CELLULAR AND MOBILE COMMUNICATIONS

Transcription

1 CELLULAR AND MOBILE COMMUNICATIONS by VIDYA SAGAR POTHARAJU Associate Professor, Dept of ECE,. 1

2 TEXT BOOKS 1.Mobile and Cellular Telecommunications-W.C.Y.Lee 2 nd Edn, Wireless Communications-Theodre.S.Rapport, Pearson education,2nd Edn., Mobile Cellular Communications-Gottapu sashibushan rao,pearson,2012. REFERENCES: 1.Principles of mobile communications-gordon L.Stuber,Springer intl,2nd Edn., Modern Wireless Communications-Simon Haykin,Michael moher,pearson Edu, Wireless Communications theory and techniques,asrar U.H.Sheikh,Springer, Wireless Communications and networking,vijay Garg,Elsevier Publications, Wireless Communications-Andrea Goldsmith,Cambridge University Press,

3 Syllabus INTRODUCTION TO CELLULAR MOBILE RADIO SYSTEMS: Limitations of conventional mobile telephone systems, Basic Cellular Mobile System, First, second, third, and fourth generation cellular wireless systems, Uniqueness of mobile radio environment. Fading-Time dispersion parameters, Coherence bandwidth, Doppler spread and coherence time. FUNDAMENTALS OF CELLULAR RADIO SYSTEM DESIGN Concept of frequency reuse, Co-channel interference, co-channel interference reduction factor, Desired C/I from a normal case in a omnidirectional antenna system, system capacity, trunking and grade of service, Improving coverage and capacity in cellular systems- Cell splitting, Sectoring, Microcell zone concept. 3

4 Conventional Mobile System 4

5 Limitations of Conventional Mobile Telephone Systems One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: Limited service capability, Poor service performance, Inefficient frequency spectrum utilization. Limited service capability : Each area is allocated with one or more channels. Which is large autonomous geographic zone. The transmitted power should be as high as the federal specification allows. The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the call will be dropped. 5

6 Poor Service Performance In the past, a total of 33 channels were all allocated to three mobile telephone systems. o Mobile Telephone Service (MTS)-40MHz o Improved MTS (IMTS) MJ-150MHz o Improved MTS (IMTS) MK -450MHz 6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list. 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The large number of subscribers created a high blocking probability during busy hours. Although service performance was undesirable, the demand was still great. A high-capacity system for mobile telephones was needed. 6

7 Inefficient frequency spectrum utilization The frequency utilization measurement (Mo), is defined as the maximum number of customers that could be served by one channel at the busy hour. Mo = Number of customers/channel Mo = 53 for MJ system,37 for MK system The offered load can then be obtained by A = Average calling time (minutes) x total customers / 60 min (Erlangs) Assume average calling time = 1.76 min. A1 = 1.76 x53 x 6 / 60 = 9.33 Erlangs A2 = 1.76 x 37 x 6 / 60 = 6.51 Erlangs (MJ system) (MK system) 7

8 If the number of channels is 6 and the offered loads are A1 = 9.33 and A2 = 6.51, then from the Erlang B model the blocking probabilities, B1 = 50 percent (MJ system) B2 =30 percent (MK system), It is likely that half the initiating calls will be blocked in the MJ system, a very high blocking probability. 8

9 9

10 If the actual average calling time is greater than blocking probability can be even higher min, the To reduce blocking probability we must decrease Mo. As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area. This is overcome by the new cellular system. 10

11 A Basic Cellular System Fig 1 : Basic Cellular system 11

12 A basic cellular system consists of three parts: a mobile unit, a cell site, and a mobile telephone switching office (MTSO) Mobile units: A mobile telephone unit contains a control unit, a transceiver, and an antenna system. Cell site: The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals. MTSO: The switching office, the central coordinating element for all cell sites, contains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, and handles billing activities. Connections: The radio and high speed data links connect the three subsystems. Each mobile unit can only use one channel at a time for its communication link. The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and cellular administration. 12

13 Uniqueness of Mobile Radio Environment 13

14 The Propagation Attenuation The propagation path loss increases not only with frequency but also with distance The propagation path loss would be 40 db/dec. This means that a 40 db loss at a signal receiver will be observed by the mobile unit as it moves from 1 to 10 km. 14

