Mobilian Corp. is sampling TrueRadio, a two-piece chip set that supports the simultaneous operation of 802.11b wireless-LAN and Bluetooth radios. First demonstrated two years ago at Fall Comdex, the chip set will enable the manufacture of a PC Card featuring the two wireless technologies with a bill of materials under $35, the company said.
Mobilian's solution uses a direct-conversion WLAN radio and a low-IF radio for the Bluetooth connection. "Bluetooth has many more dc elements than the .11b radio," so elimination of dc offsets was a main issues in going to full direct conversion, said marketing director Wade Gillham. The simultaneous operation (Simop) architecture enables higher integration, and allowed Mobilian to put most radio elements onto one chip, the MN22100. Only power amplifiers remain off-chip. "The Bluetooth radio's sensitivity is exceptionally high at -91 dBm," said Gillham, while the sensitivity of other chips typically range from -78 to -84 dBm.
The TrueRadio's MN12100 digital baseband chip performs all processing for both .11b and Bluetooth radios, and includes a 40-bit and 128-bit wired equivalent privacy encryption engine in hardware. The baseband chip also includes a direct memory access engine for each transmit/receive channel that requires about 20 percent utilization of the CPU, while Wi-Fi-only solutions typically require 80 percent utilization, Gillham said. The MN12100 includes an ARM7TDMI core performs all analog-to-digital and digital-to-analog conversions on-chip.
Gillham said the complete TrueRadio design was done in-house, which helped Mobilian achieve simultaneous operation by incorporating features that relied on the close coordination of the Wi-Fi and Bluetooth signal paths. This differs from the approach of Intersil Corp. and Silicon Wave Inc., whose Blue802 combo solution that debuted in April features Intersil's 802.11b radio and Silicon Wave's Bluetooth radio. That solution performs media-access control-layer switching between the two chips to present seemingly simultaneous operations to a user. "The TrueRadio relies completely upon highly coordinated digital and analog ends," Gillham said.
This coordination is key to the chip's operation, Gillham said. For active cancellation, the output of the Bluetooth device is tapped off and run through detection and phase-inversion circuitry called Simop-A, and is then supplied to the input of the Wi-Fi radio to cancel any interfering signals that might appear there. Gillham said the effect of a Bluetooth signal located closer than 1 meter from a Wi-Fi radio can be substantial, "so we had to offset this using the Simop-A technique," he said.
This approach essentially relies on a well-known echo-cancellation technique, said Mark Grodzinsky, product manager at Mobilian. The difference of the implementation, and the patents related to same, related to how the cancellation was done at 2.4-GHz carrier frequencies, he said. Echo cancellation is normally performed after down conversion, he said.
Deferred transmission is implemented as an enhanced version of IEEE 802.15.2 that performs MAC-layer switching. The chip uses its awareness of the packet flow and hopping sequence of respective channels to predict when 802.11b packets will be transmitted, then defers Bluetooth transmissions until those packets have been sent. "We don't alter the hop sequence of the Bluetooth radio, but we do know at least three to four hops in advance whether or not packets will be present," said Grodzinsky.
While this technique is suitable for data, Gillham acknowledged that implied latencies make it unsuitable for delivering voice over a Bluetooth connection, as voice relies more on active cancellation.
The MN22100 is built by Maxim Integrated Products in a 0.8-micron BiCMOS process; it runs off 3.3 V and comes in a 68-pin QFN package. The MN12100 is built by Taiwan Semiconductor Manufacturing Co. Ltd. in a 0.18-micron CMOS process and comes in a 208-pin FPBGA.
