Showing papers on "Multi-user MIMO published in 2005"

Fundamentals of Wireless Communication

Abstract: 1. Introduction 2. The wireless channel 3. Point-to-point communication: detection, diversity and channel uncertainty 4. Cellular systems: multiple access and interference management 5. Capacity of wireless channels 6. Multiuser capacity and opportunistic communication 7. MIMO I: spatial multiplexing and channel modeling 8. MIMO II: capacity and multiplexing architectures 9. MIMO III: diversity-multiplexing tradeoff and universal space-time codes 10. MIMO IV: multiuser communication A. Detection and estimation in additive Gaussian noise B. Information theory background.

MIMO broadcast channels with finite rate feedback

Abstract: Multiple transmit antennas in a downlink channel can provide tremendous capacity (i.e. multiplexing) gains, even when receivers have only single antennas. However, receiver and transmitter channel state information is generally required. In this paper, a system where the receiver has perfect channel knowledge, but the transmitter only receives quantized information regarding the channel instantiation is analyzed. Simple expressions for the capacity degradation due to finite rate feedback as well as the required increases in feedback load per mobile as a function of the number of access point antennas and the system SNR are provided.

Cooperative mimo in multicell wireless networks

Abstract: A method and system for cooperative multiple-input multiple output (MIMO) transmission operations in a multicell wireless network. Under the method, antenna elements from two or more base stations are used to from an augmented MIMO antenna array that is used to transmit and receive MIMO transmissions to and from one or more terminals. The cooperative MIMO transmission scheme supports higher dimension space-time-frequency processing for increased capacity and system performance.

Optimality of zero-forcing beamforming with multiuser diversity

Abstract: In MIMO downlink channels, the capacity is achieved by dirty paper coding (DPQ). However, DPC is difficult to implement in practical systems. This work investigates if, for a large number of users, simpler schemes can achieve the same performance. Specifically, we show that a zero-forcing beamforming (ZFBF) strategy, while generally suboptimal, can achieve the same asymptotic sum-rate capacity as that of DPC, as the number of users goes to infinity. In proving this asymptotic result, we propose an algorithm for determining which users should be active in ZFBF transmission. These users are semi-orthogonal to one another, and when fairness among users is required, can be grouped for simultaneous transmissions to enhance the throughput of fair schedulers. We provide numerical results to confirm the optimality of ZFBF and to compare its performance with that of various MIMO downlink strategies.

Correlation matrix distance, a meaningful measure for evaluation of non-stationary MIMO channels

Abstract: The correlation matrix distance (CMD), an earlier introduced measure for characterization of non-stationary MIMO channels, is analyzed regarding its capability to predict performance degradation in MIMO transmission schemes. For that purpose we consider the performance reduction that a prefiltering MIMO transmission scheme faces due to non-stationary changes of the MIMO channel. We show that changes in the spatial structure of the channel corresponding to high values in the CMD also show up as a significant reduction in performance of the considered MIMO transmission scheme. Such significant changes in the spatial structure of the mobile radio channel are shown to appear also for small movements within an indoor environment. Stationarity can therefore not always be assumed for indoor MIMO radio channels.

Coherent optical communication using polarization multiple-input-multiple-output

Abstract: Polarization-division multiplexed (PDM) optical signals can potentially be demultiplexed by coherent detection and digital signal processing without using optical dynamic polarization control at the receiver. In this paper, we show that optical communications using PDM is analogous to wireless communications using multiple-input-multiple-output (MIMO) antennae and thus algorithms for channel estimation in wireless MIMO can be ready applied to optical polarization MIMO (PMIMO). Combined with frequency offset and phase estimation algorithms, simulations show that PDM quadrature phase-shift keying signals can be coherently detected by the proposed scheme using commercial semiconductor lasers while no optical phase locking and polarization control are required. This analogy further suggests the potential application of space-time coding in wireless communications to optical polarization MIMO systems and relates the problem of polarization-mode dispersion in fiber transmission to the multi-path propagation in wireless communications.

