# K/Ka-Band Very High Data-Rate Receivers: A Viable Solution for Future Moon Exploration Missions
## 1 Introduction  
The current trend in space exploration is the planning of more missions to study Earth's nearest celestial bodies. To enable human expansion across the solar system, an outpost for crewed missions to the Moon, that also supports scientific discovery, and a concept of human controlled robotic lunar exploration are key passages: Moreso, they are prerequisite instances to allow a permanent presence of manned missions outside the planet Earth. Hence, cislunar space is becoming the target for a variety of projects, such as the Lunar Orbital Platform-Gateway (LOP-G) and Lunar Relay Satellites (LRS). This explains why there is a significant effort for the development of new technologies that will allow the establishment of Moon bases or stations; in this framework an appropriate telecommunications infrastructure is demanded as well. Such infrastructure should have the capability of carrying out the simultaneous multi band communications between the Earth, the Moon, including visiting (rovers) and descent/ascent (landers, terminals) vehicles, and other elements in cislunar orbit (LOP-G, LRS). Since this kind of mission employs many information intensive (i.e., high resolution, high coverage, high duty cycle) sensors demanding high speed link capabilities, data volume requirements are expected to increase [1]. Moreover, when considering human exploration, a high data rate is compulsory [2;3]. In this context the high portion of the microwave frequency band emerges from the others, mainly for three reasons: First, largest bandwidths can be exploited; second, the expected received energy is higher for the same link conditions and ground stations; and third, high performance integrated circuits (IC) operating at these frequencies are available form the cellular and base station market. Concerning the first motivation, it is evident that the 1500 MHz bandwidth allocated in the 26 GHz K-band is four times larger than the 375 MHz bandwidth allocated in the 8025-8400 MHz X-band. Assuming the same spectral efficiency in information bit/s per Hz (which is the case for a given coding, modulation and filtering scheme) the availability of an expanded spectrum results in a linear increase in the data rate and, potentially, in the data volume that can be transmitted. In other words, the K-band should allow users to transmit about four times more data than the X-band. Even the International Telecommunication Union (ITU) and the Space Frequency Coordination Group (SFCG) recommend the K-band for high data rate communications in the lunar region [4]. With respect to the second motivation, it is worth observing that, all else being equal (antenna aperture), the performance increase in the link budget is proportional to the square of the ratio of the new frequency to the former one. This increase occurs because the narrower beam concentrates more energy on the receiver. However, other factors create challenges when high frequencies are used. K-bands are the portions of the electromagnetic spectrum, in the microwave range of radio frequencies (RF), ranging between 18 and 26.5 GHz (K-band) and between 27 and 40 GHz (Ka-band). Being necessary to support human exploration, uplink frequency allocation between 22.55-23.15 GHz (K-band) and between 27.5-30 GHz (Ka-band) are of special interest as their use in cislunar applications is novel, which means that there is a need to implement space receiver technology in this band for space research (SR) applications. A main challenge affecting the use of these high frequency bands is the high Doppler frequency shift, that is directly proportional to the relative transmitter-receiver velocity, and inversely to the wavelength. This issue is not considered critical, but does need to be taken into account. In this paper, a comprehensive analysis is carried out to highlight the need for a receiver on K-bands that supports high speed links in cislunar exploration scenarios.