| In this paper, we introduce a cross-layer approach of downlink scheduling for TDM-OFDM systems, aiming at minimizing the packet loss rate. First we build a cross-layer system model for OFDM downlink systems with K users and N subcarriers. The transmitter has a finite buffer in the data link layer and employs adaptive modulation and coding (AMC) at the physical layer. Arrival data flows for different users are modeled as mutually independent random processes, i.e., Poisson or Markov arrival processes. The arrival data flows are stored in the transmitter's buffer and the transmitter allocates wireless resources to different users during each frame. In this paper, comprehensively considering the channel condition in the physical layer and the queuing behavior in the data link layer, we study both the resource allocation strategy for each frame and the steady-state statistical performance of the downlink system. With the channel state information (CSI) and queue constraint of each user, our strategy adaptively allocate subchannels and time slots to different users to maximize the throughout per frame. We utilize finite state Markov chain (FSMC) to describe the joint queuing behavior of the channel state of each subcarrier and the queue length state of each user. With its stationary distribution, we can compute the average Packet Dropping Rate (PDR), average Packet Error Rate (PER) and then the average Packet Loss Rate (PLR). The theoretical framework enables us to minimize the Packet Lose Rate by adaptively selecting system parameters such as target BER in AMC controller. However, since each state in the FSMC is a (K+KN) vector, the computational complexity is exceedingly large. By estimating the average number of leaving packets during one frame, our suboptimal method provides a good tradeoff between system performance and computational complexity. Numerical simulation results confirm our theoretical analysis and demonstrate a significant performance gain for our cross-layer optimization. |
| Yunjian Xu, IEEE student member, received B.S degree and now is a graduate student in Electronic Engineering of Tsinghua University, Beijing China, in 2006. He received Scholarship for Academic Excellence in Tsinghua University every year. His reserch interests focus on cross-layer design, OFDM systems and Wireless-MAN.
Zhigang Cao, IEEE senior member, graduated from the Department of Radio Electronics at Tsinghua University in 1962. He was a Visiting Scholar at Stanford University, Stanford, CA, from 1984 to 1986. Since 1962, he has been with Tsinghua University, where he is a Professor in the Electronic Engineering Department, Deputy Director of the State Key Laboratory on Microwave and Digital Communications, a Member of the Academic Committee, Tsinghua University. He is the author and coauthor of six books and has published more than 220 papers on Communications and Signal Processing, and holds
three patents. His current research interests include mobile communications and
digital signal processing.
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