Software-defined radio-basic and evolution to cognitive radio

We provide a brief overview over the development of software-defined or reconfigurable radio systems. The need for software-defined radios is underlined and the most important notions used for such reconfigurable transceivers are thoroughly defined. The role of standards in radio development is emphasized and the usage of transmission mode parameters in the construction of software-defined radios is described. The software communications architecture is introduced as an example for a framework that allows an object-oriented development of software-defined radios. Cognitive radios are introduced as the next step in radio systems' evolution. The need for cognitive radios is exemplified by a comparison of present and advanced spectrum management strategies.


INTRODUCTION
Reconfigurability in radio development is not such a new technique as one might think. Already during the 1980s reconfigurable receivers were developed for radio intelligence in the short wave range. These receivers included interesting features like automatic recognition of the modulation mode of a received signal or bit stream analysis. Reconfigurability became familiar to many radio developers with the publication of the special issue on software radios of the IEEE Communication Magazine in April 1995.
W er e f e rt oat r a n s c e i v e ra sasoftware radio (SR) if its communication functions are realized as programs running on a suitable processor. Based on the same hardware, different transmitter/receiver algorithms, which usually describe transmission standards, are implemented in software. An SR transceiver comprises all the layers of a communication system. The discussion in this paper, however, mainly concerns the physical layer (PHY).
The baseband signal processing of a digital radio (DR) is invariably implemented on a digital processor. An ideal SR directly samples the antenna output. A software-defined radio (SDR) is a practical version of an SR: the received signals are sampled after a suitable band selection filter. One remark This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
concerning the relation between SRs and SDRs is necessary at this point: it is often argued that an SDR is a presently realizable version of an SR since state-of-the-art analog-to-digital (A/D) converters that can be employed in SRs are not available today. This argument, although it is correct, may lead to the completely wrong conclusion that an SR which directly digitizes the antenna output should be a major goal of future developments. Fact is that the digitization of an unnecessary huge bandwidth filled with many different signals of which only a small part is determined for reception is neither technologically nor commercially desirable. 1 However, there is no reason for a receiver to extremely oversample the desired signals while respecting extraordinary dynamic range requirements for the undesired in-band signals at the same time. Furthermore, the largest portion of the generated digital information, which stems from all undesired in-band signals, is filtered out in the first digital signal processing step.
A cognitive radio (CR) is an SDR that additionally senses its environment, tracks changes, and reacts upon its findings. A CR is an autonomous unit in a communications environment that frequently exchanges information with the networks it is able to access as well as with other CRs. From our point of view, a CR is a refined SDR while this again represents a refined DR. According to its operational area an SDR can be (i) a multiband system which is supporting more than one frequency band used by a wireless standard (e.g., GSM 900, GSM 1800, GSM 1900), (ii) a multistandard system that is supporting more than one standard. Multistandard systems can work within one standard family (e.g., UTRA-FDD, UTRA-TDD for UMTS) or across different networks (e.g., DECT, GSM, UMTS, WLAN), (iii) a multiservice system which provides different services (e.g., telephony, data, video streaming), (iv) a multichannel system that supports two or more independent transmission and reception channels at the same time.
Our present discussion is on multimode systems which are combinations of multiband and multistandard systems.
The SDR approach allows different levels of reconfiguration within a transceiver.
(i) Commissioning: the configuration of the system is done once at the time of product shipping, when the costumer has asked for a dedicated mode (standard or band). This is not a true reconfiguration. (ii) Reconfiguration with downtime: reconfiguration is only done a few times during product lifetime, for example, when the network infrastructure changes. The reconfiguration will take some time, where the transceiver is switched off. This may include the exchange of components. (iii) Reconfiguration on a per call basis: reconfiguration is a highly dynamic process that works on a per call decision. That means no downtime is acceptable. Only parts of the whole system (e.g., front-end, digital baseband processing) can be rebooted. (iv) Reconfiguration per timeslot: reconfiguration can even be done during a call. Figure 1 shows an SDR transceiver that differs from a conventional transceiver only by the fact that it can be reconfigured via a control bus supplying the processing units with the parameters which describe the desired standard. Such a configuration, called a parameter-controlled (PaC) SDR, guarantees that the transmission can be changed instantaneously if necessary (e.g., for interstandard handover).
The rest of this paper is organized as follows. In Section 2 we take a look at the most important wireless transmission standards currently used in Europe and specify their main parameters. Section 3 provides an overview of design approaches for mobile SDR terminals, especially over PaC-SDRs. In Section 4 the software communications architecture (SCA), as it is used in the US Joint Tactical Radio System (JTRS), is introduced. The notion of cognitive radio (CR) is discussed in Section 5 and the need for a modified spectrum management in at least some major portions of the electromagnetic spectrum is underlined in Section 6. Finally, in Section 7 we propose the development of technology centric CRs as a first step towards terminals that may sense their environment and react upon their findings. Conclusions are drawn in Section 8.

