# Interference-Aware Radio Resource Management for Cognitive High-Throughput Satellite Systems
## 4 Interference-Aware RRM Strategy
### JSDM-Based RRM Framework  
By exploiting the aforementioned characteristics of spatial correlations in HTS systems with large-scale antennas, JSDM is an attractive means of reducing the channel training and the CSI feedback overhead while maximizing the throughput in multibeam HTS systems. The main idea of JSDM lies in partitioning users into groups with similar channel characteristics and employing a two-stage precoding scheme [34]. Different groups are sufficiently separated in the angle domain. Then, the structure of the channel covariance matrix can be used to reduce the dimension of the effective channel, resulting in reduced dimension channel training and low CSI feedback overhead. The two-stage downlink multiuser precoder can be obtained by concatenating a pre-precoding matrix that depends only on the channel's second-order statistics with a multiuser precoding matrix that depends on instantaneous realization of the resulting reduced dimensional effective channel. The pre-precoding matrix is utilized to minimize the intergroup interference, while the multiuser precoding matrix is employed to minimize the intragroup interference.  
In particular, since the transmitting directions of terrestrial and satellite user terminals are different, different terminals should be divided into different groups. Thus, \(K\) satellite users are partitioned into \(G\) groups with similar channel characteristics, and terrestrial primary receivers (PRs) are categorized into an individual group, as shown in Figure 3. These groups are served with a two-stage downlink precoding scheme. In order to protect the terrestrial PRs from inter-system interference, the satellite needs to obtain the channel state information of satellite-PR links when conducting resource allocations. Specifically, the first-stage precoder \(\mathbf{V}_{g}\in C^{M\times M_{g}}\) (\(g=1,\cdots,G\)) is designed at the satellite, where \(M\) is the number of antennas at the satellite and \(M_{g}\) is the number of beams corresponding to the \(g\)-th gateway. The second-stage precoder \(\mathbf{W}_{g}\in C^{M_{g}\times S_{g}}\) (\(g=1,\cdots,G\)) is designed at the \(g\)-th gateway, where \(S_{g}\) is the number of users scheduled by the \(g\)-th gateway.  
In this paper, \(\mathbf{V}_{g}\) is generated through an approximated block diagonalization (BD) algorithm to eliminate inter-group interference semi-statically based on long-term CSI, i.e., second-order statistics of the channel. For approximated BD algorithm, the precoding matrix of one user must be in the null space of the others, which is determined through a threshold \(\alpha\). This threshold plays a decisive role in controlling intergroup interference and allowing frequency reusing among different systems. The role of the pre-precoding is to reduce the dimensionality of the effective channel by exploiting the near-orthogonality of the eigenspaces of the channel covariance matrices of different user groups. Next, with reduced dimensions on the effective channel, the second-stage dynamic precoder \(\mathbf{W}_{g}\) can be designed based on minimum mean square error (MMSE) or zero-for  
Figure 3: Framework of the joint spatial division and multiplexing (JSDM)-based radio resource management (RRM) in cognitive HTS systems.  
distinguish users within a group by suppressing intra-group interference. Through the designed two-stage precoding, both the intra-system CCI and inter-system CCI can be effectively managed.  
As it is able to significantly reduce the overhead in both downlink training and uplink CSI feedback, JSDM enables practical applications of large antenna arrays in satellite communications, even operating in the FDD mode, where uplink/downlink channel reciprocity cannot be exploited. In addition, JSDM can be applied to an HDA implementation, where the pre-beamforming stage is implemented in the analog RF domain and the second precoding stage is implemented in the baseband. Hence, the proposed JSDM-based RRM framework is generic and can be adaptive to various communication systems.