The convergence of radar sensing and communication applications in the millimeter-wave (mmWave) and low terahertz (THz) bands has been envisioned as a promising technology, since it incorporates high-rate data transmission of hundreds of gigabits per second (Gbps) and mm-level radar sensing in a spectrum- and cost-efficient manner, by sharing both the frequency and hardware resources. However, the joint radar sensing and communication (JRC) system faces considerable challenges in the mmWave and low-THz scale, due to the peculiarities of the propagation channel and radio-frequency (RF) front ends. To this end, the waveform design for the JRC systems in mmWave and low-THz bands with ultra-broad bandwidth is investigated in this paper. Firstly, by considering the JRC design based on the co- existence concept, where both functions operate in a time-domain duplex (TDD) manner, a novel multi-subband quasi-perfect (MS-QP) sequence, composed of multiple perfect subsequences on different subbands, is proposed for target sensing, which achieves accurate target ranging and velocity estimation, whilst only requiring cost-efficient low-rate analog-to-digital converters (A/Ds) for sequence detection. Furthermore, the root index of each perfect subsequence is designed to eliminate the influence of strong Doppler shift on radar sensing. Finally, a data-embedded MS-QP (DE-MS-QP) waveform is constructed through time-domain extension of the MS-QP sequence, generating null frequency points on each subband for data transmission. Unlike the co- existence-based JRC system in TDD manner, the proposed DE-MS-QP waveform enables simultaneous interference-free sensing and communication, whilst inheriting all the merits from MS-QP sequences. Numerical results validate the superiority of the proposed waveforms regarding the communication and sensing performances, hardware cost as well as flexibility of the resource allocation between the dual functions.

TeraHertz (THz) communication has drawn increasing interest from both academic and industrial fields due to its capability of attaining data transmission over 100 Gb/s. To generate THz-band carriers, one promising low-cost electronic approach is the frequency-multiplier-last architecture, which however induces undesirable nonlinearity. Another substantial issue in THz communication systems is the residual hardware impairment in the mixer and power amplifier (PA), which are usually neglected under low-rate scenarios, but considered to be pronounced in the ultra-high-speed THz wireless links. With the existence of all these radio-frequency (RF) imperfections at the transmitter, conventional receiver design for lower-frequency bands is no longer applicable. In this paper, a single-carrier THz communication system utilizing the frequency-multiplier-last architecture is investigated. By jointly considering the overall RF imperfections of the mixer, the PA, and the frequency multiplier at the transmitter, a sophisticated mathematical model of the induced distortions on transmitted THz signals is provided. Based on the model, a low-complexity closed-form minimum mean Euclidean distance (MMED) channel estimator is firstly derived. Afterwards, the aggregate distortions plus thermal noise on the THz signals are approximated as a signal-dependent and spatially-colored noise term, where a closed-form quasi-maximum-likelihood (quasi-ML) detector is developed for the THz communication system. Simulation results demonstrate that our proposed receiver design is capable of significantly enhancing the performance of the THz system compared with its conventional counterparts, in terms of channel estimation accuracy and bit-error rate (BER).

As a complementary technology for conventional radio frequency communication, visible light communication (VLC) is a potential form of the optical wireless communication, which can provide both communication and illumination simultaneously. Since load balancing and power control for interference management are key challenges in the network deployment, we consider a joint user association and power allocation scheme in a cell-free VLC network to improve the system performance. It is mathematically formulated as a non-convex network utility maximization problem in consideration of the user fairness, load balancing, and power control. To tackle this non-convex problem, we divide it into two subproblems (i.e., the user association subproblem and the power allocation subproblem) and solve them with the dual projected gradient algorithm and successive convex approximation algorithm iteratively until a stationary point is found. Simulation results verify that significant gain can be achieved with the proposed scheme compared with the user association schemes without consideration of the power control.

