Precoding

In this paper we formulate the problem of finding the optimal precoding/multiplexing strategy in an infrastructureless multiuser scenario as a noncooperative game. We first consider the theoretical problem of maximizing mutual information on each link, given constraints on the spectral mask and transmit power. Then, to accommodate practical implementation aspects, we focus on the competitive maximization of the transmission rate on each link, using finite order constellations, under the same constraints as above plus a constraint on the average error probability. We prove that in both cases a NE always exists and the optimal precoding/multiplexing strategy leads to a (pure strategy) diagonal transmission for all the users. Thanks to this result, we can reduce both original complicated matrix-valued games to a simpler unified vector power control game. Thus, we derive sufficient conditions for the uniqueness of the NE of such a game, that are proved to have a broader validity than conditions known in the literature for special cases of our game. Finally, we show that the Nash equilibria of the vector game can be reached using the so-called asynchronous iterative waterfilling algorithm.

The downlink multiuser precoding of the multiple-input multiple-output (MIMO) method includes optimal channel state information at the base station and a variety of linear precoding (LP) schemes. Maximum ratio transmission (MRT) is among the common precoding schemes but does not provide good performance with massive MIMO, such as high bit error rate (BER) and low throughput. The orthogonal frequency division multiplexing (OFDM) and precoding schemes used in 5G have a flaw in high-speed environments. Given that the Doppler effect induces frequency changes, orthogonality between OFDM subcarriers is disrupted and their throughput output is decreased and BER is decreased. This study focuses on solving this problem by improving the performance of a 5G system with MRT, specifically by using a new design that includes weighted overlap and add (WOLA) with MRT. The current research also compares the standard system MRT with OFDM with the proposed design (WOLA-MRT) to find the best performance on throughput and BER. Improved system results show outstanding performance enhancement over a standard system, and numerous improvements with massive MIMO, such as best BER and throughput. Its approximately 60% more throughput than the traditional systems. Lastly, the proposed system improves BER by approximately 2% compared with the traditional system.

The present article carries out a review of MIMObased techniques that have been recently proposed for satellite communications. Due to the plethora of MIMO interpretations in terrestrial systems and the particularities of satellite communications, this review is built on two pillars, namely fixed satellite and mobile satellite. Special attention is given to the characteristics of the satellite channel, which will ultimately determine the viability of MIMO over satellite. Finally, some future research directions are identified.

This paper provides a comprehensive overview of critical developments in the field of multiple-input multiple-output (MIMO) wireless communication systems. The state of the art in single-user MIMO (SU-MIMO) and multiuser MIMO (MU-MIMO) communications is presented, highlighting the key aspects of these technologies. Both open-loop and closed-loop SU-MIMO systems are discussed in this paper with particular emphasis on the data rate maximization aspect of MIMO. A detailed review of various MU-MIMO uplink and downlink techniques then follows, clarifying the underlying concepts and emphasizing the importance of MU-MIMO in cellular communication systems. This paper also touches upon the topic of MU-MIMO capacity as well as the promising convex optimization approaches to MIMO system design.

An improvement to lattice-reduction-aided (LRA) vector perturbation precoding for multi-user MIMO downlink is introduced. Closest lattice point approximation by means of lattice reduction techniques can significantly lower the complexity of the closest point search compared to using a sphere encoder, but the performance of the system is also impaired. In this paper, we propose a new technique improving the suboptimal LRA closest-point approximation in a subsequent stage. This stage consists of a low-complexity candidate list generation of also likely approximations, and an evaluation step of this list. We present simulation results showing that our improvement to the LRA closest-point approximation can achieve near-optimum performance.

Multiple-input multiple-output (MIMO) has been adopted by long term evolution (LTE) and its updated version LTE-Advanced (LTE-A) to increase the spectrum efficiency. However, the benefits of multi-user (MU) MIMO highly rely on the accurate channel knowledge at the transmitter. So proper codebook design is a key problem for LTE-A FDD downlink system. In this paper, a codebook design with phase adjustment in the precoder targeting wideband channel properties is proposed. The average cell spectrum efficiency and cell-edge user spectrum efficiency of the proposed scheme are evaluated through system level simulation and compared to the discrete Fourier transform (DFT) based feedback scheme. The results show that the proposed codebooks significantly improve feedback efficiency and system performance for co-polarized linear antenna array.

