| With the proliferation of wireless intelligent product, in order to meet the demand for a wide variety of network services, data traffic of cellular network will meet huge and considerable growth. The rate and throughput of traditional cellular network cannot meet people’s pursuit of wireless networks anymore.Traditional cellular networks mainly use licensed spectrum for communication and provide users with safe and reliable service. But because of the scarce licensed spectrum resources, fewer available licensed spectrum is unable to meet the growing number of users with faster network transmit rate requirements. In order to make the development of the cellular network itself not be bounded by the limited licensed spectrum resources and to achieve higher throughput target, deploying small cells on unlicensed spectrum is a promising approach to ease the spectrum crisis. Small cells can provide greater network coverage, system capacity and more reliable transmission than WiFi networks on unlicensed spectrum. With spectrum aggregation, it will become a unified LTE network by using licensed spectrum and unlicensed spectrum resources together, and will achieve greater network capacity.For LTE technology on unlicensed spectrum(LTE Advanced in Unlicensed Band, namely U-LTE), an important issue is how to coexist with existing unlicensed spectrum networks. In the aggregation process, involving interference between different networks and interference between different base stations of the same network. Therefore, how to mitigate or even eliminate interference between small cells and the existing unlicensed access points(APs)(such as WiFi APs) is extremely pivotal since it is the root of reaching the well-coexisting goal of wireless network with multiple radio access technologies(RATs) as well as heterogeneous network. The most important and fundamental issue of different RATs is the coexistence performance between these subnetworks. Specifically, the success probability(i.e., the counterpart of outage probability) and throughput are the two most important performance matrics to indicate whether the coexisting network benefits the whole network.In order to simply and thoroughly delve the success probability and throughput performances in the multi- RAT network, in this paper established a coexisting two-tier system with cellular small cells and WiFi APs on unlicensed spectrum. Use homogeneous Poisson Points Process(PPP) to deploy two systems independently and randomly respectively. Each user will connect to the nearest base station, and all the other base stations are interference sources. In this paper, transmitting probability and success probability of small cellular networks and WiFi networks on unlicensed spectrum have been derived, respectively. In order to analyze the performance of the coexisting system, this paper introduces the concept of coexisting success probability of the whole system. Algorithm analysis and simulation results show that coexist ing success probability is a function of the number of channels and the base station deploying ratio between the two systems. With other parameters preconditions, the optimal ratio of base stations in two systems to maximize the coexisting success probability can be derived.This paper also discussed the throughput of the two systems, and gave the concept of coexistence throughput. Through simulation, the optimal base stations’ density which can maximize the coexisting throughput can be obtained, and this density is consistent with the one which can maximize the coexisting success probability. So a reasonable network deploying methods to achieve maximum system performance of the two-tiers network can get. Finally, through comparing the proposed network and the conventional WiFi system, the system throughput achieved by this paper’s algorithm is 30% greater than the traditional system throughput with only WiFi network. This also shows the algorithm proposed in this paper is a good method to improve the performance of the whole network. |
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