Transcript EEE440 Modern Communication Systems

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
Spectral allocation
Radio spectrum allocation is made by
authorities. e.g. in Malaysia, the MCMC
allocates spectrum to mobile operators
Spectral allocation
Spectral allocation
• The band is broken into a number of
frequency channels, each for one call
• 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 frequency 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?
SIR calculations
• Now include the 2nd tier of interfering cells
in your calculations
• What is the SIR ? (try for both n=3 and
n=4 as well as 3 and 4 cell reuse)
• Analyse your results, compare with the
previous results (1st tier interferers).
• What can you conclude?
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  D  n  D  n 
  1    1  4  
R 
 R  
 R 
Estimate the appropriate C for GSM with minimum required SIR of 7dB.
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?
Paper reading and presentation
assignments
• Group assignment starting on Thursday 27/8
• Each group is assigned one paper to read,
summarise and present in class.
• Group can choose any one of the eight papers.
Only one group per paper.
• The list of paper are attached on my door and
groups need to reserve the paper.
• Papers can be downloaded from my lecture
notes page