Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond Alex Brand, Hamid Aghvami Copyright  2002 John Wiley & Sons Ltd ISBNs: 0-471-49877-7 (Hardback); 0-470-84622-4 (Electronic) 1 INTRODUCTION This book focuses on issues related to multiple access for cellular mobile communications, with a specific interest in access arbitration through multiple access protocols situated at the lower sub-layer of the second OSI layer, namely the medium access control (MAC) layer. In this chapter, first an introduction to cellular mobile communication systems is provided. This introduction will be further expanded upon in Chapter 2, particularly with respect to the features which distinguish the different generations of mobile communica- tion systems, from analogue first generation (1G) systems to possible fourth generation (4G) scenarios. Next, it is discussed what impact the emergence of the Internet may have on cellular communication systems. The importance of multiple access protocols is also examined, particularly in the context of packet-based systems, and packet reserva- tion multiple access (PRMA) is considered as a case study. Finally, together with some background information, an overview of our own research efforts related to PRMA-based protocols is provided. These efforts are mainly concerned with how to combine PRMA with code-division multiple access (CDMA), when such a combination is beneficial, and more generally with different approaches to access control at the MAC layer and their respective benefits. They are documented in detail in later chapters. 1.1 An Introduction to Cellular Communication Systems 1.1.1 The Cellular Concept The first land mobile communication systems were based on wide area transmission [1]. Each base station had to provide coverage for large autonomous geographical zones. Calls of customers leaving a zone had to be dropped and re-established in a new zone [2]. Such systems suffered from low-capacity and high-transmit power requirements for mobile transceivers, shortcomings that would not have allowed us to witness the tremendous growth in mobile communications in the past few years, with penetrations now exceeding 70% in many countries. Only the introduction of cellular mobile communication systems in the late 1970s made this development possible, by enabling frequencies, used in one cell, to be reused under certain conditions in other cells to increase capacity. Nowadays, mobile communication systems are almost by implication cellular communication systems as well. We use either of these two terms interchangeably, sometimes also the full term, namely cellular mobile communication systems. Cellular mobile communication systems are designed to provide moving users (from pedestrians to travellers in high-speed trains) with a means of communication. In contrast 2 1 INTRODUCTION to (basic) cordless telephones, cellular telephones (also referred to as mobile phones, mobile stations, mobile terminals or sometimes simply handsets) are not attached to a particular base station, but may make use of any one of the base stations provided by the company that operates the corresponding network. Each of these base stations covers a particular area of the landscape, called a cell. The ensemble of base stations should cover the landscape in such a way that the user can travel around and carry on a phone call without interruption, possibly making use of more than one base station, as shown in Figure 1.1. The procedure of changing a base station at cell boundaries is called handover or handoff. We prefer the first term, since it implies (unlike the second term) that an effort is made to sustain a call across cell boundaries. Obviously, these systems can also serve stationary users, and do so increasingly, as fixed telephones are more and more substituted by wireless phones. Communication from the mobile station (MS) to the base station (BS) takes place on the uplink channel or reverse link andfromBStoMSonthedownlink channel or forward link (Figure 1.1). To enable communication, some resources need to be allocated to the base station (these may be frequency bands, time-slots, sets of codes, or any combination of the three), which in turn may assign a portion of them to individual calls to support communication on both uplink and downlink channels. The amount of resources allocated to users will depend on the current resource availability and the particular requirements of each requesting user. As the base station must be able to assign individual portions of its resources to support multiple communications, basic multiple access techniques (such as frequency-, time-, or code-division multiple access, with FDMA, TDMA, and CDMA as their respective acronyms) are required together with multiple access protocols,which govern access to these resources. The basic multiple access schemes are briefly described further below in this section, and the importance of the multiple access protocols is examined in Section 1.3. 