Daofeng Xu

A limited feedback precoding system with hierarchical codebook and linear receiver

In this paper, the conventional Grassmannian codebook for precoding is first analyzed, showing that the performance loss caused by linear receivers was not taken into account. To tackle the performance loss issue, a novel hierarchical codebook consisting of a Grassmannian subcodebook and a perturbation subcodebook is then proposed for precoding systems with linear receivers. A two-step codeword selection scheme that uses the product of two codewords selected from the subcodebooks as the precoder is also presented. Our analysis shows that the perturbation subcodebook is able to compensate for the performance loss from linear receivers. Compared with the Grassmannian codebook, the superiority of the proposed codebook in terms of search complexity as well as throughput/ BER is further confirmed by computer simulations.

Feedback of Downlink Channel State Information Based on Superimposed Coding

In closed-loop FDD MIMO system, downlink channel state information (DL-CSI) is usually feedback to base station in forms of codebook or CQI, both of which aim at lowering the feedback quantity at the cost of limited feedback precision and heavy processing complexity at mobile side. Meanwhile, the recently proposed direct channel feedback method incurs great system overhead due to its exclusive occupation of uplink bandwidth resources. We propose a low-cost feedback method for DL-CSI, which spreads unquantized and uncoded DL-CSI and superimposes it onto uplink user data sequences (UL-US). Exclusive occupation of system resources by DL-CSI can thus be avoided. Due to spreading, DL-CSI can be estimated accurately with little power allocation at the cost of some UL-USs SER performance

Improved nonlinear multiuser precoding using lattice reduction

Recent theoretical results show that multiuser techniques such as Spatial Division Multiple Access can significantly improve system capacity. In order to achieve this end, lots of precoding methods have been proposed, among which Vector Perturbation precoding has the best Symbol Error Rate performance but with the hugest complexity in searching for the optimal perturbation vector. In this paper, we propose a low complexity, near optimal multiuser precoding method by utilizing lattice reduction of the channel matrix. The perturbation vector is composed of an integer part and a non-integer part, which are designed to simultaneously improve power efficiency and minimize the mean squared error MSE. The integer perturbation vector is calculated implicitly with modulo operation, and the non-integer perturbation can be solved in closed form under MSE minimization criterion. Simulation results demonstrate the effectiveness of the proposed methods.

Linear Transceiver Design for Multiuser MIMO Downlink

An iterative linear transceiver design scheme under sum mean squared error minimization criterion is proposed. By modifying the structure of transceiver, the complex computation of Lagrangian multiplier within traditional MMSE transceiver design can be effectively obviated, and thus the whole system complexity can be greatly reduced. Because the Lagrangian multiplier has analytical solution, transmit preceding matrix also has closed-form solution, and can be solved easily with fixed- point iterations. The receiver filter is worked out independently with MMSE criterion at each terminal, and the downlink signaling of each receive filter from base station is not necessary- Simulations demonstrate that the proposed scheme is effective.

Limited Feedback Precoding Based on Hierarchical Codebook and Linear Receiver

The analysis shows that the conventional codebook construction of Grassmannian subspace packing can not control the performance loss caused by a linear receiver, while a proper unitary perturbation to the codebook is capable of compensating for this performance loss. This paper therefore proposes a novel hierarchical codebook to exploit the gain of unitary perturbation. The hierarchical codebook consists of a Grassmannian subcodebook and a proposed perturbation subcodebook. To implement precoding, this paper also presents a successive codeword selection scheme, thus the receiver would successively selects two preferred codewords from Grassmannian and perturbation subcodebooks. With the feedback binary indices of these selections, the transmitter uses the product of two preferred codewords as the precoder. The theoretical analysis of the proposed precoding technique shows that the usage of the perturbation subcodebook can improve to a certain degree the system performance in terms of throughput as well as BER, with a small additional feedback overhead, and the proposed codebook would reduce the computational and storage requirements in contrast to the conventional codebook. It is also shown via computer simulations that the proposed technique gives a better BER performance than the single Grassmannian codebook based precoding technique does, even if the feedback overhead remains the same.

Adaptive Antenna Grouping for Space-Time Block Coding and Spatial Multiplexing Hybrid System

This paper studies joint transmitter and receiver design for the space-time block coding and spatial multiplexing hybrid communication system with limited feedback. Existed QR group receiver uses QR decomposition of the channel matrix to separate the filtered versions of the multiplexed space-time coded symbol blocks followed by space-time decoder, and the performance of this scheme is usually limited due to diversity lack of the earliest detected block. To overcome this problem, in this paper each space-time data block adaptively selects the appropriate transmit antenna group according to channel state information. Furthermore, this transmit antenna grouping can be derived with low complexity using successive QR decomposition of channel matrix at the receiver, thus only the index of antenna grouping result in a predetermined codebook is needed to fed back to the transmitter. The performance of the proposed schemes is evaluated in correlated channels through computer simulations.

Energy-efficiency of Very Large Multi-user MIMO systems

Energy efficiency is becoming increasingly important for wireless communications as mobile devices are battery-constrained. Motivated by this, in this paper we consider uplink energy-efficient resource allocation in Very Large Multi-user MIMO systems. Specifically, both the number of antenna arrays at BS and the transmit power at the user are adjusted to maximize the energy efficiency, in which the power consumption accounts for both transmit power and circuit power. We demonstrate the existence of a unique globally optimal number of antenna arrays at BS and the transmit power and provide iterative algorithms to obtain it. Our simulation results show that the performance of the proposed algorithms is close to the optimum, while having low complexity. It is also found that the distance between the base station and the users plays an important role in increasing the energy efficiency, namely, the energy efficiency is increasing with the distance decreasing.

(Semi-)blind channel estimation for space time block coded system with transmit diversity

This paper deals with a blind channel estimation method for space-time coded block transmission system. By concatenating the real part and imaginary part of the received signal to form an elongated vector, we derive an equivalent input–output system model. Then channel state information (CSI) is blindly estimated using subspace method, utilizing only the redundancy inherent in space-time block coding (STBC) and cyclic prefix (CP). The estimation ambiguity, which is common to all blind methods, is analyzed in detail and we prove that there only exist four scalar indeterminacies. Three effective methods to eliminate the ambiguities are also proposed. Compared with other blind channel estimation methods for space time systems, this method needs neither redundant precoding nor oversampling, and thus has higher data rate. Besides, this method is robust to channel order overestimation, which is effectively demonstrated by numerical simulations.

Robust Precoding for Space Time Block Codes and Spatial Multiplexing Hybrid System

This paper proposes a robust precoding for the space time block codes (STBC) and spatial multiplexing (SM) hybrid system. The overall precoding is disassembled into independent precoder optimization of different STBC groups by decomposing the overall channel matrix into several independent virtual channel matrices, this method will considerably reduce the amount of the required feedback information. Also, the precoder optimization is based on a new derived SER bound and considers the uncertainty of the channel feedback. Simulation results demonstrate the performance of the proposed technique.

Subspace-Based Blind Channel Estimation for STBC-OFDM

This paper proposes a subspace-based blind channel estimation method for space-time coded OFDM system. Using only the redundancy induced by OFDM modulation and space-time block coding (STBC), channel state information (CSI) can be blindly estimated, up to two scalar ambiguities for Alamouti STBC or one for 4T4A3K STBC, even when a single receiving antenna is equipped. Compared with other blind channel estimation method for STBC-OFDM, this method needs neither pre-coding nor over-sampling, and thus has higher system data rate and lower complexity. Simulation results demonstrate the effectiveness of this method