EE424_Cellular-Concept by wuyyok

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									EE424 Communication
      Systems


  Wireless Communications
       Cellular Concept
HW: 3.1, 3.5, 3,7, 3.11, 3.13, 3.15,
                3.26
                 The Cellular Concept –
Early Mobile Radio
     Large Coverage Area using:
             Single High Power Transmitter
             Antenna Mounted on a tall Tower


    •   Good Coverage
    •   Difficult to reuse the same frequencies throughout the system
        due to significant interference (No spectrum sharing a lot of
        bandwidth is dedicated to a single call)


                       Limited capacity
•   1947 – 1977
    • 1946 FCC allocates 33 FM channels in 33, 150 , 450 MHz bands
    • 1960 Direct dialing from automobile in home area
  The Cellular Concept –                            Cont’d.

• developed by Bell Labs 1960’s-70’s
• areas divided into cells
• The cell is served by a base station with lower power
  transmitter
• Each cell gets portion of total number of channels
• Neighboring cells assigned different groups of channels, to
  minimize interference
• The available channels can be reused as many times as
  necessary as long as the interference between co-channel
  stations is kept below acceptable levels
• Cells using the same channels should be spaced enough to
  reduce co-channel interference
             The First Generation (1G)

USA    Advance Mobile Phone Service (AMPS)
        • used FDMA with 30 KHz FM-modulated voice channels.
        • The FCC initially allocated 40 MHz of spectrum to this
           system, which was increased to 50 MHz shortly after service
           introduction to support more users.
        • This total bandwidth was divided into two 25 MHz bands, one
           for mobile-to-base station channels and the other for base
           station-to-mobile channels.

Europe Total Access Communication System (ETACS)
           The Second Generation (2G)

Many of the first generation cellular systems in Europe were
incompatible, and the Europeans quickly converged on a uniform
standard for second generation (2G) digital systems called GSM.

 (GSM) Groupe Spéciale Mobile       changed to
                                    Global Systems for Mobile
                                    Communications.

In USA two standards in the 900 MHz cellular frequency
band:
      IS-54, which uses a combination of TDMA and FDMA
      and phase-shift keyed modulation

      IS-95, which uses direct-sequence CDMA with binary
      modulation and coding.
• In Japan The digital cellular standard is similar to IS-54
  and IS-136 but in a different frequency band

• The GSM system in Europe is at a different frequency
  than the GSM systems in the U.S.

• Incompatible standards makes it impossible to roam between
  systems nationwide or globally without a multi-mode phone
  and/or multiple phones (and phone numbers).

• The second generation digital cellular standards have been
  enhanced to support high rate packet data services [15]
  (2.5 G) GSM systems provide data rates of up to 100 Kbps
  by aggregating all timeslots together for a single user.
                                The 3G
•   Add broadband data to support video, internet access and other high
    speed data services for mobile devices.

•   • is based on a wideband CDMA
     It

•   The standard, initially called International Mobile
    Telecommunications 2000 (IMT-2000), provides different data rates
    depending on mobility and location, from 384 Kbps for pedestrian
    use to 144 Kbps for vehicular use to 2 Mbps for indoor office use.
•   The 3G standard is incompatible with 2G systems
•   Service providers must invest in a new infrastructure before they can
    provide 3G service.

•   The first 3G systems were deployed in Japan.

•   In fact 3G systems have not grown as anticipated in Europe, and it
    appears that data enhancements to 2G systems may satisfy user
    demands.
            The 4G
•   WiMax

•   LTE
   The Cellular Concept –

 Frequency Re-use
• Cells with the same letter use the
  same set of frequencies
• A cell cluster is outlined in bold
• A cell cluster is replicated over
  the coverage area
• Cluster size N = 7 cells
• Frequency reuse factor = 1/7
  (each cell contains one-seventh
  of the total number of channels
     Cell Shape

• The actual radio coverage of a cell is known as the
  footprint and is determined from field
  measurement or propagation prediction models

• A real footprint is amorphous in nature

• A cell must be designed to serve the weakest
  signal in the footprint.
• Regular shapes:
     Square
     Equilateral triangle and
     Hexagonal
• adjacent circles can not be overlaid upon a map without
  leaving gaps or creating overlapping regions.
  Cell Shape
Ex. hexagon geometry cell shape
• Designed to serve the weakest mobiles within the footprint
  (typically located at the edge)

• The hexagon has the largest area of the three regular shapes.