1.1.2 Propagation Phenomena in Cellular Communications The design of cellular communication systems is particularly challenging because of the adverse propagation conditions experienced on the radio channel. Without discussing the complex underlying physical mechanisms, for which the reader may consult a mobile communications handbook such as that in Reference [3] or a book dedicated to radio BS (a) (b) Handover Cell Uplink Downlink Figure 1.1 (a) Basic principle of cellular communications. (b) Uplink and downlink channel 1.1 AN INTRODUCTION TO CELLULAR COMMUNICATION SYSTEMS 3 propagation such as that in Reference [4], three main propagation effects are usually distinguished. These are the pathloss, slow fading or shadowing, and fast fading or multi- path fading. The pathloss describes the average signal attenuation as a function of the distance between transmitter and receiver, which includes the free-space attenuation as one component, but also other factors come into play in cellular communications, resulting in an environment-dependent pathloss behaviour. Shadowing or slow fading describes slow signal fluctuations, which are typically caused by large structures, such as big build- ings, obstructing the propagation paths. Fast or multipath fading is caused by the fact that signals propagate from transmitter to receiver through multiple paths, which can add at the receiver constructively or destructively depending on the relative signal phases. The received signal is said to be in a deep fade when the paths add destructively in a manner that the received signal level is close to zero. Fades occur roughly once every half wavelength [3]. Given that we are dealing with wavelengths of 30 cm and less in cellular communication systems, it is clear that multipath fading can result in relatively fast signal fluctuations; exactly how fast depends on the speed of the mobile station and on the dynamics of the surrounding environment. When designing cellular communication systems and particularly when planning the deployment of such systems (e.g. choosing suitable base station locations), one will have to account for these propagation phenomena appropriately. One way to do this is to use deterministic propagation tools such as ray tracing tools, which will calcu- late experienced signal levels for every specific location of the planned system coverage area, taking into account every structure which could affect signal levels. Another way is to resort to statistical models, which have to be established by analysing propaga- tion measurements performed in suitably chosen environments, e.g. classified as dense urban, typical urban, suburban and rural propagation environments, to name just a few. For the purposes of some of our investigations, we will deal with distance-independent pathloss coefficients and a so-called lognormal shadowing model, as outlined in detail in Chapter 5. 1.1.3 Basic Multiple Access Schemes For reasons discussed in detail in Chapter 3, we make a distinction between basic multiple access schemes, such as FDMA, TDMA, and CDMA, associated with the physical layer (PHY) on the air interface of a mobile communications system, and multiple access protocols, situated at the medium access control (MAC) layer above the PHY. Roughly speaking, the basic schemes provide the capability of dividing the total resources available to a base station into individual portions, which can be assigned to different users, and the protocols govern access to these resource portions, e.g. provide access arbitration. Analogue first generation cellular communication systems made use of FDMA as a basic multiple access scheme. In digital 2G systems, TDMA is predominant, but a CDMA-based system exists as well. CDMA is the most commonly used form of multiple access for third generation systems, in some cases complemented by a hybrid CDMA/TDMA scheme. 1.1.3.1 Frequency-Division Multiple Access In FDMA, each communication is carried over one or two (depending on the duplex scheme, see below) narrowband frequency channels. The channel bandwidth and the modulation scheme determine the gross bit-rate that can be sustained. Because of non-ideal 4 1 INTRODUCTION 3214 Channel bandwidth Guard band Frequency Figure 1.2 FDMA channels and guard bands . . . . . .n1234 n1234 . . . Frame duration Slot duration Time Figure 1.3 TDMA frames, time-slots, and bursts filters, guard bands must be introduced between these channels to avoid so-called adjacent channel interference. This is illustrated in Figure 1.2. 1.1.3.2 Time-Division Multiple Access In TDMA, rather than assigning each user a channel with its own frequency, users share a channel of a wider bandwidth, which we shall call a (frequency) carrier, in the time domain. This is achieved by introducing a framing structure with each TDMA frame subdivided into N time-slots,ifN user channels are to be supported. User i is then allowed to access the carrier only during time-slot i, by transmitting a so-called burst which fits into this time-slot, as shown in Figure 1.3. In order to sustain a continuous gross source bit-rate of R s bit/s, the transmission speed during the burst transmission must be at least NR s bit/s. Provided that enough spectrum is available, multiple carriers may be assigned to each cell. Therefore, such TDMA systems feature typically also an FDMA element, and are in reality hybrid TDMA/FDMA systems. 