Showing papers on "Multi-user MIMO published in 2003"

From theory to practice: an overview of MIMO space-time coded wireless systems

Abstract: This paper presents an overview of progress in the area of multiple input multiple output (MIMO) space-time coded wireless systems. After some background on the research leading to the discovery of the enormous potential of MIMO wireless links, we highlight the different classes of techniques and algorithms proposed which attempt to realize the various benefits of MIMO including spatial multiplexing and space-time coding schemes. These algorithms are often derived and analyzed under ideal independent fading conditions. We present the state of the art in channel modeling and measurements, leading to a better understanding of actual MIMO gains. Finally, the paper addresses current questions regarding the integration of MIMO links in practical wireless systems and standards.

Capacity limits of MIMO channels

Abstract: We provide an overview of the extensive results on the Shannon capacity of single-user and multiuser multiple-input multiple-output (MIMO) channels. Although enormous capacity gains have been predicted for such channels, these predictions are based on somewhat unrealistic assumptions about the underlying time-varying channel model and how well it can be tracked at the receiver, as well as at the transmitter. More realistic assumptions can dramatically impact the potential capacity gains of MIMO techniques. For time-varying MIMO channels there are multiple Shannon theoretic capacity definitions and, for each definition, different correlation models and channel information assumptions that we consider. We first provide a comprehensive summary of ergodic and capacity versus outage results for single-user MIMO channels. These results indicate that the capacity gain obtained from multiple antennas heavily depends on the available channel information at either the receiver or transmitter, the channel signal-to-noise ratio, and the correlation between the channel gains on each antenna element. We then focus attention on the capacity region of the multiple-access channels (MACs) and the largest known achievable rate region for the broadcast channel. In contrast to single-user MIMO channels, capacity results for these multiuser MIMO channels are quite difficult to obtain, even for constant channels. We summarize results for the MIMO broadcast and MAC for channels that are either constant or fading with perfect instantaneous knowledge of the antenna gains at both transmitter(s) and receiver(s). We show that the capacity region of the MIMO multiple access and the largest known achievable rate region (called the dirty-paper region) for the MIMO broadcast channel are intimately related via a duality transformation. This transformation facilitates finding the transmission strategies that achieve a point on the boundary of the MIMO MAC capacity region in terms of the transmission strategies of the MIMO broadcast dirty-paper region and vice-versa. Finally, we discuss capacity results for multicell MIMO channels with base station cooperation. The base stations then act as a spatially diverse antenna array and transmission strategies that exploit this structure exhibit significant capacity gains. This section also provides a brief discussion of system level issues associated with MIMO cellular. Open problems in this field abound and are discussed throughout the paper.

MIMO WLAN system

Abstract: A multiple-access MIMO WLAN system that employs MIMO, OFDM, and TDD. The system (1) uses a channel structure with a number of configurable transport channels, (2) supports multiple rates and transmission modes, which are configurable based on channel conditions and user terminal capabilities, (3) employs a pilot structure with several types of pilot (e.g., beacon, MIMO, steered reference, and carrier pilots) for different functions, (4) implements rate, timing, and power control loops for proper system operation, and (5) employs random access for system access by the user terminals, fast acknowledgment, and quick resource assignments. Calibration may be performed to account for differences in the frequency responses of transmit/receive chains at the access point and user terminals. The spatial processing may then be simplified by taking advantage of the reciprocal nature of the downlink and uplink and the calibration.

OFDM and MC-CDMA for Broadband Multi-User Communications, WLANs and Broadcasting

Abstract: From the Publisher: Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimising the use of available bandwidth. Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. Multi-Carrier (MC) CDMA is a combined technique of Direct Sequence (DS) CDMA (Code Division Multiple Access) and OFDM techniques. It applies spreading sequences in the frequency domain. Wireless communications has witnessed a tremendous growth during the past decade and further spectacular enabling technology advances are expected in an effort to render ubiquitous wireless connectivity a reality. This technical in-depth book is unique in its detailed exposure of OFDM, MIMO-OFDM and MC-CDMA. A further attraction of the joint treatment of these topics is that it allows the reader to view their design trade-offs in a comparative context. Divided into three main parts: Part I provides a detailed exposure of OFDM designed for employment in various applications Part II is another design alternative applicable in the context of OFDM systems where the channel quality fluctuations observed are averaged out with the aid of frequency-domain spreading codes, which leads to the concept of MC-CDMA Part III discusses how to employ multiple antennas at the base station for the sake of supporting multiple users in the uplink Portrays theentire body of knowledge currently available on OFDMProvides the first complete treatment of OFDM, MIMO(Multiple Input Multiple Output)-OFDM and MC-CDMAConsiders the benefits of channel coding and space time coding in the context of various application examples and features numerous complete system design examplesConverts the lessons of Shannon's information theory into design principles applicable to practical wireless systemsCombines the benefits of a textbook with a research monograph where the depth of discussions progressively increase throughout the book This all-encompassing self-contained treatment will appeal to researchers, postgraduate students and academics, practising research and development engineers working for wireless communications and computer networking companies and senior undergraduate students and technical managers.

