Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Software-defined/cognitive radio has recently made the jump from a purely research driven endeavor to one that is now being driven commercially. Such radios offer the promise of spectrum agility and re-configurability through flexibility at the MAC and physical layers. In the wireless domain, it has been shown that hybrid-MAC layer algorithms can lead to improved overall performance in varying network conditions. A hybrid MAC that uses CSMA in low-contention periods and switches to TDMA in high-contention periods can outperform CSMA or TDMA individually. However, such hybrid systems do not offer the flexibility and cognition required of dynamic spectrum networks. In this paper, we describe MultiMAC, a framework and experimental platform for evaluating algorithms that dynamically reconfigure MAC and physical layer properties. MultiMAC acts as a mediating MAC layer and dynamically reconfigures and/or selects from a collection of alternative MAC layers. As a result of monitoring current network metrics, MultiMAC chooses the MAC layer capable of achieving the best performance while ensuring that incoming frames are decoded using the correct MAC layer algorithm. MultiMAC incorporates decision processes to select the appropriate MAC component based on per-node and per-flow statistics. This engine will allow intelligent reconfiguration of the MAC and physical layers in response to changes in external conditions and/or requirements optimizing use of the available spectrum <s> BIB001 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> A number of algorithmic and protocol assumptions taken for granted in the design of existing wireless communication technologies need to be revisited in extending their scope to the new cognitive radio (CR) paradigm. The fact that channel availability can rapidly change over time and the need for coordinated quiet periods in order to quickly and robustly detect the presence of incumbents, are just some of the examples of the unique challenges in protocol and algorithm design for CR networks and, in particular, in the medium access control (MAC) layer. With this in mind, in this paper we introduce a novel cognitive MAC (C-MAC) protocol for distributed multi-channel wireless networks. C-MAC operates over multiple channels, and hence is able to effectively deal with, among other things, the dynamics of resource availability due to primary users and mitigate the effects of distributed quiet periods utilized for primary user signal detection. In C-MAC, each channel is logically divided into recurring superframes which, in turn, include a slotted beaconing period (BP) where nodes exchange information and negotiate channel usage. Each node transmits a beacon in a designated beacon slot during the BP, which helps in dealing with hidden nodes, medium reservations, and mobility. For coordination amongst nodes in different channels, a rendezvous channel (RC) is employed that is decided dynamically and in a totally distributed fashion. Among other things, the RC is used to support network-wide multicast and broadcast which are often neglected in existing multi-channel MAC protocols. We present promising performance results of C- MAC. We also describe our efforts to implement features of C- MAC in a real CR prototype with Atheros chipset, which currently includes the spectrum sensing module and preliminary features of C-MAC. <s> BIB002 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> In this paper, we investigate the channel selection strategy for secondary users in cognitive radio networks. We claim that in order to avoid the costly channel switchings, a secondary user may desire an optimal channel which maximizes the probability of successful transmissions, rather than consistently adapting channels to the random environment. We propose a stochastic channel selection algorithm based on the learning automata techniques. This algorithm adjusts the probability of selecting each available channel and converges to the e-optimal solution asymptotically. <s> BIB003 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> In this paper we design a cognitive radio that can coexist with multiple parallel WLAN channels while abiding by an interference constraint. The interaction between both systems is characterized by measurement and coexistence is enhanced by predicting the WLAN's behavior based on a continuous-time Markov chain model. Cognitive medium access (CMA) is derived from this model by recasting the problem as one of constrained Markov decision processes. Solutions are obtained by linear programming. Furthermore, we show that optimal CMA admits structured solutions, simplifying practical implementations. Preliminary results for the partially observable case are presented. The performance of the proposed schemes is evaluated for a typical WLAN coexistence setup and shows a significant performance improvement. <s> BIB004 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Radio spectrum resource is of fundamental importance for wireless communication. Recent reports show that most available spectrum has been allocated. While some of the spectrum bands (e.g., unlicensed band, GSM band) have seen increasingly crowded usage, most of the other spectrum resources are underutilized. This drives the emergence of open spectrum and dynamic spectrum access concepts, which allow unlicensed users equipped