Volume 65, Issue 3-4

In this paper, the design and numerical analysis of the interaction cavity for 1MW, 110 GHz gyrotron is presented. The in-house developed code GCOMS and the commercially available 3D particle-in-cell simulation code MAGIC have been used for the mode selection and optimization of the interaction cavity parameters respectively. TE 22,6 mode has been chosen as the operating mode. The cold cavity and the beam-wave interaction analysis have been carried out for the TE 22,6 mode and the output power and the efficiency have been computed. The sensitivity analysis of the output power and the efficiency has also been performed with respect to the cavity geometry and the electron beam parameters. More than 1MW output power has been achieved at 110 GHz frequency with the selected operating mode.

Two new configurations for the realization of two-tap transversal filters in 50 Ω delay line technique with cosine and sine frequency responses are proposed. Examples are given for the conversion of nonreturn-to-zero signals into partial response signals with duobinary, bipolar, and modified duobinary shape which can be applied in electrical and optical transmission systems. Calculated and measured results for Gbit/s signal processing are compared.

This paper presents an efficient hardware realization of multiple-input multiple-output (MIMO) wireless communication decoder that utilizes the available resources by adopting the technique of parallelism. The hardware is designed and implemented on Xilinx Virtex™-4 XC4VLX60 field programmable gate arrays (FPGA) device in a modular approach which simplifies and eases hardware update, and facilitates testing of the various modules independently. The decoder involves a proficient channel estimation module that employs matrix factorization on least squares (LS) estimation to reduce a full rank matrix into a simpler form in order to eliminate matrix inversion. This results in performance improvement and complexity reduction of the MIMO system. Performance evaluation of the proposed method is validated through MATLAB simulations which indicate 2 dB improvement in terms of SNR compared to LS estimation. Moreover complexity comparison is performed in terms of mathematical operations, which shows that the proposed approach appreciably outperforms LS estimation at a lower complexity and represents a good solution for channel estimation technique.

Fast frequency hopping OFDM (FFH-OFDM) exploits frequency diversity in one OFDM symbol to enhance conventional OFDM performance without using channel coding. Zero-forcing (ZF) and minimum mean square error (MMSE) equalization were first used to detect FFH-OFDM signal with a relatively poor bit error rate (BER) performance compared to QR-based detection algorithm. This paper proposes a parallel detection algorithm (PDA) to further improve the BER performance with parallel interference cancelation (PIC) based on MMSE criterion. Our proposed PDA not only improves the BER performance at high signal to noise ratio (SNR) regime but also possesses lower decoding delay property with respect to QR-based detection algorithm while maintaining comparable computation complexity. Simulation results indicate that at BER = 10 –3 the PDA achieves 5 dB SNR gain over QR-based detection algorithm and more as SNR increases.

The maximum Doppler spread, or equivalently, the mobile speed, is a measure of the spectral dispersion of mobile fading channel. Accurate estimation of the maximum Doppler spread is of importance in LOFDM systems which require the knowledge of the rate of channel variations to achieve its adaptive strategy. In this paper, aiming at the special characteristics of LOFDM signals, a cyclostationarity-based and superimposed pilot-aided maximum Doppler spread estimation algorithm for LOFDM systems over the doubly-dispersion channels is proposed, which avoids the waste of spectral efficiency in the traditional data-aided estimation algorithms and has a better performance and convergence speed compared with the blind estimation methods. Theory analyses and simulation results demonstrate that the proposed algorithm can obtain the effective estimation for a wide range of Doppler spreads under the condition that the power delay profile (PDP) of the channel is unknown and has a good mean square error (MSE) performance, while both the capability of anti-noise and the speed of convergence are nice.

Based on the consensus algorithm, an attack-proof cooperative spectrum sensing (CSS) scheme is presented for decentralized cognitive radio networks (CRNs), where a common fusion center is not available and some malicious users may launch attacks with spectrum sensing data falsification (SSDF). Local energy detection is firstly performed by each secondary user (SU), and then, utilizing the consensus notions, each SU can make its own decision individually only by local information exchange with its neighbors rather than any centralized fusion used in most existing schemes. With the help of some anti-attack tricks, each authentic SU can generally identify and exclude those malicious reports during the interactions within the neighborhood. Compared with the existing solutions, the proposed scheme is proved to have much better robustness against three categories of SSDF attack, without requiring any a priori knowledge of the whole network.

As a revolution in radio technologies, cognitive radio (CR) brings remarkable flexibility for spectrum access and network coordination. In cognitive radio Ad Hoc networks (CRAHNs), the distributed multi-hop architecture, mobility of CR users and the dynamic spectrum availability make it essential to construct a relatively stable network backbone for multi-hop communications. In this paper, we propose a cluster-based dynamic topology management scheme. The peculiarity of the scheme is that it takes the topology management as a dynamic optimization problem in the variable environment of CRAHNs. The scheme is composed of a Clustering algorithm based on Local Common Channels (CLCC) and the cluster maintenance phase. CLCC optimizes the cluster size while it provides robust potential for clusters. The constructed clusters are updated making use of that potential following the variation of the network. The scheme of the inter-connectivity of clusters guarantees the real-time connectivity of clusters which benefits the end-to-end performance a lot. Extensive simulation proves that the proposed solution can provide a more efficient and stable structure than existing solutions.