Software Defined Radio Architecture
Contents
1. Other methods class View superclass The View of the application def _ init _ _ self ctrl panel mainbox Constructor Other methods 40 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau class Controller superclass The controller of the application def _ init __ self frame panel vbox argv Constructor Other methods if _name__ __main__ The main program Create the Controller Controller Repeat Wait until an event and handle the event It is our first Python example A introduction to Python can be found in Appendix B The code starts with import clauses that bring in code from libraries used to build the application There are normally imports specific to the model view and controller One Python class is defined for each aspect of the application namely Model View and Controller Every such class may be built using a super class contained in one of the imported libraries Each class embeds a constructor with the signature def _init__ self The constructor is invoked automatically when an instance of the class is created The constructor of the model contains code that builds and initializes the data and logic of the application Likewise the constructor of the view assembles and starts the GUI The code of the controller creates one instance of the model and one instance of the view The controller also reg2. Software Defined Radio 2015 Michel Barbeau of forthcoming widgets is switched to horizontal by creating the layout man ager radioBox Using radioBox a widget of type float_field is created in the container panel Using the method check_input_and_call the listener SetFrequencyKV is bound to the widget Note that the listener is a method that belongs to the object denoted as ctrl The program execution is mo mentarily transferred to method BuildFrequencyTuning When the method returns the content of radioBox is added to mainbox def BuildGui self GUI builder Use the form framework bind action listeners to widgets self controls form form Create an horizontal box self radioBox wx BoxSizer wx HORIZONTAL Add space self radioBox Add 5 0 0 Create a floating point number field for frequency input and display self controls freq form float_field parent self panel sizer self radioBox label Frequency weight 1 callback self controls check_input_and_call self ctrl SetFrequencyKV Add space self radioBox Add 5 0 0 Build the frequency tunning button self BuildFrequencyTuning Add space self radioBox Add 5 0 0 Add the box radioBox to mainbox self mainbox Add self radioBox 0 wx EXPAND The method BuildFrequencyTuning creates the vertical placement layout manager tuningBox A button widget is created to provide frequency in crement value selection 100 KHz KHz or MHz T
3. The former performs updates on the latter The controller may also get data elements from the model The controller is translated to Python as the class Controller It is de rived from the GNU Radio class std_top_block which is required to make the controller instance the super node of the flow graph class Controller stdgui2 std_top_block A standard GNU Radio application must have the constructor method _init__ In addition to a self reference formal parameters consist of a reference to a frame which is a graphic container for the panel a reference to the panel of the application its layout manager vbox and a list of other arguments argv that can be parsed to extract command line parameters not used in this example def __init __ self frame panel vbox argv The constructor begins by initializing the parent class stdgui2 std_top_block __init__ self frame panel vbox argv Afterwards the initial index of the frequency tuning increment is initialized such that by default MHz is used The model is instantiated The controller is connected to the flow graph created by the model The view is created parameterized with references panel and vbox The model is then initialized to frequency 91 5 MHz in the FM broadcast range self incr 2 default is MHz self model Model self connect self model 46 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau self view Vie
4. Using the class fftsink2 from the GNU Radio API add a spectrum display widget to the SDR application developed in this section The new view should be similar to the one shown in Figure 2 12 General Coverage FM SDR Cig File Signal from USRP2 Options O Average Peak Hold Set dB div 1dB div 2 dB div AT ot 5 dB div a reas a 10 dB div l T T T 20 dB div 80 078 kHz dB 79 228 100 50 0 50 1uU kHz j dB pma ee ponpon oo NEOSCBYHGSWNE Frequency 91 5M Increment MHz Up Down Volume Figure 2 12 General coverage FM receiver with spectrum display 2 3 SDR to Computer Communications SFlexRadio Systems SDR 1000 o Figure 2 13 The Flex SDR 1000 We examine hereafter the communications between a SDR unit and a com puter There are several possibilities SDR to computer communications can be 48 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau analog or digital In the analog case on the received path the SDR generates the in phase and quadrature signals over separate lines They are represented at an IF that is within the range of a personal computer sound card The in phase and quadrature lines from the SDR are connected to the stereo line in of the computer The sound card an ADC converts the input analog signal into a stream of digital s
