Erwu Liu

Percolation and Scale-Free Connectivity for Wireless Sensor Networks

Based on complex network analysis and percolation theory, this letter proposes a method called LTPT to generate scale-free topology for wireless sensor networks (WSNs), with required connectivity and lifetime. The problem is divided into logical topology (LT) and physical topology (PT) analysis. With LTPT, we generate a robust logical topology of power-law degree distribution and build a physical topology of guaranteed connectivity and lifetime. This is the first time the connection between logical and physical connectivities is set up analytically.

A Flow-Weighted Scale-Free Topology for Wireless Sensor Networks

In this paper, complex network theory is used to generate robust scale-free topology for wireless sensor networks (WSNs). Nodes in WSNs consume energy in two stages: network generation and network operation. Existing scale-free models for WSNs focus on the energy in the first stage. However, sensors consume most energy in the second stage. This paper proposes a method called flow-aware scale-free (FASF) model to balance the energy consumption of sensors in the second stage. Taking into account the traffic flow in the network, a WSN is modeled as a weighted network and its energy usage is balanced. Both analysis and simulations indicate that FASF enhances connectivity and network lifetime, achieves high robustness against node failures, and at the same time maintains the scale-invariant property.

NLL: A Complex Network Model with Compensation for Enhanced Connectivity

The canonical scale-free model to describe complex networks is BA model with an power-law exponent γ = 3. Researchers further propose DS model (1 <; γ ≤ 4) to consider link failure besides node growth in preferential attachment. However, both models assume globally preferential attachment which is difficult to achieve in real networks. This paper proposes a new scale-free model, i.e. Neighborhood Log-on and Log-off model (NLL) which considers locally preferential connectivity. NLL incorporates both node growth and removal in topology evolvement. Unlike BA and DS, NLL adds compensation mechanism to enhance connectivity. The analysis shows that NLL has 1 <; γ ≤ 3. We conduct simulations to evaluate NLL performance and show that, NLL has short average path length and large clustering coefficient, compared with BA and DS models.

Applying LTE-D2D to Support V2V Communication Using Local Geographic Knowledge

This paper applies device-to-device technique to vehicular-to-vehicular communications (D2D-V) in an underlay fashion. Different from conventional D2D systems focusing on smart phones, whose positions are usually assumed to be statical, the D2D-V system suffers more mobility and uncertainties. These dynamic features indeed complicate the design of D2D-V system, but also, give more potential to accomplish some geographic based algorithms. Armed with the geographic knowledge of highway and the dynamic GPS information in each vehicle, we propose a geographic based reuse cellular user selection scheme and two different distributed power control schemes to serve various applications in vehicular networks. We model the problem firstly, then formulate two metrics: the sum rate and the minimum-achievable rate to evaluate the performance of the proposed schemes. Simulations are proposed to validate our schemes and analysis. Influences of different system settings are also discussed here.

Bounds on Secondary User Connectivity in Cognitive Radio Networks

We consider a large-scale cognitive radio network (CRN) in a Rayleigh fading environment. Errors of spectrum sensing by secondary users (SUs) are considered in the letter. Cumulative inter-network interference (between primary and secondary networks) is calculated and outage probability of primary users (PUs) is derived in the closed-form. The upper and lower bounds on the percolation area are obtained and analyzed, within which the secondary network percolates while the PUs outage probability is kept below a tolerable threshold. The bounds depend on SUs correct detection and false alarm probabilities, and PUs transmit power and protection radius. Further we find that the percolation area that percolates SUs and meanwhile provides protection for PUs does not always exist.

An Efficient Pilot Scheme in Large-Scale Two-Way Relay Systems

This letter considers an efficient pilot scheme for two-way relay system with massive antennas in the relay base station (BS). For such a system, we prove that user terminals (UTs) in the same communication pair can reuse the pilot sequence without performance loss. Hence, only K pilot symbols are required to support 2K UTs while 2K pilot symbols are needed in conventional cases. We analyze the asymptotic performance when the BSs antenna number grows infinite. In such a case, the inter-pair interference and noise vanish, and the self-interference determined by the second-order statistics of channel vectors can be feasibly tackled. In this way, the desired signal can be obtained by subtracting the self-interference from the received signal, and the capacity increases with the BSs antenna number.

Secondary Network Connectivity of Ad Hoc Cognitive Radio Networks

This letter investigates the connectivity of large-scale ad hoc cognitive radio networks (CRNs). Considering correct detection and false alarm probabilities ( Pd and Pf, we derive in closed-form the thinned density of active secondary users (SUs) in a secondary network. We show the active-probability and detectable radius of primary users (PUs) have a significant impact on the density of active SUs. Moreover, we find that increasing Pf of SUs considerably decreases the node density of active SUs. The connectivity of the secondary network is further analysed with the continuum percolation theory. Given the critical density of the secondary network without the impact of PUs, the critical density of the secondary network coexisting with PUs is estimated analytically. Simulations confirm our analysis.

Linear Transceiver Designs for MIMO Indoor Visible Light Communications Under Lighting Constraints

In this paper, we study linear transceiver designs for indoor visible light communications (VLCs) with multiple light emitting diodes (LEDs). Specifically, we investigate VLCs including white emitting diodes and VLCs including red/green/blue (RGB) LEDs. The transmitter precoding and the offset are jointly designed by considering certain key practical lighting constraints, such as optical power, non-negativeness, and color illumination. Various non-convex transceiver design problems are formulated aiming to minimize total mean-square-error to improve transmission reliability. We show that for multi-input single-output white VLCs, the optimal precoding reduces to a simple LED selection strategy. For multi-input multi-output (MIMO) white VLCs, we prove that the optimization problem with multiple constraints can be equivalently simplified to a problem with single constraint, which enables us to propose efficient algorithms to search local optimal solutions. For MIMO RGB VLCs, by using certain useful transformations, we show that the precoding design is equivalent to covariance matrix design of transmit signals, which can be further transformed to a convex optimization problem. To develop an algorithm to find the optimal solution, we derive the optimal structure of the covariance matrix and show that the optimal solution can be obtained via a water-filling approach. Extensive simulation results are provided to verify the performance of the proposed designs.

Energy-aware complex network model with compensation

In this paper, we use complex network theory to analyze the evolving topology of wireless sensor networks (WSNs). Based on the BA model, we propose a novel complex network model, i.e., Neighborhood Log-on and Log-off model with energy awareness (NLL-E). NLL-E assumes locally preferential attachment, which exists in many complex networks. We further consider node addition and invalidation in topology evolvement, and add compensation mechanism for network robustness. The statistical properties and dynamics of NLL-E are analytically studied. Numerical simulations indicate that, comparing with BA, NLL-E has decreased average path length and increased network connectivity.

Spectram-sculpting-aided interference avoidance for OFDM-based cognitive networks

This paper studies how to efficiently reduce the mutual interference between primary and secondary networks. Different to conventional cognitive system, in which primary and secondary networks have no information exchange, the media access control mechanism we propose allow an operating PU to share the length of its remaining occupation time to SUs. With the help of such information exchange, the primary network can be better protected and the secondary network has the potential to reduce the energy consumed by spectrum sensing. To realize such a dynamic information sharing, we propose a novel spectrum-sculpting-aided scheme in this paper. Specifically, the PU deliberately inserts one zero-subcarrier into its data carriers, and uses the zero-subcarriers position to claim its status. Through two-step sensing, each SU can attain the PUs remaining operation time, and then adjust its sensing strategy to avoid interference to the PU. Analysis of interference probability, together with simulation results, clearly exhibit the advantages of the proposed scheme in terms of the mutual interference and the amount of SUs sensing times.