Cellular Networks, Types, Analysis, Protocols (TCP/TCPIP)

INTRODUCTION TO CELLULAR SYSTEMS

The main objective of cellular system design is to handle as many calls as possible in a given bandwidth in the most efficient way with reliability and quality of service.
To achieve this objective, the cellular system employs two crucial features known as Frequency reuse and Cell splitting.

Frequency reuse refers to the usage of the same frequency carrier in different geographical locations that are distant enough so that the interference caused by using the same carrier is not a problem. The reason for the application of frequency reuse is to increase the number of simultaneous calls.
Cell splitting refers to the reconfiguration of a cell into smaller cells. This allows the system to adjust to an increase in the traffic demand in certain areas or in the whole network without any increase in the spectrum allocation.
In mobile communication “cells” represent a small geographic area which is why the technology is referred to as `cellular phone'.
Users are called Mobile stations (MS) which receive and transmit calls while moving in a cellular network.
Each cell has a Base station (BS) that supplies frequency channels to MS s. BS is also referred to as cell sites.
The Base stations are linked to a mobile switching centre (MSC) which is responsible for controlling the calls and acting as gateway to other networks.

Introduction to Cellular Geometry:

The geographical areas covered by cellular radio antennas are called cells. The cell site antenna is located at a point within the cell.
The cellular concept is a system-level idea which calls for replacing a single, high power transmitter (large cell) with many low power transmitters (small cells), each providing coverage to only a small portion of the service area.
Each base station is allocated a portion of the total number of channels available to the entire system, and nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighbouring base stations.
Neighbouring base stations are assigned different groups of channels so that the interference between base stations is minimized.
As the demand for service increases the number of base stations may be increased thereby providing additional radio capacity with no additional increase in radio spectrum.

Frequency Reuse

Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighbouring cells. The base station antennas are designed to achieve the desired coverage within the particular cell.
By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits.
The design all of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.
Figure illustrates the concept of cellular frequency reuse,
Cells labelled with the same letter use the same group of channels.
A cell cluster is outlined in bold and replicated over the coverage area.
The cluster size is 7 and the frequency reuse factor is 1/7 since each cell contains 1/7 of the total number of available channels.

Consider a cellular system which has a total of S duplex channels available for use.
Each cell is allocated a group of k channels (k < S)
The S channels are divided among N cells into unique and disjoint channel groups which each having the same number of channels.
Then the total number of available radio channels can be expressed as

S = kN

The N cells which collectively use the complete set of available frequencies is called a cluster.
If a cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of capacity and is given by

C = MkN = MS

Thus the capacity of a cellular system is directly proportional to the number of times a cluster is replicated in a fixed service area.
The factor N is called the cluster size and.is typically equal to 4, 7, or 12.
If the cluster size N is reduced while the cell size is kept constant, more clusters are required to cover a given area, and hence more capacity (a larger value of C) is achieved.
A large duster size indicates that the ratio between the cell radius and the distance between co-channel cells is small. Conversely, a small cluster size indicates that co-channel cells are located much closer together.
The smallest possible value of N is desirable in order to maximize capacity over a given coverage area (i.e., to maximize C)
The frequency reuse factor of a cellular system is given by 1/N, si each cell within a cluster is only assigned 1/N of the total available channels in the system.
The hexagonal geometry has exactly six equidistant neighbours and the lines joining the centres of any cell and each of its neighbours are separated by multiples of 60 degrees.
Hence to connect without gaps between adjacent cell, the geometry of hexagons is such that the number of cells per cluster, N, can only have values which satisfy the equation

N = i² + ij + j²

where i and j are non-negative integers

To find the nearest co-channel neighbours of a particular cell, one must
1. move i cells along any chain of hexagons and then
2. turn 60 degrees counter-clockwise and move j cells.

