What is index modulation (IM)? This is an interesting question that we have started to hear more and more frequently over the past few years. The aim of this paper is to answer this question in a comprehensive manner by covering not only the basic principles and emerging variants of IM, but also reviewing the most recent as well as promising advances in this field toward the application scenarios foreseen in next-generation wireless networks. More specifically, we investigate three forms of IM: spatial modulation, channel modulation and orthogonal frequency division multiplexing (OFDM) with IM, which consider the transmit antennas of a multiple-input multiple-output system, the radio frequency mirrors (parasitic elements) mounted at a transmit antenna and the subcarriers of an OFDM system for IM techniques, respectively. We present the up-to-date advances in these three promising frontiers and discuss possible future research directions for IM-based schemes toward low-complexity, spectrum- and energy-efficient next-generation wireless networks.

Index Modulation Techniques for Next-Generation Wireless Networks

A novel multiple-input multiple-output (MIMO) transmission scheme, called space-time block coded spatial modulation (STBC-SM), is proposed. It combines spatial modulation (SM) and space-time block coding (STBC) to take advantage of the benefits of both while avoiding their drawbacks. In the STBCSM scheme, the transmitted information symbols are expanded not only to the space and time domains but also to the spatial (antenna) domain which corresponds to the on/off status of the transmit antennas available at the space domain, and therefore both core STBC and antenna indices carry information. A general technique is presented for the design of the STBC-SM scheme for any number of transmits antennas. Besides the high spectral efficiency advantage provided by the antenna domain, the proposed scheme is also optimized by deriving its diversity and coding gains to exploit the diversity advantage of STBC. A low-complexity maximum likelihood (ML) decoder is given for the new scheme which profits from the orthogonality of the core STBC. The performance advantages of the STBC-SM over simple SM and over V-BLAST are shown by simulation results for various spectral efficiencies and are supported by the derivation of a closed form expression for the union bound on the bit error probability.

Space-Time Block Coded Spatial Modulation

In recent years, device-to-device (D2D) communication has attained significant attention in the research community. The advantages of D2D communication can be fully realized in multi-hop communication scenario. The integration of cellular and multi-hop networks not only provides guaranteed quality of service and reliability as a traditional cellular network, but also has the flexibility and adaptability as a multi-hop network. Routing in such multi-hop cellular D2D networks is a critical issue, since the multi-hop network can perform worse than a traditional cellular network if wrong routing decisions are made. This is because routing in these multi-hop networks needs to take care of the node mobility, dynamic network topology, and network fragmentation, which did not exist in traditional cellular networking. This paper provides a comprehensive survey of routing in multi-hop D2D networks. Some future research directions for the routing in D2D networks are also discussed at the end of this paper.

Routing in Multi-Hop Cellular Device-to-Device (D2D) Networks: A Survey

A comprehensive review concerning the geometry, the manufacturing technologies, the materials, and the numerical techniques, adopted for the analysis and design of wideband and ultrawideband (UWB) antennas for wireless applications, is presented. Planar, printed, dielectric, and wearable antennas, achievable on laminate (rigid and flexible), and textile dielectric substrates are taken into account. The performances of small, low-profile, and dielectric resonator antennas are illustrated paying particular attention to the application areas concerning portable devices (mobile phones, tablets, glasses, laptops, wearable computers, etc.) and radio base stations. This information provides a guidance to the selection of the different antenna geometries in terms of bandwidth, gain, field polarization, time-domain response, dimensions, and materials useful for their realization and integration in modern communication systems.

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Wideband and UWB Antennas for Wireless Applications: A Comprehensive Review

A novel dual-band substrate-integrated waveguide (SIW) antenna array topology is proposed for operation in the 28 and 38 GHz frequency bands. Four miniaturized quarter-mode SIW cavities are tightly coupled, causing mode bifurcation, and yielding an antenna topology with four distinct resonance frequencies. A pair of resonances is assigned to both the 28 and 38 GHz band, achieving wideband operation in both frequency ranges. Moreover, owing to the exploited miniaturization technique, an extremely compact array topology is obtained, facilitating easy and straightforward integration. The computer-aided design process yields a four-element antenna array that entirely covers the 28 GHz band (27.5–29.5 GHz) and 38 GHz band (37.0–38.6 GHz) with a measured impedance bandwidth of 3.65 and 2.19 GHz, respectively. A measured broadside gain of 10.1 dBi, a radiation efficiency of 75.75% and a 3 dB beamwidth of 25° are achieved in the 28 GHz band. Moreover, in the 38 GHz band, the measured broadside gain amounts to 10.2 dBi, a radiation efficiency of 70.65% is achieved, and the 3 dB beamwidth is 20°.

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Dual-Band (28,38) GHz Coupled Quarter-Mode Substrate-Integrated Waveguide Antenna Array for Next-Generation Wireless Systems

A high-performance, three-element substrate-integrated-waveguide (SIW) cavity-backed slot antenna array that covers the (5.15–5.85) GHz band is designed for integration inside or underneath the worktop of a desk, to set up a stable, high data-rate ultra-short-range $3 \times 3$ multiple-input multiple-output (MIMO) wireless communication link with a mobile user (MU) positioned on top of that worktop. The antenna topology and array geometry are carefully selected to maximally exploit the multiplexing capabilities of the ultra-short-range $3 \times 3$ MIMO channel over a wide bandwidth, yielding an increased channel capacity and/or reduced power consumption. In addition, special care was taken to guarantee a channel capacity that is less dependent on the relative orientation of the MU. Furthermore, the SIW implementation technology is combined with innovative antenna materials to guarantee a low-profile, low-cost, stable, and high-performance broadband array design that maintains its excellent performance after integration. A prototype of the antenna array was fabricated, integrated according to two different integration scenarios, and validated. Measurements prove that the antenna array allows integration into a worktop with only a minor influence on its return loss and mutual coupling, guaranteeing a bandwidth of at least 1.078 GHz and a minimum isolation between antenna elements of 30 dB within the entire (5.15–5.85) GHz band.

