Spectrum Efficiency In Wireless Communications Research Articles

A Notch-Band UWB Antenna for OAM Wave Generation

This paper investigates a trapezoidal Monopole Patch Antenna (MPA) with an Ultra-Wideband (UWB) characteristic and a dual-stage notched band at fifth generation (5G) and Wireless Local-Area Network (WLAN) bands. The 5G band is notched by inserting a U-shaped slot on the trapezoidal UWB MPA. The WLAN band is notched using five-square metallic-conductor Electromagnetic Bandgap (EBG) patches that are installed on the surface of the backside of the radiating patch and attached to the ground plane via a shorting pin. The small, proposed antenna is built upon an FR4 substrate substance, having dielectric comparative permittivity εr = 4.7, and fed by a 50 Ω coplanar waveguide. The antenna’s size is 32×42.90×0.8 mm3. The simulated results show that the antenna has an operating frequency range of 2.39–14.72 GHz for |S11|<-10 rejecting WLAN (5.15–5.825 GHz), as well as sub-6 GHz 5G (3.4–3.9 GHz) great-selectivity signals. A higher gain obtained by the UWB antenna is 6.4 dBi, while a sharp decrease occurs in the rejected bands, reaching -8.29 dBi. This study also employs the Orbital Angular Momentum (OAM) technique. However, OAM waves are generated using eight trapezoidal UWB MPAs. Creating notches reduces the interference problem in UWB frequencies, and merging the UWB with the OAM technique allows the OAM to operate on many frequencies. Moreover, notches can be generated for specific frequencies inside the UWB bands. These techniques, trapezoidal UWB MPA and OAM wave generation, can significantly increase the spectrum range and improve the spectrum efficiency in wireless communications.

Index Modulation Based Joint Mode-Frequency Hopping

Existing frequency hopping (FH) techniques mainly concentrate on how to achieve robust anti-jamming while ignoring the requirement of spectrum efficiency (SE). Orbital angular momentum (OAM) shows a great application prospect for anti-jamming and multiplexing in wireless communications. To significantly increase SE and achieve robust anti-jamming of wireless communications, in this letter we propose an index-modulation based joint mode-frequency-hopping (IM-MFH) scheme, which activates several OAM-modes and carrier frequencies to hop simultaneously. The average bit error probability (ABEP) of our proposed IM-MFH scheme is also analyzed. Numerical results verify that our proposed IM-MFH scheme outperforms existing FH schemes in terms of ABEP and SE in wireless communications.

Bifocal-Lens Converging Based OAM Wireless Communications

Orbital angular momentum (OAM) based radio vortex wireless communications have received much attention recently because it can significantly increase the spectrum efficiency. The uniform circular array (UCA) is a simple antenna structure for high spectrum efficiency radio vortex wireless communications. However, the OAM based electromagnetic waves are vortically hollow and divergent, which may result in the signal loss. Moreover, the divergence of corresponding OAM based electromagnetic wave increases as the order of OAM-mode increases. Therefore, it is difficult to use high-order OAM-mode, because the corresponding received signal-to-noise ratio (SNR) is very small. To overcome the difficulty of high-order OAM modes transmission, in this paper we propose a lens antenna based electromagnetic waves converging scheme, which maintains the angular identification of multiple OAM-modes for radio vortex wireless communications. We further develop a bifocal lens antenna to not only converge the electromagnetic wave, but also compensate the SNR loss on traditional electromagnetic waves. Simulation results show that the proposed bifocal lens can converge the OAM waves into cylinder-like beams, providing an efficient way to increase the spectrum efficiency of wireless communications.

Achieving Maximum Spectrum Efficiency for Full-Duplex MIMO: The Fractal Array-Element Based Self-Interference Mitigation Approach

Full-duplex (FD), where the terminal transmits and receives signals simultaneously using the same frequency-band and time-slots, has been treated as one of the potential candidate techniques to enhance the spectrum efficiency of wireless communications. To achieve the maximum transmission rate for FD multiple input multiple output (MIMO) terminal, all antennas equipped on the terminal need to not only work in FD mode, but also in multiplexing mode. However, the most challenging problem in wireless FD MIMO communications is how to efficiently mitigate the self-interference (SI). In this paper, we propose the correlation-based digital mitigation scheme, the positive-negative array-element (PNAE) block, and the fractal array-element-based mitigation approach to mitigate the SI within one antenna. Then, we integrate the FD into the multiplexing MIMO. The large SI among the different antennas and array-elements are mitigated using the proposed adaptive diagonal loading (ADL)-based beamforming SI mitigation scheme. The simulation results verify our theoretical analyses and show that our developed SI mitigation schemes can significantly reduce the SI for FD MIMO.

