2 Cellular Concepts R

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Mobile Communications Cellular Concept

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2 CELLULAR CONCEPTS

STRUCTURE

2.1 INTRODUCTION 2.2 CELLS : 2.3 CELLULAR SYSTEM CHARACTERISTICS 2.4 FREQUENCY REUSE :

2.1 INTRODUCTION

Traditional mobile service was structured similar to television broadcasting: One very

powerful transmitter located at the highest spot in an area would broadcast in a radius of

up to fifty kilometers. The Cellular concept structured the mobile telephone network in adifferent way. Instead of using one powerful transmitter many low-powered transmitter

were placed through out a coverage area. For example, by dividing metropolitan region

into one hundred different areas (cells) with low power transmitters using twelve

conversation (channels) each, the system capacity could theoretically be increased from

twelve conversations using one hundred low power transmitters.

The cellular concept employs variable low power levels, which allows cells to be sized

according to subscriber density and demand of a given area. As the populations grows,

cells can be added to accommodate that growth. Frequencies used in one cell cluster can

be reused in other cells. Conversations can be handed over from cell to cell to maintain

constant phone service as the user moves between cells.

Objective

The cellular system design was pioneered by during’70s by Bell Laboratories in the

United States, and the initial realization was known as AMPS (Advanced Mobile Phone

Service). The AMPS cellular service was available in United States in 1983. AMPS is

essentially generation 1 analog cellular system in contrast to generation 2 digital cellular

systems of GSM and CDMA (1S-95).

2.2 CELLS :

A cell is the basic geographic unit of cellular system. The term cellular comes from the

honeycomb areas into which a coverage region is divided. Cells are base stations

transmitting over small geographic areas that are represented as hexagons. Each cell sizevaries depending upon landscape. Because of constraint imposed by natural terrain and

man-made structures, the true shape of cell is not a perfect hexagon.

A group of cells is called a cluster. No frequencies are reused in a cluster.

Features of Digital Cellular Systems:

Small cells

Frequency reuse

Small, battery-powered handsets

Performance of handovers






2.3 CELLULAR SYSTEM CHARACTERISTICS

General Cellular radio systems allow the subscriber to place and receive

telephone calls over the wire-line telephone network where ever

cellular coverage is provided. Roaming capabilities extend service

to users traveling outside their “outside” home service areas.

characteristicsof digitalcellular systems

The distinguishing features of digital cellular systems compared to

other mobile radio systems are:

Small cells

A cellular system uses many base stations with relatively

small coverage radii (on the order of a 100 m to 30 km).

Frequency reuse

The spectrum allocated for a cellular network is limited. As a

result there is a limit to the number of channels or frequencies

that can be used. For this reason each frequency is usedsimultaneously by multiple base-mobile pairs. This frequency

reuse allows a much higher subscriber density per MHz of

spectrum than other systems. System capacity can be further

increased by reducing the cell size (the coverage area of a

single base station), down to radii as small as 200 m.

Small, battery-powered handsets In addition to supporting

much higher densities than previous systems, this approach

enables the use of small, battery-powered handsets with a radio

frequency that is lower than the large mobile units used in

earlier systems. Performance of handovers

In cellular systems, continuous coverage is achieved by executing a

“handover” (the seamless transfer of the call from one base station

to another) as the mobile unit crosses cell boundaries. This requires

the mobile to change frequencies under control of the cellular

network.

2.4 FREQUENCY REUSE :

Why frequencyreuse

The spectrum allocated for a cellular network is limited. As a result

there is a limit to the number of frequencies or channels that can be

used. A cellular network can only provide service to a large number of subscribers, if the channels allocated to it can be reused. Channel

reuse is implemented by using the same channels within cells

located at different positions in the cellular network service area.

Radio channels can be reused provided the separationbetween cells containing the same channel set is far enoughapart so that co-channel interference can be kept belowacceptable levels most of the time. Cells using the samechannel set are called co-channel cells.

Cell clustering The figure on the opposite page shows an example. Within the

service area (PLMN), specific channel sets are reused at a different

Power level should beoptimized such thatthe transmittedsignals will not crossthe radii of a cluster,beyond which the fre-quency is reused.






location (another cell). In the example, there are 7 channel sets: A

through G. Neighboring cells are not allowed to use the same

frequencies. For this reason all channel sets are used in a cluster of

neighboring cells. As there are 7 channel sets, the PLMN can be

divided into clusters of 7 cells each. The figure shows three clusters.

