Chapter 1
Introduction
1.1 Market and Technology Trends
Social, economic and political factors determine the development of the mobile communications business. Consumer demand, the economic performance of operators and government policies are some of the aspects that affect technological advances, operators' capital investments and the regulatory environment. The mobile communications sector has been characterized by a worldwide rapid increase in the number of users. During the 1980s only a handful of people had a mobile phone. At the end of the 1980s, the number of cellular subscribers was merely around 5 million. With the introduction of the Second Generation (2G) cellular systems in 1991, the ambition was to popularize progressively the usage of mobile phones by making them affordable to a large part of the population. Progress in micro electronics then made it possible to produce cheaper mobile phones. The technology advanced and gradually increasing competition between mobile vendors made it necessary to reduce the cost of cellular infrastructures. The second part of the 1990s, witnessed an extraordinary surge in the number of mobile subscribers in the developed countries. In total, the number was close to half a billion. Progress continued worldwide at a frenetic pace. According to the International Telecommunication Union (ITU), in the last seven years the number of worldwide subscribers has grown from 1.7 billion to more than 5.3 billion (75.42% of the world population), which implies growing at a compound annual growth rate of 21%. Astonishingly, in 2002 and within only two decades, mobile subscribers surpassed fixed-telephone line subscribers (ITU n.d). The evolution of the number of mobile and fixed line subscribers from the year 1996 to 2010 is shown in Figure 1.1.
Figure 1.1 Evolution of mobile and fixed phone subscriptions from 1996 to 2010
Even though these numbers are quite significant, it is worth noting that the mobile communications sector has reached a saturation point in terms of the number of subscribers in a large number of markets, but new systems result in technology upgrades of networks and devices, which offer a significantly improved user experience and capabilities and provide new business opportunities. In the European Union, mobile penetration rate is over 110% of the total population, whereas in developed Asian countries has reached 80%, as in the United States and Eastern Europe where the growth of mobile services has been quite important in recent years. There is still room for mid-term growth of less-developed markets. Operators in saturated markets need to foster demand for new services to guarantee their revenues.
That is why the mobile communications sector, today more than ever, seeks to put new telecommunication services on the market through mobile devices. Among these services social networks, location-based services, augmented reality, mobile TV, video on demand, interactive games and high quality music were applications added to mobile devices to ensure an upturn in usage of mobile services and, consequently, revenues. From 2007 there has been a quite significant increase in traffic demand. Apart from new services, several factors are fostering the mobile communications sector: the increase of Third Generation (3G) penetration as users rapidly migrate from 2G to 3G services; the increasing penetration of Universal Serial Bus (USB) modems and data cards, as well as smart phones and tablets, together with the increasing availability of easy-to-use data applications; the proliferation of flat-rate service bundles, which is also changing service mixes towards more usage-intensive services; and the increasing usage of 3G devices indoors, among other things. All these factors are making mobile data demand overload the capacity of 3G networks and will force next generation mobile systems to be designed according to take this trend into account. Rather than just requiring an increase in throughput, these developments will require improving the ubiquity of the Quality of Experience (QoE) Indicators, that is, allowing the mobile users to experience high QoE values in any geographical position, not only close to the Base Station (BS), while minimizing the radio resource and energy consumption.
In fact, current market forecasts predict mobile Internet penetration to double by 2015, which represents a real threat of congestion for current cellular networks. Indeed, recent analysis of the evolution of mobile broadband subscribers show that, starting in 2007, there has been a significant increase in their number and their traffic demands. In many countries where there are developed markets, mobile data consumption has increased from 2008 and 2010 and is growing exponentially as can be seen in Figure 1.2. Traditional asymmetric traffic – with more data in Downlink (DL) than in Uplink (UL) – is daily becoming more symmetrical. The increase in the usage of wireless systems is driving the industry to seek new methods to boost the capacity of cellular networks, that is, the number of users served or transmitted bits over the air interface.
