Cognitive Spectrum Access Control Based on Intrinsic Primary ARQ Information

We explore a flexible and non-intrusive opportunistic spectrum access control mechanism for cognitive radiosthat goes beyond the standard “Listen-Before-Talk” (LBT) strategy. We exploit the bi-directional nature of many primary communication systems to control cognitive radio access that is bas ed on both spectral sensing and primary receiver acknowledgemen ts. In this work, we limit our attention on the ACK/NAK messages in primary data-link-control. We use partially observable Markovdecision processes to devise an optimized admission contro l policy. Our new method achieves high cognitive network throughput while offering robust protection of primary user signals. I. I NTRODUCTION Cognitive radio access delineates a new paradigm for dynamically alleviating the spectrum scarcity problem. Cogn itive radios are designed to access a bandwidth already allocated to a primary user network under the constraint that the primary user communication quality of service is not heavily impair ed. Cognitive radio access control has been a popular topic of research, for example, the authors of [1] devised distribut ed spectrum sensing and access strategies under an energy constraint on secondary users. In [2] and [3], the design of sens ing policies for tracking spectrum opportunities is explored. The authors of [4] applied a partially observable Markov decisi on framework to devise an optimal sensing and channel selection policy in a multi-channel opportunistic communicatio n system. The authors of [5] and [6] studied the challenge of SNR threshold below which the sensing outcome is severely impaired regardless of the sensing time. The authors of [7] derived an optimal sensing policy for exponential idle time whereas in [8] they addressed optimal sensing strategies fo r continuous time systems, exploiting acknowledgement sign als from the SU-Rx. Also, the authors of [9] presented a cognitiv e radio system that varies transmission power according to al l the information available to the spectrum sensor. It should be noted that, thus far, “listen-before-talk (LBT)” is the sta te-ofthe-art opportunistic spectrum access approach to cogniti ve radio. LBT simply relies on spectrum sensing, thereby enablin g a secondary user (SU) to access the spectrum of a primary user (PU) only after sensing the PU spectrum unoccupied band. LBT is natural and practical, not requiring any modification to the existing PU infrastructure. However, LBT only senses primary transmission activities and it is unaware of the act u l receiver conditions. More specifically, it neither solves t he hidden receiver problem nor utilizes any capacity that robu st interference-resistant PU networks may provide. To expand the applications and to improve the efficiency of cognitive radios, we enhance the LBT learning capability by leveraging the feedback information typically availabl e in duplex PU links. Feedback signals for data-link-control ar e available in many systems such as HSDPA [10] and WiMAX [11] in the form of ACK/NAK packets and downlink/uplink profiles. Such information allows the SU to more accurately infer the effect of the SU activity on the PU receivers (PU-Rx ). In this work, we emphasize that advanced cognition should require the SUs to learn about primary network characterist ics and user interaction. Similar concept of cognition has been iscussed in [12] where a SU transmits probing signals to observe the PU transmission power changes in response to bette r estimate the SU to PU channel gain. Specifically, we utilize the ACK/NAK signals transmitted by the packet receivers for data-link-control. Exploiting the ACK/NAK signals from th e PU-Rx allows the SUs to detect the presence of any hidden PU-Rx and to assess the quality of the PU-Rx reception, thereby better providing the necessary PU protection. It al so makes it possible for the SU transmitter (SU-Tx) to optimize its access policy by estimating the PU-Rx reception quality under the SU-Tx access control policy, therefore allowing t he SU-Tx to better exploit any PU excess capacity. This paper is an extension of our preliminary work [13] where we presented an optimal channel access scheme based PU ACK/NAK information overhearing. The rest of this manuscript is divided into three sections. Section II prese nts the description of the problem and the basic formulation of the cognitive spectrum access based on acknowledgement information. Section III presents the development of our optimized spectrum access policy. Section IV presents our test results of our access algorithm. Section V summarizes our conclusions and future directions under pursuit.