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Cellular Network Concepts and Design (Ch-10 Extra )

BY LAIQ AKHTAR. Cellular Network Concepts and Design (Ch-10 Extra ). Single Cell ‘Network’. History of Cellular Networks. Why cellular networks? To address requirement for greater capacity For efficient use of frequency

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Cellular Network Concepts and Design (Ch-10 Extra )

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  1. BY LAIQ AKHTAR Cellular Network Concepts and Design (Ch-10 Extra )

  2. Single Cell ‘Network’

  3. History of Cellular Networks • Why cellular networks? • To address requirement for greater capacity • For efficient use of frequency • To address the poor quality of non cellular mobile networks and increases coverage • replaces a large transmitter with smaller ones in cells • smaller transmitting power • each cell serves a small geographical service area • each cell is assigned a portion of the total frequency

  4. Replacement of huge single cell by a number of small cells

  5. Why Hexagonal Cell Structure • No proper coverage of the area with theoretical circles. • Polygon near to the circle • Hexagon is selected for further technical simplicity.

  6. R Description of a Cell • Approximated to be a hexagonal coverage • best approximation of a circular area • Served by a base station • low powered transceiver • antenna system and it • may be divided into 6 equilateral triangles • length of base of each triangle = 0.5R (radius) • different groups of channels assigned to base stations

  7. Mathematical Description of a Cell • Area of a cell is: • Perimeter of a cell = 6R

  8. Types of Mobile Communication Cells • The size of a cell is dictated by capacity demand • Macro-cell • large, covering a wide area • range of several hundred kilometers (km) to ten km • mostly deployed in rural and sparsely populated areas • Micro-cell • medium cell, coverage area smaller than in macro cells • range of several hundred meters to a couple of metrrs • deployed mostly in crowded areas, stadiums, shopping malls

  9. Types of Mobile Communication Cells Contd. • The size of a cell is dictated by capacity demand • Pico-cell • small, covering a very small area • range of several tens of meters • low power antennas • can be mounted on walls or ceilings • used in densely populated areas, offices, lifts, tunnels etc • Mega-cell -- These cells are formed by LEO and MEO

  10. Capacity Computations • Assume there are N cells, each allocated k different frequency channels. These N cells are said to form a cluster. Total number of channels per cluster is given by • S = k N • Total capacity associated with M clusters: • C = M k N = M S • A cluster may be replicated more times in a given area if the cells are made smaller (note that power needs to be reduced accordingly). • Capacity of cellular system is directly proportional to “M”, number of times a cluster is replicated.

  11. Capacity versus interference for same size cell and power transmission • Decrease N for More Capacity: • If Cluster Size, N is decreased while cell size remains fixed, more clusters are required to cover the area (M increases). Therefore, Capacity increases. • Increase N for Less Interference: • On the other hand, if N is increased (large cluster size) means that co-channels are now farther than before, and hence we have will have less interference. • Value of N is a function of how much interference a mobile or a base station can tolerate. • We should select a smallest possible value of N but keeping S/I in the required limits.

  12. Means of Increasing System Capacity • There are several approaches for increasing cellular system capacity including: • Cell clustering • Sectoring of cells • Cell splitting • Frequency reuse • Reduction of adjacent cell interference and co-channel interference

  13. Cell Clusters • Service areas are normally divided into clusters of cells to facilitate system design and increased capacity • Definition • a group of cells in which each cell is assigned a different frequency • cell clusters may contain any number of cells, but clusters of 3, 4, 5, 7 and 9 cells are very popular in practice

  14. 2 3 7 1 6 4 5 Cell Clusters • A cluster of 7 cells • the pattern of cluster is repeated throughout the network • channels are reused within clusters • cell clusters are used in frequency planning for the network • Coverage area of cluster called a ‘footprint’

  15. 2 2 2 2 2 2 2 3 3 3 3 3 3 3 7 7 7 7 7 7 7 1 1 1 1 1 1 1 6 6 6 6 6 6 6 4 4 4 4 4 4 4 5 5 5 5 5 5 5 Cell Clusters (1) • A network of cell clusters in a densely populated Town

  16. Representation Of Cells Through BS

  17. Frequency Plan • Intelligent allocation of frequencies used • Each base station is allocated a group of channels to be used within its geographical area of coverage called a ‘cell’ • Adjacent cell base stations are assigned completely different channel groups to their neighbors. • base stations antennas designed to provide just the cell coverage, so frequency reuse is possible

  18. Frequency Reuse Concept • Assign to each cluster a group of radio channels to be used within its geographical footprint • ensure this group of frequencies is completely different from that assigned to neighbors of the cells • Therefore this group of frequencies can be reused in a cell cluster ‘far away’ from this one • Cells with the same number have the same sets of frequencies

  19. Frequency Reuse Factor • Definition • When each cell in a cluster of N cells uses one of N frequencies, the frequency reuse factor is 1/N • frequency reuse limits adjacent cell interference because cells using same frequencies are separated far from each other

  20. Factors Affecting Frequency Reuse • Factors affecting frequency reuse include: • Types of antenna used --omni-directional or sectored • placement of base stations -- Center excited or edge excited.