1.1.3.3 Code-Division Multiple Access In CDMA, narrowband signals are transformed through spectrum spreading into signals with a wider bandwidth, the carrier bandwidth. Like in TDMA, multiple users share the carrier bandwidth, but like in FDMA, they transmit continuously during the call or session. The multiple access capability derives from the use of different spreading codes for individual users. Because of the spreading of the spectrum, CDMA systems are also referred to as spread spectrum multiple access (SSMA) systems. Two basic CDMA techniques suitable for mobile communications are distinguished, namely frequency hopping (FH) and direct-sequence (DS) CDMA techniques. ‘Proper’ FH/CDMA systems have not been specified for mobile communications so far and are not discussed any further, but so-called slow frequency hopping (SFH) can also be applied in TDMA systems. The second generation Global System for Mobile Communications 1.1 AN INTRODUCTION TO CELLULAR COMMUNICATION SYSTEMS 5 (GSM) for instance features an SFH option, the benefits of which are discussed extensively in Chapter 4. For one 2G system called cdmaOne and most 3G systems, e.g. the Universal Mobile Telecommunications System (UMTS), DS/CDMA was chosen as a basic multiple access scheme. In DS/CDMA, a bit-stream is multiplied by a direct sequence or spreading code composed of individual chips. They have a much shorter duration than the bits of the user bit-stream, and this is why the original signal’s spectrum is spread. The bandwidth expansion factor, often simply referred to as spreading factor and in this book denoted by the symbol X, is equal to the duration of a bit, T b , divided by the duration of a chip, T c ,i.e.X = T b /T c . The same codes used at the transmitting side to spread the signals are used at the receiving side to de-spread them again. If codes assigned to different signals or user channels are mutually orthogonal, then these signals can be perfectly separated at the receiving side. In practise, due to multipath propagation, fully orthogonal separation at the receiving side may not be achieved even when the codes are orthogonal at the transmitting side. Provided that appropriate measures are taken, this is not really a problem, but it has an interesting consequence, which is highly relevant for some of the topics discussed in this book. Non-orthogonality creates mutual interference between all users, so-called multiple access interference (MAI). The resource assigned to an individual user in a CDMA system is therefore not so much a code, but rather a certain power level. This is illustrated in Figure 1.4, which shows sharing of resources in the time-domain, the frequency-domain, and in terms of power levels, for FDMA, TDMA and CDMA respectively. 1.1.3.4 Frequency-Division Duplex and Time-Division Duplex To sustain a bi-directional communication between a mobile terminal and a base station, transmission resources must be provided both in the uplink and downlink directions. This can either happen through frequency-division duplex (FDD), whereby uplink and downlink channels are assigned on separate frequencies, or through time-division duplex Power Power Power Time Time Time FDMA TDMA CDMA Frequency Frequency Frequency Figure 1.4 Sharing of time-, frequency- and power resources between three users in FDMA, TDMA and CDMA respectively 6 1 INTRODUCTION (TDD), where uplink and downlink transmissions occur on the same frequency, but alter- nate in time. Both methods can be applied in conjunction with any of the above-described multiple access schemes. 1G and 2G systems apply FDD. In UMTS, a 3G system, both FDD and TDD modes are supported, not least because symmetric uplink and downlink spectrum is normally required for FDD-only systems, but 3G spectrum consists of both so-called paired bands (i.e. symmetric spectrum) and unpaired bands. A description of the key features of 1G, 2G and 3G systems is provided in Chapter 2, which also considers possible 4G scenarios. Advantages and disadvantages of the different basic multiple access schemes are examined in more detail in Section 3.2. Approaches to the modelling of the physical layer performance for some of our investigations are discussed in Chapter 5. Chapter 4 on multiple access in GSM and GPRS deals also to quite a considerable extent with physical layer issues. 