Reduced-Complexity PAPR Minimization Schemes for MC-CDMA Systems

Abstract

Multicarrier Code-Division Multiple Access (MC-CDMA) (Hara & Prasad, 1997), which is based on a combination of an CDMA scheme and Orthogonal Frequency Division Multiplexing (OFDM) signaling (Fazel & Kaiser, 2008), has attracted much attention in forthcoming mobile communication systems, because of its intrinsic spectrum efficiency and interference suppression capabilities. In MC-CDMA, information symbols of many users are spread using orthogonal codes and combined in the frequency domain; this results in a relatively low symbol rate and thus non-selective fading in each subcarrier. However, one main drawback of any kind of multicarrier modulation is the inherent high value of the Peak-to-Average Power Ratio (PAPR) of the transmitted signals, because they are generated as an addition of a large number of independent signals. If low power consumption at the transmitter is a strict requirement, one would like the RF High Power Amplifier (HPA) to operate with a low back-off level (i.e. with operation point near saturation state); as a consequence of this, signal peaks will frequently enter the nonlinear part of the input-output characteristic of the HPA, thus causing severe nonlinear artifacts on the transmitted signals such as intermodulation distortion and out-of-band radiation. Therefore, reducing the PAPR is crucial in multicarrier systems, especially when transceivers are fed by batteries (such as in mobile devices), because of the intrinsic limitations in power consumption. There has been a lot of research work about PAPR reduction techniques in multicarrier systems. Among these, we have clipping and filtering schemes (Li & Cimini, 1997), block coding algorithms (Jones et al., 1994), the Partial Transmit Sequences (PTS) (Cimini & Sollenberger, 2000; Jayalath & Tellambura, 2000), and Selected Mapping (SLM) approaches (Bauml et al., 1996; Breiling et al., 2001), and the Tone Reservation (TR) (Tellado & Cioffi, 1998), and the Tone Injection (TI) techniques (Han et al., 2006). In general, reducing the PAPR is always done either at the expense of distorting the transmitted signals, thus increasing the BER at the receiver, or by reducing the information data rate, usually because high PAPR signals are somehow discarded and replace by others with lower PAPR before been transmitted. All the previously mentioned methods have been originally proposed for single-user multicarrier schemes such as OFDM. Although most of them are also applicable with minor modifications to MC-CDMA systems (Ruangsurat & Rajatheva, 2001; Ohkubo & Ohtsuki, 2002), other families of algorithms can be developed after carefully considering the different