with cognitive radios to opportunistically access the spectrum not used by primary users. Cognitive radio has many advanced features, such as agilely sensing the existence of primary users and utilizing multiple spectrum bands simultaneously. However, in practice such capabilities are constrained by hardware cost. In this paper, we discuss how to conduct efficient spectrum management in ad hoc cognitive radio networks while taking the hardware constraints (e.g., single radio, partial spectrum sensing and spectrum aggregation limit) into consideration. A hardware-constrained cognitive MAC, HC-MAC, is proposed to conduct efficient spectrum sensing and spectrum access decision. We identify the issue of optimal spectrum sensing decision for a single secondary transmission pair, and formulate it as an optimal stopping problem. A decentralized MAC protocol is then proposed for the ad hoc cognitive radio networks. Simulation results are presented to demonstrate the effectiveness of our proposed protocol. <s> BIB005 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Future wireless infrastructure networks will dynamically access spectrum for maximum utilization. However, the fundamental challenge is how to provide stable spectrum access required for most applications. Using dynamic spectrum access, each node's spectrum usage is inherently unpredictable and unstable. We propose to address this challenge by integrating interference-aware statistical admission control with stability- driven spectrum allocation. Specifically, we propose to proactively regulate nodes' spectrum demand to allow efficient statistical multiplexing while minimizing outages. Admitted nodes coordinate to adapt instantaneous spectrum allocation to match time- varying demand. While the optimization problem is NP-hard, we develop computational-efficient algorithms with strong analytical guarantees. Experimental results show that the proposed approach can provide stable spectrum usage while improving its utilization by 80-100% compared to conventional solutions. <s> BIB006 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> The scarcity of unlicensed spectrum has triggered great interest in cognitive radio (CR) technology as a means to improve spectrum utilization. An important challenge in this domain is how to enable nodes in a CR network (CRN) to access the medium opportunistically. Multi-channel MAC protocols for typical ad hoc networks assume that frequency channels are adjacent and that there are no strict constraints on the transmission power. However, a CRN may occupy a wide range of frequencies. In addition, a power mask is often enforced on the transmission power of a CR user to avoid corrupting the transmissions of spectrum-licensed primary-radio (PR) users. Obviously, CR users operating in different licensed bands will be subject to different PR-to-CR interference conditions. To avoid unnecessary blocking of CR transmissions under these constraints, we propose a novel distance-dependent MAC protocol for CRNs (DDMAC) that attempts to maximize the CRN throughput. DDMAC introduces a novel suboptimal probabilistic channel assignment algorithm that exploits the dependence between the signal's attenuation model and the transmission distance while considering the traffic profile. The protocol allows a pair of CR users to communicate on a channel that may not be optimal from one user's perspective, but that allows more transmissions to take place simultaneously, especially under moderate to high traffic loads. Simulation results indicate that compared to typical multi-channel CSMA-based protocols, DDMAC decreases the connection blocking rate of CR transmission requests by up to 30%, which improves the network throughput at no additional cost in energy consumption. On the whole, our protocol is simple yet effective. It can be incorporated into existing multi-channel systems with little extra processing overhead. <s> BIB007 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Contiguous spectrum assignment generates many small spectrum fragments that cannot be fully utilized, which leads to low spectrum utilization. With the development of advanced wireless radio technology, especially Discontiguous Orthogonal Frequency Division Multiplexing (DOFDM), discontiguous spectrum access and spectrum aggregation in a single radio has become possible. With the help of discontiguous spectrum access, the small spectrum fragments could be aggregated and further utilized, which can dramatically improve the spectrum utilization efficiency. Based on this observation, we propose Aggregation-Aware Spectrum Assignment (AASA), a spectrum assignment algorithm in cognitive ad-hoc networks. In AASA, we propose a spectrum assignment method that takes spectrum aggregation into consideration. Moreover, we prove that AASA optimizes the spectrum assignment and maximizes the number of users that can be supported in the system. Simulation result shows that AASA significantly increases the spectrum assignment efficiency compared with the existing solutions. <s> BIB008 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Driven by regulatory initiatives and radio technology advances, opportunistic spectrum access has the potential to mitigate spectrum scarcity and meet the increasing demand for spectrum. In this paper, we consider a scenario where secondary users can opportunistically access unused spectrum vacated by idle