Precoding Matrix Index Research Articles

Secret Key Exchange Using Private Random Precoding in MIMO FDD and TDD Systems

Along with the ongoing evolution of multiple antenna communication systems, new physical layer security techniques are continuing to achieve higher levels of secrecy. Most physical layer approaches, however, concern time division duplex (TDD) channels which rely on using the channel reciprocity feature as a shared randomness and tend to be associated with a large computational burden. In this paper, we propose a new physical layer method which utilizes private random precoding for exchanging the secret key bits in multiple-input–multiple-output (MIMO) systems. The principle of this method is to exploit the precoding matrix index in a manner that produces low correlation at the adversary. A robust key exchange between the transmitter and the receiver is established by uniquely relating the secret key bits to the channel precoding matrix using a private version of the universal codebooks. What is more is that the proposed method is applicable in both frequency division duplex (FDD) and TDD channels. The results demonstrate that the proposed method can offer superior performance in terms of the key agreement, secrecy level, and computational load.

Position-Based Limited Feedback Scheme for Railway MU-MIMO Systems

In future railway communications, the system architecture is highly likely to be based on the mobile relay structure. However, during the transition period, onboard passenger equipment can only directly connect to wayside public mobile networks to get wireless services. As an efficient technology to increase the capacity widely used in Long-Term Evolution (LTE) public mobile networks, multiuser multiple input–multiple output (MU-MIMO) requires channel state information (CSI) of users at base stations to suppress interuser interference, which poses a bandwidth burden on feedback links. Thus, how to reduce the MU-MIMO feedback load becomes an urgent problem. In railway scenarios, onboard passenger equipment of a user set almost has the same signal-to-noise ratio (SNR), which provides a new way to reduce the MU-MIMO feedback overheads. Accordingly, we propose a position-based limited feedback scheme for railway MU-MIMO systems, in which dedicated feedback resource positions are preassigned to codebook vectors. A resource position where users feed back channel quality indicators (CQIs) indicates a special codebook vector. Then, overheads due to precoding matrix index (PMIs) are saved. Even if bandwidth resources assigned to feedback positions may be insufficient to supply every user with a feedback chance, it still has little impact on the total capacity for whether the user with the largest CQI in an active user set is selected in highly populated railway scenarios. Moreover, a novel performance criterion called feedback efficiency is proposed. Simulation results demonstrate that the proposed scheme can achieve higher feedback efficiency compared with the conventional scheme.

Feedback Generation for CoMP Transmission in Unsynchronized Networks with Timing Offset

Coordinated multipoint (CoMP) transmission is a promising technique in long term evolution (LTE) systems to provide coverage and broadband communication for cell edge user equipments (UEs). However, as signals from multiple transmitters may reach the UE at different times, CoMP networks might experience high time offsets, leading to significant performance loss in closed-loop transmission. In this paper we show that spacing between reference signals (RSs) imposes a phase offset on the transmit covariance matrix in presence of timing offset (TO), affecting the feedback generation. The promised advantages of closed-loop CoMP transmission vanish in presence of TO due to improper precoding matrix index (PMI) selection. Keeping the phase shift close to zero, reliable PMI selection can be guaranteed and as a result near optimum performance is achieved for closed-loop CoMP transmission in unsynchronized networks with TO present.

Design of reduced complexity feedback precoding for MIMO‐OFDM

SummaryBeamforming technique is applied to significantly increase the performance of a MIMO system, if the channel information (CI) of the communication system is available at the transmitter. For the transmitter to obtain the entire CI, however, a considerable reverse‐link bandwidth is required. To save the bandwidth, a limited‐rate closed‐loop system, therefore, uses a predetermined codebook which is derived from the CI. The codebook consists of a finite number of precoders out of which the index of the best one is transmitted from the receiver to the transmitter using only a few bits saving substantial bandwidth. However, the amount of bits that need to be fed back can still be significantly large for MIMO‐OFDM systems when the precoding matrix index (PMI) for each subcarrier should be transmitted. Such per‐subcarrier precoding scheme has high feedback overhead and also incurs huge computational cost to determine the best PMI for each subcarrier. We, therefore, propose a per‐band precoding scheme that precodes a band (group) of subcarriers by only one precoder. More importantly, we develop, for the proposed per‐band scheme, reduced‐complexity precoding selection methods that lead to the design of efficient receivers. The effectiveness of the proposed scheme is investigated through computer simulations. Copyright © 2014 John Wiley & Sons, Ltd.

