Cognitive Radio Dynamic spectrum access models (shared-use)

Traditional “command-and-control” licensing scheme in which radio resources are statically assigned to a particular spectrum licensee results in spectrum under-utilization. This spectrum licensing scheme does not allow the flexible usage of frequency spectrum according to the time-varying demands of the user. To overcome such limitations, the concept of open spectrum was introduced. The open spectrum concept defines a set of techniques and models to support dynamic management of frequency bands for wireless communications systems. It has led to new spectrum access and licensing models which are able to improve the flexibility and efficiency of spectrum access, classified as exclusive-use, shared-use of the licensed spectrum, and commons models.

Dynamic spectrum access (DSA) models for cognitive radio can be categorized as exclusive-use, shared-use, and commons models.

In this article, I describe the shared-use Dynamic spectrum access model.



There are two major components of dynamic spectrum access, namely, spectrum sensing and spectrum access. Spectrum sensing, which can be implemented in both physical and MAC layers, is used to detect the presence of a licensed user. In this case, an unlicensed user observes the target frequency band and searches for a signal from a licensed user. The spectrum sensing result is used by the unlicensed user to access the spectrum without interfering with the licensed user and colliding with other unlicensed users.

In the shared-use spectrum access model, the radio spectrum can be simultaneously shared between a primary user (i.e. a licensed user) and a secondary user (i.e. an unlicensed user). In this model, unlicensed users can opportunistically access the radio spectrum if it is not occupied or fully utilized by primary users. In other words, as long as an unlicensed user does not interrupt a primary user (e.g. the collision probability is maintained below the target level), spectrum access by an unlicensed user is allowed and it remains transparent to a primary user. In a shared-use model, the spectrum can be accessed by an unlicensed user in two different modes, namely, spectrum overlay and spectrum underlay modes illustrated below.

In the case of spectrum overlay, a primary user receives an exclusive right to spectrum access. However, at a particular time or frequency, if the spectrum is not utilized by a primary user, it can be opportunistically accessed by a secondary user. Therefore, to access a spectrum band, a secondary user has to perform spectrum sensing to detect the activity of a primary user in that band. If a spectrum hole is found, a secondary user may access the spectrum. The decision of a secondary user whether to access the spectrum or not depends on constraints such as the collision probability, which is defined as the probability that the transmission from a secondary user occurs at the same time as that from a primary user. Spectrum overlay can be used for cognitive radio in FDMA, TDMA, or OFDM wireless systems.

In the case of spectrum underlay, a secondary user can transmit concurrently with a primary user. However, the transmit power of the secondary user should be limited so that the interference caused to the primary users remains below the interference temperature limit. Spectrum underlay can be used for cognitive radio systems using CDMA or UWB technology.