无线通信原理与应用英lecture2

Chapter2: The Cellular Concept -System Design Fundamentals

Lecture 2

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2.1 The Concept :Cellular Networks

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geometric shapes

Circle: Equilateral triangle: Square: Hexagon:

A c r 2 3.14r 2A t 1.3r 2

A s 2.0r 2

A h 2.6r 2

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Cellular coverage representation

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The size of a cell

Macrocell large, covering a wide are

range of several hundred kilometres (km) to ten kmmostly deployed in rural and sparsely populated areas

Microcell medium cell, coverage are a smaller than in macro cells

range of several hundred meters to a couple of metresdeployed mostly in crowded areas, stadiums, shopping malls

Picocell

small, covering a very small area range of several tens of metres low power antennas, indoor

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Three different kinds of cells

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Base station transmitters

In hexagonal cells, base stations transmitters are either: –centre-excited, base station is at the centre of the cell –edge-excited, base station at 3 of the 6 cell vertices

centre-excited

edge-excited

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2.2 Frequency Reuse and Capacity

S kNC MkN MSC can be used as a measure of capacity

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Example 1A mobile communication system uses a frequency reuse factor of1/7 and 416 channels available. If 21 channels are allocated as control channels, compute its system capacity. Assume a channel supports 8 users Channels available for allocation = 416 - 21 = 395 Channel Number of a cell = 395 / 7 = 57 Number of simultaneous users per cell = 8 x 57 = 456 Number of simultaneous users in system = 7 x 456 = 3192

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Method of locating co-channel cells

i 3, j 2

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2.3 Channel Assignment and handoffChannel Assignment Strategies: Two

assignment approachesFixed and static (most common) Dynamic

Fixed

channel assignment

all channels in a cell could be in use all the time –new calls are then blocked (no channels left) –may be solved by borrowing spare channels from nearby cells

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Dynamic channel assignment – MSC allocates frequencies when a call is made – Provides high channel utilization To do this it needs real-time information on – channel occupancy – traffic distribution

– radio signal strength indication (RSSI)– high computational load and increased storage

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Handover (Handoff)

Provides continuity of communication across cells Difficulty – dropping a call before reconnecting is unacceptable – different cells use different frequencies – mobile phone users usually move from place to place and very quickly

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Handover Process

– Received signal weakens as mobile moves out of cell – Cell site at some point requests handover to cell with

stronger signal strength– MSC switches call to new cell after allocating channels.

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Illustration of a handoff scenario at cell boundary

BS2

BS1

BS

3

PSTN

MSC Trunks

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The basic concept of handover strategies

Handover must not be too frequent or system is kept busy servicing handover requests

– handover threshold is set slightly stronger– Minimum usable signal level is normally set to be between -90 dBm and -100 dBm

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