Transform techniques for reducing the peak power in orthogonal frequency-division multiplexed systems

Parallel data transmission systems have been used to transmit data at high rates over dispersive channels with minimal equalization requirements. Examples of such systems include Discrete MultiTone (DMT), which is used in digital subscriber line (DSL) technology, and Orthogonal Frequency-Division Multiplexing (OFDM). However, these systems suffer from large peaks in the transmitted signal envelope due to the complex Gaussian distribution of the modulator output. Nonlinear system components such as analog-to-digital converters and power amplifiers distort these large peaks and force design choices that may degrade system performance. This research explores a novel method to solve this problem for phase-shift-keyed (PSK) modulation. The method involves choosing one transform from a fixed set to apply to the data vector prior to the input of the OFDM modulator in the transmitter. It is shown that the set of transforms is chosen wisely, then the receiver can recover the data vector without the aid of information identifying the transform. An example of such a receiver is presented. Two methods are presented for constructing such a set or codebook of transforms: an iterative algorithm and a direct method based on fundamental properties of the multicarrier signal. Simulation results comparing the performance with and without a transform codebook are given for an OFDM system with a nonlinear power amplifier. These results show that using a well-designed codebook improves both the bit-error-rate (BER) performance and the efficiency of the power amplifier without reducing the information bandwidth of the multicarrier transmission system.