MOBILE COMMUNICATION STANDARDS
Standards are used to publicly establish transmission methods that serve specific applications employable for mass markets. The presently most important mobile communication standards used in Europe are briefly described in the following paragraphs.

Personal area networks
Bluetooth is a short distance network connecting portable devices, for example, it enables links between computers, mobile phones or connectivity to the internet.
Cordless phone DECT (digital enhanced cordless telecommunications) provides a cordless connection of handsets to the fixed telephone system for in-house applications. Its channel access mode is FDMA/TDMA and it uses TDD. The modulation mode of DECT is Gaussian minimum shift keying (GMSK) with a bandwidth (B) time (T) product of BT = 0.5. The transmission is protected only by a cyclic redundancy check (CRC).

Wireless local area networks
Today, IEEE 802.11b installations are the most widely used in Europe. Also, IEEE 802.11a systems are in operation. If IEEE.11a is to be implemented into an SDR, it should be recognized that its modulation mode is OFDM. It should be pointed out here that there are major efforts towards the development of joint UMTS/WLAN systems which use the SDR approach.
Cellular systems GSM (global system for mobile communication) is presently the most successful mobile communication standard worldwide. Channel access is done via FDMA/TDMA and GSM uses FDD/TDD. The modulation mode of GSM is GMSK with a bandwidth time product of BT = 0.3. Error correction coding is done by applying CRC as well as a convolutional code. GSM was originally planned to be a voice communication system, but with its enhancements HSCSD, GPRS, or EDGE, it served more and more as a data system, too. In Europe, GSM systems are operating in the 900 MHz (GSM 900) as well as in the 1800 MHz (GSM 1800) bands. The North American equivalent of GSM is IS-136. Also, GSM 1900 as well as IS-95, a second-generation CDMA system, are widely used in the US. UMTS (universal mobile telecommunication system) is the European version of the third-generation family of standards within IMT-2000. One of the differences with respect to second-generation systems is that third-generation systems are mainly developed for data (multimedia) transmission. UMTS applies two air interfaces: UTRA-FDD and UTRA-TDD according to the duplex modes used. The channel access mode is CDMA. CRC, convolutional codes, as well as turbo codes [1] are employed for error protection. The basic data modulation is QPSK. Furthermore, it should be mentioned that one mobile user within an UTRA-FDD cell can occupy up to seven channels (one control and six transport channels) simultaneously. Figure 2 gives an overview over the present spectrum allocation for mobile communications in Europe. Besides the spectra of the standards mentioned above, also the spectra allocated to mobile satellite system (MSS) as well as to industrial, scientific, and medical (ISM) applications are specified. The arrows within some of the bands indicate whether uplink (mobile to base station) or downlink (base station to mobile) trafficissupported.
In connection with mobile communications, some additional groups of standards have to be discussed.

Professional mobile radio
PMR standards are developed for police, firefighters, and other administrative applications. The main difference to cellular systems is that they allow direct handheld to handheld communication. The main PMR systems in Europe are TETRA (recommended by ETSI) and TETRAPOL.

