United States Patent
`Fleming et al.
`
`[19]
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll lllll lllll lllll llllll Ill lllll llll
`US005666356A
`5,666,356
`Patent Number:
`Date of Patent:
`Sep. 9, 1997
`
`[45]
`
`[11]
`
`[54] METHOD AND APPARATUS FOR
`CONTROLLING CALLS IN A CODE
`DIVISION MULTIPLE ACCESS SYSTEM
`
`[75]
`
`Inventors: Philip Joseph Fleming, Glen Ellyn;
`Aleksandr Stolyar, Buffalo Grove, both
`of Ill.
`
`[73] Assignee: Motorola, Inc., Schaumburg, Ill.
`
`[21] Appl. No.: 539,032
`
`Oct. 4, 1995
`
`[22] Filed:
`Int. Cl.6
`•••••••••••••••••••••••••••••• H04B 7/216; H04Q 7/22
`[51]
`[52] U.S. Cl •.......................... 370/328; 370/332; 370/461;
`370/230; 455/453
`[58] Field of Search ............................ 370/18, 95.1, 328,
`370/331, 332, 461, 462, 230; 375/205,
`206; 455/33.1, 54.1, 54.2, 56.1, 67.1, 226.1
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,245,629
`5,293,641
`5,497,504
`5,530,917
`
`9/1993 Hall ......................................... 375/205
`3/1994 Kallin et al ....•..........•.•.....••••. 455/33.1
`3/1996 Acampora et al ..................... 455/33.2
`6/1996 Andersson et al ..................... 455/54.1
`
`7/1996 Bodnar ...................................... 370/18
`5,539,729
`7/1996 Bodin et al ............................ 455133.2
`5,542,093
`Primary Examiner-Douglas W. Ohns
`Assistant Examiner-Min Jung
`Attorney, Agent, or Firm-Richard A. Sonnentaq
`[57]
`ABSTRACT
`
`A method and apparatus for blocking a call attempt in a
`CDMA system. The method includes the steps of receiving
`a call attempt request to be placed over a target cell of the
`CDMA system (202), calculating an effective load for the
`cell (204), comparing the effective load to a threshold (206),
`and denying the call attempt request by sending a denial
`message if the effective load exceeds the threshold (212).
`The apparatus includes a base station (101) and a base
`station controller (200) coupled to the base station (101).
`The base station (101) includes a plurality of transceivers
`(120) providing a radio communication channel and defining
`a target cell (130). The base station controller (200) receives
`a call attempt request to be placed over one of the trans(cid:173)
`ceivers (120) of the base station (101). The base station
`controller (200) calculates an effective load for the base
`station (101), 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.
`
`7 Claims, 3 Drawing Sheets
`
`RECEIVING A CELL A ITEMPT REQUES
`TO BE PLACED OVER A B4SE SITE
`FROA PSTN OR CQ\.MJNICATION UNI
`
`202
`
`204
`
`CALCUlA TING AN EFFECTIVE LOAD
`FOR THE CELL DEF/ND BY
`THE B4SE SITE
`
`YES
`
`PROCEED TO ROJrE CALL
`208
`
`DENY THE CALL AITEMPT
`
`212
`SEND DENIAL MESS4GE TO
`CALL ORIGJN4TOR (PSTN OR
`CO/MUN/CA Tl()(IJ UNl7)
`
`YES
`
`DENY HANCOFF CALL AITEMPT
`
`218
`
`

`

`U.S. Patent
`
`Sep. 9, 1997
`
`Sheet 1of3
`
`5,666,356
`
`cx11s
`~ B4SESITE
`
`101
`
`ri
`130
`
`CEU 1
`
`112
`
`110
`'--------r------
`
`113
`
`150
`B4SE STA TICXV
`CCMROl.lER
`
`142
`
`140
`B4SESITE
`
`201
`
`113
`
`FIG.1
`120
`
`101
`
`B4SESITE
`
`CARm~1{ TRANSCEIVER
`
`TRANSCEIVER
`
`r -
`
`-
`
`-
`
`130
`- L_ - - - - - - ,
`'/ CELL
`103
`
`-
`
`COfvflAUNIG4 Tl
`UNIT
`
`122
`
`CARR/ffi2{
`
`TRANSCBVER
`
`FIG.2
`
`104
`
`COfvMUNIG4 Tl
`UNIT
`-------------
`
`

