Multicarrier orthogonal CDMA signals for quasi-synchronous communication systems

Abstract

Abstracf-We propose a multicarrier orthogonal CDMA signaling scheme for a multiple-access communication system, such as the reverse channel of a cellular network, as an alternative to the multi-user interference cancellation approach. The average variance of cross-correlations between sequences is used as a measure for sequence design. We search for sets of sequences that minimize the probability of symbol detection error, given that there is imperfect synchronization among the signals, that is, the signals are quasi-synchronous. Orthogonal sequences based on the Sylvester-type Hadamard matrices (Walsh functions) are shown to provide a significant improvement over the case where a Hadamard (orthogonal) matrix is chosen at random. Computer searches suggest that this set of codes is optimal with respect to the above measure. The issue of chip pulse shaping is investigated. Optimal pulses designed to minimize multiple-access interference in quasi-synchronous systems are obtained for various bandwidths and are shown to provide a large improvement over the raised cosine pulses. A multicarrier signaling scheme is introduced in order to reduce chip level synchronization offsets between the users.

An enhanced widely linear CDMA receiver with OQPSK modulation

“…An alternative, and yet computationally efficient, form can be expressed in the frequency domain by using the inverse Fourier representation of signals and their HSR as SNR (25) where the effect of PN spreading sequences are averaged out. Similarly, the SNR expression for the SL receiver is SNR (26) where the SL receiver is evaluated by setting in .…”

“…In the reverse link, however, only quasi-synchronism can be achieved with the aid of a timing reference. Quasi-synchronous CDMA signals can be modeled with chip delays chosen from a probability distribution function , which is nonzero in for [25]. For instance, in wideband CDMA (WCDMA) systems, all users are synchronized within in the reverse link to reduce intracell interference [26].…”

Performance of generalized multicarrier DS-CDMA over Nakagami-m fading channels

“…In these MC-CDMA schemes, the transmitter of both multitone based direct-sequence CDMA [3] (multitone DS-CDMA) and that of orthogonal multicarrier DS-CDMA (MC DS-CDMA) [2], [5] spreads the serial-to-parallel converted data streams using a given spreading code in the time domain. In a multitone DS-CDMA system the subcarrier frequencies are chosen to be orthogonal harmonics of each other with minimum frequency separation among them before DS spreading [3], which is expressed as for (1) where represents the symbol duration of the multitone DS-CDMA signal, while and are the subcarrier frequencies and initial phases, respectively.…”

“…The subcarrier frequencies hence take the form of for , where is the number of subcarriers and is the fundamental frequency. By contrast, in an orthogonal MC DS-CDMA system, the subcarrier frequencies are chosen to satisfy the orthogonality condition with the minimum possible frequency separation after DS spreading [1], [2], [5], which can be expressed as for (2) which implies that the spacing between two adjacent subcarrier frequencies is , where is the chip duration of the DS spreading codes and the subcarrier frequencies take the form of for . Let be the spreading gain of the DS-spread subcarrier signals.…”

Fractionally spread multicarrier cdma: a multiple-access scheme for various wireless channels

“…Frequency diversity in MC-CDMA systems is usually achieved by repeating the transmitted signal in the frequency (F)-domain with the aid of several subcarriers [4], [5], [7]. Classic studies of MC-CDMA have shown that the various MC-CDMA schemes proposed in the literature [2]- [7] are capable of supporting multiple users communicating over frequency-selective fading channels.…”