  Benefiting from tens of GHz bandwidth, terahertz (THz) communication is
considered to be a promising technology to provide ultra-high speed data rates
for future 6G wireless systems. To compensate for the serious propagation
attenuation of THz signals, massive multiple-input multiple-output (MIMO) with
hybrid precoding can be utilized to generate directional beams with high array
gains. However, the standard hybrid precoding architecture based on
frequency-independent phase-shifters cannot cope with the beam split effect in
THz massive MIMO systems, where the directional beams will split into different
physical directions at different subcarrier frequencies. The beam split effect
will result in a serious array gain loss across the entire bandwidth, which has
not been well investigated in THz massive MIMO systems. In this paper, we first
reveal and quantify the seriousness of the beam split effect in THz massive
MIMO systems by analyzing the array gain loss it causes. Then, we propose a new
precoding architecture called delay-phase precoding (DPP) to mitigate this
effect. Specifically, the proposed DPP introduces a time delay network as a new
precoding layer between radio-frequency chains and phase-shifters in the
standard hybrid precoding architecture. In this way, conventional
phase-controlled analog beamforming can be converted into delay-phase
controlled analog beamforming. Unlike frequency-independent phase shifts, the
time delay network introduced in the DPP can realize frequency-dependent phase
shifts, which can be designed to generate frequency-dependent beams towards the
target physical direction across the entire THz bandwidth. Due to the joint
control of delay and phase, the proposed DPP can significantly relieve the
array gain loss caused by the beam split effect. Furthermore, we propose a
hardware structure by using true-time-delayers to realize the concept of DPP.
