Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SMART FREQUENCY-HOPPING CONTROL MECHANISM FOR MITIGATING AGAINST
TRIGGERING SILENT RADIO SQUELCH CIRCUITS SY SUB-CHANNEL
TRANSMISSIONS FROM SPECTRAL REUSE TRANSCEIVER
CROSS-REFERENCE TO RELATED APPLICATIONS
[001]The present application is a continuation-in-part of and
claims the benefit of previously filed, co-pending U.S. Patent
Application, Serial No. 10/730,753, filed December 8, 2003, by
Brent Saunders et al, entitled: "Radio Communication System
Employing Spectral Reuse Transceivers" (hereinafter referred to
~~.
as the '753 application), which claims the benefit of U.S.
Patent Application Serial No. 60/432,223, filed December 10,
2002, by Gerhardt et al entitled: "Link Utilization Mechanism
for Secondary Use of A Radio Band"; and further claims the
benefit of previously filed, co-pending U.S. Patent Application
Serial No. 60/784,105, filed March 20, 2006, by E. Gerhardt et
al, entitled: "Link Utilization Mechanism for Aggregation of
Disjoint Radio Bandwidth," the disclosures of both applications
being incorporated herein.
FIELD OF THE INVENTION
[002]The present invention relates in general to communication
systems and subsystems thereof, and is particularly directed to
a 'smart' sub-channel hopping control mechanism that may be
employed by the communications controller of a spectral reuse
transceiver of a communication system of the type disclosed in
the above-identified '753 application, to delineate on which
ones of a plurality sub-channels (that are potentially available
for reuse by a secondary user due to primary channel inactivity)
the spectral reuse transceiver may transmit, in a manner that
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substantially reduces, and optimally minimizes, the likelihood
of triggering the squelch circuits of 'silent' radios of users
that have been licensed to transmit on currently inactive
primary channels.
BACKGROUND OF THE INVENTION
[003]As described in the above-identified '753 application, in
some radio bands, such as the 217-220 MHz VHF band, as a non-
limiting example, governmental licensing agencies (e.g., the
Federal Communications Commission (FCC)) customarily grant
primary licensees non-exclusive use of the band for a variety of
communication services, such as push-to-talk voice transmission.
These primary users pay for this licensed use with an
expectation that they will not encounter interference by other
users. The FCC also allows secondary users to access the same
band and the same channels within the band on a 'non-
interfering' or secondary basis, whereby a channel may be used
by a secondary, non-licensed, user, so long as the primary user
is not using that channel.
[004]The FCC and similar agencies in foreign countries are
continually looking for ways that allow expanded use of these
licensed radio frequency bands, without reducing the quality of
service available to the primary users. For secondary users,
these bands provide a cost-free opportunity with excellent radio
transmission properties for telemetry and other applications.
Because secondary users must not interfere with primary users,
complaints of interference from a primary user to the FCC may
result in its issuing an administrative order requiring that the
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secondary user move to another portion of the band or leave the
band entirely. Such a spectral transition is disruptive to the
secondary user's service and can be expensive, especially if
site visits, equipment modification, or exchange are required,
in order to implement the mandated change. It will be
appreciated, therefore, that there has been a need for a
mechanism that allows a secondary-user to employ a licensed band
on a non-interfering basis and will adapt the radio's frequency
usage should new primary users appear. It should be noted that
primary users always have priority over secondary users, there
is no first-use channel frequency right for secondary users.
[005]Advantageously, the invention described in the above-
referenced '753 application successfully addresses this need by
means a monitored spectral activity-based link utilization
control mechanism. Briefly reviewing this link utilization
control mechanism, which is intended for use with a star-
configured communication system, such as that depicted in the
reduced complexity diagram of Figure 1, a spectral reuse
transceiver installed at a master site 10 communicates with
respective spectral reuse transceivers installed at a plurality
of remote sites 12. Each spectral reuse transceiver operates
with a selectively filtered form of frequency hopping for
producing a sub-set of non-interfering radio channels or sub-
channels.
