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Channel Tracking in Wireless OFDM Systems
Heiko Schmidt, Volker K¨ hn, and Karl-Dirk Kammeyer u University of Bremen, FB-1, P.O. Box 33 04 40, D-28334 Bremen, Germany, e-mail: schmidt@comm.uni-bremen.de and Reinhard R¨ ckriem and Stefan Fechtel u Infineon Technologies AG, P.O. Box 80 09 49, D-81609 Munich, Germany e-mail: reinhard.rueckriem@infineon.com Abstract— In the presented paper, the principle of frequency domain channel estimation for wireless OFDM systems will be shown. A well known noise reduction technique will be adapted to HIPERLAN/2 and IEEE802.11a standards, and its positive effects will be demonstrated by simulation results. Channel tracking has not been considered by the WLAN standards named above, although it is well known that time-variant indoor radio channels can change their characteristics within one PHY burst. This paper presents some techniques for decision directed channel tracking, applicable in wireless OFDM systems. Keywords— HIPERLAN, IEEE802.11, OFDM, channel estimation, channel tracking, noise reduction

I. I NTRODUCTION The American IEEE802.11a standard and the European equivalent HIPERLAN/2 are two similar concepts for broadband wireless LANs (WLAN) in the 5 GHz band. Both standards are based on the multicarrier modulation technique OFDM (orthogonal frequency division multiplexing) combined with convolutional channel coding. The baseband modulation schemes of both standards are very similar, which simplifies implementation considerably. Challanges and difficulties considered in this paper regard both systems. Except for slight differences in signal mapping, most discrepancies between the standards regard the higher protocol layers. Section II presents some fundamentals of OFDM and the WLAN standards. Here we focus on the baseband modulation in the PHY layer and explain parts of the PHY burst structure relevant to channel estimation. Section III describes a frequency domain channel estimator. Assuming channel impulse responses being limited in time, correlations between adjacent subcarriers can reduce the noise influence on the estimated transfer function. Here, a new method for computing the correlations is shown. In case of time variant channel coefficients, a decision directed channel tracking algorithm for re-estimating the channel coefficients is presented in section IV. The remodulation of the detected data can be done with or without exploiting channel decoding as demonstrated in section IV. II. W IRELESS LAN OFDM SYSTEMS

multi-carrier (MC) technique OFDM [1], [2], [3]. Primarily, OFDM can be described as an analog discrete multitone technique with rectangular (orthogonal) pulse shaping filters for each subcarrier. A guard interval protects the received data against inter-symbol- (ISI) or inter-carrierinterference (ICI). Practically, discrete transmitter and receiver filter banks are used and computed by very efficient FFT algorithms. Concerning the considered standards, the total OFDM symbol duration is s including a s guard interval and the s core symbol. The active subcarriers are placed symmetrically (no DC component). With a subcarrier distance of kHz the total occupied OFDM bandwidth is about 16.5 MHz. Using an point FFT algorithm (oversampling rate ), the required time domain sample rate is exactly MHz.

Fig. 1. Time discrete OFDM system

Due to ISI- and ICI-free received symbols, the channel influence can be reduced to one complex Rayleigh fading factor (channel coefficient) on each subcarrier

As mentioned in the introduction, the new WLAN standards HIPERLAN/2 and IEEE802.11a are based on the

where denotes the data symbol of the subcarrier and the OFDM symbol ( : frequency index, : time index). Assuming a slow fading channel, the transfer function is nearly constant for the duration of one OFDM symbol. In case of a time invariant channel transfer function,

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