Efficient adaptive loading algorithms for multicarrier modulation

Communication networks rely increasingly upon wireless systems to provide transmission over the "last mile". This is due not only to the low cost of infrastructures, but also, and perhaps to a larger extent, to their flexibility. Increasingly, the current applications put a large burden on the throughput of those wireless systems. Multicarrier modulation has proven a highly effective technique to sustain such throughput and is now used in numerous current and future standards such as IEEE 802.11a. To take full advantage of that scheme, however, the multicarrier system needs to adapt to the transmission channel's time varying parameter. This can be achieved by adaptive loading algorithms, which unfortunately, are usually computationally expensive.This work addresses the issue of computational complexity within an adaptive loading algorithm by concentrating on the optimization of two algorithms crucial for a multicarrier system's performance: equalization and bit loading. Firstly, a variable-length equalizer algorithm is optimized and modified to make fast large-scale simulations possible. The algorithm is bound to significantly outperform fixed-length schemes of comparable complexity in terms of probability of error of the system. Secondly, an adaptive bit loading algorithm is implemented in real-time. The implementation target is a fixed-point DSP. The algorithm is optimized and an alternate, more computationally efficient version is proposed. The implementation is then tested for robustness and speed of convergence. Both versions of the algorithm converge to a solution well within the time constraint, with the proposed version offering a clearly better performance

WILEY WIRELESS COMMUNICATIONS AND MOBILE COMPUTING 1 DSP Implementation of a Bit Loading Algorithm for Adaptive Wireless Multicarrier Transceivers

In this paper, we present a proof-of-concept, fixed-point, DSP hardware implementation of an adaptive bit loading algorithm that is designed for wireless multicarrier transceivers. Adaptive bit loading is used to enhance the performance of multicarrier transceivers by tailoring the subcarrier signal constellations to the channel conditions, which can vary across the subcarriers. Since most bit loading algorithms possess a high computational cost and are unable to cope with rapid variations of wireless channels, they are seldom used in present wireless standards. To prove that adaptive bit loading is feasible for wireless transceivers, our work focuses on the implementation of a known bit loading algorithm that can quickly search for the final bit allocation in an iterative manner. The goal of this algorithm is to yield the largest-possible throughput while satisfying a mean BER constraint. The performance of the hardware implementation operating in time-varying channel conditions is studied in terms of the overall throughput. Furthermore, the robustness of the hardware implementation is evaluated, relative to sudden changes in the channel that interrupts the run of the algorithm. Real-time operations and fixed-point representation issues are included in the discussion. Additionally, we propose a modifie

Tap Loading of Subcarrier Equalizers for Wireless Multicarrier Transceivers

Abstract—We present a novel algorithm for defining the lengths of subcarrier equalizers employed by wireless multicarrier transmission systems operating in frequency-selective fading channels. The equalizer lengths across the subcarriers are incrementally varied in a “greedy ” fashion until the global cost function is below some prescribed threshold. By varying the equalizer lengths, the overall complexity of the equalization is constrained while the system meets a minimum error performance. Moreover, we investigate four strategies for terminating the proposed algorithm when an adequate number of equalizer taps have been allocated in this process. The results show that a system that employs variable-length equalizers defined by the proposed algorithm can achieve an improvement in error robustness of as much as an order of magnitude, relative to a system that employs constantlength equalizers with the same overall complexity. Index Terms—Adaptive allocation, equalization, loading algorithms, multicarrier modulation. I