Andreas Czylwik

Adaptive OFDM for wideband radio channels

An OFDM (orthogonal frequency division multiplexing) transmission system is simulated with time-variant transfer functions measured with a wideband channel sounder. The individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used. The simulations show that with adaptive OFDM, the required signal power for an error probability of 10/sup -3/ can be reduced by 5...15 dB compared with fixed OFDM. The fraction of channel capacity which can be achieved with adaptive OFDM depends on the average signal-to-noise ratio and the propagation scenario.

Comparison between adaptive OFDM and single carrier modulation with frequency domain equalization

The aim of the paper is to compare multicarrier and single carrier modulation schemes for radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. In the case of OFDM (orthogonal frequency division multiplexing), the inverse FFT transforms the complex amplitudes of the individual subcarriers at the transmitter into the time domain. At the receiver the inverse operation is carried out. In the case of single carrier modulation, the FFT and its inverse are used at the input and output of the frequency domain equalizer in the receiver. Different single carrier and multicarrier transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In the case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. Furthermore, a frequency-independent as well as the optimum power distribution are used.

Synchronization for systems with antenna diversity

This paper deals with the synchronization of systems with frequency domain signal processing and antenna diversity. The results can be applied for both, single carrier transmission with frequency domain equalization (FDE) and orthogonal frequency division multiplexing (OFDM). For the considered systems, carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. A synchronization concept for systems with antenna diversity is presented which uses one pilot FFT symbol per transmission frame. The pilot FFT symbol consists of two periods of a chirp signal.

Low overhead pilot-aided synchronization for single carrier modulation with frequency domain equalization

This paper deals with the synchronization of single carrier transmission with frequency domain equalization (FDE) for broadband mobile radio applications (e.g. wireless ATM). For such applications the carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. Furthermore, an estimation of the channel transfer function is necessary. A synchronization concept is presented which uses only one pilot FFT symbol for all the required synchronization tasks.

Adaptive co-channel interference (CCI) cancellation for OFDM communication systems

This paper proposes a new adaptive co-channel interference (CCI) canceller for an orthogonal frequency division multiplexing (OFDM) scheme. It employs a parametric CCI cancellation scheme, least-mean-squares (LMS) maximum-likelihood estimation (MLE), which simultaneously estimates time-varying channels and transmitted symbols both for desired and CCI signals. Computer simulations were performed on interference canceling characteristics of the proposed CCI canceller both under static and Rayleigh fading conditions, wherein a single CCI is considered. Computer simulations confirm that the proposed canceller attains a BER of 1.5/spl times/10/sup -2/ at a signal-to-interference ratio of -10 dB in Rayleigh fading condition with the maximum Doppler frequency of 52 Hz and average E/sub b//N/sub 0/ of 30 dB.

Degradation of multicarrier and single carrier transmission with frequency domain equalization due to pilot-aided channel estimation and frequency synchronization

The performance of multicarrier and single carrier modulation schemes is compared for wideband mobile radio communication systems. In both cases the fast Fourier transform (FFT) and its inverse are utilized. The transmission systems are simulated with time-variant transfer functions measured with a wideband channel sounder. In case of OFDM, the individual subcarriers are modulated with fixed and adaptive signal alphabets. In particular, the systems are compared with respect to the degradation due to pilot-aided channel estimation and carrier frequency synchronization.

Robust downlink beamforming based upon outage probability criterion [cellular radio]

We propose a new downlink beamforming algorithm that provides robustness against uncertainty of the assumed downlink spatial covariance matrix at a base station (BS) that uses antenna arrays. This uncertainty arises due to the difference between the presumed and the actual array response as well as the estimation errors of the fading coefficients. In our method, we exploit the assumed statistical distribution of the uncertainty to minimize the total downlink transmit power under the condition that the non-outage probability of each mobile station in the downlink is greater than a certain threshold value. Our approach leads to a semidefinite relaxation problem that can be solved efficiently using the well-established interior point methods. Computer simulations are used to compare the performance of our method with that of the robust transmit beamforming based upon worst-case performance optimization. Moreover, the proposed technique avoids the necessity of the knowledge of the norm bounds of the uncertainty of the downlink spatial covariance matrices.

Downlink beamforming for mobile radio systems with frequency division duplex

The capacity of cellular mobile radio systems can be increased with smart antennas. The following two concepts can be utilized. First, smart antennas enable the concept of space division multiple access (SDMA) so that transmission from a base station to several mobiles at the same time and carrier frequency is possible. Second, cochannel interference in cellular systems can be reduced so that the frequency reuse is improved. The purpose of downlink beamforming is to transmit as much power as possible to the desired mobile and as little power as possible to any undesired mobile. In the case of systems with frequency division duplex (FDD) the fast fading processes for uplink and downlink are different. But, if the frequency separation is not too large, the uplink and downlink waves exhibit the same directional dependence. Therefore, downlink beamforming has to be carried out in an average sense based on the uplink channel measurements. The paper discuss the optimization of downlink beamforming for systems with FDD. It answers the question whether beamforming should be optimized in such a way that the transmitted power is mainly sent into the direction of the path with minimum average attenuation or whether it is better to distribute the power to all desired paths. The advantage from concentrating the power to the main path is that the interference power to other mobiles may be reduced due to a higher degree of freedom for beamforming. In the case of power distribution on all paths, the advantage from higher path diversity may be obtained.

Synchronization for single carrier modulation with frequency domain equalization

This paper deals with the synchronization of single carrier transmission with frequency domain equalization (FDE) for broadband mobile radio applications. For such applications carrier frequency and temporal position of the FFT (fast Fourier transform) window have to be synchronized. Furthermore, an estimation of the channel transfer function is necessary.

Downlink beamforming for frequency-duplex systems in frequency-selective fading

Downlink beamforming is analyzed for broadband frequency-duplex systems where frequency-selective fading dominates. In order to compare different beamforming strategies, system level simulations are carried out. It is assumed that at the receiver of the mobile station, path diversity can be exploited. The performance gain from maximal ratio combining of the signal components is evaluated with a new approach that combines system level simulations with an analytical calculation of the probability density function of the signal-to-interference power ratio (SIR) at the output of the combiner.