針對多媒體資訊在正交分頻多工寬頻無線存取系統中排程技術及子載波分配方法之研究

Abstract

在此論文中，我們針對多媒體資訊在正交分頻多工(Orthogonal Frequency Division Multiple Access, OFDMA)寬頻無線存取(Broadband Wireless Access, BWA)系統作排程技術及子載波分配方法之研究。 首先，我們將驗證在正交分頻多工寬頻無線存取此種多載波的系統，使用一種最簡單的最大信號干擾比(Maximum C/I)排程法，即可同時增加系統資料流量(throughput)且維持一定的公平性。最大信號干擾比排程法在單載波分碼多工存取(Code Division Multiple Access, CDMA)系統可有效的增進系統資料流量，但一向被視為一種不公平的排程方法，在此，我們重新評估最大信號干擾比排程法在多載波的正交分頻多工存取系統中的效能。透過分析與模擬，我們發現最大信號干擾比排程法對於正交分頻多工系統的確算是一種公平的排程方法。因此，針對正交分頻多工系統存取，我們發展了一種以最大信號干擾比排程法為基礎的資源分配演算法，模擬結果顯示，最大干擾比排程法並不比比例式公平(proportional fair)排程法差很多，總結，在正交分頻多工存取系統中，最大干擾比排程法，不但可盡量使系統資料流量趨近最大，更可同時維持相當好的公平性。 然而，目前的通訊環境中，多媒體的資訊傳輸已成趨勢，因此如何對不同服務品質(Quality of Service, QoS)需求的使用者作最好的資源分配亦成一項重要的研究課題。我們在此針對了正交分頻多工存取系統發展了一套滿足服務品質的排程及通道分配的演算法。即時服務(real-time service)的使用者在意的是資料的傳輸延遲，而非即時(non-real-time)服務的使用者則是希望資料流量盡可能的越大越好。而在無線通訊的環境，通道狀況是隨時間改變的，因此，我們針對在正交分頻多工存取系統中，提出了一種考慮通道狀況和服務品質的一套排程演算法。藉著利用多載波環境中的頻率多樣性和通道變化的效應，我們提出的排程演算法可同時滿足即時與非即時使用的的服務品質要求。首先，我們藉著排隊理論中等待時間的分析來分配即時使用者的無線資源，接著使用最大信號干擾比排程法來分配非即時使用者以達最大系統流量。總而言之，我們藉著利用頻率多樣性和實體層的通道效應作跨階層的設計，便同時滿足了不同服務品質需求的使用者。

In this thesis, we first demonstrate that the simple maximum carrier to interference ratio (C/I) scheduling can both enhance system throughput and maintain fairness performances for the orthogonal frequency division multiple access (OFDMA) system. The maximum C/I scheduling has long been recognized as an effective method to enhance throughput, but it is viewed as an unfair scheduling policy in the the single carrier code division multiple access (CDMA) system. We reassess the fairness performance of the maximum C/I scheduling in the context of the multi-carrier OFDMA system. Through analysis and simulations, we find that the maximum C/I scheduling is indeed an fair scheduling for OFDMA systems. Thus, with respect to the OFDMA system, we develop a maximum C/I scheduling based resource allocation algorithm. Our results show that the fairness of the maximum C/I scheduling in OFDMA systems is comparable to that of the proportional fair scheduling scheme. To sum up, we conclude that in the OFDMA system, the maximum C/I scheduling not only can maximize system throughput, but simultaneously maintain very good fairness performance.

The orthogonal frequency division multiple access (OFDMA) is becoming an important technique for the future wireless systems. Through parallel multi-carrier transmissions, the inter-symbol interference (ISI) can be easily handled in transmitting high speed data. Furthermore, OFDMA systems bring a new dimension for allocating radio resource - subcarrier. By exploiting frequency diversity in the wide frequency spectrum, a suitable subcarrier allocation technique can further enhance throughput for the OFDMA system. This thesis addresses the issue of allocating subcarriers for providing both real-time and non-real-time traffic in the OFDMA system. We suggest a categorized subcarrier allocation (CSA) technique to improve throughput for non-real-time traffic, while satisfying the quality of service (QoS) requirements for the real-time traffic. In the proposed CSA technique, subcarriers are categorized into two groups based on their quality: good and fair. The real-time traffic will be assigned by the subcarrier with fair condition, while the non-real-time traffic will be assigned with good subcarriers. We find that such a subcarrier allocation method can apply the maximum carrier-to-interference (C/I) scheduling to maximize the throughput in good conditioned subcarriers, while the delay for the real-time traffic can be controlled by allocating enough fair-conditioned subcarriers through a queueing analytical method. Compared to dynamic subcarrier allocation (DSA) and random subcarrier allocation (RSA) methods, the CSA technique outperforms other methods in terms of throughput, blocking probability and fairness performances.