Multiuser detection techniques for TH-PPM systems

This paper investigates detection mechanisms for time hopping pulse position modulated (TH-PPM) multiple access systems in both additive white Gaussian noise and fading environments. In such systems, the raw data is not linearly related to the received signal. One technique is to express the pulse position modulated signal as the sum of linearly modulated signals with the input being a nonlinear function of the raw data. This reduces the system to an equivalent pulse amplitude modulated system with a significant increase in complexity. We present two new lower complexity detectors. The first detector, which is a frame-level maximum likelihood (ML) detector, uses the properties of the pulse correlation matrix to develop a lower complexity detection method. The second detector, which is a chip-level ML detector, relies on the knowledge of the time hopping sequence to despread the received signal. The detection is then done on a chip-by-chip basis. The performance of these detectors is compared in terms of the bit error rate for the user with weakest power and the average number of correctly decoded users. Index Terms--Bit error rate (BER), correctly decoded users, multiuser detection, pulse position modulation (PPM), time hopping (TH).