Co-existence of dissimilar wireless systems
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Co-existence of Dissimilar Wireless Systems
Jan Kruys, Cisco Systems
Third draft: December 3, 2003
This contribution has been prepared to assist the work of the Wi-Fi Alliance. It reflects the views of the
author and does not contain or represent a policy or a commitment of Cisco Systems.
Abstract
There are two dimensions to spectrum sharing: vertical sharing: between systems with different
levels of regulatory status (e.g. newcomers that have to live together with incumbents) and
horizontal sharing: between systems with equal regulatory status.
This document introduces the concept of vertical sharing but addresses horizontal sharing in
some detail.
A regulatory regime for vertical sharing of necessity has to be specific in that it deals with
protection criteria and their application for a given service or system, with the intent to protect it
from interference by other systems.
A regulatory regime for the horizontal co-existence of diverse RF systems in the same, license
exempt frequency space, is a tough nut to crack and so far it has eluded a satisfactory solution.
However, a solution becomes more urgent as the FCC and other regulators plan to make more
spectrum “commons” available under license exempt or light licensing rules. This short paper
attempts to address the problem by reducing its scope to systems that access the medium in a
non-deterministic manner. Noting that the main criterion is the receiver performance in the face of
interference, it proposes a set of simple parameters and rules that could be the basis for
regulatory measures aimed at “commons spectrum”. The rules are based on the rewarding
efficient spectrum use by good receiver design and/or short medium occupation. The reward is
given by a modification of the listen before talk criteria and the power output criteria.
Thus, systems designers have choice: limit the time on the medium or improve the receiver, or
both. Improving the receiver is understood to include the choice of modulation scheme as well as
other aspects that affect the transmitter. The required behavior, is, of necessity, statistical in
nature.
Introduction
Wireless telecommunications and wireless connectivity have seen a very high adaptation over the
last decade and have proven to be significant contributors to economic growth. Two types of
systems have dominated the scene: the mobile phone and the wireless LAN. The former relies on
licensed – that is “exclusive use” – spectrum whereas the latter operates and indeed owns its
success to “common use” of the same spectrum by all users. The merits and drawbacks have
been argued and analysed in many ways elsewhere (see [1] for an example) and therefore that
matter is not addressed here. Suffice it to state that both sorts of usage have their proper place in
society. This paper focuses on the commons problem.
There are two dimensions to spectrum sharing: vertical sharing: between systems with different
levels of regulatory status (e.g. newcomers that have to live together with incumbents) and
horizontal sharing: between systems with equal regulatory status. This document recognizes the
problem of vertical sharing but addresses only horizontal sharing in a commons setting.
The FCC and spectrum regulators outside the US are about to open up more spectrum for a
“commons” type use under license exempt or light licensing rules. This short paper attempts to
address the problem by reducing its scope to systems that access the medium in a nondeterministic manner. Noting that the main criterion is the receiver performance in the face of
interference, it proposes a set of simple parameters and rules that could be the basis for
Copyright © Cisco Systems Inc.
Co-existence of dissimilar wireless systems
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regulatory measures aimed at “commons spectrum”. Such rules and the behaviour they assume
a refine are of necessity statistical in nature. Deterministic behaviour in a shared environment is
only possible when all devices implement the same set of rules and operate on the same time
base. The obvious consequence is stifling of evolution and innovation.
Vertical Sharing
A regulatory regime for vertical sharing of necessity has to be specific in that it deals with
the protection criteria and their application to an existing or a new service or application
with the objective of protecting it from interference by other systems. A case in point is
radar systems in the 5 GHz which are protected from interference by wireless devices by
a detection and avoidance mechanism (DFS) [2] built into the latter. Other examples
include the detection of broadcast transmissions by short range devices so as to allow
them to operate in spectrum that is locally not in use by the broadcast services or that is
locally useless, e.g. because of shadowing of an area by a the horizon or a range of hills.
In both cases, the sharing takes place between very dissimilar systems with vastly
differing operating range. In both cases sensitive receivers must be protected but in one
case these receivers are few in number and co-located with powerful transmitters, in the
other case the receivers are distributed and numerous. The detection capabilities of the
“newcomer” devices must be tuned to the specific transmissions of the incumbents and
therefore they will be different from case to case. Although this is a rich field that is just
beginning to be investigated, this subject is not further addressed in this paper.