In order to enhance the secrecy performance of the generalized space shift keying (GSSK) visible light communication (VLC) system, in this paper, an optical jamming-aided secrecy enhancement scheme is proposed, in which the source transmitter (S) simultaneously sends both the confidential desired signal and optical jamming signals under the amplitude and power constraints. The optical jamming signals obey the truncated Gaussian distribution for satisfying the constraints. Given the discrete set of channel inputs, the optical jamming-aided GSSK-VLC system's secrecy performance is analyzed. Explicitly, the average mutual information (AMI), the lower bound of AMI and its closed-form approximation as well as the achievable secrecy rate are formulated analytically. Furthermore, the optimal power sharing strategy of the proposed GSSK-VLC systems relying on optical jamming is derived. Closed-form expressions are provided for the optimal power sharing in both the low- and high-SNR regions. Finally, the extensive simulation results are presented to validate our analytical results.

The concept of amorphous-boundary-based user-centric cells invoked for visible light communication (VLC) was shown to offer extra throughput benefits over the conventional network-centric VLC. However, this improvement was quantified based on a number of idealized simplifying assumptions, such as operating exactly at the Shannon capacity. Also, the light emitting diode in VLC was assumed to have no non-linear distortion and no clipping distortion. Furthermore, greedily supporting all the user equipments in the system may in fact reduce the achievable throughput, when the transmit power is restricted. To provide more practical performance estimate, in this paper, the dynamic throughput maximization of user-centric VLC (UC-VLC) systems is investigated under a range of practical considerations, where the number of served UEs, the modulation-mode assignment and the power allocation strategy are all dynamically decided by our proposed heuristic dynamic-programming-based algorithm. Our simulations indicate that both the achievable throughput and the outage probability of the proposed UC-VLC system is better than that of the conventional NC-VLC system, under a range of practical constraints.

Orthogonal frequency division multiplexing (OFDM) with index modulation has emerged as a promising complementary technique for next-generation networks due to its high energy efficiency. In this paper, zero-padded tri-mode index modulation aided OFDM (ZTM-OFDM) is proposed, where subcarriers are partitioned into subblocks. Explicitly, only a fraction of subcarriers are utilized for modulation with two distinguishable constellation alphabets in each OFDM subblock, whilst the others remain empty. By such strategy, additional bits can be conveyed by the subcarrier indices corresponding to the two constellation sets (denoted as index pattern). At the receiver, a maximum-likelihood (ML) detector and a two-stage log-likelihood ratio (LLR) detector with reduced complexity are proposed for demodulation. The proposed ZTM-OFDM is capable of enhancing the spectral and energy efficiency compared with other index modulated OFDM schemes. Theoretical analysis based on the minimum Euclidean distance and Monte Carlo simulation results validate that the proposed ZTM-OFDM can attain performance gain over conventional OFDM and other index modulated OFDM schemes.

As the flourishing of cloud services, data centers are widely invested and deployed. However, facing the hazard of Distributed Denial of Service (DDoS) attacks, legitimate users' bandwidth access to a data center is not yet a guarantee. In response, capability-based DDoS defenses provide a promising countermeasure, especially when leveraging Autonomous System (AS) as a geographic constraint to throttle attacking flows. Unfortunately, previous schemes essentially involve a source-AS fair sharing strategy, which is too coarse- grained to provide fairness among heterogeneous AS entities. This paper proposes D4, a capability-based data center protection with state-defined allocating granularity. We differentiate the states of stub ASes through diverse aspects of domain attributes. D4 enables fair bandwidth allocation among source domains and scalable data center access for users. Our scheme is integrated with Border Gateway Protocol and can be practically deployed by Internet data centers. We illustrate the effectiveness of D4 via experiments under different scenarios and comparative simulations with closely related schemes.