• Simplistic model, Universally adopted

• fewest number of cells can cover a geographic region

• Approximate circular shape
               no gaps
               no overlap    systematic system design
               equal area
      Geometry of a Hexagon




                   R
               R       R



Surface area is 6R2 3 / 4
Base Station Location


• Base station location:
   • At the center of the cell (Omni-directional antenna)
   • At the vertices of three cells (directional antennas)




  Practical considerations usually do not allow base stations to be
  placed exactly as they appear in the hexagonal layout (~1/4 cell
  radius away from the ideal location)
 Cluster Size and System Capacity


 Assume the following system parameters:
 K             Number of channels in a cell
 N             Number of cells/cluster (Cluster size)
 M             Number of times the cluster is repeated
 S = KN        Number of channels in a cluster
 C             Total number of channels
 C = MkN = MS


A cluster has N cells with unique channels
Cluster Size & System Capacity

  Cluster size N     (with cell size const)      more clusters
  are required to cover a given area C
  Co-channel cells become closer

   Cluster size N      (with cell size const) the ratio between
   cell size and the distance between co-channel cells is large

Design Objectives for Cluster Size
1. High spectrum efficiency
    • many users per cell
    • small cluster size gives much bandwidth per cell

2. High performance
   • Little interference
   • Large cluster sizes
The effect of decreasing cell size
• Increased user capacity
• Increased number of handovers per call
• Increased complexity in locating the subscriber
• Lower power consumption in mobile terminal:
   · Longer talk time,
   · Safer operation
• Different propagation environment, shorter delay spreads
• Different cell layout,
  · lower path loss exponent, more interference
  · cells follow street pattern
  · more difficult to predict and plan
  · more flexible, self-organizing system needed (cf. DECT
  vs. GSM)
      Transmit Power Constraint



•   The power transmitted by each base station
    needs to be large enough to cover its own cell,
    but small enough to not cause too much
    interference in the co-channel cells

•   As cells get smaller, transmit power is reduced
 Cluster Size and System Capacity               Cont.


• There are only certain cluster sizes and cell layout
  which are possible in order to connect without gaps
  between adjacent cells

• N = i2 + ij + j2 where i and j are non-negative integers
• Example i = 2, j = 1
   N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7

Typical Cluster Sizes
     N = 1, 3, 4, 7, 9, 12, 13, 16, 19, 21 ……………
Frequency Reuse Again

     • Frequency Reuse is the core concept
       of cellular mobile radio

     • Users in different geographical areas
       (in different cells) may
       simultaneously use the same
       frequency

     • Frequency reuse drastically increases
       user capacity and spectrum efficiency

     • Frequency reuse causes mutual
       interference (trade off link quality
       versus subscriber capacity)
Frequency Reuse
Nearest co-channel

Example N=19
    (i=3, j=2)
                     To find the nearest co-
                       channel neighbors of a
                       particular cell:
                     • move i cells along any
                       chain or hexagon.
                     • then turn 60 degrees
                       counterclockwise and
                       move j cells.
Nearest co-channel
Example 3.1

• 33 MHz bandwidth is allocated to a particular FDD
  cellular telephone system. Two 25 kHz simplex channels
  to provide full duplex voice and control channels,

(1) compute the number of channels available per cell if
   a system uses
   (a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control
    channels,
(2) determine an equitable distribution of control channels
  and voice channels in each cell for each of the three
  systems.
   Example 2.1

Total bandwidth 33 MHz
Channel BW=25 kHz×2 simplex channels =50 kHz/duplex channel
Total available channels = 33,000/50 = 660 channels

(a) For N= 4,
   total number of channels available per cell 660/4 = 165 channels.

(b) For N= 7, total number of channels available per cell = 660/7 =
    95 channels.

(c) For N= 12,
   total number of channels available per cell = 660/12 = 55
   channels.
   1 MHz = 20 control channels
Channel Assignment Strategies
 • Fixed Channel Assignments

    – Each cell is allocated a predetermined set of voice channels.

    – If all the channels in that cell are occupied, the call is
      blocked, and the subscriber does not receive service.

    – Variation includes a borrowing strategy: a cell is allowed to
      borrow channels from a neighboring cell if all its own
      channels are occupied.

    – This is supervised by the Mobile Switch Center: Connects
      cells to wide area network; Manages call setup; Handles
      mobility
Channel Assignment Strategies

• Dynamic Channel Assignments

   • Voice channels are not allocated to different cells permanently.

   • Each time a call request is made, the serving base station
     requests a channel from the MSC.

   • MSC then allocates a channel to the requested call according to
     algorithm taking into account different factors: frequency re-use
     of candidate channel and cost factors.

   • Dynamic channel assignment is more complex (real time), but
     reduces likelihood of blocking
                            Handoff

•   Reasons for handover
        Moving out of range
        Load balancing

•   Handover scenarios
    Intra-cell handover (e.g., change frequency due to narrowband
    interference)
    Inter-cell, intra-BSC handover (e.g., movement across cells)
    Inter-BSC, intra-MSC handover (e.g., movement across BSC)
    Inter MSC handover (e.g., movement across MSC)
Handoff
• Designers must specify an optimum
  signal level at which to initiate a
  handoff.
• Margin () is defined  = handoff
  threshold - Minimum acceptable signal   Improper handoff situation
  to maintain the call
• If  too small:
     Insufficient time
     to complete handoff
     before call is lost                  Proper handoff situation
     More call losses
• If  too large:
     Too many handoffs
     Burden for MSC
Call Dropped

Handoff is not made and call is dropped if:

• Large delay by the MSC in assigning a handoff.

