Cellular networks

1. Cellular networks • Cell area • Ideally, the area covered by a cell could be represented as a circle with radius R and center as base station • Actual shape of the cell is determined by the received signal strength. Factors such as reflections, refractions, elevation of the terrain, presence of a hill or valley or a tall building and presence of particles in the air determine the shape of the cell. • But in most modeling and simulations hexagons are used for cell, while a square is used as a second choice

2. Cellular networks… • Signal strength and cell parameters • As an MS moves away from the base station, the signal strength weakens and handoff (or handover in other parts of the word) from one base station to another base station takes place • When to perform handoff depends on many factors • It should not take place too quickly if the MS moves back and forth between two cells due to the underlying terrain • Area and shape of the cell • Velocity of the MS

3. Cellular networks… • Capacity of a cell • The load of a cell is characterized by the following parameters • Average number of MSs requesting service (average call arrival rate) λ • Average length of time the MS requires the service (average holding time) T • The offered traffic load is defined as • α= λT

4. Cellular networks • Frequency reuse • Earlier systems employed FDMA, and the range was limited to a radius of from 2 to 20km • The same frequency band or channel used in a channel can be reused in another cell as long as the cells are far apart and the signal strength do not interfere with each other. This enhances the available bandwidth in each cell • Distance between two cells using the same channel is called the “reuse distance”

5. Frequency reuse… • If R is the radius of each cell and N is the number of cells in a cluster, then a channel cannot be reused within this cluster. • Cochannel interference: cells that are exactly at reuse distance may experience some degree of interference, called cochannel interference.

6. Cell Splitting • All BSs transmitting information at the same power level may not be desirable in general • This is because resource demand may depend on the concentration of users in a given area. Change in number of users can also occur • Splitting cells is one way to cope with the number of users

7. Cell sectoring • Omnidirectional antennas which allow radio signals with equal power strength in all directions are difficult to design • Most of the time an antenna covers an area of 60 degrees or 120 degrees. • These are called directional antennas • Cells covered by them are called sectored cells • In general, many sectored antennas are mounted on a single microwave tower located at the center of the cell to cover the entire cell

8. Channel Allocation • Channels allocation schemes • Fixed • A fixed set of channels are allocated to a cell permanently • Dynamic • Allocation of channels to different cells is done dynamically, as needed, possibly form a central pool. • Hybrid • A combination of fixed and dynamic channel allocation schemes with channels divided into fixed and dynamic sets

9. Channel allocation • Fixed channel allocation schemes • The total number of channels in the system is divided into sets • The minimum number of channel sets required to cover the area is N in a N-cell cluster model • One approach to handle increased traffic in a cell is to borrow free channels from neighboring cells

10. Fixed channels allocation schemes… • Borrowing schemes under fixed channel allocation • Simple borrowing schemes • If all channels allocated to a cell are being used, then additional channels can be borrowed from any cell that has some unused channels. Techniques such as borrow from the richest neighbor. • Complex borrowing schemes • One strategy is to divide the channels into two groups, one group assigned to each cell permanently and the second group kept reserved as donors to be borrowed

11. Dynamic channel allocation schemes • Centralized dynamic channel allocation schemes • A channel is selected from a central pool using a specific characterizing function such as pick the first available free channel, pick a free channel that can minimize the future blocking probability in the neighborhood of the cell that needs the channel, etc • Distributed dynamic channel allocation schemes • There is no central controller. Channels are borrowed based on the current channel usage pattern of neighbors

12. {4,5,6} {1,2,3} {1,2,3} {7,8,9} {4,5,6} {4,5,6} {1,2,3} Fixed channel allocation {1,2,3}

14. Primary Source • Introduction to wireless and mobile systems --by D.P. Agrawal and Qing-An Zeng