Patent Publication Number: US-2005117533-A1

Title: Wireless communication method and apparatus for implementing call admission control based on common measurements

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. provisional application No. 60/518,380 filed Nov. 7, 2003 which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION  
      The present invention is related to a wireless communication system. More particularly, the present invention is a method and apparatus for admission control based on common measurements performed in a wireless communication system.  
     BACKGROUND  
      In wireless communication systems, a wireless transmit/receive unit (WTRU) communicates with a radio access network (RAN) via one or more radio channels which are established upon request from the WTRU or a core network. Upon receiving a call request for radio resources, a call admission control (CAC) process in a radio network controller (RNC) is invoked to process the request. The CAC process determines whether or not a call should be admitted to the system. If the call is admitted, the CAC process determines the most efficient allocation of radio resources.  
      In order to make such decisions, the CAC process must be aware of the state of the system at the time when the request is received. Power and interference measurements are typically used to characterize the current state of the system. Measurements may be made by a Node-B or a WTRU. Measurements made by a Node-B may include uplink (UL) interference, downlink (DL) carrier power level, and/or DL code transmission power. Measurements made by a WTRU may include UL total transmission power level, UL code transmission power level, DL interference, and/or path loss.  
      In many cases, measurements made by a WTRU are not available at the RNC. Thus, the CAC process must rely only on measurements made by a Node-B for admission control and resource allocation. Accordingly, a method and apparatus for implementing call admission control and resource allocation based only on measurements made by a Node-B is desired.  
     SUMMARY  
      A method and apparatus for implementing call admission control based on Node-B measurements in a wireless communication system is disclosed. The apparatus may be an integrated circuit (IC), Node-B or a wireless communication system. A coverage area of the wireless communication system is divided into a plurality of cells and each cell is served by a Node-B. Once a call request is received, a code is selected among available codes for potential allocation. A target cell load and a neighbor cell load for each of the available timeslots is calculated assuming additional allocation of the selected code to each of the timeslots using Node-B measurements. A weighted system load for the timeslot is calculated. A timeslot having a smallest weighted system load is selected for allocation of the code. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more detailed understanding of the invention may be had from the following description of a preferred example, given by way of example and to be understood in conjunction with the accompanying drawing wherein:  
       FIG. 1  is a flow diagram of a process including method steps for implementing CAC based on UL measurements in accordance with the present invention;  
       FIG. 2  is a flow diagram of a process including method steps for implementing CAC based on DL measurements in accordance with the present invention;  
       FIG. 3  is a diagram of a wireless communication system model in accordance with the present invention; and  
       FIG. 4  is a block diagram of an apparatus used to implement CAC in the system of  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention will be explained, for simplicity, in the context of a universal mobile telephone system (UMTS). However, it should be noted that the present invention may be implemented in any type of wireless communication system based on hybrid time division multiple access (TDMA)-code division multiple access (CDMA).  
      The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
      Hereafter, the terminology “WTRU” includes but is not limited to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “Node-B” includes but is not limited to a base station, a site controller, an access point or any other type of interfacing device in a wireless environment.  
      A CAC process of the present invention utilizes common measurements (i.e. measurements not dedicated to any specific radio link) made by a Node-B. The measurements may be either UL measurements or DL measurements. Optionally, the CAC process may utilize path loss information reported by a WTRU. When path loss information is available, the CAC process uses it. When path loss information is not available, a path loss parameter is used as an input, which will be explained hereinafter.  
      The UL measurement-based CAC process of the present invention uses a load metric of the target and neighboring cells in order to make a call admission decision and assign physical radio resources to the requested call.  
      With respect to load computation for target cell(s), a predicted interference level, ISCP PRED (i,t), resulting from the addition of one or more codes in timeslot t of cell i is preferably predicted using a noise rise function of the target cell, R T . 
 
 ISCP   PRED ( i,t )= ISCP ( i,t )× R   T ( ISCP ( i,t ), A ( i ), SIR );  (Equation 1) 
 
 where ISCP(i,t) is a UL timeslot interference signal code power (ISCP) measurement measured by the Node-B, A(i) is a path loss to the target cell, and SIR is a sum of the chip-level SIR targets of the added codes. The noise rise function, R T  is preferably given by:  
                 R   T     =     1     1   -       (         I   0     θ     -   1     )     ⁢       L   ⁢           ⁢   SIR       q   +     1     G   c                   ;           (     Equation   ⁢           ⁢   2     )             
 
 where θ is a thermal noise level, L is a path loss, q is a load of the cell, and G C  is a link gain. 
 
