Gunnar Fock

Optimum receiver design for wireless broad-band systems using OFDM. I

Orthogonal frequency-division multiplexing (OFDM) is the technique of choice in digital broad-band applications that must cope with highly dispersive transmission media at low receiver implementation cost. In this paper, we focus on the inner OFDM receiver and its functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. The effects of relevant nonideal transmission conditions are thoroughly analyzed: imperfect channel estimation, symbol frame offset, carrier and sampling clock frequency offset, time-selective fading, and critical analog components. Through an appropriate optimization criterion (signal-to-noise ratio loss), minimum requirements on each receiver synchronization function are systematically derived. An equivalent signal model encompassing the effects of all relevant imperfections is then formulated in a generalized framework. The paper concludes with an outline of synchronization strategies.

Optimum receiver design for OFDM-based broadband transmission .II. A case study

This paper details on the design of OFDM receivers. Special attention is paid to the OFDM-specific receiver functions necessary to demodulate the received signal and deliver soft information to the outer receiver for decoding. In part I of the paper, the effects of nonideal transmission conditions have been thoroughly analyzed. To show the impact of the synchronization algorithms-which are most critical in OFDM-on system performance and complexity we consider the design of a complete receiver consisting of symbol synchronization, carrier/sampling clock synchronization and channel estimation. The performance of the algorithms is analyzed and a qualitative estimate of the resulting complexity is given. This allows one to draw conclusions concerning the achievable system performance under realistic complexity assumptions.

An information theoretic foundation of synchronized detection

The constrained capacity of a coherent coded modulation (CM) digital communication system with data-aided channel estimation and a discrete, equiprobable symbol alphabet is derived under the assumption that the system operates on a flat fading channel and uses an interleaver to combat the bursty nature of the channel. It is shown that linear minimum mean square error channel estimation directly follows from the derivation and links average mutual information to the channel dynamics. Based on the assumption that known training symbols are transmitted, the achievable rate of the system is optimized with respect to the amount of training information needed. Furthermore, the results are compared to the additive white Gaussian noise channel, and the case when ideal channel state information is available at the receiver.

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.

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.

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.

Channel estimation for DS-CDMA with transmit diversity over frequency selective fading channels

RAKE receivers for DS-CDMA communications over fading channels require the knowledge of instantaneous complex channel phasors for each of the multipath diversity branches (or RAKE fingers). If transmit diversity is employed by using N transmit antennas in the basestation, N such fading channels need to be estimated in the mobile receiver. A channel estimation algorithm which estimates two fading channels in the case of dual TX diversity is presented, making use of known pilot symbols. The proposed scheme performs remarkably well in Rayleigh fading multipath environments with little additional complexity if compared to a scheme where only one channel needs to be estimated (i.e. no TX or RX diversity). The performance of the algorithm is assessed by means of computer simulations.

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.