| 1. |
- Mohammed, Saif Khan, 1977-, et al.
(författare)
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MIMO Precoding with X- and Y- Codes
- 2011
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Ingår i: IEEE Transactions on Information Theory. - New Jersey, USA : IEEE. - 0018-9448 .- 1557-9654. ; 57:6, s. 3542-3566
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Tidskriftsartikel (refereegranskat)abstract
- We consider a slow fading nt x nr multiple-input multiple-output (MIMO) system with channel state information (CSI) at both the transmitter and receiver. Since communication in such scenarios is subject to block fading, reception reliability, quantified in terms of the achievable diversity gain, is of importance. A simple and well known precoding scheme is based upon the singular value decomposition (SVD) of the channel matrix, which transforms the MIMO channel into parallel subchannels. Despite having low maximum likelihood decoding (MLD) complexity, this SVD based precoding scheme provides a diversity gain which is limited by the diversity gain of the weakest subchannel. We therefore propose X- and YCodes, which improve the diversity gain of the SVD precoding scheme, by jointly coding information across a pair of subchannels (i.e., pairing subchannels). In particular, subchannels with high diversity gain are paired with those having low diversitygain. A pair of subchannels is jointly encoded using a 2 x 2 real matrix, which is fixed a priori and does not change with each channel realization. For X-Codes, these matrices are 2-dimensional rotation matrices parameterized by a single angle, while for Y-Codes, these matrices are 2-dimensional upper left triangular matrices. Also, since joint coding is performed only across a pair of subchannels, the joint MLD complexity remains low. In particular, the MLD complexity of Y-Codes is even lower than that of X-Codes, and is equivalent to symbol by symbol detection. Moreover, we propose X-, Y-Precoders with the same structure as X-, Y-Codes, but with encoding matrices adapted to each channel realization. With respect to the error probability performance, the optimal encoding matrices for X-, YCodes/ Precoders are derived analytically and numerically. Whencompared to other precoding schemes reported in the literature, it is observed that X-Codes/Precoders perform better in wellconditioned channels, while Y-Codes/Precoders perform better in ill-conditioned channels.
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| 2. |
- Mohammed, Saif Khan, 1977-, et al.
(författare)
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Precoding by Pairing Subchannels to Increase MIMO Capacity with Discrete Input Alphabets
- 2011
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Ingår i: IEEE Transactions on Information Theory. - New Jersey, USA : IEEE. - 0018-9448 .- 1557-9654. ; 57:7, s. 4156-4169
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Tidskriftsartikel (refereegranskat)abstract
- We consider Gaussian multiple-input multiple-output (MIMO) channels with discrete input alphabets. We propose a non-diagonal precoder based on the X-Codes in \cite{Xcodes_paper} to increase the mutual information. The MIMO channel is transformed into a set of parallel subchannels using Singular Value Decomposition (SVD) and X-Codes are then used to pair the subchannels. X-Codes are fully characterized by the pairings and a $2\times 2$ real rotation matrix for each pair (parameterized with a single angle). This precoding structure enables us to express the total mutual information as a sum of the mutual information of all the pairs. The problem of finding the optimal precoder with the above structure, which maximizes the total mutual information, is solved by {\em i}) optimizing the rotation angle and the power allocation within each pair and {\em ii}) finding the optimal pairing and power allocation among the pairs. It is shown that the mutual information achieved with the proposed pairing scheme is very close to that achieved with the optimal precoder by Cruz {\em et al.}, and is significantly better than Mercury/waterfilling strategy by Lozano {\em et al.}. Our approach greatly simplifies both the precoder optimization and the detection complexity, making it suitable for practical applications.
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| 3. |
- Saif Khan, Mohammed, et al.
(författare)
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Modulation Diversity in Fading Channels with a Quantized Receiver
- 2012
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Ingår i: Information Theory Proceedings (ISIT), 2011 IEEE International Symposium on. - : Institute of Electrical and Electronics Engineers (IEEE). - 2157-8095. ; 11:1, s. 2949-2953
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we address the design of codes which achieve modulation diversity in block fading single-input single-output (SISO) channels with signal quantization at the receiver. With an unquantized receiver, coding based on algebraic rotations is known to achieve maximum modulation coding diversity. On the other hand, with a quantized receiver, algebraic rotations may not guarantee gains in diversity. Through analysis, we propose specific rotations which result in the codewords having equidistant component-wise projections. We show that the proposed coding scheme achieves maximum modulation diversity with a low-complexity minimum distance decoder and perfect channel knowledge. Relaxing the perfect channel knowledge assumption we propose a novel channel training/estimation technique to estimate the channel. We show that our coding/training/estimation scheme and minimum distance decoding achieves an error probability performance similar to that achieved with perfect channel knowledge.
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| 4. |
- Zaki, Ahmed, et al.
(författare)
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A Training-Based Iterative Detection/Channel Estimation Scheme for Large Non-Orthogonal STBC MIMO Systems
- 2009
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Ingår i: IEEE International Conference on Communications. ; , s. 1-5
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Konferensbidrag (refereegranskat)abstract
- In this paper, we propose a training-based channel estimation scheme for large non-orthogonal space-time block coded (STBC) MIMO systems. The proposed scheme employs a block transmission strategy where an Nt times Nt pilot matrix is sent (for training purposes) followed by several Nt times Nt square data STBC matrices, where Nt is the number of transmit antennas. At the receiver, we iterate between channel estimation (using an MMSE estimator) and detection (using a low-complexity likelihood ascent search (LAS) detector) till convergence or for a fixed number of iterations. Our simulation results show that excellent bit error rate and nearness-to-capacity performance are achieved by the proposed scheme at low complexities. The fact that we could show such good results for large STBCs (e.g., 16times16 STBC from cyclic division algebras) operating at spectral efficiencies in excess of 20 bps/Hz (even after accounting for the overheads meant for pilot-based channel estimation and turbo coding) establishes the effectiveness of the proposed scheme.
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| 5. |
- Zaki, Ahmed, et al.
(författare)
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Joint channel estimation/detection in MIMO systems using belief propagation
- 2009
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Ingår i: IEEE 20th International Symposium on Personal, Indoor and Mobile Radio Communications. ; , s. 466-470
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Konferensbidrag (refereegranskat)abstract
- We study the problem of joint channel estimation and detection in MIMO systems using belief propagation (BP). We propose a BP based algorithm which jointly estimates the MIMO channel matrix and detects the data symbols in a frame (consisting of pilot and data symbols) through message passing in two passes; one pass from the start to the end of a frame and another from the end to the start. A message comprises a small number of parameters that characterize the computed densities of the marginal probabilities. We study the performance of the proposed algorithm in V-BLAST and STBC MIMO systems for varying pilot density and pilot/data power allocation in a frame. Our simulation results show that a close to perfect CSIR performance is achieved by the proposed algorithm using a nonuniform pilot/data power allocation scheme based on the knowledge of the average channel SNR. We show that the proposed BP based joint channel estimation/detection scheme performs better than MMSE channel estimation followed by maximum-likelihood (ML) detection in slow fading.
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