IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 4, NO. 6, NOVEMBER 2005 3005 A Time–Frequency Domain Approach to Synchronization, Channel Estimation, and Detection for DS-CDMA Impulse-Radio Systems Andrea M. Tonello, Member, IEEE, and Roberto Rinaldo, Member, IEEE Abstract—This paper deals with synchronization, channel esti- mation, and detection in ultrawideband (UWB) biphase impulse– modulated systems. We address both the single-user scenario, and the multiuser scenario assuming a direct-sequence code-divi- sion multiple-access (DS-CDMA) scheme. The users’ binary-code- word elements modulate short-duration pulses. Codewords span a transmission frame. Frames are separated by a guard time to cope with the channel time dispersion. The detection approach is single-user-based and it operates in the frequency domain (FD). The algorithm first acquires frame synchronization with the de- sired user. It runs a discrete Fourier transform (DFT), and it performs FD channel estimation for the desired user via a recur- sive least-squares (RLS) algorithm. Finally, detection is directly accomplished in the FD. Frame synchronization is achieved in the time domain with a two-step procedure that first acquires coarse timing, then finely estimates where the desired user’s signal energy is located. In the presence of multiple-access interference (MAI), the algorithm is appropriately modified to include the capability of canceling the interference through the exploitation of its FD correlation. Simulation results show that the proposed approach exhibits fast convergence, and high performance with and without synchronous/asynchronous MAI. Index Terms—Channel estimation, code-division multiple access (CDMA), frequency-domain (FD) processing, impulse mod- ulation, interference cancellation, multiuser detection, timing acquisition, ultrawideband (UWB) systems. I. I NTRODUCTION T HIS PAPER deals with synchronization, channel estima- tion, and detection in impulse-radio systems [1]. Several combinations of modulation, and user-multiplexing schemes have been proposed for impulse-radio communications [2]. The common attractive feature is the carrierless baseband imple- mentation that involves transmission of short-duration pulses. This technology is commonly referred to as ultrawideband (UWB), because the pulses can occupy a very-large bandwidth [3]. Most of the work has focused so far on schemes that deploy time-hopping spreading codes with pulse-position mod- Manuscript received February 16, 2004; revised November 10, 2004; accepted January 9, 2005. The editor coordinating the review of this paper and approving it for publication is A. Molisch. Part of this paper was presented at Wireless Personal Multimedia Communications (WPMC) Symposium 2004, Abano Terme, Italy, September 12–15, 2004. This paper was supported in part by the Italian Ministry of Education, University, and Research (MIUR) under project “Reconfigurable platforms for wideband wireless communications” Prot. RBNE018RFY with the Fund for the Basic Research (FIRB). The authors are with the Dipartimento di Ingegneria Elettrica, Gestionale e Meccanica, Università di Udine, 33100, Udine, Italy (e-mail: tonello@uniud.it; rinaldo@uniud.it). Digital Object Identifier 10.1109/TWC.2005.858343 ulation [1]. Instead, in this paper, we assume the deployment of biphase pulse amplitude modulation (BPAM) in conjunc- tion with direct-sequence code-division multiple access (DS- CDMA) [2], [4], [5]. Binary codewords are assigned to users, and modulate short-duration pulses (monocycles). A user’s codeword spans a transmission frame. Frames are separated by a guard time to cope with the time dispersion that is introduced by the channel-frequency selectivity [1], [6]. When the guard time is longer than the channel time disper- sion, and only a single user accesses the medium, the optimal receiver comprises a matched filter followed by a symbol-by- symbol threshold detector [6]. The receiver filter has to be matched to the equivalent impulse response that comprises the user’s waveform, and the channel impulse response. Since UWB signals can occupy a large bandwidth, the channel is highly frequency selective, and the received signal exhibits a large number of multipath components. Potentially, high- frequency diversity gains can be achieved [6]. However, the optimal matched-filter receiver has to accurately estimate the channel, and such an estimation can be particularly complex if performed in the time domain. It has been shown in [7] that channel estimation can be partitioned into a two-step process if we model the channel as a tapped delay line. That is, we can first determine the channel ray delays, and then we can obtain an estimate of the ray amplitudes. Unfortunately, the ray search has a complexity that grows exponentially with their number. Further, false ray detection may occur in the absence of a priori knowledge about the true number of rays. Such a search can be partially simplified under the assumption of the channel to be resolvable [7]–[9]. However, this assumption can translate into deep performance losses in the nonrare event of clusters of nonresolvable rays. It has also to be emphasized that when common media is shared by multiple users, multiple-access interference (MAI) may arise at the receiver side. In a DS-CDMA system, this is due to the deployment of nonorthogonal codes, or to users that are time asynchronous, or to the presence of channel time dispersion. Assuming a single-user-detection approach, the MAI translates into performance losses, such that some form of multiuser detection is advisable [5], [10]. Motivated by the above considerations, we propose in this paper, a novel FD approach to channel estimation, detection, and MAI cancellation in impulse-radio DS-CDMA systems. It is related to the FD-detection approaches that are used in orthogonal-frequency-division-multiplexing (OFDM) systems 1536-1276/$20.00 © 2005 IEEE