Cognitive Radio

By Radoslaw Piesiewicz (CREATE-NET)

There is evidence that the economic growth of a developed community such as the European Union is strongly related to a growing base of Information and Communication Technology (ICT) resources. The radio part of the ICT is gaining increased shares in voice and broadband data communications with the sophisticated technological developments coming along in recent years. In order to keep the European gross domestic product continuously growing a variety of enabling key applications is required in significant volumes. Modern digital radio is a prerequisite for such enabling key applications by ensuring almost seamless access to broadband communications networks in a convenient and efficient manner. The essential basis for any radio communications is the radio frequency spectrum. It is important for our society to ensure that it is used as efficient as possible as it is a limited resource.

The Commission of the European Communities (CEC) has channelized the strong economic demand for extended wireless information transport resources in political statements underlining the importance of increased efficiency in the use of the radio frequency spectrum. Subsequently the CEC has called for scientific investigations to find technical solutions for the problem inside European level research and development frameworks contents and related work programmes.

Technically the radio frequency spectrum is a limited resource. Historically for this reason the global society has developed a legal framework of rules to regulate the usage rights for the frequencies already since the begin of the massive usage of the radio spectrum in 1906 during the Second International Radio Telegraphic Conference convened at Berlin on 3 October 1906, which was attended by delegations from 27 nations. At this time the global frequency management started with the introduction of the SOS and the allocation of the frequencies from 500 kHz to 1000 kHz for the marine communications, from 150 kHz to 188 kHz for costal radio stations and from 188 kHz to 500 kHz for military applications. Due to its enormous importance for the global society the spectrum management became an important task of the United Nations Organization in 1947 handled by its specialized agency International Telecommunications Union (ITU).

Basically there are two ways to increase efficiency of the radio spectrum use. First one is to enhance the performance of radio technology and therefore increase the spectrum efficiency measured in terms of transmitted information unit per bandwidth unit in a certain time. Second one is to ensure that the radio spectrum is allocated to a user service according to the instantaneous requirements in a given geographic area taking into account the current situation of exploitation and radio interference in that particular area.

Traditionally existing legacy radio communications systems are designed following the assumption that the received signal is an attenuated replica of the transmitted signal mainly disturbed by the radio channel characteristics and superseded by thermal noise in the receiver. This holds, if the number of radio services and transmitting devices is limited, which was true for a relative long period of time during the last century. Technical realisation of system separation was realised using non-adaptive traditional analog filters with their inherent low degree of flexibility. Consequently the frequency spectrum regulation policy was favouring to grant exclusive frequency use rights for dedicated radio services valid for longer periods of time. Following this historical developments current radio frequency regulation paradigm is still based on frequency separation which is defined for longer periods of time (tens of years), as described in section [1].

In the analog radio age the means to separate wanted and unwanted received signals were analog frequency filters. Based on this technology the radio systems were supposed to operate in separate frequency bands each. Consequently the multiple access technique as well as the duplex technique applied in those systems, frequency division multiple access (FDMA) and frequency division duplex (FDD) respectively, were based on the well known analog frequency filtering technique basically. This traditional approach of frequency regulation is followed still in principle until now for the separation of radio systems and services with a few very recent exceptions, namely UWB ((Ultra-Wideband) [2] and nano FM transmitters [3],[4]. Although with the begin of the digitalisation of radio communications several new and innovative options became feasible. The digitalisation of radio communications became economically and technically feasible enabled by the revolutionary advances in the semiconductor industries since the 80s allowing to implement complex digital signal processing algorithms at reasonable costs. The feasibility of time and code division multiple access (TDMA and CDMA), time division duplex (TDD) and the combination of diversity techniques with forward error correction (FEC) coding enable in principle more efficient and innovative frequency resource sharing methods. In general one can state that radio systems became more robust and it becomes feasible to accept also interference from other radio systems without resulting straight in harmful degradation of the own systems performance. Furthermore flexible air interfaces adapting to the current radio environment on a "call by call" basis are feasible now and are currently being investigated and optimised in several European research projects.

An early technical approach of air interface flexibility practiced by cellular systems operators today is the so called network roaming - where a radio communications service can be provided temporarily also using an alien network, if the native network, the user is subscribed to, is not available for whatever reason (bad radio channel, high traffic load, no native network coverage). This approach works because the user radio terminals are able to connect also to network infrastructure of alien network service provider by means of switching to other frequency bands.

