Exponential Power Delay Profile Research Articles

Efficient Beamformer for Low Mobility Indoor Communication Using Sparse Adaptive Algorithm

The proposed beamforming model exploits the underlying sparseness of the adaptive filter as impulse response of wireless channel shows some extent of sparse behavior in practice. A new formulation of cost function of fourth order instead of quadratic will help to achieve stable and faster convergence, but the computational complexity is high. Thus the filter design requires a compromise between the quadratic and fourth order cost function to achieve good estimation accuracy. Normalized least mean square fourth (NLMS/F) filter design is based on the compromise to achieve a better performance with a faster convergence. Inclusion of sparsity in the cost function of NLMS/F filter further reduces the computational complexity as less number of nonzero coefficients involve in estimation with a bounded error. IEEE 802.11 and Saleh–Valenzuela models with exponential power delay profile (PDP) are used to implement proposed beamformer for indoor application. The proposed sparse-NLMS/F beamformer is compared with its NLMS/F counterpart, and tested with practical fading condition. Different performance measures like mean square error (MSE), estimated weight convergence, beam pattern are used to test the proposed model under practical channel condition.

Block NLMS/F-based equalizer design and channel capacity analysis for indoor IEEE 802.11 fading wireless channels

In this paper, adaptive decision feedback-based model is used to design an efficient equalizer to mitigate the detrimental effects during the signal transmission through wireless channels having non-ideal frequency response. The proposed adaptive equalizer is designed using normalized block least mean square/fourth algorithm which yields low bit error rate and mean square error in spite of high noisy and fading channel conditions. The constellations as well as eye diagrams are also taken as performance measuring criteria for indoor wireless channel which is designed by using IEEE 802.11 model with exponential power delay profile. The channel capacity using the proposed model is also analyzed to verify the system performance. Combination of block processing approach and the elegant structure of the equalizer achieves higher symbol estimation accuracy and stable convergence which is quite suitable for indoor fading channel characteristics. The performance comparison of the proposed model is presented with detail simulation results.

Performance of Differential-Phase OFDM Systems over Selective Multipath Fading Channels

This paper investigates the performance of differential-phase Orthogonal Frequency Division Multiplexing (OFDM) over frequency-selective multipath Nakagami-m radio fading channels. A closed form for the average signal to noise/interference ratio in the presence of the selective multipath fading channel and additive white Gaussian noise (AWGN) is derived. The results reveal that the system performance is impacted by the interference between the adjacent OFDM frames in two successive signaling intervals which is called Inter-Symbol-Interference (ISI). In addition, the system will possibly be distorted when the orthogonality between the adjacent subcarriers is ceased, creating Inter-Channel-Interference (ICI). This paper also studies the bit-error-rate (BER) performance of the differential-phase OFDM system over the Nakagami-m channel in the presence of AWGN, ISI, and ICI under different conditions and parameters. Moreover, the effect of adding the guard period and the number of subchannels on the probability of error is analyzed. The IEEE 802.11a standard parameters with 64 subcarriers and a decaying exponential power delay profile with root-mean-square value of 129 ns are used in this study. The system performance is also simulated at different guard intervals, number of OFDM subcarriers, Nakagami severity parameter values, and different numbers of possible differential phases.

Max-SINR Based Timing Synchronization Scheme in Hexagonal Multicarrier Transmission

In this paper, the maximizing Signal-to-Interference-plus-Noise Ratio (Max-SINR) criterion is applied in Hexagonal Multicarrier Transmission (HMT) system for timing synchronization. An iterative approach for Max-SINR timing over doubly dispersive (DD) channel with exponential power delay profile and U-shape Doppler spectrum is proposed. Theoretical analysis shows that there is an allowable SINR gap between the mean delay of DD channel and the optimal timing point. Meanwhile, a low complexity suboptimal timing approach based on instantaneous channel frequency response is proposed. Theoretical analysis and simulation results show that Max-SINR timing scheme outperforms traditional timing method about 2–15 dB in SINR and the suboptimal timing algorithm achieves a smaller Mean Square Error than 10−4 over DD channels. In addition, the Iterative Projection Sequence Detection (IPSD) receiver is proposed and the Match Filter Low Bound (MFLB) in HMT system is given. Simulation results show that the Bit Error Rate (BER) of the proposed IPSD approach based on iterative Max-SINR timing approximates to the MFLB and outperforms traditional timing scheme about 3dB at BER = 10−3.