[006]For this purpose, the master site 10 periodically initiates
a clear channel assessment routine, in which the master site and
each of the remote sites 12 participate, in order to compile or
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'harvest' a list of non-interfering or 'clear' sub-channels
(such as 6.25 KHz wide sub-channels), which may be used by
participants of the network for conducting communication
sessions that do not ostensibly interfere with any licensed
user. By transmitting on only sub-channels that have been
determined to lie within clear channels, a respective site's
spectral reuse transceiver is ensured that it will not interfere
with any primary user of the band of interest.
[007]Except when it is transmitting a message to the master
site, each remote user site sequentially steps through and
monitors a current list of clear channels (that it has
previously obtained from the master site), in accordance with a
pseudo-random (PN) hopping sequence that is known a priori by
all the users of the network; looking for a message that may be
transmitted to it by the master site transceiver. During the
preamble period of any message transmitted by the master site,
each remote site's transceiver scans all frequency bins within a
given spectrum for the presence of energy. Any bin containing
energy above a prescribed threshold is marked as a non-clear
channel, while the remaining channels are identified as clear
(and therefore available for reuse) channels.
[008]Whenever a remote site notices a change in its clear
channel assessment, it reports this to the master site at the
first opportunity. As the master site has received clear channel
lists from all the remote sites, it logically combines all of
the clear channel lists, to produce a composite clear channel
list. This composite clear channel list is stored in the master
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site's transceiver and is broadcast to all of the remote sites
over a prescribed one of the clear channels that is selected in
accordance with a PN sequence through which clear channels are
selectively used among the users of the network. When the
composite clear channel list is received at a respective remote
site it is stored in its transceiver.
[009]To ensure that all communications among the users of the
network are properly synchronized (in terms of the (composite)
clear channel list and the order through which the units
traverse, or 'hop' through, the clear channel entries of the
clear channel list), the master site's transceiver transmits an
initialization message on an a priori established clear channel,
which each of the remote units monitors. This initialization
message contains the clear channel list, an identification of
the preamble channel, a PN sequence tap list, and a PN seed that
defines==the initial channel and hopping sequence for the
duration of an upcoming transmit burst. Once a remote site has
received an initialization message, that site will transition to
normal multiple access mode.
[010]For further details of the architecture and operation of
the spectral reuse link control mechanism disclosed in the
above-referenced '753 application, attention may-be directed to
that document. They will not be detailed here, in order to focus
the present description on the problem of 'silent' receiver
interference, whereby transmissions on 'clear' and thereby
potentially available secondary reuse sub-channels undesirably
cause the activation of squelch circuits of primary users'
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silent receivers, namely, those primary user receivers that are
not currently transmitting, but are monitoring primary users'
channels, containing such 'clear' sub-channels, for a
transmission (from another primary user).
SUMMARY OF THE INVENTION
[011]In accordance with the present invention, this 'silent'
receiver interference problem is successfully addressed by
equipping the spectral reuse transceiver's communications
controller with a 'smart' sub-channel hopping control mechanism,
that uses one or more prescribed sub-channel selection filters
or discriminators, to control the manner in which it hops among
'clear' or potentially available sub-channels, so as to
substantially reduce and optimally minimize the likelihood that
silent receivers of primary users will integrate sufficient
energy in the sub-channel transmissions that would otherwise
cause activation of their squelch circuits.
[012]A first of these discriminators involves limiting the
'dwell time', or duration of transmission spent, on a hopped
sub-channel, so as to reduce the energy density in a primary
user channel (such as a 12.5 KHz push-to-talk voice channel)
containing that sub-channel to a value that avoids activating of
the squelch circuit of a radio tuned to that primary user
channel.
