Unit 4 Cellular Telephony

Contents history of cellular telephony design and principle of cellular operation cellular telephony operation analog cellular telephones digital cellular telephones digital networks personal communication systems the third generation recent events in cellular telephony

Cellular Telephony System

Timing diagram illustrating how a call to a mobile user initiated by a landline subscriber is established.

Timing diagram illustrating how a call initiated by a mobile is established

Cellular concept

Cellular telephone system

Cellular telephone system Cellular telephone system provides a wireless connection to PSTN for any user location within the radio range A cellular system offers 1.high quality of service 2.high capacity 3.same radio channel may be reused 4.handoff Each BS have limited coverage area called cell

Communication between BS & MS is defined by use of two types of channels: Control Channels Voice channels

Cellular network Regions divided into “cells” modeled as hexagons. Each cell has a base station that all cell phones within the cell communicate with. Communication uses both data and control channel.

IDEAL ACTUAL DIFFERENT CELL MODEL CELL SHAPE IDEAL ACTUAL DIFFERENT CELL MODEL

Cellular network Why hexagon 1) it avoids dead spots & full area coverage is achieved 2) it requires fewer cell 3) it requires less transmitter sites 4) less expensive Group of cells is called cluster

Clusters The cells are grouped into clusters. The total number of channels are equally distributed among all cells in a cluster. The channel allocation pattern is repeated in every cluster. 1 2 3 4 5 6 7 Cluster size N = 7

Clusters (2) 1 2 4 1 3 1 2 4 2 4 3 3 1 4 2 4 3 1 3 2 4 3 1 Cluster size N = 4 2

Clusters (3) We cannot select cluster size arbitrarily. Cluster sizes must satisfy the following: N = i2+ij+j2 where i and j are non-negative integers. Thus N = 1, 3, 4, 7, 9, 12 …

1.Frequency reuse Users in different geographical areas (in different cells) may simultaneously use the same frequency Frequency reuse drastically increases user capacity and spectrum efficiency

Frequency Reuse… Process of selecting & allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning. To keep interference level within tolerable limits.

FREQUENCY REUSE The frequency spectrum is a limited resource. Therefore, wireless telephony, like radio, must reuse frequency assignments. For example, two radio stations might transmit at 91.3 FM. There is no interference as long as the radio stations are far enough apart. Problem: Limited frequency spectrum Solution: Based on the idea of splitting up coverage region into small areas referred to as cells

Frequency reuse Each cellular BS station allocated a group of radio channel to be used within small geographic area called as cell Adjacent BS are allocated different group of channel Antenna at BS are designed to achieve desired coverage in same cell The same group of channel may be used to cover different cell that are separated from one another by distance large enough to keep interference level min The process of selecting & allocating channel groups for all cellular BS is called frequency re-use

Frequency reuse distance

Frequency reuse

Fixed channel assignment strategies: Each cell is allocated a predetermined set of voice channel Any call attempt within the cell can only be served by the unused channel in that particular cell If all the channel in that cell are occupied, the call is blocked & subscriber does not Rx call

2.channel assignment Why we need channel assignment ? - to utilize the spectrum efficiently - reuse the frequency - increase the capacity - minimize interference There are Two types of channel assignment strategies - fixed or dynamic - borrowing strategies

Borrowing strategies: A cell is allowed to borrow channel from a neighboring cell if all of its own channel are already occupied MSC supervised the borrowing procedure Borrowing does not disrupt or interfere with any of the calls in progress in the donor cell

Dynamic channel assignment: Voice channels are not allocated to different cell permanently Instead each time a call request is made the BS request a channel from MSC The switch then allocates a channel to be requested cell following the algorithm

3.Handoff 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 station.

Hard Handoff :

Soft Handoff :

Umbrella Cell Approach Low speed users can be handled by microcells or picocells. High speed users are handled by a larger cell macrocell which is co-located with smaller cells. Speed of MS can be estimated by MSC by observing the rate of change of signal strength.

Macro-cellular Micro-cellular Large cells(5-20km radius) Higher TX power BS is expensive Lesser back-haul cost Low capacity Supports mobility Low bit rates Smaller cells(200m to 2km) Lower TX power BS is much cheaper Higher back-haul cost High capacity possible Pedestrian & fixed use High bit rates Macrocell Microcell

Interference and System Capacity Sources of interference another mobile in the same cell a call in progress in the neighboring cell other base stations operating in the same frequency band Non-cellular system leaks energy into the cellular frequency band Two major cellular interference co-channel interference adjacent channel interference

Cell Splitting Split congested cell into smaller cells. Preserve frequency reuse plan. Reduce transmission power. Reduce R to R/2 microcell

Practical Handoff Consideration

Expanded Signal (Increased Bandwidth) Applying Spread Spectrum Technique Expanded Signal (Increased Bandwidth) Original Signal (Narrow Bandwidth) Recovering back Original Signal

Personal Communication System Current state of cellular telephony Cellular telephony Utilizes frequency in range 800 MHz Has both Analog and Digital capabilities PCS Operates in 1900 MHz range It is purely digital Need: To deal with explosive growth of cellular telephony To avoid overloading in 800 MHz range

Advantages: PCS is developed as a digital tool that provides user with: Voice communication Paging service Fax service Mobile connection to internet

Contents history of cellular telephony design and principle of cellular operation cellular telephony operation analog cellular telephones digital cellular telephones digital networks personal communication systems the third generation recent events in cellular telephony

Q. 1 ) Explain public address system in detail. Q. 2 ) Explain the following terms related with detail N/W : Signal regeneration Echo control Crosstalk. Q. 3 ) Draw and explain the architecture of Digital Cellular Telephony. Q. 4 ) Discuss the soft hand-off in brief. Q. 5 ) What is cell? Explain the cell structure in detail. Q. 6 ) Explain any three specification of 3G in details. Q . 7 ) Explain the principle of operation of cellular telephony. Q . 8 ) Explain the medium available for connecting the cell site to MSC.