Contents
System Specifications
Working:
Wireless communication systems use E&M waves to transmit signals through the air. When subscribers share the same frequency they will create interference. The amount of interference depends on what technique is used to reduce the interference.The resource: frequency
The problem: interference
The solutions: multiple access techniques
Multiple Access Systems:
1.FDMA (Frequency Division Multiple Access method):
Users are assigned specific frequency bands. The user has sole right of using the frequency band for the entire call duration.
2.TDMA (Time Division Multiple Access):
It is an assigned frequency band shared by few users. Each user is allowed to transmit in predetermined time slots. Hence channelization of users in the same band is achieved by separation in time.
The cellular concept:
To support a very large subscriber base with a limited number of frequencies wireless Systems must reuse frequency assignments.In Order to reduce cochannel interference a cellular concept was adopted. Rather than broadcasting a signal over a very large area the radio frequency propagation was restricted to a small segment of the coverage area called a cell. Directional antennas were employed at a signal site to cover multiple cells. The same frequency is then used in another cell some distance away. The same frequency cannot be used in neighboring cells.
When directional antennas are employed at a site to segregate the local area into regions, these isolated regions now have a special relationship because their transmit and receive antennas are located very close to each other (in many case on the same tower). This property can be used to advantage in CDMA systems to communicate with a mobile through multiple sectors.
In CDMA systems then it is important to identify this special relationship. Conventional systems generally refer to each isolated region as a cell. Rather than refer to each region as a cell, in CDMA the group is collectively called a cell. Each of the subdivisions of the cell is referred to as the sector of the cell. Typically they are referred to as the alpha, beta and gamma sectors.