Performance comparison of wavelet based and conventional OFDM systems in multipath Rayleigh fading channels

In this work, the performance of wavelet based OFDM (WOFDM) is studied and compared to that of conventional OFDM over multipath Rayleigh fading channels with exponential power delay profile. Results show that WOFDM has better bit error rate (BER) performance than conventional OFDM without cyclic prefix (CP) for all signal-to-noise ratios (SNRs) and also outperforms OFDM with CP for low SNR values in indoor and outdoor environments. Therefore, WOFDM might be an alternative to conventional OFDM since it also has better bandwidth efficiency.

A Polarized Clustered Channel Model for Indoor Multiantenna Systems at 3.6 GHz

Multiple-input-multiple-output (MIMO) technologies allow high data rates to be obtained, but they suffer from interantenna correlation caused by the limits in interantenna spacing. Polarized MIMO systems resolve this problem by using colocated perpendicularly polarized antennas that have low interantenna correlation. In this paper, a polarized single-directional channel model for 2× <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> MIMO systems at 3.6 GHz in an indoor environment is presented. The wireless channel is modeled as a sum of clusters, where each cluster has specular and diffuse components. The polarization of the specular component of the clusters is included by considering a per-path polarization. The diffuse component of the clusters is modeled with a Fisher-Bingham (FB5) spectrum in the azimuth-coelevation domain and with an exponential power delay profile. Polarization is analyzed by introducing the cross-polar discrimination of the exponential power delay profile parameters. All of the parameters in the model are extracted from an experimental measurement campaign performed in an indoor environment at 3.6 GHz. Individual paths are extracted from the measurements with the space-alternating generalized expectation-maximization (SAGE) algorithm. These paths are grouped in clusters within the azimuth of arrival-elevation of arrival-delay domains at the receiver side using automatic clustering algorithms. The specular component properties of the clusters are then determined. Finally, the diffuse components of the clusters are investigated and parameterized by applying a beamforming algorithm on the diffuse part of the impulse response.

A simple pilot position detection technique for channel estimation of SC-FDE

This letter investigates the frequency domain pilot multiplexing technique (FDPMT) for channel estimation of single-carrier frequency domain equalization (SC-FDE) and proposes a simple pilot position detection technique to blindly estimate the pilot positions in the frequency domain by using the noise subspace. Simulation results show that the proposed technique is robust to multipath fading channels with exponential power delay profile.

Collision model for performance analysis of coded transmission in time hopping impulse radio over multipath nakagami-m channels

Time hopping is the strategy employed in impulse radio systems to increase the communication reliability by spreading the multiple access interference (MAI) over numerous time intervals. This results in a non-stationary interference that makes the error probability performance to be dominated by the worst-case. In this paper we employ a collision model analysis for this time-varying interference that is ruled by the system loading in term of the corresponding MAI collision probability. The model proves to be simple and accurate to characterize the decision variable conditioned to the collision events. This provides a reliable framework for evaluating the error probability in multipath faded channels with RAKE receiver either for coded and uncoded systems. In this paper we derive closed form approximations of the bit-error-rate for multipath channels with exponential power delay profile and Nakagami-m fading. Validations by numerical analysis show that the collision model provides an accurate framework to evaluate also performance of realistic channel models.

Optimum pilot-to-data power ratio for partial RAKE receiver in Nakagami-m fading channels

In this paper, we study the problem of optimizing the pilot-to-data power ratio (PDR) for uplink code-division multiple access (CDMA) systems with partial RAKE (PRAKE) receivers. Both code-division multiplex (CDM) and time-division multiplex (TDM) reference-assisted schemes are considered. We assume Nakagami-m fading channel with two types of power delay profile (PDP): uniform and exponential PDPs. The optimum PDR is derived via minimizing the upper bound of the probability of bit error. A closed-form expression for the optimum PDR is obtained for both CDM and TDM schemes with uniform PDP. For exponential PDP, we derive the condition satisfied by the optimum PDR and obtain it numerically. It is observed that the optimum PDR is noticeably affected by the decay factor, but not by the m-parameter. For exponential PDP, unlike for uniform PDP, the optimum PDR depends on the number of combined paths. We also show that the optimum PDR derived from the block-fading channel model performs favorably in the Jakes' model.

Analysis and Realization of an Exponentially-Decaying Impulse Response Model for Frequency-Selective Fading Channels

We analyze a simple frequency-selective fading model that is formulated as a sequence of independent discrete-time impulse responses, each with independent Gaussian variates scaled by an exponential power-delay profile. This model is useful for simulating propagation path responses in a scattering environment. Various expressions and relationships are derived for the frequency correlation function, correlation bandwidth, rms delay spread, and decay time. Realization techniques are also discussed, and it is concluded that the most straightforward and efficient approach is to generate a block of samples by multiplying i.i.d. complex Gaussian variates by the square root of the exponential power-delay profile and then taking the FFT.

