Browsing by Author "Lu, Hsiao-Feng (Francis)"

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An algebraic look into MAC-DMT of lattice space-time codes
(2011)
Vehkalahti, Roope
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Lu, Hsiao-Feng (Francis)
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper we are concentrating on the diversity-multiplexing gain trade-off (DMT) of some space-time lattice codes. First we give a DMT bound for lattice codes having restricted dimension. We then recover the well known results of the DMT of algebraic number field codes and the Alamouti code by using the union bound and see that these codes do achieve the previously mentioned bound. During our analysis interesting connections to the Dedekind\'s zeta-function and to Dirichlet\'s unit theorem are revealed. Finally we prove that both the number field codes and Alamouti code are in some sense optimal codes in the multiple access channel (MAC).
An Algebraic Tool for Obtaining Conditional Non-Vanishing Determinants
(2009)
Hollanti, Camilla
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Lu, Hsiao-Feng (Francis)
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Vehkalahti, Roope
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電信工程研究所
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Institute of Communications Engineering
An algebraic tool from the theory of central simple algebras is proposed to obtain families of complex matrices satisfying the conditional non-vanishing determinant (CNVD) property. Such property is of great use in e.g. the design of multiuser space-time (ST) codes, in which context it is not always crucial for the transmission matrix to be invertible. On the other hand, whenever it is invertible, it is important that it has a non-vanishing determinant. Also any submatrix of any subset of users multiplied with its transpose conjugate should preferably have a non-vanishing determinant, provided it is non-zero. In recent submissions by Lu et al. it has been shown that, with suitable multiplexing, such property yields a construction of space-time codes that achieve the optimal diversity-multiplexing tradeoff (DMT) of the multiple-input multiple-output (MIMO) multiple access channel (MAC) and outperform the previously known ST codes.
Construction Methods for Asymmetric and Multiblock Space-Time Codes
(2009-03-01)
Hollanti, Camilla
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Lu, Hsiao-Feng (Francis)
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電信工程研究所
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Institute of Communications Engineering
In this paper, the need for the construction of asymmetric and multiblock space-time codes is discussed. Above the trivial puncturing method, i.e., switching off the extra layers in the symmetric multiple-input multiple-output (MIMO) setting, two more sophisticated asymmetric construction methods are proposed. The first method, called the block diagonal method (BDM), can be converted to produce multiblock space-time codes that achieve the diversity-multiplexing tradeoff (DMT). It is also shown that maximizing the density of the newly proposed block diagonal asymmetric space-time (AST) codes is equivalent to minimizing the discriminant of a certain order, a result that also holds as such for the multiblock codes. An implicit lower bound for the density is provided and made explicit for an important special case that contains e.g., the systems equipped with 4Tx+2Rx antennas. Further, an explicit scheme achieving the bound is given. Another method proposed here is the Smart Puncturing Method (SPM) that generalizes the subfield construction method proposed in earlier work by Hollanti and Ranto and applies to any number of transmitting and lesser receiving antennas. The use of the general methods is demonstrated by building explicit, sphere decodable codes using different cyclic division algebras (CDAs). Computer simulations verify that the newly proposed methods can compete with the trivial puncturing method, and in some cases clearly outperform it. The conquering construction exploiting maximal orders improves upon the punctured perfect code and the DjABBA code as well as the Icosian code. Also extensive DMT analysis is provided.
Constructions of multiblock space-time coding schemes that achieve the diversity-multiplexing tradeoff
(2008-08-01)
Lu, Hsiao-Feng (Francis)
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電信工程研究所
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Institute of Communications Engineering
Constructions of multiblock space-time coding schemes that are optimal with respect to diversity-multiplexing (D-M) tradeoff when coding is applied over any number of fading blocks are presented in this correspondence. The constructions are based on,a left-regular representation of elements in some cyclic division algebra. In particular, the main construction applies to the case when the quasi-static fading interval equals the number of transmit antennas, hence the resulting scheme is termed a minimal delay multiblock space-time coding scheme. Constructions corresponding to the cases of nonminimal delay are also provided. As the number of coded blocks approaches infinity, coding schemes derived from the proposed constructions can be used to provide a reliable multiple-input multiple-output (MIMO) communication with vanishing error probability.
Inverse Determinant Sums and Connections Between Fading Channel Information Theory and Algebra
(2013-09-01)
Vehkalahti, Roope
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Lu, Hsiao-Feng (Francis)
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Luzzi, Laura
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電機工程學系
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Department of Electrical and Computer Engineering
This work considers inverse determinant sums, which arise from the union bound on the error probability, as a tool for designing and analyzing algebraic space-time block codes. A general framework to study these sums is established, and the connection between asymptotic growth of inverse determinant sums and the diversity-multiplexing gain tradeoff is investigated. It is proven that the growth of the inverse determinant sum of a division algebra-based space-time code is completely determined by the growth of the unit group. This reduces the inverse determinant sum analysis to studying certain asymptotic integrals in Lie groups. Using recent methods from ergodic theory, a complete classification of the inverse determinant sums of the most well-known algebraic space-time codes is provided. The approach reveals an interesting and tight relation between diversity-multiplexing gain tradeoff and point counting in Lie groups.