Optimal Power Allocation for Multi-User Linear Network Coded Cooperation System

Linear network coded cooperation (LNCC), which deeply integrates cooperative communications and linear network coding, has been proposed to increase spectrum efficiency in wireless communications. However, the issue of optimal power allocation (OPA) for multi-user LNCC systems still remains open. This paper studies the OPA problems for a multi-user two-slot LNCC system which works in a many-to-one pattern. We first analyze and present the tight approximation of the outage probability of the multi-user LNCC system by fully considering the effect of path loss, the effectiveness of which is validated by Monte Carlo simulations. On the basis of the theoretical analyses on outage performance, we further investigate the OPA strategies for multi-user LNCC systems. Two OPA strategies under different optimization criteria, i.e., minimizing the total outage probability and jointly minimizing the outage probabilities for each user under the constraint of total power available, are proposed by utilizing genetic algorithms. To the best of our best knowledge, this is the first work on OPA for multi-user LNCC systems. Numerical results demonstrate that the proposed OPA strategies do achieve the OPA and improve the system’s performance.

Beam Steering for the Misalignment in UCA-Based OAM Communication Systems

Radio frequency-orbital angular momentum (RF-OAM) is a technique that provides extra degrees of freedom to improve spectrum efficiency of wireless communications. However, OAM requires perfect alignment of the transmit and the receive antennas and this harsh precondition greatly challenges practical applications of RF-OAM. In this letter, we first investigate the effect of non-parallel misalignment on the channel capacity of the RF-OAM communication system equipped with uniform circular array. Then, we propose a transmit/receive beam steering approach to circumvent the large performance degradation in not only non-parallel case, but also off-axis and other general misalignment cases. The effectiveness of the beam steering approach is validated through both mathematical analysis and numerical simulations.

Path-Permutation Codes for End-to-End Transmission in Ad Hoc Cognitive Radio Networks

Cognitive radios (CRs) improve the spectrum efficiency in wireless communications. Nonetheless, owing to the intrinsic randomness of ad hoc cognitive radio networks (CRNs), e.g., the opportunistic links, the traditional realization of ad hoc networking that demands the end-to-end control information, is unscalable and impractical. A virtual multiple-input multiple-output (MIMO) framework has been recently developed for the realization of error-resilient end-to-end transmission without the necessity of feedback information. In this paper, we propose an end-to-end path-permutation coded (PPC) transmission in which one relay path is accessed at a time and the transmission is hopped among multiple relay paths. The hopping order is specified by a permutation array that encodes the data, meaning that the data is conveyed by the order of indices of the accessed paths. With the PPC scheme, the control overhead and data processing complexity of the end-to-end transmission become relatively low. The PPC technique can also be utilized as a multiuser technique. At the destination node, a joint sphere decoder efficiently implements the maximum a posteriori (MAP) probability decoding that simultaneously identifies the order of accessed paths and erasures. Comprehensive theoretical analyses and simulations are conducted to demonstrate the superior performance of the PPC technique in ad hoc CRNs.

Cooperative Communications for Cognitive Radio Networks — From Theory to Applications

Cognitive Radio has been recognized as a promising way to improve spectrum efficiency of wireless communications by exploiting under-utilized licensed spectrum in temporal, frequency, and spatial domains. As a remedy to the limited capability of a single node or network, cooperative communication technique has a potential to further improve the performance of cognitive networks, especially for spectrum sensing based cognitive networks. Given that spectrum efficiency of a cognitive network is determined jointly by spectrum sensing and access capabilities, it makes sense to employ cooperative techniques to enhance their performance. This article offers a tutorial on various cooperative techniques in cognitive networks, with its emphasis on spectrum sensing and access based cooperation, interference constraint based adaptive cooperative feedback, rateless network coding based cooperative transmission, and limited cooperation based interference coordination. Finally, numerical results are given to showcase advantages of the proposed cooperative schemes for cognitive networks.

Opportunities and challenges in software radio design for interference management

AbstractInterference management is a critical aspect of the design and deployment of any wireless network. Interference can reduce quality, deny service or significantly reduce the efficiency of spectrum use. Software radio holds the potential to increase the pace of innovation, flexibility and spectrum efficiency of wireless communications. Interference management is a critical aspect for devices and system design for any wireless system, and involves unique performance flexibility trade‐offs for software radio design. This paper provides a review of interference management techniques by looking at two representative communications systems: cellular networks and unlicensed wireless local area networks. These two systems are chosen since they are representative of two very different types of communications networks. Cellular systems typically operate in a licensed band and have separate frequencies for transmission and reception, while 802.11b operates in an unlicensed spectrum and utilizes the same band for transmission and reception. The paper starts with a review of the fundamental interference issues and an overview of software radio systems. Following that, for each interference issue, the approaches and constraints for each standard are reviewed and the opportunities and challenges for software radio design are discussed. Copyright © 2006 John Wiley & Sons, Ltd.