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Cellular Communications

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									                                                 Cellular Systems




                   Mobile Communications
                                 Cellular Systems


                                 Wen-Shen Wuen

                        Trans. Wireless Technology Laboratory
                           National Chiao Tung University




                           Vincent W.-S. Wuen     Mobile Communications   1




                                       Outline   Cellular Systems



Outline


  1   Cellular System Fundamentals

  2   Frequency Reuse

  3   Interference and System Capacity

  4   Trunking and Grade of Services

  5   Improving Coverage and Capacity in Cellular Systems

  6   Channel Assignment Strategies

  7   Handoff Strategies




                           Vincent W.-S. Wuen     Mobile Communications   2
                      Cellular System Fundamentals   Cellular Systems



Introdcution

  Early 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
      levels

                              Vincent W.-S. Wuen      Mobile Communications   4




                      Cellular System Fundamentals   Cellular Systems



Cellular System




      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 sets

                              Vincent W.-S. Wuen      Mobile Communications   5
                   Cellular System Fundamentals    Cellular Systems



Tesselating Cell Shapes

     To 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
     Diamond cells: better approximating contours of constant
     power in modern urban microcells




                           Vincent W.-S. Wuen       Mobile Communications         6




                               Frequency Reuse     Cellular Systems



Frequency Reuse

     S: total number of duplex channels available for use
     k: number of channels assigned to a cell (k < S)
     N : number of cells sharing the S duplex channels

                                                  S = kN                    (1)

     Cluster: a group of N cells use the complete set of available
     frequencies
     C : the total number of duplex channels with frequency reuse
     M : number of replica of a cluster

                                        C = MkN = MS                        (2)

     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         8
                             Frequency Reuse   Cellular Systems



Various Cluster Sizes for Hexagonal Cells
  Cluster sizes:
      4-cell reuse
      7-cell reuse
      12-cell reuse
      19-cell reuse
      N -cell reuse




                          Vincent W.-S. Wuen    Mobile Communications    9




                             Frequency Reuse   Cellular Systems



Locating Co-Channel Cells in Hexagonal Cells
  Example: N = 19, i = 3, j = 2




                          Vincent W.-S. Wuen    Mobile Communications   10
                                Frequency Reuse   Cellular Systems



Reuse Distance


  The distance between co-channel (frequency reuse) cells

    Origin: (0, 0)
    Nearest co-channel location
    P : (i, j)
    Reuse Distance, D

        D   =        3R   i2 + ij + j2 (3)
            =    R 3N                    (4)




                             Vincent W.-S. Wuen    Mobile Communications                          11




                                Frequency Reuse   Cellular Systems



Number of Cells Per Cluster
      Number of cells per cluster, N
                            Acluster 3 3x2 /2    3D2 /2
                 N    =             =         =
                             Acell    3 3R2 /2 3 3R2 /2
                            1 D      2
                                           1 3R2 i2 + ij + j2
                      =                  =                                 = i2 + ij + j2   (5)
                            3 R            3       R2




                             Vincent W.-S. Wuen    Mobile Communications                          12
                Interference and System Capacity   Cellular Systems



Interference


     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
     Interference effects:
         Cross talk: interference on voice channels
         Missed and blacked calls: interference on control channels
     System-generated cellular interference
         Co-channel interference
         Adjacent channel interference



                            Vincent W.-S. Wuen      Mobile Communications   14




                Interference and System Capacity   Cellular Systems



Co-channel Interference

     Cannot be combated by simply increasing transmitter power
     To reduce, co-channel cells must be separated by a minimum
     distance to provide sufficient isolation




                            Vincent W.-S. Wuen      Mobile Communications   15
                  Interference and System Capacity   Cellular Systems



Co-channel Interference, cont’d

      Assume
           the size of each cell is the same
           base stations transmit the same power
      ⇒ 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

                                                     D
                                            Q          =      3N              (6)
                                                     R
      Q ↑⇒ spatial separation of co-channel cells ↑⇒ co-channel
      interference ↓
      Q ↓⇒ N ↓⇒ M ↑⇒ C ↑ channel capacity ↑, but co-channel
      interferece ↑


                              Vincent W.-S. Wuen      Mobile Communications         16




                  Interference and System Capacity   Cellular Systems



Signal to Interference Ratio, SIR, S/I

                                          S           S
                                            =        Nco
                                                                              (7)
                                          I              I
                                                     i=1 i
  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.
                                                     −n
                                              d                   −n
                               Pr = P0                    ∴ Ii ∝ Di           (8)
                                              d0


