José J. G. Fernandes

Vectorial channel estimation for uplink MC-CDMA in beyond 3G wireless systems

In beyond 3G wireless systems the bandwidth efficiency can be increased with the use of adaptive antenna arrays. This paper focus on a key issue for adaptive antenna arrays, that is, channel parameters estimation including direction-of-arrival (DOA). In order to estimate DOA, the channel frequency responses for the links between the mobile users and each of base station array elements are estimated by pilot-aided minimum mean square error (MMSK) algorithm. This estimator is not sensitive to the channel statistics. Based on frequency response estimatives DOAs of impinging multipath components to the base station are estimated by a low complex maximum likelihood (ML) approach. Furthermore an uplink burst structure with specifically designed midamble field for multiuser channel estimation in MC-CDMA is proposed. The performance is assessed in terms of channel estimation errors for a MC-CDMA TDD system over fast and slow fading mobile channels.

Mm-wave indoor radio channel modelling vs. measurements

A propagation model for the prediction of indoor radio channel characteristics at millimetre-wave frequencies, based on geometrical optics is described. This model has been shown to agree well with wide-band measurements carried out at Eindhoven University of Technology [8]. Simulation results for 4th and 6th order reflection are compared with corresponding measured data obtained in small and medium size rooms. The influence of objects located inside the room superstructure, on the indoor radio channel characteristics, is analysed. The agreement between measured and simulated results, confirms the validity of the proposed simulation method.

Pre-equalization technique for interference cancellation in the UMTS-TDD downlink channel

This paper proposes a technique aiming to simplify the mobile terminals by transferring as much as possible the complex operations to the base station. A pre-equalization technique is proposed for the downlink channel of UMTS TDD, which allows the use of simpler receiver schemes in the mobile unit. After the mathematical background, simulation results obtained in typical scenarios, using statistical channel models, are presented to demonstrate the performance of the proposed method. Several levels of correlation between the propagation channels that can be considered to exist between the different base station antenna array elements and the desired mobile unit were considered. The results have shown that that significant performance gains can be achieved in high multiple access interference (MAI) environments.

Impact of the antenna set-up and arrays on mobile radio systems

This paper outlines the impact of the antenna set-up and antenna array configurations on mobile radio system at millimetre-wave frequencies. Simulations in typical indoor environments using directive antennas have shown a significant reduction in the channel time dispersion and in power requirements. Also in a typical city street, specially when some objects (cars, buses, etc.) are present, it gives significant performance improvements when compared with an omnidirectional antenna. The significant reduction of channel time dispersion achieved with directional antennas, associated to the fact that a mobile terminal must be able to communicate in any direction, indicates that the best performance of a broadband system is obtained with either switchable beams or antenna arrays. The propagation channel is modelled with a ray tracing tool that takes into account the propagation environment and the antennas used.

Constrained space-time zero-forcing pre-equalizer for the downlink channel of UMTS-TDD

The great diversity of services expected to be delivered by third generation mobile radio systems will impose severe operating conditions on the mobile terminal in terms of computational requirements and power consumption. Therefore, we propose to move the most demanding signal processing tasks, usually performed by the mobile unit, to the base station. This technique is developed for a UMTS-TDD downlink scenario through an equalizer synthesis method based on the redundancy between non-overlapping bands of a direct sequence spread spectrum (DS-SS) signal, with the design optimised for minimum power transmission under the zero-forcing criterion.

Complex impulse response modeling for wideband channels

The foundations of a wideband site-specific radio propagation channel model is presented. The model consists in a combination of a deterministic and statistical parts. The deterministic part generates the channel profile taking into account the influence of the propagation environment and the antenna configuration once they have a strong impact on the channel impulse response while the statistical part is used to include the small scale variations of the amplitude and phase. The model is tapped-delay-line based and it is shown that the Rice distribution is adequate to model the fast variations of the amplitude of each tap being the respective parameters extracted from the deterministic modeling part. Moreover, a statistical phase distribution about the phase of the dominant multipath component in each time bin proves to be more appropriate than the uniform distribution. Due to the lack of experimental measurements, the results of the model are compared with results obtained with a ray-tracing tool, previously validated with measurements, and have shown the validity of the proposed model.

Channel Estimation Method with Improved Performance for the UMTS-TDD Mode

Channel estimation is an essential building block for UTRA-TDD high performance receivers. Once the performance of the channel estimator algorithm proposed by 3GPP is highly dependent on the time spreading between consecutive multi-path components, a Successive Multi-path channel Estimation Technique (SMET) that improves the time resolution is proposed in this paper. A SMET based maximum likelihood approach for vectorial channel estimation, to include the estimation of the direction-of-arrival, is also proposed. This algorithm solves efficiently the complex problem of DOA estimation of multiple users in a multi path propagation environment even when the number of required DOAs exceeds the number of antenna array elements. Another property of the proposed algorithm is its ability to resolve signals from different users arriving from the same direction. This is due to processing in both time and space dimensions. The performance of these algorithms is assessed by resorting to simulations in multi-path environments using the UMTS-TDD specifications, and also by comparing the rms estimation errors against the Crámer-Rao Bound. The effect of imperfect channel estimation on the performance of RAKE and Hard-Decision Parallel Interference Canceller receivers is also analysed. The results show that a good performance can be achieved with SMET, from low to high values of Eb/n0.

Vectorial multipath channel estimation for the UTRA-TDD mode

This paper, deals with channel estimation for UTRA-TDD. Based on the algorithm suggested by the 3GPP for scalar channels, a maximum likelihood (ML) approach for vectorial channel estimation, to include also the estimation of the direction-of-arrival, is proposed. The performance of the algorithm is assessed by resorting to simulations in typical UMTS scenarios, and also by comparing the RMS estimation errors against the Cramer-Rao bound. The results show that good performance is achievable from low to high values of E/sub b//N/sub 0/, provided the path components are separated by more than 2 chips.

Constrained mean-square-error space-time pre-equalizer for the downlink channel of UMTS-TDD

This paper deals with space-time pre-equalization for UMTS-TDD systems. The motivation of this proposal lies in the fact that in TDD the up and downlink channels can be highly correlated. If this is the case, the equalization functions can be moved to the base-station (BS), therefore simplifying the mobile unit receiver design, where computational requirements and power consumption issues impose severe restrictions. A design technique, which explores both the frequency redundancy of DS-SS signals and the spatial diversity achieved through the use of an array of antennas at the BS, is derived. The used criterion intends to perform the minimization of the transmitted energy per bit, subject to the condition that signal error power spectral density (PSD) at the decision point is below a given threshold, ensuring the desired signal to distortion ratio. After the mathematical background, simulation results, obtained in typical scenarios, are presented and compared with conventional emitter and receiver based schemes using the RAKE combining principle.

Channel estimation with array processing for the uplink of UMTS-TDD

Channel estimation is critical to coherent RAKE receiver performance. This paper deals with channel estimation based on the standard algorithm suggested for UTRA-TDD by 3GPP. It proposes a spatial pre-combination technique that exploits the spatial structure of the received multipath signal to improve the channel estimator performance. Self and multiple access interference is partially cancelled before the channel estimation algorithm. The performance of the estimation algorithm is assessed by resorting to simulations in typical UMTS-TDD scenarios and the correlation coefficients between the complex channel and the estimates are computed. The effect of imperfect channel estimation on the performance of RAKE1D and RAKE2D receivers is analyzed in a multiuser scenario.