Temporal Opportunistic Spectrum Access Scheme for Wireless Networks

The current spectrum management apparently leads to the spectrum underutilization issue. To address the problem, researchers propose an approach known as dynamic spectrum access. Dynamic spectrum access is a broad paradigm that encompasses many spectrum access and management approaches. Among them, there is an approach that called spectrum overlay access approach. The spectrum overlay access (also called opportunistic spectrum access) dictates that the secondary user is allowed to access the licensed spectrum of the primary user, provided with a condition that the secondary users’ access is not intrusive to the primary users. The current researches tend to focus on the exploitation of a licensed spectrum with rather slow temporal variation, such as TV channel bands. Meanwhile, a significant licensed channel is characterized by fast and dynamic temporal variation. In this fast varying temporal character, the primary users’ spectrum condition and opportunity are changing in relatively short duration, mostly, in term of seconds or milliseconds. Our research goal is to investigate and devise more effective secondary user access scheme to exploit the short and fast varying temporal opportunities under opportunistic spectrum access approach. During the course of our study, we investigate the impact of utilizing multiple transmission rates and relays under fast varying time domain opportunistic spectrum access. From the investigation, we find the utilization of faster rate along with the relay assistance is actually lead to better temporal spectrum sharing performance compared with the direct transmission with the one hop optimal transmission rate. One caveat is that, for a system with diverse transmission rate selection, such as IEEE 802.11g or IEEE 802.11n system, not all the fast transmission rates results in the performance gain in the temporal spectrum sharing environment. The balance between a fast transmission rate and the number of the relay transmission need to be considered to improve the temporal spectrum sharing performance. As result, we observe that the proper exploitation of the iv rate and relay diversity nature has the potential to improve the efficiency of temporal spectrum sharing between the primary and secondary users. We also devise an approach to put the application of this rate and relay diversity exploitation into practical wireless networks system by proposing an adaptive rate and relay approach. The adaptive rate and relay approach are composed of three components. As the main component, we propose Auto Rate Increase (ARI) and Receiver based Auto Rate Increase (R-ARI) as the rate adjustment algorithms. Secondly, we utilize the multi-rate GPSR routing algorithm as the mean to dynamically select the relay assistance. Lastly, we propose a rate limit formula as a metric to balance the transmission rate and the number of relays. We propose novel rate adaptation algorithms to address the weakness of the conventional WLAN rate adaptation algorithm. The current dynamic rate algorithms are designed for normal wireless networks environment without ever considering the existence of primary and secondary users. Our proposed rate adaptation algorithms consider, in addition to the channel condition, the effect of primary and secondary transmission collision and adjust the transmission rate so that the secondary users are able to efficiently exploit the temporal opportunity. We evaluate the performance of our proposed approach through QualNet network simulator. The initial evaluation is aimed to compare the Auto Rate Increase (ARI) and Auto Rate Fallback (ARF) rate adaptation algorithm. The simulation parameters in this initial evaluation are modeled after the IEEE 802.11b model. The selectable rates are limited to 1, 2, 5.5, and 11 Mbps. We evaluate the algorithms under the connected network topology. The connected topology means for every transmission rate, a node is guaranteed to find the relay node to assist its transmission. The results show that the performance of ARI clearly outperforms ARF by wide margin. At most, we can expect around 56% improvement of the secondary throughput. Although the results show the superiority of Auto Rate Increase, the evaluation environment cannot be considered as