A Robust Ranging Scheme for OFDMA-Based Networks

arXiv:0903.3000v1 [cs.IT] 17 Mar 2009 A Robust Ranging Scheme for OFDMA-Based Networks Michele Morelli, Senior Member, IEEE, Luca Sanguinetti, Member, IEEE, and H. Vincent Poor, Fellow, IEEE. Abstract Uplink synchronization in orthogonal frequency-division multiple-access (OFDMA) systems is a challenging task. In IEEE 802.16-based networks, users that intend to establish a communication link with the base station must go through a synchronization procedure called Initial Ranging (IR). Existing IR schemes aim at estimating the timing offsets and power levels of ranging subscriber stations (RSSs) without considering possible frequency misalignments between the received uplink signals and the base station local reference. In this work, we present a novel IR scheme for OFDMA systems where carrier frequency offsets, timing errors and power levels are estimated for all RSSs in a decoupled fashion. The proposed frequency estimator is based on a subspace decomposition approach, while timing recovery is accomplished by measuring the phase shift between the users’channel responses over adjacent subcarriers. Computer simulations are employed to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives. Index Terms OFDMA, ranging process, timing and frequency synchronization, power estimation. M. Morelli and L. Sanguinetti are with the University of Pisa, Department of Information Engineering, Via Caruso 56126 Pisa, Italy (e-mail: michele.morelli@iet.unipi.it, luca.sanguinetti@iet.unipi.it). This work was completed while L. Sanguinetti was with Princeton University and it was supported by the U.S. National Science Foundation under Grants ANI-03-38807 and CNS-06-25637. This paper was presented in part at the IEEE International Conference on Communications (ICC), Beijing, China, 2008. H. Vincent Poor is with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA (e-mail: poor@princeton.edu) July 13, 2021 DRAFT 1 I. INTRODUCTION The demand for high data rates in wireless communications has led to a strong interest in multicarrier modulation techniques, and particularly in orthogonal frequency-division multiple- access (OFDMA), which has become part of the IEEE 802.16 family of standards for wireless metropolitan area networks (WMANs) [1]. Despite its many appealing features, OFDMA is extremely sensitive to timing errors and carrier frequency offsets (CFOs). The former give rise to interblock interference (IBI), while the latter produce interchannel interference (ICI) as well as multiple access interference (MAI). To cope with such impairments, the IEEE 802.16 standards specify a synchronization procedure called Initial Ranging (IR) by which users adjust their transmission parameters so that uplink signals arrive at the base station (BS) synchronously and with approximately the same power level. In its basic form, the IR process develops through the following steps. First of all, each ranging subscriber station (RSS) computes frequency and timing estimates on the basis of a downlink control channel. The estimated parameters are used in the subsequent uplink phase, during which each RSS transmits a randomly chosen code over a ranging time-slot. As a consequence of the different users’ positions within the cell, uplink signals arrive at the BS at different time instants. Furthermore, since the ranging code is randomly selected, several users may collide over a same time-slot. After identifying colliding codes and extracting timing and power information, the BS will broadcast a response message indicating which codes have been detected and giving instructions for timing and power adjustment. From the above discussion, the main functions of the BS during the ranging process may be classified as multiuser code detection and multiuser timing/power estimation. Some methods to accomplish these tasks were originally suggested in [2] and [3]. In these works, a long pseudo- noise (PN) code is transmitted by each RSS over all available ranging subcarriers. Code detection and timing recovery is then accomplished on the basis of suitable correlations computed in either the frequency or time domains. This approach requires huge computational complexity since one correlation must be evaluated for each possible ranging code and hypothesized timing offset. Moreover, in the presence of multipath distortions ranging subcarriers are subject to different attenuations and phase shifts, thereby leading to a loss of the code orthogonality. This gives rise to MAI, which severely degrades the system performance. Alternative solutions can be found July 13, 2021 DRAFT 2 in [4] and [5]. In particular, the method in [4] replaces the PN ranging codes with a set of modified generalized chirp-like (GCL) sequences and mitigates the effects of channel distortion through differential detection of the ranging signals. Unfortunately, this approach is still plagued by significant MAI. Th

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