Research Challenges in Wireless Communications & Networking

1. Research Challenges in Wireless Communications & Networking D. Raychaudhuri WINLAB, Rutgers University Piscataway, NJ 08854 ray@winlab.rutgers.edu

2. Introduction

3. Wireless Research: Strategic Themes (near-future) Several fundamental problems need to be solved before the “mobile Internet” can take off: • Developing PHY/MAC for broadband radios • ~Kbps Mbps  Gbps, adaptive, robust, QoS,... • Scaling wireless system capacity • widespread service implies ~Gbps/Sq-Km • Designing wireless system-on-chip (SOC) • low-cost/low-power, integrated CMOS • Unifying wireless network architectures (WLAN/IP, 2.5G, 3G cellular) & protocols • multiple radio technologies, faster/simpler standards process • Creating “useful” mobile information services • ...beyond web browsing on hand-held devices

4. Wireless Research: Strategic Themes (long-term) • Pervasive computing via large-scale sensor networks (connecting people with their physical environment) viable in 5-10 yrs • Technical challenges: • self-organizing (ad-hoc) networks • low-power/low-cost/multipurpose wireless sensors • scalable network routing and content distribution • distributed information processing in the network • end-user interfaces & applications • Above topics involve wireless, but are also inherently cross-layer or interdisciplinary...

5. Wireless Product Trends MIMO/OFDM, ATM/IP, Broadband Wireless Access (BWA) Wireless local loop (WLL) BWA/3G combo (local access providers) 3G+ or BWA+?? WCDMA, 3G.PP, etc. Integrated Cellular (3G) Digital Cellular (2/2.5G) OFDM/CDMA, MIMO, diversity, RRM,.. Public WLAN 4G: WLAN/3G/2G (cellular operators) 3G/WLAN IWF, self-org 802.11 OFDM, mob IP, security, QoS,.. Wireless LAN (802.11b) Wireless LAN (802.11x) low- tier 802.11 potentially disruptive technology areas Home LAN Home network sensor nets, etc. (consumer & verticals) WPAN (802.15.3.x) 802.15.3 WPAN, etc. Short-range radio (Bluetooth) UWB, ad-hoc nets >2005 convergence opportunities?? driver technologies 2002-03 2001

6. Wireless Research Challenges: Major Areas • Wireless research topics can be organized into following major categories • radio modems: signal processing and hardware • wireless systems: design and optimization • mobile networks & protocols • Many wireless problems of current importance are cross-layer in nature, so that a holistic approach is essential ....

7. Radio Technology

8. Radio Technology: Research Topics • Selected research topics in the radio/modem area include: • putting radio modems on “Moore’s Law” • signal processing innovations (MIMO, adaptive antennas) • flexible software-defined radios (SDR) • ultra wideband (UWB) • integrated wireless system-on-chip (sensors, etc.)

9. Radio Technology: Moore’s Law applies to wireless! • As computing and communications converge, network BW must follow CPU & memory size…. 802.11a, UWB,.., Gbps Router 802.11bWLAN, 1000 1000 1000 1000 1000 DSL Cable Modem ATM 100 100 100 100 100 CPU Local Access 3G Mobile LAN/WAN Switching Memory Size Wireless Access Local Access CPU Speed 56K modem Wireless short-range radio speeds outpacing Moore’s law over last ~5 yrs! Sw Ethernet 10 10 10 10 10 LAN/WAN CDPD Memory Mhz Kbps Kbps Mbps MB 1 1 1 1 1 1990 1995 2000 Year

10. Radio Technology: Modem Evolution Time/Frequency processing Time/Frequency + spatial processing Multicarrier Modulation (OFDM, etc.) Multiple antenna spatial processing (MIMO, etc.) DVB, 802.11a, etc.. 4G and next-gen WLL ~10-100 Mbps depending on cell size & mobility ~5-10 bps/Hz achievable with QAM QPSK/GMSK Equalized QPSK/QAM/ GMSK,.. Pulsed communication IS-136, etc. US HDTV, WLL, 802.11b UWB Spread Spectrum (CDMA) WPAN and WLAN ~100-500 Mbps no allocated spectrum no RF carrier short-range, high-data rate Wideband CDMA (w/ interference canc. & multiuser det) IS-95 UMTS/IMT-2000 ~2 Mbps depending on cell size ~0.5 bps/Hz typical for proposed systems (works at vehicular mobility speeds)

