Hybrid Wideband Beamforming for Sum Spectral Efficiency Maximization in Millimeter-Wave Relay-Assisted Multiuser MIMO Cognitive Radio Networks

Abstract

Relay-assisted hybrid beamforming plays an inevitable role in enhancing network coverage, transmission range, and spectral efficiency while simultaneously reducing hardware cost, power consumption, and hardware implementation complexity. This study investigates a cognitive radio network (CRN)-based hybrid wideband transceiver for millimeter-wave (mm-wave) decode-and-forward (DF) relay-assisted multiuser (MU) multiple-input multiple-output (MIMO) systems. It is worth mentioning that the underlying problem has not been addressed so far, which is a real motivation behind the proposed algorithm. The joint optimization of hybrid processing components and the constant amplitude constraints imposed by the analog beamforming solution make this problem non-convex and NP-hard. Furthermore, the analog beamformer common to all sub-carriers is another challenging aspect of the underlying problem. To derive the frequency-flat analog processing component in the radio frequency (RF) domain and frequency-dependent baseband processing matrices in the baseband domain, the original complicated problem is reformulated as two single-hop sum-rate maximization sub-problems. Taking advantage of this decomposition, the sum spectral efficiency is maximized through RF precoding and combining. On the other hand, the impact of interference among transmitted data streams and inter-user interference (IUI) is minimized via baseband processing matrices. Finally, computer simulations are conducted by changing system parameters, considering both perfect and imperfect channel state information (CSI). Simulation results demonstrate that the proposed algorithm achieves performance close to full-complexity precoding and outperforms other well-known hybrid beamforming techniques. Specifically, more than 95% efficiency is achieved with perfect CSI, and more than 90% efficiency is attained under the assumption of 30% error in the estimated channels.

Spectral Efficiency Maximization for Mixed-Structure Cognitive Radio Hybrid Wideband Millimeter-Wave Transceivers in Relay-Assisted Multi-User Multiple-Input Multiple-Output Systems

“…The authors of [ 54 , 66 , 67 , 68 , 69 , 70 ] primarily focus their attention on designing mm-wave relay-based hybrid precoding for MU-MIMO systems. In [ 66 ], the authors investigate the hybrid beamforming technique for sum rate maximization with mixed and fully connected structures using the DF relaying protocol.…”

“…In particular, the hybrid precoding techniques presented in [ 66 , 67 , 68 , 69 ] are not applicable when cognitive radio comes into play. The algorithm proposed in [ 70 ] is compatible with CR-based fully connected hybrid transceivers, but there is a definite need to reduce cost, energy, and hardware complexity without significant performance degradation. This work is an attempt to address this gap, considering the mixed hybrid structure to achieve the desired goal.…”

“…Contrary to the hybrid transceiver designs reported in the literature for relay-assisted MU-MIMO systems [ 54 , 66 , 67 , 68 , 69 , 70 ], there does not exist any CRN-based hybrid wideband mm-wave beamforming scheme that attempts to maximize the sum spectral efficiency, exploiting a low-complexity and energy-efficient structure. This provides motivation for the proposed hybrid transceiver design.…”

“…Furthermore, the scenario investigated in this paper is consistent with practical considerations, as the underlying network architecture attempts to utilize the spectral resources efficiently while maximizing the sum rate using a low-complexity and energy-efficient hybrid structure, taking the transmitted power and interference constraints into account. In contrast to the existing hybrid precoding techniques for relay-assisted MU-MIMO systems [ 54 , 66 , 67 , 68 , 69 , 70 ], the proposed design considers frequency-selective channels, the CR paradigm, and mixed structures simultaneously, which offers great potential for unexplored applications. It is worth mentioning that the formulated optimization problem is non-convex due to the element-wise constant amplitude constraints imposed by the analog processing components.…”

“…In contrast to the existing hybrid precoding techniques for relay-assisted MU-MIMO systems [ 54 , 66 , 67 , 68 , 69 , 70 ], the proposed design considers frequency-selective channels, the CR paradigm, and mixed structures simultaneously, which offers great potential for unexplored applications. It is worth mentioning that the formulated optimization problem is non-convex due to the element-wise constant amplitude constraints imposed by the analog processing components.…”