A 1-GHz 16-Element Four-Beam True-Time-Delay Digital Beamformer

Phased arrays are widely used due to their low power and small area usage. However, phased arrays depend on the narrowband assumption and, therefore, are not suitable for high-bandwidth applications. Emerging communication standards require increasingly higher bandwidths for improved data rates, which results in a need for timed arrays. However, high power consumption and large area requirements are drawbacks of radio frequency (RF) timed arrays. To resolve these issues, we introduce the first true-time-delay digital beamforming IC, which eliminates beam squinting error by adopting a baseband true-time-delay technique. Furthermore, we present a constant output impedance current-steering digital-to-analog converter (DAC), which improves the spurious-free dynamic range (SFDR) of a bandpass delta-sigma modulator by 7 dB. Due to the new DAC architecture, the 16-element beamformer improves SFDR by 13.7 dB from the array. Measured error vector magnitudes (EVMs) are better than 37 dB for 5-MBd quadratic-amplitude modulation (QAM)-64, QAM-256, and QAM-512. The prototype beamformer achieves nearly ideal beam patterns for both conventional and adaptive beamforming (i.e., adaptive nulling and tapering). The difference between normalized measured beam patterns and normalized simulated beam patterns is less than 1 dB within the 3-dB beamwidth. The beamformer, including 16 bandpass analog-to-digital converters (ADCs) occupies 0.29 mm² and consumes 453 mW in total power.

I. Introduction

Phased arrays use phase shifting to mimic time delay and are widely used due to their relatively simple implementation and low power consumption. However, phase shifting only approximates time delay, which is not identical to the actual time delay. Thus, phased arrays suffer from beam squinting [1] and array inter-symbol interference (ISI) [2], which is discussed further in Section II. In emerging communication applications, high bandwidths are required to support high data rates and large arrays are desirable due to the high path loss of millimeter waves [3]. However, higher bandwidths and large arrays suffer from greater levels of beam squinting [1] and array ISI [2].