In this letter, we show that the performance of recently proposed quasi-orthogonal space-time codes can be improved by phase-shifting the constellations of the symbols constituting the code. The optimal rotation of the symbols increases the minimum distance of the corresponding space-time codewords, leading to substantially improved performance.

This article originates from a panel with the above title, held at IEEE VTC Spring 2009, in which the authors took part. The enthusiastic response it received prompted us to discuss for a wider audience whether research at the physical layer (PHY) is still relevant to the field of wireless communications. Using cellular systems as the axis of our exposition, we exemplify areas where PHY research has indeed hit a performance wall and where any improvements are expected to be marginal. We then discuss whether the research directions taken in the past have always been the right choice and how lessons learned could influence future policy decisions. Several of the raised issues are subsequently discussed in greater details, e.g., the growing divergence between academia and industry. With this argumentation at hand, we identify areas that are either under-developed or likely to be of impact in coming years - hence corroborating the relevance and importance of PHY research.

In wireless communications, mobiles emit signals that arrive at a base station via multiple paths. Estimating each path's angle-of-arrival (AOA) and propagation delay is necessary for several applications, such as mobile localization for emergency services. We propose a novel subspace approach to estimate the AOA and delays of multipath signals from digitally modulated sources arriving at an antenna array. Our method uses a collection of estimates of a space-time vector channel. The Cramer-Rao bound (CRB) and simulations are provided.

In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

The consider the problem of recovering p synchronous communication signals that are transmitted through a multiple-input/multiple-output (MIMO) linear channel and are, therefore, received in the presence of both interuser (IUI) and intersymbol interference (ISI). A multichannel linear equalization approach is taken, and we propose to adjust the equalizer coefficients with a blind adaptive algorithm (without the use of training data). This multiuser constant modulus algorithm (MU-CMA) is derived from the minimization of a cost function that penalizes deviations of the equalized signals from the constant modulus property as well as cross-correlations between them. The proposed scheme appears to be an appealing technique for multiuser blind equalization that combines good convergence properties with low computational complexity.

Interference in wireless communications is traditionally treated as a cause of performance degradation. Whilst from a statistical viewpoint this is entirely justified, this article discusses scenarios where the interfering signal enhances the desired signal's useful power on an instantaneous basis and provides an unexplored source of additional signal power. The potential of harvesting this energy, which naturally exists in the communication system, is shown to be readily available in downlink systems where interference can be predicted. The main concept discussed is that instead of using knowledge of the interference to cancel, eliminate or avoid it, it is potentially more fruitful to use this knowledge to manipulate and make use of the interference towards the system's advantage. A significant source of useful signal power, which with conventional transmission techniques is left unexploited, can be used to improve the radio systems' performance and achieve reliable power-efficient communications where performance benefits are yielded without the need to increase the average transmitted power.