Chapter 1 Introduction to Cellular Radio This book is concerned with two digital mobile radio systems: the global system for mo- bile communications (GSM); and a code division multiple access (CDMA) system that was originally known as the American interim standard 95, or IS-95 and is now called cd- maOne [1–7]. While GSM was conceived and developed through the concerted efforts of regulators, operators and equipment manufacturers in Europe, cdmaOne owes its existence to one dynamic Californian company, Qualcomm Inc. The authors have been involved with both the pan-European mobile radio system, which became GSM, and the Qualcomm CDMA system for a number of years. The GSM system predates cdmaOne. The two systems are very different. The radio interface of GSM relies on time division multiple access (TDMA), which means that its radio link is very different to that of cd- maOne. Also GSM is a complete network specification, from the subscriber unit through to the network gateway. Indeed its fixed network component is perhaps its most advanced feature [1, 2]. cdmaOne, by contrast, has a more complex and advanced radio interface, and only later were fixed network issues addressed [3, 7]. In the chapters to follow, the GSM and cdmaOne systems will be described and analysed while the final chapter deals with their evolution to third generation systems. This chapter is meant to provide background information on cellular radio [1–11]. The reader who is well acquainted with the fundamentals of mobile radio communications should therefore bypass this chapter. For the reader who has elected to read this chapter we should state at the outset that our goal is to provide a clear exposition of the concepts of the subject rather than detailed analyses, which will follow in the later chapters. The first point to make is that a mobile radio network has a radio interface that enables a mobile station (MS) to communicate with the fixed part of the mobile network. Both components, the radio interface that fa- cilitates user mobility, and the fixed network that enables the mobile to communicate with 1 eter Gould Wiley & Sons Ltd GSM, cdmaOne and 3G Systems. Raymond Steele, Chin-Chun Lee and Peter Gould Copyright © 2001 John Wiley & Sons Ltd Print ISBN 0-471-49185-3 Electronic ISBN 0-470-84167-2 2 CHAPTER 1. INTRODUCTION TO CELLULAR RADIO other users via the public switch telephone network (PSTN) or the integrated services dig- ital network (ISDN), are radically dissimilar and complex. This means that to have a good appreciation of mobile radio requires a wide knowledge that includes speech coding, chan- nel coding, interleavers, radio modems, radio propagation, antennas, channel equalisation, RAKE receivers, diversity techniques, radio planning of cells, the significance of signal-to- interference ratios (SIRs), bit error rate (BER), teletraffic issues, protocol stacks, location databases, signalling systems, encryption, authentication procedures, switching, packetisa- tion techniques, and so on. If some of these subjects are dealt with from a standing start in other chapters they will not be dealt with here. Neither will they be considered if they are outside the confines of this text. What we will consider here are topics that are needed when we come to our discussions of GSM and cdmaOne. There are many ways of describing cellular radio, and the two most obvious are a bottom- up approach, or a top-down one. The former starts with the basic principles of radio prop- agation, to the concept of a cell, then clusters of cells to the radio links and multiple access methods, to setting-up, maintaining, and clearing-down of calls. The top-down approach is essentially the reverse process, starting with the big picture and ending up with radio propagation issues. We have opted for the bottom-up approach, building on concept after concept, until the overall concept of the network can be appreciated. Our starting point is the notion of a single cell. 1.1 A Single Cell Consider a base station (BS) having an antenna located on a tower radiating an electromag- netic signal to a mobile station (MS). The received signal depends on many factors. The output port of the BS equipment delivers power at the appropriate radio frequency (RF) into the cable connected to the antenna. There are losses in the cable, e.g. a 40 W RF signal at the BS equipment may yield only 16 W of radiated power. The BS antenna is usually directional, which means that power is directed over a solid angle rather than over all an- gles. This means that compared with isotropic radiator there is a gain G ( θ φ ) of