Multiple Input Multiple Output (MIMO) system provides lot of advantages when introduced in a wireless communication system. It helps in increasing system throughput and reducing error rates. Moreover, designing a MIMO system to support both multi-user and multi-cell provides larger increase in system throughput and allows an increase in number of users. This comes at the cost of how to design that system such that the interference generated is minimized. In this paper, a precoding scheme for multi-user multi-cell MIMO system is proposed to mitigate the interference generated in the downlink direction. To design the precoders, we use a metric called signal to generating interference, leakage and noise ratio (SGILNR), which is a combination of two previous metrics: one that was used to design the precoders for a multi-cell system and the other that was used for a multi-user system. Simulation results show that the proposed scheme outperform the conventional MIMO system. Also the limitations of the proposed scheme when number of users becomes higher are discussed.

LTE (Long Term Evolution) is a next major step in mobile radio communications, and will be introduced as Release 8 in the 3rd Generation Partnership Project (3GPP). The new evolution aims to reduce delays, improve spectrum flexibility and reduce cost for operators and end users [1]. To fulfil these targets, new enabling technologies need to be integrated into the current 3G radio network architectures. Multiple Input and Multiple Output (MIMO) is one of the crucial enabling technologies in the LTE system particularly in the downlink to achieve the required peak data rate. The unitary codebook based precoding technique is proposed in the standard to increase the capacity of the system. This paper presents a link level analysis of the LTE downlink and an investigation of the performance of both Single User (SU) MIMO and Multi User (MU) MIMO with codebook based unitary precoding.

In this paper, we will focus on the collaborative MIMO in combination with the LTE-advanced system. In this system, two or more users each using multiple antennas are transmitting their data within the same frequency/time grid. In this paper, we investigate the MIMO enhancements introduced to the LTE uplink of the LTE-advanced system. User equipment here is equipped by 2-4 antennas which can be used along the collaborative MIMO system to increase data rate, enhance system performance and match channel conditions according to the channel knowledge at transmitter. We propose a space frequency block codes (SFBC) precoding for the uplink to achieve space diversity in accompany with spatial multiplexing gain achieved before using the collaborative system. Then, we propose a combination between the collaborative system and the precoded MIMO whether ideally (SVD precoding) or suboptimally using codebook precoding. The results show dramatic enhancement in performance and achieves the peak spectral efficiency of the uplink mode of the LTE-advanced.

A filter-bank modulation technique called filtered multitone (FMT) and its application to data transmission for very high-speed digital subscriber line technology are described. The proposed scheme leads to significantly lower spectral overlapping between adjacent subchannels than for known multicarrier techniques such as discrete multitone (DMT) or discrete wavelet multitone. FMT modulation mitigates interference due to echo and near-end crosstalk signals, and increases the system throughput and reach. Signal equalization in an FMT receiver is accomplished in the form of per-subchannel symbol-spaced or fractionally spaced linear or decision-feedback equalization. The problem of channel coding for this type of modulation is also addressed, and an approach that allows combined removal of intersymbol-interference via precoding and trellis coding is described. Furthermore, practical design aspects regarding filter-bank realization, initial transceiver training, adaptive equalization, and timing recovery are discussed. Finally, simulation results of the performance achieved by FMT modulation for very high-speed digital subscriber line systems, where upstream and downstream signals are separated by frequency-division duplexing, are presented and compared with DMT modulation

We study a cellular system scenario where multiple data streams originating from a base station (BS) targeted to multiple cell-edge mobile stations (MSs) are transmitted via preinstalled cooperative relay stations (RSs) with multiple antennas. Our objective is to guarantee quality-of-service (QoS) in terms of predefined signal-to-noise ratio at such users within the transmit power budgets at BS and RSs while minimizing total transmit power. We propose a novel precoder design method for power allocation between multiple data streams at BS and RS by using joint zero-forcing strategy in order to avoid multiuser interference (MUI) in the signal received by MSs via both the direct and relay links. We also propose low-complexity suboptimal power allocation algorithm. We focus on analyzing the significance of direct link transmission in providing QoS to celledge MSs specially when RS is not situated directly between BS and MSs. Simulation results show that considering direct link and using the proposed scheme in such case significantly improves system outage performance compared to existing schemes in the literature which do not consider direct link.

Recent researches have greatly considered allocation of space resources in multiuser MIMO systems. The studies have proposed techniques to select the optimum group of users and their antennas greedily or fairly and transmit precoding methods to transmit multiple streams simultaneously to selected users with suppressed or minimum inter-user interference. According to our understanding, a new research trend in cellular multiuser MIMO systems is the consideration of other-cell interference in the scheduling and precoding methods. In fact, scheduling techniques have not been considered as much as precoding ones in such a system. So, in this article we propose some user and antenna selection methods in a system with other-cell interference. We also propose another modification to a modified BD precoding method for this system to lower the channel state information feedback rate.

