Cellular Concept. Cell structure

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1 Cellular Concept Dr Yousef Dama Faculty of Engineering and Information Technology An-Najah National University Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 1 Cell structure 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: Infrastructure needed 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 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 2 1

2 Cellular - Introduction Solves the problem of Spectral congestion and user capacity by means of frequency reuse. Offers high capacity in a limited spectrum allocation Offers system level approach, using low power transmitters instead of a single not interfere with the nearest location, high power transmitter (large cell) to cover larger area. A portion of the total channels available is allocated to each base station. Neighbouring base stations are assigned different groups channels, in order to minimise interference. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 3 Shape of a Cell We will assume that cellular towers use omni-directional antennas that are installed in a vertical form. Vertically, the radiated power has maximum power on the plain parallel to the earth surface. As the observation point goes above or below this plain, the radiated power drops. In fact, no power is radiated above or below the antenna directly. Assuming that no geographical features or buildings exist around the antenna and that only one cellular tower exist in the region, the radiation pattern of the omnidirectional antenna produces circles with equal power at ground level Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 4 2

3 The power drops as the radius of the circles increases. It may appear that no radiation will reach the region very close to the cellular tower because of the fact that the antenna is installed on top of a tower. But remember that distances away from the antenna are measure in kilometres while the height of the antenna usually around 10m 30m only. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 5 When two towers are close to each other, the transmitted power from each one enters the coverage area of the other. A cellular phone will connect to the cellular tower from which it receives a stronger signal. Assuming that both towers transmit equal power, the curve at which equal power is received from both towers is a straight line as shown below Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 6 3

4 When multiple towers are close to each other, a cell phone will communicate with the tower that provides it with the highest power. The curves that separate different regions belonging to different towers based on the highest power criterion are straight lines Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 7 In practice, cell boarders are never straight lines because of uneven power transmission by different towers, uneven earth surface, the existence of geographical features, trees, or buildings make the cell barriers random in shape. In fact, cell phone companies usually test cell barriers by having some drive around with sophisticated cell phones to determine were cell barriers are located for proper planning. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 8 4

5 Most Efficient Cell Shapes to Cover Large Regions What is the most efficient theoretical cell shape that will allow us to provide full coverage to a large area by stacking the minimum number of cells possible. Ideally, a circle is the best shape because it is the natural reach of electromagnetic waves when they are transmitted from an omni directional antenna. However, it is not possible to stack circles near each other and cover the whole region of interest without leaving some gaps Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 9 For proper cell shapes, let us observe the following points: o Boarders of cells are straight lines and cell shapes are polygons (Polygons are geometric shapes with all edges being straight lines like triangles, rectangles, pentagons, ). o Full coverage of the whole region is necessary without leaving any uncovered spots. o We will assume that all cells have the same shape. o Cells should have some symmetry (cells can be rotated in place at angles less than one complete rotation without affecting cells layout) Equilateral Triangles: Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 10 5

6 Square: Hexagons: (a shape with six equal sides and six equal angles) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 11 Cell Shapes R a = 2R 2 R a = 3 3/2 R 2 /16 Not suitable, (different distance from the cell s Centre to different point in the perimeter) Ideal shape, but has dead zones Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 12 6

7 Most Efficient Cell Shape: By studying the above three shapes (equilateral triangle, square, and a hexagon), we see that the HEXAGON is the most efficient cell shape because: It requires the least number of cells to cover a specific area using hexagons than using triangles or squares. It is the closest to the shape of a circle which is the natural transmission pattern of an omnidirectional antenna. Only small difference from the centre to other point in the perimeter Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 13 Cell Shapes Hexagonal Hexagonal cells are widely used to understand and evaluate system concepts. Is the basic geographic unit of a cellular system BS R : Distance from the centre to any vertex of the Actual cell shape hexagon Real Cell Shape: System planning, terrain and other effects result in cells that are far less regular, even for elevated base station antennas. Base stations location is strongly influenced by the practical problem of finding acceptable sites and may not follow the regular hexagonal grid Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 14 R 7