Due to the limited modulation bandwidth of light emitting diodes (LEDs), the imaging optical multiple-input multiple-output (MIMO) technology is applied in visible light communications (VLC) for high data rate transmission. As an imaging lens with a wide angle/field-of-view (FOV) is favorable in the imaging optical MIMO VLC system, a fisheye lens can be utilized to concentrate the light. To avoid inter-user interference among indoor users and satisfy their target bit error rate (BER) requirements with the minimum number of LEDs simultaneously, an interference-free LED allocation scheme for the fisheye lens based imaging optical MIMO VLC system is investigated in this paper. Since the formulated problem is NP-hard, a location-based greedy algorithm is proposed, where the LEDs are allocated to the users sequentially based on their distances to the center of the LED array. Simulation results verify that there exists no interference among all users while the target BER requirements for all users are satisfied with our proposed algorithm.

Visible light communication (VLC) has become a promising complement to its radio-frequency (RF) counterpart. In VLC systems, orthogonal frequency division multiplexing (OFDM) has drawn much attention due to its high data rate, simple equalization and robustness to the inter-symbol interference (ISI). In this paper, we present a comparative performance evaluation of several classical optical OFDM schemes, including DC-biased optical OFDM (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone (PAM-DMT), unipolar OFDM (U-OFDM) and Flip OFDM. Since DCO-OFDM suffers from energy efficiency loss due to addition of DC-bias, whilst ACO-OFDM, PAM-DMT and U-OFDM/Flip OFDM are spectrally inefficient due to their unique frame structures, the state-of-the-art energy- and spectrum- efficient optical OFDM schemes are investigated, including asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), hybrid ACO-OFDM (HACO-OFDM), asymmetrically clipped absolute value optical OFDM (AAO-OFDM), the spectral and energy efficient OFDM (SEE-OFDM), layered ACO-OFDM (LACO-OFDM), enhanced U-OFDM (eU-OFDM), optical OFDM with index modulation (O-OFDM-IM) and optical dual-mode index modulation aided OFDM (DM-OFDM). In this paper, their principles are firstly illustrated, then performance comparisons are conducted for those optical OFDM schemes in terms of spectral efficiency, energy efficiency and computational complexity. Since light emitting diodes (LEDs) need to support both the illumination and communication simultaneously, dimming control should be considered in VLC systems. Therefore, dimmable optical OFDM for practical VLC systems incorporating illumination is also addressed.

Compared with the conventional amplitude phase modulation (APM), spatial modulation (SM) is a low- complexity, yet energy-efficient transmission technique, whereby transmit antenna (TA) indices are utilized to convey the information. However, the number of the required TAs grows exponentially with the number of transmitted bits, which leads to unacceptable pilot overhead for channel estimation in practical systems. To reduce the number of TAs whereas keep the data rate unchanged, virtual spatial modulation (VSM) is proposed in the first time. Specifically, by activating multiple TAs with their corresponding analog phase shifters (APSs), massive equivalent channel vectors could be constructed based on the combinations of original channel vectors from different TAs and their phase rotations. By way of mapping each equivalent channel vector to a virtual transmit antenna (VTA) index which might convey the information, the number of the required TAs could grow linearly with the number of transmitted bits. Furthermore, the selection of a VTA subset from all available VTAs is formulated as a combinatorial optimization problem to maximize the minimal Euclidean distance (ED) among the equivalent channel vectors. A spatial constellation optimizing (SCO) algorithm is proposed to obtain a near-optimal solution to this problem with low-complexity. Simulation results demonstrate that the proposed VSM is able to achieve lower bit error rate (BER) under the same transmit rate compared with the conventional SM and APM schemes.

The Internet keeps flourishing and enables unexpected possibilities to all aspects of individuals' life. Such distributed networking systems as wireless sensor networks and Internet of things are widely deployed. Yet, in the meantime secure group communication remains a challenging task in these environments. To address this challenge, this paper aims at lightweight group key establishment with strong security properties. We propose a broadcast encryption scheme based on one-way function trees and specify a dual-evolving approach for dynamic group-membership update. Then we complement the scheme by a content-protection protocol and further optimize the protocol in terms of communication efficiency. As with our analysis, our scheme can successfully prevent a key leakage attack, namely, collusion attack. Through our comprehensive evaluations, we confirm the effectiveness and the adequacy of our solution in distributed networking systems.