2 research outputs found
An Improved Scheme for Initial Ranging in OFDMA-based Networks
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.Comment: 6 pages, 3 figures, To appear in the Proceedings of the 2008 IEEE
International Conference on Communications, Beijing, May 19 - 23, 200
Timing and Frequency Synchronization in Practical OFDM Systems
Orthogonal frequency-division multiplexing (OFDM) has been adopted by many broadband wireless communication systems for the simplicity of the receiver technique to support high data rates and user mobility. However, studies also show that the advantage of OFDM over the single-carrier modulation schemes could be substantially compromised by timing or frequency estimation errors at the receiver. In this thesis we investigate the synchronization problem for practical OFDM systems using a system model generalized from the IEEE 802.11 and IEEE 802.16 standards. For preamble based synchronization schemes, which are most common in the downlink of wireless communication systems, we propose a novel timing acquisition algorithm which minimizes false alarm probability and indirectly improves correct detection probability. We then introduce a universal fractional carrier frequency offset (CFO) estimator that outperforms conventional methods at low signal to noise ratio with lower complexity. More accurate timing and frequency estimates can be obtained by our proposed frequency-domain algorithms incorporating channel knowledge. We derive four joint frequency, timing, and channel estimators with different approximations, and then propose a hybrid integer CFO estimation scheme to provide flexible performance and complexity tradeoffs. When the exact channel delay profile is unknown at the receiver, we present a successive timing estimation algorithm to solve the timing ambiguity. Both analytical and simulation results are presented to confirm the performance of the proposed methods in various realistic channel conditions. ..