• Threshold margin () is set too small for the handoff time in the
  system.

•    Excessive delays may occur during high traffic conditions due to
    computational loading at the MSC

• No channels are available on any of the nearby base stations (thus
  forcing the MSC to wait until a channel in a nearby cell becomes
  free
Handoff is Necessary?

•   The base station monitors the signal level for a certain
    period of time before a hand- off is initiated. So that
    unnecessary handoffs are avoided. (signal drop may be
    due to momentary fading).

•   The length of time needed to decide if a handoff is
    necessary depends on the speed at which the vehicle is
    moving.

•   If the slope of the short-term average received signal level
    in a given time interval is steep, the handoff should be
    made quickly.
  Dwell Time
It is the time over which a call may be maintained
within a cell, without handoff

• Depends on:
    Propagation, interference, distance between
    the subscriber and the base station, and other time varying
    effects. (the speed of the user and the type of radio coverage)

• Even a stationary subscriber may have a random and finite dwell
  time due to fading effect.

• Statistics of dwell time are important in practical design of handoff
  algorithms.
Styles of Handoff
• Network Controlled Handoff (NCHO)
   – in first generation cellular system, each base station constantly
     monitors signal strength from mobiles in its cell
   – based on the measures, MSC decides if handoff necessary
   – mobile plays passive role in process
   – burden on MSC

• Mobile Assisted Handoff (MAHO)
  – present in second generation systems
  – mobile measures received power from surrounding base
    stations and report to serving base station
  – handoff initiated when power received from a neighboring cell
    exceeds current value by a certain level or for a certain period
    of time
  – faster since measurements made by mobiles, MSC don’t need
    monitor signal strength
Intersystem Handoff

 • If a mobile moves from one cellular system to
   different cellular system controlled by a different
   compatible MSC.

 • When a mobile signal becomes weak in a given cell
   and the MSC cannot find another cell within its
   system to which it can transfer the call in progress.
Prioritizing Handoff
 Dropping a call is more annoying than line busy
 • Guard channel concept (Decrease the probability of forced termination due to
   lack of available channels)

     Reserve some channels for handoffs
     Waste of bandwidth
     But can be dynamically predicted

 • Queuing of handoff requests (due to lack of available channels)
    There is a finite time interval between time for handoff and
    time to drop (signal goes below the handoff threshold).
    Better tradeoff between dropping call probability and network
    traffic.
Practical Handoff Considerations
(1) Practically, several problems arise when attempting to design for a
    wide range of mobile velocities.

   • High speed vehicles pass through the coverage region of a cell
     within a matter of seconds, whereas pedestrian users may never
     need a handoff during a call.

   • Particularly with the addition of microcells to provide capacity,
     the MSC can quickly become burdened if high speed users are
     constantly being passed between very small cells.

(2) Another practical limitation is the ability to obtain new cell sites. In
    practice it is difficult for cellular service providers to obtain new
    physical cell site locations in urban areas.
The umbrella Cell Solution
 • Is used to provide large area coverage to high speed users while
   providing small area coverage to users traveling at low speeds.

 • By using different antenna heights (often on the same building or
   tower) and different power levels, it is possible to provide large
   and small cells which are co-located at a single location.

 • # handoffs is minimized for high speed users and provides
   additional microcell channels for pedestrian users.

 • If a high speed user in the large umbrella cell is approaching the
   base station, and its velocity is rapidly decreasing, the base station
   may decide to hand the user into the co-located microcell, without
   MSC intervention.
Umbrella Cell Approach
Cell Dragging (Pedestrian users emit very strong signal
                    to the base station)

• As the user travels away from the base station at a very slow
  speed, the average signal strength does not decay rapidly.