`

`U.S. Patent
`
`Sep. 9, 1997
`
`Sheet 2 of 3
`
`5,666,356
`
`RECEIVING A CELL ATTEMPT REQUES
`TO BE PLACED OVER A B4SE SITE
`FRQ\1 PSTN OR CO/IJMJNICA TICXV UNI
`
`202
`
`204
`
`CALCULATING AN EFFECTIVE LOAD
`FOR THE CELL DEF/ND BY
`THE B4SE SITE
`
`YES
`
`YES
`
`PROCEED TO ROUTE CALL
`208
`
`DENY THE CALL AITEMPT
`
`218
`
`DENY HANOOFF CALL A7TEMPT
`FIG.3
`CALCULATING A FIRST EFFECTIVE
`LOAD FOR A FIRST CARRIER
`
`212
`SEND DENIAL MESSAGE TO
`CALL ORIGINATOR (PSTN OR
`CQ\.fiAUNICA TION UNIT)
`
`302
`
`CALCULATING A SECOND EFFECTIVE
`LOAD FOR A SEGCWD CARRIER
`
`304
`
`YES
`
`308
`SELECT SECOND CARRIER
`
`312
`SELECT FIRST CARRIER
`
`PLACE CALL OVER SEW.JD CARRIER
`
`FIG.4
`
`PLACE CALL OVER FIRST CARRIER
`
`

`

`U.S. Patent
`
`Sep. 9, 1997
`
`Sheet 3 of 3
`
`5,666,356
`
`f PREFERRED
`EM30DIMENT
`
`CAPACITY
`. (ERLANGS)
`
`12
`
`10
`ffi a: 8
`ct
`~
`~6
`~
`4
`
`2
`
`5
`
`10
`
`MJB/UTY
`
`FIG.5
`
`US. Patent
`
`Sep. 9, 1997
`
`Sheet 3 of 3
`
`5,666,356
`
`CAPACITY.
`. (ERLANGS)
`
`12 -
`
`PREFERRED
`EMBODIMENT
`
`
`
`°
`
`ROUND ROBIN
`
`Oo
`Ly Wig J
`rs
`
`a }
`
`2?
`
`4
`
`
`
`
`
`5
`
`fo
`
`MOBILITY
`
`FIGS
`
`Page 4
`
`CommScope Ex. 1015
`
`

`

`1
`METHOD AND APPARATUS FOR
`CONTROLLING CALLS IN A CODE
`DIVISION MULTIPLE ACCESS SYSTEM
`
`5,666,356
`
`2
`DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`Generally, one aspect of the present invention encom-
`5 passes a method and apparatus for blocking a call attempt in
`a CDMA system. 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 trans-
`ceivers providing a radio communication channel and defin(cid:173)
`ing a target cell. The base station controller receives a call
`attempt request to be placed over one of the transceivers of
`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
`20 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
`method includes the steps of calculating a first effective load
`for a first carrier of the group of carriers, calculating a
`second effective load for a second carrier of the group of
`carries, comparing the first effective load to the second
`effective load, and selecting one of the first and second
`carriers in response to 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 100 including a first base site 101, a second base site
`35 140, 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/EfA IS-95. However, the present invention is equally
`applicable to a frequency hopping communication system,
`such as those proposed for the domestic Personal Commu-
`nication 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 tech(cid:173)
`niquek
`The base site 101 preferably includes a plurality of
`50 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
`55 cell 130. The base site 101 also includes appropriate inter(cid:173)
`faces 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 R4400 or a Motorola 68040 type
`60 processor. Each of the communication units 103, 104, 105
`preferably comprises a mobile or portable radiotelephone, a
`mobile or portable two-way radio, or other two-way com(cid:173)
`municating device, such as a computer with radio frequency
`(RF) transmission and reception capabilities. Each of the
`65 communication units has an antenna 110 attached thereto.
`The base site 140 is substantially similar to base site 101
`except that base site 140 communicates with a controller 201
`
`CROSS REFERENCES
`
`This application is related to commonly assigned copend(cid:173)
`ing application Ser. No. 08/408,247 filed Mar. 22, 1995 by
`Labedz et al. entitled "Method and Apparatus for Limiting
`Access to a Channel." (pending). The entire contents of the
`above application is incorporated herein by reference.
`
`10
`
`FIELD OF THE INVENTION
`The present invention relates generally to wireless com- 15
`munication systems and, in particular, to code division
`multiple access communication systems.
`
`BACKGROUND OF THE INVENTION
`
`Communication systems that utilize coded communica(cid:173)
`tion signals are known in the art One such system is a direct
`sequence code division multiple access (DS-CDMA) cellu-
`lar communication system, such as that set forth in the
`Telecommunications Industry Association/Electronic Indus(cid:173)
`tries Association Interim Standard 95 (TIA/ETA IS-95), 25
`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 communication units, such as mobile or por- 30
`table radiotelephones, that are communicating in the cover(cid:173)
`age 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 com(cid:173)
`municating communication unit.
`In conventional CDMA systems, voice quality is
`degraded as more subscribers originate calls over the sys(cid:173)
`tem. 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 number of calls per 40
`cell of the CDMA system. However, this 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 pre- 45
`mium 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
`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 mul(cid:173)
`tiple carriers are used in a base site of a CDMA system.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a block diagram illustrating a CDMA commu(cid:173)
`nication system.
`FIG. 2 is a multicarrier system.
`FIG. 3 is a fiow chart illustrating a preferred embodiment
`of a method of blocking calls executed in a controller in the
`CDMA system of FIG. 1.
`FIG. 4 is a fiow 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. 5 is a graph illustrating capacity of the system of
`FIG. 1 using the method of FIG. 3.
`
`

