Showing papers on "Channel state information published in 2001"

On channel estimation and detection for multicarrier signals in fast and selective Rayleigh fading channels

Abstract: Time-domain channel estimation and detection techniques are presented for multicarrier signals in a fast and frequency-selective Rayleigh fading channel. As a consequence of the time-varying channel, the orthogonality between subcarriers is destroyed in conventional frequency-domain approaches, resulting in interchannel interference, which increases an irreducible error floor in proportion to the normalized Doppler frequency. An important feature of the proposed technique is the ability to exploit the time-selective channel as a provider of time diversity. This enables us to achieve performance superior to any other structure without increasing bandwidth or incorporating redundancy, in order to reduce the complexity of the estimator, we apply the theory of optimal low rank approximation to a minimum mean squared error channel estimator and present a theoretical calculation of mean squared error and simulations to confirm that the estimator is robust to changes in channel characteristics.

Method and apparatus for measuring and reporting channel state information in a high efficiency, high performance communications system

Abstract: Channel state information (CSI) can be used by a communications system to precondition transmissions between transmitter units and receiver units. In one aspect of the invention, disjoint sub-channel sets are assigned to transmit antennas located at a transmitter unit. Pilot symbols are generated and transmitted on a subset of the disjoint sub-channels. Upon receipt of the transmitted pilot symbols, the receiver units determine the CSI for the disjoint sub-channels that carried pilot symbols. These CSI values are reported to the transmitter unit, which will use these CSI values to generate CSI estimates for the disjoint sub-channels that did not carry pilot symbols. The amount of information necessary to report CSI on the reverse link can be further minimized through compression techniques and resource allocation techniques.

Channel estimation for OFDM transmission in multipath fading channels based on parametric channel modeling

Abstract: We present an improved channel estimation algorithm for orthogonal frequency-division multiplexing mobile communication systems using pilot subcarriers. This algorithm is based on a parametric channel model where the channel frequency response is estimated using an L-path channel model. In the algorithm, we employ the ESPRIT (estimation of signal parameters by rotational invariance techniques) method to do the initial multipath time delays acquisition and propose an interpath interference cancellation delay locked loop to track the channel multipath time delays. With the multipath time delays information, a minimum mean square error estimator is derived to estimate the channel frequency response. It is demonstrated that the use of the parametric channel model can effectively reduce the signal subspace dimension of the channel correlation matrix for the sparse multipath fading channels and, consequently, improve the channel estimation performance.

The capacity of discrete-time memoryless Rayleigh-fading channels

Abstract: We consider transmission over a discrete-time Rayleigh fading channel, in which successive symbols face independent fading, and where neither the transmitter nor the receiver has channel state information. Subject to an average power constraint, we study the capacity-achieving distribution of this channel and prove it to be discrete with a finite number of mass points, one of them located at the origin. We numerically compute the capacity and the corresponding optimal distribution as a function of the signal-to-noise ratio (SNR). The behavior of the channel at low SNR is studied and finally a comparison is drawn with the ideal additive white Gaussian noise channel.

The impact of frequency-flat fading on the spectral efficiency of CDMA

Abstract: The capacity of the randomly spread synchronous code-division multiple-access (CDMA) channel subject to frequency-flat fading is studied in the wide-band limit of large number of users. We find the spectral efficiency as a function of the number of users per chip, the distribution of the flat fading, and the signal-to-noise ratio (SNR), for the optimum receiver as well as linear receivers (single-user matched filter, decorrelator, and minimum mean-square error (MMSE)). The potential improvements due to both decentralized transmitter power control and multi-antenna receivers are also analyzed.

Design, analysis, and performance evaluation for BICM-ID with square QAM constellations in Rayleigh fading channels

Abstract: We consider bit-interleaved coded modulation with iterative decoding (BICM-ID) for bandwidth-efficient transmission over Rayleigh fading channels. We propose the design criteria that utilize a large Hamming distance inherited in a low-rate code and a new labeling technique designed specifically for fading channels. This results in a large coding gain over noniterative coded modulation and performance close to that of "turbo" coded modulation with less complexity. We also show that BICM-ID designed for fading channels usually has a very good performance over the additive white Gaussian noise (AWGN) channel while the converse is difficult to achieve. When combined with signal space diversity, diversity order can be improved to twice the diversity order of conventional BICM-ID; therefore, the code complexity can further be reduced while maintaining the same level of performance. Specifically, with the bandwidth efficiency of 2 bits/s/Hz over Rayleigh fading channels, a bit error rate (BER) of 10/sup -6/ can be achieved with 16-QAM, a four-state rate 1/2 code at E/sub b//N/sub 0/ of about seven dB. We also derive performance bounds for BICM-ID with and without signal space diversity over Rayleigh fading channels, which can be easily extended for other types of fading channels.

