Dirk Dahlhaus

Channel parameter estimation in mobile radio environments using the SAGE algorithm

This study investigates the application potential of the SAGE (space-alternating generalized expectation-maximization) algorithm to jointly estimate the relative delay, incidence azimuth, Doppler frequency, and complex amplitude of impinging waves in mobile radio environments. The performance, i.e., high-resolution ability, accuracy, and convergence rate of the scheme, is assessed in synthetic and real macro- and pico-cellular channels. The results indicate that the scheme overcomes the resolution limitation inherent to classical techniques like the Fourier or beam-forming methods. In particular, it is shown that waves which exhibit an arbitrarily small difference in azimuth can be easily separated as long as their delays or Doppler frequencies differ by a fraction of the intrinsic resolution of the measurement equipment. Two waves are claimed to be separated when the mean-squared estimation errors (MSEEs) of the estimates of their parameters are close to the corresponding Cramer-Rao lower bounds (CRLBs) derived in a scenario where only a single wave is impinging. The adverb easily means that the MSEEs rapidly approach the CLRBs, i.e., within less than 20 iteration cycles. Convergence of the log-likelihood sequence is achieved after approximately ten iteration cycles when the scheme is applied in real channels. In this use, the estimated dominant waves can be related to a scatterer/reflector in the propagation environment. The investigations demonstrate that the SAGE algorithm is a powerful high-resolution tool that can be successfully applied for parameter extraction from extensive channel measurement data, especially for the purpose of channel modeling.

Wideband angle of arrival estimation using the SAGE algorithm

In the last few years, issues concerning the spatial resolution of the received electromagnetic field have attracted a lot of attention in mobile communications. A new method for the high resolution of the electromagnetic field with respect to both the incidence direction and delay is presented. The underlying discrete propagation model relies on the assumption that a finite known number of transverse electromagnetic plane waves characterized by their complex amplitude, propagation delay, and azimuthal incidence direction are impinging in a neighborhood of the receiver position. A space-alternating generalized expectation-maximization (SAGE) algorithm is proposed to replace the high-dimensional optimization procedure necessary to compute the joint maximum-likelihood estimate of the waves parameters by several separate maximization processes which can be performed sequentially. The resolution and the mean convergence rate of the scheme are evaluated by means of Monte-Carlo simulations considering discrete synthetic propagation environments. Finally, the applicability of the SAGE algorithm to real propagation environments is demonstrated.

Optimal power adaptation for OFDM systems with ideal bit-interleaving and hard-decision decoding

We derive power adaptation strategies optimizing the error rates of broadband bit-interleaved coded OFDM systems performing hard-decisions after the demodulation and ideal interleaving. The adaptive subchannel power allocation is based on either perfect or outdated channel state information in a time-division duplex transmission. We find that for an average bit error rate level of 10/sup -6/ a gain of up to 4 dB can be achieved by the proposed adaptation in Rayleigh fading channels.

Time versus frequency domain channel estimation for OFDM systems with antenna arrays

We compare channel estimation schemes in the time and frequency domains in terms of the error performance of both the estimated channel impulse response (CIR) and transfer function (TF) as well as the resulting bit-error rate (BER) in an orthogonal frequency-division multiplexing (OFDM) system with a time-multiplexed preamble. If the total number of sub-carriers, K, exceeds the number of taps, L, in the CIR, as is the case in most practical OFDM systems, the TF estimate based on the time domain least squares (LS) scheme is more accurate than the one obtained from direct LS estimation in the frequency domain. An equivalent mean-squared error performance results for the case K=L.

Bit and power loading procedures for OFDM systems with bit-interleaved coded modulation

We present adaptive power and bit loading schemes for a bit-interleaved coded OFDM system with perfect channel state information and square quadrature amplitude modulation constellations. In order to minimize the bit error rates at the output of the Viterbi decoder subject to bit rate and power constraints, an analytical expression for the bit error rate of a hard-decision device has to minimized with respect to the number of bits and the power assigned to the subcarrier channel. The prohibitive complexity of the optimum solution can be circumvented by a simple suboptimum two-step procedure where the bit and power loading parts are decoupled. It is shown by Monte Carlo simulations that for broadband systems with Rayleigh fading the loading procedures provides a SNR gain of up to 6 dB for an average bit error rate level of 10/sup -6/.

