Cellular concepts

Name of Institution

1

Submitted to:

Mr. Sumit Singh

Dhanda

CELLULAR CONCEPTS

Name of Institution

2

Topics

• System design fundamentals

• Frequency reuse channel assignment strategies

• Hand off strategies, Interference and system

capacity

• Improving coverage and capacity in cellular

system.

Name of InstitutionBasics Early mobile telephony systems were not cellular. Coverage over a large

area was provided by a high powered transmitter mounted on a tall tower.

That resulted in very low capacity.

The cellular concept arose from the need to re-structure the radio telephone

system with the increase in demand. The increase in demand could not be

satisfied just by additional spectrum allocations.

Cell – a geographical area covered by a BS.

Base station antennas are designed to cover specific cell areas.

For convenience, the cells are shown with a hex pattern. A hex pattern

is the simplest pattern that can tessellate an area.

simple model for easy analysis → circles leave gaps

• In practice, cells are not hexagonal

• Base station location

– cell center → omni-directional antenna (360° coverage)

– not necessarily in the exact center (can be up to R/4 from the ideal

location)3

Name of Institution

4

The Cellular concept is a system level idea which calls for replacing,

a single, high power transmitter with many low power transmitters,

each providing coverage to only small portion of the service area.

Cellular Concept

Name of Institution

5

It is a design process of selecting & allocating channel groups

of cellular base stations.

Co-channel cells → cells that share the same set of frequencies

VC & CC traffic in co-channel cells is an interfering source to

mobiles in several different cells

Two competing/conflicting objectives:

—maximize frequency reuse in specified area

—minimize interference between cells\

Frequency

Reuse/Planning

Name of Institution

6

Frequency reuse

pattern

Name of Institution

7

Contd... Cells labeled with the same letter use the same group of

channels.

Cell Cluster: group of N cells using complete set of availablechannels

Each cell allocated a % of the total number of availablechannels

Nearby (adjacent) cells assigned different channel groups toprevent interference between neighboring base stations andmobile users.

As frequency reuse↑ → # possible simultaneous users↑→ #subscribers ↑→ but system cost ↑ (more towers)

To increase number of users without increasing radiofrequency allocation, reduce cell sizes (more base stations)↑→ # possible simultaneous users ↑

Name of Institution

8

S : total # of duplex channels available for use in a

given area; determined by:

– amount of allocated spectrum

– channel BW → modulation format and/or standard

specs. (e.g. AMPS)

k : number of channels for each cell (k < S)

N : cluster size → # of cells forming cluster

S = k N

System Capacity

Name of Institution

9

Contd... M : number of times a cluster is replicated over a

geographic coverage area

System Capacity = Total # Duplex Channels

C = M S = M k N

If cluster size (N) is reduced and the geographic area for each cell is kept constant:

– The geographic area covered by each cluster is smaller, so M must ↑ to cover the entire coverage area (more clusters needed).

– S remains constant.

– So C ↑

– The smallest possible value of N is desirable to maximize system capacity.

Name of Institution

10

Contd...

Frequency reuse factor = 1 / N

– each frequency is reused every N cells

– each cell assigned k ≒ S / N

N cells/cluster

– connect without gaps

– specific values are required for hexagonal

geometry

• N = i2 + i j + j2 where i, j ≧ 1

• Typical N values → 3, 4, 7, 12; (i, j) = (1,1),

(2,0), (2,1), (2,2)

Name of InstitutionContd...

11

To find the nearest co-

channel neighbors of a

particular cell

−Move i cells along any

chain of hexagons, then

−turn 60 degrees and move

j cells.

Name of Institution

12

Goal is to minimize interference & maximize use of capacity

– lower interference allows smaller N to be used → greater frequency

reuse → larger C

Two main strategies: Fixed or Dynamic

Fixed

– each cell allocated a pre-determined set of voice channels

• calls within cell only served by unused cell channels

• all channels used → blocked call → no service

– several variations

• MSC allows cell to borrow a VC from an adjacent cell

• donor cell must have an available VC to give

Channel Assignment

Strategies

Name of Institution

13

Contd...

Dynamic

− channels NOT allocated permanently

− call request → goes to serving base station → goes to MSC

− MSC allocates channel “on the fly”.

allocation strategy considers:

– likelihood of future call blocking in the cell

– reuse distance (interference potential with other cells that are

using the same frequency)

– channel frequency

Name of Institution

14

Contd...

Advantage: reduces call blocking (that is to say, it

increases the trunking capacity), and increases voice

quality

Disadvantage: increases storage & computational

load at MSC

– requires real-time data from entire network related

to:

• channel occupancy

• traffic distribution

Name of Institution

15

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

– new voice and control channel frequencies

– very important task → often given higher priority than new

call

choose a (handoff threshold) > (minimum useable signal level)

– so there is time to switch channels before level becomes

too low

– as mobile moves away from base station and toward

another base station

Handoff Strategies

Name of Institution

16

Name of Institution

17

Contd...