1.1.4 Cell Clusters, Reuse Factor and Reuse Efficiency As pointed out earlier, resources used in one cell may be reused in other cells, but this must be done in such a way that ongoing communications experience sufficient quality. Assume for now that we are dealing with frequency resources, and that the main factor affecting the quality is the so-called co-channel interference, that is, interference generated by communications in other cells transmitting on the same frequency as a desired communication link in a test cell. The required communication quality, together with other factors, e.g. related to propagation conditions, such as the pathloss coefficient, will determine the minimum distance that must be respected between two co-channel cells, the so-called reuse distance. This leads to the concept of cell clusters (or cellular reuse patterns), namely a set of neighbouring cells within the reuse distance, any two of which are not allowed to use the same frequency. The frequencies are instead reused in a cell occupying the same relative position in a neighbouring cluster, as illustrated in Figure 1.5. In other words, every cell in a cluster obtains a share of the total bandwidth available to an operator, and the same bandwidth is reused in other clusters. The number of cells within each cluster is called the (frequency) reuse factor or cluster size N f [5], which is seven in the example depicted. The reuse efficiency is the inverse of the reuse factor, hence 1/N f . With such a cellular approach, it is in theory possible to increase capacity without limit through cell splitting (i.e. deploying multiple small cells in an area previously served by a single big one), but there are certain practical constraints. 1.1.5 Types of Interference and Noise Affecting Communications The permitted co-channel interference, which depends on various physical layer aspects such as the modulation scheme employed, is a key parameter determining the minimum frequency reuse factor. Communications taking place on adjacent channels can also create notable mutual interference because of non-ideal filters both at the transmit side (resulting in some power being also radiated outside the allocated channel) and at the receive side (due to receive filters not fully rejecting out-of-band signals). This is referred to as adjacent channel interference (ACI). It is strongest between directly adjacent or neighbouring channels, and decreases as channels further apart are being considered owing to the filter 1.1 AN INTRODUCTION TO CELLULAR COMMUNICATION SYSTEMS 7 1 1 6 2 21 2 3 3 3 4 4 5 5 5 7 6 67 7 4 D Reuse distance Cell radius R Figure 1.5 Cell clusters assuming hexagonal cells with a frequency reuse factor of seven attenuation. In general, ACI is much less of a problem than co-channel interference. All the same, when several frequency channels are assigned to a cell, if they are neighbouring channels, guard bands should separate them to avoid excessive ACI, otherwise non- neighbouring channels should be assigned. Unfortunately, due to limited 3G spectrum and the fact that wideband channels are used, there are 3G scenarios for such systems where neither sufficient guard band is available nor non-neighbouring channels can be chosen, hence ACI becomes an issue, as discussed in Section 2.3. Compared to TDMA and FDMA systems, CDMA systems exhibit certain peculiarities. Firstly, the reuse factor can be set to one in CDMA systems (and in fact often is). This is also referred to as universal frequency reuse. In this case, mutual interference is generated between all cells, hence rather than referring to this as co-channel interference, the term intercell interference is used. Secondly, while user channels within a cell are separated from each other in an orthogonal manner both in TDMA systems (perfect separation between time-slots can be achieved through guard periods) and in FDMA systems (assuming sufficient guard bands to avoid ACI), this is not necessarily the case in CDMA systems. Since spreading codes do not always provide orthogonal separation, interference within a cell, so-called intracell interference, can become an issue as well. Therefore, intracell and intercell interference can both be non-negligible components of the total multiple access interference experienced by a communication link in a CDMA system. On top of interference generated by other users in the system, additional noise sources may affect the quality of a communication, for instance thermal noise. In the following, the term ‘interference’ refers to noise generated by other cellular users, and ‘noise’ to thermal noise as well as noise generated by sources outside the considered system. The communication quality in terms of bit error rate (BER) or frame erasure rate (FER) can therefore be expressed as a function of the signal-to-noise ratio (SNR), or the signal- to-interference ratio (SIR), depending on which type of signal disturbance is dominant. Typically, at the beginning of a system build out, when there are few cells, the system is 8 1 INTRODUCTION coverage-limited, and the SNR is mostly relevant. As cells are added to fill in coverage holes and reduce cell radii, and the user traffic increases, the system becomes capacity- limited and the SIR becomes more critical. In situations where neither interference nor noise can be ignored, the signal-to-interference-plus-noise ratio (SINR) may be considered as a channel measure. However, if the nature of the interference