Abstract:
A call admission control (CAC) method which operates in a network including at least one target cell and a plurality of neighboring cells under a variety of conditions. When only common measurements are available, the CAC function at a Controlling Remote Network Controller (CRNC) determines whether to accept or reject a request to setup or reconfigure a radio access bearer based on common measurements of the target cell and neighboring cells, estimated common measurements after admission, and load measurements of the target cell and the neighboring cells. When no measurements are available, the CAC function at the CRNC determines whether to accept or reject the request based on estimated load of the target cell and neighboring cells. The load is estimated based on the signal-to-interference ratio (SIR) of the requested radio access bearer, and the SIR of existing coded composite transport channels (CCTrCHs) in the target cell and neighboring cells.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application a continuation of U.S. patent application Ser. No. 10/331,442, filed Dec. 30, 2002, which claims priority from U.S. Provisional Patent Application No. 60/383,273, filed May 24, 2002, which is incorporated by reference as if fully set forth. 
     
    
     BACKGROUND  
       [0002]     The present invention is directed to strategies and algorithms by which CDMA networks perform call admission control (CAC) in three different situations: 1) when only common-measurements are available; 2) when no measurements are available; and 3) based on outage probability requirements. In particular, it is applicable to Universal Mobile Telephone System-Time Division Duplex (UMTS-TDD) systems.  
         [0003]     Call admission control (CAC) is a function responsible for deciding whether to accept or reject a request to setup or reconfigure a radio access bearer in the radio access network (RAN). CAC is performed at the Controlling Remote Network Controller (CRNC). Sometimes, although UE dedicated measurements are not available, common measurements such as uplink timeslot ISCP and downlink carrier power are available. Thus, the CRNC must have the ability to perform CAC properly with only common measurements.  
         [0004]     Sometimes, no measurements are available. In this case, the CRNC must have the ability to perform CAC properly in absence of measurements.  
         [0005]     In a UMTS-TDD system, the required signal-to-interference ratio (SIR) of a user changes with time because of fading and imperfect power control. In such a system, outage probability is a good measure of system quality of service (QoS). CAC should be designed to provide the required outage probability to the system.  
       SUMMARY  
       [0006]     The present invention is able to perform CAC under a variety of conditions. When only common measurements are available, CAC will accept or reject a request to setup or reconfigure a radio access bearer (i.e., radio link) based on: 1) common measurements of the target cell and neighboring cells; 2) estimated common measurements after admission; and the measure of the loading of the target cell and neighboring cells, (which are also estimated from common measurements). When the CAC assigns codes of the CCTrCH to different time slots, it will try to optimize the load or carrier power of the target cell and neighboring cells; whereby average or weighted average load/carrier power of the target cell and neighboring cells can be used.  
         [0007]     When no measurements are available, CAC accepts or rejects the request based on estimated load of the target cell and neighboring cells. The load may be estimated using the following information: 1) required SIR of the request, (this represents the load of the request); and 2) required SIR of existing coded composite transport channels (CCTrCHs) in the target cell and neighboring cells, (this represents the current load of the target cell and neighboring cells). When CAC assigns codes of the CCTrCH to different time slots, it will try to optimize the load of the target cell and neighboring cells; whereby average or weighted average load of the target cell and neighboring cells can be used.  
         [0008]     Finally, an alternative embodiment of the present invention can accept or reject a request based on the estimated outage probability of the target cell and neighboring cells. The estimated outage probability is based on assumption of the time-variant SIR. One alternative for estimating the outage probability is to use: 1) required SIR and SIR range of the radio link setup/reconfiguration request, (this represents the range of load of the request); 2) required SIR and SIR range of existing CCTrCHs in the target cell and neighboring cells, (this represents the current range of load of the target cell and neighboring cells); and 3) the outage probability, which is defined as the probability that the instantaneous load in a time slot exceeds a maximum allowed value. When CAC assigns codes of the CCTrCH to different time slots, it will try to minimize the total outage probability of the CCTrCH, ensuring that the outage probability of assigned timeslots in neighboring cells also meets the requirements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a flow diagram of a slot selection procedure for the uplink and downlink for a first embodiment in accordance with the present invention.  
         [0010]      FIG. 2  is a flow diagram of a slot selection procedure for the uplink and downlink for a second embodiment in accordance with the present invention.  
         [0011]      FIG. 3  is a flow diagram of a slot selection procedure for the uplink and downlink for a third embodiment in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout.  
         [0013]     A first embodiment of the present invention is directed to CAC with only common measurements available. Referring to  FIG. 1 , the basic assignment procedure  10  for CAC in the uplink is shown. The procedure  10  commences with the first code in the code set (step  12 ). The load of an uplink time slot in cell i is defined as the load generated by the codes assigned in the same time slot in this cell and in first tier cells and second tier cells, since the load generated from cells beyond second tier is negligible. Preferably the load from neighboring cells is measured using the uplink time slot ISCP. Uplink time slot Interference Signal Code Power (ISCP) contains inter-cell interference only. For the target cell, the load after admission can be calculated as follows: 
 
