# Presentation: 198800

## Bits aren't bites: Constructing a "Communications Ether" that can grow and adapt 

- David P. Reed
- Adjunct Professor, MIT Media Lab
- _[dpreed@reed.com](mailto:dpreed@reed.com)_
- March 4, 2003
- Presented at ISART 2003, Boulder, CO

## Agenda

- How can we best meet the demand for wireless communications capacity?
- Does “spectrum” have a capacity?
- “Interference” and information loss
- What’s new?
- Making capacity scale
**Viral network** architectures

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Sustaining vs. Disruptive Technology in a Regulated Industry

- 1950

- 1900

- 2000

- Shannon

- Internet

- DSP

- VLSI

- AdaptiveSystems

- Traditional capabilities

- Novel Capabilities

- Useful Wireless Communications
- Technology

- Cerf, Kahn, et al.

- Marconi

- SS,UWB, DSSS

- PSN

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Mainframe communications vs. viral communications

- Mainframe to PC evolution
- Lower economic barriers to innovative uses
- Enable new computing technologies (sound and video)
- “Mainframe communications” to viral communications
- Lower barriers to innovative uses (802.11)
- Enable new capabilities (sociable devices)

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## The big problem: scalability is starting to matter

- Pervasive computing must be wireless
- Demand for connectivity that changes constantly at all time scales
- Capacity and response time expectations evolve exponentially

- Time

- Capacity,
- Response time

- C

- tr

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## A Viral Network Architecture

- Viral network definition:
- 	each new user preserves or increases capacity and other economic value to existing users, and
- 	benefit to new user increases with scale of existing network
- Examples:
- 	Fax machines
- 	Internet
- 	“Society of Cognitive Radios”

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Does “Spectrum” have a capacity?

- The *radio tradition** *evolved from 1900-1950:
- Resonance provided a means to use multiple radio systems at one time
- As new radio based services were invented, they were given new frequencies
- Some frequencies worked to send messages farther than others.
- Power let you send the same signal farther.

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Shannon’s answer: bits and “channels”

- C = capacity, bits/sec.
- W = bandwidth, Hz.
- P = power, watts
- N0 = noise power, watts/Hz.
- Channel capacity is roughly proportional to bandwidth, and logarithm of power.

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## We don’t know the full answer.

- Sender

- Noise

- Receiver

- +

- “Standard” channel capacity is for one sender, one receiver – says nothing about the most important case: many senders, many receivers.
- “The capacity of multi-terminal systems is a subject studied in multi-user information theory, an area of information theory known for its difficulty, open problems, and sometimes counter-intuitive results.” [Gastpar & Vetterli, 2002]

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Interference and information loss

- ??

- Regulatory interference = damage
- Radio “interference” = *superposition*
- No information is* actually* lost
- Receivers may be confused
- Information loss is a systems design and architectural issue, not a physical inevitability

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Where does “interference” occur, and who causes it?

- When a new radio is added to the system, does it displace capacity? (Does it require new resources not already in use?)
- When a new radio is added to the system, does it impose costs on others, even though there is no displacement of capacity?

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## When a new radio is introduced into the system, does it displace capacity?

- The waves emitted by a new transmitter at a new point in EM space are mathematically *orthogonal* to every other such wave.
- Does the set of receivers in the space provide an adequate basis to recover the original signals?
- Spatial sampling theorem: in most cases, yes.

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## When a new radio is added to the system, does it impose net costs on others, even though there is no displacement of capacity?

- Achievable information capacity
- Increase transmitters per receiver
- 	Increases available rate region with N.
- Increase receivers per transmitter
- 	Increases available rate region with N.
- Equal transmitters and receivers???
- Achievable latency
- “Computational costs”
- Per-node cost of encoding/decoding
- “Evolutionary costs”
- Cost of sharing with legacy designs
- Growth rate

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Partitioning wasteful

- Demand is dynamic
- Bursts capped
- Random addressability & group-forming value severely reduced
- Partitioning in space, frequency, or time wasteful

- Space and
- Frequency
- Division

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Slepian-Wolf

- Frequency partitioning is optimal only when the bandwidth of each band is proportional to its power at each receiver

- A

- Noise

- Receiver

- +

- B

- B

- R2

- A

- R1

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Transport Capacity: One important measure of radio network capacity

- Network of N stations (transmit & receive)
- Scattered in a fixed space
- Each station chooses randomly to send messages to other stations
- What is achievable total *transport capacity, C**T*, in bit-meters/second?

