Note: Descriptions are shown in the official language in which they were submitted.
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STABILIZED CONTROL CHANNEL PLANNING USING LOOSELY
(,~O~JPLED DEDICATED TRAFFIC (',~HANNELS
RELATED APPLICATION
This application is related to U.S. Patent No. 5,732,353, entitled "Automatic
Control Channel Planning in Adaptive Channel Allocation Systems" and filed on
an
even date herewith.
BACKGROUND
The present invention relates generally to adaptive channel allocation in
radiocommunication systems and more particularly to automatic control channel
planning in systems which utilize adaptive channel allocation.
Various methods have been introduced to efficiently utilize the limited range
of
frequencies available for radio communications. One well-known example is
frequency reuse, a technique whereby groups of frequencies are allocated for
use in
regions of limited geographic coverage known as cells. CeIIs containing the
same
groups of frequencies are geographically separated to allow callers in
different cells to
simultaneously use the same frequency without interfering with each other. By
so
doing many thousands of subscribers may be served by a system of only several
hundred frequencies.
The design and operation of such a system is described in an article entitled
Advanced Mobile Phone Service by Blecher, IEEE Transactions on Vehicular
Technology, Vol. VT29, No. 2, May, 1980, pp. 238-244. Commonly known as the
AMPS system, this system had allocated to it by the FCC a block of the UHF
frequency spectrum further subdivided into pairs of narrow frequency bands
called
channels. At present there are 832, 30 kHz wide channels allocated to cellular
mobile communications in the United States. A table of the frequencies
dedicated to
mobile communications in the U.S. is shown in Figure 1, Of the 832 available
channels, there are Z1 control channels dedicated each to the A-carrier and
the B-
carrier. These 42 control channels provide system information and cannot be
used for
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voice traffic. The remaining 790 channels, known as voice or traffic channels,
catty
the burden of voice or data communication.
Frequency planning is a process by which individual channels are assigned to
cells within the network. Currently, most frequency planning is done a priori;
that is
a fixed frequency plan is "hard-wired" in place by each cellular system
operator.
This is known as fixed channel allocation, or FCA. However, as interference
and
traffrc load are time varying, FCA has disadvantages with regard to system
adaptability. For example, in microcells, picocells, and indoor cellular or
PCS
systems, the base stations aie located so densely and the environment is so
unpredictable and time-varying (e.g., opening a door changes the interference
conditions), that channel planning becomes nearly impossible. Because of the
time
varying nature of the interference, therefore, an adaptive scheme can offer
significant
advantages.
Adaptive channel allocation, or ACA, is a method of dynamically allocating
frequencies throughout a cellular system to increase system capacity and
adaptability.
Under an ACA scheme, more frequencies would be allocated to busy cells from
more
lightly loaded cells. In addition, the channels can be allocated such that all
links have
satisfactory quality. A common feature of ACA systems is that they allocate a
charnel out of a set of channels which fulfills some predetermined quality
criteria.
Hov~rever, different ACA schemes select channels from the set based upon
different
criteria.
The concept of ACA is well-known to those skilled in the art, and its
potential
has been described in various publications. For example, "Capacity Improvement
by
Adaptive Channel Allocation", by H~kan Eriksson, IEEE Global Telecomm. Conf.,
Nov. 28-Dec. 1, 1988, pp. 1355-1359, illustrates the capacity gains associated
with a
cellular radio system where all of the channels are a common resource shared
by all
base stations. In the above-referenced report, the mobile measures the signal
quality
of the downlink, and channels are assigned on the basis of selecting the
channel with
the highest signal to interference ratio (C/I level).
