Path 1. Optimizing the use of spectrum

Description

Spectrum usage optimization or dynamic spectrum access (DSA) is the first path of CR. Figure 10 shows an example of a cognitive cycle when spectrum use is of concern. The overall aim is to make more efficient use of spectrum to accom-modate the growing demand for future wireless services. Flexible wideband usage of the spectrum is needed to support the transfer to data-oriented services, which requires increasing transmission capacity. On the other hand, the energy efficiency of mobile devices can be enhanced by limiting the transmission time, i.e., by using a frequency band that is as wide as possible.

Identification of most vacant channels based

on history Classification of

signals from different systems

Requirements for detection

probability

Selection of transmission

channel Spectrum

sensing

Adaptive transmission

Figure 10. Cognitive cycle for optimizing spectrum use.

With regard to spectrum use, cognitive wireless networks can consist of PUs and SUs. PUs are licensed users of specific frequency bands who own the licensed bands exclusively at any time. SUs use the frequency bands of PUs opportunisti-cally whenever the PUs in the area are not operating or the interference caused by them is tolerated by PUs in their proximity. In this case, unlicensed bands are regarded as special frequency bands that have no PUs.

CR techniques can also be used to optimize spectrum use inside the network to make more efficient use of the spectrum that already belongs to the system. In this case, the CR techniques are used to improve system performance instead of sharing spectrum with other systems.

CR technologies can facilitate secondary markets in spectrum use, imple-mented by voluntary agreements between licensees and third parties. For in-stance, a licensee and third party could sign an agreement allowing secondary spectrum uses made possible only by the deployment of CR technologies.

Ulti-mately, CR devices could be developed that “negotiate” with a licensee’s system and only use spectrum if an agreement is reached between a device and the sys-tem. In this case, spectrum sensing by the user terminals is not required. Instead, the network operators need negotiation methods for managing the spectrum.

They also need to be aware of the overall spectrum allocation and policy.

Research issues

Many of the research issues related to spectrum usage optimization have been addressed in the COGNAC project. Two clearly separate cases need to be ad-dressed: DSA operation in licensed bands and in unlicensed bands. In the former case, the observations and identification of the primary system (cellular network, broadcaster, etc.) are the key issue. The issue of spectrum sensing is therefore the most important research topic. Spectrum sensing also needs to be performed with sufficient reliability to convince the regulators. Four issues, in particular, were addressed in COGNAC, though they need more attention:

 Spectrum sensing should be coupled with database-based solutions.

 The cooperative sensing needs to be surveyed and tested.

 The selection of suitable sensing method to a specific use scenario needs to be defined and demonstrated.

 Actual spectrum efficiency enhancement provided by CR systems should be clearly demonstrated.

In addition to sensing, all the aspects shown in Figure 10 are important to DSA.

The classification of signals and traffic patterns in different channels helps the sensing process and supports the learning process. The selection of a transmission channel should be made based on the requirements of the requesting user as well as the characteristics of the channel, e.g., which channel offers sufficiently long idle periods. Adaptive transmission methods, including power control methods that react to the detected changes in the environment, as well as the given a pri-ori information about the PUs in the area, also need to be researched. In the latter case, spectrum users need to organize their operation autonomously. Methods leading to fair and efficient resource use are required and, e.g., game theoretical approaches have been used to find the limits. The opening of the spectrum also brings the risk of malicious users accessing the spectrum and misusing it. Meth-ods to prevent misuse therefore need to be developed.

The current frequency regulation aims to guarantee that systems can operate free from harmful interference. To accomplish this, the operation of different systems in the same bands, following the secondary/primary usage paradigm, requires careful coexistence studies. The coexistence of different systems in the same bands could be eased by more flexible frequency allocation/reuse of meth-ods that exploit CR techniques. Current spectrum owners and operators need to be convinced that secondary spectrum use is viable and will not hinder their business. Key issues to do this include reliable demonstrations of spectrum-sensing methods, comprehensive coexistence studies, and pilot systems.

Beneficiaries and use cases

More efficient and flexible use of spectrum brings many new possibilities in terms of opening the market to new players and new advanced applications. An example of this step for licensed bands is the potential use of white spaces on TV bands. In TV whate space usage, the actual spectrum sensing is not required.

Instead, access to available frequency slots is best implemented based on spec-trum allocation information from databases or similar sources.

Network infrastructure providers benefit from easier adoption and approval of the new technology to operate in different frequency ranges. New systems that could use these spectrum reuse tools include new mobile communication sys-tems such as future releases of IMT-Advanced, especially in the femtocell-type solutions in which the operation as an SU of the spectrum may become easier due to the close ranges with the base stations (e.g., eNodeBs) and peers. An ex-ample of this is the operator deploying femtocells on the same band as its mac-rocells to provide alternative access methods for users. Another emerging possi-bility is to use unlicensed bands for cellular access in short ranges.

Beneficiaries of the new technology would be:

 Network operators: possibilities to provide better coverage and data rates for users and thus make a bigger profit; end-user level QoS may also become a reality.

 Spectrum owners: potential revenue by allowing secondary spectrum users (operators) at some frequencies/time instants; a mechanism for managing spectrum leasing dynamically is needed however.

 Operators with reserved spectrum slots may be able to sell or hire the unused spectrum locally and/or temporally, e.g., by some type of auc-tioning mechanism.

 Spectrum regulators: higher spectrum occupancy to accommodate the growing amounts of wireless traffic.

Another field that could use DSA in the near future is public safety communica-tions and military users. In these cases, the definite need for more bandwidth is motivating the adoption of novel solutions. In addition, the application of wire-less technology often takes place in areas in which wirewire-less infrastructure does not exist or has been partly or totally destroyed (natural disasters, war zones, etc…). In this case, beneficiaries of the technology are military and public safety organiza-tions that can deploy their networks more easily in any geographical area.