High Bandwidth Efficiency Digital Modulation and Its Application in Deep Space Communication

Author: (USA) Marvin K. Simon, translated by Xia Yun, Sun Wei
Publisher:
Publish Date: 2006-08-01
Features: In traditional modulation methods for transmitting telemetry and remote control data, space agencies typically use multiple subcarriers to separate different types of data and ensure that the radio frequency (RF) carrier and the modulated data spectrum do not overlap. However, subcarrier modulation has many drawbacks, such as making the aircraft more complex and introducing additional losses during modulation/demodulation. Additionally, from the perspective of this book, a key point is that subcarriers consume a significant amount of bandwidth. While replacing traditional square-wave subcarriers with sinusoidal carriers can reduce bandwidth usage, not all space exploration missions can accept this solution. In the early days of digital communication (1960s-1970s), bandwidth usage was not a problem due to low data transmission rates and the need for only a small number of data channels (subcarriers). Therefore, there was no attempt to limit bandwidth usage at that time. However, as missions became increasingly complex, the radio frequency spectrum became more crowded, data rates grew continuously, and the required subcarrier frequencies (or bandwidth occupancy) accordingly increased, making them more susceptible to interference from different spacecraft. Some even argue that subcarrier-based modulation methods are no longer viable. Fortunately, during this period, multiple improved high-bandwidth-efficiency modulation methods were developed. These methods directly modulate the carrier and are combined with improved data format arrangements (such as packetized telemetry transmission), thereby solving the multi-channel separation problem without subcarriers. Combining packetized telemetry formats with any direct modulation method, along with spectral pulse shaping for the latter, allows high data rate information to be transmitted with relatively small bandwidth. The purpose of this book is to define, explain, and demonstrate the performance (power and bandwidth) of the aforementioned high-bandwidth-efficiency digital communication systems. In addition to considering the ideal performance of these systems, we also discuss the performance of communication systems under practical transmitter and receiver characteristics, such as modulator phase imbalance, carrier incomplete synchronization, and transmitter nonlinearity. Under these practical conditions, if the transmitter is to operate with high power efficiency, i.e., the power amplifier is working in saturation or near-saturation conditions, constant envelope modulation is required. These practical constraints also limit the types of modulation methods that can be considered, which in turn limits the achievable spectral occupancy and power efficiency. If the constant envelope condition is relaxed (so that the transmitter operates linearly but its power amplifier efficiency decreases), the limitations on power and bandwidth efficiency are relaxed to the limit of Nyquist-type signals, which are theoretically strictly band-limited and can achieve maximum power efficiency. Due to the inherent trade-off between the modulation envelope fluctuations (more accurately, the instantaneous amplitude) and achievable power and bandwidth efficiency, we chose to present this issue clearly in writing this book. Specifically, we first discuss strict constant envelope modulation, then gradually consider modulations with increasingly larger envelope fluctuations, and finally discuss the modulation of signals with strictly band-limited envelopes (Nyquist-type). Following this approach, we analyze some quasi-constant envelope modulation methods that have received considerable praise in recent years, which strike a good balance between the power and bandwidth trade-off mentioned above. Finally, it should be noted that although this book strives to cover most of the published literature in this field, the focus is primarily on the research achievements of JPL. Therefore, the literature provided to readers is not an exhaustive collection of papers on high-bandwidth-efficiency modulation but rather a compendium of many technical accomplishments in deep space communication systems related to NASA-funded missions. We hope that this book will not only be informative but also inspire future engineers to continue this initiated by JPL.