• Even when the user has traveled well beyond the designed
  range of the cell, the received signal at the base station may be
  above the handoff threshold, thus a handoff may not be made.    •



Interference and traffic management problem, since the
user has meanwhile traveled deep within a neighboring cell.
To solve this problem, handoff thresholds and radio
coverage parameters must be adjusted carefully.
 Interference and System Capacity
Interference is the major limiting factor in performance of cellular
   radio systems
• Sources of interference:
    – Mobiles in the same cell
    – A call in progress in a neighboring cell
    – Other base stations operating in the same frequency band
    – Non-cellular system leaking energy into the cellular
      frequency band
• Effect of interference:
    – Cross talk in voice channels
    – For control channels              missed or blocked calls
• The two main types are:
    – co-channel interference
    – adjacent channel interference
Co-channel Interference
When the size of each cell is the same, and the BSs transmit
the same power, the co-channel interference ratio depends on:
• The radius of the cell (R)
• The distance between centers of the nearest co-channel
    cells (D)

• Co-channel reuse ratio: Q = D/R = 3N      (Hexagonal
                                              Geometry)
• Q increases        Interference decreases
• Q decreases         Interference increases (cluster size N
                     decreases and system capacity increases)
Co-channel Reuse Ratio
Signal-to-Interference Ratio

The signal-to-interference ratio (S/I or SIR) for a mobile
receiver which monitors a forward channel (Down Link
Channel) =

                           S     S
                              i0
                           I
                                I
                                i 1
                                       i



S:     The desired signal power from the desired base station
Ii :   The interference power caused by the ith interfering
       co-channel cell base station.
i0 :   The number of co-channel interfering cells.
Average Received Power

(Propagation measurements)
The average received power Pr at a distance d from the
transmitting antenna is approximated by
                            n
                     d 
             Pr  Po  
                     d 
                      0
                                           d 
             Pr (dBm)  Po (dBm)  10n log  
                                           d 
                                            0
Where P0 is the power received at a close-in reference point in
the far field region of the antenna at a small distance d0 from the
transmitting antenna, and n is the path loss exponent.
n ~ 2 to 4 in urban cellular systems.
  Co-channel Interference
Assumptions
• The interference is due to co-channel base stations.
• The transmit power of each base station is equal
• The path loss exponent is the same throughout the coverage area,
 S/I for a mobile can be approximated as

                           S            R n     ( D / R) n ( 3N ) n
                                i0
                                                          
                           I                         i0       i0
                                  ( Di ) n
                                 i 1
AMPS (FM 30 KHz channel bandwidth) S/I=18dB (sufficient quality)
  If n=4, N needs to be larger than 6.49 ~ 7

Thus a minimum cluster size of seven is required to meet S/I = 18 dB.

All the interfering cells are assumed to be equidistant from the base station
receiver. (Good for large N)
Co-Channel cell for 7 cells reuse


                        Assume n=4, the signal-to- interference
                        ratio for the worst case can be closely
                        approximated as
Co-channel Interference

In terms of co-channel reuse ratio               Q = 4.6 for N = 7

                                                        17 dB (for N = 7)


Exact solution using the equation
                        S            R n
                          
                        I     i0                               17.8 dB.
                               (D )
                              i 1
                                        i
                                            n




                 Slightly less than 18 dB
 it would be necessary to increase N to the next largest size,
 which is found to be 12 (corresponding to i = j = 2).
 Example 3.2

S/I = 15 dB , Frequency reuse factor 1/N? n=4

   (a) n=4
          Let N = 7 , then Q = D/R = 3N

    S ( 3N ) 4              Which is greater than 15
               18.66dB
    I   i0                  dB, N=7 is good value

   (b) n=3
          Let N = 7 , then Q = D/R = 3N

    S ( 3N )3           It is less than required 15 dB, N=7
              12.05dB
    I   i0                       More N should be used 9,
                        12, 19, ……. check
Adjacent Channel Interference

Origin:    Arising from signals which are adjacent in frequency
           to the desired signal

Become serious by
 • • Imperfect receiver filters which allow nearby
    frequencies to leak into the passband
     (near-far-effect)

  • A mobile close to a base station transmits on a channel close
    to one being used by a weak mobile. The base station may
    have difficulty in discriminating the desired mobile user.
Adjacent Channel Interference Example
Adjacent Channel Interference


• Adjacent channel interference can be minimized through
  careful filtering (High Q filters) and channel assignments.

• Since each cell is given only a fraction of the available
  channels, a cell need not be assigned channels which are all
  adjacent in frequency. (By keeping the frequency separation
  between each channel in a given cell as large as possible).
Example

If a mobile is 20 times as close to the base station as
another mobile and has energy spill out of its passband,
the signal-to- interference ratio at the base station for the
weak mobile (before receiver filtering) is approximately



For a path loss exponent n = 4, this is equal to -52 dB. If the
intermediate frequency (IF) filter of the base station receiver
has a slope of 20 dB/octave

Then an adjacent channel interferer must be displaced by at
least six times the passband bandwidth from the center of the
receiver frequency passband to achieve 52 dB attenuation
     Case Study (Example 3.3)

How channels of AMPS are divided into subsets to minimize adjacent
channel interference?

