The Long Term Evolution of UMTS is just one of the latest steps in an advancing series of mobile telecommunications systems. Arguably, at least for land-based systems, the series began in 1947 with the development of the concept of cells by the famous Bell Labs of the USA. The use of cells enabled the capacity of a mobile communications network to be increased substantially, by dividing the coverage area up into small cells each with its own base station operating on a different frequency. The early systems were confined within national boundaries.
They attracted only a small number of users, as the equipment on which they relied was expensive, cumbersome and power-hungry, and therefore was only really practical in a car. The first mobile communication systems to see large-scale commercial growth arrived in the 1980s and became known as the ‘First Generation’ systems. The First Generation comprised a number of independently-developed systems worldwide (e.g. AMPS (Analogue Mobile Phone System, used in America), TACS (Total Access Communication System, used in parts of Europe), NMT (Nordic Mobile Telephone, used in parts of Europe) and J-TACS (Japanese Total Access Communication System, used in Japan and Hong Kong)), using analogue technology.
Global roaming first became a possibility with the development of the digital ‘Second Generation’ system known as GSM (Global System for Mobile Communications). The success of GSM was due in part to the collaborative spirit in which it was developed. By harnessing the creative expertise of a number of companies working together under the auspices of the European Telecommunications Standards Institute (ETSI), GSM became a robust, interoperable and widely-accepted standard. Fuelled by advances in mobile handset technology, which resulted in small, fashionable terminals with a long battery life, the widespread acceptance of the GSM standard exceeded initial expectations and helped to create a vast new market. The resulting near-universal penetration of GSM phones in the developed world provided an ease of communication never previously possible, first by voice and text message, and later also by more advanced data services. Meanwhile in the developing world, GSM technology had begun to connect communities and individuals in remote regions where fixed-line connectivity was nonexistent and would be prohibitively expensive to deploy.
This ubiquitous availability of user-friendly mobile communications, together with increasing consumer familiarity with such technology and practical reliance on it, thus provides the context for new systems with more advanced capabilities. In the following section, the series of progressions which have succeeded GSM is outlined, culminating in the development of the system currently known as LTE – the Long Term Evolution of UMTS (Universal Mobile Telecommunications System).
From the technology and standards angle, there are currently three main organizations responsible for developing the standards meeting IMT requirements, and which are continuing to shape the landscape of mobile radio systems, as shown below.
The uppermost evolution track shown in the illustration is that developed in the 3rd Generation Partnership Project (3GPP), which is currently the dominant standards development group for mobile radio systems.
Within the 3GPP evolution track, three multiple access technologies are evident: the ‘Second Generation’ GSM/GPRS/EDGE family1 was based on Time- and Frequency- Division Multiple Access (TDMA/FDMA); the ‘Third Generation’UMTS family marked the entry of Code Division Multiple Access (CDMA) into the 3GPP evolution track, becoming known as Wideband CDMA (owing to its 5 MHz carrier bandwidth) or simply WCDMA;finally LTE has adopted Orthogonal Frequency-Division Multiplexing (OFDM), which is the access technology dominating the latest evolutions of all mobile radio standards. In continuing the technology progression from the GSM and UMTS technology families within 3GPP, the LTE system can be seen as completing the trend of expansion of service provision beyond voice calls towards a multiservice air interface. This was already a key aim of UMTS and GPRS/EDGE, but LTE was designed from the start with the goal of evolving the radio access technology under the assumption that all services would be packet-switched, rather than following the circuit-switched model of earlier systems. Furthermore, LTE is accompanied by an evolution of the non-radio aspects of the complete system, under the term ‘System Architecture Evolution’ (SAE) which includes the Evolved Packet Core (EPC) network. Together, LTE and SAE comprise the Evolved Packet System (EPS), where both the core network and the radio access are fully packet-switched.
The standardization of LTE and EPS does not mean that further development of the other radio access technologies in 3GPP has ceased. In particular, the enhancement of UMTS with new releases of the specifications continues in 3GPP, to the greatest extent possible while ensuring backward compatibility with earlier releases: the original ‘Release 99’ specifications of UMTS have been extended with high-speed downlink and uplink enhancements (HSDPA and HSUPA3 in Releases 5 and 6 respectively), known collectively as ‘HSPA’ (High-Speed Packet Access). HSPA has been further enhanced in Release 7 (becoming known as HSPA+) with higher-order modulation and, for the first time in a cellular communication system, multistream ‘MIMO’ operation (Multiple-Input Multiple-Output antenna system). Further enhancements of HSPA+ are being introduced in Release 8 in parallel to the first release of LTE (which for consistency is also termed Release 8). These backward-compatible enhancements will enable network operators who have invested heavily in the WCDMA technology of UMTS to generate new revenues from new features while still providing service to their existing subscribers using legacy terminals.
LTE is able to benefit from the latest understanding and technology developments from HSPA and HSPA+, especially in relation to optimizations of the protocol stack, while also being free to adopt radical new technology without the constraints of backward compatibility or a 5 MHz carrier bandwidth. However, LTE also has to satisfy new demands, for example in relation to spectrum flexibility for deployment. LTE can operate in Frequency-Division Duplex (FDD) and Time-Division Duplex (TDD) modes in a harmonized framework designed also to support the evolution of TD-SCDMA (Time-Division Synchronous Code Division Multiple Access), which has been developed in 3GPP as an additional branch of the UMTS technology path, essentially for the Chinese market.
The second path of evolution has emerged from the IEEE 802 LAN/MAN4 standards committee, which created the ‘802.16’ family as a broadband wireless access standard. This family is also fully packet-oriented. It is often referred to as WiMAX, on the basis of a socalled ‘System Profile’ assembled from the 802.16 standard and promoted by the WiMAX Forum. The WiMAX Forum also ensures the corresponding product certification. While the first version known as 802.16-2004 was restricted to fixed access, the following version 802.16e includes basic support of mobility and is therefore often referred to as ‘mobile WiMAX’. However, it can be noted that in general theWiMAX family has not been designed with the same emphasis on mobility and compatibility with operators’ core networks as the 3GPP technology family, which includes core network evolutions in addition to the radio access network evolution. Nevertheless, the latest generation currently under development by the IEEE, known as 802.16m, has similar targets to the likely future enhancements to LTE.
A third evolution track shown in the illustration above is led by a partnership organization similar to 3GPP and known as 3GPP2. Based on the American ‘IS95’ standard, which was the first mobile cellular communication system to use CDMA technology, CDMA2000 was developed and deployed mainly in the USA, Korea and Japan. Standardization in 3GPP2 has continued with parallel evolution tracks towards data-oriented systems (EV-DO), to a certain extent taking a similar path to the evolutions in 3GPP. Mirroring LTE, 3GPP2’s latest evolution is a new OFDM-based system called Ultra-Mobile Broadband (UMB), derived in part from a proprietary system known as ‘Flash OFDM’.
The overall pattern is of an evolution of mobile radio towards flexible, packet-oriented, multiservice systems. The aim of all these systems is towards offering a mobile broadband user experience that can approach that of current fixed access networks such as Asymmetric Digital Subscriber Line (ADSL) and Fibre-To-The-Home (FTTH).
