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Free BitsThe Challenge of the

Wireless Internet

Roy Yates

Rutgers University

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Wireless Data over 2G Cellular

• Cellular Voice:– Anytime Anywhere– Slow: 10K bps– High cost/bit

v cents/min voice = 13v cents/MB

• Data over Cellular:– Slow and Expensive:

20 cents/min voice = $2.60/MB

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Wireless Data over 3G Cellular

• Higher speed: 144K or 384K bps• 2X or 3X increase in BW efficiency

– Reverse link pilots– Wideband signal processing– Better multiplexing, adaptive modulation

• 2X or 3X reduced cost/bitBut voice is still 10K bps

v cents/min = 13v cents/MB

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Cellular = Expensive Bits

• v=1 cent/min = 13 cents/MB• Fill an MP3 player: 30 MB = $3.90

• Upload digital camera photos – 32 MB = $4.16

• Sync laptop disk with desktop – 50 MB = $6.50

• 144 kbps Internet access on Amtrak – 75K bps average for 3 hours = $13.16

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Internet Access = Free Bits

• Internet: Applications and attitudes are based on “free” bits

• Conventional radio access is a toll booth

• Radio bits need to be “free”

How do we deliver free bits over radio?

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ANYTIME ANYWHERE costs $…but DATA can tolerate

delay! • Information services: e-mail, voice

mail, fax, maps, non-interactive web pages, ...

• We don’t need ubiquitous coverage• Reduced coverage Higher data

rates• A little wait might be worthwhile!

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Infostations

• Network of wireless ports• Discontinuous coverage• Asymmetric link• High data rate transmission• Data services

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Isolated Infostation

•Optimization Problem:•With finite energy, how many bits can be delivered?

•Solution: Wait for the very good channel, blast away!

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Highway Infostations

•Distance attenuation time varying channel

•Transmit power profileTime Average Capacity

[Goldsmith]

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Coverage, Capacity, Delay

• Cellular Infostations

•Infostationsat BS cellsites

•Do infostations really offer more capacity?•At what delay?

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Traditional Cellular

• Modulation and BER SIR threshold

• SIR Cluster size N (frequency reuse)

• For worst-case location!

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Cell area Coverage area

• No need for ubiquitous coverage• Better Radio Channel

– Higher received power – Less interference

• Smaller cluster size• More bandwidth availableHIGHER DATA RATE!

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SIR in 2D Infostation system

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1

SIR

i

n

i

n

Dd

P

rd

P

n

r

RN

3

6

1SIR

Rr

D 1

D 3

D 6

D 4

D 2

D 5

14 Possible operating points

SIR with respect to r/R and N

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Which system is better?

2M bps (QPSK) 6M bps (64 QAM)

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Approximate Queueing Model

• Every Infostation is modeled as an M/M/1 queue with reneging

Reneging: Customersleave before sending all bits

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Queue Parameters

• User density, u• Cell radius, R• Mobile speed, v,

fV(v)• Coverage radius, r• Data rate, c• Messages per user

per second, u

• Message size, m

= (2r)(u/2) E[V]

• 1/ = E[X] = = u tc (m/c) = 2E[V]/r

Rr

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2D Cycle Time

See [Borras & Yates, WCNC 99]

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Throughput

(r/R)2

64QAM 6.25%16QAM 13.7%

QPSK 31.4%BPSK 43.6%

N=1Reuse

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Delay

Light LoadSimple Modulation

Wide Coverage

Heavy LoadComplex Modulation

Limited Coverage

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Infostations beyond Cellular

• Cellsites aren’t ideal for Infostations– Cellsites are chosen for widest coverage

• On hills, high towers

• Infostations exploit being really close• Clever (convenient) locations can

really help!

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Optimized Locations

Download a map

Internet access

Low bit-rate cellularE-mail

voice mailfax

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Cheap Bits, High Rates!

• No coverage requirements– Lots of interference range reduced.– Range may be zero in places

• Need a sufficiency of good locations • No need for licensed spectrum

– Use 100 MHz U-NII bands

• Short range wideband communications– VERY High Rates!

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Infostation Applications

• Upload digital camera photos– Walking by photo kiosk– 32 MB in 3 sec = 80 Mb/s

• Internet access on Amtrak – Infostation every 500 feet– 25 ft coverage radius at 400 Mb/s– 40 Mb/s average capacity

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Infostation Applications

• Sync laptop disk with desktop – At the airport in the X-ray

machine– 50 MB in 1 sec = 400 Mb/s

• Fill an MP3 player – While passing thru doorway– 30 MB in 2 sec = 240 Mb/s

Very high data rates

!!

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High Rates = New Applications

• Data Sprinklers (or hoses) [Badrinath]– Download a local information database

• Images of everything local– Where can I find …… ?– What’s that building?– Where am I?

• If bits are fast and free, lots of content can go unused

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Research Problems

• Network problems– Protocols for pre-fetching– Coordinated Data Delivery

• Radio Problems– 500 Mb/s in a handset!– Channel estimation

• Finding the coverage sweet spot

– Coverage/channel shaping

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Systems Research

New problems everywhere in the stack!

Freebits

LimitedCoverage

High Bit Rates

NewApplications

•Propagation •Transceiver•Link Layer

•Networks

•Applications