Multiple-input Multiple-output Radio Frequency Research Articles

On the performance of a hybrid frequency–phase-keying-based MIMO hybrid RF/FSO communication link

A hybrid technique in optical wireless communications is an effective solution to counter impairments and to improve link availability. In this work, a hybrid frequency phase keying (FPK) modulation scheme, based on frequency shift keying (FSK) and phase shift keying (PSK), for a hybrid radio frequency (RF)/free space optical (FSO) communication system is proposed. The hybrid FPK modulation scheme combines advantageous features of FSK and PSK and has the possibility to provide improved link performance in adverse channel conditions. A transmit aperture selection (TAS) and maximal ratio combining (MRC) diversity-based multiple-input–multiple-output (MIMO) hybrid RF/FSO system is considered to mitigate adverse fading effects. Analytical closed-form expressions are derived for the average symbol error probability to trace the performance of the TAS-MRC-based MIMO hybrid RF/FSO system with FPK modulation over fading and pointing error conditions. A comparative analysis of average symbol error performance is presented. The results clearly indicate that the proposed FPK modulation scheme with the MIMO hybrid RF/FSO communication system link is advantageous, as it exploits the complementary properties of both RF and FSO channel characteristics and PSK and FSK modulation schemes. In addition, the proposed MIMO hybrid RF/FSO system with FPK modulation offers superior performance, compared with MIMO RF, FSO, or hybrid RF/FSO systems applied with FSK and PSK modulation schemes. An optimum beam waist is derived analytically, and it is further verified with a numerical optimization technique. All results are validated using Monte Carlo simulation.

Interchannel Interference and Mitigation in Distributed MIMO RF Sensing.

In this paper, we analyze and mitigate the cross-channel interference, which is found in multiple-input multiple-output (MIMO) radio frequency (RF) sensing systems. For a millimeter wave (mm-Wave) MIMO system, we present a geometrical three-dimensional (3D) channel model to simulate the time-variant (TV) trajectories of a moving scatterer. We collected RF data using a state-of-the-art radar known as Ancortek SDR-KIT 2400T2R4, which is a frequency-modulated continuous wave (FMCW) MIMO radar system operating in the K-band. The Ancortek radar is currently the only K-band MIMO commercial radar system that offers customized antenna configurations. It is shown that this radar system encounters the problem of interference between the various subchannels. We propose an optimal approach to mitigate the problem of cross-channel interference by inducing a propagation delay in one of the channels and apply range gating. The measurement results prove the effectiveness of the proposed approach by demonstrating a complete elimination of the interference problem. The application of the proposed solution on Ancortek’s SDR-KIT 2400T2R4 allows resolving all subchannel links in a distributed MIMO configuration. This allows using MIMO RF sensing techniques to track a moving scatterer (target) regardless of its direction of motion.

Dynamic power allocation and scheduling for MIMO RF energy harvesting wireless sensor platforms

Radio frequency (RF) energy harvesting systems are enabling new evolution towards charging low energy wireless devices, especially wireless sensor networks (WSN). This evolution is sparked by the development of low-energy micro-controller units (MCU). This article presents a practical multiple input multiple output (MIMO) RF energy-harvesting platform for WSN. The RF energy is sourced from a dedicated access point (AP). The sensor node is equipped with multiple antennas with diverse frequency responses. Moreover, the platform allows for simultaneous information and energy transfer without sacrificing system duplexity, unlike time-switching RF harvesting systems where data is transmitted only for a portion of the total transmission duty cycle, or power-splitting systems where the power difference between the information signal (IS) and energy signal (ES) is neglected. The proposed platform addresses the gap between those two. Furthermore, system simulation and two energy scheduling methods between AP and sensor node (SN) are presented, namely, Continuous power stream (CPS) and intermittent power stream (IPS).

Performance Analysis of Mixed Interference Aligned MIMO RF/Unified FSO DF Relaying With Heterodyne Detection and Two IMDD Models

This paper considers a dual-hop decode-and-forward (DF) mixed radio frequency (RF)/free space optical (FSO) relaying scheme. In this scheme, the first hop is a multiple-input multiple-output (MIMO) RF interference channel (IC), while the second hop is an FSO channel in which heterodyne detection (HD) as well as intensity modulation-direct detection (IMDD) are considered in a unified analysis manner. It is assumed that the MIMO RF links follow Rayleigh fading, whilst the FSO link encounters Málaga ($\mathcal {M}$ ) fading with pointing errors. In this work, the Interference alignment (IA) technique is utilized in the MIMO RF IC hop to eliminate the interference, and hence this leads to enhance the multiplexing gain in this hop, and accordingly the overall system performance is improved. The design of the IA precoding matrix and receiving interference eliminating matrix is performed by using adapting orthogonalized minimum leakage (OR-ML) algorithm. For the considered scheme, exact closed-form expressions are derived for the outage probability and ergodic capacity, in these expressions the best-known FSO channel capacity results are utilized. Moreover, not only the IMDD input-independent AWGN but also the IMDD cost-dependent AWGN channel is analyzed in these investigations. In addition, the system performance is studied asymptotically at the high SNRs region, where the diversity order and coding gain analyses are illustrated. Further, we allocate simulation results that verify the deduced analytical and asymptotic expressions.

Nonreciprocal Radio System Calibration for Propagation-Based Key Generation

Propagation-based secret key generation (SKG) is a physical layer method for establishing encryption keys based on two radios observing a reciprocal propagation channel. However, nonreciprocal contributions to the channel introduced by the radio circuitry significantly degrade the ability to reliably generate keys using such methods, necessitating the development of calibration methods for removing the impact of channel nonreciprocity. This paper uses a total least squares (TLS) algorithm coupled with a detailed model of the multiple-input multiple-output radio frequency circuitry and propagation for the radio system to achieve such a calibration. Simulations show that the method is highly effective in removing the impact of nonreciprocal circuit contributions over a range of operational parameters.

All-Optical Generation of Two IEEE802.11n Signals for 2 <inline-formula> <tex-math notation="TeX">$\times$</tex-math></inline-formula> 2 MIMO-RoF via MRR System

A radio-over-fiber system capable of very spectrally efficient data transmission and based on multiple-input multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) is presented here. Carrier generation is the basic building block for implementation of OFDM transmission, and multicarriers can be generated using the microring resonator (MRR) system. A series of MRRs incorporated with an add/drop filter system was utilized to generate multicarriers in the 193.00999–193.01001-THz range, which were used to all-optically generate two MIMO wireless local area network radio frequency (RF) signals suitable for the IEEE802.11n standard communication systems, and single wavelengths at frequencies of 193.08, 193.1, and 193.12 THz with free spectral range of 20 GHz used to optically transport the separated MIMO signals over a single-mode fiber (SMF). The error vector magnitude (EVM) and bit error rate of the overall system performance are discussed. Results show that the generated RF signals wirelessly propagated through the MIMO channel using the 2 $\times$ 2 MIMO Tx antennas. There is an acceptable EVM variation for wireless distance lower than 70, 30, and 15 m. It can be concluded that the transmission of both MIMO RF signals is feasible for up to a 50-km SMF path length and a wireless distance of 15 m.