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- 1. 3.2 FREQUENCY RE USEThe key characteristic of a cellular network is the ability to re-use frequenciesto increase both coverage and capacity. AJAL.A.J Assistant Professor –Dept of ECE, Federal Institute of Science And Technology (FISAT) TM MAIL: ec2reach@gmail.com
- 2. Cellular Concept The limited capacity of the first mobile radio-telephone services was related to the spectrum used…not much sharing and a lot of bandwidth dedicated to a single call. – good coverage – interference: impossible to reuse the same frequency The cellular concept addressed many of the shortcomings of the first mobile telephones – Frequency reuse – Wasted spectrum allocated to a single user In 1968, Bell Labs proposed the cellular telephony concept to the FCC It was approved and then the work began! – FCC allocated spectrum (took away TV UHF channels 70-83) in the 825-845 MHz and 870-890 MHz bands
- 3. Cellular Concept developed by Bell Labs 1960’s-70’s areas divided into cells a system approach, no major technological changes a few hundred meters in some cities, 10s km at country side each served by base station with lower power transmitter each gets portion of total number of channels neighboring cells assigned different groups of channels, interference minimized hexagon geometry cell shape
- 4. What is a Cell?• Cell is the Basic Union in The System – defined as the area where radio coverage is given by one base station.• A cell has one or several frequencies, depending on traffic load. – Fundamental idea: Frequencies are reused, but not in neighboring cells due to interference.
- 5. Cellular Concept Limited number of frequencies => limited channels Single high power antenna => limited number of users Smaller cells => frequency reuse possible => more number of users Base stations (BS): implement space division multiplex – Each BS covers a certain transmission area (cell) – Each BS is allocated a portion of the total number of channels available – Cluster: group of nearby BSs that together use all available channels Mobile stations communicate only via the base station – FDMA, TDMA, CDMA may be used within a cell As demand increases (more channels are needed) – Number of base stations is increased – Transmitter power is decreased correspondingly to avoid interference
- 6. Cellular Concept Cell size: – 100 m in cities to 35 km on the country side (GSM) – even less for higher frequencies – Umbrella cell: large cell that includes several smaller cells • Avoid frequent handoffs for fast moving traffic Cell shape: – Hexagonal is useful for theoretical analysis – Practical footprint (radio coverage area) is amorphous BS placement: – Center-excited cell: BS near center of cell • omni-directional antenna – Edge-excited cell: BSs on three of the six cell vertices • sectored directional antennas
- 7. Cellular Concept Advantages: – higher capacity, higher number of users – less transmission power needed – more robust, decentralized – base station deals with interference, transmission area etc. locally Problems: – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells: co-channel, adjacent-channel Important Issues: – Cell sizing – Frequency reuse planning – Channel allocation strategiesBottom line: Attempt to maximize availability of channels in an area
- 8. • Cells labeled with the same letter use the same group of channels.• Cell Cluster: group of N cells using complete set of available channels• Many base stations, lower power, and shorter towers• Small coverage areas called “cells”• Each cell allocated a % of the total number of available channels• Nearby (adjacent) cells assigned different channel groups – to prevent interference between neighboring base stations and mobile users
- 9. Cell characteristics• Implements space division multiplex: base station covers a certain transmission area (cell)• Mobile stations communicate only via the base station• Advantages of cell structures: – higher capacity, higher number of users – less transmission power needed – more robust, decentralized – base station deals with interference, transmission area etc. locally• Problems: – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells• Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies
- 10. Shape of Cells Square Width d cell has four neighbors at distance d and four at distance 2d Better if all adjacent antennas equidistant Simplifies choosing and switching to new antenna Hexagon Provides equidistant antennas Radius defined as radius of circum-circle Distance from center to vertex equals length of side Distance between centers of cells radius R is 3 R Not always precise hexagons Topographical limitations Local signal propagation conditions Location of antennas
- 11. Cellular Geometries
- 12. Cell Footprint
- 13. Cellular Network Architecture Mobile Public Switching Center Telephone network and Internet Mobile Switching Center Wired network
- 14. Frequency Reuse Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells – 10 to 50 frequencies assigned to each cell – transmission power controlled to limit power at that frequency escaping to adjacent cells – the issue is to determine how many cells must intervene between two cells using the same frequency
- 15. Frequency Reuse each cell allocated a group k channels – a cluster has N cells with unique and disjoint channel groups, N typically 4, 7, 12 total number of duplex channels S = kN Cluster repeated M times in a system Total number of channels that can be used (capacity) – C = MkN = MS Smaller cells higher M higher C + Channel reuse higher capacity + Lower power requirements for mobiles – Additional base stations required – More frequent handoffs – Greater chance of ‘hot spots’
- 16. Frequency planning f3 f3 f3 f2 f2 f1 f1 f1 f3 f3 3 cell cluster f2 f2 f2 f1 f1 f3 f3 f3 f2 f3 f7 f5 f2 f4 f6 f5 7 cell cluster f1 f4 f3 f7 f1 f2 f3 f6 f5 f2 f2 f2 f2f1 f f1 f f1 f h h 3 cell cluster 3 3 3 h 2 h 2 g2 1 h3 g2 1 h3 g2g1 g1 g1 g3 g3 g3 with 3 sector antennas
- 17. Figure Frequency reuse patterns
- 18. Frequency planning• Frequency reuse only with a certain distance between the base stations• Standard model using 7 frequencies: f3 f5 f2 f4 f6 f5 f1 f4 f3 f7 f1 f2
- 19. FREQUENCY REUSE• The allotted frequency spectrum for mobile communication• during the 1 G is in the range of (800- 900 ) Mhz• but ,Using this small bandwidth, thousands of subscribers need to be served over a large area.• To manage this situation, a technique called ‘’frequency reuse ‘’ is adopted.