5. with the label RF2 Below there is a port for pluging in an Ethernet cable con necting the USRP2 unit to a computer The hardware structure of the USRP2 is modular It contains two main types of boards a motherboard and one or several daughter boards The motherboard consists of two Analog to Digital Converters ADCs two Digital to Analog Converters DACs Digital Down 32 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau Converters DDCs Digital Up Converters DUCs an interconnection unit and a network interface There are two ADC input channels Each ADC produces a stream of 14 bit samples at a rate of 100 Mega samples per second Msps According to the sampling theorem sampling rate is twice the bandwidth this means that the ADC has a bandwidth of 50 MHz The sampling is centered at the Intermediate Frequency IF of an analog radio receiver connected to the input of the ADC The ADC maps the sampled bandwidth to the range 25 MHz to 25 MHz There are two DAC output channels Each DAC accepts a stream of 16 bit samples at a rate of 100 Msps also corresponding to a bandwidth of 50 MHz The DAC produces the IF signal for the analog radio transmitter connected to its output The input channels are paired together to carry the in phase and quadrature of the same signal likewise for outpout channels In other words the USRP2 has one input complex signal channel with a bandwidth of 50 M
6. 1 The Model View Controller Pattern data updates model controller view updates actions Figure 2 8 The MVC pattern We base the overall software architecture of a SDR on the Model View Controller MVC pattern 73 see Figure 2 8 The elements of the resulting code are easily identifiable It is a pattern also adopted by most of the GUI environments although the exact terminology may vary Generally speaking the model is a source of data e g a table in a data base a file or a system variable Data sources can be multiple In a SDR application the model con sists of hardware component abstractions and software signal processing blocks The view is the GUI e g a window It is presented on a monitor The view contains buttons to control the radio operation and fields to display the radio frequency and other parameters Views can be multiple The controller insures CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 39 2015 Michel Barbeau Software Defined Radio the consistency between the model and view s The controller pulls data from the model e g the current frequency It handles actions on the view e g operator inputs It updates the model e g retune the radio and view s e g update the current frequency according to the information contained in incoming data elements and actions In other words the controller is the SDR main procedure We discuss hereafter the organization of a software applicati
7. Chapter 2 Software Defined Radio Architecture A SDR is a real time system The inputs to the system are actions performed by the radio operator and data produced by active elements present in the SDR such as analog to digital converters The outputs are graphical depictions of the radio and digital representations of radio and audio signals To prevent undesirable effects such as flickering or chopping the outputs must be pro duced promptly and consistently with the inputs Outputs must be generated to provide an effect of continuity to the radio operator even though the SDR is functioning according to a discrete model Real time performance is achieved by a combination of hardware resources and a carefully designed software ar chitecture This chapter introduces the reader to the concepts used to deal with the complexity and structure of SDRs i e the architecture of SDRs This chapter aims to provide a big picture of all the elements involved in the design of SDRs The hardware and software architectures of SDRs are discussed at length The hardware architecture is reviewed with examples in Section 2 1 From Section 2 2 the emphasis is on software architecture Both high level design and detailed design are examined with a discussion of SDR to computer communications in Section 2 3 and the structure of a signal processing block in Section 2 4 2 1 Hardware Architecture Since the early 2000s the availability at reasonable cost of
8. Hz and one output complex signal channel also with a bandwidth of 50 MHz The network interface a Gigabit Ethernet port insures the communications between the USRP2 unit and computer There is a flow of frames containing samples from the USRP2 to the computer and another flow of frames loaded with samples from the computer to the USRP2 unit The interconnection unit contains a multiplexor and a demultiplexor On the input channel there is a DDC The DDC performs decimation of the sample stream Decimation reduces the rate of the stream by a selectable integer factor m In the stream only every mth sample is retained The others are dropped Effectively decimation has a low pass filtering effect and reduces the bandwidth of the signal by the same factor m The resulting decimated signal is still centered at zero Hz The highest frequencies are filtered out Exercise 2 1 Using decimation the DDC reduces by a factor m the data rate of the input channel originally at 100 Msps What is the bandwidth of the signal produced by the DDC On the output channel there is a DUC By interpolation it increases the date rate by a selectable factor m New m 1 sample values are inserted between every mth sample from the original stream Their value is estimated The DUC maps a baseband signal to the DAC input rate i e 100 Msps The USRP2 unit has two slots for pluging in daughter boards One of them is for a radio frequency transmitter an