Another approach is described by G. Riva, "Performance Analysis of an
Improved Dynamic Channel Allocation Scheme for Cellular Mobile Radio Systems
",
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42nd IEEE Veh. Tech. Conf., Denver, 1992, pp. 794-797 where the channel is
selected based on achieving a quality close to or slightly better than a
required C/I
threshold. Furuya Y. et al., "Channel Segregation, A Distributed Adaptive
Channel
Allocation Scheme for Mobile Communications Systems ", Second Nordic Seminar
on
Digital Land Mobile Radio. Communication, Stockholm, October 14-16, 1986,
pp. 3:11-315 describe an ACA system wherein the recent history of link quality
is
considered as a factor in allocation decisions. In addition several hybrid
systems have
been presented where ACA is applied to a small block of frequencies on top of
an
FCA scheme. ~ Such an example is presented in Sallberg, K., et al., "Hybrid
Channel
Assignment and Reuse Partitioning in a Cellular Mobile Telephone System ",
Proc. IEEE VTC ' 87, 1987, pp. 405-411.
Apart from increases in system capacity, adaptive channel allocation can
obviate the need for system planning. Planning is instead performed by the
system
itself. This feature of ACA is particularly attractive when system changes are
implemented, when new base stations are added, or when the environment
changes,
for example by the construction or demolition of large buildings.
The above described adaptive channel allocation schemes, however, have
generally been utilized only in conjunction with the allocation of traffic
channels, and
not control channels. Thus, although each base station has access to all the
traffic
channels, the allocation of control channels has typically remained a fixed
allocation
in which each base station uses a certain predetermined control channel or
channels.
Since the control channels are not adaptively allocated, the operator has to
plan these
channels geographically, i.e., which base gets what control channel so as to
minimize
the amount of co-channel interference experienced on the control channels.
Thus, the
advantages of increased capacity and adaptability realized in ACA traffic
channel
allocation have generally not been achieved with respect to control channel
allocation.
Because control channels have been fixed to each base station, changes in
control
channel allocation have required costly system reconfiguration. However, only
if both
the traffic channels and the control channels are automatically allocated is
an operator
effectively relieved from planning the system.
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A partial solution to the problems of fixed control channel allocation could
be
providE:d by a system which directly incorporated the allocation of control
channels
into a ~;onventional ACA scheme. However, allocation of traffic channels in
ACA
routinea is based on certain criteria such as interference, channel success
rate,
previous performance of the channel, etc., whereas criteria for measuring
quality are
quite different for control channels. For example, there is no success rate of
previous
perfonnance for control channels since (1) a control channel cannot be allowed
to be
unsuccessful, and (2) the performance of different control channels cannot be
compared because this would require alternatively using each of the control
channels
to get an average performance measure. The latter is not desirable, since
control
channel allocation should remain reasonably stable.
Another problem with incorporating control channels directly into a
conventional ACA routine is that transmission on control channels is bursty
and
irregular, particularly on the uplink from mobile to base, because the many
mobile
stations transmit control signals over a range of different distances and
power levels.
Consequently, measurements of these bursty control signals do not provide a
reliable
indication on which to base ACA decisions. Thus, the incorporation of control
channels directly into a conventional ACA routine is not a desirable solution
to the
problem presented by the lack of a mechanism for adaptively allocating control
channels.
There is a need in the industry, therefore, for a system and method of
automatic control channel planning in ACA systems which provide reliability
and
system adaptability in the allocation of control channels.
SUMMARY
Accordingly, it is an object of the invention to provide a method and
apparatus
which enable a system using adaptive channel allocation (ACA) for allocating
traffic
or voice channels, to automatically plan the control channels as well. The
method can
allocate control channels using any existing ACA scheme, for example in the
AMPS
or ADC systems, currently used by an operator for traffic channel allocation.
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According to exemplary embodiments of the invention, a radiocommunication
system utilizes a block of control channels in the frequency spectrum to
transmit
control information between base stations and mobile stations. The system also
utilize s a set of traffic channels to transmit information such as voice
information
between bases and mobiles. Each base in the cellular system has access to all
the
traffic channels and all the control channels. Included in the set of traffic
channels is
a block of dedicated traffic channels, each one of which is associated or
coupled with
a particular control channel in the block of control channels. The particular
frequency
pairing of each dedicated traffic channel and its associated control channel
is the same
wherever these frequencies are reused in the system.
The coupling of the control channels to the dedicated traffic channels
provides
a method of allocating the control channels. By basing control channel
allocation
decisions on the base station's selection of dedicated traffic channels, ACA
is
provided for control channels without directly incorporating their into an ACA
routine.