Receivers for horizontally encoded LTE based MIMO-OFDM systems are considered in this paper. Adaptive modulation and coding (AMC) is used as well as precoding. The linear minimum mean square error (LMMSE), successive interference cancellation (SIC) and K-best list sphere detectors (LSD) are compared. The receivers were designed and implemented for 2×2 and 4×4 antenna systems and meet the decoding rate requirement in LTE, i.e, 210 Mb/s in 2×2 and 405 Mb/s in 4×4 antenna systems. The results show that the performance of the receivers is similar in low SNR but the performance difference increases when a higher rank transmission is used. The K-best LSD has the highest performance and complexity. A simpler receiver could be used in the low SNRs to save power and a more complex receiver in the high SNRs when a higher goodput is needed.

We consider a time division duplex (TDD) nt × nr multiple-input multiple-output (MIMO) system with channel state information (CSI) at both the transmitter and receiver. We propose X-and Y-Codes to achieve high multiplexing and diversity gains at low complexity. The proposed precoding schemes are based upon the singular value decomposition (SVD) of the channel matrix which transforms the MIMO channel into parallel subchannels.

Tomlinson-Harashima precoding (THP) is considered as a prominent precoding scheme due to its capability to efficiently cancel out the known interference at the transmitter side. Therefore, the information rates achieved by THP are superior to those achieved by conventional linear precoding schemes. In this paper, a new lower bound on the achievable information rate for the regularized THP scheme under additive white Gaussian noise (AWGN) channel with multiuser interference is derived. Analytical results show that the lower bound derived in this paper is tighter than the original lower bound particularly for a low SNR range, while all lower bounds converge to 0.5xlog2(6SNR/{\pi}e) as SNR approaches infinity.

In this paper, we jointly solve the problem of transmit antenna se-lection and zero-forcing (ZF) precoding in a multiple input multiple output (MIMO) system. A new problem formulation is proposed which enables efficient semi-definite programming (SDP) to solve the originally non-convex problem of antenna selection. This has been accomplished by imposing the Group Lasso sparsity promot-ing term in the precoding design criterium as a convex relaxation of the `0-norm operation. For the selected set of antennas, we then min-imize the overall transmit power, subject to a constraint on the max-imum achievable throughput. Simulation results reveal the power saving advantage of the proposed algorithm compared to a randomly selected subset of antennas. Index Terms — Multiple input multiple output (MIMO) , linear precoding, convex optimization, antenna selection, Group Lasso. 1.

Visible light communication is innovative and active technique in modern digital wireless communication. In this paper, we describe a new innovative vlc system which having a better performance and efficiency to other previous system. We used multiple input multiple output (MIMO) with orthogonal frequency division multiplexing (OFDM) technique which is enhance the data rate of wireless data transmission. Since we know the multiple transmitter and receiver having different links distances, having different temporal delays, complex channel gain and phase differences is resulting when frequency domain have transformed. In OFDM each subcarrier, the calculation of corresponding pre-coding matrix in the frequency domain for elimination of multiple interference. We first considered phase information in frequency domain, where precoding are uses of complex matrices, real, channel matrices. Which is used for reduces the channel correlation for achieve better performance. This paper describes a detailed survey of (1) VLC system and characteristics of transmitter and receiver, (2) Modulation methods, (3) SISO, MIMO, MIMO-OFDM, (4) VLC sensing and applications, (5) VLC system architecture design and programming platform. Keywords: Multiple-input multiple-output (MIMO), multiuser, orthogonal frequency-division multiplexing (OFDM), precoding, visible light communication (VLC) ________________________________________________________________________________________________________

High Peak to Average Power Ratio (PAPR) is one of the major drawbacks in Orthogonal Frequency Division Multiplexing (OFDM). The high PAPR increases the complexity of Analogue to Digital (A/D) and Digital to Analogue (D/A) converters and also ...

In this paper we examine the use of generalized singular value decomposition (GSVD) for coordinated beamforming in MIMO systems. GSVD facilitates joint decomposition of a class of matrices arising inherently in source-to-2 destination MIMO broadcast scenarios. GSVD allows two channels of suitable dimensionality to be jointly diagonalized, i.e. to be reduced to non-interfering virtual broadcast channels, through the use of jointly determined transmit precoding and receiver reconstruction matrices.