is significantly different from that of the noise and affects the signal in a different manner, then it may not be possible to lump the two together and describe the performance as a function of the SINR. Instead, one would have to use, for example, SNR curves parameterised to interference levels or SIR curves parameterised to noise levels. The signal quality can also be expressed as a function of the ratio of energy per bit E b either to the noise power per Hertz, N 0 , or the interference power per Hertz, I 0 . Finally, instead of using signal and interference levels at the ‘base-band’, the so-called carrier-to-interference ratio (CIR) at the radio frequency level is often used. According to Reference [6], CIR = E b · R b I 0 · B c ,(1.1) with R b the bit-rate in bits per second, and B c the radio channel bandwidth in Hertz. 1.2 The Emergence of the Internet and its Impact on Cellular Communications In the ‘wired world’, we are witnessing how traffic of all types is increasingly being carried on packet-switched networks using the connectionless internet protocol (IP) — or rather, the IP protocol suite, which features various other protocols on top of IP, e.g. transport protocols such as the transport control protocol (TCP) and the user datagram protocol (UDP). This development is mainly due to the tremendous success the Internet has enjoyed in recent years (incidentally, not unlike cellular communications). Initially constrained to non-real-time applications such as Telnet, file transfer, email and Web browsing, this move towards IP now embraces audio and video streaming with more stringent delay constraints, and even ‘proper’ real-time traffic such as Voice over IP (VoIP). Strictly speaking, it is typically voice over RTP (the real-time protocol, an application- level protocol), UDP and IP. It is now widely anticipated that the same will eventually happen in the wireless world as well, which has some serious technical implications on cellular communication systems. In the following, we deal with the general implications; the impact specifically on multiple access protocols is discussed in the next section. Already in the late eighties (e.g. in Reference [7]), Goodman, whom we will refer to at various other occasions in this text, suggested that both the fixed architecture and the air interface of 3G systems should be based entirely on packet-switching for all types of services. Not only would the available resources be exploited more efficiently, but also certain functions could be decentralised and distributed over many processors, which would improve the scalability of such systems. He also proposed a packet-based multiple access protocol called packet reservation multiple access (PRMA) suitable for the wireless links between mobile and base stations. Although his vision of an all-packet system was probably not driven by the Internet at that time (he suggested that ‘3G systems, in harmony with broadband integrated services digital networks, would use shared resources 1.2 THE EMERGENCE OF THE INTERNET 9 to convey many information types’), it is in some respects in tune with Internet architecture principles. Goodman’s vision has not really caught on during initial 3G standardisation efforts. True, unlike 2G systems, first releases of 3G systems have incorporated packet-data support right from the start, however, without proper support of real-time packet data services such as packet voice. For instance, the first UMTS release might well provide improved support for packet data over the air interface compared to the GSM General Packet Radio Service (GPRS). However, the packet-based infrastructure in the fixed network, which has evolved from the GPRS infrastructure, was not designed for voice. Instead, it was intended that voice would always be delivered over the circuit-switched infrastructure. But why this reluctance towards packet-voice and an all-packet system? There was a significant amount of scepticism in the industry regarding the feasibility of an all-packet system which could deliver the high-quality standards required for voice communications. Also, decentralisation, explicitly advocated by Goodman, to some extent inherent in the move to a packet-only system, and certainly consistent with the Internet architecture, means loss of operator control. This is something that operators do not like too much, as they tend to control the types of services delivered through their networks. The Internet, by contrast, is built according to the ‘end-to-end principle’, where the infrastructure in- between the end nodes is not concerned with the services to be delivered. In the case of the cellular communications industry, apart from commercial considera- tions, there are sound technical reasons for this desire to control matters. Take the issue of handovers as an example. Goodman [7] proposed to decentralise them completely, effectively placing them into full control of the mobile terminals, in order to cope effi- ciently with the large volume of handover-related signalling traffic as a