Low complexity optimal joint detection for oversaturated multiple access communications

Abstract

Abstract-Optimal joint detection for interfering (nonorthogonal) users in a multiple access communication system has, in general, a computational complexity that is exponential in the number of users. For this reason, optimal joint detection has been thought to be impractical for large numbers of users. A number of suboptimal low-complexity joint detectors have been proposed for direct sequence spread spectrum user waveforms that have properties suitable for mobile cellular and other systems. There are, however, other systems, such as satellite systems, for which other waveforms may be considered. This paper shows that there are user signature set selections that enable optimal joint detection that is extremely low in complexity. When a hierarchical crosscorrelation structure is imposed on the user waveforms, optimal detection can be achieved with a tree-structured receiver having complexity that is, in typical cases, a low-order-polynomial in the number of users. This is a huge savings over the exponential complexity needed for the optimal detection of general signals.Work in recent literature has shown that a hierarchically structured signal set can achieve oversaturation (more users than dimensions) with no growth in required signal-to-noise ratio. The proposed tree detector achieves low-complexity optimal joint detection even in this oversaturated case.

OCDMA/OCDMA Overloading Scheme for Cellular DS-CDMA Using Orthogonal Gold Codes and Complex Scrambling

“…It is observed that 28% overloading cannot be achieved, with less than 1.0 dB SNR degradation at an average BER of 1e-5. If we reduce the overloading to 25%, the SNR degradation is about 0.35 dB as shown in Figure 3 and we can ensure a BER of 5 …”

“…For example, a method of accommodating K = N + M users in an Ndimensional signal space that does not compromise the minimum Euclidean distance of the orthogonal signaling has been presented in [4] for AWGN channel. A tree-like correlation coefficient structure of user signatures suitable for optimal multiuser detection has been proposed in [5]. In another approach, two sets of orthogonal codes which are orthogonal within the sets is introduced in [6].…”

Wavelet Packets in Wideband Multiuser Communications

“…Hetling et al [23] have proposed and evaluated the use of wavelet packets as spreading codes in CDMA systems. Learned et al [24] derived an optimal joint detector for the wavelet packet based CDMA communications. The receiver design achieved low complexity detection compared with the conventional CDMA optimal receiver designs.…”

“…One obvious approach of using wavelet packets in CDMA communication is using them as signature waveforms, or non-binary spreading codes [23], [24]. In the case of directly using wavelet packets as non-binary spreading codes in a DS-CDMA system, the waveforms are usually chosen from an orthonormal basis such as wavelet packets so that there will be no interference between different users.…”

“…Hetling et al [23] first proposed user signature waveform designs using wavelet packets for improving cross-correlation properties over pseudo random codes. Learned et al [24] investigated a CDMA system using wavelet packet waveforms as spreading codes and designed an optimal joint detector that achieved a lower complexity compared with the conventional CDMA optimal receiver designs. Wong et al [25] investigated the timing error effect in a wavelet packet division multiplexing system.…”