Location and navigation
One important feature of mobile terminals is their ability to determine their own location as well as to track location information. Today many location-dependent services rely on the global positioning system (GPS). Currently the European satellite location and navigation system Galileo is under development.

Digital broadcast
There is a possibility that digital broadcast systems may be used as downstreaming media within future mobile communication infrastructures. The main developments in Europe in this area are digital audio broadcast (DAB) and digital video broadcast (DVB).
To have a sound basis for the description of a PaC-SDR that can be switched between different standards, the most important parameters of selected air interfaces are summarized in Ta b l e 1 .

MOBILE SDR TERMINALS
The general structure of a PaC-SDR terminal was already given in Figure 1. Now we are going to look into the PaC-SDR transceiver structure in a more detailed way. The main processing modules of an SDR terminal are the radio front-end, the baseband processing, and the data processing. Since a lot of information about baseband processing can be found in the literature [2,3] and since data processing is out of the scope of this paper, we are going to focus on the front-end here.
The receiver branch transforms the analog RF antenna signal into its digital complex baseband representation.   Figure 3 shows how it works: coming from the antenna, the RF signal is first bandpass filtered and then amplified. Following a two-way signal splitter, the next step is an analog mixing with the locally generated RF frequency in the inphase (I) path and with the same frequency phase shifted by −π/2 in the quadrature (Q) path. Afterwards, the I and Q components of the signal are lowpass filtered and A/D converted. The sampling rate of the A/D converters should be fixed for all signals and has to be chosen in such a way that the conditions of Shannon's sampling theorem are fulfilled for the broadest signal to be processed. Before the sampling rate can be adapted to the signal's standard, the impairments of the two-branch signal processing that come from the analog mixers and filters as well as from the A/D converters themselves have to be corrected [4].
The reason for the Sampling rate adaptation is that the signal processor should work at the minimum possible rate. For a given standard, this minimum sampling rate depends on f c = 1/T c , the symbol or chip rate, respectively. Usually a sampling rate of f s = 4 f c is sufficient for the subsequent signal processing where, after the precise synchronization, the sampling rate may be reduced once more by a factor of 4. If the fraction of the sampling rates at the adaptor's output and input is rational (or may be sufficiently close approximated by a rational number), the sampling rate adaptation can be implemented by an increasing of the sampling rate followed by an interpolation lowpass filter and a decreasing of the sampling rate. If the interpolation lowpass is implemented by an FIR filter, the impulse response usually becomes quite long. The solution is to take the up and down sampling into account within the filter process. Since the upsampled signal is usually generated by the insertion of zeros, the processing of these zeros can be omitted within the filter. This leads to the polyphase structure of Figure 4.Because different input/output ratios have to be realized for different standards, the number of filter coefficients that must be stored may become large. If necessary, a direct computation of the filter coefficients can be more efficient than their advance storage [5]. After the sampling rate adaptation, the signal is processed within the complex baseband unit (demodulation and decoding). The SDR data processing within the higher protocol layers [6] is not considered in the present paper. The SDR transmitter branch consists of the procedures inverse to that of the receiver branch. That is, the signal to be transmitted is generated as a complex baseband signal, from which, for example, the real part is taken to be shifted to the (transmission) RF.
For SDRs, reconfigurability means that the radio is able to process signals of different standards or even signals that are not standardized but exist in specific applications. One method to implement reconfigurability is parameterization of standards. We look at a communication standard as a set of documents that comprehensively describe all functions of a radio system in such a way that a manufacturer can develop terminals or infrastructure equipment on this basis. Standardization is one necessary condition to make a communication system successful on the market, as exemplified by GSM. Standardization pertains to all kinds of communication systems, that is, especially to personal, local, cellular, or global wireless networks. Of course, a standard has to contain precise descriptions of all the functions of the system. Especially for a mobile system, both the air interface and the protocol stack have to be specified. Parameterization means that every standard is looked upon as one member of a family of standards [7]. The signal processing structure of the family is then developed in such a way that this structure may be switched by parameters to realize the different standards.
When developing an SDR, one has to pay attention to the fact that there are substantial differences between the secondgeneration FDMA/TDMA standards (GSM or IS-136), the third-generation CDMA standards (UMTS or cdma2000), and the OFDM-modulated WLAN standards (IEEE 802.11a or HiperLAN/2) (cf. Ta b l e 1 ). Within UMTS, spreading at the transmitter and despreading at the receiver have to be realized. IFFT and FFT operations are necessary for WLAN transceivers. Aside from such fundamental differences, similarities among communication standards are predominant. For example, when looking at the signal processing chains, we remark that the error correction codes of all the secondgeneration standards are very similar: a combination of a block code for the most important bits and a convolutional code for the larger part of the voice bits is applied. Channel coding for data transmission is done by a powerful convolutional code. UTRA, as a third-generation air interface, offers net data rates of up to 2 Mbps and guarantees BERs, of up to 10 −6 for specific applications. To reach these BERs turbo codes are employed for data transmission. Of course, within an SDR all these procedures have to be integrated into a general encoding/decoding structure. Also a common modulator/demodulator structure has to be specified. Solutions to these tasks are given, for example, in [2,3,7].

