Wireless Application Protocol

Wireless Application Protocol

Cellular Network

Introduction

Early mobile system objective was to achieve a large coverage using single high power antenna

Impossible to reuse the same frequencies in the same coverage area.

For example, Bell mobile system in 1970 could support maximum of 12 simultaneous calls over a thousand square mile.

The Govt regulatory could not make spectrum allocation proportion to the increasing demand

Became imperative to restructure the telephone system to achieve high capacity with limited radio spectrum.

Cellular Concept

Cellular concept was a major breakthrough in solving problem of spectrum and user capacity Offers high capacity without any major change in technology.

Definition A cellular mobile communications system uses a large number of low-power

wireless transmitters to create cells—the basic geographic service area of a wireless communications system. Variable power levels allow cells to be sized according to the subscriber density and demand within a particular region. As mobile users travel from cell to cell, their conversations are handed off between cells to maintain seamless service. Channels (frequencies) used in one cell can be reused in another cell some distance away. Cells can be added to accommodate growth, creating new cells in un served areas or overlaying cells in existing areas.

Cellular System Architecture

To improve the quality of service and to support more users in their systems. Because the amount of frequency spectrum available for mobile cellular use was limited, efficient use of the required frequencies was needed for mobile cellular coverage.

Rural and urban regions are divided into areas according to specific provisioning guidelines. Deployment parameters, such as amount of cell-splitting and cell sizes, are determined by engineers experienced in cellular system architecture.

Cellular System Architecture

Cells:- A cell is the basic geographic unit of a cellular system. Shape of the 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 on the landscape. Because of constraints

imposed by natural terrain and man-made structures Clusters A cluster is a group of cells No channels are reused within a cluster.

Cellular System Architecture Example of Cluster

Cellular System Basic Concepts

High Capacity:- high capacity is achieved by limiting the coverage of each base station to a small geographic region called cell.

Same frequency/timeslots/codes are reused by spatially separate base stations.

A switching techniques called Handoff enables a call to processed uninterrupted when user moves from one cell to another.

Resolves problem of limited radio spectrum.

Cellular System Basic Concepts..

Neighboring base stations are assigned different groups of channels so that the interference between base stations (and the mobile users under their control) is minimized.

By systematically spacing base stations and their channel may be reused as many times as necessary so long as the interference between co-channel stations is kept below acceptable levels.

As the demand for service increases the number of base stations may be increased thereby providing additional radio capacity.

Frequency Reuse

Small number of radio channel frequencies were available for mobile systems, The solution the industry adopted was called frequency planning or frequency reuse. Frequency reuse was implemented by restructuring the mobile telephone system architecture into the cellular concept.

The concept of frequency reuse is based on assigning to each cell a group of radio channels used within a small geographic area. Cells are assigned a group of channels that is completely different from neighboring cells. The coverage area of cells is called the footprint. This footprint is limited by a boundary so that the same group of channels can be used in different cells that are far enough away from each other so that their frequencies do not interfere

Frequency Reuse

Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region.

Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell.

Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells.

The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning

Frequency Reuse

FrequencyReusePatterns

Cell Shape

The hexagonal shape representing a cell is conceptual and simplistic model of coverage.

The actual radio coverage is known as the footprint and is determined from field measurement, propagation prediction models

– However a regular shape is needed for systematic system design and adaptation to future growth.

It might be natural to choose a circle to represent coverage but adjacent circles cannot be overlaid upon a map without leaving gaps or creating overlapping.

Gaps

Overlapping

Case A Case B

Three possible choices of shapes: square, equilateral triangle and hexagon.

For a give distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three

Thus by using hexagon geometry, the fewest number of cells can cover a geographic region and it closely approximates circle.

Capacity of System

When using hexagon to model coverage areas– Center-excited Cell: BS depicted as being either in the center

of the cell Omni-directional antenna is used

– Edge-excited Cell: on three of the six cell vertices Sectored direction antenna is used

Consider a cellular system – which has S duplex channels available for reuse.– Each cell allocated group of k channels (k < S)– S channels divided among N cells (unique and disjoint) then

S = kN

Cluster: N cells, which collectively use the complete set of available frequencies

If a cluster is replicated M times in the system, the number of duplex channels C as a measure of capacity is

C = MkN = MS So capacity is directly proportional to the replication

factor in a fixed area. Factor N is called cluster size and is typically equal to

4, 7, 12.

