The document provides an overview of cellular engineering fundamentals, including concepts such as frequency reuse, channel assignment strategies, and the handoff process. It discusses methods for enhancing capacity and cell coverage, such as cell splitting, sectoring, and microcell zones, along with their advantages and trade-offs. Additionally, it covers handoff priorities and techniques to manage calls effectively during transitions between cells.

1. Fundamentals of Cellular
Engineering
Prepared By :
Pratik R. Detroja

2. Outline
 Introduction to cellular system
 Concept of frequency reuse
 Channel Assignment Strategies
 Handoff Process and Generation
 Handoff Priority
 Enhancing Capacity And Cell Coverage : The key Trade-off
 Cell Splitting
 Sectoring
 Microcell Zone Concept
 Reference

3. Introduction
 Goals of a Cellular System :
• High capacity
• Large coverage area
• Efficient use of limited spectrum
 Reuse of radio channel
 Enable a fix number of channels to serve an
arbitrarily large number of users by reusing
the channel throughout the coverage region

4. What is cell ?
 Each cellular base station is allocated a group of radio channels
within a small geographic area called a cell.
 Neighboring cells are assigned different channel groups.
 By limiting the coverage area to within the boundary of the cell,
the channel groups may be reused to cover different cells.

5.  Consider a cellular system which has a total of S duplex channels.
 Each cell is allocated a group of k channels, K < S
 The S channels are divided among N cells.
 The total number of available radio channels, S = kN
 The N cells which use the complete set of channels is called cluster.
 The cluster can be repeated M times within the system. The total
number of channels, C, is used as a measure of capacity.
C = MkN = MS
 The capacity is directly proportional to the number of replication M.
 The cluster size, N, is typically equal to 4, 7, or 12.
 The frequency reuse factor is given by 1/N.
Frequency reuse

6.  Hexagonal geometry has
• exactly six equidistance neighbors
• the lines joining the centers of any cell and each of its neighbors are
separated by multiples of 60 degrees.
 Only certain cluster sizes and cell layout are possible.
 The number of cells per cluster, N, can only have values which satisfy
22
jijiN 
Frequency reuse (Cont.)

7. Channel Assignment Strategies
 Goal is to minimize interference & maximize use of capacity.
 One of the Channel assignment strategies is
1. Fixed Channel Assignment :
 Channels are divided in sets.
 A set of channels is permanently allocated to each cell in the
network. Same set of channels must be assigned to cells
separated by a certain distance to reduce co-channel
interference.
 Any call attempt within the cell can only be served by the
unused channels in that particular cell. The service is blocked
if all channels have used up

8.  Most easiest to implement but least flexibility.
 An modification to this is ‘borrowing scheme’. Cell (acceptor cell) tha
has used all its nominal channels can borrow free channels from its
neighboring cell (donor cell) to accommodate new calls.
 Borrowing can be done in a few ways: borrowing from the adjacent cell
which has largest number of free channels, select the first free
channel found, etc.
 To be available for borrowing, the channel must not interfere with
existing calls. The borrowed channel should be returned once the
channel becomes free.
FCA(cont.)

9. 2. Dynamic Channel Allocation (DCA) :
 Voice channels are not allocated to any cell permanently. All channels
are kept in a central pool and are assigned dynamically to new calls as
they arrive in the system.
 Each time a call request is made, the serving BS requests a channel
from the MSC. It then allocates a channel to the requested cell
following an algorithm that takes into acount the likelihood of future
blocking within the cell, the reuse distance of the channel and other
cost functions ⇒ increase in complexity

10.  Centralized DCA scheme involves a single controller selecting a
channel for each cell. Distributed DCA scheme involves a number of
controllers scattered across the network.
 For a new call, a free channel from central pool is selected based
on either the co-channel distance, signal strength or signal to noise
interference ratio.
DCA(cont.)

11.  Handoff :
when a mobile unit moves from one cell to another while a call is in
progress, the MSC must transfer (handoff) the call to a new
channel belonging to a new base station
 very important task → often given higher priority than new call
 It is worse to drop an in-progress call than to deny a new one
 Handoff operation :
• identifying a new base station
• re-allocating the voice and control channels with the new base
station.
What is Handoff ?

