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The reliability of ultrasonic intra-body communication (IBC) is crucial for medical monitoring, diagnosis, and treatment. Recently, ultrasonic splitting receivers that jointly use coherent detection (CD) and non-coherent energy detection (ED) obtain superior bit-error rate (BER) performance compared to a single CD or ED receiver for reliable IBCs. However, this paper demonstrates that existing splitting receivers have performance gains in terms of mutual information and BER in the case of independent channel noises for CD and ED, while no performance gains exist in the case of correlated channel noises. To this end, we propose an ultrasonic index modulation-splitting-maximum ratio combination (UsIM-S-MRC) system. UsIM activates a part of transmission frames whilst S-MRC only processes active frames and treats inactive frames as zero, reducing the average channel noise in all frames. Therefore, the joint-designed UsIM-S-MRC effectively mitigates the impact of channel noise correlation on splitting detection. The theoretical mutual information, BER, and optimal splitting ratio are also derived. Extensive simulations validate the theoretical analysis and reveal that UsIM-S-MRC can attain performance gain in both independent and correlated channel noises, providing a reliable receiver structure for future ultrasonic IBCs.

Performance of wireless powered wireless systems is analyzed. The wireless devices in such systems scavenge energy from sources in downlinks and use the energy to communicate in uplinks. We introduce two new models for these energy harvesters to consider their nonlinear circuitry and their functioning over multiple line-of-sight and non-line-of-sight channels. The newly proposed Beaulieu-Xie fading model is used to characterize this manifold of channels. Performance analyses of average harvested energy and transmission outage probability show good fit between the proposed models and measured data.

Abstract The performance of a new (Beaulieu-Xie) fading model is analyzed using bounds. This recently proposed fading model can be used to describe both line-of-sight and non-line-of-sight components of a fading channel having different diversity orders. We consider the outage probability and error rate performance of maximal ratio combining, equal-gain combining, and selection combining over arbitrarily correlated Beaulieu-Xie fading channels. Closed-form expressions for upper and lower bounds to the outage probability and error rate are obtained, and it is shown that these bounds are asymptotically tight in the high signal-to-noise ratio regime. The analytical results are verified via Monte Carlo simulations. It is shown that the Beaulieu-Xie fading model can be more useful than the Ricean and Nakagami-m fading models in characterizing environments with both line-of-sight and multiple reflected specular components.

Dimmable optical orthogonal frequency division multiplexing (D-OFDM) is challenging to design for visible light communications, due to the high peak-to-average power ratio (PAPR) of OFDM signals and limited dynamic range of LEDs. Current D-OFDM schemes focused on the time domain and spatial domain designs of OFDM signals. This paper proposes a novel dynamic subcarrier activation based OFDM (DSA-OFDM) scheme. In DSA-OFDM, the number of activated subcarriers, as well as the signal's DC bias and the number of activated LEDs, is adjusted to provide both flexible dimming control and reliable communication. A generalized index modulation in both spatial domain and frequency domain is designed to select the activated subcarriers and activated LEDs. To show the influence of frequency domain design analytically, the closed-form expressions of the PAPR and the clipping noise of the DSA-OFDM signals are derived, and the optimized signal form is obtained accordingly. Simulation and numerical results show that the proposed scheme can outperform conventional dimming control schemes at various illumination levels in terms of the bit-error rate performance.

The statistical characterization of a sum of random variables (RVs) is useful for investigating the performance of wireless communication systems. We derive exact closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) of a sum of independent but not identically distributed (i.n.i.d.) Fisher-Snedecor F RVs. Both PDF and CDF are expressed in terms of the multivariate Fox's H-function. Besides, a simple and accurate approximation to the sum of i.n.i.d. Fisher-Snedecor F variates is presented using the moment matching method. The obtained PDF and CDF are used to evaluate the performance of wireless communication applications including the outage probability, the effective capacity, and the channel capacities under four different adaptive transmission strategies. Moreover, the corresponding approximate expressions are obtained to provide useful insights for the design and deployment of wireless communication systems. In addition, we derive simple asymptotic expressions for the proposed mathematical analysis in the high signal-to-noise ratio regime. Finally, the numerical results demonstrate the accuracy of the derived expressions.

