Adaptive FHSS using LBT is a mechanism
by which a given hopping frequency is made 'unavailable' because an interfering
signal was detected before any transmission on that frequency.
Adaptive FHSS equipment using LBT
should comply with the following minimum set of requirements.
- At the start of every dwell time, before transmission on a
hopping frequency, the equipment should perform a Clear Channel Assessment (CCA)
check using energy detect. The CCA observation time should not be less than 0.2%
of the Channel Occupancy Time with a minimum of 18 μs. If the equipment finds
the hopping frequency to be clear, it may transmit immediately.
- If it is determined that a signal is present with a level
above the detection threshold defined in step 5, the hopping frequency should be
marked as 'unavailable'. Then the equipment may jump to the next frequency in
the hopping scheme (even before the end of the dwell time), but in that case the
'unavailable' channel cannot be considered as being 'occupied' and should be
disregarded with respect to the requirement of the minimum number of hopping
frequencies. Alternatively, the equipment can remain on the frequency during the
remainder of the dwell time. However, if the equipment remains on the frequency
with the intention to transmit, it should perform an Extended CCA check in which
the (unavailable) channel is observed for a random duration between the value
defined for the CCA observation time in step 1 and 5% of the Channel Occupancy
Time defined in step 3. If the Extended CCA check has determined the frequency
to be no longer occupied, the hopping frequency becomes available again. If the
Extended CCA time has determined the channel still to be occupied, it should
perform new Extended CCA checks until the channel is no longer occupied.
- The total time during which an equipment has transmissions on
a given hopping frequency without reevaluating the availability of that
frequency is defined as the Channel Occupancy Time. The Channel Occupancy Time
for a given hopping frequency, which starts immediately after a successful CCA,
should be less than 60 ms followed by an Idle Period of at least 5% minimum of
the Channel Occupancy Time with a minimum of 100 μs. After the Idle Period has
expired, the procedure as in step 1 should be repeated before having new
transmissions on this hopping frequency during the same dwell time.
EXAMPLE: Equipment
with a dwell time of 400 ms can have 6 transmission sequences of 60 ms each,
separated with an Idle Period of 3 ms. Each transmission sequence was preceded
with a successful CCA check of 120 μs.
For LBT based adaptive FHSS equipment with a
dwell time < 60 ms, the maximum Channel Occupancy Time is limited by the
dwell time.
- 'Unavailable' channels may be removed from or may remain in
the Hopping Sequence, but in any case:
- Apart from Short Control Signalling Transmissions,
there should be no transmissions on 'unavailable' channels;
- A minimum of N hopping frequencies should always be
maintained.
- The detection threshold should be proportional to the transmit
power of the transmitter: for a 20 dBm e.i.r.p. transmitter the detection
threshold level (TL) should be equal to or less than -70 dBm/MHz at the input to
the receiver assuming a 0 dBi (receive) antenna assembly. This threshold level
(TL) may be corrected for the (receive) antenna assembly gain (G); however,
beamforming gain (Y) should not be taken into account. For power levels less
than 20 dBm e.i.r.p., the detection threshold level may be relaxed to:
Equation 2. TL = -70 dBm/MHz + 10 × log10 (100 mW /
Pout)
- The equipment should comply with the requirements defined in
step 1 to step 4 of the present clause in the presence of an unwanted CW signal
as defined in Table 5-9.
Table 5-9 Limits on FHSS LBT Equipment -
Unwanted Signal parameters
Wanted signal mean power from companion
device |
Unwanted CW signal frequency (MHz) |
Unwanted CW signal power (dBm) |
sufficient to maintain the link (2) |
2395 or 2488.5 (1) |
-35 (3) |
(1) The highest frequency should be used for testing operating
channels within the range 2400 MHz to 2442 MHz, while the lowest frequency
should be used for testing operating channels within the range 2442 MHz to
2483.5 MHz.
(2) A typical conducted value which can be used in most cases is
-50 dBm/MHz.
(3) The level specified is the level at the UUT receiver input
assuming a 0 dBi antenna assembly gain. In case of conducted measurements, this
level has to be corrected for the (in-band) antenna assembly gain (G). In case
of radiated measurements, this level is equivalent to a power flux density (PFD)
in front of the UUT antenna.