问题导向学习法在临床医学科研型研究生教育中的应用

问题导向学习法(problem-based learning,PBL)已成为目前西方国家医学教育的主流模式之一,其在培养学生分析问题、解决问题以及自我学习能力等方面有极大促进作用。本文结合自身实践,就PBL在临床医学科研型研究生教育过程中的应用及相关问题进行论述。

Reputation-based linear cooperation for spectrum sensing in cognitive radio networks

We propose a reputation-based cooperative spectrum sensing scheme in cognitive radio (CR) networks to solve the uncertainty resulting from the multipath fading and shadowing effect. In the proposed scheme, each cooperative CR user has a reputation degree that is initialized and adjusted by the central controller, and used to weight the sensing result from the corresponding CR user in the linear fusion process at the central controller. A simple method for adjusting the reputation degree of CR users is also presented. We analyzed and evaluated the detection performance of the reputation-based cooperative spectrum sensing scheme. Simulation results showed that our proposed scheme alleviates the problem of corrupted detection resulting from destructive channel conditions between the primary transmitter and the CR user. The performance of our proposed scheme was improved compared to the average-based linear cooperation scheme, and was similar to that of the optimal linear cooperation scheme with feasible computational complexity. Moreover, our proposed scheme does not require knowledge of channel statistics.

A novel 3780-point FFT processor scheme for the time domain synchronous OFDM system

The 3780-point FFT is a main component of the time domain synchronous OFDM (TDS-OFDM) system and the key technology in the Chinese Digital Multimedia/TV Broadcasting-Terrestrial (DMB-T) national standard.Since 3780 is not a power of 2,the classical radix-2 or radix-4 FFT algorithm cannot be applied directly.Hence,the Winograd Fourier transform algorithm (WFTA) and the Good-Thomas prime factor algorithm (PFA) are used to implement the 3780-point FFT processor.However,the structure based on WFTA and PFA has a large computational complexity and requires many DSPs in hardware implementation.In this paper,a novel 3780-point FFT processor scheme is proposed,in which a 60 63 iterative WFTA architecture with different mapping methods is imported to replace the PFA architecture,and an optimized CoOrdinate Rotation DIgital Computer (CORDIC) module is used for the twiddle factor multiplications.Compared to the traditional scheme,our proposed 3780-point FFT processor scheme reduces the number of multiplications by 45% at the cost of 1% increase in the number of additions.All DSPs are replaced by the optimized CORDIC module and ROM.Simulation results show that the proposed 3780-point FFT processing scheme satisfies the requirement of the DMB-T standard,and is an efficient architecture for the TDS-OFDM system.

On detecting primary user emulation attack using channel impulse response in the cognitive radio network

Cognitive radio is an effective technology to alleviate the spectrum resource scarcity problem by opportunistically allocating the spare spectrum to unauthorized users. However, a serious denial-of-service(DoS) attack,named the ‘primary user emulation attack(PUEA)', exists in the network to deteriorate the system performance. In this paper, we propose a PUEA detection method that exploits the radio channel information to detect the PUEA in the cognitive radio network. In the proposed method, the uniqueness of the channel impulse response(CIR) between the secondary user(SU) and the signal source is used to determine whether the received signal is transmitted by the primary user(PU) or the primary user emulator(PUE). The closed-form expressions for the false-alarm probability and the detection probability of the proposed PUEA detection method are derived. In addition, a modified subspace-based blind channel estimation method is presented to estimate the CIR, in order for the proposed PUEA detection method to work in the scenario where the SU has no prior knowledge about the structure and content of the PU signal. Numerical results show that the proposed PUEA detection method performs well although the difference in channel characteristics between the PU and PUE is small.

Funneling media access control (MAC) protocol for underwater acoustic sensor networks

Due to the characteristics of the underwater acoustic channel, such as long propagation delay and low available bandwidth, the media access control (MAC) protocol designed for underwater acoustic sensor networks (UWASNs) is quite different from that for terrestrial wireless sensor networks. In this paper, we propose a MAC protocol for the UWASNs, named the funneling MAC (FMAC-U), which is a contention-based MAC protocol with a three-way handshake. The FMAC-U protocol uses an improved three-way handshake mechanism and code division multiple access (CDMA) based technology for request-to-send (RTS) signals transmitting to the sink in order that the sink can receive packets from multiple neighbors in a fixed order during each round of handshakes. The mechanism reduces the packet collisions and alleviates the funneling effect, especially alleviating the choke point of the UWASNs. Simulation results show that the proposed FMAC-U protocol achieves higher throughput, smaller packet drop ratio, lower end-to-end delay, and lower overhead of the control packet compared to the existing MAC protocols for UWASNs.

Retransmission in the network-coding-based packet network

In this paper, retransmission strategies of the network-coding-based packet network are investigated. We propose two retransmission strategies, the packet-loss-edge-based retransmission strategy (PLERT) and the minimum retransmission strategy (MRT), which focus on optimizing the retransmission efficiency without the constraint on the encoding field size. We compared the performances of the proposed retransmission strategies with the traditional automatic repeat-request (ARQ) strategy and the random retransmission strategy. Simulation results showed that the PLERT strategy works well when the packet loss rate is small. Among these retransmission strategies, the performance of the MRT strategy is the best at the cost of the high complexity that is still polynomial. Furthermore, neither of the proposed strategies is sensitive to the encoding field size.