Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A Method and Apparatus For Controlling Calls m a Code Dmsion
Multiple Access System
Field of the Invention
1 0 The present invention relates generally to wireless
communication systems and, in particular, to code division multiple
access communication systems.
Background of the Invention
Communication systems that utilize coded communication
signals are known in the art. One such system is a direct sequence
code division multiple access (DS-CDMA) cellular communication
system, such as that set forth in the Telecommunications Industry
2 0 Association/Electronic Industries Association Interim Standard 95
(TIA/EIA IS-95), hereinafter referred to as IS-95. In accordance
with IS-95, the coded communication signals used in the DS-CDMA
system comprise DS-CDMA signals that are transmitted in a common
1.25 MHz bandwidth to the base sites of the system from
2 5 communication units, such as mobile or portable radiotelephones,
that are communicating in the coverage areas of the base sites. Each
DS-CDMA signal includes, inter alia, a pseudo-noise (PN) sequence
associated with a particular base site and an identification number of
a communicating communication unit.
In conventional CDMA systems, voice quality is degraded as
more subscribers originate calls over the system. Therefore, it is
desirable to limit the number of calls over a given cell of the CDMA
system. One suggested method would be to only allow a fixed
3 S number of calls per cell of the CDMA system. However, this
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method ignores the effects of other cell sites on system performance.
Further, to provide greater capacity, it has been suggested to allow
more than one carrier per cell of the CDMA system. In wireless
communication systems, such as CDMA systems, a premium is
placed on call capacity efficiency. Thus it would be desirable for
calls placed on one of multiple carriers within a CDMA cell to be
selected using an efficient method.
Accordingly, a need exists for a method and apparatus that
1 0 maintains voice quality in a CDMA system subjected to a high call
load that accounts for intercell interference. Further, there is a need
for improved efficiency when multiple carriers are used in a base
site of a CDMA system.
1 5 Summary of the Invention
According to one aspect of the invention a method for controlling a
call in a CDMA system is provided. The method comprises receiving a call
attempt request to be placed over a target cell of the CDMA system;
2 0 calculating an effective load based on a measurement indicative of non-
target cell interference or a number of calls in soft handoff with the target
cell
for the target cell; comparing the effective load to a threshold; and denying
the call attempt request by sending a denial message if the effective load
2 5 exceeds the threshold.
According to another aspect of the invention, a CDMA system is
provided. The system includes a base station including a plurality of
transceivers providing a radio communication channel and defining a target
cell; a communication unit located within the target cell; a base station
controller coupled to the base station; said base station controller receiving
a call attempt request from the communication unit over the radio
communication channel; said base station controller calculating an
effective load based on a measurement indicative of non-target cell
3 5 interference or a number of calls in soft handoff with the target cell,
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comparing the effective load to a threshold, and denying the call attempt
request be sending a denial message to the communication unit if the
effective load exceeds the threshold.
Brief Description of the Drawings
FIG. 1 is a block diagram illustrating a CDMA communication system.
FIG. 2 is a flow chart illustrating a preferred embodiment of a method
of blocking calls executed in a controller in the CDMA system of FIG. 1.
FIG. 3 is a flow chart illustrating a preferred embodiment of a method
of selecting a carrier from a group of carriers supported by the base site of
FIG. 1.
FIG. 4 is a graph illustrating capacity of the system of FIG. 1 using
the method of FIG. 3.
FIG. 5 is a graph illustrating the capacity of the system of FIG. 1
using the method of FIG. 3.
Description of a Preferred Embodiment
Generally, one aspect of the present invention encompasses a
method and apparatus for blocking a call attempt in a CDMA system.
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The method includes the steps of receiving a call attempt request
to be placed over a target cell of the CDMA system, calculating
an effective load for the target cell, comparing the effective
load to a threshold, and denying the call attempt request by
sending a denial message if the effective load exceeds the
threshold. The apparatus includes a base station and a base
station controller coupled to the base station. The base station
includes a plurality of transceivers providing a radio
communication channel and defining a target cell. The base
station controller receives a call attempt request to be placed
over one of the transceivers or the base station. The base
station controller calculates an effective load for the base
station, compares the effective load to a threshold, and denies
the call attempt request by sending a denial message if the
effective load exceeds the threshold.