8 Cell Size Macro cell: 10km Micro cell: 1 km Shopping centres, airports etc. Pico cells: m Inside building Small cells more bandwidth more users more base station complex networks leading to more interference and more hand overs. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 15 Are Cells Sometimes Intentionally Made Non-Hexagonal? Yes. Two of the most spread non hexagonal cell shapes are the, 1) Highway Style Cells: used to cover long stretches of highways in an almost deserted region Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 16 8

9 (2) Manhattan Style Cells: used to cover cities with high building in the shape of rectangles or blocks Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 17 Do Cells in Reality have the Hexagonal Shape? The answer is certainly NO. It is very rare that you see a cell that is close to hexagonal because of many reasons: 1) Geographical features such as mountains and valleys alter the shape of a cell significantly. Even small variations in height around the cellular tower affect the shape of the cell. 2) The inability of a cell phone company to place the cell towers in exactly the desired location due to geographical features or buildings. 3) The inefficiency of insuring hexagonal cells as sometimes the population density within the coverage area may vary making it more efficient to place more towers in regions with high population and less towers in regions with low population. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 18 9

10 Cell Cluster A cluster is a group of cells No channels are reused within a cluster BS7 BS6 BS2 BS1 BS5 BS3 BS4 A 7 cells cluster Cell Frequency (MHz) Power distribution Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University Frequency planning Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements f 4 f 3 f 5 f 1 f 2 f 3 f 6 f 7 f 2 f 4 f 5 f 1 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 20 10

11 Frequency planning II f 3 f 3 f 3 f 2 f 3 f 7 f 2 f 2 f 5 f 2 f 1 f 1 f 1 f 4 f 6 f 5 f 3 f 3 f 1 f 4 f 2 f 2 f 2 f 3 f 7 f 1 f 3 f 1 f 3 f 1 f 3 3 cell cluster with 3 sector antennas f 6 f 2 f 5 f 3 f 2 3 cell cluster Reuse factor = 3 f 2 f 2 f 1 f 3 f 1 f f 1 3 h h 2 h h h 3 h 3 g 1 g 2 g 3 g 1 g 2 g 3 f 2 f 3 g 1 g 2 g 3 7 cell cluster Reuse factor = 7 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 21 Provide additional radio capacity with no additional increase in radio spectrum Cellular concept is a system level idea which calls for replacing a single, high power transmitter with low power small transmitters with each providing coverage to only a small portion of service area Each base station is allocated a portion of total number of channels available to entire system Nearby base station are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations Nearby BS are assigned different groups of channel so that interference between BS is minimized Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 22 11

12 THE CELLULAR CONCEPT Cluster of 7 cells Cells seven groups of channel from A to G footprint of a cell - actual radio coverage omni-directional antenna v.s. directional antenna Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 23 CELLULAR NETWORK segmentation of the area into cells possible radio coverage of the cell cell idealized shape of the cell use of several carrier frequencies not the same frequency in adjoining cells cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc. hexagonal shape of cells is idealized (cells overlap, shapes depend on geography) if a mobile user changes cells handover of the connection to the neighbor cell Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 24 12

13 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 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 25 What do We Gain What do We Loose with Frequency Reuse? Reusing frequencies by dividing the allocated band by a specific integer number of cells and assigning each cell one division and then repeating the assignment over and over Produces a tradeoff between network capacity and reception quality as follows: The higher the number of divisions of the spectrum over cells (higher cell reuse factor), the lower the capacity of the network but the further away cells with similar frequency allocations are located resulting in lower interference. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 26 13

14 The lower the number of divisions of the spectrum over cells (Lower cell reuse factor), the higher the capacity of the network but the closer cells with similar frequency allocations are located resulting in higher interference. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 27 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 28 14

15 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 29 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 30 15

