Analog-to-Digital Converter Loading Analysis Considerations for Satellite Communications Systems

In contemporary communication satellite systems, uplink radio frequency (RF) signals are amplified, downconverted to intermediate frequency (IF) and/or baseband, and after appropriate filtering, are input to an analog-to-digital converter (ADC). The ADC digital output is signal processed for a variety of purposes, such as signal channelization and switching. In these systems, a foremost realization problem is the ADC, which must operate to satisfy the sampling theorem, which necessitates a sampling rate at least twice the received signal bandwidth. When the signal consists of numerous multiplexed signals, a critical matter in ADC performance is the degree of signal clipping, which arises when the instantaneous ADC input signal magnitude surpasses the maximum range of the ADC. Since at least some clipping is often present, the total ADC noise output consists of clipping plus quantization noise. A figure of merit for the ADC is the signal-to-noise ratio (SNR) of the ADC, which is defined as the ratio of input signal average power to the ADC output average noise power. The SNR is determined by, among other things, the ADC load factor, which is the ratio of the ADC input signal average power, and the ADC maximum peak power output. This paper describes analysis and simulation results on SNR versus the ADC load factor when the input signal is composed of many digitally modulated carriers. A nine-signal 8-ary phase shift key (8-PSK) modulated carrier case is considered with each signal band limited. It is important to note that for this particular ADC input, it is shown that the probability density function (PDF) is Gaussian-like. This is significant since this means that the SNR versus ADC load factor curve for the nine 8-PSK signal case will have nearly identical characteristics to that when the ADC input is white Gaussian noise. Additionally, this paper discusses what occurs when the ADC is strongly driven into the clipping region. This discussion is enhanced by comparing the ADC to a limiter in this highly distorted region. Analysis and simulation results are provided to describe ADC performance characteristics in this highly distorted region.