US20050068920A1 - Method and system for performing call admission control in the uplink for third generation wireless communication systems - Google Patents

Abstract

A method and system for performing call admission control in wireless communication systems is disclosed. Resource units required by a new user are assigned based on an outage probability of each uplink timeslot. The outage probability of each timeslot is updated as the resource units are assigned so that each resource unit assignment results in the lowest possible contribution to total outage probability. Once all of the resource units are assigned, the total outage probability is computed based on the resource allocation. If the total outage probability is below a predetermined value, the new user is admitted. If the total outage probability is above the predetermined value, the new user is rejected.

Description

CROSS REFERENCE TO RELATED APPLICATION
• This application claims priority from U.S. patent application Ser. No. 10/301,001 filed Nov. 21, 2002, which in turn claims priority from U.S. Provisional Application No. 60/365,355, filed on Mar. 14, 2002, which are incorporated by reference as if fully set forth.
BACKGROUND
• The present invention relates to the field of communications, specifically wireless communications. More specifically, the present invention relates to call admission control in third generation wireless systems.
• Third generation wireless communications, such as wideband code division multiple access time division duplex (WCDMA-TDD) systems, will support not only voice service, but also a wide range of broadband services, such as video and Internet traffic. In such a system, the goal of call admission control is to guarantee that the quality of service (QoS) is met for all users admitted into the system. Call admission control directly affects the QoS of mobile users, and the stability and capacity of the system. Therefore, call admission control is v important for the design of WCDMA-TDD systems.
• In recent years, there have been some advances regarding call admission control in WCDMA-FDD systems but few in WCDMA-TDD systems. One such system addresses the problem by making resource allocation based on a fixed required signal to interference ratio (SIR). In WCDMA-TDD systems, however, the required SIR of a user is not fixed and, in contrast, changes with time because of imperfect power control. In WCDMA-FDD systems, there are no timeslots whereas in WCDMA-TDD systems a user can use more than one timeslot.
• A need therefore exists for providing call admission control for TDD systems.
SUMMARY
• The present invention is a system and method for performing call admission control where admission decisions are based on a dynamic SIR requirement and the assumption that a user can use multiple timeslots. The present invention is implemented without using online measurement, thereby avoiding software and hardware implementation costs attributed thereto.
BRIEF DESCRIPTION OF THE DRAWING(S)
• FIG. 1 is a method for performing call admission control in the uplink for third generation wireless communication systems in accordance with the preferred embodiment of the invention.
• FIG. 2 is a call admission control system in accordance with the preferred embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
• In accordance with the present invention, call admission control is performed in WCDMA-TDD systems (where users can use multiple timeslots) while taking into account the fact that each user's required signal to interference ration (SIR) is a random variable. Resource allocation is optimized so as to yield the lowest total outage probability (P_out-total) for a new user and to ensure P_out-total is below a predetermined value.
• The present invention is preferably implemented using the following assumptions. First, as specified by the Third Generation Partnership Project (3GPP) standards, each frame is divided into 15 timeslots. Second, the chip rate of a WCDMA-TDD system is 3.84 Mcps making the equivalent chip rate in one timeslot 256 kcps (i.e. 3.84 Mcps/15=256 kcps). Third, a multi-user detection (MUD) receiver is used at the base station (BS).
• In each timeslot, Orthogonal Variable Spreading Factor (OVSF) codes are used for channelization codes. The spreading factor of a channelization code can take a value of 2, 4, 8, and 16 in the uplink. For purposes of describing the present invention, a resource unit (RU) corresponds to a particular physical channel and is defined as a channelization code having spreading factor 16 in a particular time slot. RUs therefore correspond to physical channels in a particular timeslot.
• For a new user seeking admission to a cell, the primary goal of call admission control is to properly allocate RUs (i.e. physical channels) so that QoS requirements are guaranteed, for both the new user and any users already in the cell. The number of RUs required by a new user depends on the type of call the new user has placed. For example, a new user placing a voice call requires two RUs while a new user placing a 64k data call requires five RUs.
• Decisions made by a call admission control system are based on whether RUs can be allocated successfully for the new user. Whether a RU can be allocated successfully for a new user depends on the individual outage probabilities (P_out) for all of the timeslots in which RUs have been assigned. Therefore, P_out is the probability that, in a particular timeslot, a user's required SIR will be below a certain predetermined value. In WCDMA-TDD systems, however, the required SIR of each user is not fixed, but follows a certain distribution thereby making P_out difficult to calculate. That is, even though the distribution of the SIR is known, the computation of P_out is still very complex, and cannot be done in real time.
• The Gaussian approximation, in contrast, provides a sufficiently approximate result and has relatively low computation complexity. Therefore, the Gaussian approximation approach is used to allow the RNC (Radio Network Controller) to compute P_out for each timeslot and make resource allocation decisions in real time.
• The P_out of every timeslot assigned to a new user may be combined to compute P_out-total for the new user. Assuming a new user is allocated RUs in a particular number of timeslots, the P_out-total of a new user is defined as the probability that an outage will occur in at least one of those timeslots. The P_out-total may be computed as desired. By way of example, P_out-total may be computed according to $P_{\mathrm{out}-\mathrm{total}}=1-\prod _{i=\Omega }^{}\left(1-P_{\mathrm{out}}\left(i\right)\right),$
  where Ω is the set of timeslots in which RUs have been allocated to the user.