[013]A second discriminator involves. rejecting, or not hopping
to, the most recently (immediately previously) transmitted sub-
channel, while a third discriminator involves rejecting an
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immediately 'spectrally adjacent' hopped sub-channel, namely,
one that is spectrally mutually contiguous with (one hopped sub-
channel higher or one hopped sub-channel lower than) the
previously transmitted sub-channel.
[014]Similar to the effect of the first discriminator, not
hopping to a"most recently transmitted" sub-channel, or to a
"spectrally adjacent" sub-channel, prevents the squelch circuit
of a primary user's silent receiver (that is tuned to the
channel containing such a sub-channel) from otherwise
integrating energy in that sub-channel which, when combined with
the energy in the previous, or spectrally adjacent, sub-channel,
might be sufficient to trigger the receiver's squelch circuit.
[015]A fourth discriminator involves rejecting, or not hopping
to, a sub-channel that is spectrally located at the 'center' of
a primary user channel. By 'center' of a primary user channel is
meant a sub-channel whose center frequency coincides with that
of the primary user channel, or a sub-channel that coincides
with one of those sub-channels, into which the primary channel
may be sub-divided, which overlap or are immediately adjacent to
the center frequency of the primary channel. The fourth
discriminator avoids these sub-channels, hopping instead to only
those sub-channels that are spectrally adjacent to 'edges' of
the primary channel. This selective use of only edge-adjacent
sub-channels again serves to minimize the energy density seen by
the squelch circuit in the vicinity of the center frequency of
the primary channel, and thereby reduces the likelihood that the
squelch circuit will be triggered by the energy in the sub-
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channel transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
[016]Figure 1 diagrammatically illustrates the overall
architecture of a communication network, respective terminal
unit transceiver sites of which employ the spectral reuse
transceiver of the invention disclosed in the above-referenced
'753 application;
[017]Figure 2 graphically illustrates the relationship between
silent receiver interference level and dwell time in a spectral
reuse transceiver of the type described in the above-referenced
'753 application;
[018]Figure 3 is flowchart of a routine for implementing sub-
channel discriminators that reject most recently used and
spectrally adjacent sub-channels;
[019]Figure 4 is a reduced complexity spectral diagram showing a
distribution of 6.25 KHz; 12.5 KHz, 25 KHz and 50 KHz primary
user channels;
[020]Figure 5 is a spectral diagram showing two 6.25 KHz sub-
channels spectrally abutting the edges of a 12.5 KHz primary
user channel;
[021]Figure 6 is a spectral diagram showing the band structure
of a 25 KHz primary user channel divided into four 6.25 KHz sub-
channels;
[022]Figure 7 is a spectral diagram showing the band structure
of a 50 KHz primary user channel divided into eight 6.25 KHz
sub-channels; and
[023]Figure 8 shows= the respective steps of a sub-channel
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selection routine that may be employed to implement a channel
center-avoidance discriminator.
DETAILED DESCRIPTION
[024]Before describing the details of the 'smart' sub-channel
hopping control mechanism of the present invention, it should be
observed that the invention essentially involves an augmentation
of the sub-channel hopping control mechanism executed by the
communications control processor of the spectral reuse
transceiver of the type disclosed in the above-referenced '753
application, that involves the execution of one or more
prescribed discriminators or sub-channel selection filters, so
as to effectively minimize the likelihood that the silent
receiver of a primary user will see sufficient sub-channel
energy that would otherwise cause its activation of its squelch
circuit. As will be described, these filter functions are
readily implemented by appropriately setting the configuration
parameters used by the communications controller of the
transceiver disclosed in the '753 application to control the
operation of the transceiver. The architecture of the
transceiver of the '753 application remains unchanged. As a
consequence, the present invention has been illustrated in the
drawings by readily understandable diagrammatic illustrations,
which include a generalized network architecture diagram, and a
channel sub-division diagram, that show only those details that
are pertinent to the invention, so as not to obscure the
disclosure with details which will be readily apparent to one
skilled in the art having the benefit of the description herein.
