# Presentation: 102928

## 2005 ISART Tutorial

**Institute for Telecommunication Sciences – Boulder, Colorado**

**Flexible-Use Spectrum Rights**
- Robert J. Matheson
- NTIA/ITS.M
- _[matheson@its.bldrdoc.gov](mailto:matheson@its.bldrdoc.gov)_
- Does not necessarily represent official polices of NTIA

## Spectrum Management

- 	Who gets to use spectrum?
- 	For what functions?
- 	Under what restrictions?
- 	What are the processes to decide?

**Institute for Telecommunication Sciences – Boulder, Colorado**

## Outline

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- 1. The Physical World /Engineering
-  Electrospace
-  Receivers
-  Interference
-  	 - - -  10-minute break - - -
- 2. Spectrum Management
-  General comments
-  Command-and-control
- 3. Flexible Use Rights
-  Pure electrospace rules (ideal receivers)
-  Modified electrospace rules (real receivers)
-  Extended rules for aggregation and division
-  Miscellaneous loose ends

## Radio System Basics

- Electrospace
- Receivers

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## Radio System Basics - 1

- 	Transmitter (including transmitting ant)
- 	Propagation path
- 	Receiver (including receiving antenna)

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- Sensing –
- Compare received signal with transmitted signal to study path (radars, sensors, etc.)

- Communications –
- move data from transmitter to receiver.

## Radio System Basics - 2

-   A receiver processes the electrospace to receive communications.
-   One major goal of frequency management is to arrange the signals in the electrospace so that the desired signal can be separated out by a simple and inexpensive receiver.

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## The Electrospace

- Physical location – lat., long., altitude       3-dim
- Frequency – MHz                                      1-dim
- Time - μS, hours, or years		        1-dim
- Direction-of-arrival – azimuth, elevation    2-dim
- Electrospace describes radio signals, as they appear from an ideal receiver at the physical location.  Note: receiver includes receiving antennas.

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## Spatial Dimensions

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- Spatial Dimensions

## Frequency Dimension

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## Time Dimension

- 	Months – seasonal uses
- 	Hours – to broadcast special football game
- 	Hours – midnight-to-5am daily to send 		       low cost computer updates.
- 	10 ms TDMA timeslots every 50 ms.
- Useful for serving many intermittent activities.

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## Angle-of-Arrival Dimension

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- 1. Angle-of-arrival is different from coverage area.  Consider coverage areas of 4 xmtrs.

- 4. Receiver antenna beamwidth counts, not transmitter beamwidth.

## Angle of arrival - 2 

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- Angle of arrival - 2

- 5. Future application of
- generalized angle-of-
- arrival, using less-
- expensive techniques

- 4. MIMO receiver forms multiple ortho-
-      gonal vector sums to mathematically
-                             generate paths (easier).

- 3.  NB receiver antenna isolates each path (difficult).

- 2. Multiple narrowbeam (NB) paths scattered from landscape.

## The Electrospace

-  Physical location – lat., long., altitude       3-dim
-  Frequency – MHz                                      1-dim
-  Time - μS, hours, or years		         1-dim
-  Direction-of-arrival – azimuth, elevation    2-dim
- .
- A given “region” of electrospace is considered “occupied” if the field strength exceeds “X” W/MHz/m2.

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## Same spectrum, more users

-   Point-to-point microwave uses shaped pencil beam coverage areas and divides up angle-of-arrival.
-   Trunked radio systems adaptively divide time.
-   MIMO mathematically finds multiple paths by processing angle-of-arrival part of electrospace.
-   Advanced cognitive radios will be even better at dividing up the electrospace.

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## Radio Systems

** ****Receivers** process the electrospace to give service (if successful) or interference (if not successful).
-  A **sufficiently good** receiver can separate any signals having different electrospace descriptions. Interference is caused only when a receiver is not “good-enough”. Might require adaptive antennas to null interference.
-   “Interference protection” really means “able to use a cheaper receiver.”

