arXiv subjects

Lu

Publications and source records attributed to Lu.

40 records · Page 3Linked to original sources

arXiv2015

Performance-Complexity Analysis for MAC ML-based Decoding with User Selection

This work explores the rate-reliability-complexity limits of the quasi-static K-user multiple access channel (MAC), with or without feedback. Using high-SNR asymptotics, the work first derives bounds on the computational resources required to achieve near-optimal (ML-based) decoding performance. It then bounds the (reduced) complexity needed to achieve any (including suboptimal) diversity-multiplexing performance tradeoff (DMT) performance, and finally bounds the same complexity, in the presence of feedback-aided user selection. This latter effort reveals the ability of a few bits of feedback not only to improve performance, but also to reduce complexity. In this context, the analysis reveals the interesting finding that proper calibration of user selection can allow for near-optimal ML-based decoding, with complexity that need not scale exponentially in the total number of codeword bits. The derived bounds constitute the best known performance-vs-complexity behavior to date for ML-based MAC decoding, as well as a first exploration of the complexity-feedback-performance interdependencies in multiuser settings.

cs.IT↗

arXiv2013

An error event sensitive trade-off between rate and coding gain in MIMO MAC

This work considers space-time block coding for the Rayleigh fading multiple-input multiple-output (MIMO) multiple access channel (MAC). If we suppose that the receiver is performing joint maximum-likelihood (ML) decoding, optimizing a MIMO MAC code against a fixed error event leads to a situation where the joint codewords of the users in error can be seen as a single user MIMO code. In such a case pair-wise error probability (PEP) based determinant criterion of Tarokh et al. can be used to upper bound the error probability. It was already proven by Lahtonen et al. that irrespective of the used codes the determinants of the differences of codewords of the overall codematrices will decay as a function of the rates of the users. This work will study this decay phenomenon further and derive upper bounds for the decay of determinants corresponding any error event. Lower bounds for the optimal decay are studied by constructions based on algebraic number theory and Diophantine approximation. For some error profiles the constructed codes will be proven to be optimal. While the perspective of the paper is that of PEP, the final part of the paper proves how the achieved decay results can be turned into statements about the diversity-multiplexing gain trade-off (DMT).

cs.IT↗

arXiv2009

Remarks on the Criteria of Constructing MIMO-MAC DMT Optimal Codes

In this paper we investigate the criteria proposed by Coronel et al. for constructing MIMO MAC-DMT optimal codes over several classes of fading channels. We first give a counterexample showing their DMT result is not correct when the channel is frequency-selective. For the case of symmetric MIMO-MAC flat fading channels, their DMT result reduces to exactly the same as that derived by Tse et al., and we therefore focus on their criteria for constructing MAC-DMT optimal codes, especially when the number of receive antennas is sufficiently large. In such case, we show their criterion is equivalent to requiring the codes of any subset of users to satisfy a joint non-vanishing determinant criterion when the system operates in the antenna pooling regime. Finally an upper bound on the product of minimum eigenvalues of the difference matrices is provided, and is used to show any MIMO-MAC codes satisfying their criterion can possibly exist only when the target multiplexing gain is small.

cs.IT↗

arXiv1999

Massive-Scalar Absorption by Extremal p-branes

We study the absorption probability of minimally-coupled massive scalars by extremal p-branes. In particular, we find that the massive scalar wave equation under the self-dual string background has the same form as the massless scalar wave equation under the dyonic string background. Thus it can be cast into the form of a modified Mathieu equation and solved exactly. Another example that we can solve exactly is that of the D=4 two-charge black hole with equal charges, for which we obtain the closed-form absorption probability. We also obtain the leading-order absorption probabilities for D3-, M2- and M5-branes.

hep-th↗