Figure 1.2 Exponential increase of mobile data traffic. UMTS Forum (UMTS-Forum 2010)
The decision to adopt such methods, which may involve either improving a specific cellular standard or adopting a completely new technology through a standard change, has been seen as strategic and the decision is not based purely on technical or economic grounds. The influence of economic/political factors – through influential governmental and industrial players – is undeniable.
The first commercial analog mobile communication systems were deployed in the 1950s and 1960s, although with low penetration. The year 1981 witnessed the birth of the first commercial deployments of the First Generation (1G) mobile cellular standards such as Nordic Mobile Telephone (NMT), in Saudi Arabia and the Nordic countries, C-Netz in Germany, Portugal and South Africa, Total Access Communications System (TACS) in the United Kingdom and Analog Advanced Mobile Phone System (AMPS) in the Americas. The 1G standards are called the analog standards since they utilize analog technology. The beginning of the 1990s witnessed the introduction of 2G, characterized by the adoption of digital technology. This technology allowed considerable improvements in voice quality, capacity and growth potential towards advanced applications as well as the development of Short Message Service (SMS) messaging, a form of data transmission. The European Conference of Postal and Telecommunications Administrations (CEPT) decided in 1982 to develop a pan-European 2G mobile communication system. This was the starting point of the Global System for Mobile Communications (GSM), the dominant 2G standard, which was deployed internationally from 1991. In the beginning, the main objective of GSM was the support of voice telephony and international roaming with a single-system across Europe. GSM is based on a hybrid Time Division Multiple Access (TDMA) Frequency Division Multiple Access (FDMA) method, in contrast with 1G systems based only on FDMA (Hillebrand 2002). In parallel with GSM, other digital 2G systems were developed globally and competed with each other. These other main 2G standards include IS-136, also known as D-AMPS, IS-95A also known as CDMAOne – used mainly in the Americas – and finally Personal Digital Cellular (PDC) – used exclusively in Japan. In contrast to GSM, the IS-95 technologies are based on Code Division Multiple Access (CDMA)(Viterbi 1995).
The evolution of the 2G, called 2.5G, allowed the introduction of packet-switched services in addition to voice, the most significant circuit switched service. The main 2.5G standards, General Packet Radio Service (GPRS) and IS-95B, are basically an extension of GSM and IS-95A, respectively.
Shortly after the 2G became operational, industrial players were already preparing and discussing the next wireless generation standards. In January 1998, CDMA under two variants – Wideband Code Division Multiple Access (WCDMA) and Time Division CDMA (TD-CDMA) – was adopted by the European Telecommunications Standards Institute (ETSI) as a Universal Mobile Telecommunication System (UMTS) as the 3G mobile communication system, also called International Mobile Telecommunications 2000 (IMT-2000). As a member of the IMT-2000 family of standards, the Third Generation Partnership Project (3GPP) developed UMTS technology using both WCDMA and TD-CDMA modulation schemes (Holma and Toskala 2000) and is generally favored in Japan and countries using GSM. On the other hand CDMA2000, initially an outgrowth of the 2G CDMA standard IS-95, is mainly dominant in the Americas and Korea.
New specifications have been developed within the framework of 3GPP together known as 3G Evolution. For this evolution, two Radio Access Network (RAN) approaches and an evolution of the core network have been suggested. The first RAN approach is High-Speed Packet Access (HSPA) – referred to as a 3.5G technology. HSPA comprises High-Speed Downlink Packet Access (HSDPA), added in Release 5 (Rel-5), and High Speed Uplink Packet Access (HSUPA), added in Release 6 (Rel-6) of UMTS. Both enhance the packet data rate, respectively to 14.6 Mbps in DL and to 5.76 Mbps in UL. Again, HSPA is based on WCDMA and is completely backward compatible with UMTS. The philosophy behind this radio network approach is to add new features while still serving the old mobiles, and is further applied in HSPA Evolution, also known as HSPA+. This is a good solution for the mid-term future. The equivalent evolution in CDMA2000 are 1xEV-DO and 1xEV-DV. While CDMA 1xEV-DO started deployment in 2003, HSPA and CDMA 1xEV-DV entered into service in 2006.