Minimal-Rate Description for Multiple-Access Channels
(2012)
Huang, Sue-May
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Lu, Hsiao-Feng (Francis)
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Moser, Stefan M.
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傳播研究所
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Institute of Communication Studies
In this paper we consider the problem of a two-user multiple-access channel that is different from the conventional one. We require the receiver to provide only a reliable information-mixture of the incoming messages from the two users, not the exact contents. The minimum rate of such mixture is termed the minimal description rate of the channel output. Answers to this problem have many potential applications to the latest network-coding-based wireless communication networks. Exact capacity regions of a class of fading multiple-access channels are determined, along with the corresponding deterministic capacity-achieving coding schemes.
Optimal Distributed Codes for Feedback-Aided Cooperative Relay Networks
(2016-07)
Lu, Hsiao-Feng (Francis)
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電機學院
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College of Electrical and Computer Engineering
A novel transmission scheme for cooperative relay networks is presented in this paper. The proposed scheme is based on the non-orthogonal selection decode-and-forward protocol with an additional assumption of having a low rate feedback channel from the destination to relays. Benefited from the feedback information, an optimal distributed code that has an extremely short delay equal to four is constructed, and the same code is applicable to networks with the arbitrary number of relays to yield optimal cooperative diversity. The proposed code is sphere decodable with a decoding complexity again independent of the number of relays in high SNR regime. In particular, when operating at multiplexing gain >=(1/2), the lattice decoder at the destination has a zero complexity exponent, meaning a constant decoding complexity and independent of transmission rate. Analyses for the decoding complexity of other existing diversity-optimal distributed codes are also provided. It is shown that these codes have a linear growth in delay and an exponential growth in decoding complexity as the number of relays increases.
Optimal Diversity-Multiplexing Tradeoff and Code Constructions of Some Constrained Asymmetric MIMO Systems
(2010-05-01)
Lu, Hsiao-Feng (Francis)
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Hollanti, Camilla
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電機工程學系
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Department of Electrical and Computer Engineering
In multiple-input-multiple-output (MIMO) communications, the notion of asymmetric channel refers to the situation when the number of transmit antennas is strictly larger than the number of receive antennas. Such channels can often be found in MIMO downlink transmissions. While existing cyclic-division-algebra (CDA)-based codes can still be employed to achieve the optimal diversity-multiplexing tradeoff (DMT) at high signal-to-noise ratio (SNR) regime, such codes cannot be directly decoded using, for example, the pure sphere decoding method. Although other means of decoding methods such as minimum mean square error generalized decision feedback equalizer (MMSE-GDFE) with lattice search and regularized lattice decoding are available, an alternative approach is to constrain the number of active transmit antennas in each channel use to be no larger than the number of receive antennas. The resulting system is coined constrained asymmetric MIMO system. Two general types of asymmetrical channels are considered in this paper, namely, 1) when there are two receive antennas and the number of transmit antennas is arbitrary, and 2) when the number of transmit antennas is one larger than the number of receive antennas. Explicit optimal transmission schemes as well as the corresponding code constructions for such constrained asymmetric MIMO channels are presented, and are shown to achieve the same DMT performance as their unconstrained counterparts.
Remarks on Diversity-Multiplexing Tradeoffs for Multiple-Access and Point-to-Point MIMO Channels
(2012-02-01)
Lu, Hsiao-Feng (Francis)
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電機工程學系
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Department of Electrical and Computer Engineering
In this paper, we answer several open questions related to diversity-multiplexing tradeoffs (DMTs) for point-to-point and multiple-access (MAC) MIMO channels. By analyzing the DMT performance of a simple code, we show that the optimal MAC-DMT holds even when the channel remains fixed for less than K n(t) + n(r) - 1 channel uses, where K is the number of users, n(t) is the number of transmit antennas of each user, and n(r) is the number of receive antennas at receiver. We also prove that the simple code isMAC-DMT optimal. A general code design criterion for constructing MAC-DMT optimal codes that is much more relaxed than the previously known design criterion is provided. Finally, by changing some design parameters, the simple code is modified for use in point-to-point MIMO channels. We show the modified code achieves the same DMT performance as the Gaussian random code.
Space-Time Storage Codes for Wireless Distributed Storage Systems
(2014-01-01)
Hollanti, Camilla
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Karpuk, David
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Barreal, Amaro
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Lu, Hsiao-Feng (Francis)
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電機資訊學士班
Distributed storage systems (SDSSs) have gained a lot of interest recently, thanks to their robustness and scalability compared to single-device storage. Majority of the related research has exclusively concerned the network layer. At the same time, the number of users of, e.g., peer-to-peer (p2p) and device-to-device (d2d) networks as well as proximity based services is growing rapidly, and the mobility of users is considered more and more important. This motivates, in contrast to the existing literature, the study of the physical layer functionality of wireless distributed storage systems. In this paper, we take the first step towards protecting the storage repair transmissions from physical layer errors when the transmission takes place over a fading channel. To this end, we introduce the notion of a space-time storage code, drawing together the aspects of network layer and physical layer functionality and resulting in cross-layer robustness. It is also pointed out that existing space-time codes are too complex to be utilized in storage networks when the number of helpers involved is larger than the number of receive antennas at the newcomer or data collector, hence creating a call for less complex transmission protocols.