Petros Elia

Explicit Space–Time Codes Achieving the Diversity–Multiplexing Gain Tradeoff

A recent result of Zheng and Tse states that over a quasi-static channel, there exists a fundamental tradeoff, referred to as the diversity-multiplexing gain (D-MG) tradeoff, between the spatial multiplexing gain and the diversity gain that can be simultaneously achieved by a space-time (ST) code. This tradeoff is precisely known in the case of independent and identically distributed (i.i.d.) Rayleigh fading, for Tgesnt+nr-1 where T is the number of time slots over which coding takes place and nt,nr are the number of transmit and receive antennas, respectively. For T nt case, we present two general techniques for building D-MG-optimal rectangular ST codes from their square counterparts. A byproduct of our results establishes that the D-MG tradeoff for all Tgesnt is the same as that previously known to hold for Tgesnt+n r-1

Perfect Space–Time Codes for Any Number of Antennas

In a recent paper, perfect (n times n) space-time codes were introduced as the class of linear dispersion space-time (ST) codes having full rate, nonvanishing determinant, a signal constellation isomorphic to either the rectangular or hexagonal lattices in 2n 2 dimensions, and uniform average transmitted energy per antenna. Consequence of these conditions include optimality of perfect codes with respect to the Zheng-Tse diversity-multiplexing gain tradeoff (DMT), as well as excellent low signal-to-noise ratio (SNR) performance. Yet perfect space-time codes have been constructed only for two, three, four, and six transmit antennas. In this paper, we construct perfect codes for all channel dimensions, present some additional attributes of this class of ST codes, and extend the notion of a perfect code to the rectangular case.

What Else Does Your Biometric Data Reveal? A Survey on Soft Biometrics

Recent research has explored the possibility of extracting ancillary information from primary biometric traits viz., face, fingerprints, hand geometry, and iris. This ancillary information includes personal attributes, such as gender, age, ethnicity, hair color, height, weight, and so on. Such attributes are known as soft biometrics and have applications in surveillance and indexing biometric databases. These attributes can be used in a fusion framework to improve the matching accuracy of a primary biometric system (e.g., fusing face with gender information), or can be used to generate qualitative descriptions of an individual (e.g., young Asian female with dark eyes and brown hair). The latter is particularly useful in bridging the semantic gap between human and machine descriptions of the biometric data. In this paper, we provide an overview of soft biometrics and discuss some of the techniques that have been proposed to extract them from the image and the video data. We also introduce a taxonomy for organizing and classifying soft biometric attributes, and enumerate the strengths and limitations of these attributes in the context of an operational biometric system. Finally, we discuss open research problems in this field. This survey is intended for researchers and practitioners in the field of biometrics.

Perfect space-time codes with minimum and non-minimum delay for any number of antennas

We here introduce explicit constructions of minimum-delay perfect space-time codes for any number n/sub t/ of transmit antennas and any number n/sub t/ of receive antennas. We also proceed to construct non-minimal delay perfect space-time codes for any n/sub t/, n/sub r/ and any block length T /spl ges/ n/sub t/. Perfect space-time codes were first introduced in F. Oggier et al. (2004) for dimensions of 2 /spl times/ 2, 3 /spl times/ 3, 4 /spl times/ 4 and 6 /spl times/ 6, to be the space-time codes that have full rate, full diversity-gain, non-vanishing determinant for increasing spectral efficiency, uniform average transmitted energy per antenna and good shaping of the constellation. The term perfect corresponds to the fact that the code simultaneously satisfies all the above mentioned important criteria. As a result, perfect codes have proven to have extraordinary performance. Finally, we point out that the set of criteria in F. Oggier et al. (2004) of non-vanishing determinant, full diversity, and full rate, is a subset of the more general and more strict set of criteria for optimally in the diversity-multiplexing gain (D-MG) tradeoff [Elias, P. et al., 2004], [Kumar, K.R. et al., 2005], an approach that takes special significance when constructing non-minimal delay perfect codes. Both minimum and non-minimum delay perfect codes are shown to be D-MG optimal.

Toward the Performance Versus Feedback Tradeoff for the Two-User MISO Broadcast Channel

For the two-user MISO broadcast channel with imperfect and delayed channel state information at the transmitter (CSIT), the work explores the tradeoff between performance on the one hand, and CSIT timeliness and accuracy on the other hand. This paper considers a broad setting where communication takes place in the presence of a random fading process, and in the presence of a feedback process that, at any point in time, may provide CSIT estimates-of some arbitrary accuracy - for any past, current or future channel realization. This feedback quality may fluctuate in time across all ranges of CSIT accuracy and timeliness, ranging from perfectly accurate and instantaneously available estimates, to delayed estimates of minimal accuracy. Under standard assumptions, the work derives the degrees-of-freedom (DoF) region, which is tight for a large range of CSIT quality. This derived DoF region concisely captures the effect of channel correlations, the accuracy of predicted, current, and delayed-CSIT, and generally captures the effect of the quality of CSIT offered at any time, about any channel. This paper also introduces novel schemes which-in the context of imperfect and delayed CSIT-employ encoding and decoding with a phase-Markov structure. The results hold for a large class of block and nonblock fading channel models, and they unify and extend many prior attempts to capture the effect of imperfect and delayed feedback. This generality also allows for consideration of novel pertinent settings, such as the new periodically evolving feedback setting, where a gradual accumulation of feedback bits progressively improves CSIT as time progresses across a finite coherence period.