666 duplex channels

In 1989, the FCC allocated an additional 10 MHz (166 new channels)
There are now 832 channels

Forward channels      1     666 (870.030         889.98 MHz)
Reverse channel s     1     666 (825.030         844.98 MHz )

Extended channels 667          799
and               990          1023.
Case Study   cont.
       Case Study                                cont.
Two operators               (416 channels for each)

Channels distinguished as block A and block B

•416 channels               395    voice &       21       control
channels
•                                                     block A
       1             312    voice channels            channels
       313           333    control channels
                                                      block B
       334           354    control channels          channels
       355           666    voice channels

       667           716 & 991       1023
                     extended Block A voice channels
       717           799 extended Block A voice channels
     Case Study                                    cont.

395 voice channels         =      21 subsets x ~ 19 channels

    In each subset, the closest adjacent channel is 21 channels
    away.

    In a 7-cell reuse system, each cell uses 3 subsets of channels

    The 3 subsets are assigned such that every channel in the cell is
    assured of being separated from every other channel by at least
    7 channel spacing.
     Case Study                                         cont.

In the following Table:

Each cell uses channels in the subsets, iA + iB + iC,
               where i is an integer from 1 to 7.

•The total number of voice channels in a cell is about 57

The channels listed in the upper half of the chart belong to
block A and those in the lower half belong to block B.

•The shaded set of numbers correspond to the control
channels which are standard to all cellular systems in North
America
    Power Control for reducing Interference

The power levels transmitted by every subscriber unit are
under constant control by the serving base stations.

•   Each mobile transmits the smallest power necessary to
    maintain good quality link

•   Power control prolong battery life for the subscriber unit

•   Power control reduces the reverse channel S/I in the system.

•   Power control is especially important for emerging CDMA
    spread spectrum systems that allow every user in every cell
    to share the same radio channel.
           Trunking and Grade of Service

• Trunking is the aggregation of multiple user circuits into a
  single channel.
• The aggregation is achieved using some form of multiplexing.
• Cellular radio systems rely on trunking to accommodate a
  large number of users in a limited radio spectrum.




                                                 (SLC) Subscriber line
                                                 concentrator
     The Concept of Trunking
• Large number of users share small number of channels.

• Assigning users channels on demand

• Each cell has pool of channels

• When user requires service, channel allocated to user

• When user no longer requires service, channel returned to pool
  to be allocated to next user

• The user is blocked (denied access) when all radio channels are
  already in use.

• A queue may be used to hold the requesting users until a
  channel becomes available.
      Trunking Theory

Important to design trunked radio systems that can handle a
specific capacity at a specific grade of service, GOS

Trunking theory was developed by Erlang

• Erlang based his studies of the statistical nature of the arrival and
  the length of calls. The measure of traffic intensity bears his name

• One Erlang represents the amount of traffic intensity carried by
  a channel that is completely occupied (i.e. 1 call-hour per hour
  or 1 call-minute per minute).

• For example, a radio channel that is occupied for thirty minutes
  during an hour carries 0.5 Erlangs of traffic
    The Grade of Service (GOS)

•   The grade of service (GOS) is a measure of the ability of a
    user to access a trunked system during the busiest hour

•   It is used to define the desired performance of a particular
    trunked system.

•   GOS is typically given as the probability that a call is
    blocked, or the probability of a call experiencing a delay,
    greater than a certain queuing time.
       Some Definitions used in Trunking Theory
• Set-up Time: The time required to allocate a trunked radio channel to
  requesting user

• Blocked Call: Call which cannot be completed at the time of request

• Holding Time (H) : Average duration of a typical call (H in seconds)

• Traffic Intensity (A): Measure of channel time utilization (average
  channel occupancy measured in Erlangs)

• Load: Traffic intensity across the entire trunked radio system (Erlangs)

• Grade of service (GOS): A measure of congestion which is specified as
  the probability of a call being blocked (Erlang B), or the probability of
  a call being delayed beyond a certain amount of time (Erlang C)

• Request Rate (l): The average number of call request per unit time (S-1 )
                      Traffic Intensity (A)
The traffic intensity offered by each user (Au) is
  Au = call request rate × Holding time     Au = l H

Total offered traffic intensity (A) is      A = U Au
where U is the number of users in a given system

In a C channel trunked system, and if the traffic is equally distributed
   The traffic intensity per channel          AC = U Au/C

At a given time, if the offered traffic exceeds the capacity of the
system (e.g., UAu > C), calls are blocked

The AMPS is designed for 2% GOS. I.e, 2 out of 100 calls will be
blocked due to channel occupancy during the busiest hour
            Types of trunked systems

•There are two systems commonly used.