      The CAC process of the present invention may operate using only the measurements made by the Node-B, and does not have to use a path loss measurement reported from a WTRU. However, if a path loss measurement reported by the WTRU is available, such as during a handover, the path loss measurement is used as an input to the noise rise function, R T . Otherwise, a path loss value parameter is used instead of a path loss measurement. The path loss value parameter should be determined from the distribution of path losses measured throughout the cell through operation, administration and maintenance (OA&amp;M). For example, the 50th percentile path loss for a given cell deployment may be used.  
      The estimated load in a particular timeslot t of cell i is preferably computed as follows:  
                 L   ⁡     (     i   ,   t     )       =     1   -       N   O         ISCP   PRED     ⁡     (     i   ,   t     )             ;           (     Equation   ⁢           ⁢   3     )             
 
 where N O  represents the receiver noise level. The estimated load, L(i,t), is used to evaluate the admission of the requested resource units in the timeslot. 
 
      With respect to load computation for neighboring cells, the load of timeslot t in neighboring cell j is computed as follows:  
                 L   ⁡     (     j   ,   t     )       =     1   -       N   O       ISCP   ⁡     (     j   ,   t     )             ;           (     Equation   ⁢           ⁢   4     )             
 
 for all j ≠i. The current ISCP measurement of Node B j is available to the target cell and used as an input for the load computation. The resulting load, L(j,t), is used to evaluate the admission of the requested resource units in the timeslot. 
 
      In an alternate embodiment, the load of timeslot t in neighboring cell j may be computed using the noise rise in neighboring cell j. In this embodiment, a noise rise function of neighboring cells may be estimated using a noise rise function of the target cell to estimate the increase of interference in neighboring cells assuming a code(s) is assigned thereto as follows: 
 
 R   N   =R   T (1 +G   C   ×A ( i )× SIR );  (Equation 5) 
 
 where R T  is given in Equation 2, G C  is a calibration parameter, A(i) represents the path loss to the target cell and SIR is the sum of the chip-level SIR targets of the added codes. The derivation of a noise rise function of neighboring cells from a noise rise function of a target cell is explained in more detail with reference to  FIG. 3 . In this embodiment, Equation 4 is replaced with:  
               L   ⁡     (     j   ,   t     )       =     1   -         N   O         ISCP   ⁡     (     j   ,   t     )       ×     R   N         .               (     Equation   ⁢           ⁢   6     )             
 
      The allocation of one or more codes in timeslot t of cell i is accepted if and only if the following conditions are satisfied: 
 
 L ( i,t )&lt; LT   MAX ; and  (Equation 7) 
 
 L ( j,t )&lt; LN   MAX ;  (Equation 8) 
 
 for all neighboring cells j under consideration. L(i,t) and L(j,t) are computed as described in Equation 3 and Equation 4 (or alternatively, Equation 6), respectively. LT MAX  and LN MAX  represent the load thresholds for the target cell and neighboring cells. 
 