9. SDR platforms for experimenters has considerably progressed Such platforms are ideal for exper imentation when the hardware and software are open That is to say when hardware schematics are available and involved firmware and software are open source When these conditions are absent then sooner or latter the experi menter faces obstacles hardly surmountable because of a lack of information When detailed hardware design and source code are available everything can 31 2015 Michel Barbeau Software Defined Radio be understood Any solvable problem can be solved It is only a matter of time Two SDR platforms that meet these conditions are the Universal Software Ra dio Peripheral and High Performance Software Defined Radio They are both introduced hereafter This section also includes an overview of SDRs used for tactical communications 2 1 1 Universal Software Radio Peripheral Figure 2 1 The USRP In this subsection we review the hardware architecture of the Universal Soft ware Radio Peripheral version 2 USRP2 14 The USRP2 bridges the analog world of electromagnetic radio waves and digital world of computers on which the actual processing of signals is done e g demodulation and modulation The USRP2 is an independent hardware box connected to a computer using a point to point network connection actually an Ethernet cable Figure 2 1 is a photo of a USRP2 unit Top right there is a SMA connector for the antenna
10. amples that are processed on the computer On the transmit path the computer generates a digital signal in phase and quadrature at an IF within the range of the sound card The sound card a DAC converts the digi tal IF into an analog IF The computer stereo line out is connected to the SDR input On the SDR an analog up converter up shifts the signal to the desired frequency Figure 2 13 pictures the FlexRadio Systems SDR 1000 It is a 100 watt transceiver covering all HF amateur radio bands plus the six meter and two meter bands with all options installed SDR to computer communications are analog The sound card does the ADC and DAC functions POWER USB PHONES MIC elehadio yaken FLEX 1500 Figure 2 14 The Flex SDR 1500 In the digital case the SDR has its own sound card i e ADC and DAC On the receive path the SDR produces the digital in phase and quadrature signals at an IF On the transmit path the SDR accepts digital in phase and quadrature signals at an IF SDR to computer communications are achieved using a digital interface The SDR and computer are linked either using a USB a FireWire or an Ethernet point to point line We hereafter discuss the case where an Ethernet point to point line is used for SDR to computer communications Figure 2 14 pictures the FlexRadio Systems SDR 1500 It is a five watt transceiver covering all HF amateur radio bands plus the six meter band The SDR to computer communications are digital usi
11. cation waits grabs all the replies and builds a list of discovered USRP units using their MAC address By MAC address the application may also select a particular unit among all the replies On the control channel the computer successively sends the commands to set the 3 receiver gain 4 center frequency 5 decimation rate and 6 trigger the transmission of the flow of samples 7 Samples arrive in frames The payload part of each frame contains a number of samples Frames are handled individually in the sequence of arrival 2 4 Signal Processing Block Detailed Design In this section we explain how a GNU Radio signal processing block works Two programming languages are involved Regarding importation of blocks from the GNU Radio API the importer i e the program using them is written in Python as shown in Section 2 2 The blocks are however implemented in C The reason being that Python is a script programming language relatively easier to use than C The latter however does achieve much CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 51 2015 Michel Barbeau Software Defined Radio better performance This is important in digital signal processing involving a lot of repetitive calculations The GNU Radio class gr flow_graph_block is the base abstraction of signal processing blocks It has ports that may be of type float or complex Every specific block is defined as a specialization class derived directly or indi
12. d the other is for a radio frequency receiver The receiver is connected to the two ADCs Similarly the transmitter CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 33 2015 Michel Barbeau Software Defined Radio Figure 2 2 A USRP2 unit with TVRX daughterboard is connected to the two DACs Figure 2 2 shows a photo of a USRP2 unit with the top cover removed The TVRX daughterboard is mounted It is a receiver originally designed for TV sets covering from 50 MHz to 860 MHz The actual signal bandwidth is six MHz much lower than the 50 MHz that can be handled by the USRP2 The flow of an input signal in a USRP2 unit is pictured in Figure 2 3 The signal flows from left to right An antenna captures the electromagnetic signal A receiver converts the electromagnetic analog signal into an electric analog signal at the IF Two ADCs convert the IF analog signal into a digital representation i e two streams of samples Each stream is at 100 Msps 14 bits per sample One branch produces the in phase signal while the other outputs the quadrature signal The bandwidth of the signal is centered at zero Hz and spans from 25 MHz to 25 MHz The DDCs reduce both the sample data rate and bandwidth by a factor m The bandwidth of the resulting signal is also centered at zero Hz and spans from 25 m MHz to 25 m MHz The samples are then buffered and sent to the computer using an Ethernet ETH network interface The flow of an output signal is si
