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For wireless communications using linear large-scale antenna arrays, we define a receiving coordinate system and parameterization strategy to facilitate the study of the impact of three-dimensional position and rotation of the arrays on the achievable spatial degrees of freedom (DoF) in line-of-sight (LOS) channels. An analytical framework based on spatial bandwidth analysis is developed, under which three elementary problems corresponding to three basic orthogonal receiving directions are investigated. For each of them, accurate, simple, and interpretable closed-form approximations for the achievable spatial DoF are derived, and the spatial region where a sufficient amount of spatial DoF is expected available is determined. The expressions can easily be integrated into large-scale system-level simulations. Some interesting and surprising observations are made from simulation studies based on the analytical results. For instance, the spatial bandwidth is shown to be approximately constant in almost the entire spatial multiplexing region. Moreover, in significant parts of this region, the optimal receive array orientation is not parallel with the transmitting array. [ABSTRACT FROM AUTHOR]

User-centric ultra-dense network (UDN) has been considered a key enabler of high data rate and high reliable vehicle-to-infrastructure (V2I) communication. In this paper, an analytical framework is developed based on stochastic geometry to characterize the uplink V2I transmission performances, in terms of the outage probability, the average vehicle throughput and the average spatial throughput, particularly resolving two important but not well-understood factors in such networks: the spatial correlation in the interference powers among the cooperative roadside units (RSUs) as caused by the shared set of co-channel vehicles, and the combined vertical distance (CVD) owing to the lane width and the antenna height difference between the vehicle and the RSUs. Exact analytical expressions of the outage probability are derived for the general multiple association cases, a far-distance approximation method is introduced to reduce the computational complexity, and lower and upper bounds are obtained corresponding to two extreme assumptions of the spatial interference correlation. We show that the impacts of the spatial interference correlation can be significant and ignoring the correlation can lead to non-optimal system deployment, and that the far-distance approximation can approach well with the exact results. Meanwhile, although CVD diminishes the V2I uplink transmission performances, the diminution can be mitigated through RSU cooperation. [ABSTRACT FROM AUTHOR]

A novel generalized polarization-space modulation (GPSM) is proposed for polarized multiple-input multiple-output (MIMO) systems with a limit number of radio frequency (RF) chains. In the spatial domain, multiple dual-polarized (DP) transmit antennas are activated, and then combinations of those indices are used to convey information. While in the polarization domain, depending on the random input bits, only one polarization state is selected for each active DP transmit antenna to transmit information following the rule of the polarized shift keying. At the receiver, the maximum likelihood detector is employed as a benchmark to detect information bits being used to select the polarization state and activated DP antennas. In the detector, imperfect channel state information (CSI) is taken into account. Two less computationally complex detectors, i.e., a linear detector and a sphere decoding (SD) detector are proposed to relieve the computational burden. Sacrificing the average bit error probability (ABEP) performance, the proposed linear detector can reduce the computational complexity significantly. The proposed SD detector can achieve the optimum ABEP performance, while reducing computational complexity by reducing the search space. A closed-form union upper bound (UUB) on the ABEP of the GPSM system with imperfect CSI at the receiver is analytically derived and validated through simulations. From the UUB, a loose asymptotic bound on the ABEP, which sheds light on deriving the diversity gain and the coding gain, is derived. Numerical results show that the signal-to-noise ratio loss caused by increasing the number of transmit antennas is less than 3 dB while the spectral efficiency is increased by 7 b/z/Hz. Therefore, the GPSM can be a promising candidate of down link massive MIMO systems to achieve a high spectral efficiency with a limit number of RF chains. [ABSTRACT FROM AUTHOR]

The single-radio-frequency (RF) multiple-input-multiple-output (MIMO) system has been proposed to pursue a high spectral efficiency while keeping a low hardware cost and complexity. Recently, the available degrees of freedom (DoF) in the polarization domain has been exploited to reduce the size of the transmit antenna array and provide 1 bit per channel use (bpcu) multiplexing gain for the single-RF MIMO system. Nevertheless, the polarization domain resource still has the potential to provide a higher multiplexing gain in the polarized single-RF MIMO system. In this paper, we propose a generalized polarization shift keying (PolarSK) modulation scheme that uses polarization states in the dual-polarized transmit antenna as an information-bearing unit to increase the overall spectral efficiency. At the receiver, the optimum maximum likelihood (ML) detector is employed to investigate the ultimate performance limit of the PolarSK. A closed-form union upper bound on the average bit error probability (ABEP) of the PolarSK with the optimum ML receiver is derived. Inspired by the analytic ABEP, a constellation diagram optimization algorithm is proposed. To reduce the computational complexity of the receiver, a linear successive interference cancellation (SIC) detection algorithm and a sphere-decoding (SD) detection algorithm are introduced. Through numerical results, performances of the proposed PolarSK in terms of ABEP and computational complexity are analyzed. Furthermore, the PolarSK is analyzed over measured indoor channels. Numerical and measurement results show that the PolarSK scheme outperforms the state-of-the-art dual-polarized/unipolarized SM schemes. [ABSTRACT FROM PUBLISHER]

