this ppt useful for knowing all basic level of mobile communication technologies.

1. Fundamentals of Cellular
Engineering
Prepared By :
Maulik Patel

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
N=i2
+ij+ j2
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 user moves from one cell to the other, to keep the
communication between the user pair, the user channel has to
be shifted from one BS to the other without interrupting the
call
• when a MS moves into another cell, while the conversation is
still in progress, the MSC automatically transfers the call to a
new FDD channel without disturbing the conversation. This
process is called as handoff.
 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
 Once a signal level is set as the minimum acceptable for good
voice quality (Prmin), then a slightly stronger level is chosen as the
threshold (PrH)at which handoff has to be made.
 A parameter, called power margin, defined as
Δ = PrH − Prmin
 If Δ is too small, then there may not be enough time to complete
the handoff and the call might be lost even if the user crosses the
cell boundary.
 If Δ is too high o the other hand, then MSC has to be burdened
with unnecessary handoffs. This is because MS may not intend to
enter the other cell.

15. Handoff Generation
 Handoff for first generation analog cellular systems
• 10 secs handoff time
• is in the order of 6 dB to 12 dB
• Handoff for second generation cellular systems, e.g., GSM
• 1 to 2 seconds handoff time
• mobile assists handoff
• is in the order of 0 dB to 6 dB
• Handoff decisions based on signal strength, co-channel
interference, and adjacent channel interference.
• 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

16.  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

17. 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

18. 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

19. 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, P
Guard Channel Scheme

20.  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
Guard Channel Scheme (Cont.)

21. 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

22.  The Trade-off :
Enhancing Capacity And Cell
Coverage
range
(km)
Throughput
/cell
(Mbps) 802.11b
Noise
Limited
Interference
Limited
A-MAS
Benefit
Technical Interpretation
 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

23.  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

24. Cell Description
 Why cell shape is hexagonal…?
 Why Cell Splitting , Sectoring and Microcell zone…..?
 As users increases per cell the channel capacity decreases
 Techniques needed to provide extra channel

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

26. Cont..
 A new cell site must be constructed when the cell is
split
 Such that the radio frequencies are reassigned, and
transmission power is reduced
 Each with its own base station and a corresponding
reduction in antenna height
 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

27. 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

28. 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

29. 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

30. 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 load
and switching caused by sectoring
 Large control base system is replaced by several lower
powered transmitters on the edge of the cell

31. 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

32. Reference
 Evolved universal terrestrial radio access (E-UTRA), physical
channelsand modulation.3GPP TR 36.211, V.8.5.0, 2008.
 Channel assignment strategies; Srilasak , Wongthavarawat,
Limmongkol; wireless Innovation & security Lab., Nat. Electron,.&
Comput. Technol. Center, Pathymthani, Thailand..
 Influence of the Handoff Process on the Channel Holding Time
Distribution for Cellular Systems.Doiningo Lara-Rodriguez. Center
for Resenrcli and Advanced Studies. Electricill Engineering
 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
 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.

33. Thank You