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MIMO Technologies for the Wireless Future
Abstract- Future wireless systems are expected to support high data rates of 1 Gbit/s or more in a variety of scenarios. A
key technology in order to achieve the required high spectral efficiency is the application of multiple input multiple output (MIMO) techniques, which exploit spatial diversity, array gain or spatial multiplexing gain. Anothersource of diversity – inherent to wireless systems- is that of the multiuser diversity. Multiuser (MU) MIMO algorithms combine both MIMO gains with multiuser diversity benefits. Although MU MIMO techniques have been extensively studied and were shown to provide considerable average cell throughput gains, they often prove inadequate to cope with intercell interference and can only offer poor cell edgeperformance. Network coordination (multisite MIMO) can be applied in this case, which can achieve significant improvements for the users including those at the cell edge, based on coordinated transmission and reception by multiple base stations. In this paper we present an overview of the most promising MIMO technologies and discuss their relative merits and requirements.

THE successful adoption of advanced technologies, such as MIMO, in future wireless systems design, as a means

to address the challenging spectral efficiency, flexibility and adaptability requirements, is not only a matter of devising sophisticated signal processing, resource allocation or cross layer techniques but also and most importantly a matter of realistic consideration of the overallnetwork performance dynamics and the overhead signaling bandwidth constraints. Following this line of thought, we present in this paper a brief overview of the MIMO techniques currently considered in the evolving standards (such as 3GPP-LTE), namely open and closed loop single user (SU) MIMO techniques. Performance targets to address IMT-Advanced requirements are discussed and promising candidate MIMOtechnologies for future wireless systems design are explained, namely multiuser MIMO for average cell throughput improvements and multisite MIMO for average cell and cell edge throughput enhancements. II. MIMO 3GPP-LTE MIMO is an essential ingredient of 3GPP-LTE [1], where a 2 transmit and receive antenna scheme is considered to be the baseline downlink configuration. Four transmit and receiveantennas are also supported. For the uplink, transmission with only one transmit antenna including antenna selection is supported. For the downlink, the standard contains both transmit diversity and spatial multiplexing. For open loop transmit diversity, basically a spacefrequency Alamouti scheme is used as depicted in Fig. 1 [3]. Together with a simple linear combiner at the receiver, this schemeessentially produces an effective single-input singleoutput channel, the channel coefficient of which is given by the sum of the squared magnitudes of the channel coefficients from all transmit to all receive antennas. The constructive interference leads to an effective channel, which is more stable than an individual channel from a transmit to a receive antenna. GRAFICO 1 In case of 4 transmitantennas, the LTE standard just uses different subcarriers and antenna switching with two Alamouti schemes. Alamouti-based transmit diversity does not directly increase the data rate by adding simultaneously transmitted spatial data streams. True spatial multiplexing with linear precoding and a variable number of spatial streams (layers) is also supported in 3GPP-LTE. The terminal chooses thepreferred precoder from a codebook and feeds the respective codebook index back to the base station. As an example, the codebook for 2 Tx antennas and the respective beampattems for an antenna spacing of half wavelength are depicted in Fig. 2. GRAFICO2 A third MIMO variant called large delay cyclic delay diversity (CDD) precoding is used for open loop precoding. Here, the precoder is determined by P(i)...
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