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## Interference - 1

- Interference – any distortion of the processed desired signal caused by unwanted extraneous radio signals (excludes multipath, internal noise).
- Interference can be caused by co-channel operation, excessive sidebands, intermodulation in receiver, receiver overload, etc.     Note: not by lack of signal.
- No sharp line between acceptable and unacceptable interference.  All interference is unwanted. Even the possibility of interference is unwanted, since it requires more robust system design.

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## Interference - 2

-   Externalized cost is either cost of interference, or cost of better receivers to prevent interference.  Interference rules establish expected receiver capabilities.
-   A faulty receiver causes interference only to receiver user, who is well-motivated to fix receiver.  No external controls are needed.

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## Interference – 3

- Interference is **always** caused by an inadequate receiver and could be fixed by a “good-enough” receiver (though “good-enough” for some situations might require adaptive antennas to null out interference, or other very complex and expensive components).
- Using better receivers would decrease interference, and/or allow more signals to be transmitted before interference occurred.
- Therefore, using better receivers is always expected to improve spectrum efficiency.

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## Better Receivers

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-  Better IF filters to remove signals on adjacent channels
-  Better dynamic range to reduce IM and overload from strong 	signals
-  Better directional antennas to isolate desired signal from 	others
-  Better intelligence to adaptively re-tune to less crowded frequency
-  Better intelligence to change modulation and power
-  Etc.   e.g.,  cognitive radios

## Receiver standards?

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- “A major goal of spectrum management is to make the world safe for cheaper receivers.”

-  A major objective of good spectrum management is to develop rules that allow the use of cheap equipment (receivers),  e.g. :
- 	Keep low power and high power bands separated,
- 	Use large duplex band structures,
- 	Limit maximum transmitter power, etc.
- Such features are intended to allow the use of cheaper receivers.

- Yes, but ...

## Receiver standards

**Procurement** of equipment often simplified by referring to receiver procurement standards.
- Receiver standards valuable to **educate user** about typical receiver performance requirements.

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- None of above primarily concern spectrum management.

## Receiver Summary

-   All interference is caused when receiver performance is not sufficient for the given electrospace environment.  Better receivers can improve spectrum efficiency, at a cost.  Poor spectrum management rules may make it necessary to use better receivers.
-   The receiver user is usually well-motivated to get a “good-enough” receiver.

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## - - - 5-minute break - - -

- 5-minute break
- Next: Regulatory Concepts and flexible-use rules

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## Map of Spectrum Rights

-    Horizontal axis shows decision-making rights, from all-Govt to all-user.
-    Vertical axis shows how much a frequency is preferentially given to  specific users versus general users.
-    Many areas of model are currently in widespread productive use.

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-    Traditional
- licenses (C&C)

- Auctioned
-    bands

- ISM bands

- General users (Commons)

- Specific Users (exclusive licenses)

-  Cordless phones, mesh networks (?)

- Flexible-use rights

-        Part 15
- Spread spectrum

- Amateurs

- Govt. Regulations

- User Decisions

## Map of Spectrum Rights

- Licensed systems with high-power transmitters (traditional C&C)
- Non-licensed systems with low-power transmitters (Part 15)

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- Govt. Regulations

- User Decisions

-    Traditional
- licenses (C&C)

- Auctioned
-    bands

- ISM bands

- General users (Commons)

- Specific Users (licenses)

-      FRS, CB, Cordless phones

- Spectrum use rights

-        Part 15
- Spread spectrum

- Amateurs

**Greater uncertainty**
**Lower transmit power**

## Complementary Concepts

-   Will always be a requirement for multiple spectrum regulatory environments, because of different system technical and business requirements.
-   Real estate analogy -  different kinds of property with different rules.
-   Publicly-owned public spaces (parks, highways)
-   Publicly-owned private spaces (classrooms, hospital rooms)
-   Privately-owned public spaces (stores, amusement parks)
-   Privately-owned private spaces (houses, hotel rooms).

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## Choose the best fit

-   Actual band rules often mix rules from multiple concepts.  Few pure concept bands.  Figure out which mix of rules is best for each intended service and technology.
-   Easy to apply different rules to different bands.  No special advantages to having same rules for all bands, but many disadvantages.