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## “Spectrum capacity” model under static partitioning

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## One example of an architectural improvement: hop-by-hop repeating

- Energy/bit reduced by 1/hops.

- Many paths can operate concurrently.

- What is repeater network’s capacity
-  as radios are added?

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Repeater Network Capacity

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Spatial organization

- Directional antennas provide fixed allocation
- Smart antennas provide dynamic allocation
- A single smart antenna can receive two different signals in two directions at once

- B

- R2

- A

- R1

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Another spatial organization approach: Spatially organized waveforms

- BLAST - diffusive medium & signal processing
- 	(“exploiting multipath”)
- Cellular telephone systems
- MIMO systems
- Cooperative signal regeneration

*G*

*S*

*R*

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Another spatial organization approach: When propagation gets worse, system capacity can go up

- Indoor environments
- Trees
- Hills
- Urban landscapes

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Does adding new radios impose other costs?

- Three ways forward:
- Obsolescence – better systems replace old ones
- Upward compatible evolution – newer systems compensate for old ones
- Upgrade existing systems – existing systems adapt to new ones

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Software Defined and Cognitive Radios

- DSP Generates and Recognizes Waveforms
- Adaptive Control Algorithms
- MEMS/Nanotech “Software Antennas”
- System adaptation and evolution costs drop to near-zero

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## UWB and “VWB”

- Impulse radio uses coded sequences of extremely short high energy pulses to achieve high-rate communications
- Pulses have energy in very wide bandwidths, very low average energy
- Can coexist invisibly with many radio services
- Non-impulse-based “Very Wide” band is more costly, and certainly more legacy-compatible.

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Costs in security, robustness?

- End-to-end encryption can assure private and authenticated communications as needed
- Dynamic and adaptive reconfiguration enhances security against attack, robustness against failure
- Spatial spreading of signals (lower energy, more spatial diversity) helps dramatically

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## A Society of Cognitive Radios

- Viral network definition:
- 	each new user preserves or increases capacity and other economic value to existing users, and
- 	benefit to new user increases with scale of existing network
- Cognitive radios that can cooperate to extract the maximum capacity from the medium, while behaving politely to radio systems with more limited capabilities

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## The Viral Communications PrinciplesVersion 0.2

- Each radio brings its own orthogonal space
- Each radio brings its own computational capacity
- Cooperation allows the combined capacity of all radios to be dynamically allocated, and thus benefits all in available capacity to individuals – “Cooperation gain”
- Disperse communications load widely

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Some research problems in “society of cognitive radios”

- Discovery problem – how does a new radio discover existing “Society” to join
- Internetworking problem – how do two “Viral networks” decide to interconnect, and what framework is used for interconnection?
- “Etiquette” problem – how does a society of cognitive radios know when and how to be polite to legacy radios

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Discovery problem

- Related to the problem of bootstrapping IP connection (DHCP, ...), modem training sequences...
- Discovery happens in the RF –
- 	options include beacons, ..., but how can they be standardized

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## Internetworking problem

- Distinct from wired Internet
- 	Potential low cost gateways “everywhere”
- Tightly coupled with problem of managing coexistence when partitioned.

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003

## What are the technical opportunities and challenges before us?

- To achieve scalability and evolvability, centrally designed/regulated must become self-regulating
- Internetworking creates flexibility of configuration
- Develop coexistence strategies based on scalability, not fixed capacities.
- A Society of Cognitive Radios that can assist each other when appropriate and feasible
- Open architecture to reduces barriers to interconnection and upward compatible evolution
- Expect rapid, technology and demand driven evolution and obsolescence – we don’t know what will be the best technologies, the best architectures, and the dominant applications

- Copyright © 2002, 2003 David P. Reed

- 3/4/2003