  21. Excitation of Cells • Once a frequency reuse plan is agreed upon overlay the frequency reuse plan on the coverage map and assign frequencies • The location of the base station within the cell is referred to as cell excitation • In hexagonal cells, base stations transmitters are either: • centre-excited, base station is at the centre of the cell or • edge-excited, base station at 3 of the 6 cell vertices

  22. Finding the Nearest Co-Channel After selecting smallest possible value of N we should see that N should follow the following eq. N= i2+j2+ij (1) Move i cells along any chain of hexagons (2) Turn 600 counter-clockwise and move j cells, to reach the next cell using same frequency sets • this distance D is required for a given frequency reuse to provide enough reduced same channel interference • ie, after every distance D we could reuse a set of frequencies in a new cell

  23. Freq Reuse ( N=7 , i=2 j=1)

  24. Freq Reuse ( N=19 , i=3 j=2)

  25. How frequency Reuse Increases Capacity • Example: A GSM communication system uses a frequency reuse factor of 1/7 and 416 channels available. If 21 channels are allocated as control channels, compute its system capacity. Assume a channel supports 20 users • Channels available for allocation = 416 - 21 = 395 Number of channels per cell = 395 / 7 = 57 Number of simultaneous users per cell = 20 x 57 = 1140 Number of simultaneous users in system = 7 x 1140 = 7980

  26. Fixed Channel Allocation Techniques • Available spectrum is W Hz and each channel is B Hz. Total number of channels: Nc = W/B • For a cluster size N, the number of channels per cell: Cc = Nc/N • To minimize interference, assign adjacent channels to different cells.

  27. Features of Fixed Channel Allocation Techniques • FCA is the optimum allocation strategy for uniform traffic across the cells. • A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.

  28. Channel Borrowing • Borrow frequencies from low traffic cells to high traffic cells. • Temporary channel borrowing: channel is returned after call is completed. • If channels from cell E are borrowed by cell A, then neighboring cells E cannot use those channels.

  29. Dynamic Channel Allocation • All channels are placed in a pool, and are assigned to new calls according to the reuse pattern. Signal is returned to the pool, when call is completed. • Issues related to channel allocation are still under research.

  30. Comparison of Channel Allocation Techniques Fixed Channel Allocation • Advantages: --- Less load on MSC --- Simple • Disadvantages: Blocking may happen Dynamic Channel Allocation • Advantages: Voice channels are not allocated permanently. That is, resource is shared on need-basis • Disadvantages: --- Requires MSC for processing---burden on MSC --- May be very complicated

  31. Handoff • Do we need hand off in old mobile telephony and why. • No because of single huge cell covering the whole service area. • What is handoff. • When a mobile moves into a different cell while a conversation is in progress,the MSC automatically transfers the call to a new channel belonging to the new base station • What are important considerations to design a handoff process. • Handoff must be performed successfully. • Handoff must occur infrequently. • Handoff should be smooth and the users must not be able to detect it.

  32. Handoff Contd. • When to handoff. • Signal level drop is not due to momentary fading and mobile is actually moving away from the BS. • For this the BS monitors the signal level for a certain period of time before a handoff is initiated.This period of time varies with the speed of MS. • Perfect relationship is required between speed of signal drop and required handoff time.

  33. Handoff Contd. • Who detects the need for handoff. • Network Initiated Handoff: • Signal measurements by BS & supervised by MSC. • BS monitors its all RVCs to determine the relative location of each mobile user with respect to the BS. • Mostly implemented in 1G systems. • This type of handoff takes almost 10 sec as in AMPS. • Two separate receivers on BS • One is used to measure RSSI(Radio Signal Strength Indication) of calls in progress with in the cell. • Second is used to scan and determine signal strengths of mobile users which are in neighboring cells. • Both the signals are monitored by the MSC which decides when to handoff.