15 Mobile Communication Operation 15

16 Evolution of Mobile Radio Communications Major Mobile Radio Systems Police Radio uses conventional AM mobile communication system Edwin Armstrong demonstrate FM First public mobile telephone service - push-to-talk Improved Mobile Telephone Service, IMTS - full duplex Bell Lab introduce the concept of Cellular mobile system AT&T propose the concept of Cellular mobile system to FCC Bell Mobile Phone service, poor service due to call blocking Advanced Mobile Phone System (AMPS), FDMA, FM Global System for Mobile (GSM), TDMA, GMSK U.S. Digital Cellular (USDC) IS-54, TDMA, DQPSK IS-95, CDMA, QPSK, BPSK 16

17 Examples of Mobile Communication Systems Pagers-Simplex Hand held Walkie-Talkies-Half duplex Cordless phones-full duplex Cellular telephones-full duplex pager Walkie-Talkie Cordless phone Cellular telephone 17

18 Classification of mobile radio transmission system Simplex: communication in only one direction Half-duplex: same radio channel for both transmission and reception (push-to-talk) Full-duplex: simultaneous radio transmission and reception (FDD, TDD) Frequency division duplexing uses two radio channel Forward channel: base station to mobile user Reverse channel: mobile user to base station Time division duplexing shares a single radio channel in time. Forw ard C hannel R everse C hannel 18

19 19

20 DUPLEXING In wireless communication systems,it is often desirable to allow the user to send simultaneously information to the base station while receiving information from the base station. Duplexing is done either using frequency or time domain techniques: Frequency division duplexing (FDD) Time division duplexing (TDD) FDD - is more suitable for radio communication systems, TDD- is more suitable for fixed wireless systems 20

21 Data Rates 2 Mbps Overview 1 Mbps 100 Kbps 10 Kbps 1 Kbps 1G (<1Kbps) 2G (9.6Kbps) 2.5G (10-150Kbps) 3G (144Kbps to 2Mbps) Years 21

22 Cellular networks: From 1G to 5G 22

23 Introduction to Radio Wave Propagation The mobile radio channel places fundamental limitations on the performance of wireless communication systems Paths can vary from simple line-of-sight to ones that are severely obstructed by buildings, mountains, and foliage Radio channels are extremely random and difficult to analyze The speed of motion also impacts how rapidly the signal level fades as a mobile terminals moves about. 23

24 Problems Unique to Wireless systems Interference from other service providers Interference from other users (same network) CCI due to frequency reuse ACI due to Tx/Rx design limitations & large number of users sharing finite BW Shadowing Obstructions to line-of-sight paths cause areas of weak received signal strength 24

25 Problems Unique to Wireless systems Fading When no clear line-of-sight path exists, signals are received that are reflections off obstructions and diffractions around obstructions Multipath signals can be received that interfere with each other Fixed Wireless Channel random & unpredictable must be characterized in a statistical fashion field measurements often needed to characterize radio channel performance 25

26 Mechanisms that affect the radio propagation.. Reflection Diffraction Scattering In urban areas, there is no direct line-of-sight path between: the transmitter and the receiver, and where the presence of high- rise buildings causes severe diffraction loss. Multiple reflections cause multi-path fading 26

27 Reflection, Diffraction, Scattering Reflections arise when the plane waves are incident upon a surface with dimensions that are very large compared to the wavelength Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction occurs according to Huygens's principle when there is an obstruction between the transmitter and receiver antennas, and secondary waves are generated behind the obstructing body Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave. (Waves bending around sharp edges of objects) Scattering occurs when the plane waves are incident upon an object whose dimensions are on the order of a wavelength or less, and causes the energy to be redirected in many directions. Scattering occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal or less 27

28 Ground Reflection (2-Ray) Model a model where the receiving antenna sees a direct path signal as well as a signal reflected off the ground. In a mobile radio channel, a single direct path between the base station and mobile is rarely the only physical path for propagation Hence the free space propagation model in most cases is inaccurate when used alone Hence we use the 2 Ray GRM It considers both- direct path and ground reflected propagation path between transmitter and receiver This was found reasonably accurate for predicting large scale signal strength over distances of several kilometers for mobile radio systems using tall towers ( heights above 50 m ) 28

29 Ground Reflection (2-Ray) Model Good for systems that use tall towers (over 50 m tall) Good for line-of-sight microcell systems in urban environments E TOT is the electric field that results from a combination of a direct line-ofsight path and a ground reflected path The maximum T-R separation distance ( In most mobile communication systems ) is only a few tens of kilometers, and the earth may be assumed to be flat. ETOT =The total received E-field, ELOS=The direct line-of-sight component E g =The ground reflected component, 29

