Peter Schulz-Rittich

Channel tracking for RAKE receivers in closely spaced multipath environments

This paper deals with the problem of channel tracking for RAKE receivers in propagation environments characterized by closely spaced multipath components. After outlining why conventional single-path channel tracking algorithms fail in such scenarios, several new estimation algorithms are developed that are tailored to channels with closely spaced multipaths. This is achieved by removing or minimizing self-interference caused by multipath components. Other interfering users are treated as noise. Both timing tracking and phasor tracking and their interaction are covered in this paper. The derived algorithms are benchmarked against perfect channel knowledge on one hand and conventional tracking algorithms on the other hand, both in a UMTS test scenario. In moderate scenarios, the use of these new algorithms leads to performance improvements of up to 2 dB, in terms of signal-to-noise ratio (SNR) at moderate bit error rates, and even manages to track the channel in conditions where conventional tracking algorithms fail completely.

Performance analysis of phasor estimation algorithms for a FDD-UMTS RAKE receiver

The performance of phasor estimation algorithms for the UMTS RAKE receiver are analysed. Due to the need of covering a wide range of possible mobile speeds and spreading factors, two different algorithms are considered, namely a LMS-Kalman type algorithm, and a Wiener filter. The Wiener filter is considered both, as a predictor and a smother. It is shown that the simpler LMS-Kalman predictor is particularly suited for low mobile speeds and low spreading factors, whereas higher speeds and/or higher spreading factors require the usage of the more complex Wiener filter. Furthermore, the very high spreading factors (128-512) require the usage of the common pilot channel in order to facilitate smoothing instead of prediction.

Low complexity adaptive code tracking with improved multipath resolution for DS-CDMA communications over fading channels

A new adaptive timing error detector (TED) embedded in a code-tracking loop for RAKE reception of direct sequence-CDMA signals is presented. The loop consists of a digital coherent TED and a loop filter with lowpass characteristic. In a multipath fading environment, one such loop can be allocated for each finger in a RAKE receiver and the interference from adjacent paths can be mitigated by adaptively prefiltering the signal prior to the correlation process. The filter coefficients are computed online in order to minimize an interference cost function. Multipaths whose delays differ by as little as one chip duration become resolvable and can be tracked, resulting in significant performance gains of the overall system, especially if the channel delay spread is small. The tracking performance of the proposed loop is assessed by computer simulation.

Random beamforming in correlated MISO channels for multiuser systems

We examine the technique of random beamforming to exploit multiuser diversity in the downlink of a wireless cellular communication system with an antenna array at the basestation. In random beamforming systems, the scalar signal is multiplied by a weight vector and the resulting signal vector is transmitted by the antenna array. By varying the weight vector in time, one can increase the dynamic of the effective channel resulting in faster fading and a larger variance of the effective channel. This can be exploited by a scheduler at the basestation which selects users for transmission that momentarily have a good channel. Our work focusses on the generation of the weight vectors for a uniform linear array in the basestation. We especially consider correlation between the antenna elements, but assume that the correlation matrices for the different users are not known to the basestation. Three random beamforming approaches are compared with the simplest possible case of using equal weights, where the weight vector is a constant which serves only to normalize the radiated power. We derive the mean of the received power for the different beamforming techniques and confirm our results by Monte Carlo simulations, where we evaluate the mean power of the actually scheduled user, if a proportional fair scheduling algorithm is used. One important result is the good performance of the rotating beam approach even in a not fully correlated environment.

Low complexity high resolution subspace-based delay estimation for DS-CDMA

We consider the problem of estimating the propagation delays of a synchronous direct-sequence code division multiple access (DS-CDMA) system operating over a multipath fading channel. In a mobile receiver, the task of delay estimation can be divided into an acquisition of all multipath delays and subsequent tracking of the individual delays e.g. with a RAKE structure. Both tasks are especially challenging in indoor scenarios which commonly exhibit low delay-spread and thus require a high resolution of the estimation algorithms. A novel delay acquisition algorithm is presented, which is able to resolve multipaths whose delay difference is below one chip duration with high probability of acquisition and low computational complexity. It is based on a decomposition of the time-averaged correlation matrix of the output of a sliding correlator into signal and noise subspaces, with subsequent MUSIC spectrum computation and maximum search. The performance of the algorithm is assessed by means of computer simulations.