11. z W Offset w trajectory d Short-range radio channels Example opportunistic transmission scenario: : vehicular user passes by an “Infostation”

12. Short-range radio channel Initial results show that channel is well-behaved for distance ~5m  100’s of Mbps readily achieved with various modem techniques Data from Domazetovic & Greenstein [2001]

13. Radio Technology: UWB Source: J. Foerster, Intel Research, 2001 “sweet spot” for use as nx100 Mbps WPAN UWB appropriate for energy-efficient radio links, typically short-range Also has potential hardware complexity advantages... Pragmatic bit-rate comparison between UWB and 802.11x options

14. Radio Technology: Hardware Innovations • As wireless modems become faster and more ubiquitous, key hardware innovations urgently needed: • compact RF components, including MEMS • mixed signal design & testing • silicon integration and packaging • UWB radio architecture • software-defined radio @ 10-100 Mbps • integrated wireless sensors (low-power)

15. Wireless Systems

16. Wireless Systems:Research Topics • Designing and optimizing wireless systems via radio resource management (power control, interference avoidance, scheduling, etc.) • Selected research topics in the wireless systems area include: • scaling cellular system capacity • scaling ad-hoc network capacity & throughput per user • radio resource management for 3G and ad-hoc nets • interference avoidance • spectrum sharing in unlicensed bands

17. Wireless Systems: Increasing the scale of networks • Rapidly increasing use of untethered data devices implies that wireless access network capacity (bps/sq-Km) will soon have to scale to “gigabit” levels... Growing proportion of all computing devices --> 50% +? Telecom Network Mobile Comm Devices Sensors/ low-tier data Wireless Access Networks Mobile PDA/PIA Semi-mobile Laptop, etc. Internet Example: ~10,000 devices/sq-Km @1 Mbps peak and 0.1 Mbps avg implies system capacity ~Gbps/sq-Km Fixed PC/WS

18. Wireless Systems: Increasing the scale of networks • Consider first the scaling limits of existing and emerging wireless network standards... • 2G cellular/PCS: • cell size ~ 3-5 Km, avail BW ~ 5 Mhz, spectral eff ~ 0.2-0.3 bps/Hz • max capacity ~ 100 Kbps avg, 1 Mbps peak (with packet MAC) per sq-Km • off by 3 orders-of-magnitude! • 3G Cellular/PCS: • cell size ~ 3-5 Km, avail BW ~ 25 Mhz, spectral eff ~ 0.3-0.5 bps/Hz • max capacity ~ 1 Mbps avg, 10 Mbps peak (with packet MAC) per sq-Km • still off by 2 orders-of-magnitude! • Wireless LAN (802.11x, Hiperlan): • cell size ~ 0.1-0.5 Km, avail BW ~ 100 Mhz, spectral eff ~ 0.2-0.3 bps/Hz • max capacity ~ 100 Mbps avg, 1 Gbps peak per sq-Km • correct order-of-magnitude, but too many access points & limited mobility

19. Wireless Systems: Architecture Evolution Gigabit Metro Area Network (w/ integrated mobility support) Location-aware information services, mcast, cache, etc. Standard IP, ATM, etc. Standard IP + M interface Dynamic provisioning/ QoS Mobile/Wired Network GW Regulated spectrum, static freq co-ord High-speed radio hot spot Radio macrocell WAP services. etc. Static provisioning Radio Microcell (~0.5-1 Km radius) Unregulated spectrum, dynamic freq coordination Custom wireless protocol AP/ mini-BTS BTS WPAN Mbps/Km2 Gbps/Km2 2G/3G end-users IP end-users Cellular Macrocell (~5-10 Km radius) Faster radio PHY’s with high interference rejection & bps/Hz efficiency 2G/2.5G/3G radio access (single standard) WLAN+ or “4G” or new radio access (multiple standards) WLAN Microcell (~100m radius) Current Wireless Network Scalable Heterogeneous Pico/Micro/Macrocellular Wireless Network Model IP end-users

20. BS k BS 1 Wireless Systems: RRM Model for Cellular systems Source: Prof. R. Yates, Rutgers U • Multiple cell scenario with desired and interfering signals • Algorithms for allocation of bit-rate, base station, channel, tx schedule, power • Common theme: reduce interference, transmit when the channel is “good”

21. Wireless Systems: RRM in 3G – adaptive incremental redundancy example Source: Dr. L. Razoumov, Rutgers U

22. Wireless Systems: Efficient Spectrum Use • Scaling of wireless services will need new spectrum (~Ghz) particularly for new high-speed data services • Need to rethink traditional approach to spectrum regulation • More unlicensed spectrum (e.g. 5 Ghz U-NII) • Market mechanisms other than one-time spectrum auctions? • Spectrum etiquette procedures for coexistence of QoS-based wireless services (beyond “LBT”) • Incentives for efficient utilization of spectrum resources? • Relationship to property rights?