Horizontal Sharing
1 Problem Summary
Unlicensed spectrum, more recently more fashionably known as spectrum commons,
pose an interesting problem: how to efficiently share spectrum without imposing
technically unfair and/or stifling regulation.
The following concentrates on the issue of sharing a “channel”. Although avoiding cochannel operation is high useful and in fact mandatory in some cases [2], doing so in a
commons spectrum context is at once obvious (look for a clean piece of spectrum) and
complex – in the commons spectrum, channels do not exist and any system can use any
piece of spectrum. In fact, further work may well show that avoidance of co-channel
operation in a commons world is a trivial and a reasonable on its own only if the
commons a large – or sparsely used.
The sharing study done by Microsoft [3] sums up the problem: interference varies with
power levels, bandwidth ratios, and medium access methods and without any means of
policing how systems behave, wide bandwidth devices with listen before talk medium
access procedures, typically hold the short end of the stick. This means that
applications that require significant wireless bandwidth are subject to the threat of
punishing interference that turns the users experience into something not worth
repeating.
The broader view is that different applications require different bandwidth – or at least
different signalling. It will be clear that some constraints are required on the type of
systems that are to co-exist in the same spectrum: operating range has to be of the
same order of magnitude and two way communication is required.
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In the absence of deterministic sharing mechanisms used in the “exclusive use” domain,
the problem is finding means to encourage efficient use of spectrum without regulating
specific parameters, or at least minimizing the number of regulated parameters.
This is known as the problem of “the tragedy of the commons”. In the commons of old,
the tragedy was overgrazing of the available shared pasture. In the spectrum domain
today, the tragedy is too much interference – made worse by a wide disparity between
the systems using the spectrum. In the commons of old, social control made sure that
the tragedy was avoided. No such tools are available to the wireless systems designer
or user.
There are two categorically different approaches to solving the commons sharing
problem: the heads up approach and the heads down approach.
The heads-up approach requires a separate channel on which stations post their actual
and intended use of a given piece of spectrum and all adjust their usage accordingly.
The problem here is how to handle individual reductions to allow large numbers of users.
This problem is not addressed here. Instead, we focus on the heads down approach: all
stations look only after their own interests – but they play by the rules.
Finally, it should be noted that there are two types of commons – single species
commons and multi-species commons. Whereas the former allows sophisticated and
efficient solutions - see IEEE802.11 and offspring – the latter, the broader common
problem, is less tractable and cannot be mastered with the same efficient results as its
narrower variant. Thus the rules developed here are rules of last resort, needed to
address the hard problem of a heads-down, multiple species commons, suitable for the
ISM and the U-NII band.
2 Analysis of the broader spectrum commons problem
Interference is not a transmitter issue and it is not solved by listen before talk rules.
Instead, it is a receiver issue. If receivers are able to distil the wanted signal from strong
interference, there would be no problem. In fact, there would be no interference. The
real problem is that receivers need a positive operating margin (or signal to noise +
interference ratio). The higher the signalling rate, the higher the receiver’s operating
margin has to be. The available margin, in turn, is determined by the strength of the
received signal and the relative strength of the interference at the receiver.
Note: even in small scale systems like RLANs the above is true – it is the reason for the
RTS/CTS protocol of IEEE802.11.
The better receiver is able to correctly receive weaker signals in the presence of more
interference. However, better receivers can be costly (MIMO leads to higher power
consumption, beam forming leads to higher device cost and operational limitations). To
avoid that cost, the receiver can move closer to the sender and achieve the same result.
The level of interference is determined by the power output of transmitters. That power
in turn is driven by the need to deliver a certain power level at the intended receiver.
Typically, the transmitter power is fixed by regulatory means or by convention (as in the
case of Wi-Fi devices). Since the conventional wisdom is “more is better”, transmitter
power is driven up by considerations of range and data rate. In fact, higher date rates
are more energy efficient for both sender and receiver. A prisoner dilemma results:
reducing power voluntarily will not benefit the volunteer so why should he do so.
