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Claims  |
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What is claimed is:
1. In a CDMA communication system having a plurality of communication
resources within an allocated frequency spectrum, a method of identifying
a proximal base site of a plurality of base sites by a communication unit
through detection and measurement of identification signals received from
at least some of the plurality of base sites on the plurality of
communication resources where each base site transmits an identification
signal containing at least an ID on a dedicated resource of the plurality
of resources with each resource of the plurality of resources defined by
parameters stored within a resource table in a memory unit with the
communication unit and base sites with such stored parameters of each
resource including a code key of a plurality of code keys, a bandwidth of
a plurality of bandwidths, and a frequency of a plurality of frequencies
and where at least some resources of the plurality of resources may, be
frequency coextensive, such method including the steps of:
A) selecting a resource from the resource table within the memory of the
communication unit;
B) detecting, and decoding to produce an ID from a detected identification
signal by the communication unit on the resource;
C) measuring signal strength by the communication unit of the detected
identification signal to produce a magnitude value and storing the value
and the ID in a signal strength table within the memory of the
communication unit;
D) incrementing to a next resource within the resource table;
E) repeating setps A-D with remaining resources within the resource table;
F) identifying the proximal base site by determining a highest relative
signal strength value stored within the signal strength table and
recovering the ID stored therewith.
2. The method as in claim 1 further including the step of selecting a
communication resource by application of an algorithm for identifying
communication resources between a lowest and a highest frequency in an
allocated spectrum using a multiple of a base bandwidth and a code key
table.
3. In a CDMA communication system having a plurality of comunication
resources within an allocated frequency spectrum, an apparatus for
identifying a proximal base site of a plurality of base sites by a
communication unit through detection and measurement of identification
signals received from at least some of the plurality of base sites on the
plurality of communication resources where each base site transmits an
identification signal containing at least an ID on a dedicated resource of
the plurality of resources with each resource of the plurality of
resources defined by parameters stored within a resource table in a memory
unit within the communication unit and base sites with such stored
parameters of each resource including a code key of a plurality of code
keys, a bandwidth of a plurality of bandwidths, and a frequency of a
plurality of frequencies and where at least some resources of the
plurality of resources may, be frequency coextensive, such apparatus
comprising
A) means for selecting a resource from the resource table within the memory
of the communication unit;
B) means for detecting, and decoding to produce an ID from a detected
identification signal by the communication unit on the resource;
C) means for measuring signal strength by the communication unit of the
detected identification signal to produce a magnitude value and storing
the value and the ID in a signal strength table within the memory of the
communication unit;
D) means for incrementing to a next resource within the resource table;
E) means for repeating steps A-D with remaining resources within the
resource table
F) means for identifying the proximal base site by determining a highest
relative signal strength value stored within the signal strength table and
recovering the ID stored therewith.
4. The apparatus as in claim 3 further including means for identifying
communication resources between a lowest and a highest frequency in an
allocated spectrum using a multiple of a base bandwidth and a code key
table.
5. In a CDMA communication system having a plurality of communication
resources within an allocated frequency spectrum having a lowest and a
highest frequency, a plurality of base sites having transmitters, at least
one of which transmits an identification signal on a communication
resource of the plurality of communication resources for the benefit of an
at least one communication unit for identification of a proximal base site
through detection of the identification signal, with the plurality of
communication resources having an at least first group of communication
resources defined by a plurality of code keys, a group bandwidth, with the
first group bandwidth defined to be a base bandwidth and subsequent group
bandwidth to be twice previous group bandwidth, and an at least first
frequency group, with the first frequency in each group being one-half
group bandwidth above the lowest frequency of the allocated frequency
spectrum and remaining group frequencies in incremental steps of one group
bandwidth above the first frequency in the group up to a frequency
one-half group bandwith below the highest frequency in the allocated
spectrum, a method of identifying a proximal base site of the plurality of
base sites by the at least one communication unit, such method including
the steps of:
A) selecting a resource having a lowest frequency and narrowest group
bandwidth in the at least first group of communication resources from a
resource table within a memory unit within communication unit;
B) decoding the information signal by the communication unit using a code
key of the plurality of code keys entered into a code key table within the
memory;
C) measuring signal strength and storing magnitude in a proximal base site
storage table within the memory upon detection of the information signal;
D) incrementing code keyws within the code key table;
E) repeating steps B through the present step with the remaining code key
within the code key table;
F) incrementing to the next comunication resource wihtin the at least first
group of communication resources within the at least first group of
communication resources;
G) repeating steps B through the present step until the group bandwidth
equals the allocated frequency spectrum; and,
H) identifying the communication resource within the proximate base site
storage table having the highest relative strength measurement magnitude
as from the proximal base site.
