Preamble Design and Analysis for Channels with Significant Doppler Variation

Abstract

Modern iterative Forward Error Correction (FEC) techniques have dramatically increased the coding gain applicable to satellite channels with continuous and long packet message traffic. To maximize the benefit of these gains, longer preambles and better synchronization techniques are required to leverage these gains at lower C/kT (Eb/No). However, as network layer gains in system capacity require more packet oriented traffic, preambles from multiple transmitters can constitute a larger portion of limited channel resources. This is the case with packet-oriented systems such as Demand Assigned Multiple Access (DAMA). The effect of more costly preambles is exacerbated in channels that consists of shorter asynchronous message traffic such as is widely used for channel control signaling information in many military systems. Preambles are especially important to coherently received waveforms because in addition to setting the start of message position, they are used to determine the initial parameters of carrier phase, carrier frequency, and symbol timing, which are then tracked through various estimation and control loops. This paper will show that in many ways, preamble detection can be considered a problem in non-coherent detection of Frequency Shift Keying (FSK) from a large alphabet with Ricean channel statistics. This paper presents an analysis of existing MIL-STD-188-181C[1] preamble techniques that are widely used in UHF SATCOM. Suggested compatible improvements to those techniques are presented along with more efficient Minimum Shift Keying (MSK) waveform preambles that lend themselves to acquisition while still providing sufficient estimates of frequency, carrier phase and symbol timing in demanding channels.