A multi user beamforming scheme for downlink MIMO channels based on maximizing signal-to-leakage ratios

Abstract: Multi-user multiple-input multiple-output (MU-MIMO) wireless systems can provide a substantial gain in network downlink throughput by allowing multiple users to communicate in the same frequency and time slots. The challenge is to design transmit beamforming vectors for every user while limiting the co-channel interference (CCI) from other users. One approach is to perfectly cancel the CCI at every user, which requires a relatively large number of transmit antennas. In this paper, we consider an alternative approach based on maximizing the signal-to-leakage ratio (SLR) for designing transmit beamforming vectors in a multi-user system. One advantage of the proposed scheme is that it does not impose a restriction on the number of available transmit antennas; it also outperforms the conventional beamforming scheme.

Multimode precoding for MIMO wireless systems

Abstract: Multiple-input multiple-output (MIMO) wireless systems obtain large diversity and capacity gains by employing multielement antenna arrays at both the transmitter and receiver. The theoretical performance benefits of MIMO systems, however, are irrelevant unless low error rate, spectrally efficient signaling techniques are found. This paper proposes a new method for designing high data-rate spatial signals with low error rates. The basic idea is to use transmitter channel information to adaptively vary the transmission scheme for a fixed data rate. This adaptation is done by varying the number of substreams and the rate of each substream in a precoded spatial multiplexing system. We show that these substreams can be designed to obtain full diversity and full rate gain using feedback from the receiver to transmitter. We model the feedback using a limited feedback scenario where only finite sets, or codebooks, of possible precoding configurations are known to both the transmitter and receiver. Monte Carlo simulations show substantial performance gains over beamforming and spatial multiplexing.

Coherent optical MIMO (COMIMO)

Abstract: We present a multiple-input multiple-output (MIMO) optical link based on coherent optics and its ability to exploit the inherent information capacity of multimode fiber. A coherent implementation differs from previous work in optical MIMO by allowing the system to tolerate smaller modal delay spreads, because of a much larger carrier frequency, and yet maintain the necessary diversity needed for MIMO operation. Furthermore, we demonstrate the use of MIMO adaptive equalization to mitigate intersymbol interference when exceeding the bandwidth-length product of the link. The impact of phase noise is studied with numerical simulation.

Construction and capacity analysis of high-rank line-of-sight MIMO channels

Abstract: This paper describes a technique for realizing a high rank channel matrix in a line-of-sight (LOS) multiple-input-multiple-output (MIMO) transmission scenario. This is beneficial for systems which can not make use of the originally derived MIMO gain given by independent and identically distributed (i.i.d.) flat Rayleigh fading channels. Typical applications are fixed wireless access and radio relay systems. The technique is based on optimization of antenna placement in a uniform linear array. By introducing a new and more general geometrical model than that applied in earlier works, additional insight into the optimal design parameters is gained. A novel analysis of the sensitivity of the optimal design parameters is performed. The LOS transmission matrix is used in a Rician fading channel model, and performance is evaluated with respect to ergodic capacity, outage capacity, and effective degrees of freedom. The results show that even with some deviation from optimal design, the LOS MIMO case outperforms the i.i.d. Rayleigh case in terms of Shannon capacity.

