With the rapid deployment of new wireless devices and applications, the last decade has witnessed a growing demand for wireless radio spectrum. However, the policy of fixed spectrum assignment produces a bottleneck for more efficient spectrum utilization, such that a great portion of the licensed spectrum is severely under-utilized. The inefficient usage of the limited spectrum resources has motivated the regulatory bodies to review their policy and start to seek innovative communication technology that can exploit the wireless spectrum in a more intelligent and flexible way. The concept of cognitive radio was proposed to address the issue of spectrum efficiency and has been receiving increasing attention in recent years, since it equips wireless users with the capability to optimally adapt their operating parameters according to the interactions with the surrounding radio environment. There have been many significant developments in the past few years concerning cognitive radios. In this chapter, the fundamentals of cognitive radio technology, including the architecture of a cognitive radio network and its applications, are introduced. The existing works on spectrum sensing are reviewed, and important issues in dynamic spectrum allocation and sharing are discussed in detail. Finally, an overview on implementation of cognitive radio platforms and standards for cognitive radio technology is provided.

Introduction

The usage of radio spectrum resources and the regulation of radio emissions are coordinated by national regulatory bodies such as the Federal Communications Commission (FCC).

Recent increases in demand for cognitive radio technology have driven researchers and technologists to rethink the implications of the traditional engineering designs and approaches to communications and networking. One issue is that the traditional thinking is that one should try to have more bandwidth, more resources, and more of everything, while we have come to the realization that the problem is not that we do not have enough bandwidth or resources. It is rather that the bandwidth/resource utilization rates in many cases are too low. For example, the TV bandwidth utilization nowadays in the USA is less than 6%, which is quite similar to that in most developed countries. So why continue wanting to obtain more new bandwidth when it is indeed a scarce commodity already? Why not just utilize the wasted resource in a more effective way?

Another reconsideration is that often one can find the optimization tools and solutions employed in engineering problems being too rigid, without offering much flexibility, adaptation, and learning. The super highway is a typical example in that, during traffic hours, one direction is completely jammed with bumper-to-bumper cars, while the other direction has few cars with mostly empty four-lane way. That is almost the case for networking as well. Rigid, inflexible protocols and strategies often leave wasted resources that could otherwise be efficiently utilized by others. It was recognized that traditional communication and networking paradigms have taken little or no situational information into consideration by offering cognitive processing, reasoning, learning, and adaptation.

Since in ad hoc networks nodes need to cooperatively forward packets for each other, without necessary countermeasures, such networks are extremely vulnerable to traffic-injection attacks, especially to those attacks launched by insider attackers. Injecting an overwhelming amount of traffic into the network can easily cause network congestion and decrease the network lifetime. In this chapter we focus on traffic-injection attacks launched by insider attackers. After investigating the possible types of traffic-injection attacks, we present two sets of defense mechanisms to combat such attacks. The first set of defense mechanisms is fully distributed, whereas the second is centralized with decentralized implementation. The detection performance of each of the mechanisms is also formally analyzed. Both theoretical analysis and experimental studies have demonstrated that, with such defense mechanisms, there is hardly any gain to be obtained by launching traffic-injection attacks from the attackers' point of view.

Introduction

In this chapter, we study a class of powerful attacks: traffic-injection attacks. Specifically, attackers inject an overwhelming amount of traffic into the network in an attempt to consume valuable network resources, and consequently degrade the network performance. Since, in ad hoc networks, nodes need to cooperatively forward packets for other nodes, such networks are extremely vulnerable to traffic-injection attacks, especially those launched by insider attackers.

Roughly speaking, traffic-injection attacks can be classified into two types: query-flooding attacks and injecting-data-packets attacks (IDPAs). Owing to the changing topology or traffic pattern, nodes in ad hoc networks may need to frequently update their routes, which may require broadcasting route-query messages.