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[025]As pointed out briefly above, an essential objective of
each of the discriminators of the augmentation to the sub-
channel hopping control mechanism in accordance with the
invention is to substantially reduce, or minimize, the
likelihood that any sub-channel transmitted by the spectral
reuse transceiver will present sufficient energy, in the channel
to which a silent receiver of a primary user is tuned, to
trigger the receiver's squelch circuit. Non-limiting, but
preferred, examples of such discriminators include: 1- limiting
the 'dwell time' (duration of transmission) on a hopped sub-
channel; 2- rejecting (not hopping to) the most recently
transmitted sub-channel;--3- rejecting an immediately 'spectrally
adjacent' sub-channel; and 4- rejecting a sub-channel that is
spectrally located at the 'center' of a primary user channel.
The operation and effect of each of these discriminators will be
discussed individually below.
[026]To facilitate an understanding of the functionality arid
effect of the first discriminator - dwell time - attention may
be directed to Figure 2, which graphically illustrates the
relationship between silent receiver interference level and
dwell time in a spectral reuse transceiver of the type described
in the above-referenced '753 application. As pointed'out above,
by 'dwell time' is meant the length of time that the
transmission section of the spectral reuse transceiver remains
on a selected sub-channel before hopping to a new sub-channel.
Typically, dwell time is measured in numbers of symbols, bits,
or milliseconds. In the graph 20 of Figure 2, dwell time is a
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point along the horizontal axis 22, while interference is a
point along the vertical axis 24. As can be seen from the graph,
decreasing dwell time on a sub-channel serves to reduce the
energy density in a primary user channel (such as a 12.5 KHz
push-to-talk voice channel) containing that sub-channel to a
value that avoids activating =of the squelch circuit of a radio
tuned to that primary user channel.
[027]The graph 20 of Figure 2 applies to frequency-hopping
radios in general; the actual values of points on the graph will
depend upon the particular implementation of the frequency-
hopping dwell time control mechanism used by the radio. It
should be noted that there is a trade-off between dwell time and
each of complexity of implementation and sub-channel
availability. Reducing the dwell time to an extremely short
interval can be non-trivial, as it increases the complexity of
the design. In addition, depending on the implementation of the
dwell time control mechanism, there may be a reduction in
throughput, as the dwell time decreases. Thus, the actual dwell
time on a hopped sub-channel is a choice of the manufacturer,
based upon intended performance, implementation complexity and
regulatory constraints.
[028]As a non-limiting, but practical, illustration, reducing
the dwell time to a value on the order of five symbols (or less)
provides a significant reduction in the likelihood that the sub-
channel transmission will trigger silent receiver squelch noise.
This is due to the fact that the principal effect of decreasing
the dwell time or duration of transmission at a respective
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hopped sub-channel is a reduction in the energy density in a
user channel (such as a 12.5 KHz voice radio user channel) to
which a silent receiver of a primary user is tuned, and thereby
reduces the likelihood that the constrained dwell time sub-
channel transmission will trigger a receiver's squelch circuit.
[029]A second discriminator or filter that may be used by the
transceiver's communications controller to reduce or minimize
the amount of sub-channel energy present in the bandwidth of the
channel to which a silent receiver is tuned, is operative to
cause the communications controller to reject, or not hop to,
the most recently used, or immediately precedent, hopped sub-
channel. A relatively simple filtering mechanism, shown in the
flowchart of Figure 3, described below, involves incrementally
'sweeping' the transmission frequency - for example, from the
lowest sub-channel to the highest sub-channel (or conversely
from -the highest sub-channel to the lowest sub-channel), which
ensures that no sub-channel will be used for two immediately
successive hops.
