Showing papers on "Channel state information published in 2002"

Free-space optical communication through atmospheric turbulence channels

Abstract: In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.

Capacity scaling in MIMO wireless systems under correlated fading

Abstract: Previous studies have shown that single-user systems employing n-element antenna arrays at both the transmitter and the receiver can achieve a capacity proportional to n, assuming independent Rayleigh fading between antenna pairs. We explore the capacity of dual-antenna-array systems under correlated fading via theoretical analysis and ray-tracing simulations. We derive and compare expressions for the asymptotic growth rate of capacity with n antennas for both independent and correlated fading cases; the latter is derived under some assumptions about the scaling of the fading correlation structure. In both cases, the theoretic capacity growth is linear in n but the growth rate is 10-20% smaller in the presence of correlated fading. We analyze our assumption of separable transmit/receive correlations via simulations based on a ray-tracing propagation model. Results show that empirical capacities converge to the limit capacity predicted from our asymptotic theory even at moderate n = 16. We present results for both the cases when the transmitter does and does not know the channel realization.

Communication over fading channels with delay constraints

Abstract: We consider a user communicating over a fading channel with perfect channel state information. Data are assumed to arrive from some higher layer application and are stored in a buffer until transmitted. We study adapting the user's transmission rate and power based on the channel state information as well as the buffer occupancy; the objectives are to regulate both the long-term average transmission power and the average buffer delay incurred by the traffic. Two models for this situation are discussed; one corresponding to fixed-length/variable-rate codewords and one corresponding to variable-length codewords. The tradeoff between the average delay and the average transmission power required for reliable communication is analyzed. A dynamic programming formulation is given to find all Pareto optimal power/delay operating points. We then quantify the behavior of this tradeoff in the regime of asymptotically large delay. In this regime, we characterize simple buffer control policies which exhibit optimal characteristics. Connections to the delay-limited capacity and the expected capacity of fading channels are also discussed.

Multiple-access multiple-input multiple-output (MIMO) communication system

Abstract: Techniques to achieve better utilization of the available resources and robust performance for the downlink and uplink in a multiple-access MIMO system. Techniques are provided to adaptively process data prior to transmission, based on channel state information, to more closely match the data transmission to the capacity of the channel. Various receiver processing techniques are provided to process a data transmission received via multiple antennas at a receiver unit. Adaptive reuse schemes and power back-off are also provided to operate the cells in the system in a manner to further increase the spectral efficiency of the system (e.g., reduce interference, improve coverage, and attain high throughput). Techniques are provided to efficiently schedule data transmission on the downlink and uplink. The scheduling schemes may be designed to optimize transmissions (e.g., maximize throughput) for single or multiple terminals in a manner to meet various constraints and requirements.

A space-time correlation model for multielement antenna systems in mobile fading channels

Abstract: Analysis and design of multielement antenna systems in mobile fading channels require a model for the space-time cross correlation among the links of the underlying multiple-input multiple-output (MIMO) channel. In this paper, we propose a general space-time cross-correlation function for mobile frequency nonselective Rice fading MIMO channels, in which various parameters of interest such as the angle spreads at the base station and the user, the distance between the base station and the user, mean directions of the signal arrivals, array configurations, and Doppler spread are all taken into account. The new space-time cross-correlation function includes all the relevant parameters of the MIMO fading channel in a clean compact form, suitable for both mathematical analysis and numerical calculations/simulations. It also covers many known correlation models as special cases. We demonstrate the utility of the new space-time correlation model by clarifying the limitations of a widely accepted correlation model for MIMO fading channels. As another application, we quantify the impact of nonisotropic scattering around the user, on the capacity of a MIMO fading channel.

Method and apparatus for utilizing channel state information in a wireless communication system

Abstract: Techniques for transmitting data from a transmitter unit to a receiver unit in a multiple-input multiple-output (MIMO) communication system. In one method, at the receiver unit, a number of signals are received via a number of receive antennas, with the received signal from each receive antenna comprising a combination of one or more signals transmitted from the transmitter unit. The received signals are processed to derive channel state information (CSI) indicative of characteristics of a number of transmission channels used for data transmission. The CSI is transmitted back to the transmitter unit. At the transmitter unit, the CSI from the receiver unit is received and data for transmission to the receiver unit is processed based on the received CSI.