A new composite fading model is introduced. This shadowed Beaulieu-Xie model is developed to characterize wireless communication in an environment with an arbitrary number of line-of-sight and non-line-of-sight signals, in contrast to the existing Rayleigh, Ricean, generalized Ricean (i.e., κ - μ), and Nakagami-m models. The proposed model benefits from four parameters that characterize a wide range of fading conditions, unlike existing composite models such as the shadowed Ricean model, the two-wave with diffuse power model, and the fluctuating two-ray model. The proposed shadowed Beaulieu-Xie model is used here to characterize experimental data obtained from fading measurements in 28 GHz outdoor millimeter-wave channels, and it is found to describe the communication environment accurately. We conclude that the proposed composite fading model is particularly useful for characterizing emerging (millimeter-wave and terahertz) wireless communication systems.

We analyze a secure unmanned aerial vehicle-assisted two-hop mixed radio frequency (RF) and underwater wireless optical communication (UWOC) system using a fixed-gain amplify-and-forward (AF) relay. The UWOC channel was modeled using a mixture exponential-generalized Gamma distribution to consider the combined effects of air bubbles and temperature gradients on transmission characteristics. Both legitimate and eavesdropping RF channels were modeled using flexible α-μ distributions. Specifically, we first derived both the probability density function (PDF) and cumulative distribution function (CDF) of the received signal-to-noise ratio of the system. Based on the PDF and CDF expressions, we derived the closed-form expressions for the tight lower bound of the secrecy outage probability (SOP) and the probability of non-zero secrecy capacity (PNZ), which are both expressed in terms bivariate Fox’s H-function. To utilize these analytical expressions, we derived asymptotic expressions of SOP and PNZ using only well-known functions. We also used asymptotic expressions to determine the suboptimal transmitting power to maximize energy efficiency. Furthermore, we investigated the effect of levels of air bubbles and temperature gradients in the UWOC channel, and studied the nonlinear characteristics of the transmission medium and the number of multipath clusters of the RF channel on the secrecy performance. Finally, all analyses were validated using a simulation.

In this paper, we study the utility fairness resource allocation in a multi-user orthogonal based cognitive radio network with cochannel interference (CCI) mitigation. In our proposed system model, we introduce the correct reception probability (CRP) model as a network utility metric. Furthermore, useful bounds on CRP are derived to analyze the performance of proposed allocation schemes. The optimal resource allocation is formulated as a worst-case user CRP maximum problem with both average CCI and average power budget constraints. However, this problem is non-convex and generally challenging to solve. Therefore, we solve this problem by successively performing subchannel allocation and power allocation. Firstly, a k-means clustering inspired subchannel allocation strategy is proposed to divide secondary users (SUs) into multiple groups by minimizing the average mutual-signal-to-interference-ratio degree between any two SUs. The concept of reference user is employed to guarantee the quality of service of the primary user. In each subchannel, we formulate a max-min utility optimal power allocation problem. The non-linear Perron Frobenius theory is applied to solve this power allocation problem. Simulation results show that the proposed resource allocation scheme is fair and has relatively fast convergence.

Abstract Micromanipulation for applications in areas such as tissue engineering can require mesoscale structures to be assembled with microscale resolution. One method for achieving such manipulation is the parallel actuation of many microrobots in parallel. However, current microrobot systems lack the independent actuation of many entities in parallel. Here, the independent actuation of fifty opto-thermocapillary flow-addressed bubble (OFB) microrobots in parallel is demonstrated. Individual microrobots and groups of microrobots were moved along linear, circular, and arbitrary 2D trajectories. The independent addressing of many microrobots enables higher-throughput microassembly of micro-objects, and cooperative manipulation using multiple microrobots. Demonstrations of manipulation with multiple OFB microrobots include the transportation of microstructures using a pair or team of microrobots, and the cooperative manipulation of multiple micro-objects. The results presented here represent an order of magnitude increase in the number of independently actuated microrobots in parallel as compared to other magnetically or electrostatically actuated microrobots, and a factor of two increase as compared to previous demonstrations of OFB microrobots.

