Adaptive threshold control scheme in a centralized packet radio network with common direct-sequence spread-spectrum modulation

The authors describe an automatic threshold control scheme for the detection of a header which is a frame synchronization word in an unslotted centralized packet radio network in which all communicators employ a common direct-sequence spread-spectrum modulation format. Adaptive threshold control is done by setting the header-detection threshold proportional to the maximum-likelihood estimate of the multiple-access noise level. The design value of false alarm probability determines the constant of proportionality under the assumption that the multiple-access noise is a complex Gaussian random process. Computer simulation is carried out to show that this Gaussian assumption is reasonably valid. This scheme yields a false alarm probability which is nearly invariant to the changes in the number of actual transmissions in the channel, and a detection efficiency essentially free of loss with respect to the ideal detection where the multiple access noise level is known to the receiver. Throughput bounds for an unslotted ALOHA system with this scheme are analyzed