Another early approach of spectrum sharing is the so called secondary usage of radio frequencies based on a non-protection, non-interference scenario enabling secondary services to use the frequency spectrum primarily allocated to other radio services under certain technical conditions. Therefore the interference to other systems measured in terms of in band and in terms of out-of-band emissions are important means of measure of mutual influence of the radio systems. Even if it is basically not allowed for primary radio services to share radio spectrum (except of ISM bands) each radio system operator has to accept by law that there is unintentional out-of-band emission from the other radio systems in operation and has to cope with such interference. From legal point of view the allowed out-of-band emission for radio systems is defined in Europe in the appropriate ETSI standards and CEPT recommendations [5],[6]. These standards are implemented or revisited under the R&TTE directive released by the Radio Spectrum Committee (RSCOM) of the European Commission in 1999 [7].

A first approach of dealing with such situation of allowing access to exactly the same radio spectrum resource to several different services is the introduction of various kind of non data-aided mitigation techniques [8]-[11] such as activity factor limitation (like the initial low duty cycle, LDC, approach [9],[10]), enhancement of sensing techniques like listen before talk (LBT) and detect and avoid (DAA) as investigated in CEPT [11] and ETSI [8]. One further possibility is the introduction of spectrum avoidance algorithms based on absolute geographic location, the so called location based recognition of exclusion zones.

Static spectrum assignment, commonly used in radio communications, is one of the main reasons behind the commonly shared feeling of the lack of spectrum resources. This is especially true at frequencies below 3 GHz for which intense competition for spectrum has occurred over the last few years. In practical cases, the real spectral utilization is in the order of few percent and may be as low as a fraction of a percent in large spectrum regions. This is in evident contradiction with respect to concerns regarding spectrum shortage. A second, much more sophisticated approach to overcome these insufficiencies is the development of a "cognitive radio" (CR) concept allowing a highly efficient data-aided spectrum sharing between several different heterogeneous radio systems (for example by defining a standardised Cognitive Signalling Channel as one possibility for universal recognition of a radio terminal and its radio isolation to the current receiver) [12]-[17].

Fig. 1  Allocated spectrum and possible bands of operation for CR-based systems in Europe.

CR is currently regarded as the most promising solution to the aforementioned problem by allowing smart spectrum management in future wireless communication systems. CR is a radio that can adapt its transmission or reception parameters based on cognitive interaction with the wireless environment in which it operates. This interaction may involve passive spectrum sensing or active communication and negotiation with other spectrum users based on reasoning procedures which represent the "intelligence" of the CR itself. The Active CR aims to improving spectrum efficiency, capacity and fairness by coordinating the radio behaviour in time, location and frequency. Active CR could potentially break the ever-serious bottleneck of limited spectrum availability and open up new frontiers and opportunities for wireless system designers and application developers. Moreover, Active CR with intelligence in both the radio link and network layers is capable of optimising its transmission across all available signal dimensions allowing a huge increase in the prospects for spectrum efficiency, co-existence, compatibility and coordination among the ever-proliferating wireless communication systems and devices. Active CR may evolve spectrum management from the frequency domain only, to dynamic spectrum management and interference avoidance in both frequency and time domains. Currently there is a drive for more innovative use of radio spectrum across the whole European area. Fig. 1 shows the area of interest for the cellular applications (top) and the white zones which need a new approach for spectrum/interference management (bottom).

With the start of UWB regulation process in Europe [18]-[20], in Asia and on ITU level it became obvious that there is a number of challenges associated with the regulation of such a kind of modern example of spectrum sharing technology. The modern radio technology is able and required to operate based on co-existence and cognitive radio principles. Besides technical challenges there are also administrative ones such as convincing the stakeholders in the European regulation process that the CR approach will provide a more efficient spectrum use from an overall economic point of view and will open a new market for several applications enabled by those new modern "intelligent" radio technologies. A prominent example is the Ultra-Wideband Radio Technology and the associated applications.

In the global communications community, four application scenarios are conceived as possible targets for CR:

• CR can be applied within a licensee's network to increase the efficiency of use.
• CR can facilitate secondary markets in spectrum use, implemented by voluntary agreements between licensees and third parties.
• CR can facilitate automated frequency coordination among licensees of co-primary services.
• CR can be used to enable non-voluntary third party access to spectrum, for instance as an unlicensed device operating at times or in locations where licensed spectrum is not in use.

