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INFO-H-507 Mobile and Wireless Networks Cellular Systems EngineeringCellular Concept •Proposed by Bell Labs in 1971•Geographic Service divided into smaller cells •Neighboring cells do not use same set of frequencies to prevent interference •Often approximate coverage area of a cell by an idealized hexagon •Increase system capacity by frequency reuse Cellular Concept! Proposed by Bell Labs in 1971! Geographic Service divided into smaller “cells”! Neighboring cells do not use same set of frequencies to prevent interference! Often approximate coverage area of a cell by an idealized hexagon ! Increase system capacity by frequency reuse 2Less colours as possible -> the available BW is fixed -> BW of each div is limitedmodular -> extendablethe capacity can be expressed: bps/cell | bps/km^2 | Erlang/cell | Erlang/km^2•Deploy a large number of low-power transmitters(Base Stations) each having a limited coverage area•Reuse the spectrum several times in the area to be covered to increase capacity•issues•Capacity(traffic load)in a cell•One measure=number of communication channels that are available •Performance•Call blocking probability,handoff dropping probability,throughput etc.•Interference•Why not a large radio tower and large service area?•Number of simultaneous users would be very limited(to total number of traffic channels T)•Mobile handset would have greater power requirement •Cellular concept-small cells with frequency reuse•Advantages•Lower power handsets•Increases system capacity with frequency reuse•Drawbacks•Cost of cells•Handoffs between cells must be supported•Need to track user to route incoming call/message/dataT ypes of Interference•TDMA/FDMA based systems•Co-channel interference•Interference from signals transmitted by another cell using the sameradio spectrum•Adjacent channel interference•Interference from signals transmitted in the same cell with overlappingspectral sidelobes•CDMA systems•Interference from within the cell•Interference from outside the cellClustering in TDMA/FDMA•Adjacent cells CANNOT use the same channels•Co-channel interference will be too severe•The available spectrum is divided into chunks(sub-bands)that are distributed among the cells•Cells are grouped into clusters•Each cluster of cells employ the entire available radio spectrum•The spatial allocation of sub-bands has to be done to minimizeinterferenceCellular Concept(cont’)•Let T=total number of duplex channelsK cells=size of cell cluster(typically4,7,9,12,21)N=T/K=number of channels per cell•For a specific geographic area,if clusters are replicated M times, then total number of channels•System capacity=M⇥T•Choice of K determines distance between cells using the samefrequencies termed co-channel cells•K depends on how much interference can be tolerated by mobilestations and path lossCell Design -Reuse Pattern•Example:cell cluster size K =7,frequency reuse factor =1/7;•Assume T =490total channels,N =T /K =70channels per cell9! Example: cell cluster size K = 7, frequency reuse factor = 1/7; ! Assume T = 490 total channels, N = T/K = 70 channels per cell B A E C D G F B A E CD G F BA E C D G FAssume T = 490 total channels,K = 7, N = 70 channels/cellClusters are replicated M=3timesSystem capacity = 3x490 = 1470total channelsHex is used -> approximation of circle -> seamlessly fitGeometry of a hexagonal cell (1)π/6RSource: Ian Groves, Fundamentals of Communications (lecture notes)•Unit scale is the distance between neighboring cell centers •For cell radius R,one has2R cos(⇡/6)=1and R=1/p3.•T ofind the distance r to any point P(u,v),do(u,v)to(x,y) transformation:x=u cos(⇡/6)y=v+u sin(⇡/6)r2=x2+y2=u2cos2(⇡/6)+v2+u2sin2(⇡/6)+2uv sin(⇡/6) which yieldsr=p u2+v2+uv•Hexagonal cells will be grouped in clusters of cells•clusters pave the surface as a repetitive pattern of order6(hexagonalsymmetry)•the coordinates,in the(u,v)system,of any adjacent cell is of the form(i,j)where i,j2Z.•an equivalent cluster radius R c can be derived from the distance D •start from a reference cell and move i hexagons along the u-axis thenj hexagons along the v-axis.g Co-channel cells (continued)distance ihe u directiondistance j alongrectionter size K = i2 2,AuAAAAAAD=r(i,j)=p i2+j2+ijR c=Dp3=p i2+j2+ij p3•The number of cells in a cluster is given by N=Area(cluster)Area(cell)•Single cell area is6times the area of a triangle with equal edges R:Area(cell)=6⇥12✓R d2◆=6⇥12R p3R2!