Second-generation (2G) digital cellular systems constitute the majority of cellular communication infrastructures deployed today. 2G systems such as GSM, whose rollout started in 1987, signaled a major shift in the way mobile communications is used worldwide. In part they helped fuel the transition of a mobile phone from luxury to necessity and helped to drive subscriber costs down by more efficient utilization of air interface and volume deployment of infrastructure components and handsets.
Major geographical regions adopted different 2G systems, namely TDMA and CDMA in North America, GSM in Europe, and Personal Digital Cellular (PDC) in Japan. Figure 3.3 depicts the worldwide subscriber numbers for major 2G cellular systems. It effectively shows how the GSM system has been successful and why it is now being adopted in geographical areas other than Europe (such as North America, China, the Asia-Pacific region, and more recently, South America). CDMA, which originated in North America, has also proliferated in South America and later in the Asia-Pacific region. TDMA remains to be widely deployed in North and South America regions, but it is expected to decline mostly because of the decisions taken by few major North American carriers to convert their TDMA networks to GSM.
This second-generation system, widely deployed in the United States, Canada, and South America, goes by many names, including North American TDMA, IS-136, and D-AMPS (Digital AMPS). For the sake of clarity, we will refer to it as North American TDMA, as well as simply TDMA, when the context makes it clear. TDMA has been used in North America since 1992 and was the first digital technology to be commercially deployed there. As its name indicates, it is based on Time Division Multiple Access. In TDMA the resources are shared in time, combined with frequency-division multiplexing (that is, when multiple frequencies are used). As a result, TDMA offers multiple digital channels using different time slots on a shared frequency carrier. Each mobile station is assigned both a specific frequency and a time slot during which it can communicate with the base station, as shown in Figure 3.4.
The TDMA transmitter is active during the assigned time slot and inactive during other time slots, which allows for power-saving terminal designs, among other advantages. North American TDMA supports three time slots, at 30 kHz each, further divided into three or six channels to maximize air interface utilization. A sequence of time-division multiplexed time slots in TDMA makes up frames, which are 40 ms long. The TDMA traffic channel total bit rate is 48.6 Kbps. Control overhead and number of users per channel, which is greater than one, decrease the effective throughput of a channel available for user traffic to 13 Kbps. TDMA is a dual-band technology, which means it can be deployed in 800-MHz and 1900-MHz frequency bands. In regions where both AMPS and TDMA are deployed, TDMA phones are often designed to operate in dual mode, analog and digital, in order to offer customers the ability to utilize coverage of the existing analog infrastructure.
There are still some analog cellular systems in operations in Europe, but their number is declining, and some regional networks are being completely shut down or converted to Global System for Mobile Communications. The GSM cellular system initiative was initiated in 1982 by the Conference of European Posts and Telecommunications Administrations (CEPT) and is currently governed by European Telecommunications Standards Institute (ETSI), which in turn has delegated GSM specifications maintenance and evolution to 3GPP (reviewed in part in Chapter 1). The intent behind GSM introduction was to have a common approach to the creation of digital systems across European countries, to allow—among other advantages of a common standard—easy international roaming and better economies of scale by decreasing handset and infrastructure components costs through mass production. In hindsight, this was a smart political decision, which contributed to the worldwide success of European cellular infrastructure providers and equipment manufacturers.
Let's look at some details of the GSM air interface technology. The GSM standard, similarly to North American TDMA, is based on the use of two simultaneous multiplexing technologies, TDMA and FDMA. Each radio frequency (RF) channel in GSM supports eight time slots (compared to three for North American TDMA) grouped into TDMA frames, which are in turn grouped into multiframes consisting of 26 TDMA frames carrying traffic and control channels. Multiframes are built into superframes and hyperframes. This yields an 8-to-1 capacity increase over NMT or TACS in the same RF spectrum. The allocation of the time slots is essentially static on a short-term basis; for instance, the eighth time slot of a given RF channel is assigned to the same user each time it comes around, whether or not the user has voice or data to send.
The GSM system, emphasizing not only physical properties but also service definitions (unlike some 1G systems), supports three major types of services: bearer services, tele-services, and supplementary services. GSM bearer services allow for transparent or acknowledged user data transfer and define access attributes, information transfer attributes, and general attributes with specific roles. Access attributes define access channel properties and parameters such as bit rate; transfer attributes define data transfer mode (bidirectional, unidirectional), information type (speech or data), and call setup mode; general attributes define network-specific services such as QoS and internetworking options. Tele-services are what GSM subscribers actually use. They are based on the foundation provided by bearer services and govern user-to-user communications for voice or data applications. Examples of tele-services include Group 3 Fax, telephony, Short Message Service (SMS), and circuit data IP and X.25 communications. GSM supplementary services provide additional value-added features such as call waiting, call forwarding, call barring, and conference calling used by wireless operators to further differentiate their offerings. Further information about GSM can be obtained from a variety of sources such as [Eberspacher 2001].
