1. Cellular Systems

2. Frequency AssignmentsUK
890 MHz
915
935
960 US
825
845
870
890
Japan
870
885
925
940

3. Frequency usage in GSM at Europe
960 MHz
124
Downlink
200 kHz
935.2 MHz 1
20 MHz
915 MHz
124
Uplink
890.2 MHz 1 t
Bandwidth per channel is 200 kHz
Each user is assigned channel for an uplink and a downlink
So at most 124 simultaneous calls.

4. Goals Low power transmitter system Increase network capacity
Frequency reuse
Build robust scaleable system
Architecture to deal with different user densities at different places

5. Idea! Partition the region into smaller regions called cells.
Each cell gets at least one base station or tower
Users within a cell talks to the tower
How can we divide the region into cells?

6. “Cell”ular Structure

7. Properties of Cell structure
Typical Cell sizes
some cites few hundred meters
country side few tens of kilometers
Advantages of cell structures:
more capacity due to frequency reusage
less transmission power needed
more robust, tolerate failures
deals interference, transmission area locally
Problems:
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells

8. Inside a cell
Center-excited cell where the tower is placed somewhat near the center with a omni-directional antenna
Edge-excited cell where the towers are placed on three of the six corners with sectored directional antennas.

9. Channels Reuse
Cell structure can reuse frequency only when certain distance is maintained between cells that use the same channels.
Each base station is given a group of radio channels to be used within a cell.
Base stations in neighboring cells are assigned completely different set of channels.
By limiting the coverage areas, called footprints, within cell boundaries, the same set of channels may be used to cover different cells separated from one another by a distance large enough to keep interference level within tolerable limits.
Cells that use the same set of frequency channels are called co-channel cells and the distance between them is called co-channel reuse distance.

10. Interference Co-channel interference Adjacent channel interference
Signals from cells that share a channel cause co-channel interference
Can’t remove it by increasing power.
Adjacent channel interference
Signals from adjacent cells cause this.
Use filter to reduce it
But, available channels decrease for incoming calls.

11. Frequency reuse factor
Total available channels = S
N “adjacent” cells (called a cluster) share S channels
System has M clusters
Each cell gets k channels
S = k N
Capacity of the system is C = MkN
Frequency reuse factor is 1/ N

12. Common Channel Sharingf1 f2 f3
3 cell cluster f2 f3 f7 f5 f2 f4 f6 f5
7 cell cluster f1 f4 f3 f7 f1 f2 f3 f6 f5 f2 f1 f2 f3 h1 h2 h3 g1 g2 g3
3 cell cluster
with 3 sector antennas

13. Improving coverage and capacity in cellular systems
Cell splitting: Reduce radius of cell to increase frequency reuse.
Sectoring: Uses directional antennas to control interference and frequency reuse.
Repeaters for range extension: Use retransmitters to cover areas subjected to fading.
Zone microcells: Distributes the coverage of a cell.

14. Cell splitting
Cell splitting is the process of subdividing a congested cell into smaller cells, each with its own BS and a corresponding reduction in antenna height and transmitter power.
Cell splitting increases the capacity of a cellular system since it increases the number of times that channels are reused.

15. Sectoring
In sectoring, a cell has the same coverage space but instead of using a single omni-directional antenna that transmits in all directions, either three or six directional antennas are used and each with beamwidth of about 120° or 60° as shown in Figure.
When sectoring is employed, the channels allocated to a particular cell are divided among the different sectors. It is done in such a way that channels assigned to a particular sector are always at the same direction in the different cells.

16. Range extension by the use of repeaters
Repeaters are bidirectional in nature and simultaneously send signals to and receive signals from a serving BS.
Upon receiving signals from BSs in forward link, the repeater amplifies and reradiates the BS signals to the specific coverage region.
Repeaters are being widely used to provide coverage into and around buildings, where coverage has been traditionally weak.
However, repeaters do not add any capacity to the system, they just increase the reach of a BS or MS into “shadowed” areas.

17. Microcell zone concept
By the use of sectorization technique, we can increase the system performance (i.e. quality of the signal) but side by side, there will be a large increment of handoffs which results in the increment of load on the switching and control link elements of the mobile system.
So a microcell zone concept is introduced which leads to an increased capacity without any degradation in trunking efficiency caused by sectoring.
The microcell zone concept is related to sharing the same radio equipment by different microcells. This results in reduction of cluster size and hence increase in system capacity.
The microcell coverage zone approach is used in practice to expand the capacity of cellular communication systems.

19. For example, each of the three or more zones (represented as Tx/Rx) in figure are connected to a single BS and share the same radio equipment (same frequency).
Generally, the zones are connected by coaxial cable, fiber optic cable, or microwave link to the BS.
As a mobile travels within a cell, it is served by the zone with the strongest signal.
As the mobile travels from one zone to another within the cell, it retains the same channel.
Thus, unlike in sectoring, a handoff is not required at the MSC when the mobile travels between zones within the cell since all the three zones have the same frequency.
The BS simply switches the channel to a different zones’ site. Hence the BS radiation is localized and interference is reduced.

20. Advantages of microcell zone concept
A given channel is active only in a particular zone. Thus, interference is reduced and capacity is increased.
Handoffs are reduced (also compared to decreasing the cell size) since the microcells within the cell operate at the same frequency; no handover occurs when the mobile unit moves between the microcells.
Size of the zone apparatus is small. The zone site equipment being small can be mounted on the side of a building or on poles.