 The load from its own cell is called the intra-cell load Load UL     —     Intra (i) and is given by:  
                 Load   UL_Intra     ⁡     (   i   )       =       α   UL     ·       ∑     h   ∈     Ω   ⁡     (   i   )           ⁢     S   ⁢           ⁢   I   ⁢           ⁢     R   h                   Equation   ⁢           ⁢   1             
 
 where Ω(i) is the set of codes assigned in this time slot in the cell i. Since the load is defined based on noise rise, then intra-cell interference I intra  may be given by:  
               Intra   ⁢     -     ⁢   cell   ⁢           ⁢   noise   ⁢           ⁢   rise     =         I   intra       N   0       =     1     1   -       Load   UL_Intra     ⁡     (   i   )                     Equation   ⁢           ⁢   2             
 
 Which can be rewritten as:  
               I   intra     =       N   0       1   -       Load   UL_Intra     ⁡     (   i   )                   Equation   ⁢           ⁢   3             
 
 The total interference can be found by:  
               Total   ⁢           ⁢   noise   ⁢           ⁢   rise     =           I   Intra     +   ISCP       N   0       =     1     1   -       Load   UL     ⁡     (   i   )                     Equation   ⁢           ⁢   4             
 
 Then, the total load is given by:  
                 Load   UL     ⁡     (   i   )       =       1   -     1     Total   ⁢           ⁢   noise   ⁢           ⁢   rise         =     1   -       N   0         I   Intra     +   ISCP                   Equation   ⁢           ⁢   5             
 
         [0019]     For neighboring cells, (tier one or tier two cells only), the load after admission can be calculated as follows: 
 
 The load of a time slot in cell j before admission is calculated using Equations 1-5, and denoted by Load UL     —     Before (j). If SIR t  is the required SIR target of the code to be assigned in this time slot in the target cell i, the load of a time slot in cell j after admission, denoted by Load UL (j), is given by:  
                 Load   UL     ⁡     (   j   )       =     {               Load   UL_Before     ⁡     (   j   )       +       β   UL     ·     SIR   t                 if   ⁢           ⁢   j     ∈     Tier   ⁢           ⁢     One   ⁡     (   i   )                         Load   UL_Before     ⁡     (   j   )       +       σ   UL     ·     SIR   t                 if   ⁢           ⁢   j     ∈     Tier   ⁢           ⁢     Two   ⁡     (   i   )                           Equation   ⁢           ⁢   6             
 
 Where Tier One(i) is the set of codes assigned in this time slot in tier-one neighboring cells of the cell i, and Tier Two(i) is the set of codes assigned in this time slot in tier-two neighboring cells of the cell i. 
 
         [0022]     For CAC, a code will be admitted into a time slot in cell i only if, after admission, the following conditions can be satisfied in this time slot: 
 
 Load   UL ( i )≦ CAC   —   Target   —   Load   —   Thres   —   UL   Equation 7 
 
and 
 
 Load   UL ( j )≦ CAC   —   Neighbor   —   Load   —   Thres   —   UL,  ∀jεTier One(i)∪Tier Two(i)  Equation 8 
 
 Where CAC_Target_Load_Thres_UL is the admission threshold of load in the target cell, and CAC_Neighbor_Load_Thres_UL is the admission threshold of load in the neighboring cells. 
 
         [0024]     A measure of the quality after admission is the average load, {overscore (Load UL )}, which is defined as the average load of target cell and neighboring cells. It is given by:  
                 Load   UL     _     =       1   N     ⁢       ∑     j   =   1     N     ⁢       Load   UL     ⁡     (   j   )                   Equation   ⁢           ⁢   9             
 
         [0025]     An alternative measure is the weighted average load in the uplink, {overscore (Load UL     —     Weighted )}, which is similar to Equation 9, but gives priority to the target cell load by using a weight factor w (w&gt;1) for the target cell. It is given by:  
                 Load   UL_Weighted     _     =       1   N     ⁢     (         ∑     j   =       1   ⁢   j     ≠   1       N     ⁢       Load   UL     ⁡     (   j   )         +     w   ·       Load   UL     ⁡     (   i   )           )               Equation   ⁢           ⁢   10             
 
         [0026]     Slot Selection Procedure for Call Admission Control in the Uplink  
         [0027]     Suppose that the new CCTrCH seeking admission has M codes in its code set to be assigned. These M codes are arranged in the order of increasing spreading factor, (decreasing required SIR target). The slot selection follows the procedures below: 
        1. Start with the first code in the code set (step  12 — FIG. 1 ).     2. For each uplink time slot, compute the load of target cell and neighboring cells in this time slot using Equation 1 as if this code is assigned into this time slot (step  14 ).     3. For each uplink time slot, judge if this code can be assigned to this time slot by checking if load of target cell and neighboring cells in this time slot after assignment satisfies conditions in Equations 7 and 8 (steps  16 ,  17 ). 
            a. If yes, this time slot will be considered as possible time slot for assignment in step  22  (step  18 ).     b. Otherwise, this time slot will not be considered as possible time slot for assignment in step  22  (step  20 ).    
            4. Check if there are any possible time slots for assignment (step  22 ). 
            a. If there is at least one possible time slot for assignment, among all possible time slots, select the time slot that yields the lowest average load as defined in Equation 9 or the lowest weighted average load as defined in Equation 10 (step  24 ). This code will be assigned to this selected time slot. Go to step  28 .     b. Otherwise, this code cannot be assigned in the target cell (step  26 ). It also means the CCTrCH with this code set cannot be accommodated in the target cell. The slot selection procedure ends (step  33 ).    
            5. Check if there are any more codes to be assigned (step  28 ). 
            a. If yes, select the next code in the code set (step  30 ) and go to step  14 .    
            b. Otherwise, the assignment of the CCTrCH is done (step  32 ). The slot selection procedure ends (step  33 ).        
 
         [0039]     Basic Assignment Procedure for Call Admission Control in the Downlink  
         [0040]     Let CaPwr(i) denote the carrier power of a downlink time slot in the cell i. Let SIR t  denote the required SIR target of the code to be assigned in this time slot in the target cell i. Let PL(k) denote the pathloss of this UE between BS of cell k, k=1, 2, . . . , N. The code TX power for this new code, denoted by TX code , is given by: 
 
 TX   code   =SIR   t   ·PL ( i )· I   total   Equation 11 
 
 and I total  is given by:  
               I   total     =         α   DL     ·       CaPwr   ⁡     (   i   )       /     PL   ⁡     (   i   )           +       ∑     j   ∈     Tier   -     One   ⁡     (   i   )             ⁢       CaPwr   ⁡     (   j   )       /     PL   ⁡     (   j   )           +       ∑     j   ∈     Tier   -     Two   ⁡     (   j   )             ⁢     CaPwr   ⁢       (   j   )     /     PL   ⁡     (   j   )             +     N   0               Equation   ⁢           ⁢   12             
 
         [0042]     When the system is at moderate or high load, (where call admission control is really put in use), background noise N 0  is negligible. Therefore, Equation 11 is rewritten as:  
               TX   code     =       SIR   t     ·     (         α   DL     ·     CaPwr   ⁡     (   i   )         +       ∑     j   ∈     TierOne   ⁡     (   i   )           ⁢       CaPwr   ⁡     (   j   )       ·       PL   ⁡     (   i   )       /     PL   ⁡     (   j   )             +       ∑     j   ∈     TierTwo   ⁡     (   j   )           ⁢       CaPwr   ⁡     (   j   )       ·       PL   ⁡     (   i   )       /     PL   ⁡     (   j   )               )               Equation   ⁢           ⁢   13             
 
         [0043]     If  
         X   =       ∑     j   ∈     TierOne   ⁡     (   i   )                   ⁢           ⁢       CaPwr   ⁡     (   j   )       ·       PL   ⁡     (   i   )       /     PL   ⁡     (   j   )               ,       
 
 for UE at different locations, (which implies different pathloss to BS), X is a random variable. Define ω DL  as:  
               ω   DL     =       arg   min     ⁢     {       ω   DL     :       Pr   ⁡     (     X   ≤       ω   DL     ·       ∑     j   ∈     TierOne   ⁡     (   i   )                   ⁢           ⁢     CaPwr   ⁡     (   j   )             )       &gt;   θ       }               Equation   ⁢           ⁢   14             
 
 Where θ is a predefined percentage, for example, 90%. 
 
         [0046]     Similarly, if  
         Y   =       ∑     j   ∈     TierTwo   ⁡     (   i   )                   ⁢       CaPwr   ⁡     (   j   )       ·       PL   ⁡     (   i   )       /     PL   ⁡     (   j   )               ,       
 
 for UE at different locations, (which implies different pathloss to BS), Y is a random variable. Define ξ DL  as  
                 ξ   DL     =       arg   min     ⁢     {       ξ   DL     :       Pr   (     Y   ≤       ξ   DL     ·       ∑     j   ∈     TierTwo   ⁡     (   i   )                   ⁢     CaPwr   ⁡     (   j   )             )     &gt;   θ       }         ⁢                   Equation   ⁢           ⁢   15             
 
 Then, Equation 13 can be written as:  
               TX   code     =       SIR   t     ·     (         α   DL     ·     CaPwr   ⁡     (   i   )         +       ω   DL     ·       ∑     j   ∈     Tier   -     One   ⁡     (   i   )                     ⁢     CaPwr   ⁡     (   j   )           +       ξ   DL     ·       ∑     j   ∈     Tier   -     Two   ⁡     (   j   )                     ⁢     CaPwr   ⁡     (   j   )             )               Equation   ⁢           ⁢   16             
 
         [0049]     After the new code is added, the sum of code transmit power of existing codes will increase by ΔTX(i). The value of ΔTX(i) is estimated to be:  
               Δ   ⁢           ⁢     TX   ⁡     (   i   )         =           α   DL     ·     TX   code                   α   DL     ·     CaPwr   ⁡     (   i   )         +       ω   DL     ·       ∑     j   ∈     Tier   -     One   ⁡     (   i   )                     ⁢     CaPwr   ⁢     (   j   )           +                 ξ   DL     ·       ∑     j   ∈     Tier   -     Two   ⁡     (   i   )                     ⁢     CaPwr   ⁡     (   j   )                   ⁢     CaPwr   ⁡     (   i   )                 Equation   ⁢           ⁢   17             
 
 Therefore, the carrier power of cell i after admission is estimated to be: 
 
 CaPwr ( i )= CaPwr ( i )+ TX   code   +ΔTX ( i )+ Margin   target cell   Equation 18 
 
 Where Margin target cell  is the margin used for call admission control in the target cell. 
 