High-Speed Circuit-Switched Data (HSCSD) is an option in GSM that allows combining multiple GSM time slots (traffic channels) each capable of a 14.4-Kbps data rate. The resulting bit rate made available for a single user might reach as high as 56 Kbps, although probably at a steep price tag. In fact, owners of the mobiles capable of HSCSD support will have to pay for the combined GSM time slots being used.
Wireless carriers can achieve the migration to HSCSD by upgrading GSM Mobile Switching Center (MSC) and Base Transceiver Station (BTS) software. Wireless carriers also have to distribute handsets capable of receiving HSCSD transmission or firmware upgrades for the GSM mobiles based on Personal Computer Memory Card International Association (PCMCIA) and CompactFlash (CF) cards (such as those produced by Nokia). HSCSD can be supported within the existing GSM mobility management infrastructure, which also enables roaming and other familiar GSM services at higher data rates.
Code Division Multiple Access (CDMA) IS-95—or cdmaOne—is one of the popular 2G technologies being used in the Americas, Asia, and Eastern Europe. CDMA is based on the technique in which each subscriber is assigned a unique code, also known as pseudorandom code that is used by the system to distinguish that user from all other users transmitting simultaneously in the same frequency band. CDMA belongs to the class of systems called spread spectrum systems, and more specifically to the Direct Sequence Spread Spectrum (DSSS) family. Physical channels in CDMA are defined in terms of radio frequency of the carrier and a code—that is, a sequence of bits. The digital signal resulting from the encoding of voice or data, after the application of appropriate framing (or radio link layers), is digitally scrambled before it modulates the carrier frequency. This is accomplished by digitally (base 2) adding the signal to the pseudorandom code that is used to distinguish the user. The entire carrier spectrum is available to each single user, hence the name spread spectrum.
The receiver, which has a pseudorandom signal decoder, reproduces the original signal by demodulating the RF and adding (base 2) the same pseudorandom signal used by the transmitter, thus obtaining the original signal. CDMA is an interference-limited system, meaning that anytime a user is not transmitting and thereby not interfering with other users sharing the same spectrum, the effective bandwidth, and hence signal-to-noise ratio, available to other users will increase to some degree. CDMA properties are as follows:
Multiple voice channels are available for each radio channel.
To prevent interference, callers are assigned to different radio frequency channels (or, if sharing a radio channel, different pseudorandom codes).
The same radio channel can be used in adjacent cells.
The number of calls in a sector is "soft" limited, not hard limited.
Bandwidth usage influences the number of simultaneous users.
To better visualize the CDMA concept, imagine a room filled with pairs of people talking to each other, each couple in their own language. They would only be able to understand their counterparts but not the rest of the conversations in the room. As the number of pairs with unique language increases, the noise level will reach its maximum, after which no conversations will be possible (not unlike in some trendy restaurants and pubs).
The CDMA cellular technology also comes with soft handoff capability— that is, the system specifies a receiver (RAKE receiver) capable of receiving up to three signals related to the same channel, because of multipath effects or to multiple sources transmitting the same signal. The system allows the mobile station to send and receive simultaneously with three base stations, which are defined as belonging to the "active set" of base stations. This allows for the avoidance of handoff Ping-Pong effects and also allows for improved performance against multipath or adverse radio conditions.
CDMA was originally deployed under the commercial name cdmaOne based on TIA [IS95], a mobile-to-base-station compatibility standard for wideband spread-spectrum systems. It is a direct sequence CDMA scheme in which users are differentiated by unique codes known to both transmitter and receiver. The IS-95A version of the standard allows for circuit-switched data service up to 14.4 Kbps. The next generation of IS-95, called IS-95B, requires software and hardware change in CDMA system elements and mobile stations but will support packet data at a sustained bit rate of 64 to 115 Kbps. This is achieved mostly through the use of advanced channel and code aggregation techniques and other modifications to IS-95A. In IS-95B, up to eight CDMA traffic channels can be aggregated for use by a single subscriber—not unlike HSCSD used in GSM.