...... AN OPEN RF-DIGITAL INTERFACE FOR SOFTWARE-DEFINED RADIOS
...... THIS ARTICLE PRESENTS AN OPEN, COMPLETE RF-DIGITAL INTERFACE APPROPRIATE FOR
SOFTWARE-DEFINED RADIOS (SDRS). THE INTERFACE INCLUDES DATA AND METADATA
(CONTROL AND CONTEXT) PACKETS.THE CONTROL AND CONTEXT PACKETS DESCRIBE THE
ENTIRE RF FRONT END.THE PROPOSED DESCRIPTION HAS A HIERARCHICAL STRUCTURE
AND IS A HARDWARE ABSTRACTION.THE INTERFACE SUPPORTS ADVANCED
ARCHITECTURES SUCH AS SDR CLOUDS.THE METADATA PACKETS CAN BE REPRESENTED
USING FORMAL, COMPUTER-PROCESSABLE SEMANTICS.
...... Wireless signals are centered at a RF front-end details certain RF. The signal-processing operations— As Figure 1 shows, the RF front end is not synchronization, modulation, and so on—are entirely analog; it consists of an analog front implemented on baseband signals centered at end and a digital front end (DFE).1 The DFE zero frequency. Therefore, receivers must does interpolation or decimation to increase or down-convert from RF to baseband. Trans- decrease the sampling frequency. Furthermore, mitters perform the opposite process of the down-conversion and up-conversion can up-conversion. The RF front end, located be implemented partially or even entirely between the antenna and the digital subsys- with digital signal processing (DSP). (See the Todor Cooklev tem, performs the tasks of down-conversion “Glossary” sidebar for a list of terms.) and up-conversion and also does filtering for In a software-defined radio (SDR), the Indiana University–Purdue frequency band selection and amplification. radio access technologies (RATs) at the base- In this article, we advance an RF-digital band level are implemented entirely in soft- University Fort Wayne interface that is open, which allows the RF ware. A RAT can be called a radio protocol, front end and the digital hardware to be although more accurately it is an aggregation seamlessly replaced independently of each of protocols. For radio protocols imple- Akinori Nishihara other. This property is not achieved by pre- mented entirely in software, the digital hard- vious attempts at defining this interface. ware platform must be programmable. Such Tokyo Institute of Technology The developed interface is a stream of platforms can be built with general-purpose data and metadata (context and control) processors, digital signal processors, or field- packets. The proposed interface completely programmable gate arrays (FPGAs). describes the RF front end and can serve as a The International Telecommunication hardware abstraction language for it. The Union (ITU) defines SDR as a radio “that metadata packets can be described using a allows the RF operating parameters includ- simple ontology, such as RDF,and exchanged ing, but not limited to, frequency range, among different cognitive radio platforms. modulation type, or output power to be set ......
0272-1732/13/$31.00 c 2013 IEEE Published by the IEEE Computer Society 47 ...... SOFTWARE-DEFINED RADIO
...... Glossary Baseband: digitally modulated signal that is low-pass in nature Up-conversion: the process of moving a signal’s spectrum con- (that is, not up-converted onto an RF carrier) tent from zero to RF (or to IF) Interpolation: increasing the sampling frequency Waveform software: software that implements a radio-access Decimation: decreasing the sampling frequency technology IF: intermediate frequency Ontology: a general mechanism to describe objects in a certain Down-conversion: the process of moving a signal’s spectrum domain and the relationships between them content from RF to zero frequency (or to IF) Metadata: data that helps interpret signals (data about data) N...... Application Application 1 RAT s/w 1 RAT s/w RAT s/w N Cognitive engine Real-time kernel Drivers
Amplify Antenna ADC Digital User Filter DFE system hardware I/O Convert DAC
Figure 1. Block diagram of a software-defined radio (SDR) system. The architecture of an SDR includes hardware, system software, and service software layers.