         [0052]     The increase to the carrier power of cell j (if jεTier One(i)) after admission, ΔTX(j), is estimated to be:  
               Δ   ⁢           ⁢     TX   ⁡     (   j   )         =         (       ω   DL     ·     (       TX   code     +     Δ   ⁢           ⁢     TX   ⁡     (   i   )           )                     α   DL     ·     CaPwr   ⁡     (   j   )         +       ω   DL     ·     
     ⁢       ∑     j   ∈     Tier   -     One   ⁡     (   j   )                     ⁢     CaPwr   ⁡     (   k   )           +                 ξ   DL     ·       ∑     k   ∈     Tier   -     Two   ⁡     (   j   )                     ⁢     CaPwr   ⁡     (   k   )                   ⁢   C   ⁢           ⁢     aPwr   ⁡     (   i   )                 Equation   ⁢           ⁢   19             
 
         [0053]     The increase to the carrier power of cell j (if jεTier Two(i)) after admission, ΔTX(j), is estimated to be:  
               Δ   ⁢           ⁢     TX   ⁡     (   j   )         =           ξ   DL     ·     (       TX   code     +     Δ   ⁢           ⁢   T   ⁢           ⁢     X   ⁡     (   i   )           )                   α   DL     ·     CaPwr   ⁡     (   j   )         +       ω   DL     ·                         ⁢         ∑     k   ∈     Tier   -     One   ⁡     (   j   )                     ⁢     CaPwr   ⁡     (   k   )         +       ξ   DL     ·                           ⁢       ∑     k   ∈     Tier   -     Two   ⁡     (   j   )                     ⁢     CaPwr   ⁡     (   k   )                   ⁢     CaPwr   ⁡     (   j   )                 Equation   ⁢           ⁢   20             
 
 Therefore, the carrier power of cell j after admission is estimated to be: 
 
 CaPwr ( j )′= CaPwr ( j )+Δ TX ( j )+ Margin   neighbor cell   Equation 21 
 
 Where Margin neighbor cell  is the margin used for call admission control in neighboring cells. 
 
         [0056]     At call admission control, a code will be admitted into a time slot in cell i only if after admission the following conditions can be satisfied in this time slot: 
 
 CaPwr ( i )′≦ CaPwr   maximum   Equation 22 
 
and 
 
 CaPwr ( j )′≦ CaPwr   maximum , ∀jεTier-One(i)∪Tier-Two(i)  Equation 23 
 
 Where CaPwr maximum  is the maximum allowed carrier power at Node-B. 
 
         [0058]     A measure of the quality after admission is the average carrier power, {overscore (CaPwr)}, which is defined as the average carrier power of target cell and neighboring cells. It is given by  
               CaPwr   _     =       1   N     ⁢       ∑     j   =   1     N     ⁢           ⁢     CaPwr   ⁡     (   j   )                   Equation   ⁢           ⁢   24             
 
         [0059]     An alternative measure is the weighted average load in the uplink, {overscore (CaPwr Weighted )}, which is similar to the definition in Equation 24, but gives priority to the target cell&#39;s carrier power by using a weight factor w (w&gt;1) for the target cell. It is given by  
                 CaPwr   Weighted     _     =       1   N     ⁢     (         ∑       j   =   1     ,     j   ≠   i       N     ⁢           ⁢     CaPwr   ⁡     (   j   )         +     w   ·     CaPwr   ⁡     (   i   )           )               Equation   ⁢           ⁢   25             
 
         [0060]     Slot Selection Procedure for Call Admission Control in the Downlink  
         [0061]     The flowchart of the slot selection procedure in the downlink is the same as in the uplink (shown in  FIG. 1 ), except that call admission control in the downlink tries to minimize the average carrier power instead of average load. Suppose that the new CCTrCH seeking admission has M codes in its code set to be assigned. Since the direction is downlink, the M codes have the same spreading factors 16 or 1. Therefore, the order of assignment for codes does not matter in the downlink. The slot selection follows the procedures below: 
        1. Start with the first code in the code set (step  12 ).     2. For each downlink time slot, estimate the carrier power of target cell and neighboring cells in this time slot using Equations 16-21 as if this code is assigned into this time slot (step  14 ).     3. For each downlink time slot, judge if this code can be assigned to this time slot by checking if carrier power of target cell and neighboring cells in this time slot after assignment satisfies conditions in Equations 22 and 23 (steps  16   m    17 ). 
            a. If yes, this time slot will be considered as a possible time slot for assignment in step  22  (step  20 ).     b. Otherwise, this time slot will not be considered as a possible time slot for assignment in step  22  (step  20 ).    
            4. Check if there are any possible time slots for assignment (step  22 ). 
            a. If there is at least one possible time slot for assignment, among all possible time slots, select the time slot that yields the lowest average carrier power as defined in Equation 24 or the lowest weighted average carrier power as defined in Equation 25 (step  24 ). This code will be assigned to this selected time slot. Go to step  28 .     b. Otherwise, this code cannot be assigned in the target cell (step  26 ). It also means the CCTrCH with this code set cannot be accommodated in the target cell. The slot selection procedure ends (step  33 ).    
            5. Check if there is any more code to be assigned (step  28 ). 
            a. If yes, select the next code in the code set (step  30 ), and go to step  14 .     b. Otherwise, the assignment of the CCTrCH is done (step  32 ). The slot selection procedure ends (step  33 ).    
               