Threefold Rotationally Symmetric SIW Antenna Array for Ultra-Short-Range MIMO Communication

Reliable, high-data rate indoor communication is essential to transfer crucial information between firefighters for improving their safety and decreasing the number of casualties caused by indoor fires. Since electronic monitoring systems, including antennas implemented inside the firefighter jacket, should provide high data rates, communication over a wideband channel is required. We study an 80-MHz-wide body-to-body channel at 3.6 GHz between two firefighters of a Rapid Intervention Team (RIT) performing the primary search for victims, by static and dynamic channel sounder measurements. Two ultra-wideband (UWB) substrate integrated waveguide (SIW) cavity-backed slot textile antennas were unobtrusively deployed in the front and the back sections of the firefighters’ jackets, providing up to $2\times 2$ MIMO communication. We calculate the achievable single-input single-output (SISO)-, single-input multiple-output (SIMO)-, multiple-input single-output (MISO)-, and multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) capacities for realistic indoor broadband body-to-body communication channels between two firefighters. Furthermore, we analyze implementations of one-dimensional (1-D) spatial waterfilling and two-dimensional (2-D) space-frequency waterfilling, studying their ability to further enhance transmission of live sensor data, pictures or videos between mobile firefighters.

Capacity of Broadband Body-to-Body Channels Between Firefighters Wearing Textile SIW Antennas

Given that the market of wearables is in so-called hypergrowth mode, more and more of these on-body devices will interact with each other. These body-to-body, device-to-device links should not only provide reliable but also secure communication of personal user data. Therefore, we have analyzed the potential of using the unique reciprocal body-to-body channel between two legitimate parties, to create a high-level security key that is unknown to an eavesdropper. Both randomly moving legitimate parties, typically called Alice and Bob, were equipped with low-power wireless on-body sensor nodes, which collect the Received Signal Strength values. Additionally, the eavesdropper Eve, who is continuously sniffing the body-to-body channel using a third sensor node, collects her own sequence of RSS values, which are expected to be highly decorrelated from the RSS values from both Alice and Bob. Based on a statistical analysis, applied to Received Signal Strength values to verify the correlation, entropy and mutual information, the body-to-body link seems very suitable for RSS-based secret key generation in indoor and outdoor Wireless Body Area Networks. Moreover, this practical and lightweight alternative for secret key generation ensures low on-chip complexity and, hence, low computational power consumption.

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RSS-based secret key generation for indoor and outdoor WBANs using on-body sensor nodes

High data-rate wireless communication for in-body human implants is mainly performed in the 402–405 MHz Medical Implant Communication System band and the 2.45 GHz Industrial, Scientific and Medical band. The latter band offers larger bandwidth, enabling high-resolution live video transmission. Although in-body signal attenuation is larger, at least 29 dB more power may be transmitted in this band and the antenna efficiency for compact antennas at 2.45 GHz is also up to 10 times higher. Moreover, at the receive side, one can exploit the large surface provided by a garment by deploying multiple compact highly efficient wearable antennas, capturing the signals transmitted by the implant directly at the body surface, yielding stronger signals and reducing interference. In this paper, we implement a reliable 3.5 Mbps wearable textile multi-antenna system suitable for integration into a jacket worn by a patient, and evaluate its potential to improve the In-to-Out Body wireless link reliability by means of spatial receive diversity in a standardized measurement setup. We derive the optimal distribution and the minimum number of on-body antennas required to ensure signal levels that are large enough for real-time wireless endoscopy-capsule applications, at varying positions and orientations of the implant in the human body.

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Improved Reception of In-Body Signals by Means of a Wearable Multi-Antenna System

The emergence of miniaturized flexible electronics enables on-duty first responders to collect biometrical and environmental data through multiple on-body sensors, integrated into their clothing. However, gathering these life-saving data would be useless if they cannot set up reliable, preferable high-data-rate, wireless communication links between the sensors and a remote base station. Therefore, we have developed a four-element ultrawideband textile cross array that combines dual-spatial and dual-polarization diversity and is easily deployable in a first responder's garment. The impedance bandwidth of the array equals 1.43 GHz, while mutual coupling between its elements remains below -25 dB. For a maximal bit error rate of 1e-4, the array realizes a diversity gain of 24.81 dB. When applying adaptive subcarrier modulation, the mean throughput per orthogonal frequency division multiplexing (OFDM) subcarrier increases by an extra bit/symbol when comparing fourth- to second-order diversity.

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Thijs Castel

Papers

Threefold Rotationally Symmetric SIW Antenna Array for Ultra-Short-Range MIMO Communication

Capacity of Broadband Body-to-Body Channels Between Firefighters Wearing Textile SIW Antennas

RSS-based secret key generation for indoor and outdoor WBANs using on-body sensor nodes

Improved Reception of In-Body Signals by Means of a Wearable Multi-Antenna System

Four-Element Ultrawideband Textile Cross Array for Dual-Spatial and Dual-Polarization Diversity