The benefits of FHSS
Introduction
A previous paper from Silver Spring (FHSS Systems - SE24 WI41 Input Paper 1v1) set out a general
background to the history and development of frequency hopping systems, but may have left the
reader uncertainty as to the nature of the technology proposed.
This paper attempts to clarify the type of frequency hopping system proposed for Smart
Meter/Smart Grid applications in the band 870-876MHz described in the SR document submitted to
the SE24 WI41 study TS 102 886, and to be embodied in the emerging PHY and MAC specifications,
TS 102 887.
The paper also sets out the benefits inherent in the use of this type of technology, both to the user
and other users of the band, leading to a proposal for a predictable sharing environment.
The established use of frequency hoppers in the 2.4GHz band is noted and the regulations set out for
their use in that band suggested as potential prototype for similar regulation in this band.
Finally, the paper sets out how these systems can be realistically modelled in SEAMCAT.
Slow frequency hopping, device-centric, spread spectrum systems
Worldwide Smart Metering/Smart Grid deployments over the last decade have predominantly
utilized a form of slow frequency hopping developed to robustly and fairly share unlicensed
spectrum. With ‘slow’ frequency hopping data frames are sent between pairs of devices as full
packets. Smart Meter/Smart Grid transmissions are typically 5-50ms in duration – easily enough time
to pass the sensor information. These short packets are long enough for other SRD devices
operating in the same band to see them as quasi stationary – exhibiting the same channel duty cycle
as non-hopping SRDs.
Sending larger amounts of information entails fragmenting the data. Regulations and good
engineering practice steer subsequent packets onto different channels – spreading the energy from
a single device equally across the available spectrum.
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In operation, the targeting transmitter switches to the single channel it calculates the targeted
receiver to be receiving. Each device has its own pseudo random channel sequence consistent with
the algorithm set out in EN 300 220 which regulates a per channel duty cycle restriction.
If energy is already on that channel it may postpone the transmission until the ‘next hop’. FHSS
devices actively avoid channels with energy – ‘skipping’ over channels with fixed channel SRD traffic.
This can also be achieved by continuously tracking the ‘link success percentage per channel’ and
avoiding channels which are not highly successful.
Additionally, a ‘four way handshake’ can be used wherein a short POLL packet is used to sound the
channel before the longer, more disruptive DATA packet is sent. If the POLL fails, the DATA is not
sent, avoiding busy channels.
Like other modern SRD devices, the devices send the data packets to the receiver and await an
acknowledgement
Benefits of frequency hoppers
Overview
FHSS devices, as the name implies, spread their energy evenly across the available spectrum. The
spreading does not occur ‘all at once’ as with DSSS1 devices but rather statistically over many
transmissions. In this way they act (and may be modelled) as a single-channel SRD that changes its
channel after every session (e.g. after every two packets).
This spreading ‘by-packet’ has many advantages for both the FHSS devices as well as those fortunate
fixed channel devices sharing the spectrum with a neighbour device that can change channel.
The manifold FHSS benefits include:
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No channel lies fallow – leading to much greater spectral efficiency than fixed channel
devices
Conversely, there are no ‘hot spot’ channels – as with popular single channel applications
such as Z-Wave (868.42 MHz) where blocking congestion is inevitable.
FHSS systems ‘find a clear channel, use it, then quiesce’. FHSS devices realize much higher
performance; the spectrum can support much higher duty cycles from all devices without
undue degradation.
Fixed channel SRDs sharing the spectrum with FHSS devices ‘see’ only a fraction of their
transmissions. The more available channels, the smaller the fraction.
FHSS has proven itself as a risk-free spectrum sharing technique for more than 2 decades.
Fair spectrum sharing creates a ‘predictable sharing environment’; since interference is
equally unlikely on every channel, there is no requirement for allocating dedicated (and
largely unused) channels to ostensibly deliver reliability through underuse.
FHSS with sub-25 ms channel occupancy has proven non-interfering to GSM-R systems
which takes advantage of both the frequency agility and ‘smart channel selection’ of modern
FHSS devices.
Traffic patterns in shared bands
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Direct sequence spread spectrum
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Occupancy (%)
Occupancy (%)
Occupancy (%)
Modelling in WI41 to date has assumed that all traffic is evenly distributed across a shared band,
despite there being no centralised management function to achieve this result – an ‘ideal’ case.
When considering the impact of a frequency hopper on interference to devices operating in the
band, the figure below illustrates the assumptions made. The first diagram shows the assumed
traffic generated by the population of fixed-channel devices; the second shows the interference that
would be caused by a single, fixed interfering device (red bar), say, a smart meter; and the third
shows the impact of deploying a frequency hopper.
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This illustrates the way in which, according to this simplistic approach, frequency hopping reduces
interference by the reciprocal of the number of transceive channels – the more ‘hops’ off your
channel, the more time available for fixed channel SRDs.
Non-hopping, AFA, or otherwise static co-spectrum devices within range have their interference
reduced in direct proportion to the number of channels the local FHSS devices occupy.
Occupancy (%)
Experience around the world, however, shows that, in shared bands, traffic from different types of
devices tends to ‘clump’ in some popular channels. The reason for this is that, partly, for certain
technologies and/or applications manufacturers establish default channels for their operation and,
secondly, devices will, statistically tend to clump. The traffic in the channels (averaged over a
suitable timescale), therefore tends to look like that shown in the figure below, which shows the
typical occupancy of a set of shared channels used by fixed or slowly adapting (AFA) devices.
