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39
Dual methods for nonconvex spectrum optimization of multicarrier systems
 IEEE TRANS. COMMUN
, 2006
"... The design and optimization of multicarrier communications systems often involve a maximization of the total throughput subject to system resource constraints. The optimization problem is numerically difficult to solve when the problem does not have a convexity structure. This paper makes progress ..."
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Cited by 202 (7 self)
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The design and optimization of multicarrier communications systems often involve a maximization of the total throughput subject to system resource constraints. The optimization problem is numerically difficult to solve when the problem does not have a convexity structure. This paper makes progress toward solving optimization problems of this type by showing that under a certain condition called the timesharing condition, the duality gap of the optimization problem is always zero, regardless of the convexity of the objective function. Further, we show that the timesharing condition is satisfied for practical multiuser spectrum optimization problems in multicarrier systems in the limit as the number of carriers goes to infinity. This result leads to efficient numerical algorithms that solve the nonconvex problem in the dual domain. We show that the recently proposed optimal spectrum balancing algorithm for digital subscriber lines can be interpreted as a dual algorithm. This new interpretation gives rise to more efficient dual update methods. It also suggests ways in which the dual objective may be evaluated approximately, further improving the numerical efficiency of the algorithm. We propose a lowcomplexity iterative spectrum balancing algorithm based on these ideas, and show that the new algorithm achieves nearoptimal performance in many practical situations.
Crosslayer optimization for OFDM wireless network Part I: Theoretical framework
 IEEE TRANS. WIRELESS COMMUN
, 2005
"... In this paper, we provide a theoretical framework for crosslayer optimization for orthogonal frequency division multiplexing (OFDM) wireless networks. The utility is used in our study to build a bridge between the physical layer and the media access control (MAC) layer and to balance the efficien ..."
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Cited by 129 (3 self)
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In this paper, we provide a theoretical framework for crosslayer optimization for orthogonal frequency division multiplexing (OFDM) wireless networks. The utility is used in our study to build a bridge between the physical layer and the media access control (MAC) layer and to balance the efficiency and fairness of wireless resource allocation. We formulate the crosslayer optimization problem as one that maximizes the average utility of all active users subject to certain conditions, which are determined by adaptive resource allocation schemes. We present necessary and sufficient conditions for utilitybased optimal subcarrier assignment and power allocation and discuss the convergence properties of optimization. Numerical results demonstrate a significant performance gain for the crosslayer optimization and the gain increases with the number of active users in the networks.
Lowcomplexity nearoptimal spectrum balancing for digital subscriber lines
 IN IEEE INTERNATIONAL CONFERENCE ON COMMUNICATIONS (ICC ’05), SEOUL, KOREA
, 2005
"... This paper investigates the multiuser spectrum optimization problem for digital subscriber lines. We propose an iterative and lowcomplexity spectrum optimization technique that improves upon the recently proposed optimal spectrum balancing (OSB) algorithm. In the optimal spectrum balancing algori ..."
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Cited by 30 (1 self)
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This paper investigates the multiuser spectrum optimization problem for digital subscriber lines. We propose an iterative and lowcomplexity spectrum optimization technique that improves upon the recently proposed optimal spectrum balancing (OSB) algorithm. In the optimal spectrum balancing algorithm, the Lagrange multipliers are used to decouple the constrained optimization problem into a series of pertone unconstrained optimization problems. However, each pertone problem still has a computational complexity that is exponential in the number of users. This paper proposes an iterative algorithm for the pertone optimization problem to further reduce the computational complexity of spectrum balancing. The essential idea resembles that of iterative waterfilling. In each step of the algorithm, each individual user iteratively optimizes the joint objective function with a fixed set of Lagrange multipliers. The new algorithm has a computational complexity that is polynomial in the number of users. Simulation results show that the new algorithm has a nearoptimal performance.
