Semester Thesis

Efficient Bit Allocation Scheme for Precoded MIMO BICM-OFDM Systems

bit allocation

Orthogonal frequency-division multiplexing (OFDM) in combination with bit-interleaved coded modulation (BICM) has turned out a robust yet implementation efficient technique for reliable communication over fading channels without channel state information (CSI) at the transmitter [1]. Diversity is achieved in BICM by an interleaving at bit-level, in a separate operation between the encoding and the modulation. OFDM with BICM is part of the IEEE 802.11a standard. If the transmitter has CSI obtained e.g. from a time-division duplex transmission [2] with/without feedback, the negative effects of the fading can be further alleviated by an adaptation of the signaling to the varying channel gain.

Adaptive power loading (APL) strategies of the waterfilling (WF) type, being optimal from an information theoretic point of view, were derived in [3,4] under various constraints. In adaptive bit loading (ABL) schemes, it is difficult to compute the optimum bit allocation for each subcarrier. The reason is the frequently assumed infinite granularity in the constellation size with a non-integer number of bits per symbol, which is not practically realizable. Some suboptimum algorithms to reduce the complexity have been proposed in the context of digital subscriber lines [5,6]. Recently, different algorithms have been proposed regarding the discrete bit loading problem in wireless channels [7,8].

OFDM can be combined in a straightforward manner with multiple-input multiple-output (MIMO) techniques to increase the spectral efficiency and/or robustness against fading. Furthermore, the use of adaptive techniques to select the subcarrier power and/or modulation seems to be essential to achieve these objectives.

In this project, we study an efficient bit allocation scheme for a precoded MIMO BICM-OFDM system, where the optimization criterion is the bit-error rate (BER) at the decoder output. The adaptive subcarrier bit allocation is based on either perfect or outdated CSI at the transmitter. The performance improvement using multiple transmit antennas and the sensitivity of the ABL scheme to outdated CSI are analyzed.

[1] G. Caire, G. Taricco, Bit-Interleaved Coded Modulation, IEEE Trans. Inf. Theory, Vol. 44, No. 3, Mai 1998.
[2] T. Hunziker, D. Dahlhaus, Optimal Power Adaptation Bit-Interleaved Coded OFDM Systems with Hard-Decision Decoding, Proc. IEEE Int. Conf. Comm. (ICC), pp. 3392-3397, May 2003.
[3] G. Caire, G. Taricco, and E. Biglieri, Optimum Power Control Over Fading Channels, IEEE Trans. Inform. Theory, vol. 45, pp. 1468-1489, July 1999.
[4] A. J. Goldsmith and P. P. Varaiya, Capacity of fading channels with channel side information, IEEE Trans. Inform. Theory, vol. 43, pp. 1986-1992, Nov. 1997.
[5] R. F. H. Fischer and J. B. Huber, A new loading algorithm for discrete multitone transmission, in IEEE Proc. Globecom 1996, (London, UK), pp. 724-728, Nov. 1996.
[6] P. S. Chow, J. M. Cioffi, and J. A. C. Bingham, A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels, IEEE Trans. Commun., vol. 43, pp. 773-775, Feb./Mar./Apr. 1995.
[7] A. N. Barreto and S. Furrer, Adaptive bit loading for wireless OFDM systems, in 12th IEEE Int. Symp. on Personal, Indoor and Mobile Communications (PIMRC) 2001, pp. G88-G92, 2001.
[8] C. Mutti, D. Dahlhaus, T. Hunziker, and M. Foresti, Bit and power loading procedures for OFDM systems with bit-interleaved coded modulation, in IEEE Int. Conf. on Telecommunications (ICT) 2003, (Papeete, French Polynesia), pp. 1422-1427, Feb. 2003.

Type Semester Thesis
Subject area Bit Allocation Scheme, Precoding, MIMO BICM-OFDM Systems
Type of work 60% Theory, 40% Software (Matlab)
Student Luciano Leins
Supervisor Dr. Carlo Mutti, Dr. Etienne Auger
Professor Prof. Dr. Armin Wittneben