Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD FOR ACCESSING A MICROCELL U~;ING
ANALOG CONTROL CHANNELS
BACKGROUND OF THE INnrENTION
I. FIELD OF THE IN~rENTION
The present invention relates to radio communications. More
particularly, the present invention relates to cellular radiotelephone systems.
II. DESCRIPTION OF THE REI~TED ~RT
A cellular radio communication system is typically comprised of a
number of cells covering a geographic region. Each cell is comprised of a base
station that is allocated a number of radio ~h~nnel.q for transmitting from a
number of directional antennas. The cell may also be divided up into sectors,
each sector having a number of different ~hz~nnels.
The cell shapes are determined by both the radiation pattern of the
antennas and the local conditions at the cell site. Cells, however, are typically
i~e~li7ed as hexagonal patterns since such a pattern closely approximates the
ideal antenna r~ tifln pattern.
Cells are typically organized in clusters. Each cluster has a
predetermined number of cells (e.g., an N=7 system refers to each cluster in thesystem having 7 cells) with each cell being ~R.qigned different frequency groups.
The same frequency groups are reused in corresponding cells of different
clusters. These clusters are repeated as needed to cover a geographic area. An
example of this frequency layout concept is illustrated in FIG. 1.
The numbers illustrated in the cells of FIG. 1 indicate the frequency
groups ~R.qigned to each 120 sector of the cell. In this example, there are 21
different frequency groups that are used in a cluster. The cluster is reused
multiple times within the geographic region depicted in FIG. 1.
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FIG. 2 illustrates a prior art cell site connected to the mobile telephone
~rr.hz~nge (MTX~. The antenna (200) at the cell site is connected to the base
station (2~0) that contains the receivers and tr~n~mit.ters for the cell site tocomml7niç~te with the mobile radiotelephone (230). The base station is
5 connected to and controlled by a base station controller (BSC) (215). The BSC
(215) is connected to the MTX (220) that is then connected to the public
switched telephone network (PSTN). The MTX h~n~lles all of the swit~.hin~ for
a number of cell sites and BSCs, routing calls from the PSTN to the
appropriate cell site and routing calls from the cell site to the PSTN.
In a typical radiotelephone system, a su~scriber is not configured to a
specific cell. A radio ch~nnel is allocated to the radiotelephone for call
initiation based on a number of factors including: the Received Signal Strength
Indicator (RSSI) (measured during access), the type of radiotelephone (analog
or digitaV, interference conllit.ior ~, and the availability of radio ~.h~nnel.c. As
15 the radiotelephone moves through different cells of the cellular system, the
radio r.h~nnels are dyn~mic~lly allocated to the radiotelephone in response to
changes in these factors (see Electronic Industry Association Recommended
Standard-553 (EIA RS-533) for a detailed e~rl~n~tion of the RSSI).
Microcells are lower power cells that cover a .~m~ller area than
20 macrocells, as illustrated in FIG. 3. FIG. 3 shows a microcell (300) that is
located within the bounds of a macrocell (305). In a typical urban setting, the
~ radius of the macrocell (305) is approximately 500 meters while the radius of a
microcell (300) is typically less than 200 meters.
Microcells are typically deployed in a cellular system as a cost e~~ective
25 solution for at least two applications: cold and hot spot coverage. Cold spotcoverage occurs in a macrocell system that has radio signal coverage holes. In
this situation, the radiotelephone either cannot communicate with any base
station or the quality of the communication signal is substantially reduced due
to lack of coverage.
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In a hot spot coverage situation, a macrocell or macrocells experience a
high concentration of traffic. In this case, the base station may run out of
available frequencies and not be able to h~n-lle additional radiotelephone
traffic.
In both situations, microcells are typically added as underlay cells to the
existing macrocell system. Once underlaid, the microcells require a frequency
ignment The typical methods for ~igning frequency groups to microcells
are reusing frequencies and reserving or segregating frequencies from the
macrocell frequency groups.
In the frequency reuse scheme, a microcell reuses the frequencies
assigned to the macrocell system. The frequency reservation or segregation
scheme reserves a block of frequencies normally used by the macrocell system.
In theory, a microcell h~n~ s any traffic within its defined boundaries.
This includes calls ori~in~.ing or terminating within its range as well as hand-16 offs of radiotelephones passing through its coverage area. In reality, however,
this operation is difficult to achieve.
A problem comes from the limit~tions of the cellular air interface
standards for analog control ~h~nnel.~ (EIA RS-553, IS-91, and IS-54). These
standards were not designed to work with overlay/underlay schemes. For
example, they lack the ability to steer radiotelephones to the desired control
~h~nnel. As a consequence, the cellular system must attempt to compensate for
the behavior of the radiotelephones, as dictated by the standards, and must
make fast and accurate decisions on the dispositions of calls based on
inadequate, and potentially inaccurate, information.
