Cellular Communication

Definition Uses a large number of low power

wireless transmitters to create cells. Cells geographic service area of wireless

communication. Variable power levels allow cells to be

sized. Mobile user can travel from cell to cell. Conversation are handed off between

cells.

Mobile Communication Principle Each mobile uses a separate, temporary

radio channel to talk to cell site. Channel use a pair of frequencies for

communication. Forward link one frequency for

transmitting from the cell site. Reverse link one frequency for the cell

site to receive calls from the users. Structure of mobile networks includes

telephone system and radio services.

Contd.

Early Mobile Architecture Structured in a fashion similar to

television broadcasting. One very powerful transmitter located at

the highest spot. Broadcast radius of up to 50 kilometers. E.g. metropolitan region into one

hundred different area. Using twelve conversational channels

each.

Contd.

Mobile Cellular Concept Interference problem due to same

channel in adjacent areas. So all channels could not be reused in

every cell. Frequency reuse was still viable

solution. Interference were not due to the

distance between areas. Due to the transmitter power (radius) of

areas.

Contd.

If we reduce the area by 50%. Increase the number of potential

customers. Reduce 10 kilometer radius into 1

kilometer radius would have one hundred times more channels.

Contd.

Cellular System Architecture Cells Clusters Frequency Reuse Co-Channel Interference Adjacent Channel Interference Cell Splitting Dimensioning Handoff

Cells

Basic geographic unit of cellular system.

Cellular comes from the honeycomb shape of the areas.

Cell size varies depending on the landscape.

In physical the true shape of cells in not perfect hexagon.

Clusters Cluster is a group of cells, denoted by N. Which can be formed from regular

patterns of cells is given by. N=i2+ij+j2 I,j=0,1,2,3,…… Hence only cluster sizes of 3,4,7,9,12,…… The Re-Use distance D, and cell radius R. The ratio D/R is a function of cluster size

and is known as Re-Use Ratio.

Contd. The values of D/R are shown below N 3 4 7 12 D/R 3 3.464 4.580 6 Values for D/R are approximately equal to Re-Use distance will determine the

amount of co-channel interference. The cluster size and D/R ratio therefore

determine the co-channel interference.

NRD 3/

19-cell reuse example (N=19)

Frequency reuse plan for C = 3, with hexagonal cells. (i=1, j =1)

Frequency Reuse

Small number of radio channel were available for mobile systems.

Find way to reuse radio channels. Mobile telephone system

architecture is restricted into cellular concept.

Coverage area of cells is called Footprint.

Contd.

Numerical Total Bandwidth 33MHz. Uses two 25Khz simplex channel to provide full

duplex voice and control channels. Compute the total number of channels

avaliable per cell if a system uses: 4 cell/cluster 7 cell/cluster 12 cell/cluster If 1 MHz of the allocated spectrum is dedicated

to control channels and voice channels in each cell for each of three systems.

Self practice question 3.4 page no.97.

Co-Channel and Adjacent Channel Interference

CCI is interference from two different radio stations on the same frequency.

ACI is interference caused by extraneous power from a signal in an adjacent channel.

Caused by inadequate filtering. ACI is distinguished from crosstalk.

Smaller N is greater capacity

Co-channel cells for 7-cell reuse

Signal to Interference Formula

S is desired signal power. I Interference power. i 0 number of co-channel interfering

cells. D/R co-channel reuse ratio. S/I signal-to-interference ratio.

S/I = (D/R)n/i 0

Numerical S/I ratio of 15 db is required for

satisfactory forward channel performance of a cellular system, what is the frequency reuse factor of cluster size that should be used for maximum capacity if the path loss exponent is n = 4 and n = 3? Assume that there are six co-channel cells in the first tier, and all of them are at the same distance from the mobile. Use suitable approximations.

Practical Handoff When mobile user traveled from one

cell to another during call. Adjacent areas using another radio

channel. Call either be dropped or transferred. Handoff occurs mobile telephone

network automatically transfer a call from adjacent cell radio channel.