- 20. FREQUENCY REUSE
- 21. Cellular Networks• Propagation models represent cell as a circular area• Approximate cell coverage with a hexagon - allows easier• analysis• Frequency assignment of F MHz for the system• The multiple access techniques translates F to T traffic channels• Cluster of cells K = group of adjacent cells which use all of the systems frequency assignment
- 22. Frequency reuse conceptALPHABETICALREPRESENTATION
- 23. Wrong implementation
- 24. END Result of Wrong implementation
- 25. Correct implementation
- 26. Freq reuse Advantage Cluster3 Freq reuse: Several cells in coverage area that use same set of frequencies Cochannel cells: Those cellsCluster1 using same freqs Cochannel interference: The mobile in cell A also gets signals from other Co channel cells Cluster2 31
- 27. Cellular Concepts In this case N=19 (i=3, j=2)
- 28. Frequency Reuse Power of base transceiver controlled Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations 10 – 50 frequencies per cell E.g. The pattern consists of N cells K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average
- 29. Characterizing FrequencyReuse distance between centers of cells that use the sameD = minimum band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern Reuse factor Each cell in pattern uses unique band of frequencies Hexagonal cell pattern, following values of N possible N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, … Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … D/R= 3N D/d = N
- 30. Distance R D R 35
- 31. R 3 D 3 6 1 6 1 4 4 7 2 7 2 5 3 5 3 6 1 3 6 1 46 1 4 7 2 4 7 2 57 2 5 5 36
- 32. Example of frequency reuse factor or pattern 1/4The frequency reuse factor is the rate at which the same frequency canbe used in the network. It is 1/K (or K according to some books) whereK is the number of cells which cannot use the same frequencies fortransmission. Common values for the frequency reuse factor are1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12 depending on notation).
- 33. Reuse Ratio : q Assuming hexagonal shape cells of equal size D = q = 3N R where: D: Distance between the centres of cells R: Radius of the cell q: Reuse ratio N: Cluster size 38
- 34. Example For N = 7 and R 5 km D = 3N R D = 3× 7 × 5 D = 4.583 × 5 = 22.91 The minimum distance at which the same frequency can be reused is approximately 4.6 times R, which is in this case 22.91 km 39
- 35. FrequencyReusePatterns
- 36. N=7, 32 cells, R=1.6km, in total 336channels
- 37. So what is FREQUENCY REUSE ?• After allotting the available total bandwidth among a set of cells , the same frequency band will be used in another set of cells .• This kind of reuse can be adopted until the entire area to be covered is exhausted .• Note• Seven cells in a set is the most frequently used configuration.this configuration usually operates with a cell diameter of 1- 3 km range
- 38. Cellular architecture One low power transmitter per cell B Frequency reuse–limited spectrumA Cell splitting to increase capacity Reuse distance: minimum distance between two cells using same channel for satisfactory signal to noise ratio Measured in # of cells in between
- 39. Reuse distance 2 – reuse patternOne frequency can be (re)used in all cells of the same color
- 40. Reuse distance 3 – reuse pattern
- 41. Signal-to-interference ratio S/I or SIR
- 42. Capacity calculation—FDMAn: capacity (number of total users)m: number of cells to cover the areaN: frequency reuse factor (# cells/cluster)B: bandwidth per userW: total available bandwidth (spectrum) mW n= N B 50
- 43. ProblemIn an FDMA system calculate frequency re use factor ? • m= 70 • W= 100 • n=10 • B=100 • N = ? cells/cluster
- 44. Capacity calculation—FDMAIn the previous example,• m=20,• W=25 MHz,• N=4, and• B=30 KHz. m W 20 25000 n= = = 4,166 N B 4 30 52