primaries. We introduce two metrics to protect primary performance, namely collision probability and overlapping time. We present three spectrum access schemes using different sensing, back-off, and transmission mechanisms. We show that they achieve indistinguishable secondary performance under given primary constraints. We provide closed form analysis on secondary user performance, present a tight capacity upper bound, and reveal the impact of various design options, such as sensing, packet length distribution, back-off time, packet overhead, and grouping. Our work sheds light on the fundamental properties and design criteria on opportunistic spectrum access. <s> BIB009 </s> Advances in cognitive radio networks: A survey <s> A. Medium Access Control in Cognitive Radio Networks <s> Dynamic spectrum access has become a promising approach to fully utilize the scarce spectrum resources. In a dynamically changing spectrum environment, it is very important to consider the statistics of different users' spectrum access so as to achieve more efficient spectrum allocation. In this paper, we propose a primary-prioritized Markov approach for dynamic spectrum access through modeling the interactions between the primary and the secondary users as continuous-time Markov chains (CTMC). Based on the CTMC models, to compensate the throughput degradation due to the interference among secondary users, we derive the optimal access probabilities for the secondary users, by which the spectrum access of the secondary users is optimally coordinated, and the spectrum dynamics are clearly captured. Therefore, a good tradeoff can be achieved between the spectrum efficiency and fairness. The simulation results show that the proposed primary-prioritized dynamic spectrum access approach under proportional fairness criterion achieves much higher throughput than the CSMA-based random access approaches and the approach achieving max-min fairness. Moreover, it provides fair spectrum sharing among secondary users with only small performance degradation compared to the approach maximizing the overall average throughput. <s> BIB010
Medium access control (MAC) refers to the policy that controls how a secondary user should access a licensed spectrum band. Various medium access control protocol have been proposed in wireless networking such as carrier sense multiple access and slotted ALOHA. Due to the new features of CR networks, such as the collision avoidance with a primary user and dynamics in spectrum availability, new medium access protocols need to be designed to address the new challenges in CR networks. A cognitive medium access protocol with stochastic modeling is proposed in BIB004 , which enhances the coexistence of CR with WLAN systems based on sensing and prediction. A primary-prioritized Markov approach for dynamic spectrum access is proposed in BIB010 , which models the interactions between the primary users and the secondary users as continuous-time Markov chains. By designing appropriate access probabilities for the secondary users, a good tradeoff can be achieved between spectrum efficiency and fairness. A cognitive MAC (C-MAC) protocol for distributed multi-channel wireless networks is introduced in BIB002 . Since the C-MAC operates in multiple channels, it is able to deal with the dynamics of channel availability due to primary users' activity. A stochastic channel selection algorithm based on learning automata is proposed in BIB003 , which dynamically adapts the probability of access one channel in real time. It is shown that the probability of successful transmissions is maximized using the proposed selection algorithm. A MultiMAC protocol that can dynamically reconfigure MAC and physical layer properties based on per-node and per-flow statistics is proposed in BIB001 . Considering the limited capability of spectrum sensing and limited bandwidth, a hardware-constrained cognitive MAC is proposed in BIB005 , which optimizes the spectrum sensing decision by formulating sensing as an optimal stopping problem. Secondary users also need to be aware of their surrounding environment in allocating and accessing the spectrum. Considering that each node's spectrum usage is unpredictable and unstable, the work in BIB006 proposes to integrate interference-aware statistical admission control with stability-oriented spectrum allocation. The nodes' spectrum demand is regulated to allow efficient statistical multiplexing while the outage is minimized. Since secondary users operating in different frequency bands at different locations are constrained by different interference requirements, a distance-dependent MAC protocol is proposed BIB007 to optimize the CR network throughput subject to a power mask constraint to protect the primary user. The idea of how to utilize location awareness to facilitate spectrum sharing between secondary and primary users is illustrated in  . An aggregation-aware spectrum assignment scheme is proposed in BIB008 to optimize the spectrum assignment when the available spectrum band is not contiguous. Collision probability and overlapping time are introduced in BIB009 to evaluate the protection of a primary user. Different spectrum access schemes using different sensing, back-off, and transmission mechanism are considered, which reveal the impact of several important design criteria, such as sensing, packet length distribution, back-off time, packet overhead, and grouping.