=3p32R2•Following similar reasoning,the cluster area is:Area(cluster)=3p32(R c)2•And,finally,N=R2c R2=i2+j2+ijExamplesMore Examples171324 3 1 4 212 3 4 1 3 1 4 2 6 7 5 1 11111K = 4 (i =2, j =0)K = 7 (i =2, j =1)2 9 8 67 1 310111245 6586798 12453 10 1112491011 K = 12 (i =2, j =2)interference:x6only count the 1^st tierReuse factor•The frequency reuse factor Q isgiven by:Q =D R=p 3Nand can interpreted as the rate at which the same frequency can be used in the network.•NB:reuse factor is sometimesdefined as 1/N directlyPossible Cluster Sizes (N ) and Frequency reuse factor (Qi j N Q 101 1.7321133204 3.464217 4.583309 5.196221263113 6.2454016 6.92832197.5541217.93750258.663327942289.165•In general,SIR =PdesiredP i P interf ,iSIR Calculation 21SITE ASITE BRSSI, dBmC/I Distancerd-60-90 -120signal-interference-ratio•Now consider a mobile at the edge of cell,distance R from transmitter(downlink only)with power P t.•Number offirst-tier(dominant)co-channel cells is i=6(always)•Averagefirst tier co-channel cell is distance D awayThe SIR can approximated as followsSIR[dB]⇡10log✓P t R ↵6P t D ↵◆=10log✓16Q↵◆=10↵log(Q) 7.86:Hex faces D: distance between same cells, z.B 1 to 1 in 1st tier•The SIR is independent of cell size!SIR Calculation (2)•For ↵=4,one has the following values2011 / 201217S /I for different cluster sizesFor ν=4i j N Q S/I 101 1.732 1.742113311.28204 3.46413.78217 4.58318.64309 5.19620.832212623.333113 6.24524.024016 6.92825.8232197.5527.3241217.93728.1950258.6629.73327930.3742289.16530.693 4 712 13195101520 2530cluster size KS r i n d BS r as a function of the cluster size23•trade-off to be found in cellular systems:•if N decreases,the SIR drops and the quality decreases•if N increases,less channels are available per cell (see also “trunkingefficiency”in Erlang chapter)more cells -> better S/I-> less resource / area (density) z.B bps/cell=> tradeoffSIR Calculation (exercises)•Consider a GSM cellular system•typical SIR requirement of 18dB•Suburban propagation environment with ↵=4•solving the SIR for D /R givesDR=(6SIR )1/↵.Remember D /R =p 3N ,which results in N =13(6SINR )2/↵.•a margin of 18dB =10log (x )and x =63.0957.ThereforeN =6.4857•Since N must be an integer,you round up to nearest feasible clustersize N =7.SIR Calculation (3)•T ypical values for GSM system•one cell with a SIR 18dB,the average N is between 7and 9•sometimes N =3with dynamic frequency re-allocation•If the traffic load increases in a given zone•increase the available channels (buy more frequencies)•decrease N while keeping the SIR acceptable•reduce cell size through the value of R (increases the number of basestations and cost of deployment/operation)for GSM, for a communication, a freq is occupied -> more densely a cell is -> more communication is supported<= low speed <= low SIRlarge cell => low density cell => low resource for each cellumbrella cellumbrella cell is a big cell, which covers a group of cell,in case the resource is saturated by one of the covered cell,Cell Sectoring •Use directional antennas toreduce interference•Divide each BS (i.e.,hexagon)in k of sectors•Radio propagation is focused in certain directions•increases the number of cells Nbut also cuts the interferers in k •No new base station has to beset upCell Sectoring! Use directional antennas to reduce interference ! Radio propagation isfocused in certaindirections ! Antenna coverage isrestricted to part of acell called a sector ! By reducinginterference, the cluster size can be reduced (J s is reduced, and so wecan reduce N ) 33Cell Sectoring •For a 3sector antenna:•each sector uses 1/3of the allocated channels •mobile is interfered by 2first-tier base stations instead of 6,i.e.,SIR sectorized =SIR omni +10log 3=SIR omni +4.8[dB]NETWORKS OB_Propa +Cellular eng_ d. 2./0age 6tiq e l stimation précéd n e est simpliste et I affectés par problèmes de propagation ondes directes, réfléchies,….fading)/I égradé par les autres cellules des clusters voisins qui tilisent la m me fréqu nce et nterfèrent ./I amélioré par ifférentes fonctionnalités (e.g. supportées par le GSM)(1)contrôle de puissance des mobiles 2) DTX : “discontinuous transmission”3) Frequency hopping 4) HO and ver)BS3 BS secteurmportant économie en sites (BS)F2,F5F1,F4 F3,F6 3 secteursid d beille éalité•Results for a 18.6[dB]of SIR requirement:•omni antenna:N =7•3-sectors antenna N =4•The capacity increase is 7/4⇡1.75times 21/21the antenna is directional != omni=> 6/3 = 2 interference ONLY drawbacks:1) handover between freqs 2) splitting the freqs => reduce the local cap 3) for a whole cell{f_1 … f_6} <= 7 Erlangs for a 3BS cell{f_1,f_2; f_3,f_4; f_5, f_6} <= 3*3 erlangsresource problem。