Software defined radio in the battlefield: Multiband analogue front end brings one chip radio closer to reality

The network of advanced wireless communication technologies is a labyrinth of complex radio frequency (RF) and signal processing technologies, all working in harmony to produce a reliable communications network delivering information at the touch of a button. The history of military communications has been a proliferation of different incompatible radios, where a team may need units for airborne links, satellite communications, a base relay station and an emergency transmitter, as well as application specific roles such as for UAV operation. Each of these radio links serves a vital purpose and leaving one out of the mix would put the operational team at a disadvantage. Yet each radio carries a cost in size, weight and spare battery needs. The problem is further complicated as new requirements and links are added to the list. A universal full duplex radio module is required which can be used across all platforms and dynamically reconfigured in the field. This would lessen the load, provide flexibility and versatility, be efficient and provide longer operating life from a single set of batteries, all enabling significant SWaP (size, weight, power) advantages. But making the 'universal' radio concept into a reality has proven harder than envisioned and providing a suitable analogue front end (AFE) has been very difficult. Until recently, a practical AFE for this type of versatile radio required an array of overlapping parallel channels, each designed to cover a particular segment of the RF spectrum and with bandwidth matched to the intended signal format. This approach is costly in terms of final printed circuit board footprint, weight and power. Software defined solution A software defined radio (SDR) can accommodate various physical layer formats and protocols, encrypt data and convert analogue to digital with software running on a processor, making it perfect for military requirements. Users can control the frequency, modulation, bandwidth, encryption functions and waveform requirements dynamically. SDR also has the flexibility to incorporate new waveforms and functionality into the system without the need to upgrade or replace hardware components. These SDRs would be used not only by the soldier for data access and communicating back to the command centre, but also to create a wide area sensor/mesh networks for position/detection and communication among combat soldiers. The most important component in SDR is the transceiver, which needs to have an exceptionally wide RF range that can be tuned rapidly via software. It also needs to support frequency division multiplexing (FDD) and time division multiplexing (TDD), whilst having a high level of performance in terms of range and reliability, even under noisy conditions. At the same time, such a device should be able to operate under reduced power to minimise the drain on the soldier's battery pack. A highly integrated mixed signal RF IC would make broadband SDR designs smaller, lighter and less power hungry. But the real challenge is the extremely broadband nature of the AFE in the SDR; many spectrum specific front ends would be needed, each of which is a challenge to design and evaluate. Not only that, the final product would fall short in terms of SWaP. Component requirements A new generation of wideband, programmable front end transceivers that supports dual independent transceiver channels is available, including the AD9361 RF Agile Transceiver from Analog Devices. These parts have the potential to meet the SDR challenges. The system processors can reconfigure key parameters (such as bandwidth and RF frequency) dynamically to match the application needs and to deliver optimum results. In addition, it has the potential to serve the fast growing multiple input, multiple output (MIMO) segment, as well as those applications which don't require MIMO. This 10 × 10mm chip scale device has user tuneable bandwidth ranging from 200kHz to 56MHz, along with other features and performance attributes which are needed to build a signal chain spanning 70MHz to 6GHz. Using this 2 × 2 direct conversion component reduces the entire AFE into a single, relatively simple circuit. It interfaces with the host processor via an LVDS or CMOS port for speed and simplicity. The device integrates 12bit A/D and D/A converters, fractional-N synthesisers, digital and analogue filters, automatic gain control (AGC), transmit power monitoring, quadrature correction and other critical functions. The receiver noise figure is also less than 2.5dB, whilst the transmitter's error vector magnitude (EVM) is better than -40dB and the transmitter noise floor is below -157dBm/Hz. For both transmit and receive paths, the local oscillator step size is 2.5Hz, allowing precise tuning. Despite all the functions integrated into the part power consumption is generally around 1W. System design Since a complex design, such as a flexible wideband SDR, involves major circuitry design effort, along with algorithm development and tradeoffs, the AD9361 comes with a reference design optimised for use with Xilinx FPGAs. The board is fully customisable in software without any hardware changes and provides additional options for various MIMO configurations. Analog Devices has also collaborated with The MathWorks, allowing the AD9361 and the AD9361 Filter Design Wizard to be incorporated into its design and simulation tools. The Mathworks has also included the device in its SimRF library, allowing its performance to be simulated in a wireless system design. Since this small, high performance and flexible IC replaces a considerable amount of discrete circuitry, it may seem the need for such discrete designs has been eliminated. This is not necessarily the case; a well designed, carefully debugged and properly laid out discrete transceiver signal chain design for a given segment, format and bandwidth of the SDR's total range may still be able to out perform the AD9631 IC for that particular application, albeit in a larger footprint. Whether system engineers prefer to design and develop SDRs using the Analog Devices FMC platform or to use a commercially available SDR as the platform, the overall product package and performance of the AD9361 may provide a major head start. The IC has been released and has already been designed into two available open market SDR products: the Universal Software Radio Peripheral (USRP) from Ettus Research (http://www.ettus.com); and the Maveriq Multichannel Reconfigurable RF Transceiver from Epiq Solutions (http://www.epiqsolutions.com). Duncan Bosworth is a segment marketing engineer with Analog Devices.