Optimal transmitter eigen-beamforming and space-time block coding based on channel mean feedback

Abstract: Optimal transmitter designs obeying the water-filling principle are well-documented; they are widely applied when the propagation channel is deterministically known and regularly updated at the transmitter. Because channel state information is impossible to be known perfectly at the transmitter in practical wireless systems, we design, in this paper, an optimal multiantenna transmitter based on the knowledge of mean values of the underlying channels. Our optimal transmitter design turns out to be an eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the correlation matrix of the estimated channel at the transmitter and with proper power loading across beams. The optimality pertains to minimizing an upper bound on the symbol error rate, which leads to better performance than maximizing the expected signal-to-noise ratio (SNR) at the receiver. Coupled with orthogonal space-time block codes, two-directional eigen-beamforming emerges as a more attractive choice than conventional one-directional beamforming with uniformly improved performance, without rate reduction, and without essential increase in complexity. With multiple receive antennas and reasonably good feedback quality, the two-directional eigen-beamformer is also capable of achieving the best possible performance in a large range of transmit-power-to-noise ratios, without a rate penalty.

Method and apparatus for processing data in a multiple-input multiple-output (MIMO) communication system utilizing channel state information

Abstract: Techniques to “successively” process received signals at a receiver unit in a MIMO system to recover transmitted data, and to “adaptively” process data at a transmitter unit based on channel state information available for the MIMO channel. A successive cancellation receiver processing technique is used to process the received signals and performs a number of iterations to provide decoded data streams. For each iteration, input (e.g., received) signals for the iteration are processed to provide one or more symbol streams. One of the symbol streams is selected and processed to provide a decoded data stream. The interference due to the decoded data stream is approximately removed (i.e., canceled) from the input signals provided to the next iteration. The channel characteristics are estimated and reported back to the transmitter system and used to adjust (i.e., adapt) the processing (e.g., coding, modulation, and so on) of data prior to transmission.

Localization in sensor networks with fading and mobility

Abstract: In sensor networks the device localization is an interesting topic due to its relationship with routing and energy consumption. We propose a scheme to perform localization, based on the estimation of the power received by only two beacons placed in known positions. By starting from the received powers, eventually averaged on a given window to counteract interference and fading, the actual distance between the sensor and the beacons is derived and the position obtained by means of triangulation. The paper shows the effectiveness of this approach in different environments, by including the possible disturbance due to fading channels and sensor mobility.

MIMO transmission over a time-varying channel using SVD

Abstract: The performance of a system using singular value decomposition (SVD) over a multiple-input multiple-output (MIMO) channel is dependent on the accuracy of the channel state information (CSI) at the transmitter and the receiver. In time division duplex (TDD) systems, the channel is reciprocal, hence the CSI can be retrieved through estimation of pilot symbols and applied to transmission. However, for a time-varying channel, the estimated CSI at the transmitter is incorrect due to an inherent delay between the estimation of the CSI and the transmission of data. A typical system employing SVD suffers degradation in capacity when incorrect CSI is used to transmit data. This paper proposes a new linear processing architecture, which reduces the effect of incorrect CSI at the transmitter. No additional pilot signals are required. It is shown at terminal velocity of 2 m.s/sup -1/ for the Hiperlan 2 standards, that the proposed architecture has a capacity of one bit lower than the theoretical channel capacity. At low velocities, this architecture outperforms (in terms of capacity) simpler systems that do not require CSI at the transmitter.

Rayleigh fading multi-antenna channels

Abstract: Information theoretic properties of flat fading channels with multiple antennas are investigated. Perfect channel knowledge at the receiver is assumed. Expressions for maximum information rates and outage probabilities are derived. The advantages of transmitter channel knowledge are determined and a critical threshold is found beyond which such channel knowledge gains very little. Asymptotic expressions for the error exponent are found. For the case of transmit diversity closed form expressions for the error exponent and cutoff rate are given. The use of orthogonal modulating signals is shown to be asymptotically optimal in terms of information rate.