16 How Often Are Frequencies Reused (Frequency Reuse Factor)? The band of frequency allocated for cellular system use can be reused with different CLUSTERS. We mean by cluster here the configuration of cells over which the complete frequency band is divided and this configuration of cells is repeated over and over. The frequency reuse factor is defined as 1 over the number of cells in the cluster of the system. Valid clusters are those that result in 6 cells with the same frequency of a particular cell located at equal distance from it Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 31 1) 1-Cell Frequency Reuse Cluster ( Frequency Reuse Factor = 1) [ Valid]: This means that the whole band of frequency of used in a cell and reused in each of the adjacent cells. Because the same frequency is used in all cells, high interference occurs in this system making it impractical. Note that there are 6 cells with the same frequency band around each cell. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 32 16

17 2) 2-Cell Frequency Reuse Cluster [Invalid] In this cluster, the allocated band is divided into 2 bands and the two sub bands are reused in an alternating fashion somehow. Clearly, only two cells with the same frequency as a particular cell are at equal distance from it Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 33 3) 3-Cell Frequency Reuse Cluster ( Frequency Reuse Factor = 1/3) [ Valid]: Here, the allocated band is divided into 3 bands (possibly with equal bandwidth) and the three sub bands are reused in an alternating fashion. No neighboring cells have the same frequency in this configuration resulting in it being the cluster with the least number of cells that is provides practical frequency reuse. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 34 17

18 4) 4-Cell Frequency Reuse Cluster ( Frequency Reuse Factor = 1/4) [ Valid]: Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 35 6) 5-Cell Frequency Reuse Cluster (Frequency Reuse Fact or = 1/5) [Invalid] This configuration is invalid because as you see in the figure above, a cell does not have 6 co channel cells at equal distances from it Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 36 18

19 What Makes a Cell Frequency Pattern Valid or Invalid? It is not wither you can stack clusters near each other to cover the whole desired coverage area or not. For example, 2 Cell and 5 cell frequency reuse clusters can cover the whole area without gaps. However, if you look at either the 2 or 5, you note that to each cell there are some close co channel cells (not equal to 6) and there are some co channel cells at a farther away distance (also not equal to 6). This makes the interference be dominated by the close co channel cells. So, we are splitting the frequency band into smaller regions in the hope of reducing the interference but we are not necessarily getting the benefits of this. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 37 Possible Cluster Sizes Is any cluster size possible? The answer is NO. The number of cells N per cluster is given by, Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 38 19

20 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 39 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 40 20

21 If we repeat this cluster 6 times around the current cluster, we got the following structure Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 41 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 42 21

22 Distance between Co-Channel Cell Centers Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 43 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 44 22

23 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 45 Distance between a Cell Corner and its Co- Channel Cell Centers Note that this is exactly the same as finding the distance between all corners of a cell and its co channel cell center. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 46 23

24 How to Form a Cluster Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University Select a cell, make the center of the cell as the origin, and form the coordinate Plane. 2. The positive half of the u-axis and the positive half of the v-axis intersect at a 60-degree angle 3. Define the unit distance as the distance of centers of two adjacent cells 4. for each cell center, we can get an ordered pair (u, v) to mark the position 5. Since j=1 6. we can obtain the label L for the cell whose center is at (u, v). 7. Then we compute the labels of all adjacent cells 8. The cells with labels from 0 through N 1 form a cluster of N cells. 9. The cells with the same label can use the same frequency bands Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 48 24

25 Example N = 7, j=1 We can compute label L for any cell using its center s position (u, v). We can compute label L for any cell using its center s position (u, v). u v L mod(x,y) is x - n.*y where n = floor(x./y) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 49 For each cell, we use its L values to label it. The cells with labels 0 through 6 form a cluster of 7 cells Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 50 25

26 Interference and System Capacity There are different types of interference that cellular calls suffer from Two major cellular interference Co-channel interference: interference that results from the transmission of signals at the same frequency of a particular channel transmitted from a nearby co channel cell Adjacent channel interference: interference that results from transmitting a signal with a nearby frequency but from the same tower or another mobile unit in the same cell. Because of the limited capabilities of analog and digital filters, it may be very difficult to remove adjacent channels completely. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 51 Co-channel Interference and System Capacity Frequency reuse - there are several cells that use the same set of frequencies co-channel cells co-channel interference To reduce co-channel interference, co-channel cell must be separated by a minimum distance. When the size of the cell is approximately the same co-channel interference is independent of the transmitted power co-channel interference is a function of R: Radius of the cell D: distance to the center of the nearest co-channel cell Increasing the ratio Q=D/R, the interference is reduced. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 52 26