Several high-speed wireless systems use Orthogonal Frequency Division Multiplexing (OFDM) due to its advantages. 5G has adopted OFDM and is expected to be considered beyond 5G (B5G). Meanwhile, OFDM has a high Peak-to-Average Power Ratio (PAPR) problem. Hybridization between two PAPR reduction techniques gains the two techniques’ advantages. Hybrid precoding-companding techniques are attractive as they require small computational complexity to achieve high PAPR reduction gain. Many precoding-companding techniques were introduced to increasing the PAPR reduction gain. However, reducing Bit Error Rate (BER) and out-of-band (OOB) radiation are more significant than increasing PAPR reduction gain. This paper proposes a new precoding-companding technique to better reduce the BER and OOB radiation than previous precoding-companding techniques. Results showed that the proposed technique outperforms all previous precoding-companding techniques in BER enhancement and OOB radiation reduction. The proposed technique reduces the Error Vector Magnitude (EVM) by 15 dB compared with 10 dB for the best previous technique. Additionally, the proposed technique increases high power amplifier efficiency (HPA) by 11.4%, while the best previous technique increased HPA efficiency by 9.8%. Moreover, our proposal achieves PAPR reduction gain better than the most known powerful PAPR reduction technique with a 99% reduction in required computational complexity. View Full-Text

This paper considers the multiuser multiple-input multiple-output (MIMO) broadcast channel. We consider the case where the multiple transmit antennas are used to deliver independent data streams to multiple users via vector perturbation. We derive expressions for the sum rate in terms of the average energy of the precoded vector, and use this to derive a high signal-to-noise ratio (SNR) closed-form upper bound, which we show to be tight via simulation. We also propose a modification to vector perturbation where different rates can be allocated to different users. We conclude that for vector perturbation precoding most of the sum rate gains can be achieved by reducing the rate allocation problem to the user selection problem. We then propose a low-complexity user selection algorithm that attempts to maximize the high-SNR sum rate upper bound. Simulations show that the algorithm outperforms other user selection algorithms of similar complexity.

In this paper, upper and lower bounds on the transmission capacity of spread-spectrum (SS) wireless ad hoc networks are derived. We define transmission capacity as the product of the maximum density of successful transmissions multiplied by their data rate, given an outage constraint. Assuming that the nodes are randomly distributed in space according to a Poisson point process, we derive upper and lower bounds for frequency hopping (FH-CDMA) and direct sequence (DS-CDMA) SS networks, which incorporate traditional modulation types (no spreading) as a special case. These bounds cleanly summarize how ad hoc network capacity is affected by the outage probability, spreading factor, transmission power, target signal-to-noise ratio (SNR), and other system parameters. Using these bounds, it can be shown that FH-CDMA obtains a higher transmission capacity than DS-CDMA on the order of 1 , where is the spreading factor and 2 is the path loss exponent. A tangential contribution is an (apparently) novel technique for obtaining tight bounds on tail probabilities of additive functionals of homogeneous Poisson point processes.

This paper proposes an efficient sub-optimal MIMO linear precoder based on the maximization of minimum distance for three virtual suchannels. A new virtual MIMO channel representation with two channel angles allows the parameterization of the linear precoder and the optimization of the distance between signal points at the received constellation. To illustrate the optimization process, a precoder is derived for BPSK and QPSK modulation following the max-SNR approach, which consists in pouring power only on the most favored virtual sub-channel. Simulation results over a Rayleigh channel confirm the interest of this new precoder in terms of bit-error-rate.

Massive MIMO systems are now well known for their energy efficiency. The ability to focus the energy to a targeted user not only increases the gain of useful signal but also reduces inter-user interference to the point where Spatial Division Multiple Access becomes possible.
Using such systems at high frequencies, thus greatly reducing the array form factor, seems a smart mix and we give an insight into how well an angle based precoder fares compared to well known massive MIMO linear precoders in a highly Line-of-Sight (LOS) environment.

In this paper we prove the optimality of this angle based precoder, called Digital Beamsteering (DBS), in an ideal pure LOS configuration and then we show how it performs in a more realistic environment. Furthermore, its robustness against angle estimation error is illustrated for a specific scenario and serves to back up the importance such a solution represents for future dense 5G networks, angular information being easier to get and especially to keep track of.