13. e stored in Ack In other words the field Ack contains the number of the next expected frame The Fixed payload header contains a Channel number three bit flags used for computer to USRP frames only and a Timestamp The field Channel is used for multiplexing different flows of frames on the same Ethernet link For example there is a flow for computer to USRP control commands There are also data flows i e the streams of samples Each of them is identified by a logical channel name Immediate is the first of the three bit flags When set it indicates that the frame should be sent now Start of burst is the second flag and marks the beginning of a sequence of frames logically related together The end of the sequence of frames is marked with the flag End of burst set Timestamp contains either the time when the frame should be sent by the sender or when it was received by the receiver 50 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau The payload is a variation point Figure 2 16 illustrates a frame where the payload starts with a Configure receiver command The first line starts with a one byte field indicating the exact type of command opcode In this example the opcode would be CONFIG_RX Next there is a one byte field indicating the length in bytes of the command Len The one byte RID field is a request identifier incremented from one command to another On the second line the
14. ent of the function is the IP address of the USRP unit The second parameter is the data type of the samples generated by the USRP unit i e 32 bit float complex numbers The third parameter indicates that the USRP is the source of one stream of sample numbers The sample rate is set to 250 Ksps USRP set up self anUSRP uhd usrp _source device_addr addr 192 168 10 2 hardware identity io_type uhd io_type COMPLEX_FLOAT32 input data type num_channels 1 number of stream from the device Set the sample rate self anUSRP set_samp_rate 250e3 250 Ksps Secondly using the function blks2 wfm_rcv the demodulator is created named aDemodulator The first argument is the input sample rate The second number is a decimation factor This means that the sample rate at the output of the demodulator is 15 625 sps 250 Ksps divided by 16 The bandwidth is lowered to 7 812 5 Hz aDemodulator blks2 wfm_rcv 250e3 16 Thirdly using the function audio sink a sound card is instantiated The sink is named aSoundCard The first argument is the input sample rate in sps The second argument is the label of the sound card default is used if left unspecified aSoundCard audio sink 15625 Finally the last step consists of interconnecting all together the nodes of the flow graph self connect self anUSRP aDemodulator aSoundCard CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 43 2015 Michel Barbeau Software Defined Radi
15. espect to the latest versions of GNU Radio The automatically generated documentation GNU Radio X Y Z C API 19 where X Y Z is the number of the release of GNU Radio is up to date but mainly covers the material from the C perspective There are rules facilitating the mapping of C elements to Python Given a GNU Radio C class with a named such as gr class there is a friend constructor function of the form gr_make_class that is mapped by SWIG to a Python function of the form gr class Methods defined in C are mapped to Python under the same names and analogous signatures The similarity of C and Python helps a lot in understanding the automatically generated documentation In the Python environment help can be obtained about what is available in a module e g audio by first importing it i e with the statement from gnuradio import audio and entering the following CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 53 2015 Michel Barbeau Software Defined Radio command help audio Q 54 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau Ethemet header Destination address Source address USRP2 transport header no Fixed payload header Frequency Decimation factor Generic request reply command Op cote S Figure 2 16 A computer to USRP frame A J RX GAIN gt ff 4 RX FREQ gt ID REPLY 2 5 RX DECIM
16. ety of Great Britain 13 contains a detailed review of the hardware architecture of the HPSDR Also check the web site openhpsdr org Documentation about the GNU Radio framework including examples and source code can be obtained at gnuradio org GNU Radio Python examples have been published in Chapter 18 of the book Cognitive Radio Com munications and Networks edited by Wyglinski et al 83 Computer network concepts are required to reach a full understanding of the protocol architecture issues related to the communications between a computer and a SDR using a network interface such as Ethernet On computer networks we refer the reader to the books of Peterson and Davie 57 Stallings 69 and Tanenbaum and Wetherall 74 Youngblood s series of articles A Software Defined Radio for the Masses are pioneering works on SDR for experimenters They cover SDR to computer analog communications and sound card programming 86 87 88 89 Critcal reviews of the FlexRadio SDR 1000 have been authored by Ford 17 and Lindquist 39 Silver wrote a critical review of the FlexRadio Systems SDR 1500 65 Lindquist wrote a review for the FlexRadio Systems FLEX 5000A SDR part is the same at he FLEX 5000c 40 These reviews reflect pretty well the benefits and challenges associated with the utilization of SDRs The GNU Radio classes are documented in the Simple User Manual for GNU Radio 3 1 1 19 Unfortunately this useful manual is not up to date with r
