 3.2 a variety of cognitive radio applications and the main highlevel actions associated with them were presented. On examining the table we noted that many of the actions, whether commercial, public safety or military based, centre on two activities:

1. The cognitive radio shapes its transmission profile and configures any other relevant radio parameters to make best use of the resources it has been given or identified for itself, while at the same time not impinging on the resources of others.

2. If and when those resources change, it reshapes its transmission profile and reconfigures any other relevant operating parameters, and in doing so it redirects resources around the network.

A re-examination of Table 3.2 will confirm that these actions are standard throughout a whole variety of applications. It therefore comes as no surprise that two kinds of decisions that regularly need to be made are decisions that map to these two activities, namely decisions about how resources are distributed and decisions about how those resources are exactly used.

During the production phase of this book, the FCC released two reports that are of relevance to this book. At that stage it was too late to include details of the reports in the main body of the text. This short appendix addresses the issues briefly.

On 15 October 2008 the FCC released their report (FCC/OET 08-TR-1005) on the Evaluation of the Performance of Prototype TV-Band White Space Devices Phase II. The opening paragraph of the report summarises what the report shows:

The Federal Communications Commission's Laboratory Division has completed a second phase of its measurement studies of the spectrum sensing and transmitting capabilities of prototype TV white space devices. These devices have been developed to demonstrate capabilities that might be used in unlicensed low power radio transmitting devices that would operate on frequencies in the broadcast television bands that are unused in each local area. At this juncture, we believe that the burden of ‘proof of concept’ has been met. We are satisfied that spectrum sensing in combination with geo-location and database access techniques can be used to authorize equipment today under appropriate technical standards and that issues regarding future development and approval of any additional devices, including devices relying on sensing alone, can be addressed.

The report goes on to state that

All of the devices were able to reliably detect the presence a clean DTV signal on a single channel at low levels in the range of – 116 dBm to – 126 dBm; the detection ability of each device varied little relative to the channel on which the clean signal was applied.

Introduction

In Chapter 1 the working definition for cognitive radio used throughout this book was presented. That definition ended with the statement ‘A cognitive radio is made from software and hardware components that can facilitate the wide variety of different configurations it needs to communicate.’ In this chapter we look at the hardware involved. There is no one right way to build a cognitive radio so the chapter merely aims to give a sense of what kind of hardware can be used and some of the related performance issues.

A complete cognitive radio system

In a cognitive radio receiver, the antenna captures the incoming signal. The signal is fed to the RF circuitry and is filtered and amplified and possibly downconverted to a lower frequency. The signal is converted to digital format and further manipulation occurs in the digital domain. On the transmit side the opposite occurs. The signal is prepared and processed and at some stage is converted from digital to analogue format for transmission, upconverted to the correct frequencies and launched on to the airwaves via the antenna.

Throughout this book we have been using the terms ‘cognitive radio’ and ‘cognitive node’ interchangeably. The reason for this is that a cognitive radio will almost all of the time function as a node in a network. Therefore it is useful to think of the complete cognitive radio system in terms of a communication stack.

Having covered the fundamentals of meshes, we now arrive at the point where we may begin to consider the big and often asked questions about mesh, four of which we consider together, via our list of hypotheses. As a reminder, these are that

1. meshes self-generate capacity,

2. meshes improve spectral efficiency,

3. directional antennas help a mesh, and

4. meshes improve the overall utilisation of spectrum.

We will examine them formally, via analysis of existing peer reviewed publications, followed by some more recent analysis and insight of our own [1, 2]. A key problem in assessing the published literature is that different assumptions are made in different published papers; a direct comparison is thus at risk of being inconsistent. We spend some time at the outset to ensure we avoid this issue.

We will bear in mind that we are predominantly interested in our six application examples of Chapter 2. This will set helpful bounds to our scope for testing the hypotheses.

When we look at Hypothesis 1 which is concerned with capacity, we form our initial viewpoint via a simple thought experiment, which looks at how we expect the capacity of a mesh might behave versus demand, relative to the known case of cellular. This is followed by a summary of four important peer reviewed research papers in the field, which concern system capacity. We contend that the important conclusions presented in these papers were never intended to be used by readers as evidence that a real-world mesh can self-generate capacity.