Another aspect of the present invention is directed to a
method and apparatus for selecting a carrier from a group of
carriers supported by a target cell of a CDMA system. The
2 0 method includes the steps of calculating a first effective load
for a first carrier of the group of carriers, calculating a second
effective Ioad for a second carrier of the group of carriers,
comparing the first effective load to the second effective load,
and selecting one of the first and second carriers in response to
2 5 the comparison. The apparatus includes means for performing
the above method steps.
The present invention can be more fully described with
reference to FIGS. 1-3. FIG. 1 illustrates a communication system
3 0 100 including a first base site 101, a second base site 140, and one or
more communication units 103, 104, 105. The communication
system 100 preferably comprises a direct sequence code division
multiple access (DS-CDMA) cellular communication system, such as
that set forth in TIA/EIA IS-95. However, the present invention is
3 5 equally applicable to a frequency hopping communication system,
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such as those proposed for the domestic Personal Communication
System (PCS). In addition, those skilled in the art will appreciate
that the CDMA system may be used as a wireless local loop system.
In a cellular communication system, the base site 101 is coupled to a
mobile switching center (MSC) 113 that is in turn coupled to the
public switched telephone network (PSTN).115 using known
techniques.
The base site 101 preferably includes a plurality of
transceivers 120 and 122 (only two shown) that receives coded
communication signals from the communication units 103, 104
within a coverage area defined by a first cell 130 of the base site 101
and transmits coded communication signals to the communication
units 103, 104 within the first cell 130. The base site 101 also
includes appropriate interfaces between the MSC 113, the
transceivers 120 and 122, and an antenna 112. In addition, the base
site 101 includes a controller 200 which preferably includes a
programmable processor such as a MIPS 84400 or a Motorola
68040 type processor. Each of the communication units 103, 104,
2 0 105 preferably comprises a mobile or portable radiotelephone, a
mobile or portable two-way radio, or other two-way communicating
device, such as a computer with radio frequency (RF) transmission
and reception capabilities. Each of the communication units has an
antenna 110 attached thereto. The base site 140 is subst~.ntially
2 5 similar to base site 101 except that base site 140 communicates with a
controller 201 via a communication link 142 since the controller 201
is within a base station controller 150 that is in turn connected to the
MSC 113. Base site 140 has an antenna 114 and has a coverage area
defined by a second cell 132.
In the preferred DS-CDMA system 100, the coded
communication signals comprise DS-CDMA communication signals
107, 108 that are conveyed between the communication units 103,
104, and 105 and the base sites 101, 140 through a respective RF
3 5 channel within each cell 130, 132. In an alternate frequency hopping
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communication system, the coded communication signals might
comprise slow frequency hopping (SFH) communication signals
(multiple modulation symbol time intervals per hop) or fast
frequency hopping (FFH) communication signals (multiple hops per
modulation symbol time interval). The RF channel includes an
uplink (communication units 103, 104 to base site 101 ) and a
downlink (base site 101 to communication units 103, 104). In a
preferred embodiment, the uplink comprises a prescribed bandwidth
(e.g., 1.25 MHz for IS-95) collectively used by the communication
1 0 units 103, 104 to transmit multiple coded communication signals
107, 108 (DS-CDMA signals in this case) toward the base site 101.
Each DS-CDMA communication signal 107, 108 includes, inter alia,
a pseudo-noise sequence associated with the base site 101 and an
identification code for the particular communication unit 103, 104.
1 5 In the preferred embodiment, the base sire 101 and the controller
200 is part of a Motorola Supercell 9600*base station. Alternatively,
the controller 200 may instead be disposed within a mobile switch,
such as a Motorola EMX2500*switch, connected to the base site 101.