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## Regulatory Concepts

- and
- Control

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## Command and Control

-   Which bands are used for which services, who is eligible to use band.
-   Completely blended engineering and regulation.  Definitions for transmitter sidebands, receiver off-frequency rejection, etc.
-    Complete band recipe:  Service provided, technical parameters, base station sites, service area, frequency re-use distance,  def. of harmful interference, etc.
-   Guaranteed service, if you follow the recipe.

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## Band Allocations

-   Band allocations include:  Mobile, broadcasting  (AM, FM, TV), point-to-point microwave, ISM, satellite,  PCS, Radar, paging, MMDS, LMDS, etc.
- Each band allocation contains detailed, specialized rules for successful operations in that band.

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## C&C Receivers

-   If you operate a receiver meeting the allocated band receiver specifications, you should not get interference.
-   If you do get interference, there must be something wrong with receiver or transmitters.  Possible need for a (new) transmitter to change operations, even though transmitter meets all nominal specs.

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## In favor of C&C

-   Highly-differentiated services – mobile, microwave, satellite, radar, broadcasting, Part 15, PCS/cellular, etc.

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## Problems with C&C

-   Consensus mode of public review slows down any detailed planning.  Tend to design conservative, worst-case, less-productive systems.
-   Almost impossible to keep up with technological changes in many rapidly-changing systems.
-   How to overcome disadvantages of C & C regulations?

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## Regulatory Concepts

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## Flexible Spectrum Use Rights

- The rights to use spectrum can be bought and sold on a secondary market, including the ability to divide and aggregate spectrum rights.
- The user has great flexibility to provide a wide range of services without asking the permission of regulators.
- The user has a reasonable expectation of operating without interference from other users.
- Note: Spectrum use rights (in this context) do not concern how one initially obtains these rights (license, auction, etc.), or whether possession is temporary or permanent.

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## Pure Electrospace Use Rights

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**   ****Electrospace **defines how spectrum could be unambiguously described, licensed, used.
-   A **ideal receiver** will separate any signals having different Electrospace descriptors.

## The Electrospace

-  Physical location – lat., long., altitude       3-dim
-  Frequency – MHz                                      1-dim
-  Time - μS, hours, or years		         1-dim
-  Direction-of-arrival – azimuth, elevation    2-dim
- .

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- A ideal receiver will separate any signals having different electrospace descriptions.

## Pure Electrospace Rules

- Keep your signals within your licensed electrospace region.  (no signals permitted outside region)
- Use ideal receivers to avoid interference from electrospace neighbors.

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- Any services, technologies, architectures, transmitter powers, modulations, etc. are permitted, as long as these two rules are obeyed.   (replaces all of 47CFR and NTIA Manual)

**Only two rules:**

## Division and Aggregation

- Electrospace regions can be freely divided or combined along any combination of electrospace dimensions, using unregulated secondary markets.

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**Approach: ** Electrospace description uniquely describes specific arbitrary regions where signals are allowed, using orthogonal coordinate systems.  Therefore, no problems anticipated in dividing or aggregating multiple electrospace regions in whatever arbitrary ways seem useful.

## Perfection, except ...

**But...**
-   I can’t reduce signals to “zero” outside licensed electrospace region (frequency or geography).
-   I can’t buy an ideal receiver.
-   (Otherwise, everything is fine.)
- Are there some reasonable approximations that still leave us with most of the advantages of pure Electrospace rules?

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## Modified Electrospace Rules

- Define a very small signal, X, as being close enough to “zero.”
- X = “minimum” signal or “allowable leakage” signal.
- X is power spectral density (PSD) = W/m2/MHz.  Anything above X is defined as a “signal”.  All “signals” must remain within the licensed electrospace region (frequency, location, time, direction-of-arrival).
- Choose X to be small enough that it would not ordinarily cause interference to normal system performance.  X can have different values in different frequency bands.

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## X = minimum “signal”

**For transmitter**:   X = maximum power spectral density (PSD) allowed outside the licensed electrospace region.  Design your transmitters so that your signal is always below X outside licensed electrospace region.
**For receiver**:   X is guaranteed maximum unwanted signal PSD “leaking” from other users at your desired frequency of operation.         (X from each user.  Make X small enough, so that leakage from several users will not cumulatively cause interference.)

