I. Introduction
A. Abbreviations
B. Background
C. Potential Sensing Applications of PMNs
D. Contributions and Structure of this Paper
II. Three Types of JCAS Systems
A. Major Differences between C&S Signals
B. Radar-Centric Design: Realizing Communication in Primary Radar Systems
D. Joint Design Without an Underlying System
E. Advantages of JCAS Systems
F. Summary of Key Research Problems
III. Framework for a PMN
A. System Platform and Infrastructure
B. Three Types of Sensing Operations
C. Signals Usable from 5G NR for Radio Sensing
D. Summary and Insights
V. Major Research Challenges for PMN
A. Sensing Parameter Extraction from Sophisticated Mobile Signals
B. Joint Design and Optimization
VI. Detailed Technologies and Open Research Problems
A. Performance Bounds
B. Waveform Optimization
C. Antenna Array Design
D. Clutter Suppression Techniques
E. Sensing Parameter Estimation
F. Resolution of Sensing Ambiguity
G. Pattern Analysis
H. Networked Sensing under Cellular Topology
I. Sensing-Assisted Communications
VII. Concluding Remarks