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RAKE Receivers

Synchronization Algorithms for RAKE W-CDMA Receivers in UMTS User Equipment

Advanced Synchronization Algorithms for RAKE Receivers for WCDMA

Joint project with Agere Systems

Introduction

The third-generation cellular mobile communication system UMTS is based on direct-sequence code division multiple access (DS-CDMA). DS-CDMA is well suited for transmission over the typical multipath fading channel, as it allows a high resolution of distinct multipaths due to the large signal bandwidth. This is particularly the case for UMTS with its signal bandwidth of 5 MHz (wideband CDMA or WCDMA). For the first generation of UMTS handsets, the well-known RAKE receiver will be the structure of choice for efficient and affordable single-user detection. The RAKE receiver structure can be derived directly from the multipath channel model: ideally, one RAKE finger is allocated to each multipath, maximizing the amount of received signal energy. A prerequisite is, however, that the RAKE has corresponding knowledge about the current channel parameters such as number of paths, their location (in the delay domain) and current (complex-valued) attenuation.

One crucial task in every digital receiver implementation is the synchronization, meaning the estimation and compensation of the relevant channel parameters. The concept of estimating those parameters and then using them as if they were the true values is called synchronized detection and is used in virtually every digital receiver implementation. In our case, the necessary channel parameters need to be estimated and tracked throughout the transmission.

Throughout the investigation of the synchronization tasks in a RAKE receiver for WCDMA, the necessity of high resolution multipath acquisition and tracking algorithms for channels which exhibit small delay spreads, i.e. with closely spaced multipaths, was identified. Several novel algorithms and resulting structures for path delay and chanel phasor tracking in such environments were derived and the findings have been published in international conference proceedings (see bibliography at the end of this page). Current work in this area focuses on the task of channel acquisition for closely spaced multipath scenarios.

The Multipath Fading Channel

The UMTS radio channel is characterized by multipath propagation where a number of reflected or scattered radio rays arrive at the receiving end. Such a propagation environment is illustrated in Figure 1. Mathematically, the multipath Rayleigh fading channel with time varying delays can be described by its time-varying channel impulse response (CIR)

with being the relative path delay and t the absolute time variable, respectively. Each of the channel taps ck(t) is assumed to be Rayleigh distributed; furthermore, the channel is assumed to be subject to a doppler spread being proportional to the mobile speed.

 

Signal Model and the RAKE Receiver

Assume that a sequence of independent QPSK-modulated symbols {ak} is transmitted. The baseband-equivalent transmitted CDMA signal for one user can be expressed as

with Nc being the spreading factor, Ns the length of the (long) scrambling sequence,
d = [d0 ... dNs-1] the effective spreading sequence, g(t) the root-raised cosine (RRC) transmit pulse and Tc the chip duration, for UMTS 244.14 ns. The received signal r(t) is then modelled as the convolution of <nobr>s(t)</nobr> with the channel CIR plus additive white gaussian noise (AWGN) n(t). This signal is pulse-matched filtered by the same root-raised cosine pulse and is given by

Rg(t) is the pulse autocorrelation function of the RRC pulse, which is raised-cosine shaped and happens to be a Nyquist pulse.

Figure 2 depicts the basic RAKE structure. The received signal is first sampled at the sampling rate 1/Ts, which must be at least equal to 2/Tc to guarantee sufficient statistics for detection and synchronization, and subsequently fed to all M RAKE fingers. In each finger, an interpolation/decimation unit provides a data stream on chiprate 1/Tc which is sampled at the estimated timing instant. The latter is provided by the code tracking loop, one for each finger. After correlation with the complex conjugate of the spreading sequence and summation over one symbol interval, the symbol estimates in each finger are weighted according to the maximum-ratio criterion (MRC). The weighting factors q are just the channel phasors c, normalized to the total signal power.

Figure 2. RAKE receiver model

It is immediately clear from Figure 2 that applying the concept of synchronized detection to the RAKE receiver means estimating the channel path delays as well as the phasors q.

A more detailed view of a RAKE finger with the necessary synchronization tasks is given in Figure 3. The main datapath with timing compensation, descrambling/despreading and MRC is recognized from Figure 2. Furthermore, the red units indicate synchronization algorithms which are needed in each finger. Also, the blocks at the bottom (scrambling code acquisition, RAKE finger control and multipath identification) are central units which are displayed here along with the necessary control flow for interaction with the datapath and the synchronization path.
 

