Advances in MIMO Techniques for Mobile Communications—A Survey <s> Link Adaptation without Precoding <s> Multiple antennas at the transmitter and receiver can achieve enormous capacities by transmitting on the channel's eigenmodes when the channel realization is known at the transmitter. If the transmitter has no knowledge of the channel, a significant fraction of the eigenmode capacity can be achieved in an open-loop mode, but multi-dimensional coding is required. We show how the open-loop capacity can be achieved with conventional single-dimensional coding using optimum successive decoding (OSD) and simple per-antenna rate control. Using power allocation, the capacity can be further increased, although only slightly. <s> BIB001 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> Link Adaptation without Precoding <s> MIMO systems exploit the multipath propagation in rich scattering environment using multiple transmit and receive antennas to increase the capacity of a link. The matrix channel plays a pivotal role in the throughput of a MIMO link since the modulation, data rate, power allocation and antenna weights are dependent on the channel gain. This paper evaluates adaptively modulated open loop MIMO system with limited feedback and adaptively modulated closed loop MIMO systems. For open loop MIMO a modified V-BLAST system with limited feedback is considered. This is compared with a rate maximization algorithm for closed loop MIMO using iid flat-fading Rayleigh channel, spatially correlated channel and realistic indoor channel. It is shown that when realistic models are used the difference in performance between closed loop MIMO and limited feedback V-BLAST systems is greater. At a noise-normalized power of 25 dB the percentage improvement in the throughput of CL-MIMO over OL-MIMO is 15.1% for Rayleigh fading channel, 48.1% for the spatially correlated channel and 104% for the realistic channel model. It is also shown that the throughput reduces in presence of channel correlation and that this affects the performance of the limited feedback V-BLAST system to a greater extent in comparison with CL-MIMO system. The average SNR penalty for OL-MIMO to achieve same throughput as CL-MlMO is 4dB for the Rayleigh fading model, 7dB for the spatially correlated model and 10 dB for the realistic channel model. <s> BIB002
In addition to the precoding matrix, other information e.g. the received signal to interference and noise ratio (SINR) may also be included in the feedback for link adaptation. However, some MIMO schemes like the modified V-BLAST schemes in BIB001 BIB002 rely solely on this type of feedback without any precoding information. Another example is the 2-codeword multiple codewords (2CW-MCW) scheme for FDD MIMO-OFDM cellular systems proposed in  that uses SINR feedback for each stream to select a suitable modulation and coding scheme (MCS) for each of the two simultaneously transmitted codewords. The two codewords are mapped onto 2 and 4 streams respectively for 2 u 2 and 4 u 4 antenna configurations. The mapping may either be fixed or adaptive. Adaptive mapping also makes use of the SINR feedback. Precoding is not used in this scheme resulting in reduced feedback overhead.