To provide stability in control channel allocation, the control channels are
preferably coupled in an average sense to the dedicated traffic channels.
Thus, the
recent history of a base station's selection of dedicated traffic channels can
be
monitored and averaged by the base station, the average behavior of the
dedicated
traffic channels being used to control allocation of a control channel for
that base
station. This monitoring and averaging can be implemented, for example, with
an
accumulator for each dedicated channel. From these accumulators an ordered
list of
the most frequently selected dedicated traffic channels at a particular base
station can
be produced. When the most frequently selected dedicated traffic channel is
changed,
a new control channel, coupled to the new most frequently selected dedicated
traffic
channel, is allocated to the base station. Thus, by allocating the control
channels
based upon an average past performance of dedicated traffic channels, the
invention
can provide stability in control channel allocation, ensuring that control
channels
remain relatively stationary with respect to the base stations, adapting to
slow varying
changes, but not often being reallocated from frequency to frequency. The
control
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channels are thus loosely coupled to the dedicated traffic channels and do not
follow
sudden changes in the allocation of dedicated traffic channels.
Because the frequency pairing between the control channels and the dedicated
traffic channels is the same wherever the frequencies are reused, there is a
high
correlation between the quality in the coupled channels throughout the system.
This
correlation in quality thus allows control channels to be adaptively allocated
based on
the allocation of the associated dedicated traffic channels without directly
incorporating the control channels into an ACA routine.
The invention provides several other advantages over prior
radiocommunication systems. For example, the invention allows both the traffic
chamlels and the control channels to be adaptively allocated, which fully
relieves an
operator from system planning. The benefit of not having to fixedly associate
control
channels with base stations, and the resultant ability to adapt to slow
changes in the
environment such as new buildings and large constructions, or changes in the
infrastructure such as the addition of more base stations in "hot spots", is
of prime
importance. The invention thus provides a significant advantage over systems
which
employ ACA on traffic channels only.
In addition, the invention provides the advantage of operating through the
allocation of the traffic channels based on measurements of the traffic
channels.
These measurements are significantly more reliable and easy to determine than
measurements of the control channels.
Finally, the invention provides the ACA benefit that the system can adapt to
changing traffic conditions. Peak traffic conditions can be accommodated by
temporarily allocating more traffic channels in a restricted area. For control
channels, this adaptation to non-uniform traffic is generally less of a
concern.
However, the present invention allows the usage of more than one control
channel in
a b~~se when required by traffic conditions.
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According to an aspect of the present invention there is provided a method for
allocating control channels in a radiocommunication system comprising the
steps of
providing a plurality of traffic channels in the radiocommunication system,
providing a
plurality of control channels in the radiocommunication systems, associating
each of a
dedicated subset of the plurality of traffic channels with a respective one of
the plurality
of control channels, accumulating a number of times that each of the dedicated
traffic
channels is allocated to a base station over a period of time, and allocating
a control
channel to the base station based upon the counted numbers of times.
According to another aspect of the present invention there is provided a base
1o station comprising a transmitter for transmitting signals on traffic
channels and control
channels, means for counting a number of times each of the traffic channels is
allocated
to the base station, and means for informing a system of the counted number of
times.
According to a further aspect of the present invention there is provided in a
radiocommunication system having base stations which transmit information on
traffic
channels and control channels, a method for allocating control channels to the
base
stations comprising the steps of associating at least one of the traffic
channels with one of
the control channels, allocating the traffic channels to base stations, and
allocating the
control channels to the base stations based on the allocation of the
associated traffic
channels to the base stations.
2o According to a further aspect of the present invention there is provided a
base
station in a radiocommunication system comprising a transmitter for
transmitting signals
on traffic channels and control channels, means for counting a number of times
that each
of the traffic channels is allocated to the base station, and means for
selecting a control
channel to be allocated to the base station based on the counted numbers of
times.
