Multiple-input multiple-output (MIMO) systems have potential to provide the capacity needed for future generation wireless systems and have been adopted by 3GPP Long Term Evolution (LTE) system. However, due to several practical limitations the theoretical benefits of MIMO are not fully achieved. The degree of channel state information (CSI) at the transmitter is one key limiting factor when aiming at high performance. In this thesis, the performance possible to achieve under different CSI assumptions at the transmitter is studied. In the beginning, full CSI is assumed at the receiver and the problem of sum rate maximization for two multiplexed users with linear receiver and transmit processing is investigated. A new simple transmission scheme is presented for the case when the users have more than one receive antennas and the MIMO channels are rank deficient. Next, an LTE-type system is considered with a limited rate feedback link. The tradeoff between system performance and the number feedback bits is considered. Feedback schemes related to closed loop precoding and adaptive modulation and coding (AMC) for single user MIMO are studied. By optimizing the feedback, performance improvements or savings in feedback bits are achieved. For closed loop precoding, the precoding matrix is partitioned into a Grassmannian part, which steers the transmit power, and an orthogonalizing part, which controls the inter-stream interference. The optimality of orthogonalization has been investigated and a codeword selection metric is introduced for the orthogonalization part. For AMC, a relative channel quality indication (CQI) report optimization has been considered. In the last part, a system where dynamic mode switching is possible between single user and multiuser transmissions without higher layer signaling is considered. The feasible feedback to support such a system in LTE context is studied. It is investigated how feedback derived under single user transmission assumption can support the dynamic switching and whether multiuser specific additional precoding or CQI feedback is needed.
|Translated title of the contribution||Lineaariset lähetysmenetelmät ja signalointi moniantennijärjestelmän laskevassa siirtotiessä|
|Publication status||Published - 2012|
|MoE publication type||G5 Doctoral dissertation (article)|