Asymptotic analysis of proportional fair algorithm

Abstract: It was previously shown (under simplifying assumptions) that the proportional fair algorithm gives equal power and time to users who only differ in their distance from the BS, their fading characteristics being the same (see Holtzman, J.M., Proc. VTC2000-Spring, p.1663-7, 2000). We give here a more general result for two classes of users with different fading characteristics. All other things being equal, the user class with more fading variability gets more throughput with a lower (but not much lower) fraction of time transmitting. We also demonstrate a separability property-the effects of variability and distance from the base station are separable.

Channel capacity and beamforming for multiple transmit and receive antennas with covariance feedback

Abstract: We consider the capacity of a narrowband point to point communication system employing multiple-element antenna arrays at both the transmitter and the receiver with covariance feedback. Under covariance feedback the receiver is assumed to have perfect channel state information (CSI) while at the transmitter the channel matrix is modeled as consisting of zero mean complex jointly Gaussian random variables with known covariances. Specifically we assume a channel matrix with i.i.d. rows and correlated columns, a common model for downlink transmission. We determine the optimal transmit precoding strategy to maximize the Shannon capacity of such a system. We also derive closed form necessary and sufficient conditions on the spatial covariance for when the maximum capacity is achieved by beamforming. The conditions for optimality of beamforming agree with the notion of water-filling over multiple degrees of freedom.

Limitations of sum-of-sinusoids fading channel simulators

Abstract: Rayleigh signal fading due to multipath propagation in wireless channels is widely modeled using sum-of-sinusoids simulators. In particular, Jakes' (1994) simulator and derivatives of Jakes' simulator have gained widespread acceptance. Despite this, few in-depth studies of the simulators' statistical behavior have been reported in the literature. Here, the extent to which Jakes' simulator adequately models the multipath Rayleigh fading propagation environment is examined. The results show that Jakes' simulator does not reproduce some important properties of the physical fading channel. Some possible improvements to Jakes' simulator are examined. The significances of the number and the symmetries of the Doppler frequency shifts on the validity of the simulator's reproduction of the physical fading channel are elucidated.

Turbo-coded modulation for systems with transmit and receive antenna diversity over block fading channels: system model, decoding approaches, and practical considerations

Abstract: We study the use of turbo-coded modulation for wireless communication systems with multiple transmit and receive antennas over block Rayleigh fading channels. We describe an effective way of applying turbo-coded modulation as an alternative to the current space-time codes with appropriate interleaving. We study the performance with the standard iterative turbo decoding algorithm, as well as the iterative demodulation-decoding algorithm. In addition to the introduction of the turbo-coded modulation scheme, we consider a variety of practical issues including the case of large number of antennas, the effects of estimated channel state information, and correlation among subchannels between different transmit-receive antenna pairs. We present examples to illustrate the performance of the turbo-coded modulation scheme and observe significant performance gains over the appropriately interleaved space-time trellis codes.

Transmit-Antennae Space-Time Block Coding for Generalized OFDM in the Presence of

Abstract: — Transmit antenna diversity has been exploitedrecently to develop high-performance space–time coders andsimple maximum-likelihood decoders for transmissions over flatfading channels. Relying on block precoding, this paper developsgeneralized space–time coded multicarrier transceivers appro-priate for wireless propagation over frequency-selective multipathchannels. Multicarrier precoding maps the frequency-selectivechannel into a set of flat fading subchannels, whereas space–timeencoding/decoding facilitates equalization and achieves perfor-mance gains by exploiting the diversity available with multipletransmit antennas. When channel state information is unknown atthe receiver, it is acquired blindly based on a deterministic variantof the constant-modulus algorithm that exploits the structureof space–time block codes. To benchmark performance, theCramer–Rao bound of channel estimates is also derived. Systemperformance is evaluated both analytically and with simulations.Index Terms— Blind channel estimation, multipath fading chan-nels, space–time coding, transmit diversity.

