Published papers

Quantization is an important operation in digital communications systems. Quantizer not only introduces quantization noise but it also changes the statistical properties of the quantized signal. Furthermore, quantization noise cannot be always considered as an additive source of Gaussian noise. It depends on the input signal probability density function. In an orthogonal-frequency-division-multiplexing transmission the signal undergoes different operations which change its statistical properties. In this paper we theoretically analyze the statistical transformations of the signal from the transmitter to the receiver and determine how these effect the quantization. The discussed process enables the design of a quantizer with an optimal resolution for the required performance metrics. Simulations relate the effect of quantizer resolution and the channel statistics to the bit error rate at the receiver.

We propose a gateway selection scheme that considers multiple end-to-end QoS parameters such as route availability period, route capacity and route latency, to select a potential gateway node. The precise estimation and propagation of end-to-end QoS metrics is proposed in this paper. We simulate our gateway selection scheme with a simple hybrid gateway discovery algorithm and it shows that our gateway selection scheme improves throughput, packet delivery ratio with less energy consumption per node. Also it improves the end-to-end delay compared to the single QoS gateway selection schemes.

We propose a new method of the moving target detection with MUSIC(MUltiple SIgnal Classification), processed at only receiver. That offers much narrower width than that of cross-correlation. The proposed method can distinguish the moving targets from the stationary targts, though the conventional MUSIC can not. This is realized by the decorrelation of the time delay-derived phase rotation factors caused by moving targets. The performances using ISDB-T(Integrated Service Digital Broadcasting - Terrestrial), the bandwidth of which is 5.57MHz, are evaluated by the computer simulations. The results show that two close moving targets, separated by 0.1 micro sec, with SNR=-10dB each can be resolved even if the stationary target is located around the one moving target.

To achieve a traceback method that some nodes act as units by using clustering technique, we propose the schemes to manage information about traceback in each cluster and reconstruct a trace-path efficiently, and a traceback method between each cluster head. In our scheme, we can avoid degradation of success rate of the trace-path reconstruction caused by mobility of each node and because traceback is handled by only cluster heads we can also reduce the traffic and time for traceback. By a computer simulation, we evaluate the number of queries and success rate of traceback and show that the proposed scheme can reduce the both number of queries and achieve high success rate of traceback.

This paper proposes two energy-efficient standby mode algorithms in short-range one-to-one 60 GHz millimeter-wave (mmWave) communications. Among the many usage scenarios for mmWave radio, file downloading from kiosk terminals or peer-to-peer sync service with portable terminals are of great interest. The proposed two energy efficient standby mode algorithms for one-to-one communications get assured connection setup time and take each terminal’s different requirement for reduction of its power consumption into consideration. This experimental result is the same as the analysis result and it is verified that our proposed standby algorithm works as designed.

In this paper, optimal physical carrier sensing range is calculated based on the signal to interference and noise ratio and distance between transmitter and receiver nodes in the ad hoc network to avoid the packet collision in ongoing receiver node. After that, the adaptive usage of polarized antenna will be applied to further increase the aggregate throughput in ad hoc network. The simulation results show that the proposed methods can give the higher aggregate throughput for ad hoc network.

In this paper, we propose a lightweight grid-based relay slot selection scheme in EH-WSNs. In our scheme, we use square-shaped grid and divide the network virtually to give each node primitive coordination and each intermediate node decides its relaying slot by calculating the differential of grid-index between a sender and a receiver. Based on grid-based coordination, our scheme turns multiplications into lightweight subtractions and this reduces the computational complexity and required memory. In addition, we also propose a probabilistic relaying scheme to reduce collisions in each relaying slot. We evaluate the conventional and proposed schemes through the computer simulation and show that our scheme reduces the computational complexity and achieves comparable throughput to the conventional scheme

Recently, forest fire monitoring system in wireless sensor networks has received much attention. The conventional scheme receives fire alert data quickly to inform fire forest event to the sink. However, since two or more nodes may detect a fire, high priority fire detection data frequently collide. In this paper, 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 received 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, and have less effect of wind compared with the conventional scheme.

In this paper, we propose a method that reduces the complexity on the relocation of initial deployment and coverage hole healing of mobile sensor nodes in the hybrid Wireless Sensor Networks (WSNs). Our method finds the best-ordered ways of mobile nodes’movement to give the shortest distance movements for mobile nodes in WSN. Adaptive threshold distance is used to eliminate the consideration of some mobile nodes, which are already occupied or situated within the threshold distance from optimal new positions. The simulation results show that the proposed methods can give much smaller calculation time to find the shortest paths for the movements of mobile nodes in WSN.

The IPv6 Routing Protocol for Low-power and Lossy networks (RPL) is a standard routing protocol to realize the Internet of Things (IoT). We evaluate the false detection rates by computer simulation and we show that the proposed scheme reduces the false detection rate in comparison with the conventional scheme.

In this paper, we propose a deterministic BT detection by letting tags respond in the specified slots and comparing the expected and observed status of each slot. Response slot is determined by using hash function, a random number, frame length and tags ID. This way enables the reader to predict the status of each slot. Our scheme immediately detects BT if the expected status is different from the observed status. Our scheme can also detect even though BT jams low-rate. Simulation results show that our scheme performs better than the conventional BT detection scheme in terms of the number of required slots to detect BT.