Multiple-input multiple-output (MIMO) radar and imaging: degrees of freedom and resolution

Abstract: In this paper, radar is discussed in the context of a multiple-input multiple-output (MIMO) system model. A comparison is made between MIMO wireless communication and MIMO radar. Examples are given showing that many traditional radar approaches can be interpreted within a MIMO context. Furthermore, exploiting this MIMO perspective, useful extensions to traditional radar can be constructed. Performance advantages in terms of degrees of freedom and resolution are discussed. Finally, a MlMO extension to space-time adaptive processing (STAP) is introduced as applied to ground moving-target indication (GMTI).

MIMO capacity with interference

Abstract: System capacity is considered for a group of interfering users employing single-user detection and multiple transmit and receive antennas for flat Rayleigh-fading channels with independent fading coefficients for each path. The focus is on the case where there is no channel state information at the transmitter, but channel state information is assumed at the receiver. It is shown that the optimum signaling is sometimes different from cases where the users do not interfere with each other. In particular, the optimum signaling will sometimes put all power into a single transmitting antenna, rather than divide power equally between independent streams from the different antennas. If the interference is either sufficiently weak or sufficiently strong, we show that either the optimum interference-free approach, which puts equal power into each antenna, or the approach that puts all power into a single antenna is optimum and we show how to find the regions where each approach is best.

On the capacity of MIMO broadcast channel with partial side information

Abstract: Since having full channel state information in the transmitter is not reasonable in many applications and lack of channel knowledge does not lead to linear growth of the sum rate capacity as the number transmit antennas increases, it is therefore of interest to investigate transmission schemes that employ only partial CSI. In this paper, we propose a scheme that constructs M random beams and that transmits information to the users with the highest signal-to-noise-plus-interference ratios (SINRs), which can be made available to the transmitter with very little feedback. For fixed M and n increasing, the sum-rate capacity of our scheme scales as M log log n, which is precisely the same scaling obtained with perfect channel information. We furthermore show that linear increase in capacity can be obtained provided that M does not grow faster than O(log n). We also study the fairness of our scheduling scheme and show that, when M is large enough, the system becomes interference-dominated and the probability of transmitting to any user converges to 1/n, irrespective of its path-loss. In fact, using M = /spl alpha/ log n transmit antennas emerges as a desirable operating point, both in terms of providing linear increase in capacity as well as in guaranteeing fairness.

Pilot-aided channel estimation for OFDM in wireless systems

Abstract: A method and apparatus for pilot-symbol aided channel estimation in a wireless digital communication system which transmits packets of N OFDM data blocks, each data block comprising a set of K orthogonal carrier frequencies. At the transmitter, pilot symbols are inserted into each data packet at known positions so as to occupy predetermined positions in the time-frequency space. At the receiver, the received signal is subject to a two-dimensional inverse Fourier transform, two-dimensional filtering and a two-dimensional Fourier transform to recover the pilot symbols so as to estimate the channel response.

Fading correlations in wireless MIMO communication systems

Abstract: We investigate the effects of fading correlations on wireless communication systems employing multiple antennas at both the receiver and the transmitter side of the link, so called multiple-input multiple-output (MIMO) systems. It turns out that the amount of transmitter sided channel knowledge plays an important part when dealing with fading correlations. Furthermore, the possible availability of time diversity in a time-selective channel can have essential influence on performance. To study the influence of time-selectivity, the concept of sample-mean outage is introduced and applied to information theoretic measures, like capacity or cutoff rate. It will be shown, that in some cases correlated fading may offer better performance than uncorrelated fading permits, which is due to exploitable antenna gain, that will also be defined in a general form for MIMO systems.