The number of channel sets is called K. K is also called the reusefactor. In the figure, K=7. Valid values of K can be found using

equation (where i and j are integers):

K=i²+j²+I*j

Explaining this equation is beyond the scope of this course. Some

constraints to K are provided later in this chapter. Note that in the

example: Cells are shaped ideally (hexagons). The distance between

cells using the same channel set is always the same.

Other cellclusters

The figure on the opposite page shows some examples of possible

clusters. The more cells in a cluster, the greater the separation

between co-channel cells when Other clusters are deployed. Theidea is to keep co-channel cell separation the same throughout the

system area for cells of the same size. Some valid cluster sizes that

allow this are: 1, 3, 4, 7, 9 and 12.

Procedure for locating co-channel cells

It is always possible to find cells using the same channel set, if only

the value of K is known. The following procedure is used.

In the figure on the opposite page an example is shown with K = 19.

SignalattenuationWith distance

Frequencies can be reused throughout a service area because radio

signals typically attenuate with distance to the base station (or

mobile station). When the distance between cells using the same

frequencies becomes too small, co-channelInterference might occur and lead to service interruption or

unacceptable quality of service.

Step Action

1 Use the integer values i and j from the equation, and start

With the upper left cell. Through this cell, draw the j-axis.

2 Draw the i-axis. To find the starting point for the i-axis, count j cellsdown the j-axis. In the example, one has to count 2 cells down (j=2).The positive direction of the i-axis is always two cell faces (120degrees) relative to the positive direction of the j-axis.

3 Find the first co-channel cell. It is found by counting i cells in thepositive i-axis direction. In the example, i = 3.

4 Find the other co-locating cells by repeating the previous steps. TheStarting point is again at the upper left cell, but now choose another

Direction for the j-axis (e.g. rotate the j-axis with 60 degrees, which isone cell face). As each cell has 6 faces, one will find 6 co-channelcells around the starting cells. These are the nearest located co-channel cells.






Capacity/Performance Trade-offs : n If K increases, then performance increases n If K increases, then call capacity decreases per cell The number of sites to cover a given area with a given high traffic density, and hence the

cost of the infrastructure, is determined directly by the reuse factor and the number of

traffic channels that can be extracted from the available spectrum. These two factors arecompounded in what is called spectral efficiency of the system. Not all systems allow the

same performance in this domain: they depend in particular on the robustness of the radio

transmission scheme against interference, but also on the use of a number of technical

tricks, such as reducing transmission during the silences of a speech communication. The

spectral efficiency, together with the constraints on the cell size, determines also the

possible compromises between the capacity and the cost of the infrastructure. All this

explains the importance given to spectral efficiency.

Many technical tricks to improve spectral efficiency were conceived during the system

design and have been introduced in GSM. They increase the complexity, but this is

balanced by the economical advantages of a better efficiency. The major points are the

following:The control of the transmitted power on the radio path aims at minimizing the average

power broadcast by mobile stations as well as by base stations, whilst keeping

transmission quality above a given threshold. This reduces the level of interference

caused to the other communications;

Frequency hopping improves transmission quality at slow speeds through frequency

diversity, and improves spectral efficiency through interferer diversity;

Discontinuous transmission, where by transmission is suppressed when possible, allows a

reduction in the interference level of other communications. Depending on the type of

user information transmitted, it is possible to derive the need for effective transmission. In

the case of speech, the mechanism called VAD (Voice Activity Detection) allows

transmission requirements to be reduced by an important factor (typically, reduced by

half);

The mobile assisted handover, whereby the mobile station provides measurements

concerning neighboring cells, enables efficient handover decision algorithms aimed at

minimizing the interference generated by the cell (whilst keeping the transmission quality

above some threshold).

References:

1. The GSM system for mobile communication-Michel Mouly & Marie-

Bernadette Pautet.

2. GSM system Engineering-Asha Mehrotra (Artech House Publisher).

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2 Cellular Concepts R