Resource Allocation in OFDMA Systems Sample Clauses

Resource Allocation in OFDMA Systems. OFDMA is a multiple access version of the popular OFDM and inherits OFDM’s resistance to frequency selective fading and inter-symbol interference [68–70]. In OFDMA, in order to deal with frequency-selective fading and support a high data rate, an entire channel is divided into a large number of orthogonal narrow-band sub-channels (subcarriers) [71]. In an OFDMA-based network, different subcar- riers can be allocated to different users to provide a flexible multiuser access scheme [72, 73] and exploit multiuser diversity [74]. There is plenty of room to exploit the high degree of flexibility of radio resource management in the context of OFDMA. Due to different channel frequency responses at different frequencies and for different users, data rate adaptation over each subcarrier, adaptive power allocation (APA) and dynamic subcarrier assignment (DSA) result in a consid- erable improvement in the performance of OFDMA networks. Using data rate adaptation [75, 76], the transmitter can send higher transmission rates over the subcarriers with better conditions to improve throughput and ensure an accept- able bit-error rate (BER) at each subcarrier. Furthermore, a data transmission rate close to the channel capacity is achievable by optimal power allocation with dynamic subcarrier assignment [77]. However, deep fading on some subcarriers still results in low channel capacity. On the other hand, channel characteristics for different users in multiuser environments are almost independent. Hence, the subcarriers in a deep fade for one user may not be experiencing deep fading for other users [78]. By dynamically assigning subcarriers, the network can benefit from multiuser diversity, which can be exploited by scheduling transmissions when a user has favorable channel conditions [79]. Using this approach, the system capacity in- creases with the number of users [80]. Exploiting multiuser diversity can also lead to a considerable enhancement in the spectral efficiency. Apart from the spectral efficiency, QoS and fairness are of great importance for resource alloca- tion in wireless networks. Achieving optimality for spectral efficiency, fairness and QoS is usually unfeasible [78]. For instance, throughput-optimal scheduling schemes are unfair to those users faraway from a base station or with bad channel conditions, whereas the absolute fairness may result in low bandwidth efficiency. Thus, it is desirable to achieve an optimal trade-off among efficiency, fairness and ...