Practical Physical Layer Security Schemes for MIMO-OFDM Systems Using Precoding Matrix Indices

In physical-layer security, secret bits are extracted from wireless channels. With the assumption of channel reciprocity, the legitimate users share the same channel which is independent of the channels between the legitimate users and the eavesdropper, leading to secure transmissions. However, practical implementation of the physical layer security faces many challenges. First, for the correlated channel such as the multiple-input and multiple-output (MIMO) channel, the security is decreased due to the correlation between the generated secret bits. Second, the nearby eavesdropper posts a security threat due to observing the same channel as the legitimate user's. Third, the eavesdroppers might try to reconstruct the wireless environments. In this paper, we propose two practical physical layer security schemes for the MIMO orthogonal frequency-division multiplexing (MIMO-OFDM) systems: the precoding matrix index (PMI)-based secret key generation with rotation matrix (MOPRO) and the channel quantization-based (MOCHA) scheme. The former utilizes PMI and rotated reference signals to prevent the eavesdroppers from learning the secret key information and the latter applies channel quantization in order to extract more secret key bits. It is shown that not only the secure communication but also the MIMO gain can be guaranteed by using the proposed schemes.

Enhanced Limited Feedback Schemes for DL MU-MIMO ZF Precoding

This paper proposes new limited-feedback Channel State Information (CSI) calculation schemes for Zero Forcing (ZF)-precoded downlink Multi-User Multiple-Input Multiple-Output (MU-MIMO) systems. It is a common understanding that the feedback quantized by the codebook limits the performance of MU-MIMO. In this paper, through a quasi-ZF or a quasi-Minimum Mean-Squared Error (MMSE) weight, the channel matrix is transferred to one of the codebook vectors, based on which, the CSI is calculated. Thus, the quantization error is minimized. Meanwhile, the selection for the codebook vector guarantees the maximizing of the estimated Signal to Interference plus Noise Ratio (SINR). We verify that the proposed scheme obtains accurate feedback information, and the predicted weight can be the same as the optimal linear decoder as if the receiver knew all the precoder information that is fed-forward from the BS, as long as the number of antennas for each receiver equals that for the transmitter, and equals that for the total transmit data streams. Compared to the commonly used Precoding Matrix Index (PMI) based method, which uses rank-one single user (SU)-MIMO feedback, simulation results show that the proposed schemes achieve higher sum capacities in different scenarios. Moreover, since the weight can be directly used as the decoder, the feed-forward overhead is reduced.

MIMO precoding and mode adaptation in femtocellular systems

Hierarchical femtocellular architectures have become popular recently because of their potential to provide increased coverage and capacity in cellular systems. However, introduction of femtocells might reduce the overlay macrocellular system performance due to increased interference caused to macrocellular users. In this article, two MIMO precoding techniques are considered at the femtocellular base stations (FBSs) to control the interference to the macrocellular users: precoding matrix index (PMI), and least interference (LI). With MIMO precoding, the limited CSI at the transmitter is the index of the precoder chosen from the codebook fed back by the receiver. The LI technique can be employed at the FBSs to maximize the macrocellular throughput, but it also results in significant reduction in femtocellular throughput. The PMI approach can maximize the signal power at a desired receiver, with minimal feedback. In this article, we develop algorithms that adapt at the FBSs between the LI and PMI schemes to increase both the macrocellular and femtocellular throughputs. We show that allowing for mode adaptation at each FBS improves the system performance when compared with using the same mode across the system, and a simple binary choice at each FBS can nearly achieve the optimum mode-adaptation performance. Analysis and simulation results in a multicell environment are presented to illustrate the improvement in system performance with the proposed techniques.