 
         [0073]     The second embodiment of the present invention is directed to call admission control in the absence of measurements.  
         [0074]     Basic Assignment Procedure for Call Admission Control in the Uplink  
         [0075]     The load of an uplink time slot in a cell is defined as the load generated by the codes assigned in the same time slot in this cell and in first tier cells and second tier cells (load generated from cells beyond second tier is negligible). Then, the load in a cell k is:  
                   Load   UL     ⁡     (   k   )       =         α   UL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h             ,     
     ⁢     
     ⁢     k   =   1     ,   2   ,   …   ⁢           ,   N           Equation   ⁢           ⁢   26             
 
 Where α UL  is the average MUD residual factor in the uplink, β UL  is the weight factor for codes in the tier-one cells in the uplink, σ UL  is the weight factor for codes in the tier-two cells in the uplink, Ω(k) is the set of codes assigned in this time slot in the cell k, Tier One(k) is the set of codes assigned in this time slot in tier-one neighboring cells of the cell k, Tier Two(k) is the set of codes assigned in this time slot in tier-two neighboring cells of the cell k. 
 
         [0077]     At call admission control, a code will be admitted into a time slot in cell i only if after admission the following conditions can be satisfied in this time slot: 
 
 Load   UL ( i )≦ CAC   —   Target   —   Load   —   Thres   —   UL   Equation 27 
 
and 
 
 Load   UL ( j )≦ CAC   —   Neighbor   —   Load   —   Thres   —   UL,  ∀jεTier One(i)∪Tier Two(i)  Equation 28 
 
 Where CAC_Target_Load_Thres_UL is the admission threshold of load in the target cell, and CAC_Neighbor_Load_Thres_UL is the admission threshold of load in the neighboring cells. 
 
         [0079]     A measure of the quality after admission is the average load in the uplink, {overscore (Load UL )}, which is defined as the average load of the target cell and neighboring cells. It is given by:  
                 Load   UL     _     =       1   N     ⁢       ∑     j   =   1     N     ⁢       Load   UL     ⁡     (   j   )                   Equation   ⁢           ⁢   29             
 
         [0080]     An alternative measure is the weighted average load in the uplink, {overscore (Load UL     —     Weighted )}, which is similar to the definition in Equation 29, but gives priority to the target cell load by using a weight factor w (w&gt;1) for the target cell. It is given by:  
                 Load   UL_Weighted     _     =       1   N     ⁢     (         ∑       j   =   1     ,     j   ≠   i       N     ⁢       Load   UL     ⁡     (   j   )         +     w   ·       Load   UL     ⁡     (   i   )           )               Equation   ⁢           ⁢   30             
 
         [0081]     Slot Selection Procedure for Call Admission Control in the Uplink  
         [0082]     The flowchart of the slot selection procedure in the uplink is shown in  FIG. 2 . Suppose that the new CCTrCH seeking admission has M codes in its code set to be assigned. These M codes are arranged in the order of increasing spreading factor (decreasing required SIR target). The slot selection follows the procedures below: 
        1. Start with the first code in the code set (step  12 ′).     2. For each uplink time slot, compute the load of target cell and neighboring cells in this time slot using Equation 26 as if this code is assigned into this time slot (step  14 ′).     3. For each uplink time slot, judge if this code can be assigned to this time slot by checking if load of target cell and neighboring cells in this time slot after assignment satisfies conditions in Equations 27 and 28 (steps  16 ′,  17 ′). 
            c. If yes, this time slot will be considered as possible time slot for assignment in step  22 ′ (step  18 ′).     d. Otherwise, this time slot will not be considered as possible time slot for assignment in step  22 ′ (step  20 ′).    
            4. Check if there are any possible time slots for assignment (step  22 ′). 
            e. If there is at least one possible time slot for assignment, among all possible time slots, select the time slot that yields the lowest average load as defined in Equation 29 or the lowest weighted average load as defined in Equation 30 (step  24 ′). This code will be assigned to this selected time slot. Go to step  28 ′.     f. Otherwise, this code cannot be assigned in the target cell (step  26 ′). It also means the CCTrCH with this code set cannot be accommodated in the target cell. The slot selection procedure ends (step  33 ′).    
            6. Check if there are any more code to be assigned (step  28 ′). 
            a. If yes, select the next code in the code set, and go to step  14 ′ (step  30 ′).     b. Otherwise, the assignment of the CCTrCH is done (step  32 ′). The slot selection procedure ends (step  33 ′).    
               
