Inside the Softransceiver
A sidebar accompanying our September 2008 cover story, "An RFIC for the world."
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 9/1/2008 2:00:00 AM
RFICs used in today’s cellphones are designed for a specific cellular protocol. That limits a phone’s usefulness. “Cellphone users want their phone to work everywhere,” noted BitWave cofounder and chief marketing officer Russell Cyr. In order to operate in multiple networks, a phone needs additional RFICs, which adds to the phone’s bill of materials. It also means that only one RFIC is in use at any time.
| See the full article, "An RFIC for the world." |
With the Softransceiver RFIC, the engineers at BitWave have designed a device that can adapt itself to whichever wireless technology it encounters, including GSM, EDGE, WCDMA, HSDPA, EVDO, and LTE. To provide seamless coverage between standards such as GSM and WCDMA, the Softransceiver must perform a compressed mode handoff in 125 µs. Additionally, the Softransceiver must be able to switch modes fast enough that end users don’t perceive a delay.
 Figure A. A traditional SDR moves the ADC as close to the antenna as possible. |
The Softransceiver will typically switch frequencies and protocols in about 250 µs, but the time depends primarily upon the synthesizer settling time. In addition to changing wireless protocols, the Softransceiver must also work at frequencies from 700 MHz to 3.8 GHz. To make the transformation, the Softransceiver contains programmable RF function blocks such as RF amplifiers, VCOs, mixers, filters, LOs, baseband amplifiers, and ADCs.
The BitWave device isn’t a typical SDR. Most SDRs use a high-speed ADC to digitize the incoming signal as close to the antenna as possible. Depending on the carrier frequency, an ADC may be able to digitize an RF signal or it may digitize a signal mixed down to an IF. Then, a DSP demodulates the digitized signal (Figure A).
 Figure B. The Softransceiver consists of a programmable RF transceiver that includes functional blocks from the front end through a digital I/Q interface to the baseband processor. |
BitWave takes a different approach. The company’s RFIC replaces a traditional transceiver and includes all the RF blocks from a low-noise amplifier to a digital I/Q interface and includes blocks such as mixers and filters. These subsystems produce a digital I/Q signal that is passed to a baseband modem and demodulated. The baseband modem can then use the information to connect with speakers, microphones, and keyboards (Figure B). The device also connects to an RF front end that contains filters and amplifiers.
To reconfigure itself, the BitWave device contains a database of configuration data. An 8051 microcontroller core, embedded in the chip, manages the device’s configuration and calibration whenever the phone changes wireless interfaces. Built on a 130-µm CMOS process, the BitWave device takes up no more space, nor does it use any more power, than a dedicated RFIC.
Wireless communication standards
EDGE: enhanced data rates for GSM evolution
EVDO: evolution data only
GSM: Global System for Mobile Communication
HSDPA: High-Speed Downlink Packet Access
LTE: long term evolution; part of UMTS (universal mobile telecommunications system)
WCDMA: wideband code-division multiple access
WiFi: wireless local area network (WLAN) products that are based on the IEEE 802.11 standards
WiMAX: Worldwide Interoperability for Microwave Access
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