30 Diffraction Occurs when the radio path between sender and receiver is obstructed by an impenetrable body and by a surface with sharp irregularities (edges) The received field strength decreases rapidly as a receiver moves deeper into the obstructed (shadowed) region, the diffraction field still exists and often has sufficient strength to produce a useful signal. Diffraction explains how radio signals can travel urban and rural environments without a line-of-sight path The phenomenon of diffraction can be explained by Huygen's principle, which states that all points on a wave front can be considered as point sources for the production of secondary wavelets, and that these 'wavelets combine to produce a new wave front in the direction of propagation The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacle. 30

31 Scattering The medium which the wave travels consists of objects with dimensions smaller than the wavelength and where the number of obstacles per unit volume is large rough surfaces, small objects, foliage, street signs, lamp posts. Generally difficult to model because the environmental conditions that cause it are complex Modeling position of every street sign is not feasible. 31

32 Illustration.. 32

33 Mobile Radio Propagation Environment The relative importance of these three propagation mechanisms depends on the particular propagation scenario. As a result of the above three mechanisms, macro cellular radio propagation can be roughly characterized by three nearly independent phenomenon; Path loss variation with distance (Large Scale Propagation ) Slow log-normal shadowing (Medium Scale Propagation ) Fast multipath fading. (Small Scale Propagation ) Each of these phenomenon is caused by a different underlying physical principle and each must be accounted for when designing and evaluating the performance of a cellular system. 33

34 Path Loss: Models of "large-scale effects" location 1, free space loss (Line of Sight) is likely to give an accurate estimate of path loss. location 2, a strong line-of-sight is present, but ground reflections can significantly influence path loss. The plane earth loss (2-Ray Model) model appears appropriate. location 3, plane earth loss needs to be corrected for significant diffraction losses, caused by trees cutting into the direct line of sight. location 4, a simple diffraction model is likely to give an accurate estimate of path loss. location 5, loss prediction fairly difficult and unreliable since multiple diffraction is involved 34

35 Radio Propagation Mechanisms Line Of Sight (LOS) Non Line Of Sight (NLOS) 35

36 Line of Sight (LOS) Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. The received signal is directly received at the receiver the effects such as reflection, diffraction and scattering doesn t affect the signal reception that much. Radio signals can travel through many non-metallic objects, radio can be picked up through walls. This is still line-of-sight propagation. Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter. 36

37 Free Space Propagation Model Free space propagation model is used to predict: Received Signal Strength when the transmitter and receiver have a clear, unobstructed LoS between them. The free space propagation model assumes a transmit antenna and a receive antenna to be located in an otherwise empty environment. Neither absorbing obstacles nor reflecting surfaces are considered. In particular, the influence of the earth surface is assumed to be entirely absent. Satellite communication systems and microwave line-of-sight radio links typically undergo free space propagation. 37

38 Free Space Propagation Model Path Loss Signal attenuation as a positive quantity measured in db and defined as the difference (in db) between the effective transmitter power and received power. Friis is an application of the standard Free Space Propagation Model It gives the Median Path Loss in db ( exclusive of Antenna Gains and other losses ) clear, unobstructed line-of-sight path satellite and fixed microwave Friis Transmission Equation (Far field) 38

39 Friis Free Space Equation Pt Transmitted power, Pr(d) Received power Gt Transmitter antenna gain, Gr Receiver antenna gain, d T-R separation distance (m) L System loss factor not related to propagation system losses (antennas, transmission lines between equipment and antennas, atmosphere, etc.) L = 1 for zero loss Signal fades in proportion to d 2 The ideal conditions assumed for this model are almost never achieved in ordinary terrestrial communications, due to obstructions, reflections from buildings, and most importantly reflections from the ground. The Friis free space model is only a valid predictor for P r for values of d which are in the far-field of the Transmitting antenna 39

40 Example (a) If a transmitter produces 50 watts of power, express the transmit power in units of dbm, and dbw. (b) If 50 watts is applied to a unity gain antenna with a 900 MHz carrier frequency, find the received power in dbm at a free space distance of 100 m from the antenna, What is P r (10 km)? Assume unity gain for the receiver antenna. Solution: (a) TX power in dbm = 10 log 10 (Pt/1mW) = 10 log 10 (50/1mW)=47 dbm (b) Tx power in dbw = 10 log 10 (Pt/1W) = 10 log10(50)=17 dbw Rx power = Pr(d) = Pt Gt Gr 2 / (4) 2 d 2 L Wavelength, = , GT=Gr = 1, D=100 m, L=1 Pr(100 m) = x10-06 W = 3.5x10-3 mw =10log (3.5*10-3 ) = dbm Pr(10*1000 m) = 3.5*10-3 /10^4 = 3.5*10-7 mw 40

