Time-varying Mobile Channels Research Articles

Mobile channel estimation based on decision feedback in vehicle-to-infrastructure visible light communication systems

Visible light communication (VLC) emerges as a promising candidate in vehicle-to-infrastructure (V2I) networks. For VLC downlink between street lamps and a mobile vehicle, received signals tend to experience a time-varying multi-source and multi-path channel condition. This will affect the effectiveness of channel equalization when assuming constant channel characteristics within each data packet. In this paper, in order to track the time-varying mobile VLC channel effectively, we propose a channel estimation method for V2I downlink based on decision feedback with orthogonal frequency division multiplexing. Considering different system parameters, extensive numerical experiments are carried out. Results show that compared with the conventional scheme using only training sequences, adopting decision feedback can achieve higher channel estimation accuracy, thus improving link performance significantly. For vehicle velocity at 15 m/s, bit error rate (BER) < 10 −4 can be achieved at ∼199 Mbps if transmit power per lamp is > 30 W. Moreover, the proposed method is almost unaffected by the change of vehicle moving state, with stable BER achieved.

Channel Estimation for TDS-OFDM Systems in Rapidly Time-Varying Mobile Channels

This paper explores the performance of time-domain synchronous orthogonal frequency-division multiplexing (TDS-OFDM) systems operated under rapidly time-varying mobile channels. Since a rapidly time-varying channel contains more unknown channel coefficients than the number of observations, the mobile channel can conveniently be modeled with the discrete Legendre polynomial basis expansion model to reduce the number of unknowns. The linear minimum mean square error (LMMSE) estimate can be exploited for channel estimation on inter-block-interference-free received signal samples owing to transmitting pseudo-noise (PN) sequences. In conventional TDS-OFDM systems, the channel estimation performance is limited due to estimating channel responses only from the beginning part of the channel. Therefore, a new system model named “ partitioned TDS-OFDM system ” is proposed to improve the system performance by inserting multiple PN sequences to the middle and end parts of the channel as well. In addition to providing the reconstruction error performance, Bayesian Cramer–Rao lower bound is derived analytically. Also, the LMMSE-based symbol detection is employed. To alleviate the negative effects of inter-carrier-interference (ICI) occuring in mobile channels, ICI cancellation is applied to enhance the detection performance. The simulation results demonstrate that the proposed TDS-OFDM system is superior to the conventional system and its corresponding performance is able to approach the achievable lower performance bound.

Interpolation based pilot‐aided channel estimation for STBC spatial modulation and performance analysis under imperfect CSI

A combination of space-time block coding and spatial modulation (STBC-SM) has been recently proposed for multiple input-multiple output systems to obtain both spatial diversity gain and more capacity simultaneously while assuming the perfect channel state information (P-CSI) was available at the receiver. However, in practical scenarios the CSI is unknown to the receiver and should be estimated in order to detect the transmitted data in a reliable way. Therefore, channel estimation (CE) is a major challenge in designing the STBC-SM systems. In this study, the problem of CE is investigated and a new pilot-aided channel estimation (PA-CE) technique, coupled with an interpolation, is proposed for the STBC-SM systems operating in the presence of rapidly time-varying mobile channels. Several interpolation schemes such as the linear, nearest neighbour, piecewise cubic Hermite and the low-pass interpolations are applied and their performances are compared to determine the best suitable interpolation technique to be employed in STBC-SM systems. Bit error rate (BER) performance of the proposed CE technique is then investigated in time-varying channels with different modulation. Moreover, the pairwise error probability of the STBC-SM scheme is derived and its average bit error probability is evaluated analytically in the presence of CE errors.

