Capture Effect in Packet Radio
Packet radio multiple access techniques are based on contention within a channel.When used with FM or spread spectrum modulation, it is possible for the strongest user to successfully capture the intended receiver, even when many other users are also transmitting
- The closest transmitter is able to capture a receiver because of the small propagation path loss.This is called the near-far effect
- The capture effect offers both advantages and disadvantages in practical systems.
- Because a particular transmitter may capture an intended receiver, many packets may survive despite collision on the channel
Hidden transmitter problem:
- A strong transmitter may make it impossible for the receiver to detect a much weaker transmitter which is attempting to communicate to the same receiver. This problem is known as the hidden transmitter problem.
- A useful parameter in analyzing the capture effects in packet radio protocols is the minimum power ratio of an arriving packet, relative to the other colliding packets, such that it is received. This ratio is called the capture ratio
- It is dependent upon the receiver and the modulation used.
- The Table 1below shows the multiple access techniques which should be used for different types of traffic conditions