Diversity Techniques for Multipath Fading Channels between Earth and the Moon

Abstract

In this paper we design and analyze a communication system between a lander/rover on the surface of the lunar South Pole and an Earth station. To communicate to and from the lander/rover on the lunar South Pole, low and/or medium directional antennas onboard the lander/rover will have to be pointed at low elevation angles, thus causing multipath fading effects due to reflection of a portion of the transmitted electromagnetic waves from the surface of the Moon. These effects are not commonly encountered in traditional deep space communications between a spacecraft and a ground station. We model the fading channel based on existing and simulated data, using a Rician fading channel with time-varying Rician parameter depending on the Earth elevation in the lunar sky. Our channel model also includes the effect of the Doppler and delay spreads, which produce time-varying fading events and intersymbol interference (ISI), respectively. Under these channel conditions, and for coherent reception typical in space communications, the acquisition and tracking loop of the receiver must acquire and track the incoming carrier phase in presence of Rician multipath fading. It must also estimate the amplitude of the fading signal, which is then used as an input to soft decoder. We consider simple modulation and coding schemes, in particular those specified in the CCSDS standard for space applications, and study their performance when the system is capable of taking advantage of spatial diversity to mitigate the effects of fading, either using two antennas on Earth or two antennas on surface of Moon. We also consider using frequency diversity, i.e., data is transmitted simultaneously via two frequency bands. After designing various components of the communication system, we use Simulink models to obtain the end-to-end performance of the communication link under investigation. Using the simulation results, we investigate the performance of low-complexity combining and selection methods when they are applied before and after the decoders and propose a new method that allows users to select the output from the most reliable receiver during fading fluctuations. This technique is particularly interesting for scenarios where two independent and possibly non-collocated receivers are used to achieve spatial diversity, or the user has two receivers operating at different frequencies.