17. field Bitmask contains bits indicating which of the following fields apply For instance if this command is issued to set the frequency then the bitmask has the configure frequency bit on There is a two byte field to store the ra dio gain Gain An eight byte field is loaded with a desired radio frequency Frequency A four byte field specifies a decimation factor Decimation The end of the payload is marked by a Generic request reply command In that case the operation code is end of operation The field Len is interpreted as in a Con figure receiver command The fields left blank are not used 2 3 2 Receiving Samples Figure 2 17 illustrates a scenario in which an application running on a computer communicates with a USRP unit to trigger the transmission of an incoming flow of samples These are typically the communications involved when the execution of a receiver application is launched Both the computer and USRP unit have an Ethernet interface and are connected by an Ethernet cable Each of them has a unique six byte MAC address A standard broadcast MAC address is also involved in the scenario The application searches the Ethernet for all USRP units 1 On the control channel a command is sent with the opcode ID The command is encapsulated in a frame sent using the broadcast MAC destination address 2 Every present USRP unit may reply with a frame encapsulating a command with the opcode ID REPLY For 50 milliseconds the appli
18. gr_vector_void_star typedef std vector lt const void gt gr_vector_const_void_star class gr_block public virtual int general_work int noutput_items gr vector_int amp ninput_items gr_vector_const_void_star amp input_items gr_vector_void_star amp output_items 0 i class my_block public gr_block public int general_work same as above signal processing return noutput_items i The parameter noutput_items designates the number of samples that are pro duced in each output stream upon completion of the execution of the method 52 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau The parameter ninput_items specifies the number of samples present in each input stream The pointer input_items refers to the arrays of input samples one array for each input stream The pointer output_items refers to the arrays of output samples one array for each output stream Upon completion of its execution the method general_work returns the number of samples actually produced in each output stream 2 5 Further Reading Mitola s book Software Radio Architecture Object Oriented Approaches to Wireless Systems Engineering was the first to address the SDR architecture topic 50 Detailed information about the architecture of the USRP family of SDRs is available at the web site www ettus com Chapter 8 of the Radio Com munication Handbook from the Radio Soci
19. gt 6 START FRAME 7 lt Figure 2 17 Receiving samples from a USRP unit CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 55
20. he event handler named SetIncrValue is associated to the widget Using the function forms incr_decr_buttons the widget idb is instan tiated It comprises a plus button and a minus button for incrementing or decrementing the frequency The event handlers IncrementFrequency and DecrementFrequency are associated to the two buttons All the aforemen tioned handlers are methods that belong to the controller The widget idb is added to the tuningBox In turn tuningBox is added to radioBox def BuildFrequencyTuning self Tuning button builder Create an vertical box tuningBox wx BoxSizer wx VERTICAL Create a button for increment value selection forms forms button CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 45 2015 Michel Barbeau Software Defined Radio sizer tuningBox parent self panel choices 0 1 2 labels 100 Hz KHz MHz value 2 label Increment callback self ctrl SetIncrValue Create and Up Down buttons for in decrementing the frequency idb forms incr_decr_buttons self panel self ctrl IncrementFrequency self ctrl DecrementFrequency label Up Down tuningBox Add idb 0 wx EXPAND Add the box tuningBox to radioBox self radioBox Add tuningBox 0 wx EXPAND The Controller The controller is connected to the view It provides event handlers to the view They handle events generated by the GUI following actions executed by the user The controller is also connected to the model
