在無線寬頻網路中，正交分頻多工系統(OFDM)被視為是一個具有潛力的無線通訊技術。雖然在多用戶正交分頻多工(MU-OFDM)系統中，其系統的容量可以藉由指派每一個子通道到具有最好通道的用戶，且其通道上的功率分配是採用water-filling的方式而達到最大化，但是它的複雜度過高，而不被實際的系統所接受。為了避免同時對子通道以及功率做最佳化分配所帶來的高運算複雜度，我們通常會採用次佳化的演算法來代替。最常見到的做法是將子通道分配與功率分配分開做。一般來說，這樣的次佳化資源分配方法必須要在系統的總容量與系統的複雜度之間做取捨。
本篇論文主要針對合作式多用戶正交分頻多工系統提出一個次佳化的資源分配演算法，同時觀察空間分集(spatial diversity)對系統容量的影響。藉由合作式通訊(cooperative communications)的協定方式，將三點的中繼網路(three-node relay networks)簡化成兩點的網路(two-node network)，那麼對於合作式多用戶正交分頻多工系統來說，其資源分配的問題相當於多用戶正交分頻多工系統。在合作式的網路中，我們採用了decode-and-forward機制。如此一來，在資源分配的問題會多增加一個額外的限制條件。從模擬的結果可以看出提出的演算法能增加系統的總容量，且運算複雜度與未加入合作式通訊的多用戶正交分頻多工系統具有相同的階層。
再者，為了提升頻寬效益，我們將感知無線電(cognitive radio)加入合作式多用戶正交分頻多工系統。在感知無線電系統裡，為了維持主用戶(primary user)的通訊，其資源分配的問題必須包含最大功率的限制。從模擬的結果可以得知加入合作式通訊的多用戶正交分頻多工感知無線電系統比起未加入合作式通訊的多用戶正交分頻多工感知無線電系統具有較高的系統總容量，且其運算複雜度具有相同階層。

Multiuser orthogonal frequency division multiplexing (MU-OFDM) is one of the promising techniques for next-generation wireless broadband networks. Although the overall capacity of an MU-OFDM system can be maximized by assigning each subchannel to the user with the best channel-to-noise ratio for that subchannel and distributing power subsequently by water-filling, the complexity is too high to be acceptable for practical applications. To avoid the prohibitive complexity for solving the joint subchannel and power allocation problem, many suboptimal algorithms have been studied in the literature. Among them, separating the subchannel allocation from the power allocation is one of the common schemes. In general, such suboptimal resource allocation methods have a tradeoff between the system capacity and computational complexity.
This thesis presents a suboptimal resource allocation algorithm for cooperative MU-OFDM systems, where the effect of the spatial diversity on the system capacity is investigated. By simplifying a three-node relay network to a two-node network, the resource allocation problem for cooperative MU-OFDM systems is equivalent to that for the traditional MU-OFDM systems. To use the decode-and-forward scheme in the cooperative network, an additional constraint is put on the resource allocation problem. It is shown that the proposed algorithm increases the system capacity with the same order of computational complexity as a non-cooperative MU-OFDM system.
To further improve the spectrum efficiency, we employ the concept of cognitive radio to cooperative MU-OFDM systems. In such cognitive radio systems, the resource allocation problem must include a peak power constraint to protect the primary users. It is demonstrated that the proposed cooperative multiuser OFDM-based cognitive radio system has higher system capacity than that without combining cooperative communications under the same order of computational complexity.

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