power in the θ and φ directions, where θ and φ are angles measured in the vertical and horizontal directions, respectively. As the transmitted energy spreads out from the BS, the amount of power the MS antenna can receive diminishes [12, 13]. The mobile’s antenna is usually located only one to two metres above the ground whereas the BS antenna may be at a height from several metres to in excess of a hundred metres. The heights of the antenna affect the path loss, i.e. the difference in the received signal power at the MS antenna compared with the BS transmitted power. The path loss (PL) is usually measured in decibels (dB). As an example, for the plane earth model there are two paths, a direct line-of-sight (LOS) path and a ground-reflected 1.1. A SINGLE CELL 3 path. The expression for PL is PL = h T h R d 2 2 G T ( θ φ ) G R ( θ φ ) (1.1) where h T and h R are the heights of the transmitting and receiving antennas, respectively, d is the distance between the two antennas, and G T ( θ φ ) and G R ( θ φ ) are the gains of the transmitter and receiving antenna, respectively. When written in decibels, the path loss, L p , becomes L p = 10log 10 PL = 20log 10 h T + 20log 10 h R 40log 10 d + 10log 10 G T ( θ φ )+ 10log 10 G R ( θ φ ): (1.2) This equation is only valid when d > 2πh T h R λ (1.3) where λ is the wavelength of the radiated wave. The plane earth model is useful but may deviate significantly from reality. In the plane earth model, L p decreases at 40 dB per decade increase in distance, i.e. if the distance increases by 10 times, the path loss will increase by 40 dB. This rate is often used in prac- tical situations, although measurements show it may be closer to 35 dB per decade. If the transmitted power is sufficiently high a MS will often travel beyond the LOS of the BS antenna. When a mobile goes behind a large building the average received power will decrease and when it emerges from the building that casts the electromagnetic shadow, the average received power will rise. The fading due to large obstacles that produce electromag- netic shadows is called shadow fading. As a result of this fading effect, as the MS travels away from the BS the received power at the MS and the BS is subjected to considerable variations. These variations due to shadowing effects can be represented by a log-normal distribution of a shadow fading random variable ζ. Specifically we introduce this variable into Equation (1.2) to give L p = 20log 10 h T + 20log 10 h R 40log 10 d + ζ + 10log 10 G T ( θ φ )+ 10log 10 G R ( θ φ ) (1.4) where ζ is measured in decibels and may be positive or negative. In this book we will often use the expression for received signal power as S ( dB )= 10log 10 P 10n log 10 d + ζ (1.5) 4 CHAPTER 1. INTRODUCTION TO CELLULAR RADIO or, when not in decibels, the expression becomes S = Pd n 10 ζ = 10 (1.6) where P is the transmitted power from the BS and n is called the exponent of the PL. Observe that when we employ Equations (1.5) or (1.6), the terms relating to antenna heights and antenna gains are absent. This is because we often ignore the effects associated with the antennas on the path loss when we are concerned with signal-to-interference ratios (SIRs) since these parameters tend to cancel out on the signal and interference paths. Equation (1.6) is used extensively in Chapters 3 and 5. The MS is not only subjected to shadow fading, but also to small scale fading, i.e. due to the received signal changing in amplitude and phase as a consequence of a small change in the spatial separation (e.g. fraction of a wavelength) between the MS and its BS [4]. This occurs because the MS is travelling through an electromagnetic field, receiving more than one version of the same transmitted signal that travelled via different paths. Each path results in a component of the received signal that has a specific attenuation and phase orientation. The received signal at the MS is therefore the vector sum of all these multipath signals. The vector sum may be large at one instant and a small movement of the MS may result in the multipath signal being very small. This variation often takes place over a distance of half a wavelength which is only ( 3 10 8 )=( 2 10 9 )= 15 cm for a 1 GHz radio frequency carrier. If the received paths are close together in time, we may represent the channel impulse function by a single delta function whose amplitude is Rayleigh distributed while its phase has a uniform distribution. The Fourier transform of a delta function is a flat spectrum. Since the weighting of the delta function varies due to the fading, the magnitude