6. The method as in claim 5 further including the step of selecting a
communication resource by application of an algorithm for identifying
communication resources between a lowest and a highest frequency in an
allocated spectrum using a mulitple of a base bandwidth and a code key
table.
7. In a CDMA communication system having a plurality of communication
resources within a allocated frequency spectrum having a lowest and a
highest frequency, a plurality of base sites having transmitters, at least
one of which transmits an identification signal on a communication
resource of the plurality of communication resources for the benefit of an
at least one communication unit for identification of a proximal base site
through detection of the identification signal, with the plurality of
communication resources having an at least first group of communication
resources defined by a plurality of code keys, of group bandwidth, with
the first group bandwidth defined to be a base bandwidth and subsequent
group bandwidth to be twice previous group bandwidth, and an at least
first frequency group, with the first frequency in each group being
one-half group bandwidth above the lowest frequency of the allocated
frequency spectrum and remaining group frequencies in incremental steps of
one group bandwidth above the first frequency in the group up to a
frequency one-half group bandwidth below the highest frequency in the
allocated spectrum, an apparatus for identifying a proximal base site of
the plurality of base sites by the at least one communication unit, such
apparatus comprising:
A) means for selecting a resource having a lowest frequency and narrowest
group bandwidth in the at least first group of communication resource from
a resource table within a memory unit within the communication unit;
B) means for decoding the information signal by the communication unit
using a code key of the plurality of code keys entered into a code key
table within the memory;
C) means for measuring signal strength and storing magnitude in a proximal
base site storage table within the memory upon detection of the
information signal;
D) means for incrementing code keys within the code key table;
E) means for repeating steps B through the present step with the remaining
code keys within the code key table;
F) means for incrementing to the next communication resource within the at
least first group of communication resources;
G) means for repeating steps B through the present step until the group
bandwidth equals the allocated frequency spectrum; and,
H) means for identifying the communication resource within the proximate
base site storage table having the highest relative strength measurement
magnitude as from the proximal base site.
8. The apparatus as in claim 7 further including means for identifying
communication resources between a lowest and a highest frequency in an
allocated spectrum using a multiple of base bandwidth and a code key
table.
9. In a CDMA communication system having a plurality of communication
resources within an allocated frequency spectrum a plurality of base sites
having transmitters, at least one of which transmits an identification
signal on a communication resource of the plurality of communication
resources for the benefit of an at least one communication unit for
identification of a proximal base site for hand-off of a communication
transaction on a home resource from a home base site to the proximal base
site through detection of the identification signal from the proximal base
site, with the plurality of communication resources having an at least
first group of resources defined by a plurality of code keys, an at least
first group bandwidth and an at least first group of frequencies, a method
for achieving hand-off between base sites having a plurality of
bandwidths, such method including the steps of:
A) identifying a proximal base site having a group bandwidth through a
suitable algorithm comparing signal strengths;
B) comparing home base site signal bandwidth with the group bandwidth of
the proximal base site and, upon occasion, determining that the home site
bandwidth is larger than proximal base site group bandwidth and
transmitting a request to a home site resource controller requesting the
home site bandwidth be reduced to the proximal base site group bandwidth
thereby defining a new home resource; and
C) achieving hand-off by transmitting a resource request to the proximal
base site requesting access to the communication transaction through the
proximate base site on the home communication resource.