CHANNEL BASED DESIGN OF SYSTEMS WITH MULTIPLE ANTENNAS

In this article a method of optimizing wireless communication systems using multiple antennas is presented. This method focuses on the synthesis of antenna radiation patterns that are optimized in terms of mutual information, taking into account the specific limitations of the antenna design, such as the available space (for the antenna structure), polarization, number and arrangement of the antennas. Multiple-input-multiple-output (MIMO) antenna systems have the potential to increase the mutual information of wireless communication, through additional antennas, hence more channels and hardware. In contrast to the antennas, which are usually quite inexpensive, the size, complexity and price of the front-ends scale with the number of antennas. Accordingly, it is of interest for commercial applications, to use as few system inputs and outputs as possible to reduce the hardware for cost saving. The performance of a MIMO-antenna system is a direct result of the channel transfer matrix H, which in turn depends on the propagation environment and the antennas. Since the environment usually cannot be changed, the choice of antennas is crucial for the overall system behavior. The antennas determine inherently the correlation between the transmission coefficients of the channel matrix and, consequently, the mutual information of the system. The number of degrees of freedom (NDF) in the antenna design are: the spatial separation of the antennas, their polarization and their respective complex directional characteristics. The interaction of the propagation environment with the antennas and the high NDFs make the development of such a system especially time-consuming and expensive. This is the reason why, in most cases, a heuristic approach is used. This means that a certain antenna configuration is chosen based on general assumptions about the propagation environment, knowing that by doing so it is not guaranteed whether this choice increases the potential mutual information or not. Afterwards a prototype set-up with several possible combinations and a subsequent verification by measurements is omitted typically due to cost and time constraints. In contrast to the heuristic approach, our proposal includes full consideration of the propagation environment. The simulations performed during the proposed antenna synthesis method are reproducible and we can expect a resulting antenna system that is optimized in terms of mutual information in time variant mobile channels. Our promise is that this will allow the use of MIMO systems in industrial applications which are (typically) not realizable due to cost and space reasons and due to the high complexity. The basic idea behind the synthesis presented in this article is to optimize antenna systems by using channel knowledge already in the design process of the system. This enables us to determine fixed directional patterns that are optimized in terms of mutual information for any given set of constraints such as the number of antennas, their geometrical sizes and/or their polarization. By doing so hardware costs can be reduced since a system with fewer but more optimized antennas may have the same performance as a heuristically designed system with multiple antennas.

Adaptive generalised selection-combining receiver over time-varying mobile communication channel

An adaptive generalised selection-combining (GSC) receiver is proposed for time-varying frequency selective fading channel. The receiver adaptively selects and combines a subset of diversity branches out of the available branches and discards the others. This is performed by comparing the maximum likelihood (ML) metric of each diversity branch with a threshold. The branches that are lower than the threshold are selected and combined using maximal ratio combiner. The proposed adaptive GSC receiver dynamically adjusts the threshold value according to the time variations of the channel. Equalisation and data detection are performed after combining using ML sequence estimation implemented by Viterbi algorithm. The minimum survivor technique is employed to reduce the complexity of the receiver. An upper bound for the probability of error of the proposed receiver is derived. The performance of the proposed adaptive GSC receiver is compared with the performance of different threshold GSC receivers.

A dual combining based decoding scheme for two-branched mobile STBC

In this paper, a dual combining based decoding scheme is proposed to enhance the detection performance of space-time block coding with two transmit antennas in a highly time-varying mobile channel. The proposed scheme analyzes the signal-to-noise ratio and signal-to interference plus noise ratio of decision variables for a zero-forcing combiner (ZFC) and a matched filter combiner (MFC), respectively. Based on the analysis, we derive a sophisticated decoding scheme which utilizes tentative detection symbols of the ZFC based decoder to cancel inter-symbol interference from the decision variables of the MFC. Computer simulations show that the proposed scheme outperforms the conventional decoding scheme with the ZFC or the MFC.

Genetic algorithm-based MMSE receiver for MC-CDMA systems transmitting over time-varying mobile channels

A novel multi-user detector, based on a genetic algorithm-assisted per-carrier MMSE criterion, is proposed for MC-CDMA systems transmitting over time-varying multipath fading channels. The analysed multi-user detector outperforms state-of-the-art adaptive receivers based on deterministic gradient algorithms, particularly for an increasing number of users.

Spectrally Efficient Communication over Time-Varying Frequency-Selective Mobile Channels: Variable-Size Burst Construction and Adaptive Modulation

Methods for providing good spectral efficiency, without disadvantaging the delivered quality of service (QoS), in time-varying fading channels are presented. The key idea is to allocate system resources according to the encountered channel. Two approaches are examined: variable-size burst construction, and adaptive modulation. The first approach adapts the burst size according to the channel rate of change. In doing so, the available training symbols are efficiently utilized. The second adaptation approach tracks the operating channel quality, so that the most efficient modulation mode can be invoked while guaranteeing a target QoS. It is shown that these two methods can be effectively combined in a common framework for improving system efficiency, while guaranteeing good QoS. The proposed framework is especially applicable to multistate channels, in which at least one state can be considered sufficiently slowly varying. For such environments, the obtained simulation results demonstrate improved system performance and spectral efficiency.

