SPACE-TIME CODING FOR ULTRA-WIDE BANDWIDTH SIGNAL F Héliot, M Ghavami Centre for Telecommunications Research, King’s College London, United Kingdom ABSTRACT Ultra Wide Band (UWB) systems have attracted a lot of research interest lately, owing to their appealing features in short-range mobile communications. This includes low power peer-to-peer transmission, multiple access communications, high data rates, and precise positioning capability. Space-Time Coding (STC) techniques, such as the block coding scheme, or the trellis coding scheme, are known to be simple and practical ways to increase the spectral efficiency in wireless communications. This work presents two different types of space-time coding techniques wedded with Pulse Position Modulation (PPM) Impulse Radio Multiple Access (IRMA) system. A space-time block codes and a space-time trellis codes technique with coherent reception are proposed. The spatial and temporal diversity achieved by theses techniques is exploited to enhance the performance of UWB systems, in typical UWB environment and with fractionally- spaced (FS) coherent Rake detection. INTRODUCTION Impulse radio (IR), is defined as a form of ultra-wide bandwidth spread-spectrum signalling which is well designed for base-band asynchronous multiple access (MA), short distance high data rate multimedia services, and tactical wireless communications [1]. Additionally, the use of Multiple Input Multiple Output (MIMO) systems are known to enhance capacity in wireless communication systems by employing multiple transmit, and optionally, multiple receive antennas. Several architectures have been proposed so far to exploit the potential of MIMO systems, amongst them are space-time trellis coded modulation (STTCM) [2], space-time block codes (STBC) [3] and layered space- time architectures (BLAST), all of which provide high data rates with a given transceiver complexity. The work reported in [5], proposed an IR system exploiting multiple number of transmit antennas and employing a STBC scheme. The scheme is based on an Alamouti- type encoder [3], adapted to an analog non-linear PPM multi-antenna IR system. This work shows a considerable improvement in the bit-error-rate (BER). In this paper we improve the work in [6], and combine an IR system with orthogonal PPM and coherent reception. We also give result about a traditional narrow-band STTCM encoder [2] adapted to impulse radio signalling. Where data is trellis encoded across the transmit antennas thus providing an extra gain in addition to the diversity gain obtained by the use of multiple antennas. Investigations of possible performance enhancement of the two Space-Time Coding (STC) techniques compare to a single link system are undergo, in order to challenge their relevance in typical UWB environment. The rest of the paper is organized as follows, Section II briefly present the PPM-IRMA Model. Section III provides basics knowledge about Hermitian pulses [7]. In Section IV, our STBC scheme is explained. Then section V brings indication about the system model and results are given. Eventually a conclusion is given. PPM-IRMA MODELLING This section briefly describes the PPM-IRMA waveform model [4]. The waveform transmitted by the u-th user during a typical transmission of N s symbols per user, using Time Hopping (TH) sequence for accommodated these users is given by: () ( ) ∑ ∑ − = − =         − + − + − = 1 0 1 0 ) ( ) ( ) ( s f N i i u I c f u f f N k u u T T k iN C T k iN t w P t ω (1) Where each symbol is transmitted over a symbol period T s , T s = N f T f . N f is the number of frame per symbol and T f is the frame period. w(t) is a UWB transmitted pulse with width T w . N u users is accommodated using TH sequence, thus each frame contains N c chips of duration T c , T f = N c T c +T g . T g is a guard time for processing delay. Each frame contains only one pulse per user. C u (k) is the TH pseudo-random sequence N p periodic, C u (k) Є [0,Nc - 1]. P u denotes the u-th user transmit power, and I u (i) represents the information bearing the i- th transmitted symbol sent by the u-th user, I u (i) Є [0,M - 1]. T Iu(i) is the delay related to I u (i). This delay represents the shift in position of the monocycle pulse in the set of all possible position-shifting, according to the M-ary PPM. The modulation is set to be orthogonal, so each time interval T c is equally sliced in M possible position-shifting T Iu(i) = (I u (i)/M)T c . HERMITIAN PULSES At the first place, the set of Modified Hermitian Pulse (MHP) retained our attention because of the attractive