Analog radiotelephones are required to lock onto the strongest control
channel received. In an underlay/overlay arrangement, that control ~h~nn~l is
almost always the macrocell control channel. Even if the mobile is physically
closer to the microcell, the macrocell transmits at a higher power. There is no
information in the mobile access messaging that tells the system how close the
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radiotelephone is to any underlaid microcells and, hence, no effective method
for redirecting the access from the macrocell
Additionally, since there is little reliable information available from the
~Toice channel serving a radiotelephone as to its distance and direction from the
5 macrocell, there is no way of directly determining that the mobile has enteredthe coverage area of the microcell. Most systems rely on continuous polling of
the radiotelephones in the macrocell to evaluate which ones may be ~nfli~tes
for h:~ntling down to the microcell. This ties up the base station's resources that
could be better used servicing additional tra~tlc. There is a resulting need for a
10 more efficient and inexpensive analog cellular system and process that
increases call capacity with a minim~l impact on call quality
SU M~L~RY OF THE IN~JENTION
The present invention encompasses a process for accessing a microcell by
a radiotelephone. In the preferred embodiment, the microcell is underlaid to a
macrocell. The process first determines the proximity of the radiotelephone to
the microcell. If t~e proximity is less than or equal to a predetermined
distance, the radiotelephone is ~ ign~d to the microcell. If the proximity is
20 greater than a predetermined distance, the radiotelephone is ~qsigned to the
overlying macrocell.
In the preferred embodiment, the proximity of the radiotelephone to the
microcell is determined by the microcell using a control channel with the same
frequency and digital color code as the macrocell's control channel. This allows25 the microcell to monitor the RSSIDf the radiotelephone. If the RSSI is above a
predetermined threshold, and the microcell has available voice channels, the
radiotelephone is assigned to the microcell.
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BRI3 :F DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a typical prior art cellular system frequency layout.
FIG. 2 shows a typical prior art cell site.
FIG. 3 shows a macrocell with an underlaid microcell.
FIG. 4 shows a flowchart of the process of the present invention.
FIG. 5 shows a block diagram of a typical prior art radiotelephone of the
present invention.
DETATT.F.n DESCRIPTION OF THE PREFERRED EMBODIMENT
The process of the present invention determines the radiotelephone's
proximity to the microcell using the microcell's control ~ nnel If the RSSI of
the radiotelephone is greater than or equal to a predetermined threshold and a
15 voice c hzlnnel is available from the microcell, the radiotelephone is ~igned to
the microcell.
A typical prior art analog radiotelephone of the present invention is
illustrated in FIG. 5. This radiotelephone is comprised of a microphone (510)
that takes the voice signal from the user and converts it to an analog signal.
20 The tr~n.~mit,ter (520) modulates the voice signal to the proper .frequency
~;igned by the system and tr~n~ fi it, through the duplexer (530) to the
antenna (535) for radiation to the cell site.
The antenna (535) also receives radiotelephone ~ from the cell site.
The duplexer couples the antenna (535) to the receiver (525). The receiver (525)25 then demodulates the received signal into an analog signal for conversion to a
sound signal by the speaker (505).
The radiotelephone user inputs telephone numbers and generally
controls the operation of the radiotelephone through the keypad and display
(500). This information is used by the radiotelephone controller (515) to control
30 transmitter (520) and receiver (525) and to determine what telephone number
is transm-itted.
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As is well known in the art, the radiotelephone accesses a cell's control
~h~nn~l when it sends an origination message to a cell or responds to a page
fro~n a cell. The cell measures the radiotelephone's RSSI during the access to
determine if it is strong enough for communication with that particular cell.
Control (~.hslnn~ls are typically reused within a cellular system requiring thatthe radiotelephone identify itself with the cell since it cannot ~e identified on
the basis of frequency. This is accomplished by the radiotelephone sending its
electronic serial number and assigned mobile identific~ )n number over the
control r.hzlnn~l.
In the preferred embodiment, the microcell of the present invention uses
a receive-only control (~.hs3nn~1 tuned to the same frequency as the overlaid
macrocell. If the microcell were allowed to transmit on this frequency, the
macrocell coverage area would be subjected to intolerable levels of interference.
The microcell also uses the same digital color code (I~CC) value as the
15 macrocell. As is well known in the AMPS art, up to three different DCC's are
used to differentiate between reuses of the control channe~ frequencies. For
example, if a radiotelephone accesses a cell having control channel fl and
DCCl, then another cell having the same control channel fl but DCC2 will
ignore the access. In a typical frequency plan, 21 different control ~h~nnel
20 frequencies are used.
