Channel-independent synchronization of orthogonal frequency division multiple access systems

We develop synchronization algorithms for both the downlink and the uplink of quasi-synchronous and asynchronous orthogonal frequency division multiple access systems. Unlike existing alternatives, the proposed time- and carrier-offset estimators do not require the transmission of known sequences and exhibit performance independent of the underlying channel zero locations. The only necessary assumption is that there are virtual subcarriers which are not occupied by any user. We derive a closed-form variance expression for the carrier-offset estimator at high signal-to-noise ratio (SNR), as a function of the number of active users and the SNR. We compare our method with alternative ones and validate our theoretical derivations with simulation results.

I. Introduction

Multiple access systems based on orthogonal frequency division multiplexing (OFDM) or, more generally, multicarrier (MC) modulation, are capable of insuring perfect elimination of multiuser interference (MUI) and intersymbol interference (ISI) in transmissions over time dispersive channels, using a very simple receiver. As opposed to CDMA systems, where MUI can be cancelled completely only resorting to computationally demanding multiuser detection (MUD) schemes [24], MC systems are able, in principle, to suppress MUI and ISI completely using a simple fast Fourier transform-based receiver [25]. This property is achieved by selecting a duration of the OFDM/MC block smaller than the channel coherence time, so that the channel can be considered stationary within one blocklength, and by inserting between any two successive transmitted blocks a cyclic prefix of duration at least equal to the channel order. However, different from CDMA, OFDM systems are much more sensitive to synchronization errors and to the nonlinearities of the transmitter high power amplifiers. This explains why, although OFDM has been successfully used in broadcasting systems, its generalization to multiple access systems, namely OFDMA, has not seen an equally widespread application yet, although it has been proposed for cable TV [18], broadband radio access networks [3] and for multiuser communications via satellite links [27].