We consider the problem of maximizing the harvested power in Multiple Input Multiple Output (MIMO) Simultaneous Wireless Information and Power Transfer (SWIPT) systems with power splitting reception. Different from recently proposed designs, we target with our novel problem formulation at the jointly optimal transmit precoding and receive uniform power splitting (UPS) ratio maximizing the harvested power, while ensuring that the Quality-of-Service (QoS) requirement of the MIMO link is satisfied. We assume generic practical Radio Frequency (RF) Energy Harvesting (EH) receive operation that results in a non-convex optimization problem for the design parameters, which we then solve optimally after formulating it in an equivalent generalized convex form. Our representative results including comparisons of achievable EH gains with benchmark schemes provide key insights on various system parameters.

In video transcoding, pre-encoded frames may be arbitrarily dropped to freely adjust the video to meet the network and client requirements. Since transcoding is carried out in real-time, incoming motion vectors are reused to reduce the transcoding latency. In this paper, we propose a new motion vector composition scheme for arbitrarily dropping any frame from incoming video bit-stream comprising I, B and P frames. The transcoded bit-stream retains the I-B-P frame structure. Experimental results are presented and compared to show the efficacy of the proposed scheme.

A simple linear precoding technique is proposed for multiple input multiple output (MIMO) broadcast systems using phase shift keying (PSK) modulation. The proposed technique is based on the fact that, on an instantaneous basis, the interference between spatial links in a MIMO system can be constructive and can contribute to the power of the useful signal to improve the performance of signal detection. In MIMO downlinks this co-channel interference (CCI) can be predicted and characterised prior to transmission. Contrary to common practice where knowledge of the interference is used to eliminate it, the main idea proposed here is to use this knowledge to influence the interference and benefit from it, thus gaining advantage from energy already existing in the communication system that is left unexploited otherwise. The proposed precoding aims at adaptively rotating, rather than zeroing, the correlation between the MIMO substreams depending on the transmitted data, so that the signal of interfering transmissions is aligned to the signal of interest at each receive antenna. By doing so, the CCI is always kept constructive and the received signal to interference-plus-noise ratio (SINR) delivered to the mobile units (MUs) is enhanced without the need to invest additional signal power per transmitted symbol at the MIMO base station (BS). It is shown by means of theoretical analysis and simulations that the proposed MIMO precoding technique offers significant performance and throughput gains compared to its conventional counterparts.

We address the possibility of overloaded vector precoding in single user MIMO channels, i.e. the number of data streams is larger than the minimum of the number of antennas at transmitter and receiver side. We find that the convex vector precoding introduced in [1] allows for overloading while, with a certain probability, keeping the received signal free of interference. We find that the probability that overloading is not possible decays exponentially with the size of the system as long as the number of data streams is less than twice the minimum number of antennas. We give an explicit formula to calculate this probability for any antenna configuration in presence of correlated Rayleigh fading. Although overloading comes with the need for increased transmitted power, we show by means of the replica method that overloading up to 22% yields better spectral and power efficiency than without overload and spatial matched filter processing at the receiver.

Previous designs of Tomlinson-Harashima precoding (THP) for the broadcast channel with multiple (cooperative) transmitters and noncooperative (decentralized) scalar receivers assume complete channel state information (CSI) at the transmitter. Due to its nonlinearity, THP is more sensitive to errors in CSI at the transmitter than linear precoding, which may be large in wireless communications. Thus, THP performance degrades severely-and it is even outperformed by corresponding linear precoders. We propose a novel robust optimization for THP with partial CSI based on mean-square error (MSE), which performs a conditional mean (CM) estimate of the cost function including a novel model of the receivers. This enables a smooth transition of robust THP from complete, via partial, to statistical CSI. The performance is now always superior or in the worst case equal to corresponding linear precoding techniques. With these features, the proposed robust THP is very attractive for application in the wireless broadcast channel.

This paper provides a comprehensive overview of critical developments in the field of multiple-input multiple-output (MIMO) wireless communication systems. The state of the art in single-user MIMO (SU-MIMO) and multiuser MIMO (MU-MIMO) communications is presented, highlighting the key aspects of these technologies. Both open-loop and closed-loop SU-MIMO systems are discussed in this paper with particular emphasis on the data rate maximization aspect of MIMO. A detailed review of various MU-MIMO uplink and downlink techniques then follows, clarifying the underlying concepts and emphasizing the importance of MU-MIMO in cellular communication systems. This paper also touches upon the topic of MU-MIMO capacity as well as the promising convex optimization approaches to MIMO system design.