12. Handoff

13. Handoff Process
 Handoff Threshold
 Minimum usable signal for acceptable voice quality (-90dBm to -
100dBm)
 Handoff margin cannot be too large
or too small.
 If is too large, unnecessary handoffs burden the MSC
 If is too small, there may be insufficient time to complete
handoff before a call is lost.
usableminimum,, rhandoffr PP 

15.  Handoff must ensure that the drop in the measured signal is not
due to momentary fading and that the mobile is actually moving
away from the serving base station.
 Running average measurement of signal strength should be
optimized so that unnecessary handoffs are avoided.
• Depends on the speed at which the vehicle is moving.
• Steep short term average -> the hand off should be made quickly
• The speed can be estimated from the statistics of the received short-
term fading signal at the base station
 Dwell time: the time over which a call may be maintained within a
cell without handoff.
Handoff Process (cont.)

16. Handoff Generation
 Handoff measurement
 1st Generation Cellular (Analog FM → AMPS)
• Received signal strength (RSS) of RVC measured at base station &
monitored by MSC
• A spare Rx in base station (locator Rx) monitors RSS of RVC's in neighboring
cells
 Tells Mobile Switching Center about these mobiles and their channels
• Locator Rx can see if signal to this base station is significantly better than
to the host base station
• MSC monitors RSS from all base stations & decides on handoff
• 10 secs handoff time

17. Handoff Generation (Cont.)
 2nd Generation Cellular w/ digital TDMA (GSM, IS-136)
• Mobile Assisted HandOffs (MAHO)
• important advancement
• The mobile measures the RSS of the FCC’s from adjacent base stations &
reports back to serving base station
• if Rx power from new base station > Rx power from serving (current) base
station by pre-determined margin for a long enough time period → handoff
initiated by MSC
• 1 to 2 seconds handoff time
 IS-95 CDMA spread spectrum cellular system
• Mobiles share the channel in every cell.
• No physical change of channel during handoff
• MSC decides the base station with the best receiving signal as the service
station

18.  Dropped call is considered a more serious event than call blocking.
Channel assignment schemes therefore must give priority to
handover requests.
 A fraction of the total available channels in a cell is reserved only
for handover requests. However, this reduces the total carried
traffic. Dynamic allocation can improve this.
 It reduces rate of handoff failure
 It is desirable from user’s point of view
 Prioritizing Handoff
1. Guard channels concept
2. Queuing handoff requests
Handoff Priority

19. A Cell
New Calls
Handoff Calls
From
neighboring cells
Common
Channel Pool
Call completion
Handoff out
To neighboring
cells
Wireless Cellular System Traffic
in a cell

20. Guard Channel Method
A fraction of available channels is reserved exclusively for
handoff requests
It has disadvantage of reducing total carried traffic
It Offers efficient spectrum utilization when dynamic channel
assignment strategies by minimizing number of required guard
channels
It reduces number of blocked handoffs
It reduces system capacity

21. Handoff dropping less desirable than new call blocking!
Handoff call has Higher Priority: Guard Channel Scheme
GCS: g channels are reserved for handoff calls.
g trade-off between Pb & Pd
Here, New call blocking probability, Pb
Handoff call dropping probability, Pd
Guard Channel Scheme

22.  When a new call (NC) is attempted in an cell covered by a base station
(BS), the NC is connected if an idle channel is available in the cell.
Otherwise, the call is blocked
 If an idle channel exists in the target cell, the handoff call (HC)
continues nearly transparently to the user. Otherwise, the HC is
dropped
 Loss Formulas
 New call blocking probability, Pb : Percentage of new calls rejected
 Handoff call dropping probability, Pd : Percentage of calls forcefully
terminated while crossing cells
Guard Channel Scheme (Cont.)

