Timing estimation for quasi-synchronous SDMA systems

In this paper, timing estimation for quasi-synchronous SDMA systems is considered. The problem arises in the uplink of a mobile communication system where precise synchronization among all mobiles is very difficult to be achieved and signals of all mobiles are time-aligned at the receiver within a small synchronization window. An efficient timing estimation method is proposed. It can simultaneously estimate the channel responses and the time delays which are required for signal detection. Simulation results show that the time delays can be accurately estimated and the performance of MMSE multi-user detection with the proposed method is very close to that with the perfect timing and channel information.published_or_final_versio

Two-step signal detection for MIMO-OFDM systems without cyclic prefix

In this paper, a MIMO-OFDM system without cyclic prefix (CP) is considered and a two-step signal detection algorithm is proposed. The algorithm is based on some structural properties derived from shifting the received OFDM symbols. The first step cancels inter-carrier interference (ICI) and inter-symbol interference (ISI) with an equalizer designed using second-order statistics of the shifted received OFDM symbols. The second step detects the signals from the equalizer output in which the signals are still corrupted with multiantenna interference (MAI). In the proposed algorithm, precise knowledge of the channel length is unnecessary and only one pilot OFDM symbol is utilized to estimate the required channel state information, assuming the number of transmit antennas is smaller than the number of subcarriers in one OFDM symbol. Simulation results show that the proposed algorithm achieves comparable performance to algorithms for MIMO-OFDM system with cyclic prefix and it is robust against channel length overestimation. © 2009 IEEE.published_or_final_versionThe IEEE Wireless Communications and Networking Conference (WCNC) 2009, Budapest, Hungary, 5-8 April 2009. In Proceedings of WCNC, 2009, p. 1-

Semi-blind time-domain equalization for MIMO-OFDM systems

In this paper, a semi-blind time-domain equalization technique is proposed for general multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The received OFDM symbols are shifted by more than or equal to the cyclic prefix (CP) length, and a blind equalizer is designed to completely suppress both intercarrier interference (ICI) and intersymbol interference (ISI) using second-order statistics of the shifted received OFDM symbols. Only a one-tap equalizer is needed to detect the time-domain signals from the blind equalizer output, and one pilot OFDM symbol is utilized to estimate the required channel state information for the design of the one-tap equalizer. The technique is applicable irrespective of whether the CP length is longer than, equal to, or shorter than the channel length. Computer simulations show that the proposed technique outperforms the existing techniques, and it is robust against the number of shifts in excess of the CP length. © 2008 IEEE.published_or_final_versio

Rake-based multiuser detection for quasi-synchronous SDMA systems

In this letter, a Rake-based multiuser detection technique, consisting of multiuser single-path signal separation, time-delay estimation, and multipath combining, is proposed for quasi-synchronous spatial-division multiple-access (SDMA) systems. Time diversity is achieved for performance improvement. In addition, only the upper bounds of the channel length and the time delays are required. Simulation results verify the effectiveness of the proposed technique, as well as its robustness against overestimation of the maximum channel length and the maximum time delay. © 2007 IEEE.published_or_final_versio

Low complexity near-maximum likelihood decoding for MIMO systems

In recent literature, an increasing radii algorithm (IRA) is introduced to decode the signals in multiple-input multiple-output (MIMO) systems. It is developed from the idea of sphere decoder (SD) and can achieve near-maximum likelihood (ML) decoding performance with relatively lower complexity than the SD by taking the noise statistics into consideration. In this paper, an improved IRA (IIRA) is proposed to further reduce the complexity. Apart from the noise statistics, channel information is taken into consideration as well. Additionally, a radii update scheme is introduced which enables the search space of the proposed algorithm to be further pruned. As a result, the proposed algorithm achieves a performance close to that of the IRA but with substantial computational savings. The effectiveness of the proposed decoder is verified by simulations. ©2009 IEEE.published_or_final_versionThe IEEE 20th International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Tokyo, Japan, 13-16 September 2009. In Proceedings of 20th PIMRC, 2009, p. 2429-243

Blind MIMO channel estimation with an upper bound for channel orders

Many known second-order statistics based blind algorithms for MIMO channel estimation are sensitive to channel order overestimations. To overcome this problem, an algorithm is proposed in [1] for SIMO system only, and then a simple generalization of it to MIMO system is presented in [2]. In this paper, improvements and refinements on the algorithm in [2] are given, which makes the method robust to noise and round-off error. The method can give estimations of all channel impulse responses subject to a scalar matrix ambiguity when only an upper bound for all MIMO channel orders is known. © 2005 IEEE.published_or_final_versio

Policy Implications of Demographic Changes in the VHA Veteran Population Following OEF/OIF

Includes article and tables.The US Department of Defense (DoD) and the US Department of Veteran’s Affairs (VA) are the two institutions responsible for the medical care of military personnel and Veterans. DOD major policy and operational personnel changes impact the VA by modifying the characteristics of Veterans presenting at the VA for medical health care. The challenges to VA resulting from DOD changes over the past four decades include managing the large influx to the VA of aging Veterans; accommodating the Veterans from OEF/OIF; responding to the expanded role of female service members in DoD; accepting increased numbers of National Guard and Reserve Components resulting from extended deployment and expanded operational role

Semi-blind CFO, channel estimation and data detection for ofdm systems over doubly selective channels

Proceedings of the IEEE International Symposium on Circuits and Systems, 2010, p. 1887-1890Semi-blind joint CFO, channel estimation and data detection for OFDM systems over doubly selective channels (DSCs) is investigated in this work. A joint iterative algorithm is developed based on the maximum a posteriori expectation-maximization (MAP-EM) algorithm. In addition, a novel algorithm is also proposed to obtain the initial estimates of CFO and channels. Simulation results show that the performance of the proposed CFO and channel estimators approaches to that of the estimators with full training at high SNRs. Moreover, after convergence, the performance of data detection is close to the ideal case with perfect CFO and channel state information. ©2010 IEEE.published_or_final_versionThe IEEE International Symposium on Circuits and Systems (ISCAS), Paris, France, 30 May-2 June 2010. In Proceedings of ISCAS, 2010, p. 1887-189

Blind symbol synchronization based on cyclic prefix for OFDM systems

In this paper, a blind symbol synchronization algorithm is presented for orthogonal frequency-division multiplexing (OFDM) systems, and a new timing function based on the redundancy of the cyclic prefix (CP) is introduced. It proves that the maximum of this function necessarily points to the correct timing offset, irrespective of channel conditions when the signal-to-noise ratio is high. Using the timing function, the timing offset is estimated through a searching algorithm. Channel power profile and channel length information are unnecessary. Simulation results show that the proposed algorithm is robust and outperforms the existing CP-based algorithms, particularly in frequency-selective fading channels. © 2008 IEEE.published_or_final_versio