or altered by software, excluding changes to The radio in Figure 1 can operate at any operating parameters which occur during the one of N different RATs, such as Wi-Fi, normal preinstalled and predetermined oper- GSM, and LTE-Advanced. The center fre- ation of a radio according to a system specifi- quency and power at the RF level can be con- cation or standard.”2 Therefore, SDR can be sidered completely independent of the radio defined simply as a radio that can implement protocol. A special element is necessary to radio protocols to be defined in the future. determine which radio protocol is active at Therefore, SDR architecture is the science any one time, and at what parameters (such and art of interconnecting hardware and soft- as center frequency and power) this protocol ware components to create such “future- will operate. This element in Figure 1 is the proof” radios. To become future-proof, cognitive engine (CE). (The architecture in radios should have upgradable software and Figure 1 is similar to that in previous work by hardware. Architectures that allow the add- the SDR Forum,3 where the term “switcher” ing, upgrading, and swapping of hardware is used instead of a CE.) Figure 1 can also and software components can be called open. describe radios that aren’t software-defined, If one makes a parallel comparison with com- in which the radio protocols are imple- puter architecture, an SDR’s architecture mented with dedicated hardware that isn’t includes hardware, system software, and serv- programmable. In such hardware radios, usu- ice software layers (see Figure 1). ally every radio protocol is connected using a ...... 48 IEEE MICRO closed RF-digital interface to a separate RF The RF-digital interface front end. The RF-digital interface in Figure 1 In both SDR and non-SDR, there are includes both high-speed data and low-speed other software applications. The difference metadata. Unlike conventional radios, in SDR between these software applications and radio the metadata can’t be assumed to be constant protocols implemented in software (or “radio and known. Figure 2 shows a way to imple- software,” for short) is that the radio software ment the RF-digital interface where the meta- has much greater execution-speed require- data is defined explicitly. In general, the main ments. As a result, the radio software can’t be blocks of a transmitter include a digital-to- made completely independent of the digital analog converter (DAC), followed by an up- hardware, which is indicated by the dotted converter, power amplifier (PA), and transmit lines in Figure 1. This is the main barrier to RF filter. An up-converter or down-converter implementing radio protocols entirely in generally consists of a local oscillator, mixer, software. Another, more subtle barrier to and a filter. The local oscillator’s frequency is achieving SDR is the lack of an open RF- software-defined. After up-conversion, the RF digital interface. If the RF-digital interface is signal is amplified using the PA, for which closed, the radio protocols become hardware gain and output power are the main program- dependent. mable parameters. To adjust the transmit In pursuing an open architecture, several power level, there might be a programmable horizontal and vertical interfaces in Figure 1 attenuator (not shown). The receiver consists can be defined. The Software Communica- of a channel-select RF filter, amplifier, down- tions Architecture (SCA) defines a set of converter, and analog-to-digital converter interfaces that isolate the radio applications (ADC). The important parameters of pro- 4 from the hardware. Another related work grammable RF filters are RF,bandwidth, out- specifies a programming interface of C++, of-band attenuation, and stop-band edge. A VHDL (VHSIC Hardware Description Lan- programmable impedance-matching circuit guage), and IDL (Interface Definition Lan- (not shown) can also be used. A duplexer sepa- guage) that is a set of APIs between the rates the transmit and receive paths in time or waveform application and the rest of the frequency. The antenna and the duplexing radio.5 The goal is to make the waveform technique must also be software-defined in software portable onto different platforms. general, although this isn’t easy to achieve in While these interfaces are also important, practice. In any practical SDR device, not all the RF-digital interface is critical in achieving parameters will be software-defined. Which hardware modularity. Early attempts at defin- parameters are programmable and the specific ing the RF-digital interface for SDR consid- way they are programmed is hardware- ered a hardware bus-type interface.3 One dependent and should be hidden from the obvious limitation of this solution is that a RF-digital interface. hardware bus can operate only at specific To achieve these properties, we propose a clock speeds and data rates. Furthermore, the packet connection between the RF and digital work completely ignored the metadata, subsystems (Figure 2). Over this packet connec- including parameters such as frequency and tion, there are six packet types that implement power.3 These parameters were defined for the interface: data, context, control, extension thefirsttimebyVITA49.6 However, the data, extension context, and extension control VITA 49 standard is incomplete because it packets. The data, context, and control packet doesn’t specify the transmitter side or how to types are mandatory. Each of the extension control the receiver. In this article, we packet types is optional. The packet header describe a complete packet-based interface conveys the packet type. If a decoder does not that lets the RF subsystem and the digital support a particular extension packet type, it subsystem be replaced independently of each ignores its content and could provide feedback other. This interface completely describes the indicating a potential problem. RFfrontend—thatis,itcanbeusedasa In this article, we focus on the case in which hardware abstraction language for the RF the RF front end generates context packets and front end. the digital hardware side generates control ...... NOVEMBER/DECEMBER 2013 49 ...... SOFTWARE-DEFINED RADIO
Upconverter Signal Control amplifier DAC DFE packet packet filter decoder encoder
RF parser Encoder SW control SW control SW control Control Signal packet packet SW decoder encoder control Antenna system Duplexing Digital hardware
Context Signal packet packet GPS encoder decoder SW control SW control SW control RF encoder Parser
Downconverter Signal Context amplifier ADC DFE packet packet filter encoder decoder
Figure 2. The packet-based RF-digital interface. The metadata is defined explicitly.