      It is noted that the allocation of a code(s) to a timeslot must satisfy WTRU capability requirements; otherwise, the allocation of the set of codes is rejected. For example, the UMTS standard defines a plurality of different classes of WTRUs. Each class is defined by a different set of capabilities. One of the capability requirements of a WTRU is the number of codes that the WTRU supports in a single timeslot, as well as the number of different timeslots the WTRU can simultaneously support. The lower class WTRUs support less codes per timeslot, whereas the higher class WTRUs support more codes per timeslot. A Node-B is aware of the WTRU class and hence, of the WTRU&#39;s capabilities in terms of the number of supported codes per timeslot and the number of supported timeslots. Therefore, before actually allocating codes to a particular WTRU in a given timeslot, it should be confirmed that the WTRU can handle the number of allocated codes in the timeslot.  
       FIG. 1  is a flow diagram of a process  100  including method steps for implementing CAC based on UL measurements in accordance with the present invention. When a wireless communication system receives a call request for a WTRU, a code is selected from a list of available code sets (step  102 ). The selected code is preferably the code with the smallest spreading factor (SF) in the code set. A first timeslot is also selected for potential allocation amongst available timeslots (step  104 ). The set of available timeslots consists of all timeslots that are available for the requested service type, (e.g., real time (RT) or non-real time (NRT)), and direction, (i.e., UL or DL). The set of available timeslots is set through OA&amp;M.  
      The process computes a target cell load and a neighboring cell load for the selected timeslot assuming the selected code is added to the selected timeslot in accordance with Equation 3 and Equation 4 (or alternatively, Equation 6) (step  106 ). In Equation 3, the load computation considers all codes from the code set that have already been allocated to the selected timeslot.  
      The process  100  then verifies CAC by determining whether the estimated target cell load and a neighboring cell load are below predetermined thresholds, respectively (step  108 ). If either the estimated target cell load or the estimated neighboring cell load is not below the thresholds, the code is not added to the timeslot for allocation, and the process proceeds to step  114 . If both the estimated target cell load and the estimated neighboring cell load are below the thresholds, the selected code is added to the timeslot, at which point the timeslot becomes a candidate timeslot for potential allocation of the selected code and is added to a list of candidate timeslots (step  110 ). Once the code is added to the timeslot, a weighted system load is computed for the timeslot at step  112  as follows:  
                   L   SYSTEM     ⁡     (   t   )       =         L   ⁡     (     i   ,   t     )       +       ∑     j   =   1       𝔍   1       ⁢       α   1     ⁢     L   ⁡     (     j   ,   t     )           +       ∑     j   =   1       𝔍   2       ⁢       α   2     ⁢     L   ⁡     (     j   ,   t     )               1   +     η   ⁢           ⁢     N   ⁡     (   t   )               ;           (     Equation   ⁢           ⁢   9     )             
 
 where ℑ 1  and ℑ 2  define respectively the set of tier one and tier two neighboring cells to be included in the overall system load. α 1  and α 2  represent weighting factors to be applied to tier one and tier two cell loads. The denominator, 1+ηN(t), is a fragmentation adjustment factor, where η corresponds to the fragmentation adjustment parameter and N(t) corresponds to the number of codes already assigned to the timeslot. Once the weighted system load has been computed, the process  100  proceeds to step  114 . 
 
      If it is determined that there are more available timeslots at step  114 , the next timeslot is selected from the list of available timeslots (step  116 ), and the process  100  returns to step  106 . If there are no available timeslots for computing a weighted system load, the process  100  determines whether there are any candidate timeslots (step  118 ). If there are no candidate timeslots, the process  100  indicates a failure of allocation of resources and rejects the requested code set (step  130 ). If there are candidate timeslots, a timeslot having a smallest weighted system load, L SYSTEM (t) is selected thereby resulting in allocation of the selected code in the selected candidate timeslot (step  120 ). The allocated code is removed from a list of available code sets (step  122 ), and a list of candidate timeslots is reset (step  124 ). If there are more available codes in a code set, as determined in step  126 , the process  100  returns to step  102 . If not, the process  100  proceeds to step  128  where the process  100  indicates a successful allocation of resources and returns a resource assignment solution for the call request (step  128 ).  
      The DL measurement-based CAC process of the present invention uses a transmit carrier power of the target cell and neighboring cells in order to make an admission decision and assign physical resources to a requested call. The DL ISCP is predicted using carrier powers of neighboring cells. The DL ISCP in timeslot t of a WTRU located in cell i, I DL (i,t), can be expressed according to:  
                   I   DL     ⁡     (     i   ,   t     )       =       N   O     +       ∑     j   ∈     𝔍   1         ⁢         P   T     ⁡     (     j   ,   t     )         A   ⁡     (   j   )           +       ∑     j   ∈     𝔍   2         ⁢         P   T     ⁡     (     j   ,   t     )         A   ⁡     (   j   )               ;           (     Equation   ⁢           ⁢   10     )             
 
