True-Time-Delay Beamforming Receiver With RF Re-Sampling

Analog domain true-time-delays (TTD) are desired in hybrid beamforming receivers with large relative bandwidths to mitigate the problem of beam squint. We propose a true-time-delay beamforming receiver architecture which enables squint-free wideband spatial filtering prior to the A/D conversion. The receiver implements true-time-delay with delayed re-sampling of the discrete-time output of a passive mixer. The receiver has the capability to extend the range of the beamforming delays from one to several carrier periods of the RF signal with pulse-skipped local oscillator (LO) signals, thereby enabling TTD beamforming with large antenna arrays. Further, a polyphase structure with parallel mixers is proposed to prevent spectral aliasing resulting from the lowered sample rate of the pulse-skipped LO signals. In addition, the maximum beamforming delay scales with the LO frequency, supporting large arrays also at low frequencies where the antenna separation set by the wavelength is large. We verify the proposed concepts with transistor-level simulation of the receiver implemented with a 28-nm CMOS process. The design achieves a squint-free beamforming for a 400 MHz RF bandwidth, and a maximum beamforming delay of three carrier time periods. The power consumption for a 3 GHz carrier frequency is 4 mW per antenna.

I. Introduction

Electronic beamforming with antenna arrays required for radars and upcoming communication systems with spatial filtering, like the 5G systems [1], can be implemented either in the analog [2]–[4] or the digital [5]–[8] domain. In analog beamforming receivers the signals from different antennas are combined to form one beam before the analog-to-digital converter (ADC). This beamforming creates spatial filtering prior to the ADC, which improves the receiver chain’s dynamic range performance in the presence of in-band blockers [7]. Analog beamforming limits the number of supported simultaneous beams, since implementing parallel analog reception paths for each beam becomes prohibitive due to increased hardware complexity leading to large area and power consumption [9]. In digital beamforming, signals from different antennas are digitized separately and the beams are formed in the digital domain. As digital signal processing supports efficient hardware paralellization, multiple simultaneous beams in different directions are feasible [8]. On the other hand, digital beamforming does not provide spatial filtering at the ADC inputs, which limits dynamic range in the case of in-band blockers [10]–[12]. Hybrid beamforming can alleviate the blocker problem by means of analog spatial filtering while retaining the computational flexibility of digital signal processing [1], [9], [13].