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 two-

dimensional 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 allocationRadio 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

intP

PtRSIR

n

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

nn

n

RDRD

RSIR

)()(

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 byD3 =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 bySIR = PT / (6PT√3C R-n)

= 1/ (6√3C R-n)

• At the edge of the cell, the are many proposed approximations

SIR calculations

nnn

R

D

R

D

R

DSIR

411

1

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