| With the fast development of mobile communication services,the conflict between spectrum resources and customer's demand grows more and more rapidly. For CDMA system,the contradiction between the system capacity and link interference is greatly incisive,too. To solve the former problem,hiarachical cellular system (HCS) could be applied,where macrocells cover low-teletraffic areas and serve slow calls,while microcells overlay high-teletraffic areas and serve fast calls. By this means,better spectrum efficiency can be achieved and the number of handoffs can be decreased. To solve the latter problem,fast cell selection (PCS) can be used to reduce its interference effectively and then improve the link capacity and data throughput in CDMA system whose capacity is mainly restricted by the downlink.In this thesis,theoretical analysis and computer simulation are carried out in the following aspects:initial cell selection (ICS),analysis and design of call-overflow scheme,design of bidirectional call-overflow scheme based on dynamic guard channels assignment (DGCA),analysis and design of PCS algorithm.Based on the analysis of various existing problems in HCS,this thesis puts forward ICS. Then,ICS in the HCS is analyzed with respect to various parameters such as the mobile velocity,the residence time in different cells,network response time and the call lifetime. In the thesis,the effect of network response time (NRT) on HCS grade of service (GoS) is discussed. On the basis of a certain GoS requests,the method of calls' access is proposed according to the maximum speed of the mobile. The analysis shows that there is a close relationship between the number of handoffs and GoS,and ICS can reduce the number of handoffs and unsuccessful probability of the calls.According to Markov process,a general model for call flow in HCS is introduced,which is utilized to compare different call-overflow schemes. The analysis shows that bidirectional call-overflow scheme can lead to the lowest call blocking probability and the lowest handoff failure probability. The reason for this is that the scheme can balance the teletraffic between the microlayer and macrolayer,and both the slow and fast calls can share all the channel resources. To verify our algorithm,parts of theoretical and simulated results are compared.Based on bidirectional call-overflow scheme,the author then analyzes the velocity threshold and the maximum mobile velocity's influence on the system GoS. That is,when the teletraffic is in a certain range,both increasing the velocity threshold and decreasing the maximum velocity can reduce the unsuccessful probability of calls.In most existing schemes,the calls usually request the access to microcells firstly. This thesis proposes an improved bidirectional call-overflow scheme,i.e. slow calls request the access to microcell firstly while fast calls request the access to macrocells firstly. In this way,the total number of handoffs can be reduced.Compared with the blocking of new calls,the users are more dissatisfied with the existing call interrupt. So the handoff calls should have higher priority,which is ensured by assigning the guard channels in the HCS. However,the introduction of guard channels will have quite different effects on blocking probability and handoff failure probability. Therefore,based on DGCA,the improved bidirectional call-overflow scheme can allocate the teletraffic to different layers. While considering the tradeoff between two different probabilities,it can make the dissatisfaction grade of users the lowest.In comparison with the soft handoff,PCS in high-speed downlink packet access (HSDPA) can reduce the downlink interference effectively,and therefore improve the link capacity. However the current PCS also has shortcomings,i.e. increase of unnecessary handoff and unbalance of the teletraffic among different cells. Using fuzzy mathematical idea,the thesis proposes a fuzzy PCS (FFCS) algorithm,which takes into account of three main factors,i.e. the signal strength of common pilot channe...
|
PDF Full Text Request