An Improved Scheme for Initial Ranging in OFDMA-based Networks

📝 Abstract

An efficient scheme for initial ranging has recently been proposed by X. Fu et al. in the context of orthogonal frequency-division multiple-access (OFDMA) networks based on the IEEE 802.16e-2005 standard. The proposed solution aims at estimating the power levels and timing offsets of the ranging subscriber stations (RSSs) without taking into account the effect of possible carrier frequency offsets (CFOs) between the received signals and the base station local reference. Motivated by the above problem, in the present work we design a novel ranging scheme for OFDMA in which the ranging signals are assumed to be misaligned both in time and frequency. Our goal is to estimate the timing errors and CFOs of each active RSS. Specifically, CFO estimation is accomplished by resorting to subspacebased methods while a least-squares approach is employed for timing recovery. Computer simulations are used to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives.

💡 Analysis

An efficient scheme for initial ranging has recently been proposed by X. Fu et al. in the context of orthogonal frequency-division multiple-access (OFDMA) networks based on the IEEE 802.16e-2005 standard. The proposed solution aims at estimating the power levels and timing offsets of the ranging subscriber stations (RSSs) without taking into account the effect of possible carrier frequency offsets (CFOs) between the received signals and the base station local reference. Motivated by the above problem, in the present work we design a novel ranging scheme for OFDMA in which the ranging signals are assumed to be misaligned both in time and frequency. Our goal is to estimate the timing errors and CFOs of each active RSS. Specifically, CFO estimation is accomplished by resorting to subspacebased methods while a least-squares approach is employed for timing recovery. Computer simulations are used to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives.

📄 Content

arXiv:0802.4344v1 [cs.IT] 29 Feb 2008 An Improved Scheme for Initial Ranging in OFDMA-based Networks Luca Sanguinetti* and Michele Morelli Department of Information Engineering University of Pisa Pisa, Italy luca.sanguinetti, michele.morelli@iet.unipi.it H. Vincent Poor Department of Electrical Engineering Princeton University Princeton, NJ USA poor@princeton.edu Abstract— An efficient scheme for initial ranging has recently been proposed by X. Fu et al. in the context of orthogonal frequency-division multiple-access (OFDMA) networks based on the IEEE 802.16e-2005 standard. The proposed solution aims at estimating the power levels and timing offsets of the ranging subscriber stations (RSSs) without taking into account the effect of possible carrier frequency offsets (CFOs) between the received signals and the base station local reference. Motivated by the above problem, in the present work we design a novel ranging scheme for OFDMA in which the ranging signals are assumed to be misaligned both in time and frequency. Our goal is to estimate the timing errors and CFOs of each active RSS. Specif- ically, CFO estimation is accomplished by resorting to subspace- based methods while a least-squares approach is employed for timing recovery. Computer simulations are used to assess the effectiveness of the proposed solution and to make comparisons with existing alternatives. I. INTRODUCTION The main impairment of an orthogonal frequency-division multiple-access (OFDMA) network is represented by its re- markable sensitivity to timing errors and carrier frequency offsets (CFOs) between the uplink signals and the base station (BS) local references. For this reason, the IEEE 802.16e-2005 standard for OFDMA-based wireless metropolitan area net- works (WMANs) specifies a synchronization procedure called Initial Ranging (IR) where subscriber stations that intend to establish a link with the BS can use some dedicated subcarriers to transmit their specific ranging codes [1]. Once the BS has revealed the presence of ranging subscriber stations (RSSs), it has to estimate some fundamental parameters including timing errors, CFOs and power levels. Two prominent schemes for initial synchronization and power control in OFDMA were proposed in [2] and [3]. In these works, a long pseudo-noise sequence is transmitted by each RSS over the available ranging subcarriers. Timing recov- ery is then accomplished on the basis of suitable correlations computed in the frequency- and time-domain, respectively. The main drawback of these methods is their sensitivity to *This work was completed while the author was with Princeton University and it was supported by the U.S. National Science Foundation under Grants ANI-03-38807 and CNS-06-25637. multipath distortion, which destroys orthogonality among the employed codes and gives rise to multiple access interference (MAI). Better results are obtained in [4] by using a set of generalized chirp-like (GCL) sequences, which echibits increased robustness against the channel selectivity. A differ- ent approach to managing the IR process has recently been proposed in [5]. Here, the pilot streams transmitted by RSSs are spread in the time-domain over adjacent OFDM blocks using orthogonal codes. In this way, signals of different RSSs can be easily separated at the BS as they remain orthogonal after propagating through the channel. Timing information is eventually acquired in an iterative fashion by exploiting the autocorrelation properties of the received samples induced by the use of the cyclic prefix (CP). Unfortunately, this scheme is derived under the assumption of perfect frequency alignment between the received signals and the BS local reference. Actually, the occurrence of residual CFOs results into a loss of orthogonality among ranging codes and may lead to severe degradations of the system performance in terms of mis- detection probability and estimation accuracy. In the present work we propose a novel ranging scheme for OFDMA systems that is robust to time and frequency misalignments. The goal is to estimate timing errors and CFOs of all active RSSs. The number of active codes is found by resorting to the minimum description length (MDL) principle [6] while the multiple signal classification (MUSIC) algorithm [7] is employed to detect which codes are actually active and to determine their corresponding CFOs. Timing estimation is eventually achieved through least-squares (LS) methods. Although the proposed solution allows one to esti- mate the timing errors of each RSS in a decoupled fashion, it may involve huge computational burden in applications characterized by large propagation delays. For this reason, we also present an alternative scheme derived from ad hoc- reasoning which results into substantial computational saving. It is worth noting that timing synchronization in OFDMA uplink transmissions has received little attention so far. A well-established way to handle timing errors is to design th

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