The new issue of Xcell Journal carries a fascinating article about the use of TV White-Space spectrum to provide medium-range (1 to 5 miles) wireless Internet services at data rates to 16Mbps. The Xcell Journal article “Kintex-7 FPGA Receiver Mines TV ‘White Space’ for new Comms Services” profiles the efforts of a new company named Adaptrum, which is developing base stations and client equipment to use the recently abandoned 400-700MHz spectral band originally reserved for analog UHF TV channels in the US. With the advent of digital HDTV broadcast, this spectral band is now available for other uses and a sort of “long-distance” WiFi is definitely one of the uses being eyed by companies such as Google and Microsoft. Adaptrum is targeting underserved and low-density areas that require low infrastructure costs. Some of the company’s key target markets are in Africa, Asia, the UK, and Singapore. CEO Haiyun Tang founded Adaptrum in 2005 with Berkeley Professor Emeritus Bob Brodersen. It’s a bootstrap story, starting with an Air Force SBIR grant.
Adaptrum has now built its Mark II model client, the ACRS 2.0. It’s a rugged device built in a cast aluminum shell that can be mounted on a pole or a wall. Here’s a photo:
Inside is an RF modem and baseband unit. Gokhan Cosgul, Director of Hardware Systems at Adaptrum, evaluated several design approaches for creating the base station and client hardware. He was already familiar with the use of FPGAs for system prototyping and his search led him to Xilinx just as the company’s Kintex-7 FPGAs were starting to appear. Cosgul chose the Xilinx Kintex-7 XC7K355T FPGA for prototyping Adaptrum’s second-generation design, which uses cognitive radio techniques. The FPGA performs physical-layer signal processing.
In the lab, the choice of an FPGA allowed Adaptrum to bring up its second-generation hardware quickly. Originally, Adaptrum planned to use FPGAs in production for its White Space base stations but has now decided to use them for production client hardware as well. This choice meets the company’s BOM cost goals while vastly simplifying country-by-country customization to meet differing national White-Space standards.
There are some key system-level design lessons to be gleaned from Adaptrum’s experience:
The Kintex-7 FPGA’s power dissipation is low enough to allow the Adaptrum units to operate without fans. This is a huge advantage for hardware designed to be used outdoors.
Use of FPGAs for the final design allows continual design tweaks to be made long after the pcbs have been designed, manufactured, and even fielded. In these early days of White Space testing, there are frequent changes to the White Space standards and to Adaptrum’s designs. FPGAs easily handle this sort of situation.
Adaptrum says that the use of FPGAs “absolutely” gave it a huge time-to-market advantage.
If Adaptrum had used an ASIC as planned for its client hardware design, there would have been a huge up-front cost and it would not be nearly so easy to change the design to accommodate changing standards and continual design improvements.