Research On Spectrum Sharing Strategies Based On The Frequency Fragmentation In Cognitive Radio Networks

Today, in the new Internet epoch, the people’s demand for network is no longer limited to the traditional fixed cable access, but transformed into the wireless network which can be accessed in any time, any place, and any condition. Therefore, the development of the wireless network is particularly important. Faced with the booming increase of the demand for the wireless network and the popularization of mobile devices, the spectrum resources which is regard as the strategic resources of the wireless network had been out of the high-speed growth and turn to a scare resource. At the same time, with the inefficiency of traditional static spectrum division, the problem of resource shortage is further exacerbated. In this case, the cognitive radio arises at the historic moment.In the cognitive radio network, the cognitive users can share the spectrum unused by the authorized users dynamically, so the utilization of spectrum resource can be improved rapidly. However, because of the random presence of primary users and the CR users, the available spectrums are fragmented with the size of each fragment varying from one channel to several channels. Due to the spectrum fragmentation, the cross-band interference between spectrally adjacent users is considered harmful, especially in the distributed network where there is no central controller to control the CR users’ parameters. Therefore, the spectrum fragmentation and the cross-band interference will directly affect the efficient use of the spectrum resources. Thus, to improve the spectrum utilization, this paper carries out an in-depth research on dynamic spectrum sharing strategies based on the spectrum fragmentation and the cross-band interference in the distributed CR network. And the main contents are as follows:1) Because of the random behavior of the authorized users in the distributed CR network based on the TV band, the available spectrum is discrete. Therefore, there are many spectrum fragmentations which cannot satisfy the CR users’ demand. Here, through the Markov process which analyzes the random behavior of the authorized users, the two new performance indicators and the average statistical outage probability are defined to study the influence of fragmentation on the system performance. And based on the efficiency optimization, two strategies are present. The first strategy involves adjustment factor to set aside more resources for the next node, in order to reduce the occurrence of interrupt. The second strategy based on the k-agile receiver involves the constraint of agility and guard bands, and selects the minimum non-contiguous fragments to solve the fragments problem in the physical layer and improve the flexibility of the access strategy. The simulations show that both of the strategies are superior to the existing strategies.2) Due to the spectrum fragmentation, the cross-band interference between spectrally adjacent users is considered harmful in the distributed cognitive radio networks. To eliminate the interference, frequency guard-bands are inserted between spectrum fragments. The guard-bands not used for transmission are considered as the spectrum consumption. Accordingly, the achieved throughput is at the cost of the guard-bands consumption. Thus, to utilize the spectrum fragments efficiently, the ratio of throughput to spectrum consumption is defined for the dynamic spectrum access problem. Therefore, under the conditions of prescribed user quality-of-service requirements and the frequency agility limit, a novel mathematic model in which the ratio of throughput to spectrum consumption is maximized is proposed. The problem is a traditional combinatorial optimization problem. To this end, several spectrum access strategies with lower complexities than the traditional exhaustive search are proposed. Simulation results show that the ratio can be used as the index whether the spectrum fragments are effectively utilized. Meanwhile, the performances of these strategies are different with the various complexities, which provides us an option to choose the proper strategy for the practical scenario.