41 Small Scale Multipath fading Multipath creates small scale fading effects: Rapid changes in signal strength over a small travel distance or time interval Random frequency modulation due to varying Doppler shifts on different multipath signals Time dispersion (echoes) caused by multipath propagation delays Factors influence small scale fading Multipath propagation result in multiple version of transmitted signal Speed of mobile result in random frequency modulation due to different Doppler shifts Speed of surrounding if the surrounding objects move at a greater rate than the mobile The transmission bandwidth of the signal if the transmitted radio signal bandwidth is greater than the bandwidth of the multipath channel 41

42 Doppler Effect Doppler effect occurs when transmitter and receiver have relative velocity away towards 42

43 Resting sound source f s f o Frequency f s V=340m/s Frequency f o source at rest observer at rest 43

44 Sound source moving toward observer f o f s Frequency f s Observer hears increased pitch (shorter wave length) Frequency f o source Observer at rest 44

45 Sound source moving away from observer f o f s Observer hears decreased pitch (longer wave length) Frequency f s Frequency f o source Observer at rest 45

46 Doppler Shift Calculation Δl is small enough to consider v = speed of mobile, λ= carrier wavelength = 1 f d is +/-ve when moving towards/away the wave 2l 2 dcos( ) Phase difference, 1 v Doppler Shift, fd cos( ) 2 t 46

47 Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source. When they are moving toward each other, the frequency of the received signal is higher than the source. When they are opposing each other, the frequency decreases. Doppler Shift in frequency: where v is the moving speed, is the wavelength of carrier. f f R R f f f D C C f f D D v cos Signal MS Moving speed v 47

48 Example Consider a transmitter which radiates a sinusoidal carrier frequency of 1850 MHz. For a vehicle moving 96 km/h, compute the received carrier frequency if the mobile is moving (a) directly towards transmitter (b) Directly away from the transmitter (c) In a direction perpendicular to the direction of arrival of the transmitted signal 48

49 Solution: fc = 1850 MHz λ= c / f λ = m v = 96 km/h= m/s (a) f = fc+ fd = MHz (b) f = fc fd = MHz (c) In this case, θ =90 o, cos θ = 0, And there is no Doppler shift. f = fc (No Doppler frequency) 49

50 Syllabus FUNDAMENTALS OF CELLULAR RADIO SYSTEM DESIGN Concept of frequency reuse, Co-channel interference, co-channel interference reduction factor, Desired C/I from a normal case in a omnidirectional antenna system, system capacity, trunking and grade of service, Improving coverage and capacity in cellular systems- Cell splitting, Sectoring, Microcell zone concept. 2-50

51 The information from sender to receiver is carried over a well defined frequency band. This is called a channel. Frequency Carries/Channels Each channel has a fixed frequency bandwidth (in KHz) and Capacity (bit-rate) Different frequency bands (channels) can be used to transmit information in parallel and independently. Replacing a single, high power transmitter (large cell) with many low power transmitters (small cells). Each providing coverage to only a small portion of the service area. Each base station is allocated a portion of the total number of channels available to the entire system, Nearby base stations are assigned different groups of channels. All the available channels are assigned to a relatively small number of neighboring base stations. 2-51

52 2-52

53 Assume a spectrum of 90KHz is allocated over a base frequency b for communication between stations A and B Assume each channel occupies 30KHz. There are 3 channels Each channel is simplex (Transmission occurs in one way) For full duplex communication: Use two different channels (front and reverse channels) Use time division in a channel Example Station A Channel 1 (b - b+30) Channel 2 (b+30 - b+60) Channel 3 (b+60 - b+90) Station B 2-53

54 2-54

55 Frequencies for communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible UV light VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency Frequency and wave length: = c/f UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light wave length, speed of light c 3x10 8 m/s, frequency f 2-55

56 2-56

57 2-57

58 Coverage Aspect of Next Generation Mobile Communication Systems Satellite In-Building Urban Suburban Global Picocell Microcell Macrocell Global 2-58

59 Cellular Geometries 2-59

60 Cellular Telephone Systems Provide connection to the PSTN for any user location within the radio range of the system. Characteristic Large number of users Large Geographic area Limited frequency spectrum Reuse of the radio frequency by the concept of cell. Basic cellular system: mobile stations, base stations, and mobile switching center. 60

