Free Bits
The Challenge of the
Wireless Internet
Roy Yates
Rutgers University
1
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
2
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/bit
But voice is still 10K bps
 v cents/min = 13v cents/MB
3
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
4
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?
5
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!
6
Infostations
•
•
•
•
•
Network of wireless ports
Discontinuous coverage
Asymmetric link
High data rate transmission
Data services
7
Isolated Infostation
•Optimization Problem:
•With finite energy, how
many bits can be delivered?
•Solution: Wait for the very
good channel, blast away!
8
Highway Infostations
•Distance attenuation
 time varying channel
•Transmit power profile
Time Average Capacity
[Goldsmith]
9
Coverage, Capacity, Delay
• Cellular Infostations
•Infostations
at BS cellsites
•Do infostations really
offer more capacity?
•At what delay?
10
Traditional Cellular
• Modulation and BER  SIR threshold
• SIR  Cluster size N
(frequency reuse)
• For worst-case
location!
11
Cell area
 Coverage area
• No need for ubiquitous coverage
• Better Radio Channel
– Higher received power
– Less interference
• Smaller cluster size
• More bandwidth available
HIGHER DATA RATE!
12
SIR in 2D Infostation system
n
d
P 
r

SIR 
n
6
 d 
P  

i 1
 Di 
1  3N R 

SIR  
6  r 
n
D1
D2
D6
R
D3
r
D5
D4
13
SIR with respect to r/R and N
Possible operating points
14
Which system is better?
2M bps (QPSK)
6M bps (64 QAM)
15
Approximate Queueing Model
• Every Infostation is modeled as an
M/M/1 queue with reneging



Reneging: Customers
leave before sending
all bits
16
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
•  = (2p r)(u/2) E[V]
• 1/  = E[X] =
= u tc (m/c)
  = 2E[V]/p r
q
f
R
r
17
2D Cycle Time
See [Borras & Yates, WCNC 99]
18
Throughput
N=1
Reuse
(r/R)2
64QAM 6.25%
16QAM 13.7%
QPSK 31.4%
BPSK 43.6%
19
Delay
Light Load
Simple Modulation
Wide Coverage
Heavy Load
Complex Modulation
Limited Coverage20
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!
21
Optimized Locations
Download
a map
E-mail
voice mail
fax
Low bit-rate
cellular
Internet access
22
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!
23
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
24
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!!
25
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
26
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
27
Systems Research
New problems everywhere in the stack!
Free
bits
Limited
Coverage
High Bit
Rates
New
Applications
•Propagation
•Transceiver
•Link Layer
•Networks
•Applications
28