With increasing demands for varied wireless services, the ease with which radio devices can be modified has become highly important. One solution to this is the use of SDR. Wireless Innovation Forum, which was founded in 1996, works in collaboration with IEEE to develop and promote streamlined work in the field of SDR. Software Defined Radio (SDR), in the simplest of terms, may be understood as a communication system where processing functions like modulation, filtering, amplifying and so on is performed by means of software either completely or partially. Some of the most influential projects which have contributed in giving the current shape to SDR technology are tabulated in Table B.1.

Advantages provided by SDR:

• • SDR can easily upgrade to support any future technique. This permits new technologies to be easily implemented and introduced in the market.

• • SDR allows remote re-programming and remote software downloads, which in turn reduces the cost and time of maintenance while significantly increasing the capacity.

• • As processing is implemented using software, SDR can help in establishing an advanced testing environment without the need of building any physical circuit.

• • SDR can offer capabilities of multi-band, multi-standard and multi-service system just by modifying the software package.

• • SDR enables addition of new features to the already existing infrastructure, thus, reducing the cost of new infrastructure establishment.

Types of SDR:

SDR can be one of the following types depending upon the area in which SDR is implemented:

• 1. Multi-channel systems: Systems that can support the processing of more than one transmit or receive channels simultaneously are multi-channel systems

Communication industry is one of the fastest growing industries all over the world. Since its introduction, the system components have evolved dramatically. Owing to this rapid change/enhancement in technologies, the study of communication principles and systems is extensive and skills in this field are in high demand. Incorporation of wireless technology in any communication system provides added advantages in terms of flexibility and mobility. Wireless voice and data services are fast replacing their wired counterparts. Several researchers have contributed in this domain and with every passing day, new information is being added to this vast pool.

Topics covered

The authors have endeavoured to include a large number of topics on wireless communication in a single book. The book has been divided into two parts. Chapter 1 to 6 explain the fundamental principles and basics used for designing any wireless system, whereas chapters 7–12 throw light on popular wireless systems. Chapter 1 provides an overview of the wireless system highlighting topics like advantages and challenges of wireless communication, functional blocks that make up the transmitter and receiver entities in the wireless system and frequency allotment techniques used to avoid inter and intra system interference. The chapter ends with a discussion on generations and standards proposed for popularly employed wireless communication systems.

The Global System for Mobile is a 2G cellular system standard developed in Europe for overcoming the diversity present in 1G cellular system standards. Although initially adopted by European nations, GSM has become a highly popular technology with a very high user base all over the world. The first GSM system was designed to operate in the 900 MHz band, but owing to increasing demand, it was extended to 850 MHz, 1800 MHz and 1900 MHz bands. Different countries use different GSM variants for operation. Europe, Australia, Middle East, Asia and Africa mostly use GSM 900 and GSM 1800, whereas United States, Mexico, Canada and most countries in South America use GSM 850 and GSM 1900. This 2G cellular system provides voice service and circuit switched data service up to 9.6 kbps.

GSM system uses the Gaussian Minimum Shift Keying modulation technique. The access technique used is a combination of TDMA and FDMA. These techniques have already been described in Chapters 2 and 3. This chapter will discuss in depth the architecture, spectrum allocation, interfaces, radio frequency channels, signalling models and basic processes involved in GSM.

GSM Architecture

Any mobile network can be logically divided into three parts namely the access part, the core part and the maintenance part. The access part of the GSM network is also known as the Base Station Sub-system. It comprises the Base Transceiver Station (BTS) and the Base Station Controller (BSC).

Introduction

With growth in Internet usage along with mobile telephony, the requirement of providing multimedia services to registered cellular subscribers led to the development of higher generations of wireless system. In Chapter 8 and 9, 2G, 2.5G and 2.75G standards were discussed. This chapter will focus on UMTS [a 3G standard which is considered as the 3G successor of GSM and uses the WCDMA technique for radio access], 3.5G standard [HSDPA], 3.6G standard [HSUPA] and LTE. These generations aim at providing varied services including voice, video, multimedia, location-based and high speed data service.