A dimming control scheme that combines analog dimming and spatial dimming is proposed. The basic idea of the combined scheme is that both the direct current bias level of each individual light-emitting diode (LED) and the number of glared LEDs are simultaneously adjusted to achieve a required dimming level. Besides, we design a space constellation to transmit additional information in spatial domain without compromising uniform illumination constraint, and this essentially improves the performance of the system. A maximum-a-posteriori probability estimator is also designed to estimate the received time domain and spatial domain signals. Since two degree of freedoms are simultaneously exploited for dimming control, the proposed scheme outperforms conventional dimming schemes, which only focus on signal domain design, and this is verified by simulation results. To the best of authors' knowledge, this is the first design that effectively utilizes multidomain resources to achieve dimming control for visible light communications.

Ambient backscatter is a new technology that uses ambient signals to enable communication between battery-free tags (or sensors) and readers. Previous studies on ambient backscatter focused on scenarios with only one tag. In this paper, we investigate the ambient backscatter communication system with multiple tags and analyze the performance of tag selection. Specifically, we formulate a tag selection scheme and design the corresponding detection method. Our results indicate that obtaining the closed-form bit-error rate (BER) in such cases is challenging; hence, we provide a solution for deriving an approximate BER. The simulation results show that the approximated BER curves are in agreement with the exact curves.

In visible light communication (VLC) systems, optical orthogonal frequency-division multiplexing (O-OFDM) is an appealing modulation scheme. Recently, a number of O-OFDM schemes have been proposed. Among them, direct-current O-OFDM (DCO-OFDM) is a widely used scheme for its high spectral efficiency and low complexity. Since VLC involves a combination of illumination and communication, different optical power is often required to achieve a certain illumination level. However, clipping noise will dominate a severe performance degradation of DCO-OFDM when a relatively high or low illumination level is imposed, and it restricts applications of DCO-OFDM in future VLC systems. To address this problem, an enhanced DCO-OFDM (eDCO-OFDM) scheme is proposed. By introducing a piecewise function with adaptive slopes according to the required optical power, the proposed eDCO-OFDM scheme has the potential to effectively eliminate the clipping noise. Furthermore, two parameter selection mechanisms with different complexities and performance gains are designed for the piecewise function in the eDCO-OFDM scheme. Simulation results verify the energy and spectral efficiency of the proposed scheme under the constraints of optical power.

Free-space optical communication links operating over lognormal turbulence channels using on-off keying (OOK) are studied in this paper. Such systems can suffer from irreducible error floors that result from the use of demodulation with fixed and unoptimized detection thresholds. The resulting error floors are analyzed for the general case of low- and high-state offsets (i.e., finite extinction ratios). An electrical signal-to-noise ratio (SNR) optimized detection system is applied. The system uses the electrical SNRs to implement adaptive detection thresholds and eliminate the error floors. The system can accommodate operation with finite extinction ratios, as it uses the method of moments and maximum likelihhod estimation techniques to estimate the low- and high-state offsets and electrical SNR. Numerical results show that the SNR gap between the electrical-SNR-optimized detection system and the adaptive detection system is 2.3 dB at a bit-error rate of 10-5 without a state offset. The SNR gap increases to 4.5 dB with a state offset of ξ=0.2 .

Alamouti-type space-time block coding is studied for optical wireless communication systems using coherent detection over atmospheric turbulence channels. Atmospheric turbulence-induced fading and phase noise are known to impair the performance of coherent optical wireless systems. Two new Alamouti-type space-time coded architectures are proposed to overcome turbulence-induced fading and phase noise from transmitter lasers, receiver local oscillators, and turbulence channels. Their error rate performance is analyzed for a wide range of turbulence conditions. With developed analytical error rate expressions, the usefulness of the proposed Alamouti-type space-time block coding is demonstrated for coherent optical wireless communication systems. Numerical results confirm the performance improvements of the proposed systems over that of single-input-single-output systems.