I. Introduction  
II. Ambient Backscatter Communications: An Overview  
A. Backscatter Communications Systems  
C. Antenna Design  
D. Channel Coding and Decoding  
E. Modulation and Demodulation  
F. Backscatter Communications Channels  
III. Bistatic Backscatter Communications Systems  
A. Overview of Bistatic Backscatter Communications Systems  
IV. Ambient Backscatter Communications Systems  
A. Overview of Ambient Backscatter Communications Systems  
C. Potential Applications  
D. Discussion  
V. Emerging Backscatter Systems  
B. RF-Powered Cognitive Radio Networks and Backscatter Communication  
C. Wireless-Powered Communication Networks and Backscatter Communication  
D. Backscatter Relay Networks  
E. Visible Light Backscatter Communications  
F. Long-range LoRa Backscatter Communications  
VI. Open Issues and Future Research Directions  
A. Heterogeneity of Ambient Signals  
B. Interference to Licensed Systems  
D. Security and Jamming Issues  
E. Millimeter-wave-based Ambient Backscatter  
VII. Conclusion