Transmit antennae space-time block coding for generalized OFDM in the presence of unknown multipath

Abstract: Transmit antenna diversity has been exploited to develop high-performance space-time coders and simple maximum-likelihood decoders for transmissions over flat fading channels. Relying on block precoding, this paper develops generalized space-time coded multicarrier transceivers appropriate for wireless propagation over frequency-selective multipath channels. Multicarrier precoding maps the frequency-selective channel into a set of flat fading subchannels, whereas space-time encoding/decoding facilitates equalization and achieves performance gains by exploiting the diversity available with multiple transmit antennas. When channel state information is unknown at the receiver, it is acquired blindly based on a deterministic variant of the constant-modulus algorithm that exploits the structure of space-time block codes. To benchmark performance, the Cramer-Rao bound of the channel estimates is also derived. System performance is evaluated both analytically and with simulations.

Error rates for Nakagami-m fading multichannel reception of binary and M-ary signals

Abstract: This paper derives new closed-form formulas for the error probabilities of single and multichannel communications in Rayleigh and Nakagami-m (1960) fading. Closed-form solutions to three generic trigonometric integrals are presented as part of the main result, providing a unified method for the derivation of exact closed-form average symbol-error probability expressions for binary and M-ary signals with L independent channel diversity reception. Both selection-diversity and maximal-ratio combining (MRC) techniques are considered. The results are generally applicable for arbitrary two-dimensional signal constellations that have polygonal decision regions operating in a slow Nakagami-m fading environments with positive integer fading severity index. MRC with generically correlated fading is also considered. The new expressions are applicable in many cases of practical interest. The closed-form expressions derived for a single channel reception case can be extended to provide an approximation for the error rates of binary and M-ary signals that employ an equal-gain combining diversity receiver.

Joint iterative channel estimation and decoding in flat correlated Rayleigh fading

Abstract: This paper addresses the design and performance evaluation with respect to capacity of M-PSK turbo-coded systems operating in frequency-flat time-selective Rayleigh fading. The receiver jointly performs channel estimation and turbo decoding, allowing the two processes to benefit from each other. To this end, we introduce a suitable Markov model with a finite number of states, designed to approximate both the values and the statistical properties of the correlated flat fading channel phase, which poses a more severe challenge to PSK transmission than amplitude hiding. Then, the forward-backward algorithm determines both the maximum a posteriori probability (MAP) value for each symbol in the data sequence and the MAP channel phase in each iteration. Simulations show good performance in standard correlated Rayleigh fading channels. A sequence of progressively tighter upper bounds to the capacity of a simplified Markov-phase channel is derived, and performance of a turbo code with joint iterative channel estimation and decoding is demonstrated to approach these capacity bounds.

On optimality of beamforming for multiple antenna systems with imperfect feedback

Abstract: We determine a necessary and sufficient condition under which beamforming (scalar coding) achieves Shannon capacity in a narrowband point to point communication system employing multiple transmit antennas and a single receive antenna. We assume perfect channel state information at the receiver (CSIR) and either channel mean or channel covariance feedback from the receiver to the transmitter. We find that better feedback quality not only increases the system capacity but also reduces the required decoding complexity at the receiver.

Space-time turbo codes with full antenna diversity

Abstract: In attempting to find a spectrally and power efficient channel code which is able to exploit maximum diversity from a wireless channel whenever available, we investigate the possibility of constructing a full antenna diversity space-time turbo code. As a result, both three-antenna and two-antenna (punctured) constructions are shown to be possible and very easy to find. To check the decodability and performance of the proposed codes, we derive non-binary soft-decoding algorithms. The performance of these codes are then simulated and compared with two existing space-time convolutional codes (one has minimum worst-case symbol-error probability; the other has maximal minimum free distance) having similar decoding complexity. As the simulation results show, the proposed space-time turbo codes give similar or slightly better performance than the convolutional codes under extremely slow fading. When fading is fast, the better distance spectra of the turbo codes help seize the temporal diversity. Thus, the performance advantage of the turbo codes becomes evident. In particular, 10/sup -5/ bit-error rate and 10/sup -3/ frame-error rate can be achieved at less than 6-dB E/sub b//N/sub 0/ with 1 b/s/Hz and binary phase-shift keying modulation. The practical issue of obtaining the critical channel state information (CSI) is also considered by applying an iteratively filtered pilot symbol-assisted modulation technique. The penalty when the CSI is not given a priori is about 2-3 dB.

