Experimental Validation of the OFDM Bit Error Probability for a Moving Receive Antenna

Abstract

For an orthogonal frequency-division multiplexing system with pilot-symbol aided channel estimation, we compare the measured bit error ratio to the theoretical bit error probabil- ity. In order to measure mobile physical systems, we utilize the Vienna Wireless Testbed which has been augmented by a rotation wheel unit. The analytical solution assumes Rayleigh fading, additive Gaussian noise, and an arbitrary linear interpolation method to estimate the unknown channel taps. Our measurements confirm our assumptions and demonstrate convincingly that our theoretical expressions accurately model the true physical behavior, even for speeds of up to 100 km/h. I. INTRODUCTION Nowadays, most wireless communication standards (DAB, LTE, WIFI 802.11) employ Orthogonal Frequency Division Multiplexing (OFDM) as their modulation technique. While we have a good knowledge of what to expect in coherent transmissions over wireless channels in idealistic setups, the implications of practical systems are by far not so well understood. In particular high speed scenarios as they appear in car/train to infrastructure communications by LTE require a solid understanding of the transmission chain. Coherent trans- mission requires channel knowledge which typically comes by estimation based on training or so-called pilot symbols, i.e., known symbols at transmitter and receiver side. Pilot-symbol- Aided Channel Estimation (PACE) is usually considered in this context whereby the channel at data positions is estimated by interpolation. The optimal linear interpolation in terms of Minimum Mean Squared Error (MMSE) was derived in (1). However, an MMSE solution requires a-priori knowledge of the channel statistics and a matrix inversion. This motivated other authors to investigate different interpolation methods e.g., spline (2), 2D Deslauriers-Dubuc (3) and 2D low-pass (4). As a comparison measure the Bit Error Ratio (BER) was obtained through Monte Carlo simulations. Such simulations require a long simulation time and do not offer analytical insights. This recently motivated us to derive an analytical Bit Error Probability (BEP) expression for arbitrary linear interpolation methods (5) which can, for example, be used to find an improved pilot pattern design or to determine the optimal trade-off between pilot and data symbol power (6). Mobile wireless communication channels are characterized by time-varying multipath propagation (7), i.e., due to multiple scatterers, the electromagnetic signal can propagate along several different paths which causes frequency-selectivity and time-selectivity. The first can be combated by inserting a cyclic prefix while the latter leads to Inter-Carrier Interference (ICI)