適應性OFDM子載波的配置和位元的分配

Abstract

本篇論文考慮在下傳多用戶正交分頻多工通訊系統中，適應性子載波配置和位元分配的演算法。假設每個使用者的通道狀況已知下，適應性調變位階可以根據每位使用者子載波上的通道狀況，和每位使用者傳輸率的需求而做適應性的調整。 我們所提及的演算法，是希望能在錯誤率的要求下，和滿足每位使用者不同傳輸率的限制下，去最小化整體系統所需的傳送功率。本篇論文的主要貢獻是改良王所提出的Lagrange演算法，藉由找出一組Lagrange係數的最低邊界值來作為初始的子載波配置。 演算法藉由Lagrange係數去決定子載波的配置和位元的分配。因此演算法有較快的收斂速度和較少的遞迴次數。 要同時解決子載波和位元的配置問題，有以下兩個步驟被採用。第一，先決定好子載波的分配。第二再決定出每根子載波上的調變模式。最後較快的收斂速度演算法，被發展出來完成解決子載波和位元分配的問題。

In this paper, the adaptive subcarrier assignment and bits loading algorithm for downlink communication to multiple users in an orthogonal-frequency division multiplexing (OFDM) system are proposed. Assuming that the channel condition for each user is already known, the adaptation of modulation level on each subcarrier based on each user’s channel condition and required transmission rate is proposed. In effect, the proposed algorithm minimizes the system total transmit power under the given BER while still satisfying each user’s transmission rate constraints. The major contribution of this paper is to improve the algorithm that Wong proposed by giving the formula to determine the lower bound of the Lagrangian multipliers for each user. Specifically, this algorithm uses the Lagrangian multipliers to decide the subcarrier assignment and bits allocation. Therefore, a faster convergence and smaller number of recursions can be found. To solve the problem of joint allocation, the following two steps are taken. First, the subcarriers to users are assigned. Second, the modulation level on each subcarrier is decided. As a result, a faster algorithm to accomplish the task is developed.