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If the transmitter has no inherent incentive to reduce power and the receiver has no
inherent incentive to improve its operating margin, one cannot rely on self interest to
solve the commons problem and the only recourse is regulation.
3 What sort of rules are feasible?
The first question is “what sort of rules are possible?” The nature of the problem is not so
much complexity but lack of complete information at any point in time. Since the coexisting systems are assumed to be diverse, exact behaviour like in IEEE 802.11 can
not be specified. In addition, the medium is not reliable and its properties as seen by any
device, can change rapidly. Therefore a deterministic approach is not possible and the
rules have to be aim at controlling the statistics of device behaviour and they have to be
make use of input parameter values who values are (short term) averages – like duty
cycles or transmission error rates. That statistical approach should be kept in mind in the
following. The integration period for these statistics has to be long compared to the
events being controlled.
4 What to regulate – what is a minimum set of rules for the spectrum commons?
Which parameters should be regulated? Transmitter power and time patterns are major
factors in creating interference and those have to be used as the targets of the
regulatory constraints. Ways must be found to limit both with undue restrictions on
technical solutions. Systems designers must be encouraged to trade time in the air for
power output and vice versa – in other words the power limit must contain a duty cycle
factor, e.g. TXout*Duty Cycle = constant. Systems that can do with a lower TXout can be
in the air longer and systems that are in the air for a short time can use a higher TXout.
The fact that broadband systems suffer more from narrowband interference than vice
versa suggests that the appropriate power limit is power spectral density, not power as
such. The spatial aspect also should not be ignored. Interference is caused over a
certain area and that area is largely independent of the antenna gain – if one ignores the
vertical dimension. Therefore the proper power measure is PSD at the transmitter output.
The next step is put constraints on when a transmitter may be turned on. Although the
PSD/duty cycle limit motivates designers to be frugal with power, it does not help to
separate transmissions in time. Therefore a Listen-Before-Talk requirement is useful.
The main rule is that if the spectrum to be used is sensed to have energy in it, the
transmitter will wait a random time period before trying again, e.g. n*2 usec where n = a
random variable. This variable too can be made flexible: at low duty cycles, a fixed backoff can be considered sufficient, at high duty cycles, a longer back-off would prevent high
duty cycle systems from locking out low duty cycle systems.
Since we have to make rules covering a very wide range of systems, it is hard to set a
single limit for all. Again, the usage patterns can be used to increase inter-system
fairness (= to increase the freedom of designers). However pure LBT is inefficient at high
utilization levels and if we want to avoid getting into specification of medium access rules,
the LBT principle has to be modified slightly: if a transmitter expects that a transmission
will be effective, it need not wait for a free channel. This is achieved by a modification of
the LBT threshold (higher success expectation should allow the threshold to be set
higher = less constrained access).
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Finally, for some applications it may be advantageous to lump a short sequence of
transmissions as one LBT event. To make this possible without undue burden on other
users, the duration of such a burst has to be limited. Considering the broad trend
towards higher transmission rates, that burst period can be kept short.
In summary we have three basic rules:
a) All stations observe a common PSD limit - modified by their on airtime
- less time in the air means more power
b) All stations shall use LBT – modified by their PSD and receiver success rate
- a lower PSD output allows a higher LBT threshold and higher probability of
successful receive allows a being less sensitive to other transmissions.
c) The maximum time a station can transmit or otherwise occupy the medium is
limited
In the following we look at parameter values for a simple set of Spectrum Commons
rules and their consequences. Note that we only consider systems with equal status.
National Security, safety of life and other considerations will affect the values of the
regulated parameters. However, the principle can remain the same.
5 Example of Spectrum Commons Rules and Parameter values
- Parameter description
PSD limit:
Power spectral density in mW – the average power while the
transmitter is on, measured at the antenna connector. In
operations, the PSD limit is modified by the duty cycle of the
transmitter and the receive failure expectation to yield the
actual PSD value for a given transmission.
