Oscillator mismatch and jitter compensation in concurrent codecs

Abstract

The advent of concurrent coding theory means that omnidirectional communication systems can possess a level of keyless jam-resistance comparable to that of traditional spread spectrum systems, all of which rely on shared secret keys. To achieve this, concurrent codecs possess the ability to efficiently separate multiple legitimate codewords that have been superimposed. This is achieved by leveraging a highly asymmetric sensitivity to bit errors and, consequently, a reliance on communication channels having correspondingly high degrees of asymmetry in their bit error probabilities. While suitable physical channels must possess inherently high degrees of asymmetry, this asymmetry can be artificially enhanced using post processing techniques with the effect that system designers can trade small amounts of jam-resistance for increases in noise immunity. Furthermore, to rob potential adversaries of the option of attacking the receiverpsilas ability to synchronize with the transmitted signal, concurrent codecs do not perform real-time adaptive synchronization and instead use asynchronous protocols. To avoid bit misalignments over the length of the packet, such protocols normally require that transmitters and receivers have oscillators with frequency tolerances on the order of one part in ten times the packet length. However, a concurrent codec can use simple post-processing techniques to exploit the asymmetry in bit error sensitivity to give receivers high degrees of immunity to timing jitter as well as high tolerances to oscillator mismatch. This has implications not only for processing gain, but also for implementation cost since transceivers can utilize oscillators having greatly relaxed specifications compared to that required by traditional systems. This paper presents these techniques and analyzes their impact on jam-resistance and oscillator performance requirements.