AD9361 SDR: software-designed radio introduction

Today’s highly integrated solutions have exceeded the wildest dreams of the originators of a technology that started out as a military program to provide a multiprotocol, wide spectrum single radio architecture. Although an SDR was theoretical at the time, technology and techniques have been developed that make it a practical strategy in simplifying hardware, introducing new features and supporting multiple modulation techniques. This includes upgrades to new methods as they are invented. Today’s SDR is low cost, highly integrated and versatile – a far cry from the cumbersome and expensive discrete designs of the past  Now, thanks to advances that include the revolutionary capability of the AD9361 and the AD9364 - Analog Devices’ single chip RF Agile Transceivers, SDR can also make the claim of high performance. To cap this off, Analog Devices has strived to make the technology accessible to designers with a complete ecosystem for hardware and software based around well-documented, easy to use reference designs.

What is Software Defined Radio (SDR)?

An idea before its time, SDR has a rich history that commenced in 1970 with the concept of a digital receiver. Considerable technical challenges have caused it to struggle to maintain momentum against the rapid transformation in modern wireless systems. The dream of a universal radio was made a reality by Advanced Defense Research (DARPA) in the 90s in an attempt to future proof military communications and foster interoperability. This culminated in the multi-billion dollar US Defense program known as the Joint Tactical Radio System (JTRS) and an abstraction layer called the Software Communications Architecture (SCA). This ambitious project ultimately failed after over a decade of research and development but led to huge steps forward in the underlying technology. Despite these advances, SDR could not truly find a foothold in the commercial consumer product arena due to system cost, size and power consumption. It remained a military and infrastructure oriented technology until recent innovations, including the AD9361 have enabled SDR to be a practical and affordable reality.

 

Figure 1: Introducing the AD9361 Agile Transceiver

The tipping point for SDR use in commercial communications products was the arrival of low cost digital signal processors (DSP) and advances in analog and RF integration in CMOS process. These innovations enabled intermediate frequency (IF) and baseband subsystems to become digitized in second generation cellular systems. Digitization placed elements of a radio architecture onto a Moore’s Law trajectory. Sophisticated algorithmic functionality like error correction, advanced modulation schemes, efficient data coding methods and channel equalization were swiftly introduced as processing power advanced. Increases in processing capability and improvements in reconfigurable logic have led these elements to become soft implementations able to be updated and changed. The AD9361 is the perfect companion to these digital platforms due to its highly flexible configurability, convenient CMOS or LVDS interfacing and proven Linux driver support.  

Advances in SDR

Adaptive radio and cognitive radio are the latest advances in the SDR arena. They are similar concepts and need the AD9361 to achieve the required power and integration.  Radios dynamically adjust transceiver configuration for waveform, protocol, frequency and networking to make best use of the available spectrum rather than being allocated a fixed frequency band or protocol to use. As a device moves around, its environment changes and the RF connection dynamically responds to this by trying to make optimum use of the best service immediately available. This can be accommodated for by the AD9361’s wide frequency band and modulation bandwidths. Some smart radio proposals incorporate adaptive mesh networking, and yet others propose using orthogonal frequency division multiple access (OFDMA) to utilize unused spectrum in a spectrum pooling approach. The ultimate direction of the next generation is still being defined, but when wideband cognitive radio becomes generally adopted, it will revolutionize wireless communication. Regardless of the ultimate direction, all potential strategies share the same challenges associated with flexibility and performance that can be met with the AD9361.

Despite digital processing advances, transceivers still need high performance RF analog stages for the front end amplification, filtering, frequency generation and down-conversion. Difficulty in integrating RF functionality with sufficient performance and flexibility fundamentally limited SDR. Attempts to integrate the RF elements led to tradeoffs in sensitivity, selectivity, linearity and isolation due to limitations in the underlying semiconductor performance. This led the industry to adopt the general belief that gains in flexibility and integration could not be obtained without a sacrifice in performance and functionality. Analog Devices AD9361 changed the paradigm with performance that meets specs like 4G LTE. It provides designers with game changing reductions in size and component count without the compromises in performance designers have come to expect. It is fully configurable and scalable – able to be used synchronously in multi-chip systems for extended capability. The result of the high level of flexibility and integration is improved time to market and reduced power consumption and board area. Target applications include P2P comms systems, femtocell/picocell/microcell base stations, general purpose radio systems, wideband cognitive radio and MIMO. Its key hardware features include:

•    Two fully independent transceivers with separate signal paths able to be used in single chip 2x2 configuration or be synchronized in 4x4, 8x8 or larger systems for applications including beam-forming and MIMO. Each receiver incorporates up to 3 differential/6 single ended inputs. The AD9364 Agile Transceiver is a single transceiver version of the AD9361.
•    Two integrated independent local oscillators (LO) that can enable the transceiver to operate in frequency division duplex (FDD) or time division duplex (TDD) modes. Integrated fractional-n synthesizers supporting a 2.5Hz frequency tuning resolution.
•    Frequency range (70MHz – 6000MHz).
•    Software configurable sampling rates from 547kSPS* to 61.44MSPS with on chip 12 bit ADCs.
•    Integrated AGC, DC offset correction and quadrature correction.
•    Excellent receiver noise figure (2dB @800MHz LO).
•    Excellent transmitter noise floor (-157dBm/Hz).
•    CMOS and LVDS interface options for convenient interface to baseband processor.

*This number is currently under revision

Figure 2: AD9361 Evaluation Boards

(a)    Analog Devices AD-FMCOMMS2-EZB board

 
(b)    Arrow Electronics HSMC ARRADIO board

The AD9361 is enveloped in a complete ecosystem to enable rapid evaluation of the AD9361 and development of SDR products. This is truly a revolutionary way to develop communication systems as it avoids the need for upfront development of a working hardware prototype and software drivers. This enables the development team to focus on the differentiating features of the design rather than the underlying architecture. The SDK has comprehensive software support and simulation modelling. Analog Devices has a long history in RF and SDR, with a simulation environment for the AD9361 based on MathWorks SimRF toolbox Although not specifically for the AD9361, Analog Devices has many technical references concerning SDR like techniques for maximizing dynamic range in receivers. These references discuss important calculations associated with SDR like ADC noise figure (NF) and signal to noise ratio (SNR) as a function of clock jitter. The AD9361 is also supported directly by an Analog Devices Wiki that contains everything from driver source code to step by step guides for worked examples documentation on evaluation circuit boards. From a software perspective, the AD9361 and AD9364 device and ecosystem features include:

•    Easy to configure with software commands – Not intimidating to use.
•    Complete transceiver development ecosystem – Suite includes Linux user application, Linux and bare-metal/no-OS device drivers and reference hardware is available to simplify design.
•    The ARRadio hardware reference design system from Arrow is available. An HSMC mezzanine card that is compatible with the low cost SoCKit Development Kit built around the Cyclone V SOC dual ARM Cortex-A9s.
•    The AD-FMCOMMS2 – EZB EMC hardware reference design is available. An FMC mezzanine card that is compatible with the FMC based carrier cards.
•    SDK has agnostic FMC connectors for easy connection to any baseband processing system.
•    SDK has a user application and reference design that can generate DDS for continuous tones for test outputs, transmit directly from streaming files and capture receiver outputs to display on screen.
•    Linux Industrial input output (IIO) oscilloscope application. See receiver information in time domain, constellation and spectrum FFT. Allows low level peak and poke of registers inside the AD9361 due to its debug option.
•    Modification of device characteristics thanks to Linux is by simple file open, read, write, close operations.
•    A validated Matlab Simulink SimRF model is available that accurately represents the transceiver noise and nonlinearities at different power levels and frequencies. These can be used to predict design performance and tweak settings in a virtual environment for close approximation to actual hardware. The MathWorks Instrument Control Toolbox™ can automate RF measurements, connect to spectrum analyzers and signal generators to test the device using the reference hardware.

