  Channel sounding is a vital step in understanding wireless channels for the
design and deployment of wireless communication systems. In this paper, we
present the design and implementation of a coherent distributed massive MIMO
channel sounder operating at 5-6 GHz with a bandwidth of 400 MHz based on the
NI USRP X410. Through the integration of transceiver chains and RF switches,
the design facilitates the use of a larger number of antennas without
significant compromise in dynamic capability. Our current implementation is
capable of measuring thousands of antenna combinations within tens of
milliseconds. Every radio frequency switch is seamlessly integrated with a
16-element antenna array, making the antennas more practical to be transported
and flexibly distributed. In addition, the channel sounder features real-time
processing to reduce the data stream to the host computer and increase the
signal-to-noise ratio. The design and implementation are verified through two
measurements in an indoor laboratory environment. The first measurement entails
a single-antenna robot as transmitter and 128 distributed receiving antennas.
The second measurement demonstrates a passive sensing scenario with a walking
person. We evaluate the results of both measurements using the super-resolution
algorithm SAGE. The results demonstrate the great potential of the presented
sounding system for providing high-quality radio channel measurements,
contributing to high-resolution channel estimation, characterization, and
active and passive sensing in realistic and dynamic scenarios.