Fundamentals of Wireless Communication: The wireless channel

Abstract: A good understanding of the wireless channel, its key physical parameters and the modeling issues, lays the foundation for the rest of the book. This is the goal of this chapter. A defining characteristic of the mobile wireless channel is the variations of the channel strength over time and over frequency. The variations can be roughly divided into two types (Figure 2.1): Large-scale fading , due to path loss of signal as a function of distance and shadowing by large objects such as buildings and hills. This occurs as the mobile moves through a distance of the order of the cell size, and is typically frequency independent. Small-scale fading , due to the constructive and destructive interference of the multiple signal paths between the transmitter and receiver. This occurs at the spatial scale of the order of the carrier wavelength, and is frequency dependent. We will talk about both types of fading in this chapter, but with more emphasis on the latter. Large-scale fading is more relevant to issues such as cell-site planning. Small-scale multipath fading is more relevant to the design of reliable and efficient communication systems – the focus of this book. We start with the physical modeling of the wireless channel in terms of electromagnetic waves. We then derive an input/output linear time-varying model for the channel, and define some important physical parameters. Finally, we introduce a few statistical models of the channel variation over time and over frequency.

Increasing wireless channel capacity through MIMO systems employing co-located antennas

Abstract: Wireless networks consisting of compact antennas find applications in diverse areas such as communication systems, direction of arrival estimation, sensor networks, and imaging. The effectiveness of many of these systems depend on maximizing the reception of RF power and extracting maximum information from the incident electromagnetic (EM) wave. Traditionally, this has been achieved through multiple-input multiple-output (MIMO) systems employing a spatial array of antennas that enhance the channel capacity. In this paper, we report similar increases in channel capacity obtained through the use of vector antennas consisting of co-located loops and dipoles, which can respond to more than one component of the EM field. It is shown that systems with three- and four-element vector antennas at both the transmitter and receiver operating around the frequency of 2.25 GHz support three and four times more information, respectively, as compared to conventional systems consisting of sensors with single antennas. Comparison with a simplified theoretical model of a MIMO system with co-located antennas in a rich multipath environment shows good agreement.

Smart antenna engineering

Abstract: The increasing demand for wireless services and the growing number of wireless users are pushing networks to their capacity limits. Smart antenna technology can overcome these capacity limits as well as improve signal quality and let mobile telephones operate on less power. This book arms wireless telecom engineers with a powerful design methodology that allows them to select the smart antenna approach most suitable for a particular application. It also allows wireless engineers to evaluate systems performance of a particular design and engineer smart antenna designs for optimal performance. A comprehensive and practical reference, this book offers guidance in the application of smart antennas in 2G, 3G, and advanced Mimo (multiple input and multiple output) wireless systems.

Adaptive mimo systems

Abstract: This invention is generally concerned with apparatus, methods and processor control code for MIMO (multiple-input multiple-output) communications systems. An adaptive MIMO communications system comprising a MIMO transmitter and a MIMO receiver and having a feedback path from the receiver to the transmitter, the MIMO transmitter including a receiver to receive a feedback signal from the receiver sent via the feedback path and a controller responsive to the feedback signal to configure a MIMO transmission from the transmitter to the receiver, the MIMO receiver being configured to receive the MIMO transmission, and wherein the MIMO receiver includes a decision module, the decision module having a power indicating input to input a signal indicating a desired receiver power consumption, and an output for sending a MIMO configuration signal to the MIMO transmitter, the decision module being configured to determine a MIMO transmission configuration responsive to the power indicating input and to output a MIMO configuration signal for feedback to the transmitter to configure the transmitter in accordance with the determination.

What makes a good MIMO channel model

Abstract: Using different meaningful measures of quality, the paper investigates the accuracy of analytical MIMO channel models. Different metrics should be applied if the underlying MIMO channel supports predominantly beamforming, spatial multiplexing or diversity. The number of envisaged antennas plays an important role. By comparing the results of an extensive indoor measurement campaign at 5.2 GHz, we find the following main conclusions: (i) the recently developed Weichselberger model predicts capacity for any antenna number and represents diversity best of all three models, but still not satisfactorily; (ii) except for 2/spl times/2 MIMO systems, the Kronecker model fails to predict capacity, joint angular power spectrum, and diversity; (iii) the virtual channel representation should only be used for modeling the joint angular power spectrum for very large numbers of antennas. The answer to the question posed in the title: the appropriate model has to be chosen according to the considered application.