THE SOFTWARE COMMUNICATIONS ARCHITECTURE
The Joint Tactical Radio System (JTRS) represents the future (mobile) communications infrastructure of the US joint forces. Introducing JTRS stands for an essential step towards the unification of radio communication systems, the transparency of services, and the exchangeability of components. The development of the JTRS is accompanied and supervised by the US forces' Joint Program Office (JPO). Development, production, and delivery continue to be the tasks of competing industrial communications software and hardware suppliers. An important new aspect added by the JTRS set-up is that the suppliers are guided to aim for a most perfect interchangeability of components due to the supervision function of the JPO. The tool used by the JPO is the software communications architecture (SCA) [8], an open framework that prescribes the developing engineers how the hardware or software blocks have to act together within the JTRS. The communication devices emerging from this philosophy are clearly SDRs.
A major group of suppliers and developers of communication software and hardware founded the SDR Forum [9]to promote their interests. The importance of the SDR Forum, however, reaches well beyond the application of SDRs in the JTRS. This is underlined by the SDR Forum membership of European and Asian industrial and research institutions that usually are mainly interested in the evolution of commercial mobile communication networks.
The SCA describes how waveforms aretobeimplemented onto appropriate hardware devices. A waveform is defined by the determination of the lower three layers (network, data link, physical) of the ISO/OSI model. Therefore, waveform is a synonym of standard or air interface. Based on the waveform definition, a transmission method is completely determined. The definition of a waveform, therefore, lays down the modulation, coding, access, and duplex modes as well as the protocol structure of the transmission method.
The SCA defines the software structure of an SDR that may be usable within the JTRS. The underlying hardware as well as the software is described in object-oriented terms. Moreover, the structures of application program interfaces (APIs) and of the security environment are described. Each component has to be documented in a generally accessible form.
The JTRS operating environment (OE) defined in the SCA consists of three main components: (i) a real-time operating system, (ii) a real-time request broker, (iii) the SCA core framework.
When developing an SCA compliant radio device the supplier gets the operating system and the CORBA middleware from the commercial market. The core framework as well as the waveform is developed by him or he also gets it from the market or (in future) it may be contributed by the JPO.
The SCA is the description of an open architecture with distributed components. It strictly separates applications (waveforms) from the processing platform (hardware, operating system, object request broker, core framework). It segments the application functions and defines common interfaces for the management and the employment of software components. It defines common services and makes use of APIs to support the portability of hardware and software components and of applications.
The connections between the applications and the core framework within the SCA are given by the APIs. Standardized APIs are essential in assuring the portability of applications as well as for the exchangeability of devices. APIs guarantee that application and service programs may communicate with one another, independent of the operating system and the programming language used. APIs are waveform specific since uniform APIs for all waveforms would be inefficient for implementations with bounded resources. Therefore, the goal is to have a standard set of APIs for each waveform. The single APIs are essentially given by the layers of the ISO/OSI model.
(i) A PHY API supports initialization and configuration of the system in non-real-time. In real-time it takes care of the transformation of symbols (or bits) to RF in the transmitter branch. In the receiver branch it transforms RF signals to symbols (bits).
(ii) A MAC API supports all the MAC functions of the ISO/OSI layer model (e.g., timeslot control in TDMA or FEC control).
(iii) An LLC API makes available an interface for the waveform's link layer performance (according to the ISO/OSI layer model: data link services) on component level.
(iv) A network API makes available an interface for the waveform's network performance on component level.
(v) A security API serves for the integration of data security procedures (INFOSEC, TRANSEC).
(vi) An input/output API supports the input and output of audio, video, or other data.
The security relevant SCA aspects are written down in the SCA security supplement [8]. The SCA security functions and algorithms are of course defined with respect to the military security requirements of JTRS.