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## X - Tradeoffs

- Smaller X means smaller interfering signals from other users, less-expensive receivers.
- Where is “sweet spot” in the trade-offs?
- Differences between X’s in various bands is possible.  Might be one of the ways that different flexible-use bands are “optimized” for different classes of applications.

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## Ideal Receivers?

- Note that an “ideal” receiver is not needed for interference-free performance, but only a “good-enough” receiver is needed.  In many cases, a “good-enough” receiver will be quite simple.
- How can electrospace rules be modified to ensure that a “good-enough” receiver is always relatively simple?
- Consider how interference is caused in receivers and put reasonable limits on those cases.  Limits could be different in various bands.

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## Causes of interference -1

- 1.    Intentionally-radiated co-channel signal, from transmitters using same frequency at distant locations, or
- Unintentionally-radiated sidebands/spurious signals from nearby transmitters operating at other frequencies.
- If permitted, intentionally-radiated low-power underlay signals (Part 15).
**Controlling mechanism (pure electrospace rules):**
- Cannot be more than X dBm/MHz/m2 interference from each source outside the licensed electrospace.  Falls off rapidly with distance from transmitter.

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## Causes of Interference - 2

**Non-linear:  **Intermodulation products, desensitization.
**Linear:**  bandpass filter feed-through (worse for close-in strong signals), shielding leakage, image frequencies, LO sidebands and spurs.
- A signal environment with strong unwanted signals requires a **better-quality receiver** (higher cost, larger, heavier, greater complexity, more power consumption, etc.)    Strong unwanted signals are expensive for receiver owners (just like co-channel interference).   Therefore, should try to control strong signals.

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## Ideal Xmtrs and Receivers

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- Ideal transmitter

## Real Receivers

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## Real Receivers and Xmtrs

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- No interference when xmtr and rcvr curves do not intersect.

- Linear receiver
- responses

- Non-linear receiver
- responses

- Interfering signal
- spectrum

- Small signals cause interference only when interfering signal is at receiver tuned frequency.  Large signals cause interference at many frequencies.  Note: Small signals act like ideal electrospace model.

## Control of Strong Signals

- Control maximum **field-strength** near receivers,
-  - - - or - - -
- Control maximum **transmitter power**.
- 		Either – or both – limit(s) could be used.
- No limit on number of transmitters, sites, etc.  Limit only on maximum power at a location, which aggravates receiver performance requirements.
- Completely technology-dependent.  High-performance cheap receivers (e.g., receivers with cheap superconducting tracking filters) could greatly change limits.

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## Field Strength Limit -1

-   No limit on EIRP or transmitter power, as long as strong fields don’t reach public locations (or cause signals outside licensed electrospace).

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## Field Strength Limit -2

- Transmitters can broadcast as much power as wanted, but areas of high field strength can cause interference to other radios – an externalized cost to other radio users – which must be controlled.
- No limit on transmitter power, but more powerful transmitters will require better control of vertical antenna patterns.
- Frequencies near to bands having many battery-powered portable receivers are particularly important to control.

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## Transmitter Power Limits

-   Licenses specify maximum power = Y Watts / MHz
-   Maximum power is proportional to bandwidth.  E.g. - adding two adjacent licenses doubles Bandwidth and doubles power.
-   Transmitter power limits are not as effective as field strength limits, but they are simpler to manage and they have a nice “feel” to them.  (However, this rule would not have prevented 800 MHz interference.)
-   A little surprising that this “obvious” rule to prevent off-frequency interference actually does not work very well by itself.

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## Changes in X, Emax, Y

-   X = W/MHz/m2 is already proportional to bandwidth, so allowable leakage power scales linearly with bandwidth of electrospace region.
-   EMAX is an absolute value.  Does not change (receiver overload is not affected by bandwidth of overload signal).
-   Y = w/MHz transmitter power is already proportional to bandwidth, so allowable power scales linearly with bandwidth of electrospace region.

