Fengzhong Qu

Intelligent transportation spaces: vehicles, traffic, communications, and beyond

Recent years have witnessed numerous technical breakthroughs in electronics, computing, sensing, robotics, control, signal processing, and communications. These have significantly advanced the state of applications of intelligent transportation systems. More recently, as one leading effort toward the cyber-physical-social system, the concept of intelligent transportation spaces was proposed to further improve the vehicles, traffic, and transportation safety, efficiency and sustainability. ITSp integrate not only various ITS modules, but also pedestrians, vehicles, roadside infrastructures, traffic management centers, sensors, and satellites. With distributed and pervasive intelligence, ITSp clearly impose some stringent requirements on the information exchange among all entities within the ITSp, in terms of the information availability, reliability, fidelity, and timeliness. These requirements, together with the high mobility of vehicles and the highly variable network topology, make the communications and networking for ITSp very challenging. This article will introduce the concept of ITSp and analyze possible communication technology candidates for ITSp. Further discussions will also be provided at the end of this article.

Basis expansion model for underwater acoustic channels

Underwater acoustic communications (UAC) channels are widely perceived as being doubly selective in both time and frequency domain. Basis expansion model (BEM) is an approximation of the time-varying channel with a parsimonious set of BEM coefficients, which facilitates coherent and differential schemes resilient to doubly selectivity of UAC channels. Different BEMs not only approximate the channels with different accuracy levels, but also induce different effects on the model fitting bias and the additive noise. In this paper, we will first show how our previously proposed coherent and differential schemes based on discrete Fourier transform (DFT) BEM can be modified to accommodate other BEMs. We will then analyze the tradeoff between the channel modeling accuracy and bias/noise effect. Our analyses, simulations and experiment results confirm that BEM is a powerful tool in UAC. Additionally, in the choice among various BEM options, not only the modeling accuracy has to be considered, the nature of the model fitting bias and the noise effect also have to be taken into account.

On the estimation of doubly-selective fading channels

Coherent communications over doubly-selective fading channels are attracting increasing research interests because of the performance advantage over their noncoherent counterparts. In this paper, we use a simple windowing and dewindowing technique to improve the accuracy of an existing basis expansion model (BEM) and develop a windowed least-squares (WLS) estimator for doubly-selective fading channels. We also design the optimum pilot pattern for the WLS estimator. Our designs can considerably improve the channel estimation performance. Simulations are provided to corroborate our theoretical analysis.

On the Capacity and System Design of Relay-Aided Underwater Acoustic Communications

In underwater acoustic communications (UAC), frequency-dependent signal attenuation, long propagation delay and doubly-selective fading channels render reliable communications a challenging problem, especially at long distances. To enhance reliability and to extend range, relay communications have been extensively studied in terrestrial environments. However, their application to UAC has not been thoroughly explored. In this paper, we analyze the capacity of relay-aided (RA- )UAC. The result shows a prominent capacity increase in RA-UAC systems, when compared with traditional direct-link UAC. In addition, effects of various system parameters on capacity are also evaluated. These parameters include source-to-destination distance, transmit power allocation and relay location. To realize the benefits of RA-UAC, special considerations are to be taken in practical RA-UAC system designs. To account for and to take advantage of the unique characteristics of UAC channels, we develop a practical asynchronous amplify-and-forward (AF) relay system for UAC. To collect the ample multipath energy and diversity enabled by this relaying protocol, we also employ the precoded orthogonal frequency division multiplexing (OFDM) as the basic physical layer module. Our system resolves both the time synchronization difficulty and frequency selectivity of UAC. Simulations and comparisons are presented to verify our analysis and design.

A Two-Stage Approach for the Estimation of Doubly Spread Acoustic Channels

In this paper, the estimation of doubly spread acoustic channels is investigated. By parameterizing the amplitude variation and delay variation of each path with polynomial approximation, this paper derives a mathematical model for the discrete-time channel input-output relationship tailored to single-carrier block transmissions. Based on the mathematical model, the channel estimation problem is transformed into estimation of the low-dimensional parameter sets (amplitude, delay, Doppler scale) that characterize the channel. A two-stage sparse channel estimation technique is then developed, which estimates the delay and Doppler scale sequentially. Compared to the one-stage joint estimation, the two-stage estimation approach greatly reduces the number of candidates on the delay-Doppler scale grid searched by the orthogonal matching pursuit (OMP) algorithm, that is, the dictionary size is reduced dramatically. As a result, the computational complexity is much lower. Further, the two-stage approach demonstrated higher levels of accuracy in computer simulations and led to better detection performance when applied to experimental data.

