# Interference Alignment for Cognitive Radio Communications and Networks: A Survey
## 4 Interference Alignment in Cognitive Radio: Second Paradigm
### Leakage Interference
#### 4.2.2 Space-Time Water-Filling (ST-WF)
Even though the blocks interval is moderately insignificant compared to the involved channels coherence time, the SU only needs to know which timeslot is used for a regular PU communication (i.e., when the PU-Tx forwards data to PU-Rx) and which one is used for a reverse PU communication (i.e., when the PU Rx transmits back to the PU Tx). This is because it is the only knowledge that the SU requires to compute the pre/post-coding matrices, resulting in a novel blind CSI estimation scheme that achieves better performance. Similar to ref. [127], the work done in ref. [60] extends SVD for the PU link based on the ST-WF technique to achieve better channel capacity from the PU. However, this work employs multiple SUs in order to take advantage of the gains in performance for using cooperative spectrum sensing (CSS) [57; 64; 75]. Each SU senses the absence or presence of the TOs by the binary hypothesis test [128], and then sends a summary of its own observations to a fusion centre (FC) in the form of probabilities of missed detection and false alarm respectively. The FC uses the hard combination fusion rule [129] for making the final decision on the state of TOs, and relays these decisions to the SUs. The SUs perform linear pre-coding to provide orthogonality and ensure multiple SUs have no interference effect on PU-Rx. Unlike [127], this research considered that multiuser SUs use a time division duplex (TDD) scheme hence assumption of channel reciprocity between the onwards Tx and Rx channels. The work in ref. [79], specifies that reciprocity is essential towards increasing sum rates, thus, the SU receivers will periodically transmit feedback through reverse channel in timeslots with channel parameters indicating a loss of fidelity or a change in the PUs optimization parameters. To attain the maximum multiplexing gain of a multi-user MIMO CR network, the work in ref. [73] proposed a distributed OIA (DOIA) technique using a threshold-based beamforming (TBF) algorithm. Instead of using the SWF or ST-WF, this scheme makes use of a maximum eigenmode beamforming (MEB) technique for optimizing transmission at the PUs to enable some of its eigenmodes for SUs. While its operation is very close to the ST-WF scheme, the key benefit of adopting the MEB protocol is so that the PU puts all its power on the transmit antenna corresponding to the maximum eigenmode of its transmission channel, thus by default, the rest of the eigenmodes are left idle for the SUs' transmission. The benefit of its slightly lower computational complexity when compared with the existing water-filling methods that is still very much debatable as it also suffers from this rigid allocation of power on the largest eigenmode because the largest eigenmode mode might not be the optimum for the SUs transmission. For this virtual cooperation in which local CSI is available, the SUs sense for the unused eigenmodes in the manner of CSS. After the fusion centre (FC) has decided which eigenmodes to use, the SUs align the signals transmitted to the SDs related with the PUs unused eigenmodes to make sure of orthogonality between the PU and the SUs. The TBF algorithm, which is a distributed power-allocation strategy, was used to maximize the SUs transmission rates. It enables the SU links with a maximum eigenvalue above a certain threshold to transmit data at full power, while the rest remain silent, thus enabling the CR network to maximize the sum rate of both the PU and SUs. This MEB scheme has been extended to the work in ref. [69], for femto-cells, which consists of an overlay MIMO cognitive femtocell network with macrocell users equipment (MUEs), taken as the PUs, and femto-cell access points (FAPs), which are taken as the SUs. In this work, the MUEs base station puts all its power on the antenna corresponding to the largest eigenmode of the channel matrix, leaving the rest free. An effective CSS technique is then proposed, where the FAPs detect these unused TOs.