# K/Ka-Band Very High Data-Rate Receivers: A Viable Solution for Future Moon Exploration Missions
## 4 Receiver Architecture
### New Receiver Features  
From a digital point of view, the receiver will be equipped with new features, namely: Autonomous Doppler shift compensation, autonomous determination of the uplink signal characteristics and autonomous handover between base stations.  
A first estimation of the Doppler shift is reported in Table 3. From the analysis of these data it is clear that any new functionality of the lunar receiver should consider the presence of a Doppler shift and of the associated Doppler compensation methods. Recently investigated techniques in the field of Doppler correction adopt a combination of search-range techniques and fast Fourier transform (FFT) [22; 23]. These techniques, based on open-loop architectures and maximum likelihood estimation, can increase the acquisition probability and reduce the carrier frequency estimation error. A possible solution makes use of Doppler estimation and compensation techniques originally developed for high-mobility digital video broadcasting terrestrial receivers based on multi-carrier techniques [24]. These techniques can be adapted to a single-carrier satellite scenario by inserting a specially designed preamble that facilitates the estimation of the Doppler shift. Special preambles have been exploited, for instance, also in the DVB-S2 standard [25]. This Doppler compensation technique allows the receiver to acquire the signal lock even in the presence of extreme values of frequency shifts.  
The lunar receiver should be able to detect the presence of a first signal, for instance of an E-LS link, and of a second signal, for instance due to an LO-LS link. In some cases, one of the two signals could be an unwanted interference to be suppressed. Therefore, it is important that the lunar receiver is able to distinguish two signals transmitted at the same time and to determine the characteristics (such as, for instance, central frequency, SNR, Doppler shift, symbol rate, modulation, code rate) of both signals. The determination of the presence or absence of a signal is a spectrum sensing problem that is also the first task of a cognitive radio network. In this case the proposed solution can be obtained by a combination among energy detection strategy (used to evaluate the power of the two signals), FFT and correlation techniques (to determine modulation and rate parameters).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Symbol Rate** & \(\Phi_{rms}^{2}\) & \(P_{RX}\) & \(k_{B}\,T_{0}\,B_{IF}\) & \(F_{RX}\) & \(F_{RX}^{eff}\) & \(C/N\) & \(\text{SNR}_{eff}\) \\
**(Msym/s)**  
Finally, in order to maintain an E-LS communication link, handover is required. In this way, the lunar receiver can be served by the best-located Earth transmitting station. In satellite networks, several categories of handover do exist. In the present lunar scenario, the lunar receiver does not transmit the received signal quality to the Earth stations. Therefore, the lunar receiver can neither initiate nor control the handover. Consequently, the lunar receiver should be able to autonomously detect when the Earth stations perform the handover [26]. Here the key idea is to allow the lunar receiver to detect two or more signals, and to successively select or combine the signals themselves. Two solutions are possible. In both solutions, the Earth stations initiate the handover by allowing, during the handover, the simultaneous transmission of the same signal from two different Earth stations. In the first solution, called a hard handover, the lunar receiver detects the two signals separately and then selects the most powerful one based on a link quality criterion. In the second solution, called soft handover, the lunar receiver jointly detects the two received signals, combining them after Doppler estimation and correction. Such a solution may require a specially designed preamble (like the one of DVB-S2) or a cyclic prefix and additional processing at the lunar receiver to constructively combine the two signals without link loss.