In this paper, we consider the downlink of a multiuser wireless communication system with multiple antennas at the base station and users each with a single receive antenna. It is known that when channel state information (CSI) is available at the transmitter a large performance gain can be achieved. In a system employing time-division duplexing (TDD), CSI can be obtained at the base station if there is reciprocity between the forward and reverse channels. CSI can also be conveyed from the users to the base station via a limited-rate feedback channel in a frequency-division duplexing (FDD) system. In any case, channel estimation errors are inevitable due to the presence of background noise in the estimated signal and due to the finite number of feedback bits used in a limited-rate feedback system model. In this paper, we first consider the general case when partial CSI is available at the transmitter. We derive an MMSE based precoding technique that considers channel estimation errors as an integral part of the system design. Using rate-distortion theory and the generalized Lloyd vector quantization algorithm, we then specialize our results for the more practical limited-rate feedback system model. Compared to previously proposed precoding techniques such as channel inversion and regularized channel inversion, it is shown that the proposed precoding technique significantly improves the average bit error rate (BER) in the system. Furthermore, the performance of the proposed technique is investigated in the high signal-tonoise ratio (SNR) regime, and similar to [1], [2], it is shown that the proposed technique suffers from a ceiling effect that asymptotically limits the system performance. Index Terms-MIMO broadcast channels, downlink precoding, channel inversion, regularized channel inversion. Amir D. Dabbagh was born in Haifa, Israel. He received the B.Sc. (with honors) and M.Sc. degrees in electrical engineering from the Technion, Israel Institute of Technology in 1998 and 2002 respectively. During the years 2000-2004, he worked as a systems engineer in SmartLink, Netanya, Israel, where he designed and implemented different ITU-T modem related standards. In 2007, he earned a Ph.D. degree in electrical engineering from Purdue University at West Lafayette. From February 2007 until June 2008 he worked with the Networks Advanced Technology group in Motorola, Arlington Heights, IL. Since July 2008, he has been with Qualcomm, Inc., San Diego, CA. His research interests include communications theory, coding theory, and information theory.

The forthcoming wireless communication networks, commonly referred to as fourth generation (4G) systems, are expected to support extremely high data rates as close as possible to the theoretical channel capacity while satisfying quality of service (QoS) constraints. The development of these systems must take into account the problem of limited radio resources and the harshness of wireless channel conditions. Two emerging technologies that are potential candidates for 4G wireless networks are multiuser multiple-input multiple-output (MU-MIMO) wireless systems and orthogonal frequency division multiplexing (OFDM). The MU-MIMO technique allows the spatial multiplexing gain at the base station to be obtained without the need for multiple antenna terminals, thereby allowing multiple users to receive data over the downlink simultaneously. The use of OFDM provides protection against intersymbol interference (ISI) and allows high data rates to be achieved. Linear precoding schemes for MU-MIMO wireless systems, e.g., zero forcing beamforming (ZF-BF) and minimum mean squared error beamforming (MMSE-BF), have been widely concerned for their high performance in single-carrier MU-MIMO networks where a base station attempts to communicate simultaneously with multiple users. In this paper, we evaluate and extend the ZF-BF and MMSE-BF schemes from single-carrier MU-MIMO to multicarrier MU-MIMO architecture based on OFDM, i.e., MU-MIMO-OFDM system, assuming the availability of channel state information (CSI) at the transmitter. Numerical results demonstrate that both introduced linear precoding strategies provide a higher sum-rate capacity improvement compared to a conventional MU-MIMO-OFDM system where the users are served on a time division multiple access (TDMA) basis.

Block diagonalization (BD) based precoding techniques are well-known linear transmit strategies for multiuser MIMO (MU-MIMO) systems. By employing BD-type precoding algorithms at the transmit side, the MU-MIMO broadcast channel is decomposed into multiple independent parallel single user MIMO (SU-MIMO) channels and achieves the maximum diversity order at high data rates. The main computational complexity of BD-type precoding algorithms comes from two singular value decomposition (SVD) operations, which depend on the number of users and the dimensions of each user's channel matrix. In this work, low-complexity precoding algorithms are proposed to reduce the computational complexity and improve the performance of BD-type precoding algorithms. We devise a strategy based on a common channel inversion technique, QR decompositions, and lattice reductions to decouple the MU-MIMO channel into equivalent SU-MIMO channels. Analytical and simulation results show that the proposed precoding algorithms can achieve a comparable sum-rate performance as BD-type precoding algorithms, substantial bit error rate (BER) performance gains, and a simplified receiver structure, while requiring a much lower complexity.

