高速無線網路下短距離通訊協定之設計與分析

Abstract

在過去十年裡，如何設計無線短距離通訊協定以達到高資料傳輸速率一直備受關注。在眾多不同的標準裡，由於設計和佈建上的成功，IEEE 802.11 和毫微微蜂巢式基地台(femtocell) 被視為是兩個最適合採用的技術。跟傳統蜂巢式基地台相比，IEEE 802.11 和femtocell 系統因為連線品質較好的關係，可以提供更高的傳輸速度給使用者。然而在這些技術中仍然存在一些問題導致系統效能無法進一步提升。

分散式協調功能(distributed coordination function, DCF) 是IEEE 802.11 媒體存取控制(media access control, MAC) 層中的基本機制。在無線區域網路(wireless local area networks, WLANs) 裡，它被用來管理無線站台(wireless stations, WSs) 之間隨機且複雜的通道存取。更進一步，為了支援服務品質(quality-of-service, QoS)，IEEE 802.11e 提出了增強分散式通道存取 (enhanced distributed channel access, EDCA) 機制以提供不同的通道存取優先權給不同的資料傳輸型態。在資料封包傳輸前，隨機後退(random backoff)機制和request-to-send (RTS)/clear-to-send (CTS) 交握需要分別被執行來減少封包碰撞機會和可能的終端機隱匿問題。然而眾多的開支(overhead) 會導致系統的效能明顯降低。因此，本論文首先提出一個動態保留-解決碰撞(adaptive reservation-assisted collision resolution, ARCR) 的協定。藉著在競爭式(contention-based) 通道存取網路中採用動態的保留區間(adaptive reservation period)，ARCR 機制可以明顯改善封包碰撞和減少後退延遲(backoff delay)。更進一步，本論文再提出一個增強式的動態保留-解決碰撞(enhanced-ARCR, E-ARCR) 協定，提供多種不一樣優先權的流量來支援服務品質。

接著，IEEE 802.11n 從媒體存取控制層的觀點提出了封包聚合(frame aggregation) 技術來改善系統效能。為了達到可靠的資料傳輸和較高的系統吞吐量(system throughput)，兩個自動重傳請求(automatic repeat reQuest, ARQ) 機制在本論文被提出。聚合選擇性重送-自動重傳請求(aggregated selective repeat ARQ, ASR-ARQ) 機制考慮了封包選擇性重送的自動重傳請求。另一方面，對於較差的通道狀況，藉由採用里德所羅門碼(Reed-Solomon code, RS code) 充當前向錯誤更正(forward error correction, FEC) 機制，聚合混合式自動重傳請求(aggregated hybrid ARQ, AH-ARQ)可以改善系統效能。

更進一步，對於femtocell的應用，如何選取適當的存取策略一直是個重要的議題。femtocell 考慮了兩個主要的策略：開放式存取模式(open access mode)與封閉式存取模式(closed access mode)。當femtocell 處於開放式存取模式時，所有的使用者都可以使用femtocell。另一方面，當封閉式存取被採用時，femtocell只允許註冊者(subscribers) 連線，其他的非註冊者(nonsubscribers) 只能和傳統基地台連線。明顯地，封閉式存取可以提供較多的資源給註冊者，但不利於整體網路。反之，開放式存取能可以提升整體網路的效能，但不利於註冊者。為了使存取策略有更高的彈性，本論文提出了混合式存取模式(hybrid access mode)，讓非註冊者可以有限度地存取femtocell。為了描述使用者的自私行為，我們建立了兩個基地台選擇的賽局(cell selection games)，並在一些合理的假設下證明這些賽局有純策略納許均衡(pure strategy Nash Equilibrium) 存在。藉由觀察在納許均衡下的結果，我們建議採用混合式存取模式，以達到在註冊者、整體網路和營運商(operator)之間較好的效能權衡。

此外，本論文也建立效能分析模型，以此來驗證所提出演算法的效率。由分析跟模擬的結果可發現藉著採用本論文所提出演算法，IEEE 802.11 和femtocell 的網路效能可以進一步被提升。

Wireless short-range communication protocols for high data rate have attracted a lot of attentions over the past decade. Among different standards, IEEE 802.11 and femtocell techniques are considered the most two well-adopted suites due to their remarkable success in both design and deployment. Compared to conventional cellular network, higher data rate can be achieved for users in the IEEE 802.11 and femtocell systems due to better link qualities. However, some crucial issues still exist in these techniques such that system performance cannot be further enhanced.

Served as basic access mechanism in the IEEE 802.11 media access control (MAC) layer, distributed coordination function (DCF) is employed to regulate the random and complex medium accessing behaviors among the wireless stations (WSs) within the same wireless local area networks (WLANs). Moreover, in order to support quality-of-service (QoS) requirements, the enhanced distributed channel access (EDCA) mechanism is proposed in the IEEE 802.11e standard to provide prioritized channel accesses for different types of traffics. Before the data transmission, random backoff and request-to-send (RTS)/clear-to-send (CTS) handshaking are respectively executed in order to resolve packet collision and potential hidden terminal problems. Nevertheless, the system performance will be significantly decreased owing to heavy overhead in the DCF/EDCA. Therefore, an adaptive reservation-assisted collision resolution (ARCR) protocol modified from conventional DCF is proposed in this dissertation. By adoption of adaptive reservation periods within the contention-based channel access networks, proposed ARCR algorithm can significantly improve packet collision and reduce the backoff delays compared to other existing works. Furthermore, in order to support QoS requirements for multiple prioritized traffics, the enhanced-ARCR (E-ARCR) protocol is further proposed as the enhanced version of the ARCR scheme.

Next, frame aggregation techniques are proposed in the IEEE 802.11n standard to improve the system performance of WLANs from the MAC perspective. In order to achieve both reliable data transmission and high system throughput, two MAC-defined automatic repeat reQuest (ARQ) mechanisms are proposed to consider the effect from frame aggregation. An aggregated selective repeat ARQ (ASR-ARQ) scheme is proposed which incorporates the selective repeat ARQ scheme with the consideration of frame aggregation. On the other hand, for worse channel quality, the aggregated hybrid ARQ (AH-ARQ) mechanism is proposed to further improve the throughput performance by adopting the Reed-Solomon block code as forward error correction scheme.

Moreover, how to choose a feasible access strategy for femtocell is considered an important issue. Two major access policies are considered in femtocell network, including the open access mode and closed access mode. All users are allowed to connect to the fBS by adopting the open access mode; while the closed access mode only permits authorized subscribers to utilize the femtocell BSs (fBS). Closed access will intuitively be advantageous to the femtocell subscribers; on the other hand, system performance of the entire system network can be enhanced if open access mode is adopted. In order to relax the inflexible access strategies, hybrid access policy is considered in this dissertation which allows nonsubscribers to possess limited connections to the fBS. Two cell selection games for distinct scenarios are theoretically modeled to formulate the behaviors of nonsubscribers, and the existences of pure strategy Nash equilibria (NEs) are also proven under feasible utility functions. By the observation at NEs, it is suggested to adopt hybrid access mode to achieve better tradeoff for performance enhancement from the perspectives of subscribers, heterogeneous network system, and operator.

Besides, in order to validate the effectiveness of proposed algorithms, analytical models are also constructed in this dissertation. Both analytical and simulation results show that performance of IEEE 802.11 and femtocell techniques can be further enhanced by adopting proposed algorithms in this dissertation.