A NEW TIME-FREQUENCY SYNCHRONIZATION SCHEME FOR OFDM-TDMA SYSTEMS zyx Jose M. Paez-Borrallo, Santiago zyxwv Zazo, M. Julia Fernandez-Getino ETS Ingenieros de Telecomunicacion zyxwv - Universidad Politkcnica de Madrid. Ciudad Universitaria zyxwv sh, Madrid 28040, Spain Phone: 34-91-3367280; Fax: 34-91- 3367350; e-mail: paez@gaps.ssr.upm.es Abstract - Bursty data transmission over OFDM systems requires a rapid time-fiequency synchronization using a desirable short-time training sequence. Several sophisticated methods provide frame and carrier recovery in two symbols. We formulate these methods with a common notation and this leads us to address an oversampled scheme which allows synchronization in only one symbol. In our proposal, the limit of the acquisition range for the carrier fiequency offset can be extended to zyxwvu *N the subcarrier spacing instead of zyxwvut fl the subcarrier spacing as previous methods. Discussions about its robustness against channel and noise are included and supported by several representative computer simulations. I. INTRODUCTION Orthogonal Frequency-Division Multiplexing (OFDM) systems is a large interesting alternative to single carrier schemes using either a continuous stream of data as in a broadcast application (DVB, DAB) or a burst operation mode as in a Wireless Local Area Network over a fiequency selective channel. The main advantage of the OFDM transmission technique is the fact that there is no intersymbol interference (ISI) at all, if a long enough Cyclic Prefix ,CP, is included. This effectively simulates a channel performing cyclic convolution which implies orthogonality over dispersive channels. Synchronization of the symbol timing and carrier frequency offset is probably the most difficult task in these systems because they must be performed very accurately or there will be loss of orthogonality between the subsymbols placed at the different fiequencies. It is well known that OFDM systems are very sensitive to carrier frequency o&ets since they can only tolerate which are a small fiaction of the subcarrier spacing without a large degradation in system performance [I]. Several methods for time-frequency acquisition have been developed in the recent years (see [2] and references therein). Usually, symbol-timing search is performed first by using correlation methods; in practice, time searching must be evaluated accurately in order to set optimal conditions for fiequency acquisition. In our approach, we will propose a two steps procedure which obtain a very accurate time estimation. Frequency offset compensation in burst operation mode is addressed, in most of the methods, on the phase comparative between two identical halves in one training symbol; these schemes provide the correction within a fiactional part of the carrier spacing and includes an extra symbol to correct the remaining frequency offset as an integer number of the subcarriers frequency separation [2,3]. Our proposal is suitable for burst mode applications where using more that one training symbol is not recommended because the loss in the system throughput. Our approach formulates the basic equations of methods dealing with the duplication of the first symbol in two identical halves with matrix notation and the concept of Decimation in Frequency DFT Algorithms zyxw [4]. This formulation improves the knowledge about the performance of these methods and it also allows a new synchronization scheme by means of the concept of Decimation in Time DFT Algorithms. Our final scheme extends the acquisition range over the transmission bandwidth by just oversampling the received sequence by a factor of two. The behavior of this method and, in particular, its robustness against additive white Gaussian noise and channel characteristics are also discussed and supported by several representative computer simulations. 11. FORMULATION OF THE OFDM SYNCHRONIZATION SYSTEM Let us consider the following block diagram in Fig. 1 which remarks the main topics related to the 0-7803-5565-2/99/$10.00 0 1999 IEEE 2408