 where N O  represents a receiver noise level, A(j) represents a path loss between a WTRU and a cell j, and P T (j,t) represents a total DL transmit power of cell j in timeslot t. All quantities are expressed using a linear scale. ℑ 1  and ℑ 2  define respectively the set of tier one and tier two neighboring cells to be included in the interference prediction. The information about carrier transmission powers of neighboring cells is available to a target cell. However, the information about a path loss from the WTRU to neighboring cells is not available to the target cell. Therefore, the DL ISCP is estimated as follows:  
               E   ⁡     [       I   DL     ⁡     (     i   ,   t     )       ]       =       N   O     +       ∑     j   ∈     𝔍   1         ⁢       E   ⁡     [     X   1     ]       ⁢       P   T     ⁡     (     j   ,   t     )           +             (     Equation   ⁢           ⁢   11     )                       ⁢       ∑     j   ∈     𝔍   2         ⁢       E   ⁡     [     X   2     ]       ⁢       P   T     ⁡     (     j   ,   t     )                                           ⁢       =       N   O     +       μ   1     ⁢       ∑     j   ∈     𝔍   1         ⁢       P   T     ⁡     (     j   ,   t     )           +       μ   2     ⁢       ∑     j   ∈     𝔍   2         ⁢       P   T     ⁡     (     j   ,   t     )               ;                           
 
 where X 1  is a random variable corresponding to a link gain (i.e. inverse of a path loss) between the WTRU and a neighboring tier 1 cell Node B, X 2  is a random variable corresponding to a link gain between the WTRU and a neighboring tier 2 cell Node B, and μ i  and μ 2  represent the mean link gains between the WTRU located in the target cell and the Node Bs serving tier 1 and tier 2 cells. The mean link gains are cell deployment-specific parameters which are set through OA&amp;M. 
 
      Once the expected interference level is calculated, the interference resulting from the addition of one or multiple codes in timeslot t of cell i is predicted as follows using the noise rise function of the target cell described in Equation 2: 
 
 I   DL   PRED ( i,t )= E[I   DL ( i,t )]×R T ( E[I   DL ( i,t )], A ( i ), SIR );  (Equation 12) 
 
 where A(i) represents a path loss to the target cell and SIR represents a sum of the chip-level SIR targets of the added codes. 
 
      If the WTRU path loss measurement is available to the target cell, such as during a handover, the WTRU path loss measurement is used as an input for calculating the target cell noise rise function. Otherwise, a path loss value parameter is used, which is set through OA&amp;M. The path loss value parameter should be determined from the distribution of path losses measured throughout the target cell.  
      The carrier power resulting from the addition of one or multiple codes in timeslot t of cell i is predicted as follows: 
 
 P   T   PRED ( i,t )= P   T ( i,t )× R   T ( E[I   DL ( i,t )], A ( i ), SIR )+I DL   PRED ( i,t )× A ( i )× SIR;   (Equation 13) 
 
 where A(i) and SIR represent respectively the path loss to the target cell and the sum of the chip-level SIR targets of the added codes. The increase of interference resulting from the addition of the code is applied to existing codes as well. This is achieved by multiplying the current transmission power by the noise rise. The resulting predicted carrier transmission power, P T   PRED (i,t), is expressed in Watts. 
 
      In an alternate embodiment, the carrier power in neighboring cells can be predicted according to: 
 
 P   T   PRED ( j,t )= P   T ( j,t )× R   N ;  (Equation 14) 
 
 where R N  is calculated according to Equation 5. 
 
      The allocation of a set of codes in timeslot t of cell i is accepted if and only if the following conditions are satisfied: 
 
(10 log 10 ( P   T   PRED ( i,t ))− M   T )&lt; P   T   MAX ; and  (Equation 15) 
 
(10 log 10  ( P   T ( j,t ))− M   N )&lt; P   T   MAX ;  (Equation 16) 
 
 for all neighboring cells j under consideration. P T   PRED  (i,t) is computed as described in Equation 13. M T  and M N  represent respectively CAC power margins for the target and neighbor cells. P T   MAX  corresponds to the maximum Node-B timeslot carrier power, expressed in dB, which is set through OA&amp;M. 
 