Planning the Number of Taps in a Delay Line for Negative Exponential Delay Profile

In real radio-communication systems, information about the time-varying channel is required for planning tasks. From such information, tapped delay lines are used to model the impulse response of the channel. This letter analyzes the relative error between the theoretical RMS (Root Mean Square) delay spread of negative exponential power delay profiles and that obtained from the tapped delay line. A method using a closed expression for calculating this error is then proposed for the planning of the number of taps in different environments.

Multipath Delay Profile Acquisition for Ultra-Wideband PPM Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The acquisition of a multipath channel profile (including the leading delay) for ultra-wideband pulse position modulation (PPM) communication systems is considered in the limit of large bandwidth. Optimal acquisition is defined by the maximum likelihood detector whose performance can be assessed via order statistics. Four channel scenarios are examined: deterministic or Gaussian channel taps, uniform or exponential power delay profiles. For these four channels, the rate of growth of the multipath as a function of bandwidth which leads to acquisition failure is determined. Acquisition is not even partially possible on multipath channels with deterministic path amplitudes, if the number of paths diverges too fast. Furthermore, if the number of independent Gaussian paths increases without bound, but slower than the bandwidth, then the system cannot even partially acquire in the limit. These negative results are shown for somewhat idealized environments, implying that multipath delay profile acquisition will fail under more realistic conditions. </para>

Time Delay Estimation Bounds in Convolutive Random Channels

We develop bounds on time delay estimation (TDE) in wideband convolutive random channels. The improved Ziv-Zakai bound (ZZB) is adapted to this case, resulting in mean-square error (MSE) bounds that are independent of estimator bias, and avoid regularity conditions. We assume a known transmitted signal, a tapped delay line random channel model, and a uniform prior on the delay. The taps are modeled as Gaussian, including non-zero mean and possible correlation between taps. The channel model may be tuned to a variety of environments through the choice of mean and covariance, and models wideband and ultra-wideband line of sight (LOS) and non-LOS (NLOS) cases. Flat fading is a simple special case with one tap. The channel power delay profile is incorporated through the tap covariance matrix, and results for an exponential power delay profile are given. We derive the ZZB for a given channel realization, and then average over the random channel model. The resulting bound reflects the impact of the random channel, providing an average MSE performance bound, that assumes perfect channel estimates are available to the receiver. Specific cases and examples illustrate the impact of the key system, channel, and signal parameters.

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

We present an optimum diversity receiver called blind ratio combining (BRC), which minimizes the average symbol error probability or maximizes the average output signal-to-noise ratio, where the channels' time delays and the random phases are known while the fading amplitudes are unknown. In contrast to previous works where efforts were made to find a posteriori probabilities at the receiver, the BRC simply calculates the optimum weights, which depend on the channel's statistics, avoiding continuous channel estimation, and thus, it significantly reduces the system's complexity. In nonidentical multipath fading channels with power delay profile (PDP), the BRC receiver performs between maximal ratio combining (MRC) and equal gain combining (EGC), and attains to keep its performance comparable—and in some cases superior—to that of generalized selection combining (GSC), while for large values of the decay factor it approaches MRC. Moreover, in the important practical case of exponential PDP—common in RAKE receivers modeling and adopted for the Universal Mobile Telecommunications System (UMTS) spatial channel modeling by the European Telecommunications Standards Institute-Third Generation Partnership Project (ETSI-3GPP)—the optimum weights can be accurately approximated by simple elementary functions. Furthermore, it is proved that the utilization of these weights ensures an error performance improvement over EGC for arbitrary PDPs. The proposed BRC receiver can be efficiently applied in wireless wideband communication systems, where a large number of diversity branches exists, due to the strong multipath effects.

Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems With Virtual Carriers

In this paper, we propose adaptive beamforming schemes for orthogonal frequency-division multiplexing (OFDM) communication systems. After applying the discrete-Fourier-transform operation to the output of each antenna, one estimates the signal-plus-interference spatial correlation matrix by selecting the samples at ldquononzero-valuedrdquo subcarriers and the interference-alone spatial correlation matrix by choosing those samples at ldquozero-valuedrdquo subcarriers (also known as virtual carriers). The beamforming weight vector is the ldquolargestrdquo generalized eigenvector of the resulting matrix pencil, wherein the solution is computed in a batch fashion. By using a limited number of samples, the proposed scheme offers competitive performance compared with that of the scheme using optimal beamforming weight vectors that are calculated for each subcarrier under small-angular-spread scenarios. The adaptive solution for calculating the beamforming weight vector is also derived. The proposed schemes are shown to be suitable in scenarios of narrow angular spread with exponential power delay profile. The simulations that are presented here employ the OFDM signal model to illustrate the efficacy of the proposed algorithms.