Introduction to sensor networks

Sensors have been widely used for many decades. Types of sensors range from thermostats in homes, to control central-heating systems, through to sensors in cars to warn of a lack of engine oil. Over time there has been a tendency towards an ever-increasing amount of sensors as we seek greater control and convenience. For example, many cars now include sensors to check tyre pressure, windscreen-washer fluid, temperature at multiple points within the cabin, the presence of a passenger of sufficient weight to make the airbag active and so on. In urban areas there is a growing number of closed-circuit television (CCTV) cameras, sensors in buildings to detect occupants and sensors on mass-transit systems to identify users for payment purposes.

Some envisage a world in which sensors are much more widespread. They predict the deployment of additional sensors around cities to measure pollution levels, traffic congestion, temperature and more, or sensors scattered around fields of crops providing information on local growing conditions and allowing precisely tailored application of fertilizers, irrigation, etc. Sensors around the home might detect temperature, light and movement to precisely control the home environment. Sensors in industrial buildings might monitor every aspect of the production process, ensuring a higher-quality output and reduced wastage.

Sensors systems to date have mostly been wired – that is, there is a wire from the sensor to the control system.

Introduction

To make use of wireless services, users need devices. These are generally some sort of mobile phone, although increasingly devices include laptop computers with datacards. The capabilities of these devices are an important factor in determining the manner in which they are used and the amount of data they generate – this was clearly shown when the Apple iPhone was introduced and those using it generated some 50 times more data traffic than the average customer.

The phone is an area of intense competition, and innovation and advances in many aspects of its performance can be expected. In this section we consider a number of the key components of the phone, including the screen, input devices, batteries and storage.

Screens

The screen on a mobile device tends to be a compromise between something small that will allow the device to be portable and something large that will provide a good user experience. Different devices embody different compromises depending on their function and the perceived user preference. (This is less relevant when using a device such as a wirelessly enabled laptop.) Over time, screens have added colour and then become ever more vivid and brighter, with better resolution and more depth of colour. Most screens on mobiles, though, still remain poor for watching video and can be difficult to see in bright daylight.

A key breakthrough in mobile devices would be for the screen not to be constrained by the size of the device.

Introduction

We start our look at wireless technology with what are, to date, the most important wireless systems – the cellular communications networks. They are the most important because they create by far the greatest value from the use of wireless communications to date [1] (although some suggest that the role of short-range unlicensed communications will become increasingly valuable over the next decade).

Cellular communications has been one of the great success stories for wireless since the early 1990s. The number of subscribers, networks and mobile phones has grown very quickly and the networks have gone through three major generations of technology. However, this is now changing and network operators face the following key challenges.

• Growth in subscriber numbers is mostly at an end, except in developing countries, and for the first time annual revenue has started falling as competition has reduced prices.

• Traffic levels across wireless networks are growing rapidly as mobile broadband becomes popular, causing network congestion but not resulting in the increased revenue needed to invest in additional resources.

• Services and applications are increasingly being provided by others, sidelining the operators, who are becoming ‘bit pipes’, reducing their potential for increased revenue and changing the value chain.

Whether 4G, sometimes known as ‘long-term evolution’ (LTE), can help operators address these challenges and indeed how it might change the cellular industry is the focus of this chapter; other possible solutions such as femtocells are discussed in subsequent chapters.

The rationale for this book

Wireless communications seems to be an area of frequent and rapid change. New concepts such as updating a Twitter account from a mobile phone arise and become pervasive in less than a year. New devices like the iPhone capture the public imagination within weeks of being launched and in turn change the relationships between the key players in the industry. Satellite navigation seems to be rapidly incorporated into most mobile devices, which themselves are typically replaced within 18 months. Compared with most other industries and consumer products the rate of change is startling. Even in other industries such as the automotive industry, some of the new features such as adaptive cruise control, advanced satellite navigation and collision-control radars are due to advances in wireless technology.

Understanding what is on the ‘wireless horizon’ – namely what developments are now being considered, developed or trialled – can help make sense of how the wireless world is likely to evolve. This book is about scanning that horizon, identifying the important developments and discussing how they will impact on the world of wireless communications over the next decade or so.