- 45. Capacity calculation—TDMA• n: capacity (number of total users)• m: number of cells to cover the area• N: frequency reuse factor (# cells/cluster)• B: bandwidth per user• W: total available bandwidth (spectrum)• Nu: number of time slots per carrier mW n= Nu N B 53
- 46. Capacity calculation—TDMA (contd.)Assuming again,• m=20,• W=25 MHz,• N=4,• B=200 KHz,• Nu=4. mW 20 25000 n= Nu = 4 = 2,500 N B 4 200 54
- 47. Capacity of CDMAn: number of usersW: total bandwidthR: data rateSr: signal to noise ratio W n= R × Sr 55
- 48. Capacity per cell (CDMA)Assume:W=1.25MHz=1,250,000 HzR=9600 bpsSr should be larger than 3dB => 2 times W 1250000 n= = = 65 users R × S r 9600 × 2 56
- 49. System architecture• A set of seven cells will be controlled by a BSC base station controller• The terminal station located in every cell will have access to the BSC.• 7 base terminal stations will have one BSC• For every frequency reuse , 1 BSC is required. The elements that determine frequency reuse are the reuse distance and the reuse factor
- 50. Scenario• For 100 BTS we require more than 14 BSC’s• All BSC are connected to a Mobile Switching centre.• Here mobile stations are connected to BTS by wireless means• All Base stations are connected to their respective BSC’sby cables.• In modern communication , fibre optic cables are used to connect BTS
- 51. @ 3G Fig: Advanced cellular mobilecommunication system Architecture
- 52. Cell Planning (1/3)• The K factor and Frequency Re-Use Distance 7 6 2 K = i2 + ij + j2 1 K = 22 + 2*1 + 12 5 3 j K=4+2+1 7 R 2 K=7 6 1 i D 5 3 4 D= √3K * R D = 4.58R Frequency re-use distance is based on the cluster size K The cluster size is specified in terms of the offset of the center of a cluster from the center of the adjacent cluster
- 53. Cell Planning (2/3) 7-cell reuse pattern A1 A3 G1 A2 B1 A1 G3 B3 A3 A2 B1 G2 C1 B2 G1 B3 C3G3 C1 B2 F1 C2 G2 F3 D1 C3 D3 C2 D1 F2 E1 D2 F1 D3 E3F3 D2 E2 F2 E1 E3 Frequency E2 reuse
- 54. Cell Planning (3/3)• Cell sectoring – Directional antennas subdivide cell into 3 or 6 sectors – Might also increase cell capacity by factor of 3 or 6• Cell splitting – Decrease transmission power in base and mobile – Results in more and smaller cells – Reuse frequencies in non-contiguous cell groups – Example: ½ cell radius leads 4 fold capacity increase
- 55. BTS • BTS acts as an interface between the Mobile unit & BSC • BTS connects mobile units by wireless means to the BSC BTS Antenna• BTS has got an antenna usually at an elevation location.• Invariantly , Roof tops,• Small steel towerare used for erecting the BTS antenna.
- 56. Omni directional coverage= coverage in all directions• Compact power supply systems (some even chargeable batteries) are used at the base terminal stations.• BTS is usually located at the centre of the cell area for omni directional coverage. INDIAN ANALOGY
- 57. Bandwidth vs mobility• The wireless interface at BTS and wired topologytopology between BTS- BSC & BSC – MSC necessitates certain translation in protocols.• Upto the stage of BTS , air interface protocols are used. After BTS , the transmission media happen to be wired one , mostly fibre optic communication.• Hence at the BTS or BSC stage , there is a need for the translation of these protocols
- 58. Illustration – of translation requirement• In mobile communication at the mobile user’s unit the speech signal is connected into 13 kbps digitized voice. This conversion is is required to have an air interface with bandwidth efficiency.• But the backbone network through which the voice has to travel in certain applications can provide 64 kbps. Contd…
- 59. Contd…• PCM digitization is adopted in fixed telephone network which make use of these backbone network.• When mobile unit converts the analog information into 13kbps digital information , the BSC converts the same into 64 kbps information.
