A.Bastug, "Advanced Receivers for High Speed Packet Data Access..."
Faculty of Engineering and Natural Sciences
FENS SEMINARS
Advanced Receivers for High Speed Packet Data Access in UMTS Downlink
Ahmet Baştuğ
Abstract: The talk is oriented towards advanced signal processing techniques at UMTS user equipment receivers, which are particularly suited for the so-called 3.5G high speed packet data services (HSDPA).
First we give a brief description of the UMTS downlink system and the HSDPA specifications.
In the second part we consider a family of user dedicated downlink channel estimation methods, which are especially beneficial when there is dedicated channel transmit beamforming. The methods do not assume any a priori knowledge of the path delays and the beamforming parameters and they exploit all the transmitted pilot sequences as well as the structured dynamics of the channel. First we build least squares (LS) estimates of the channels associated with dedicated and common pilots in every slot. Then we optimally improve the dedicated channel estimate quality by jointly Kalman filtering the two LS estimates or alternatively (suboptimally) Kalman filtering them separately and combining via weighted LS. In the suboptimal case, the order of Kalman filtering and weighted LS combining results in differing performance and complexity in different conditions. In order to estimate the missing model parameters we incorporate the expectation maximization (EM) algorithm to the Kalman mechanism expanded with one-lag smoothing.
In the third part we consider a chip level decision-directed NLMS equalization scheme which targets estimating the total transmitted base station chip sequence in a decision-directed manner and using it as the desired response for equalizer adaptation. For this purpose, we explicitly use only the knowledge of the user-assigned HSDPA codes in order to obtain reliable signal components by hard decisions. By exploiting the equivalence between the actual multi-rate transmission in the sense of containing multiple spreading factors and the multi-code pseudo-transmission at the single HSDPA spreading level, we also use the estimated pseudo-symbols of other active codes via LMMSE weightings. In addition to its reasonable complexity and Max-SINR achieving performance in realistic HSDPA working regimes, the proposed scheme also has the advantage of not requiring the channel parameters. We evaluate its performance by extensive simulations vis-à-vis the Griffiths equalizer. Since the latter requires channel parameters, in order to make a fair comparison we perturb the correct channel parameters by some amounts compliant with the performances of different channel estimation methods considered in the second part.
In the last part we assess the benefits of using chip level equalizers w.r.t. the usage of the conventional Rake receiver and using hard decision or hyperbolic tangent symbol nonlinearities w.r.t. the usage of linear feedback operations in the context of an iterative parallel interference cancellation receiver that we derive from the polynomial expansion of the symbol level covariance matrix inverse after the first stage equalization. Since the equalizers at different stages of the considered interference canceller are to be different as well, in order to estimate the essential equalizer parameters of a particular stage we use the analysis results from the preceding stages.
Biography: Ahmet Baştuğ is a UMTS algorithm development engineer at Philips Semiconductors in Sophia Antipolis, France. He is also following a joint PhD program from E.N.S.T Paris and Eurecom Mobile Communications Department in Sophia Antipolis under the supervision of Prof. Dirk Slock. He got his BSc degree from Middle East Technical University, Ankara in 1999 and his MSc degree from Boğaziçi University, Istanbul in 2002, both in electrical engineering. For his MSc thesis he worked on the application of channel coding and advanced detection techniques for watermarking of still images. From 1998 to 1999 he was with Karel Electronics, Ankara as an FPGA designer of digital PBX system components and from 1999 to 2002 with ST-Microelectronics, Istanbul (formerly Alcatel Microelectronics) as an ASIC design engineer where he participated in the designs of Bluetooth, IEEE 802.11a & HIPERLAN-2 Wireless-LAN and ADSL/VDSL modem chip sets. He has 10 filed or pending patents and 15 papers in peer reviewed journals and conferences related mostly to several UMTS physical layer reception techniques at the user equipment.
April 5, 2006, 13:40, FENS G032