[show abstract][hide abstract] ABSTRACT: We consider a coordinated multi-cell (CMC) system and the associated problem of independent carrier frequency offsets (CFOs) at the basestations (BSs). These BS CFOs cause accumulated phase errors that compromise downlink beamforming accuracy, and consequently degrade the spectral efficiency of the CMC system. Since the inherent structure of coordinated downlink beamforming techniques makes it impossible to correct for the BS CFOs at the mobile subscriber (MS), our topic is estimation and correction of the BS CFOs at the BSs. Our method begins with the formation of MS-side estimates of the BS CFOs, which are then fed back to the coordinated BSs. We then derive an optimum maximum likelihood (ML) estimator for the BS CFOs that uses independent MS-side CFO estimates and average channel signal-to-noise ratios. However, it is demonstrated that the CFOs of the MSs themselves introduce a bias to the optimal BS CFO estimator. To compensate for this bias, a joint BS and MS CFO estimator is derived, but shown both to require high computation and have rank deficiency. This motivates an improved technique that removes the rank problem and employs successive estimation to reduce computation. It is demonstrated to overcome the MS CFO bias and efficiently solve the joint BS and MS CFO problem in systems that have low to moderate shadowing. We term the full BS CFO estimation and correction procedure "BS CFO tightening".
IEEE Journal on Selected Areas in Communications 01/2011; · 3.12 Impact Factor
[show abstract][hide abstract] ABSTRACT: The joint estimation of multiple carrier frequency offsets (CFOs) and channel state information (CSI) in a multibase MIMO downlink system is considered here for the case of a fading channel. The multiple CFOs result from independent local oscillators at each basestation (BS), and cause accumulation of phase errors that compromise beamforming accuracy. Using previously devised multiple CFO estimation techniques for simultaneously transmitted training sequences in a non-fading channel, we begin by examining the effects of fading on a linear channel state estimate. An example illustrates that a fade rate threshold can be defined after which linear channel estimation is infeasible. Because of the dependence of the CFO estimators on the CSI estimate, this threshold applies to the CFO estimates as well. As a way to combat the channel variation, the maximum likelihood metric is modified to accommodate a parameterized fading channel estimate.
[show abstract][hide abstract] ABSTRACT: We consider downlink MIMO beamforming from several coordinated basestations (BSs), and the associated problem of in dependent carrier frequency offsets (CFOs) at the BSs which cause accumulated phase errors to compromise beamforming accuracy. Correction of the CFOs requires estimation of their values, so our topic is multiple CFO estimation, a little-explored area. We present a robust and easily generalized estimator that accounts for the training sequence (TS) correlations caused by the CFOs, and show that it meets the Cramer-Rao lower bound (CRLB) at moderate signal-to-noise ratios (SNRs). The performance of the estimator is contingent upon TSs short enough to ensure convexity of the log-likelihood over the allowable CFO ranges. For combinations of TS length and CFO range that violate this constraint, we present two suboptimal estimators based on segmentation of the TS, both of which also meet the CRLB at moderate to high SNRs.
IEEE Journal on Selected Areas in Communications 09/2008; · 3.12 Impact Factor
[show abstract][hide abstract] ABSTRACT: The effect of carrier frequency offset (CFO) in a coordinated multibase MIMO downlink system is considered. We begin with evidence that uncompensated base station (BS) CFOs can significantly reduce the achievable capacity of such systems. Correction of the CFOs is desirable, and a critical step in doing so is estimation of their values. We present a method by which mobile subscribers (MSs) can estimate multiple CFOs from training sequences received from the BSs. Compared to existing estimators, the proposed estimator gives better accuracy with a lower transmit power.
Vehicular Technology Conference, 2008. VTC Spring 2008. IEEE; 06/2008
[show abstract][hide abstract] ABSTRACT: We consider a narrowband multiuser system in which several transmitting users are received through a synchronous, flat fading channel at an antenna array. The paper introduces a multiuser detection technique that combines group wise detection with iteration. It is well suited to overloaded conditions, where there are more transmitter antennas than receive antennas. The soft decisions it uses internally also make it suitable as a detector in a concatenated structure. Its performance and computation can be traded off through selection of group size. A variant of the algorithm achieves further computation reduction by incorporating a soft sphere detection core.
IEEE Transactions on Wireless Communications 03/2007; · 2.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper applies extrinsic information (EXIT) analysis to an internally iterative multiuser detector (IMUD) in serial concatenation with convolutional codes. The focus is overloaded conditions, in which the number of transmitters exceeds the number of receiver antennas. The study seeks to verify if EXIT charts can be used as a design tool under these circumstances. The use of non-Gaussian LLR distributions in performance estimation is discussed. The mutual information of the extrinsic information from IMUD is characterized under a variety of situations. It is shown that the EXIT charts do provide a means to determine both the minimum SNR and necessary number of iterations for convergence. However, estimating the bit error rate from the EXIT chart does not seem to be possible except in special circumstances.
Communications, 2006. ICC '06. IEEE International Conference on; 07/2006
[show abstract][hide abstract] ABSTRACT: We consider a multiuser system in which the interfering users have the same pulse shape and are received in a synchronous, flat fading channel at an antenna array. This paper introduces a new iterative group-wise multiuser detection technique (IMUD) that gives a tradeoff of complexity and performance between joint maximum likelihood (JML) detection and zero forcing (ZF) MUD. It is shown that in the overloaded case the new IMUD algorithm significantly outperforms MMSE V-BLAST. Even with an equal number of users and receive antennas, IMUD gives better performance within two iterations. An additional feature of IMUD is that it inherently produces soft-output decisions.
Personal, Indoor and Mobile Radio Communications, 2004. PIMRC 2004. 15th IEEE International Symposium on; 10/2004