The second UMTS evolution is called Long Term Evolution (LTE) (Dahlman et al. 2008; Sesia et al. 2011) and the evolved core network is known as Evolved Packet Core (EPC). The target of LTE is high performance and reduced cost for the radio access. LTE is a radio interface designed from scratch. Hence, in contrast to HSPA, LTE is not backward compatible with UMTS. However, the design is clearly influenced by earlier specification work done by 3GPP. At the end of 2007 first LTE specifications were approved. The LTE system has peak data rates of around 326 Mbps, increased spectral efficiency and significantly shorter latency than previous systems. LTE is based on Orthogonal Frequency Division Multiple Access (OFDMA) and advanced spatial processing Multiple-Input Multiple-Output (MIMO). The Next Generation Mobile Network (NGMN) initiative formulated requirements on further developments of mobile communications NGMN. Such requirements are mainly related to a flat network architecture based on the Internet Protocol (IP) for cost reduction, higher spectral efficiency for better use of the available frequency spectrum, lower latency and higher peak data rates with flexible allocation of data rates to users. Additional requirements are a high cell average throughput and sufficiently high cell edge capacity in order to cover the expected increasing data traffic with growing user density. LTE was developed to meet these requirements, and this was an important step towards the next International Mobile Telecommunications Advanced (IMT-Advanced) standard (see section 1.3).
With regard to the cellular systems market, today, the GSM family (GSM, GPRS and Enhanced Data rates for GSM Evolution (EDGE)) is the dominant second-generation mobile communication standard with a global market share – at the end of 2010 – of more than 79% and 4.18 billion subscribers in more than 200 countries (GSA n.d.). On the other hand, the number of 3G subscribers including HSPA has risen to 619 million subscribers, which represents 11.7% of the market (GSA n.d.). The main competitor to GSM is IS-95 CDMA that serves the rest of the market. Currently, the main subscriber growth markets for the GSM system are emerging markets, such as China with about seven million new subscribers per month and India with about 16 million new subscribers per month. With respect to LTE, at the end of 2010 seven LTE networks were commercially launched, being still an incipient technology. The evolution of cellular mobile systems is shown in Figure 1.3.
Figure 1.3 Evolution of wireless communication systems
With regard to the peak data rates of cellular systems, from the onset of the introduction of cellular systems and until the mid-1990s the data peaked at approximately around 10 kbps. The peak data rate was lifted to 160 kbps with the introduction of GPRS. Only few years later, the first UMTS systems supported peak data rates of 384 kbps. Nowadays, HSDPA supports peak data rates from 7.2 Mbps to about 14.6 Mbps (by using adaptive modulation and coding with higher-order modulation and multicode transmission (Holma and Toskala 2007). HSPA-Evolved specified by 3GPP Release 7 (Rel-7), the second phase of HSDPA, can achieve data rates of up to 42 Mbps (assuming 64-Quadrature Amplitude Modulation (QAM)). The coming technology, LTE, will see the peak data rate reaching 326 Mbps. Finally, in a few years, IMT-Advanced will theoretically push the peak rate to attain the huge throughput rate of 1.6 Gbps. The evolution of the peak data rate from years 1990 to 2015 is shown in Figure 1.4.
Figure 1.4 Evolution of downlink peak rate from years 1990 to 2015
In parallel with these developments in the telecommunications industry, the wireless information and telecommunications sector provides different IP-based access systems for different application areas. Wireless Local Area Network (WLAN) systems, Institute of Electrical and Electronics Engineers (IEEE) standard 802.11, are used for local and short-range applications without mobility. WLAN systems are widely available globally. Wireless Personal Area Networks (WPAN) are standardized by IEEE 802.15 for very short ranges and high throughput. Broadband Wireless Access (BWA) systems, according to IEEE 802.16, target higher ranges including the support of user mobility (IEEE-ISTO n.d.; IEEE-SG n.d.). The BWA Worldwide Interoperability for Microwave Access (WiMAX) system is a member of the IMT-2000 family (WiMAX n.d.). As with UMTS and LTE, an evolution process has taken place within IEEE. In fact, the IEEE wireless system will evolve from the current WiMAX toward the new IEEE 802.16m standard, which is an IMT-Advanced technology.
