The ITA 9 Analog F-Digital Interface
FeatureThe VITA 49Analog RF-DigitalInterfaceTodor Cooklev,Robert Normoyle,and David Clendenen photodiscAbstractOver the last several years, the interface between the digitaland analog RF subsystems of a radio (the analog RF-digitalinterface) has become increasingly important. This article firstdiscusses the different technology alternatives that exist for thisinterface and the rationale behind them. The emergence of cognitive radio technology is identified as a trend that increases theimportance of the analog RF-digital interface. Then the articlediscusses in detail the VME bus International Trade Association(VITA) 49 standard. VITA 49 is a packet-based protocol to convey digitized signal data and metadata (or context data) pertaining to different reference points within a radio receiver. Themetadata includes analog front-end parameters such as RFcenter frequency, bandwidth, IF center frequency, sampling rate,gain, location, etc. This standard allows all information aboutthe spectrum as seen by a radio at its antenna to be described,stored, and transported to other systems. VITA 49 is identified asa hierarchical radio description language that is a new paradigmfor radio architectures.Digital Object Identifier 10.1109/MCAS.2012.2221520Date of publication: 27 November 2012fourth QUARTER 2012I. Introductionost wireless communication systems consistof a digital hardware section (digital back-end)and an analog RF section (analog front-end)located between the digital section and the antenna.This article is focused on the interface between the digital and the analog RF sections of a radio, the RF-digitalinterface. Our perspective is from the viewpoint ofsoftware-defined radios (SDRs)—radios for which thephysical-layer parameters can be set or altered by software [1]. SDRs require reconfigurable hardware andhardware modularity—the ability to change portions ofthe system.Before examining the analog RF-digital interface, letus briefly examine the analog RF and digital sectionsof a radio. The digital section implements the digitalsignal processing at the physical layer (coding, digitalmodulation) and some higher-layer protocols such asaccess protocols, encryption, etc. There is no universal design approach for the digital section of a radio—there is only a set of technology alternatives. In general,the available methods to implement the digital hardware are specialized processors such as Digital SignalM1531-636X/12/ 31.00 2012IEEEIEEE circuits and systems magazine21
The design of the analog RF and the digital subsections of a radio have beenthe subject of considerable research over the years, while much lessattention has been paid to the interface between them.Processors (DSPs), general-purpose processors (GPPs),various multiprocessors, and field-programmable gatearrays (FPGAs) [2]. Application-specific integrated circuits (ASICs), which otherwise achieve the lowest costand lowest power consumption, are not reconfigurableand therefore, in principle, are not applicable to SDRs.These technologies are well-known and each is characterized by a different design approach. The digitalsubsection of a radio may consist of multiple types ofreconfigurable hardware. For example, a DSP, a GPP, andan FPGA might all be present in one radio.The basic functions of the analog RF section arefrequency band selection, amplification, and up/downconversion. Certain key radio parameters such as RFcenter frequency and power are determined entirely bythe RF section. Like the digital section, the RF sectionis similarly characterized by the lack of general designapproaches. Older designs were often implementedwith a large number of discrete components locatedbetween the antenna and the digital hardware. Modernimplementations have far fewer external parts and, inmost cases, use CMOS integrated circuit (IC) technology which has become the least expensive alternative,due to high volume use by the computer industry. It ismuch more challenging to build reconfigurable analog RF components; nevertheless, certain solutionsare available [3]. Overall, the design of the analog RFand the digital subsections of a radio have been thesubject of considerable research over the years [3, 4],while much less attention has been paid to the interfacebetween them.Digital and analog RF are different technologies thathave different evolution paths. As a result, they havebecome increasingly encapsulated in separate modules.These separate modules are, in most cases, ICs. Insome systems, these separate modules are boards, oreven chassis. Because these technologies have different evolution paths, it has become desirable to be ableto replace them separately. This can be accomplishedonly if there is interoperability. Therefore, the interface between them has become a design problem withimportance for both sides.Since the focus here is a standard open interface forwireless devices, it is well worth clarifying that thereare multiple different types of standards that affect