Enabling coherent Ka-band downlink with a software-defined radio

The migration to Ka-band for science downlink on deep space missions increases data rates significantly, but also presents new challenges to radio and RF system designers. One challenge is to maintain low carrier phase noise on a coherent downlink. Thermal noise on the X-band uplink that is within the bandwidth of the carrier recovery process modulates the phase of the coherent downlink. For missions that use X-band for command uplink and Ka-band for science downlink, such as the NASA Solar Probe Plus mission, the ratio of downlink to uplink frequency acts as a phase noise multiplier on the coherent downlink. Analysis and prototype tests revealed that the additional phase noise degraded both telemetry and navigation performance significantly. Accordingly, an additional software filter is inserted into the Ka-band coherent turnaround path. This filter constrains the phase noise sufficiently to meet all communication and navigation requirements. In this paper we describe the phase noise on a coherent downlink due to additive noise that is tracked by the uplink carrier recovery process. We present simulated and measured phase noise performance, with and without the additional filter. Measured frame-error rate performance is presented and the impact on radio navigation due to increased delay through the turnaround channel is discussed. This paper describes the filter implementation and results obtained with an engineering model of the SPP Frontier Radio. A companion paper describes the analytic formulation and considers other phase noise contributions such as solar scintillation [1].