61 FREQUENCY REUSE Each cellular base station is allocated a group of radio channels within a small geographic area called a cell. Neighboring cells are assigned different channel groups. By limiting the coverage area to within the boundary of the cell, the channel groups may be reused to cover different cells. Keep interference levels within tolerable limits. Frequency reuse or frequency planning The design process of selecting and allocating channel groups for all of the cellular base station within a system is FREQUENCY REUSE/PLANNING 2-61

62 Cell Cell is the small geographic area covered by the base station. The area around an antenna where a specific frequency range is used. Cell is represented graphically as a hexagonal shape, but in reality it is irregular in shape. cell 2-62

63 Cell Shape Cell R R R R R (a) Ideal cell (b) Actual cell (c) Different cell models 2-63

64 Cellular Geometries The most common model used for wireless networks is uniform hexagonal shape areas A base station with omni-directional antenna is placed in the middle of the cell d 3R 2-64

65 Fundamentals of Cellular Systems Ideal cell area (2-10 km radius) Cell BS MS Alternative shape of a cell MS Hexagonal cell area used in most models Illustration of a cell with a mobile station and a base station 2-65

66 Why hexagon for theoretical coverage? For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three Thus by using hexagon geometry, the fewest number of cells can cover a geographic region, and hexagon closely approximates a circular radiation pattern which would occur for an omnidirectional BS antenna and free space propagation When using hexagons to model a coverage areas, BS transmitters are depicted as either being in the center of the cell (center-excited cells) or on the three of the six cell vertices (edge-excited cells) Normally omnidirectional antennas are used in center-excited cells and directional antennas are used in corner-excited cells 2-66

67 Signal Strength Signal strength (in db) Cell i Cell j Select cell i on left of boundary Select cell j on right of boundary Ideal boundary 2-67

68 Frequency Reuse An efficient way of managing the radio spectrum is by reusing the same frequency, within the service area, as often as possible This frequency reuse is possible thanks to the propagation properties of radio waves 2-68

69 How Often Are Frequencies Reused (Frequency Reuse Factor)? Cells with the same number have the same set of frequencies Frequency Reuse 2-69

70 2-70

71 Cluster: A cluster is a group of adjacent cells. No frequency reuse is done within a cluster. Number of cells in cluster N=i 2 +ij+j cell cluster 4-cell cluster 7-cell cluster 2-71

72 2-72

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74 2-74

75 Frequency Allocation Concepts Consider a cellular system which has a total of S duplex channels. Each cell is allocated a group of k channels,. k S The S channels are divided among N cells. The total number of available radio channels S kn The N cells which use the complete set of channels is called cluster. The cluster can be repeated M times within the system. The total number of channels, C, is used as a measure of capacity C MkN MS The capacity is directly proportional to the number of replication M. The cluster size, N, is typically equal to 4, 7, or 12. Small N is desirable to maximize capacity. The frequency reuse factor is given by 1/ N 2-75

76 Frequency Reuse The frequency reuse concept can be used in the time domain and the space domain. Frequency reuse in the time domain results in the occupation of the same frequency in different time slots. It is called time-division multiplexing (TDM). Frequency reuse in the space domain can be divided into two categories. 1. Same frequency assigned in two different geographic areas, such as AM or FM radio stations using the same frequency in different cities. 2. Same frequency repeatedly used in a same general area in one system2 the scheme is used in cellular systems. There are many cochannel cells in the system. The total frequency spectrum allocation is divided into K frequency reuse patterns, as illustrated in Fig. for K = 4, 7, 12, and 19. F6 F7 F2 F1 F5 F4 F3 F6 F7 F7 F2 F1 F5 F4 F2 F3 F6 F7 F1 F2 F3 F6 F1 F3 F5 F4 F5 F4 Fx: Set of frequency 7 cell reuse cluster 2-76

77 Hexagonal geometry has exactly six equidistance neighbors the lines joining the centers of any cell and each of its neighbors are separated by multiples of 60 degrees. Only certain cluster sizes and cell layout are possible. The number of cells per cluster, N, can only have values which satisfy N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 28,, etc. The popular value of N being 4 and 7. N i 2 ij where i and j are integers. j 2 j 60 o i 2-77

78 1 2 3 i j=2 j direction 60 i direction (a) Finding the center of an adjacent cluster using integers i and j (direction of i and j can be interchanged). j=1 j=1 i=2 i=2 j=1 i=2 i=2 j=1 j=1 i=2 i=2 j=1 (b) Formation of a cluster for N = 7 with i=2 and j=1 i=3 j=2 i=3 j=2 i=2 i=2 i=2 j=2 j=2 i=2 i=2 i=2 j=2 i=3 j=2 j=2 j=2 j=2 i=3 j=2 i=3 j=2 i=3 (c) A cluster with N =12 with i=2 and j=2 (d) A Cluster with N = 19 cells with i=3 and j=2 2-78

