Study On The Technology Of Call Admission Control In WCDMA System

1. IntroductionThe mobile communications system has transmitted from the first generation which was analogue mobile communications system to the third generation which is developed hopefully now. WCDMA has been accepted widely as the air interface criterion in 3GPP, which is the steadiest and widely used 3G criterion. But compared with the perfect 2G network which has been developed for 10 years, the steady degree of WCDMA is much lower. Furthermore, there is a growing conflict between the requirement of data service and the limited resource. So it is necessary to study how to provide service of higher quality to users and thus improve their perceived benefits with limited resources.Radio resource management (RRM) can use limited wireless resources rationally and effectively, which includes power control, handover, call admission control (CAC), congestion control and pocket control. Among them, CAC is used to prevent the decline of the quality of service (QoS) which is existent or the reduction of overcast area of the cell to be lower than the planning area as too many newly-accessed users pour into the net. In the light of user sensible degree (USD), it is more important to reduce the blocking rate and dropping rate of real time connection compared to ensure the performance of non-real time services because of the multi-service characteristics of 3G.. So we can use CAC arithmetic to protect the performance of real time service in order to improve USD.There is retrial phenomenon in the cellular mobile networks, which means that some users will always choose to retrial after being blocked or stopped because the wireless network resources is limited and the users are moving. Retrial phenomenon, which has a significant negative impact on the network performance, cannot be neglected. We reserve guard channel (GC) beforehand for retrial and upgrade its priority to avoid that users couldn't get in when they have called many times because it will influent USD seriously.2. Retrial Priority Admission-Multimedia (RPA-M)We divide the services of WCDMA into two types based on the different QoS requirement of services in 3G: Class I is real time connection, Class II is non-real time connection. Class II can temporarily save the data on some network node (e.g. node B) when the network is crowded, and can be transferred at a lower rate. So we don't consider the retrial and handover of Class II, and set their priority at lowest position. At the same time, we divide Class I connection requirement into four parts: new call requirement in the cell, retrial of new call requirement, handover requirement, and retrial of handover requirement. We set handover requirement and retrial of handover requirement of Class I which have the highest priority. These requirement will be rejected only when all of the channels in the cell have been used; the second is retrial of new call requirement of Class I, which will be rejected when there are more than N r (include N r) channels used, otherwise they will be accepted; the third is new call requirement of Class I, which will be rejected when there are more than N n (include N n) channels used, otherwise they will be accepted; the last is call requirement of Class II, which will be rejected when there are more than N II (include N II) channels used, otherwise they will be accepted.This paper compared RPA-M with the arithmetic without retrial priority. The simulation indicate that: RPA-M makes the retrial priority access come true, improve the service accessibility and retainability and heighten the USD. But the system resource is fixed, so the decline of the retrial blocking rate will raise the blocking rate of lower priority services at the cost.3. Dynamic Retrial Priority Admission-Multimedia(DRPA-M) This paper dynamic adjusts the thresholds of each service based on call blocking rate and handover dropping rate which are system level indications, and tries to further improve the performance of highest priority service, in addition, give attention to the higher priority service.We adjust the service thresholds from high priority to low. For the Nth service which has the highest priority. Its service thresholds is fixed, as: TTH N = THN=C. When we adjust the ith (1≤ii+1> U_i+1 and TTH_i+1 > TH_i+1,then TTH_i = TH_i; if B_i+1> U_i+1 and TTH_i+1 = TH_i+1,then TTH_i = TH_i-1; if B_i+1> U_i+1 and TTH_i+1 < TH_i+1,then TTH_i = TH_i-2; if B_i+1< D_i+1 and TTH_i+1 > TH_i+1,then TTH_i = TH_i+2; if B_i+1< D_i+1 and TTH_i+1 = TH_i+1,then TTH_i = TH_i+1; if B_i+1< D_i+1 and TTH_i+1 < TH_i+1,then TTH_i = TH_i; }To prevent heavily adjustment which cause the thresholds of services too high or too low caused by the different service arrival rate, author set upper limit( UTTH_i) and lower limit( DTTH_i) for every service. TTH_i should be adjusted between UTTH_i and DTTH_i.From the simulation results, compared with the TCAA, DRPA-M can improve the performance of handover and retrial requirement, and promote USD, while cannot infect other indications.