What are the key concepts in network information theory?

Network information theory extends Shannon's work to multi-user scenarios. Key results: Multiple access channel (MAC) capacity region defines achievable rate pairs for multiple transmitters to one receiver, Broadcast channel capacity for one transmitter to multiple receivers, Relay channel strategies (decode-forward, compress-forward, amplify-forward), Interference channel (capacity unknown in general, strategies include treating interference as noise, decoding interference, Han-Kobayashi). Practical implications: NOMA (non-orthogonal multiple access), cooperative communications, and network coding. Capacity achieving often requires sophisticated coding across users.