Wireless Networks Should Spread Spectrum On Demand Ramki Gummadi (MIT) Joint work with Hari...
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Transcript of Wireless Networks Should Spread Spectrum On Demand Ramki Gummadi (MIT) Joint work with Hari...
Wireless Networks Should Spread Spectrum On
DemandRamki Gummadi (MIT)
Joint work with Hari Balakrishnan
HotNets 2008 2
The problem: Bursty traffic
Demand variability observable even at short (30 s) time scales• From OSDI 2006 traces
• Five APs, three orthogonal channels
• Spatio-temporal demand variations common
Next 30 seconds
First 30 seconds
HotNets 2008 3
Today: Static spectrum allocation
Partitioned into non-interfering channels• Avoid CSMA hidden and exposed terminals
• Avoid back-offs
X
HotNets 2008 4
Insight: Spectrum tracks demand
Spectrum tracking demand achieves higher SINR than shifting demand to where spectrum is
HotNets 2008 5
ODS: On-Demand Spectrum
Demand-based spectrum to nodes Uses spread-spectrum codes Allocates multiple codes to transmitters
• A single transmitter can use entire spectrum
HotNets 2008 6
Key challenge
Avoid inter-AP coordination• Different admin domains
• Demand-communication overhead
X
HotNets 2008 7
Mechanism: Spread-spectrum codes
Data
Code
Signal
Received signal
Copy of receivedsignal
Alice’s code
Bob’s code
Concurrent
HotNets 2008 8
Roadmap
ODS design• Determine demands
• Allocate codes
• Ensure conflict-freedom
• Use multiple codes concurrently
ODS evaluation
HotNets 2008 9
Determining demands
An AP computes demands of its own clients• Averaged over last 30 s
Demand if queue length qi, bit-rate ri
• For uplink, a client tells its queue length to AP
di =qiri
d2=1d1=3
HotNets 2008 10
Allocating codes
Large (128) codebook c of random codes• Same at each AP
AP allocates transmitter codes • Minimizes mean transmission time. (Fairness?)
ith ci =lc diP
i dj
m
c1=96c2=32
HotNets 2008 11
Code assignment
Each AP assigns codes to transmitters from the codebook randomly• No coordination among APs
.
.
.
.
.
.
HotNets 2008 12
Code selection
Each transmitter selects up to k (=11, say) codes from its allocation randomly
With 2 tx, 1 code, no-conflict probability:
With n transmitters, 1 code, If n tx, k codes, conflict-free code number:
Optimum code number as
p= 1¡ kc
p=(1¡ kc)n
¸ =k(1¡ kc)n
¸opt =cne n! 1
The optimum conflict-free code number under random selection within factor e of centralized
The optimum conflict-free code number under random selection within factor e of centralized
HotNets 2008 13
Random code selection performance
High throughput at low contention Non-zero throughput even with 128 interferers
Random selection policy can be both efficient and robust
Random selection policy can be both efficient and robust
HotNets 2008 14
Finding conflict-free codes
Transmitter uses feedback from receiver• Assign success probability p {0,1} per code
• Toggle p based on receiver feedback p=0 at tx whose hashed id closest to code
.
.
.
.
.
.
p=1p=0 p=1
2
id=100id=010
code=101
HotNets 2008 15
Using codes concurrently Divide packet into sub-packets Use one code per sub-packet Transmit all coded sub-packets concurrently
• Packet header tells receiver which codes are used
• Codes in conflict easy to identify at receiver
Packet
HotNets 2008 16
Recap: Avoid inter-AP coordination
Two key mechanisms• Random code selection
Efficient and robust
• Feedback-based conflict detection Decentralized
HotNets 2008 17
Roadmap
ODS design• Determine demands
• Allocate codes
• Ensure conflict-freedom
• Use multiple codes concurrently
ODS evaluation
HotNets 2008 18
Challenge: Data reduction
USRP/GNURadio USB throughput-limited Two steps needed for data reduction
• De-spreading and synchronization
FPGA de-spreads, followed by synchronization Transmitter design similar
Q Convolution Filter
I Convolution FilterRx I/Q Modem
I2+Q2 Peak Detector
Peak I,QSamples
(USB)
PC
FPGA De-spreadingSynchronization
HotNets 2008 19
Preliminary evaluation
0
0.5
1
1.5
2
2.5
3
3.5
4
1
Number of interferers
Link
thr
ough
put
ODS, two bonded 2 Mbps links
No ODS, two bonded 2 Mbps links
ODS improves link throughput by 75%ODS improves link throughput by 75%
HotNets 2008 20
Related work Plain CDMA
• Inefficient spectrum usage with bursty traffic• Sub-optimal
Load-aware spectrum distribution (MSR)• Uses channel-widths instead of codes• Inter-AP coordination (10-minute updates)
CDMA
X
log2(1+P1
P2+N)
log2(1+P2
P1+N)
VWID
TDMAR1
R2(bits/s/Hz)
A
B
log2(1+P1N )
log2(1+P2N )
HotNets 2008 21
Contributions
Exploit bursty demands to improve spectrum usage• Demand-based code allocation
Challenge: Avoid inter-AP coordination• Random code selection• Feedback-based conflict detection
Future work: Better implementation, evaluation• Need high-throughput, low-latency radios