Showing papers on "Multi-user MIMO published in 2002"

Space-Time Codes for High Data Rate Wireless Communications

Abstract: Space-time codes (STC) are a class of signaling techniques, offering coding and diversity gains along with improved spectral efficiency. These codes exploit both the spatial and the temporal diversity of the wireless link by combining the design of the error correction code, modulation scheme and array processing. STC are well suited for improving the downlink performance, which is the bottleneck in asymmetric applications such as downstream Internet. Three original contributions to the area of STC are presented in this dissertation. First, the development of analytic tools that determine the fundamental limits on the performance of STC in a variety of channel conditions. For trellis-type STC, transfer function based techniques are applied to derive performance bounds over Rayleigh, Rician and correlated fading environments. For block-type STC, an analytic framework that supports various complex orthogonal designs with arbitrary signal cardinalities and array configurations is developed. In the second part of the dissertation, the Virginia Tech Space-Time Advanced Radio (VT-STAR) is designed, introducing a multi-antenna hardware laboratory test bed, which facilitates characterization of the multiple-input multiple-output (MIMO) channel and validation of various space-time approaches. In the third part of the dissertation, two novel space-time architectures paired with iterative processing principles are proposed. The first scheme extends the suitability of STC to outdoor wireless communications by employing iterative equalization/decoding for time dispersive channels and the second scheme employs iterative interference cancellation/decoding to solve the error propagation problem of Bell-Labs Layered Space-Time Architecture (BLAST). Results show that remarkable energy and spectral efficiencies are achievable by combining concepts drawn from space-time coding, multiuser detection, array processing and iterative decoding.

A fourth-generation MIMO-OFDM broadband wireless system: design, performance, and field trial results

Abstract: Increasing demand for high-performance 4G broadband wireless is enabled by the use of multiple antennas at both base station and subscriber ends. Multiple antenna technologies enable high capacities suited for Internet and multimedia services, and also dramatically increase range and reliability. In this article we describe a multiple-input multiple-output OFDM wireless communication system, lab test results, and field test results obtained in San Jose, California. These are the first MIMO system field tests to establish the performance of MIMO communication systems. Increased capacity, coverage, and reliability are clearly evident from the test results presented in this article.

Adaptive modulation and MIMO coding for broadband wireless data networks

Abstract: Link adaptation techniques, where the modulation, coding rate, and/or other signal transmission parameters are dynamically adapted to the changing channel conditions, have emerged as powerful tools for increasing the data rate and spectral efficiency of wireless data-centric networks. While there has been significant progress on understanding the theoretical aspects of time adaptation in LA protocols, new challenges surface when dynamic transmission techniques are employed in broadband wireless networks with multiple signaling dimensions. Those additional dimensions are mainly frequency, especially in multicarrier systems, and space in multiple-antenna systems, particularly multiarray multiple-input multiple-output communication systems. We give an overview of the challenges and promises of link adaptation in future broadband wireless networks. We suggest guidelines to help in the design of robust, complexity/cost-effective algorithms for these future wireless networks.

Equal gain transmission in multiple-input multiple-output wireless systems

Abstract: Wireless systems with multiple-transmit and multiple-receive antennas (MIMO systems) are of interest due to their ability to provide substantial gains in capacity and quality. In this paper we propose equal gain transmission (EGT) as a technique to provide diversity advantage in MIMO systems. Using bounds on the error rate we show that EGT obtains a diversity gain on the order of the product of the number of transmit and receive antennas. Since in practice full channel knowledge at the transmitter is difficult to realize, we propose a technique using quantized phase information at the transmitter that achieves a full diversity advantage given enough bits of feedback. Monte Carlo simulation comparisons with other systems show the performance as a function of quantization.

Wireless OFDM Systems: How to Make Them Work?

