An alternative receiver structure is presented for discrete multitone-based systems. The usual structure consisting of a (real) time-domain equalizer in combination with a (complex) 1-tap frequency-domain equalizer (FEQ) per tone, is modified into a structure with a (complex) multitap FEQ per tone. By solving a minimum mean-square-error problem, the signal-to-noise ratio is maximized for each individual tone. The result is a larger bit rate while complexity during data transmission is kept at the same level. Moreover, the per tone equalization is shown to have a reduced sensitivity to the synchronization delay.

We investigate detectors for transmultiplexer-based high bit rate transmission over dispersive channels like telephone wires. The transmitter is based on a synthesis stage. At the receiver side, we design, for an MMSE criterion, multiple-input/multiple-output (MIMO) linear detectors applied after an analysis stage and upgraded with a MIMO decision-feedback (DF) section. The performance of the system is investigated for filter banks over copper wires with different noises (AWGN, FEXT and RFI). The influence of the forward section rate is investigated. The potential of these detectors is clearly demonstrated. Besides, reduced complexity solutions are also put forward.

We consider a fractionally spaced decision feedback joint detector (FS-DFJD) for multitone code division multiple access (CDMA) systems. We first derive the structure of the receiver for an minimum mean-squared error (MMSE) criterion. Then we investigate the bit-error rate (BER) performance achieved by this detector for binary phase-shift keying (BPSK) modulation in steady-state conditions. We consider an asynchronous scenario where the signal of each user propagates over a two-path channel. The resistance of the detector against timing errors is studied, and the resistance against a near-far scenario is also demonstrated.

The combination of multitone modulation with direct sequence spectrum spreading (DS/SS) has been introduced in the past. The performance of a correlation receiver has been evaluated for a multipath channel and in the presence of an additional multiple access interference. We analyze the problem of decision feedback equalization (DFE) for such a system. In order to understand the potential of the system with equalization, we first study the steady-state behavior of the equalizer for a minimum mean square error (MMSE) criterion. The investigation is carried out for a receiver made of a bank of filters matched to both the symbol shape and the channel, and for a two path channel. Assuming transmission of binary phase shift keying (BPSK) symbols, an exact expression of the bit error probability is obtained in the form of an integral. Then adaptive least mean square (LMS) and recursive least square (RLS) structures are derived. The performance of the adaptive RLS algorithm is demonstrated by means of computer simulations.

We analyze the problem of joint equalization and cancellation of the multipath and multiple access interference for multitone CDMA introduced by Vandendorpe (see IEEE Transactions on Vehicular Technology, vol.44, no. 2, p.327, 1995). A linear structure is investigated for an MMSE criterion. In order to understand the potential of the system we first study the steady-state behaviour of the joint equalizer-canceller. The investigation is carried out for a joint device following a receiver made of a bank of matched filters and assuming perfect carrier phase and timing recovery. The scenario is asynchronous. An exact expression in the form of an integral is obtained for the bit error probability of the BPSK symbols before and after cancellation, by means of the characteristic function method. The performance of different systems is obtained for a constrained comparison and the near-far resistance of the detector is demonstrated.

The combination of multitone transmission (also known as orthogonal frequency division multiplexing (OFDM)) with direct sequence spread spectrum (DS/SS) is suitable for the design of a multiuser system robust against channel impairments. For high bit rate applications in indoor channels, the need for equalization is high, because guard intervals would lead to poor spectral efficiency. This paper presents a comparison between a recursive least square (RLS) scheme and a least mean square (LMS) algorithm, both extended for the linear equalization of OFDM/DS systems.