If cluster size N is reduced while cell size is kept constant

– more clusters are required – More capacity is achieved

Large cluster size indicates that co-channel cells are far from each other

Conversely, small cluster size means co-channel cells are located much closer together

The value of N is a function of how much interference a mobile or BS can tolerate

Clusters are inversely proportion to N– Capacity is directly proportional to Clusters– Thus frequency reuse factor is given by 1/N.

In last fig, each hexagon has exactly six equidistant neighbors and that the lines joining the centers of any cell and its neighbors are separated by multiple of 60 degrees.– There are only certain cluster sizes and layouts

possible

Locating co-channel neighbors

To connect hexagons without gaps,– The geometry of hexagon is such that the number of cells per

cluster N can only have valuesN = i2 + ij + j2

where i and j are non-negative integers. To find out the nearest co-channel neighbors of a

particular cell, do the following– Move I cells along any chain of hexagon– Then turn 60 degree counter clockwise and move j cells

Example: Locating co-channel cells

In this example N=19, i=3, j=2

Example

BW = 33 MHz allocated to particular FDD cellular system, where two 25 KHz simplex channel to provide full-duplex for voice/data.

Compute the number of channels per cell if a system uses

– Four-cell reuse– Seven-cell reuse– Twelve-cell reuse.

If 1 MHz is dedicated to control channels, determine equitable distribution of control and voice channels per cell for above three systems?

Solution: Part I

TotalBW = 33 MHz, ChannelBW = 25 KHz x 2 = 50 KHz/duplex channelS = 33,000 / 50 = 660 channelsFor N = 4

k = 660 / 4 ≈ 165 channelsFor N = 7

k = 660 / 7 ≈ 95 channelsFor N = 12

k = 660 / 12 ≈ 55 channels

Solution: Part II

Sc = 1000 / 50 = 20 channelsSv = S – Sc = 660 – 20 = 640 channelsFor N=4,

5 control channels + 160 voice channel. For N=7,

4 cells with 3 control + 92 voice channels2 cells with 3 control + 90 voice channels1 cell with 2 control + 92 voice channelsIn practice, 1 control/cell and 4x91 + 3x92 voice channels

For N = 12, 8 cells with 2 control + 53 voice channels4 cells with 1 control + 54 voice channelsIn practice, 1 control and 8x53 + 4x54 voice channels

Operation of Cellular Systems Base station (BS) at center of each cell

– Antenna, controller, transceivers Controller handles call process

– Number of mobile units may in use at a time BS connected to mobile telecommunications switching office (MTSO)

– One MTSO serves multiple BS– MTSO to BS link by wire or wireless

MTSO:– Connects calls between mobile units and from mobile to fixed telecommunications

network– Assigns voice channel– Performs handoffs– Monitors calls (billing)

Fully automated

Overview of Cellular System

Channels

Control channels– Setting up and maintaining calls– Establish relationship between mobile unit and

nearest BS Traffic channels

– Carry voice and data

Typical Call in Single MTSO Area (1)

Mobile unit initialization– Scan and select strongest set up control channel– Automatically selected BS antenna of cell

Usually but not always nearest (propagation anomalies)– Handshake to identify user and register location– Scan repeated to allow for movement

Change of cell– Mobile unit monitors for pages (see below)

Mobile originated call– Check set up channel is free

Monitor forward channel (from BS) and wait for idle– Send number on pre-selected channel

Paging– MTSO attempts to connect to mobile unit– Paging message sent to BSs depending on called mobile number– Paging signal transmitted on set up channel

Typical Call in Single MTSO Area (2) Call accepted

– Mobile unit recognizes number on set up channel– Responds to BS which sends response to MTSO– MTSO sets up circuit between calling and called BSs– MTSO selects available traffic channel within cells and notifies BSs– BSs notify mobile unit of channel

Ongoing call– Voice/data exchanged through respective BSs and MTSO

Handoff– Mobile unit moves out of range of cell into range of another cell– Traffic channel changes to one assigned to new BS

Without interruption of service to user

Other Functions Call blocking

– During mobile-initiated call stage, if all traffic channels busy, mobile tries again

– After number of fails, busy tone returned Call termination

– User hangs up– MTSO informed– Traffic channels at two BSs released

Call drop– BS cannot maintain required signal strength– Traffic channel dropped and MTSO informed

Calls to/from fixed and remote mobile subscriber– MTSO connects to PSTN– MTSO can connect mobile user and fixed subscriber via PSTN– MTSO can connect to remote MTSO via PSTN or via dedicated lines – Can connect mobile user in its area and remote mobile user

Call Stages