21. ichel Barbeau Software Defined Radio hopping an important threat in tactical communications The achievable data rate is 4 Mbps enough for digital voice It operates in the 225 MHz to 440 MHz 610 MHz to 960 MHz and 1 35 GHz to 1 85 GHz bands Figure 2 6 The AN GRC 245A V HCLOS Radio Picture courtesy Ultra Elec tronics Figure 2 6 pictures the model AN GRC 245A V HCLOS Radio 76 HC LOS means High Capacity Line Of Sight Radios It can operate in the 225 MHz to 440 MHz 1 25 GHz to 2 69 GHz and 4 4 GHz to 5 GHz bands Over distances of up to 40 kilometers it can achieve a data rate up to 34 Mbps This data rate is enough for real time video streaming It integrates networking support using the Internet Protocol This unit is designed for reliability very robust and can sustain very harsh environmental conditions Figure 2 7 features the next generation HCLOS radio It integrates features of the two aforementioned models 2 2 Software Architecture This section is about the software architecture of SDR applications both high level and low level The model view controller pattern the foundation of several software applications comprising a Graphical User Interface GUI is used and discussed at length 38 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau Ext gapt INTERUNIT Figure 2 7 The next generation HCLOS Radio Picture courtesy Ultra Elec tronics 2 2
22. isters with the view handlers of events Each event handler may be implemented as an individual method The registration typically involves binding an event handler to a GUI element called a widget Hence when this widget is activated e g a button is pressed the corresponding event handler is invoked Events may be registered in the controller When supported by the API events handlers are registered when each widget is created The controller may need to invoke explicitly a method to provoke the display of the view not required in this example Every class may also enclose other methods needed for its implementation The main part of the program delimited by the statement if _mname__ _main__ creates an instance of the controller and contains the main pro gram loop The loop is designed such that it repeatedly blocks and waits for an event to occur When an event occurs the execution is transferred to the corresponding handler The code of the handler is run to completion before any other event is handled If necessary events may be queued It is an event based execution model The complete Python code of the application can be found in Appendix C In the following three sections we discuss some details of the implementation of the three main classes CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 41 2015 Michel Barbeau Software Defined Radio The Model The model represents the data and logic of the application do
23. main The con troller may perform updates on the model such as changing the frequency Upon request the model may return data elements to the controller such as the current frequency In principle the model should be independent of the view and controller On the other hand the view is by nature dependent on the model while the controller depends on both the model and view N Model aDemodulator aSoundCard Ne b Figure 2 9 Model of a SDR FM Broadcast Receiver The logical model of the SDR general coverage FM receiver leveraging the GNU Radio API and USRP SDR is pictured in Figure 2 9 The radio is con structed according to a hierarchical flow graph model The nodes of the graph are signal processing blocks The links are data flows between signal processing blocks A flow graph has a source and a sink Blocks have input ports and output ports Ports are typed short float or complex Ports are connected to gether with links In a general a link is point to multipoint i e an output port can be linked to several input ports Linked ports must all be of the same type The signal processing blocks are implemented in C libraries The graph is programmed in Python The GNU Radio API includes glue code that bridges the Python and C worlds In Figure 2 9 the signal flows from left to right The node anUSRP represents the source of signal For processing the output port of node anUSRP is connected to the input
24. milar In Subsection 2 2 we carry on with the architecture of software for the USRP2 34 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau Antenna Buffer Figure 2 3 USRP2 input signal flow Exercise 2 2 Assuming that four bytes are required to represent every signal sample two bytes for the in phase and two bytes for the quadrature and an overhead of approximately 20 is needed for headers and control signals estimate the maximum data rate needed to support the communications between a USRP2 unit and a computer Redo the same calculations assuming a sample rate of 250 Ksps 2 1 2 High Performance Software Defined Radio Figure 2 4 shows a High Performance Software Defined Radio HPSDR unit with the top cover removed The HPSDR is a modular system with several possible configurations Every module is named after a character of the Greek mythology that evokes its function This particular unit includes a Metis Eth ernet interface a Mercury receive module a Penelope transmitter module the first three vertical cards the Alex RF bandpass filters grey module placed right and a PennyWhistle power amplifier placed behind the vertical cards Using a Gigabit Ethernet link the Metis interface insures the communications between the HPSDR unit and a computer Using a 130 Msps 16 bit ADC the Mercury receive module directly converts from analog to digital the zero to 55 MHz
