Rapid changes and improvements of recent mobile and network technologies have been realizing ubiquitous network services. Even though the present wireless access technologies already provide wireless data communication services, there still exists the request for the high-speed wireless data transmission. To satisfy this request, researches on broadband wireless access technologies are ongoing. Broadband wireless access systems include various technologies such as random access process, radio resource management, interference cancelation, power control, and so on. Among them, this dissertation focuses on the random access process because it is one of the fundamental technologies for the initialization of the broadband wireless access systems.

Random access process in the broadband wireless access systems refers to the contention-based random access, and provides functions of the initial network entry, the bandwidth request, and the network entry during handover. The procedure of random access process in the broadband wireless access systems is conducted as follows. First, the base station (BS) periodically broadcasts the necessary information for random access process such as the location of random access channel. Second, multiple mobile stations (MSs) simultaneously transmit randomly chosen random access code to the BS. Third, the BS carries out the multiuser resolution process to identify transmitted random access codes. If BS successfully identifies random access codes, it allocates some resources to each MS and notifies this information to MSs. Fourth, each MS transmits the detailed information to the BS using the allocated channel. Random access process in the broadband wireless access systems have following two key features. First, it is performed by the code-based multiple random access. Polling or CSMA-CA based random access scheme cannot be adopted because the number of MSs in typical broadband wireless access systems such as WiMAX and LTE systems is significantly large. Second, random access signals experience the multipath fading channel environment due to the relatively large cell size. This dissertation analyzes the performance of random access process in the broadband wireless access systems considering above stated two features.

In the mean time, the interference problem in the relay-deployed network, which is caused by the unbalanced transmit power between the BS and the relay station (RS), should be taken into account as well. Hence, this dissertation also scrutinizes the interference problem of random access process in the relay-deployed network. and provides a novel transmit power control algorithm. Followings are detailed descriptions of this dissertation.

This dissertation provides the mathematical modeling of random access process in the broadband wireless access systems and derives the probability density function of received random access signals. Using the characteristics of random access signal and the central limit theorem, received random access signals are modeled into Rayleigh random variable. By this modeling, the coarse performance analysis of random access process is feasible, which is considerable beneficial for the initial system design.

This dissertation also provides the finite performance analysis of random access process in the broadband wireless access systems. For the thorough performance analysis, four performance evaluation metrics are defined: 1) the probability of detection success, which refers to the probability for a MS to succeed random access process, 2) the probability of detection miss, which refers to the probability that the transmitted random access code is not detected, 3) the probability of detection false alarm, which refers to the probability that not transmitted random access code is detected as the transmitted code, 4) the average necessary random access success time, which refers to the average necessary time for a MS to succeed random access process including retransmission, random backoff, and penalty time. Based on above defined evaluation metrics, the performance analysis of random access process in the broadband wireless access systems is conducted with various system parameters including the number of MSs and SNR under various channel environments. Moreover, the comparison between the localized FDMA and the interleaved FDMA, which are two different mapping methods of single-carrier FDMA systems, is provided. The localized FDMA is determined to show better performance for the random access process.

This dissertation reveals the interference problem of random access signals in the relay-deployed network. Since the existing random access transmit power control algorithm is based on the compensation of the distance-based path loss between the BS and the MS, it cannot be directly applied to the relay-deployed network. When the existing transmit power control algorithm is adopted, the transmit power of random access signal between the RS and the MS is set to relatively lower than the transmit power of random access signal between the BS and the MS. It is derived from the fact that the distance between the RS and the MS is relatively shorter than that of the BS and the MS. Therefore, the random access signal between the BS and the MS may cause the interference to the random access signal between the RS and the MS. As the number of RSs is increased, this interference problem becomes more severe. This dissertation provides the probability and the amount of the interference problem in terms of the number of RSs, the number of MSs, and the location of RSs, which validates that the interference problem degrades the performance of random access process significantly.