1.3 Development of IMT-Advanced and Beyond
The radio spectrum is a scarce resource that has considerable economic and social importance. In general, governments of every country decide on the spectrum allocation. On the other hand, global coordination of spectrum usage is in the responsibility of ITU, which, through spectrum regulation, aims to facilitate spectrum harmonization for global roaming to reduce equipment cost by means of global economies of scale. Since 1992, and in the framework of the International Telecommunication Union – Radiocommunication Sector (ITU-R), the World Administrative Radiocommunications Conference (WARC 1992) has reached quite significant agreements at a global level to designate specific frequency bands to International Mobile Telecommunications (IMT) standards. The objective of this initiative is to specify a set of requirements in terms of transmission capacity and Quality of Service (QoS), in such a way that if a certain technology fulfills all these requirements then the technology is included by ITU in the IMT-2000 standards. This inclusion is an official endorsement of the technologies that might motivate the concerned players (e.g. operators, telecommunications providers, etc.) to take the technologies into account and to consider investing in them. Furthermore it allows these standards to make use of the frequency bands designated for IMT. With the aim of coordinating the global use of spectrum, every three to four years ITU-R holds the World Radiocommunication Conference (WRC), where ITU radio regulations that govern spectrum distribution are adopted.
World Radiocommunication Conference (WRC)-07 identified additional frequency spectrum for mobile and wireless communications. The first step towards this new spectrum allocation was performing an in-depth study of the mobile market forecast and the development of spectrum requirements for the increasing service demand. Reports predicted the total spectrum bandwidth requirements for mobile communication systems in the year 2020 to be 1280 MHz and 1720 MHz for low and high user-demand scenarios, respectively. Bearing in mind that the spectrum bandwidth designed by ITU as IMT was much lower than this forecast (693 MHz in Region 1 (Europe, Middle East and Africa, and Russia), 723 MHz in Region 2 (Americas) and 749 MHz in Region 3 (Asia and Oceania)), and given that the time elapsed between the adoption of the radio regulations and the definitive allocation of a frequency band to operators takes from 5 to 10 years, the WRC-07 that took place in Geneva ended with the identification of new frequency bands for IMT technologies.
Figure 1.5 depicts the current state of the frequency bands reserved for IMT. Despite not fully corresponding to what was targeted, the new spectrum allocated for mobile communications will allow operators to satisfy the initial needs with the deployment of technologies towards IMT-Advanced. Furthermore, the increasing demand for mobile services has been progressively recognized with additional spectrum, a trend that is expected to be maintained in future WRCs.
Figure 1.5 Mobile frequency bands allocated for IMT technologies
With this strong endorsement, the race towards IMT-Advanced successfully reached its end in the autumn of 2010. Anticipating the invitation from ITU, in March 2008 3GPP initiated a study item on LTE-Advanced (LTE-A) (also called LTE-Release 10 (Rel-10)). At IEEE, the IMT-Advanced (candidate) – IEEE 802.16m – was finalized in September 2009. These two technologies, LTE-A (3GPP 2010) and IEEE 802.16m (IEEE 2010) were submitted to ITU as IMT-Advanced technology candidates. Based on the evaluation results submitted to ITU-R in June 2010, ITU-R announced in October 2010 that both LTE-A and IEEE 802.16m proposals successfully met all of the criteria for the first release of IMT-A.
Compared to its predecessor, the IMT-Advanced technologies rely on several new features such as Carrier Aggregation (CA), improved MIMO support, relaying and improved support for heterogeneous deployments. These features are described and analyzed in this book. The most promising ideas within those features for IMT-Advanced and beyond are explained and illustrated.
