A communications satellite is an artificial satellite sent to space for the purpose

of telecommunications.

Communications satellites are commonly used for mobile phone signals, weathertracking, or broadcasting television programs. Communications satellites are artificialsatellites that receive signals from an earth station and then retransmits the signal toother earth stations. They commonly move in a geostationary orbit.

Satellite

Frequency band Uplink frequency (GHz)

Downlink frequency (GHz)

Major applications

S - band 3.2 – 3.7 1.8 – 2.3 TV transmission used by Doordarshan

C - band(Commercial)

5.9 – 6.4 37 – 42 Television broadcast,fixed point to point ground stations, nonmilitary applications.

X - band(Military)

7.9 – 8.4 7.25 – 7.75 Mobile (ships,aircrafts), radio relay.

Ku - band(Commercial)

14 – 14.5 11.7 – 12.2 Broadcast and fixed point service, non military applications.

Ka - band(Commercial)

27 – 30 17 – 20 Broadcasts.

Allocated frequency bands and uplink/downlink frequencies

Cellular System

• The design objective of earlier mobile radio system was to achieve large coverage area by using single high powered transmitter with antenna mounted on tall building.

• This approach achieve large coverage but same frequency can not be used ,if attempt to do that it my lead to interference.

• But the demand for mobile service increased and spectrum for it is limited so, it become necessary to restructure the radio mobile system to achieve high capacity with large coverage area

Why we go for cellular system?

• Each high power transmitter is replaced by many low power transmitter providing coverage for small service area.

• Each base station is allocated portion of the total no. of channel allocated to entire system.

• Neighboring base station is allocated different group of radio channel.

• All the radio channels are distributed among small number of near by base station.

Cellular System

Block diagram of cellular system

Cellular System

• Each base station is allocated fixed number of channel among allocated channel to be used within geographical area called ‘CELL’.

• The radio coverage of cell is known as ‘FOOTPRINT’.

• Neighboring base station have different group of channel.

• In figure total area is divided in to 3(three) cells A,B,C and each cell have assigned different group of channel

BS

AA

B

C

Cellular System

Cellular Concept

• Consider cellular system having Sduplex channel.

• If total area is divided in to N cells.

• The channel available to each cell is K

• K= S/N or S=NK

• N Cells use all available frequency so N is called as cluster

AA

B

C

• Allocation of same frequency channel to another cell in other cluster bytaking care of enough separation between them is called frequencyreuse.

• The cells with same letter use same group of channel.

Frequency Reuse

• Allocation of same frequency channel to another cell in other cluster by taking care of enough separation between them is called frequency reuse.

• The cells with same letter use same group of channel.

• If C capacity of system then C=MNK.

• If cluster size is large capacity is high.

Frequency Reuse

Cell Splitting….

Large cell(low density)

Small cell(high density)

• Cell splitting increases capacity by increase in number of cells

• More number of cell increases interference.

• More cells require handoff many times.

• So, we pay for hand off.

• Cells splitting is process of subdividing the congested cells in to smaller cell with

I. A corresponding reduction in antenna height

II. Corresponding reduction in transmitter power

III. their own base station.

• Splitting cell reduces cell size and more number of the cell to use.

• More number of cells – more no. of clusters-more no. of channels – more capacity

• It replaces large cell with small cell without upsetting channel allocation

Cell Splitting….

Hand Off

• The process of transferring call which is in progress from one channel or BS to another is called hand off.

• Priority must be given to handoff request for channel assignment rather than new call initiation request.

Call processing

1. Mobile to mobile (handset to handset)

2.Mobile to land line

3.Land line to mobile

Mobile to mobile

Mobile1Base

station of mobile1

Mobile switching

center

Base station of mobile2

Mobile2

cell1 cell2

BS1Mobile1 BS2

Mobile2

Landline to mobile phone

Public switched telephone network(PSTN)

Mobile switching

center

Base station of mobile2

Landline Mobile

Mobile To Landline Phone

Public switched telephone network(PSTN)

Mobile switching

center

Base station of

mobile

LandlineMobile

cell splitting, cell sectoring, handoff

cell splitting, cell sectoring, handoff

cell splitting, cell sectoring, handoff