Hard Time Meets a Soft World - Management of Time Concepts in Software-Defined Radios

Many military, government and civil radio communications waveforms rely on a precise concept of time for proper operation. Without proper time coordination, frequency hopping, time-division multiplexing, synchronous data passing or precise positioning waveforms will be unable to properly align data in time, find peers on the proper frequency / time, or will interfere with other station´s communications. Such time dependencies can either be between the platform hardware and the waveform, between local and remote nodes, between a node and a central time server or any combination of the above. Commercial and military radio hardware has in the past twenty years undergone an evolution in technology, and with that evolution the means of handling time-based concepts has changed as well. The present generation of software-defined radios strongly emphasizes waveform-platform independence and cross-vendor waveform portability between diverse platforms. This has necessitated a re-examination on how time concepts are communicated and used by waveforms and platforms alike. In this paper, several techniques for representing platform time, and communicating / using that time for precise frequency hopping and TDMA waveforms are discussed, as well as considerations when communicating time between SDRs and external equipment. These methods can be used to implement platforms and waveforms that support the cause of portability, while retaining the performance of the past proprietary implementations.