Future Wireless Networks Course Overview Potential Course Topics Broadcast Channels MAC Channels Duplexing FD, TD, and CD

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

Design 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

Broadcast and Multiple Access Channels Broadcast (BC): One Transmitter to Many Receivers. Multiple Access (MAC): Many Transmitters to One Receiver. R1R1 R2R2 R3R3 x h 1 (t) x h 21 (t) x h 3 (t) x h 22 (t)

Wireless Local Area Networks (WLANs) WLANs connect “local” computers (100m range) Breaks data into packets Channel access is shared (random access) Backbone Internet provides best-effort service Poor performance in some apps (e.g. video) Internet Access Point

Wireless LAN Standards b (Older Generation) Standard for 2.4GHz ISM band (80 MHz) Frequency hopped spread spectrum Mbps, 500 ft range a (Newer Generation) Standard for 5GHz NII band (300 MHz) OFDM with time division Mbps, variable range Similar to HiperLAN in Europe g (New Standard) Standard in 2.4 GHz and 5 GHz bands OFDM Speeds up to 54 Mbps New WLAN cards have all three standards

Cellular Systems: Reuse channels to maximize capacity Geographic region divided into cells Frequencies/timeslots/codes reused at spatially-separated locations. Co-channel interference between same color cells. Base stations/MTSOs coordinate handoff and control functions Shrinking cell size increases capacity, as well as networking burden BASE STATION MTSO

Cellular Phone Networks BS MTSO PSTN MTSO BS San Francisco New York Internet

3G Cellular Design: Voice and Data Data is bursty, whereas voice is continuous Typically require different access and routing strategies 3G “widens the data pipe”: 384 Kbps. Standard based on wideband CDMA Packet-based switching for both voice and data 3G cellular struggling in Europe and Asia Evolution of existing systems (2.5G,2.6798G): l GSM+EDGE l IS-95(CDMA)+HDR l 100 Kbps may be enough What is beyond 3G? The trillion dollar question

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

Ad-Hoc Networks Peer-to-peer communications. No backbone infrastructure. Routing can be multihop. Topology is dynamic. Fully connected with different link SINRs

Sensor Networks Energy is the driving constraint Nodes powered by nonrechargeable batteries Data flows to centralized location. Low per-node rates but up to 100,000 nodes. Data highly correlated in time and space. Nodes can cooperate in transmission, reception, compression, and signal processing.

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

Potential Topics Multiuser information theory Access techniques in wireless networks CDMA (interference, MUD, coding/spreading tradeoffs, power control, …) Multiuser multicarrier/OFDM systems Multiuser Ultrawideband (UWB) systems Cellular systems (CDMA/TDMA, adaptive techniques, power control, …) Ad hoc networks (link, MAC, routing, adaptivity, power control, throughput/delay,…) Sensor networks (cooperative signalling, signal processing, etc.) Dynamic resource allocation in wireless networks Energy efficient protocols for wireless networks Cross layer design in wireless networks Space-time processing/MIMO for multiuser systems Multirate/multimedia over wireless networks Opportunistic scheduling and transmission Efficient spectrum utilization via adaptive radios Other topics?

Broadcast Channels Synchronization easy. Interference signals follow same path as desired signal (no near-far problem) Complexity/power at transmitter less restricted than at receiver. Broadcast channel is limiting factor in systems with asymmetric traffic (e.g. web browsing, video/music downloads).

Multiple Access Channels Synchronization overhead required. Interference signals follow different paths than desired signal (near-far problem) Complexity/power at receiver less restricted than at transmitter. MAC channel is limiting factor in cellular systems with symmetric traffic.

Frequency Division Advantages Narrowband channels (no ISI) Low complexity Allows cts. time transmission and channel estimation. Disadvantages Radios must be frequency-agile Handoff complicated by continuous transmission Dedicated channels (idle ones wasted) Difficult to allocate multiple channels per user Total system bandwidth divided into orthogonal channels assigned to different users. FD alone not used in current digital systems

Time Division Advantages No need for frequency agility Discontinuous transmission facilitates handoff and reduces power consumption. Easy to allocate multiple channels/user Disadvantages Synchronization required Multipath destroys slot orthogonality Typically requires ISI mitigation (short timeslots) Idle channels may be wasted Short transmissions make equalization and dynamic resource allocation hard. Time divided into orthogonal slots, with different timeslots assigned to different users. TD used (with frequency hopping) in GSM

Code Division Advantages With semi-orthogonal codes, no hard limit to # of users in system (soft capacity - system is interference-limited) Interference reduction techniques increase capacity Synchronization not required Can allocated multiple “channels”/user using multicode or multirate techniques. No near-far problem on downlink Disadvantages Complexity Multipath creates multiple interferers Orthogonal or semi-orthogonal codes used to modulate each users signal. Code properties used to separate users at the receiver. CD used in IS-95 and 3G

Hybrid Techniques Multiuser OFDM Time and frequency divided into orthogonal slots Different users assigned different orthogonal slots Very flexible technique Usual problems with OFDM (peak-to-average power,…) Multicarrier OFDM OFDM signal modulated with a CDMA code across frequency Users separated via CDMA code properties Semi-orthogonal codes introduce interference Multiuser OFDM used in Flarion system

Duplexing Separation of uplink and downlink traffic Frequency Division Duplexing-FDD: uplink and downlink traffic sent in different frequency bands. No synchronization issues. Uplink and downlink channels may fade indepently. Duplexor required to separate signals Time Division Duplexity- TDD: uplink and downlink traffic sent on different timeslots. Simple duplex equipment requires synchronization need guard bands to prevent overlap transmission in one direction can be used to measure channel in other direction. Flexible bandwidth allocation between uplink and downlink.

Code Division Duplexing Separate uplink and downlink using orthogonal or semiorthogonal codes Semi-orthogonal codes have insurmountable near-far problems. Orthogonal codes destroyed by multipath channel Not used in any existing systems.

Examples AMPS FDMA/FDD GSM (EDGE)TDMA/FDD IS-54 and IS-136TDMA/FDD JDCTDMA/FDD IS-95CDMA/FDD IMT-2000CDMA/FDD 3GWCDMA/FDD bCDMA/FDD a,gOFDM/FDD