  The time division multiple access (TDMA) technique has been applied in
automotive multiple-input multiple-output (MIMO) radar. However, it suffers
from the transmit energy loss, and as a result the parameter estimation
performance degradation when the number of transmit elements increases. To
tackle these problem, a transmit beamspace (TB) Doppler division multiple
access (DDMA) approach is proposed. First, a phase modulation matrix with empty
Doppler spectrum is introduced. By exploiting the empty Doppler spectrum, a
test function based on sequential detection is developed to mitigate the
Doppler ambiguity in DDMA waveform. Then, a discrete Fourier transform
(DFT)-based TB in slow-time is formed.The proposed method can achieve waveform
diversity in Doppler domain and generate a TB in slow-time that concentrates
the transmitted power in a fixed spatial region to improve the transmit energy
distribution for automotive MIMO radar, which is favored by medium/long range
radar (MRR/LRR) applications. As compared to the conventional TDMA technique,
the proposed TB DDMA approach can fully exploit the transmission capabilities
of all transmit elements to ensure that the emitted power is efficiently used
and inherits easy implementation. Moreover, the proposed TB DDMA method avoids
the trade-off between the active time for each transmit antenna and the frame
time. Simulation results verify the effectiveness of the proposed TB DDMA
approach for automotive MIMO radar.