In this paper, we propose a concurrent moving-based connection restoration scheme between actors to ensure the continuous connectivity. By sending a message to neighbor actor before moving, an actor can inform when it starts to move and where it moves to. Thus, neighbor actors which receive a message can calculate when it starts to move for concurrent move of actors. An actor starts to move when the communication with neighbor actors is unavailable due to neighbor actor’s move. Further, when the actor starts to move, it creates sub-paths to its moving neighbor actor for continuous connectivity. The network restoration time is shortened because of concurrently moving instead of moving one by one. We show that our scheme restores the network connectivity much robuster and faster than the conventional scheme by computer simulation.

マルチキャストトラヒックに対応可能なクロスバーATMスイッチを提案した。まず、従来のクロスバーATMスイッチのルーチングクロスバーにセルコピー機能をもつスイッチエレメントの状態を新たに付加し、またコピー要求数分のセルコピー及びセルルーチングが完了した不要なセルをカットし、コピー要求数分のセルコピーが完了せずにセルルーチングされたマルチキャストセルのヘッダ変換を行うためにヘッダ変換回路を設置する構成のクロスバーATMスイッチを提案モデル1として提案した。しかし、提案モデル1では、マルチキャストトラヒックを扱うことにより、同一の出力ポートを目指す出力競合が起こりやすくなる。そこで提案モデル1から集線網を取り除き、ルーチングクロスバーの全出線を共有バッファに接続する構成のクロスバーATMスイッチを提案モデル2として提案した。ユニキャストセルとマルチキャストセルが混在したトラヒックに対して、提案モデル1と提案モデル2のセル棄却率特性及び平均遅延特性の比較を計算機シミュレーションにより行なった結果、提案モデル2は提案モデル1と比較して低中負荷時においてセル棄却率特性を大幅に改善できることを示した。

符号化率可変パンクチャド畳込み符号と適応$L$値パルス位置変調(PPM)を用いた適応ビットレート室内赤外線無線通信方式を提案した.提案方式では, 伝送開始時に 端末からの通信路のSNRに応じて符号化率と変調指数を決定し, 所要ビット誤り率を満たしつつ, できるだけ高いビットレートでの通信を実現できる. 提案方式の特性をハイブリッド直射通信路において理論解析により評価した結果, 提案方式は通信路状態が良い場合には低い変調指数と高い符号化率を, また通信路状態が悪い場合には高い変調指数と低い符号化率を用いることで, 適応的にビットレートを変えて所要ビット誤り率を満たしつつ, 畳込み符号, 繰り返し符号を用いた方式に比べ, より高いビットレート, より長い水平距離での通信を実現できることを示した.

クロスレンジ誤差低減に有効な移動局を用いた複数レーダによる目標追随方式を提案した。ここで移動局とは航空機にレーダを搭載したもので、空中を飛行しながらレーダを用いて目標を追随する。提案するアルゴリズムに従い移動局の動きを制御することにより、従来の固定局のみを用いた複数レーダによる目標追随方式に比べて、移動する目標に対して各レーダからのビームを直交または直交に近い配置にすることができる。目標へのビームを直交または直交に近い配置にした各レーダからの観測値を統合することにより、クロスレンジ誤差が増大する影響を低減することができ、良好な追随精度を得ることができる。計算機シミュレーションを行い、提案方式は従来の固定局のみを用いた複数レーダによる目標追随方式と比べて、遠方目標に対して良好な追随精度が得られることを示した。

ユーザ識別に用いられる拡散符号に埋め込んだエンベット変調方式した。エンベット変調方式では、拡散符号の重み位置に伝送ビットを送信するので、従来の光CDMA方式よりも伝送レートを向上することができる。しかしながら、相関処理によるビット判定を用いることができないため、多元接続干渉の影響が大きくなるという問題がある。そこで、伝送ビットとして非対称誤り訂正符号による符号語を送信し、受信側で非対称誤り訂正符号の復号処理により，多元接続干渉の影響を緩和する。結果として、従来の光CDMA方式と比べて、ビットレートの低下や伝送帯域の増大をさせることなく、ビット誤り率特性を改善できることを明らかにした。

受信側で複数回チャネル推定を繰り返し、推定誤差を減少させるEM (expectation-maximization)アルゴリズムに注目し、その繰り返し数を減少させ、推定をより簡単にし、かつフレーム間のフェージング相関の大きい場合には特性を改善する新しいチャネル推定及び送信信号検出方法を提案した。提案方式では、初期フレームは基本的な従来のEMアルゴリズムを用いて、チャネル推定及び送信信号検出を行ない、それに続くフレームでは,自フレーム内のパイロット信号と前フレームにおける推定送信信号を重みづけをして用いて推定を行なう。計算機シミュレーションの結果、前フレームの推定送信信号に基づくチャネル推定と自フレームのパイロット信号に基づくチャネル推定を共に用いるほうが、どちらかの情報のみを用いるよりも特性が良いことを示した。さらに、提案推定法は繰り返しなしで,複数回推定を行なう従来のEMアルゴリズムによる推定と、初期フレーム以外においてほぼ同等な特性が得られることを示した。