Overview and Basics of Software Defined Radios INSTRUCTOR: Dr. George Collins By Praveen Kumar Chukka

Software Defined Radio  Software-Defined Radio (SDR) is the technology wherein software modules running on a generic hardware platform consisting of DSPs and general purpose microprocessors.  Which are used to implement radio functions such as generation of transmitted signal (modulation) at transmitter and tuning/detection of received radio signal (demodulation) at receiver.

Architecture  The digital radio system consists of three main functional blocks:  1)RF (Radio Frequency) section 2)IF (Intermediate Frequency) section and 3)baseband section.  The RF section consists of essentially analog hardware modules while IF and baseband sections contain digital hardware modules.

Block diagram

 The RF section is responsible for transmitting/receiving the radio frequency (RF) signal from the antenna via a coupler and converting the RF signal to an intermediate frequency (IF) signal.  The RF front-end on the receive path performs RF amplification and analog down conversion from RF to IF.  On the transmit path, RF front-end performs analog up conversion and RF power amplification.

Contd..  The ADC/DAC blocks perform analog-to-digital conversion (on receive path) and digital-to-analog conversion (on transmit path), respectively.  ADC/DAC blocks interface between the analog and digital sections of the radio system.  DDC/DUC blocks perform digital-down conversion (on receive path) and digital-up-conversion (on transmit path), respectively.  DUC/DDC blocks essentially perform modem operations, i.e., modulation of the signal on transmit path and demodulation (also called digital tuning) of the signal on receive path.

 The baseband section performs baseband operations (connection setup, equalization, frequency hopping, timing recovery, correlation) and also implements the link layer protocol (layer 2 protocol in OSI protocol model).

Benefits of using SDR  Flexibility.  High performance.  Software reusability.  Being software already, It is easy to test individual signal processing blocks, simulate performance and test behavior.

Problems faced by SDR  High power consumption.  The cost of creating ASIC mask is getting more expensive.

GNU Radio Design  GNU Radio is one of the most complete and widely used SDRs for cognitive radio development.  GNU Radio is a software package that provides signal processing blocks, discrete components to perform a specific task.  In GNU Radio, each separate signal processing block is implemented in C++ and gets built into a library.  Python is the language to connect these blocks and generate a flow graph to run a GNU radio application program.  This is performed by “wrapping” the C++ library into a python module so that python can call the C++ functions through the “wrapper”.

Universal Software Radio Peripheral  The USRP is a board that does basic intermediate frequency(IF) processing of up and down conversions, decimation, interpolation and filtering.  Along with the USRP board are a set of daughterboards which serve as RF front end. They allow the output signal to be modulated to a higher frequency and an input signal to be stripped of its carrier.  The USRP provides analog to digital and digital to analog sampling.  It can hold transmitter and receiver daughterboards at a time.  While the USRP and GNU Radio are parallel developments projects, they do not necessarily depend on one another as other SDR platforms use the USRP and other RF front ends(Daughterboards) can use GNU Radio.

Parallel programming in GNU Radio  GNU Radio blocks operate on chunks of data to where large blocks are passed around in memory. This is important to reduce memory and bus access overhead.  A thread-per-block model is now available, where each block runs its own thread as its name might suggest.  When the processing resources are available, each thread is loaded in to its own processor core.  The thread-per-block model is not considered currently because each block may have different computational requirements.  Still, we see a significant improvement in processing capabilities of GNU radio which indicate a positive trend for SDR.

Conclusion  The technology can be used to implement wireless network infrastructure equipment as well as wireless handsets, PDAs, wireless modems and other end-user devices.  However, factors like higher power consumption, increased complexity of software and the benefits offered by the technology should be carefully considered before using SDR technology to build a radio system.

Acronyms  ADC Analog-to-Digital Conversion  DAC Digital-to-Analog Conversion  DDC Digital Down Conversion  DUC Digital Up Conversion  IF Intermediate Frequency  RF Radio Frequency  Rx Receive  SDR Software Defined Radio  Tx Transmit  USRP Universal Software Radio peripheral

References  [1] Artificial Intelligence in Wireless Communications by Thomas W.Rondeau and Charles W. Bostian, Artech House,2009  [2] Software Defined Radio, [URL: broadcastpapers.com]  [3] Overview of Software Defined Radio, [URL: