Multi-user Multiple-input Multiple-output Communication Systems Research Articles

Codesign for Hybrid MU-MIMO Communication and MIMO Radar Systems Based on Mutual Information

To enhance the coexistence performance of multiple-input multiple-output (MIMO) radar system and multiuser MIMO communication system with hybrid beamforming architecture, this work proposes a codesign problem regarding the digital and analog beamformers of communication base station, the receive beamformers of downlink users, and the transmit waveform of radar. An information-theoretic criterion is adopted to improve the radar detection performance, while the signal-to-interference-plus-noise ratio requirement is imposed on each downlink user to ensure communication quality. In addition, similarity restriction is also involved to manage some significant radar waveform characteristics. An iterative algorithm is introduced to tackle the resulted nonconvex and NP-hard problem. In each iteration, several subproblems are developed to decompose the original problem and are solved by the lower bound maximization and the alternating direction method of multiplier algorithm. Numerical simulations show that the proposed algorithm allows the objective function to increase monotonically and finally converge to a finite value. It also shows that the codesign scheme can effectively optimize the radar detection performance while ensuring the achievable sum rate and the bit error rate of the communication system.

Interference Utilization for Spectrum Sharing Radar-Communication Systems

This paper proposes a new transmitter precoding approach for co-existence between multiple-input multiple-output (MIMO) radar and multi-user MIMO communication systems employing phase shift keying (PSK) modulation. To enable the satisfactions of radar communication functionalities, the common practice is to suppress the mutual interferences, while our proposed scheme is to use the known interferences existing already in radar and communications systems and make the most of them: selectively reverse the destructive interferences to the negative direction while retaining the interferences which are beneficial to the desired signal. The resulting interferences are always kept constructive and can contribute to the power of the useful signal. Numerical results validate that the effective signal to noise ratio (SNR) can be enhanced by 7 dB with no additional power-peruser investment.

Spatial Constellation Design-Based Generalized Space Shift Keying for Physical Layer Security of Multi-User MIMO Communication Systems

We propose a novel spatial constellation design (SCD) method with generalized space shift keying (GSSK-SCD) signaling for physical layer security (PLS) in multi-user (MU) multiple-input multiple-output (MIMO) communication systems. In GSSK-SCD, the received spatial constellations are optimized through a novel precoding scheme, which minimizes the BERs at legitimate users and significantly worsens eavesdroppers' BER. In addition to extensive BER simulations, we also provide analytical expressions for the secrecy rate regions for the proposed GSSK-SCD. Both analytical derivations and simulation results, including comparisons with artificial noise aided conventional GSSK, reveal that the GSSK-SCD approach provides significant PLS improvements for MU-MIMO systems.

Mutual interference alignment for co-existing radar and communication systems

In this paper, we propose a mutual interference alignment (IA) method for interference elimination in co-existing multiple-input multiple-output (MIMO) radar and multi-user MIMO communication systems, namely, radar-communication systems. Traditional IA methods proposed for communication systems cannot be directly adopted in the radar-communications due to failure to fulfill the requirements of radar target detection. In contrast, our proposed mutual IA method can efficiently improve the signal-to-noise ratio (SNR) needed by radar functionalities without reducing the degrees-of-freedom (DoFs) of communication system, which implies no degradation for the communication performance. The algorithm idea is first to align all interferences at every receiver to one signal space, then the interferences can be easily canceled by receive filter. It is examined by the Neyman-Pearson (NP) test for radar functionality, while the communication performance is evaluated by DoFs and bit error rate (BER) analysis. Both theory analysis and numerical results are provided to validate the effectiveness of the proposed mutual IA method for co-existing radar and communication systems.

A Novel Downlink Interference Alignment Method for Multi-User MIMO System With No CSIT: A Space-Time Coding Approach

The problem of interference alignment in multi-user multiple-input multiple-output communication systems is considered, in this paper. A specific transmission strategy is proposed to transmit the symbols of multiple users which enables a significant saving of transmission time slots as compared to a trivial time division multiplexing based strategy. In order to detect the symbols at different receivers, the concept of left null of channel vector is utilized. The statistical properties of received signal-to-noise ratio at receiving end are studied by using probability density function, cumulative distribution function. Further, analytical performance of the system is discussed in terms of symbol error rate and diversity order by using moment generating function based approach. It is observed through theoretical analysis that the error performance of the proposed scheme degrades with increasing number of users; but this loss in performance can be compensated by deploying more antennas at the receivers. It is also shown through simulations that the computational complexity of the proposed scheme is also very much less as compared to the existing schemes.

Secure multiuser MIMO communication systems with imperfect channel state information

In this paper, we propose a secure wireless communications system through a multiple input multiple output (MIMO) channel which includes a multiple antenna base station and multiple single antenna legitimate users that are overheard by a multiple antenna eavesdropper. By assuming that the eavesdropper’s channel is unknown by the base station, an artificial noise beamforming is used to prevent this eavesdropper to decode legitimate users’ message in the downlink. Additionally, the base station has only imperfect channel state information (CSI) of legitimate users which is the practically relevant case. Under the condition of imperfect CSI, a noise leakage on legitimate users’ signal is occurred and it degrades the achievable average secrecy sum rate. In order to reduce this noise leakage, the semi-orthogonal selection having a rotated codebook is proposed to establish a secure communications link. We demonstrate the average secrecy sum rate results of the proposed algorithm for secure multiuser MIMO systems under imperfect CSI through extensive simulation results.

