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

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.

.

.

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.

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

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.

.

.

.

.

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