thecellular, and more generally, wireless industries. Thebest known cellular standards are Radio Access Technologies (RATs), such as GSM, and the more recentLong-Term Evolution (LTE), and LTE-Advanced [5].Other well-known physical layers are Wireless LocalArea Networking (WLAN) technologies such as IEEE802.11n, Worldwide Interoperability for MicrowaveAccess (WiMAX) or IEEE 802.16, etc. [6]. These accesstechnologies have been making steady progress andcontinue to do so. On the other hand, the developmentof what can be called infrastructure standards has beenmarkedly slower. These infrastructure standards aim todevelop open interfaces between various componentsof wireless systems.These infrastructure standards have been significantly affected by the state of the wireless industry.Traditionally, the wireless industry has been dominatedby a relatively small number of big companies. Thesedominating companies considered proprietary interfaces as a sustainable competitive advantage. Theycould develop all enabling technologies on their own,while smaller companies could develop only someenabling technologies. Anytime new entrants sought todevelop open interface standards, the dominant suppliers worked just as hard to prevent, or at least delay, thedevelopment of such interfaces. Since standardizationrequires some consensus, until the last ten years therewas little progress.Over the last ten or so years, the evolution of thewireless industry has been especially fast. This evolution has been shaped up by several factors. The timeto market for wireless products has been getting muchshorter. The time interval between standardizing onemobile communication system and standardizing thefollow-up system has been getting shorter too. Anothertrend that has affected the industry at the same timeis the success of WLAN technology, offering very highdata rates over unlicensed (and therefore low-cost)spectrum. Consumers demand personal portabledevices that integrate computing and communicationsfunctionalities. These systems require multiple RATs,while state-of-the-art analog RF and digital ICs are optimized for a single RAT. Communication devices startedTodor Cooklev and David Clendenen are with Indiana University—Purdue University Fort Wayne, Fort Wayne, Indiana. Robert Normoyle is with DRS,Gaithersburg, Maryland.22IEEE circuits and systems magazinefourth QUARTER 2012
to be built with multiple pairs of digital baseband ICsand RF ICs, one pair for every RAT that is supported.Because it is very easy to add and remove a pair of ICssupporting one RAT, the resulting architecture can becalled a “Velcro” architecture, illustrated in Fig. 1. Notethat, in the Velcro architecture, there is a separate analog RF-digital interface between every pair of digital andanalog RF ICs.To the extent that they can offer complete solutions,the large vendors can be expected to resist open standards. However, because of this rapid technologicalchange and short time-to-market, the large vendorshave been increasingly able to develop on their ownonly some, but not all, of the enabling technologies.In this environment, these companies have becomeinterested in “open innovation” intellectual propertymodels that would allow them to acquire those piecesof the solution that they do not have, and also sell portions of their technology to any other vendor as longas it is for a profit. Since any company can acquire allthe enabling technologies, competition is based onwho is first to market with a complete solution. Shorttime-to-market requires maximum flexibility and reusability of modules. The standards development process has become another area of competition, sincecompanies are trying to impact the standards processin a direction that maximizes the value of their technology offering.Overall, the environment today is more receptiveto open infrastructure standards and several standardtechnologies have emerged.In this article, we first discuss the evolution ofthe analog RF-digital interface in Section II. Then wedescribe the American National Standards Institute(ANSI)-approved VME bus International Trade Association (VITA) 49 standard in Section III. Section IV isdevoted to both established and promising emergingapplications of this standard. Section V contains theconcluding discussion.II. Evolution of the Analog RF-Digital InterfaceThe analog RF-digital interface is significantly affectedby the placement of the analog-to-digital converter(ADC)/digital-to-analog converter (DAC) pair. For mostof the wireless systems designed in the 1990s, the ADC/DACs were implemented separately or placed on thebaseband chip. This meant that the interface betweenthe baseband and the analog RF ICs had to be analogor mixed-signal. In a mixed-signal interface the transmitand receive I and Q signals are analog, but the controlsignals (typically clock, Tx/Rx enable) are digital. Morerecently, it became apparent that there are benefits toa completely digital interface, meaning that the ADC/fourth QUARTER 2012RF ICDigital ICRAT 1RF