WIRELESS COMMUNICATION

THE CELLULAR CONCEPT- SYSTEM DESIGN FUNDAMENTALS

LECTURE 4

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Tanvir Ahmad NiaziTanvir.niazi@mail.au.edu.pk

Air University, Islamabad

The Cellular Concept

1. Introduction

2. Frequency Reuse

3. Channel Assignment Strategies

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Early Mobile Telephone Systems

One high-power transmitter was used to cover a large area--- approx. 50km. Located at a very high spot.

The mobiles were

simultaneously connected

using different Frequency

channels. Capacity of such systems

was very limited.

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Basics

Early mobile telephony systems were not cellular. Coverage over a large area was provided by a high powered transmitter mounted on a tall tower. Frequency reuse was not employed. That resulted in very low capacity.

The cellular concept arose from the need to restructure the radio telephone system with the increase in demand. The increase in demand could not be satisfied just by additional spectrum allocations.

Cellular Concept: Replace large transmitters with many smaller transmitters. Neighboring base stations (BS) are assigned different sets of channels. Capacity can be increased by additional partitions.

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Cell – a geographical area covered by a BS.

Frequency Reuse – the frequency channels allocation scheme.

For convenience, the cells are shown with a hex pattern. A hex pattern is the simplest pattern that can tessellate an area.

In practice, cells are not hexagonal and BS are not exactly in the center of the cell.

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An Example of a Cellular Cluster

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Capacity Computations

Assume there are N cells, each allocated k different frequency channels. These N cells are said to form a cluster. Total number of channels per cluster is given by

S = k N

Total capacity associated with M clusters:

C = M k N = M S A cluster may be replicated more times in a given area if the cells are made

smaller (note that power needs to be reduced accordingly).

Capacity of cellular system is directly proportional to “M”, number of times a cluster is replicated.

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Capacity versus interference for same size cell Decrease N for More Capacity: If Cluster Size, N is decreased

while cell size remains fixed, more clusters are required to cover the area (M increases). Therefore, Capacity increases.

Increase N for Less Interference: On the other hand, if N is increased (large cluster size) means that co-channels are now farther than before, and hence we have will have less interference.

Value of N is a function of how much interference a mobile or a base station can tolerate.

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Frequency Reuse

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Frequency Reuse

Geometry of the hexagonal cells is such that to cover adjacent areas completely, N can have only some of the values.

N should satisfy the relationship,

where i and j are positive integers. Typical values of N are 4,7,12 etc.

To draw the cell pattern given i and j:1. Move i cells across hexagons.

2. Turn 60º CCW and move j cells.

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Frequency Reuse (N=7, i=2, j=1)

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Frequency Reuse (N=19, i=3, j=2)

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Example:A total of 33 MHz are allocated to a system which uses 2x25 kHz for full duplex (i.e., each channel is 50 kHz). What is the number of channels per cell?

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Number of channels per system

Now assume 1 MHz of the 33 MHz is allocated to control channels. Each control channel is still 50 kHz Total number of voice (traffic) channels is now

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Channel AssignmentStrategies

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Channel Allocation Techniques

To satisfy the user, a channel needs to be available on request. Reasonable probability of call blockage (GOS) is 2%. GOS fluctuate with location and time. The goal is to keep a uniform GOS across the system. Reduction of variations in GOS allow more users – an increase in

capacity. Three types of algorithms for channel allocation:

Fixed channel allocation (FCA) Channel Borrowing Dynamic channel allocation (DCA)

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Fixed Channel Allocation Techniques

Available spectrum is W Hz and each channel is B Hz. Total number of channels:

Nc = W/B

For a cluster size N, the number of channels per cell:

Cc = Nc/N

To minimize interference, assign adjacent channels to different cells.

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Features of Fixed Channel AllocationTechniques

FCA is the optimum allocation strategy for uniform traffic across the cells.

A non uniform FCA strategy, when it is possible to evaluate GOS in real time and adjust the FCA accordingly. This requires a more complex algorithm.

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Channel Borrowing

Borrow frequencies from low traffic cells to high traffic cells. Temporary channel borrowing: channel is returned after call is

completed. If all the channels in a cell are occupied, channels are borrowed

from neighboring cells. The MSC supervises such borrowing procedures and ensures

disruption free service.

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Dynamic Channel Allocation

All channels are placed in a pool, and are assigned to new calls according to the reuse pattern. Signal is returned to the pool, when call is completed.

Issues related to channel allocation are still under research.

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Comparison of Channel Allocation Techniques

Fixed Channel Allocation Advantages:

Less load on MSC Simple

Disadvantages: Blocking may happen

Dynamic Channel Allocation Advantages:

Voice channels are not allocated permanently. That is shared on need-basis

Disadvantages: Requires MSC for processing---burden on MSC May be very complicated

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Hand off

HANDOFF: The process of transferring a call across the cell boundaries. Handoffs are prioritized over new calls. Handoffs need to be performed infrequently.

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Assignment # 1:

Due date – 02 February, 2012

Problems: 1.3, 1.9, 1.10, 1.13, and 1.18,

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