packets, but more generally, all metadata pack- RF front-end parameters that are software- ets are bidirectional. Context packets include defined. The control packet decoder is tightly parameters pertaining to both the transmitter coupled to the analog front end and trans- and receiver such as RF, IF, bandwidths, ADC lates the metadata to each programmable and DAC sample rates, ADC, and DAC num- device’s hardware settings. ber of bits, gain, voltage full-scale range, filter In addition to the data, context, and con- order, out-of-band attenuation, stop-band trol packets, our proposed solution includes edge, transmit output power, reference point, their extension counterparts. The interface is timestamp, timestamp delay, and location. extensible, and arbitrary additional parame- Collectively, these parameters describe the RF ters can be defined in extension control pack- front end completely. Every time one or more ets. For example, extension data packets of these parameters changes, the RF encoder could carry spectrum information. The sends a context packet containing the current extension context packets can be used to con- value of the parameters that have changed. vey bit error rates, power supply voltages, and The control packets contain metadata receiver noise figures. Note that there might specifying the reference point, timestamp, be power-management circuitry that can and output power pertaining to the transmit- turn off parts of the radio when not in use. ter, and the DAC and ADC sample rate, Although Figure 2 doesn’t show power man- DAC and ADC number of bits, reference agement, if this feature is desired, power- level, voltage full-scale range, RF, IF, local management information can be specified in oscillator (LO) power, bandwidth, gain, filter extension control packets. order, out-of-band attenuation, and stop- At the RF-digital interface, encoders and band edge pertaining to both the transmitter parsers exchange multiplexed streams of and receiver. The control packets include all packets (Figure 2). The interface to the RF ...... 50 IEEE MICRO transmitter has a packet encoder on the digi- as overhead. However, the metadata packets tal hardware side and a packet parser on the need not be sent as often as the data packets. RF side. The output of the packet encoder is Control and context packets need to be sent a stream of multiplexed signal and control only when the relevant metadata changes— packets (and optionally their extensions). that is, the metadata is understood to be per- The parser on the RF side receives this stream sistent between updates. This makes the over- and separates the control packets from the head due to the control and context packets data packets. The payload of the transmit proportional to how often parameters such as data packets is fed to the DAC. The parame- the center radio frequency and bandwidth ters specified in the control packets are trans- change. Because sampling periods are typi- lated into a hardware-specific format for cally on the order of nanoseconds, while every software-defined element. metadata parameters such as center frequency Every circuit element of the RF front end change on the order of milliseconds, the time in Figure 2 can be controlled individually to transmit the metadata will be negligible using a control packet. However, every compared to the time to transmit the data parameter has minimum and maximum val- packets. ues. For example, the RF front end should The proposed interface promotes modu- not be told to tune to a particular frequency if larity by being completely independent of it can’t operate at that frequency. Within one the PHY and MAC used by the wireless sys- radio, the tuning range (if any) is always tem, and by being independent of the specific known, but like all metadata in previous radio techniquetocarrythesepackets.Forexam- designs, it has never been explicitly defined. ple, the connection between the RF front end Now, the control packet encoder on the digi- and the digital system can be implemented tal side must be made aware of every compo- using Gigabit Ethernet. Although the con- nent’s tuning range to produce proper control nection could reside at layer 2 and below, it packets. This awareness can be provided by could also be implemented as a full network context packets or extension context packets. stack with either TCP or User Datagram Pro- If a parameter is fixed, this can be communi- tocol, for example. cated by the tuning range information. Typi- cally, the tuning range must be updated when the RF front end is replaced. Because this Reference points, stream IDs, happens less often than other contexts need to and timestamps be changed, it’s appropriate to specify tuning We specify where the metadata applies range in extension context packets transmit- using the concept of reference points. For ted by the context packet encoder in Figure 2. instance, the center frequency or power-level The RF side also has a packet encoder characteristics are unclear if the reference that’s connected to a packet parser on the digi- point is unknown. Therefore, metadata pack- tal hardware side. The elements in the receiver ets include reference points. chain are connected in a