UMTS

Javier Sanchez, Mamadou Thioune

• Publisher: Wiley-ISTE
• Year: 2013
• ISBN:
  9781118615744
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Mobile & Wireless Communications
• Index: Technology & Engineering

During the first decade of this new millennium, it is estimated that more than €100 billion will be invested in the third generation (3G) Universal Mobile Telecommunications System (UMTS) in Europe. This fact represents an amazing challenge from both a technical and commercial perspective. Written by experts in the field, this book gives a detailed description of the elements in the UMTS network architecture: the User Equipment (UE), the UMTS Radio Access Network (UTRAN) and the core network. The completely new protocols based on the needs of the new Wideband Code Division Multiple Access (WCDMA) air interface are highlighted by considering both Frequency- and Time-Division Duplex modes. The book further introduces the key features of existing topics in Releases 5, 6 and 7.

Chapter 1

Evolution of Cellular Mobile Systems

The purpose of this chapter is to describe the milestones in the evolution of cellular mobile systems. Particular attention is paid to the third generation (3G) systems to which the UMTS belong.

The performance of mobile cellular systems is often discussed with respect to the radio access technology they support, thus neglecting other important aspects. However, a cellular mobile communication system is much more than a simple radio access method, as illustrated in Figure 1.1. The mobile terminal is the vector enabling a user to access the mobile services he subscribed to throughout the radio channel. The core network is in charge of handling mobile-terminated and mobileoriginated calls within the mobile network and enables communication with external networks, both fixed and mobile. Billing and roaming functions are also located in the core network. The transfer of users’ data from the terminal to the core network is the role of the radio access network. Implementing appropriate functions gives to the core network and to the terminal the impression of communicating in a wired link. One or several radio access technologies are implemented in both the radio access network and the mobile terminal to enable wireless radio communication.

Figure 1.1 Basic components of a mobile communications network

1.1. Multiple-access techniques used in mobile telephony

Surveying the different multiple-access techniques is equivalent to describing the key milestones in the evolution of modern mobile communication systems. In the past, not all users of the radio spectrum recognized the need for the efficient use of the spectrum. The spectrum auctions for UMTS licenses have emphasized the fact that the radio spectrum is a valuable resource. Thus, the major challenge of multiple-access techniques is to provide efficient allocation of such a spectrum to the largest number of subscribers, while offering higher data rates, increased service quality and coverage.

1.1.1. Frequency division duplex (FDD) and time division duplex (TDD)

Conventional mobile communication systems use duplexing techniques to separate uplink and downlink transmissions between the terminal and the base station. Frequency division duplex (FDD) and time division duplex (TDD) are among the transmission modes which are the most commonly employed. The main difference between the two modes, as shown in Figure 1.2, is that FDD uses two separate carrier bands for continuous duplex transmission, whereas in TDD duplex transmission is carried in alternate time slots in the same frequency channel. In order to minimize mutual interference in FDD systems, a guard frequency is required between the uplink and downlink allocated frequencies (usually 5% of the carrier frequency). On the other hand, a guard period in TDD systems is required in order to reduce mutual interference between the links. Its length is decided from the longest round-trip delay in a cellular system (in the order of 20-50μs).

Figure 1.2. Duplexing modes used in modern mobile communications systems

1.1.2. Frequency division multiple access (FDMA)

FDMA is the access technology used for first generation analog mobile systems such as the American standard AMPS (Advanced Mobile Phone Service). Within an FDMA system, each subscriber is assigned a specific frequency channel as illustrated in Figure 1.3a. No one else in the same cell or in a neighboring cell can use the frequency channel while it is allocated to a user - when an FDMA terminal establishes a call, it reserves the frequency channel for the entire duration of the call. This fact makes FDMA systems the least efficient cellular systems since each physical channel can only be used by one user at a time. Far from having disappeared, the FDMA principle is part most of modern digital mobile communication systems where it is used as a complement to other radio multiplexing schemes.