23. Queuing Handoff Requests
 First, Put handoff requests in a queue
 Then Serves handoffs on a FCFS basis
 It reduces number of failed handoffs
 It reduces system capacity

24. Queuing Handoff Requests (Cont.)
 Possible due to time interval elapsed when the signal level drops
below to threshold until minimum signal level
 Decrease probability of forced termination due to lack of
available channels
 Tradeoff between decrease in probability of forced termination
and total traffic
 The delay time and queue size is determined from traffic pattern
 Queuing does not guarantee zero probability of call termination
since large delays will signal level to drop min

25.  The Trade-off :
Enhancing Capacity And Cell
Coverage
range
(km)
Throughput
/cell
(Mbps) 802.11b
Noise
Limited
Interference
Limited
A-MAS
Benefit
Technical Interpretation
 Gain vs. noise, fading, ... expands envelope to
right
 Interference mitigation (+ gain) expands it
upwards
Economic Interpretation
 Coverage improvements reduce CapEx, OpEx (esp. backhaul, sites)
 Capacity improvements reduce delivery cost, spectrum requirements

26.  The number of channels available to customers (equivalently,
the channel density per square kilometer) could be increased
by decreasing the cluster size.
 It might be that an increase in channel density is required only
in specific parts of the system to support an increased demand
in those areas.
 Cell-splitting is a technique which has the capability to add new
smaller cells in specific areas of the system.
 Sectoring is basically a technique which can increase the SIR
without necessitating an increase in the cluster size.
 Microcell zone
Trade-off

27. Introduction
 Why cell shape is hexagonal…?

28. Cont…
 Why Cell Splitting , Sectoring and Microcell zone…..?
 As users increases per cell the channel capacity
decreases
 Techniques needed to provide extra channel
 Cell Splitting, Sectoring and Microcell zone increases the
capacity

29. Cell Splitting
 In base stations where usage of cellular network is
high, these cell split into smaller cell

30. Cont..
 A new cell site must be constructed when the cell is
split
 Each with its own base station and a corresponding
reduction in antenna height
 Such that the radio frequencies are reassigned, and
transmission power is reduced
 The process of subdividing a congested cell into
smaller cell leads to increase in capacity
 Cell splitting is one of the easy and less costly solution
when increasing the capacity of cellular network

31. Cell Sectoring
 Sectorization consist of dividing an omnidirectional
(360 degree)view of cell site into non overlapping
slices called sectoring
 To overcome some limitations like co-channel
interference, cell sectoring is done
 There are 2 methods for cell sectoring
 1) 60 degree
 2) 120 degree

32. Cont…
 Replacing a single omnidirectional antenna at base
station with several directional antenna achieves
capacity improvement by essentially rescaling the
system
 Advantages
 It reduces interference which increases capacity
 It enables to reduce the cluster size and provides an
additional freedom in assigning channels
 Limitations
 Increased number of antennas at each base station
 Loss of traffic
 Since sectoring reduces the coverage area of a
particular group of channels, the number of handoffs
increases as well

33. Microcell
 As the splitting of cell idea evolves, the usage of
smaller cell becomes efficient and it leads the
creation of microcell
 The aim of creating a microcell are increasing the
capacity of cellular networks in areas where
population is very high

34. Microcell Zone Concept
 By the use of sectorization technique, we can increase
the system performance but there will be a large
increment of handoffs which results in the increment of
load on the switching and control link elements of the
mobile system
 So a Microcell Zone Concept is introduced which
leads to an increased capacity without any
degradation in trunking efficiency caused by
sectoring
 Large control base system is replaced by several lower
powered transmitters on the edge of the cell

35. Cont…
 The mobile retains in the same channel and the base
station simply switches to a different zone site and the
mobile moves from zone to zone
 A channel is active only in a particular zone in which
mobile is travelling , base station radiation is localized
and interference is reduced

36. Reference
 Ray, S; Pawlikowski, K; Sirisena, H; , ”Handover in Mobile WiMAX
Networks: The State of Art and Research Issues,” IEEE Commun.
Surveys & Tutorials , vol.PP, no.99, pp.1-24, 2010
 Channel assignment strategies; Srilasak , Wongthavarawat,
Limmongkol; wireless Innovation & security Lab., Nat. Electron,.&
Comput. Technol. Center, Pathymthani, Thailand..
 Adaptive cell sectoring using fixed overlapping sectors in CDMA
networks; alagan S. Anpalagan elvino S. Sousa; Department of
electrical and computer engineering; University of toronto.
 Evolved universal terrestrial radio access (E-UTRA), physical
channelsand modulation.3GPP TR 36.211, V.8.5.0, 2008.