1.    System with no queue for call requests.
      - If a channel is available, no setup time and the user is
           given immediate access to the available channel

      - If no channels are available, the requesting user is
        blocked without access and is free to try again later

        -This type of trunking is called
              (blocked calls cleared) - Erlangs B

2.    System which a queue is provided to hold calls which
      are blocked. (Blocked Calls Delayed) Erlangs C
                 Erlang B formula

• Blocked calls cleared
• The probability of blocking during the busy hour

                                       AC
                    Pr( blocking)  C C! k  GOS
                                         A
                                     k!
                                    k 0




   – Can use plot of Erlang B formula to determine one of the
     parameters: Pr(blocking), C, A

    Au = l H    &       A = U Au      &       AC = U Au/C
                          Erlang B Plot
                          Number of trunked channels C
Probability of Blocking




                           Traffic Intensity in Erlangs
Erlang B Example
                    Blocked Calls Delayed
• Blocking calls are delayed until channels are available, queuing
• Erlang C
                                                           AC
The probability of a call not       Pr(delay  0)                C 1
                                                                        Ak
                                                    A  C!(1  C )
                                                     C         A
having immediate access to a channel                              k  0 k!

• The probability that a delayed call will have to wait longer than t
             = e-(C-A)t/H
• Probability of delay larger than t
                     Pr[delay>t]= Pr[delay>0] e-(C-A)t/H

• The average delay D           D = Pr[delay>0] H/(C-A)

GOS is defined in this case as the probability that a call is
blocked after waiting a specific length of time in the queue.
                       Erlang C Plot
                       Number of trunked channels C
Probability of Delay




                        Traffic Intensity in Erlangs
                  Some Examples

3.4 How many users can be supported for .5% blocking probability
   for the following number of trunked channels in a blocked calls
   cleared system? 1, 5,10, 20, 100. Assume each user generates
   0.1 Erlangs of traffic (Au = 0.1).

For GOS = 0.005, then use Fig. 3.9 U = A/Au = A/0.1

C = 1, U = 1 User            C = 5, U = 11Users

C= 10 , U = 39 Users         C= 20, U = 110 Users

C = 100,U = 809 Users
    Ex.     3.5 (2 million residents in an urban area)
•    Pr(blocking) = 2%
•    Each user averages 2 calls per hour at an average s duration of 3 min./call
      • System A: 394 cells, 19 channels/cell
      • System B: 98 cells, 57 channels/cell
      • System C: 49 cells, 100 channels/cell

• Find number of subscribers U that can be supported in each cell

     • Traffic intensity by user Au = l H = (2/60)(3) = 0.1 & Pr(blocked) = 0.02

• System A: C = 19,     From Erlang B plot, A ~ 12 Erlangs
      U = A/Au = 12/0.1 = 120 users/cell, N = 120 users/cell * 394 cells = 47,280

• System B: C = 57,     From Erlang B plot, A ~ 45 Erlangs
      U = A/Au = 45/0.1 = 450 users/cell N = 450 users/cell * 98 cells = 44,100

• System C: C= 100,    From Erlang B plot, A ~ 88 Erlangs
       U = A/Au = 88/0.1 = 880 users/cell N = 880 users/cell * 49 cells = 43,120

Percentage market penetration for systems A, B, and C and the market
   penetration of the three systems
Ex.    3.6
 A certain city has an area of 1,300 square miles and is covered by a cellular
 system using a seven-cell reuse pattern. Each cell has a radius of four miles
 and the city is allocated 40 MHz of spectrum with a full duplex channel
 bandwidth of 60 kHz. Assume a GOS of 2% for an Erlang B system is
 specified. If the offered traffic per user is 0.03 Erlangs, compute:
 (a) the number of cells in the service area,
      = Total area/area of cell = 31 cells
 (b) the number of channels per cell,
      =Total channels/7 = 40MHz/(7x60KHz) = 95 Channels/cell
 (c) traffic intensity of each cell,
      From Erlangs B graph GOS=0.02, Au = 0.03, C=95 channels A …
 (d) the maximum carried traffic,
       = number of cells × traffic intensity per cell = 31 × 84 = 2604 Erlangs.
 (e) the total number of users that can be served for 2% GOS,
      = Total traffic (Atot)/trafic intensity per user = 2604/0.03 = 86800
 (f) the number of mobiles per unique channel (where it is understood that
 channels are reused),= number of users/number of channels = 86800/666
 (g) the theoretical maximum number of users that could be served at one
      time by the system = Number of the available channels in the system
Ex.   3.7
  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 that has a 5% probability of a delayed call:
     (a) How many users per square kilometer will this system support?
     (b) What is the probability that a delayed call will have to wait for more
         than 10 s?
     (c) What is the probability that a call will be delayed for more than 10
         seconds?