Blind channel identification and equalization in OFDM-based multiantenna systems

Abstract: Wireless systems employing multiple antennas at the transmitter and the receiver have been shown to have the potential of achieving extraordinary bit rates. Orthogonal frequency division multiplexing (OFDM) significantly reduces the receiver complexity in multiantenna broadband systems. We introduce an algorithm for blind channel identification and equalization in OFDM-based multiantenna systems. Our approach uses second-order cyclostationary statistics, employs antenna precoding, and yields unique channel estimates (up to a phase rotation for each transmit antenna). Furthermore, it requires only an upper bound on the channel order, it does not impose restrictions on channel zeros, and it exhibits low sensitivity to stationary noise. We present simulation results demonstrating the channel estimator and the corresponding multichannel equalizer performance.

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.

Feedback gain in multiple antenna systems

Abstract: Multiple antenna transmission and reception have been shown to significantly increase the achievable data rates of wireless systems. However, most of the existing analysis assumes perfect or no channel information at the receiver and transmitter. The performance gap between these extreme channel assumptions is large and most practical systems lie in between. Therefore, it is important to analyze multiple antenna systems in the presence of partial channel information. We upper bound the outage probability performance of multiple antenna systems with preamble-based channel estimation and quantized feedback. We design causal feedback and power control schemes to minimize this upper bound on outage probability. We consider the following practical issues in our analysis and design: (1) the channel information is imperfect both at the receiver and at the transmitter and (2) part of the total available resources for the system need to be used for estimation and feedback. Our results demonstrate that for block fading channels, sending a periodic preamble and causally receiving channel state information via a feedback channel can lead to substantial gains in the outage performance over any nonfeedback scheme. Most of the gains achieved by perfect feedback can be achieved by very few bits of feedback. Furthermore, it is demonstrated that these outage probability gains can be translated into improvements in frame error rate performance of systems using space-time codes. Thus, implementing a power control, even at the cost of reduced spectral resources for the forward channel is beneficial for block fading channels.

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.

Exact pairwise error probability of space-time codes

Abstract: We describe a simple technique for the numerical calculation, within any desired degree of accuracy, of the pairwise error probability (PEP) of space-time codes over fading channels. This method applies also to the calculation of E[Q(/spl radic//spl xi/)] for any nonnegative random variable /spl xi/ whose moment-generating function /spl Phi//sub /spl xi//(s)=E[exp(-s/spl xi/)] is known. Its application to the multiple antenna independent Rayleigh-fading channel and to the Rayleigh block fading channel is discussed, and illustrated by two simple examples.

Iterative receivers for space-time block-coded OFDM systems in dispersive fading channels

Abstract: We consider the design of iterative receivers for space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) systems in unknown wireless dispersive fading channels, with or without outer channel coding. First, we propose a maximum-likelihood (ML) receiver for STBC-OFDM systems based on the expectation-maximization (EM) algorithm. By assuming that the fading processes remain constant over the duration of one STBC code word and by exploiting the orthogonality property of the STBC as well as the OFDM modulation, we show that the EM-based receiver has a very low computational complexity and that the initialization of the EM receiver is based on the linear minimum mean square error (MMSE) channel estimate for both the pilot and the data transmission. Since the actual fading processes may vary within one STBC code word, we also analyze the effect of a modeling mismatch on the receiver performance and show both analytically and through simulations that the performance degradation due to such a mismatch is negligible for practical Doppler frequencies. We further propose a turbo receiver based on the maximum a posteriori-EM algorithm for STBC-OFDM systems with outer channel coding. Compared with the previous noniterative receiver employing a decision-directed linear channel estimator, the iterative receivers proposed here significantly improve the receiver performance and can approach the ML performance in typical wireless channels with very fast fading, at a reasonable computational complexity well suited for real-time implementations.

