Evolution of Wireless Networks
Table 1.1. summarizes the history of cellular networks. Through the generations, emphases have been made on different design objectives, ones that best served the requirements of the time.
Interest in the First Generation (1G) cellular, for example, focused on mobilizing landline telephony. The outcome networks, Advanced Mobile Phone Systems (AMPS) and Total Access Communication Systems (TACS), were circuit switched with analog voice transmission over the air. A definite drawback of analog transmission was a generally degraded quality and an extreme sensitivity to basic mobility and medium conditions. Hence, the main design objective in Second Generation (2G) cellular networks was to enhance voice quality. The standards responded by replacing analog voice transmission with digital encoding and transmission, immensely improving voice communication. Improvements to the network core also facilitated the introduction of basic digital messaging services, such as the Short Messaging Service (SMS). The two main standards comprising 2G networks were Global System for Mobile Communications (GSM) and Interim Standard 95 (IS-95), commercially called (cdmaOne). GSM relied mostly on Time Division Multiple Access (TDMA) techniques, while cdmaOne, as the name suggests, utilized Code Division Multiple Access (CDMA). Such division, in addition to variation in the spectrum bands utilized for deployments in different regions, would mark a characteristic interoperability problem that was to be witnessed for a substantial period of time afterwards.
Table 1.1 Generations of cellular technologies [2]
The introduction and the increasing popularity of the 2G technologies coincided with the early years of the Internet. As the Internet experienced an exponential growth in usage, interest in having digital and data services of wireless and mobile devices began to materialize. Evolutions for the two main 2G technologies, GSM into General Packet Radio Services (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) and cdmaOne into cdmaTwo (IS-95b), enhanced the network cores to be able to handle simple data transfers. For example, GPRS introduced two components, the GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN). The objectives of these components was to augment the existing GSM infrastructure to facilitate data access at the RIT level (SGSN), and to facilitate interconnecting the GPRS network with other data networks, including the Internet (GGSN). Basic email and mobile web access were enabled, but the sophistication of the general mobile Internet experience did not allow popular access, and restricted its usage to the enterprise.
In 1999, the ITU approved five radio interfaces comprising the IMT-2000 technologies. These were the EDGE, cdma2000, Universal Mobile Telecommunication System (UMTS) (Wideband—CDMA (W-CDMA), Time-Division—CDMA (TD-CDMA) and Time Division-Synchronous CDMA (TD-SCDMA)) and Digital Enhanced Cordless Telecommunications (DECT). In 2007, Worldwide Interoperability for Microwave Access (WiMAX) was also recognized as an IMT-2000 technology. These technologies make up the 3G networks. In their design, great emphasis was given to enhance the support for voice services, expand and enhance the support for data services, and enable multimedia to the mobile handset. 3G technologies are sometimes classified based on their nature, with EDGE and CDMA2000 recognized as being evolutionary technologies, that is, enhancing their 2G predecessor technologies, and UMTS and WiMAX as revolutionary, that is, based on completely new radio interfaces. In the case of UMTS, it was WCDMA, while WiMAX relied on Orthogonal Frequency Multiple Access (OFDMA). As will be illustrated in the next chapter, the viability of sub-carrier allocation facilitated by OFDMA has made it the multiple access technique of choice in 4G networks.
3G technologies displayed, and still display, that Internet access through a mobile handset can provide users with a rich experience. The recent widespread of smart phones and pads offered by various vendors indicates the strong demand for such services. However, 3G technologies have faced certain challenges in accommodating the increasing demand. These include deteriorating quality of indoor coverage, unsustainable data rates at different mobility levels, roaming difficulties (incoherent spectrum allocation between different countries), and infrastructure complexity. While some of these challenges could be efficiently mitigated by denser deployments, the associated cost and complexity made this an unattractive solution. As for network performance, signaling overhead in 3G networks has been observed to consume substantial bandwidths—even more than the requirements of the multimedia being transferred.
The latter revolutions in 3G technology, namely the LTE from 3GPP and the WiMAX 1.5 from the WiMAX Forum, directly addressed these and other issues. Parting away from the RITs that have been used in 2G and early 3G technologies (TDMA and W-CDMA), LTE and WiMAX are based on OFDMA. This facilitated delivering high data rates while being robust to varying mobility levels and channel conditions. The two networks also introduced other technologies, such as using advanced antenna techniques, simplified network core, the usage of intelligent wireless-relay network components, and others.
In early 2008, the ITU-R issued a circular letter initiating the proposal process for candidates for IMT-Advanced technologies. The requirements set for IMT-Advanced were made to address the outstanding issues faced by operators, vendors and users in 3G networks, and were made to accommodate the expanding demand for mobile broadband services. The requirements were set with the general framework of the IMT objectives (i.e., per Recommendation ITU-R M.1645 [3]), which set the desired objectives for users, manufactures, application developers, network operators, content providers, and services providers. Both the 3GPP and IEEE responded with candidate proposals in October 2009, the 3GPP with LTE-Advanced, an evolution of LTE, and the IEEE with the WirelessMAN-Advanced air interface (IEEE 802.16m). Currently, deployments of LTE and WiMAX have already started. The Global mobile Suppliers Association (GSA) indicates commitments by 128 operators in 52 countries [4] in addition to 52 pre-commitments (trial or test) deployments [5]. Meanwhile, the WiMAX forum in its most recent Industry Research Report (IRR) indicates that there are currently 582 WiMAX deployments in 150 countries [6, 7]. Note that these deployments are not IMT-Advanced, that is, are not 4G networks. However, given the ease of upgrade from LTE to LTE-Advanced and from WiMAX 1.5 to WiMAX 2.0, these deployments are indicative of how future deployments will play out.
The initial timeline set by the ITU-R Working Party 5D, the party overseeing IMT-Advanced systems, is shown in Figure 1.2. At the moment, the standardization of both technologies has passed Step 7 which entails the consideration of evaluation results in addition to consensus building and decision. The working party met in October 2010 to decide on the successful candidates and decide on future steps. Both LTE-Advanced and WirelessMAN-Advanced have been recognized as IMT-Advanced technologies. Both standardization bodies are now in Step 8, which entails the development of the radio interface recommendations.