Fundamental Limits of Random Downlink Integrated Sensing and Communication over Rician Channels
2026-07-01 • Information Theory
Information Theory
AI summaryⓘ
The authors study how a wireless system that both talks to users and senses targets performs when signals experience Rician fading, which includes a strong direct path. They analyze two ways of focusing signals (beamforming) and calculate how often communication or sensing fails. Their results show that the presence of a direct line-of-sight path affects communication more than sensing and that using a special coding method improves reliability in strong direct-path conditions. They also provide formulas and performance insights useful for designing such systems.
Rician fadingintegrated sensing and communication (ISAC)multiple-input multiple-output (MIMO)beamformingline-of-sight (LoS)Cramer-Rao bound (CRB)communication outage probabilitydirty paper coding (DPC)subspace joint beamforming (SJB)linear beamforming (LB)
Authors
Marziyeh Soltani, Mahtab Mirmohseni, Rahim Tafazolli, Mark F. Flanagan
Abstract
This paper studies the stochastic performance of a downlink multiple-input multiple-output integrated sensing and communication (ISAC) system over Rician fading channels. Rician fading is important in line-of-sight (LoS)-dominated deployments, where a deterministic propagation component can strongly affect sensing and communication reliability. The base station (BS) simultaneously serves a user and senses a target. The BS-user channel contains LoS and non-line-of-sight components. The user LoS angle may be fixed or random, and the target angle may follow an arbitrary distribution potentially correlated with the user angle. Compared with Rayleigh fading, the deterministic LoS component introduces angle-dependent terms and leads to generally independent but non-identically distributed random vectors, requiring new analysis. We analyze two beamforming strategies: subspace joint beamforming (SJB), optimal for the shared waveform structure, and linear beamforming (LB), a practical alternative using separate sensing and communication beamformers. For both schemes, we derive communication outage probability (OP) and sensing OP based on the Cramer--Rao bound (CRB). We also identify special cases with simpler expressions. For LB, we derive upper and lower bounds on sensing OP and a tractable approximation. We characterize large-system and high-power scaling laws. LB without dirty paper coding (DPC) is interference-limited at high power due to radar self-interference. Results show the Rician K-factor affects communication more strongly than sensing, with non-monotonic behavior across regimes. LB with DPC achieves the best overall performance in strong LoS environments and is the only scheme achieving ultra-high communication reliability in Rayleigh fading, while SJB provides a robust lower-complexity alternative across operating conditions.