result of smaller and smaller cell sizes due to ever increasing traffic density. Cellular operators, however, like to control which terminal is served by which cell and thus prefer network-controlled handovers. This is firstly because normally only the network has a complete view of the communication quality on both up- and downlink (the latter through measurement reports sent by terminals), which may be different. Secondly, the quality of individual communications has to be traded off against system capacity and the quality of other communications, requiring careful admission control and load balancing by the network. In general, centralised algorithms exploiting the global view of a matter perform better than decentralised ones with only local information available. Obviously, they are also more complex, and therefore sometimes inappropriate, but when it comes to efficient use of scarce and precious air-interface resources, it is often worth the effort. In spite of the initial scepticism by the cellular industry towards packet-based systems, the power of the Internet is proving too strong, and things are moving on. Subsequent releases of 3G systems will be capable of supporting voice over the packet-switched infrastructure as well. This does not necessarily imply a complete decentralisation of the architecture of cellular systems, at least not in the beginning. Operators driving these developments are predominantly interested in the new services they hope to deliver over their networks, as discussed in somewhat more detail in Section 2.4 and again in Chapter 11, and most of them are not (yet?) prepared to give up control. In terms of our main topics of interest for this book, such developments will primarily impact multiple access protocols, less the basic multiple access schemes. However, as decentralisation goes further (assuming that it will eventually), autonomous ‘plug-and-play’ base stations 10 1 INTRODUCTION become desirable, something which could affect the choice of preferred basic multiple access schemes as well. For instance, in CDMA systems, to improve the transmission quality, terminals are often connected to the network via multiple base stations. This implies some co-ordination between these base stations. Furthermore, entities are needed that can process the signals of multiple base stations. Dealing with such matters through packet-based systems is by no means impossible, but it is somewhat of an obstacle to full decentralisation. 1.3 The Importance of Multiple Access Protocols in Cellular Communications In 2G systems, which were designed to carry voice and some low-bit-rate data services by setting up ‘circuits’ or dedicated channels for the duration of a call, access arbitration is only required at the time of setting up a call to request such dedicated channels. With the advent of advanced 2G systems, e.g. GSM complemented by GPRS, and first releases of ‘true’ 3G systems such as UMTS, non-real-time data carried on common or shared channels becomes increasingly important, which calls for more sophisticated multiple access protocols. As just discussed, these systems will further evolve to support real-time IP traffic. What does this mean in terms of choosing appropriate multiple access protocols? We are continuing to use Reference [7] as a case study, since packet reservation multiple access, the multiple access protocol proposed by Goodman for the air-interface uplink channel, was a subject of extensive research efforts by the authors, as documented in this book (see Section 1.4). Consider a traffic source which alternates between ‘off’ or ‘silent’ (no packets are generated) and ‘on’ or ‘active’ (packets are generated at a rate matching the channel transmission rate, e.g. one packet per TDMA frame fitting into one time-slot). A typical example would be a voice source subject to voice activity detection. By reserving resources on the air interface only during ‘on’ phases, when packets need to be transmitted, rather than hanging on to them for the entire duration of a call (as would be the case in a ‘circuit-switched model’), PRMA attempts to make efficient use of air-interface resources. Compared to a conventional TDMA air interface, where N time- slots can sustain N calls, with PRMA M calls can be multiplexed onto N now shared time-slots, with M>N; how many exactly depends obviously on the so-called activity factor, i.e. the fraction of time the traffic source is in ‘on’ state. In other words, PRMA allows for a certain degree of statistical multiplexing over the air. One could therefore conclude that in conjunction with a packet-switched infrastructure, a ‘packet-switched air interface’ such as PRMA would also make sense. However, while the split between packet-switching and circuit-switching is fairly evident in the fixed network infrastructure, when dealing with the air interface, the situation is a little bit more complicated. Essentially, on the air interface, we can distinguish between dedicated channels on one hand and common or shared channels on the other. Typically, dedicated channels, which are set apart for the sole use of one communication link between a mobile terminal and a base station, are associated with the ‘circuit-switch model’. Conversely, shared channels (shared between a limited number of users) or common channels (common to the whole cell population), for which appropriate multiple access protocols are crucial, are often associated with the ‘packet-switch model’. This is indeed often appropriate, particularly for packet-based services which exhibit very bursty traffic characteristics, i.e. traffic sources which alternate between short activity periods (e.g. at high bit-rates) [...]