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Special QPSK has been demonstrated to have superiority of spectral efficiency. However, due to its scatter of constellations and no pilots inserted, the channel estimation is a challenge. In this Letter, clustering based two-stage blind channel estimation is proposed for special QPSK transmission. For the scheme, k-means clustering is applied to reduce the complexity and improve the accuracy of channel estimation in the first stage, while frame synchronisation helps to confirm the estimated result. Simulation results show that this scheme presents small mean square error and good BER performance. Furthermore, this scheme can implement frame synchronisation and channel estimation simultaneously.

The analytical approximation of the Ricean channel (non-central Wishart) matrix is often analysed with the central Wishart matrix. This method falls short of exploiting the non-zero Ricean channel mean to calculate the average zero-forcing (ZF) massive multiple-input and multiple-output(MIMO) ergodic rate. In this Letter, the authors develop a new approach to accurately approximate the inverse Ricean channel, and determine the main diagonal. After which, optimal power allocation is used to maximise the ergodic rate. Numerical results validate the accuracy of the proposed approximation with the ideal channel.

In multi-untrusted relay networks, the transmission should be carefully devised to guarantee both security and performance. A novel scheme with symbol separation and beamforming is proposed to improve secure transmission. First, the best two relays with the strongest channel gains are selected to deliver the separated real and imaginary parts of the confidential symbols. Then, directional beamforming is carefully designed to maximise the receptions of the real and imaginary parts at the two selected relays and to zero force the receptions at the other relays. The proposed scheme ensures a perfect physical layer security with high BER at untrusted relays, and an improved BER performance at the destination. Numerical simulations show the correctness and efficiency of the proposed scheme.

For an energy harvesting relay network, an optimal secure transmission scheme is proposed. In the case of limited harvesting energy and total power, destination-based jamming technique is applied to protect the confidential information from relay's jamming and eavesdropping. Moreover, the penalty function algorithm is utilised to obtain the optimal power allocation factor to maximise secure rate. The impacts of power allocation factor and energy transformation efficiency on the system secure rate are analysed. Simulation results validate the correctness and efficiency of the proposed scheme.

For multi-user MIMO downlink transmission, a block-diagonal zero-forcing (BD-ZF) precoder is usually used to suppress multi-user interference. In the case of a large number of transmitted antennas installed at the base station, there exists a larger nullspace than the precoder requires. An optimal BD-ZF precoder is developed to maximise throughput using the determinant maximisation technique. It helps to gather all gain for the transmission of each user. Numerical results validate that the precoder obtains throughput improvement.