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 time-sharing condition, the duality gap of the optimization problem is always zero, regardless of the convexity of the objective function. Further, we show that the time-sharing 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 low-complexity iterative spectrum balancing algorithm based on these ideas, and show that the new algorithm achieves near-optimal performance in many practical situations

/pdf/dual-methods-for-nonconvex-spectrum-optimization-of-45uov3hj65.pdf

Dual methods for nonconvex spectrum optimization of multicarrier systems

In this paper, we develop a transmit power adaptation method that maximizes the total data rate of multiuser orthogonal frequency division multiplexing (OFDM) systems in a downlink transmission. We generally formulate the data rate maximization problem by allowing that a subcarrier could be shared by multiple users. The transmit power adaptation scheme is derived by solving the maximization problem via two steps: subcarrier assignment for users and power allocation for subcarriers. We have found that the data rate of a multiuser OFDM system is maximized when each subcarrier is assigned to only one user with the best channel gain for that subcarrier and the transmit power is distributed over the subcarriers by the water-filling policy. In order to reduce the computational complexity in calculating water-filling level in the proposed transmit power adaptation method, we also propose a simple method where users with the best channel gain for each subcarrier are selected and then the transmit power is equally distributed among the subcarriers. Results show that the total data rate for the proposed transmit power adaptation methods significantly increases with the number of users owing to the multiuser diversity effects and is greater than that for the conventional frequency-division multiple access (FDMA)-like transmit power adaptation schemes. Furthermore, we have found that the total data rate of the multiuser OFDM system with the proposed transmit power adaptation methods becomes even higher than the capacity of the AWGN channel when the number of users is large enough.

/pdf/transmit-power-adaptation-for-multiuser-ofdm-systems-2iopjrpfqx.pdf

Transmit power adaptation for multiuser OFDM systems

This paper investigates the problem of dynamic multiuser subchannel allocation in the downlink of OFDM systems. The assumptions are that the channel model is quasi-static and that the base station has perfect channel information. In traditional TDMA or FDMA systems, resource allocation for each user is non-adaptively fixed, and the water-filling power spectrum is known to be optimal. Since the subchannel allocations among the users are not optimized, a group of users is likely to suffer from poor channel gains resulting from large path loss and random fading. To resolve this problem, we derive a multiuser convex optimization problem to find the optimal allocation of subchannels, and propose a low-complexity adaptive subchannel allocation algorithm. Simulation results show that the proposed algorithm performs almost as well as the optimal solution. Also, a higher spectral efficiency is achieved for a larger number of users in a cell due to the multiuser diversity.

Increase in capacity of multiuser OFDM system using dynamic subchannel allocation

Mobile-edge computation offloading (MECO) off-loads intensive mobile computation to clouds located at the edges of cellular networks. Thereby, MECO is envisioned as a promising technique for prolonging the battery lives and enhancing the computation capacities of mobiles. In this paper, we study resource allocation for a multiuser MECO system based on time-division multiple access (TDMA) and orthogonal frequency-division multiple access (OFDMA). First, for the TDMA MECO system with infinite or finite cloud computation capacity, the optimal resource allocation is formulated as a convex optimization problem for minimizing the weighted sum mobile energy consumption under the constraint on computation latency. The optimal policy is proved to have a threshold-based structure with respect to a derived offloading priority function , which yields priorities for users according to their channel gains and local computing energy consumption. As a result, users with priorities above and below a given threshold perform complete and minimum offloading, respectively. Moreover, for the cloud with finite capacity, a sub-optimal resource-allocation algorithm is proposed to reduce the computation complexity for computing the threshold. Next, we consider the OFDMA MECO system, for which the optimal resource allocation is formulated as a mixed-integer problem. To solve this challenging problem and characterize its policy structure, a low-complexity sub-optimal algorithm is proposed by transforming the OFDMA problem to its TDMA counterpart. The corresponding resource allocation is derived by defining an average offloading priority function and shown to have close-to-optimal performance in simulation.

Energy-Efficient Resource Allocation for Mobile-Edge Computation Offloading

Multiuser orthogonal frequency division multiplexing (MU-OFDM) is a promising technique for achieving high downlink capacities in future cellular and wireless local area network (LAN) systems. The sum capacity of MU-OFDM is maximized when each subchannel is assigned to the user with the best channel-to-noise ratio for that subchannel, with power subsequently distributed by water-filling. However, fairness among the users cannot generally be achieved with such a scheme. In this paper, a set of proportional fairness constraints is imposed to assure that each user can achieve a required data rate, as in a system with quality of service guarantees. Since the optimal solution to the constrained fairness problem is extremely computationally complex to obtain, a low-complexity suboptimal algorithm that separates subchannel allocation and power allocation is proposed. In the proposed algorithm, subchannel allocation is first performed by assuming an equal power distribution. An optimal power allocation algorithm then maximizes the sum capacity while maintaining proportional fairness. The proposed algorithm is shown to achieve about 95% of the optimal capacity in a two-user system, while reducing the complexity from exponential to linear in the number of subchannels. It is also shown that with the proposed resource allocation algorithm, the sum capacity is distributed more fairly and flexibly among users than the sum capacity maximization method.