  C = 60 channels, Au = 0.029, λ = 1/60 × 60               H = Au/ λ
          C/cell 60/4 = 15
  (a) U = A/Au ( From Erlangs C graph find A for C = 15 and probability of
      delay = 0.05)
  (b) The probability that a delayed call will have to wait longer than 10 s is
          Pr [delay >t |delay]= e(–(C – A)t /H) = e(–(15 – 9.0)10/104.4) =

          56.29%
  (c)     Probability that a call is delayed more than 10 seconds,
          Pr [delay >10] = Pr [delay >0]Pr [delay >t |delay]
          = 0.05 x 0.5629 = 2.81%
 Improving Coverage & Capacity Increasing
   Demand of Services Increases?
   More channels per unit coverage area are needed

Techniques to expand the capacity of cellular systems:

    1.   Cell splitting – cells in areas of high usage can be split into smaller
         cells.
         Increase capacity by increasing the number of base stations.

    2.   Cell sectoring – cells are divided into a number of wedge-shaped
         sectors, each with their own set of channels (Directional antennas to
         control interference) . Reducing trunking efficiency.

         Improve capacity by reducing co-channel interference.

    3.   Microcell zone (distributes the coverage of a cell and extends the cell
         boundary to hard -to-reach places.)
         Improve capacity by reducing co-channel interference.
                        Cell splitting

Cell splitting is achieved through:
    • Subdividing a congested cell into smaller cells (reducing cell radius
      and keeping the D/R ratio unchanged)
    • Reduction in antenna height and transmitter power (different cells
      will have different transmit power requirements to support cells of
      different sizes)

Some properties
   – Cell splitting enables more spatial reuse (greater system capacity)
   – Cell splitting preserves original frequency reuse plan
   – In practice, cells might have different coverage areas due to practical
      BS placement issues
   – Cell splitting causes increased handoff
        • Can use “umbrella” cells where fast-moving mobiles covered by
          original cell and slower mobiles covered by microcells
Cell splitting                                     Cont.

   If we assume that the radius of every cell is reduced to half
                        of its original value


         Four times as many cells would be required
                    to cover the same area


                 Number of clusters over the
                  coverage region increases


                 Number of channels increases
Example
• The base stations are placed at corners of the cells
• The original base station A is surrounded by six new
  microcells
• In this example the smaller cells added in such a way as to
  preserve the frequency reuse plan of the system
• Each microcell base station is placed half way between two
  larger stations utilizing the same channel

 Cell splitting simply scales the
 geometry of the cluster

 The radius of each new microcell
 is half that of the original cell
            G
        F       B       G
            A       F       B

        E       C       A

    G       D       E       C
F       B       G       D
    A       F       B
E       C       A
    D       E       C
                D
            G
        F       B       G
            A       F       B

        E       C       A

    G       D       E       C
F       B       G       D
    A       F       B
E       C       A
    D       E       C
                D
Example                                             Cont.
 • The transmit power of the new cells must be reduced

 • The transmit power of the new cells with radius half that of the
   original cells can be found by examining the received power Pr
   at the new and old cell boundaries and setting them equal to
   each other
 •               Pr [at old cell boundary]  Pt1R-n

 •              Pr [at new cell boundary]  Pt2 (R/2)-n

 • For n = 4 and set the received powers equal to each other

     Pt2 = Pt1/16 the transmit power must be reduced by 12 dB in
                  order to fill in the original coverage area with
                  microcells, while maintaining the S/I requirement.
Practical problems in Cell splitting                            Cont.


    Channel Assignment
•   Not all cells are split at the same time
•   It is often difficult to find real estate that is perfectly situated
    for cell splitting
•   Different cell sizes will exist simultaneously
•   Special care needs to be taken to keep the distance between co-
    channel cells at the required minimum, and hence channel
    assignments become more complicated

    Handoff:
    High speed and low speed traffic should be simultaneously
    accommodated (the umbrella cell approach is commonly used).
Practical problems in Cell splitting                                Cont.


  In practice different cell sizes will exist simultaneously

 • If the larger transmit power is used for                 D
   all cells, some channels used by the             E               F
   smaller cells would not be sufficiently      C           B           E
                                                    G       F
                                                                    C
   separated from co-channel cells              D
                                                        E
                                                            A
                                                                G
                                                                        G
                                                        D       B

 • If the smaller transmit power is used for        F       C       D
                                                B           E           F
   all the cells, there would be parts of the       C               B
   larger cells left unserved                               G
Practical problems in Cell splitting                             Cont.

 Channels in the old broken into two channel groups:
    1. The first one corresponds to the smaller cell
    reuse requirements                                           D
    2. The second corresponds to the larger cell         E               F
                                                     C           B           E
    reuse requirements.                                  G   E
                                                                 F
                                                                     G
                                                                         C
 The larger cell is usually dedicated to high speed D        D
                                                                 A
                                                                     B
                                                                             G
 traffic so that handoffs occur less frequently.         F       C
                                                                         D
                                                     B           E           F
 At the beginning: fewer channels in the small           C               B
 power groups                                                    G
 demand grows: smaller groups will require more
 channels. splitting process continues until all the
 channels in an area are used in the lower power
 group cell splitting is complete within the region,
 and the entire system is rescaled to have a smaller
 radius per cell.
Antenna downtilting