[030]A third discriminator involves rejecting an immediately
'spectrally adjacent' hopped sub-channel, namely, a sub-channel
that is spectrally mutually contiguous with (one hopped sub-
channel higher or one hopped sub-channel lower than) the
previously transmitted sub-channel. Similar to the effect of the
first discriminator, not hopping to a most recently transmitted
sub-channel in accordance with the second discriminator, or not
hopping to a spectrally adjacent sub-channel in accordance with
the third discriminator prevents the squelch circuit of a
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primary user's silent receiver that is tuned to a channel
containing such a sub-channel from otherwise integrating energy
in that sub-channel which, when combined with the energy in the
previous or spectrally adjacent sub-channel, might be sufficient
to trigger the receiver's squelch circuit.
[031]As in the case of the second discriminator, rejecting an
immediately 'spectrally adjacent' hopped sub-channel may be
readily accomplished by incrementally sweeping the transmission
frequency and skipping every nth sub-channel, and repeating as
necessary, using the next value of n. For example, with n having
a value of two, the spectral reuse transceiver may use even-
numbered sub-channels and then odd-numbered sub-channels,
repeating as necessary. Depending on the implementation chosen,
more-random selections may be used. The sub-channel selection
distribution function (such as a uniform distribution or
Gaussian distribution) may be further constrained by a density
function that rejects selections which increase the energy
density in any one or more user channels in the band. For
example, the sub-channel rejection filter may prohibit 'n'
transmissions per period 't', where 'n' and 't' are parameters
of the filter. After such a rejection, the distribution function
may be used again to find a new 'candidate' sub-channel.
[032]More particularly, as shown in Figure 3, a filter routine
that is effective to implement the second and third
discriminator functions, described above, has an initialized
entry point 31, wherein the designation of the last hopped sub-
channel is set at an-invalid value (e.g., -1), so that the first
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selected hopped sub-channel will always be valid (not filtered
out). A next hopping channel step 32 then causes the next hopped
sub-channel to be selected, by invoking the operation of a
pseudo-random number sequence generator (PNSEQ) step 33. When
invoked, PNSEQ step 33 outputs a pseudo-randomly based number
that specifies one of the available hopped sub-channels as a
'candidate' hopped channel.
[033]This candidate hopped. sub-channel is= then coupled to a
channel compare step 34, which determines whether the candidate
hopped sub-channel, as specified by the PNSEQ step 33, is the
'same' hopped sub-channel as, or is 'adjacent' to the last valid
hopped sub-channel. If the candidate hopped channel is the 'same
as', or 'adjacent' to, the last valid hopped channel, the
channel compare step 34 provides an output, 35 to the PNSEQ step
33 to select a new candidate hopped sub-channel. Otherwise, the
channel compare step 34 supplies the candidate= hopped sub-
channel to a hop step 36, which outputs the candidate hopped
sub-channel to the frequency hopping control mechanism of the
transceiver, causing the transceiver to be tuned to that sub-
channel. (It should be noted that although selection of the next
candidate hopped sub-channel in the routine of Figure 3 is
pseudo-randomly based, as an equivalent alternative, it may be
non-randomly based (e.g., an immediately successive hopped sub-
channel, or every nth hopped sub-channel).)
[034]As interfering traffic increases, the number of available
sub-channels will decrease and, at some point, the distribution
filter may be unable to find any candidate sub-channels that
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satisfy the discriminator function. In this case, the quality of
service will be reduced, as a result of complying with an
interference-avoidance policy, which is a filter configuration
parameter, such as flow-controlling lower-priority radio traffic
or flow-controlling all traffic in the network. If the policy
permits, the frequency-hopping sequences may be repeated, in
which case the transmission density may increase (with an
associated increase in interference), but throughput will remain
unchanged. The frequency-hopping sequences may be managed in the
frequency and time domains to minimize energy density over time
in the various user channels, thereby minimizing the likelihood
of activating legacy squelch circuits.