As will be seen in the chapters of this book, simply identifying interesting new technologies is far from sufficient. There have been many ‘interesting’ new wireless technologies that have failed to live up to their initial promise – mesh wireless networking is one – for a variety of reasons, many of which are not technical.

Introduction

Healthcare is an area of rapidly growing importance. Many studies have shown clearly how the population is ageing, needs for healthcare are growing and yet the number of people in work (and hence paying for the healthcare system) is falling. Under current extrapolations, healthcare will increasingly become unaffordable and impossible to implement, with an unfeasibly large percentage of the population engaged in caring for others. Either the quality of healthcare provided will fall or new means must be found to care for those who currently rely on the support of others.

Wireless communications provide one possible part of the solution. Through a system of monitors, alerts and the provision of information, it might be possible for electronic systems to allow people to monitor their health more effectively or to generate better information that remote medical professionals can monitor and analyse. The benefit of both scenarios is that the users of wireless medical infrastructures would be able to stay in their homes and look after themselves for longer. A downside is that the increasing medical monitoring will uncover mild forms of sickness and ailments that have hitherto remained invisible, or at least have been dealt with by the natural defence mechanisms of the body without medical intervention. Another, related to this, is making individuals excessively health conscious, hypochondriacs in other words.

An ageing society

The statistics around the age and well-being of our societies are clear. There will be an ever-increasing percentage of older people whose healthcare needs will grow. At the same time, the number of people available to act as carers will fall, as will the number of people in work and paying taxes to pay for healthcare. Without changes to the manner in which we deliver healthcare, the situation will become increasingly problematic.

It is likely that there will be many elements to the solution. One of these may be the use of technology to allow people to stay in their homes for longer and to live healthier lives. Wireless may have a role to play as discussed in this chapter, which builds on earlier chapters looking at technology and wireless in the healthcare arena.

Assisted-living solutions can be divided into the following categories.

• Telehealth – delivering medical care, treatment, or monitoring services to old and disabled people at home from a remote location.

• Telecare – delivering social care/monitoring services to old and disabled people at home from a remote location. This includes preventative care.

• Healthy-living services – delivering services for healthier lifestyles to old and disabled people at home from a remote location.

• Engagement services – delivering services into the home from a remote location, to engage older and disabled people in terms of social, educational or entertainment activities.

• Teleworking – working remotely from home for an employer, or voluntary work, or as a self-employed person who needs remote computing to work successfully with others.

The software-defined handset

Mobile phones are becoming increasingly complex. Over time they have added both additional cellular frequencies and standards and also other wireless technologies. A high-specification phone might now support 2G at 900 MHz, 1800 MHz and 1900 MHz, 3G at 2.1 GHz, Bluetooth and WiFi (both at 2.4 GHz) and GPS (at 1.4 GHz). Some even support mobile TV (DVB-H) at about 700 MHz. Phones in future years will need to support additional technologies and frequency bands.

The traditional way to design a phone supporting multiple standards has been to add an additional chipset to the phone for each standard, although, as time has progressed, some chipset vendors have developed single chips combining some of the most popular combinations of standards. This has the advantage of being relatively simple but the disadvantage of proliferating chips with associated cost. The radio-frequency (RF) design of the phone also becomes ever more complicated. As more frequency bands are supported, the possibility of interference between then increases and the difficulty of designing antennas and filters that can isolate multiple bands grows. Some have suggested that the growth in complexity might be closer to exponential than linear since each new band introduced has to be designed to work with all the other bands already used by the device. To date, these problems have been soluble, with the costs absorbed in the large quantities of handsets sold and with a degree of integration of multiple standards into one chipset.