Cooperative relaying for ad-hoc ground networks using swarm UAVs

Abstract: Low cost UAVs have proved valuable in their use as sensor data collection systems in tactical situations. In this paper we examine their use as communication relays to improve range and reliability of ad hoc ground based network. MIMO based wireless communications are well known to improve capacity and reliability of point-to-point communication links. Recently, cooperation between distributed wireless nodes has been studied to exploit multi user macro diversity in an adhoc network. Application of MIMO techniques using distributed wireless nodes are also envisioned, where the cooperating nodes become the virtual antennas of a MIMO transceiver. Inspired by the recent advances in multi-UAV formation flights, we study the application of such distributed MIMO based relay schemes on a cluster of multiple UAVs. Specifically we look at the performance of distributed transmit beamforming and distributed OSTBC schemes under ideal and non ideal conditions of UAV flight. Simulation parameters are chosen based on literature study of air-to-ground propagation models. The results from our simulations show tremendous improvements in range and reliability when compared to direct ground based communication links

Downlink Sum-MSE Transceiver Optimization for Linear Multi-User MIMO Systems

Abstract: We address the problem of Mean Square Error (MSE) transceiver design for point-to-multipoint transmission in multi-user Multiple-Input-Multiple-Output (MIMO) systems. We solve the problem of minimizing the downlink sum MSE under a sum power constraint by jointly optimizing the linear transmitters and receivers. Our result is based on a "duality" between downlink and uplink MSE regions. It states that downlink and uplink share the same achievable MSE region under a sum power constraint. Thus, the problem of downlink design can be solved efficiently by focusing on an equivalent uplink problem.

A novel tree-based scheduling algorithm for the downlink of multi-user MIMO systems with ZF beamforming

Abstract: Spatial multiplexing in the downlink of wireless multiple antenna communications promises high gains in system throughput. However, spatially correlated users and a limited number of antennas at the base station motivates the need for a scheduling algorithm which efficiently arranges users into groups to be served in different time or frequency slots. In this paper we propose a novel tree-based scheduling algorithm which successfully solves this problem achieving a close to optimum grouping strategy. The algorithm has been tested with zero forcing beamforming techniques and is based on a new metric for the user performance considering the effect of other users present in the same group analyzing their spatial features.

Rate bounds for MIMO relay channels using precoding

Abstract: Relay channels plays a central role in next-generation multihop wireless systems This paper considers the MIMO relay channel where multiple antennas are employed by each terminal New lower bounds on the capacity of a Gaussian MIMO relay channel are derived under the assumption that the transmitter employs either superposition coding or dirty-paper coding The proposed lower bounds improve on a previously proposed lower bound that arises from a simple transmit strategy

Lattice array receiver and sender for spatially orthonormal MIMO communication

Abstract: In this paper, line-of-sight (LoS) oriented multiple input multiple output (MIMO) communication is examined. It is shown that two opposing rectangular (as well as square and linear) lattice antenna arrays achieve the maximum attainable channel capacity, when a certain distance-wavelength condition is fulfilled. A relationship for a large number of antenna elements is derived that connects LoS MIMO with LoS SISO channel capacity, through array aperture, wavelength and number of array antenna elements. Furthermore, a sensitivity study is performed for the proposed MIMO array arrangement that reveals high robustness against various array and antenna element position errors. Sensitivity against reflections is also examined, with the conclusion that the studied MIMO scheme appears less sensitive to reflections than a SISO system.

Routing in ad-hoc networks with MIMO links

Abstract: Smart antennas include a broad variety of antenna technologies ranging from the simple switched beams to the sophisticated digital adaptive arrays. While beam-forming antennas are good candidates for use in strong line of sight (LOS) environments, it is the multiple input multiple output (MIMO) technology that is best suited for multipath environments. In fact, the MIMO links exploit the multipath induced rich scattering to provide high spectral efficiencies. The focus of this work is to identify the various characteristics and tradeoffs of MIMO links that can be leveraged by routing layer protocols in rich multipath environments to improve their performance. To this end, we propose a routing protocol called MIR for ad-hoc networks with MIMO links, that leverages the various characteristics of MIMO links in its mechanisms to improve the network performance. We show the effectiveness of the proposed protocol by evaluating its performance through ns2 simulations for a variety of network conditions.