To overcome above stated interference problem, this dissertation proposes a novel random access transmit power control algorithm for the relay-deployed broadband wireless access systems. The proposed algorithm is based on the random access transmit power boosting. The transmit power of the random access signal within the RS coverage is properly enhanced considering the location and the coverage of the RS. Moreover, the transmit power control is conducted through RS's broadcast message setting for the backward compatibility, which means that the modification of the MS is not necessary The performance evaluation of the random access process in the relay-deployed network is also conducted based on the previously defined four metrics. Results of the performance analysis show that the above stated interference problem is efficiently overcome by the proposed random access transmit power control algorithm.

Achievement of this dissertation will provide benefit as a guideline for the design of broadband wireless access systems.

本論文「Random Access Process in Broadband Wireless Network(広帯域無線通信システムにおけるランダムアクセス方式に関する研究)」は,広帯域無線通信システムにおけるランダムアクセス方式の性能分析,中継器システムにおける中継器と基地局間の干渉問題を解決する送信電力制御機構について論じている.

第1章は,「Introduction」であり,インターネットの普及と通信トラフィックの増加による広帯域無線通信への期待,広帯域無線通信システムの実現において核心技術となるランダムアクセス技術の重要性,中継器システムの導入の必要性について触れ,本論文の背景と目的について述べている.

第2章「Background」では,広帯域無線通信システムの伝送技術として有望なOFDMA及びSC-FDMA技術に基づき,広帯域無線システムの特性とランダムアクセス方式の仕組みについて述べている.ランダムアクセスの性能分析においては,ランダムアクセスコードの特性とランダムアクセスコードの検出機構が最も大きな課題となることから,ランダムアクセスコードの相互相関性の特徴とその相互相関性を用いるピーク値検出機構を示している.特に,今後広帯域無線通信システムに採択される予定のPNコードとZadoff-Chuコードの構成と特徴を述べている.また,ランダムアクセス信号の受信電力を同程度にするための送信電力制御機構について述べるとともに,同時に受信される複数のランダムアクセスコードを検出する機構について述べている.

第3章「Random Access Process in Cellular Network」では,広帯域無線通信システムにおけるランダムアクセスの性能分析に必要な評価基準を定義し,ランダムアクセスの性能評価を行っている.当該評価基準には,信号検出の成功率,信号の誤り検出確率,信号の未検出確率,ランダムアクセスの成功までかかる平均時間などが含まれ,広帯域無線通信システムの設計において考慮すべきシステムパラメータを明らかにしている.また,信号の未検出確率をランダムアクセス信号の確率変数として捉えることで数学的に分析している.ここで,ランダムアクセスの成功までにかかる平均時間の導出にあたっては,PHY層とMAC層の双方を同時に考慮している.さらに, SC-FDMA伝送技術のサブキャリアマッピング方式であるLFDMA方式とIFDMA方式とを数学的に比較し,ランダムアクセス方式においては,LFDMA方式がより効率的であることを示している.すべての評価はマルチパスフェージング環境下で行い,理想的なチャネル環境を仮定した従来の研究と比較してより現実的な結果を提供している.

第4章「Random Access Process in Relay Network」では,中継器システムが導入された広帯域無線通信システムにおけるランダムアクセスの性能分析と送信電力制御機構について述べている.まず,多数の中継器の導入による中継器と基地局間で生じる干渉問題を示し,新たな送信電力制御機構の必要性を明らかにしている.次いで,中継器や基地局からのブロードキャスト信号に基づき送信電力を制御する新たな送信電力制御機構を提案している.基地局からの信号により生じる干渉を低減させるのに必要な受信電力値を中継器が計算し,ブロードキャスト信号を介して端末に通知する機構である.基地局に変更を加えることなく,送信電力制御を行うことができるとともに,端末は基地局と中継器とに関わらずより強いブロードキャスト信号にあわせて送信電力を制御することが可能である.

第5章「Summary and Conclusion」は論文全体を総括しており,本論文の成果をまとめるとともに,広帯域無線通信システムにおけるランダムアクセス機構の実現に向けて残された課題と,今後の関連研究分野における研究開発の方向性について述べている.

以上,本論文は,広帯域無線通信システムにおけるランダムアクセス方式の性能を分析するとともに,中継器システムでの伝送電力制御機構を導入することで中継器と基地局間で生じる干渉問題を解決したものであり,情報学の基盤に貢献するところが少なくない.

したがって,博士(科学)の学位を授与できると認める.