10. In a CDMA communication system having a plurality of communication
resources within an allocated frequency spectrum, a plurality of base
sites having transmitters, at least one of which transmits an
identification signal on a commmunication resource of the plurality of
communication resources for the benefit of an at least one communication
unit for identification of a proximal base site for hand-off of a
communication transaction on a home resource from a home base site to the
proximal base site through detection of the identification signal from the
proximal base site, with the plurality of communication resources having
an at least first group of resources defined by a plurality of code keys,
an at least first group bandwidth and an at least first group of
frequencies, an apparatus for achieving hand-off between base sites having
a plurality of bandwidths, such apparatus comprising:
A) means for identifying a proximal base site having a group bandwidth
through a suitable algorithm comparing signal strengths;
B) means for comparing home base site signal bandwidth with the group
bandwidth of the proximal base site and, upon occasion, determining that
the home site bandwidth is larger than proximal base site group bandwidth
and transmitting a request to a home site resource controller requesting
the home site bandwidth be reduced to the proximal base site group
bandwidth thereby defining a new home resource; and
C) means for achieving hand-off by transmitting a resource request to the
proximal base site requesting access to the communication transaction
through the proximate base site on the home communication resource |
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Claims |
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Description |
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FIELD OF THE INVENTION
The field of the invention relates to communication system and more
specifically to trunked mobile communication systems.
BACKGROUND OF THE INVENTION
Trunked mobile communication systems are known. Such systems typically
allocate communication resources (such as a frequency pair or TDM time
slot) upon perception of a need for communication services. Such
communication resources typically allocated on a 30 KHz bandwidth channel
in allocated areas of an assigned frequency spectrum.
In some systems perception of a need for communication services occurs when
a mobile communication unit transmits a resource request to a resource
controller. In other systems a communication unit simply seizes control of
a communication resource and begins transmitting a signal.
In either case the geographical range of a mobile communication unit is
limited. The limited range often requires that communication transactions
between units, or between a mobile unit and a PSTN subscriber, be
supported through central site communication support facilities. Central
site communication support facilities (base sites) provide communication
support functions within a service coverage area proximal to a base site
antenna.
Upon entering the service coverage area of a base site a communication unit
may request service through a resource controller that may be located at
the base site. The communication unit in desiring service may first search
a designated frequency spectrum for a control resource (an uplink) over
which to transmit a service request. The communication unit then monitors
the resource (a downlink) for a response. Where the control resource
consists of a frequency pair the uplink typically consists on the first of
the pair and the downlink is the second frequency of the frequency pair.
Upon activation a communication unit searches for a control resource. Upon
finding a control resource, on the other hand, the communication unit may
wish to determine whether the resource represents the nearest resource
controller thereby insuring adequate reception. The communication unit
makes this determination by measuring the signal strength of the
transmitted signal on the control resource. The communication unit may
make such a measurement on a number of frequencies (from differing base
sites) before selecting the resource offering the strongest signal,
thereby insuring access to the nearest base site.
After transmitting a resource request on an uplink through the nearest
resource controller the communication may wait for a response. The
resource controller may respond with an information signal identifying a
different communication resource over which the communication unit may
conduct a communication transaction. Upon receiving such information the
communication unit tunes to the assigned frequency and begins
communicating.
Subsequent to initiating a communication transaction, and as the
transaction continues, a communication unit may move outside the service
coverage area of the base site. The communication unit detects such
condition by monitoring the signal received from the base site. As the
signal level received from the base site exceeds a threshold, the
communication unit scans communication resources for a control resource
used by another nearby base site. Upon detecting another base site, the
communication unit requests a hand-off to the adjacent base site. If a is
available within the other system a hand-off may occur, if not, the call
may be dropped.
Transimssions between communication units and base sites are, in many
systems, under a frequency division multiple access (FDMA) format. Other
systems use time division multiplexing (TDM) in conjunction with the FDMA
format.
Spread spectrum communication systems are also known. Spread spectrum
systems are communication systems with a high inherent degree of
resistance to interference from other transmitters. Spread spectrum
systems achieve their resistance to interference by transmitting an
information signal distributed over a relatively wide frequency bandwidth.
Use of the wide bandwidth avoids a total blocking of signals from
relatively narrowband interferers present on individual frequencies or
limited bands of frequencies.
Two examples of spread spectrum systems are direct sequence spread spectrum
systems (DSSS) and frequency hopping. DSSS systems achieve the wide band
distribution of a message signal by encoding an information signal with an
output from a pseudo random number generator. Frequency hoppers, on the
other hand, achieve a distribution of signals through an indexing format.
The pseudo random number generator used by a DSSS system is a digital
circuit used to generate a repeating sequence of digital numbers under the
control of a code key. The use of a code key allows a receiver having an
substantially identical pseudo random number generator as the transmitter,
and the appropriate code key, to decode a transmitted signal. Other
receivers without the proper codes are unable to decode the signal.