ICDigital ICRAT 2RF ICDigital ICRAT NFigure 1. Velcro architecture.DACs are placed on the RF IC. In this way, the digitalchip can be changed to accommodate different wirelessstandards independently of the radio front-end. Placingthe ADC/DAC on the RF IC is more appropriate for complex physical layers such as multiple-input multipleoutput (MIMO), where the number of spatial streamsmay be different from the number of antennas (and RFfront-end chains).Over the past few years, several specificationshave been developed defining the interface betweenthe RF front-end and the digital baseband processing sub-system. These interfaces include the Reference Point 3 specification developed under the OpenBase Station Architecture Initiative (OBSAI) [7], theDigRF interface specification from the Mobile Industry Processor Interface (MIPI) alliance [8, 9], and theCommon Public Radio Interface (CPRI) [10]. There aretechnical and business reasons that several specifications exist. The OBSAI and CPRI specifications are forbase stations, while the DigRF is for handsets. Thesestandards have allowed an ecosystem of vendors toappear, offering interoperable RF and baseband processing modules.To avoid confusion, we note that, in some systemimplementations, there is a finer granularity of interface definitions including such interfaces as the baseband/network interface, and baseband/applicationprocessor interface. While these interfaces can also beimportant, they are not discussed here. From a radioarchitecture perspective the RF-Digital interface ismuch more critical.As mentioned above, CPRI and OBSAI are majorindustry standards in place for cellular radio base stations. Both standards allow splitting a base station intofurther sub-units, particularly into a system unit andIEEE circuits and systems magazine23
Recently, other technological trends have emerged such as DistributedAntenna Systems and cognitive radio, which require re-evaluationof the analog RF-digital interface.into a remote radio head. CPRI is an interface betweenblocks referred to as Radio Equipment Control (REC)and Radio Equipment (RE). The parties behind the CPRISpecification are Ericsson AB, Huawei Technologies Co.Ltd, NEC Corporation, and Siemens AG.The OBSAI standard is broader in scope than theCPRI specification. It specifies the electrical andmechanical characteristics of multiple interfaces insidea base station, developed within the Technical WorkingGroup of the Open Base Station Architecture InitiativeSpecial Interest Group (OBSAI SIG). The OBSAI interfacespecifications covering the radio base station interfacesfor Transport, Clock/Control, Base Band and Radio,together with hardware connection specifications.OBSAI includes also test specifications. OBSAI specifications define a mapping of the base station functionalities onto Control and Clock, Transport, Baseband, RF,and General Purpose blocks for all air interface standards, but the internal structure of the blocks is not inthe scope of OBSAI specifications.Another related work is the IQ interface developedat the Fraunhofer Institute in Germany [11]. The interface enables the bidirectional transmission of digitalIQ baseband signals as well as the transmission ofcontrol and management information. The digital interface allows a transmission of IQ samples with ratesbetween 1.2 and 72 MSamples per second. Thus, waveforms with a bandwidth of approximately 57 MHz canbe transmitted. Moreover, the specification supports adivision of the IQ samples into up to eight transmittersor receivers in the transceiver module, for example inmultiple antenna systems. Thus, the maximum samplerate per transmitter or receiver is reduced, and alsothe maximum bandwidth for the waveform is reducedin proportion to the number of addressed transmittersand receivers. This IQ interface is developed on thebasis of the CPRI and OBSAI specifications and is alsopoint-to-point.DigRF is a series of specifications for the interfacebetween the baseband and analog RF chips for mobilehandsets. These interfaces generally consist of twodifferential digital paths, one for transmit and one forreceive, which are capable of handling several prioritized local channels for configuration, timing controland data. These interfaces are software-defined tosome extent, since they allow some software control24over parameters such as RF center frequency, bandwidth, and power. The main limitation of the DigRFseries of interfaces is that they are designed only forspecific air interface standards, with specific clockspeeds, etc. They are even not backwards compatibleamong themselves. The latest version 4 is for devicesthat implement air interfaces such as LTE and mobileWiMAX and is not backwards compatible with DigRFv3 developed for 3G cellular [9], which in turn is notbackwards compatible with DigRF 1.12 developed for2G [8]. Backwards compatibility is not even a designrequirement because both the analog RF IC and thedigital hardware IC (which is an ASIC) used in handsets are not reconfigurable and are