In this paper, we study the performance of the Regularized Channel Inversion (RCI) precoder in a multi-user MIMO system with malicious users. We consider the general case when the number of users per transmit antenna β can take any value. We derive the optimal regularization parameter that maximizes the achievable secrecy sum-rate via large-system analysis, which we show to be accurate via simulations. We find that the secrecy requirements limit the number of users per transmit antenna that can be served with non-zero sum-rate. We show that for large signal-to-noise ratio, RCI can achieve secrecy without reducing the sum-rate if β < 1. If β = 1, this is still possible at a cost of less than 4dB in terms of transmitted power. If β > 1, then the secrecy requirements result in a poor sum-rate.

As the wireless communication network is driven
toward densification with small cell deployments, massive MIMO technology shows great promises to boost capacity and energy efficiency. This paper aims at showing how raising carrier
frequencies impacts the performance of the linear precoders used in Massive MIMO systems. By means of a geometrical deterministic channel model, we simulate a dense outdoor scenario and highlight the influence of the direct and multi-paths components.
More importantly we prove that, in a Line-of-Sight configuration, the discriminating skills of the well known Zero Forcing precoder is much more sensitive to the antenna array architecture and the user location than the Conjugate precoder.

In spectrum pooling scenario, the spectral leakage of DFT-based OFDM signal can be divided into in-band-out-ofsubband (IBOSB) radiation and out-of-band (OOB) radiation. A precoding scheme is proposed to suppress the IBOSB sidelobes. The precoding design is based on the generalized eigenvalue problem. Simulation results demonstrate that the proposed scheme can suppress the sidelobes significantly in contrast to the uncoded schemes. At the same time, the Bit-Error-Rate (BER) performance of the precoded system over multipath fading channel is improved due to the frequency diversity.

In this paper, we consider distributed estimation of an unknown random vector by using wireless sensors and a fusion center (FC). We adopt a linear model for distributed estimation of a vector source where both observation models and sensor operations are linear and the multiple access channel (MAC) is coherent. Two cases are considered: Noiseless fusion center and Noisy fusion center. In the case of a Noiseless fusion center (where there is no noise at the fusion center), the sensor precoders are designed to minimize the mean square error (MSE) at the fusion center. A closed form solution is found and it is shown that the system performance approaches the benchmark as long as the number of messages transmitted by each sensor is equal to the length of the vector source. Subsequently, if there is noise at the fusion center and one is interested in a closed form solution, a filter is designed to cancel out the effect of noise at the fusion center. This separate design will come at the expense of losing performance. An alternative iterative solution can be found when considering noise at the fusion center where sensor precoders are designed to minimize the MSE at the fusion center subject to transmit power constraints at each sensor. It is shown that the proposed scheme always converges. Finally, simulations are provided to verify the analysis and present the performance of the proposed schemes.

In this paper we consider the problem of lossy coding of correlated vector sources with uncoded side information available at the decoder. In particular, we consider lossy coding of vector source x ∈ R N which is correlated with vector source y ∈ R N , known at the decoder. We propose two compression schemes, namely, distributed adaptive compression (DAC) and distributed universal compression (DUC) schemes. The DAC algorithm is inspired by the optimal solution for Gaussian sources and requires computation of the conditional Karhunen-Loève transform (CKLT) of the data at the encoder. The DUC algorithm, however, does not require knowledge of the CKLT at the encoder. The DUC algorithms are based on the approximation of the correlation model between the sources y and x through a linear model y = Hx + n in which H is a matrix and n is a random vector and independent of x. This model can be viewed as a fictitious communication channel with input x and output y. Utilizing channel equalization at the receiver, we convert the original vector source coding problem into a set of manageable scalar source coding problems. Furthermore, inspired by bit loading strategies employed in wireless communication systems, we propose for both compression schemes a rate allocation policy which minimizes the decoding error rate under a total rate constraint. Equalization and bit loading are paired with a quantization scheme for each vector source entry (a slightly simplified version of the so called DISCUS scheme). The merits of our work are as follows: 1) it provides a simple, yet optimized, implementation of Wyner-Ziv quantizers for correlated vector sources, by using the insight gained in the design of communication systems; 2) it provides encoding schemes that, with or without the knowledge of the correlation model at the encoder, enjoy distributed compression gains.

This paper presents a novel method for blind carrier-frequency-offset (CFO) estimation in linearly precoded OFDMA uplink. Our investigation starts from the single-user equivalent (SUE) scenario presented in [2] where active users in the network are categorized into a number of reference (synchronized) users (RUs) and a new (asynchronous) user (NU). The major idea is to take advantage of time correlation induced by the linear precoder, which offers a second-order moments-based blind CFO estimation. The precoder design is carefully performed in terms of CFO identifiability, estimation accuracy and overall system performance.