Non-reciprocal transceivers in OFDM/SDMA systems: impact and mitigation

Abstract: Channel reciprocity is needed in SDMA or MIMO downlink pre-filtering when the channel knowledge is acquired in the uplink. We first show analytically that the non-reciprocity of the base station analog hardware, which is part of the channel, introduces a very high level of multi-user interference and quantify the effect of the non-reciprocity by means of simulations. We then propose a novel calibration technique at the base station that enables to compensate for the non-reciprocity and reduce the MUI to a negligible value while having a low implementation cost.

Prototype experience for MIMO BLAST over third-generation wireless system

Abstract: In this paper, a multiple-input-multiple-output (MIMO) extension for a third-generation (3G) wireless system is described. The integration of MIMO concepts within the existing UMTS standard and the associated space-time RAKE receiver are explained. An analysis is followed by a description of an actual experimental MIMO transmitter and receiver architecture, both realized on digital signal processors (DSPs) and FPGAs within a precommercial OneBTS base station. It uses four transmit and four receive antennas to achieve downlink data rates up to 1 Mb/s per user with a spreading factor of 32 and the UMTS chip rate of 3.84 MHz. Furthermore, different MIMO detectors are evaluated, comparing their performance and complexity. System performance is evaluated through simulations and indoor over-the-air measurements. Capacity and bit-error rate measurement results are presented.

Multiple antenna systems: their role and impact in future wireless access

Abstract: Multiple antennas play an important role in improving radio communications. In view of this role, the area of multiple antenna communication systems is in the forefront of wireless research. This article reviews two key related aspects of multiple antenna communication systems: multiple access interference mitigation at the receiver via multi-user beamforming; and space-time modulation and coding for MIMO systems. It is shown that both multi-user and MIMO receivers share similar signal processing and complexity tradeoffs.. Following that, a general unified framework for assessing different types of space-time modulation for MIMO systems is introduced. These space-time modulation methods are then compared in terms of Shannon capacity over multipath channels. Key MIMO system performance and implementation issues are also highlighted.

Frequency synchronization for MIMO OFDM wireless LAN systems

Abstract: The paper proposes an accurate and time-efficient technique for frequency synchronization of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The technique uses a preamble and is thus especially suitable for burst mode communication. The preamble consists of training sequences simultaneously transmitted from the various transmit antennas. From analysis, it is shown that the accuracy of frequency synchronization is close to the Cramer-Rao lower bound and increases for increasing RMS delay spreads and number of receive antennas. Furthermore, application of the proposed algorithm in MIMO OFDM wireless local-area-network (WLAN) systems leads to a BER that is only slightly higher than that of a perfectly synchronized system, making it highly applicable.

Introducing Space into MIMO Capacity Calculations

Abstract: The large spectral efficiencies promised for multiple-input multiple-output (MIMO) wireless fading channels are derived under certain conditions which do not fully take into account the spatial aspects of the channel. Spatial correlation, due to limited angular spread or insufficient antenna spacing, significantly reduces the performance of MIMO systems. In this paper we explore the effects of spatially selective channels on the capacity of MIMO systems via a new capacity expression which is more general and realistic than previous expressions. By including spatial information we derive a closed-form expression for ergodic capacity which uses the physics of signal propagation combined with the statistics of the scattering environment. This expression gives the capacity of a MIMO system in terms of antenna placement and scattering environment and leads to valuable insights into the factors determining capacity for a wide range of scattering models.

Interference Analysis and Reduction for Wireless Systems

Abstract: Throughout time, people have and will continue to use communications at an ever-increasing pace in an interference environment [1]. In addition to the widespread use of satellite systems during the decades of the 1970s and the 1980s, we are now living through the mobile revolution. A large percentage of the communications needs can be carried out satisfactorily, even in a bad interference situation, and people are willing to show moderation. For example, hearing a distant cochannel repeater when your local repeater is not active, while annoying, is not ‘‘unacceptable interference.’’ Hearing adjacent channel splatter while carrying on a conversation on simplex or your local repeater, while affecting the quality of the conversation, is not truly unacceptable interference. If it makes communication completely impossible, then it should be considered interference, although it still may not necessarily be harmful or willful. Take note at this point that many of the noise sources to be defined here do not affect FM/PM type radio operation except to cause desensing of the radio, possibly masking the desired signal. This is the reason we strive to define and derive the qualitative measures by which we can design modern wireless system in an ever-increasing interference background. Up to this point, we have examined and analyzed distortion mainly in the form of fading that is caused to information signals by the wireless channel for the types of wireless systems currently being used. In this and the following chapters, we shall analyze and study interference and include