[035]A fourth, 'channel center-avoidance', discriminator
involves rejecting (not hopping to) a sub-channel that is
spectrally located at, or in the vicinity of, the 'center' of a
primary user channel. Being 'centrally' located means that
transmission of, the sub-channel would inject energy into a
spectrally center portion of the primary user channel, where the
silent receiver is particularly sensitive. As an example, the
center frequency of the sub-channel may coincide with that of
the primary user channel, or may overlap or be immediately
adjacent to the center frequency of the primary channel. The
fourth discriminator avoids these sub-channels, by hopping,
instead, to only those sub-channels that may be considered to be
spectrally displaced from the center of the primary channel, so
as to be close or adjacent to 'edges' of the primary channel.
This selective use of only edge-adjacent sub-channels again
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serves to mitigate against the sub-channel injecting inject
energy into the spectrally center portion of the primary user
channel, so as to minimize the energy density seen by the
squelch circuit in the vicinity of the center frequency of the
primary channel, and thereby reduces the likelihood that the
squelch circuit will be triggered by the energy in the sub-
channel transmission.
[036]In order to appropriately choose such 'edge'-associated
sub-channels, 'the fourth discriminator relies upon the spectral
structure of the band's channel plan. As a non-limiting
illustration, consider the spectral structure of the licensed
217-220 MHz band referenced above, which, as illustrated in the
reduced complexity spectral diagram of Figure 4, contains a
distribution of 6.25 KHz, 12.5 KHz, 25 KHz and 50 KHz user
channels, respectively shown at 41, 42, 43 and 44. The spectral
disparity among these channels results from the fact that, over
time, they have been sequentially licensed to various primary
users in response to incremental allocation requests, on the one
hand, and due to the evolution of tighter spectral efficiency
requirements that have been promulgated by the FCC to meet the
continuously increasing demand for bandwidth.
[037]For the band structure example of Figure 4, the filtering
mechanism employed by the fourth discriminator selects hopping
sub-channel sequences that give preference to those 6.25 KHz
sub-channels that are located at or coincide with edges of the
respective 12.5 KHz and 25 KHz primary user channels 42 and 43,
or which are contained within either of two pairs of spectrally
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contiguous 6.25 KHz sub-channels, that spectrally abut the edges
of a 50 KHz primary user channel 44. This may be readily
understood by reference to the spectral diagrams of Figures 5, 6
and 7.
[038] In particular, Figure 5 shows two 6.25 KHz sub-channels 51
and 52 spectrally abutting the edges 55, 56, respectively of a
12.5 KHz primary user channel 50, so that the center frequencies
of the 6.25 KHz sub-channels 51 and 52 are spectrally displaced
from the center frequency of the 12.5 KHz primary user channel
50. Therefore, selecting either of sub-channels 51 and 52 will
cause the spectral reuse transceiver to transmit on a sub-
channel whose center frequency is spectrally spaced apart from
the center frequency of the 12.5 KHz primary user channel 50;
also, as shown in Figure 5, the energy in either sub-channel
51/52 is minimal at the center frequency 53 of the 12.5 KHz
channel 50.
[039]Figure 6 shows the band structure of a 25 KHz primary user
channel 60, in which four 6.25 KHz sub-channels 61, 62, 63 and
64 are available. Of these four sub-channels, sub-channels 61
and 64 spectrally abut the outer edges 66 and 67, respectively,
of the 25 KHz primary user channel 60, and may therefore be
denoted as''outer' sub-channels; on the other hand, while sub-
channels 62 and 63 are spectrally adjacent to the center
frequency 65 of the 25 KHz primary user channel 60, and may
therefore be denoted as 'inner' sub-channels. As can be seen
from Figure 5, transmitting on only the edge-adjacent or 'outer'
sub-channels 61 and 64 will inject less energy into the
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spectrally central portion of the 25 KHz user channel 60, and
therefore will typically have less impact on a squelch circuit
of a silent receiver tuned to the 25 KHz user channel than would
transmitting on either of the 'inner' sub-channels 62 and 63
near the center frequency of the user channel 60. As a result,
transmitting on either of the 'outer' sub-channels 61 and 64
will reduce the likelihood that the squelch circuit will be
triggered by the energy in either of these two 'outer' sub-
channels.