hardware-dependent The ability to multiplex different packet way to the context packet encoder, which in types onto the same connection is supported turn produces proper context packets with by the concept of Stream Identifiers or IDs. information such as RF, IF, bandwidth, and Data and metadata packets that share a sample rate. The context packet encoder can stream identifier have a one-to-one relation- also keep track of GPS data and includes loca- ship, referred to as data-metadata pairing. A tion information in the context packets. The data packet stream and a paired metadata RF packet encoder’s output is a stream of mul- packet stream form an information stream. tiplexed signal and context packets (and For example, a data packet will be transmit- optionally, their extensions). From this stream, ted with the RF parameters specified in the the digital-side parser separates the packet most recent paired control packet. There types and processes them appropriately. could be multiple information streams, for It becomes clear that the metadata (con- example, if there are multiple spatial streams. trol and context) packets are multiplexed All six packet types can include timestamps with the data packets and can be considered to enable synchronization. For example, control ...... NOVEMBER/DECEMBER 2013 51 ...... SOFTWARE-DEFINED RADIO
Bit 31 Bit 0 Bit 31 Bit 0 Bit 31 Bit 0 7 8 9 Signal PA Upconverter DAC packet Header Header Header decoder Stream ID 8 Stream ID 9 Stream ID 9 Ref Point 7 Ref Point 8 Encoder RF parser Timestamp SW control SW control SW control Timestamp Timestamp Signal data Control RF frequency Sample rate packet payload Other Other decoder Translation to hardware settings metadata metadata Trailer DAC Upconverter Data control control packet packet packet
Figure 3. Radio transmitter operating with the proposed control and data packets. Three reference points—a digital IF signal, the DAC output, and an RF signal—describe the RF front end’s topology at a coarse level.
packets are time-stamped so that the software- for clock and time-of-day synchronization, defined elements in the RF front end are con- even without GPS.7 In this way, even radios figured at the specified time instant. that are at geographically different locations The timestamp in data packets sent from can operate synchronously. the RF encoder indicates the local time at the Figure 3 illustrates the concepts discussed time of timestamping. However, it’s often so far. The figure has three reference desirable to use the antenna as a reference points—the digital IF signal fed to the DAC, point and convey accurately the instant a sig- the DAC output, and the RF signal at the nal is impinging on the antenna. A signal that output of the up-converter, which are arrives at the antenna will emerge from the assigned IDs of 9, 8, and 7, correspondingly. RF encoder with a certain delay due to the These reference points describe the RF front delays introduced by the various electronic end’s topology. Two control packets are circuits in the RF front end. This is why there shown—one for the DAC and one for the is a timestamp adjustment field to account for up-converter, which is described by RF, IF, all delays prior to attaching the timestamp. If and bandwidth. An IF frequency of zero can we want the reference point to be the antenna, indicate up-conversion from baseband. We then negative timestamp adjustments can be can describe the DAC using sample rate, included, indicating that events at the refer- number of bits, and reference level. ence point occurred earlier than the time- Figure 3 also shows the format of the stamp specifies. The timestamp adjustment is packets. The first word is a header that con- a single value that’s specific for the RF front tains information about the packet type and end and is useful because it does not have to packet size. The header is followed by a be equal to a multiple of the sampling period. stream identifier, a reference point, and an For data packets sent to the RF front end, the optional timestamp. For data packets, the timestamp refers to the time to start emission time stamp is followed by the payload. For of the data, keeping in mind that the actual context and control packets, the timestamp is transmission will start with a delay that is followed by metadata parameters. equal to the timestamp adjustment. Note that for synchronization timestamps are required, but not sufficient. Together Examples with timestamps, GPS can achieve synchro- We can combine the metadata descrip- nization. However, there are other techniques tions of several circuit elements; ultimately, ...... 