Dual optimization methods for multiuser orthogonal frequency division multiplex systems
 in Proc. IEEE Globecom, Nov 2004
"... Abstract — The design and optimization of orthogonal frequency division multiplex (OFDM) systems typically take the following form. The design objective is to maximize the total data rate which is the sum of individual rates in each frequency tone. The design constraints are usually linear constrain ..."
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Cited by 28 (5 self)
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Abstract — The design and optimization of orthogonal frequency division multiplex (OFDM) systems typically take the following form. The design objective is to maximize the total data rate which is the sum of individual rates in each frequency tone. The design constraints are usually linear constraints imposed across all tones. This paper shows that regardless of whether the objective and the constraints are convex, the duality gap for this class of problems is always zero in the limit as the number of frequency tones goes to infinity. As the dual problem typically decouples into many smaller pertone problems, solving the dual problem is much more efficient. This observation leads to an efficient method to find the global optimum of nonconvex optimization problems for the OFDM system. Multiuser optimal power allocation, optimal frequency planning and optimal lowcomplexity crosstalk cancellation for vectored DSL are used to illustrate this point. I.
Joint subcarrier pairing and power allocation for OFDM transmission with decodeandforward relaying
 IEEE Trans. Signal Processing
, 2011
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Resource allocation for multiuser cooperative OFDM networks: Who helps whom and how to cooperate
 IEEE Transactions on Vehicular Technology
, 2009
"... Abstract—Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network perf ..."
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Cited by 17 (1 self)
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Abstract—Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network performance can be improved. Two important questions are answered—who should help whom among the distributively located users, and how many resources the users should use for cooperation to improve the performance. To answer these questions, a poweroptimization, subcarrierallocation, and relayselection problem is formulated over a multiuser orthogonal frequencydivision multiplexing (OFDM) network, which is applicable to systems such as wireless local area networks (WLANs). In the multiuser OFDM network, cooperation among different users is conducted by assigning the subcarriers of the helping users to relay a certain part of the helped users ’ data, while maintaining the desired rates of both helping users and helped users by means of power control and rate adaption. This way, the bandwidth efficiency of the multiuser OFDM system with cooperation is the same as that of the noncooperative OFDM system. The formulated optimization problem is an assignment problem for subcarrier usage and corresponding bit loading as well as power control. We provide an approximate closedform solution for a twouser twosubcarrier case. Then, a suboptimal heuristic algorithm for a multipleuser multiplesubcarrier case is proposed and implemented in the base station to solve the formulated NPhard problem. From the simulation results, the proposed scheme achieves up to 50 % overall power saving for the twouser system and 19%–54 % overall power saving for the multiuser case with random locations, compared with the current multiuser OFDM system without cooperative diversity. The proposed scheme is also compared to a much more complicated orthogonal frequencydivision multiple access (OFDMA) system. Index Terms—Combinatorial mathematics, cooperative systems, power control, relays, resource management.
Transmit optimization for relaybased cellular OFDMA systems
 in Proc. IEEE Int. Conf. Commun. (ICC
, 2007
"... Abstract—This paper considers a broadband cellular orthogonal frequencydivision multipleaccess (OFDMA) system with relay nodes operating in decodeandforward and halfduplex mode. Two transmit resource allocation problems for sumrate maximization are formulated for such a system. The first one ..."
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Cited by 14 (0 self)
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Abstract—This paper considers a broadband cellular orthogonal frequencydivision multipleaccess (OFDMA) system with relay nodes operating in decodeandforward and halfduplex mode. Two transmit resource allocation problems for sumrate maximization are formulated for such a system. The first one is the optimization of subcarrier allocation with predetermined power assignment for each subcarrier, and the other is the joint optimization of power and subcarrier allocation. Since these problems can’t be solved easily in direct forms, we make continuous relaxation and solve the dual problems using a subgradient method. Numerical results show that a remarkable increment in sumrate is achieved, with the aid of relay nodes and sophisticated resource allocation, compared to a system without relay nodes. I.
Distributive subband allocation, power and rate control for relayassisted OFDMA cellular system with imperfect system state knowledge
 IEEE Trans. Wireless Commun
, 2009
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