Figure 3. RAKE finger architecture

Further issues analyzed throughout the course of the project are

  • digital interpolation and decimation for sampling rate conversion and timing compensation
  • digital rate adaptation for D/A conversion in the uplink
  • frequency offset tracking
  • sampling clock offset tracking
  • open-loop and closed-loop transmit diversity reception
  • high-resolution channel identification

Contact

Meik Dörpinghaus, Peter Schulz-Rittich, Andreas Senst, Gunnar Fock, Jens Baltersee

Figure 1. The multipath propagation scenario

Publications

Schulz-Rittich, P., Baltersee, J., Fock, G. and Meyr, H.: Channel Estimation for DS-CDMA with Transmit Diversity, in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), Sep. 2001
Baltersee, J., Fock, G., Schulz-Rittich, P. and Meyr, H.: RAKE Channel Estimation in Closely Spaced Multipath Environments, in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), Sep. 2001
Fock, G., Schulz-Rittich, P., Baltersee, J. and Meyr, H.: Timing-Error-Detector Optimization in Closely Spaced Multipath Scenarios, in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), in Proceedings of the VDE Aachen Symposium on Signal Theory(Aachen, Germany), Sep. 2001
Baltersee, J., Fock, G. and Schulz-Rittich, P.: A Novel Multipath Interference Cancellation Scheme for RAKE Channel Estimation, in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), May. 2001 ©2001 IEEE
Fock, G., Schulz-Rittich, P. and Baltersee, J.: Optimized Timing-Error-Detector for DS-CDMA Applications in Multipath Scenarios, in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), May. 2001 ©2001 IEEE
Schulz-Rittich, P., Baltersee, J. and Fock, G.: Channel Estimation for DS-CDMA with Transmit Diversity over Frequency Selective Fading Channels, in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), in Proceedings of the IEEE Semiannual Vehicular Technology Conference VTC2001 Spring(Rhodes, Greece), May. 2001 ©2001 IEEE
Fock, G., Baltersee, J., Schulz-Rittich, P. and Meyr, H.: Channel Tracking for RAKE Receivers in Closely Spaced Multipath Environments, in (IEEE COML), 2001
Senst, A., Fock, G. and Meyr, H.: Comparison of Rate Adaptation and Modulation vs. Interpolation and Decimation for Uplink Rate Conversion, in Proceedings of Aachen Symposium on Signal Theory, 2001
Schulz-Rittich, P., Fock, G., Baltersee, J. and Meyr, H.: Low Complexity Adaptive Code Tracking with Improved Multipath Resolution for DS-CDMA Communications over Fading Channels, in Proceedings of the IEEE Sixth International Symposium on Spread Spectrum Techniques & Applications ISSSTA(Parsippany), Sep. 2000 ©2000 IEEE
Fock, G., Schulz-Rittich, P., Baltersee, J. and Meyr, H.: Multipath Resistant Coherent Timing-Error Detector for DS-CDMA Applications, in Proceedings of the IEEE Sixth International Symposium on Spread Spectrum Techniques & Applications ISSSTA(New Jersey, USA), Sep. 2000
Baltersee, J., Fock, G., Schulz-Rittich, P. and Meyr, H.: Performance Analysis of Phasor Estimation Algorithms for a FDD-UMTS RAKE Receiver, in Proceedings of the IEEE Sixth International Symposium on Spread Spectrum Techniques & Applications ISSSTA(Parsippany), Sep. 2000 ©2000 IEEE
Fock, G., Baltersee, J., Schulz-Rittich, P. and Meyr, H.: Exploring the UMTS WCDMA-Receiver Design Space Using a Semianalytical Approach, in Proceedings of the IEEE Global Telecommunications Conference GLOBECOM(Rio de Janeiro, Brasil), in Proceedings of the IEEE Global Telecommunications Conference GLOBECOM(Rio de Janeiro, Brasil), Dec. 1999
Baltersee, J., Fock, G., Simon, V. and Meyr, H.: Performance Bounds for a UMTS RAKE Receiver with Imperfect Timing Synchronisation, in Proceedings of the IEEE Global Telecommunications Conference GLOBECOM(Rio de Janeiro, Brasil), in Proceedings of the IEEE Global Telecommunications Conference GLOBECOM(Rio de Janeiro, Brasil), Dec. 1999
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