25. ng a USB point to point line Figure 2 15 shows the FlexRadio Systems SDR 5000c It is a 100 watt transceiver covering all HF amateur radio bands plus the six meter two meter and 70 centimeter bands with all options installed SDR to computer communications are digital using CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 49 2015 Michel Barbeau Software Defined Radio Figure 2 15 The Flex SDR 5000c a FireWire point to point line 2 3 1 Ethernet based SDR to computer Communications The USRP and HPSDR use an Ethernet line to interconnect with a computer Ethernet is a multiple access medium analogous to a radio channel The data is sent in units called frames that are transmitted one by one The frame format used for the communications between a USRP unit and a computer is pictured in Figure 2 16 A frame consists of four parts Ethernet header USRP transport header Fixed payload header and payload a Config ure receiver command and a Generic request reply command in this example The Ethernet header is 14 bytes and consists of a six byte Destination address a six byte Source address and a two byte Type field The latter indicates the kind of data encapsulated in the Ethernet frame The flow of packets is con trolled The USRP transport header contains a one byte sequence number field seqno numbering the current frame and a one byte acknowledgement field Ack confirming to the peer reception of all frames up to the valu
26. o The View The view renders the graphical representation of the application and enables interaction with the user From the model it gets and displays data elements There are clickable widgets The view waits for user actions When one occurs an event reflecting the action done by the user is generated float_field File Frequency incr_decr_buttons Figure 2 10 General coverage FM receiver view The MVC view aspect is coded in Python as the class View The constructor method init__ has three formal parameters in addition to a self reference The parameter ctrl is a reference to the controller of the application The parameter panel refers to a graphical area in which are placed the upcoming widgets see Figure 2 10 The parameter mainbox represents a panel layout manager By definition the widgets are placed vertically one after the other The values of the formal parameters are stored in attributes The constructor transfers the control to method BuildGui def _ init __ self ctrl panel mainbox Constructor Store parameters into attributes self ctrl ctrl self panel panel self mainbox mainbox Build the user interface self BuildGui The method BuildGui uses a mechanisms called form An attribute named controls stores a mapping of widget to listener bindings A listener is a method invoked when an action is performed on the associated widget Placement 44 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE
27. omputer The demodulated audio is returned to the HPSDR for digital to analog conversion and speaker output That is to say the audio is captured and rendered on the HPSDR unit 36 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau Therefore in addition to the two radio double signal streams there are two 48 Ksps streams for audio output and one 192 Ksps stream for audio input at 16 bits per sample in all cases Assuming that an overhead of approximately 20 is needed for headers and control signals estimate the data rate required to support the communications between a HPSDR unit and a computer 2 1 3 Tactical Communications Systems The coverage of the topic SDR hardware architecture wouldn t be complete without a look at some high end platforms Although they may be not within the means of the average experimenters they do illustrate what the state of the art is in this field We introduce SDR systems used in tactical communications systems all featured radios are from Ultra Electronics In all the models the computer and radio frequency hardware are integrated within the same unit Figure 2 5 shows the AN GRC 512A V Radio Relay 77 This unit includes Figure 2 5 The AN GRC 512A V Radio Relay Picture courtesy Ultra Elec tronics facilities for dealing with radio jamming e g spectrum analysis and frequency CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 37 2015 M
28. on for the USRP The application is a FM receiver It is assumed that the daughterboard TVRX2 50 to 860 MHz receiver is plugged into the receiver slot of the USRP The application runs on the computer connected with an Ethernet cable to the USRP unit The development is based in on the GNU Radio Application Programming Interface API and framework 20 Under the Linux operating system this API is available in the Python programming language In the sequel we outline a typical architecture of an SDR application in that environment We then walk through the code of its detailed implementation in the Python programming language 2 2 2 Architecture Outline The architecture of the application software is according to the MVC concept It is a way to deal with complexity It acknowledges and separates three as pects the domain specific logic GUI and flow of execution Separate aspects individually represent smaller problems that are easier to address and solve A MVC based application is best programmed with code units directly cor responding to the three MVC aspects The code is just easier to read assuming that the reader as a good grasp of the MVC pattern In an object oriented language such as Python each aspect may be coded as an individual top level class The program is outlined as follows Import the GNU Radio modules impo aii class Model superclass The model of the application def _ init _ _ self Constructor
