I. Introduction  
A. Application Types  
B. Access Networks  
C. 6G Vision and Requirements  
D. Survey Scope and Related Works  
E. Contributions  

II. Key Performance Indicators  
A. Latency and Reliability  
B. Survival Time and Service Availability  
C. Age of Information  
D. Battery Lifetime and Energy Efficiency  
E. Connection Density  
F. Data Rate and Spectral Efficiency  

III. Enabling Access Solutions  
A. IoT Communications over Licensed Spectrum  
B. IoT Networks over Unlicensed Spectrum  
C. Domain of Future Access Solutions  

IV. Channel Code Design  
A. State-of-the-Art Channel Coding Techniques for IoT  
B. Low-capacity Communications  
C. Channel Coding for CSI-free Communications  
D. Decoder Design  

V. Massive Connectivity  
A. Multiple Access Solutions  
B. Random Access Solutions  

VI. IoT Security  
A. IoT Security Attacks Based on Their Characteristics  
B. SDN and Blockchains in IoT  

VII. Deep Learning for IoT  
A. Neural Networks  
B. Deep Learning Models  
C. Autoencoder  
D. Deep Reinforcement Learning  

VIII. NTN Integration into IoT Networks  
A. UAVs Integration into IoT Networks  
B. Satellite Integration into IoT Networks  

IX. Remaining Challenges and Future Directions  
A. Limited Battery Lifetime  
B. mmWave Access  
C. Network-Device Cooperation  
D. Massive Connectivity  
F. Risk-aware Learning for IoT  
G. Security Issues  

X. Conclusions