2 0 In an alternative preferred embodiment, each base site 101,
140 in the CDMA system 100 includes a plurality of RF channels
referred to as carriers. Multiple carriers are typically added in
CDMA systems to increase capacity of the system. Each carrier
comprises prescribed bandwidths (e.g., 1.25 MHz for IS-95) for
2 5 uplink (communication unit 103 to base site 101) and downlink (base
site 101 to communication unit 103). At a call origination, the call is
assigned to one of the carriers available in the cell of origination
(target cell). A particular embodiment of the multicarrier system is
illustrated in FIG. 2: Differences between the multicarrier
3 0 embodiment of FIG. 2 and the embodiment of a single carrier system
illustrated in FIG. 1 are as follows. Communication units 103, 104
have a capability to use different frequency bands, corresponding to
different carriers, for transmitting and receiving. In the forward
direction, radio signals corresponding to different carriers are
3 5 combined by a combiner 160 before they are transmitted by the
* Trademark
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antenna. In the reverse direction, the radio signal received by the
antenna is split into signals corresponding to different carriers. FIG.
2 shows different groups of transceivers 120 and 122 processing
signals of different carriers. Modifications of the particular
embodiment of a multicarrier system, described above, are also
possible. For example, the transceivers 120, l22 do not have to be
"dedicated" to specific carriers but can be shared by the carriers.
Further detail of a suitable multicarrier system is described in the IS-
95 standard.
FTG. 3 is a i~Iow chart illustrating a preferred embodiment of a
call blocking routine that may be programmed into the controller
200 or 201 in communication with the base site 101 or 140. For
purposes of illustration, reference will hereafter be made only to
1 5 base site 101 and controller 200 although the method of either FIG.
or FIG. 3 is equally applicable to base site 140 and controller 201.
Referring now to FIG. 3, a call attempt request to be placed
over the base site 101 is received from either the PSTN 115 via the
2 0 MSC 113 or one of the communication units 103, 104, at 202. At
204, an effective load is calculated by the controller 200 for the
target cell 130 defined by the coverage area of the base site 101. A
preferred method of calculating effective load for the target cell 130
will be described in detail below. At decision step 214, a
2 5 determination is made whether the call attempt is an original call
attempt or a handoff call attempt. A handoff call attempt may occur
when a communication unit, such as communication 105, moves
from a non-target cell 132 into the target cell 130. If the call
attempt is an origination, then processing continues at decision step
3 0 206; otherwise, processing continues at decision step 216.
At decision step 206, the calculated effective load is compared
to a first threshold, referred to as an origination threshold. If the
effective load exceeds the first threshold then the call attempt is
3 5 denied, at 208, and the base site 101 sends a denial message to the
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call originator, either to the PSTN 115 via the MSC 113 or to a
communication unit 103 or 104, at 212. However, if the effective
load does not exceeds the first threshold then the call is routed at 210
and normal call processing is performed.
At decision step 216, the calculated effective load is com~~ared
to a second threshold, referred to as a handoff threshold. If the
effective load exceeds the second threshold then the handoff call
attempt is denied at 218. Otherwise, the handoff call is allowed at
1 0 220.
A preferred embodiment of a method of calculating the
effective load and the thresholds is now described. The effective
load for a target cell, such as cell 130 for base site 101, may be
1 5 calculated in the controller 200 by adding the number of calls in the
target cell to a weighted summation of the number of calls in non-
target cells, such as adjacent cells. In a particular embodiment where
there are a plurality of adjacent cells, and the cells are approximately
the same size, each of the weights in the weighted summation is set
2 0 equal to a non-target cell interference factor divided by the number
of adjacent cells. It should be understood that the number of calls in
a cell includes calls that are in soft handoff with the cell. The non-
target cell interference factor may be calculated by an estimate such
as .55 as described in "Other-Cell Interference in Cellular Power-
2 5 Controlled CDMA", by Viterbi et al., IEEE Transactions on
Communications, Vol. 42, No. 2/3/4, p. 1501-1504 ( 1994)
In an exemplary system where
each cell m the system can support a maximum of eighteen calls and
where the propagation law exponent is four, a preferred value for
3 0 the origination threshold would be about 25, and a preferred value
for the handoff threshold would be about 28.