Duty cycle:
The ratio between on-air time and the separation between onair events. The events considered are events in one direction
between a pair of devices. Either device may communicate
with other devices – the duty cycles of those events are
considered separately. The on-air time used to determine the
duty cycle covers all the time between gaining access to the
spectrum after an LBT sensing until the spectrum is released.
LBT threshold
Listen before talk reduces unnecessary interference. However,
LBT can lead to inefficiency – sometimes waiting for a quiet
channel is not needed - e.g. if the receiver is nearby or has a
low (=good) operating margin. Thus a high actual PSD would
need a lower, more sensitive threshold and vice versa.
LBT Back-off
When the spectrum is occupied, the transmitter has to wait for
a new opportunity. However, the wait period until that new
attempt should reflect the failure expectation (lower means
less back-off).
Rx Failure Expectation
Many systems are designed with an acceptable “receive
event” error rate in mind. This value can be used as
parameter to drive transmit decisions: if the expectation of
failure is high, transmission should be discouraged.
Transmitters can be informed by receivers of the RFE –
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instantaneous accuracy is less important - statistical accuracy
is sufficient.
- Regulated Parameters, values and modifiers
PSD limit and LBT threshold are defined in terms of a reference case: 16Mb/s in a 20
MHz channel over 32 m in free space with a good, conventional receiver.
The modifier definitions describe how the nominal values of the regulated parameters
are converted into actual values, depending on the local conditions.
Absolute Tx out limit
Absolute air time limit
Integration period
30 dBm (or less, as in the case on the lower two U-NII bands)
2 msec (one transmission or burst of transmissions not separated by
LBT medium sensing)
> 1 second
Nominal PSD limit
Nominal LBT threshold
Nominal LBT back-off
PSDnom = 0 dBm/MHz (-10 dBm/100KHz, etc)
LBTnom = -80 dBm/MHz
LBOnom = (r+s*n)usec, r < LBOnom < 15
s - is a scale factor that allows the rules to be adapted to various
operating range conditions, e.g. for 100 m, s=1 usec.
r is a constant that models device delay in sensing the medium –
5 usec is a suitable value.
Nominal Duty Cycle
Nominal RFE value
DCnom = .1
RFEnom = .1
DC modifier
RFE Modifier
DCmod = (DCact/DCnom)
RFEmod = (RFEact/RFEnom)
Actual RFE
is determined dynamically from receiver feedback which is
system specific.
At duty cycles below DCnom, RFEmod may be set to .1
Actual PSD value
Actual LBT threshold
Actual LBT back-off
PSDact = PSDnom - 10logDCmod
LBTact = LBTnom - 10logRFEmod - 10logDCmod + (PSDnom-PSDact)
LBTact = (LBOnom*RFE mod)usec, r < LBTact < 63
The effect of these values and rules is that:
- no device can monopolize the medium
- shorter duty cycles facilitate access to the RF spectrum and/or higher RF power
- better receivers give advantageous channel access.
- channel hogging to get a message or packet through is discouraged.
These rules do not prevent systems to use fast acknowledgment nor do they prevent a
reasonable measure of QoS – schemes like WME of IEEE802.11 will work, as will
bursting, high gain antennas and MIMO schemes.
6 Some test cases
a) Telemetry system, 1 MHz, 1Mb/s, 2.4GHz, 10% duty cycle, 128 m range indoor:
Available link budget: 0 dBm – 114 dBm noise floor = 114 dB
Required link budget: pathloss + SNR +Implementation margin = 41+58+6+6 =
111 dB.
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b) Same system, duty cycle = 1% allows 10dBm duty cycle advantage, doubles the
range to 256 m in an indoor environment or more in an outdoor environment.
c) RFID system, 2,4 GHz duty cycle = 2%, range is 4 m, passive tag with signal
gain of -50 dB (assumed!), Rx sensitivity = -114 + 12 dB = -102 dBm
Txout = 0 dBm with a 7dBm DC advantage (2% instead of 10% duty cycle)
Link budget: 7 – (-102) = 109 dBm
At 4 m, two way pathloss = 65 dB; margin for tag is 44dB but it needs 50dB. This
requires quadrupling the bandwidth to make it work at the targeted range.
d) RLAN, Wi-Fi, 5GHz, 10 stations, 24 Mb/s, 200mW, average duty cycle per
station is 2 %, SNR = 18dB, impl. margin is 8 dB.