Adaptive modulation with imperfect channel information in OFDM

Abstract: Adaptive modulation in OFDM is a means by which the OFDM transmitter adapts the subchannel bit and power allocation to the amplitude response of the frequency selective channel for improved performance in high data rate wireless communications. Previous studies demonstrating the performance gains of adaptive modulation have assumed the availability of perfect channel state information. This paper examines the impact on performance of an adaptive OFDM system due to imperfect channel information arising from realistic channel estimators and time-varying channels. The latter is of particular relevance to mobile wireless applications, for which OFDM has received much attention. The performance of a typical channel estimator is found to be sufficient to justify the use of adaptive modulation in wireless OFDM systems, but in mobile situations the effect of outdated channel information in the presence of quickly time-varying channels can be significant. The use of channel prediction is shown to mitigate the impact of outdated channel information and improve performance when the Doppler spread, or mobile velocity, is high.

Iterative maximum-likelihood sequence estimation for space-time coded systems

Abstract: In previous work on decoding space-time codes, it is either assumed that perfect channel state information (CSI) is present, or a channel estimate is obtained using pilot symbols and then used as if it were perfect to extract symbol estimates. In the latter case, a loss in performance is incurred, since the resulting overall receiver is not optimal. We look at maximum-likelihood (ML) sequence estimation for space-time coded systems without assuming CSI. The log-likelihood function is presented for both-quasi-static and nonstatic fading channels, and an expectation-maximization (EM)-based algorithm is introduced for producing ML data estimates, whose complexity is much smaller than a direct evaluation of the log-likelihood function. Simulation results indicate the EM-based algorithm achieves a performance close to that of a receiver which knows the channel perfectly.

An information theoretic foundation of synchronized detection

Abstract: The constrained capacity of a coherent coded modulation (CM) digital communication system with data-aided channel estimation and a discrete, equiprobable symbol alphabet is derived under the assumption that the system operates on a flat fading channel and uses an interleaver to combat the bursty nature of the channel. It is shown that linear minimum mean square error channel estimation directly follows from the derivation and links average mutual information to the channel dynamics. Based on the assumption that known training symbols are transmitted, the achievable rate of the system is optimized with respect to the amount of training information needed. Furthermore, the results are compared to the additive white Gaussian noise channel, and the case when ideal channel state information is available at the receiver.

Differential space-time block codes

Abstract: We propose a new differential modulation scheme for multiple-antenna systems based on square space-time block codes (STBC) when neither the transmitter nor the receiver knows channel state information. Compared with the known differential unitary space-time modulation (DUSTM), the proposed constellations generally have multiple amplitudes and do not have group properties. This generalization potentially allows the spectral efficiency to be increased by carrying information not only on orientations (or phases) but also on amplitudes. Two non-coherent decoders, optimal differential decoder (DD) and near-optimal DD, are derived for flat Rayleigh fading channels. Particularly, the near-optimal DD inherits the decomposition decoding property retained by STBC with coherent receiver and thus has linear complexity. Compared with the best-known cyclic group constellations designed for DUSTM, our proposed constellations with the near-optimal DD have significantly lower probability of error and lower decoding complexity.

Capacity and optimal power allocation for fading broadcast channels with minimum rates

Abstract: We derive the capacity and optimal power allocation scheme for a multiuser fading broadcast channel in which minimum rates must be maintained for each user in all fading states, assuming perfect channel state information at the transmitter and at all receivers. We show that superposition coding can achieve the capacity of such channels and explicitly characterize the boundary of the capacity region. The optimal power allocation scheme is a two-step process: we first allocate the minimum power required to achieve the minimum rates in all fading states, and we then optimally allocate the excess power to maximize the ergodic rates averaged over all fading states in excess of the minimum rate requirements. The optimal allocation of the excess power is a multi-level water-filling relative to effective noise that incorporates the minimum rate constraints. Numerical results are provided for different fading broadcast channel models.

Unconditionally secure communications over fading channels

Abstract: In this paper, we propose a novel method for two communicates to exchange a secret over a public wireless fading channel. Unlike conventional computationally secure public key methods, this technique is information theoretic and unconditionally secure provided that a component of the reciprocal channel fading over time between the two communicates is statistically independent with the channel fading from either communicate to the eavesdropper. This technique may be particularly well suited to secure tactical mobile communications. A simple protocol suitable for a lognormal shadowed fading channel is described and its key exchange rate is derived.

Channel estimation and adaptive power allocation for performance and capacity improvement of multiple-antenna OFDM systems

Abstract: We describe several techniques for the analysis and design of OFDM multiple-antenna systems, which include channel estimation and tracking, optimal training sequences for initial channel acquisition, and adaptive power allocation with closed-loop updating of channel state information for capacity improvement.

Analysis of an up/down power control algorithm for the CDMA reverse link under fading

Abstract: We analyze the performance of a code division multiple access (CDMA) reverse link a with an up/down power control algorithm in the presence of fading. We derive a stochastic nonlinear feedback control system model for the power controlled reverse link, and study the power control performance based on the nonlinear model using the technique of statistical linearization. We provide a general analysis framework that allows us to study the effect of mobile speed, power control step size, and fading channel parameters such as correlation coefficient and rate of fading on power control errors. Numerical results show excellent accuracy of our analysis, which can be used to design and optimize the system parameters without going through lengthy simulations. For example, in the presence of shadow fading, it is seen that a power control step size in the range of 0.5-1.3 dB is sufficient to keep power control errors near a minimum. In the case of Rayleigh fading, the standard deviation of power control errors grows quickly even at moderate mobile speeds.

Efficient use of fading correlations in MIMO systems

Abstract: We investigate the effects of both fading correlations and transmitter channel knowledge in multiple element antenna (MEA) communication systems. While, for independent and identically distributed fades between receive and transmit antennas, pioneering work showed that a huge increase in capacity is possible for MEA compared to a single antenna system, recent contributions warn that fading correlations destroy most of this advantage. While this is true for zero transmitter channel knowledge, we show however that long-term average channel state information enables the transmitter to efficiently use the fading correlations to its advantage and offers the potential to even increase capacity beyond the one possible for independent fading. A conceived transmit technique is presented that efficiently makes use of fading correlations, and also provides optimum choice of digital modulation schemes that carry the information.

A space-time block-coded OFDM scheme for unknown frequency-selective fading channels

Abstract: We introduce a space-time block-coded orthogonal frequency-division multiplexing (STBC-OFDM) selective fading channels which does not require channel knowledge either at the transmitter or at the receiver. The decoding algorithm is based on generalized maximum-likelihood sequence estimation. We investigate the performance of the proposed scheme over two-tap Rayleigh fading channels. Simulation results show the performance to be near optimum.

Prediction of fast fading mobile radio channels in wideband communication systems

Abstract: The fast fading mobile radio channels place fundamental limitations on the performance of wireless communication systems, such that the prediction of the changing channel behaviors is of interest. For the frequency-selective fading channels in wideband systems, the knowledge of the channel transfer function over the entire frequency band is required. We propose an ESPRIT-type algorithm to model and further predict the wideband time-varying channel at different frequencies jointly, assuming that the scatterers remain constant. Simulation results show that the joint-frequency prediction scheme has superior performance over conducting the channel prediction on a single frequency.

An adaptive antenna selection scheme for transmit diversity in OFDM systems

Abstract: In wireless communication systems, as one of the effective techniques for combating fading, transmit diversity has attracted much attention, especially when receive diversity is expensive or impractical due to the constraint of terminal size. In this study we consider an adaptive carrier-by-carrier basis antenna selection scheme for transmit diversity in OFDM systems under the assumption that a transmitter has priori knowledge of the channel state information (CSI). In the proposed schemes transmit antennas are selected adaptively for each subcarrier according to CSI.

Minimum duration outages in Rayleigh fading channels

Abstract: Minimum duration outages have been introduced for lognormal shadow fading where the durations of signal fades were considered in evaluating outages. We develop and analyze such minimum duration outages for channels susceptible to Rayleigh fading. The results show that under typical Doppler frequencies the outages due to Rayleigh fading are more likely to cause frame or packet errors rather than call dropping due to the short time scales in effect.

Effect of spatial fading correlation on performance of space-time codes

Abstract: The performance of space-time codes is investigated over Rayleigh fading channels with spatially correlated fading between transmit antennas. An exact pairwise error probability is derived based on which an analytical estimate for bit error probability is computed. The analytical results are verified through computer simulation.