 
         [0094]     Basic Assignment Procedure for Call Admission Control in the Downlink  
         [0095]     The load of a downlink time slot in cell i is defined as the load generated by the codes assigned in the same time slot in this cell and in first tier cells and second tier cells (load generated from cells beyond second tier is negligible). Therefore, the load in the downlink is similar to the load in the uplink. However, there is a difference between them. In the uplink, there is only one receiver, the BS. In the downlink, there are several receivers, UEs, scattered in the cell. To compensate for this difference, a scale factor is added into the load calculation. Then, the load is given by:  
                   Load   DL     ⁡     (   k   )       =     Scale   ·     (         α   DL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   DL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   DL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h           )         ,     
     ⁢     
     ⁢     k   =   1     ,   2   ,   …   ⁢           ,   N           Equation   ⁢           ⁢   31             
 
 Where α DL  is the average MUD residual factor in the downlink, β DL  is the weight factor for codes in the tier-one cells in the downlink, σ DL  is the weight factor for codes in the tier-two cells in the downlink, Ω(k) is the set of codes assigned in this time slot in the cell k, Tier One(k) is the set of codes assigned in this time slot in tier-one neighboring cells of the cell k, Tier Two(k) is the set of codes assigned in this time slot in tier-two neighboring cells of the cell k. 
 
         [0097]     At call admission control, a code will be admitted into a time slot in cell i only if after admission the following conditions can be satisfied in this time slot: 
 
 Load   DL ( i )≦ CAC   —   Target   —   Load   —   Thres   —   DL   Equation 32 
 
and 
 
 Load   DL ( j )≦ CAC   —   Neighbor   —   Load   —   Thres   —   DL,  ∀jεTier-One(i)∪Tier-Two(i)  Equation 33 
 
 Where CAC_Target_Load_Thres_DL is the admission threshold of load in the target cell, and CAC_Neighbor_Load_Thres_DL is the admission threshold of load in the neighboring cells. 
 
         [0099]     A measure of the quality after admission is the average load in the downlink, {overscore (Load DL )}, which is defined as the average load of target cell and neighboring cells. It is given by:  
                 Load   DL     _     =       1   N     ⁢       ∑     j   =   1     N     ⁢       Load   DL     ⁡     (   j   )                   Equation   ⁢           ⁢   34             
 
         [0100]     An alternative measure is the weighted average load in the uplink, {overscore (Load DL     —     Weighted )}, which is similar to the definition in Equation 34, but gives priority to the target cell load by using a weight factor w (w&gt;1) for the target cell. It is given by:  
                 Load   DL_Weighted     _     =       1   N     ⁢     (         ∑       j   =   1     ,     j   ≠   i       N     ⁢       Load   DL     ⁡     (   j   )         +     w   ·       Load   DL     ⁡     (   i   )           )               Equation   ⁢           ⁢   35             
 
         [0101]     Slot Selection Procedure for Call Admission Control in the Downlink  
         [0102]     The flowchart of slot selection procedure is the same as in  FIG. 2 . Suppose that the new CCTrCH seeking admission has M codes in its code set to be assigned. Since the direction is downlink, the M codes have the same spreading factors 16 or 1. Therefore, the order of assignment for codes does not matter in the downlink. The slot selection follows the procedures below: 
        1. Start with the first code in the code set (step  12 ′).     2. For each downlink time slot, compute the load of target cell and neighboring cells in this time slot using Equation 31 as if this code is assigned into this time slot (step  14 ′).     3. For each downlink time slot, judge if this code can be assigned to this time slot by checking if load of target cell and neighboring cells in this time slot after assignment satisfies conditions in Equations 32 and 33 (steps  16 ′,  17 ′). 
            a. If yes, this time slot will be considered as possible time slot for assignment in step  22  (step  18 ′).     b. Otherwise, this time slot will not be considered as possible time slot for assignment in step  22  (step  20 ′).    
            4. Check if there are any possible time slots for assignment (step  22 ′). 
            a. If there is at least one possible time slot for assignment, among all possible time slots, select the time slot that yields the lowest average load as defined in Equation 34 or the lowest weighted average load as defined in Equation 35 (step  24 ′). This code will be assigned to this selected time slot. Go to step  28 ′.     b. Otherwise, this code cannot be assigned in the target cell (step  26 ′). It also means the CCTrCH with this code set cannot be accommodated in the target cell. The slot selection procedure ends (step  33 ′).    
            5. Check if there are any more codes to be assigned (step  28 ′). 
            a. If yes, select the next code in the code set, and go to step  14 ′ (step  30 ′).     b. Otherwise, the assignment of the CCTrCH is done (step  32 ′). The slot selection procedure ends (step  33 ′).    
               
 
         [0114]     The third embodiment of the present invention is directed to call admission control based on outage probabilities  
         [0115]     Definition of Outage Probability for Call Admission Control in the Uplink  
         [0116]     The load of an uplink time slot in a cell is defined as the load generated by the users assigned in the same time slot in this cell and in first tier cells and second tier cells (load generated from cells beyond second tier is negligible). In most technical literature, the load from neighboring cells is assumed to be a fixed ratio of the load from its own cell based on the assumption of homogeneous system. However, in a heterogeneous system, the load cannot be modeled in such a way. We compute the load from neighboring cells based its actual traffic. Then, the load in a cell k is given by:  
                   Load   UL     ⁡     (   k   )       =         α   UL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h             ,     
     ⁢     
     ⁢     k   =   1     ,   2   ,   …   ⁢           ,   N           Equation   ⁢           ⁢   36             
 
 Where α UL  is the average MUD residual factor in the uplink, β UL  is the weight factor for users in the tier-one cells in the uplink, σ UL  is the weight factor for users in the tier-two cells in the uplink, Ω(k) is the set of users assigned in this time slot in the cell k, Tier One(k) is the set of users assigned in this time slot in tier-one neighboring cells of the cell k, Tier Two(k) is the set of users assigned in this time slot in tier-two neighboring cells of the cell k. 
 
         [0118]     Since the load is defined based on noise rise, we have:  
               Noise   ⁢           ⁢   rise     =         I   total       N   0       =     1     1   -       Load   UL     ⁡     (   k   )                     Equation   ⁢           ⁢   37             
 
         [0119]     Because of the dynamic range limitation and for the purpose of power control stability, the noise rise at the BS should be limited a maximum value of NR max . Then, we have:  
                 I   total       N   0       ≤     NR   max             Equation   ⁢           ⁢   38             
 
 Therefore, Equation 38 can written as:  
                   Load   UL     ⁡     (   k   )       ≤     1   -     1     NR   max           ⁢     
     ⁢   or           Equation   ⁢           ⁢   39                     α   UL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h           ≤     1   -     1     NR   max                 Equation   ⁢           ⁢   40             
 
         [0121]     The probability of outage in a TDD time slot i, denoted by P out , is defined as the probability that inequality in Equation 40 does not hold. It is given by  
               P   out     =     Pr   ⁢     {           α   UL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   UL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h           &gt;     1   -     1     NR   max           }               Equation   ⁢           ⁢   41             
 
         [0122]     Computation of Outage Probability  
         [0123]     Because of fading and imperfect power control, the value of SIR h  is a random variable that follows a lognormal distribution. Therefore, SIR h  can be expressed as: 
 
 SIR   h =10 N(μ     h     ,σ     h     2 )  Equation 42 
 
 Using ψ to replace  
         1   -     1     NR   max         ,       
 
 Equation 41 can be written as:  
               P   out     =     Pr   ⁢     {         ∑     h   =   1     N     ⁢       SIR   h     ·     A   h         &gt;   ψ     }               Equation   ⁢           ⁢   43             
 
 Where A h  is given by:  
               A   h     =     {           α   UL           h   ∈     Ω   ⁡     (   k   )                   β   UL           h   ∈     TierOne   ⁡     (   k   )                   σ   UL           h   ∈     TierTwo   ⁡     (   k   )                         Equation   ⁢           ⁢   44             
 
 Then, we have: 
 
 SIR   h   ·A   h =10 (μ     h     ,σ     h     2 )·10 logA     h   =10 N(μ     h     +logA     h     ,σ     h     2 )  Equation 45 
 
 Let X h  denote SIR h ·A h , then X h  is still a lognormal random variable. Its mean μ X     h    and variance σ X     h     2  are given by: 
 
μ X     h   =10 μ     h     +logA     h   ·10 ln10σ     h     2/2   Equation 46 
 
σ X     h     2 =10 2(μ     h     +logA     h     ) ·10 ln10σ     h     2 ·(10 ln10σ     h     2 −1)  Equation 47 
 
 Equation 43 becomes:  
               P   out     =     Pr   ⁢     {         ∑     h   =   1     N     ⁢     X   h       &gt;   ψ     }               Equation   ⁢           ⁢   48             
 
         [0130]     Even though the distribution of X h  is known, the computation of P out  in Equation 48 is still very complex, and cannot be done in real time. At moderate or high system load, value of N in Equation 48 is large. Therefore, the Gaussian approximation will have both good approximation result and low computation complexity. Here, we choose the Gaussian approximation approach to allow the Radio Network Controller (RNC) to compute the outage probability and make a decision of resource allocation in real time.  
         [0131]     Consider that we have a random variable  
         Y   =       ∑     h   =   1     N     ⁢     X   h         ,       
 
 where {X h } are N independent identical random variables, each with mean μ X     h   , and variance σ X     h     2 . Then:  
               μ   Y     =       ∑     h   =   1     N     ⁢     μ     X   h                 Equation   ⁢           ⁢   49                   σ   Y   2     =       ∑     h   =   1     N     ⁢     σ     X   h     2         ⁢     
     ⁢     and:             Equation   ⁢           ⁢   50                 P   out     =       Pr   ⁢     {     Y   &gt;   ψ     }       =     Q   ⁡     (       ψ   -     μ   Y         σ   Y       )                 Equation   ⁢           ⁢   51             
 