SR ARQ for Adaptive Modulation Systems Combined With Selection Transmit Diversity

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. In this paper, both throughput and packet error rate are analyzed for a selective-repeat ARQ scheme based on a constant-power variable-rate adaptive M-QAM system combined with selection transmit diversity over multiple-input multiple-output Markovian-Nakagami channels. In this analysis, the impact of using outdated and/or imperfect channel state information on the performance of the system is considered.

Impact of Channel Prediction on Adaptive Coded Modulation Performance in Rayleigh Fading

Adaptive coded modulation (ACM) is a promising tool for increasing the spectral efficiency of time-varying mobile channels while maintaining a predictable bit-error rate (BER). An important restriction in systems with such a transmission scheme is that the transmitter needs to have accurate channel-state information (CSI). Earlier analysis of ACM systems usually assumes that the transmitter has perfect knowledge of the channel or that the CSI is accurate but outdated. In this paper, we investigate the effects of predicting the CSI using a linear fading-envelope predictor in order to enhance the performance of an ACM system. For the case in which multidimensional trellis codes are used on Rayleigh-fading channels, we obtain approximative closed-form expressions for BER and average spectral efficiency. Numerical examples are given for the case of Jakes correlation profile and maximum a posteriori-optimal predictor coefficients.

Blind decision feedback equalization of time-varying channels with DPSK inputs

This paper presents a new method to the blind equalization of a class of linear time-varying mobile channels for differentially encoded channel input signals. Through reparameterization, the equivalent discrete system can be modeled as a single-input multiple-output system. Unlike some existing methods, this new algorithm does not require prior knowledge on the cyclic frequency of the channel response. The algorithm is simple and relies only on a decision feedback structure and the differential encoding nature of the channel input signals. The convergence analysis of the decision feedback system is presented along with simulation examples.

On ARQ scheme with adaptive error control

Adaptive automatic repeat request (ARQ) schemes are quite effective for throughput enhancement in time-varying mobile channel environments. An ARQ scheme with adaptive error-correcting codes is considered. A robust channel sensing algorithm by making use of XOR-ing is proposed for link adaptation. With the proposed channel estimation algorithm, an adaptive scheme that adapts its error-correcting codes according to channel conditions is presented. The efficiency of the proposed adaptive ARQ scheme is evaluated in terms of throughput performance by computer simulation for a time-varying mobile channel characterized by Rayleigh fading and log-normal shadowing. As an enhancement in counteracting long deep fades, the incorporation of frequency diversity into an adaptive ARQ scheme is also investigated and the simulation results show that this scheme is quite efficient in counteracting long deep fades.

Excess delay estimation from time-varying mobile radio channel impulse response measurements

This letter proposes a method for estimating the instantaneous excess delay in time-varying mobile radio channel impulse response measurement data using image processing techniques. The method is shown by example to produce more accurate mean power delay profiles than those produced assuming either constant or linearly varying excess delay.

Adaptive multilevel code selection and MUI cancellation for DS/CDMA over fading channel

For transmission of speech or multimedia information in a time-varying mobile channel, fixed rate codes are normally used designed for average or worst channel conditions. However, fixed rate codes fail to explore the time-varying nature of the mobile channel. In their previous work the authors have shown that a multilevel code, when used in conjunction with multiuser interference (MUI) cancellation in cascade, yields better performance than a conventional coded modulation scheme applied to CDMA. They show that, when multilevel codes are changed with varying channel conditions (i.e. are chosen adaptively according to the channel state information (CSI)), throughput (average number of information bits per symbol duration) can be increased without too much performance degradation. Code selection criteria as well as some codes with appropriate thresholds are also presented.

Simulation of digital transmission over mobile channels at 300 kb/s

Studies of digital transmission over typical urban and suburban mobile channels using simulations that employ a local area model for the time varying mobile channel impulse response are discussed. The digital transmission techniques of coherently detected and differentially coherent detected versions of quadrature phase-shift keying (QPSK), Gaussian minimum shift keying (GMSK), and coherently detected binary phase-shift keying (BPSK) over example mobile channels are presented. Two measures of performance are considered; the mean bit error ratio (and irreducible bit error ratio), which is used to compare the robustness of the various modulation methods to delay spread, and the outage probability, which provides a measure of the overall transmission quality as would be perceived by a user. Emphasis is placed on results obtained for GMSK, which is the modulation scheme to be employed in the Pan-European digital cellular mobile system. The effects of RMS delay spread on the mean bit error ratio, mean irreducible bit error ratio, and probability of outage are considered for different channel types.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>