An alternative preferred embodiment of the method of
calculating the effective load is now described. The effective load for
3 5 a target cell may be calculated as a function of the number of calls in
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the target cell and not in soft handoff with any other cell (Ko), the
number of calls that are in soft handoff with the target cell and a
non-target cell (Kh), and the number of calls in a non-target cell that
are not in soft handoff with the target cell (Kn). In particular, a
weighted summation of the above factors, Ko, Kh, and Kn as set '
forth below is preferred:
Load=Ko+wl xKh+w2xKn
The first weight, wl, should be less than one and the second
weight, w2, should be Iess than the non-target cell inlerferemce
divided by the number of non-target cells. In a particular example,
w 1 = 1 and w2 = .09. The origination and handoff thresholds in this
embodiment are typically less than 20. For example, the origination
threshold may be about 16 and the handoff threshold may be about
19.
Another alternative preferred embodiment of the method of
calculating the effective load is now described. This embodiment is
2 0 suitable to applications where CDMA system forward link overload
protection is used. The effective load for a target cell may be
calculated as a function of the number of calls in the target cell and
not in soft handoff with any other cell (KO), the number of calls that
are in soft handoff with the target cell and one non-target cell (Kl ),
2 5 and the number of calls that are in soft handoff with the target cell
and two non-target cells (K2). In particular, a weighted summation
of the above factors, K0, Kl, and K2 as set forth below is preferred:
Load = v0*KO + vl*Kl + v2~'K2
The weights v0, v1, and v2 are proportional to the average
power to be allocated in the target cell to mobiles in the various
types of a soft handoff. For example, the weights may be set as
follows: vl=l, v2=0.9, and v3=0.8. The origination and handoff
3 5 thresholds may be set to 15 and 18 respectively.
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FIG. 4 is a flow chart illustrating a preferred embodiment of a
' carrier selection routine that may be programmed into the controller
200 in communication with the base site 101 in a system that has
" 5 multiple carriers per cell 130. Referring to FIG. 3, the controller
200 calculates a first effective load for a first carrier, at 302, and
calculates a second effective load for a second carrier, at 304. Both
the first and second carriers are supported by the base site 101 and
are used to communicate with communication units 103, 104. At
1 0 306, a comparison is made between the first effective load and the
second effcctiv a load. If the first effective load exceeds the second
effective load, then the controller 200 selects the second carrier, at
308, and places a call over the second carrier, at 310. However, if
the first effective load does not exceed the second effective load, then
1 5 the controller 200 selects the first carrier, at 312, and places the call
over the first carrier. A person having ordinary skill in the art will
appreciate that the selected carrier may either be used for a call
origination or for a hard handoff.
2 0 The preferred embodiment described with respect to FIG. 4
has many benefits. For example, by using the calculated effective
load to select a carrier from a group of potential carriers the call
load over the various carriers is more efficiently balanced. As a
result, the capacity of the CDMA system is advantageously
2 5 improved. A comparison of capacity between the preferred
embodiment and conventional systems is illustrated in FIG. 5. As
shown in FIG. 5, the preferred embodiment has greater capacity
regardless of the mobility of the communication units within the
target cell 130.
In addition to those described herein, further advantages
and modifications of the above described apparatus will readily
occur to those skilled inthe art. For example, both reverse and
J
forward link power measurements in target and non-target
3 5 cells may be used in the effective Ioad calculations. In
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addition, although the preferred effective load is calculated
using a weighted sum, alternative mathematical combinations
r
may also be used.
The invention, in its broader aspects, is therefore not
limited to the specific details, representative apparatus, and
illustrative examples shown and described above. Various
modifications and variations can be made to the above
specification without departing from the scope or spirit of the
present invention, and it is intended that the present invention
cover all such ~nodi4'lca~tie~ns and v ~.1'laL1~17tS provided they come
within the scope of the following claims and their equivalents.
What is claimed is:
