- 學位論文
Distributed Channel-Aware Transmission Strategies for Cooperative and Cognitive Wireless Networks
應用於合作式與感知無線電網路之分散式通道感知傳輸技術
摘要
在這篇論文中我們針對分散式無線網路提出具通道或頻譜感知能力的傳輸機制，利用所提出的傳輸機制，使用者將根據自己的通道狀態或頻帶使用狀態來決定傳輸策略，將系統總流量最大化，藉由多重使用者多樣性(multiuser diversity)、合作式中繼(cooperative relaying)、與隨機地存取閒置頻帶來避免通道衰減(fading)與干擾(interference)的傷害。這些傳輸策略，傳統上是由網路中的中央控制者來決斷，但在本文中，我們將讓每位使用者分散式地根據自己所能獲取的資訊、利用通道與頻帶感知能力、以及合作式的傳輸方式來傳送資料。具體而言，我們首先考慮一個時槽式ALOHA網路，每位使用者自行丟擲銅板來決定是否要傳輸。為了使系統的總流量最大化，我們讓每位使用者根據自己到基地台的通道狀態來決定最佳的傳輸機率、速率與能量。若通道資訊有誤差，我們提出的傳輸機制將考量誤差的機率分布而設計，比起沒有考量通道資訊或是沒有考量通道資訊誤差時，我們的方法能有效地提升系統流量。在一個使用者能互相合作、轉傳資料的時槽式ALOHA網路中，我們也提出具通道感知能力的傳輸與合作伙伴選擇策略。相較於使用者間不合作的網路，合作式網路中的傳輸策略會同時考量使用者到基地台的通道狀態與使用者之間的通道狀態。這些傳輸策略也將被應用於感知無線電網路(cognitive radio network)中，此時傳輸策略的決斷將同時考慮通道資訊與頻帶的使用狀態，利用類似的概念來設計具通道與頻譜感知能力的通道存取技術，將感知無線電系統的總流量最大化。然而，感知無線電系統中的使用者將不再直接傳送自己的封包，而是在每個時槽的一開始先發送預約封包來競爭通道，利用通道資訊分裂演算法(splitting algorithm)來解析預約封包，當預約封包的碰撞被解析成功時，就可以讓通道狀態最好的使用者來傳送資料。最後，藉由大量的電腦模擬來展示這些傳輸策略與演算法的成效。
並列摘要
Channel and/or sensing aware transmission policies for distributed wireless networks are proposed in this dissertation. In the proposed policies, users are allowed to make transmission decisions based on local channel state information (CSI) and/or spectrum occupancy information (SOI) to maximize the sum throughput of the system. These policies are able to overcome limitations due to fading and interference by allowing users to exploit multiuser diversity, request cooperative relaying from each other, and opportunistically access vacant spectrum. These techniques have been considered in the past but mostly from a centralized perspective, where a central controller is used to coordinate transmissions. This dissertation instead exploits the advantages of channel-aware, spectrum-aware, and cooperative transmissions in a decentralized fashion, allowing users to make independent transmission decisions based only on local information. Specifically, we first consider a slotted ALOHA network where each user determines whether to transmit or not according to a local coin toss. With the goal of maximizing the sum throughput of the network, we derive the optimal channel access policy which determines the transmission probability, rate, and power, by exploiting the uplink CSI. By assuming that the CSI is imperfect, the proposed policy is derived by taking into consideration the statistics of the channel estimation error. The throughput can be increased substantially compared to the case with no CSI or the case where error statistics are not taken into consideration. By allowing users to help by relaying each other's packets, channel-aware transmission control and partner selection policies are also derived for slotted ALOHA networks with cooperative users. In contrast to the policies derived for non-cooperative systems, the transmission control and partner selection are determined based on both the uplink and the inter-user CSI. These concepts can also be applied to cognitive radio environments where transmission decisions should not only be made based on the channel quality but also on the spectrum occupancy. Following similar concepts, a channel and sensing aware channel access policy is proposed to maximize the throughput of secondary users in cognitive radio systems. However, instead of allowing users to transmit their packets directly, a reservation period is employed at the beginning of each time slot for users to compete for transmission. In the reservation period, a channel-aware splitting algorithm is employed to resolve the collision among users and to schedule the user with the best channel quality to transmit if collision is resolved. The efficacy of the proposed transmission policies are demonstrated through extensive computer simulations.