Using the same control channel and DCC as the overlaid macrocell
enables the microce~l to monitor the macrocell control r.hzlnnel for access
attempts by a radiotelephone. When the access attempt is detected, the
microcell measures the RSSI of the radiotelephone, thus enabling the BSC to
25 determine whether the access shoilld be served by the macrocell or the
microcell.
In the preferred embodiment, the macrocell and microcell of the present
invention are both controlled by the same BSC. This enables the system to
better coordinate ch~nnf~l allocations between the two cells, thus minimi7.ing
.
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delays and race conditions between cells. Alternate embodiments could be
controlled by multiple BSCs.
A ~Lowchart of the process of the present invention is illustrated in FIG
4 The process begins with the macrocell and microcell monitoring their control
channels (401). If an access attempt is detected on either control ~h~nnel (402),
the control ~h~nnf?l measures the radiotelephone's signal strength (RSSIm)
during the access (403) and reports the access to the BSC (405). The BSC then
determines if the access was reported by the macrocell or the microcell (440).
If the access was reported by the macrocell, the BSC waits for a
10 duplicate access message from the microcell (410). In the preferred
embodiment, this message must be received in the range of 20 - 200 ms to be
considered a valid message. Alternate embo(liments use other time thresholds
for determining that a valid access message exists.
If the BSC receives a duplicate access message from the microcell within
15 the range of time allowed (415), the radiotelephone's RSSI, as measured by the
microcell's control t-h~nnel (RSSI~), is compared to a predetermined RSSI
threshold (425). If the RSSIm is greater than or equal to the RSSI threshold
(425) and the microcell has available voice channels, the radiotelephone is
~.qJgn~d to the microcell (430) This predetermined threshold, as well known in
20 the art, is determined during RF pl:~nnin~ of the cellular system. It is the
threshold below which a quality comml1ni~z~tion cannot take place between the
radiotelephone and the microcell.
If the BSC~ does not receive a duplicate message within the required time
'' (415), the radiotelephone is assigned to the macrocell (435), assuming the
2~ macrocell has available voice channels If there are no available voice channels,
the radiotelephone is not allowed access to the macrocell If the BSC does not
receive a duplicate message, this implies that the radiotelephone is outside themicrocell's coverage area and could not be heard by the microcell.
If an access is detected and it is not from the macrocell (440), then the
3n access me~,sa~e is from the microcell. The BSC waits for 20 - 200 ms for a
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duplicate message from the macrocell (4~0). If the duplicate message is
received in time (455), then the radiotelephone's RSSI, as measured by the
microcell's control channel (RSSIm) is compared to a predetermined RSSI
threshold (460).
If the RSSIm is greater than or equal to the RSSI threshold and the
microcell has available voice ~h~nnelR, the radiotelephone is assigned to the
microcell (430). Otherwise, the radiotelephone is assigned to the macrocell
(435), assuming it has available voice channels. If the macrocell does not have
voice ch~nn~ls available, the radiotelephone is denied access to the macrocell.
If the BSC does not receive a duplicate access message in the allotted
time, the access attempt is discarded (470). This may have been caused by a
collision on the macrocell control r.h~nnel due to more than one radiotelephone
attempting to access the control channel at the same time. This cannot be
detected by the microcell.
The goal of the process of the present invention is to use the macrocell
control ~h~nn~l to m~n~Fe the communications between the radiotelephone
and the MTX. The microcell's control channel is used to assess the
radiotelephone's proximity to the microcell.
In general, the radiotelephone is assigned to the microcell only if the
access is seen by both the macroceLI and the microcell, and the microcell RSSI
reading is high enough to indicate that the radiotelephone is within the
microcell's coverage area. In the preferred embodiment, the microcell's
coverage area is 200 meters from the microcell. Alternate embodiments have
other coverage areas.
26 The above described embodiment assumes there is only one microcell in
a given macrocell. However, alternate embodiments use multiple microcells
deployed in a single macrocell's coverage area. With several control channels
tuned to the same frequency and DCC, the BSC~ collects all of the duplicate
accesses received within the delay period and determines to which cell the
radiotelephone will be ~ Rsign~d.
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The above described embodiment also assumes that the microcell and
the macrocell are controlled by the same BSC. However, alternate
embodiments use different B~Cs to control the cells. If the radiotelephone is
going to be assigned to a microcell, the macrocell BSC requests the microcell
BSC to identify the microcell voice ~hz~nnel to be allocated to the
radiotelephone .
Other alternate embodiments associate a microcell with more than one
macrocell. In this case, the microcell must be equipped to monitor multiple
control r~h~nnels.
The preferred embodiment was discussed using AMPS as an example. It
should be clear that the present invention can be used with other cellular
systems that use analog access ~h~nnels.
VVE CLAIM:
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