Stream control in networks with interfering MIMO links

Abstract: A distributed algorithm, which exploits channel state information (CSI) at the transmitter, is presented for determining the maximum number of independent data streams for each transmitting node in a network of interfering multiple-input multiple-output (MIMO) links. Simulated throughputs for two simple network topologies show that the algorithm yields nearly optimal stream control. These closed-loop MIMO throughputs are compared to those of open-loop MIMO, with and without optimal stream control, and to the throughput when the links operate in a non-interfering, TDMA fashion.

New transmit schemes and simplified receivers for MIMO wireless communication systems

Abstract: In this paper, optimized transmit schemes for multiple-input multiple-output (MIMO) systems with simplified receivers are proposed for the downlink of high-speed wireless communication systems. In these systems, MIMO signal preprocessing is performed at the transmitter or base station with the receiver at the mobile station having a simplified structure that requires only limited signal processing. An important potential application for our transmit MIMO techniques is in the downlink of high-speed wireless communication systems with Vertical Bell Laboratories Layered Space-Time (V-BLAST) or a similar technique utilized in the uplink, creating a high-speed duplex system with a simplified mobile station transceiver structure. Two approaches are introduced and these depend on whether or not receive diversity is employed at the receiver. Both methods require that channel state information be available at the transmitter. In addition, some important associated issues such as peak-to-average power ratio requirements at the transmitter and robustness to downlink channel errors are also investigated and various solutions are proposed. Simulation results are provided and these show that performance improvement can be achieved when compared with other MIMO transmit schemes.

Per-tone equalization for MIMO OFDM systems

Abstract: This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.

An FPGA based rapid prototyping platform for MIMO systems

Abstract: Multiple input multiple output (MIMO) techniques hold the potential of dramatically increasing the data rates and spectral efficiency of wireless communications systems. Even with extensive research on the design of transmission and reception algorithms, little is known as to how much of the predicted gains are actually achievable on real wireless channels. In this paper, we present a MIMO testbed, which enables the rapid prototyping of MIMO transceivers for wideband channels. Such prototypes provide experimental quantification of achievable gains from MIMO algorithms. The testbed design allows real-time operation of baseband processing and RF up/down-conversion. The choice of testbed components is made to allow maximum, flexibility for research purposes, including monitoring and control of all subsystems. In addition to discussing the testbed's design, we present the implementation of two wireless systems. The first is a spread-spectrum system based on IEEE 802.11b. The second is an implementation of Alamouti's transmit diversity scheme.

Adjusting transmit power in a wireless communication system

Abstract: A method and apparatus are provided for determining adjustments to be made to a transmitter based on the velocity or location of a wireless communication device in relation to a wireless network infrastructure. The transmitter to be adjusted can be located in either the wireless communication device, or in a device in the wireless communication infrastructure, or both.

Smart geometrical antenna design exploiting the LOS component to enhance a MIMO System based on Rayleigh-fading in indoor scenarios

Abstract: In this paper we propose a special antenna design for indoor applications which enables superior transmission properties within a designated and fixed area due to a line of sight (LOS) based MIMO antenna configuration. In the rest of the room the MIMO-system relies on the multi-path signal from the Rayleigh channel background. We derive a simple design recipe to achieve a nearly perfect antenna configuration for a LOS-based MIMO-system and apply this to an indoor application example. Assuming a typical office scenario, capacity issues are discussed and a very simple transmission system is simulated. In the designated area a very simple transmission system delivers nearly the maximum data rate and hence, the application of successive interference (SIC) cancellation is not necessary here. For the rest of the room SIC yields a significant improvement. At last we propose a more symmetrical antenna structure, which reduces the angular dependence of the transmission quality.

Performance analysis and design optimization of LDPC coded MIMO OFDM systems

Abstract: The performance analysis and design optimization of low density parity check (LDPC) coded multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems for high speed wireless transmission is considered. The tool of density evolution with mixture Gaussian approximations is used to optimize irregular LDPC codes and to compute minimum operational signal-to-noise ratios for ergodic MIMO OFDM channels. In particular, the optimization is done for various MIMO OFDM system configurations which include different number of antennas, different channel models and different demodulation schemes; and the optimized performance is compared to the corresponding channel capacity.