High data-rate transmission with eigenbeam-space division multiplexing (E-SDM) in a MIMO channel

Abstract: When channel state information is known at a transmitter in multiple-input multiple-output systems, the optimum capacity is given by eigenmode channel division with water-pouring power control. In this eigenbeam-space division multiplexing (E-SDM), bit assignment to substreams based on the capacity is not optimum due to the fact that the number of assigned bits is expressed by a discrete quantity. In the paper, a method to assign both bit and transmit power to each substream based on the criterion minimizing total bit error rate (BER) is developed, and the BER performance is numerically analyzed in comparison to spatial division multiplexing (SDM). The simulation results assuming 5-transmit and 2-receive antennas show that the E-SDM provides about 10 dB gain compared to the conventional SDM at average BER of 10/sup -3/.

Delay-constrained capacity with causal feedback

Abstract: A block-fading channel model is considered, and a K-block delay constraint is imposed on data transmission. The key consideration is that the channel state information is fed back to the transmitter in a causal manner. A general cost function /spl mu/(x) is considered in solving the delay-constrained transmission problem, under the short-term and the long-term power constraints. A causal power adaptation strategy is needed to maximize the cost function, hence dynamic programming is found to give the optimum solution. The general cost function is then specialized to the cases of expected and outage capacities. In the case of expected capacity, it is observed that optimizing the transmitted power does not give much benefit at high signal-to-noise ratio (SNR), but provides a substantial gain at low SNR. At low SNR, it is proved that the capacity increases by a factor of approximately log K/m, due to power adaptation, when the channel fades according to the /spl chi//sub 2m//sup 2/ statistics. In the case of outage capacity, it is shown that the optimum power adaptation solution to the long-term constraint problem provides a substantial SNR gain at both low and high values of SNR. Random coding bounds are derived for the outage capacity algorithms.

Optimal transmitter eigen-beamforming and space-time block coding based on channel correlations

Abstract: Optimal transmitter designs obeying the water-filling principle are well-documented, and widely applied when the propagation channel is deterministically known and regularly updated at the transmitter. Because channel state information may be costly or impossible to acquire in rapidly varying wireless environments, we develop in this paper statistical water-filling approaches for stationary random fading channels. The resulting optimal designs require only knowledge of the channel's second order statistics that do not require frequent updates, and can be easily acquired. Optimality refers to minimizing a tight bound on the symbol error rate. Applied to a multiple transmit-antenna paradigm, the optimal precoder turns out to be a generalized eigen-beamformer with multiple beams pointing to orthogonal directions along the eigenvectors of the channel's covariance matrix, and with proper power loading across the beams. Coupled with orthogonal space time block codes, two-directional eigen-beamforming emerges as a more attractive choice than conventional one-directional beamforming, with uniformly better performance, and without rate reduction or complexity increase.

Reverse link power control in 1xEV-DV systems

Abstract: The transmit power of a mobile station on the reverse link channel that carries channel state information, rate selection, and/or sector selection information is power controlled separately from the reverse link traffic channels when the mobile station is in soft handoff. The serving base station in the active set for the mobile station controls the mobile station transmit power on the rate control channel. The non-serving base stations control the transmit power on the reverse traffic channel.

Model-based channel estimation for OFDM signals in Rayleigh fading

Abstract: This paper proposes a robust pilot-assisted channel estimation method for orthogonal frequency division multiplexing (OFDM) signals in Rayleigh fading. Our estimation method is based on nonlinear regression channel models. Unlike the linear minimum mean-squared error (LMMSE) channel estimate, the method proposed does not have to know or estimate channel statistics like the channel correlation matrix and the average signal-to-noise ratio (SNR) per bit. Numerical results indicate that the performance of the proposed channel estimator is very close to the theoretical bit error propagation lower bound that is obtained by a receiver with perfect channel response information.

Digital communication over generalized fading channels: a unified approach to performance analysis

Abstract: Signal fading has been an enduring problem since the advent of modern communication systems and most recently has received considerable attention relative to the wireless communication channel. Literally hundreds of authors over the past forty years have contributed to the literature dealing with this problem in one form or another. A large number of these contributions fall into the category of performance evaluation of systems impaired by this phenomenon. Traditional analytical solutions to this problem have, in many instances, taken the form of complicated expressions that provide little or no insight into the dependency of the system behavior on the various system parameters that characterize it. In other cases, closed-form solutions were not possible at all.