29. port of node aDemodulator The output port of node aDemodulator is connected to the input port of the node aSoundCard All three nodes are embedded in a super node of type Model This is translated in Python as a class named Model class Model gr hier_block2 Class Model is defined as a specialization of class hier_block2 a kind of signal processing block that can combine other blocks The core of the logic of the class Model is in method __init__ Let us look at the details of this method To perform complete initialization the method starts with a call to the constructor of the parent class 42 CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE Software Defined Radio 2015 Michel Barbeau def _ init __ self gr hier block2 _ init _ self General Coverage FM Receiver gr io_signature 0 0 0 input signature gr io_signature 0 0 0 output signature Every block has a set of input streams and output streams For a hierarchical block the input signature and output signature gr io_signature 0 0 0 in both cases specify the number of input ports and number of output ports and their data type For this block there are none It is solely a container The construction of the flow graph of the model starts hereafter In this example there are only three nodes and one super node Firstly a source node is created named anUSRP using the GNU Radio function uhd usrp_source The USRP unit is selected by IP address The first argum
30. rectly from the base class gr flow_graph_block A specific block is coded as three related pieces of code The first piece is stored in a C header file with extension h and declares the interface of the block e g the methods and their signature The second piece is stored in a C program file with extension cc and defines the implementation of the block e g the algorithms of the methods The program is compiled into a dynamic library The third piece specifies which methods from the C declaration of the block should be visible to the Python importer i e it is a view on the block in Python A tool dubbed Simplified Wrapper and Interface Generator SWIG produces the code in Python interfacing with the C code in the dynamic library Let s now have a closer look at an outline of the definition of a block in C The use of arrays to buffer samples is an important aspect of the implementation in C Conceptually the flows of samples entering and leaving a block are non interrupted streams In reality samples enter and leave in arrays Every signal processing block declares and defines the method general_work The runtime environment repeatedly calls this method Every call is parameterized with an array of input samples and outputs an array of samples Here is an outline of the code of a block include lt vector gt include lt stddef h gt typedef std vector lt int gt gr_vector _int typedef std vector lt void gt
31. spectrum A DDC reduces the sample rate to 192 Ksps i e a bandwidth of 96 KHz The Penelope transmitter module converts signals from digital to analog RF It consists of a DUC and a DAC The RF is in the 1 8 MHz to 55 MHz range at 500 milliwatts It is very low power in the HF range The PennyWhistle power amplifier raises the level of the signal up to 20 watts The Alex RF bandpass filter on the receive path consists of high pass and low pass CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 35 2015 Michel Barbeau Software Defined Radio Figure 2 4 The HPSDR with top cover removed hardware filters that select solely the signals falling within the HF amateur ra dio bands The signals outside the HF amateur radio bands are attenuated On the transmit path low pass filters attenuate the levels of the harmonics falling outside the amateur radio bands Exercise 2 3 In a HPSDR system there are four streams of samples There are two 192 Ksps streams representing the in phase and quadrature of a received radio signal at 24 bits per sample There are two 48 Ksps streams representing the in phase and quadrature of a transmitted radio signal at 16 bits per sample In contrast to the USRP2 the HPSDR has an onboard mono ADC connected to a microphone and an onboard stereo DAC connected to a speaker For example on the receive path the signal samples are sent from the HPSDR unit to the computer The signal is demodulated on the c
32. w self panel vbox self SetFrequency 91 5 1le6 The sequel of the code of the controller consists of event handlers There is a handler processing a type of events generated when the plus button is pressed i e IncrementFrequency The handler DecrementFrequency runs when the minus button is pressed Handling of a change of frequency requires glue code to connect the event listener SetFrequencyKV to the event handler SetFrequency See the Appendix C for details The main part of the program creates the standard GNU Radio applica tion stdapp actualized with the class Controller The standard application whose code is imported from the GNU Radio library instantiates the controller Method MainLoop is invoked on the application instance The loop blocks and waits for the next event occurrence calls the corresponding handler and waits again app stdgui2 stdapp Controller General Coverage FM SDR app MainLoop Exercise 2 4 Using the classes multiply_const_ff and quantized slider field from the GNU Radio API add volume control to the SDR application developed in this section The new view should be similar to the one shown in Figure 2 11 General Coverage FM SDR File Frequency Increment MHz 147 3M a Up Down B Figure 2 11 General coverage FM receiver with volume control CHAPTER 2 SOFTWARE DEFINED RADIO ARCHITECTURE 47 2015 Michel Barbeau Software Defined Radio Exercise 2 5
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