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<p>Cellular Systems</p> <p>Mobile CommunicationsCellular Systems</p> <p>Wen-Shen WuenTrans. Wireless Technology Laboratory National Chiao Tung University</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>1</p> <p>Outline</p> <p>Cellular Systems</p> <p>Outline</p> <p>1</p> <p>Cellular System Fundamentals Frequency Reuse Interference and System Capacity Trunking and Grade of Services Improving Coverage and Capacity in Cellular Systems Channel Assignment Strategies Handoff Strategies</p> <p>2</p> <p>3</p> <p>4</p> <p>5</p> <p>6</p> <p>7</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>2</p> <p>Cellular System Fundamentals</p> <p>Cellular Systems</p> <p>IntrodcutionEarly mobile radio systems: Cover a large area by using a single, high powered transmitter with an antenna mounted on a tall tower. No frequency reuse, no interference Limited user capacity Cellular concept: Based on power fall off with distance of signal propagation and reuse the same channel frequency at spatially separated locations Sovling problem of spectral congestion and user capacity Replacing a single, high power transmitter (large cell) with many low power transmitters (small cells) Available channels can be reused as many times as necessary so long as the co-channel interference is kept below acceptable levelsVincent W.-S. Wuen Mobile Communications</p> <p>4</p> <p>Cellular System Fundamentals</p> <p>Cellular Systems</p> <p>Cellular System</p> <p>Each cell is assigned to a unique channel set, Cn Adjacent cells: cells assigned to a different channel sets Co-channel cells: cells using the same channel setsVincent W.-S. Wuen Mobile Communications</p> <p>5</p> <p>Cellular System Fundamentals</p> <p>Cellular Systems</p> <p>Tesselating Cell ShapesTo approximate the contours of constant received power around the base station Hexagonal cells:Having largest area for a given distance between the center of a polygon and its farthest perimeter points Approximating a circular radiation pattern for an omnidirectional base station antenna and free space propagation</p> <p>Diamond cells: better approximating contours of constant power in modern urban microcells</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>6</p> <p>Frequency Reuse</p> <p>Cellular Systems</p> <p>Frequency ReuseS: total number of duplex channels available for use k: number of channels assigned to a cell (k &lt; S) N : number of cells sharing the S duplex channels S = kNCluster: a group of N cells use the complete set of available frequencies (1)</p> <p>C : the total number of duplex channels with frequency reuse M : number of replica of a cluster C = MkN = MS(2)</p> <p>Cluster size: N is typically 4, 7 or 12 for hexagonal cell shape. Frequency reuse factor: 1/N For the same cell size at a given area, N M C Vincent W.-S. Wuen Mobile Communications</p> <p>8</p> <p>Frequency Reuse</p> <p>Cellular Systems</p> <p>Various Cluster Sizes for Hexagonal CellsCluster sizes: 4-cell reuse 7-cell reuse 12-cell reuse 19-cell reuse</p> <p>N -cell reuse</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>9</p> <p>Frequency Reuse</p> <p>Cellular Systems</p> <p>Locating Co-Channel Cells in Hexagonal CellsExample: N = 19, i = 3, j = 2</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>10</p> <p>Frequency Reuse</p> <p>Cellular Systems</p> <p>Reuse DistanceThe distance between co-channel (frequency reuse) cells Origin: (0, 0) Nearest co-channel location P : (i, j) Reuse Distance, D</p> <p>D</p> <p>= =</p> <p>3R R 3N</p> <p>i2 + ij + j2 (3)(4)</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>11</p> <p>Frequency Reuse</p> <p>Cellular Systems</p> <p>Number of Cells Per ClusterNumber of cells per cluster, N</p> <p>N</p> <p>= =</p> <p>Acluster 3 3x2 /2 3D2 /2 = = Acell 3 3R2 /2 3 3R2 /2 1 D 3 R2</p> <p>1 3R2 i2 + ij + j2 = 3 R2</p> <p>= i2 + ij + j2</p> <p>(5)</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>12</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Interference</p> <p>Major limiting factor in the performance and major bottleneck in increasing capacity Sources of interference:anothr mobile in the same cell a call in progress in a neighboring cell other base station operating in the same frequency band any noncellular system which leaks energy into the cellular frequency band</p> <p>Interference effects:Cross talk: interference on voice channels Missed and blacked calls: interference on control channels</p> <p>System-generated cellular interferenceCo-channel interference Adjacent channel interference</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>14</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Co-channel InterferenceCannot be combated by simply increasing transmitter power To reduce, co-channel cells must be separated by a minimum distance to provide sufcient isolation</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>15</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Co-channel Interference, contdAssumethe size of each cell is the same base stations transmit the same power</p> <p> co-channel interference ratio is independent of TX