27 For a hexagonal geometry A small value of Q provides large capacity A large value of Q improves the transmission quality - smaller level of co-channel interference A tradeoff must be made between these two objectives Cluster Size (N) Co-Channel Reuse Ratio (Q) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 53 d 0 P :measued power 0 TX Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 54 27

28 Computation of Co-Channel Interference in Different Configurations Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 55 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 56 28

29 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 57 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 58 29

30 Consider only the first layer of interfering cells S I 0 n ( D / R) 3N 6 i i i 0 0 n Example: AMPS requires that SIR be greater than 18dB N should be at least 6.49 for n=4. Minimum cluster size is 7 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 59 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 S I 2( D R) 4 ( D R / 2) 4 4 R ( D R / 2) 4 ( D R) 4 D 4 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 60 30

31 Notes on Co-Channel Signal to Interference Ratio 1. The radius of the cell has no effect on the SIR because as the radius of the cell increases, the distance of the cell increases, for example, the location of the corners of the cell move away by the same ratio that the co channel cells move away. 2. The path loss exponent n has a profound effect on the SIR. The higher the value of n, the higher the SIR and vice versa. So, the fact that in urban regions the value of n is greater than the free space n of 2 is unexpectedly beneficial. 3. To get an acceptable value of the SIR around 18 db or so, we need a cluster size of at least 7 and path loss exponent of around The distances between one of the corners of the central cell and the interfering co channel towers become simpler to compute when i is equal to zero because of symmetry where each two cells will have the same distance to the corner at which the mobile phone is located. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 61 Adjacent Channel Interference (ACI) Adjacent Channel Interference occurs: Not because of mobile phones or cellular towers in different co channel cells are transmitting signals at the same frequency. but it occurs because of mobile phones in the same cell transmitting signals at different but close (adjacent) frequencies that the cellular tower has difficulty in filtering out the different channels from each other. We generally study co channel interference that occurs in the forward channel received by the mobile unit. Although co channel interference occurs in the reverse channel received by the tower, it is harder to study the co channel interference in the reverse channel because of the difficulty of locating the mobile phones in the different co channel cells. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 62 31

32 1. Al l Mobile Phones are at Equal Distance from the Tower Consider the case where there are 10 mobile phones that are communicating with a particular tower using channels consecutive channel Because all mobile phones are located at equal distances from the tower, the power received at the tower due to each mobile is almost the same. The received signal at the tower may look like the following (assuming all channels are adjacent) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 63 To extract any of the channels (for example the channel of Mobile 5), and reject all other channels by attenuating them by around 20 db, for example, a simple filter can be used in the following form: The resulting signal at the output of the filter will be Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 64 32

33 2. Some Mobile Phones are Very Close to Tower and Some are Very Far Consider now the second case where the same 10 mobile phones that are communicating with a particular tower, but 9 of these phones are located at equal distance and very close from the tower while one of them is located at a very far away point from the tower such that the ratio of power received from the nearby phones to the power received by the far away phone is 60 db. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 65 This situation produces a phenomenon called the near far effect where the powers received at the tower for different channels varies significantly. The received signal at the tower may look like the following (assuming all channels are adjacent): Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 66 33

34 Trying to extract the channel of Mobile 5 (which is the weak signal) using the same filter above with the 20 db passband to stopband attenuation ratio shown below: Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 67 Clearly, the filter failed to extract the channel of the faraway mobile (Mobile 5) because the adjacent channels are much stronger than it. We would need several similar filters (3 more similar filters making a total of 4 filters) to extract the channel of Mobile 5. This results in a complicated filter and therefore a complicated mobile system. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 68 34

