Research On Network Selection Strategies Of UMTS Cellular And WLAN Heterogeneous System

The existing communication networks are consisted of multiple kinds of networks:wired network and wireless network; same networks of different generations; different kinds of networks. Different networks make a complicated heterogeneous network environment, in the common area of different networks, users’ candidate networks change from only cellular network to multiple networks (the same kind or different kinds of networks). Thanks to the large deployment of WLAN (Wireless Local Area Network) access point and the different frequency bands from cellular networks WLAN adopts, it causes no interference of the same frequency and is suitable to be merged with cellular networks and form heterogeneous networks to provide users with high data rates. An appropriate network selection strategy under heterogeneous environment will not only can supply users with high quality of service and ensure the best connection, but also improve the network resource utilization.Currently, cellular networks use TDMA, CDMA and OFDMA as its multiple access methods; however WLAN uses CSMA/CA. With multiple access methods cellular network adopts, the throughput calculation in cellular networks should take the same frequency interference into consideration. WLAN has a different multiple access method based on contention, the throughput calculation should allow for the status of users including collision, successful transmission and idle status. Apart from that, the throughput is affected by the performance anomaly phenomenon because of the different channel condition among users and access points. In this paper, the different kinds of multiple access methods are taking into consideration and a maximizing utility problem with the constraints of minimum rate requirement and network throughput limit is formed. Due to the non-convexity of the problem, we transform non-convex constraints into fractional constraints by using the approximation between Poisson’s distribution and binomial distribution and the relationship between the throughput and the collision probability. And then with mathematic manipulation, the fractional constraints are converted into linear ones. Finally the problem is solved with dual-decomposition method and a distributed algorithm is proposed to achieve the network selection result. Simulation results show that the proposed algorithm can enhance the system’s performance and improve the utilization of WLAN under both the normal distribution and hot-spot distribution of users. In heterogeneous networks, WLAN can provide high data rates support, but high data rates also means high power consumption. In the heterogeneous network of UMTS cellular network and WLAN, the power model of WLAN has to take the transmission probability, collision probability and idle probability into consideration. Firstly, in this paper, we analyze the power model of UMTS and WLAN. And based on the models, a problem of minimizing users’power consumption is formed. It has constraints of minimum user rate requirement and network throughput limit. Due to the non-convex constraints, it cannot be solved directly using optimization methods. We use inequality scale transformation to transform the multiplicative constraints into linear ones and solve the problem with standard optimizing methods. The tendency between users’ power consumption and the user’s number and minimum rate requirement is simulated when users are normally distributed or hot-spot distributed. Under both distributions, the proposed algorithm can effectively reduce users’ power consumption.