      If the carrier power is predicted in neighboring cells according to Equation 14, then Equation 16 is replaced by: 
 
(10 log 10 ( P   T   PRED ( j,t ))− M   N )&lt; P   T   MAX .  (Equation 17) 
 
      Moreover, the allocation of the set of codes must satisfy WTRU capability requirements; otherwise, the allocation of the set of codes is rejected.  
       FIG. 2  is a flow diagram of a process  200  including method steps for implementing CAC based on DL measurements in accordance with the present invention. When a wireless communication system receives a call request for a WTRU, a code is selected from a list of available code sets (step  202 ). Under the current third generation partnership project (3GPP), only SF 16 codes are used for DL. However, other SF codes may be used for DL. Thus, a code may be selected, starting from a code having a smallest spreading factor (SF) in the code set. A first timeslot is also selected for potential allocation amongst available timeslots (step  204 ). The set of available timeslots consists of all timeslots that are available for the requested service type, (e.g., RT or NRT), and direction, (i.e., UL or DL). The set of available timeslots is set through OA&amp;M.  
      The process  200  computes a predicted interference level and carrier transmission power of a target cell and a predicted interference level and carrier transmission power of neighboring cells for the selected timeslot assuming the selected code is added to the selected timeslot in accordance with Equation 12 and Equation 13 (or alternatively, Equation 14) (step  206 ). In Equations 12 and 13, the computation considers all codes from the code set that have already been allocated to the selected timeslot.  
      The process  200  then verifies admission control by determining whether the estimated target cell carrier transmission power and a neighboring cell carrier transmission power are below predetermined thresholds, respectively (step  208 ). If both the estimated target cell carrier transmission power and the estimated neighboring cell carrier transmission power are below the thresholds, the selected code is added to the timeslot, at which point the timeslot becomes a candidate timeslot for potential allocation of the selected code and is added to a list of candidate timeslots (step  210 ). If either the estimated target cell carrier transmission power or the estimated neighboring cell carrier transmission power is not below the thresholds, the code is not added to the timeslot for allocation, and the process proceeds to step  214 .  
      Once the code is added to the timeslot, a weighted interference level is computed for the timeslot at step  212  as follows:  
                 I   DL   W     ⁡     (     i   ,   t     )       =           I   DL   PRED     ⁡     (     i   ,   t     )         1   +     γ   ⁢           ⁢     N   ⁡     (   t   )             .             (     Equation   ⁢           ⁢   18     )             
 
 The denominator, 1+γN(t), is a fragmentation adjustment factor, where λ corresponds to the fragmentation adjustment parameter and N(t) corresponds to the number of codes already assigned to this timeslot. 
 
      If it is determined that there are more available timeslots at step  214 , the next timeslot is selected from the list of available timeslots (step  216 ), and steps  202 - 214  are repeated. If there are no available timeslots for computing a weighted interference level, the process  200  determines whether there are any candidate timeslots (step  218 ). If there are no candidate timeslots, the process  200  indicates a failure of allocation of resources and rejects the requested code set (step  230 ). If there are candidate timeslots, a timeslot having a smallest weighted interference level, I DL   W (i,t) is selected thereby resulting in allocation of the selected code in the selected candidate timeslot (step  220 ). The allocated code is removed from a list of available code sets (step  222 ), and a list of candidate timeslots is reset (step  224 ). If there are more codes in a code set, the process returns to step  202  for evaluation of each code, and if not, the process proceeds to step  228  (step  226 ). In step  228 , the process  200  indicates a successful allocation of resources and returns a resource assignment solution for the call request.  
      The derivation of the noise rise function for neighboring cells from a noise rise function of the target cell is explained in more detail with reference to  FIG. 3 .  FIG. 3  is a diagram of a wireless communication system model  300  in accordance with the present invention. There are a total of N+1 cells C 0 -C N  and the number of WTRUs m il -m iN  in cell C 1  is N i +1. The WTRUs m il -m iN  served by cell C i  are denoted by {m ij }. The analysis presented hereinafter applies for both UL and DL.  
      I ij  is an interference level seen by WTRU m ij  (for DL) or by a Node-B serving WTRU m ij  (for UL). The required transmission power for serving a WTRU m ij  is equal to: 
 
 P   ij   =I   ij   SIR   ij   L   ij   (Equation 19) 
 
 where L ij  is a path loss between a cell C i  and a WTRU m ij , and SIR ij  is a required signal-to-interference ratio to adequately serve the WTRU m ij . This power is transmitted either by the WTRU m ij  (in case of UL) or by its serving Node-B (in case of DL). 
 