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BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the present
invention will be more readily understood upon reading the following detailed
description in conjunction with the drawings in which:
Figure I is an illustration of the allocated frequency spectrum as per the
U.S.
standard IS-54B;
Figure 2 is a diagram of an exemplary radiocommunication network;
Figure 3 is a schematic diagram of an exemplary base and mobile station;
Figure 4 is a diagram of traffic and control channels in a frequency spectrum
according to an exemplary embodiment of the invention; and
Figure 5 illustrates accumulators in a base station according to an exemplary
embodiment of the present invention.
DETAILED DESCRIPTION
Before describing the details of the present invention, an example of the
construction of a cellular mobile radio system in which the present invention
can be
utilized will be described. While the construction shown depicts a digital
system,
those skilled in the art will appreciate that it is also possible to implement
the present
invention on other types of systems such as analog or dual-mode systems.
Fig. 2 is a schematic diagram illustrating ten cells, C1 to C10, in a cellular
mobile radio telephone system. Normally, methods according to the present
invention
would be implemented in a cellular mobile radio system comprising many more
cells
than ten. For purposes of this discussion, the system depicted herein is
considered to
be an isolated portion of a larger system which has been fragmented.
For each cell C 1 to C 10, there is a respective base station B 1 to B 10.
Fig. 2
illustrates base stations situated in the vicinity of cell centers and having
om.ni-directional antennas. The base stations of adjacent cells may however be
co-
lorated in the vicinity of cell borders and have directional antennas.
Also illustrated in Figure 2 are ten mobile stations M1 to M10, which are
movable within a cell and from one cell to another cell. The method according
to the
present invention may be implemented in a cellular mobile radio system
comprising
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many more mobile stations than ten. Normally, there are many more mobile
stations
than there are base stations.
A mobile switching center MSC as illustrated in Fig. 2 is connected to all ten
illustrated base stations, for example by cables or other media such as fixed
radio
links. The mobile switching center is also connected by cables or other media
to, for
example, a public switching telephone network or similar fixed network with
ISDN
facilities. To simplify the illustration, not all connections from the mobile
switching
center to base stations and connections to the fixed network are illustrated
in Figure 2.
An exemplary base station 110 and mobile 120 are illustrated in Figure 3. The
base station includes a control and processing unit 130 which is connected to
the MSC
140 which in turn is connected to the public switched telephone network (not
shown).
The base station 110 for a cell includes a plurality of traffic or voice
channels
handled by traffic channel transceiver 150 which is controlled by the control
and
processing unit 130. Also, each base station includes a control channel
transceiver
160 which may be capable of handling more than one control channel. The
control
channel transceiver 160 is controlled by the control and processing unit 130.
The
control channel transceiver 160 broadcasts control information over the
control
channel of the base station or cell to mobiles locked to that control channel.
The
traffic channel transceiver broadcasts the traffic or voice channels which can
also
include digital control channel location information.
When the mobile 120 first enters idle mode, it periodically scans the control
channels of base stations such as base station 110 to determine which cell to
lock on
to. 'The mobile 120 receives the absolute and relative information broadcast
on a
conrsol channel at its traffic and control channel transceiver 170. Then, the
processing unit 180 evaluates the received control channel information which
includes
the characteristics of the candidate cells and determines which cell the
mobile should
lock to. The received control channel information not only includes absolute
information concerning the cell with which it is associated, but can also
contain
relative information concerning other cells proximate to the cell with which
the
control channel is associated. These adjacent cells are periodically scanned
while
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monitoring the primary control channel to determine if there is a more
suitable
candidate.
In the above described radiocommunication system, the frequency spectrum
according to an exemplary embodiment of the invention is divided into two
parts, one
part for the control channels and one part for the traffic channels. Figure 4
shows a
set 50 of N control channels F cl to F cN. Under the AMPS and IS-54 systems,
for
example, a block of 21 frequencies located in a dedicated part of the
frequency
speclxum can be set aside for control channels so that the mobiles know where
in the
frequency spectrum to scan for the control channels. According to other
schemes, the
cont~,-ol channels may be disposed on channels which are not adjacent to one
another
and may be located by mobile stations using a variety of mechanisms, e.g., by
location information transmitted on traffic channels. Those skilled in the art
will
appreciate that the present invention is applicable to any system in which
control
channels are employed. '
Figure 4 also shows sets 60 and 70 of N+M channels used for traffic,
including a set 60 of N dedicated traffic channels F dtl to F dtN. For
example, a
21-channel, dedicated traffic channel block 60 can be specified somewhere in
the
channel space, such as adjacent to block 50, although this particular
arrangement is
not required. Finally, Figure 4 shows a set 70 of M ordinary traffic channels
F tl to
F_tlVi.