Abstract: From the Publisher: Orthogonal Frequency Division Multiplexing (OFDM) has become the technology of choice for broadband communication in a wireless multipath environment. For instance, it will be used in next generation wireless local area networks (WLANs) and broadband fixed wireless access networks. Although the theory of OFDM is well developed, implementation aspects of OFDM systems remain a challenge. Wireless OFDM Systems brings together the broad and extensive experience of the editor and contributors by providing a comprehensive overview of OFDM implementation issues and OFDM based WLAN systems. The book starts with a discussion of indoor propagation channels, followed by an overview of the basics of OFDM, HIPERLAN/2 and IEEE 802.11a standards. Next, readers are lead through a complete design cycle of an OFDM transceiver, starting with channel estimation and synchronization functionality, and carried through to actual realizations. The different influences of the radio front-end on OFDM communication performance are detailed in the next chapters as well as many OFDM practical problems related to non-linear power amplifiers. The authors conclude with illustrations of several practical implementations involving wireless OFDM transceivers. Wireless OFDM Systems is a must have reference work for wireless communication designers who are building wireless OFDM transceivers.

Performance of MIMO systems with channel inversion

Abstract: The paper discusses channel inversion which is a spatial equalization technique when channel state information is available at the transmitter. Channel inversion is a straightforward concept without iterations and it might be useful when the data transmission is critical with time e.g. high data rate applications. We discuss performance degradation caused by channel estimation errors, clipping due to the limited range of the transmitted power and the effect of cochannel interference. These results give an insight into the technical constraints of this transmission technique and show how these critical issues can be limited or reduced.

Performance enhancement of multiuser MIMO wireless communication systems

Abstract: This paper describes a new approach to the problem of enhancing the performance of a multiuser multiple-input-multiple-output (MIMO) system for communication from one base station to many mobile stations in both frequency-flat and frequency-selective fading channels. This problem arises in space-division multiplexing systems with multiple users where many independent signal streams can be transmitted in the same frequency and time slot through the exploitation of multiple antennas at both the base and mobile stations. Our new approach is based on maximizing a lower bound for the product of signal-to-interference plus noise ratio (SINR) of a multiuser MIMO system. This provides a closed-form (noniterative) solution for the antenna weights for all the users, under the constraint of fixed transmit power. Our solution is shown by simulation to have better performance than previously proposed iterative or noniterative solutions. In addition, our solution requires significantly reduced complexity over a gradient search-based method that directly optimizes the product SINRs while still maintaining similar performance. Our solution assumes channel state information is present at the base station or transmitter.

On the capacity of multiple input multiple output broadcast channels

Abstract: We consider a multiuser multiple input multiple output (MIMO) Gaussian broadcast channel (BC), where the transmitter and receivers have multiple antennas. Since the MIMO broadcast channel is in general a non-degraded broadcast channel, its capacity region remains an unsolved problem. We establish a duality between what is termed the "dirty paper" achievable region (the Caire-Shamai achievable region) for the MIMO broadcast channel and the capacity region of the MIMO multiple-access channel (MAC), which is easy to compute. Using this duality, we greatly reduce the computational complexity required for obtaining the dirty paper achievable region for the MIMO BC. The duality also enables us to translate previously known results for the MIMO MAC to the MIMO BC. We also show that the dirty paper achievable region achieves the sum-rate capacity of the MIMO BC by establishing that the maximum sum rate of this region equals an upper-bound on the sum rate of the MIMO BC.

A single coefficient spatial correlation model for multiple-input multiple-output (MIMO) radio channels

Abstract: Spatial fading correlation is one of the impairments practical Multiple-Input Multiple-Output (MIMO) wireless communication systems have to cope with. The designer of such systems should take into account that the system performance can substantially degrade when correlation is present. Therefore, we introduce an easy-to-use single parameter MIMO spatial correlation model that nevertheless reflects the relevant characteristics of the real-life propagation phenomena appropriately. We demonstrate how an excellent match between the BER performance based on measurements and that based on the introduced model can be achieved, as well as its flexibility to model a wide range of propagation environments.