      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
                     S       R−n        R−n       3N
                       = N           =         =                    (9)
                     I        co  −n
                                 Di    Nco D−n   Nco
                            i=1

                              Vincent W.-S. Wuen      Mobile Communications         17
                  Interference and System Capacity   Cellular Systems



Co-channel Interference For N =7



    Consider first tier of
    co-channel cells:
 S               R−4
   ≈
 I 2(D − R)−4 + 2(D + R)−4 + 2D−4
                               (10)
 S                1
   ≈
 I 2(Q − 1)−4 + 2(Q + 1)−4 + 2Q−4
                               (11)
where Q = D/R and assume n = 4.




                              Vincent W.-S. Wuen      Mobile Communications   18




                  Interference and System Capacity   Cellular Systems




  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 first 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
  < 15 dB, therefore a larger N should be used.
  N = 12 ⇒ D/R = 6, S/I = 63 /6 = 36 = 15.56 dB > 15 dB, therefore N = 12
  should be used.




                              Vincent W.-S. Wuen      Mobile Communications   19
                   Interference and System Capacity   Cellular Systems



Channel Planning of Wireless Systems

     Typically 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 traffic intensity

                               Vincent W.-S. Wuen      Mobile Communications   20




                   Interference and System Capacity   Cellular Systems



Adjacent Channel Interference

     results from imperfect receiver filters 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 filtering and channel
     assignments.
         Keeping frequency separation between each channel as large as
         possible
         Avoiding the use of adjacent channels in neighboring cell sites
     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
     filtering is approximately
                                                    S
                                                      = X −n
                                                    I
                   S
     for n = 4 ⇒   I   ≈ −40 dB
                               Vincent W.-S. Wuen      Mobile Communications   21
                  Trunking and Grade of Services   Cellular Systems



Definition of Common Terms in Trunking Theory

     Set-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).
     Traffic Intensity: Measure of channel time utilization, which is
     the average channel occupancy measured in Erlangs.
     Load: Traffic intensity across the entire trunked radio system,
     measured in Erlangs.
     Grade of Service (GOS): A measure of congestion specified 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 λ second−1 .
                           Vincent W.-S. Wuen       Mobile Communications   23




                  Trunking and Grade of Services   Cellular Systems



Trunking Theory

     Each user generates a traffic intensity of Au Erlangs:

                                                Au = λH

     The total offered traffic intensity A for a system containing U
     users:
                                  A = UAu

     In a C channel trunked system, if the traffic is equally
     distributed, the traffic i ntensity per channel, Ac :

                                            Ac = UAu /C

     Erlang: the amount of traffic intensity carried by a channel that
     is completely occupied (1 Erlang = 1 call-hour / hour).
     Busy hour traffic, Ab = call/busy hour × mean call holding time.


                           Vincent W.-S. Wuen       Mobile Communications   24
                   Trunking and Grade of Services   Cellular Systems




  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 traffic if the call is terminated at 2:45am?
  Solution:
  Traffic=(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 traffic per subscriber?
  Solution:
  Traffic=(50 calls)×(3min)×(1 hour/60 min)=2.5 Erlangs
  2.5/20=0.125 Erlangs per subscriber.




                            Vincent W.-S. Wuen       Mobile Communications   25




                   Trunking and Grade of Services   Cellular Systems



Erlang B: Blocked Calls Cleared



                                                     AC
                                                      C!
                         p [blocked] =                         = GOS
                                                    C    Ak
                                                    k=0 k!
  where C : the number of trunked channels offered by a trunked radio
  system; A: the total offered traffic.
  Assumptions of Erlang B:
      There are memoryless arrivals of requests.
      The probability of a user occupying a channel is exponentially
      distributed.
      There are a finite number of channels available in the trunking
      pool.




                            Vincent W.-S. Wuen       Mobile Communications   26
                  Trunking and Grade of Services   Cellular Systems



GOS of an Erlang B System




  Trunking efficiency: a meaure of the number of users which can be
  offered a particular GOS with a particular configuration of fixed
  channels.
                           Vincent W.-S. Wuen       Mobile Communications   27




                  Trunking and Grade of Services   Cellular Systems



Erlang B Chart




                           Vincent W.-S. Wuen       Mobile Communications   28
                  Trunking and Grade of Services   Cellular Systems



Erlang C: Blocked Calls Delayed

     Probability of a call not having immediate access to a channel
     and being queued:

                                                       AC
                                                       C!
                p [delay > 0] =                                                = GOS
                                                         A            C−1 Ak
                                         AC + C!      1− C            k=0 k!