Rationale – home coverage and capacity

The capacity of any network is determined by the spectrum available, the efficiency of the technology and the number of cells. Historically, the vast majority of the gains in the capacity of cellular networks came from ever decreasing cell sizes. The logical conclusion of this is to implement small cells in each building – indeed, in some cases, in each room. If this were done, many have suggested that there would no longer be a capacity problem since many hundreds of Mbits/s would be available in each room, with only a few people in the room to share it between. For many years the cellular community has been working towards very small cells, partly to realise this vision.

Another reason for small cells is to enhance coverage within buildings. For most, cellular coverage indoors is adequate, but there are some situations where homes are on the edge of coverage or have particularly high building-penetration losses. A small cell in the home would provide high-quality coverage.

A final reason is associated with current business models. Cellular operators in many countries are different entities from the fixed-network operators. There is often competition between the two. When cellular users return home they may switch to the home fixed connection, perhaps because it is less expensive or the quality is better. Providing a home cell with associated modified billing could result in any call revenues from home being received by the mobile operator.

Defining location

Location is broadly about knowing where someone or something is. It may be about knowing where you are in order to navigate to somewhere else or find something. Or it may be about knowing where other people are, for all sorts of reasons. At different times differing types of location information are useful. For example, when using a car satnav the precise position of the car in terms of its geographic coordinates is important. In another situation it may be sufficient to know whether a person is at home or not. Sometimes location is relative, such as wanting to know how far away the nearest restaurant is. Sometimes it is helpful to know also in which direction a person is orientated, for example in order to provide them with information on the building which they are viewing. Sometimes precision is important; in other situations location within a kilometre or two may be adequate. Hence, understanding location is not just as simple as assuming that once a GPS fix has been achieved the problem is solved.

This section looks first at the different methods which a device can use to locate itself or be located by networks. Then we discuss some of the location-based applications that might be of interest. Finally, we look at why many of these applications are not widespread and note the implications of this for the wireless industry and other applications.

The scale of emissions from wireless systems

Across all areas of human activity there is increasing realisation that the emission of greenhouse gases is causing undesirable global warming effects and coupled with this is a desire, and in some cases a requirement, to reduce emissions. This applies to the wireless sector as much as to any other.

It is not entirely clear exactly what contribution wireless makes to greenhouse-gas emissions. Doing such calculations is always very difficult, in particular in defining what should be within their scope. For example, it is clear that power consumed by base stations and mobile-handset chargers should be included, but what about power consumed by laptops that have embedded wireless devices? Deciding whether to include manufacturing and recycling costs can be problematic, as can attribution of costs in areas such as the paper used by cellular operators in the normal course of their business. All of this means that any attribution will be approximate and may change over time, both as more information becomes available and as decisions on the elements to include change.

There is basic agreement on the broad contribution of information and communication technology (ICT), with the total global carbon footprint in 2008 estimated to be about 800 MtCO2e (mega-tons of carbon dioxide or equivalent) or approximately 2% of global emissions. This is predicted to grow to about 1,400 MtCO2e by 2020, or approximately 2.8% of global emissions at that date.

A world of multiple different networks

There is a key trade-off in the design of wireless networks. Lower frequencies are generally preferred for their better propagation, making coverage simpler and cheaper. However, at lower frequencies there is less spectrum available, with the result that networks tend to have a lower capacity. This is a basic law of physics and not something that can be changed in the future with technological advances (although the capacity per unit amount of spectrum does tend to improve over time).

This trade-off means that there is rarely one perfect system. Instead, lower-frequency systems are used to provide coverage across much of the country but with relatively low capacity, while high-frequency systems provide much greater capacity in high-density areas. Hence, we have the current hierarchy of wireless systems, where 2G technology at 900 MHz provides wide-area coverage at data rates of about 100 kbits/s, 3G coverage at 2.1 GHz covers about 30% of a country (depending on the topography and population distribution) at data rates of about 1 Mbits/s and WiFi at 2.4 GHz or 5 GHz covers buildings and very dense urban areas at data rates of about 10 Mbits/s. Such a hierarchy has evolved over time as the most economic way to provide a service as close as possible to the user requirements.