Cellular Communications_2_ by hcj


Cellular Basics
Spectrum Reuse
   Earlier systems: single central transmitter
   Cover wide area
   Single channel per user
   25kHz for sufficient audio quality and guard
   40 users in 1MHz, 400 users for 100MHz
   Modern systems have millions of subscribers
Spectrum Reuse
   Several transmitters,
    each having only
    certain coverage area

   Cell==coverage area

   Reuse same spectrum in
    many transmitters
   Often shown as hexagonal shapes
   In reality, very irregular boundaries
   Signal strength decreases gradually=>no exact cell
   Some cell areas may overlap
   Allocate different spectrum to adjacent cells
   Can overlap without causing interference
   Cells with different spectrum
    grouped together as cluster
   Often clusters of size 7
Theoretical Network Planning

Honeycomb (hexagonal) cell structure

 Cluster: set of different frequencies used in group of cells

Cluster is repeated by linear shift
    i steps along one direction
    j steps in the other direction

How many different frequencies does a cluster contain?
Reuse Distance

Distance between cell centers = 3 × Cell Radius

Reuse distance
       distance between the centers of two co-channel cells

                           2        p
            Ru =    i 2 + j + 2ij cos    3R

            R    is Cell Radius
            Ru   is Reuse Distance
        and          p
                 cos( /3) = 1/2
Cluster Radius

Radius of a cluster

                   Ru     i 2 + j + ij
              Rc =    =                R
                    3           3
                         C: number of channels needed for (i,j) grid
Cluster Size
                         is proportional to surface area of cluster

      Surface area of one hexagonal cell is

                                    3 3 2
                             SR =      R

      Surface area of a (hexagonal) cluster of C cells is
                    SRu = C SR   =      { }
                                   3 3 Ru
                                    2   3

   Combining these two expressions gives Ru = R 3C
Possible Cluster Sizes

  We have seen
          Ru = R 3C

  and also

                        2   2
             Ru =      i + j + ij   3R


             C = i 2 + j 2 + ij

  with integer i and j .
Cellular Telephony

    Chose C to ensure acceptable link quality at cell boundary

Typical Cluster Sizes
    Cluster size C = i 2 ij + j 2= 1, 3, 4, 7, 9, ...

                     C= 1      i = 1, j = 0    } Cluster size for CDMA net
                     C= 3      i = 1, j = 1
                     C= 4      i = 2, j = 0
                     C= 7      i = 2, j = 1    } Usual cluster sizes for TDMA
        ·            C= 9      i = 3, j = 0    } cellular telephone nets
                     C = 12    i = 2, j = 2
Reuse distance 2 – reuse pattern

 One frequency can be (re)used in all cells of the same color
Reuse distance 3 – reuse pattern
 Design Objectives for Cluster Size

•High spectrum efficiency
    many users per cell
    Small cluster size gives much bandwidth per cell

• 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)
   Macrocells
     10km,    sparsely populated area

   Microcells
     1km,   densely populated area

   Picocell
     200m,    particular buildings, streets
Reuse distance 3 – reuse pattern
Fixed and Dynamic assignment
   Fixed frequency assignment: permanent
     certain frequencies are assigned to a certain cell
     problem: different traffic load in different cells

   Dynamic frequency assignment: temporary
     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
Increasing Capacity

   Add new channels
   Dynamic channel allocation – frequencies can be
    taken from adjacent cells by congested cells
   Cell splitting – cells in areas of high usage can be
    split into smaller cells
   Cell sectoring – cells are divided into a number of
    wedge-shaped sectors, each with their own set of
    channels (typical: 3)
   Microcells – antennas move to buildings, hills, and
    lamp posts

Cell sectorization
   Use directional
   Collocate cell antenna
    at the cell edges
   Reduce cost
   Maintain call while moving
Basic Network Architecture
Basic Architecture
   Base Station Controller (BSC)
            each base station
     Control

     Manage hand-off of a call from one base station to
   Mobile Switching Center(MSC)
     Manages  setup and tear down of calls to and from
      mobile subscribers
   Home Location Register (HLR)
     HLR   subscriber database including location
   Base Transceiver
    Station (BTS)
     Antenna  Tower
     Radio transceivers

     Power Supply

     Link to BSC (land lines
      or microwave)
Setting up calls/registration
   Make a call originated from mobile handset
     Allocate   resources (channel)

   Receive a call
       Locate the current cell

   After the telephone is switched on
     Contact base station
     Register to use a network
   Authenticate (e.g. for billing)
     Authentication   Center (AuC)
   Store my location
     HLRfor “home” subscribers
     VLR for “visiting”/roaming subscribers

   Mobile communicates with the network to update
   Network keeps last known location
Receiving a calls
   Network should send a notification to a mobile
   Network send to the area where mobile is located
   Mobile listen to a “paging” channel
   Examine each message on the paging channel and
    compares number with his own
   Respond if match
Paging channel
   Always listening to the paging channel drains the
   Divide paging channel into 10 subgroups according
    to a last digit of mobile phone number
   Mobile has to listen only 1/10 of time
   Longer call setup time
Random Access Channel(RACH)
   Respond to call /paging channel message
   Initiate a call
   “Access” message
   Request a channel/slot/resources for further
   Slotted ALOHA
   Mobile monitor signal strength
   Network knows about availability of channels
     Mobile monitors strength of signal from current and
      adjacent cells and sends this information to network
     When signal drops below certain level, network
      reserved new channel at adjacent cell
     Mobile switch channel, network shuts down old channel
                               Handoff Region
33   Signal strength                                               Signal strength
       due to BSi                                                    due to BSj

                       Pi(x)                               Pj(x)


          BSi                    MS                                      BSj
                X1        X3              X5     Xth       X4         X2

     • By looking at the variation of signal strength from either base station it is
     possible to decide on the optimum area where handoff can take place.
Types of Handoffs
     Hard handoff
         A hard handoff is a “break before make” connection.
         MS is linked to no more than one BS at any given time.
         Hard handoff is primarily used in FDMA and TDMA.

  Soft handoff
      It isn't a “break before make” transition.
      The call can be carried on both cells simultaneously.

      Soft handoff is used in CDMA.
Handoff Decisions
   Decision-making process of handoff may be
    centralized or decentralized
   Three different kinds of handoff decisions
     Network-Controlled Handoff
     Mobile-Assisted Handoff

     Mobile-Controlled Handoff
Umbrella Cells
Operation Support Systems
   Network Management Systems
   Service Delivery
   Service Fulfillment, including the Network Inventory,
    Activation and Provisioning
   Service Assurance
   Customer Care
   Billing

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