Without the proper code, in fact, a receiver may not even by able to
detect the presence of the spread spectrum signal.
The inability of a spread spectrum receiver to detect a spread spectrum
signal transmitted under a different code key and the immunity of spread
spectrum systems to interference are together the greatest asset of the
spread spectrum technology. Because of the great immunity of spread
spectrum receivers to competing signals relatively large numbers of
transmitters and receivers using different code keys can share the same
frequency spectrum with relatively little mutual interference but with a
significant increase in spectral efficiency resulting from the greater
number of users.
Because of the inherent resistance of spread spectrum communication systems
to interference a need exists for a way of applying spread spectrum to
trunked communication systems. Such a system, on the other hand, must
provide a methodology of detecting spread spectrum signals transmitted
under a number of different code keys.
SUMMARY OF THE INVENTION
Under one embodiment of the invention, in a CDMA communication system
having a number of communication resources within an allocated frequency
spectrum, a method is offered of identifying a proximal base site of a
number of base sites by a communication unit. The communication unit
identifies the proximal base site through detection and measurement of
identification signals received from at least some of the number of base
sites on the number of communication resources where each of the base
sites transmits an identification signal containing at least an ID on a
dedicated resource of the number of resources.
Each resources of the number of resources is defined by parameters stored
within a resource table in a memory unit within the communication unit and
the base sites. The parameters of each resource include a code key of a
number of code keys, a bandwidth of a number of bandwidths, and a
frequency of a number of frequencies. Some of the resources of the number
of resources may be frequency coextensive.
The method of identifying the proximal base site includes the step of
selecting a resource from the resource table within the memory unit. The
communication unit, upon occasion, detects and decodes an ID within an
identification signal on the resource. The signal strength of the detected
identification signal is measured by the communication unit to produce a
magnitude value. The value and ID are stored in a signal strength table
within memory. The communication unit increments to the next resource in
the resource table within memory and repeats the previous steps. The
communication unit then identifies the proximal base site by determining
the highest relative value stored within the signal strength table the
recovers the ID stored therewith.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 comprises a block diagram of a base site transmitter.
FIG. 2 comprises a block diagram of a communication unit receiver.
FIG. 3 depicts service coverage areas of three adjacent base sites.
FIG. 4 depicts a communication unit passing from a first service coverage
area to a second.
BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT
Pursuant to one embodiment of the invention a code division multiple access
(CDMA) communication system (10, FIG. 3) is constructed serviced by base
sites (11, 12, and 13). A communication unit (20) with such system (10)
may request service from and be serviced by any base site (11, 12, or 13)
depending on proximity and the ability of the base sites (11, 12, and 13)
to detect a signal from the communication unit (20). Each of the base
sites (11, 12, and 13) has a service coverage area (depicted by the
hexagonal figure (14, 15, and 16) surrounding the base sites (11, 12, and
13) shown in FIG. 3). While a communication unit (20) is within the
service coverage area (16) of a base site (13) and desires to initiate a
communication transaction the communication unit first scans a set of
communication resources to locate a control communication resource used by
the base site (13).
Shown (FIG. 1) is a block diagram of a base site transmitter (60) shown
generally. Include within the transmitter (60) is a vocoder (51), a
resource controller circuit (50), a memory (52), a clock (53), a code key
(55), an encoder (54), a carrier frequency generator (57), a mixer (56),
and an amplifier (58). The resource controller (50) may be a general
purpose computer and may have a memory (52) associated with or installed
within the resource controller (50). The clock (53) is programmable and a
clock output to the encoder (54) is dependent upon a clock speed loaded
into the clock from memory (52) under the control of the resource
controller (50). The code key (55) is a buffer containing a code loaded
from memory (52) also under control of the resource controller (50). The
carrier frequency generator (57) is likewise programmable dependent upon a
value loaded from memory (52). The encoder (54) together with an input
from the code key (55) provides a pseudo random number generator by which
a spread spectrum signal may be generated from an input from the vocoder
(51) or resource controller (50). The input to the encoder (54) from a
vocoder (51) may be a voice signal converted into a data stream within the
vocoder (51) or the input to the encoder (54) may be an information signal
from the resource controller (50).
