13.2 A digital multimode polar transmitter supporting 40MHz LTE Carrier Aggregation in 28nm CMOS

The evolving trend for increasing data rates in cellular communication systems with limited and fragmented frequency spectrum requires enhanced spectral efficiency of today's and future communication standards. The resulting need for high-order modulation schemes with large peak-to-average power ratio results in stringent requirements on in-channel linearity and SNR. At the same time FDD operation in SAW filter-less designs enforces very demanding limits for out-of-band and spurious emissions. Digital polar TX concepts have demonstrated low power consumption combined with low out-of-band noise in moderate bandwidth applications like WCDMA [1]. However, the implementation of 4G wideband polar systems with 20MHz RF bandwidth and above is very challenging due to the nonlinear conversion from cartesian to polar coordinates extending the effective signal bandwidth even further. A digital quadrature TX concept with up to 80MHz bandwidth, low EVM and moderate power efficiency has been shown in [2]. More than 30% power-added efficiency has been demonstrated in a 20MHz polar TX based on a switched-capacitor digital PA [3]. However, [2,3] suffer from low resolution and limited out-of-band noise performance.