Published papers

パルス位置変調(PPM)を用いたスペクトラル位相符号化-光空間CDMAシステムを提案し，ビット誤り率(BER)特性を理論的に導出した．提案システムは，スペクトラル位相符号化方式を用いることにより，時間符号化方式を用いた従来の光空間PPM-CDMAシステムと比較し，符号拡散利得および雑音に対する耐性を向上できる．提案システムのBERを，ビットレートおよびチップレート一定下で，従来システムのBERと比較した．その結果，提案システムは，多元接続干渉とシンチレーションの緩和技術を用いた従来システムと比較し，それら緩和技術を用いずにBER特性を改善できることを示した．

検証に伴う演算量を制御できるようにチャレンジを分割することによって攻撃の影響を低減する方式を提案する．さらに，検証に伴う演算は対応するチャレンジの要素が1 である場合に1024 ビット長の乗算剰余演算が発生することが主な原因であるため，チャレンジの生成時にチャレンジの要素が1 としてよい数に上限値を設けることにより演算量を低減させる．チャレンジを複数に分割した際の計算時間およびメモリ量について評価を行い，本方式の有効性を示す.

We propose a new forest fire monitoring system in order to reduce dropped rate of high priority fire detection data, by specifying a high priority on data only immediately after fire detection and just before the destruction by fire. Furthermore, the node only transmits high priority data to a node which has a low possibility of destruction by fire for low end-to-end delay of high priority fire detection data. The simulation results show that our proposed scheme can reduce dropped rate of high priority data and the end-to-end delay compared with the conventional scheme.

Frequent topological changes due to high speed is one of the main issues in Vehicular Ad-hoc NETworks (VANETs). In this paper, we model transmission probabilities of 802.11p for VANETs and effect of these probabilities on average transmission time. To evaluate the performance of routing protocols in VANETs, we select three routing protocols; Dynamic Source Routing (DSR), Fish-eye State Routing (FSR) and Optimized Link State Routing (OLSR). Framework of these selected routing protocols with respect to their packet cost is also a part of this work. Moreover, the chosen protocols are enhanced to obtain efficient behavior by modifying their default parameters. Simulation work is done in NS-2 for evaluation and comparison of original protocols with modified ones. For this comparison, we choose three performance metrics; throughput, End-to-End Delay (E2ED) and Normalized Routing Load (NRL) in varying mobilities and scalabilities. Finally, we deduce that modified DSR (DSR-mod) outperforms other protocols by achieving 16% more packet delivery for all scalabilities and 28% more throughput.

In general, automobiles travel from the origin to the destination using a shortest route. However, the shortest route may not be a highest wireless connection-capacity route, because of availability of wireless services (base station and access points etc.) along the route. To the best of our knowledge, currently no algorithm exists for selecting a route that maximizes wireless connection-capacity, while keeping route length shortest and close to shortest. In this paper, we propose two modified version of Dijkstra route selection algorithms: one for selecting a maximum connection capacity shortest route, and the other is for discovering higher wireless connection-capacity routes; the length of the route could be larger than a shortest route, but no larger than predetermined bound. The second proposed algorithm exploits the state change of the intersection to broaden the search range of possible routes. Results from our extensive simulation for a Manhattan-street type grid network with the heterogeneous IEEE 802.11a wireless access, show that for a 50% increase in route length and 15 Access Points (APs), the proposed algorithm can increases wireless connection-capacity by 35.67% and 31.27% compared to the shortest and random route selection algorithms, respectively.

Wireless system capacity can be increased enormously by using multiple antennas at transmitter and receiver sides in the system. Among them, maximum likelihood (ML) receiver is an optimal receiver, but it is difficult to implement due to high complexity arising from exhaustive searches over all candidate vector symbols. On the other hand, zero forcing ordered successive interference cancellation (ZF-OSIC) receiver can reduce the complexity greatly although its performance is less than ML receiver. Performance of ZF-OSIC receiver is seriously affected by the lower SNR layers. Therefore, in our proposed detection scheme, we will choose the optimal number of transmit antennas in the high SNR transmit layers, to be detected by the ZF-OSIC scheme and the remaining transmit antennas in the low SNR trasnmit layers will be detected by ML scheme to give the better BER performance for the spatial multiplexing MIMO system. Simulated results show that the complexity of the proposed system is less than that of conventional ZF-OSIC receiver under the same or better BER performance condition. The processing time of proposed method is significantly reduced when compared with conventional ML detection scheme as well as ZF-OSIC detection scheme.