Umic Announcement: Past And Future Challenges In Synchronization And Channel Estimation For Ofdm Systems

UMIC

Ultra high-speed Mobile Information and Communication

UMIC/Mobile Network Performance guest seminar

Prof. Christian Oberli Pontificia Universidad Católica de Chile

Past and future challenges in synchronization and channel estimation for OFDM systems Commercial deployment of wireless communication systems based on the Orthogonal Frequency Division Multiplexing (OFDM) modulation technique has been going on for nearly 15 years. Since its early use in point-to-multipoint digital television and single-user, single-antenna wireless local area networks (WLANs), OFDM has become part of multi-antenna WLANs and is at the heart of future multi-user cellular systems with base station coordination. Throughout this evolution, a key condition for the viability of OFDM has been -and continues to be- the need of highly accurate synchronization and channel estimation. The performance of OFDM gets degraded in two ways by impairments such as carrier frequency offset, channel variation over time and, to a lesser extent, sampling frequency offset: they distort the transmitted symbols in magnitude and phase and spread their energy as Inter-Carrier Interference (ICI) over the remaining subcarriers. The literature reports many signal models for describing these effects for each kind of impairment. A few models describe OFDM signals with two of the three impairments. But despite the variety of approaches, the derivations frequently use approximations and the models are often incompatible with each other, thus making most of them finally unfit for designing algorithms with the high level of accuracy needed. This talk will present the steps that lead to the derivation of a general and mathematically exact signal model for the joint effect of the three impairments mentioned. Following OFDM's own evolution, the first part of the talk will focus on the problem of synchronization tracking in singleuser, multi-antenna 802.11n-like transmissions. An extension of the model that includes the effect of time-varying channels is presented in the second part, along with a vis-a-vis comparison of signal-to-ICI ratios caused by each impairment. The third part of the talk will outline some of the challenges that the development of future OFDM-based systems faces, and will give hints at how the general signal model will be a valuable tool in that quest.

Tuesday 27 January 2009, 3:00 p.m. UMIC Research Centre, Room 025, Mies-van-der-Rohe-Str. 15