Contents I. INTRODUCTION
SPACE TIME PROCESSING has gained a lot of attention over the last years as it exploits extra degrees of freedom stemming from the precious spatial dimension [1]. Space-time coding is a space-time mapping approach that transforms the fluctuating multi-input-multi-output (MIMO) channel into a number of independent subchannels [2]. An optimal full-rate full-diversity space-time code is the one proposed by Alamouti [3] regarding a transmitter equipped with two antennas attached to two RF chains. However, while multiple radios can be afforded at the base station or at the access point, the number of radios in the user mobile equipment is limited due to cost, space and DC power constraints. For example, in the 3GPP long term evolution (LTE) system, multiple antennas will be used by the mobile terminal for the downlink reception while a asingle antenna will be used for the uplink transmission [4]. The asymmetry of the two operational modes is mainly intended for avoiding the costly power amplifiers (PA) in the transmit RF chains. Although antenna selection is a proposed option for such standards [5] so as to capture most of the diversity out of the available transmit antennas, it requires instantaneous channel feedback (e.g. the antenna index) from the receiver back to the transmitter, which is a heavy burden on the communication link especially when considering high mobility scenarios. On the other hand, feeding back the channel information after several frames may provide no gain as the channel might change dramatically over such time [6]. It is the subject of this paper to propose a novel open-loop space-time technique suitable for next generation mobile terminals. The novelty of the proposed scheme is that it can be implemented using a single radio, hence most of the troubles related to integrating multiple radios within a limited area are avoided. In addition to the orthogonal structure of the transmitted block of signals, the encoded signals are mapped onto an orthonormal space of basis functions, hence independent fading is guaranteed for most channel distributions
Obviously channels with very narrow angular spreads or keyhole channels will always result in correlated channel responses.
. The proposed scheme is superior to the Alamouti code proposed in [7] using a compact three element switched parasitic antenna system in the following sense 1) two antenna elements rather than three elements are used in this paper, thus leading to further reduction in size and cost 2) the proposed scheme is implemented using a decoupling and matching network (DMN) which is already proposed in the literature for enhancing the performance of closely spaced-antennas. By this way, parasitic antennas which are not common in standard portable units are avoided. Throughout the paper, a small letter designates a scalar; a bold small letter designates a vector; a bold big letter designates a matrix; an Euler-font letter designates a complex beampattern. The operators designate complex conjugate and transpose operators, respectively and operator returns the real part of the complex value . The rest of the paper is organized as follows: in section III we describe a novel scheme for mapping two BPSK signals within Alamouti code from one orthonormal basis onto an alternative orthonormal one using a single RF source. An antenna system example is proposed in section IV and used for evaluating the performance of the proposed technique via Monte-Carlo simulations in a realistic propagation channel. Finally section VI concludes the paper and discusses some future-work issues.