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DYNAMIC SPECTRUM MANAGEMENT

COGNITIVE FEMTOCELL NETWORKS:
AN OVERLAY ARCHITECTURE FOR
LOCALIZED DYNAMIC SPECTRUM ACCESS
.

GÜRKAN GÜR, SUZAN BAYHAN, AND FATIH ALAGÖZ, BOGAZICI UNIVERSITY

ABSTRACT

Center femtocell {n, σ}SU-FPU

{n, σ}SU-SU

}SU-SU

{n, σ

The authors define and propose a femtocell-based cognitive radio architecture for enabling multitiered opportunistic access in next-generation broadband wireless systems.

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We define and propose a femtocell-based cognitive radio architecture for enabling multitiered opportunistic access in next-generation broadband wireless systems. This architecture combines the conventional femtocell idea with an infrastructure-based overlay cognitive network paradigm. The cognitive femtocell concept leads to simpler and easier proliferation of cognitive radio into practical systems. We highlight the drawbacks and advantages of the proposed network structure with a discussion on research directions for cognitive femtocell architecture.
We also provide experimental results to illustrate a general proof of concept for this new modality. INTRODUCTION
The need for anytime-anywhere service access over wireless networks has spurred a great demand for wireless radio resources. Additionally, the emerging web-based services such as
YouTube and Facebook require broadband access due to multimedia-rich content. Hence, efficient spectrum utilization is of great concern for two reasons: providing spectrum opportunities for emergent technologies/services and utilizing already crowded lower frequency bands with better spectrum efficacy. Dynamic spectrum access (DSA) facilitates the so-called cognitive radio (CR) devices to analyze the spectrum bands and access them if unoccupied until the arrival of an incumbent transmitter. In CR terminology, a primary user (PU) refers to a user who has higher priority or legacy rights for utilizing a specific part of the spectrum, whereas a secondary user (SU) equipped with CR exploits the unused parts of this spectrum in such a way that PU communications are not disturbed.
Since a PU has the right to access the spectrum
(depending on its contract with its service provider) whenever it needs, a CR node must vacate those primary bands as fast as possible.
Otherwise it would harm the primary communications, which is a violation of CR operation

1536-1284/10/$25.00 © 2010 IEEE

rules. However, it is nontrivial for an SU to detect the existence of a higher-priority PU in these spectrum bands of interest [1]. This requirement is challenging since there is usually no interaction between the primary and secondary (cognitive) networks. In order to detect the spectrum opportunities or the PUs, SUs sense the spectrum with sensors. Depending on sensor readings and the spectrum decision algorithm, the SU decides on the state of a frequency band as idle or busy.
Reliable PU detection is the fundamental step in the realization of DSA and the opportunistic use of white spectrum. Depending on the PU signal knowledge and the radio environment, various transmitter detection schemes can be applied such as matched filter detection and energy detection [1]. A detection scheme must be reliable such that some degree of probability of detection (Pd) must be met (e.g., 0.9 in IEEE
802.22 [2], which is the first CR-based wireless network standard). Moreover, probability of false alarm (Pf), which is defined as the probability that the detection scheme incorrectly declares the spectrum as occupied when it is not, should be sufficiently low. In case of false alarm, these spectrum holes cannot be identified for SU use, thereby resulting in lower spectrum efficiency.
Performance of primary signal detection schemes may significantly degrade in case of severe channel impairments. In order to cope with this issue, robust spectrum sensing can be achieved by cooperation among SUs at the expense of message exchange overhead and cooperation delay.
SUs can share