Base site transmitters (60) within the base sites (11, 12, and 13) transmit
identification signals over a communication resource dedicated to the
transmitting base site (11, 12, or 13). A communication resource for
purposes of the invention is defined by a code key, a bandwidth, and a
carrier frequency (frequency). In the preferred embodiment of the
invention a set of code keys are available to each base site (11, 12, and
13) and are listed within a code key table within memory (52). Code keys
may be entered into the code key table on startup or otherwise developed
through a suitable algorithm.
A set of frequencies defining communication resources may likewise exist
within a frequency table within memory. The frequency table may be created
upon startup depending on an allocated frequency spectrum (and bandwidth)
within which the base is constructed to operate. Also since different,
non-interfering spread spectrum signals may be transmitted on the same
frequency under different code keys a number of communication resources
may be frequency coextensive without mutual interference.
Bandwidth under the preferred embodiment is 1.2 MHz (base bandwidth), or a
multiple of 1.2 MHz. (Communication resource under the invention having
the same bandwidth will be part of the same group of communication
resources.) Bandwidth, as with code keys and frequencies, may be entered
within a bandwidth table within the memory unit (52) the bandwidth table,
and code key table within memory (52) together comprise a table of
communication resources. To accommodate a varying bandwidth (under the
CDMA format) a programmable clock (53) is provided within the base site
transmitter (50) providing a transmitted signal with a bandwidth as small
as the base bandwidth up to a multiple of the base bandwidth.
In some service coverage areas (14, 15, or 16) the base site infrastructure
may be constructed to service frequency division multiple access (FDMA) or
time division multiple access (TDMA) signals simultaneously with spread
spectrum signals. In such a system an available base site bandwidth may be
divided up for use under different transmission formats.
As previously mentioned, each base site transmits an identification signal
(70, 71, and 72) over a dedicated communication resource also, sometimes,
used as a control resource. A control resource, for purposes of the
invention, is defined as an RF link used in the two-way exchange of
control information between base sites (11, 12, and 13) and communication
units (20). In one embodiment of the invention the base sites (11, 12, and
13) and the communication unit (20) exchange control information over the
control resource with a spread spectrum signal under a TDM format with two
slots per frame. The first slot on a control resource is allocated to base
sites for the transmission of identification signals. The second slot is
reserved to communication units (20) for the transmission of resource
requests.
For purposes of simplicity it will be assumed that each base site (11, 12,
and 13) use a first frequency (f1) as a control resource. It will also be
assumed that a first base site (11) uses a first code key (C1), a second
base site (12) uses a second code key (C2), and a third base site (13)
also uses code key (C1). It will also be assumed that base sites one and
two (11 and 12) operate with a spread spectrum bandwidth of 2.4 MHz. Base
site three (13) will be assumed to be a base site that is required to
support FDMA communication transactions simultaneously with spread
spectrum transmissions. Base site three (13) will be assumed to operate
with a spread spectrum bandwidth of the base bandwidth.
Even through base sites one and three (11 and 13) use the same code key
(C1), and frequency, the bandwidths of spread spectrum transmission from
base sites one and three (11 and 13) are different (2.4 MHz verses 1.2
MHz). Transmissions from base sites one and three (11 and 13),
consequently, do not significantly interfere because base sites one and
three (11 and 13) are transmitting on different communication resources.
In operation each base site continuously transmits an identification
signal. The identification signal may include an ID of the base site,
information of signalling formats, and information on formats under which
resource requests may be transmitted to the base site (11, 12, and 13).
The identification signal (70, 71, or 72) transmitted by base site
transmitter (60) originate as a data signal from within the resource
controller (50) and shifted into the encoder (54). The resource controller
(50) in base site one (11) programs a clock rate into the clock (53) to
produce a 2.4 MHz bandwidth signal and a code key (C1) into the code key
buffer (55) through memory (52). The resource controller (50) in base site
three (13) likewise programs a clock rate into the clock (53) to produce a
1.2 MHz bandwidth signal and a code key (C1) into the code key buffer (55)
through memory (52).
Upon programming the clock (53) and code key buffer (55) by the resource
controller an identification signal shifted into the encoder (54) is
spread over a 2.4 MHz bandwidth for base sites 11 and 12 or 1.2 MHz for
base site 13. The signal is then shifted by a carrier frequency (fl)
within mixer (56), amplified (58), and transmitted as an identification
signal (70, 71, and 72) unique to the base site from which it orginated
(11, 12, or 13).
