EEE440 Modern
Communication Systems
Cellular Systems
Introduction
• The geographical area of coverage is organised
into cells
• Each cell is controlled by a base station
• A common model of cellular structure in a twodimensional case is to consider all cells to be
hexagonal in shape and all of the same size
• In real systems, cells have complex shapes
depending on antenna directivity and location,
propagation conditions and terrain topography
Structure of a cellular system
Structure of a cellular system
• BS-controls multiple MS
• MSC – controls multiple BS
– Responsible for intercellular handover, mobile location, paging
and mobility management
• HLR – contains reference and profile information
for all mobile users registered with the MSC as
their home location
• VLR- registers visiting MS
Spectral allocation
Radio spectrum allocation is made by
authorities. e.g. in Malaysia, the MCMC
allocates spectrum to mobile operators
Spectral allocation
Channel allocation
• The band is broken into a number of channels
• Channels in a wireless communication system typically
consist of time slots in TDMA , frequency bands in
FDMA and/or CDMA pseudo noise sequences, but in an
abstract sense, they can represent any generic
transmission resource
• The number of channels limit the number of
simultaneous users
• To increase the capacity a given service area is divided
into a number of cells
• The channels can be reused in different cells
Channel reuse
• Different cells can use the same frequency
channel
• However, adjacent cells cannot be
assigned the same frequency because of
inter-channel inteference
• The assignment must be spaced far
enough apart to keep interence to
tolerable levels
Channel reuse
• For example in a one dimensional cell
structure, the total number of channels can
be divided into 4 groups (4-reuse)
• There are three-cells separating cells with
the same set of frequencies
Channel reuse
Channel reuse
• The assignment strategy depends on the
tolerable interference which is quantified by
calculating the signal-to-interference ratio (SIR)
or also called carrier-to-interference ratio (CIR)
SIR = desired average signal power at a receiver
total average interference power
• The SIR should be greater than a specified
threshold for a proper signal operation
• For GSM the desired SIR is 7-12dB
SIR calculations
• Calculated on an average power basis
• Focus on the distance-dependent part of
the received power equation (ignores
shadow and multipath fading)
• Assume g(d)=kd-n; n = 3 or 4
SIR calculations
• Consider 1-dimensional cell structure
– D= spacing between interfering cells
– R=the half width (center to edge) of each cell
• Consider downlink power receive at a mobile
located at the edge of a cell (worst situation) at
point P
• Say each base station located at the centre of its
cell transmits with the same average power, PT
SIR calculations
• The average received power at distance d
meter from a base station is given by
PTd-n ; n = 3 or 4
• The SIR at the mobile at point P is given
by
n
PtR
SIR 
 Pint
Sum of all interfering base stations
SIR calculations
• Theoretically all base stations transmitting
at the same frequency will interfere with
the home base station transmission
• However, in reality only a relatively small
number of nearby interferes need be
considered because of the rapidly
decreasing received power as the
distance, d increases
SIR calculations
• Consider the first tier interferers only
• The two interfering base stations closest to
the mobile at point P are located at (D+R)
and (D-R) respectively from the mobile
• The corresponding SIR is given by
R n
SIR 
( D  R) n  ( D  R) n
SIR calculations
• Calculate the SIR in dB for different values
of n (3 or 4) and different cell reuse (3 or
4)
• What can you conclude?
• Individual assignment questions.
SIR calculations
• Consider 2-dimensional cell structure
• All hexagonal cells of same size
• The number of cells for an area is given generally by,
C=i2 + j2 + ij ; i, j = integers 1,2,3…
• For GSM C=3 or 4
SIR calculations
• Consider a typical hexagonal cell
• The distance from the center of the cell to any vertex is
the radius R
• Each edge is of length R
• The distance across the cells = √3R
SIR calculations
• There are 6 interfering base stations
around the home base station
• The spacing between the closest
interfering base stations is given by
D3 =3R for 3-cell reuse (c=3)
D4=2√3R for 4-cell reuse (c=4)
– In general for C-cell reuse, Dc=√3C R
SIR calculations
• Consider the case when the mobile is at
the middle of the cell
• The SIR is given by
SIR = PT / (6PT√3C R-n)
= 1/ (6√3C R-n)
• At the edge of the cell, the are many
proposed approximations
SIR calculations
SIR 
1
 D  n
  1
 R 
D 
   1
R 
n
D
 4 
R
n 


Estimate the appropriate C for GSM with minimum required SIR of 7dB.
Channel Allocation Schemes
• Allocate channels to base stations and access points
and to avoid co-channel interference among nearby cells
• Fixed Channel Allocation
– requires manual frequency planning to allocate specific channels to specific cells
– This allocation is static and can not be changed
– the number of channels in the cell remains constant irrespectively of the number
of customers in that cell.
– For efficient operation, FCA systems typically allocate channels in a manner that
maximizes frequency reuse.
– Thus, in a FCA system, the distance between cells using the same channel is the
minimum reuse distance for that system.
– The problem with FCA systems occurs whenever the offered traffic to a network
of base stations is not uniform.
– Consider a case in which two adjacent cells are allocated N channels each.
There clearly can be situations in which one cell has a need for N+k channels
while the adjacent cell only requires N-m channels (for positive integers k and
m).
– In such a case, k users in the first cell would be blocked from making calls while
m channels in the second cell would go unused.
– Clearly in this situation of non-uniform spatial offered traffic, the available
channels are not being used efficiently. This result in traffic congestion and some
calls being lost when traffic gets heavy in some cells, and idle capacity in other
cells.
Channel Allocation Schemes
• Dynamic
– handles bursty cell traffic and utilizes the cellular radio resources more
efficiently.