Unlike conventional systems, the control channels according to exemplary
embodiments of the invention can be used by any base station, and no fixed
allocation
of control channels to base stations is performed a priori. instead, each
control
channel is coupled to or associated with one of the dedicated traffic
channels, as
shown in Figure 4, resulting in N pairs of control/dedicated traffic channels,
F_ci/F_dti, where i ranges from 1 to N. The method of frequency allocation as
well
as the division of frequencies used for control channels, dedicated traffic
channels,
and ordinary traffic channels can be the same in every base station in the
cellular
system. In addition, the particular frequency pairing of each dedicated
traffic channel
and its associated control channel can be the same in every base station in
the system.
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The traffic channels, including the dedicated traffic channels, can be
incorporated directly into an ACA scheme, whereby they are allocated to base
stations
according to changing interference conditions, for example. The ACA scheme
used
for the traffic channels is preferably a distributed scheme, i.e., the ACA
method uses
local information and is carried out in the base stations or in the MSC. Under
such
an ACA scheme, a reallocation of a dedicated traffic channel might occur, for
example, because of an unacceptably high co-channel interference level from a
nearby
base station transmitting on the same dedicated traffic channel frequency.
Because the
interfering neighboring base 'station uses the same pairing of dedicated
traffic channel
and associated control channel, it is also likely that there will be an
unacceptably high
interference on the control channel frequency. In other words, because all
control
chann els in all bases are coupled in the same way to the dedicated traffic
channels in
the traffic block 60, there is a strong correlation between the quality and
interference
level in the coupled channels. Thus, optimizing the dedicated traffic channels
in
block 60 through the ACA scheme will automatically optimize the control
channels in
block 50 as well. Since the most successful dedicated traffic channel is the
one with
the lowest co-channel interference, it is desirable to use its associated
control channel
as the: control channel of the cell. This is because the coupling between the
dedicated
traffic. channels and control channels is the same in all bases throughout the
system.
Therefore, the co-channel interference conditions in each cell for the
dedicated traffic
channel and its associated control channel are very similar in quality.
Moreover, by
basing the channel allocation decisions on measurements of the traffic
channels rather
than measurements of the control channels, system reliability is enhanced.
Because
transmission on control channels is bursty and irregular, particularly on the
uplink
from mobile to base, measurements of the traffic channels provide a more
reliable
indication on which to base ACA decisions. The way in which the coupling or
association between traffic channels and control channels is used to allocate
control
channels according to the present invention will now be described.
Stability in control channel allocation (i.e., ensuring that control channels
remain relatively stationary with respect to the base stations, adapting to
slow varying
changes, but not frequently hopping from frequency to frequency) is
advantageous
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because mobiles commonly use control channels as references, for example to
make
decisions about base selection, adaptive power control, and adaptive tragic
channel
allocation. Mobile-assisted ACA and APC are discussed further in U.S. Patent
Serial
No. 5,491,837 entitled "Method and System for Channel Allocation Using
S Power Control and Mobile-Assisted Handover Measurements ", filed on
March 7, 1994. Thus, control channel allocation according to the present
invention is based upon the usage by each cell of the dedicated traffic
channels over a period of time. Given the afore-described quality correlation
between
the dedicated control channels and their respective, coupled control channels,
a
control channel associated with a most frequently used dedicated control
channel will
be allocated to a cell or base station.
An exemplary embodiment can thus include the step of monitoring the
allocation of dedicated traffic channels in each base station. In the
controller of the
base station, for example, there can be provided N accumulators for the N
dedicated
traffic channels as shown in Fig. 5. Each time a dedicated traffic channel is
allocated
for a connection, its corresponding accumulator can be incremented. After a
certain
period of operation, the accumulators' contents thus indicate which dedicated
traffic
channels were most successful in the ACA routine, for example by which were
allocated most frequently. Therefore, an ordered list with preferred dedicated
traffic
channels can be provided, starting with the most successful dedicated traffic
channel.
