Abstract
Cognitive radio (CR) systems are potentially capable of mitigating the spectrum shortage of contemporary wireless systems. In this paper, we provide a brief overview of CR systems and the important research milestones of their evolution, along with their standardization activities, as a result of their research. This is followed by the detailed analysis of the interweave policy-based CR network (CRN) and by a detailed comparison with the family of underlay-based CRNs. In the interweave-based CRN, sensing of the primary user’s (PU) spectrum by the secondary user’s (SU) has remained a challenge, because the sensing errors prevent us from fulfilling the significant throughput gains that the concept of CR promises. Since missed detection and false alarm errors in real-time spectrum sensing cannot be avoided, based on a new approach, we quantify the achievable rates of the interweave CR by explicitly incorporating the effect of sensing errors. The link between the PU transmitter and the SU transmitter is assumed to be fast fading. Explicitly, the achievable rate degradation imposed by the sensing errors is analyzed for two spectrum sensing techniques, namely, for energy detection and for magnitude squared coherence-based detection. It is demonstrated that when the channel is sparsely occupied by the PU, the reusing techniques that are capable of simultaneously providing low missed detection and false alarm probabilities cause only a minor degradation to the achievable rates. Furthermore, based on the achievable rates derived for underlay CRNs, we compare the interweave CR and the underlay CR paradigms from the perspective of their resilience against spectrum sensing errors. Interestingly, in many practical regimes, the interweave CR paradigm outperforms the underlay CR paradigm in the presence of sensing errors , especially when the SNR at the SU is below 10 dB and when the SNR at the PU is in the range of 10–40 dB. Furthermore, we also provide rules of thumb that identify regimes, where the interweave CR outperforms the underlay CR.
Highlights
While certain parts of the frequency spectrum are crowded by users, most part of the spectrum still remains largely unoccupied [1]
Diverse cognitive radio approaches have been suggested in the literature, but the three most popular ones are the interweave Cognitive radio (CR) based on opportunistic spectrum access, the underlay CR and the overlay CR based spectrum sharing [5], [6]
The achievable rate regions are plotted for Rayleigh fading communication links covering all possible combinations of pfa and pmd for the above detectors, where the probability of the Primary User (PU) channel being free was set to p = 0.25, 0.5 and 0.75 in Fig. 6 at the average received Signal to Noise Ratio (SNR) values of −24dB and −26dB for both the energy detector (ED) and the MSC
Summary
While certain parts of the frequency spectrum are crowded by users, most part of the spectrum still remains largely unoccupied [1]. To the best of our knowledge there have been no comparative studies of all of the above paradigms in the presence of diverse channel parameters and user constraints, albeit the authors of [10] provided comparisons of the interweave CR and the underlay CR, while ensuring the minimum outage probability for the PU. A characteristic of these achievable rates derived for the underlay CR are that they ensure maintaining its original full single-link rate for the PU, while limiting the SUs transmission rate This makes our comparison a fair one, since no new interference temperature or no different power constraints is involved. We will demonstrate that interestingly, there are various regimes of practical interest, where despite encountering spectrum sensing errors, the interweave CR provides a higher rate for the SU as well as a higher sum rate to the SU and PU together, than the underlay CR.
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