A Multi-Channel Sigma-Delta Modulator for Subband Digital Predistortion with LTE signals.
Paper selected for NEWCAS’16.
With the increasing bandwidth of communication signals, the challenges on the feedback path used for digital predistortion (DPD) of power amplifiers are getting significantly stringent. A subband approach has been recently proposed by the authors in order to relax the design constraints on the feedback path ADC. This paper presents a new step toward a practical implementation of the concept with LTE signals. A continuous-time multi-channel modulator is proposed to address wider bandwidths and sizing choices are discussed with simulation results. Some nonlinear effects of the feedback path such as I/Q imbalance, pure nonlinearity and quantization are simulated to define high level design parameters. Results show that for a distorted signal with 30 dB ACPR, 60 dB SNR is sufficient for the principal subband and 22 dB SNR is required for the adjacent subbands to perform as good as an ideal fullband DPD. Continuous-time simulation using a 2-4-4 configuration of the zero IF MSNBC with 4-bit quantizers achieves the required subband SNRs.