Statistical Channel State Information Aided Proportional Fair Scheduling Scheme for Highly Transmit Correlated Channels

In this paper, we consider the downlink heterogeneous scenario with highly transmit correlated channels, and propose two kinds of the statistical channel state information (SCSI) aided proportional fair scheduling (PFS) schemes: (1) signal-to-noise ratio based PFS (SNR PFS) scheme and (2) SNR PFS scheme with threshold, which determine the preferred beamforming vector and proportional fair index by exploiting the SCSI and only send back the instantaneous channel quality indicator (CQI) without the corresponding precoding matrix index at each timeslot. Although the SNR PFS scheme only requires little feedback overhead and low computational complexity, the performance of the SNR PFS scheme is nearly the same as the conventional scheme (i.e., codebook-based PFS scheme). However, with the increasing number of users, a large number of CQI feedbacks will increase the signaling burden. To overcome this obstacle, the proposed SNR PFS scheme with threshold which sets an appropriate feedback threshold to limit the CQI feedbacks with poor channel quality would hardly affect the performance of the scheme. We further derive the approximate rate expressions of the SNR PFS scheme by using probability statistics and the theory of order statistics, whose result is nearly approximated to that of the Monte Carlo simulation. Finally, our simulation results show that the performance of the SNR PFS scheme with β th = ln(K/5) is close to that of the SNR PFS scheme, while it has less limited feedback capacity than the SNR PFS scheme.

CSI Feedback for Dynamic Switching Between Single User and Multiuser MIMO

We consider channel state information (CSI) feedback in 3GPP Long Term Evolution (LTE)-Advanced context. In LTE-Advanced, switching between single user and multiuser transmission schemes is possible without higher layer signaling, which means that the feedback should support both single user and multiuser transmissions. Typically, the CSI feedback consists of a precoding matrix index (PMI) and channel quality indication(s) (CQI). For PMI feedback, we consider different PMI selection schemes and study whether there is a tradeoff between single user and multiuser specific codeword selection metrics. For multiuser CQI, we consider different CQI estimation strategies for two paired users, which is the primary case in LTE-Advanced. The schemes include single user single stream and two stream CQIs and several multiuser specific CQI estimation options. We find that estimating the multiuser CQI as an average over unitary pairs or as the minimum of the signal-to-interference and noise ratios of the unitary pairs offers a stable, well-performing options for different signal-to-noise ratio (SNR) ranges and antenna correlation values.

Vector perturbation based adaptive distributed precoding scheme with limited feedback for CoMP systems

A downlink adaptive distributed precoding scheme is proposed for coordinated multi-point (CoMP) transmission systems. The serving base station (BS) obtains the optimal precoding vector via user feedback. Meanwhile, the precoding vector of each coordinated BS is determined by adaptive gradient iteration according to the perturbation vector and the adjustment factor based on the vector perturbation method. In each transmission frame, the CoMP user feeds the precoding matrix index back to the serving BS, and feeds back the adjustment factor index to the coordinated BSs, which can reduce the uplink feedback overhead. The selected adjustment factor for each coordinated BS is obtained via the precoding vector of the coordinated BS used in the previous frame and the preferred precoding vector of the serving BS in this frame. The proposed scheme takes advantage of the spatial non-correlation and temporal correlation of the distributed MIMO channel. The design of the adjustment factor set is given and the channel feedback delay is considered. The system performance of the proposed scheme is verified with and without feedback delay respectively and the system feedback overhead is analyzed. Simulation results show that the proposed scheme has a good trade-off between system performance and the system control information overhead on feedback.

Evaluation of PMI Feedback Schemes for MU-MIMO Pairing

The status of codebook based MU MIMO feedback schemes in different wireless standards are briefly overviewed. Based on the Normal Feedback Scheme to feedback the preferred Precoding Matrix Index (PMI), a Best Companion Pairing (BCP) scheme and a Best Companion Cluster (BCC) scheme are introduced to make more efficient pairing. Besides the preferred PMI, UE need to feedback more PMIs for the BCP scheme or a cluster index for the BCC scheme to minimize the intracell or intercell interference. System level simulation results show that the BCC scheme can make a reasonable tradeoff between feedback overhead and throughput.