`

`5,666,356
`
`4
`PSTN 115 via the 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
`5 calculating effective load for the target cell 130 will be
`described in detail below. At decision step 214, a determi(cid:173)
`nation 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
`10 105, 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 206; otherwise, processing con(cid:173)
`tinues 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 denied, at 208, and the base site 101
`sends a denial message to the 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
`compared 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 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 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 ion the weighted summation is set
`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 0.55 as described in
`"Other-Cell Interference in Cellular Power-Controlled
`CDMA", by Viterbi et al., IEEE Transactions on
`Communications, Vol. 42, No. 21314, p. 1501-1504 (1994),
`incorporated herein by reference. In an exemplary system
`where each cell in the system can support a maximum of
`eighteen calls and where the propagation law exponent is
`four, a preferred value for 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 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 a non-target cell (Kh), and
`the number of calls in a non-target call that are not in soft
`handoff with the target cell (Kn). In particular, a weighted
`60 summation of the above factors, Ko, Kh, and Kn as set forth
`below is preferred:
`
`3
`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 com(cid:173)
`munication signals comprise DS-CDMA communication
`signals 107, 108 that are conveyed between the communi(cid:173)
`cation units 103, 104, and 105 and the base sites 101, 140
`through a respective RF channel within each cell 130, 132.
`In an alternate frequency hopping communication system,
`the coded communication signals might comprise slow
`frequency hopping (SFII) communication signals (multiple
`modulation symbol time intervals per hop) or fast frequency
`hopping (FFII) communication signals (multiple hops per
`modulation symbol time interval). The RF channel includes 15
`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 units 103, 104 to transmit 20
`multiple coded communication signals 107, 108 (DS(cid:173)
`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 communi- 25
`cation unit 103. 104. In the preferred embodiment, the base
`site 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. 30
`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 typi(cid:173)
`cally added in CDMA systems to increase capacity of the
`system. Each carrier comprises prescribed bandwidths (e.g., 35
`1.25 MHz for IS-95) for uplink (communication unit 103 to
`base site 101) and downlink (base site 101 to communica(cid:173)
`tion 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 40
`illustrated in FIG. 2. Differences between the multicarrier
`embodiment of FIG. 2 and the embodiment of a single
`carrier system illustrated in FIG. 1 are as follows. Commu(cid:173)
`nication units 103, 104 have a capability to use different
`frequency bands, corresponding to different carriers, for 45
`transmitting and receiving. In the forward direction, radio
`signals corresponding to different carriers are combined by
`a combiner 160 before they are transmitted by the antenna.
`In the reverse direction, the radio signal received by the
`antenna is split into signals corresponding to different car- 50
`riers. 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 trans(cid:173)
`ceivers 120, 122 do not have to be "dedicated" to specific 55
`carriers but can be shared by the carriers. Further detail of
`a suitable multicarrier system is described in the IS-95
`standard.
`FIG. 3 is a flow 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 base site 101 and controller 200
`although the method of either FIG. 2 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
`
`Load=Ko+wlxKh+w2xKn
`
`The first weight, wl, should be less than one and the
`65 second weight, w2, should be less than the non-target cell
`interference divided by the number of non-target cells. In a
`particular example, wl=l and w2=0.09. The origination and
`
`