Setting out from these application scenarios, we can anticipate several typical application examples:

• CR technologies are useful for the internal optimization of the networks, which are essential in solving the every-serious spectrum resource scarcity problem while the current 3G telecommunications networks are evolving to the all IP-based 4G telecommunications networks aiming at providing seamless, ubiquitous, end-to-end, and quality-guaranteed services inside a specific operator and carrier.
• CR technologies can be applied to the interference detection, interference avoidance/mitigation, and, moreover, the interference exploitation setting out from the concept of linear and non-linear network coding.
• CR technologies lend themselves to the application in heterogeneous communications networks [21],[22], encompassing different service providers and radio standards like WiMAX BWA (World-wide Interoperability for Microwave Access Broadband Wireless Access), e.g. 802.16e/h, 3G/B3G/4G 3GPP mobile operators (e.g., UMTS/HSDPA/LTE/DVB-H).
• CR technology can become a key enabler for true heterogeneous communication environment where data-aided mitigation techniques, such as physical or logical layer cognitive pilot channel (CPC) will be implemented for optimal sharing of the spectrum and coexistence with least interference among various radio access nodes. These ideas gain in importance especially with respect to the vision of Future Internet or Internet of Things, where a multitude of different devices are expected to communicate seamlessly and rearrange their network configuration in an autonomous fashion in order to route/exchange the information in most efficient way. The cognitive and reconfigurable radio paradigms with CPC and cognitive routing schemes are expected to contribute greatly to the realization of this vision, as forecasted by the Future Internet Assembly and reflected in the 7^th Framework Program (2007-2013) of the European Community for research, technological development and demonstration activities.

The technical developments are challenging as well in several areas and need still a considerable amount of research and development effort in various basic disciplines still considered as pre-competitive R&D activities. Just to mention some, without claiming completeness here:

• Design of radio communications systems based on CR principle and having no exclusive spectrum usage rights WHILE still ensuring certain Quality of Service for the application
• Provision of sufficient protection to indoor fixed wireless access terminals operating close to the sensitivity limits without introducing harmful interference
• Protection of strategic services such as military radar and military sensing and communications without disclosing details of their system specification
• Provision of many potential radio services addressed due to the wide spectrum band targeted, which is partly even not completely specified by now.

From current point of view the challenges listed can be dealt with applying the techniques introduced, such as mitigation techniques and creation of common signalling carriers which are currently basically visions. First steps have been taken to verify the effectiveness of those techniques and intensive research work is ongoing to further develop these visions and turn them into reality [8],[12]-[14].

References

[1]                    URL of the International Telecommunication Union, Radiocommunication Sector (ITU-R) history, as of 2008-05-23: http://www.itu.int/net/about/history.aspx

[2]                    2007/131/EC: Allowing the use of the radio spectrum for equipment using ultra-wideband technology in a harmonised manner in the Community. 21 February 2007 - Commission Decision 2007/131/EC on allowing the use of the radio spectrum for equipment using ultra-wideband technology in a harmonised manner in the Community.  URL of the CEPT ERO mirroring the official EC document, as of 2008-05-23: http://www.erodocdb.dk/doks/filedownload.aspx?fileid=3398&fileurl=http://www.erodocdb.dk/Docs/doc98/official/pdf/2007131EC.PDF

[3]                    German national administration Bundesnetzagentur Vfg 7 / 2006: Allgemeinzuteilung von Frequenzen in den Frequenzbereichen 87,5- 108 MHz, 863- 865 MHz und 1795- 1800 MHz für drahtlose Audio-Funkanwendungen, Germany.

[4]                    Ofcom: http://www.ofcom.org.uk/consult/condocs/exemption/statement/

[5]                    CEPT/ERC/RECOMMENDATION 74-01E (Siófok 98, Nice 99, Sesimbra 02, Hradec Kralove 05): UNWANTED EMISSIONS IN THE SPURIOUS DOMAIN. URL of the CEPT ERO, as of 2008-05-23: http://www.erodocdb.dk/doks/filedownload.aspx?fileid=1695&fileurl=http://www.erodocdb.dk/Docs/doc98/official/pdf/REC7401E.PDF

[6]                    ECC/REC/(02)05, Revised ECC RECOMMENDATION, 21/10/2002: UNWANTED EMISSIONS. URL of CEPT ERO, as of 2008-05-23: http://www.erodocdb.dk/doks/filedownload.aspx?fileid=2658&fileurl=http://www.erodocdb.dk/Docs/doc98/official/pdf/REC0205.PDF

[7]                    Directive 1999/5/EC of the European Parliament and of the Council of 9 March 1999 on radio equipment and telecommunications terminal equipment and the mutual recognition of their conformity.