Advanced Radio Resource Management (RRM) While, from technological point of view, physical layer improvements are already close to their upper limit and only advanced antenna systems seem to be able to improve system performance, there is still a high potential to maximize efficiency in radio resource and interference management. Medium Access Control (MAC) aspects are attracting huge attention. Chapter 2 presents some innovative concepts for advanced RRM that have been identified by the research community for potential inclusion in IMT-Advanced and beyond.
Spectrum and Carrier Aggregation (CA) IMT-Advanced requirements establish a minimum support of 1 Gbps and 100 Mbps peak rates for low-mobility and high-mobility users, respectively. In order to fulfill these challenging requirements, wider channel bandwidth than legacy 3G systems have to be supported. However, as shown in Figure 1.5, the available spectrum resources are spread out over different frequency bands and with different bandwidths. Hence, CA, the concept of aggregation of continuous or discontinuous spectrum will be necessary in order to achieve wider effective carrier bandwidth. In addition, spectrum sharing, due to the scarcity of wide contiguous frequency bands, is crucial in order to optimize spectrum usage. These two concepts, CA and spectrum sharing, will be treated in Chapters 3 and 4, respectively.
MIMO In MIMO communications, multiple antenna elements are employed both in the transmitter and the receiver, in order to obtain increased data rates or improved reliability compared to single-antenna transmission. In order to address the challenge to offer very high data peak rates, IMT-Advanced has moved the emphasis from simple transmit diversity modes to spatial multiplexing and beamforming. In fact, LTE-A and IEEE 802.16m have evolved in the same direction: up to eight transmit antennas at the BS and up to four transmit antennas at the User Equipment (UE) are supported. Further, the standards are progressing toward the full adoption of Multi-User (MU)-MIMO transmissions, that is, the BS can spatially multiplex data streams intended for different UEs. Chapter 5 describes the latest advances in MIMO techniques.
Relaying With the growth of data traffic and the emergence of new services, there is a need to enhance coverage and/or capacity in specific locations. In addition, fast roll-outs are sometimes required to extend a network, implying a very dynamic and fast change of operator's infrastructure. Relaying techniques are emerging as an attractive solution to fill these needs. In fact, they are by construction characterized by ease of deployment (due to in-band backhauling) and reduced deployment cost compared to a regular BS. Chapter 7 overviews relaying techniques in general and describes them within the IMT-Advanced framework in particular.
Although IMT-Advanced systems will offer high data rates, even higher bit-rate demands and hungry applications such as high-definition TV are foreseen. Hence, novel techniques are required to meet the expected performance in terms of higher cell edge throughput and increased average cell capacity. Some of the candidate techniques include Coordinated MultiPoint transmission or reception (CoMP), Network Coding (NC) and Device-to-Device (D2D) communications. These techniques are seen as necessary to further extend IMT-Advanced capabilities. Chapters 6, 7 and 9 will overview and describe them.
Coordinated Multipoint transmission or reception (CoMP) Future cellular networks will need to provide high data-rate services for a large number of users, which requires a high spectral efficiency over the entire cell area. Hence it is important to ensure that the radio interface is robust to interference, especially the presence of Inter-Cell Interference (ICI), which degrades the performance of users located in the cell-edge areas. Recently, CoMP has attracted interest in its attempt to curb the ICI by relying on tight interference coordination. CoMP refers to a system where the transmission and/or reception at multiple, geographically separated antenna sites is dynamically coordinated in order to improve system performance. Chapter 6 treats thoroughly CoMP systems.
Network Coding (NC) In a classical network, data streams originating from a source and intended for a desired destination are routed through intermediate nodes before reaching their final destination. By contrast, NC manipulates those data streams at an intermediate node by combining them before forwarding the resulting data to the destination. Network Coding is well suited to broadcast and cooperative wireless networks. Chapter 8 deals with NC in general and with its application to wireless communication in particular.