 Let P out (i) denote the outage probability of time slot i. If a user is allocated to use L slots (l=1, 2, . . . , L), the total outage probability of the allocation, denoted by P out     —     total , is defined as the probability that outage occurs in at least one time slot. It is given by:  
               P   out_total     =     1   -       ∏     l   =   1     L     ⁢     (     1   -       P   out     ⁡     (   l   )         )                 Equation   ⁢           ⁢   52             
 
         [0134]     Slot Selection Procedure for Call Admission Control in the Uplink  
         [0135]     The call admission control function will try to minimize the total outage probability of the CCTrCH while making sure that the outage probability of assigned timeslots in neighboring cells also meets the requirements. The flowchart of the call admission control algorithm is shown in  FIG. 3 .  
         [0136]     Suppose that the new CCTrCH seeking admission in the target cell k has M codes in its code set to be assigned. These M codes are arranged in the order of increasing spreading factor (decreasing required SIR target). The slot selection follows the procedures below: 
        1. Start with the first code in the code set, (step  36 ).     2. Compute the current outage probability of each time slot in the target cell (step  38 ). Also compute the outage probability of each time slot in the neighboring cells as if this code is assigned into the time slot (step  38 ). 
            a. If the outage probability of all neighboring cells are less than the maximum allowed outage probability, say τr, then this time slot can be considered for assignment.     b. Otherwise, this time slot cannot be considered for assignment.    
            3. Among possible time slots for assignment, start with time slot with the lowest outage probability, say time slot i (step  40 ).     4. Assign the code into the time slot i and compute the updated outage probability of the time slot (step  42 ).     5. Check if there are still more codes not assigned for the user (step  44 ). 
            a. If no, all codes are already assigned. Go to step  46 .     b. Otherwise, continue to step  52  to assign the next code in the code set.    
            6. Branching at step  44 B, compute the outage probability of each time slot in the neighboring cells as if this code is assigned into the time slot (step  52 ). Check if time slot i is still among those possible time slots (step  54 ). 
            a. If no, find the time slot with the lowest outage probability among those possible time slots, say slot j. Set i=j (step  56 ), and go to step  42 .     b. Otherwise, check if the outage probability of time slot i is still the lowest among those possible time slots (step  58 ). 
                i. If yes, go to step  42 .     ii. Otherwise, compute if it is worthy to assign the next code into the time slot with the lowest outage probability, say slot j (step  60 ). This is done by comparing the contribution to the total outage probability by those codes already assigned to time slot i and this code. The contribution to total outage probability if this code is put into slot j, denoted by P contribution  is given by: 
 
 P   contribution =1−(1 −P   out ( i ))·(1 −P   out ( j )) 
     The contribution to total outage probability if this RU is still assigned into slot i, denoted by P′ contribution , is same as the outage probability in slot i. That is, P′ contribution =P out (i)′. Check if P contribution ≧P′ contribution  ( 62 ). 
                    1. If no, go to step  40 .     2. Otherwise, set i=j (step  64 ), and go to step  42 .    
                   
               
            7. Compute the total outage probability of the allocation (step  46 ), P out     —     total , as in Equation 52. Check if P out     —     total ≦θ (step  48 ). 
            a. If yes, the user will be admitted (step  50 ).     b. Otherwise, the user will be rejected (step  51 ).    
               
 
         [0157]     Call Admission Control in the Downlink  
         [0158]     The call admission control function in the downlink is similar to uplink. However, there are some differences in load definition and its physical meaning. In the uplink, there is only one receiver, the BS. In the downlink, there are several receivers, UEs, scattered in the cell. To compensate this difference, a scale factor is added into the load calculation. Then, the load is given by:  
                   Load   DL     ⁡     (   k   )       =     Scale   ·     (         α   DL     ·       ∑     h   ⁢           ∈           ⁢     Ω   ⁡     (   k   )           ⁢     SIR   h         +       β   DL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     One   ⁡     (   k   )             ⁢     SIR   h         +       σ   DL     ·       ∑     h   ⁢           ∈           ⁢     Tier   ⁢           ⁢     Two   ⁡     (   k   )             ⁢     SIR   h           )         ,     
     ⁢     
     ⁢     k   =   1     ,   2   ,   …   ⁢           ,   N           Equation   ⁢           ⁢   53             
 
         [0159]     In the uplink, the load is defined based on total noise rise at the BS, the common receiver. In the downlink, multiple receivers are scattered in the cell. Therefore, the downlink load is defined based on average downlink noise rise, we have:  
               Noise   ⁢           ⁢   rise     =           I   DL     _       N   0       =     1     1   -       Load   DL     ⁡     (   k   )                     Equation   ⁢           ⁢   54             
 
         [0160]     Other than the difference in load definition and physical meaning, outage probability computation and slot selection in the downlink are the same as in the uplink as shown in  FIG. 3 .