Performing link adaptation in a multiantenna and multiuser system is challenging because of the coupling between precoding, user selection, spatial mode selection and use of limited feedback about the channel. The problem is exacerbated by the difficulty of selecting the proper modulation and coding scheme when using orthogonal frequency division multiplexing (OFDM). This paper presents a data-driven approach to link adaptation for multiuser multiple input mulitple output (MIMO) OFDM systems. A machine learning classifier is used to select the modulation and coding scheme, taking as input the SNR values in the different subcarriers and spatial streams. A new approximation is developed to estimate the unknown interuser interference due to the use of limited feedback. This approximation allows to obtain SNR information at the transmitter with a minimum communication overhead. A greedy algorithm is used to perform spatial mode and user selection with affordable complexity, without resorting to an exhaustive search. The proposed adaptation is studied in the context of the IEEE 802.11ac standard, and is shown to schedule users and adjust the transmission parameters to the channel conditions as well as to the rate of the feedback channel.

In this two-parts paper we propose a decentralized strategy, based on a game-theoretic formulation, to find out the optimal precoding/multiplexing matrices for a multipoint-to-multipoint communication system composed of a set of wideband links sharing the same physical resources, i.e., time and bandwidth. We assume, as optimality criterion, the achievement of a Nash equilibrium and consider two alternative optimization problems: 1) the competitive maximization of mutual information on each link, given constraints on the transmit power and on the spectral mask imposed by the radio spectrum regulatory bodies; and 2) the competitive maximization of the transmission rate, using finite order constellations, under the same constraints as above, plus a constraint on the average error probability. In Part I of the paper, we start by showing that the solution set of both noncooperative games is always nonempty and contains only pure strategies. Then, we prove that the optimal precoding/multiplexing scheme for both games leads to a channel diagonalizing structure, so that both matrix-valued problems can be recast in a simpler unified vector power control game, with no performance penalty. Thus, we study this simpler game and derive sufficient conditions ensuring the uniqueness of the Nash equilibrium. Interestingly, although derived under stronger constraints, incorporating for example spectral mask constraints, our uniqueness conditions have broader validity than previously known conditions. Finally, we assess the goodness of the proposed decentralized strategy by comparing its performance with the performance of a Pareto-optimal centralized scheme.

Multi-user MIMO communication can provide significant gains by exploiting spatial multiplexing. However, it requires better feedback to provide accurate channel state information at the transmitter (CSIT) for minimizing the multiuser interference. 3GPP LTE provides support for MU-MIMO, but it is not sufficient to extract sizable gains. In this paper, our primary goal is to efficiently exploit the system's resources for MU-MIMO in LTE. In the existing 3GPP LTE standard (Rel. 8), we observed that the transmission mode 5 (TM5) dedicated for MU-MIMO utilizes wideband feedback method for providing channel directional information/ precoding matrix indicator (CDI/PMI). The standard supports finer granularity feedback i.e. sub-band feedback method, but it's not utilized for MU-MIMO and restricted only to SU-MIMO. Therefore in this work, we propose to exploit the sub-band feedback for providing more frequent update of PMI. However, in order to support this feedback method, we need to propose a new downlink control information (DCI) format for TM5 that will contain additional fields in comparison to the existing DCI format 1D. Furthermore, to extract optimal performance at the system level, we also propose a MAC-layer scheduling algorithm that deals with resource management on sub-band basis. Utilizing these proposed methods, we show considerable gains in MU-MIMO for 3GPP LTE at the system level.

Block transmission with cyclic prefix is a promising technique to realize high-speed data rates in frequency selective fading channels. Many popular linear precoding schemes, including orthogonal frequency-division multiplexing (OFDM), single-carrier (SC) block transmission, and time-reversal (TR), can be interpreted as such a block transmission. This paper presents a unified performance analysis which shows how the optimal precoding strategy depends on the receiver type and the optimization criterion (capacity and mean-square error). We analyze three variants of TR methods (based on maximum-ratio combining, equal-gain combining and selective combining) and two-types of pre-equalization methods (zero-forcing and minimum mean-square error). As one application of our framework, we derive optimal power control for OFDM in the presence of interference limitation for distributed antenna systems; we find that without power control, OFDM does not have any capacity advantage over SC transmissions. When comparing SC and TR, we find that for single-antenna systems at high SNRs, SC has a capacity advantage; however, TR performs better in the low SNR regime. For multiple-antenna systems, TR always provides higher capacity, and the capacity of TR can approach that of optimal precoders with a number of distributed antennas.