Iterative decoding and channel estimation for space-time BICM over MIMO block fading multipath AWGN channel

Abstract: In this paper, we consider a generic model of space-time bit-interleaved coded modulation (ST-BICM) on a multiple-input multiple-output (MIMO) Rayleigh fading multipath channel. A practical low-complexity receiver structure performing iteratively MIMO data detection, channel decoding and channel estimation, is presented. The MIMO data detection, employing a reduced-state list-type soft output Viterbi algorithm enables to cope with severe channel intersymbol interference (ISI) without MIMO prefiltering. Among other results, simulations show that our approach can dramatically improve the downlink performance of time-division multiple access (TDMA) systems with high order modulation, keeping a reasonable complexity at the receiver side.

Spatially greedy scheduling in multi-user MIMO wireless systems

Abstract: This paper considers the benefit of opportunistic channel state dependent scheduling in a multi-user MIMO wireless system. Channel state dependent scheduling can provide significant performance gains for wireless networks by exploiting the independence of fading statistics across the user population. While opportunistic schedulers typically pick the single "best" user for transmission, MIMO systems can support transmissions to multiple users simultaneously. Consequently, greedy schedulers in MIMO systems can either transmit only to a single-user transmission or allow multiple users to transmit simultaneously. This paper analytically characterizes the performance gains achieved by both single-user and multi-user MIMO schedulers in a dynamic scenario with a time-varying number of users. In particular, the relation between throughput (or equivalently, ergodic capacity) and user experienced file-transfer delay for both single-user and multi-user MIMO schedulers are evaluated. Our main result indicates that multi-user greedy MIMO scheduling lead to lower average user experienced delays compared to single- user greedy scheduling.

Symbol timing for MIMO OFDM and other wireless communication systems

Abstract: Symbol timing for a wireless communication system, such as a multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) wireless LAN system, is determined by summing the powers for an appropriate set of channel impulse responses, integrating this power summation over an appropriate window (e.g., equivalent to the guard interval), and identifying the time at which the maximum integration occurs. Depending on the implementation, symbol timing can be determined for each receiver branch individually or for all receiver branches jointly. In either case, the determined symbol timing(s) can minimize the amount of inter-symbol and inter-channel interferences that are invoked in the system.

2-hop distributed MIMO communication system

Abstract: The concept of multi-hop distributed multiple-input, multiple-output (MIMO) communication scenarios is introduced and analysed for the 2-hop case in terms of capacity benefits over the direct and single-input, single output (SISO) 2-hop communication scenarios. To this end, an explicit capacity allocation strategy for the distributed MIMO case is deduced.

A VLSI implementation of MIMO detection for future wireless communications

Abstract: This paper describes a VLSI implementation of V-BLAST detection for future multiple-input-multiple-output (MIMO) wireless communications. This design is implemented using a 0.35-/spl mu/m 5-layer metal 3.3 V CMOS technology. For a 4-transmit and 4-receive antennas system using QPSK modulation scheme, a detecting throughput of 128 Mb/s can be achieved. Furthermore, it is shown that the implementation complexity can be reduced further to meet the requirements for future high speed downlink packet access (HSDPA) systems with MIMO extensions in 3/sup rd/ generation (3G) mobile wireless systems.

Spatial decomposition of MIMO wireless channels

Abstract: In this paper a novel decomposition of spatial channels is developed to provide insight into spatial aspects of multiple antenna communication systems. The underlying physics of the free space propagation is used to model the channel in scatterer free regions around the transmitter and the receiver, and the rest of the complex scattering media is represented by a parametric model. The channel matrix is separated into a product of known and random matrices where the known portion shows the effects of the physical configuration of antenna elements. We use the model to show the intrinsic degrees of freedom in a multiantenna system. Potential applications of the model are briefly discussed.

Precoding and loading for BLAST-like systems

Abstract: In this contribution we study transmission over MIMO channels, where channel state information available at the transmitter enables the use of precoding as weak as an optimization of rate and power distribution over the parallel subchannels in the transmission system (loading). It is shown that spatial loading provides substantial gains and a revised version of the V-BLAST algorithm is given to calculate the matrix filters required for Tomlinson-Harashima precoding, or equivalently decision-feedback equalization.