35 Solving Adjacent Channel Interference The solution to the problem of ACI is to insure that non of the channels assigned to a specific cell are adjacent. For a cluster size of N = 7, for example, distribute the channels of the system among different cells in the cluster such that the following channels are allocated to the different cells: Cell 1 gets Channels 1, 8, 15, 22, 29, 36, Cell 2 gets Channels 2, 9, 16, 23, 30, 37, Cell 3 gets Channels 3, 10, 17, 24, 31, 38, Cell 4 gets Channels 4, 11, 18, 25, 32, 39, Cell 5 gets Channels 5, 12, 19, 26, 33, 40, Cell 6 gets Channels 6, 13, 20, 27, 34, 41, Cell 1 gets Channels 7, 14, 21, 28, 35, 42, Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 69 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 70 35

36 In this case, although there was a 60 db difference in power between the near channels and far channel, the wide guardband between the channels allowed us to use a simple filter (low order) that has a very high passband to stopband attenuation ratio of 80 db to extract the very weak signal and reject the very strong signals. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 71 Note that even if the transmission of a mobile phone in another cell reaches the tower and causes ACI, because the mobile phone is located in another cell, its power would certainly be limited and would not cause significant ACI. Therefore, to solve the ACI problem, the channels of a cellular system are always distributed such that no adjacent channels are assigned to the same cell Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 72 36

37 Power Control for Reducing Interference Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the reverse channel long battery life increase SIR solve the near-far problem Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 73 Handoff Strategies Handoff (H.O.) is the process of transferring an active call from one cell to another as the mobile unit moves from the first cell to the other cell without disconnecting the call. The amount of received power by the mobile phone or the amount of received power by the tower or both are usually used to determine whither a H.O. is necessary or not. The following points are put into mind: 1. Most systems give higher priority to H.O. over call initiation (it is more annoying to have an active call disconnected than to have a new call blocked), 2. Handoffs must be completed successfully as much as possible as infrequently (as few times) as possible and must be unnoticeable to the user (the user should not feel the H.O) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 74 37

38 To meet these requirements, two power levels are defined: Minimum acceptable signal to maintain the call PMinimum to maintain call : this is the minimum power received by the mobile phone or tower that allows the call to continue. Once the signal drops below this level, it becomes impossible to maintain the active call because the signal is too weak (noise level becomes high relative to signal level). This is a fundamental power level that is usually in the range of 100 dbm to 90 dbm ( mw to mw). Handoff Threshold P Threshold : this power limit is usually selected to be few db s (5 db to 10 db) above the minimum acceptable signal to main the call level. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 75 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 76 38

39 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 77 Speed of mobile unit determines time needed for handoff: o For slow drop in power, slow handoffs is possible, o For fast drop in signal power, fast handoff is required to insure the signal power does not reach the minimum power to maintain a call. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 78 39

40 Dwell Time Dwell time is the time that an active call is maintained within a cell without handing it off to another cell or dropping it. Dwell time is a form of statistics that are very useful for determining the proper time of performing a handoff. Consider for example the case where the dwell time of all calls in a specific cell is 5 minutes (this is just a theoretical assumption). Such information tells us that any call will certainly have to be handed off after 5 minutes. If the dwell time of the cell is a random number between 4 and 5 minutes, this will inform us that no handoffs should take place before 4 minutes and all remaining calls must be handed off once they reach 5 minutes. This is a clear indication of the importance of the dwell time in determining proper handoff times. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 79 The dwell time is determined by: Propagation path Interference Distance to base station (tower) Speed of mobile station Many other time varying factors We usually notice two dwell time situations: 1. For mobile units traveling on a highway These mobile units usually receive good coverage Speeds are usually constant Mobile units usually face no obstacles Mobile units travel over fixed and well defined paths Because of these, the towers covering highways usually have random dwell times that are concentrated around the average dwell time. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 80 40