/pdf/adaptive-resource-allocation-in-multiuser-ofdm-systems-with-441ykegom1.pdf

Adaptive resource allocation in multiuser OFDM systems with proportional rate constraints

Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimize the total transmit power. This is done by assigning each user a set of subcarriers and by determining the number of bits and the transmit power level for each subcarrier. We obtain the performance of our proposed algorithm in a multiuser frequency selective fading environment for various time delay spread values and various numbers of users. The results show that our proposed algorithm outperforms multiuser OFDM systems with static time-division multiple access (TDMA) or frequency-division multiple access (FDMA) techniques which employ fixed and predetermined time-slot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the bit-error rate (BER), or the area of coverage for a given outage probability.

/pdf/multiuser-ofdm-with-adaptive-subcarrier-bit-and-power-qfzvyfs4s3.pdf

Multiuser OFDM with adaptive subcarrier, bit, and power allocation

In this paper, we propose sub-carrier allocation algorithms for a multiple access scheme in downlink OFDM transmission. Knowing the channel characteristics of all the users at the base station, the sub-carrier allocation algorithm assigns sub-carriers to the users in a way that the total transmit power is minimized. An optimal algorithm is presented to provide the best sub-carrier assignment. The complexity of the optimal solution renders it impractical for real-time application. To deal with the frequently changing channel characteristic of a fast time-varying multipath fading channel, a heuristic algorithm based on constructive assignment and iterative improvement is proposed which can give out a valid solution in real time. Experimental results show that the performance of this real-time algorithm is close to that of the optimal allocation.

/pdf/a-real-time-sub-carrier-allocation-scheme-for-multiple-3vx00l3cx4.pdf

A real-time sub-carrier allocation scheme for multiple access downlink OFDM transmission

Disclosed are systems and methods which provide high bandwidth data communication with respect to a large coverage area using smart antenna and/or directional antenna (referred to herein as multi-beam antenna) technology. Circuitry may be provided at a WLAN AP to provide selection of particular antenna beams used in the downlink and/or uplink, control of multicast transmission, control of unicast transmission, and to provide antenna pattern shaping techniques. Embodiments implement multi-beam antenna technology with little or no hardware modifications to AP circuitry. Other embodiments implement multi-beam technology using radio front-end and/or radio hardware modifications to AP circuitry. Various diversity techniques may be implemented, such as selection diversity, maximum ratio combining, and equal gain combing. To provide desired antenna pattern shaping, phase offsets with respect to a signal as transmitted in each antenna beam may be employed.

Wlan access point with extended coverage area

Embodiments of systems and methods for efficient hand-off in a wireless network employ a neighbor report which may include, in addition to more convention information about neighboring access point, search thresholds, target beacon transmission time of neighboring access points, and execution thresholds. The present systems and methods also provide a mechanism of updating neighbor report and its elements. A faster and lower spectrum cost active search scheme based on sending a null packet may also be used by the systems and methods.

Systems and methods for efficient hand-off in wireless networks

A multiuser OFDM (orthogonal frequency division multiplexing) downlink transmission with adaptive modulation is considered. Assuming that the base station knows the instantaneous channel gains for all users, we propose an algorithm which minimizes the total transmit power by assigning, to each user, a set of subcarriers within an OFDM symbol and by determining, for each sub-carrier, the number of bits to be transmitted and the power level used. We obtained the performance of our proposed algorithm in a multiuser frequency selective fading environment with various root-mean-square (RMS) delay spread. The results show that our proposed algorithm outperforms conventional OFDM systems with or without optimal bit allocation using TDMA or FDMA as multiaccess techniques. We have also quantified the improvement in terms of the required transmit power, for fixed bit error rate.

Cheong Yui Wong

Papers

Multiuser OFDM with adaptive subcarrier, bit, and power allocation

A real-time sub-carrier allocation scheme for multiple access downlink OFDM transmission

Wlan access point with extended coverage area

Systems and methods for efficient hand-off in wireless networks

Multiuser subcarrier allocation for OFDM transmission using adaptive modulation