Energy-Efficient Multicarrier and Multicarrier-CDMA Systems
Abstract
Design, analysis, and performance of Multicarrier (MC) and Multicarrier-Code Division Multiple Access (MC-CDMA) systems are considered. Specifically, Multicarrier Modulation (MCM) such as Orthogonal Frequency Division Multiplexing (OFDM) is used in these systems with emphasis on energy efficiency. These systems are examined for their performances over frequency-selective and frequency-non-selective channels. Methods are proposed for reduction of energy costs of an OFDM system. They are: i) Clipping (CL) based on symbol statistics; ii) Partial Transmit Sequence (PTS) using Magic square pattern and adjacent partitioning; and iii) Selective Mapping (SLM) and Tone Reservation (TR) with reduced complexity. These methods are described and algorithms are presented for computation of Peak-to-Average Power Ratio (PAPR). Using extensive simulations, Complementary Cumulative Distribution Functions (CCDFs) of PAPR of OFDM signals for Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), and -QPSK are obtained as a function of number of subcarriers in the system. It is shown that PAPR reductions up to can be achieved using the proposed methods for a 128-subcarrier OFDM system. These reductions amount to increased energy efficiency of High Power Amplifier (HPA) up to 23.794% in an OFDM system.
Bit Error Rate (BER) performances of OFDM system with and without the proposed PAPR reduction techniques are considered over both frequency-selective and frequency-non-selective Rayleigh fading channels. Power Penalty (PP) required over these channels are assessed for BPSK, QPSK, and -QPSK mappers in an OFDM system. It is observed that at BER, PP required is of the order of for frequency-non-selective and for frequency-selective Rayleigh fading channels.
Finally, using a complex spreading sequence, a hybrid of OFDM and CDMA systems referred to as MC-CDMA system is proposed and is investigated for its PAPR properties using the three proposed PAPR reduction techniques. BER performance of MC-CDMA system is determined over frequency-selective and frequency-non-selective Rayleigh fading channels. It is shown that PAPR of MC-CDMA system is a function of the spreading sequence, and the PAPR reduction method used. A PAPR of can be achieved when the proposed complex pseudo-random (PN) sequence is used.
Keywords: Energy-Efficient, OFDM, MC-CDMA, Clipping, PTS, SLM, TR, PAPR, PSD, BER.