Device-to-Device (D2D) Communications One aspect of the design of IMT-Advanced systems that has not received sufficient attention so far is the emergence of high data-rate local services. Such local services can provide the high data rates needed to consume rich multimedia services through mobile computers such as tablets, laptops, netbooks and smart phones. Cellular operators may offer such cheap access to spectrum with controlled interference enabled D2D communication as underlay to the cellular network. The licensed spectrum may be used as the only resource for communication or it may be complemented by license-exempt spectrum. The D2D concept is presented and studied in Chapter 9.
IMT-Advanced Performance Evaluation The basic framework for IMT-Advanced performance evaluation has been well defined by ITU-R R (ITU-R 2009). These guidelines have been published by ITU-R to perform the assessment of IMT-Advanced technology candidates by external evaluation groups. In this framework system parameters, channel models and deployment scenarios are defined. Moreover, the IMT-Advanced technology proponents have presented their own self-evaluation based on these scenarios. External evaluation groups like the Wireless World Initiative New Radio + (WINNER) European Eureka Celtic project have performed an extensive performance evaluation of IMT-A. In Chapter 10 the end-to-end performance of the LTE-A candidate is carefully presented, including the entire calibration process followed in WINNER+.
Future Directions The technologies above offer further potential to significantly improve system performance. Therefore, research is needed in areas such as radio resource allocation, heterogeneous networks, MIMO and CoMP, relaying, network coding, device-to-device communications, green and energy efficient communications. Chapter 11 discusses these research trends.
3GPP 2010 Further Advancements for E-UTRA Physical Layer Aspects. Technical Specification Group Services and System Aspects 36.814, 3rd Generation Partnership Project (3GPP), http://www.3gpp.org/ftp/Specs/html-info/.
Dahlman E, Parkvall S, Sköld J and Beming P 2008 3G Evolution: HSPA and LTE for Mobile Broadband 2nd edn. Academic Press, New York.
GSA n.d. The Global Mobile Suppliers Association, http://www.gsacom.com.
Hillebrand F 2002 GSM and UMTS: The Creation of Global Mobile Communication. John Wiley & Sons, Ltd, Chichester.
Holma H and Toskala A 2000 WCDMA for UMTS- Radio Access for Third Generation Mobile Communications. John Wiley & Sons, Ltd, Chichester.
Holma H and Toskala A 2007 HSDPA/HSUPA for UMTS. John Wiley & Sons, Ltd, Chichester.
IEEE 2010 IEEE 802.16m System Description Document (SDD). Technical Specification 09/0034r3, Broadband Wireless Access Working Group, http://ieeexplore.ieee.org.
IEEE-ISTO n.d. The IEEE Industry Standards and Technology Organization (IEEE-ISTO) http://www.ieee-isto.org
IEEE-SG n.d. IEEE Standards Groups, http://grouper.ieee.org/groups/802/.
ITU n.d. Mobile Cellular: Subscribers per 100 People, International Telecommunication Union, http://www.itu.int/ITU-D/ict/statistics/.
ITU-R 2009 Guidelines for Evaluation of Radio Interface Technologies for IMT-Advanced. Report ITU-R M.2135-1, International Telecommunications Union Radio (ITU-R), http://www.itu.int/publ/R-REP/en.
NGMN Ltd n.d. The Next Generation Mobile Networks, www.ngmn.org.
Sesia S, Baker M and Toufik I 2011 LTE - The UMTS Long Term Evolution: From Theory to Practice 2nd edn. John Wiley & Sons, Ltd, Chichester.
UMTS-Forum 2010 Recognising the Promise of Mobile Broadband. White paper, UMTS-Forum, http://www.umts-forum.org.
Viterbi AJ 1995CDMA: Principles of Spread Spectrum Communication. Addison Wesley Longman Publishing Co., Inc., Redwood City, CA.
WiMAX n.d. Worldwide Interoperability for Microwave Access (WiMax) Forum, www.wimaxforum.org/home/.