UWB antenna performance evaluation from the communication system point of view Alain Sibille, Serge Bories, Raffaele D'Errico, and Christophe Roblin Unité Electronique et Informatique (UEI) Ecole Nationale Supérieure de Techniques Avancées (ENSTA) Paris, France (alain.sibille, serge.bories, raffaele.derrico, christophe.roblin)@ensta.fr Abstract—This work discusses the performance of ultra wide band antennas in the context of communication systems. An analysis of the physical origins of antenna imperfections is first presented, followed by the definition of modulation and detection scheme dependent radiation patterns. The extension of the analysis to radio links involving a multipath channel is then carried out. The discussion is supported by a variety of antenna measurements and simulations, and makes use of an angular statistical multipath channel model. Keywords-antenna, distortion, gain, radio channel, dispersion, impulse radio, multiband OFDM I. INTRODUCTION The role of antennas in the quality of the radio link is more difficult to assess in ultra wide band (UWB) than in narrowband or wideband communications. This stems from the extra complexity implied by the time (impulse response) or frequency (transfer function) dependence of the antenna characteristic. For instance to what extent is a dip in the antenna gain at some specific frequency detrimental, or to what extent is a late ripple in the temporal response really unfavorable, are questions which have no immediate and intuitive answers. It seems likely in addition that the answer is not unique, since the way the signals are processed in a receiver (Rx) affects the influence of imperfections on the detection performance. Furthermore, the complexity of ultra wide band signals mainly comes from the large number of degrees of freedom in the radio channel, and the mixing of (moderate) antenna imperfections and (strong) channel non idealities renders the case even more difficult to understand. Given this situation, the work presented here first discusses the origins of antenna imperfections, in relation with the main specifications and conditions of use of the antennas. Secondly, it relates the effect of these imperfections to the signal to noise ratio (SNR) of the output signal, by taking into account the main candidate UWB transceiver architectures (impulse radio, DS-UWB, pulsed multibands, multiband-OFDM, coherent and non coherent detection). Thirdly it analyses the effect of the radio channel on the output SNR, in relation with the major characteristics of the channel like the multipath density, and the nature of the system architecture. a 1 b 2 1 2 Tx Antenna Rx Antenna k -k r Fig. 1: free space radio link between two antennas II. CLASSIFICATION OF ANTENNA IMPERFECTIONS A. Generalities Starting with the antenna operation in transmission the wave vector dependent radiated electric field is written below, where ) (k A is dimensionless and η 0 = 120π Ohms is the free space impedance. The antenna vector transfer functions in transmission (VTFT) ) (k H T and in reception (VTFR) ) ( k H R are defined accordingly, with ) (k E pw a received plane wave electric field, and ) ( 1 f a , ) ( 2 f b incident and received wave amplitudes at the antenna connectors.: ) ( 4 ) ( 0 k A k E π η r e jkr - = (129 ) ( / ) ( ) ( 1 f a k A k H T = (229 ) ( ) ( ) ( 2 f f b pw A k H R ⋅ = (329 ) ( ) ( 0 f e pw j pw A k E r k η ⋅ = (429 Due to reciprocity, we also have ) ( 4 ) ( k H k H T R π λ j - = , where λ is the wavelength. Since an antenna on the transmitter (Tx) side is assumed to radiate a waveform as close as possible to the incoming signal, we will define below a perfect antenna accordingly. Since the fed signal is scalar while the electromagnetic fields are vectorial, this simple definition immediately raises a fundamental problem. Let us restrict ourselves to a purely linearly polarized antenna. The first condition for a perfect antenna is then to reproduce the signal waveform at its feeding point in the electric far field. The perfect radiator has a frequency independent VTFT in amplitude and a linear frequency dependence of the radiated This work was in part supported by the European Commission under IST integrated project PULSERS (FP6)