17 Jan 2016

Open Source Comes to Software Defined Radio SDR

Josh Mickolio of Digi-Key & Dr. Ebrahim Bushehri, CEO, Lime Microsystems look at the way open source software is being used for software defined radio design.

It may seem a little strange, given the maturity of the cellular telecoms industry, but there is currently a swell of interest in the design of small base stations at many different frequencies around the world. As networks upgrade from 3G to LTE, and other requirements such as the Internet of things become more important, there is increasing interest in flexible RF hardware and open source software.

This is a global challenge – there are many different bands for LTE and even more for LTE Advanced, and system designers want to have as much flexibility as possible without incurring extra costs. Carriers across Europe as well as Verizon and AT&T are looking to significantly expand their network coverage using a mix of frequencies, particularly for carrying data, leading to significant opportunities for equipment makers.

But there are significant challenges in making these systems cost effective and yet flexible enough to meet all the different protocols and frequencies without having to design, build, test and certify a separate system for each region and each carrier.

Flexible SDR hardware

One of the ways forward for this is field programmable RF, FPRF and software defined radio, SDR. FPRF devices such as the coat reduced LMS7002M from Lime Microsystems provide a chip that is flexible not just in the RF frequencies it supports but also in the signal processing capabilities (Figure 1). The 65nm CMOS process allows a wideband RF transceiver to support frequencies from 100MHz up to 3.8GHz in a cost effective chip. The 2 x 2 MIMO antenna support allows for higher link budgets and full or half duplex to reduce the cost of the end node, allowing the overall system cost to be optimised.

SDR system

Figure 1: Lime Microsystems’ LMS7002M SDR

But the opportunity is also available to make use of the high performance 12bit data converters, highly linear low noise amplifiers and variable gain amplifiers contained in the chip, and even the on-board 8051 processor core. If not being used for a particular signal chain, these are available for designers to use as well, reducing the complexity needed on a board design. This opens up many more opportunities for designers of low cost systems.

Open source software

The availability of FPRF devices is also changing the way the SDR software is developed. Now open source software can be developed and shared by the industry. Lime makes the drivers available as open source so that they can be included in projects that are running Windows or Linux for the baseband.

This is also encouraging different ways to implement the baseband capability. Where the SDR range is restricted to a particular set of modulation schemes and protocols, high-end microcontrollers with communications-oriented accelerators have shown surprising capabilities. For example it is possible to implement an LTE baseband using devices such as QorIQ microcontroller from Freescale Semiconductor.

But for more general SDR designs with a wider range of possible interfaces, design teams have used the same underlying architecture, but with the accelerators implemented in an FPGA.

Software requirements

Supporting a range of OFDM LTE implementations will need fast Fourier transforms and turbo coding, as well as offload for the protocol stack, which can be done in a general-purpose DSP chip, in an FPGA, or in an ASIC. The FPGA gives designers the flexibility to experiment and explore different air interfaces. Open source implementations of the software for different protocols and frequencies can be shared between projects, both inside a company and throughout a wider ecosystem, to accelerate the system design and get products to market faster.

But open source is now not just about software. Myriad has also developed the first open-source interface board to connect field programmable RF transceiver systems directly to FPGA boards. The Myriad RF group is supporting open source implementations of these air interfaces, sharing designs and code to encourage a wider range of system designs and to reduce the barrier to entry. A standard low cost reference platform brings the chip onto a flexible hardware platform that can be used in a wide range of applications.

Multi-standards

The Myriad–RF 1 board is a multi-band, multi-standard RF module, based on Lime’s LMS6002D transceiver. It has one RF broadband output and one RF broadband input with digital baseband interface through a standard FX10A connector, and is set up for UMTS band 1, the most common for 3G implementations (Figure 2). The prototyping board also provides pin headers for power supply, reference clock, analog I/Q input/output and SPI interface connections.

Myriad-RF 1 SDR Board

Figure 2: Myriad-RF 1 SDR Board

The Zipper mezzanine board supports HSMC, FMC and USB connections so that it can connect to both Altera and Xilinx FPGA development platforms – and enables direct connection with a PC, turning it into the baseband controller for almost any size and complexity of network. The hardware is also available as open source, as the design files can be downloaded in KiCad format free of charge, although pre-built versions of the interface board are available via Myriad’s distribution partners.

Lime has also released a new version of the control utility for use with the Zipper board, which is supported on both Windows and Linux. The utility allows for the calibration of the LMS6002D and the LMS7002M transceivers, along with configuration of the chip and the clock generator and PLL. All of this opens up the possibility of compact small cell SDR basestations for a wide range of new applications.

The way forwards

As a result the telecoms industry has been pushing low cost baseband implementations to go with the flexible frontend. But all of this needs a broad ecosystem of hardware and software, and that’s what the open source approach brings. The open source software can be run on the increasing range of baseband processors, with the drivers available for the flexible front end transceivers. Add on boards that connect with standard interfaces further simplify the system development, allowing the designer to concentrate on the details of the specific applications.

Open source software combined with software defined radios or field-programmable RF devices enables manufacturers to serve a number of wireless markets, such as various kinds of IoT hubs, from a single, cost optimized hardware design. This could also allow a system developer to produce a single box such as a femtocell or white-space transceiver that could be quickly configured for any of a wide range of known field environments.

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About the author

Dr. Ebrahim Bushehri (pictured) is CEO, Lime Microsystems and his experience spans over 15 years in directing and managing of design teams for the implementation of high performance ICs within the wireless communication market. He was with Middlesex University Microelectronics Centre, MUMEC collaborating with top tier organizations such as Nokia, Qinetiq (formerly Defence Evaluation Research Agency) and Fraunhofer IAF. He was a professional group committee member of Institution of Electrical Engineering, IEE and is a member of Institution of Electrical and Electronic Engineering, IEEE.

Josh Mickolio, Wireless and RF Product Manager at Digi-Key Corporation, is focused on the wireless industry and its emerging markets. Working in this role for over 10 years has offered the opportunity to engage in an exciting technology sector while supporting the latest engineering developments in Bluetooth, Cellular, WLAN, Zigbee and discrete RF for connected devices and infrastructure design.

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