• Referring now to FIG. 1, a method 10 is shown wherein call admission control is performed in the uplink for third generation wireless communication systems. Assuming, purely for purposes of describing the invention, that a new user requires two RUs (i.e. the new user has placed a voice call), the method 10 begins in step 12 by computing the current P_out of each uplink timeslot. Again, P_out is the probability that a new user's SIR is below a predetermined value in a particular timeslot and is computed for each uplink timeslot. Therefore, in step 12, the probability of the new user's SIR being below the predetermined value is computed for each timeslot. As explained, P_out accounts for the fact that the user's SIR changes with time and is computed by the RNC using the Gaussian approximation to reduce computation complexity.
• Once P_out has been computed for each timeslot, the timeslot having the lowest P_out, say timeslot i, is selected in step 14. Since timeslot i is the timeslot with the lowest P_out, the P_out in timeslot i is denoted P_out(i). In step 16, one RU is assigned to timeslot i and P_out(i) is updated accordingly. Once the first RU has been assigned, the method proceeds to step 18. In step 18, the method determines whether additional RUs need to be assigned. As mentioned, for purposes of describing the invention, it can be assumed that the new user requires two RUs. Therefore, the determination in step 18 will be positive and the method will proceed to step 20.
• In step 20, the method determines whether P_out(i) is still the lowest P_out (i.e. the method determines whether, despite being assigned a RU, timeslot i still has the lowest P_out) If P_out(i) is still the lowest P_out, the method goes back to step 16 and the second RU is assigned to timeslot i and continues as indicated. If, in contrast, P_out(i) is no longer the lowest P_out, the method proceeds to step 22. In step 22, P′_contribution is computed. The P′_contribution is the contribution to P_out-total assuming the next RU (i.e. the second RU according to the assumption noted above) is accepted to timeslot i despite the fact that P_out(i) is no longer the lowest P_out. The P′_contribution is the same value as the new P_out of timeslot i. That is, P′_contribution is equal to P_out(i).
• In step 24, P_contribution is computed. The P_contribution is the contribution to P_out-total assuming the next RU (i.e. the second RU according to the assumption noted above) is accepted to the timeslot having the lowest P_out, say timeslot j. The P_contribution is given by P_contribution=1−(1−P_out(i))·(1−P_out(j)) Once P_contribution and P_contribution have been computed, the method proceeds to step 26 where it determines whether P_contribution is greater than or equal to P_contribution (i.e. P_out(i)′). If P_contribution is greater than or equal to P′_Contribution, the method proceeds to step 16 wherein the next RU will be assigned to timeslot i despite the fact that timeslot i no longer has the lowest P_out. That is, even though timeslot i no longer has the lowest P_out, assigning the next RU to timeslot i will result in a lower P_out-total than assigning the next RU to timeslot j, which actually has the lowest P_out. If, in contrast, P_contribution is less than P′_contribution, i is set equal to j in step 28 and the method proceeds to step 16. The method sets i equal to j so that, in step 16, the next RU is assigned to timeslot j because assigning the next RU to timeslot j will result in the lowest P_out-total.
• From step 16, the method again proceeds to step 18. Note, steps 20 through 28 would not have been necessary where the new user only needed one RU. But, because in the assumption of the example the user needed two RUs, one run through steps 20 through 28 was necessary in order to determine the optimal allocation of the second RU. Steps 20 through 28 are performed, as needed, for every RU required by the user. Once all of the RUs have been assigned, the method proceeds to step 30. In step 30, P_out-total is computed to determine the outage probability of the new user based on the allocation of RU(s), as allocated in steps 12 through 28.
• In step 32, the method determines whether P_out-total is less than or equal to a predetermined value, say θ. The predetermined value θ is an operator dependent parameter and may be any value, as desired, depending on the desired level of network stability. If P_out-total is less than θ, the new user is admitted (step 34). If not, the new user is rejected (step 36).
• Pursuant to the present invention, P_out-total increases as the number of users increases and saturates around the predetermined value θ thereby dramatically improving system stability (i.e. the number of dropped calls). Due to the stringent admission standards, the present invention also results in a dramatic increase in blocking probability (which also increases as the number of users) in comparison to static sequential and random call admission control methods. The combination of increased stability and blocking probability significantly improves users QoS as, from a user's perspective, having a call blocked is much more preferable than having a call dropped.
• Referring now to FIG. 2, a system 100 is shown for implementing call admission control according to the present invention. The system 100 comprises a RNC 102, a BS or Node-B 104 and user equipment (UE) 106 wherein the BS and UE each have a multi-user detection (MUD) receiver 103, 108, respectively.
• When the UE 106 is used by a user to place a call, the RNC 102 will perform call admission control and allocate RUs required by that new call to appropriate timeslots so as to ensure the lowest possible P_out-total and to ensure that P_out-total remains below the predetermined threshold θ.
• To perform call admission control, the RNC 102 computes P_out for every uplink timeslot and assigns a RU to the timeslot with the lowest P_out. If there are additional RUs required by the new user that need to be allocated, the RNC 102 will assign subsequent RUs to the same timeslot the previous RU was assigned to, so long as that timeslot still has the lowest P_out. If that timeslot no longer has the lowest P_out, the RNC 102 will determine whether it still should assign subsequent RUs to that timeslot or to the timeslot now having the lowest P_out. To make that determination the RNC 103 determines which timeslot results in the lowest contribution to P_out-total. The RNC repeats this analysis for every RU required by the new call.
• Once all of the RUs that are required by the new user have been allocated to particular timeslots, the RNC 103 determines whether the allocation results in P_out-total being below the predetermined θ. If P_out-total is below θ, the new user is admitted. If not, the new user is rejected.
• Although the present invention has been described in detail, it is to be understood that the invention is not limited thereto, and that various changes can be made therein without departing from the spirit and scope of the invention, which is defined by the attached claims.