Shown (FIG. 2) is a block diagram of a receiver (100) within the
communication unit (20). Shown within the diagram (FIG. 2) is a carrier
frequency generator (101), memory (102), a clock (103), code key buffer
(105), decoder (104), controller (106), a signal strength measurement
device (109), and an audio amplifier/speaker unit (107). The blocks shown
in the receiver n100) have a similar functionality as the transmitter. The
carrier frequency generator (101) shifts a received spread spectrum signal
to a zero IF level within a mixer (108). A second pseudo random number
generator (decoder (104), clock (103), and code key buffer (105) decode a
received (zero-IF) spread spectrum signal and provides an output to the
controller (106) or to an audio output (107) and the signal strength
measurement device (109).
To detect an identification signal (70, 71, or 72) a receiver (100) must
sequence through a table of communication resources incrementing to a next
resource within the resource table within memory (102) testing on each
resource for a received identification signal before incrementing to the
next communication resource within the resource table. Upon detecting an
identification signal (70, 71, or 72) the controller (106) causes a signal
strength measurement to be taken by the signal strength measurement device
(109). A signal strength magnitude is then stored within a signal strength
table within memory (102). A measurement of signal strength is performed
whenever an identification signal is detected on each communication
resource listed within the table of communication resources within memory
(102). A request for communication access is then transmitted to a
proximal base site base upon the highest relative value of signal strength
magnitude stored within the signal strength table within memory (102).
Stored within the signal strength table, in addition to signal strength
magnitude, are parameters definding the communication resource (control
resource) of the proximal base site. In the example shown (FIG. 3) the
closest base site (13) to the communication unit (20) would be identified
as the proximate base site through signal strength measurements. The
stored parameters in the example would include a code key (C1), a
frequency (f1) and bandwidth (base bandwidth) used by the base site (13).
Stored parameters may also include information on transmission of resource
requests (resource request channel) to the proximal base site. Information
within the identification signal may include instructions to the
communication unit for transmitting resource requests such as transmitting
on the dedicated resource of the proximal base site in the second slot
under a TDM format or may include an ID of another resource used by the
proximal base site for receipt of resource requests.
Upon identification of the proximal base site (13) the communication unit
(20) transmits a resource request (73) to the base site (13) which request
is acknowledged and a resource granted upon a resource allocated for use
by that base site (13). The resource granted may have a frequency (f2), a
bandwidth (base bandwidth), and a code key (C3).
The communication unit tunes to the allocated resource and begins
transmitting a communicated signal. As the communications transaction
continues through the home base site (13), the communication unit (20) may
move to the edge, or outside, the service coverage area (16) of base site
(13) (FIG. 4). As the communication unit (20) reaches the edge of the
service coverage area (16) such movement must be detected and the
transaction handed off to a nearby base site or the transaction will be
lost. Detection of movement may occur through signal strength measurements
performed by the home base site (13) or by the communication unit (20).
Handoff, likewise, may be initiated by the home base site (13) or by the
communication unit (13).
In one embodiment of the invention the communiaction unit (20) detects such
movement through signal strength measurements of an audio signal received
through the base site (13) and performed within the communication unit
(100) by the signal strength measurement device (109). As the measured
signal strength measured by the signal measurement device (109) exceeds a
threshold the communication unit seeks a second proximal base site (12).
The communication unit detects the second proximal base site (12) by again
indexing through the table of communication resource measuring detected
identification signals. Upon detection (by a determination of highest
relative signal strength stored within the signal strength table) of a
second proximal base site (12) the communication unit transmits a shared
access request on the resource request channel of the second proximal base
site (12). The shared access request contains an information signal
requesting that the second proximal base site (12) provide simultaneous
access to the communication transaction then being served through the
first proximal base site (13) on the same resouce.
Since the first base site (13) may operate on the base bandwidth and the
second base site (12) may operate on a multiple of the base bandwidth the
resource requested through the second base site (12) is specific in
content. The resource request transmitted to the second base site (12)
requests service the second base site (12) on the communication resource
defined by frequency f2, with a base bandwidth, and under code key C3.
