Wireless Communications

1. 121

2. Wireless Communications 121

3. Course Material • Wireless Communications: Principles and Practice by T. Rappaport • Mobile Communications: Jochen Schiller • References • Wireless Communications and Networks by W. Stallings • Wireless Communication by Roy Blake. • Internet. 121

4. CELLULAR CONCEPTS 121

5. Single Cell ‘Network’ 121

6. 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 121

7. Replacement of huge single cell by a number of small cells 121

8. 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. 121

9. 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 • may be divided into 6 equilateral triangles • length of base of each triangle = 0.5R (radius) • different groups of channels assigned to base stations 121

10. Mathematical Description of a Cell • Area of a cell is: • Perimeter of a cell = 6R 121

11. 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 meters • deployed mostly in crowded areas, stadiums, shopping malls 121

12. 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 121

13. 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. 121

14. 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. 121

15. 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 121

16. 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 121

17. 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’ 121

18. 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 121

19. Representation Of Cells Through BS 121

20. 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 121

21. 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 121

22. 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 121

23. 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. 121

24. 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 121

25. 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

26. Freq Reuse ( N=7 , i=2 j=1) 121

27. Freq Reuse ( N=19 , i=3 j=2) 121

28. 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 cells = 395 / 7 = 57 Number of simultaneous users per cell = 20 x 57 = 1140 Number of simultaneous users in system = 7 x 1140 = 7980 121

29. Channel Allocation Techniques • To satisfy the user, a channel needs to be available on request. • Reasonable probability of call blockage (GOS) is 2%. • GOS fluctuate with location and time. The goal is to keep a uniform GOS across the system. • Reduction of variations in GOS allow more users – an increase in capacity. • Three types of algorithms for channel allocation: • Fixed channel allocation (FCA) • Channel Borrowing • Dynamic channel allocation (DCA) • Targets to achieve through the different channel allocation techniques. 121

30. 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. 121

31. 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. 121

32. 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. 121

33. 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. 121

34. 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 121