Claims (6)

1. A method for performing call admission control in a time-slotted wireless communication system, the method comprising:

(a) calculating an individual outage probability of each of a plurality of time slots, the individual outage probability being a probability that a signal-to-interference ratio (SIR) in a time slot falls below a predetermined SIR value; and

(b) allocating for a user one or more radio resource units to one or more time slots such that a total outage probability becomes the lowest, the total outage probability being a probability that an SIR in any allocated time slot falls below the predetermined SIR value.

2. The method of claim 1 further comprising:

determining whether the total outage probability is below a predetermined threshold; and

admitting the user when the determination is positive and rejecting the user when the determination is negative.

3. The method of claim 1 wherein step (b) comprises:

selecting a time slot having the lowest individual outage probability;

allocating the time slot one of a plurality of radio resource units; and

allocating subsequent radio resource units in a time slot in which a contribution to the total outage probability of each subsequent radio resource unit is lowest.

4. An apparatus for performing call admission control in a time-slotted wireless communication system, the apparatus comprising:

an outage probability calculator for calculating an individual outage probability of each of a plurality of time slots, the individual outage probability being a probability that a signal-to-interference ratio (SIR) in a time slot falls below a predetermined SIR value; and

a radio resource unit allocator for allocating for a user one or more radio resource units to one or more time slots such that a total outage probability becomes the lowest, the total outage probability being a probability that an SIR in any allocated time slot falls below the predetermined SIR value.

5. The apparatus of claim 4 further comprising:

a controller for determining whether the total outage probability is below a predetermined threshold, whereby admitting the user when the determination is positive and rejecting the user when the determination is negative.

6. The apparatus of claim 4 wherein the radio resource unit allocator allocates one of a plurality of radio resource units to a time slot having the lowest individual outage probability, and subsequent radio resource units in a time slot where a contribution to the total outage probability of each subsequent radio resource unit is lowest.

Images