Cross Layer Design in Wireless Networks Andrea Goldsmith Stanford University Crosslayer Design Panel ICC May 14, 2003 Future Wireless Networks Ubiquitous Communication Among People and Devices Wireless Internet access Nth generation Cellular Wireless Ad Hoc Networks Sensor Networks Wireless Entertainment Smart Homes/Spaces Automated Highways All this and more… •Hard Delay Constraints •Hard Energy Constraints Challenges Wireless channels are a difficult and capacitylimited broadcast communications medium Traffic patterns, user locations, and network conditions are constantly changing Applications are heterogeneous with hard constraints that must be met by the network Energy and delay constraints change design principles across all layers of the protocol stack These challenges apply to all wireless networks, but are amplified in ad hoc/sensor networks Evolution of Current Systems Wireless systems today Next Generation 2G Cellular: ~30-70 Kbps. WLANs: ~10 Mbps. 3G Cellular: ~300 Kbps. WLANs: ~70 Mbps. Technology Enhancements Hardware: Better batteries. Better circuits/processors. Link: Antennas, modulation, coding, adaptivity, DSP, BW. Network: Dynamic resource allocation. Mobility support. Application: Soft and adaptive QoS. “Current Systems on Steroids” Future Generations Rate 4G 802.11b WLAN 3G Other Tradeoffs: Rate vs. Coverage Rate vs. Delay Rate vs. Cost Rate vs. Energy 2G 2G Cellular Mobility Fundamental Design Breakthroughs Needed Design objective Want to provide end-to-end “QoS” The challenge for this QoS is the system dynamics Scheduling can help shape these dynamics Adaptivity can compensate for or exploit these dynamics Diversity provides robustness to unknown dynamics Scheduling, adaptivity, and diversity are most powerful in the context of a crosslayer design Energy must be allocated across all protocol layers Crosslayer Design Hardware Link Delay Constraints Rate Constraints Energy Constraints Access Network Application Adapt across design layers Reduce uncertainty through scheduling Provide robustness via diversity Crosslayer Techniques Adaptive techniques Diversity techniques Link, MAC, network, and application adaptation Resource management and allocation (power control) Synergies with diversity and scheduling Link diversity (antennas, channels, etc.) Access diversity Route diversity Application diversity Content location/server diversity Scheduling Application scheduling/data prioritization Resource reservation Access scheduling Key Questions What is the right framework for crosslayer design? What are the key crosslayer design synergies? How to manage its complexity? What information should be exchanged across layers, and how should this information be used? How do the different timescales affect adaptivity? What are the diversity versus throughput tradeoffs? What criterion should be used for scheduling? How to balance the needs of all users/applications? Single User Systems Receiver Traffic Generator Channel Data Buffer Source Coder Channel Coder Modulator (Power) Cross-Layer System Stringent QoS constraints require that the full dynamics of the system be represented Average performance metrics are misleading The “Myth” of Averages: Delay in milliseconds Minimizing Average Delay What if we constrain delay across all channel gains? Average Delay Channel Gain in dB Power in milliwatts Hard Delay Constraints (50 ms) 50ms constraint on delay across all channel gains with power adaptation only 200mW Joint source-channel coding power saving Source data rate in bits per second Crosslayer design in multiuser systems • Users in the system interact (interference, congestion) • Resources in the network are shared • Adaptation becomes a “chicken and egg” problem • Protocols must be distributed Energy-Constrained Nodes Each node can only send a finite number of bits. Short-range networks must consider transmit, circuit, and processing energy. Transmit energy minimized by maximizing bit time Circuit energy consumption increases with bit time Introduces a delay versus energy tradeoff for each bit Sophisticated techniques not necessarily energy-efficient. Sleep modes save energy but complicate networking. Changes everything about the network design: Bit allocation must be optimized across all protocols. Delay vs. throughput vs. node/network lifetime tradeoffs. Optimization of node cooperation. Distributed Control over Wireless Links Automated highways, factories, and homes Network design must meet control requirements. Automated highway controllers unstable with any delay Controller design should be robust to network faults. Need joint application and communication network design. Design Approach Theory Optimization Simulation Testbeds Application Design Network Design Link Design Device Design Summary Crosslayer design needed to meet requirements and constraints of future wireless networks Key synergies in crosslayer design must be identified The design must be tailored to the application Crosslayer design should include adaptivity, scheduling and diversity across protocol layers Energy can be a precious resource that must be shared by different protocol layers Lots of fun and challenging research problems