One of the bottlenecks that stops anyone developing their own small cell has been the lack of an open and flexible RF hardware platform. Lime Microsystems aims to provide that as part of a wider ecosystem, enabling experimentation, innovation and customisation for both licensed and unlicensed spectrum. I spoke with the CEO, Dr Ebrahim Bushehri, to discuss this project and his long-term vision.
Quick recap of Lime Microsystems
The company is a UK startup which developed their own RF transceivers chip. They worked with baseband chip suppliers to become incorporated into small cell reference designs and have some commercial deployments.
They are now shipping their second generation fully programmable chip which accommodates a wide range of frequencies from 100kHz through 3.8GHz.
Enabling the Open Source Community
For some time, Lime have been working closely with the Open Source community, specifically focussing on basestations, access points and repeaters serving areas overlooked and underserved in the developing world. They launched MyriadRF.org as a focal point for Open Source RF design to stimulate innovation and accessibility.
About two years ago, Altera became interested. They are a global supplier of FPGA (Field Programmable Grid Array) chips, and invested in Lime to provide a fully flexible platform that can be deployed by anybody.
The platform hardware design is also open source, so anybody could design and build their own board if they wanted to. Even the firmware code loaded into the FPGA is also public, so users can change or resynthesize the design and function if they were so inclined. Most users want a ready built platform, so a standard set of software with APIs is provided as a starting point.
Lime have found people in the community who have taken the code, optimised it, added functionality and then republished it back for others to use. For example, the RF used to manage low power/low bandwidth IoT devices is radically different from that for high speed LTE. The platform supports both.
Crowdfunding the next generation
Lime ran a Crowdfunding project in June 2016 which was successful and exceeded their $500K target by 25% so far. This raised funds to build around 2000 boards for delivery in November. At $249 ($499 including antennas and case), it’s within reach of many developers. UK network EE has bought corporate sponsorship, for which 100 boards will be donated to educational institutions throughout the country.
The hardware could support a wide range of radio protocols: cellular (2G/3G/4G), Wi-Fi (802.11), IoT (Lora, Zigbee etc.) and receive broadcast (Satellite TV, Satellite Weather, GPS, Digital TV, Digital Radio etc.).
2x2 MIMO capability is embedded.
Could you build a small cell with this?
You would need to combine the RF board with a suitable motherboard via the USB 3.0 socket. The amount of processing power will depend on the application – significantly more is needed to support high speed LTE than GSM for example. Baseband processing is also performed on the general purpose processor in the motherboard.
Open source basestation software is available for GSM and LTE, and it is expected that those projects will be supported. Boards have already been shipped to advanced developers, who are expected to seed the community with compatible open source software. We reviewed available OpenSource basestation software last year, finding that the lowest cost hardware development boards cost from around $1,000 to over $2,000.
The higher range of 3.8GHz will allow it to be used with the US shared spectrum scheme proposed for their CBRS band.
A partnership with Canonical (who distribute Ubuntu Linux) plans to introduce an App store with software modules than run on the platform. Just like Apple or Google, some may be free of charge while others are commercial. You can expect announcements to follow about this in the coming weeks, and that the store would be up and running before the boards are delivered. Software may be open or closed source determined by each vendor.
Canonical has created a stripped down variant of the Ubuntu operating system called Snappy, designed to run securely on autonomous machines and other IoT devices – anything from Drones to household appliances. The most radical idea suggested by Maartin Ectors, Canonical’s VP of IoT, is to build a small cell with this module and attach to a balloon or drone, but he does also suggest these could be embedded in street furniture, vending machines, digital signage etc.
Cellular software vendor Quortus has announced they will support the scheme, making a version of their ECX software available. This can provide full core network functionality for 2G, 3G and LTE as well as host Mobile Edge Computing applications. Feature scope, pricing and commercial licensing terms are yet to be made public.
Ebrahim thought that in the future it may not be unreasonable to download an App to run the cellular technology of your choice. While this might be great for lab work, I think it may take a little more than that to design and build a reliable commercial network.
Wide range of RF applications
The board isn’t just designed for small cells of course. Applications on the Myriad website include anything from Ham radio and Satellite weather to Low Energy Bluetooth and especially IoT. There are many programmers worldwide who are familiar with Linux and Open Source programming.
What’s different is that it allows two way communication – there are already low cost SDRs available for receive only – and across a very wide range of spectrum.
There isn’t really anything available elsewhere that matches this two way capability for developers with such wide ranging scope.
Commercial RF equipment generally uses custom designed ASIC chips rather than FPGA for anything other than low volume/niche products. This has been due to lower unit costs and lower power consumption, but at the expense of investing considerable time and money to design the ASIC chip.
Ebrahim notes that the cost of FPGAs has come down dramatically in recent years (we’re talking 10 to 25 dollars when bought in volume now), and that the flexibility of a common platform for so many different applications reduces cost through higher volume of production.
Operation at higher frequencies up to 12GHz is possible with a new extension chip (LMS8001+). The prototype and engineering samples are already available and expected to be in production before the end of the year. This would allow support for the unlicensed Wi-Fi band at 5GHz including the use of MulteFire.
If you want to play with your own Small Cell and have full control of the features and functionality, then this relatively small investment of money (along with probably more costly investment in your time) may be worthwhile. It should certainly prove popular with educational institutions and labs to test and trial new designs and concepts.