power and is a function of the radius of the cell, R, and the distance between centers of nearest co-channel cells, D.Co-channel reuse ratio, Q</p> <p>Q</p> <p>D = R</p> <p>3N</p> <p>(6)</p> <p>Q spatial separation of co-channel cells co-channel interference Q N M C channel capacity , but co-channel interferece </p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>16</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Signal to Interference Ratio, SIR, S/IS = I SNco I i=1 i</p> <p>(7)</p> <p>S: desired signal power from the desired station Ii : the interference power caused by the i-th interfering co-channel cell base station Di : the distance of the i-th interferer from the mobile. Pr = P0 d d0n n Ii Di</p> <p>(8)</p> <p>Assume transmit power of each base station is equal and the path loss exponent is the same, the S of for a mobile at cell I boundary: n 3N S Rn Rn = N = = (9) n co I Nco Dn Nco Di i=1Vincent W.-S. Wuen Mobile Communications</p> <p>17</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Co-channel Interference For N =7</p> <p>Consider rst tier of co-channel cells:</p> <p>S R4 I 2(D R)4 + 2(D + R)4 + 2D4 (10) S 1 I 2(Q 1)4 + 2(Q + 1)4 + 2Q4 (11) where Q = D/R and assume n = 4.</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>18</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Example 1 If signal-to-interference ratio of 15 dB is required for satisfactory forward channel performance of a cellular system, what is the co-channel reuse factor and cluster size that should be used for maximum capacity if the path loss exponent is (a) n=4, (b)n=3? Assume there are six co-channel cells in the rst tier and all of them are at the same distance from the mobile. Solution: (a) Consider 7-cell reuse pattern: Q = D/R = 3N = 4.583, S/I = ( 3N)n /Nco = 4.5834 /6 = 75.3 = 18.66 dB N = 7 can be used. (b) Consider 7-cell reuse pattern: S/I = 4.5833 /6 = 16.04 = 12.05 dB &lt; 15 dB, therefore a larger N should be used. N = 12 D/R = 6, S/I = 63 /6 = 36 = 15.56 dB &gt; 15 dB, therefore N = 12 should be used.</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>19</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Channel Planning of Wireless SystemsTypically 5% of the entire mobile spectrum is devoted to control channels and 95% of the spectrum is dedicated to voice channels. Air interface standards ensure a distinction between voice and control channels and control channels are not allowed to be used as voice channels and vice versa. Different frequency reuse strategy is applied to control channels to ensure greater S/I protection in control channels. For propagation consideration, most practical CDMA systems limits frequency reuse with f 1/f 2 cell planning. CDMA system has a dynamic, time-varying coverage region depending on the instantaneous number of users on the radio channel. breathing cell dynamic control of power levels and thresholds assigned to control channels, voice channels for changing trafc intensityVincent W.-S. Wuen Mobile Communications</p> <p>20</p> <p>Interference and System Capacity</p> <p>Cellular Systems</p> <p>Adjacent Channel Interferenceresults from imperfect receiver lters which allows nearby frequency to leak into the passband. causes near-far effect, a nearby TX captures the receiver of the subscriber. ACI can be minimized through careful ltering and channel assignments.Keeping frequency separation between each channel as large as possible Avoiding the use of adjacent channels in neighboring cell sites</p> <p>For a close-in mobile (MS1) is X times as close to the BS as another mobile (MS2) and has energy leaks to the passband, the S/I at the BS for the weak mobile (MS2) before receiver ltering is approximately</p> <p>S = X n Ifor n = 4 S I</p> <p> 40 dBVincent W.-S. Wuen Mobile Communications</p> <p>21</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Denition of Common Terms in Trunking TheorySet-up Time: The time required to allocated a trunked radio channel to a requesting user. Blocked Call (Lost Call): Call which cannot be completed at time of request, due to congestion. Holding Time: Average duration of a typical call. Denoted by H (in seconds). Trafc Intensity: Measure of channel time utilization, which is the average channel occupancy measured in Erlangs. Load: Trafc intensity across the entire trunked radio system, measured in Erlangs. Grade of Service (GOS): A measure of congestion specied as the probability of a call being blocked (for Erlang B), or the probability of a call being delayed beyond a certain amount of time (for Erlang C). Request Rate: The average number of call requests per unit time. Denoted by second1 .Vincent W.-S. Wuen Mobile Communications</p> <p>23</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Trunking TheoryEach user generates a trafc intensity of Au Erlangs:</p> <p>Au = HThe total offered trafc intensity A for a system containing U users: A = UAu In a C channel trunked system, if the trafc is equally distributed, the trafc i ntensity per channel, Ac :</p> <p>Ac = UAu /CErlang: the amount of trafc intensity carried by a channel that is completely occupied (1 Erlang = 1 call-hour / hour). Busy hour trafc, Ab = call/busy hour mean call holding time.