      Equation 19 can be re-written: 
 
P ij =I ij q ij   (Equation 20) 
 
 where q ij ≡SIR ij  L ij  is defined as the “load” of the WTRU m ij . The load q i  of cell C i  is defined as follows:  
               q   1     ≡       ∑     j   =   0       N   i       ⁢       q   ij     .               (     Equation   ⁢           ⁢   21     )             
 
      The interference level I ij  can be calculated, for a system where same-cell WTRUs cause negligible interference, as follows:  
               I   ij     =       θ   +       ∑         i   ′     =   0         i   ′     ≠   i       N     ⁢       ∑       j   ′     =   0       N     i   ′         ⁢       P       i   ′     ⁢     j   ′           L       i   ′     ⁢     j   ′     ⁢   ij               =     θ   +       ∑         i   ′     =   0         i   ′     ≠   i       N     ⁢       ∑       j   ′     =   0       N     i   ′         ⁢         q       i   ′     ⁢     j   ′         ⁢     I       i   ′     ⁢     j   ′             L       i   ′     ⁢     j   ′     ⁢   ij                         (     Equation   ⁢           ⁢   22     )             
 
 where θ is a thermal noise level, and L i′j′ij  is a path loss between the WTRU m ij  and the cell C i′ (for DL) or between the WTRU m i′j′  and the cell C i  (for UL). 
 
      A link gain (inverse of a path loss) between a cell and a WTRU connected to another cell is equal to G c .  
               L       i   ′     ⁢     j   ′     ⁢   ij       =         1     G   c       ⁢           ⁢   if   ⁢           ⁢     i   ′       ≠     i   .               (     Equation   ⁢           ⁢   23     )             
 
      With this assumption, Equation 22 can be re-written as follows:  
               I   ij     =     θ   +       G   c     ⁢         ∑       i   ′     =   0     N         i   ′     ≠   i       ⁢       ∑       j   ′     =   0       N     i   ′         ⁢       q       i   ′     ⁢     j   ′         ⁢       I       i   ′     ⁢     j   ′         .                       (     Equation   ⁢           ⁢   24     )             
 
      The right term is independent of j. Therefore, I i ≡I ij  ∀j, and Equation 23 can be re-written as follows:  
                     I   i     =       ⁢     θ   +       G   c     ⁢         ∑       i   ′     =   0     N         i   ′     ≠   i       ⁢       I     i   ′       ⁢       ∑       j   ′     =   0       N     i   ′         ⁢     q       i   ′     ⁢     j   ′                               =       ⁢     θ   +         G   c     ⁡     (         ∑       i   ′     =   0     N     ⁢       I     i   ′       ⁢     q     i   ′           -       I   i     ⁢     q   i         )       ⁢     ∀   i                       (     Equation   ⁢           ⁢   25     )             
 
      From this set of equations (valid for any cell C i ) it is possible to express the interference of any cell, say cell C 0 , as a function of the loads qi of all cells and the constant G c . This can be achieved by first considering Equation 24 for i=0 specifically:  
               I   0     =     θ   +       G   c     ⁢       ∑       i   ′     =   0     N     ⁢       I     i   ′       ⁢     q     i   ′             -       G   c     ⁢     I   0     ⁢     q   0                 (     Equation   ⁢           ⁢   26     )             
 
      Then, combining it with the general equation in i, the following equations are obtained: 
 
 I   i   =I   0   +G   c   I   0   q   0   −G   c   I   i   q   i   , or   (Equation 27) 
 
               I   i     =       I   0     ⁢       1   +       G   c     ⁢     q   0           1   +       G   c     ⁢     q   i           ⁢     ∀     i   .                 (     Equation   ⁢           ⁢   28     )             
 
      Let C 0  represent the target cell to which codes are being allocated to and C i  represent a neighboring cell. As such, the load q 0  of C 0  will change following the allocation of the codes.  
      Let q 0   in  represent the initial load of C 0 , prior to the allocation of codes. Let q 0   f  represent the final load of C 0 , following the allocation of codes. Then, 
 
 q   0   f   =q   0   in   +L×SIR   (Equation 29) 
 