  34. Handoff Contd • Who detects the need for handoff. 2) Mobile Assisted Handoff( MAHO) • Every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. • A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base station (serving base station) by a certain level or for a certain period of time. • This handoff technique has been implemented in most of the 2G systems. • This type of handoff takes almost 1- 2sec as in GSM. • Preferred for microcellular system

  35. Handoff Contd. • What are the types of handoff. • Hard Handoff • Soft Handoff • What are different levels of handoff. • (1) Intra Cell (2) Inter cell (3) Inter system • Importance of handoff. • When no priority to handoff call blocking would be equal for call initiation and call handoff.

  36. Priority To Handoff • There are two strategies to give a priority to handoff. • (1) Guard Channel: 100% guaranty for successful handoff but It will cause low trunking efficiency. • (2) Queuing Of Handoff Request: There can be unsuccessful handoffs due to long delay in queue. --“Probability of forced termination” decreases at the cost of reduced Total Carried Traffic. -- Queuing is possible because of the time available between the Threshold power level and the Hand off power level.

  37. Practical Handoff Problems • (1)Problem Caused by high speed mobility: • More handoffs are required to handle high speed mobility of MS during a call. It will cause load on the system as well as call drops. • (2) Problem Caused by low speed mobility: Cell dragging: • In the line of sight and smooth area signal does not drop sharply for pedestrian users so user goes on using the frequency of the previous cell in to the new cell. This causes increase in the co-channel interference.

  38. Solution For More Handoffs • Umbrella Cell Approach: • Micro cells inside A macro cell. ---- Macro cell is defined by high power and lengthy tower. ---- Micro cells are defined inside the macro cell with less power and less height towers. ---- High speed MS are handled by macro cell and low speed subscribers are handled by micro cells. ---- This strategy increases the no of capacity channels per unit area and decreases the no of handoffs.

  39. Umbrella Cell Approach

  40. Solution For Cell Dragging • Handoff threshold ----and radio coverage parameters must be adjusted carefully according to the environment .

  41. Assignment No.1 • There are 50 cells in the city. We have only 36 carrier frequencies . • Perform freq planning using N=4/12 and N= 4/12 + 8 + 8+8 Discuss and compare the capacity and interference in both the cases. • Given : Freq. hopping is allowed in for TCH but not for BCCH. Note: If you are supposing some thing else mention it. Due Date: Before 1/2/05

  42. INTERFERENCE AND SYSTEM CAPACITY

  43. Interference • It is a major limiting factor in the performance of cellular radio systems. (In comparison with wired comm. Systems, the amount and sources of interferences in Wireless Systems are greater.) • Creates bottleneck in increasing capacity • Sources of interference are: 1. Mobile Stations 2. Neighboring Cells 3. The same frequency cells 4. Non-cellular signals in the same spectrum • Interference in Voice Channels: Cross-Talk • Urban areas usually have more interference, because of: a)Greater RF Noise Floor, b) More Number of Mobiles

  44. Major Types Of Interference • Co-Channel Interference (CCI) • Adjacent Channel Interference (ACI) • Other services: like a competitor cellular service in the same area • The cells that use the same set of frequencies are called co-channel cells. • The interference between signals from these cells is called Co-Channel Interference (CCI). • Cannot be controlled by increasing RF power. Rather, this will increase CCI. • Depends on minimum distance between co-channel cells. 1) Co-Channel Interference and System Capacity

  45. The yellow cells use the same set of frequency channels, and hence, interfere with each other. In case of N=7, there are 6 first-layer co-channels. • In constant cell size and RF power, CCI is a function of Distance between the co-channel cells(D), and the size of each cell (R). • Increasing ratio D/R, CCI decreases. • Define Channel Reuse Ratio = Q = D/R

  46. Signal-to-interference ratio • S is the power of the signal of interest and Ik is the power of kth interference. • The signal strength at distance d from a source is • That is, received signal power is inversely related to nth power of the distance. • where n = path loss exponent

  47. R D • For hexagonal geometry, D/R can be calculated: • Smaller Q provides larger capacity, since that would mean smaller N. (Capacity 1/N). • Larger Q improves quality, owing to less CCI. • for N=3, Q=3, N=7, Q=4.58, N=12, Q=6, N=13, Q=6.24

  48. Then we can express the SIR in terms of distance • where the denominator represents the users in neighboring clusters using the same channel. • Let D k=D be the distance between cell centers. Then • Note how C/I improves with the frequency reuse N. • Analog systems: U.S. AMPS required C/I ~= 18dB For n = 4, the reuse factor for AMPS is N  6.49, so N =7. • Now, let us consider the worst case for a cluster size of N= 7. The mobile is at the edge of the cell. Express C/I as a function of actual distances.

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