Convex primal decomposition for multicarrier linear MIMO transceivers

Abstract: The design of linear transceivers for multiple-input-multiple-output (MIMO) communication systems with channel state information is particularly challenging for two main reasons. First, since several substreams are established through the MIMO channel, it is not even clear how the quality of the system should be measured. Second, once a cost function has been chosen to measure the quality, the optimization of the system according to such criterion is generally difficult due to the nonconvexity of the problem. Recent results have solved the problem for the wide family of Schur-concave/convex functions, resulting in simple closed-form solutions when the system is modeled as a single MIMO channel. However, with several MIMO channels (such as in multi-antenna multicarrier systems), the solution is generally more involved, leading in some cases to the need to employ general-purpose interior-point methods. This problem is specifically addressed in this paper by combining the closed-form solutions for single MIMO channels with a primal decomposition approach, resulting in a simple and efficient method for multiple MIMO channels. The extension to functions that are not Schur-concave/convex is also briefly considered, relating the present work with a recently proposed method to minimize the average bit error rate (BER) of the system.

Adaptive MIMO transmission scheme: exploiting the spatial selectivity of wireless channels

Abstract: We present a novel adaptive transmission technique for MIMO systems with the aim to enhancing the spectral efficiency for a target error rate performance and transmit power. This adaptive method employs the condition number of the spatial correlation matrix as an indicator of the spatial selectivity of the MIMO channel. The distribution of the condition number is then used to identify the prevailing channel environment. Depending on the identified channel state, our adaptive algorithm chooses the MIMO transmission method, among spatial multiplexing, D-STTD, and beamforming, which maximizes the spectral efficiency. Performance results show significant gains in throughput and reduced error rate compared to conventional fixed transmission schemes.

Successive optimization Tomlinson-Harashima precoding (SO THP) for multi-user MIMO systems

Abstract: Multi-user multiple-input multiple-output (MU MIMO) systems have the advantage of combining the high capacity achievable with MIMO processing and the benefits of space division multiple access. Previously proposed techniques that use the channel state information at the transmitter to improve the performance of the downlink suffer from a capacity loss due to a zero multi-user interference (MUI) constraint or they have to allow some MUI to improve the system performance. In this paper we propose a combination of one linear precoding technique named successive optimization (SO) and a nonlinear precoding technique, Tomlinson-Harashima precoding (THP). It uses all of the subspaces available in a MU MIMO system and can completely eliminate multi-user interference. We show that SO THP provides a very high capacity and a better BER performance than similar minimum mean-square-error (MMSE) THP precoding techniques at low SNR especially for the users equipped with multiple antennas. SO THP is also less sensitive to channel estimation errors than MMSE THP precoding. Thereby, we minimize the capacity loss due to the MUI cancellation and we reduce the complexity of the receiver since there is no MUI in the system.

Cross-layer MAC design for wireless networks using MIMO

Abstract: We address the problems of the IEEE 802.11 MAC for wireless networks by designing a MAC protocol jointly with a multiple-input multiple-output (MIMO) technique in the physical layer. We propose a new MAC protocol, mitigating interference using multiple antennas MAC (MIMA-MAC), which mitigates interference from neighboring nodes by employing the spatial multiplexing capability of MIMO. Both theoretical analysis and simulation results clearly show that the proposed MIMA-MAC system outperforms the 802.11 system in terms of the aggregate throughput of the network by mitigating interference from neighboring nodes and thereby increasing simultaneous transmissions in the network