Max Tx out is 12dBm + 7dBm Duty Cycle advantage
Required indoor range: 64m
Available link budget: 19dBm – (101dBm noise floor) = 120dB.
Required link budget: pathloss + SNR + Impl margin = 46+48+18+8 = 120 dB.
e) Same RLAN, 2x2 MIMO, 10 dB Rx improvement
Range increases to 128m or LBT threshold can be increased by 10dB to gain
throughput advantage.
7 Implementation aspects
The proposed co-existence rules above may seem complex and relatively costly to
implement. However, when compared to low cost devices on the market today like Wi-Fi
and Bluetooth, the above is seen to be simpler and relatively easy to implement.
Designers can make choices that obtain the maximum performance possible or the can
limit device performance and behavior so to reduce cost. For example, devices with a
constant traffic rate need not calculate their own statistics and a fixed duty cycle means
the Tx power can be fixed as well.
Designers may also opt for higher performance, more costly solutions such as
beamforming antennas. Such antennas allow higher data rates as well as a lower RFE –
both factors allow them to decrease their LBT threshold and LBT back-off and thus get
preferential spectrum access.
Receiver performance can be ignored but in that case performance is limited by the
nominal power and duty cycle values.
8 Conclusion
The parameters and rules given in this paper offer the prospect of improving spectrum
sharing among diverse systems. By linking power spectral density, duty cycle and
medium access constraints, this approach forces designers to make choices that
optimize the behavior of a system in a given application and cost profile but that are
neutral from the co-existence point of view. A small set of “test cases” confirms that
proposed regime does not constrain systems design or applications unduly.
References:
1] Werbach Paper on Commons versus Exclusive rights spectrum rules
2] ITU-R Recommendation M1562 – Characteristics of DFS for the protection of the
radio-determination service.
3] Microsoft …..paper on co-existence…
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Co-existence of dissimilar wireless systems
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Appendix: Determining RFE
First of all it should be noted that the underlying assumption in all of the preceding is that
it is not possible to regulate device behavior in a deterministic, case specific manner.
Interference in common spectrum is statistical in nature and therefore the RFE
parameter can never be an accurate predictor of the actual chance of communication
failure. Thus the means of determining RFE need not be accurate and microsecond
accurate either. This argument can be taken a step further: there is no need to specify a
mechanism for determining RFE. Each system with have its preferred solution or method
that give optimum performance for that system.
Important from a regulatory viewpoint is that the mechanism is present and effective.
Implementors have the option of setting RFE at .1 in case they want to avoid the
expense of implementing a dynamic mechanism.
The following gives some examples.
1) In-band signaling with each transmission
This method adds a bit of overhead to each transmission but it has the advantage
that is keeps a running tab on actual conditions. This method does not work well
with very bursty traffic.
2) Simple acknowledgement.
Keeping acknowledgement statistics will work in many cases where changes in the
RF channel occur slowly relative to the channel occupancy. Many indoor
environments have sufficient reflectivity to “smear out” RF fields so that the above
condition is likely to occur.
3) Broadcasting local interference statistics.
Where communications are intermittent, a feed forward mechanism may be
employed: receivers can broadcast their typical or averaged interference levels so
that transmitters have a basis for picking a reasonable RFE value for a given
transmitter. Notably in the case of RLAN access points this may be a suitable
mechanism - the interference data can be sent out with each beacon.
Note that it is possible to verify claims about a dynamic RFE implementation: by setting
up a communication between two devices and injecting interference at the receiver, the
behavior of the transmitter can be made to change in predictable ways.
References
[1] Werbach, Open Spectrum: The New Wireless Paradigm. Spectrum Series Working
Paper #6 October 2002.
[2] ITU-R Recommendation M1562 – Characteristics of DFS for the protection of the
radio-determination service.
[3] Pierre De Vries & Amer Hassan, “Spectrum Sharing Rules for New Unlicensed
Bands,” unpublished, Dec 2003.
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