Lars Schmitt

Maximum ratio combining of correlated diversity branches with imperfect channel state information and colored noise

Recently, an analytical expression for the mean bit error probability of a binary modulated signal has been derived for a receiver performing maximum ratio combining of correlated diversity branches based on imperfect channel state information. We extend these results to the general case of colored additive noise. Furthermore, an alternative solution is derived which yields numerically stable results also in the case of the respective channel and interference covariance matrices having closely-spaced or multi-fold eigenvalues. Finally, the results are applied to a spatio-temporal W-CDMA RAKE receiver, where the effects of multiple-access-interference (MAI) and interpath interference (IPI), due to multipath propagation, on the mean bit error probability are investigated.

Prediction of Downlink SNR for Opportunistic Beamforming

Opportunistic beamforming (OBF) is a technique to jointly increase the throughput and quality of service (QoS) in a multiuser system with fair and channel-aware scheduling. With this technique, the channel dynamics are influenced such that fading peaks occur more often within a given period of time. Channel-aware scheduling requires reliable SNR information in the base station that is obtained via feedback information from the users. Since this feedback is not instantaneous, the problem caused by a feedback delay are exacerbated by OBF because of higher channel dynamics. In this paper, a SNR predictor is derived for OBF that is matched to the beamforming process.

Initial synchronization of W-CDMA systems using a power-scaled detector with antenna diversity in frequency-selective Rayleigh fading channels

An analytical evaluation of the performance in terms of detection probability and mean detection time of a noncoherent detector at the base station is presented for a wideband CDMA system, where the mobile terminal transmits a pilot signal in the form of bursts of modulated chips, which are transmitted periodically and separated by long silent intervals. The performance analysis is carried out for a power-scaled detector employing temporal and spatial noncoherent averaging in frequency-selective fading channels taking channel dynamics and initial frequency offsets into account. With the presented analysis, which is verified by means of simulations, it is possible, and convenient, to trade off temporal noncoherent versus coherent averaging, depending on the length of the pilot bursts and channel dynamics and to examine the improvement of using multiple diversity antennas at the base station.

Enhanced Predictive Up/Down Power Control for CDMA Systems

In this paper we derive an enhanced power control algorithm, fitting into the up/down control scheme, as it is considered in the frequency division duplex (FDD) mode of the current 3GPP standard. Analysis of the classical up/down power control scheme unveils, that with increasing velocities the power control performance degrades, as the fixed step size power control is not able to track the channel fading properly. For the uplink we derive a nonlinear control algorithm generating the up/down power control commands accounting for the future of the channel fading process. Simulations show that this algorithm in combination with perfect future channel state information can partially mitigate the drawbacks of a fixed step-size up/down power control. A prerequisite for predictive power control is the acquisition of the future channel state information. In this paper we deduce a robust and adaptive structure for the prediction of the channel fading process in the context of a power controlled code division multiple access (CDMA) system based on least mean square (LMS) adaptation. Link level simulations show a signal to noise and interference ratio (SINR) gain in terms of the block error rate, enabling a decrease of the target SINR and thus leading to an enhanced spectral efficiency.

Performance of initial synchronization schemes for WCDMA systems with spatio-temporal correlations

Two different power-scaled noncoherent detection schemes are analyzed and compared in terms of detection probability and mean detection time in the uplink of a wideband CDMA system, where the mobile terminal transmits a pilot signal in the form of bursts of modulated chips, which are transmitted periodically and separated by long silent intervals. Both detection schemes employ temporal noncoherent averaging but differ in the way of spatial processing. One of the schemes, which is well suited for spatially uncorrelated scenarios, employs spatial noncoherent averaging, whereas the other scheme, better suited for scenarios with a distinct spatial structure, employs fixed beamforming. The performance analysis is carried out for spatially correlated frequency-selective fading channels taking channel dynamics and initial frequency offsets into account. With the presented analysis it is possible to investigate the effects of spatial correlation on the detection performance and to examine the improvement of using multiple antenna elements at the base station depending on the respective detection scheme.

A rake finger grid for asynchronous DS-CDMA systems using LMMSE tap weight estimation

In this paper a Rake finger grid structure for direct-sequence code division multiple access (DS-CDMA) systems is analyzed as an alternative to the conventional Rake structure using individual fingers with timing- and phasor tracking. The grid uses a group of equispaced Rake fingers in the delay domain. Each of the Rake fingers uses linear minimum mean-squared error (LMMSE) tap weight estimation needed for the subsequent maximum-ratio combining (MRC). It is pointed out during the paper that the use of a grid is advantageous compared to the conventional Rake receiver in some scenarios where the channel forms so called clusters, consisting of many unresolvable, closely spaced multipaths. It is also shown that when using fractionally-spaced grid fingers, the task of fine timing tracking can be omitted. Only coarse timing estimation is necessary, which can be performed by a suitable acquisition unit.