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Channel
Introducing Frequency Hopping devices in to the band allows additional utilisation to be achieved
whilst minimising the interference to the existing devices. Two regimes can be described
In lightly loaded areas, frequency hoppers tend to find lightly loaded channels and transmit
preferentially in those. In the extreme, the amount of additional interference caused is zero, whilst
utilisation of the band can increase many fold.
Occupancy (%)
Occupancy (%)
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Channel
Channel
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Occupancy (%)
Occupancy (%)
In more heavily loaded areas, many channels will be used by all systems, and the presence of
frequency hoppers allows those channels that are currently in heavy use by less agile systems to be
avoided.
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Statistically, the amount of additional interference introduced is still minimised, as the utilisation –
and hence spectral efficiency of the band – increases. The additional interference (caused by the
hoppers) will be less than that calculated for the ‘ideal’ case, above.
This apparent paradox can be understood by observing that an unmanaged population of fixed
devices will cause more aggregate interference across the band because of this tendency to clump.
Frequency hoppers rectify this imbalance, which is good for the operation of the hoppers and good
for the population of fixed channel devices.
Only frequency hopping devices can avoid heavily used channels. While not all FHSS systems
actively employ LBT, many do. Additionally, channels that have a poor ‘history’ are often grey/black
listed until they become less heavily used or otherwise less contended.
In summary, in both cases, the additional interference added to the ensemble of users is minimized
as a ratio of the additional utility derived from the band.
Towards a Predictable Sharing Environment
This ability for frequency hopping devices to ‘smooth’ traffic across bands across over wide
geographic areas should be the basis for a predictable sharing environment. When coupled with
work being carried out in ETSI SF411 to, similarly, smooth traffic in the time domain – by defining
duty cycle limits over varying time frames (eg hours, minutes and seconds) – this type of technology
promises to offer the SRD community a way of maximising utilisation of bands (ie sharing), whilst
both minimising and making predictable the interference with which devices are likely to need to
contend.
Experience in other shared bands around the world shows that the benefits of frequency hoppers
means that they become the dominantly used technology in the band, and in such an environment,
duty cycle limits are defined on a per-channel basis to encourage the use of this benevolent
technology.
Regulation
The rules embodied in EN 300 3282 (the relevant paragraphs being shown I the annex to this paper)
recognise the benefits of frequency hopping in the 2.4GHz band and should be adopted when
considering rules for the 870-876MHz band.
The standard recognises the need for frequency hoppers to spread their traffic across the band,
whilst allowing (lightly utilised) channels to be used preferentially (in effect, up to four times more
than average utilisation). Coupled with a suitable restriction placed on the overall duty cycle allowed
to individual devices (calculated by the compatibility study, but taking account of the beneficial
nature of frequency hoppers to the community), this could form the basis of an equitable sharing
arrangement.
Such an arrangement has at least two advantages: 1) it places a known limit on the amount of traffic
any device will place upon a single channel, and 2) it motivates those that can, to move their
operation across all available channels. As has been noted, frequency agile technologies aid
incumbent or less-sophisticated devices by dramatically limiting the interference they will cause to
devices that cannot change channels.
Modelling frequency hoppers in SEAMCAT
Mesh Smart Metering and Smart Grid systems are networks and so traffic will be, at an application
level, be coordinated in time and space. The device-centric frequency hopping model, however,
disperses the traffic across the frequency/time space, breaking the coordination of transmissions in
frequency vs space vs time.
In an geographic area where traffic is highest - and that will be around collector points – traffic will
be higher, but evenly dispersed across the spectrum. In fact, better still, actively avoiding less agile
systems, as shown above.
Therefore, as a first approximation, it is reasonable to calculate the traffic generated by hoppers by
considering a single 200-kHz channel and divide the traffic generated by each device by the number
of channels available, as illustrated below.
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Wideband transmission systems - Data transmission equipment operating in the 2,4 GHz
ISM band and using wide band modulation techniques.
This calculation, however, will underestimate the utility of hoppers, as described above. The
increased interference calculated will, in fact, be the maximum additional interference that will be
introduced. This is because frequency hoppers maximise the real spectrum efficiency whilst
minimising collisions (where fixed/less agile systems will have been colliding with one another
more).
Annex: FHSS regulations contained in EN 300 328
4.2.1 FHSS modulation
FHSS modulation shall:
either:
a) make use of at least 15 well defined, non-overlapping hopping channels separated by the channel bandwidth
as
measured at 20 dB below peak power;
or if capable of adaptive frequency hopping:
b) at least be capable of operating over a minimum of 90 % of the band specified in table 1, from which at any
given time a minimum of 20 channels or hopping channels shall be used.
For both cases, the minimum channel separation shall be 1 MHz, while the dwell time per channel shall not
exceed
0,4 s.
While the equipment is operating (transmitting and/or receiving) each channel of the hopping sequence shall be
occupied at least once during a period not exceeding four times the product of the dwell time per hop and the
number of
channels. Systems that meet the above constraints shall be tested according to the requirements for FHSS
modulation.