Multiuser TH-precoding for TDD–CDMA over multipath channels

Abstract

Nonlinear precoding schemes for downlink time-division duplex -CDMA systems over multipath fading channels, are considered. First, the capacity results of a downlink CDMA system with either multiuser detection or precoding, were obtained and compared. It is seen that the two schemes exhibit similar capacity regions for both sum rate and maximum equal rate, which motivates the development of efficient nonlinear transmitter precoding techniques to reduce the receiver complexity at the mobile units without degrading the system performance. We then develop both bit-wise and chip-wise Tomlinson -Harashima (TH) multiuser precoding methods for downlink CDMA with multipath, to remove multi-user interference, inter-chip interference and inter-symbol interference. Efficient algorithms for multiuser power loading and ordering are also developed. Implementation of the proposed TH-precoding schemes in time-varying channels based on channel prediction is addressed as well. Simulations results are provided to demonstrate the effectiveness of the proposed techniques in suppressing interference in downlink CDMA. IntroductionMultiuser detection (MUD) techniques are considered powerful for interference suppression in CDMA systems, especially in uplinks, where the base-station receiver has the knowledge of all users' spreading sequences and channel states, and can perform sophisticated signal processing [1]. In the downlinks, however, the mobile receiver typically has only the knowledge of its own spreading sequence and channel state. Although adaptive linear MUD (either training-based or blind) can be employed for such scenario, the performance can be limited because of the linear constraint on the detector. Moreover, the power constraint of the mobile calls for simple receiver processing. On the other hand, the precoding schemes for downlink CDMA effiectively transfer the signal processing for interference suppression from the mobile receiver to the base-station transmitter. This approach is feasible if the base-station can estimate the downlink channels of all users [e.g. in systems employing time-division duplexing (TDD) where the uplink and downlink channels are reciprocal]. In [2], a precoding method was proposed which is essentially an implementation of the RAKE receiver at the transmitter. Hence this approach does not attempt to mitigate the multiple-access interference (MAI). Recently, different linear precoding techniques have been proposed to combat MAI and inter-chip interference but without considering inter-symbol interference (ISI) [3,4]. If the ISI is present then the complexity of these techniques becomes prohibitive since the dimension of the matrix filter is proportional to the data frame length multiplied by the number of users (i.e. block processing). More recently, bit-wise linear precoding methods have been proposed to reduce the precoding complexity in the presence ISI [5]. The downlink CDMA is a special case of broadcast channels. There has been significant recent interest in characterising the ca...