Tao Luo

A Cooperative Double-Threshold Energy Detection Algorithm in Cognitive Radio Systems

As is well-known, cooperation among cognitive users can improve the accuracy of spectrum detection in cognitive radio systems. In this paper, based on OR-rule, a cooperative double-threshold energy detection algorithm is proposed. Theoretical analysis and simulation results indicate that this strategy can improve the spectrum detection probability significantly.

Channel estimation for the downlink of 3GPP-LTE systems

In this paper, we study the channel estimation algorithms for the downlink of 3GPP LTE systems. By using computer simulation, the pilot signal assisted channel estimation algorithms based on least square (LS) and linear minimum mean square error (LMMSE) criteria, together with channel interpolation based on piecewise-linear interpolation and DFT based interpolation are studied. By analyzing simulation results in terms of Mean Square Error (MSE) and un-coded Bit Error Rate (BER), we conclude that the channel frequency responses of pilot tones are estimated by using LS estimator, and the channel frequency responses of data tones are interpolated by DFT based interpolation method is appropriate for the downlink of 3GPP LTE systems.

Cooperative spectrum allocation with QoS support in cognitive cooperative vehicular ad hoc networks

To solve the contradiction between the increasing demand of diverse vehicular wireless applications and the shortage of spectrum resource, a novel cognitive cooperative vehicular ad-hoc network (CC-VANET) framework is proposed in this paper. Firstly, we develop an adaptive cognitive spectrum sensing (ACSS) mechanism which can help to trigger and adjust the spectrum sensing window according to network traffic load status and communication quality. And then, Generalized Nash Bargaining Solution (GNBS), which can achieve a good tradeoff between efficiency and weighted fairness, is proposed to formulate the asymmetric inter-cell resource allocation. Finally, GNBS-Safety (GNBS-S) scheme is developed to enhance the Quality of Service (QoS) of safety applications, especially in the heavy load status, where the bandwidth demanded and supplied cannot be matched well. Furthermore, the primary user activity (PUA) which can cause rate loss to secondary users, is also considered to alleviate its influence to fairness. Simulation results indicate that the proposed CC-VANET scheme can greatly improve the spectrum efficiency and reduce the transmission delay and packet loss rate on the heavy contention status. And GNBS spectrum allocation scheme outperforms both the Max-min and Max-rate schemes, and can enhance the communication reliability of safety service considerably in CC-VANET.

The Energy-Aware Controller Placement Problem in Software Defined Networks

Controller placement is an important problem in software defined networks. Previously, some works addressed the problem by taking into account primary factors, such as propagation delay of control paths and capacity of controllers. Here, the problem is addressed from a novel standpoint of energy consumption, which is modeled as a binary integer program (BIP). In the BIP model, the energy consumption of the network that serves for the control traffic is minimized under the constraints of the delay of control paths and the load of controllers. In consideration of the high complexity of the BIP in large networks, a genetic heuristic algorithm is designed to find an effective sub-optimal solution. Simulation results show that the energy consumption of the heuristic algorithm is close to that of the BIP solution. No more than 4%, additional links are used by the heuristic algorithm if all the links have the same energy consumption.

Myopic sensing for opportunistic spectrum access using channel correlation

We consider secondary user performing opportunistic spectrum access (OSA) over multiple correlated channels occupied by primary users. The channel occupancy states can be modeled as a discrete-time Markov process. Due to hardware and energy limitations, secondary user can choose only one channel to sense in each slot. Based on the sensing outcome, it decides its access action. Hence, channel selection scheme (i.e. sensing policy) at the MAC layer is of vital importance for secondary users throughput and sensing policy design can be formulated as a partially observable Markov decision process (POMDP). Unlike other research, in which case the channel occupancy states (induced by primary users) evolving independently among multiple channels, our work emphasizes on the case that multiple channels are not independent any more, then they evolve dependently. By adopting myopic sensing, which is an approach with low complexity to implement POMDP, we propose a myopic sensing policy exploiting the channel correlation in this paper. Numerical results illustrate that the proposed sensing policy improves the throughput of the secondary user either with or without the presence of spectrum sensing error.