      Equation 28 must be satisfied both prior to and following the allocation of codes to C 0 . That is,  
                 I   i   in     =       I   0   in     ⁢       1   +       G   c     ⁢     q   0   in           1   +       G   c     ⁢     q   i           ⁢     ∀   i         ⁢     
     ⁢   and           (     Equation   ⁢           ⁢   30     )                 I   i   f     =       I   0   f     ⁢       1   +       G   c     ⁢     q   0   f           1   +       G   c     ⁢     q   i           ⁢     ∀   i               (     Equation   ⁢           ⁢   31     )             
 
 where I 0   in  and I 0   f  represent respectively the initial and final interference in target cell C 0 , and I i   in  and I i   f  represent respectively the initial and final interference in neighbor cell C i . 
 
      The noise rise in neighbor cell C i  is then given by:  
               R   N     =         I   i   f       I   i   in       =         I   0   f       I   0   in       ×         1   +       G   c     ⁢     q   o   f           1   +       G   c     ⁢     q   o   in           .                 (     Equation   ⁢           ⁢   32     )             
 
      Equation (32) can be rewritten as:  
                     R   N     =       ⁢         I   0   f       I   0   in       ×       1   +       G   c     ⁡     (       q   o   in     +     L   ×   SIR       )           1   +       G   c     ⁢     q   o   in                         =       ⁢         I   0   f       I   0   in       ×     (     1   +         G   C     ×   L   ×   SIR       1   +       G   c     ⁢     q   o   in             )                     (     Equation   ⁢           ⁢   33     )             
 
      When the initial load of C 0  is unknown, Equation 33 can be simplified to:  
               R   N     =         I   0   f       I   0   in       ×     (     1   +       G   C     ×   L   ×   SIR       )               (     Equation   ⁢           ⁢   34     )             
 
 by setting q 0   in  to zero. R T  corresponds to the noise rise calculated according to Equation 2. 
 
       FIG. 4  is a block diagram of an apparatus  400  used to implement CAC in accordance with the present invention. The apparatus  400  communicates with a core network  420  and a WTRU  430 , and may reside in an RNC or a Node-B, or any other network entity which is responsible for CAC and radio resource allocation.  
      The apparatus  400  includes a receiver  402 , a code selector  404 , a first calculation unit  406 , a comparator  408 , a second calculation unit  410 , and a controller  412 . Once a call request is received from the WTRU  430  or the core network  420 , the controller  412  initiates a CAC process in accordance with the present invention. The code selector  404  selects a code among available codes in response to the controller  412 . The selected code is evaluated for potential allocation to each of available timeslots through calculation of an estimated target cell load and neighbor cell loads based on UL interference, or through calculation of an estimated target cell transmission power and neighbor cell transmission power based on DL interference.  
      If the CAC process is based on UL interference, the first calculation unit  406  calculates a target cell load and a neighbor cell load for each available timeslot using Node-B measurements and assuming addition of the selected code. The comparator  408  compares the target cell load and the neighbor cell load with predetermined thresholds, respectively. If both the target cell load and the neighbor cell load are below the thresholds, respectively, the code is added to the timeslot for potential allocation. The second calculation unit  410  calculates a weighted system load for the timeslot. The controller  412  controls the overall process and selects a timeslot having a smallest weighted system load among candidate timeslots to allocate for the call request.  
      If the CAC is based on DL interference, the first calculation unit  406  calculates a target cell transmission power and a neighbor cell transmission power for each available timeslot using Node-B measurements and assuming addition of the selected code. The comparator  408  compares the target cell transmission power and the neighbor cell transmission power with predetermined thresholds, respectively. If both the target cell transmission power and the neighbor cell transmission power are below the thresholds, respectively, the code is added to the timeslot for potential allocation. The second calculation unit  410  calculates a weighted interference for the timeslot. The controller  412  selects a timeslot having a smallest weighted interference among candidate timeslots to allocate for the call request. It is noted that the functions performed by the components with the apparatus  400  may be performed by more or less components as desired.  
      Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.