Cross Layer Design in Wireless Networks Andrea Goldsmith Stanford University Crosslayer Design Panel ICC May 14, 2003

Future Wireless Networks Wireless Internet access Nth generation Cellular Wireless Ad Hoc Networks Sensor Networks Wireless Entertainment Smart Homes/Spaces Automated Highways All this and more… Ubiquitous Communication Among People and Devices Hard Delay Constraints Hard Energy Constraints

Challenges Wireless channels are a difficult and capacity- limited 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 2G Cellular: ~30-70 Kbps. WLANs: ~10 Mbps. Next Generation 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 Mobility 2G3G4G b WLAN 2G Cellular Other Tradeoffs: Rate vs. Coverage Rate vs. Delay Rate vs. Cost Rate vs. Energy 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 Access Network Application Delay Constraints Rate Constraints Energy Constraints Adapt across design layers Reduce uncertainty through scheduling Provide robustness via diversity

Crosslayer Techniques Adaptive techniques Link, MAC, network, and application adaptation Resource management and allocation (power control) Synergies with diversity and scheduling Diversity techniques 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? l What are the key crosslayer design synergies? l How to manage its complexity? l 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 Traffic Generator Data Buffer Source Coder Channel Coder Modulator (Power) Receiver Stringent QoS constraints require that the full dynamics of the system be represented Channel Cross-Layer System Average performance metrics are misleading

The “Myth” of Averages: Minimizing Average Delay What if we constrain delay across all channel gains? Average Delay Channel Gain in dB Delay in milliseconds

Hard Delay Constraints (50 ms) Power in milliwatts Source data rate in bits per second 50ms constraint on delay across all channel gains with power adaptation only Joint source-channel coding power saving 200mW

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. Transmit energy minimized by maximizing bit time Circuit energy consumption increases with bit time Introduces a delay versus energy tradeoff for each bit Short-range networks must consider transmit, circuit, and processing energy. 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 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. Automated highways, factories, and homes

Design Approach Theory Optimization Simulation Testbeds Network Design Optimization Simulation Link Design Device Design Application 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