23. Packet service Channel: #1 ..... #2 Streaming service A #3 .... #4 Streaming service B #5 .... #6 Periodic announcements incl..: Service type, User #, Channel #, service params, Priority, Cost/Price Bids, etc. #N Wireless Systems: Efficient Spectrum Use • Spectrum etiquette procedure a key issue for U-NII scenario • “CSCC” approach proposed as possible solution... • Coordination channel using simple standard protocol at edge of band • Semantics of higher layer coordination protocol TBD... • Support arbitrary spectrum policies based on user priority, cost bids, etc. Common Spectrum Coord Channel (CSCC)

24. A wins contention ( B records & reports transaction!) Service Type User ID Price Bid $.05/hr Wireless Systems: Efficient Spectrum Use Example of CSCC etiquette used for “dynamic pricing” based spectrum allocation: A fn B fn B contends for fn A raises bid on fn channel channel fn fn A B A CSCC Price Bid $.09/hr Price Bid $.07/hr …e-cash exchange ?

25. Mobile Networks

26. Mobile Networks: Some Research Topics • Selected research topics in the mobile networks area include: • new MAC protocols: 802.11x, 803.15.x, sensor nets • “4G” network architectures • mobility protocols: beyond mobile IP • new architectures (WLAN hot-spots, Infostations, ..) • self-organizing wireless networks (sensors, etc.) • ad-hoc network routing • multicasting and mobile content delivery • wireless network security

27. Mobile Networks: “4G” Protocol Evolution 2.5G/3G Services 4G Services PSTN IP uniform service API (Internet+) GSM/ GPRS 3G Access Network service feature modules Security QoS VPN Content Delivery 2.5G/3G Radio generic network API WLAN Services Unified IP-based mobile network Low-tier services Mobile Service Middleware incl support for multihop, mcast, etc, Generic Radio Access Network IP IP uniform radio API’s WPAN network layer (e.g. Bluetooth) Ethernet 3G/4G Radio WLAN radio WPAN/low- tier radio WPAN radio 802.11 Radio Radio-specific vertically integrated systems with complex intetworking gateways Radio Independent modular system architecture for heterogeneous networks The Future Today’s Wireless Systems

28. Mobile Networks: Protocols beyond mobile IP Global Internet Mobile IP overlay network • Mobile IP provides a permanent IP address • for users moving between wireless AP’s • Desired RAN features for ad-hoc WLAN, • sensor nets, 4G: • handoff support (micro-mobility) • discovery and self-organization • ad-hoc routing, integrated with MAC • peer-to-peer modes • multicast, QoS, security, etc. •  closer layer 2/3 coupling needed access point radio bridge/ router (forwarding node) Radio Access Network 1 IP extensions or generalized L2 MAC??

29. Mobile Networks: 3G/WLAN interworking Cellular/2.5G,3G Unified Mgmt Layer Bluetooth UWB, Bluetooth<-> 3G IWF net link PHY IWF1 IWF2 WLAN 3G BT Protocol stacks Bluetooth<->WLAN IWF Techniques for seamless service: - Authentication, global roaming - Security issues - Dynamic handoff - End-to-end QoS control - Network management - Service level agreements WLAN, HiperLAN, UWB, WLAN<->3G IWF 3G/WLAN interworking Multiple devices with various radio interfaces

30. Mobile Networks: Hot-Spot MAC • Mobile user passes through hot-spot (Infostation) in sec during which ~MB files are downloaded/uploaded • Requires modifications to conventional WLAN MAC, incl fast synch, pre-authentication, etc. • Motivates 2-tier arch with ~10m service zone (for high-speed data transfer) and ~50m access control zone Infostations access point Data cache Low-speed control channel (for synch & service setup) ~100 MB/s Fast transfer Service Zone Access Control Zone Transit time ~sec Total transit time ~10sec

