A 24-to-30GHz Double-Quadrature Direct-Upconversion Transmitter with Mutual-Coupling-Resilient Series-Doherty Balanced PA for 5G MIMO Arrays

Abstract

The performance and robustness of millimeter-wave (mm-wave) phased-array transmitters (TXs) define, to a large extent, the quality of a high-data-rate 5G link. In practical situations, however, this TX performance is strongly affected by mutual coupling among the closely-spaced radiating elements in the phased-array antenna, yielding a beam-steering angle-dependent and time-varying loading or VSWR condition. Furthermore, 5G mm-wave systems typically employ spectrally efficient modulation schemes with high peak-to-average power ratios (PAPRs). This requirement demands the TX power amplifier (PA) to operate in power back-off (PBO), thus degrading its average efficiency. To alleviate this issue, outphasing or Doherty PAs (DPAs) can be adopted [1-4]. However, as depicted in Fig. 14.4.1 (Top left), these efficiency-enhancement techniques will worsen the output reflection coefficient of the PA (\Gamma_{{\mathrm {PA}}}). Consequently, the unwanted 'element-to-element' coupled signal reflects back to the antenna and will deteriorate the phased-array beam pattern and its TX linearity. A previously promoted solution for this antenna VSWR problem is load mismatch detection, followed by tuning of the output matching network (self-healing). However, this requires the use of a reconfigurable and inevitably lossy matching network [5]. Also, active load pulling [1] and using a reconfigurable series/parallel DPA configuration [2] have been proposed to realize a VSWR resilient efficiency-enhanced TX. Nevertheless, all these techniques are only suitable when dealing with a known and stable antenna impedance mismatch, which is, unfortunately, not the case in practical situations.