1. Cellular Concept

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 a different 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.

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).

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 size varies 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:

n Small cells n Frequency reuse n Small, battery-powered handsets n Performance of handovers

1

Cellular System Characteristics

General

characteristics of digitalcellular systems

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.

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 reuseThe 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 used simultaneously 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 furtherincreased 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.

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

2

Cell clustering

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 separation between cells containing the same channel set is far enough apart so that co-channel interference can be kept below acceptable levels most of the time. Cells using the same channel set are called co-channel cells.

The figure on the opposite page shows an example. Within the service area (PLMN), specific channel sets are reused at a different 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 reuse factor. 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 cell clusters

Procedure for

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. The idea 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.

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

3

locating co-channel cells

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.

Step Action1 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 cells down

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 (120 degrees) relative to the positive direction of the j-axis.

3 Find the first co-channel cell. It is found by counting i cells in the positive 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 anotherDirection for the j-axis (e.g. rotate the j-axis with 60 degrees, which is one cell face). As each cell has 6 faces, one will find 6 co-channel cells around the starting cells. These are the nearest located co-channel cells.

Signal attenuationWith 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.

As long as the ratio

Frequency reuse distance = D Cell radius R

Is greater than some specified value, the ratio

Received radio carrier power = C Received interferer radio carrier power I

Will be greater than some given amount for small as well as

4

large cell sizes when all signals are transmitted at the same power level. The average attenuation of radio signals with distance in most cellular systems is a reduction to about 1/16 of the received power for every doubling of distance (1/10000 per decade).

The frequency reuse distance is also known as separation distance. is also known as the signal-to-noise ratio.

The figure on the opposite page shows the situation. At the base station, both signals from subscribers within the cell covered by this base station and signals from subscribers covered by other cells are received. Interference is caused by cells using the same channel set.

The ratio D/R needs to be large enough in order for the base station to be able to cope with the interference.

Relationshipbetween K and D/R

Capacity/performance trade-offs

There is a relationship between K and ratio D/R, shown by the following equation: ___ D/R= 3KExplaining this equation is beyond the scope of this course.

Note that there is a direct relationship between K and ratio D/R. The result is that when the reuse factor K, and so the shape of the cluster is chosen, ratio D/R is fixed.

When engineering a cellular network, the most important trade-off to make is the one between call capacity and performance:

Relationship between K and PerformanceThe performance of a cellular network can be expressed in quality of service. An acceptable quality of service means a low (co-channel) interference level in the network.

The relationship between the reuse factor K and the network performance is: if K increases, then the co-channel interference decreases, and so the performance increases (note that there is a fixed relationship between Kand ratio D/R).

Relationship between K and Cell Capacity

The other key relationship in cellular networks is the one

5

between the reuse factor K and call capacity. First of all, call capacity depends on the number of available channels. In GSM, a limited number of frequencies is available (for GSM: 124 frequencies, and for GSM-1800: 374 frequencies). The frequencies are grouped into frequency sets. If K increases, the number of frequencies per set (and so per cell) decreases, and so the call capacity per cell.

The value of K in GSM cellular networks varies between 4 and 21. Note that in real networks, K is not constant within the whole PLMN area, but varies depending on the traffic capacity needed in certain regions. Typically, K is high in urban regions and low in rural regions.

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 are compounded 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;

6

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).

7