 

Figure 3: AD-FMCOMMS2 and ARRadio Evaluation Systems

The AD9361 SDR Signal Path

Dual receiver sections convert RF signals into digital data before being passed to the baseband processor (BBP). Two independent channels allow for multiple input, multiple output (MIMO) systems while sharing a common frequency synthesizer. Three inputs can be multiplexed per receiver, enabling the AD9361 for use in receiver diversity systems where more than one antenna is required. The direct conversion receiver inputs data from the antenna and passes it to the Low Noise Amplifier (LNA). This is followed by matched quadrature amplifiers and mixer elements. Bandpass filters shape signals and remove aliasing spectrum as RF is decimated to baseband. If additional amplification or selectivity is required, external LNA or filtering can be incorporated prior to the device. Automatic Gain Control (AGC) can adjust signal levels automatically or by BBP control. Received Signal Strength Indication (RSSI), DC offset tracking and circuitry necessary for self-calibration are also incorporated. Sample rates of the 12 bit ADCs can be adjusted. Digitized signals are able to be further decimated by a series of filters and a 128 tap FIR filter. The dual direct conversion transmitters receive digital data from the BBP where it is interpolated through a programmable 128-tap FIR filter and a series of interpolation filters. A 12bit DAC with an adjustable sampling rate converts the digital signal to analog. The resulting quadrature channels are upconverted to RF by mixers. The quadrature signals are combined and passed through bandpass filters for shaping. The RF signal is sent to the output amplifier for transmission. Each channel has adjustable attenuators, real-time self-calibration and a Tx power monitor

Cognitive Radio – Military SDR of Today and the Future

Soldiers currently utilize JTSR SDR radios like the AN/PRC-154 Rifleman from Thales.

 

These are clever radios with the ability to self-form and self-heal ad hoc, simultaneous voice and data networks and have the software-defined capability for upgradeability and interoperability. They can also act as repeaters for other radios. Military SDR radios can adapt to a wide variety of protocols and interoperate between military and civilian organizations. In the future, SDR capability will enable the military communications to dynamically adapt to congested and jammed spectral environments and deliver improvements in efficiency and bandwidth by utilizing the best connection available - cognitive radio. The best connection could be using a currently unused spectrum – a dynamically changing condition. As with cellphone networks and narrow band trunking radio systems, civil defense and government authorities can lock users out of the system to ensure service. This is critical in times of civil defense emergency. Military cognitive radios may not just passively utilize spectrum – they may aggressively capture spectrum when required. The way that a cognitive radio will interact with its spectral environment is termed an etiquette.

SDR is seen as offering considerable advantage in situational awareness by optimizing broadband connectivity. Another important area the military sees SDR enabling is update and upgrade. Being able to upgrade radios with the latest advances in coding, feature sets and databases enables the system to be able to adapt to new requirements and capabilities. This should also lead to extension of the life expectancy of the radio system.  The military sees cognitive radio as an extension of their process called the OODA Loop – observe, orient, decide and act. They are an integral part of this process

The military is trending towards the concept of a tactical cellphone as the features associated with modern wireless network connectivity become increasingly important for communication on the battlefront. As with all infantry equipment there is considerable pressure on reducing weight and size while increasing battery life, driving the need for higher levels of integration. These needs can be met by SDR transceivers based on the AD9361. This future tactical cellphone will require cognitive radio capability – it may even be required to be an intelligent radio with inbuilt machine learning to dynamically optimize performance. As with consumer electronics, soldiers also need to be enabled with a variety of connected devices like tablets, laptops and cameras. Smart vehicles, aircraft and autonomous systems like drones and robotics – even exoskeletons may one day require cognitive (and hence SDR) radio capability to interoperate and network on the battlefront. Research is being conducted to enable military SDR radios with the ability to perform electronic countermeasures such as jamming. They are also looking at adding control links for robotics and sensor data monitoring. Background sensor monitoring by cognitive radio networks can provide dynamic terrain data for chemical, biologic or even radiation levels and provide a better total picture of the battlefield.

The Future of SDR Technology

SDR is still rapidly evolving as the need for smarter devices that better use limited and congested spectrum increases. The AD9361 and its lower cost single transceiver cousin, the AD9364, are perfectly positioned to meet the needs of the current and next generation of SDR. The AD9361 is an excellent enabling device for cognitive radio systems. Its high level of integration and flexibility help enable reductions in component count, size and power consumption. Development has never been easier for transceiver products with a comprehensive and easy to use development kit and well documented reference designs. Hardware reference designs are available that are compatible with both Xilinx and Altera FPGA SOC Development Kits. This provides a rapid development path for both RF and BBP based upon low cost FPGA based SOCs equipped with dual ARM Cortex A9. Proven Linux drivers complete the design suite and simplify the software development process.

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