The accumulators can have a finite memory, for example, accumulating traffic
channel information over a period of hours or days. In this way, the system
adapts to
significant system changes, such as the addition of a new base station nearby,
without
permitting statistical aberrations to cause changes in control channel
assignments. The
preferred list can be made using average performance of the dedicated traffic
channels, resulting in a form of low pass filtering or integration of channel
allocation
over time. In this way, when handovers, lost calls, or call terminations
occur, such
events do not on average influence the preferred list and thus the control
channel
allocation.
Since the average in the accumulator is taken over a relatively long time
interval, for example, hours or days, only slow changes alter the ordering of
the
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dedicated traffic channel list, and therefore the allocation of control
channels. The
response time to a change in the environment, however, can be reduced by the
finite
nature of the accumulator. For example, if a dedicated traffic channel has a
very
good history at a particular base station, its accumulator contents will be
very high.
If this dedicated traffic channel subsequently becomes unsuccessful and
remains
unsuccessful, another dedicated traffic channel will take its place. If the
accumulators
were not finite, this transition might take a very long time. The smaller the
accumulators are in size, the faster their response to a "step" input.
However, the
less integration or low pass filtering that is done, the greater the
sensitivity of the
accumulator to aberrations. The finite accumulator thus provides a leaky
integrator.
Because there is an ordered list of dedicated traffic channels, there is also
a
corresponding ordered list of control channels.which a base station may use to
allocate
one or more control channels. In the event of a peak traffic load which
requires more
than one control channel in a cell, more control channels may be allocated,
starting
with the second control channel from the top of the ordered control channel
list and
proceeding downwards. This additional allocation of control channels, however,
does
not necessarily require the allocation of the associated dedicated traffic
channels.
To obtain high quality measurements of dedicated traffic channels, long
periods of time in which the ACA scheme allocates only ordinary traffic
channels
should be avoided. This can, for example, be achieved by requiring the ACA
scheme
to allocate at least one dedicated traffic channel as along as there are users
active in
the cell. In case the user on the dedicated traffic channel hangs up or is
handed over
to another base, the next traffic channel allocation (at call set-up or
handover) should
preferably use a dedicated traffic channel.
The foregoing description focuses on characteristics of the present invention.
Those skilled in the art will readily appreciate that the present invention is
applicable
to any ACA scheme, that is, adaptive channel allocation based upon any quality
criteria selection scheme. Moreover, while the illustrative embodiments have
been
described in terms of mobile stations and cellular systems generally, it will
be
understood that the present invention is applicable to any type of wireless
remote
device (e.g., PCS, PDA, modems, data terminals, portable units) and any type
of
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system (e.g., satellite transmission system, hybrid satellite and land-based
transmission system, indoor system, etc.).
The foregoing description focuses on characteristics of the present invention.
Those skilled in the art will readily appreciate that the present invention is
applicable
to an.y ACA scheme, that is adaptive channel allocation based upon any quality
critexia selection scheme. Although these exemplary embodiments assume a fixed
set
of frequencies allocated for control channel usage (e.g., the 21 control
channels
allocated for AMPS and IS-54), those skilled in the art will recognize that
the present
invention is also applicable to systems in which the control channel
frequencies are
not fixed. For example, the digital control channel (DCC) scheme in IS-136
allows a
digital control channel to be allocated anywhere in the spectrum. However,
since
each carrier that supports a DCC also supports two traffic channels in the
three slot
IS-136 TDMA scheme, one of these traffic channels can be coupled to the DCC on
the shared carrier.
The above-described exemplary embodiments are intended to be illustrative in
all respects, rather than restrictive, of the present invention. Thus the
present
invention is capable of many variations in detailed implementation that can be
derived
from the description contained herein by a person skilled in the art. All such
variations and modifications are considered to be within the scope and spirit
of the
present invention as defined by the following claims.
