JPL's Wireless Communication Reference Website

Chapter: Data Networks
Chapter: Analog and Digital Transmission

Capture in Random Access Networks

The name 'capture effect' was originally used to describe the effect of the threshold in analogue non-linear modulation (such as FM). In studies of collision-type random- access networks for data communication, the description 'capture effect' has been adopted to describe the fact that any practical radio receiver may successfully receive a message, despite the presence of interference from other (weaker) signals. If packets compete for successful reception at the receiver, one signal may 'capture' the receiver. In this context, however, the word 'effect' has lost its meaning of indicating a remarkably rapid impairment of the quality of reception with only a slight decrease of the C/I-ratio.

In network planning, but also in many studies into the performance of wireless networks, a receiver threshold or protection ratio is considered.

The performance of random-access protocols strongly depends on the probability of a successful transmission of an individual data packet over the common radio channel. We can make a subtle distinction between receiver capture and successful reception. With receiver 'capture' we typically mean that a strong signal dominates the decision variables in the receiver detector. Despite such receiver capture, the signal may nonetheless contain bit errors that incidentally leads for failure of correct packet detection.

The probability of successful reception depends on

In theoretical analyses, basically two classes of models exist:

Capture Ratio Models

Examples of models that belong to former category are

BER Models

Models of the second category, based on bit error probabilities, are

Hybrid models

Comparison of Models

The probability of capture has to be computed for instance if one is interested in The figure below compares several models. It shows that the results are very critical to assumptions on the detection process in the receiver. We believe that a simple model that requires the C/I ratio to be sufficiently large, but ignores details of bit error probabilities is fairly realistic. Results from this model are presented by black bars. More involved models do not necessarily yield more realistic results.

Estimates of the probability that one out of n packets successful captures the receiver in a random access network. Packet length is taken 16 bits with BPSK modulation. All signals are assumed to be perfectly bit aligned, and receiver bit synchronization is perfect.

JPL's Wireless Communication Reference Website 1993, 1995.