Abstract

We consider the idea of rate-adaptive multiple-subcarrier-based transmission to achieve broadband communication between fixed access-point and mobile user terminal in an indoor non-directed infrared wireless system.Given the high channel dynamics (both bandwidth and gain) in non-directed IR links [1], adaptive system design allows to exploit channel capacity most efficiently. Here, high data-rates achieved under good channel conditions, could be under adverse conditions suitably lowered to maintain reliable transmission (predefined QoS).To avoid complex optics often accompanied by various pointing and tracking mechanisms (involved with angle and space diversity systems, so far proposed for IR), an adaptive system can be realized by signal processing. This would allow deployment of simple optical components. Many performance-improvement techniques considered lately in other areas (e.g. wireless and wireline transmission) may also prove beneficial in IR. However, specific conditions of the IR channel impose evaluation of any existing technique anew [2]. Moreover, in order to deal with both channel dynamics and multipath propagation (major obstacle for high-speed broadband transmission in IR) at the same time, it makes sense to consider adaptive system based on multiple-subcarrier (MS) transmission. Namely, it is known from radio domain that multicarrier modulation techniques deal inherently with multipath distortion, and eliminate the need for adaptive equalization at the receiver. Therefore, the concept of a rate-adaptive MS-based IR system, which adjusts the transmission rate to the channel state by assigning suitable modulation formats to individual subcarriers, promises to efficiently exploit channel capacity, while allowing simple optics and relatively simple electronic signal processing.In [3] we introduced the general idea behind such innovative system for IR and performed theoretical channel capacity analysis. It was shown that data rates of 100 Mb/s and more could be expected under good channel conditions. Results of such theoretical analysis, which assumed an ideal system (infinite granularity of modulation formats, use of different error correcting codes), can serve as a reference for practical solutions.In this paper, we will explore how such reference values can be approached in a practical system with help of bit-loading. •Suitable parameters for optical components and optical power budget will be discussed for a realistic indoor scenario. To find favorite adaptive system parameters, theoretical capacity will be discussed with respect to the number of supposed subcarriers and signal bandwidth. Additionally, we will stress differences to the radio channel. •Based on the subcarrier SNR values for different channel states, and suitable choice for bandwidth and number of subcarriers, we will derive and discuss a span of appropriate modulation formats. Finally, an example for bit-loading will be given, and results will be discussed and compared to those of the ideal adaptive system (upper limit) and the one with static-design (lower limit).[1]Jungnickel, et al.: A Physical Model of the Wireless Infrared Communication Channel, IEEE JSAC, 2002[2]Barry: Wireless Infrared Communication, Kluwer, 1994[3]Grubor, et al.: Capacity Analysis in Indoor Wireless Infrared Communication using Adaptive Multiple Subcarrier Transmission, Proc. IEEE ICTON, 2005

© 2006 Optical Society of America

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