     The probability that the delayed call is forced to wait more than
     t second:

            p [delay > t]       =      p [delay > 0] p [delay > t|delay > 0]
                                                            (C − A)t
                                =      p [delay > 0] exp −                             (12)
                                                               H

     Average delay D for all calls in a queued system

                                                                H
                                  D = p [delay > 0]
                                                               C −A

                           Vincent W.-S. Wuen       Mobile Communications                     29




                  Trunking and Grade of Services   Cellular Systems



Erlang C Chart




                           Vincent W.-S. Wuen       Mobile Communications                     30
                    Trunking and Grade of Services   Cellular Systems




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 traffic.
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



                             Vincent W.-S. Wuen       Mobile Communications      31




                    Trunking and Grade of Services   Cellular Systems




Example 5
Trunked 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 = λH = 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 = λH = 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 = λH = 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%


                             Vincent W.-S. Wuen       Mobile Communications      32
                   Trunking and Grade of Services   Cellular Systems



Example 6
Given 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 traffic per user is 0.03
Erlangs, compute (a) the no. of cells in the service area (b) the no. of
channels per cell (c) traffic intensity of each cell (d) the maximum carried
traffic (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 channels/cell.
(c) C = 95, GOS = 0.02 ⇒ traffic intensity per cell A = 84 Erlangs/cell.
(d) Maximum carried traffic=no. of cells × traffic intensity per cell =
31 × 84 = 2604 Erlangs.
(e) Traffic/user=0.03 Erlangs ⇒ Total no. of users = 2604/0.03=86800 users
(f) no. of mobiles per channel= no. of users/no. of channels =86800/(40
MHz/60 kHz)=130 mobiles/channel.
(e) The theoretical maximum no. of served mobiles (all channels are
occupied)= C × Nc = 95 × 31 = 2945 users
                            Vincent W.-S. Wuen       Mobile Communications       33




                   Trunking and Grade of Services   Cellular Systems




Example 7
A hexagonal cell within a four-cell system has a radius of 1.387 km. A total
of 60 channels are used within the entire system. If the load per user is
0.029 Erlangs and λ = 1 call/hour, compute the following for an Erlang C
system which has a 5% probability of delayed call: (a) how many user per
square kilometer will the system support? (b) the probability that a delayed
call will have to wait for more than 10 seconds? (c) the probability that a
call will be delayed for more than 10 seconds?
Solution:
Cell area=2.598 × (1.387)2 = 5km2 . no. of channel per cell C = 60/4 = 15
channels.
(a) For Erlang C of 5% probability of delay with C = 15, the traffic
intensity=9.0 Erlangs.
no. of users=total traffic intensity/traffic per user = 9/0.029=310 users for
5 km2 or 62 users/km2
(b) H = Au /λ = 0.029hour = 104.4 second.
p[delay > 10|delay] = exp (−(C − A)t/H) = exp(−(15 − 9)10/104.4) = 56.29% (c)
p[delay > 0] = 5% = 0.05
p[delay > 10] = p[delay > 0]p[delay > 10|delay] = 0.05 × 0.5629 = 2.81%


                            Vincent W.-S. Wuen       Mobile Communications       34
                       Improving Coverage and Capacity   Cellular Systems



Cell Splitting



      Let R ↓ and keeps D/R
      unchanged
 Pr [at old cell boundary] ∝ Pt1 R−n

 Pr [at new cell boundary] ∝ Pt2 (R/2)−n

 for n = 4
                       Pt1
               Pt2 =
                       16




                                  Vincent W.-S. Wuen      Mobile Communications   36




                       Improving Coverage and Capacity   Cellular Systems



Cell Splitting


 Example 8
 Assume each BS uses 60
 channels and large cell radius of 1
 km and microcell radius of 0.5
 km. Find the number of channels
 in a 3 km by 3 km square around
 A when (a) without the use of
 microcells (b) the labeled
 microcells are used (c) all original
 BS are replaced by microcells.
 Solution:
 (a) 5 × 60 = 300 (b) (5 + 6) × 60 = 660
 (2.2x) (c) (5 + 12) × 60 = 1020 (3.4x)




                                  Vincent W.-S. Wuen      Mobile Communications   37
               Improving Coverage and Capacity   Cellular Systems



Sectoring


     Increasing S/I ratio, keeping cell radius R the same and
     decreasing D/R ⇒ D ↓⇒ N ↓⇒ frequency reuse ↑ ⇒ cluster size
     N can be reduced because of S/I is improved.