Modulation Selection from a Battery Power Efficiency Perspective

In this paper, we compare the battery power efficiencies of various pulse-based modulations widely adopted for their low complexity. Taking into account circuit modules and battery imperfectness, we establish simple closed-form analytical formulas which can be used to conveniently determine the relative preference between arbitrary pulse-based modulation pairs in terms of their actual average battery energy consumption.

Joint user scheduling and channel allocation for cellular networks with full duplex base stations

Full-duplex communication (FDC) can potentially double the network capacity by allowing a device to transmit and receive simultaneously on the same frequency band. In this study, a novel resource allocation and user scheduling algorithm is proposed to maximise the network throughput for a cellular network with full-duplex (FD) base stations (BSs). The authors consider that FDC is utilised at the BS with imperfect self-interference (SI) cancellation while user devices only work in the traditional half-duplex (HD) way. In addition, to potentially cancel co-channel interference caused by other users, the opportunistic interference cancellation (OIC) technique is applied at user side. Since FDC does not always perform better than HD due to residual SI (RSI), a joint mode selection, user scheduling, and channel allocation problem is formulated to maximise the system throughput. The optimisation problem is non-convex and NP-hard, thereby a suboptimal heuristic algorithm with low computational complexity is proposed. Numerical results demonstrate that user diversity gain, FD gain, and OIC gain can be achieved by the proposed algorithm, respectively. The performance of FDC depends on the intensity of RSI and the distribution of user devices.

Underwater Laser Communication Using an OFDM-Modulated 520-nm Laser Diode

In this letter, we propose and experimentally demonstrate an underwater wireless optical communication system based on a simple and cost-effective TO56 transversal multi-mode green-light laser diode (LD) and a low-cost 150-MHz positive-intrinsic-negative detector. We use quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing modulation, with fine frequency granularity and high spectral efficiency, to tackle the challenging issue imposed by the ragged and uneven frequency response of the green LD within a limited bandwidth. We have demonstrated that a bit rate of 1.118 Gb/s (net bit rate: 0.927 Gb/s) can be achieved at a bit-error-rate of 2.98 × 10-3, with the assistance of bit loading (using both 16-QAM and 256-QAM signals). A spectral efficiency as high as 6.18 bit/s/Hz is also obtained in the proposed underwater wireless optical communication system.

A journey toward modeling and resolving doppler in underwater acoustic communications

Underwater acoustic (UWA) communications is the only reliable method for long-distance communications underwater, and is widely used in commercial, scientific, and military scenarios. However, the UWA channel is most challenging due to its double dispersion property in both long time delay and large Doppler spread, resulting in severe multipath spread and time variation. Among these, Doppler spread is one of the most critical challenges, and researchers have proposed various models in order to resolve Doppler spread in UWA channels. In this article, an overview of Doppler modeling and resolving is provided, divided into four stages: the quasi- static model of the mid-1980s, the uniform Doppler shift model in the 1990s, the basis expansion model and uniform path speed models of the late 1990s, and the recently developed non-uniform path speed model. Furthermore, the UWA channel sparsity property utilized by each of those models will also be discussed.

Single carrier FDMA over underwater acoustic channels

Underwater acoustic sensor networks (UWASN) have been attracting growing research interests in recent decades. However, underwater acoustic (UWA) communications suffer from the long propagation delay, the limited bandwidth, multipath, and the Doppler effect. Orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) are attractive to UWA communications because of their robustness against frequency selective distortion. Nevertheless, OFDM and OFDMA have several drawbacks: high sensitivity to the frequency offset, large peak-to-average power ratio (PAPR), and subcarrier nulls. Single carrier frequency division multiple access (SC-FDMA) which is a DFT-precoded version of OFDMA has stimulated wide interests in cellular wireless communication systems because of the inherent merits. However, SC-FDMA over UWA channels has not been investigated in the existing literature. In this paper, we apply SC-FDMA to UWA channels and mainly focus on the receiver design of the localized FDMA (LFDMA) scheme. Furthermore, our proposed receiver algorithm is tested using the experimental data recorded from the ACOMM10 experiment. The experimental results show that with multiple hydrophones combined, an average uncoded bit error rate (BER) of 10−3 can be achieved.