The system operation for downlink power and rate assignments for mixed-size WCDMA cellular systems is shown in Fig. 3.1. The base station allocates power to handoff users based on the joint power and rate assignment (JPRA) algorithm firstly, then the non-handoff users based on the multi-quality balancing power allocation (MQBPA) algorithm. If the system resource is insufficient to support all the users with the allocated rates the required signal qualities, a multirate removal algorithm (MRV) is activated to release system resources by reducing users’ service rate or even suspending users’ transmission. The total transmission power of the base stations are adjusted based on the tuning factor obtained from MQBPA algorithm. The procedure of the radio resource allocation is done as soon as the stop criterion is satisfied.
3.2.1 System Model
In the multirate mixed-size WCDMA cellular system, the received interference of user m served by base station b, determined by Ib,m, is
Ib,m = (1 − fα)PbTLb,m+X
k6=i
PkTLk,j+ ηo, (3.1)
where fα is the orthogonality factor; Lb,m is the link quality from cell b to user m, which includes effects of both pathloss and shadowing; ηo is background noise. Note that the first and second terms in (4.1) denote the intra-cell and inter-cell interferences, respectively, in which the first term is caused by imperfect orthogonality of channel codes. Moreover, the received bit-energy-to-noise ratio (Eb/No) of user m in base station b and with service rate r, denoted by γb,m(r), must be larger than or equal to the required signal quality, denoted by γ∗(r). For bandwidth W , the γb,m(r) can be expressed as
γb, m(r) = pb,m(r) · Lb,m· GP(r) Ib,m
≥ γ∗(r) , (3.2)
where pb,m(r) is the transmission power from base station b to user m, and GP(r) = W/r is the processing gain of service rate r. Furthermore, for a soft handoff user h with service
rate r, its received Eb/No, γh(r), can be obtained by using the maximum ratio combining (MRC) method to combine signals from all serving base stations in the active set Dh, i.e.,
γh(r) = X
b∈Dh
γb,h(r). (3.3)
3.2.2 The MQBPA algorithm
The multi-quality balancing power allocation (MQBPA) algorithm is to provide each non-handoff user the required signal quality of itself. Assume each service rate r has the required signal quality γ∗(r); denote Ab (Bb) as the total transmission power for non-handoff (soft handoff) in cell b such that Ab+ Bb = PbT. The MQBPA algorithm assigns the non-handoff user m in cell b with service rate r an amount of power, pb,m(r), by
Substituting (3.4) and (3.5) into (3.2), the received signal quality of the non-handoff user m in cell b can be yielded as
γb,m(r) = PAb m∈Ub
wb,m γ∗(r). (3.6)
The goal of the quality balancing power allocation is to make each user with service rate r have its required signal quality such that γb,m(r) ≥ γ∗(r). That is, Ab/P
m∈Ubwb,mshould be larger or equal to 1. Otherwise, the total allocation power Abof base station b for non-handoff users should be adjusted by tuning factor ψb, which is given by
ψb = γ∗(r)
γb,m(r). (3.7)
This is because, from (3.6), all non-handoff users in cell b have exactly the same value of γb,m(r)/γ∗(r), no matter what kind of service rate r is allocated. The MQBPA algorithm is described in the following.
[The MQBPA Algorithm]
Step 1: [Initialize]
• Initialize the total transmission power PbT of the traffic channel to the maximum total transmission power ePb for each cell b.
• Calculate the total allocation power Bb for handoff users in each cell b after executing JPRA algorithm.
Step 2: [Calculate wb,m]
• Calculate wb,m, based on (3.5), for user m in cell b.
Step 3: [Calculate allocation power]
• Calculate the total transmission power Ab for non-handoff users in each cell b, which is equal to (PbT − Bb).
• Calculate allocation power pb,m(r)= min( pb,m(r), epb) for each non-handoff user m with service rate r in cell b based on (3.4).
Step 4: [Calculate tuning factor]
• Calculate tuning factor ψb for each cell b based on (3.6) and (3.7).
Step 5: [Check Stop Criterion for each cell b]
• IF any ψb 6= 1.0 and the convergence is not met, THEN
− Adjust total transmission power as PbT = min( ψb× Ab+ Bb, ePb ).
− Goto Step 2.
ELSE DONE. ¥
The proposed MQBPA algorithm will converge to a desired solution if there exists an effective individual power allocation for all users and they can obtain their required signal qualities. If the solution does not exist, the MRV algorithm will be activated, which is stated in the next subsection.
NO YES the service rate of the
selected data user
Remove the selected data user
END START
Figure 3.2: The flowchart of the MRV algorithm
3.2.3 The MRV algorithm
The multirate removal (MRV) algorithm defines a novel removal index for user m with service rate r, denoted by Jm(r), as
Jm(r) = γ∗(r)
Peb· Lb,m· GP(r), (3.8)
where ePb is pilot power of base station b, which is related to the cell size. The removal index shows how much the system resource is required to serve user m. The worse the received signal strength, the higher the service rate and the required signal quality are, and the larger the removal index value will be.
In order to provide higher priority for voice users, the proposed MRV algorithm removes system resource from data users first unless all the data users are reduced to basic service rate. The flowchart of MRV algorithm is shown in Fig. 3.2. At first, the MRV scheme will
check if all data users are with basic rate. If there exists at least one data user not with the basic rate, the MRV scheme will choose the data user with the maximum removal index. If the service rate of the selected user is with the basic rate, then the system will remove it directly, otherwise reduce its rate to the next lower service rate. If all data users are with basic rate, the system will remove the user which is with the maximum removal index.