USER CENTRIC AND TECHNOLOGY CENTRIC COGNITIVE RADIO PROPERTIES
The description of CR given by Mitola and Maguire in their seminal paper [10] mainly focuses on the radio knowledge representation language (RKRL). CR is looked upon as a small part of the physical world using and providing information over very different time scales. Equipped with various sensors, a CR acquires knowledge from its environment. Employing software agents, it accesses data bases and contacts other sources of information. In this context, CR seems to become the indispensable electronic aid of its owner. Reading [10] leads to the impression that a CR must be a complex device that helps to overcome all problems of everyday life, all the same whether they are recognized by the CR's owner or not. Of course, these visions as well as the recognition cycle for CRs in [11] are strongly intended to stimulate new research and development. For a more pragmatic point of view, however, we approach CR in a different way. The properties of CRs may be divided into two groups: (i) user centric properties that comprise support functions like finding the address of an appropriate restaurant or a movie theater, recommendation of a travel route, or supervision of appointments, (ii) technology centric properties like spectrum monitoring, localization, and tracking, awareness of processing capabilities for the partitioning or the scheduling of processes, information gathering, and knowledge processing.
From our point of view, many of the user centric properties can be implemented by using queries to data bases. This type of intelligence can be kept in the networks and activated by calls. In transceiver development, much more difficult design choices need to be made to realize the wanted technology centric properties of a CR. Therefore, we concentrate on the latter in the following sections.

THE NEED FOR ADVANCED SPECTRUM MANAGEMENT
Today, spectrum is regulated by governmental agencies. Spectrum is assigned to users or licensed to them on a longterm basis normally for huge regions like countries. Doing this, resources are wasted, because large-frequency regions are used very sporadically. The vision is to assign appropriate r esour c est oendusersonlyaslongastheyar eneededfora geographically bounded region, that is, a personal, local, regional, or global cell. The spectrum access is then organized by the network, that is, by the users. First examples for selfregulation in mobile radio communications are to be found in the ISM (2400-2483.5 MHz) and in the WLAN (5150-5350 MHz and 5470-5725 MHz) bands. Future advanced spectrum management will comprise [12] the following.
(i) Spectrum reallocation: the reallocation of bandwidth from government or other long-standing users to new services such as mobile communications, broadband internet access, and video distribution. (ii) Spectrum leases: the relaxation of the technical and commercial limitations on existing licensees to use their spectrum for new or hybrid (e.g., satellite and terrestrial) services and granting most mobile radio licensees the right to lease their spectrum to third parties. (iii) Spectrum sharing: the allocation of an unprecedented amount of spectrum that could be used for unlicensed or shared services.
If we look upon the users' behavior in an FDMA/TDMA system over the time/frequency plane (cf. Figure 5), we may find out that a considerable part of the area remains unused [12,13]. This unused area marks the pool from which frequencies can be allocated to secondary users (SUs), for example, in a hotspot. In the following we denote the FDMA/TDMA users as primary users (PUs). In order to make the implementation of the SUs' system into the PUs' system feasible, two main assumptions should be fulfilled: (i) the PUs' system is not disturbed by the SUs' system, (ii) the PUs' system remains unchanged (i.e., all signal processing that has to be done to avoid disturbances of the PUs communications must be implemented in the SUs' system).
Now we assume that the transmission method within the SUs' system is OFDM. Figure 6 gives a brief overview over an OFDM transmitter: the sequential data stream is converted to a parallel stream, the vectors of which are interpreted as signals in the frequency domain. By applying an inverse fast Fourier transform (IFFT), these data are transformed into the time domain and sent over the air on a set of orthogonal carriers with separation ∆ f on the frequency axis. If some carriers should not be used, it is necessary to transmit zeros on these carriers. This is the strategy to protect the PUs' system from disturbances originating from the SUs' system. In order to make the SUs' system work, the following problems have to be solved.
(i) The reliable detection of upcoming PUs' signals within an extremely short time interval. (This means that the detection has to be performed with a very high detection probability ensuring a moderate false alarm probability.) (ii) The consideration of hidden stations.
(iii) The signaling of the present transmission situation in the PUs' system to all stations of the SUs' system such that these do not use the frequencies occupied by the PUs.
The solutions to these problems have recently been found [13]. The keywords for these solutions are distributed detection, boosting of the detection results and combining them in the hotspot's access point to an occupancy vector, and distributing the occupancy vector to all mobile stations in the hotspot. (a) The central point in our present discussion is that the SUs system's transceivers have in some sense to act like CRs. They have to sense their spectral neighborhood for PUs' signals and to react upon their findings.

