CELLULAR NETWORK

Early mobile system Cellular Network Use of one powerful transmitter located on high location. Range of signals was upto 50km. These system achieve good coverage. Frequency can’t be reused throughout the system Transmitter need to be very powerful to cover entire area.

Cellular Network Instead of using single powerful transmitter, use multiple low power transmitter. Multiple Transmitters

Cellular Network These transmitters known as Base Station (BTS). Each base station allocated portion of allocated channels of entire system. Example: if a telephone system has bandwidth of 3MHz. One channel needs 30KHz. Channel per BTS = Available bandwidth no. of station X per channel capacity

Cellular Network Coverage range of base station also known as cell.

Cellular Network Channels of neighboring cells are different in cellular network. If same frequencies used in neighboring cells then interference will be there.

Cellular Network Instead of using single powerful transmitter, use multiple low power transmitter. These transmitters known as Base Station (BTS). Each base station allocated portion of allocated channels of entire system. Coverage range of base station also known as cell. Channels of neighboring cells are different in cellular network.

Cellular Network- Cell Cellular Network In Cellular system land area is divided into regular shaped cells, Which can be hexagonal, square, circular or some other irregular shapes. What will be shape of cell? Criteria for shape of cell are. Criteria for shape of cell are. Area without overlap Geometric shape Area of cell

Shape of cell Cellular Network Area overlap No overlapping Criteria not fulfilled Area to circle 17.7% Area to circle 83% Area to circle 63.7%

Why Cellular ? Cellular Network Scalability: As the demand for service increases the number of base stations may be increased thereby providing additional radio capacity Scalability: As the demand for service increases the number of base stations may be increased thereby providing additional radio capacity This enables a fixed number of channels to serve an arbitrarily large number of subscribers by reusing the channel throughout the coverage region Same frequenc ies can be used.

Why Cellular ? Cellular Network Distributed throughout the geographic region frequency channels can be re-used.

Cellular Network

Cellular Network- Points Cellular Network possible radio coverage of the cell idealized shape of the cell cell segmentation of the area into cells – use of several carrier frequencies – not the same frequency in adjoining cells – cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc. – hexagonal shape of cells is idealized (cells overlap, shapes depend on geography) – if a mobile user changes cells  handover of the connection to the neighbor cell – use of several carrier frequencies – not the same frequency in adjoining cells – cell sizes vary from some 100 m up to 35 km depending on user density, geography, transceiver power etc. – hexagonal shape of cells is idealized (cells overlap, shapes depend on geography) – if a mobile user changes cells  handover of the connection to the neighbor cell

Cellular Network Problems: Fixed network needed for the base stations Handover necessary Interference with other cells Advantage: Higher capacity, higher number of users Less transmission power needed More robust, decentralized. Base station deals with interference, transmission area etc.

Hand Off Cellular Network A handoff refers to the process of transferring an active call or data session from one cell in a cellular network to another or from one channel in a cell to another.

Types of Hand-off Cellular Network Hard handoff Soft handoff

Hard Hand Off Cellular Network Characterized by an actual break in the connection while switching from one cell or base station to another. The switch takes place so quickly that it can hardly be noticed by the user. Because only one channel is needed to serve a system designed for hard handoffs, it is the more affordable option. Before Hand off

Hard Hand Off Cellular Network First connection will be broken. Then new connection will be established. First connection will be broken. Then new connection will be established. X During Hand off Break before make

Soft Hand Off Cellular Network Entails two connections to the cell phone from two different base stations. This ensures that no break ensues during the handoff. Naturally, it is more costly than a hard handoff. Soft Hand off make before break

Problem with traditional approach Cellular Network 100 Khz 130 Khz 110 Khz 120 Khz Every user needs 2Khz Total 15 Users can use the service at a time.

Cellular Network Total frequency = 30 Khz 10Khz Every user needs 2Khz Per cell 5 users Tota users= 15 10Khz Khz Khz +15

Cellular Network Total frequency = 30 Khz 10Khz The N cells which collectively use the complete set of available frequencies is called a Cluster.

Cluster Cellular Network N Cell which utilize complete frequency of system. Certain value of N are possible, which allow repeating of those cluster without repeating. N=i 2 +ij+j 2 i and j are non negative integers.

Cluster Cellular Network Find nearest co-channel. move i cells along any chain of hexagons then, turn 60 degrees counter-clockwise and move j cells. i=2 j=1 N=7

Cluster Size Cellular Network Calculate Size of Cluster ? i=3 j=2 N=19

Cluster Size Cellular Network Calculate Size of Cluster ? i=1 j=2 N=7

Frequency Reuse Factor Cellular Network it is portion of total frequency allotted to one cell. Total frequency = 30 Khz 10Khz Total frequency Frequency in one cell Frequency reuse factor= 1 Size of Cluster Frequency reuse factor=

Frequency Reuse Cellular Network Frequency reuse is a method used by service providers to improve the efficiency of a cellular network and to serve millions of subscribers using a limited radio spectrum A transmitter transmitting in a specific frequency range will have only a limited coverage area Beyond this coverage area, that frequency can be reused by another transmitter

Frequency Reuse distance Cellular Network R

Frequency Planning Cellular Network Static Frequencies assignment Dynamic Frequencies assignment

Static Frequency Assignment Cellular Network 50KHz Certain frequency assigned to certain cell. Different cells have different traffic load. Ex. first cell having only two users, so allotted frequencies to first cell is not fully utilized. Second cell having high traffic. Different cells have different traffic load. Ex. first cell having only two users, so allotted frequencies to first cell is not fully utilized. Second cell having high traffic.

Dynamic Frequency Assignment Cellular Network base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements

Increasing Capacity-Cell Splitting Cellular Network 10Khz Cell can be again divided into smaller cell. Capacity will be increased. Problem with this approach is increased load for hand-off. Need more infrastructure (base stations)

Increasing Capacity-Cell Splitting Cellular Network Femto Cell: Indoor, Home Pico Cell: In buildings, range: meters Micro cell:200-2KM Macro cell:1KM-30KM

Increasing Capacity-Cell Sectorization Cellular Network Omni directional antenna radiate in all directions. Instead of omni direction antenna, single direction antenna is used to divided cell into sector. Omni Direction antenna. Neighboring cell can’t use same set of frequency because of interferences. Cell sectorization allow use of same set of frequency in adjacent cell.

Frequency Reuse Cellular Network 10Khz Reuse Distance : The closest distance between the centers of two cells using the same frequency (in different clusters) Also known as frequency re-use distance

How to calculate frequency reuse distance? Cellular Network 10Khz reuse= sqrt(3 N)R N : size of cluster 10Khz

Traffic intensity Cellular Network a =λ ·T traffic intensity is the average channel occupancy used to measure the time of channel utilization. Unit is Erlang. λ = Average number of calls T = average time of call

Traffic intensity- Numericals Cellular Network Q. In a cellular mobile system, each mobile subscriber average two calls per hour at an average call duration of three minutes. Determine the traffic intensity per mobile subscriber.

Numerical Cellular Network Q. If a total of 33MHz bandwidth is allocated to particular cellular telephone system which uses two 25kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) 4-cell reuse, (b) 7-cell reuse (c) 12-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.