Contd.

Handoff-the basics

Cellular Network Structure

Radio Base Station Mobile Switching Centre. Public Switching Telephone

Network.

Radio Base Station

Two way radio installation in a fixed location.

Communicate with one or more mobile or portable transceivers.

Typically used low power two way radios.

Mobile Switching Centre

Sophisticated telephone exchange. Provides circuit-switched calling,

Mobility management and mobile phone roaming.

Means Voice, data, fax and SMS and call divert.

Public Switching Telephone Network

PSTN is concentrated of the world’s public circuit-switched telephone network.

Same way that the Internet is the concentration of the world’s public IP-based packet-switched networks.

PSTN is entirely digital.

Coverage and Capacity Cell Splitting: allows orderly growth

of the cellular system. Sectoring: uses directional antennas

to further control the interference and frequency of reuse channels.

A Micro cell zone concept: distributes the coverage of a cell and extends the cell boundary to hard-to-reach place.

Cell Splitting Process of subdividing a congested cell

into smaller cell. Reduction in antenna height and

transmitter power. Increase capacity of cellular system by

channel reuse. In practice not all cells are split at same

time. Special care needs for co-channel cells

and handoff issues.

Before Cell Splitting

Cell Splitting

Numerical

Contd. Assume each base station uses 60

channels, regardless of cell size. If each original cell has a radius of 1 Km and each micro-cell has a radius of 0.5 Km, find the number of channels contained in a 3 Km square centered around A under the following condition: (a) without the use of micro cell (b) when the lettered micro cells as shown in figure is used; and (c) if all the original base station are replaced by micro cells. Assume cells on the edge of the square to be contained with the square.

Sectoring Increase capacity keep cell radius unchanged

decrease the D/R ratio. SIR is improved using directional antenna,

cluster size may be reduced. Necessary to reduce the relative interference

without the decreasing the transmit power. Cell is normally partitioned into three 120 and

six 60 sectors. Modern base station support sectorization,

handoff occurs without intervention from MSC.

Sectoring Improves S/I

Sectoring Improves S/I

Microcell Zone Concept Increase number of handoffs required

when sectoring is employed. Proposal is based on a microcell

concept for seven cell reuse. Each of three or possible more zone

sites are connected single base station.

Multiple zones and single base station make up a cell.

The Zone Cell Concept

Zone Cell Concept

Cellular Radio Network Planning

Dimensioning Coverage Planning Capacity and Frequency Planning Parameter Planning Troubleshooting

Dimensioning First phase of radio network planning. Main objective is plan of network and

calculate the network elements. Includes radio link power budget

calculation, rough cell radius estimation, and capacity number of calls estimation, transmission network estimation and network pricing.

Geographical information is another option to improve the dimension result because data is usually land use data, not elevation.

Coverage Planning For optimizing the locations and

the number of network sites. Phase network planner find good

site location and suitable geographical information.

Network planner see map. If the area is large paper sheet

map is not practical.

Capacity and Frequency Planning

In this plan many spatial analyses are used and also visualized on the map.

When the mobile phone user is moving, what will happen………

Ready plan of cellular network includes spatial data of network components: base station, antennas and repeaters.

Antenna has direction, height, tilt angle so on.

Cellular Traffic Tele-traffic necessary field in

telecommunications network planning. Ensure network costs are minimized

without compromising the quality of service of user network.

Includes Quality of service targets, traffic capacity and cell size, spectral efficiency and sectorization, traffic capacity versus coverage and channel holding time analysis.

Assignment Brief Description of mobile

communication principle and why we need mobile communication.

Can we resolve the problem of co-channel interference and adjacent channel interference? How it is possible justify?

Why we need re-use distance and cell radius in clusters of cell?

Contd.

What’s the value of D/R when the cluster size are 27,48 and 75? Explain with calculation?

Electromagnetic Radiation can effect human body. If its effect, so how can we resolve?