Upon receipt of the request at the second base site (12), the second base
site (12) grants access on the communication resource defined by frequency
f2, with the base bandwidth, and under code key C3. The communication
transaction is then, for a time, simultaneously supported from both base
sites (12 and 13).
As the communication transaction continues signal strength measurement
devices (not shown) at base sites (12 and 13) monitor the signal received
from the communication unit (20). At an appropriate signal level (second
threshold), and as the communication unit (20) passes into the service
coverage area (14) of the second base site (12) the declining signal level
detected by base site (13) decreases below the second threshold and
simultaneous service may be discontinued.
After service is discontinued from the original home base site (13) the new
proximal base site (12) may transmit instructions to the communication
unit (20) assigning a new resource of a wider bandwidth, or, different
frequency (f3two times base bandwidth, and code key C4). Such a
re-assignment may be consistent with signal quality parameters within the
service coverage area (14). The resource re-assignment also preserves
adequate resource availability in the original home base site (13).
If after re-assigning a new communication resource to the communication
unit (20) communication unit (20) should now move back into the original
service coverage area (16) of the original base site (13) then the process
must be reversed. Upon detecting the deteriorating signal level from the
second base site (12) the communication unit (20) scans for another
proximal base site. The nearest base site detected (13) in this case is
the original base site (13). In detecting and identifying the new proximal
base site (13) the communication unit (20) now detects that the new
proximal base site (through receipt and decoding the identification
signal) has a narrower bandwidth than the base site (12) currently
supporting the communication transaction.
Before transmitting a shared access request to the new proximal base site
(13) the communication unit must first transmit a request to the current
base site (12) requesting a resource of the same bandwidth of the new
proximal base site (13). The current base site acknowledges the request
with a resource allocation. The communication unit then transmits a
resource request to the new proximal base site (12) in the form of a
shared access request on the resource then in use with the current base
site (12). As before, when the communication unit passes entirely into the
service coverage area (16) of the new proximal base site (13), service
through the previous base site (12) is terminated.
In above embodiment of the invention a resource table is created within
memory of the communication unit containing in sequence, every combination
of code keys, frequencies, and bandwidths. In the process of detecting
identification signals from base sites a communication unit (20)
increments through the resource table testing each resource for the
presence of an identification signal.
In another embodiment of the invention the controller (106) identifies
resources based upon an algorithm using base bandwidth. In the above
described example containing base sites (11, 12, and 13) where base site
(13) operates with a base bandwidth and the base sites (11, 12 and 13)
operate within an allocated frequency spectrum having a highest and lowest
frequency the controller (106) begins searching for identification signals
(70, 71, or 72) at the lowest frequency and the base bandwidth. The
algorithm identifying such parameters requires as an input a base
bandwidth, and a lowest and highest frequency of an allocated frequency
spectrum.
The controller (106) identifies a lowest frequency as being one-half base
bandwidth above the lowest frequency in the allocated frequency spectrum.
The controller (106) then tests the lowest frequency of a first group of
resources at the lowest (first) frequency, the base bandwidth, and a first
code key within the code key table. The controller (106) tests subsequent
resources by incrementing through the code key table testing each resource
in sequence.
Upon reaching the end of the code key table the resource controller
increments to a second frequency, one base bandwidth above the first
frequency, and tests again with the base bandwidth and each code key
within the code key table. Upon reaching the end of the code key table the
process repeats up to a highest frequency equal to one-half base bandwidth
below the highest frequency in the allocated frequency spectrum.
Upon reaching the highest frequency in the allocated frequency spectrum the
resource controller increments to a second group of communication
resources and again tests for an identification signal. The resource
controller identifies the first frequency in the second group of
communication resources by doubling the base bandwidth (subsequent
bandwidth) and calculating a first frequency for the second group of
resources as above. The resource controller then tests each resource in
the second group of resources using each code key as above up to a highest
frequency in the second group determined as for the first group.
The resource controller repeats the process of testing subsequent groups of
communication resources up to a bandwidth that substantially equals the
allocated frequency spectrum. Upon reaching a bandwidth equaling the
frequency spectrum the resource controller then selects the resource for
use having the highest relative signal strength magnitude.
In another embodiments of the invention a control resource within CDMA base
sites is provided at a base bandwidth and at the lowest frequency within
the allocated frequency spectrum. Different code keys are provided to
minimize mutual interference.
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