designed for specific air interfaces. If one is replaced, it is very likelythat the other one will be replaced as well. Thereforethe DigRF specifications are close to a coupled interface. They are appropriate only for handsets supporting specific wireless standards, but not appropriateas an interface between the analog front-end andthe digital hardware for software-defined radios.The interface design developed in [12] is improvedcompared to the DigRF specifications, but it is alsonot intended for SDRs. The main conclusion is thatthere was no RF-digital interface for software-definedradios prior to the development of VITA 49, althoughsome have recognized the importance of the RFdigital interface and have observed that a solution isnotably missing [13].Recently, other technological trends have emergedsuch as Distributed Antenna Systems (DAS) and cognitive radio, which require re-evaluation of the analog RFdigital interface.According to DAS there is a central digital signal processing facility (a DSP fabric) and a set of distributedantennas and analog front-ends connected to the central facility by a high-bandwidth network such as optical fiber. DAS is now being considered as a base stationtechnology. One advantage of DAS is that it may becheaper to have one central site for digital signal processing. Another advantage is that an antenna (with ananalog front-end) may be provided at a location whereit may be difficult to put an entire base station. TheDAS is, in fact an SDR cloud, where all the digital signalprocessing is done remotely in a “cloud,” much like theprocessing in cloud computing [14].IEEE circuits and systems magazinefourth QUARTER 2012
The connection between the analog front-end andthe distributed facility may be analog or digital. Thefirst implementations have been analog with proprietary schemes (a familiar evolution path!). Unfortunatelyanalog transmission over fiber suffers from variousdrawbacks such as attenuation, etc. It is also difficultto make multiple signals share the same cable. If thereare multiple antennas, analog DAS will require multiplefibers, while digital DAS can multiplex all the signalsonto one link.The other powerful technological trend is theadvent of cognitive radio. In turn, cognitive radiorequires complete reconfigurability, provided onlyby software-defined platforms [1]. Solutions builtusing the Velcro architecture, sometimes referred toas multi-mode and multi-band, are software-definedto some extent. They can switch between bands andmodes, but once the device is built, the bands andmodes cannot be changed. Cognitive radios requireseamless switching among RATs, reconfigurability andupgradability. Cognitive radios have a functional blockthat can be called a cognitive engine (CE) [1]. The CEmust be aware of all communication parameters, suchas RF center frequency, bandwidth, etc. Some of theseparameters pertain only to the baseband subsystem,some parameters pertain to the analog RF subsystem,and some pertain to both the analog RF and the digitalsubsystems. It is best if the analog RF-digital interfaceis completely independent from any RAT and from anyparticular analog RF or digital hardware technology. Inthis way the interface can support any wireless physical layer, including all currently deployed and futureRATs, as well as real time switching between them.None of the specifications considered so far satisfiesthis requirement. Note that even though these interfaces may include various “control” circuitry, thisdoes not make a radio implementing such interfacessoftware-defined.A software-defined radio (SDR) is a radio in whichthe physical layer functions are software-defined [1].One of the keys to enabling SDR technology is having RFfront-end that is software-defined, i.e. whose main characteristics are software-controlled or programmable.The analog RF-digital interface in Fig. 2 consists oftwo types of signals: high-speed and low-speed. The IFand baseband signals use high-speed connections. Thesoftware control of the analog front-end provided bythe digital hardware uses a low-speed connection. It isclear why this connection is low-speed: the parametersof the analog RF change at a much lower rate than the IFor baseband signal.In most cases, even when the interface is said to be“open,” only the signal data interface is specified andfourth QUARTER lHardwareFigure 2. Block diagram of a software-defined radio transceiver.Front-End BridgeEthernetBridgeDigitalHardwareFigure 3. Ethernet network between the analog front-endand the digital baseband.the control interface is left proprietary. Such interfacedefinitions also do not achieve hardware modularity. Ifone component on either side of the interface changes,the entire interface to it has to be redesigned. Therefore, the low-speed connection must also be includedin the interface definition, and this inclusion should bein a way that is completely independent of any radioaccess technology and any particular analog RF or digital hardware technology.It is possible to