A Comparative Analysis of Spatial Multiplexing Techniques for Outdoor MIMO-OFDM Systems with a Limited Feedback Constraint.

Stefano Savazzi , Politecnico di Milano, Monica Nicoli , Politecnico di Milano, and Mikael Sternad, UU

IEEE Transactions on Vehicular Technology, vol. 58, no, 1, January 2009, pp. 218-230.   © IEEE

Outline:

The use of multiple antennas enables also space-division multiple access (SDMA), which allows intra-cell bandwidth reuse by creating spatially multiplexed channels.

A fundamental challenge to be overcome is how the scheduler should separate and group the users. Recent studies focused on the uplink case and suggested clustering of users based on their mutual spatial correlation, or alternatively based on their main direction of arrival in case of moderate angular spread at the base station.

If channel state information (CSI) is perfectly known to the transmitter then interference-free spatially multiplexed channels could be set up through appropriate precoding matrices or vectors. However, for frequency division duplex (FDD) systems, this assumption leads to an unacceptable feedback rate requirement for users at vehicular velocities.

Abstract:

In this paper we analyze spatial multiplexing techniques for the downlink of a multiple-input-multiple-output orthogonal-frequency-division-multiplexing system.

Our study is focused on outdoor environments characterized by moderate angular spread. We consider two techniques that are able to separate the downlink data streams associated to different users and to guarantee a fixed error probability by exploiting a limited feedback from each user.

The proposed Adaptive Grid of Beams (AGoB) and Grid of Beams (GoB) differ in the way the precoders are designed (by adaptive or fixed processing) and in the scheduling policy. The new AGoB is able to harness a partial knowledge of the downlink channel spatial structure to better select the users and adjusts their precoders for downlink transmission.

The performances of GoB and AGoB are compared in this paper in terms of throughput and cell coverage capability. The radio interface is adapted to fit the requirements for the AMC-AAS (adaptive modulation and coding with advanced antenna system) mode of the IEEE 802.16-2005 standard [9].

Numerical results show that, as far as the channel exhibits a limited angular spread at the base station, the AGoB technique is able to provide significant throughput gains compared to the fixed GoB approach. On the other hand, large angular spreads are proved to have a substantial impact on the system performance as the benefits of the adaptation are significantly reduced.

Related publications:

Earlier conference paper at ICC 2006, Istanbul.

Reduced-rank Channel Estimation and Tracking in Time-slotted CDMA Systems,

Reduced-rank channel estimation, method by Erik Lindskog and Claes Tidestav, IEEE VTC'99.

Source:

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