Pranav Dayal

An optimal two transmit antenna space-time code and its stacked extensions

A space-time code is proposed that exhibits the highest coding gain between competing full rate full transmit diversity space-time codes for the two transmit antenna wireless channel. The proposed code is derived from a layered architecture with real rotation of QAM information symbols in two dimensions. The existing codes of similar architecture concentrate on application of complex full modulation diversity rotations or asymmetric real rotations for the two layers. An analytic evaluation illustrates the significant improvement in coding gain achieved with the proposed code. Moreover, the value of the unnormalized coding gain for the new code is independent of the rate of the code. A stacked extension of the proposed code offers a reduced complexity capacity optimal alternative to the full diversity codes for larger number of transmit antennas. Performance enhancement in several scenarios is verified through simulations.

Interference avoidance for in-device coexistence in 3GPP LTE-advanced: challenges and solutions

With the ever growing usage of various wireless technologies and services, more and more handheld devices are equipped with multiple radio transceivers (e.g., LTE, WiFi, Bluetooth, and GNSS). As a result, in-device coexistence interference has become a serious problem due to the extreme proximity of multiple radio transceivers within the same device. It has been shown by the studies of 3GPP that for some specific frequency bands, concurrent operations of LTE and ISM/GNSS radios working in adjacent or sub-harmonic frequencies will result in significant in-device coexistence interference that cannot be completely eliminated by filter technology. This motivates 3GPP to introduce signaling mechanisms and procedures, which enable the devices to effectively solve the indevice coexistence problems with help from the LTE network. This article first discusses the scenarios in which coexistence interference may happen, and then provides an overview of main solutions and related procedures specified in 3GPP Release-11 standards for LTE. In addition, some aspects of device-internal coordination and implementation to facilitate the coexistence scenarios are also discussed.

An algebraic family of complex lattices for fading channels with application to space-time codes

A new approach is presented for the design of full modulation diversity (FMD) complex lattices for the Rayleigh-fading channel. The FMD lattice design problem essentially consists of maximizing a parameter called the normalized minimum product distance d/sub p//sup 2/ of the finite signal set carved out of the lattice. We approach the problem of maximizing d/sub p//sup 2/ by minimizing the average energy of the signal constellation obtained from a new family of FMD lattices. The unnormalized minimum product distance for every lattice in the proposed family is lower-bounded by a nonzero constant. Minimizing the average energy of the signal set translates to minimizing the Frobenius norm of the generator matrices within the proposed family. The two strategies proposed for the Frobenius norm reduction are based on the concepts of successive minima (SM) and basis reduction of an equivalent real lattice. The lattice constructions in this paper provide significantly larger normalized minimum product distances compared to the existing lattices in certain dimensions. The proposed construction is general and works for any dimension as long as a list of number fields of the same degree is available.

Efficient multiuser cooperation strategies using QAM space-time block codes

Practical space-time coding strategies are proposed for multiple users cooperating to communicate to a destination in a wireless channel. For a system with m single antenna users and a destination with N receive antennas, a two-phase cooperative scheme is proposed that achieves a diversity order of m - 1 + N for the error probability. This effectively proves the achievability of the diversity order implied by the outage probability for this model. This two-phase scheme employs a short full diversity space-time block code over QAM symbols. Even though the suboptimum decoder used by the destination assumes that the inter-user communication is successful, the corresponding analysis does account for the decoding errors in the inter-user communications phase. The key to the diversity order analysis is an appropriate modification of the rule for deciding whether or not a user acts as a relay for another user. The performance analysis technique presented here also extends to another practical cooperation strategy, proposed specifically for the case of m = 2 and N = 1, that is inspired from an optimal diversity-multiplexing tradeoff curve achieving protocol.

Generalized multiuser QAM based cooperation strategy and an optimal two-user scheme

A two user cooperation strategy based on QAM information symbols, known as the self-information canceling linear (SCL) scheme, was recently proposed by the authors. The SCL scheme achieves the full diversity order of 2 and the full multiplexing gain of 1 inherent in the system. However, the diversity versus multiplexing gain tradeoff (DMT) of the SCL scheme was not shown to be optimal. In the first part of this work, we propose a new two user cooperation strategy that achieves the optimal DMT curve for the two user cooperative diversity system. This scheme is obtained by a suitable modification of the original SCL scheme. The modified SCL scheme is also a practical cooperation strategy based on QAM information symbols and the present analysis accounts for the inter-user communications errors. The modified SCL scheme is, thus, established as a specific coding scheme to achieve the information theoretic limits within the DMT framework. In the second part of this work, we generalize the original SCL scheme to the case of m users for any m > 2. The generalization leads to a practical multiuser cooperative scheme based on QAM symbols that provides the full diversity order of m, fair rate allocation among the users and potential for achieving the full multiplexing gain.

Unified multi-antenna code design for fading channels with spatio-temporal correlations

A unified framework for multiple antenna space-time coding is presented leading to a general code design criterion valid for arbitrary spatial and temporal fading correlations. This framework provides insights into the effect of each of spatial and temporal correlations on space-time code design. The single code design expression encompasses quasi-static, fast fading, multiple block fading and other general correlations in the temporal dimension as well as arbitrary rank correlations in the spatial dimension. The general coding strategy proposed consists of pre-coding a space-time code with its size determined by the rank of transmit correlation, the structure determined by the Cholesky factorization of the temporal correlation and the precoding matrix obtained from the eigenvectors of the transmit correlation.

Unified multi-antenna code design for fading channels with spatio-temporal/spectral correlations

A unified framework for coherent multiple antenna communications is presented leading to a general space-time code design criterion valid for arbitrary spatial and temporal fading correlations. This framework provides insights into the effect of each of spatial and temporal correlations on space-time code design. The single code design expression applies to quasi-static, fast fading and multiple block fading channels and more generally also to channels with arbitrary correlations in the temporal dimension as well as in the spatial dimension with arbitrary rank. The general coding strategy proposed consists of precoding a space-time code with its size determined by the rank of transmit correlation, the structure determined by the Cholesky factorization of the temporal correlation, and the precoding matrix obtained from the eigenvectors of the transmit correlation matrix

Diversity order analysis of training codes for MIMO block fading channels

A general class of training-based space-time codes is considered for the noncoherent block correlated Rayleigh fading channel. Such codes can be efficiently decoded by first forming the MMSE estimate of the channel and then decoding the underlying coherent space-time code using the channel estimate as if it were perfect. The diversity order of this (in general, suboptimal) estimator-decoder is obtained, and it is shown that under certain conditions on the channel correlation matrix, training codes inherit the diversity order of the underlying coherent code.