On the capacity of wireless relaying

Abstract: We consider wireless relaying: one or more nodes in a wireless (ad-hoc) network assist other nodes in their transmission by partially retransmitting messages. A characteristic of wireless relays - as compared to the work by T.M. Cover and A.A. El Gamal on the relay channel (see IEEE Trans. on Inf. Theory, vol.25, no.5, p.572-84, 1979) - is that they cannot transmit and receive simultaneously at the same frequency. We derive new upper and lower bounds for the capacity of this wireless relay channel. We then apply these result to a 4 terminal network, and show that the gain (considering outage capacity) of using wireless relaying is of the order of 8-9 dB.

Environmental issues for MIMO capacity

Abstract: Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Increased capacity is achieved by introducing additional spatial channels that are exploited using space-time coding. In this paper, the environmental factors that affect MIMO capacity are surveyed. These factors include channel complexity, external interference, and channel estimation error. The maximum spectral efficiency of MIMO systems in which both transmitter and receiver know the channel (using channel estimate feedback) is compared with MIMO systems in which only the receiver knows the channel. Channel complexity is studied using both simple stochastic physical scattering and asymptotic large random matrix models. Both uncooperative (worst-case) and cooperative (amenable to multiuser detection) interference are considered. An analysis for capacity loss associated with channel estimation error at the transmitter is introduced.

Wireless network scheduling data frames including physical layer configuration

Abstract: A wireless network is disclosed in which individual wireless stations can be configured to implement any of a plurality of physical configurations including antenna configurations. Such antenna configurations may include, without limitation, multiple input multiple output (MIMO) and single input single output (SISO). Different types of MIMO configurations can also be implemented such as open loop MIMO and closed loop MIMO.

Capacity and downlink transmission algorithms for a multi-user MIMO channel

Abstract: Downlink beamforming in a multi-user MIMO channel can provide significant gain in system throughput by allowing space division multiple access (SDMA). The exact solution for the sum capacity of such channels does not exist in closed form, but requires an expensive iterative algorithm. By imposing certain constraints on the capacity equation, a sub-optimal closed-form solution can be obtained. The paper presents two such solutions. The first, referred to as "block-diagonalization" arises from forcing all inter-user interference to zero. The second - "successive optimization" - is an alternative method of minimizing transmit power for an arbitrary information rate per user, and allows solutions which, under certain circumstances, are superior to the block-diagonalization approach. Both algorithms have sub-optimal performance, but they lead to simpler transmitter and receiver structures, and allow a tradeoff between performance and complexity.

Adaptive resource allocation in SDMA-based wireless broadband networks with OFDM signaling

Abstract: The increasing popularity of wireless broadband access in local and wide area networks is the main expression of the need for flexible and ubiquitous wireless connectivity. In order to satisfy user resource requirements in the presence of volatility of the wireless medium, sophisticated multiple access and adaptation techniques are required, which alleviate channel impairments and increase system throughput. The use of multiple antennas at the base station allows intra-cell channel reuse by multiple spatially separable users through space division multiple access (SDMA) and hence enhances cell capacity. However, the employment of antennas in the physical layer raises significant issues in the medium access control (MAC) layer. We investigate the impact of antenna arrays on MAC layer channel allocation in the context of orthogonal frequency division multiplexing (OFDM), which is the predominantly proposed signaling scheme for wireless broadband access. We propose an algorithm to allocate channels to users based on their spatial separability properties, while appropriately adjusting beamforming weights and transmission rates for each user in a channel. The unified consideration of such adaptive techniques yields significant throughput benefits.

Multi-user downlink beamforming with multiple transmit and receive antennas

Abstract: Downlink multiple-input, multiple-output (MIMO) systems assuming a large number of base station antennas, a small number of mobile station antennas, and rich-scattering, quasi-stationary, and flat-fading channel environments are considered. A MIMO space-division multiple access (SDMA) technique is proposed which enables concurrent transmission of multiple data streams to each multiple mobile station. The proposed method can achieve much higher downlink channel capacity than conventional MIMO techniques.