Method for supporting various multi-antenna schemes in BWA system using multiple antennas

Abstract: Disclosed is a method for using various multiple antenna schemes in a baseband wireless access system is provided. According to the method, a downlink MAP message is constructed in order to support various multiple antenna schemes based on a multiple-input multiple output (MIMO), which is one of the multiple antenna schemes, so that compatibility with exiting MIMO technology having no MIMO feedback can be achieved and overhead occurring in transmission of an MAP information element can be reduced. Further, it is possible to efficiently support spatial multiplexing technology capable of transmitting multiple layers having different modulation and coding in a MIMO system.

MIMO communications in ad hoc networks

Abstract: We consider MIMO (multiple-input multiple output) communications in an ad hoc network with K simultaneous communicating transceiver pairs. In particular, we compare channelized MIMO transmission and the MIMO interference transmission using the inherent multiplexing gain of multiple transmit/receive antennas. The relative performance in terms of their limiting spectral efficiencies largely hinges on the power constraint used in the system. In particular, for a MIMO network with per-user power constraint, we show that the channelized transmission has unbounded limiting spectral efficiencies while the interference transmission is inherently limited by the degrees of freedom of the system, namely the transceiver antenna numbers. On the other hand, with a total power constraint, both transmission schemes have saturating asymptotic spectral efficiencies as the number of users increases. For MIMO interference transmission, we also point out the importance of channel state information (CSI) at the transmitter. In particular, we show that with finite transmitter-receiver pairs, better throughput may result with stronger CSI assumption at the transmitter

Energy efficiency of MIMO transmissions in wireless sensor networks with diversity and multiplexing gains

Abstract: The energy efficiency of MIMO transmissions in wireless sensor networks is analyzed considering the trade-off between diversity and multiplexing gains. Various MIMO are studied with noncooperative, half-cooperative or cooperative realizations. Energies consumed in transmission, processing circuitry and cooperation are obtained, which show that the optimal energy efficiency requires both the diversity and the multiplexing gains be exploited.

Joint Estimation of Channel Parameters for MIMO Communication Systems

Abstract: In the next generation mobile communication systems, high data rates and high capacity are expected if multiple antennas are used at both receive and transmit sides. Such a radio propagation channel constitutes a multiple-input multiple-output (MIMO) system. In a wireless MIMO system, it is possible to estimate channel parameters in a multipath environment by extending the classical parameter estimation methods to the joint space and time domain. In this paper, we propose a subspace-based approach to jointly estimate the angle-of-arrival (AOA), angle-of-departure (AOD) and delay-of-arrival (DOA) of digitally modulated multipath signals in MIMO communication systems. The novel approach uses a collection of estimates of a space-time manifold vector of the channel which utilizes a Khatri-Rao product to transfer the estimated channel response matrix to the classical model. Simulation results show that the proposed methods can achieve high resolution of channel parameters and resolve more multipath components than the number of array elements

A comparative study of MIMO detection algorithms for wideband spatial multiplexing systems

Abstract: The implementation of wideband MIMO systems poses a major challenge to hardware designers due to the huge processing power required for MIMO detection. To achieve this goal with a complete VLSI solution, channel coding and MIMO detection are preferably separated so that each of them can be fitted into a single chip. In this paper, a comparative study is presented regarding various uncoded adaptive and non-adaptive MIMO detection algorithms. Intended to serve as a reference for system designers, this comparison is performed from several different perspectives including theoretical formulation, simulated BER/PER performance, and hardware complexity. All the simulations are conducted within MIMO-OFDM framework and with a packet structure similar to that of the IEEE 802.11a/g standard. As the comparison results show, the RLS algorithm appears to be an affordable solution for a wideband MIMO system targeted at gigabit wireless transmission. As a direct result of this work, an ASIC for a 25 MHz wideband 8 /spl times/ 8 MIMO-OFDM system using RLS has been designed and fabricated.