41 2. For mobile units in dense environments: o Mobile units often move randomly inside the cell o No clear paths for mobile units are defined o Some mobile phones may never leave the coverage area of the cell for very long periods of time (the owner of the phone lives in the cell) For these points, the towers covering dense environments have dwell times that are widely spread around the average dwell time. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 81 Trunking and Grade of Service (GoS) Trunking is the concept that allows large number of users to use a smaller number of channels as efficiently as possible. Trunking is based on statistics. The number of available channels in a trunked system is directly related to the probability of call blocking during peak time In some systems, because of high system demand, calls that cannot be initiated are o Blocked (caller will have to make the call later with not priority at all). Such systems are sometimes called Blocked Calls Cleared systems o Queued (call is placed in a queue for several seconds until a free channel becomes available). Such systems are sometimes called Blocked Calls Queued systems. Trunking and Queuing theories were first studied by a mathematician called Erlang Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 82 41

42 Erlangs: One Erlangs represents the amount of traffic density carried by a channel that is completely occupied. 1 Erlang = 1 call with a duration of 1 hour over a channel every hour = 2 calls with a duration of 0.5 hours over the channel every hour = 30 calls with a duration of 4 minutes over the channel every 2 hours (120 minutes) A channel that carries 2 calls of duration 5 minutes each per hour carries (2*5 min/60 min = 1/6 Erlangs) The grade of service (GOS) is related to the ability of a mobile phone to access the trunked mobile phone system during the busiest hour. To meet a specific GOS, the maximum required capacity of the system must be estimated and the proper number of channels must be allocated for the system GOS is a measure of the congestion of the system which is specified as the probability of a call being blocked (Erlang B system) or the probability of a call being delayed beyond a certain amount of time (Erlang C system). Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 83 Traffic Intensity Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 84 42

43 When offered traffic intensity ( A ) > Maximum capacity of system carrier traffic becomes limited due to limited capacity of the system To study the traffic capacity of a trunked system, we will assume the following three assumptions: A) There are memoryless arrivals of call requests: all users including users who had blocked called may request a channel at any time. Also, because a user has just had a call blocked, does not affect his decision in making another call or the time to make that other call. B) The probability of a user occupying a channel is exponentially distributed. So, longer calls have lower probability C) There are a finite number of channels available in for trunking. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 85 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 86 43

44 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 87 Probability of Call Blocking in an Erlang B System Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 88 44

45 Probability of Call Delay in an Erlang C System Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 89 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 90 45

46 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 91 Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 92 46

47 Cells System Capacity To calculate number of channels: Bsys Ncarr Bcarr where B sys is system bandwidth and B carr is carrier bandwidth. Given network size, Z (in square miles) and cell size, C (in square miles): number of cells in network = Z/C System capacity which is number of available channels (A net ): Z Anet Acell C where A cell is number of channels/cell as given below, with N slot as number of slots per carrier and N being frequency reuse factor. A cell N slot N N Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 93 carr Example a. What frequency reuse ratio should be used for this system? Required S/I = 16 db Path loss exponent of 3.8 Assume 6 co-channel interfering cells b. If there are 400 frequency channels in the system, assigned equally, how many channels per cell would there be? Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 94 47

48 solution a. Use equation for S/I to calculate N 1 n S R N I i i D n 3 N n S i 0 I N Choose N = 7 obeys rule N= i 2 +i*j+j 2, i>=0, j>=0 b. Equal assignment: 400 Nc Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University Advantages Solves the problem of spectral congestion and user capacity. Offer very high capacity in a limited spectrum without major technological changes. Reuse of radio channel in different cells. Enable a fix number of channels to serve an arbitrarily large number of users by reusing the channel throughout the coverage region. Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 96 48

49 Capacity Expansion in Cellular System Cell splitting 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) Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 97 Cell Sectoring Directional antennas Cells served by them are called sectored cells Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 98 49

50 Cell breathing CDM systems: cell size depends on current load Additional traffic appears as noise to other users If the noise level is too high users drop out of cells Mobile communications Lecture Notes, prepared by Dr Yousef Dama, An-Najah National University 99 50

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