The IMT-2000 standard of ITU-R [International Telecommunications Union – Radio Communications section] defines the outline and requirements for standards that can be placed within the family of 3G standards (Table 10.1). Although the research work to develop future public land mobile telecommunication system by ITU-R started in the 1980s, it was only in the early 1990s that some progress was noted. In 1992, at WARC [World Administrative Radio Conference] 230MHz of spectrum in the 2 GHz band was allocated for third generation system development. Visions that led to development of the third generation system standard included:

• • Increased data rate up to 2 Mbps for stationary subscribers, minimum of 384 kbps for pedestrian subscribers and 144 kbps for subscribers at high mobility.

• • Common frequency band all over the world enabling global roaming with a single handset.

• • Improved scalability.

• • Increased user handling capacity.

• • Backward and forward compatibility to reduce installation cost as much as possible.

• • Support of high quality voice and various customized services including data, multimedia and location based service.

Introduction

Wireless systems provide high flexibility and mobility support to their users. Each wireless system is allocated a fixed part of the available frequency spectrum for communication purposes. Table 3.1 lists the range of frequency allocated for some significant wireless services. A user in any wireless system is assigned a particular frequency within this pre-determined band either on permanent or on demand basis. With ever-increasing user density, planning the allocation of the available spectrum to users within the system has become important. Proper resource planning gives the following benefits: First, it aids in avoiding congestion and interference amongst the users. Second, efficient utilization of the otherwise limited frequency spectrum is obtained.

Vast varieties of multiplexing and access techniques are employed for efficient utilization of resources. Multiplexing is predominantly a physical layer function while access technique selection is a data link layer function of the OSI [Open Systems International] model. Multiplexing techniques help to divide the fixed system resource into multiple non-overlapping channels while access techniques are used to accommodate multiple users in these multiple channels, thus increasing the system user capacity Duplexing is another important process involved in preventing interference between uplink and downlink transmission from the same system entity. As multiplexing, duplexing and access methods are all interlinked in planning the resource allocation, they will be discussed together in this chapter.

General Packet Radio Service

Introduction

GSM standard included voice and circuit switched data service. In circuit switched data service, the bandwidth is dedicatedly allocated to a user before data transfer. With increasing user base, the demand for new and higher data rate non-voice services grew. A new switching technique had to be designed. For the same GPRS [General Packet Radio Service] standard with packet switched data functionality was introduced in GSM networks. Table 9.1 lists the significant differences between circuit switched data service of GSM and GPRS.

The development of GPRS standard, which is often referred to as 2.5 G, was started by ETSI/SMG in 1994. As the packet switched principle was employed, this network had the capability to easily communicate with packet-based protocols like IP and X.25. Some new network entities were added in the already existing GSM network to allow the GPRS users to remain connected to the network and on the basis of requirement enjoy the data service. This led to a change in charging principle employed in the GPRS system. Several channel coding schemes were also used to increase per user data rate. Although theoretical data rate for GPRS user is high, in practice, it is averaged around 56 kbps. This standard of wireless communication is often referred to as the first step towards third generation.

Both the subscribers and the service provider have benefitted by the introduction of GPRS services. Some of the noteworthy advantages offered to the subscribers are as follows:

• • Subscribers can enjoy services at higher data rate. Theoretically, the maximum achievable speed is 115.2 kbps. However, in practice, the maximum data rate enjoyed by the end-user is 56 kbps.

Introduction

The cellular telephone system is one of the fastest advancing and popularly used wireless systems with an ever increasing user base. Subscribers can enjoy a wide range of services extending from high clarity voice service to high definition video service and finally high speed data service on the move. Owing to this immense benefit offered, wireless cellular systems are replacing their wired telephony counterparts. Today, the subscriber base of cellular telephone systems has crossed the billions user mark throughout the world. This chapter will begin with a brief outline of the development trend of cellular systems from the early analog communication system to the present UMTS and LTE systems. The rest of the chapter will throw light on the fundamentals of the cellular system design focusing on principles of channel reuse, clustering, handover, cause of interference and mitigation procedures.