Low complexity LMMSE turbo equalization for combined error control coded and linearly precoded OFDM

The turbo equalization approach is studied for Orthogonal Frequency Division Multiplexing (OFDM) system with combined error control coding and linear precoding. While previous literatures employed linear precoder of small size for complexity reasons, this paper proposes to use a linear precoder of size larger than or equal to the maximum length of the equivalent discrete-time channel in order to achieve full frequency diversity and reduce complexities of the error control coder/decoder. Also a low complexity Linear Minimum Mean Square Error (LMMSE) turbo equalizer is derived for the receiver. Through simulation and performance analysis, it is shown that the performance of the proposed scheme over frequency selective fading channel reaches the matched filter bound; compared with the same coded OFDM without linear precoding, the proposed scheme shows an Signal-to-Noise Ratio (SNR) improvement of at least 6dB at a bit error rate of 10−6 over a multipath channel with exponential power delay profile. Convergence behavior of the proposed scheme with turbo equalization using various type of linear precoder/transformer, various interleaver size and error control coder of various constraint length is also investigated.

Theoretical analysis of reverse link capacity for an SIR-based power-controlled cellular CDMA system in a multipath fading environment

Capacity estimation in a code-division multiple-access system is closely related to power control schemes, which complicates the analysis due to the interaction between the signal power and the interference from other users and from other paths. For a signal-to-interference ratio (SIR)-based power control scheme, most previous work has been restricted to a single-cell system or to a multiple-cell system neglecting the effect of multipath fading. This paper is to give a theoretical foundation to the possible reverse link capacity of a multiple-cell system with perfect SIR-based power control, assuming two different multipath Rayleigh fading channel models: uniform and exponential power delay profiles. The effects of the numbers of resolvable propagation paths and RAKE fingers, and other system parameters such as the required E/sub b//I/sub 0/, the processing gain, and the maximum allowable transmit power of a mobile station, are investigated. The results are compared between single- and multiple-cell systems. When the number of resolvable paths is one or the number of Rake fingers is one, the link capacity becomes zero in a multiple-cell environment. This can be avoided by the use of antenna diversity. Antenna diversity reception is found to linearly increase the link capacity as the number of antennas increases.

A statistical approach to the analysis of DS/CDMA cellular systems employing RAKE receivers and sectorized antennas

RAKE receivers and sectorized antennas are used in direct-sequence/code-division multiple-access (DS/CDMA) cellular systems to improve the system performance. This paper presents a statistical method for analyzing the performance of DS/CDMA cellular radio systems employing RAKE receivers and sectorized antennas. Average bit error rates in the system are estimated considering the multipath fading effects of the environment. (The fast fading is assumed to be Rayleigh distributed, and the distance-dependent means of the multipath components have an exponential power delay profile.) The analysis of RAKE receivers quantifies the performance improvement that could be achieved by increasing the number of RAKE fingers. Sectorized antennas improve the system performance by reducing the interference at the receiver. In a perfectly sectorized system, assuming three sectors per cell, the capacity of the system can be improved by a factor of three. However, due to the imperfection in practical antennas, it is not possible to achieve this improvement. The performance of systems employing practical sectorized antennas (with finite front-to-back ratios and overlapping sectors) is compared with the performance of perfectly sectorized systems. The analysis shows that the incremental performance improvement diminishes with each incremental increase in the number of RAKE fingers. Performance degradation due to finite front-to-back ratio is shown to be insignificant for practical values of the front-to-back ratio of sectorized antennas. However, the reliability of mobile reception can be degraded significantly in areas where adjacent sectors overlap.

Performance of coherent QPSK communications over frequency-selective fading channels for broadband PCS

The fading caused by multipath and the effect of delay spread affect broadband wireless personal communications channels by introducing intersymbol interference, which appear when the root-mean-squared (rms) delay spread is significant in comparison to the duration of the transmitted symbols. This results, in the absence of equalizing, in an irreducible error probability. In this paper, we show that it is easy to obtain, for given power-delay profile models, the variance of the interference components at the output of a QPSK receiver. Using Gaussian approximations, a general form for a “fading factor” is defined. This approach is applied, in the particular cases of exponential and Maxwell power-delay profiles, to analyze the performance of coherent QPSK modulation over frequency-selective fading channels.