[8]                    URL of ETSI STF 350 on DAA parameters and test procedures for UWB communications applications (ETSI STF 350), as of May 19th 2008: http://portal.etsi.org/STFs/STF_HomePages/STF350/STF350.asp

[9]                    ECC Report 094: Technical requirements for UWB LDC devices to ensure the protection of FWA systems. URL of CEPT ERO, as of 2008-05-23: http://www.erodocdb.dk/doks/filedownload.aspx?fileid=3299&fileurl=http://www.erodocdb.dk/Docs/doc98/official/pdf/ECCREP094.PDF

[10]               ECC/DEC/(06)12: Low Duty Cycle UWB in the band 3.4-4.8 GHz. ECC Decision of 1 December 2006 on the harmonised conditions for devices using Ultra-Wideband (UWB) technology with Low Duty Cycle (LDC) in the frequency band 3.4-4.8 GHz. URL of CEPT ERO, as of 2008-05-23: http://www.erodocdb.dk/doks/filedownload.aspx?fileid=3287&fileurl=http://www.erodocdb.dk/Docs/doc98/official/pdf/ECCDEC0612.PDF

[11]               TG3#23_03R0: Draft ECC Report 120 on DAA, Technical requirements for UWB DAA (Detect And Avoid) devices to ensure the protection of radiolocation in the bands 3.1-3.4 GHz and 8.5-9 GHz and BWA terminals in the band 3.4-4.2 GHz (under public consultation until 19 May 2008). URL of CEPT ERO, as of 2006-05-23: http://www.ero.dk/C1C484C5-26C4-490F-B70A-391DE79F1DB1?frames=no&

[12]               URL of ETSI RRS, as of May 19th 2008: http://portal.etsi.org/rrs/rrs_tor.asp

[13]               URL of Radio Access & Spectrum cluster of the EC DG INFSO and MEDIA of the European Commission, as of May 19th 2008: http://www.newcom-project.eu:8080/Plone/ras

[14]               URL of IEEE SCC 41, as of May 19th 2008: http://www.scc41.org/

[15]               J. Mitola: Cognitive Radio. Ph.D. Thesis, Royal Institute of Technology (KTH), Stockholm, 2000.

[16]               F.H.P. Fitzek, M.D. Katz (Editors): Cooperation in Wireless Networks: Principles and Applications -  Real Egoistic Behavior is to Cooperate. Springer, 2006.

[17]               F.H.P. Fitzek, M.D. Katz (Editors): Cognitive Wireless Networks: Concepts, Methodologies and Visions Inspiring the Age of Enlightenment of Wireless Communications. Springer, 2006.

[18]               RSCOM 04-08: MANDATE TO CEPT TO HARMONISE RADIO SPECTRUM USE FOR ULTRA-WIDEBAND SYSTEMS IN THE EUROPEAN UNION. 2004-02-18. URL of RSCOM, as of 2008-05-23: http://ec.europa.eu/information_society/policy/radio_spectrum/docs/current/mandates/rscom0408_mandate_uwb.pdf

[19]               RSCOM: SECOND MANDATE TO CEPT TO IDENTIFY THE CONDITIONS NECESSARY FOR HARMONISING RADIO SPECTRUM USE FOR ULTRA-WIDEBAND SYSTEMS IN THE EUROPEAN UNION, 2005-06-06. URL of the EC RSCOM, as of 2008-05-23: http://ec.europa.eu/information_society/policy/radio_spectrum/docs/by_topics/final_second_uwb_mandate.pdf

[20]               RSCOM: MANDATE TO CEPT TO IDENTIFY THE CONDITIONS RELATING TO THE HARMONISED INTRODUCTION IN THE EUROPEAN UNION OF RADIO APPLICATIONS BASED ON ULTRAWIDEBAND (UWB) TECHNOLOGY (MANDATE 3), 2006-07-15. URL of RSCOM, as of 2008-05-23: http://ec.europa.eu/information_society/policy/radio_spectrum/docs/current/mandates/3_ec_to_cept_uwb_06_06.pdf

[21]               B. Ahlgren, L. Eggert, B. Ohlman, A. Schieder: Ambient Networks: Bridging Heterogeneous Network Domains. Proceedings of the IEEE International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC2005), Berlin, September 2005.

[22]                           http://www.ambient-networks.org/publications_newversion.html