[Paper Review] A Real-Time Millimeter-Wave Phased Array MIMO Channel Sounder

C. Umit Bas, Rui Wang+6 more|arXiv (Cornell University)|Mar 15, 2017

Millimeter-Wave Propagation and Modeling16 references4 citations

TL;DR

This paper presents a real-time millimeter-wave MIMO channel sounder at 28 GHz using phased array beamforming for simultaneous directional transmission and reception. By enabling sub-microsecond beam switching and phase-coherent measurements without cabled synchronization, it reduces measurement time from hours to 1.44 ms per MIMO snapshot, enabling high-resolution, dynamic channel characterization in real time.

ABSTRACT

In this paper, we present a novel real-time MIMO channel sounder for 28 GHz. Until now, the common practice to investigate the directional characteristics of millimeter-wave channels has been using a rotating horn antenna. The sounder presented here is capable of performing horizontal and vertical beam steering with the help of phased arrays. Thanks to fast beam-switching capability, the proposed sounder can perform measurements that are directionally resolved both at the transmitter (TX) and receiver (RX) as fast as 1.44 milliseconds compared to the minutes or even hours required for rotating horn antenna sounders. This does not only enable us to measure more points for better statistical inference but also allows to perform directional analysis in dynamic environments. Equally importantly, the short measurement time combined with the high phase stability of our setup limits the phase drift between TX and RX, enabling phase-coherent sounding of all beam pairs even when TX and RX are physically separated and have no cabled connection for synchronization. This ensures that the measurement data is suitable for high-resolution parameter extraction algorithms. Along with the system design and specifications, this paper also discusses the measurements performed for verification of the sounder. Furthermore, we present sample measurements from a channel sounding campaign performed on a residential street.

Motivation & Objective

• To overcome the long measurement times of traditional rotating horn antenna sounders, which can take minutes to hours per location.
• To enable real-time, directionally resolved channel measurements at mmWave bands for dynamic environments.
• To achieve phase-coherent sounding between physically separated transmitters and receivers without cabled clock synchronization.
• To support high-resolution parameter extraction algorithms like RIMAX and SAGE by minimizing phase drift across beam pairs.
• To validate the system's performance through outdoor measurements on a residential street.

Proposed method

• The system uses phased array antennas at both transmitter and receiver, with phase shifters enabling electronic beam steering across 90-degree sectors.
• A custom FPGA-based control interface switches beams in less than 2 μs, enabling rapid MIMO snapshot acquisition.
• The sounder operates without a shared reference clock by leveraging stable voltage-controlled oscillators and phase-locked loops with consistent random phase offsets across beam pairs.
• Directional power delay profiles (PDPs) are estimated via inverse Fourier transform of calibrated frequency responses: $ P( heta_{TX}, heta_{RX}, au) = ig| ilde{oldsymbol{F}}^{-1} igackslash \{ H_{\theta_{TX},\theta_{RX}}(\vec{f}) / H_{\text{cal}}(\vec{f}) \big\} \big|^2 $.
• 360-degree coverage is achieved by rotating the receiver and combining measurements from four 90-degree sectors, with angular power spectra computed via maxima over TX and RX beam angles.
• High-resolution parameter extraction is enabled by phase stability, allowing use of advanced algorithms such as RIMAX and SAGE to recover true double-directional channel characteristics.

Experimental results

Research questions

• RQ1 Can a real-time mmWave MIMO channel sounder be developed that reduces measurement time from hours to milliseconds?
• RQ2 Can phase-coherent measurements be achieved between separated transmitters and receivers without cabled clock synchronization?
• RQ3 Can the system enable high-resolution parameter extraction for double-directional channel models in dynamic environments?
• RQ4 How does the system perform in real-world outdoor environments with non-line-of-sight and long-distance propagation?
• RQ5 What is the achievable path-loss dynamic range and measurement fidelity with electronic beamforming at 28 GHz?

Key findings

• The system achieves a measurement time of 1.44 milliseconds per MIMO snapshot, representing a reduction of several orders of magnitude compared to rotating horn antennas.
• The sounder achieves a measurable path-loss of 159 dB over a 400 MHz bandwidth without waveform averaging, demonstrating high sensitivity and dynamic range.
• Phase drift between transmitter and receiver is limited to within a single MIMO snapshot, enabling phase-coherent measurements even without cabled synchronization.
• Sample measurements on a 150 m non-line-of-sight residential link show consistent directional power delay profiles aligned in the delay domain with no correction needed.
• The path-loss curve from 360-degree coverage matches the theoretical free-space path-loss model, validating the accuracy of the omnidirectional PDP synthesis.
• The system enables high-resolution parameter extraction suitable for advanced algorithms like RIMAX and SAGE, which recover the true channel response independent of hardware effects.