1 free bits the challenge of the wireless internet roy yates rutgers university
Post on 15-Jan-2016
214 views
TRANSCRIPT
1
Free BitsThe Challenge of the
Wireless Internet
Roy Yates
Rutgers University
2
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
3
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
4
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
5
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?
6
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!
7
Infostations
• Network of wireless ports• Discontinuous coverage• Asymmetric link• High data rate transmission• Data services
8
Isolated Infostation
•Optimization Problem:•With finite energy, how many bits can be delivered?
•Solution: Wait for the very good channel, blast away!
9
Highway Infostations
•Distance attenuation time varying channel
•Transmit power profileTime Average Capacity
[Goldsmith]
10
Coverage, Capacity, Delay
• Cellular Infostations
•Infostationsat BS cellsites
•Do infostations really offer more capacity?•At what delay?
11
Traditional Cellular
• Modulation and BER SIR threshold
• SIR Cluster size N (frequency reuse)
• For worst-case location!
12
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!
13
SIR in 2D Infostation system
6
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
15
Which system is better?
2M bps (QPSK) 6M bps (64 QAM)
16
Approximate Queueing Model
• Every Infostation is modeled as an M/M/1 queue with reneging
Reneging: Customersleave before sending all bits
17
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
18
2D Cycle Time
See [Borras & Yates, WCNC 99]
19
Throughput
(r/R)2
64QAM 6.25%16QAM 13.7%
QPSK 31.4%BPSK 43.6%
N=1Reuse
20
Delay
Light LoadSimple Modulation
Wide Coverage
Heavy LoadComplex Modulation
Limited Coverage
21
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!
22
Optimized Locations
Download a map
Internet access
Low bit-rate cellularE-mail
voice mailfax
23
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!
24
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
25
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
!!
26
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
27
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
28
Systems Research
New problems everywhere in the stack!
Freebits
LimitedCoverage
High Bit Rates
NewApplications
•Propagation •Transceiver•Link Layer
•Networks
•Applications