of the flat spectrum changes, and the condition is known as flat fading. This means all the frequencies in the received signal fade together and by the same amount. Often we have a path arriving in the vicinity of the MS and subjected to local scattering producing a single delta function that is Rayleigh distributed. Then another ray arrives yield- ing another delta function that is also Rayleigh distributed. This process of each received ray causing a group of scattered rays that can be represented by a Rayleigh distributed delta function yields a channel impulse response that is itself made up of a number of impulses or delta functions at epochs 0, τ 1 , τ 2 ::: , as shown in Figure 1.1. Since each delta function is fading independently the spectrum of the radio channel no longer fades uniformly for all frequencies. This type of fading is called frequency selective fading, which means that in the time domain the depths of the fades are, in general, much less than for flat fading. In the latter case the fading can be very deep, typically up to 40 dB, and this may cause bursts of symbol errors. As a consequence, having a wideband channel means that the signal is less likely to drop below the receiver sensitivity for a given transmitted power compared 1.1. A SINGLE CELL 5 with a narrow band channel. However, the wideband channel has a wider impulse response, and since the received signal is the convolution of the transmitted signal with the impulse response of the radio channel, one data symbol is smeared into other symbols. This effect, called intersymbol interference (ISI), requires the receiver to un-smear the symbols. This is achieved using a channel equaliser in GSM and a RAKE receiver in cdmaOne. We will return to channel equalisation and RAKE receivers in some detail in later sections. As a MS travels away from the BS, the received signal at the MS decreases as the path loss increases. The received signal will also exhibit large scale (shadowing) fading and small scale fading. Figure 1.2 shows an example of the variations in the received signal level (in dBs) as the MS travels. The dotted line represents the change in received signal level due to shadow fading. The rapid changes in the received signal level are the consequence of small scale fading, which for a particular carrier frequency depends on the MS speed. The faster the MS travels, the more rapid is the fading. A stationary MS may be in a deep fade. Fortunately the effect of small scale fading can be effectively combatted in modern digital mobile radio systems. Shadow fading and path loss is another matter. Having passed through the radio channel, the RF signal transmitted by the BS will arrive at the MS antenna. This will usually connect directly into the receiver input but, unlike the BS, there are no cable losses. The antenna will be omni-directional whereby it is able to capture signal energy equally from all directions in the horizontal plane. In the case of a handheld MS, the signal may be attenuated by the user’s body before arriving at the antenna, and network operators generally include a margin in their planning procedures to account for body loss. As the MS travels there is a change in the frequency of the received carrier on each path due to the Doppler effect. For a MS travelling in a direction making an angle α i with respect to a signal received on the ith path, the carrier frequency is changed from f c to τ 0 ττ 21 Magnitude τ 34 time Figure 1.1: Magnitude of wideband channel impulse response, measured from the arrival of the first path. 6 CHAPTER 1. INTRODUCTION TO CELLULAR RADIO -130 -120 -110 -100 -90 -80 -70 -60 -50 -40 0 0.2 0.4 0.6 0.8 1 Time (s) Received signal level (dBm) Figure 1.2: Combined shadow and fast fading. f c +( ν = λ ) cosα i ,whereν is the speed of the MS and λ is the wavelength of the carrier ( = 3 10 8 = f c m). Therefore, not only does each path in the received signal experience a different attenuation and phase shift, it is also subjected to a change in carrier frequency that can be positive or negative depending on α i . The Doppler power spectral density (PSD) is parabolic about the carrier to frequencies f c ( ν = λ ) when the probability density function (PDF) of α i is uniform, i.e. rays arrive at the MS from all directions with equal probability. In general there are only a few rays and their direction is often restricted, e.g. by local buildings. In this case the Doppler spectrum will be non-monotonic and rapidly changing. Fast changes in the Doppler spectrum manifest themselves as fast changes in the radio channel impulse