</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>24</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Example 2 Call established at 2 am between a central computer and a data terminal. Assuming a continuous connection and data transferred at 34 kbit/s what is the trafc if the call is terminated at 2:45am? Solution: Trafc=(1 call)(45 min)(1 hour / 60 min) =0.75 Erlangs Example 3 A group of 20 subscribers generate 50 calls with an average holding time of 3 minutes, what is the average trafc per subscriber? Solution: Trafc=(50 calls)(3min)(1 hour/60 min)=2.5 Erlangs 2.5/20=0.125 Erlangs per subscriber.</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>25</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Erlang B: Blocked Calls Cleared</p> <p>p [blocked] =</p> <p>AC C! Ak C k=0 k!</p> <p>= GOS</p> <p>where C : the number of trunked channels offered by a trunked radio system; A: the total offered trafc. Assumptions of Erlang B: There are memoryless arrivals of requests. The probability of a user occupying a channel is exponentially distributed. There are a nite number of channels available in the trunking pool.</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>26</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>GOS of an Erlang B System</p> <p>Trunking efciency: a meaure of the number of users which can be offered a particular GOS with a particular conguration of xed channels.Vincent W.-S. Wuen Mobile Communications</p> <p>27</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Erlang B Chart</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>28</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Erlang C: Blocked Calls DelayedProbability of a call not having immediate access to a channel and being queued:AC C! A 1 C</p> <p>p [delay &gt; 0] =</p> <p>AC + C!</p> <p>C1 Ak k=0 k!</p> <p>= GOS</p> <p>The probability that the delayed call is forced to wait more than t second:</p> <p>p [delay &gt; t]</p> <p>= =</p> <p>p [delay &gt; 0] p [delay &gt; t|delay &gt; 0] (C A)t p [delay &gt; 0] exp H</p> <p>(12)</p> <p>Average delay D for all calls in a queued system</p> <p>D = p [delay &gt; 0]</p> <p>H C A29</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Erlang C Chart</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>30</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Example 4 How many users can be supported for 0.5% blocking probability for the following number of trunked channels in a blocked calls clear system? (a) 1, (b) 5, (c) 10, (d) 20, (e) 100. Assume each user generate 0.1 Erlangs of trafc. Solution: (a) C = 1, Au = 0.1, GOS = 0.005, from the chart, A = 0.005 U = A/Au = 0.005/0.1 = 0.05 users (b) C = 5, Au = 0.1, GOS = 0.005, from the chart, A = 1.13 U = A/Au = 1.13/0.1 11 users (c) C = 10, Au = 0.1, GOS = 0.005, from the chart, A = 3.96 U = A/Au = 3.96/0.1 39 users (d) C = 20, Au = 0.1, GOS = 0.005, from the chart, A = 11.1 U = A/Au = 11.1/0.1 111 users (e) C = 100, Au = 0.1, GOS = 0.005, from the chart, A = 80.9 U = A/Au = 80.9/0.1 809 users</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>31</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Example 5Trunked mobile networks A, B, and C provide cellular services in an urban area with 2 million residents. The (no. of cells, no. channels/cell) for the three providers are (394,19), (98,57) and (49,100). Find the number of users that can be supported at 2% blocking if each user averages two calls/hour at an average call duration of 3 min. Find the percentage market penetration for each provider. Solution: System A: GOS = 0.02, C = 19, Au = H = 2(3/60) = 0.1 Erlangs. For GOS = 0.02 and C = 19 A = 12 Erlangs U = A/Au = 12/0.1 = 120 total number of subscribers is 120 394 = 47289 System B: GOS = 0.02, C = 57, Au = H = 2(3/60) = 0.1 Erlangs. For GOS = 0.02 and C = 57 A = 45 Erlangs U = A/Au = 45/0.1 = 450 total number of subscribers is 450 98 = 44100 System C: GOS = 0.02, C = 100, Au = H = 2(3/60) = 0.1 Erlangs. For GOS = 0.02 and C = 100 A = 88 Erlangs U = A/Au = 88/0.1 = 880 total number of subscribers is 880 49 = 43120 Market penetration: A: 47280/2,000,000=2.36%; B: 44100/2,000,000=2.205%;C: 43120/2,000,000=2.156%</p> <p>Vincent W.-S. Wuen</p> <p>Mobile Communications</p> <p>32</p> <p>Trunking and Grade of Services</p> <p>Cellular Systems</p> <p>Example 6Given a city area: 1300 mile2 , with 7-cell reuse pattern, cell radius=4 miles and frequency spectrum: 40MHz with 60KHz channel bandwidth. Assume GOS=2% for an Erlang B system, if the offered trafc per user is 0.03 Erlangs, compute (a) the no. of cells in the service area (b) the no. of channels per cell (c) trafc intensity of each cell (d) the maximum carried trafc (e) the total no. of users can be served for the GOS (f) the no. of mobiles per unique channel (g) the theoretical maximum no. of users that could be served at one time by the system. Solution: (a) Acell = 1.5 3R2 = 2.5981 42 = 41.57 square mile. Total no. of cells Nc = 1300/41.57 = 31 cells. (b) Total no. of channels per cell C = 40MHz/(60kHz 7) = 95 channe...</p>