[040]It may be noted that, for the case of using 12.5 KHz and 25
KHz primary user channels, shown in Figures 5 and 6, if more
(6.25 KHz) sub-channels are required, gradual selections may be
made, in order to uniformly distribute sub-channel selections
across the primary user channels. Preference may be given to a
second one of the two 'outer' 6.25 KHz sub-channels 61 and 64
within a 25 KHz user channel 60 in a given amount of time,
versus using the second.of the two 6.25 KHz sub-channels 51, 52
within a 12.5 KHz user channel 50. Also, the choice of sub-
channels may be such that the spectral reuse transceiver will
not make adjacent sub-channel hops within any 25 KHz voice
channel, thereby minimizing the energy-integrating effect of
legacy squelch circuits.
[041]Figure 7 shows the band structure of a 50 KHz primary user
channel 70, in which eight 6.25 KHz sub-channels 71, 72, 73, 74,
75, 76, 77 and 78 are available. Of these eight sub-channels;
two pairs of mutually contiguous sub-channels 71, 72 and 77, 78
spectrally abut the respective edges 70-1 and 70-2 of the 50 KHz
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primary user channel 70, and may therefore be denoted as 'outer'
pairs of sub-channels, while two pairs of mutually contiguous
sub-channels 73, 74, 75 and 76 are spectrally adjacent to the
center frequency 79 of the 50 KHz primary user channel 70, and
may therefore be denoted as 'inner' pairs of sub-channels. As
can be seen from Figure 7, transmitting on only the edge-
adjacent pairs of sub-channels 71, 72 and 77, 78 will
substantially reduce (optimally minimize) the energy density
seen by a silent receiver that is tuned to (the center frequency
of) the 50 KHz primary channel 70, and thereby reduce the
likelihood that its squelch circuit will be triggered by the
energy in any sub-channel of either of the two 'outer' pairs of
sub-channels.
[042]Figure 8 shows the respective steps of a filter routine
that may be employed to implement the 'channel center-avoidance'
discriminator described above with reference to Figures 5-7. In
particular, the routine of Figure 8 serves to mitigate against
silent receiver interference by avoiding transmitting on those
(6.25 KHz) sub-channels that encroach upon the 'center' of an
allocated user channel. As described above, the center of a
primary user channel may overlap one or more candidate sub-
channels, depending on the plan used for the radio band. Also,
it is again to be noted that the term 'center' is not limited to
a particular band location, and may encompass one or more sub-
channels, such as three hopped sub-channels as the 'center'.
[043]The filter routine of Figure 8 has an initialized entry
point 81, wherein the designation of the last hopped sub-channel
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used is defaulted to an invalid value (e.g., .-1), so that the
first selected hopped sub-channel will always be valid (not
filtered out). A next hopping channel step 82 causes the next
hopped sub-channel to be selected by invoking the operation of a
pseudo-random number sequence generator (PNSEQ) step 83. When
invoked, PNSEQ step 83 outputs a pseudo-randomly based number
that 'specifies one of the available (6.25 KHz) hopped sub-
channels as a 'candidate' hopped channel.
[044]This candidate hopped sub-channel is then coupled to a
center frequency compare step' 84, which determines whether the
PNSEQ step 83 has selected a next to-be-hopped-to sub-channel
candidate that overlaps the center of the user's channel. If the
candidate sub-channel overlaps the center of the user's channel,
the center frequency compare step 84 provides an output 85 that
causes the PNSEQ step 83 to select a new candidate to-be-hopped-
to sub-channel. Otherwise, the center frequency compare step 84
presents the candidate sub-channel to a hop step 86, which
outputs the candidate sub-channel to the frequency hopping
control mechanism of the spectral reuse transceiver radio,
causing its transmitter to be tuned to that sub-channel. Again,
as in the filter routine of Figure 3, selection of the next
candidate sub-channel in the routine of Figure 8 need not be
pseudo-randomly based, but may be non-randomly based (e.g., an
immediately successive hopped sub-channel, or every nth hopped
sub-channel).