Estimation of continuous flat fading MIMO channels

Abstract: Multiple input multiple output (MIMO) systems can provide high data rate wireless services in a rich scattering environment. We study one of the proposals for MIMO systems, the Bell Labs Layered Space-Time (BLAST) architecture. Channel estimation using training sequences is required for coherent detection in BLAST. We apply the maximum likelihood (ML) channel estimator and the optimal training sequences for block flat fading channels to continuous flat fading channels, and analyze the estimation error. The optimal training length and training interval that maximize the throughput for a given target bit error rate are found by computer simulations as functions of the Doppler frequency and the number of antennas.

On the performance of orthogonal space-time block coding with quantized feedback

Abstract: Orthogonal space-time block coding (OSTBC) is a recent technique that provides maximal diversity gains on a space-time channel at a very modest computational cost. Recently, several authors have suggested to improve the performance of an OSTBC system by using a feedback of channel state information from the receiver to the transmitter. In this letter, we study the performance of an OSTBC system with quantized low-rate feedback. We establish conditions under which the system achieves full diversity and we also analyze the performance of a method that employs a feedback consisting of only one information bit.

Adaptive modulation for variable-rate OFDM systems with imperfect channel information

Abstract: Adaptive modulation has been shown to have significant benefits for high-speed wireless data transmission when orthogonal frequency division multiplexing (OFDM) is employed. However, accurate channel state information (CSI) is required at the transmitter to achieve the benefits. Imperfect CSI arises from noisy channel estimates, which may also be outdated due to a delay in getting the CSI to the transmitter. We study adaptive OFDM with both perfect and imperfect CSI for the variable bit rate case, where a target bit error rate is set. Performance results are provided for adaptive OFDM with imperfect CSI. The use of multiple estimates is shown to mitigate the effect of CSI delay. In addition, the fundamental error mechanisms which result in performance degradation are studied. Robust approaches are then proposed that are less sensitive to CSI errors.

Effect of channel estimation error in OFDM-based WLAN

Abstract: We examine the performance degradation due to the channel estimation error in orthogonal frequency division multiplexing (OFDM)-based wireless LAN (WLAN). The average effective SNR and average bit error probabilities (BEPs) are derived in a Rayleigh fading channel.

Tomlinson-Harashima precoding in space-time transmission for low-rate backward channel

Abstract: In this paper, Tomlinson-Harashima precoding, a nonlinear pre-equalization technique, is proposed for transmission over multiple-input/multiple-output channels. Instead of equalizing intersymbol interference (temporal equalization) here spatial equalization, i.e., equalization of multi-user interference is performed. If only a low-rate backward channel is available for communicating channel state information back from the receiver to the transmitter, a compromise precoder setting, calculated from (medium-term) average channel knowledge in combination with linear residual equalization at the receiver side is proposed. Compared to an optimal adjustment of the precoder, i.e., perfect channel state information at the transmitter, only small losses have to be accepted.

Optimal rate allocation for superposition coding in quasi-static fading channels

Abstract: Coding with a single fixed rate may not achieve the maximum throughput of a compound channel if the transmitter only has access to channel state statistics. Superposition coding for broadcast channels can be employed to increase the throughput. We study the optimal rate allocation for finite level superposition coding to maximize the throughput. Results show 2-level superposition coding is adequate to achieve most of the throughput gain in a quasi-static Rayleigh fading channel.

On design criteria and construction of non-coherent space-time constellations

Abstract: We consider the problem of digital communication in a Rayleigh flat-fading environment using a multiple-antenna system, when the channel state information is available neither at the transmitter nor at the receiver. It is known that at high signal-to-noise ratio (SNR), or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the noncoherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In this work, inspired by the Stein's lemma, we propose to use the Kullback-Leibler (KL) distance between conditional distributions to design space-time constellations for noncoherent communication. In fast fading, i.e., when the coherence interval is equal to one symbol period and the unitary construction provides only one signal point, the new design criterion results in pulse amplitude modulation (PAM)-type constellations with unequal spacing between constellation points. We also show that in this case, the new design criterion is equivalent to design criteria based on the exact pairwise error probability and the Chernoff information. When the coherence interval is larger than the number of transmit antennas, the resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when an appropriately designed outer code or multiple receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.