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- Changes in X, Emax, Y

## Miscellaneous Loose Ends

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## Division and Aggregation

**Primary Rule:**  Electrospace regions can be freely divided or combined along any dimension.  However, interference conditions must not be made worse by any transactions.

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**Approach:**  Identify factors that make interference worse (i.e., require better receivers), and adopt rules limiting these factors.
- Does not guarantee to prevent interference, but only says that total interference after transaction should not be worse than before.

## Shape of power distribution

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- Note: A neighbor would barely notice whether two 1-MHz signals (50 watts each) had been replaced with one 2-MHz signal of 100 watts.
-       50    +     50      =            100                (okay)
- But:
-       50    +    50       ≠            100               (not okay)
-   Signals cannot push power to the edges of bandwidth, creating worse adjacent-channel rejection problems for neighbors.

-  Possible Rule:  Average power to signal band edge cannot be more than twice the average power density of the entire signal, for all frequencies in band.

## Multiple Signal Problem - 1

** ****Solution:**   Require all “related” signals to count as a single signal.  “Related” means signals that act together, including a common signal radiated from multiple antennas, multiple carriers of a COFDM signal, etc.
**Problem:**** ** If each signal can produce a ground-level signal of Emax, arbitrarily dividing a signal into two signals could allow 2 x Emax at ground level.
**Solution:**  Require all “related” signals to count as a single signal.

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## Multiple Signal Problem - 2

**Solution:**  Require all signals at a single site to jointly meet the EMAX limit.  This requirement exists for signals regulated under the transmitter power limits or the EMAX limits.
- Note that this is one of the problems that also exists for sites with   C&C rules today.

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## Relaxing the boundaries?

**Solution:**  Establish a emission mask for such situations that would represent a legal “safe harbor.”  If the relative spectrum of the signal remained within the emission mask, it would be deemed in compliance, even if the absolute levels were above X.
- Note:  This is similar to many situations solved by a “site manager,” because normal rules don’t work when multiple radio systems are physically very close.

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## Interference?

- 1.   Show that the cause is a transmitter violating rules.
-                         If not #1, then you can:
- Improve your own system.  Better receivers, more powerful transmitter (for better S/I).
- Accept interference, live with it.  Partial refund to clients, accept smaller area of operation, etc.
- Negotiate with interfering transmitter for mutually acceptable business deal.
- Note:  Much cleaner determination of responsibility than C&C.

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## Modified Electrospace Rules

- Keep your signals within your licensed electrospace region (no signals greater than X outside region).
- Restrict strong signals (limit EIRP or field strength), so that ideal receivers are not needed.

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- Any services, technologies, architectures, transmitter powers, modulations, etc. are permitted, as long as the two rules are obeyed.   (replaces all of 47CFR and NTIA Manual)

## Possible Downsides - 1

-    More engineering expertise required?
-    Possibly more violations, due to inexpert designs
-    A more rapidly-changing signal environment
-   no expectation of protecting earlier users from more recent users.  Everyone has same rules.
-    Receivers must reject a wider variety of interference.
-   Receivers may be more expensive and complex compared to conventional bands, which only have a few types of interfering signals.

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## Possible Downsides - 2

-  Possibly, spectrum users would need to hire measurement “guns” to enforce rules in cases of interference.
-   Will flexible-use bands incur higher system costs
-  need to reject mix of interfering signals increases cost?
-  efficiencies from greatly increased flexibility significantly reduce costs?
-   Will flexible-use typically be used to develop a new service then be abandoned for special-purpose (C&C?) bands whenever a new service gets big enough?

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## Flexible-Use Summary

-  Responsibility for interference is well-defined.
-  Market forces can provide efficient distribution of frequencies on a commodity basis via flexible secondary markets.
-  Major problems seem to be completely solvable in logical ways.  Some loose ends still remaining, including setting numerical values for X (leakage) and maximum power.

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## Final Comments

- Currently unknown how the choice of numerical limits would make flexible-use bands particularly suitable (or unsuitable) for specific types of services.
- Currently unknown what eventual mix and proportion of systems in C & C, non-licensed, Part 15 underlays, and flexible-use, if all options were available.

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## The End

- Questions
-  or
- comments?

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