Flexible Digital Payloads : Supporting RF Equipment Considerations

摘要

The world is becoming ever hungrier for wide bandwidth mobile communications. Where the primary role of satellite telecommunications was once static TV broadcasting to millions of fixed users, new demands are leading to the merging of traditional FSS and mobile systems. This in turn introduces a wave of innovation in the design and implementation of telecommunications payloads. This demand is driven by applications including broadband to aeroplanes, cruise ships, oil rigs and governmental users. Such applications require a quantum leap in the overall cost per bit of satellite based communications if the overall system business case is to be realised. In order to meet this challenge spacecraft operators and manufacturers are increasingly implementing communications systems based upon High Throughput or HTS satellite systems. Such systems produce a lower cost per communications bit by amortizing the satellite launch, platform and operational costs over an increased communications capacity, compared to a classical non-HTS equivalent payload. This increase in system capacity is achieved by implementing a multi-beam antenna system based on providing the required market coverage with many (typically 20 to 200) narrow spot beams rather than a small number of wide beams. Such an approach has the advantage that frequency re-use can be achieved between non-adjacent beams and beam directivity is improved leading to better G/T and EIRP. However one disadvantage of an HTS system is that achieving communications between users in different beams requires payloads with Forward and RTN transponder paths both of which are hubbed to central communications gateways which manage the system connectivity. In such a system beam to beam communications between beams which do not include gateways, can be implemented only by using the so called double hop approach and routing the communication path via a gateway. For some services this double hop approach introduces a too high communications latency and waste of spectrum, and as such flexible beam to beam connectivity needs to be provided within the satellite payload itself. Study has shown that for complex multi-beam payloads with greater than a few tens of beams digital processors are the best means of providing this flexible connectivity. Recent advances in digital processing technology allow for potentially very high levels of processed bandwidth with minimal impacts to payload mass, cost and DC power. However the emergence of digital processing technology does not completely infer the demise of traditional analogue equipment. In fact to extract the best flexibility, dynamic range, noise figure, phase noise and overall system performance careful consideration must be placed on the design and implementation of the supporting RF equipment's needed to embed a digital processor within a typical communications payload. This paper will consider the design drivers for the RF equipment needed to support the next generation of digital processed payloads. The paper considers the process of digital sampling and its impact on the RF up and downconverter equipment required to map spectrum to the digital processor ports. It discusses the merits of baseband vs IF sampling and its implications for the RF converter design and system linearity. The paper also discuss the Master Reference Oscillators, Digital Processor clock oscillator and Converter local oscillators to examine the impact of phase noise and aperture jitter on processed payload dynamic range.