response. Again, mobile radio equipment is well able to combat Doppler effects, unless the MS speed is excessive, e.g. in very high-speed trains. So far we have considered a mobile travelling away from a BS and the received signal level decreasing with increasing BS to MS distance. The MS continues its travels with its receiver combating the fast fading, Doppler effects and channel dispersion due to its good design. The MS has a noise floor, which is reached when the mobile has travelled sufficiently far from the BS such that the receiver noise dominates the received signal level and the receiver behaves as if no signal is being received. Before this extreme condition is reached there is a received signal threshold known as the receiver sensitivity.When the received signal level is above this level, the bit error rate (BER) is acceptably low. Conversely, when the received signal level drops below the receiver sensitivity, the MS is 1.2. MULTIPLE CELLS 7 no longer able to receive signals of an acceptable quality from the BS. The point in space at which this threshold occurs represents a boundary point for the down-link or forward link, i.e. the transmissions from the BS to the MS. What about the up-link or reverse link, i.e. the transmission from the MS to the BS? The two links are never the same. They are similar in GSM and radically different in cdmaOne. The MS transmitter operates at significantly lower power levels than the BS and so the maximum radiated power levels are lower than those at the BS. The BS is able to compensate for the MS deficiencies by being able to operate at a lower receiver sensitivity and by employing techniques such as space diversity to enhance the received signal from the MS. It is important to note that the signal characteristics that we have already discussed in relation to the down-link (i.e. path loss, fast and slow fading, Doppler shift and ISI) will also be present in the received up-link signal. To simplify our discussion, we will assume that our boundary point is the same for either link, unless specifically stated. If the MS takes a number of different routes away from the BS and on each route notes the location where the received signal goes below the receiver sensitivity, then by joining up these location points on a map we will form a contour around the BS. A stylised arbitrary irregularly shaped contour is shown in Figure 1.3. The area enclosed within the boundary is called a cell. 1.2 Multiple Cells The dimensions of a cell are limited by the transmitter and receiver performances, the path loss, shadow fading and other factors described in the previous section. If we are going to cover wide areas we will need to tessellate cells, and switch a MS between BSs as it roams throughout the network. If hundreds or thousands of cells are required, then some cells must operate with the same carrier frequencies. This phenomenon is called frequency reuse. BS Figure 1.3: A single cell. 8 CHAPTER 1. INTRODUCTION TO CELLULAR RADIO Let us consider the situation where each radio carrier supports N traffic channels, and the spacing between adjacent carriers is B c Hz. Consequently, a traffic channel occupies an equivalent bandwidth of B = B c = N Hz. Suppose the spectrum regulator assigns W Hz for the up-link transmissions and W Hz for the down-link transmissions. The number of carriers for each down-link is approximately W = B c . If we are going to reuse carriers in other cells we must ensure that each receiver can operate with an SIR that will give a sufficiently low BER. Let us for the moment consider that the only form of interference is from either users or BSs in other cells that are using the same traffic channel as a particular mobile in the zeroth cell, say. This interference is called co-channel interference or intercellular interference.To ensure that the interference is sufficiently low compared with the required signal power S (i.e. the SIR is sufficiently high) the interfering cells must be spaced sufficiently far apart. This may mean that other cells must be spaced between the zeroth cell and the interfering cells. Each cell is given a different channel set until all the bandwidth W is used. If this means M cells consume the bandwidth W ,thenwehaveM contiguous cells that form a cluster of cells. We now form another cluster of M cells and tessellate it with the first cluster. In each cluster all the channels are used, and the clusters are arranged such that two cells that use the same channel set are spaced as far apart as possible. Figure 1.4 shows two four-cell clusters where the cells marked A, B, C and D in each