                          Vincent W.-S. Wuen      Mobile Communications   38




               Improving Coverage and Capacity   Cellular Systems



Sectoring, cont’d




                          Vincent W.-S. Wuen      Mobile Communications   39
             Improving Coverage and Capacity   Cellular Systems



Microcell Zone




                        Vincent W.-S. Wuen      Mobile Communications   40




             Improving Coverage and Capacity   Cellular Systems



Microcell Zone




                        Vincent W.-S. Wuen      Mobile Communications   41
                   Channel Assignment Strategies   Cellular Systems



Channel Assignment Strategies

     Fixed channel assignment
          each cell is allocated to a predetermined set of voice channels ⇒
          the call is blocked is all the channels are occupied.
          borrowing strategy: a cell is allowed to borrow channels from a
          neighboring cell if all of its own channels are occupied.
          MSC supervises the borrowing procedure to ensure no disrupting
          calls or interference with any of the calls in progress in the donor
          cell.
     Dynamic channel assignment
          the serving BS request a channel from MSC whenever a call
          request is made.
          following an algorithm considering the likelihood of future
          blocking in the cell, the frequency of use of the candidate cell, the
          reuse distance of the channel and other cost functions.
          MSC needs to collect real-time data on channel occupancy, traffic
          distribution, and radio signal strength indicator (RSSI) of all
          channels on a continuous basis. ⇒ increasing storage and
          computational load on the system.

                            Vincent W.-S. Wuen      Mobile Communications         43




                              Handoff Strategies   Cellular Systems



Handoff


     When a mobile moves into a different cell when a conversation
     is in progress, the MSC automatically transfer the call to a new
     channel belonging to a new BS.
     Many handoff strategy prioritize handoff requests over call
     initiation requests when allocating an unused channel.
     Handoff threshold: a signal level slightly stronger than the
     minimum usable signal for acceptable voice quality.

                               ∆ = Pr,handoff − Pr,min.usable

     ∆ too large ⇒ unnecessary handoffs burden MSC
     ∆ too small ⇒ may be insufficient time to complete a handoff
     before a call is lost



                            Vincent W.-S. Wuen      Mobile Communications         45
                           Handoff Strategies   Cellular Systems



Handoff Scenario at Cell Boundary




                         Vincent W.-S. Wuen      Mobile Communications   46




                           Handoff Strategies   Cellular Systems



Handoff Decision
  Monitor the signal level of MS for a period of time
      to ensures MS is actually moving away from the serving BS.
  Dwell time
      The time over which a call may be maintained within a cell,
      without handoff, depending on propagation, interference,
      distance between the MS and BS, and other time varying
      effects
  Monitor RSSI
      BS monitors the signal strengths of all its reverse voice
      channels to determined the relative location of each MS.
      Locator receivers monitor the signal strength of users in
      neighboring cells need of handoff and report RSSI to MSC.
  Mobile assisted handoff (MAHO)
      MS measures the received power from the surrounding BS’s
      and continuously reports to the serving BS.
      Faster handoff time than first generation analog system
      Suited for microcellular environments
                         Vincent W.-S. Wuen      Mobile Communications   47
                            Handoff Strategies   Cellular Systems



Handoff Considerations



  Prioritizing Handoffs
      Guard channel concept: reserves a fractional of total available
      channels exclusively for handoff ⇒ reducing total carried traffic
      ⇒ combining with dynamic channel assignment to offer
      efficient spectrum utilization
      Queuing of handoff requests: using the finite time interval
      between the time the received signal levels drops below the
      handoff threshold and the time the call is terminated ⇒ not
      guarantee a zero probability of forced termination




                          Vincent W.-S. Wuen      Mobile Communications   48




                            Handoff Strategies   Cellular Systems



Handoff Considerations


      Umbrella cells




      Cell dragging
      Hard handoff
      Soft handoff



                          Vincent W.-S. Wuen      Mobile Communications   49

								
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