Dumitru Mihai Ionescu

Improved 8- and 16-state space-time codes for 4PSK with two transmit antennas

New space-time codes for 4PSK constellations, designed via a modified determinant criterion, send 2 b/s/Hz and show improved performance in quasi-static flat fading.

On space-time code design

It is shown that the separation between space-time code matrices can be described in terms of a metric of Euclidean type, which is defined via the singular values of difference code matrices, and arises naturally from a minimization of the pairwise error probability. Essentially, the distance between complex space-time code matrices is the Euclidean distance between the respective - demultiplexed and concatenated - transmit antenna streams, expressed in terms of the structure inherent to the multiple antenna arrangement. It is further shown that the determinant criterion can be strengthened, in a manner that not only suggests an optimum space-time code matrix structure, but also outlines the central role played by the Euclidean distance in quasi-static fading. Theorem 5 - which claims that in order to optimize the product distance one must optimize the Euclidean distance -establishes a close interdependence between product and Euclidean distances; it thereby links the performance determining factors in quasi-static and independent fading, and rigorously establishes the relevance of combining space-time coding and modulation in fading environments. A multidimensional space-time constellation for two transmit antennas, and its coset partitioning-based on traces of differences between constellation matrices-are described. Example codes constitute the first reporting of a space-time coded modulation scheme for fading channels, whereby a space-time constellation is partitioned in cosets.

New results on space-time code design criteria

A sub-class of space-time codes, defined via compliance with a new criterion of optimality, is introduced. A new distance between two codewords is defined and leads to new criteria for code design. It is shown that, unlike the non-Euclidian metrics currently in use, the new distance measure is a true metric, its square has a useful additive property and, in itself, preserves the traditional form of the upper bound to the probability of mistaking two codewords, as encountered in the union bound. The resulting new criteria for space-time code design are presented.

A class of nonorthogonal rate-one space-time block codes with controlled interference

Complex, rate one, orthogonal block space-time codes (STCs) exist only for two transmit antennas and were proposed by Alamouti (1998). Designing rate-one block STCs for more than two transmit antennas implies giving up orthogonality. For more than two transmit antennas a class of rate one block STCs with a controlled number of interference terms per detected symbol is presented. Conditions that link the matrix formulation of the receive equation to the matrix formulation of a multiuser detection scheme are discussed. The block STCs design problem is closely related to the class of linear real Hadamard codes.

Predictive closed-loop power control for frequency-division duplex wireless systems

A novel method for performing uplink closed-loop power control is proposed, namely with uplink prediction and downlink-derived predictor coefficients. The lack of correlation between the fading processes on uplink and downlink (due to frequency division duplex and frequency selective fading) is circumvented by using the uplink fading history, together with a set of link-independent, autoregressive coefficients.

Predictability of uplink channels from downlink channels in frequency division duplex systems: a power control application

A novel method for performing predictive closed-loop power control is used to illustrate the predictability of uplink channels from downlink channels in a frequency division duplex (FDD) wireless link. In an uplink power control scenario, the proposed scheme performs uplink prediction using downlink-derived (linear) predictor coefficients. Essentially, the lack of correlation between the fading processes on uplink and downlink (due to FDD and frequency selective fading) is circumvented by using the uplink fading history, together with a set of link-independent, autoregressive (AR) coefficients. An AR model is used to formally describe the frequency selective fading process on either link, regardless of the number of Rayleigh paths. It is also proven that the relevant AR coefficients are common to both links, up to a proper normalization, irrespective of whether the number of paths on the uplink differs from that on the downlink. Both an uncoded and a coded system are simulated, and their performance results are compared, on equal grounds, with those of the corresponding nonpredictive systems.

Design of space-time codes with optimal coding gain

Space time codes have been proposed in the literature as an efficient means for improving the data rates over fading channels with multiple transmit antennas. In particular, the rank and the determinant of code difference matrices have been shown to be important in the design of space time codes for fading channels. In the present work, we investigate the problem of maximizing the coding gain of space-time codes, given by the minimum of the determinants of all the code difference matrices. We rely on equality of the singular values of the code difference matrices as a necessary and sufficient condition for obtaining the optimal coding gain. Finally, we discuss the construction of trellis codes and present simulation results.

Fading-Resilient Super-Orthogonal Space-Time Signal Sets: Can Good Constellations Survive in Fading?

In this correspondence, first-tier indirect (direct) discernible constellation expansions are defined for generalized orthogonal designs. The expanded signal constellation, leading to so-called super-orthogonal codes, allows the achievement of coding gains in addition to diversity gains enabled by orthogonal designs. Conditions that allow the shape of an expanded multidimensional constellation to be preserved at the channel output, on an instantaneous basis, are derived; this property, referred to as fading resilience, is first discussed in the context of quasi-static fading, then generalized to coordinate interleaved transmissions for rapid fading. In both cases - and despite of piecing together (in the latter case) a constellation point as a result of coordinate deinterleaving - the channel alters neither the relative distances nor the angles between signal points in a fading-resilient signal constellation. It is shown that nonorthogonal MIMO constellations having a geometric structure that is resilient to fading do exist, and their further study is warranted.

SISO APP Searches in Lattices With Tanner Graphs

An efficient, low-complexity, soft-output detector for general lattices is presented, based on their Tanner graph (TG) representations. Closest-point searches in lattices can be performed as nonbinary belief propagation on associated TGs; soft-information output is naturally generated in the process; the algorithm requires no backtrack (cf. classic sphere decoding), and extracts extrinsic information. A lattices coding gain enables equivalence relations between lattice points, which can be thereby partitioned in cosets. Total and extrinsic a posteriori probabilities at the detectors output further enable the use of soft detection information in iterative schemes. The algorithm is illustrated via two scenarios that transmit a 32-point, uncoded super-orthogonal (SO) constellation for multiple-input multiple-output (MIMO) channels, carved from an 8-dimensional nonorthogonal lattice D4⊕D4: it achieves maximum likelihood performance in quasistatic fading; and, performs close to interference-free transmission, and identically to list sphere decoding, in independent fading with coordinate interleaving and iterative equalization and detection. Latter scenario outperforms former despite absence of forward error correction coding-because the inherent lattice coding gain allows for the refining of extrinsic information. The lattice constellation is the same as the one employed in the SO space-time trellis codes first introduced for 2 × 2 MIMO by Ionescu et al., then independently by Jafarkhani and Seshadri. Algorithmic complexity is log-linear in lattice dimensionality versus cubic in classic sphere decoders.