cluster, respectively, use the same channel sets. The number of cells in the cluster, M, is called the reuse factor. The value of M depends on the SIR. If, for an acceptable BER, the SIR is required to be high, then we must have many cells in the cluster in order to space the ‘reuse cells’ sufficiently far apart such that the interference is low enough to satisfy the minimum SIR requirement. We will see that GSM requires M 3, while cdmaOne can operate with M = 1. Why is a low cluster size good? By operating with a smaller number of cells in a cluster the number of channels per cell, equal to ( N = M )( W = B c ) , is high, since M is low. The carried C D A B Cluster 1 C D A B Cluster 2 Figure 1.4: Two tesselated four-cell clusters. 1.2. MULTIPLE CELLS 9 traffic in Erlangs for a given blocking probability has a non-linear relationship with the number of channels per cell such that for more channels there is a disproportionate increase in the traffic that may be supported. We now observe an important aspect of cellular radio, i.e. for a mobile radio system that employs clusters of cells. If the radio link equipment is capable of operating with a low SIR, the cluster size becomes small and the carried traffic high. Another important point to note is that a cell becomes smaller in the presence of cochannel interference. By this we mean that the area around the cell site where the SIR is high enough to yield a sufficiently low bit error rate (BER) is decreased due to the presence of the interfering cells. This is illustrated in Figure 1.5. We also note that as the levels of interference power alter, so does the SIR, and so does the effective cell boundary for an acceptable BER. The cell boundaries shown in Figure 1.5 relate to a specific BER. For a higher BER, the cell size increases and vice versa. It is important to avoid the simple notion that a cell has a fixed area. It is better to think of it as breathing, i.e. changing its size as the traffic conditions within the network vary. Cell breathing is a feature of both GSM and cdmaOne, although it is more acute in the CDMA system. For analysis reasons we generally consider fixed cells and often worse case conditions. Newcomers to cellular radio often consider spectral efficiency in terms of the number of channels, N, a carrier can support in a given bandwidth. This notion is related to modulation efficiency in terms of bits per second per Hertz of RF bandwidth. Since cellular radio must operate in an interference-limited environment, the crucial factor is not the modulation efficiency. For example, employing quadrature amplitude modulation (QAM), where each symbol carries multiple bits, gives a high modulation efficiency [10, 14]. However, QAM requires a high SIR value and hence large cluster sizes, resulting in low values of carried traffic per cell site, for a given bandwidth allocation. The choice of modulation and multiple access scheme is complex and will be addressed at a later stage. What we must note is that, given a modulation and multiple access scheme resulting in a cluster size of M, the number of users on the network is greatly increased if the cells, and thereby the clusters, are small. This is because each cluster carries a traffic of MA c Erlangs, where A c is the carried traffic at each BS, and if a cluster occupies an area S c then the traffic carried per km 2 is MA c = S c Erlangs/km 2 for a bandwidth W . Using small cells, often called microcells, means S c is small and the traffic density that may be supported is high. 1.2.1 Hexagonal cells These types of cells are conceptual. The cell site is located at the centre of each hexagon, and the hexagonal cells are tessellated to form clusters [15]. Although these cells are fic- titious, they are often used for comparing the performances of different cellular systems. Figure 1.6 shows clusters of tessellated hexagonal cells. Observe that for hexagonal cells there are always six near cochannel cells, irrespective of the cluster size. This is because 10 CHAPTER 1. INTRODUCTION TO CELLULAR RADIO without cochannel interference Cell boundary Cell boundary in the presence of cochannel interference 0th Cell Figure 1.5: The zeroth cell and nearest cells using the same channel sets. The shaded areas are the cells after co-channel interference is introduced. a hexagon has six sides. Figure 1.7 shows two co-channel cells shaded. From the figure, h = j sin60 = j p 3 = 2 , sinψ = j p 3 = 2 D (1.7) cos ψ = i +( j = 2 ) D (1.8) and as sin 2 ψ + cos 2 ψ = 1 (1.9) D = i 2 + ij + j 2 1 2 (1.10) where D is the distance between cochannel BSs. From Figure 1.8 the distance between two cell sites is 2µ = p 3R (1.11) where R is the distance from the centre of a cell to its apex, and from Figure 1.7, i = l 2µ (1.12) and j = m 2µ (1.13) [...]