  Antenna downtilting,
  Focuses radiated energy from the base station toward
  the ground (rather than toward the horizon), to limit
  the radio coverage of newly formed microcells.
Example 2.8
 Each base station uses 60 channels, regardless of cell size
Rorig cell = 1 km &           Rmic cell = 0.5 Km

Find the number of channels (N) contained in a
3 km by 3 km square centered around A under
the following conditions:
(a) without the use of microcells
(b) when the lettered microcells as shown in
     the figure are used
(c) If all the original base stations are replaced
     by microcells
(a) 5 cells are included N = 5×60 = 300 channels
(b) Number of cells = 5 + 6 = 11
                                                         Assume cells on the edge of
       N = 11×60 = 660 channels
                                                         the square to be contained
(c) Number of cells = 5 + 6 + 5 = 17                     within the square.
        N = 17×60 = 1020 channels
                      Cell Sectoring

The uses of directional antennas improve S/I, then capacity
improvement is achieved by reducing the number of cells in a
cluster, thus increasing the frequency reuse. It is necessary to reduce
the relative interference without decreasing the transmit power.

 Keeping the cell radius unchanged and decreasing the D/R ratio


                Number of clusters over the
                 coverage region increases


               Number of channels increases
       Reduction of Co-channel interference
              using sector antennas
• The factor by which the co-channel interference is
  reduced depends on the amount of sectoring used
• A cell is normally partitioned into three 120° sectors or
  six 60° sectors as shown below




      120° sectoring                      60° sectoring
         How 120° sectoring reduces interference
                 from co-channel cells

• Out of the 6 co-channel cells in the first tier, only two of
   them interfere with the center cell
A mobile in the center cell will
experience interference on the
forward link from only these two
sectors. The resulting S/I for this case
can be found from
to be 24.2 dB which is
a significant improvement
 over the omnidirectional where
the worst case S/I was shown to be 17
dB
This S/I improvement allows the decreasing the cluster size N in
order to improve the frequency reuse, and thus the system capacity.
Antenna downtilting


  In practical systems, further improvement in S/I is
  achieved by downtilting the sector antennas such that the
  radiation pattern in the vertical (elevation) plane has a
  notch at the nearest co-channel cell distance.
• The S/I improvement is achieved at the cost of the
  number of antennas at each base station

• Sectoring decreases trunking efficiency due to
  channel sectoring at the base station

• Since sectoring reduces the coverage area of a
  particular group of channels, the number of handoffs
  increases

• Handed off from sector to sector within the same cell
  without intervention from the MSC
Example 3.9
Consider a cellular system:
H = two minutes        GOS = less than 1%.
l = one call per hour Total traffic channels = 395
N = 7 blocked calls are cleared (Erlang B distribution)

channels/cell C = 395/7 = 57 traffic channels.

Unsectored (C=57) the system may handle 44.2 Erlangs
or 1326 calls per hour.

120° sectoring, C = 57/3 = 9 channels per antenna
Each sector can handle 11.2 Erlangs or 336 calls per hour
Cell capacity of 3 × 336 = 1008 calls per hour (24% decrease)
Thus, sectoring decreases the trunking efficiency while
improving the S/I
                 A Microcell Zone Concept
    To solve the handoff and trunking efficiency problems raised
    due to sectoring option
• Large central base station is
  replaced by several lower
  powered transmitters (zone
  transmitters) on the edges of
  the cell.

• Each of the three zone
  sites are connected to a
  single base station and
  share the same radio
  equipment.
A Microcell Zone Concept                              Cont.
• Travel mobile is served by the zone with the strongest
  signal

• Any base station channel may be assigned to any zone by
  the base station

• As a mobile travels from one zone to another within the
  cell, it retains the same channel and the base station simply
  switches the channel to a different zone site

• Decreased co-channel interference improves the signal
  quality and also leads to an increase in capacity without the
  degradation in trunking efficiency caused by sectoring.
  A Microcell Zone Concept        Cont.

Dz /Rz of 4.6 can achieve the
required link performance

The capacity of the system
depends on the ratio D/R (not
zones dependent)

Dz/Rz = 4.6 ~ equivalent to
D/R = 3 which correspend to N =
3 system (table 2.1)

Capacity increases by about 7/3
A Microcell Zone Concept   Cont.
Repeaters
Repeaters
Repeaters


 Outdoor
 Antennas
                  LNA
            BPF




                  LNA
            BPF

                         Indoor
                        Antenna
Repeaters


Outdoor
Antennas
                                           LNA
                    BPF




                                          LNA
                    BPF

                                                                                      Indoor
                                                                                     Antenna




           Fig. 3.3 Repeater bidirectional amplifier using duplexer and automatic
                                                                gain control (AGC)

								
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