connect the analog RF and digitalhardware subsections with gigabit Ethernet [15], asillustrated in Fig. 3. Note that this is just Ethernet,without a higher-layer protocol such as TCP/IP on topof it. Ethernet packets are used to carry digital signalpackets (at IF or baseband level). The use of Ethernet, while low in cost, has significant disadvantagesincluding lack of clock synchronization. It is difficultto keep the radio front-end and the baseband processors synchronized. One solution to this problem isto use an extra wire for clock distribution, but thislargely defeats the original idea of using Ethernet. Aneven bigger disadvantage is that this solution is onlyappropriate for digital radios that do not need the lowspeed control connection and therefore are not fullysoftware-defined.III. Overview of Vita 49VITA 49 is a new standard [16], and is not the resultof the evolution of a previously existing standard.The VME bus International Trade Association (VITA)develops interface standards. It was felt that whatIEEE circuits and systems magazine25
ContextPacketEncoderVRT EncoderAntennaSystemAmplify FilterDownconvertADCSignalPacketEncoderFigure 4. Radio receiver architecture based on VITA 49.eventually became the ANSI-approved VITA 49 standard would be best developed under the umbrella ofVITA. This effort initially was targeted at the defensecommercial off-the-shelf (COTS) industry with anemphasis on the needs of the signal intelligence(SIGINT) community.III.1. Data and MetadataA block diagram of a receiver using VITA Radio Transport (or VRT) is illustrated in Figure 4. VRT definesseparate packets for signal data and context information. Note that the digital interface between the baseband and the ADC includes both data and metadata.Table 1.Standard metadata in VITA 49.Reference Point IdentifierBandwidthIF Reference FrequencyRF Reference FrequencyRF Reference Frequency OffsetBand Frequency OffsetReference LevelGainOver-Range CountSample RateTimestamp AdjustmentTimestamp Calibration TimeTemperatureDevice IdentifierState and Event IndicatorsIF Data Packet Payload FormatFormatted GPS (Global Positioning System) GeolocationFormatted INS (Intertial Navigation System) GeolocationECFF (Earth-Centered, Earth-Fixed) EphemerisRelative EphemerisEphemeris Reference IdentifierGPS ASCII26The data are the ADC samples that carry the downconverted RF signal. These datasamples are time-stamped.DigitalHardwareThe metadata is data aboutVRTthese samples, therefore jusDecodertifying the name metadata(data about data). The metadata is defined to conveyinformation about the radio,MultiplexedPayload andits location and the processContext Packetsing done on the signal priorto generating the signal datapacket. The list of metadata parameters is given inTable 1.The Context Packets convey information about howthe signal was processed by the radio. It contains information to represent the modifications to the signalthat is represented by a physical transform, such asfrequency translation, gain, time delay. The data andcontext packet types are mandatory. There are optionalextension packet types for data and context. If a VRTdecoder that does not support extension packet typesreceives an extension packet, it will only identify thepackets as being of extension type and simply ignoreits content.The metadata packets are multiplexed on the digitalconnection to the baseband and need only to be sentwhen the relevant metadata changes. The metadata isunderstood to be persistent between updates. The overhead due to the context packets is proportional to howoften parameters such as the RF center frequency andbandwidth change. Since parameters such as frequencyand bandwidth change at a much lower rate than the IFsignal, the metadata packets will be a small percentageof all packets.A set of constituent data and context packet streamsis called an information class. Different types of radioswill produce different data and context packet streamsand will be separated according to the informationclass. A VRT Packet Class is the specification of thename, structure, and function of the packets in a VRTPacket Stream.A device may have multiple RF chains. In this case,there would be multiple signal packet streams andmultiple context packet streams, which may have to bemultiplexed, i.e. transmitted over a single shared link.There must be a way to associate each data packetstream with its corresponding context packet stream.This is accomplished with stream identifiers. Data andcontext packets that share a stream identifier can beidentified as paired. Pairing establishes a one-to-oneIEEE circuits and systems magazinefourth QUARTER 2012