D-MG Tradeoff and Optimal Codes for a Class of AF and DF Cooperative Communication Protocols

Cooperative relay communication in a fading channel environment under the orthogonal amplify-and-forward (OAF), nonorthogonal and orthogonal selection decode-and-forward (NSDF and OSDF) protocols is considered here. The diversity-multiplexing gain tradeoff (DMT) of the three protocols is determined and DMT-optimal distributed space-time (ST) code constructions are provided. The codes constructed are sphere decodable and in some instances incur minimum possible delay.

Sphere Decoding Complexity Exponent for Decoding Full-Rate Codes Over the Quasi-Static MIMO Channel

In the setting of quasi-static multiple-input multiple-output channels, we consider the high signal-to-noise ratio (SNR) asymptotic complexity required by the sphere decoding (SD) algorithm for decoding a large class of full-rate linear space-time codes. With SD complexity having random fluctuations induced by the random channel, noise, and codeword realizations, the introduced SD complexity exponent manages to concisely describe the computational reserves required by the SD algorithm to achieve arbitrarily close to optimal decoding performance. Bounds and exact expressions for the SD complexity exponent are obtained for the decoding of large families of codes with arbitrary performance characteristics. For the particular example of decoding the recently introduced threaded cyclic-division-algebra-based codes—the only currently known explicit designs that are uniformly optimal with respect to the diversity multiplexing tradeoff—the SD complexity exponent is shown to take a particularly concise form as a non-monotonic function of the multiplexing gain. To date, the SD complexity exponent also describes the minimum known complexity of any decoder that can provably achieve a gap to maximum likelihood performance that vanishes in the high SNR limit.

Person recognition using a bag of facial soft biometrics (BoFSB)

This work introduces the novel idea of using a bag of facial soft biometrics for person verification and identification. The novel tool inherits the non-intrusiveness and computational efficiency of soft biometrics, which allow for fast and enrolment-free biometric analysis, even in the absence of consent and cooperation of the surveillance subject. In conjunction with the proposed system design and detection algorithms, we also proceed to shed some light on the statistical properties of different parameters that are pertinent to the proposed system, as well as provide insight on general design aspects in soft-biometric systems, and different aspects regarding efficient resource allocation.

Fundamental limits of cache-aided wireless BC: Interplay of coded-caching and CSIT feedback

Building on the recent coded-caching breakthrough by Maddah-Ali and Niesen, the work here considers the K-user cache-aided wireless multi-antenna (MISO) symmetric broadcast channel (BC) with random fading and imperfect feedback, and analyzes the throughput performance as a function of feedback statistics and cache size. In this setting, our work identifies the optimal cache-aided degrees-of-freedom (DoF) within a factor of 4, by identifying near-optimal schemes that exploit the new synergy between coded caching and delayed CSIT, as well as by exploiting the unexplored interplay between caching and feedback-quality. The derived limits interestingly reveal that — the combination of imperfect quality current CSIT, delayed CSIT, and coded caching, guarantees that — the DoF gains have an initial offset defined by the current CSIT quality, and then that the additional gains attributed to coded caching are exponential, in the sense that any linear decrease in the required DoF performance, allows for an exponential reduction in the required cache size.

Explicit space-time codes that achieve the diversity-multiplexing gain tradeoff

In the recent landmark paper of Zheng and Tse it is shown for the quasi-static, Rayleigh-fading MIMO channel with nt transmit and nr receive antennas, that there exists a fundamental tradeoff between diversity gain and multiplexing gain, referred to as the diversity-multiplexing gain (D-MG) tradeoff. This paper presents the first explicit construction of space-time (ST) codes for an arbitrary number of transmit and/or receive antennas that achieve the D-MG tradeoff. It is shown here that ST codes constructed from cyclic-division-algebras (CDA) and satisfying a certain non-vanishing determinant (NVD) property, are optimal under the D-MG tradeoff for any nt,nr. Furthermore, this optimality is achieved with minimum possible value of the delay or block-length parameter T = n t. CDA-based ST codes with NVD have previously been constructed for restricted values of nt. A unified construction of D-MG optimal CDA-based ST codes with NVD is given here, for any number nt of transmit antennas. The CDA-based constructions are also extended to provide D-MG optimal codes for all T ges nt, again for any number nt of transmit antennas. This extension thus presents rectangular D-MG optimal space-time codes that achieve the D-MG tradeoff. Taken together, the above constructions also extend the region of T for which the D-MG tradeoff is precisely known from T ges nt + nr - 1 to T ges nt