SURVEY OF MOBILE TELECOMMUNICATION SYSTEMS
This section provides a technical summary of current mobile telecommunication systems, to the extent that this is necessary for understanding the use of smart cards in this area. Significantly more detailed descriptions of all of the technical aspects of currently used mobile telecommunications networks can be found in J¨org Ebersp¨acher et al. [Ebersp¨acher 00], BernhardWalke [Walke 00] and Raymond Steele et al. [Steele 2001]. In this chapter, the term ‘mobile telecommunication system’ is used instead of ‘mobile telephone system’, since in all recent systems simple voice transmission is only one of many possible services, with the transmission of various types of data becoming increasingly more prominent.

Multiple-access methods
The frequency bandwidth available to a mobile telecommunication system, which is also called its frequency spectrum, is typically limited to a few tens of megahertz. In order to make this limited bandwidth quasi-concurrently available to as many subscribers as possible, ‘multipleaccess’ methods must be used. The purpose of such methods is to allow the greatest possible number of mobile stations within a cell to access the network with acceptable quality by suitably exploiting radio transmission techniques and information technology. There are basically four different types of multiple access methods. They differ in their cost of implementation and the efficiency with which they utilize the available bandwidth. These four methods are called frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA) and space-division multiple access (SDMA). They are briefly described below.

FDMA (frequency-division multiple access)
With frequency-division multiple access, each transmitter is assigned a reserved frequency band within the total available frequency range. The transmitter is allowed to continuously and exclusively transmit within its assigned frequency band. With FDMA, each transmitter within a cell transmits on a different frequency. Incidentally, this is also the most commonly used method for conventional radio equipment, which uses a single common channel (a halfduplex link) for communications. If a full-duplex link is used (i.e., simultaneous uplink to the base station and downlink to the mobile station), which is usually the case for telephony, two frequency channels are naturally required to handle each call. Due to its limited technical complexity, FDMA is relatively well suited to mobile telecommunications using analog data transmission. For instance, frequency-division multiple access was used for the air interface between fixed and mobile stations in the German C-Netz. In this system, separate 4.44-MHz frequency bands were reserved for uplink and downlink, with each band being divided into 222 frequency channels, each 20 kHz wide.

TDMA (time-division multiple access)
With time-division multiple access, data are transmitted quasi-concurrently from several transmitters to a single receiver on a single frequency. Each transmitter is assigned a particular time slot, within which it is allowed to transmit exclusively but not continuously. In theGSMsystem, for example, the time slot available for a signal burst is 577 μs (15/26 ms), of which 546 μs are occupied by the signal burst to be sent within this interval. The difference between these twovalues (31 μs) is used as a guard time to accommodate small timing variations. Maintaining the necessary exact timing of the time slots requires very precise and technically complex synchronization between the transmitter and the receiver. Furthermore, the signal propagation time between the transmitter and the receiver must be taken into account when time-division multiple access is used. For example, the difference in signal propagation time between mobile stations in the immediate vicinity of a base station and mobile stations 30 km from the base station is approximately 100 μs. In practice, these propagation time differences must be offset by ‘premature’ transmission, so that the signals transmitted by the mobile stations always arrive at the base station exactly within the time slots reserved for them. Incidentally, the need to offset the transmission time in order to compensate for propagation time differences is what determines the maximum diameter of a cell in the GSM system. The maximum allowable interval for equalizing propagation times between the base station and the mobile station is 116.3 μs. This is the maximum time that a transmission can be sent prematurely and still arrive at the receiver within the prescribed time slot. This yields a maximum cell radius in the GSM system of approximately 35 km. Premature transmission is also called ‘timing advance’. In order to reduce the effects of frequency-selective interference, time-division multiple access can be combined with frequency hopping, in which both the transmitter and the receiver change frequency channels after each time slot in a predefined sequence. As a result, there is a high probability that interference in particular frequency ranges will only affect isolated signal bursts. In many cases, the results of such interference can be compensated using errorcorrecting transmission codes. An example of the use of time-division multiple access in combination with frequencydivision multiple access is the air interface between fixed and mobile stations in the GSM system. In this case, the available frequency band of 25 MHz is divided into 24 individual channels, each having a bandwidth of 200 kHz. Each of these frequency channels in turn is allocated eight call channels. This means that up to eight mobile stations can concurrently transmit on a single frequency channel, with each mobile station having access to the frequency channel for an interval of 0.577 ms every 4.615 ms.

CDMA (code-division multiple access)
Code-division multiple access is a multiple access method in which data are transmitted to a receiver by multiple transmitters that concurrently transmit signals within the entire available frequency spectrum. Code-division multiple access is based on spread-spectrum technology, in which an original narrow-band signal is expanded into a wide-bandwidth radio signal using a transmitter-specific mapping law and then transmitted as a wideband signal. This wideband signal is received by the receiver, where it can be transformed back into the original narrow-band signal by employing the known mapping law used by the transmitter. In the wideband code-division multiple access (WCDMA) variant, two separate frequency bands are used for uplink and downlink, for which reason this CDMA variant is often referred to as frequency-division/code-division multiple access (FD/CDMA). In the time-division/codedivision multiple access (TD/CDMA) variant, the uplink and downlink are separated by using different time slots. Code-division multiple access has the advantage of bring highly insensitive to frequencyselective interference. It also provides weak protection against unauthorized eavesdropping if the transmitter-specific mapping law is not known to the attacker. CDMA is used in the UMTS system in the WCDMA variant, using a bandwidth of 5 MHz each for uplink and downlink.

SDMA (space-division multiple access)
Space-division multiple access is a multiple access method for transmitting data in parallel from multiple transmitters to a receiver using a single frequency. For this purpose, the transmitters use directionally selective (adaptive) aerials aimed at specific receivers. This requires a relatively high level of technical complexity, so this method is presently used only to a limited degree for base stations in the mobile telecommunications sector. The directionally selective aerials are usually antenna arrays with electronic beam-steering capability. This makes it unnecessary to physically aim the aerial towards the receiver. Space-division multiple access can basically be combined with other multiple access methods, but it is presently seldom used in the mobile telecommunications sector due to its unfavorable cost/benefit ratio.