Ergodic Interference Alignment for Spectrum Sharing Radar-Communication Systems

In this paper, we propose an ergodic interference alignment (IA) method for interference elimination in spectrum sharing multiple-input multiple-output (MIMO) radar and multi-user MIMO communication systems. In existing ergodic IA methods, the channel state information (CSI) is generally assumed to be constant, but time-varying channels should be considered in practice. Differently, our proposed ergodic IA allows to time-varying and/or frequency selective channels for spectrum sharing radar-communication systems. The essence is equivalent to design a simple transmit beamforming, which helps to find a pair of desired complementary channels easily without long-time system delay, therefore, the interferences can be effectively cancelled. The proposed method is evaluated by generalized likelihood ratio test (GLRT) and signal to interference plus noise ratio (SINR) for the radar functionality, while the communication functionality is examined by bit error rate (BER) performance. Both theoretical analysis and simulation results validate that the effectiveness of the ergodic IA method for spectrum sharing radar-communication systems.

Solving Fractional Polynomial Problems by Polynomial Optimization Theory

This work aims to introduce the framework of polynomial optimization theory to solve fractional polynomial problems (FPPs). Unlike other widely used optimization frameworks, the proposed one applies to a larger class of FPPs, not necessarily defined by concave and convex functions. An iterative algorithm that is provably convergent and enjoys asymptotic optimality properties is proposed. Numerical results are used to validate its accuracy in the non-asymptotic regime when applied to the energy efficiency maximization in multiuser multiple-input multiple-output communication systems.

An Overview on Resource Allocation Techniques for Multi-User MIMO Systems

Remarkable research activities and major advances have been occurred over the past decade in multiuser multiple-input multiple-output (MU-MIMO) systems. Several transmission technologies and precoding techniques have been developed in order to exploit the spatial dimension so that simultaneous transmission of independent data streams reuse the same radio resources. The achievable performance of such techniques heavily depends on the channel characteristics of the selected users, the amount of channel knowledge, and how efficiently interference is mitigated. In systems where the total number of receivers is larger than the number of total transmit antennas, user selection becomes a key approach to benefit from multiuser diversity and achieve full multiplexing gain. The overall performance of MU-MIMO systems is a complex joint multi-objective optimization problem since many variables and parameters have to be optimized, including the number of users, the number of antennas, spatial signaling, rate and power allocation, and transmission technique. The objective of this literature survey is to provide a comprehensive overview of the various methodologies used to approach the aforementioned joint optimization task in the downlink of MU-MIMO communication systems.

Energy‐efficient pilot and data power allocation in massive multi‐user multiple‐input multiple‐output communication systems

This study aims to improve the energy efficiency of time-division duplexing massive multi-user multiple-input multiple-output communication systems for both uplink and downlink transmissions. By using minimum mean square error channel estimation, two novel pilot–data power allocation schemes are proposed to minimise the total uplink and downlink transmit power under per-user signal-to-interference-plus-noise ratio (SINR) requirement and per-user power consumption constraints. The proposed schemes take into account the maximum-ratio combining and zero-forcing (ZF) detectors in the uplink transmission together with maximum-ratio transmission and ZF precoder in the downlink transmission. In order to simplify the proposed optimisation problems, lower bounds of the average SINR are derived and used in the power allocation algorithms. The key contribution of this study lies in formulating the original energy-efficient power allocation problem and converting such a complicated optimisation problem to a geometric programming problem. Computer simulation validates the tightness of the derived SINR lower bounds and shows that the proposed schemes can save up to 78% of the total power as compared with the equal power allocation among all the mobile users.

Max–min fair wireless energy transfer for multiple‐input multiple‐output wiretap channels

In this study, the authors study the max–min fairness for wireless energy transfer in a multiuser multiple-input multiple-output communication system with simultaneous wireless information and power transfer. In particular, they aim to maximise the minimum harvested energy among the multiple multi-antenna energy receivers while guaranteeing secure communication for multi-antenna information receiver. The dual use of artificial noise to facilitate both wireless energy transfer and secure communication is exploited in the authors’ proposed problem. Both scenarios of perfect and imperfect channel state information (CSI) known at the transmitter are considered. For the perfect CSI case, the formulated max–min energy harvesting (MM-EH) problem is non-convex and intractable. To circumvent it, an iterative optimisation algorithm based on Taylor series expansion is proposed. Then, they turn their attention to the imperfect CSI case, where a max–min robust energy harvesting (MMR-EH) problem is considered. Though the MMR-EH problem is more complicated than the MM-EH problem, they reveal that the iterative optimisation method can be extended to the solution of the former, wherein the S-procedure is introduced. Simulation results show the efficiency of their proposed solutions in terms of energy harvesting.