  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.
