A K -Band Four-Beam Transmitter With Decoupled Phase/Gain Control and Enhanced Power Back Off Efficiency for SATCOM

This article presents a 17.7–20.2 GHz eight-element four-beam RF-beamforming transmitter in 65-nm CMOS for satellite communication (SATCOM). The transmitter utilizes an analog scheme in the variable-gain amplifier (VGA) to achieve dB-in-linear gain control with a high dynamic range (DR), together with a current combining crossbar for a compact footprint. The vector-modulation-based (VM-based) phase shifter (PS) adopts the cancellation-based $G_{\mathrm {m}}$ units for low switching variation. Furthermore, eight PSs of each beam are split into two groups, with each group sharing the same tunable I/Q generator for low power consumption. Considering the increased peak-to-average power ratio (PAPR) caused by high-order modulation schemes and multibeam operation, the transmitter incorporates compact transformer-based Doherty power amplifiers (DPAs) with adaptive bias control to improve the power efficiency at back-off. The chip achieves a 31-dB gain-tuning range with a 0.283-dB step and a 360° phase-shifting range with a calibrated 7-bit resolution. The measured root-mean-square (RMS) phase/gain errors induced by the VGA/PS are below 2.05°/0.147 dB, respectively, which decouples phase/gain control. The transmitter achieves a maximum output 1-dB gain compression point (OP $_{\mathrm {1 dB}}$ ) of 17.9 dBm and a maximum saturated power ( $P_{\mathrm {sat}}$ ) of 18.9 dBm. The output element exhibits 29.3% drain efficiency (DE) at 6-dB power back off (PBO) from $P_{\mathrm {sat}}$ , which is 1.56/3.12 times over the normalized class-B/class-A implementations, demonstrating Doherty PBO efficiency enhancement. The chip occupies an area of $8.59\times 4.17$ mm2 and consumes 398 mW per element at OP $_{\mathrm {1~dB}}$ . Additionally, a 16-element linear array has been developed to showcase the capabilities of the multibeam operation and gain tapering.

I. Introduction

The increasing demand for high-speed, low-latency global connectivity is driving the development of low-Earth-orbit (LEO) and medium-Earth-orbit (MEO) satellite communications (SATCOMs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. To address the significant free-space path loss, phased-array systems are widely used to create steerable directional beams, enhancing the high signal-to-noise ratio (SNR) [10]. Moreover, the equivalent isotropically radiated power (EIRP) improved by the spatial power combining in large transmitter phased arrays relieves the need for output power per element, facilitating the use of cost-effective, high-integration CMOS technologies with limited output power capacity [11]. However, the satellite’s physical size limits the dimensions of the phased arrays, thereby constraining the number of beamformers. Consequently, a coaperture multibeam phased array is proposed to process independent data streams from different directions concurrently with a reduced antenna array size, as illustrated in Fig. 1. Fig. 1.

Multibeam application of the SATCOM.