All Rights Reserved © Alcatel-Lucent 2006, #####
Design Issues for Wireless Networks Across Diverse and Fragmented Spectrum
Collaborators:Bell Labs India: Supratim Deb, Kanthi NagarajBell Labs USA: Piyush Gupta
All Rights Reserved © Alcatel-Lucent 20092 | Connecting the Next Billion 2010 |
Mobile Data Explosion Will Result in Diverse & Fragmented Spectrum
Examples: AT&T Spectrum in New York – 700MHz band, 800MHz, 1.7GHz and 2.1GHz Unlicensed Spectrum in US – DTV Whitespaces (500-700MHz), 2.4GHz and 5.1GHz
All Rights Reserved © Alcatel-Lucent 20093 | Connecting the Next Billion 2010 |
Research Goal
PHY + Sensing
MAC + Spectrum
Selection
Routing + Flow
Control
Design a network stack that operates across
1) Fragmented and spatially varying spectrum with diverse propagation
2) Devices with different tunable center freq. and b/w range
New Design Perspective
Lot of research
All Rights Reserved © Alcatel-Lucent 20094 | Connecting the Next Billion 2010 |
Designing MAC and above: Unique Challenges Devices can tune frequency and
bandwidth Non-channelized system complex MAC
No fixed interference graph Frequency dependent propagation Complicates spectrum allocation and MAC
design
No fixed communication graph Next-hop (hence, routing) depends on
frequency
Leakage power at a distance depends on frequency Adjacent channel interference (hence, guard
band) varies with frequency
Cross layer optimization is highly complex
log fc
Rece
ived P
ow
er
at
fixed d
ista
nce
, pow
er
(dB
)
slope = - 20
Pow
er
Sp
ectr
al
Den
sit
y
All traditional network stack design issues require a fresh look.
All Rights Reserved © Alcatel-Lucent 20095 | Connecting the Next Billion 2010 |
Interference Management
Standard Approach: Measurement based interference map
But, if the spectrum is diverse … Two devices may interfere only in certain frequency bands
Design Principle: Generate different interference maps for different bands
Ideally a single/small number of control channels Can use measurements over a single control channel
Pr(f1) - Pr(f2) = 20 log(f2/f1)
Interference maps in a higher freq. band can be deduced from interference map in a lower band
(and knowledge of ambient interference)
All Rights Reserved © Alcatel-Lucent 2006, #####
Agile and Efficient MAC forUnlicensed Access in TV
Whitespaces and ISM Bands
All Rights Reserved © Alcatel-Lucent 20097 | Connecting the Next Billion 2010 |
Gist of FCC Mandate
Usable Free Spectrum: Unused TV channels between 500-698 MHz for unlicensed portable access Varies from city to city
Limit Interference to DTV receiver: Tx power: 16 dBm in adjacent band, 20 dBm elsewhere
Out of Band Emission: Has to fall by 55 dB in the adjacent 6 MHz band Spectrum Mask
All Rights Reserved © Alcatel-Lucent 20098 | Connecting the Next Billion 2010 |
Single Link Capacity With ACI Constraints Fundamental Question: What’s the optimal transmit power anyway?
High power-> lower bandwidth Guard band required to prevent interference to the TV channel
Low power -> reduced system capacity
Observations:
• Optimal capacity is independent on center frequency
• For FCC’s spectrum mask, the power cap is not too sub-optimal
Design Implications:
• Choose fixed power (marginal loss in capacity)
• Ensure a minimum bandwidth (leakage depends on PSD)
20 dBm
DTV
Chann
el
DTV
Chann
el
Freq
16 dBm
DTV
Chann
el
DTV
Chann
el
Freq
All Rights Reserved © Alcatel-Lucent 20099 | Connecting the Next Billion 2010 |
MAC Design Considerations
Fragmented spectrum + limitations on maximum tunable bandwidth of a radio implies
Multi radio AP, single radio client (for cost considerations)
Need to account for frequency dependent propagation
Resilient to disruptions E.g. wireless microphones
Evolution over WiFi Easy adoption path, quick time to
market, can interoperate with ISM bands
IEEE 802.11af standard already in progress
All Rights Reserved © Alcatel-Lucent 200910 | Connecting the Next Billion 2010 |
Joint Spectrum Selection and Client Allocation
1 2 3 4 5 6 7
(1,2,3,4,5,6,7)
(1,2,3,4,5,6,7)(1,2,3,4,5,6,7)
(1,2,3,4,5,6,7)
Joint Spectrum Selection and Client Assignment is a non-trivial problem
All Rights Reserved © Alcatel-Lucent 200911 | Connecting the Next Billion 2010 |
Capturing the Utility of a Band
Important Observation: For most technologies, data
rate/Hz depends on SNR as
Data Rate / Hz = a × (SNR in dB) - b
Client-1
Client-2 ASE = Data rate averaged over all clients / Hz
= a × (SNR averaged over all clients in dB) – bSNR(f1) - SNR(f2) = 20 log(f2/f1)ASE in f1 can be generated for ASE in f2
Design Implication: Two step ASE generation: Each AP generates ASE in control channel band. Use frequency dependence and ambient interference
measurements to compute ASE in all bands.
All Rights Reserved © Alcatel-Lucent 200912 | Connecting the Next Billion 2010 |
Proportional Fairness is Key for Whitespaces
6Mbps
2.4GHz
6Mbps600MHz
More likely that far
away client will bring
down performance
of system
Very simple design with
minimal information overhead on
top of 802.11 ensures
proportional fairness
All Rights Reserved © Alcatel-Lucent 200913 | Connecting the Next Billion 2010 |
Simulation Results
Comparison scheme White space selection algorithm is
optimal Number of clients assigned per
band is proportional to number of clients
60-90% improvement in total throughput
Proportionally fair
Frequency agnostic schemes do not work well!
All Rights Reserved © Alcatel-Lucent 200914 | Connecting the Next Billion 2010 |
Making it Work in Practice
Designed frequency translator with < 2 microsecond switching delay
Dynamic range of 100- 900 MHz
Integrates with a WiFi card on Sokeris box
Extensive indoor trials done
Waiting for experimental license for outdoor trials
Top Related