Joint Subcarrier Pairing and Power Allocation in OFDM Two-Hop systems
Date Issued
2009
Date
2009
Author(s)
Hsu, Chih-Ning
Abstract
Orthogonal Frequency Division Multiplexing (OFDM) can overcome multi-path effect and provide frequency diversity in wireless transmission. Frequency diversity can be further utilized when relayed transmission is employed. In this thesis, it is assumed that the channel state information (CSI) is known to the source. We aim to maximizehe weighted sum rate of OFDM relaying system under total power constraint by jointly optimizing subcarrier pairing in two consecutive time slots and power allocation on eachubcarrier in each time slot. We restrict that there is only one node transmitting on each subcarrier. Decode-and-forward (DF) strategy is considered when the relay is used. Besides, there are two types of transmissions when relay is not used, the difference is whether the source is permitted to transmit in the second time slot or not. First we formulate the problem as a mixed integer programming (MIP) problem which is NP-hard. Then we make continuous relaxation and solve the relaxed problem in dual domain by sub-gradient method. The relaxed problem is convex for one type and non-convex for the other type. Although one primal problem is convex, the constraint qualification is not satisfied. It means that the duality gaps for both types are not zero, and the dual optimal solution for each type provides an upper bound of the achievable rate. However, the duality gaps are virtually zero when the number of subcarriers goes to infinity. Thus we can achieve nearptimal solution. Finally we extend the system to have individual power constraint.
Subjects
OFDM
power allocation
subcarrier pairing
optimization
continuous relaxation
duality gap
Type
thesis
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