79 Reuse Distance R Cluster For hexagonal cells, the reuse distance is given by F7 F2 D 3NR F6 F1 F5 F4 F3 F6 F7 F2 F1 F3 where R is cell radius and N is the reuse pattern (the cluster size or the number of cells per cluster). Reuse factor is D q 3N R F5 F4 2-79

80 Frequency Reuse For hexagonal cells, the number of cells in the cluster is given by N N I 2 J 2 ( I J ), I, J 1,2,3,4... {1,3,4,7,9,12,16,19,21,...} D R 3N 2-80

81 Smaller N is greater capacity 2-81

82 D R 3N 2-82

83 Geometry of Hexagonal Cells (1) How to determine the DISTANCE between the nearest co-channel cells? Planning for Co-channel cells D is the distance to the center of the nearest co-channel cell R is the radius of a cell D j 3R i 30 o R 0 3R 2-83

84 where D = Distance between the cells using the same frequency, R = Center to vertex distance, N = Cluster size, q = Reuse frequency. 2-84

85 2-85

86 D 2 = 3 * R 2 * (i 2 + j 2 + i * j) As N = i 2 + j 2 + i * j D 2 = 3 * R 2 * N D 2 /R 2 = 3*N D/R = 3 N As q = D/R q = 3N 2-86

87 Co-channel cells for 7-cell reuse 2-87

88 Cochannel Interference Second tier cochannel Base Station First tier cochannel Base Station D 5 D 6 R D 4 D 2 D 1 Mobile Station D 3 Serving Base Station 2-88

89 2-89

90 Cochannel Interference Cochannel interference ratio is given by C I Carrier Interference M C k1 I k where I is co-channel interference and M is the maximum number of co-channel interfering cells. For M = 6, C/I is given by C I M k 1 C D k R g where g is the propagation path loss slope and g = 2~

91 Co-Channel Interference Consider only the first tier of interfering cells, if all interfering base stations are equidistant from the desired base station and if this distance is equal to the distance D between cell centers, then the above equation can be simplified to: (i.e., r=r and assume D i =D and use q=d/r): S I N I r i1 D i R N D I ( D / R) N I q N I 2-91

92 DESIRED C/I FROM A NORMAL CASE IN AN OMNIDIRECTIONAL ANTENNA SYSTEM Assume that all Dk are the same for simplicity, as shown in Fig; then D = Dk, and q = qk, and the value of C/I is based on the required system performance and the specified value of γ is based on the terrain environment. With given values of C/I and γ, the cochannel interference reduction factor q can be determined. Normal cellular practice is to specify C/I to be 18 db or higher based on subjective tests. this acceptance implies that both mobile radio multipath fading and cochannel interference become ineffective at that level. The path-loss slope γ is equal to about 4 in a mobile radio environment 1/4 q = D/R = (6 63.1) = 4.41 The 90th percentile of the total covered area would be achieved by increasing the transmitted power at each cell; increasing the same amount of transmitted power in each cell does not affect the result of Eq. (2.7-4). This is because q is not a function of transmitted power. The computer simulation described in the next section finds the value of q = 4.6, which is very close to Eq. The factor q can be related to the finite set of cells K in a hexagonal-shaped cellular system by 2-92

93 2-93

94 Example: Co-Channel Interference If S/I = 15 db required for satisfactory performance for forward channel performance of a cellular system. a) What is the Frequency Reuse Factor q (assume K=4)? b) Can we use K=3? Assume 6 co-channels all of them (same distance from the mobile), I.e. N=7 2-94

95 Example: Co-Channel Interference a) NI =6 => cluster size N= 7, and when =4 The co-channel reuse ratio is q=d/r=sqrt(3n)=4.583 S I q N I 1 4 (4.583) Or db greater than the minimum required level ACCEPT IT!!! b) N= 7 and =3 S I q N I 1 3 (4.583) Or db less than the minimum required level REJECT IT!!!

