Hybrid Delay-Phase Precoding in Wideband UM-MIMO Systems under True Time
Delay and Phase Shifter Hardware Limitations
Abstract
This paper introduces a novel hybrid precoding architecture, along with
several hardware-aware delay-phase precoding schemes, that address the
beam squint effect while complying with the limitations of true time
delay (TTD) elements and phase shifters (PSs). The proposed architecture
significantly alleviates the burden of the maximum delay range
constraint imposed on TTDs. Additionally, we address the time resolution
constraint of TTDs by analyzing the resulting resolution error and
compensating for it in the phase domain. Similarly, we account for the
resolution constraint of PSs by proposing a practical precoding scheme
that jointly optimizes TTDs and PSs, taking into consideration their
hardware limitations. Specifically, we formulate the design of the
hybrid analog precoder under TTDs and PSs constraints as a mixed-integer
optimization problem and propose an alternating minimization-based
iterative algorithm to solve it. To reduce the computational complexity
of this algorithm, we eventually propose a low-complexity precoding
algorithm that maintains satisfactory performance with minimal
computational overhead.
We would like to mention that a part of this work has been accepted for
presentation at IEEE ICC 20231. In our previous work at IEEE ICC 2023, a
preliminary investigation of the hybrid analog precoding under the
hardware limitations of TTDs is presented, where the finite-resolution
limitation of TTDs is primarily addressed in the design of the hybrid
analog precoder. The work in IEEE ICC 2023 proposes an iterative
delay-phase precoding algorithm, that achieves remarkably higher array
gain compared to a recent state-of-art work in the literature. The
efficacy of this algorithm is validated in 2D scenario assuming uniform
linear arrays (ULAs) in IEEE ICC 2023, whereas it is further evaluated
in 3D scenario assuming uniform planar arrays (UPAs) in a conference
paper, submitted to IEEE IWMTS 20232. Nevertheless, this algorithm
suffers array gain loss as the maximum delay range supported by TTDs
decreases. In the same vien, a remarkable gain loss occurs with the
adoption of higher number of antenna elements, which makes this
algorithm unscalable for UM-MIMO systems. Motivated by these drawbacks,
we propose in this work a novel delay-phase precoding architecture that
alleviates the effect of the maximum delay range limitation of TTDs to
high extent surpassing our work in IEEE ICC 2023. The proposed
architecture does not require higher time delay values as the number of
antennas increases, which makes it suitable for UM-MIMO systems.
Moreover, in addition to the hardware limitations of TTDs, we consider
in this work the hardware limitations of PSs where impractical
infinite-resolution PSs are assumed.