  This paper presents the design and implementation of a novel SDR based
massive MIMO testbed with up to 70 nodes built at Tennessee Technological
University. The deployment can reach a $30 \times 30$ antenna MIMO scheme. With
this testbed, we are able to measure the channel matrix and compute the
achievable rate of the massive MIMO system using experimental data. The
measured channel capacity is linearly increasing with the number of antennas of
the base station. We also demonstrate the channel reciprocity including the
circuits impact from the transmitter and receiver. We show that the Vandermonde
channel model is more realistic to describe the massive MIMO architecture than
the widely used Gaussian channel model, in terms of capacity. By adjusting the
range for angle of arrival $\alpha$ and the base station antenna distance $d$
during the simulation, we find out the Vandermonde model agrees with our
measured capacity at a certain $\alpha$ for each selected $d$ and the $\alpha$
is very close to that of the experiment deployment. It is the first time that
the feasibility of Vandermonde channel model is demonstrated by the experiment
for massive MIMO.