96 Example: co-channel Interference We need a larger N (thus q is larger). Use eq. N =i 2 +ij+j 2, for i=j=2 next possible value is N=12. q=d/r=sqrt(3n) =6 and =3 S I q N I 1 3 (6) Or db N=12 can be used for minimum requirement, but it decreases the capacity (we already gave an example: when cluster size is smaller, the capacity is larger). 2-96

97 Worst Case Co-Channel Interference i.e., mobile terminal is located at the cell boundary where it receives the weakest signal from its own cell but is subjected to strong interference from all all the interfering cells. We need to modify our assumption, (we assumed D i =D). The S/I ratio can be expressed as S I S I N I r i1 D i 2( q 1) 2( D R) 4 1 2q R 2D 2( q 1) 4 Used D/R=q and =4. Where q=4.6 for normal seven cell reuse pattern. 2( D R) D D-R R D+R D+R D D-R 2-97

98 For hexagonal geometry with 7-cell cluster, with the mobile unit being at the cell boundary, the signal-to-interference ratio for the worst case can be approximated as ) ( 2) / ( 2) / ( ) 2( D R D R D R D R D R I S 2-98

99 Example: Worst Case Cochannel Interference A cellular system that requires an S/I ratio of 18dB. (a) if cluster size is 7, what is the worst-case S/I? (b) Is a frequency reuse factor of 7 acceptable in terms of co-channel interference? If not, what would be a better choice of frequency reuse ratio? Solution (a) N=7 q = 3N 4.6 S/I = 54.3 or 17.3 db. (b) The value of S/I is below the acceptable level of 18dB. We need to decrease I by increasing N =9. The S/I is or 19.8dB.. If a path loss component of =4, the worst-case signal-to-interference ratio is 2-99

100 Example: Worst Case Cochannel Interference For a conservative estimate if we use the shortest distance (=D-R) then S I 6( q 1 1) (3.6) 28 Or db. REMARK: In real situations, because of imperfect cell site locations and the rolling nature of the terrain configuration, the S/I ratio is often less than 17.3 db. It could be 14dB or lower which can occur in heavy traffic. Thus, the cellular system should be designed around the S/I ratio of the worst case. REMARK: If we consider the worst case for a 7-cell reuse pattern We conclude that a co-channel interference reduction factor of q=4.6 is not enough in an omnidirectional cell system. In an omnidirectional cell system N=9 (q=5.2) or N=12 (q=6.0) the cell reuse pattern would be a better choice. These cell reuse patterns would provide the S/I ratio of db and db, respectively

101 CAPACITY EXPANSION IN CELLULAR SYSTEM Techniques to provide more channels per coverage area is by Cell splitting Cell sectoring Coverage zone approches 2-101

102 CELL SPLITTING Cell splitting increases the capacity of cellular system since it increases the number of times the channel are reused Cell splitting - defining new cells which have smaller radius than orginal cells by installing these smaller cells called MICROCELLS between existing cells Capacity increases due to additional number of channels per unit area Cell splitting is process of subdividing a congested cell into smaller cells each with its own base station(with corresponding reduction in antenna height and tx power) 2-102

103 Cell Splitting Large cell (low density) Small cell (high density) Smaller cell (higher density) Depending on traffic patterns the smaller cells may be activated/deactivated in order to efficiently use cell resources

104 CELL SPLITTING Split congested cell into smaller cells. Preserve frequency reuse plan. Reduce transmission power. Reduce R to R/2 microcell 2-104

105 Transmission power reduction from to Pt1 P t 2 Examining the receiving power at the new and old cell boundary P Pr [at old cell boundary] Pt 1 r[ at new cell boundary] Pt 2 If we take n = 4 (path loss) and set the received power equal to each other P t2 The transmit power must be reduced by 12 db in order to fill in the original coverage area. Problem: if only part of the cells are splited Different cell sizes will exist simultaneously Handoff issues - high speed and low speed traffic can be simultaneously accommodated P t1 16 R n ( R / 2) n 2-105

106 CELL SPLITTING Splitting cells in each CELL Antenna downtiliting Illustration of cell splitting within a 3 km by 3 km square 2-106

107 Sectoring Decrease the co-channel interference and keep the cell radius R unchanged Replacing single omni-directional antenna by several directional antennas Radiating within a specified sector 2-107

108 Cell Sectoring by Antenna Design c 120 o b a b c 120 o a (a). Omni (b). 120 o sector (c). 120 o sector (alternate) c d 90 o a b e d f 60 o c a b (d). 90 o sector (e). 60 o sector 2-108

109 Cell Sectoring by Antenna Design Placing directional transmitters at corners where three adjacent cells meet B C X A 2-109

