Advances in MIMO Techniques for Mobile Communications—A Survey <s> V-BLAST <s> Information theory research has shown that the rich-scattering wireless channel is capable of enormous theoretical capacities if the multipath is properly exploited. In this paper, we describe a wireless communication architecture known as vertical BLAST (Bell Laboratories Layered Space-Time) or V-BLAST, which has been implemented in real-time in the laboratory. Using our laboratory prototype, we have demonstrated spectral efficiencies of 20-40 bps/Hz in an indoor propagation environment at realistic SNRs and error rates. To the best of our knowledge, wireless spectral efficiencies of this magnitude are unprecedented and are furthermore unattainable using traditional techniques. <s> BIB001 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> V-BLAST <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> BIB002 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> V-BLAST <s> In recent years, considerable attention has been paid to the joint optimal transceiver design for multi-input multi-output (MIMO) communication systems. In this paper, we propose a joint transceiver design that combines the geometric mean decomposition (GMD) with either the conventional zero-forcing VBLAST decoder or the more recent zero-forcing dirty paper precoder (ZFDP). Our scheme decomposes a MIMO channel into multiple identical parallel subchannels, which can make it rather convenient to design modulation/demodulation and coding/decoding schemes. Moreover, we prove that our scheme is asymptotically optimal for (moderately) high SNR in terms of both channel throughput and bit error rate (BER) performance. This desirable property is not shared by any other conventional schemes. We also consider the subchannel selection issues when some of the subchannels are too poor to be useful. Our scheme can also be combined with orthogonal frequency division multiplexing (OFDM) for intersymbol interference (ISI) suppression. The effectiveness of our approaches has been validated by both theoretical analyses and numerical simulations. <s> BIB003 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> V-BLAST <s> High-Speed Downlink Packet Access (HSDPA) using Multiple-Input Multiple-Output (MIMO) techniques received increasing attention recently, because MIMO-HSDPA allows capacity enlargement and, therefore, supports the demand of high data services, which are expected to become the dominant source of 3G traffic and of revenue in cellular communications in the next few years. This contribution will discuss one particular MIMO-HSDPA scheme - the Per Antenna Rate Control (PARC) - from the viewpoint of conventional receiver architectures. In addition, the Signal-to-Interference-Ratio (SIR) is introduced as a basic metric for the channel quality indication required in a PARC-based system, by briefly reviewing blind and data-aided SIR estimation methods. <s> BIB004 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> V-BLAST <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> BIB005
The vertical Bell Laboratories Layered Space-Time (V-BLAST) BIB001 is one of the very first open-loop spatial multiplexing MIMO systems which has been practically demonstrated to achieve much higher spectral efficiencies than SISO systems, in rich scattering environments. In V-BLAST, a single data stream is demultiplexed into multiple substreams which are mapped on to symbols and then transmitted through multiple antennas. Intersubstream coding is not employed in V-BLAST, however channel coding can be applied to the individual substreams for reduction of bit error rate (BER). CSI in a V-BLAST system is available at the receiver only by means of channel estimation. Figure 1 shows the simple block diagram of a V-BLAST system. V-BLAST detection can be accomplished by using linear detectors like zero-forcing (ZF) or minimum mean square error (MMSE) detector along with symbol cancellation (also called successive interference cancellation). Symbol cancellation is a nonlinear technique which enhances the detection performance by subtracting the detected components of the transmit vector from the received symbol vector BIB001 . This technique, however, is prone to error propagation. The QR decomposition of the MIMO channel matrix can be used to represent the ZF nulling in V-BLAST BIB003 . Assuming a frequency-flat fading MIMO channel, the corresponding sampled baseband received signal for a V-BLAST system with M transmit and N receive antennas (M N) is therefore given by  ii discrete-time index is dropped to simplify notation. Multiplying both sides of Equation (3) by Q H gives y Rx n (4) The sequential signal detection in V-BLAST can be accomplished as follows BIB003 : for : 1: where represents mapping to the nearest modulation symbol. The results for an initial V-BLAST prototype mentioned in BIB001 yielded spectral efficiencies of 20-40 bps/Hz in indoor scenarios which is quite impressive. However, later has shown that V-BLAST also performs reasonably well in mobile scenarios and can be employed for MIMO-OFDM systems as well and further improvements have been suggested in the literature. BIB002 proposes an extension of V-BLAST incorporating power and rate feedback which approaches closed-loop MIMO capacities. Equal power allocation with per-antenna rate control (PARC) produces the best results for the proposed system. PARC enables the transmitter to select the appropriate data rate and the associated modulation and coding scheme (MCS) for each transmit antenna based on the feedback of channel quality information from the receiver BIB004 . It presents a comparison between a modified V-BLAST system with limited feedback (including the modulation index and the number of streams to be used) and closed-loop MIMO (CL-MIMO) in BIB005 . CL-MIMO shows 15.1% throughput improvement for Rayleigh fading channel, 48.1% for spatially correlated channel and 104% for the case of a realistic channel model, at SNR of 25 dB. Figure 2 shows these results. 