Digital Communications with Emphasis on Data Modems

11WAVEFORM ACQUISITION

11.1 INTRODUCTION

Communication link budgets typically focus on the waveform detection requirements; however, an equally important consideration is the message acquisition link budget. The acquisition processing must detect the presence of the received signal and estimate the necessary parameters with sufficient accuracy to provide for message synchronization [1]. The message acquisition is typically designed to operate in a one or two decibel lower signal‐to‐noise ratio than that required for the message detection. An example link acquisition budget is given in Section 15.15. The more restrictive acquisition performance requirement and the necessity to estimate various received waveform parameters is usually offset by providing a unique preamble¹ to the message that is tailored to expedite the estimation processing. An important aspect of the preamble is that it provides the necessary integration time for parameter estimation. The fundamental issue in the acquisition processing is the time and frequency error of the received signal relative to the receiver and demodulator clocks and oscillators [2–4]. Time and frequency precorrection requires accurate estimation of the line of sight (LOS) range and range rate and of the receiver/demodulator oscillator and clock accuracies. In ground‐to‐satellite links, some degree of precorrection is usually required by the ground station to aid in the satellite’s uplink acquisition and tracking and thereby to reduce ...