`

`5,666,356
`
`5
`handoffthresholds 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 5
`embodiment is 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 10
`handoff with the target cell and one non-target cell (Kl), 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, KO, Kl, and K2 as set forth
`below is preferred:
`
`15
`
`6
`the present invention, and it is intended that the present
`invention cover all such modifications and variations pro(cid:173)
`vided they come within the scope of the following claims
`and their equivalents.
`What is claimed is:
`1. A method of controlling a call in a code division
`multiple access, CDMA, system comprising the steps of:
`receiving a call attempt request to be placed over a target
`cell of the CDMA system;
`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 exceeds the threshold.
`2. A method of selecting a carrier from a group of carriers
`supported by a target cell of a code division multiple access,
`20 CDMA, system comprising the steps of:
`calculating a first effective load based on a measurement
`indicative of non-target cell interference or a member
`of calls in soft handoff with the target cell for a first
`carrier of the group of carriers;
`calculating a second effective load based on a measure(cid:173)
`ment indicative of non-target cell interference or a
`number of calls in soft handoff with the target cell 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 the comparison.
`3. An apparatus for selectively blocking a call attempt in
`a code division multiple access, CDMA, system base station
`comprising:
`a base station including a plurality of transceivers pro(cid:173)
`viding a radio communication channel and defining a
`target cell;
`a base station controller coupled to the base station;
`said base station controller receiving a call attempt
`request to be placed over one of the transceivers of the
`base station;
`said base station controller 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, 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.
`4. The apparatus of claim 3, wherein the non-target cell
`interference is estimated based on an inverse of a propaga(cid:173)
`tion law exponent.
`5. A code division multiple access, CDMA, system com-
`55 prising:
`a base station including a plurality of transceivers pro(cid:173)
`viding 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
`interference or a number of calls in soft handoff with
`
`Loa~Ko+vlKl +v2* K2
`
`The weights vO, vl, 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 thresholds may be set to 15 and 18
`respectively.
`FIG. 4 is a flow chart illustrating a preferred embodiment 25
`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 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 30
`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 306, a
`comparison is made between the first effective load and the
`second effective load. If the first effective load exceeds the 35
`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 the controller 200
`selects the first carrier, at 312, and places the call over the 40
`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.
`The preferred embodiment described with respect to FIG.
`4 has many benefits. For example, by using the calculated 45
`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 improved. A comparison of capac-
`ity between the preferred embodiment and conventional 50
`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 in the art. For example, both
`reverse and forward link power measurements in target and
`non-target cells may be used in the effective load calcula(cid:173)
`tions. In addition, although the preferred effective load is 60
`calculated using a weighted sum, alternative mathematical
`combinations 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 65
`modifications and variations can be made to the above
`specification without departing from the scope or spirit of
`
`

`

`5,666,356
`
`7
`the target cell, comparing the effective load to a
`threshold, and denying the call attempt request by
`sending a denial message to the communication unit if
`the effective load exceeds the threshold.
`6. An apparatus for selecting a carrier from a group of 5
`carriers supported by a target cell of a code division multiple
`access, CDMA, system comprising:
`means for calculating a first 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 a first carrier of the group of carriers;
`means for calculating a second effective load based on a
`measurement indicative of non-target cell interference
`
`8
`or a number of calls in soft handoff with the target cell
`for a second carrier of the group of carriers;
`
`means for comparing the first effective load to the second
`effective load; and
`
`means for selecting one of the first and second carriers in
`response to the comparison.
`7. The apparatus of claim 6, wherein the CDMA system
`10 comprises one of a personal communication system and a
`wireless local loop system.
`
`* * * * *
`
`

`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENT NO.
`
`DATED
`
`INVENTOR(S) :
`
`5,666,356
`September 9, 1997
`Fleming, et al.
`
`It is certified that error appears in the above-indentified patent and that said Letters Patent is hereby
`corrected as shown below:
`
`Column 6, line 22 reads "member" should be --number--.
`
`Column 6, line 35 reads "system base station comprising" should
`be --system comprising--.
`
`Signed and Sealed this
`
`Twenty-third Day of December, 1997
`
`Attest:
`
`Attesting Officer
`
`Commi.\'.Yiont'r of Patent.\' and Trademarks
`
`BRUCE LEHMAN
`
`