Media access control for MIMO wireless network

Abstract: An apparatus and method for operating a Multiple Input Multiple Output (MIMO)-capable subscriber node are disclosed. In one embodiment, an apparatus includes a MIMO-capable physical layer transceiver and a Medium Access Control (MAC) layer block that measures a received transmission to determine channel conditions. Upon receiving a transmission wherein said channel conditions meet specified conditions, the MAC layer block transmits a message to an access point indicating that the apparatus wants to switch between a non-MIMO mode and a MIMO mode, and upon receiving an acknowledgement, the MAC layer block switches between the non-MIMO mode and the MIMO mode. An apparatus for operating an access node is also disclosed.

On MIMO channel capacity, correlations, and keyholes: analysis of degenerate channels

Abstract: It has been demonstrated that zero correlation of a random multiple-input multiple-output channel is not a guarantee of its high capacity. Degenerate channels exist, which have zero correlation and still low capacity. We provide a statistical analysis of this phenomenon, formulate the general condition for a channel to be degenerate, and propose a method to estimate its capacity.

Capacity limits of dense palm-sized MIMO arrays

Abstract: We address the capacity limits of size-constrained multi-input multi-output (MIMO) arrays. While most work on MIMO focuses on arrays with of a small number of sufficiently spaced, low correlated antenna elements, we look at the case where a fixed small space is being filled up with antenna elements. We obtain a dense MIMO system whose rate performance is analyzed. We establish capacity limits and an equivalence with the capacity of MIMO arrays with unlimited aperture.

Multiple-input multiple-output wireless communication systems using antenna pattern diversity

Abstract: Multiple-input multiple-output (MIMO) wireless communication systems employ multiple transmit and multiple receive antennas to obtain significant improvement in channel capacity. However, the capacity is limited by the correlation of subchannels in non-ideal scattering environments. In this paper, we investigate MIMO systems that use antennas with dissimilar radiation patterns to introduce decorrelation, hence increasing channel capacity. We develop a ray tracing model that takes into account both the propagation channel and the transmit and receive antenna patterns. Using a computational electromagnetic simulator, we show that: (1) MIMO systems that exploit antenna pattern diversity allow for improvement over dual-polarized antenna systems; and (2) the capacity increase of such MIMO systems depends on the characteristics of the scattering environment.

Influence and modelling of mutual coupling in MIMO and diversity systems

Abstract: MIMO (multiple input multiple output) and diversity systems can increase distinctly the system performance of future wireless communication systems. The correlation of signals, depending on the power azimuth spectrum, between single transmit and receive antennas is a criterion to asses smart antenna systems. The assumption of independent antennas often made is not vindicated any more for small antenna spacing, since mutual coupling between single antenna elements occurs. This paper discusses the influence of mutual coupling on the correlation and shows the consequences for MIMO systems.

Capacity of Wireless Communication Systems Employing Antenna Arrays, a Tutorial Study

Abstract: A tutorial study is performed on the capacity of multiple antenna wireless communication systems. Multiple antenna structures can be classified into single-input multiple-outputs (SIMO), multiple-inputs single-output (MISO), and multiple-inputs multiple-outputs (MIMO) systems. Assuming that the channel is known at receiver, capacity expressions are provided for each structure, under the conditions of quasi-static flat fading. Also, information rate limits are provided in each case for some suboptimal structures or detection techniques that may be used in practice. Using simulations for the case of flat Rayleigh fading, capacities of optimal/suboptimal implementations are contrasted for each multi-antenna structure. Discussions are made on system design, regarding implementation complexity and practical limitations on achieving these capacities. In particular, the problem of fading correlation and required antenna spacing, effect of fast channel fading, and lack of channel knowledge at receiver are discussed. Providing the results of the most recent researches considering the capacity of multi-antenna systems, as well as some new results, this paper can give a good perspective for designing appropriate architectures in different wireless communication applications.