DownconverterContext Packet StreamStream ID: 200Reference Point: 100RF Reference FrequencyIF Reference FrequencyBandwidthADCContext Packet StreamStream ID: 300Reference Point: 200Reference LevelSample Rate Other Metadata Other 0200Reference PointADCData Packet StreamStream ID: 300Data PayloadFigure 5. Block diagram of a radio receiver that produces VITA 49 data and metadata packets.relationship between a data packet and a contextpacket stream. Pairing is the only instance in which itis allowable to use the same Stream Identifier for twopacket streams. There are other associations thatallow additional context to be associated with the datapacket stream.For example, bit error rates, spectral occupancyinformation (FFT values), power supply voltages, etc.,can be associated with signal data packets.III.2. Reference Points and Time-StampsA primary reason VITA 49 is useful for spectrumsensing is because it uses reference points and timestamps. It is important to specify a point in the systemwhere the metadata applies, referred to as a referencepoint. For instance, the center frequency or powerlevel characteristics are ambiguous without knowing if the reference point is the input RF signal or theIF signal.Figure 5 illustrates the concept of a referencepoint. The downconverter converts the analog RFsignal from an antenna to IF or baseband. There arethree reference points in the figure—the RF signal,fourth QUARTER 2012the IF signal, and the output of the ADC, which areassigned IDs 100, 200, and 300, correspondingly. Forthe downconverter the reference point is the RF signal. For the ADC the reference point is the ADC input,while the data packet contains samples from the ADCoutput. After the ADC, a VRT-compliant packet streamis produced, consisting of data packets and metadatapackets. There are two metadata packets, one forthe downconverter, and one for the ADC, each onecontaining the parameters of the respective block.The downconverter is described by RF reference frequency, IF reference frequency and bandwidth, andthese parameters are included in the downconvertercontext packet.The values of the IF reference frequency and RFreference frequency are equal to the center of thecorresponding band. Downconversion to basebandcan be indicated by IF frequency of zero. The ADC canbe described by sample rate and reference level. TheReference Level field allows associating signal valuesto power values and is equal to the power (in dBm)of a sinewave at the reference point (the ADC input)that results in a digitized sinewave of amplitude 1.IEEE circuits and systems magazine27
data is also valuable wheninformation from multiplechannels or multiple radiosVRT Systemat different locations needs toContext Packet StreamStream ID: 300be processed.Reference Point: 100It should be noted that theReference Leveltimestamp attached at digitiSample RateRF Reference Frequencyzation reflects all delays priorIF Reference Frequencyto digitization. The timestamp Other Metadatais adjusted taking into accountthe group delay of the analogfilter, the sampling delay ofthe ADC, the digital down300converter (DDC), any otherVRTAnalogVRTSystemgroup delay due to other com300ponents (a cable, for example)that may be present. It should100VRT Systembe noted that while in generalData Packet Streamgroup delay varies with freStream ID: 300Reference Pointquency, the timestamp adjustData Payloadment is constant. Thereforean implicit assumption ismade that the group delay willFigure 6. Block diagram of a radio receiver encapsulated as a VITA 49 system.be constant in the frequencyband of interest.Context information isNote that nonlinearities are not taken into account also time-stamped, but the time-stamp has slightly difwhen specifying the Reference Level.ferent interpretation. When the timestamp mode is fineIf the downconverter and ADC were considered com- resolution, the context packet timestamp conveys theponents encapsulated in a single receiver module then precise timing of events related to the described sigthe same system could be described with the block nal. When the timestamp mode used is coarse resoludiagram shown in Figure 6.tion, the context packet contains events that occurredOne may aggregate the context from each compo- sometime within the data packet. In this way the VRTnent and generate a summary context packet stream compliant radio would know the exact time instantfor the entire system to be sent along with the system’s when parameters such as RF center frequency, bandfinal data packet stream. This aggregate context packet width, power, etc. have been changed. Time-stampingdescribes the entire system with the reference point enables algorithms such as beamforming, direction ofbeing the antenna.arrival, etc.These examples illustrate that VRT can be viewed asAn example of a multi-radio device is shown ina hierarchical system description language. Each com- Figure 7. The IF data packet stream from the VRT packetponent through which the signal flows can be described encoder includes context from the combiner and fromwith respect to how
II. Evolution of the Analog RF-Digital Interface The analog RF-digital interface is significantly affected by the placement of the analog-to-digital converter (ADC)/digital-to-analog converter (DAC) pair. For most of the wireless systems designed in the 1990s, the ADC/ DACs we
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