– DCA allows the number of channels in a cell to vary with the traffic load,
hence increasing channel capacity with little costs.
– In DCA systems, no set relationship exists between channels and cells.
Instead, channels are part of a pool of resources.
– Whenever a channel is needed by a cell, the channel is allocated under
the constraint that frequency reuse requirements can not be violated.
– There are two problems that typically occur with DCA based systems.
– First, DCA methods typically have a degree of randomness associated
with them and this leads to the fact that frequency reuse is often not
maximized unlike the case for FCA systems in which cells using the
same channel are separated by the minimum reuse distance.
– Secondly, DCA methods often involve complex algorithms for deciding
which available channel is most efficient. These algorithms can be very
computationally intensive and may require large computing resources in
order to be real-time.
Channel Allocation Schemes
• Hybrid
– Combined FCA and DCA
– Channel Borrowing is one of the most straightforward hybrid allocation
schemes.
– Here, channels are assigned to cells just as in fixed allocation schemes.
– If a cell needs a channel in excess of the channels previously assigned
to it, that cell may borrow a channel from one of its neighboring cells
given that a channel is available and use of this channel won't violate
frequency reuse requirements.
– Note that since every channel has a predetermined relationship with a
specific cell, channel borrowing is often categorized as a subclass of
fixed allocation schemes.
– The major problem with channel borrowing is that when a cell borrows a
channel from a neighboring cell, other nearby cells are prohibited from
using the borrowed channel because of co-channel interference.
– This can lead to increased call blocking over time. To reduce this call
blocking penalty, algorithms are necessary to ensure that the channels
are borrowed from the most available neighboring cells; i.e., the
neighboring cells with the most unassigned channels.
Channel Allocation Schemes
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Two extensions of the channel borrowing approach are Borrowing with
Channel Ordering (BCO) and Borrowing with Directional Channel
Locking (BDCL).
Borrowing with Channel Ordering was designed as an improvement over
the simpler Channel Borrowing approach
BCO systems have two distinctive characteristics
– The ratio of fixed to dynamic channels varies with traffic load.
– Nominal channels are ordered such that the first nominal channel of a cell has
the highest priority of being applied to a call within the cell.
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The last nominal channel is most likely to be borrowed by neighboring
channels.
Once a channel is borrowed, that channel is locked in the co-channel cells
within the reuse distance of the cell in question.
To be "locked" means that a channel can not be used or borrowed.
From a frequency reuse standpoint, in a BCO system, a channel may be
borrowed only if it is free in the neighboring cochannel cells.
Channel Allocation Schemes
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In Borrowing with Directional Channel Locking, borrowed channels are only locked in
nearby cells that are affected by the borrowing.
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This differs from the BCO scheme in which a borrowed channel is locked in every cell
within the reuse distance.
The benefit of BDCL is that more channels are available in the presence of borrowing
and subsequent call blocking is reduced.
A disadvantage of BDCL is that the statement "borrowed channels are only locked in
nearby cells that are affected by the borrowing" requires a clear understanding of the
term "affected.“
This may require microscopic analysis of the area in which the cellular system will be
located. Ideally, a system can be general enough that detailed analysis of specific
propagation measurements is not necessary for implementation.
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Channel Allocation Schemes
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A natural extension of channel borrowing is to set aside a portion of the
channels in a system as dynamic channels with the remaining (nominal)
channels being fixed to specified cells.
If a cell requires an extra channel, instead of borrowing the channel from a
neighboring cell, the channel is borrowed from the common "bank" of
dynamic channels.
An important consideration in hybrid systems of this type is the ratio of
dynamic channels to fixed channels.
The optimum ratio depends upon the traffic load
Locally Optimized Dynamic Assignment Strategy (LODA), this method is
best described as a purely dynamic channel allocation procedure as
opposed to a hybrid method.
In this strategy there are no nominal channels; all channels are dynamic.
When a given cell needs to accommodate a call, it chooses from among the
bank of available channels according to some cost criteria.
The channel with minimum cost is assigned.
In a general sense, the cost is a measure of the future blocking probability
in the vicinity of the cell given that the candidate channel is assigned.
Traffic handling capacity
• The number of channels available per cell is given by the
total number of channels divided by the cell reuse
parameter, C
• System performance is measured by the probability of
call blocking which describes the chance that a user
attempting to place a call receives a busy signal.
• The measure depends on the number of channels
available to handle simultaneous calls and the traffic
expected to utilise the system
• With a specified call blocking probability (e.g. 1% or 5%)
a limit must be put on the amount of traffic expected to
use the cell
Traffic handling capacity
• Traffic intensity or traffic load is commonly
defined as the product of the average number of
call attempts per unit time(λ) and the average
call length (1/µ)
• Traffic intensity, A = λ/µ in unit Erlangs
• The statistical model assumes that the pattern of
call attempts or arrival obeys a Poisson
distribution with average rate of arrival λ and the
call lengths are exponentially distributed with
average length 1/µ
Traffic handling capacity
• With N channels available, the cell
blocking probability, PB is given by the
Erlang-B formula
• A table or plot of PB vs A (Erlang-B
function) is used to find the number of
channels required for a given traffic load
and PB
Cell size
• Asssume that users are uniformly distributed
over the cell
• The area of the hexagonal cell of radius R is
(3√3R2)/2
• Say there is 1 call every 15 minutes and a
typical call last for 200 seconds on average
• The load for 1 user is given by
• For a total cell load, A=101 the number of users
is about 450 users
Cell size
• The user density for 450 users is given by
450 / (3√3R2)/2 = 173/R2 mobiles per unit
area
• Consider a rural area with density of
mobile = 2 terminals per km2. What is the
cell radius
• For suburban = 100 mobiles per km2?
• For urban = 1000 mobiles per km2?