Transmission capacity analysis for linear VANET under physical model

In this study, the transmission capacity of VANETs in a highway scenario is analysed on the basis of a 1D line model and the carrier sense multiple access with collision avoidance (CSMA/CA) protocol. We describe the CSMA/CA protocol used in IEEE802.11p from the perspective of the geometric relationship amongst simultaneous transmitters. The desired channel and interfering channels are assumed to experience the same amount of path-loss and Rayleigh fading. On the basis of the proposed model, we analyse the attempted transmission probability of each road segment and the maximum intensity of active transmitters, including their theoretical values. Then, we employ the physical model to obtain the outage probability and derive the upper bound of the transmission capacity of a VANET, which is defined as the average spatial density of successful transmissions in the network. Simulation results indicate that the theoretical value offers a good bound on network capacity.

Estimating the upper bound of transmission capacity in linear VANET

The IEEE 802.11p VANET has been studied by academia and industry for many years. In this paper the upper bound of transmission capacity in linear VANET is deduced and evaluated. The maximum of simultaneous transmitters within dedicated areas, which is widely accepted as the indicator of transmission capacity, is constrained by the CSMA/CA or EDCA mechanism in 802.11p. To estimate the upper bound, a linear road scenario is built and one determined uniform distribution of transmitting vehicles is proved to contain maximal number of simultaneous transmitters; then the upper bound of transmission capacity is proposed; finally simulation results show that the bound offers a good constraint for the transmission capacity in linear VANET.

Fair resource allocation for cooperative relay OFDMA networks

In this paper, in order to improve the efficiency of wireless resource utilization, power control and subcarrier allocation are optimized jointly in a three-node symmetric cooperation orthogonal frequency-division multiple access uplink system. Equivalent direct channel gain is introduced to simplify relay links. Transmission mode selection strategy guarantees an optimal operation mode on the subcarriers allocated to each user node. Nash bargaining solution is used for fairness. The problem is formulated to maximize the overall system rate, under each user nodes maximal power and minimal rate constraints, while considering the fairness between the two user nodes. The joint resource allocation is decomposed and solved by dynamic subcarrier allocation algorithm and adaptive power allocation algorithm. Simulation results indicate that the sum rate of the system, the rate of user node and the fairness have been improved comparing to the non-cooperative strategy.

Multi-channel myopic sensing for opportunistic spectrum access using channel correlation

We consider the issue of opportunistic spectrum access (OSA) in cognitive radio network. By modeling the channel occupancy states induced by the primary users as a discrete-time Markov process, the channel selection scheme of the secondary users at the MAC layer can be formulated as a partially observable Markov decision process (POMDP). Under the POMDP framework, myopic sensing (which is a kind of channel selection scheme) is adopted with the assumption that occupancy of each channel evolves independently. In this paper, differing from former research assuming independent channel occupancy, we investigate into the case when correlation exists across different channels. By exploiting the channel correlation information, we propose two multi-channel myopic sensing policies: one with full-dimension belief vector, and the other with reduced-dimension belief vector. The sensing policies are given out under different PHY layer conditions: with or without the presence of spectrum sensing error. Simulation results illustrate that the proposed sensing policies can improve the throughput of the secondary users as time increases.

Randomized space-time block coding with limited feedback

In this paper, a decode-and-forward (DF) cooperative diversity scheme is considered for dynamic relay networks, where each node is equipped with a single antenna. A randomized space-time block coding (STBC) cooperative relaying scheme has been proposed to get spatial diversity gains without centralized control information. However, it can not get maximum diversity gain when the number of active relay nodes is small. In this paper, we propose a limited feedback scheme to improve the bit error rate (BER) performance of the randomized STBC scheme. Simulation results reveal that the proposed feedback scheme can achieve almost the same BER performance as the centralized STBC scheme in some channel conditions. In addition, it does not degrade the performance of the randomized STBC scheme even with a large feedback delay.