A dropped handoff can be caused by two factors

– not enough time to perform handoff

a. delay by MSC in assigning handoff

b. high traffic conditions and high

computational load on MSC can cause

excessive delay by the MSC

– no channels available in new cell

Name of Institution

18

Contd... 2nd Generation Cellular digital TDMA (GSM, IS-136)

a. Mobile Assisted Hand Offs (MAHO)

− important advancement

− The mobile measures the RSS of the FCC’s from

adjacent base stations & reports back to serving base

station

− if Receiver 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

A mobile may move into a different system controlled by a different MSC

– Called an intersystem handoff

Name of Institution

19

Guard channel

% of total available cell channels exclusively set

aside for handoff requests

makes fewer channels available for new call

requests

a good strategy is dynamic channel allocation

(not fixed)

− adjust number of guard channels as needed by demand

− so channels are not wasted in cells with low traffic

Name of Institution

20

Queuing Handoff

Requests

use time delay between handoff threshold and

minimum useable signal level to place a blocked

handoff request in queue

a handoff request can "keep trying" during that

time period, instead of having a single block/no

block decision

prioritize requests (based on mobile speed) and

handoff as needed

calls will still be dropped if time period expires

Name of Institution

21

Problems occur because of a large range of mobile

velocities

– pedestrian vs. vehicle user

Small cell sizes and/or micro-cells → larger number

of handoffs

MSC load is heavy when high speed users are passed

between very small cells

Practical Handoff

Considerations

Name of Institution

22

use different antenna heights and Tx power levels

to provide large and small cell coverage

large cell → high speed traffic → fewer handoffs

small cell → low speed traffic

example areas: interstate highway passing through

urban center, office park, or nearby shopping mall

Umbrella Cells

Name of Institution

23

Name of Institution

24

Cell Dragging

low speed user with line of sight to base station

(very strong signal)

strong signal changing slowly

user moves into the area of an adjacent cell

without handoff

causes interference with adjacent cells and other

cells

Name of Institution

25

Co-Channel

Interference

Interference is the limiting factor in performance of allcellular radio systems

What are the sources of interference for a mobilereceiver?

Interference is in both

– voice channels

– control channels

Two major types of system-generated interference:

1) Co-Channel Interference (CCI)

2) Adjacent Channel Interference (ACI)

Name of Institution

26

Contd... Possible Solutions:

1) Increase base station Tx power to improve radio

signal reception

− this will also increase interference from co-channel

cells by the same amount

2) Separate co-channel cells by some minimum

distance to provide sufficient isolation from

propagation of radio signals

− if all cell sizes, transmit powers, and coverage patterns

≈ same → co-channel interference is independent of Tx

power

Name of Institution

27

Contd...

co-channel interference depends on:

– R : cell radius

– D : distance to base station of nearest co-channel

cell

if D / R ↑ then spatial separation relative to cell

coverage area ↑

– improved isolation from co-channel RF energy

Q = D / R : co-channel reuse ratio

– hexagonal cells → Q = D/R =√3N

Name of Institution

28

When the demand for wireless service increases, the number

of channels allotted to a cell becomes insufficient to support

the required number of users.

Certain design techniques are used to provide more number of

channels per unit coverage area, thus increasing the capacity:

– Cell Splitting : It allows an orderly growth of the cellular

system

– Sectoring : It uses Directional antennas to control the

interference and frequency reuse of channels.

– Zone Microcell: It distributes the coverage of the Cell.

– More bandwidth

– Borrow channel from nearby cells

Improve Capacity

Name of Institution

29

It is the process of subdividing the congested cell into smaller

cells.

Each of the smaller cells will have their own base station with a

reduction in antenna height and transmitted power.

The smaller cells are known as Microcells.

Cell Splitting increases the capacity of the cellular system as it

increases the number of times the channels are reused

The increased number of cells would increase the number of

clusters over the coverage region which in turn increase the

number of channels and thus capacity in the coverage area

Cell Splitting allows the system to grow by replacing large cells

with smaller cells without changing the co-channel re-use ratio

(Q).

Cell Splitting

Name of Institution

30

Cells are split to add channels with no new spectrum

usage

Name of Institution

31

Name of Institution

32

Sectoring

The co-channel interference in a cellular system can be decreased by replacing the Omni directional antenna at the base station by several directional antennas, each radiating within a specified sector.

The process of reducing the co-channel interference and thus increasing the capacity of the system by using directional antennas is known as Sectoring.

In general a cell is partitioned into three 120 degree sectors or six 60 degree sectors.

When sectoring is employed, the channels used in a particular cell are broken down into sectored groups and are used only in a particular sector.

Name of Institution

33

Name of Institution

THANK YOU

34