Visible Light Communications towards 5G

5G networks have to offer extremely high capacity for novel streaming applications. One of the most promising approaches is to embed large numbers of co-operating small cells into the macro-cell coverage area. Alternatively, optical wireless based technologies can be adopted as an alternative physical layer offering higher data rates. Visible light communications (VLC) is an emerging technology for future high capacity communication links (it has been accepted to 5GPP) in the visible range of the electromagnetic spectrum (~370–780 nm) utilizing light-emitting diodes (LEDs) simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz. However, myriad gigabit speed transmission links have already been demonstrated. Non line-of-sight (NLOS) optical wireless is resistant to blocking by people and obstacles and is capable of adapting its’ throughput according to the current channel state information. Concurrently, organic polymer LEDs (PLEDs) have become the focus of enormous attention for solid-state lighting applications due to their advantages over conventional white LEDs such as ultra-low costs, low heating temperature, mechanical flexibility and large photoactive areas when produced with wet processing methods. This paper discusses development of such VLC links with a view to implementing ubiquitous broadcasting networks featuring advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) or carrier-less amplitude and phase modulation (CAP) in conjunction with equalization techniques. Finally, this paper will also summarize the results of the European project ICT COST IC1101 OPTICWISE (Optical Wireless Communications - An Emerging Technology) dealing VLC and OLEDs towards 5G networks

A New Look at Physical Layer Security, Caching, and Wireless Energy Harvesting for Heterogeneous Ultra-dense Networks

Heterogeneous ultra-dense networks enable ultra-high data rates and ultra-low latency through the use of dense sub-6 GHz and millimeter wave (mmWave) small cells with different antenna configurations. Existing work has widely studied spectral and energy efficiency in such networks and shown that high spectral and energy efficiency can be achieved. This article investigates the benefits of heterogeneous ultra-dense network architecture from the perspectives of three promising technologies, i.e., physical layer security, caching, and wireless energy harvesting, and provides enthusiastic outlook towards application of these technologies in heterogeneous ultra-dense networks. Based on the rationale of each technology, opportunities and challenges are identified to advance the research in this emerging network.Comment: Accepted to appear in IEEE Communications Magazin

Measurement techniques for the characterization of radio frequency gallium nitride devices and power amplifiers

The rapid growth of mobile telecommunications has fueled the development of the fifth generation (5G) of standards, aiming to achieve high data rates and low latency. These capabilities make use of new regions of spectrum, wider bandwidths and spectrally efficient modulations. The deployment of 5G relies on the development of radio-frequency (RF) technology with increased performance. The broadband operation at high-power and high-frequency conditions is particularly challenging for power amplifiers (PA) in transmission stages, which seek to concurrently maximize linearity and energy efficiency. The properties of Gallium Nitride (GaN) allow the realization of active devices with favorable characteristics in these applications. However, GaN high-electron mobility transistors (HEMTs) suffer from spurious effects such as trapping due to physical defects introduced during the HEMT growth process. Traps dynamically capture and release mobile charges depending on the applied voltages and temperature, negatively affecting the RF PA performance. This work focuses on the development of novel measurement techniques and setups to investigate trapping behavior of GaN HEMTs and PAs. At low-frequency (LF), charge dynamics is analyzed using pulsed current transient characterizations, identifying relevant time constants in state-of-the-art GaN technologies for 5G. Instead, at high-frequency, tailored methods and setups are used in order to measure trapping effects during the operation of HEMTs and PAs in RF modulated conditions. These RF characterizations emulate application-like regimes, possibly involving the control of the device’s output load termination. Therefore, an innovative wideband active load pull (WALP) setup is developed, using the acquisition capabilities of standard vector-network-analyzers. Moreover, the implications of performing error-vector-magnitude characterizations under wideband load pull conditions are studied. Finally, an efficient implementation of a modified-Volterra model for RF PAs is presented, making use of a custom vector-fitting algorithm to simplify the nonlinear memory operators and enable their realization in simulation environments

3-Way Doherty Power Amplifiers: Design Guidelines and MMIC Implementation at 28 GHz

This article presents the design strategy and the implementation of a three-way Doherty power amplifier (DPA3W) to enhance the efficiency at deep power back-off. Theoretical design equations are derived, based on which design charts are drawn to explore the available design space, accounting for practical constraints related to the available technology and selected application. The proposed design strategy is demonstrated by the design, fabrication and experimental characterization of a three-way multistage Doherty amplifier optimized for efficiency peaks at 6 and 12 dB back-off. The amplifier is realized on the WIN Semiconductors 150 nm GaN-SiC high-electron-mobility transistor (HEMT) monolithic process at 28 GHz, targeting 5G applications. The prototype achieves saturated output power in excess of 34 dBm and power added efficiency of the order of 15% from 6 to 12 dB back-off, demonstrating competitive performance and a good agreement between simulations and measurements, thus validating the approach

GaN-based HEMTs on Low Resistivity Silicon Technology for Microwave Applications

This paper investigates the effect of insertion AlN spacer between the GaN channel and buffer in a sub-micron gate (0.3 μm) AlGaN/GaN HEMTs on a low-resistivity (LR) (σ < 10 Ω.cm) silicon substrates on RF performance. Enhancement in short circuit current gain (fT) and maximum frequency of oscillation (fMAX) was observed in the HEMT with a 1 nm AlN spacer, where (fT) and (fMAX) were increased from 47 GHz to 55 GHz and 79 GHz to 121 GHz, respectively. Small-signal-modelling analysis was carried out to study this improvement in performance. We found that the AlN interlayer played a crucial role in reducing the gate-source capacitance, Cgs, by 36 % and delay, τ, by 20 % under the gate, as a result of an increase in mobility and a reduction in trap-related effects