Development Trend in Cellular Systems

The idea of a cell-based mobile radio system was first proposed at Bells Laboratory in the early 1970s but the commercial deployment of the system was done only in the 1980s after FCC allocated the proposed spectrum. Since then the wireless cellular telephone system has evolved to introduce new services and features. Table 7.1 lists some of the milestones that led to the development of the present cellular system.

The first generation systems were based on the analog communication system. Many standards were developed by different countries–AMPS in the United States, NMT in Scandinavia, NTT in Japan, RC2000 in France and TACS in Europe.

Introduction

In Chapter 1, the block diagram of a basic wireless communication system was explained.

In this chapter, the principle behind the working of a spread spectrum modulator and demodulator will be explained. Spreading as the name suggests is a technique by which a narrowband signal is transformed to a noise-like signal that has a wider bandwidth. Pseudo-Noise (PN) sequence is used for spreading or de-spreading the information signal. The transmission bandwidth allocated for any system is limited. Thus, using a spread signal for transmission of single user information can seem to be an inefficient technique. However, when a multi-user environment is considered, then the use of a spreading technique is beneficial in multiple ways. First, it permits multiple users information to spread and occupy the same bandwidth with least interference between each other. Second, on spreading, the signal becomes noise-like and can be de-spread only by the correct PN sequence. If de-spreading is attempted with any other PN sequence, the signal remains noise-like. Thus, use of the spreading technique ensures secured data transmission between the sender and the desired receiver. Third, it helps to prevent multi-path fading. Fourth, in multi-user systems, as all spread signals use the same bandwidth, there is no requirement of any frequency planning. Last but not the least, spreading a signal makes it immune to jamming.

Radio jamming is a collective term used for methods used to deliberately block or degrade the working of any wireless system including mobile telephony, satellite communication, CCTV systems and the like.

Introduction

Wireless communication involves transmission of messages over radio channels which are prone to noise and interference. In order to ensure reliable transmission, several processes are involved both at the transmitting and receiving end. In Chapter 1, the block diagram of a wireless communication system was explained. This chapter will focus on coding, modulation, diversity and equalization techniques.

Source Coding

The end-user generates information that has to be transmitted towards the receiver. The generated user message may contain several redundancies that can be easily omitted such that the user message still remains in understandable format. Removal of redundancies to an acceptable limit is the main function of a source encoder. This main criterion behind the source coding principle leads to two types of coding techniques, namely lossless coding and lossy coding. The former type of source coding techniques ensures that the original user message is reconstructed at the receiver end. These coding variants are also referred to as entropy coding or noiseless coding techniques. On the other hand, in the latter type, the source coding is done in such a way that at the receiver end, only an approximation of the original user message can be reconstructed.

Depending upon the code length of the source encoder output and the corresponding user message length, lossless coding can be classified into four types:

• • Fixed-to-fixed mapping: In this technique, a fixed number of user message symbols are mapped to codeword of fixed length.

• • Fixed-to-variable mapping: In this technique, a fixed number of user message symbols are mapped to codeword of varying length. All the symbols in the user message do not have equal probability of occurrence.

Printed antennas are the most commonly used antennas in wireless applications where size is a constraint. The most basic form of printed antenna is the microstrip antenna that comprises a printed patch on the grounded substrate (Fig. A.1). In this text, a microstrip antenna will be designed. An antenna's performance is controlled by various parameters. The overall goal of any design is to obtain a stable output in the desired band. In order to achieve this goal, different factors on which the characteristic depends need to be examined and decided properly

The important considerations in the design of microstrip antenna are as follows

I. Substrate selection: Substrate in antenna design is principally needed for mechanical support of the antenna metallization. However, it also affects the performance of the antenna. There is no ideal substrate that will work equally well for all applications; in fact, the choice is completely determined by the application. The following points should be kept in mind before selecting any substrate.

• • Substrate thickness (h): A thicker substrate, besides being mechanically strong, will increase the radiated power, reduce conductor loss and improve impedance bandwidth. However, it will also increase the weight, dielectric loss, surface wave loss and extraneous radiation from probe feed.

• • Substrate dielectric constant (er): Low value of dielectric constant for substrate will increase the fringing field at the patch periphery and thus, the radiated power.

• • Loss tangent: A high loss tangent will increase dielectric loss and therefore reduces antenna efficiency.