Co-channel interference cancellation based on MIMO OFDM systems

Abstract: This article deals with the exploitation of multiple input multiple output (MIMO) systems for broadband wireless indoor applications. Two systems, the Wind-Flex and ubiquitous antenna, are considered. Both aim to increase the system capacity by different approaches, to increase the single link data rate and the number of users in the whole system, respectively. Computer simulation results show the effectiveness of both MIMO systems.

Joint beamforming strategies in OFDM-MIMO systems

Abstract: Orthogonal Frequency Division Multiplexing (OFDM) has been recently established for several systems such as HIPERLAN/2 and Digital Video and Audio Broadcasting. The success of this modulation is due to the easy implementati9n of the modulator and demodulator, including the equalization process. Moreover, in last years also increasing interest has been put on the study of Multi-Input-Multi-Output (MIMO) channels, based on the use of arrays of antennas at both the transmitter and the receiver. In this paper we propose two new and low computational cost joint beamforming strategies for communication systems that combine OFDM and MIMO channels. An asymptotic analysis of the proposed methods is carried out, providing a complete comparative study among other classical strategies. To demonstrate the potential of the proposed techniques, we present MonteCarlo simulation results of the system BER and make comparisons with other methods.

Intrinsic capacity of the MIMO wireless channel

Abstract: This paper presents a new framework for the information-theoretic analysis of multipath multiple-input multiple-output (MIMO) wireless communications channels. The approach defines an upper bound on transmission that depends on the physical propagation scenario and transmit/receive antenna apertures but is otherwise independent of antenna properties. Transmit sources and receive sensors are abstracted in terms of spatial basis functions. The "intrinsic capacity" of the channel is defined as the maximum mutual information of the transmit and receive vectors over all possible modulations as well as transmit and receive basis functions and constrained by fixed average radiated power, limited transmit and receive volumes, and sensor noise. Definition of a "coherence matrix" allows a closed-form solution for the capacity under a new radiated power constraint.

Delay spread measurements on a wideband MIMO channel at 3.7 GHz

Abstract: This paper summarizes the delay spread characteristics of a 20 MHz MIMO channel at a carrier frequency of 3.676 GHz. The signals are transmitted from a two-antenna base to a two-antenna mobile receiver, and the experiments were conducted in a suburban setting outside Chicago, Illinois. The results include a statistical description of the delay spread environment. In addition, a case study of the temporal behavior of the channel is presented along with a one-bounce analysis to locate macro scatterers.

Analysis of two receiver schemes for interleaved OFDMA uplink

Abstract: In the uplink of OFDMA (orthogonal frequency division multiple access, a combination of OFDM and FDMA), the interleaved carrier assignment scheme is preferred for its maximum frequency diversity. Two receiver schemes are presented. The first is a group synchronization scheme, in which the uplink receiver compensates the effect of carrier frequency offsets and tries to recover the synchronized spectrum of one OFDMA block. The other one, a user separation scheme, exploits the signal's algebraic structure on the uplink of the interleaved OFDMA system, by which the signal waveform of each single user is separated and synchronized at the same time. We present the algorithms for each scheme and analyze their performance in terms of signal-to-interference-plus-noise ratio and residual interference.

On MIMO Systems and Adaptive Arrays for Wireless Communication : Analysis and Practical Aspects

Abstract: This thesis is concerned with the use of multiple antenna elements in wireless communication over frequency non-selective radio channels. Both measurement results and theoretical analysis are prese ...

Evaluation of capacity of indoor wireless MIMO channel using ray tracing

Abstract: This paper uses a ray-tracing approach to estimate the Shannon capacity of a MIMO wireless system. Several very simple scenarios are modeled, to determine the principles that limit the capacity.