31. Mobile Networks: Hot-Spot MAC • 802.11a MAC can be used for opportunistic service • Pre-authenticate user in low-bit rate mode (~50m range) • Mobile terminal waits for modem to reach max 54 Mbps (~10m range) • High priority access mode used for Infostations access normal channel activity AP Beacon Infostations file transfer* .. A1 ........ A3 PIFS IS Control packet A2 PIFS IS transfer request* ACK Mobile requests advance authentication Authentication message exchange Terminal enters max PHY speed zone Terminal enters WLAN coverage area Priority Access initiated *RTS/CTS msgs not shown time

32. Mobile Networks: UWB Sensors • UWB potentially well-suited for sensor networks • Bit-rate readily traded off against range • Energy efficient modulation • Robust to interference • Multiple radio links supported by single UWB RF • Low cost silicon for integrated sensor device UWB (R23, code 23) UWB (R12, code 12) S2 S1 UWB (R13, code 13) S3

33. Mobile Networks: UWB Sensor MAC • Potential MAC/link layer based on DS/CDMA UWB PHY: • Continuous beacon for synchronization & sensor ID broadcast • Low bit-rate, high-spreading gain common link establishment channel with a single code used in random access mode • Handshake protocol for setting achievable link bit-rate with dedicated code Beacon S1 Beacon S2 S1 S2 Link establishment signal (S1,S2, C12) Link ACK (S1,S2, C12) S1 S2 Rate adaptation, ARQ Common code Control Code B Code A

34. Mobile Networks: Ad-hoc Networks • Ad-hoc network ideas proposed for tactical and sensor scenarios, with potential applications to WLAN/4G: • flat network model with multi-hop routing radios • on-demand routing protocols (DSR, AODV, etc.) designed for high node mobility (...fairly mature topic) • enhancements via MAC clustering, energy-efficient routing, .. • application-level data aggregation (diffusion routing, XML,..) • geographically constrained routing • Active problem areas: • Scaling of capacity • Dynamic behavior • Energy efficiency • MAC/routing interactions • QoS routing • Geo routing • Security of ad-hoc nodes • Integration with WLAN, etc. SN radio links for multi-hop routing MAC cluster (optional)

35. Mobile Networks: Hierarchical Ad-Hoc Net • Hierarchical, self-organizing network currently under consideration, based on: • 3 service tiers (cellular, WLAN, personal area) • BS’s, AP’s, FN’s (forwarding radio nodes), user devices • automatic discovery and power mgmt protocols • hierarchical, ad-hoc multihop routing and spatial MAC Internet BTS AP Access Point WLAN micro-cell Forwarding node FN 3G cell personal-area pico-cell low-tier (e.g. sensor) user nodes

36. Mobile Networks: Higher Layers • Research issues which arise in connection with information delivery over wireless nets: • Qos with heterogeneous & time-varying radios • transport layer problems (TCP timeouts, etc.) • need for services such as reliable multicast • information “pull” model vs. multicasting model • opportunistic services (hot-spots, caching,..) • delivery of the “right information” at the “right time and place” (location/content aware) • media scaling to match radio and terminal capabilities • sensor network & pervasive computing software models

37. User XML Descriptor Mobile Networks: Content Multicast • New real-time, context- and location-aware information delivery paradigms under consideration ... • Content multicasting based on XML investigated as possible option for delivering relevant info to mobiles. Interest profile SX Semantic Router B SX Mobile interest profile contains: (user, location, terminal capability,..) Semantic content multicast Router A Content Provider

38. Mobile Networks: Experimental Research • A flexible, open-architecture mobile/ad-hoc sensor network testbed recently established at WINLAB • open-source Linux routers and AP’s (commercial hardware) • Linux and embedded OS forwarding and sensor nodes (custom) • radio link and global network monitoring/visualization tools 802.11b PDA Management stations Radio Monitor 802.11b Linux PC Forwarding Node/AP (custom) AP Commercial 802.11 Router network with arbirtrary topology Compute & storage servers Sensor Node (custom) PC-based Linux router

39. Wireless Research: Multidisciplinary Research Topics • In conclusion, we mention some wireless-related multidisciplinary research topics: • spectrum regulation principles (...economics, policy) • integrated wireless sensors (...materials, semiconductor) • software models for pervasive computing (..CE, CS) • dynamics of large-scale ad-hoc sensor nets (...math, control) • security in ad-hoc sensor networks (...CS) • new applications of sensors: environmental, medical, public safety, etc. (..CS, domain experts from various disciplines) • robotics (..mechanical, controls)