[045]As described above, the communications control processor
for a spectral reuse transceiver of a communication system of
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the type disclosed in the above-identified '753 application may
employ one or more of the discriminators described above. The
choice of which discriminators are to be used is up to the user.
For optimal performance, employing all four discriminators will
minimize the likelihood of triggering the squelch circuits of
most, if not all, currently employed receivers. However, even if
all of the above discriminators are employed, there is still the
possibility that the squelch circuit of silent radios of some
primary users may be triggered by a sub-channel transmission.
This possibility is due to the fact that, over the years, such
users have purchased their (push-to-talk) radios from different
manufacturers, whose products vary in cost and do not
necessarily adhere to a common set of quality and performance
standards. As a result, the sensitivities of the squelch
circuits of these radios are not the same.
[046] For example, some squelch circuits do not have significant
selectivity (or can be adjusted arbitrarily low), and will
respond to true random noise, if the squelch threshold is very
close to the noise floor. As the squelch circuit's threshold
approaches the noise floor, it ceases to provide differentiation
between a useful signal and noise. Also, while it is important
to avoid successive (hopped) transmissions on spectrally
adjacent sub-channels and successive transmissions on the same
sub-channel within a given user channel, as described above with
reference the second and third discriminators, it is also
important to manage sub-channel transmissions with a greater
granularity, which can be carried out in the time domain. Thus,
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using multiple discriminators should effectively prevent the
energy in each sub-channel transmission from significantly
accumulating in time in any user channel, thereby mitigating
against an undesirable increase in sub-channel energy density
seen by a silent receiver.
[047]However, if the receiver of a primary user's (push-to-talk)
radio has relatively low signal processing/filtering capability,
its squelch circuit may be triggered by a secondary user's sub-
channel transmission, even if all of the discriminators are
used. Should this occur, it can be expected that the primary
user will complain to the FCC. In this case, the practical
recourse for the secondary user is to set the configuration
parameters of the spectral reuse transceiver, so that the
encroaching sub-channel is permanently excluded, or 'notched'
out, from the clear channel assessment routine. This will
prevent that routine from ever looking for activity in the
offending sub-channel, and thereby effectively eliminate the
possibility that such a permanently excluded sub-channel will
activate the squelch circuit of the complaining user's silent
radio.
[048]As will be appreciated from the foregoing description, the
likelihood of triggering the squelch circuit of silent receivers
of licensed primary channel users as a result of transmitting of
'clear' sub-channels employed in a spectral reuse communication
system of the type disclosed in the above-referenced '753
application is substantially reduced, and optimally minimized,
in accordance with the present invention, by means of a 'smart'
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hopping control mechanism comprised of one or more or sub-
channel selection prescribed discriminators or filters, that
enables a transmitting spectral reuse transceiver to
substantially reduce the amount of transmitted sub-channel
energy that may be sensed by a silent receiver, as the spectral
reuse transceiver sequentially hops to and transmits on
potentially available sub-channels. Optimally, this serves to
minimize the likelihood that silent receivers will integrate
sufficient energy in sub-channel transmissions that would
otherwise cause activation of their squelch circuits.
[049]While we have shown and described several embodiments in
accordance with the present invention, it is to be understood
that the same is not limited thereto but is susceptible to
numerous changes and modifications as known to a person skilled
in the art, and I therefore do not wish to be limited to the
details shown and described herein, but. intend to cover all such
changes and modifications as are obvious to one of ordinary
skill in the art.
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