1.1.3. Time division multiple access (TDMA)

GSM, TDMA/136 and PCS are second generation mobile standards based on TDMA. The key idea behind TDMA relies on the fact that a user is assigned a particular time slot in a frequency carrier and can only send or receive information in those particular times (see Figure 1.3b). When all available time slots in a given frequency are used, the next user must be assigned a time slot on another frequency. Information flow is not continuous for any user, but rather is sent and received in “bursts”. The important factor to be considered while designing is that these time slices are so small that the human ear does not perceive the time being divided. In GSM up to 8 users may in theory share the same 200 kHz frequency band almost simultaneously, whereas in IS-136 different users can be allocated to 3 time slots within a 30 kHz frequency channel. The capacity of TDMA is about 3 to 6 times as much as that of FDMA [RAP 96].

1.1.4. Code division multiple access (CDMA)

In a CDMA system, unique digital codes, rather than separate radio frequencies, are used to differentiate users (see Figure 1.3c). The codes are shared by the terminal and the base station. All users access the entire spectrum allocation all of the time, that is, every user uses the entire block of allocated spectrum space to carry his/her message. CDMA technology is used in 2G IS-95 (cdmaOne) mobile communication systems and is also part of UMTS and cdma2000 3G standards.

Figure 1.3 FDMA, TDMA and CDMA multiplex access principle

A very popular example used to stress the differences between FDMA, TDMA and CDMA is as follows.

Imagine a large room (frequency spectrum) intended to accommodate many pairs of people. Due to dividing walls, individual offices can be created within the room. They are then allocated to each pair of people so that their conversation can be isolated from noise generated by the other parties. Each office is like a single frequency/channel (principle of FDMA). No one else could use the office until the conversation was complete, whether or not the different pairs were actually talking. A better usage of each office can be achieved by accommodating multiple pairs of people within the same room. For this to work, each party shall respect a rule that is to keep silent while one pair is talking (principle of TDMA). The important factor to be considered is that these silence periods shall be small enough that the human ear cannot perceive the time slicing.

In the analogy with CDMA technology, all the offices are eliminated to create “open-spaces” instead, so that conversation can be carried out at any time. The rule is now for all pairs to hold their conversations in a different language - the brains of contiguous pairs of people being able to naturally filter interference from the other pairs. The languages are analog to the codes assigned by the CDMA system. In theory, it should be possible to accommodate in the large room as much pairs as permitted by the cubic-volume of each person and provided that the number of available languages is enough. Unfortunately, even if the parties speak in different languages, a sudden raise of the voice volume of one couple may disturb the conversations of all the neighboring pairs. This problem may be overcome by implementing an automatic mechanism to control the voice volume of each party. In a CDMA system, this is actually a power control scheme whose performance is of paramount importance for the system to operate properly. However, despite such mechanism, adding the contribution of all the voices, no matter how low their level is, may produce an overall background noise in the room that makes it too difficult to hold a clear conversation. In such a case, some couples may be required to get out from the room or just to remain silent.

1.1.5. Space division multiple access (SDMA)

SDMA technique is based on deriving and exploiting information on the spatial position of mobile terminals. The radiation pattern of the base station is adapted in both uplink and downlink directions to each different user in order to obtain, as illustrated in Figure 1.4, the highest gain in the direction of the mobile user. At the same time, radiation which is zero shall be positioned in the directions of interfering mobile terminals, the ultimate goal being the overall enhancement of capacity and coverage within the mobile system [RHE 96]. SDMA approach can be integrated with different multiple access techniques (FDMA, TDMA, CDMA) and therefore it can be optionally used in all modern mobile communications systems.

Figure 1.1 Space division multiple access (SDMA) principle

1.1.6. Orthogonal frequency division multiplexing (OFDM)

OFDM is a special case of multi-carrier modulation. The main idea is to split a data stream into N parallel streams of reduced data rate and transmit each of them on a separate sub-carrier. High spectral efficiency is achieved in OFDM since a large number of sub-carriers where overlapping spectra is used. OFDM can be combined with FDMA, TDMA and CDMA methods in order to obtain the access schemes referred to as MC-FDMA, MC-TDMA and MC-CDMA, respectively (see Figure 1.5).

OFDM has been adopted in the terrestrial digital video broadcasting (DVB-T) standard and in the digital audio broadcasting (DAB) standard followed by the wireless local area network standards IEEE 802.11a/g, IEEE 802.16, BRAN, HIPERLAN/2 and HIPERMAN. Although not used in current 2G/3G mobile radio systems, the successful deployment of the OFDM technique has encouraged several studies intended to design new...