On the throughput of proportional fair scheduling with opportunistic beamforming for continuous fading states [cellular wireless systems]

We examine the throughput of an opportunistic beamforming system with proportional fair scheduling and show for normally distributed channel fading states that for large numbers of users the average throughput of each user multiplied by the number of users approaches the maximum possible throughput of this user achievable by coherent beamforming, if round robin scheduling was used. Thus, we extend a proof by Viswanath et al. who showed this for discrete fading states. We give the average SNR of the scheduled user (averaged across the fading states) in closed form and the average throughput in the form of an integral as a function of the number of transmit antennas and users. Simulations of this system confirm the analytical results. Finally, we show that for a large number of transmit antennas, the probability density function of the SNR of the scheduled user and therefore also the throughput asymptotically approach those of a system with a max SNR scheduler that always transmits to the user having the largest SNR and thus maximizes the total throughput.

Multipath resistant coherent timing error detector for DS-CDMA applications

A new coherent timing error detector (TED) for timing/code tracking loops used inside RAKE receivers in CDMA systems is presented. In contrast to the conventional TED it is well suited for the case of multipath propagation channels. In order to accomplish this task a compensation term is introduced inside the tracking loop directly behind the conventional TED. This compensation term is calculated using the knowledge on the relative delays of all paths and their respective channel coefficients. A compensation scheme like the one described here becomes necessary whenever closely spaced paths have to be tracked. This fact makes this algorithm a favorable candidate for indoor scenarios where individual paths can be spaced even more closely than one chip. The performance of the presented scheme is assessed by means of simulation.

A novel multipath interference cancellation scheme for RAKE channel estimation

A novel channel estimation scheme is proposed for a RAKE receiver operating in the downlink of a mobile communication scenario. The approach is an extension of the well known Wiener channel estimator and partly cancels multipath interference, with the additional advantage of being able to cope with physically closely spaced multipaths which arise, for example, in an indoor scenario. It is demonstrated by means of simulation that the algorithm improves the quality of the channel estimates significantly. Little additional computational effort is necessary to implement the algorithm, making it an ideal candidate for improving reception capabilities of the mobile user equipment.

The effect of imperfect SNR knowledge on multiantenna multiuser systems with channel aware scheduling

We analyze a cellular communication system in which a basestation (BTS) or access point transmits packet data to several mobile data users by means of a TDMA scheme. All users estimate their instantaneous signal-to-noise-ratio (SNR) in each slot and feed this information back to the BTS. A scheduler in the BTS then uses this information to allocate the channel resource to the user which maximizes a certain metric. We are interested in assessing the sensitivity of the system performance in terms of spectral efficiency per cell with respect to an imperfect knowledge of the multiuser channel, expressed by the estimated SNR of all users. By assuming a block fading channel model for each user, data-aided maximum-likelihood intra-slot SNR estimation can be performed if known pilot symbols are transmitted in each slot. We derive a novel SNR estimator for the block fading channel, which takes the channel statistics into account. The new estimator clearly outperforms the AWGN ML estimator in terms of system performance. Not only is the spectral efficiency larger for the system, but the optimum spectral efficiency is also achieved with fewer pilot symbols per slot.

Optimized timing error detector for DS-CDMA applications in multipath scenarios

An extension to the timing error detector (TED) for timing/code tracking loops used inside RAKE receivers in CDMA systems is presented. In contrast to the conventional TED, it is well suited for the case of multipath propagation channels. In order to accomplish this task, the conventional early-late TED is replaced by an FIR filter which is optimized for the instantaneous multipath scenario. This optimization minimizes the overall distortions inside the timing tracking loop of each of the fingers of the RAKE receiver. The filter coefficients are calculated using the knowledge on the relative delays of all paths and their respective powers. An optimization becomes necessary whenever closely spaced paths have to be tracked. This fact makes this algorithm a favorable candidate for indoor scenarios where individual paths can be spaced even more closely than one chip. The performance of the scheme is assessed by means of simulation.