To obtain the optimal coefficient matrix for the integer-forcing (IF) linear receiver, the successive minima problem (SMP) on lattices needs to be solved. By decomposing SMP into a series of subspace avoiding problems (SAP) and developing a modified sphere-decoding (SD) algorithm based on the Schnorr-Euchner (SE) enumeration strategy to solve each instance of SAP, two practical algorithms are constructed to solve SMP exactly for real and complex lattices respectively. The initial radius and the starting position on the search-tree of the SAP algorithm are optimized by exploiting the intermediate results obtained in previous rounds. As compared to the Minkowski reduction algorithm, the proposed complex SMP algorithm brings not only more freedom in lattice code design, but also computation reduction in finding the coefficient matrix. Moreover, benefiting from the tree-search initialization optimization, the proposed real SMP algorithm can bring even more computation reduction when the system size is large. However, the proposed algorithms achieve practically the same performance as Minkowski reduction for the IF linear receiver, as the Minkowski-reduced basis approximates successive minima very closely. [ABSTRACT FROM PUBLISHER]

The Spatial Modulation (SM) transmission scheme boosts the spectral efficiency and achieves the multiplexing gain by activating a single transmit antenna in each time slot. Radio wave propagation characteristics determined by the environment is a decisive factor for the SM system. In this paper, we investigate the performance of the SM scheme over real-world 4x4 Multi-Input Multi-Output (MIMO) channels measured in typical indoor scenarios. Firstly, a MIMO channel sounder is established. Based on the sounder, 15150 complex 4x4 MIMO channel matrices are measured inside a typical teaching building under both Line of Sight (LOS) and None Line of Sight (NLOS) scenarios. Secondly, by comparing the SM system over the measured channel and commonly-used channel models, we prove that both the Independently and Identically Distribute (i.i.d.) Rayleigh and the Spatial Correlation (SC) channel model are oversimplified, and that only practical experiences can yield definitive answers to the achievable real-world system performance. Thirdly, the Average Bit Error Probability (ABEP) performance of the SM system is studied based on the measured data under a variety of system configurations. The study of different receive antenna array settings (4x4, 4x2 and 4x1 MIMO setups) approves the significance of combining scheme at the receiver. SM systems employing different signal constellations (Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) and 16 Quadrature Amplitude Modulation (16QAM)) are also investigated and some interesting results are revealed. Lastly, performance assessment of SM against State-Of-The-Art (SOTA) MIMO schemes (Space-Time Block Code (STBC) and Vertical Bell Labs Layered Space-Time (V-BLAST) code) is conducted. Results show that for a 4x4 MIMO, the low-complexity SM scheme outperforms both STBC and V-BLAST. [ABSTRACT FROM PUBLISHER]

The keyhole effect causes a dramatic performance degradation to Multiple-Input Multiple-Output (MIMO) systems. The main contribution of this letter is the analytical performance evaluation of Spatial Modulation MIMO (SM-MIMO) systems over the keyhole channel. The closed-form Pairwise Error Probability (PEP) is derived with accuracy verified by simulations. Based on the PEP, the achievable diversity gain is extracted and shown to be no more than 1 under any configuration. In addition, a tight Average Bit Error Probability (ABEP) upper bound is given. Numerical results demonstrate that compared to independent fading scenarios, the keyhole channel causes a significant performance degradation to the SM-MIMO system. It is shown that SM-MIMO schemes outperforms Space-Time Block Code (STBC) and V-BLAST schemes over the keyhole channel. Therefore, among multiple antenna transmission schemes, SM-MIMO schemes are better choices for systems that may experience the keyhole effect. However, owing to the inherent unfitness of the keyhole channel for multiple antenna transmission, SM-MIMO schemes are still inferior to SISO/SIMO schemes. [ABSTRACT FROM PUBLISHER]