  In integrated sensing and communication (ISAC) systems, random signaling is
used to convey useful information as well as sense the environment. Such
randomness poses challenges in various components in sensing signal processing.
In this paper, we investigate quantizer design for sensing in ISAC systems.
Unlike quantizers for channel estimation in massive multiple-input-multiple-out
(MIMO) communication systems, sensing in ISAC systems needs to deal with random
nonorthogonal transmitted signals rather than a fixed orthogonal pilot.
Considering sensing performance and hardware implementation, we focus on
task-based hardware-limited quantization with spatial analog combining. We
propose two strategies of quantizer optimization, i.e., data-dependent (DD) and
data-independent (DI). The former achieves optimized sensing performance with
high implementation overhead. To reduce hardware complexity, the latter
optimizes the quantizer with respect to the random signal from a stochastic
perspective. We derive the optimal quantizers for both strategies and formulate
an algorithm based on sample average approximation (SAA) to solve the
optimization in the DI strategy. Numerical results show that the optimized
quantizers outperform digital-only quantizers in terms of sensing performance.
Additionally, the DI strategy, despite its lower computational complexity
compared to the DD strategy, achieves near-optimal sensing performance.