110 Microcell Zone Concept Antennas are placed at the outer edges of the cell Any channel may be assigned to any zone by the base station Mobile is served by the zone with the strongest signal. Handoff within a cell No channel re-assignment Switch the channel to a different zone site Reduce interference Low power transmitters are employed Microcell Micro cells can be introduced to alleviate capacity problems caused by hotspots. By clever channel assignment, the reuse factor is unchanged. As for cell splitting, there will occur interference problems when macro and micro cells must co-exist. Microcells-Reduced power, Good for city streets, along roads and inside large buildings 2-110

111 Terminology : Cellular traffic Trunking : Trunking is the concept that allows large number of users to use a smaller number of channels(or phone lines, customer service representatives, parking spots, public bathrooms, ) as efficiently as possible. Grade of service (GoS) : A user is allocated a channel on a per call basis. GoS is a measure of the ability of a user to access a trunked system during the busiest hour. It is typically given as the likelihood that a call is blocked (also known as blocking probability mentioned before). Trunking theory : is used to determine the number of channels required to service a certain offered traffic at a specific GoS. Call holding time (H) : the average duration of a call. Request rate (λ) : average number of call requests per-unit time

112 Trunking and Grade of Service Erlangs: One Erlangs represents the amount of traffic density carried by a channel that is completely occupied. Ex: A radio channel that is occupied for 30 minutes during an hour carries 0.5 Erlangs of traffic. Grade of Service (GOS): The likelihood that a call is blocked. Each user generates a traffic intensity of Erlangs given by H: average duration of a call. : average number of call requests per unit time A u A u H For a system containing U users and an unspecified number of channels, the total offered traffic intensity A, is given by A UA u For C channel trunking system, the traffic intensity, A is given as c Ac UAu / C 2-112

113 Traffic flow or intensity A Measured in Erlang, which is defined as the call minute per minute. Offered traffic for a single user is given as λ average number of call request H duration of a call A u = λ H For a system containing U user, total offered traffic A = U A u Exercise : There are 3000 calls per hour in a cell, each lasting an average of 1.76 min. Offered traffic A = (3000/60)(1.76) = 88 Erlangs If the offered traffic exceeds the maximum possible carried traffic, blocking occurs. There are two different strategies to be used. Blocked calls cleared Blocked calls delayed Trunking efficiency : is defined as the carried traffic intensity in Erlangs per channel, which is a value between zero and one. It is a function of the number of channels per cell and the specific GoS parameters

114 Capacity S = no of duplex channels available K = no of channel in one cell N = no of cell/cluster M = no of cluster in a given system C = total no of duplex channel available in a cellular system (capacity) C = M * K * N = M * S Example: For K = 100, N = 7, calculate system capacity for M = 6 and M = 4 i) C = 6 * 100 * 7 = 4200 channels ii) C = 4 * 100 * 7 = 2800 channels 2-114

115 Example 1 If total of 44 MHz of bandwidth is allocated to a particular FDD cellular radio system which uses two 25kHz simplex channels to provide full duplex voice and control channel, calculate the number of channels available per cell, k for (a) 3 cell reuse (b) 7 cell reuse (c) 12 cell reuse 2-115

116 Formula (Cellular Traffic) i) Total number of channel per cell (NC) NC = (Allocated spectrum) / (channel BW x Frequency reuse factor) unit = channel / cell ii) No.of cell in the service area = Total coverage area / area of the cell. Unit = cell Traffic intensity of each cell can be found from table or Erlang B chart. Depend on NC and GOS Traffic capacity = # of cell x traffic intensity /cell ( Erlang ) Iii) Total no.of user (U) = total traffic (A) / traffic per user ( A u ) Iv) number of call that can be made at any time = NC x no.of cell 2-116

117 Example 2 How many users can be supported for 0.5% blocking probability for the following number of trunked channels in a blocked calls cleared system. (a) 1 (b) 5 (c) 10 (d) 20 (e) 100 Assume each user generates 0.1 Erlangs of traffic 2-117

118 Example 3 A city has an area of 3000 sq.km and is covered by a cellular system using 7 cell per cluster. The area of a cell is 100 sq.km. The cellular system is allocated total bandwidth of 40 MHz of spectrum with full duplex channel bandwidth of 200 KHz. For the GOS of 2 % and the offered traffic per user is 0.03 Erlangs, calculate; a) The number of cell in the city b) The number of channels per cell c) Traffic intensity of each cell d) Traffic intensity for the city e) The total number of users that can be served in the city f) The number of mobiles per channel g) Number of call that can be made at any time in the city 2-118

119 Erlang B Trunking GOS 2-119

120 Erlang B 2-120

121 121 Thank you