... were published in Reference [55] In GPRS, discrimination between the access delay performance experienced by different services is possible through computation of different permission probabilities for contending terminals according to the delayclass they belong to, e.g by using a prioritised version of pseudo-Bayesian broadcast In MD PRMA, on top of access delay discrimination for non-real-time services,... BCH code probability of contending users (and thus Pdrop ) and the quality of service of users holding a reservation, the latter by erasing some of their packets As an example, Qpe [K] for a (511, 229, 38) BCH code and a spreading factor X = 7 is shown in Figure 1.7 The BCH code is used for forward error correction coding For more explanations and details on the exact conditions considered, refer to... of contending users not holding, but wishing to obtain, a reservation In order to protect the reservation mode users from excessive MAI and to stabilise the protocol, the access permission probability p of contending users is dynamically controlled The core contribution of our research is the investigation of different load-based and backlogbased access control algorithms in this context, as discussed... relevant for hybrid CDMA/TDMA air interfaces, when time-slots may simultaneously be accessed by contending users and carry traffic of users holding a reservation However, it is also relevant for UTRA FDD, where users holding a reservation (e.g on dedicated channels) are not separated in time from contending users Therefore, in Chapter 10, where the multiple access concepts relevant for UTRA are introduced,... (where UTRA stands for UMTS Terrestrial Radio Access), for instance, they could be applied both on the Common Packet Channel (CPCH) and on dedicated channels, as discussed in more detail in Chapter 10 For an air interface based on hybrid CDMA/TDMA, combining PRMA with CDMA is straightforward The channel structure is as in PRMA; that is, the time axis must be divided into slots, which are grouped into... (e.g GSM [10]) or on CDMA (cdmaOne) for multiple access The main research efforts towards 3G were also either directed to TDMA-only systems, such as the European RACE Advanced TDMA (ATDMA) collaborative research project [11], or to CDMA-only systems (e.g the RACE Code-division test-bed or Codit project [12]) However, hybrid CDMA/TDMA schemes had already been proposed in the late 1980s for GSM, and were... forward for personal communications systems in the US In this proposal, standardised as IS-661 [14], but never deployed commercially outside New York, multiple access within a cell is provided by means of TDMA only, but the TDMA channel is spread in order to decrease the frequency reuse factor, and different codes are assigned to different cells In some ways, a GSM system can provide the same feature through... third hybrid approach referred to as code-time-division multiple access (CTDMA) was proposed by Massey in 1989 and investigated at the Swiss Federal Institute of Technology (ETH) in Zurich [16] In CTDMA, again only one code per cell is used, and the time-division element is provided through staggering of users in intervals of a few chips rather than providing ‘proper’ time-slots Other publications dealing... restricting access for contending users to time-slots with high load (by choosing low p values) and setting p (close) to one for time-slots with low load, which, compared to unconstrained channel access, balances the load between time-slots In these investigations, no hard limit was assumed for the number of packets that can be carried in a single time-slot, and distinct codes were not distinguished Instead,... system operation In Chapter 10, the different options available on the UMTS air interface for the support of packet traffic are described In Chapter 11, the issue of interference-limited operation versus blocking-limited operation specifically for supporting VoIP in enhanced 12 1 INTRODUCTION GPRS systems is re-examined and the overheads associated with VoIP are discussed Additionally, our own research efforts . propagation effects are usually distinguished. These are the pathloss, slow fading or shadowing, and fast fading or multi- path fading. The pathloss describes. access capability derives from the use of different spreading codes for individual users. Because of the spreading of the spectrum, CDMA systems are also