Joint Kalman channel estimation and equalization for the UMTS FDD downlink

The performance of a digital mobile receiver operating in time-varying frequency-selective propagation environments is significantly affected by the accuracy of the employed channel estimation. In the frequency-division duplex (FDD) downlink of the universal mobile telecommunication system (UMTS) a code-multiplexed pilot signal is continuously broadcast in order to enable the mobile terminal to estimate the parameters of the transmission channel. However, multipath propagation destroys the code-orthogonality between user and pilot signals resulting in strong multiple-access interference (MAI). Conventional channel estimation techniques do not take into account this impairment. An algorithm consisting of a modified extended Kalman filter is presented which estimates the channel parameters as well as the MAI jointly. The output of the filter can directly be used to retrieve the equalized desired user signal. It is shown by simulations that this algorithm has superior performance compared to the standard Rake receiver as well as decoupled Kalman channel estimation and equalizer structures, especially in fast fading frequency-selective environments.

Joint demodulation in DS/CDMA systems exploiting the space and time diversity of the mobile radio channel

A demodulation algorithm for a base station receiver in a direct sequence (DS) spread spectrum code division multiple access (CDMA) communication system is proposed which performs joint multiuser detection and estimation using the output signals of an antenna array in the absence of a tight power control. The scheme is a combination of a multistage (MS) detector for data recovery and a space-alternating generalized expectation maximization (SAGE) algorithm for channel parameter estimation. Monte-Carlo simulations show that the scheme is able to overcome the near-far problem while simultaneously combining the signal energy spread over time and space in a nearly optimum fashion.

Multiple-Antenna Signaling Over Fading Channels With Estimated Channel State Information: Capacity Analysis

Multiple-antenna concepts for wireless communication systems promise high spectral efficiencies and improved error rate performance by proper exploitation of the randomness in multipath propagation. In this paper, we investigate the impact of channel uncertainty caused by channel estimation errors on the capacity of Rayleigh and Ricean block-fading channels. We consider a training-based multiple-antenna system that reserves a portion of time to sound the channel. The training symbols are used to estimate the channel state information (CSI) at the receiver by means of an arbitrary linear estimation filter. No CSI is assumed at the transmitter. Our analysis is based on an equivalent system model for training-based multiple-antenna systems which specifies the channel by the estimated (and hence, known) channel coefficients and an uncorrelated, data-dependent, multiplicative noise. This model includes the special cases of perfect CSI and no CSI. We present new upper and lower bounds on the maximum instantaneous mutual information to compute ergodic and outage capacities, and extend previous results to arbitrary (and possibly mismatched) linear channel estimators and to correlated Ricean fading. Several numerical results for single- and multiple-antenna systems with estimated CSI are included as illustration.

Optimal power loading for multiple-input single-output ofdm systems with bit-level interleaving

We derive a power loading procedure optimizing the bit-error rates of multiple-input single-output (MISO) bit-interleaved coded modulation (BICM) orthogonal frequency-division multiplexing (OFDM) systems performing hard-decisions at the receiver and ideal interleaving. The adaptive subcarrier power allocation is based on either perfect or outdated channel state information at the transmitter. The scheme has the same complexity as the one for the single transmit antenna case. For a standard BICM-OFDM system in Rayleigh fading, Monte Carlo simulations show that the relative signal-to-noise ratio gain by the adaptation is up to 4 dB at an average bit-error rate level of 10-66. The relative gain in MISO systems decreases to 1.05 dB for increasing the number of transmit antennas. The achievable gain decreases for decreasing cross-correlation of the outdated and the actual channel state

Simple ergodic and outage capacity expressions for correlated diversity ricean fading channels

Multiple-antenna systems have been shown to achieve very high spectral efficiencies. In this paper, we derive simple single-integral expressions for the ergodic and outage capacity of a diversity system in correlated Ricean fading channels, where the channel coefficients are assumed to be known to the receiver only. For illustration purpose, we present numerical results showing the effect of channel correlation, Ricean components, angular spread and multipath components in an orthogonal frequency-division multiplexing (OFDM) system