the hierarchy of wireless data networking (an rf view) dan dobkin and titus wandinger wic 2002

The Hierarchy of Wireless Data Networking

(An RF View)

Dan Dobkin and Titus WandingerWIC 2002

The Communications EdgeTM

OverviewOverview

• Performance of wireless links is limited by fundamental and practical constraints:– “beam” capacity BW*log(S/N) [Shannon]– Geographic reuse and interference management– Watts and the cost to deliver them

• Different technologies are optimal for different needs– UWB for short range ultrahigh rate– WLAN for moderate range high rate– WAN for long range low rate– SatCom for coverage

• Each technology has a different set of problems• Ideal future network will combine where

applicable


UltrawidebandUltrawideband• UWB is just spread-spectrum with an attitude

– Transmit signals spread across several GHz of bandwidth

• Right on top of existing licensed and unlicensed users

– Keep power low enough to avoid excessive interference to other users of the same spectrum


Ultrawideband Performance Limits

Ultrawideband Performance Limits

• UWB range limited to about 2x(distance to nearest non-participating radio) for high datarates– Note that at low data rate you get more processing gain

• 100 Kbps = 45 dB => much longer range

• CONCLUDE: UWB is ideal for short distances and high datarates; low power = low cost (someday)


Ultrawideband: RF Challenges

Ultrawideband: RF Challenges

• Interference:– UWB “stomps on” users in many bands– FCC constrained operation to >3 GHz to avoid

interference with GPS and air-traffic-control radar– Adaptive operation may enable higher power

operation => greater range at high datarate• Is this desirable?

– Dithering of pulsed signals necessary to avoid narrow band radiation at multiples of (1/pulse rate)

• UWB self-interference– Code-division multiplexing is also versatile solution

for high processing gain systems

• Decoding– Power needed to encode / decode the signal remains

large even if RF power is very small• Ex: Xtreme Networks chipset 200 mW for < 1 mW RF


Wireless LANWireless LAN• WLAN = Wireless Ethernet

– Data-only in unlicensed bands (2.4 or 5.x GHz); nominally spread-spectrum [but not really]

– Range limited not by technology but (mostly) by regulatory limits on transmission power

• COMMERCIAL EQUIPMENT TODAY:– 6-30 Mbps TCP thruput, 50 meter indoor range– VERY low cost due to simplicity, high volume, low power

– Conclude: WLAN provides high data rates over moderate distances at low capital cost


WLAN RF ChallengesWLAN RF Challenges

• Partitioning / coverage:– RF propagation in buildings is complex– Roaming adaptation in 802.11 networks is limited– Installation should be simple and inexpensive– Constraints are not consistent!

• Interference:– Multiple users (esp. in a building) may interfere– 802.11 (CSMA/CA) is robust to interference but rate

drops– Smart antennas would help:

• Most transmissions from MS are directed to a particular AP

• Some transmissions from AP are directed to a particular MS

• In each case an adaptive antenna will deliver more signal and less interference with less power consumed


WAN: “cellular” communicationsWAN: “cellular” communications

• Voice network modified for data communications– EXAMPLE: Qualcomm’s CDMA2000-EVDO

• 1.25 MHz channel devoted to data only; forward link is time-domain multiplexed, reverse link is code domain multiplexed

• Forward link capacity 2.5 Mbps/basestn for slow-moving terminals; reverse link capacity about 0.5 Mbps

• Typical range 1 km at high rate (adaptive)

– Sophisticated handoff, authentication, billing– High capital cost– Key problem is economic not technical:

• Low-revenue-per-bit data displaces high-revenue-per-bit voice

• Conclude: good coverage, low data rate, high capital cost


WAN RF ChallengesWAN RF Challenges

• Power consumption:– Large range achieved by use of high transmit power– High data capacity achieved by use of advanced

modulations• Stringent requirements on linearity => serious backoff

from maximum output power

– Result: use 1 KW to transmit 100 W RF• High hardware cost, high operating cost, cooling

challenge, size challenge


Satellite CommunicationsSatellite Communications• Various system choices trading off # of satellites, radio

complexity, system complexity– Geosynchronous: a few satellites a LONG way away– Low earth orbit: 1000 km orbits, 30-80 satellites

• Coverage is wonderful– Except inside any structure!

• Problem: capacity where you need it– Aggregate capacity of network several Gbps– HOWEVER: capacity/km2 limited; can’t sell service to densely

populated areas (where people who could pay for the cost live)

• To deliver 1 Mbps/square km from 1500 km up we need 10 Gbps and 1 degree beamwidth (1.5 meter antenna, can be built)

• Covers 10,000 square km; similar coverage 3000 small bstn, $140M capital cost

• BUT cellular basestations are granular: low entry cost, adjustable to localized demand, reconfigurable network

• Satellite universal communications is a proper sphere for government action: serves legitimate social need but not a viable business if self-funded


SatCom RF ChallengesSatCom RF Challenges

• Receiver:– Low-cost, high-sensitivity mm-wave receive easily

achieved with outdoor directional antenna pointed at station

– Not so easy with omnidirectional antenna:• Omni collection area goes as square of wavelength• At = 2.5 cm (12 GHz), 24 dB penalty vs. 900 MHz

cellphone with dipole – Assume 10 W at satellite 1500 km up, 1 degree beamwidth– Power delivered per cm: -99 dBm– Omni collection area 0.132 = 0.8 cm2; power collected =-

100 dBm– Noise floor at 1 MHz bandwidth =-114 dBm; if we need 5

dB (S/N) we have 9 dB margin

• Conclude: system may work outdoors but probably not indoors

– Can fix by using large processing gain but wastes bandwidth


End-result: heterogeneous network

End-result: heterogeneous network

• No one approach can or should support all requirements– UWB: in-home and short-range high-rate

• Standalone • Integrate with fiber backhaul for ultrahigh datarate future network

– WLAN: general-purpose high data rate in areas of high user density

• Standalone• Integrate public hotspots with WAN for low-cost high-rate data

– WAN: voice, essential / low rate data• If you can use a handset you’re fine

– SatCom: only the lonely (really need it)• Integration with WAN not economically justified• Universal connectivity is a public good, justifies public investment

• The real need (and opportunity) is interworking software so the user doesn’t know or care how the data is shipped– Loose integration via Internet is adequate for most purposes

the hierarchy of wireless data networking (an rf view) dan dobkin and titus wandinger wic 2002

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