Weighted RLS channel estimators for DS/CDMA signals in multipath

2009 IEEE 70th Vehicular Technology Conference Fall, 2009

Recently, we have proposed an adaptive channel estimation (CE) scheme using one-tap recursive least square (RLS) algorithm (adaptive RLS-CE), where the forgetting factor is adapted to the changing channel condition by the least mean square (LMS) algorithm, for direct sequence-code division multiple access (DS-CDMA) with frequency-domain equalization (FDE). However, the tracking ability for adaptive RLS-CE is limited since the channel estimate obtained from the previously received block is used. In this paper, we introduce the polynomial prediction to improve the tracking ability. We evaluate the bit error rate (BER) performance of DS-CDMA using polynomial prediction RLS-CE in a frequency-selective fast Rayleigh fading channel by computer simulation.

IEEE Transactions on Signal Processing, 2002

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation.

IEICE Transactions on Communications, 2009

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can increase the downlink bit error rate (BER) performance of DS-CDMA beyond that possible with conventional rake combining in a frequency-selective fading channel. FDE requires accurate channel estimation. Recently, we proposed a pilot-assisted channel estimation (CE) based on the MMSE criterion. Using MMSE-CE, the channel estimation accuracy is almost insensitive to the pilot chip sequence, and a good BER performance is achieved. In this paper, we propose a channel estimation scheme using one-tap recursive least square (RLS) algorithm, where the forgetting factor is adapted to the changing channel condition by the least mean square (LMS) algorithm, for DS-CDMA with FDE. We evaluate the BER performance using RLS-CE with adaptive forgetting factor in a frequency-selective fast Rayleigh fading channel by computer simulation.

IEEE International Conference on Communications, 2002

This paper investigates channel estimation in the uplink of a DS-CDMA system. The transmission medium is characterized by the presence of multipath components and the parameters of interest are the delays and complex gains of the propagation paths. We propose a new method for estimating the channel parameters of a user entering the system. The method is based on the

1998

This paper focuses on the design of a multiuser receiver structure for the reverse link of a code-division multiple-access communication system, in the presence of multipath effects and using an antenna array at the base station receiver. The algorithm presented in this paper solves the complex multidimensional problem of channel estimation in this complex scenario using a maximum-likelihood approach. This channel estimation technique requires the transmission of a training sequence or feedback of detected data. Once a composite channel-impulse response of each user is estimated, it is directly used in the detection process instead of first extracting the individual channel parameters, such as path delays and attenuation factors. The paper presents a framework that facilitates a computationally efficient solution to the combined problem of channel estimation and detection in a scenario involving multiple users, multiple paths, and multiple sensors at the receiver.

Wireless Personal Communications, 1998

A direct sequence (DS) spread spectrum code division multiple access (CDMA) communication system is considered where several users transmit data symbols over different multipath channels. The main objective of this work is the proposal of a sequential algorithm for joint parameter estimation and multiuser data detection. The computationally prohibitive maximization of the log-likelihood function is replaced by a sequential scheme using the multistage detector for recovering the data symbols and the expectation-maximization algorithm for estimating the channel parameters. The performance of the resulting multiuser receiver is evaluated via Monte Carlo simulations using both synthetic as well as measured channel impulse responses. A comparison of the performance with analytical expressions for the single-user single-path case shows that the proposed system is capable of eliminating the near-far effect existing in conventional DS/CDMA systems.

IEICE Transactions on Communications, 2007

Frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace the conventional rake combining to significantly improve the bit error rate (BER) performance in a frequency-selective fading channel. MMSE-FDE requires an accurate estimate of the channel transfer function and the signal-to-noise power ratio (SNR). Direct application of pilot-assisted channel estimation (CE) degrades the BER performance, since the frequency spectrum of the pilot chip sequence is not constant over the spreading bandwidth. In this paper, we propose a pilot-assisted decision feedback frequencydomain MMSE-CE. The BER performance with the proposed pilot-assisted MMSE-CE in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that MMSE-CE always gives a good BER performance irrespective of the choice of the pilot chip sequence and shows a high tracking ability against fading. For a spreading factor SF of 16, the E b /N 0 degradation for BER=10 −4 with MMSE-CE from the ideal CE case is as small as 0.9 dB (including an E b /N 0 loss of 0.28 dB due to the pilot insertion).

2005

A general method for user dedicated downlink channel estimation in WCDMA receivers is addressed, particularly suited in the presence of dedicated channel transmit beamforming. A three-step dedicated channel estimation procedure is derived which exploits all the existing pilot sequences as well as the structured dynamics of the channel. In the first step, least squares (LS) estimates of the channels associated with dedicated and common pilots are built. In the second step, an improved unbiased minimum mean square error (UMMSE) estimate of the dedicated channel is obtained by optimally combining the initial LS estimates exploiting the correlation between dedicated and common pilot channels. In the last step, the improved dedicated channel estimate is further refined via Kalman filtering by exploiting the channel temporal correlation.

Wireless Personal Communications, 2010

In order to attain near-single user performance in uplink multicarrier code- division multiple-access (MC-CDMA) systems, multiuser detection (MUD) methods may be employed which rely on simultaneous estimation of the channel frequency responses of multiple users. Pilot symbol assisted (PSA) channel estimation is needed in fast fading channels and it can be performed either by applying maximum likelihood (ML) criterion or minimum mean-squared error (MMSE) criterion. The performance of ML estimation technique degrades significantly in the case of fractionally spaced (FS) multipath channels where dominant paths are closely spaced with respect to the time resolution of the system. In such situation, the number of effective paths (which contribute more towards signal power) becomes considerably less than the actual number of multipaths at low and moderate SNR values. We propose an improved ML estimation method which considers only effective paths during the estimation process. The proposed method performs nearly identical to the MMSE estimation method and it can also provide significant reduction in the computational complexity when a large number of users are accommodated in the system.

A pilot-aided joint channel estimation and data detection method for uplink MC-CDMA mobile system is proposed. Conventional pilot-aided estimation with low pilot overhead is used to obtain an initial estimate of channel. The information bits obtained from the first iteration are subsequently used to improve channel estimates. The second iteration uses a Parallel Interference Canceller (PIC) scheme followed by a frequency-time domain Wiener filter. Results show that the proposed pilot-and-data-aided channel estimator allows, at price of some complexity increase, the use of short pilot sequences and increases the number of uplink radio channel that can be simultaneously estimated.