- 60. Frequency reuse revisited: cluster size and reuse distance Cluster with cluster size N D frequency group A Co-channel cells frequency group B [ 68 ]
- 61. The geometry of a hexagonal cellUnit scale is distance betweenneighboring cell centers.For cell radius To find the distance from the origin, , of point , convert axis: [ 69 ]
- 62. The geometry of a hexagonal cell [continue]– So,– Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the u-axis and then j hexagons along the v-axis.– The distance between co-channel cells in adjacent clusters is– The number of cells in a cluster, N, is hence since i and j can only take integer values.– The frequency reuse factor , Q, is defined by [ 70 ]
- 63. Frequency Reuse Factor Effective reuse of resources can highly enhance the system capacity Frequency reuse factor (FRF) K defines frequency reuse pattern With a smaller frequency reuse factor (FRF), more available bandwidth can be obtained by each cell
- 64. Previous Frequency ReuseSchemes With the usage of FRF-1, the most user terminals (UTs) are afflicted with heavy Inter-cell interference (ICI) Especially near the cell edge The conventional method to figure out this problem is by increasing the FRF mitigate the ICI efficiently but decrease on available bandwidth The most representative approaches improving cell-edge performance while retaining spectrum efficiency Soft Frequency Reuse (SFR) scheme Incremental Frequency Reuse (IFR) scheme
- 65. FREQUENCY REUSE SCHEME Soft Frequency Reuse (SFR) SchemeIncremental Frequency Reuse (IFR) SchemeEnhanced Fractional Frequency Reuse (EFFR)
- 66. Soft frequency Reuse CCU: cell-centre users CEU: cell-edge users
- 67. Soft Frequency Reuse (SFR)Scheme CCU: cell-centre users CEU: cell-edge users
- 68. Limitations of SFR How to define the borderline to divide cell area for CCUs and CEUs is a key issue Generally, there are more CEUs than CCUs in a cell since the outer surface area is much larger than the inner part CEUs have maximum one third of the entire bandwidth to utilize, which results in lower spectrum efficiency More ICI could happen even in a low traffic-load situation, while there are still subchannels in idle and underutilized in the system The resource allocation via the SFR scheme starts always from the first subchannel up
- 69. Incremental Frequency Reuse(IFR) Scheme (1) The only difference between the IFR design and the classical reuse-1 Classical reuse-1: allocate resources always from the first subchannel IFR: start dispensing resources from different points
- 70. Incremental Frequency Reuse(IFR) Scheme (2) IFR scheme can overcome the low spectrum reuse efficiency problem and the more ICI at low loading traffic problem IFRscheme only performs better when just fewer traffic exists in a system When the loading factor is greater than 0.3, it is inferior to the SFR scheme
- 71. Enhanced Fractional FrequencyReuse (EFFR) (1) HYBRIDS Enhanced Fractional Frequency Reuse (EFFR) scheme intends to retain the advantages of the both approaches Concept Define 3 cell types for directly contiguous cells in a cellular system Reserves for each cell-type a part of the whole frequency band named Primary Segment The Primary Segments among different type cells should be orthogonal The Primary Segment of each cell will be further divided into reuse-3 part: cannot be reused by directly neighboring cells reuse-1 part: is at the same time a part of the Secondary Segments belonging to the other two cell-types
- 72. Enhanced Fractional FrequencyReuse (EFFR) (2)
- 73. Power Allocation and SINREstimation Transmission Power Allocation Any cell-type is not allowed to use the reuse-3 subchannels dedicated to the other two cell types The power allotted to the reuse-3 subchannels can be tripled Signal-to-Interference-Ratio (SINR) Estimation A cell acts on the Secondary Segment as a guest, and occupying secondary subchannels is actually reuse the primary subchannels belonging to the directly adjacent cells Reuse on the Secondary Segment should conform to two rules: monitor before use resource reuse based on SINR estimation Each cell listens on every secondary subchannel all the time Before occupation, a cell makes SINR evaluation and chooses resources with best estimation values for reuse If all available secondary resources are either occupied or not good enough to a link, this cell will give up scheduling resources
- 74. Worst Case Interference S/I ~ R-4 /[2(D-R)-4 + 2(D+R)-4 + 2D-4]
- 75. Frequency Reuse @ CDMAIn CDMA reuse patterns are not required.Subscriber in every cell can use the same frequencyat the same time. Subscriber is discriminated fromanother by the assignment of a unique code to everyconversation.In GSM freq. Reuse pattern of 7 is used.
- 76. Problems – Propagation path loss for signal power: quadratic or higher in distance – fixed network needed for the base stations – handover (changing from one cell to another) necessary – interference with other cells:• Co-channel interference: Transmission on same frequency• Adjacent channel interference: Transmission on close frequencies
- 77. Solution: Topology of Different Areas 20 20 20 40 20 100 60 60 60 20 100 100 20 60 100 100 Town 20 20 Suburb Highway 20 Rural
- 78. Q UERRIES ?

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