... vectors g1 = 101] and g2 = 111] and the corresponding code word c2 c1 ], where c1 2 6 =4 b2 b1 b0 3 7g 5 1 and c2 2 6 =4 b2 b1 b0 (1.17) 3 7g 5 2 (1.18) and where b0 , b1 , and b2 are the three bits in the shift register An equivalent form of the generator vectors is two generator polynomials g1(z) = 1 + z2 and g2(z) = 1 + z + z2 (1.19) where 1 denotes the present input bit, and z and z2 represent the... second row and the second column The 1.4 THE CDMA RADIO INTERFACE 35 Hadamard matrix of order four is H4 = 2 " # H2 H2 H2 H2 6 =6 6 4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 7 7 7 5 (1.45) where row 1 and column 1 are Walsh code 0, or w0 ; row 3 and column 3 are w1 ; row 4 and column 4 are w2 ; and row 2 and column 2 are w3 The Walsh code number w j is the average number of zero crossings between +1 and 1 If... Both GSM and cdmaOne use the analysis-by-synthesis type of codecs The GSM codec is a regular pulse excited linear predictive codec (RPE-LTP) that generates bits at a fixed rate A variable rate code excited linear predictive codec (CELP) is used in cdmaOne Since our analysis of both GSM and cdmaOne presupposes that the speech is in a coded format, we advise the interested reader to read Chapters 3 and 8... a band-pass filter that will pass the frequencies within the operating band of the TDMA system, but reject the signals from systems operating in adjacent bands After the RF front-end, the signal is down-converted to an intermediate frequency (IF), before being narrowly filtered at the bandwidth of the 1.3 THE TDMA RADIO INTERFACE 25 TDMA carrier The signal is next converted into its in-phase (I) and. .. used by both cdmaOne and GSM A convolutional coder accepts the latest k-bit and the previous (K 1)k-bit inputs to generate an n-bit code word, where K indicates the number of k-bit inputs required to produce a code CHAPTER 1 INTRODUCTION TO CELLULAR RADIO 20 word, and is referred to as the constraint length [2] A convolutional code can basically be defined by the three parameters, n, k, and K, and is denoted... these W j , then W1 = w0 , W2 = w3 , W3 = w1 and W4 = w2 In cdmaOne the row numbering in the Hadamard matrix is used and not the Walsh number Higher order Hadamard matrices are built up using the same procedure, i.e # " and the codes used in cdmaOne are H4 H4 H32 H32 H8 = H4 H4 H32 H32 (1.46) " H64 = # : (1.47) Figure 4.1 shows the set of 64 Walsh codes used in cdmaOne, where the numbering relates to W... multiple access (CDMA) [3, 7] This preamble is to introduce the words spread spectrum and CDMA, to whet your appetite before we open Pandora’s box and see what is inside Suffice to say at this juncture that spread spectrum communication systems have their origins in military communications because of their ability to withstand high levels of jamming interference They also have the virtue that the spread... integrate and dump process Figure 1.22 shows a more general BPSK system Another way of representing a BPSK signal is s(t ) = p 2Pb(t ) cosωct (1.33) where P is the power in s(t ) Note that we are considering power and not energy as s(t ) is not now confined to one bit duration and b(t ) is a sequence of data bits We have, for simplicity, omitted the transmitter and receiver filters If b(t ) is a random signal,... unlike the situation in cdmaOne, then a user would do better not to use Walsh codes [19] However, as used in cdmaOne they are effective Walsh codes are easy to generate using a Hadamard matrix The matrix of order two contains the zero-order and first-order Walsh codes, namely H2 = " 1 1 1 1 # : (1.44) The matrix is square and the zero-order Walsh code is found in the first row and in the first column The... radio channel will cause a number of deleterious effects to the transmitted signal Its amplitude will vary with the path loss and fast and slow fading, and ISI may be introduced as a result of multiple, different length paths between the transmitter and receiver In the case of GSM, any ISI introduced by the radio channel will be in addition to the ISI already introduced by the modulation scheme The RF . cilitates user mobility, and the fixed network that enables the mobile to communicate with 1 eter Gould Wiley & Sons Ltd GSM, cdmaOne and 3G Systems. Raymond. follow, the GSM and cdmaOne systems will be described and analysed while the final chapter deals with their evolution to third generation systems. This chapter