52 IEEE MICRO all transmit or receive circuit elements of the SDR architecture can be described with a single Control paired Data packet. Therefore, this technique is in fact a hier- packet packet archical description language for the RF front Header Header end. Then, we can aggregate the control and Stream ID: 9 Stream ID: 9 context to and from each component and gener- Ref. point: 1 Timestamp ate aggregate control or context packets. This RF frequency aggregate control or context packet describes the Signal data IF frequency payload entire RF front end with the reference point being the antenna. For example, we can com- RF power Trailer bine the up-converter and the DAC control Other metadata packets in Figure 3 in a single control packet. Furthermore, the ongoing evolution of 9 RF Digital wireless technology is leading to systems in front-end hardware which there is coordination and cooperation 1 5 among radios to reduce the probability of error. paired For example, if data is relayed, two or more Context Data independent copies are received at the destina- packet packet tion, improving the robustness. Figure 4 illus- Header Header trates the operation of a relay with aggregate Stream ID: 5 Stream ID: 5 context and control packets that are paired with Ref. point: 1 Timestamp the respective data packets. Both amplify-and- RF frequency Signal data forward and decode-and-forward relaying tech- IF frequency payload niques can be used. Other metadata Trailer A related development is distributed antenna systems or SDR clouds,8 in which the RF front end is collocated with the antenna, Figure 4. Wireless relay. According to the whereas the digital signal processing is per- proposed architecture, an RF front end can formed remotely at a data center, conceptually implement a repeater by itself, without the similar to cloud computing. Figure 5 illustrates digital hardware. an SDR cloud that has two information
Header Header Header Header 91 Stream ID 91 Stream ID 91 Stream ID 92 Stream ID 92 RF front end Ref. point 1 Timestamp Ref. point 2 Timestamp 1 51 RF frequency Signal data RF frequency Signal data payload payload ...... Signal- processing cloud Header Header Header Header 92 RF Stream ID 51 Stream ID 51 Stream ID 52 Stream ID 52 front end Ref. point 1 Timestamp Ref. point 2 Timestamp 2 52 RF frequency Signal data RF frequency Signal data ... payload ... payload
Figure 5. A multiple-antenna SDR cloud. The proposed RF-digital interface becomes the interface between the cloud and the RF front end.
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streams, one for every spatial stream. This parameters that must be described. The SDR cloud supports advanced wireless techni- parameters defined in the metadata packets ques—such as joint transmission—that require must be part of any radio ontology, because coordination among the spatial streams. In these parameters describe the entire RF front joint transmission, multiple devices jointly end. transmit data to another device to improve the A radio might be asked, “What are the received signal’s quality, and the defined meta- minimum and maximum RF at which you data packets can accomplish the required coor- can transmit?” or “What are the minimum dination. Note that in SDR clouds, the and maximum bandwidths at which you can distinction between radio and RF front end is operate?” For radios to understand such ques- becoming blurred, and RF-digital interface tions, they must not only speak a common becomes the interface between the RF front language10 but they must also have explicitly end and the cloud. defined metadata. To enable such interac- tions, it is convenient to use a simple ontol- ogy scheme such as the Resource Description Metadata packet descriptions Framework (RDF).10 It describes things using RDF ontology using triplets—for example, subject, predi- The ITU defines cognitive radio as a radio cate, and object. The subject is the resource that can “obtain knowledge of its operational being described. The predicate is a property and geographical environment, established pol- of the subject, and the object field contains icies and its internal state; to dynamically and the value of this property. The mapping autonomously adjust its operational parameters between such ontology descriptions and the and protocols according to its obtained knowl- context and control packets is one to one. edge in order to achieve predefined objectives; This description method allows queries to and to learn from the results obtained.”2 be made and answered. The query is a series Therefore, a cognitive radio must have of triplets in which some of the slots contain domain knowledge of radio communication. variables. For example, a query about the On the basis of this knowledge, the CE can minimum and maximum RF would be: optimize the various parameters and protocols. ThereisadifferencebetweentheCEandother PREFIX sdr:
...... Akinori Nishihara is a professor at the References Tokyo Institute of Technology, where he has 1. T. Hentschel, M. Henker, and G.P. Fettweis, held several academic appointments. His “The Digital Front-End of Software Radio research interests include digital signal proc- Terminals,” IEEE Personal Communica- essing, circuits and systems, and educational tions, vol. 6, no. 4, 1999, pp. 40-46. technology. Nishihara has a PhD from the 2. Definitions of Software Defined Radio Tokyo Institute of Technology. He is a (SDR) and Cognitive Radio System (CRS), fellow of IEEE and the IEICE. report ITU-R SM.2152, International Tele- Direct questions and comments about communication Union, 2009. this article to Todor Cooklev, IPFW, 2101 3. Software Defined Radio Commercial Hand- East Coliseum Blvd., ET 229, Ft Wayne, IN set Guidelines, SDRF-04-S-006-V1.0.0, SDR 46805; [email protected]...... NOVEMBER/DECEMBER 2013 55