Adaptive Bit and Power Allocation for Indoor Wireless Multicarrier Systems Alexander M. Wyglinski Peter Kabal Fabrice Labeau Department of Electrical & Computer Engineering McGill University, 3480 University St. Montr´ eal, QC, Canada H3A 2A7 Abstract We propose an adaptive bit and power al- location algorithm for indoor wireless systems employing multicarrier modulation. The pro- posed scheme maximizes throughput via an incremental allocation algorithm while oper- ating under a maximum mean bit error con- straint. Unlike other bit allocation algorithms, which allocate a continuous distribution of bits followed by quantization, the proposed algo- rithm allocates a discrete distribution of bits. Moreover, the proposed algorithm employs a stricter subband power constraint to limit the interference to other users and satisfy govern- ment regulatory requirements, unlike other al- gorithms which only employ a total power con- straint. Finally, the assumption of flat sub- channels is dropped, thus a subcarrier mini- mum mean-squared error equalizer is applied to the case of orthogonal frequency division multiplexing systems employing a cyclic pre- fix. The performance of the proposed system is evaluated in terms of throughput and bit al- location and compared with an IEEE 802.11a- compliant system. The results show that the proposed system outperforms the IEEE 802.11a-compliant system when transmitting at lower signal-to-noise ratios. Furthermore, the benefits of power allocation are noticeable at low signal-to-noise ratios. I. Introduction The development of high speed wireless net- works over the past several years has resulted in the implementation of systems which are capable of transmitting at data rates of up to 54 megabits per second (Mb/s) [1]. Although much work has gone into making these systems reliable, robust in fading channels, and spectrally efficient when transmitting at high data rates, several problems still need to be addressed. In this paper we ap- proach the problem of a frequency-selective fad- This research was partially funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Le Fonds de Recherche sur la Nature et les Technolo- gies du Qu´ ebec. ing channel by using adaptive bit and power al- location in multicarrier systems. In multicarrier modulation, data is transmit- ted in parallel subcarriers at a lower data rate than the serial input and output of the system. This effectively transforms the frequency selec- tive fading channel into a collection of flat fad- ing subchannels. An efficient version of multicar- rier modulation is orthogonal frequency division multiplexing (OFDM), which has no intersymbol interference (ISI) when a sufficiently long cyclic prefix is used. As a result, many wireless local area network (WLAN) standards, such as IEEE 802.11a [1] and HIPERLAN/2 [2], use OFDM systems at the core of their design. Conventional wireless OFDM systems uses a fixed signal constellation size across all subcar- riers. Their overall error probabilities are dom- inated by the subcarriers with the worst perfor- mance. One solution to this problem is adaptive bit and power allocation, where the signal constella- tion size and power distribution across all the sub- carriers vary according to the estimated channel conditions in order to minimize the overall error probability. In Kalet [3], Chow et al. [4], Fis- cher & Huber [5], Hughes-Hartog [6], and Leke & Cioffi [7], the power and bit allocation are opti- mized in order to either achieve a maximum bit rate for a given probability of error or to mini- mize the probability of error given a target bit rate; the power level of each subcarrier sums up to a constant total power while the bit allocation can be non-integer with no maximum limit on the constellation size. On the other hand, in the work by Schmidt & Kammeyer [8], Czylwik [9], and Keller & Hanzo [10], adaptive modulation is used, where the subcarriers are adaptively modu- lated with a fixed number of signal constellation sizes. Although these methods either offer low complexity or near-optimal bit allocations, none offer a balance between these two criteria. Fur- thermore, none of these algorithms impose practi- cal constraints on the power allocation that meet regulatory requirements. In this paper, we pro- Proceedings of the 15th International Conference on Wireless Communications, (Calgary, AB, Canada), July 2003