  Time-division multiplexed (TDM) channel sounders, in which a single RF chain
is connected sequentially via an electronic switch to different elements of an
array, are widely used for the measurement of double-directional/MIMO
propagation channels. This paper investigates the impact of array switching
patterns on the accuracy of parameter estimation of multipath components (MPC)
for a time-division multiplexed (TDM) channel sounder. The commonly-used
sequential (uniform) switching pattern poses a fundamental limit on the number
of antennas that a TDM channel sounder can employ in fast time-varying
channels. We thus aim to design improved patterns that relax these constraints.
To characterize the performance, we introduce a novel spatio-temporal ambiguity
function, which can handle the non-idealities of real-word arrays. We formulate
the sequence design problem as an optimization problem and propose an algorithm
based on simulated annealing to obtain the optimal sequence. As a result we can
extend the estimation range of Doppler shifts by eliminating ambiguities in
parameter estimation. We show through Monte Carlo simulations that the root
mean square errors of both direction of departure and Doppler are reduced
significantly with the new switching sequence. Results are also verified with
actual vehicle-to-vehicle (V2V) channel measurements.