TECHNOLOGY CENTRIC COGNITIVE RADIO
In a more advanced spectrum sharing system, CRs have to apply more advanced algorithms. If a portion of the spectrum may be accessed by any access mode, the following procedure becomes imaginable: starting from the transmission demand of its user, the CR decides about the data rate, the transmission mode, and therefore about the bandwidth of the transmission. Afterwards it has to find an appropriate resource for its transmission. This presumes that the CR knows where it is (self-location), what it is able to do (selfawareness), and where the reachable base stations are. To get more information about possible interferences it should, for example, be able to detect signals active in adjacent frequency bands and to recognize their transmission standards [14].
Summing up, a CR should have implemented the following technologies (possibly among others): (i) location sensors (e.g., GPS or Galileo); (ii) equipment to monitor its spectral environment in an intelligent 2 way; 2 Intelligent means that searching for usable frequency bands is not done by just scanning the whole spectrum.

Control
SDR core

Spectrum monitoring Localization
Information and knowledge processing (iii) in order to track the location's or the spectral environment's developments, learning and reasoning algorithms have to be implemented; (iv) when complying with a communications etiquette, it has to listen before talk as well as to prevent the disturbance of hidden stations; (v) in order to be fair it has to compromise its own demands with the demands of other users, most probably in making decisions in a competitive environment using the results of game theory [15]; (vi) it has to keep its owner informed via a highly sophisticated man-machine interface.
A first block diagram of a technology centric CR is given in Figure 7. One of the most important decisions that have to be made in an open access environment is whether a control channel is to be implemented or not. The most challenging development is that of the information and knowledge processing.

CONCLUSIONS
Standardization of a transmission mode is necessary to ensure its success on the market. From standards we can learn about the main parameters of a system and, by comparing different standards, we may conclude about similarities and dissimilarities within their signal processing chains. Keeping this knowledge in mind, we are able to construct PaC-SDRs. A far more general setup is given by the SCA which is a framework for the reconfigurablity of transceivers and for the portability of waveforms from one hardware platform to another. Starting from SDRs, the next step in the evolution of intelligent transmission devices leads to CRs that may be looked upon as a small part of the physical world using and providing information over very different time scales. Since this approach seems to be very futuristic, we take a look at the urgent problem of efficient spectrum usage. In order to introduce advanced spectrum management procedures (e.g., spectrum pooling), the employment of CRs that at least are able to monitor their electromagnetic environments and to track their own locations is necessary. Therefore, the development of technology centric CRs is proposed here as a first step towards general CRs.