Handoff Strategies

When a mobile moves into a different cell while a conversation is in progress, the MSC automatically transfers the call to a new channel belonging to the new base st. This handoff operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station.
Handoffs must be performed successfully and as infrequently as possible.
System designers must specify an optimum signal level at which to initiate a handoff.
A particular signal level is specified as the minimum usable signal for acceptable voice quality at the base station receiver.
A slightly stronger signal level is used as a threshold at which a handoff is made.
This margin, given by Δ = P_{r handoff} – P_{r minimum usable}’ cannot be too large or too small.
If Δ is too large, unnecessary handoffs which burden the MSC may occur, and if Δ is too small, there may be insufficient time to complete a handoff before a call is lost due to weak signal conditions.
Handoffs are broadly of two categ4ies: Hard Handoff and Soft Handoff
Hard handoff is break-before -make. The connection to the old BS is broken before a connection to the new BS is made. Hard handoff is further divided into intercellular and intracellular types.
Soft handoff is make-before-break. The connection to the old BS is not broken until connection to the new BS is made.
The time over which a call may be maintained within a cell, without handoff, is called the dwell time.
Mobile assisted Handoff (MAHO): In today's second generation systems, handoff decisions are mobile assisted. In mobile assisted handoff (MAHO), every mobile station measures the received power from surrounding base stations and continually reports the results of these measurements to the serving base station. A handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the 'current base station by a certain level or for a certain period of time. The MAHO method enables the call to be handed over between base stations at a much faster rate than in first generation analog systems since the handoff measurements are made by each mobile, and the MSC no longer constantly monitors signal strengths. MAHO is particularly suited for microcellular environments where handoffs are more frequent.

Improving Coverage and Capacity in Cellular Systems

As the demand for wireless service increases, the number of channels assigned to a cell becomes insufficient to support the required number of users. Hence design techniques that provide more channels per unit coverage area are required.
Techniques such as Cell splitting, Sectoring, and Coverage zone approaches are used in practice to expand the capacity of cellular systems
Cell splitting allows an orderly growth of the cellular system where capacity is increased by increasing the number of base stations.
Sectoring uses placement of directional antennas to further control the co-channel interference and frequency reuse of channels.
The zone microcell concept distributes the coverage of a cell and extends the cell boundary to hard-to-reach places':
Cell splitting and zone microcell techniques do not suffer the trunking inefficiencies experienced by sectored cells. The base station takes care of handoff of microcells. Thus the computational load at the MSC is reduced.

Cell Splitting

Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own base station and a corresponding reduction in antenna height and transmitter power.
Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused.
By defining new cells which have a smaller radius than the original cells and by installing these smaller cells (called microcells) between the existing cells, capacity increases due to the additional number of channels per unit area.

The base stations are placed at corners of the cells, and the area served by base station A is assumed to be saturated with traffic.

New base stations are therefore needed in the region to increase the number of channels in the area and to reduce the area served by the single base station.
Thus smaller cells are added in such a way as to preserve the frequency reuse plan of the system.
The cell A is split into several cells called Microcells
The area of a cell is proportional to R². Thus splitting reduces the new cell radius to one-half a its original value. This Team that the area of cell reduces to one quarter of its original value. Theoretically 4 quarter-size cells can fit into one full size hexagonal cell.

Sectoring

Sectoring consists of increasing capacity by using directional antennas and decrease the co-channel interference.
Sectoring increases SIR so that the cluster size may be reduced.
First, the signal to interference ratio (SIR) is reduced by using directional antennas
Then the number of cells is decreased thereby increasing frequency reuse and capacity.
The co-channel interference in a cellular system may be decreased by replacing a single omnidirectional' antenna at the base station with several directional antennas, each radiating within a specified sector.

• By using directional antennas, a given cell will receive interference and transmit with only a fraction of the available co-channel cells. The factor by which the co-channel interference is reduced depends on the amount of sectoring used. A cell is normally partitioned into three 120° sectors or six 60° sectors.

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