iccit_ofdm_paper1 submitted to ICCIT’2001 8/16/01 : 9:28 PM 1/9 MODIFIED HIGH-EFFICIENCY CARRIER ESTIMATOR FOR OFDM COMMUNICATIONS WITH ANTENNA DIVERSITY UFUK TURELI AND PATRICK J. HONAN Department of Electrical Engineering and Computer Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 E-mail: utureli@stevens-tech.edu Orthogonal frequency division multiplexing (OFDM) based wireless communication systems combined with coding and antenna diversity techniques operate at very low signal-to-noise ratio (SNR) levels. Receivers are generally coherent demodulators implemented around a fast Fourier transform (FFT), which is the efficient implementation of the Discrete Fourier Transform (DFT). Carrier synchronization is critical to the performance of an OFDM system. Tone or sub-carrier orthogonality is lost do to frequency offset error. OFDM carrier frequency offset estimation and subsequent compensation can be performed from the received signal without periodic pilots or preambles. OFDM algebraic structure can be exploited in a blind fashion to estimate carrier offset. However, the performance degrades at low SNR. The algorithm here will allow highly accurate synchronization by exploiting maximum diversity gains to increase effective SNR without reference symbols, pilot carriers or excess cyclic prefix. Furthermore, diversity gains overcome lack of identifiability in the case of channel zeros on the DFT grid. 1 Introduction Next generation wireless communication systems will handle broadband applications and OFDM coupled with antenna diversity has been proposed [1]. Carrier frequency synchronization is critical to the performance of an OFDM system. Tone or sub-carrier orthogonality is lost do to frequency offset error. This results in higher inter-channel-interference (ICI) levels thus lowering signal-to- interference-and-noise ratio (SINR). For OFDM, frequency offsets of as little as 1% begins to result in noticeable penalties in SINR [2]. These effects can be countered by correcting for frequency offset prior to the FFT. This requires the accurate estimation of the frequency offset. OFDM is the standard modulation scheme used in Europe for Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) [9,10]. In addition, local area networks (LANs) such as IEEE 802.11a are OFDM based. These systems are based on earlier developed synchronization methods using known preambles and/or periodic pilots. Pilot based carrier synchronization systems consume bandwidth and power and result in significant intra cell interference. These methods spend valuable bandwidth and power resources and require channel estimation. Since the performance of OFDM is severely degraded in the presence of carrier frequency offset, reliable channel estimation is difficult to perform before carrier frequency offset compensation. Blind synchronization that exploits the structure of