Practical Chip Design

As if Nintendo didn’t have enough excitement with flying game controllers, conversations with Broadcom, supplier of the Bluetooth and WiFi chips in the Wii suggests that as use of the game console expands another serious issue will confront advanced users. This is once again a system design problem, but it should be laid at the feet not of Nintendo or Broadcom, but of the standards committees who formulated the Bluetooth and 802.11 specifications.

The problem is that when used simultaneously in close proximity the two radios interfere with each other. In theory this is absolutely not true: 802.11 uses a wide frequency band that should make it relatively insensitive to narrow-band interference, and Bluetooth—with Advanced Frequency Hopping (AFH) enabled, is supposed to gracefully move away from any frequency band with a high level of energy in it. Trouble is, the standards folks with their superb modeling skills apparently never actually tried this in the real world, with both radios in the same box. Bluetooth and WiFi don’t coexist well at all.

Unfortunately for Nintendo, the Wii system design relies on Bluetooth to transmit control information from the game controller to the console. Setting aside a very pertinent discussion over the wisdom of putting a Bluetooth link in a control loop, this means that when someone is playing a game, the Bluetooth radio is essentially always on. There is no downtime in which to perform time-division multiplexing with another radio.

That is only a problem if the Nintendo user is also moving a significant amount of data—a download, for instance—over the WiFi port. The yammering Bluetooth radio in the controller will cause serious interference with the WiFi signal, sharply reducing the effective data rate. Not a problem for the occasional packet, but quite noticeable, sources say, for file downloads, and a critical problem if future use models attempt to stream video over the WiFi port.

The designers have done pretty much everything possible under the respective radio standards to lessen the impact on users. The media access controllers behind the two radios negotiate with each other over a proprietary, Broadcom-designed bus, examining the packet queues and priorities, and attempting to create windows in the traffic for multiplexing. This requires some fairly sophisticated quality-of-service decisions, such as looking for natural gaps in speech so that lower-priority data or best-effort packets can be swept through.

The two radios also try to stay out of each other’s way in the frequency band. If the WiFi chip is on, the Bluetooth radio, which normally uses a center band in the Bluetooth spectrum, moves to outer bands, in effect spreading its spectrum a little. This seems to help. But if the Bluetooth radio turns on enhanced data rate 2.0, the power level goes up considerably, and the WiFi error rate becomes an issue again. Similarly if the WiFi radio is trying to carry streaming video—which along with game-control data and speech has top priority—there’s just not much room for negotiation.

Nintendo and Broadcom are looking for specific solutions. But there’s a larger issue here. If contention problems are hard to resolve within one vendor’s box, how is the digital living room every going to function? Some people in the industry are suggesting that there will have to be an industry-wide standard for quality-of-service negotiations between competing radios operating in different boxes in the same general area. This protocol would not only allow Bluetooth and WiFi radios in the same box—potentially on the same SoC—to both function, but would allow what today are nightmare scenarios for wireless designers, such as multiple Bluetooth links between devices and remotes, 802.11n WiFi, and several varieties of cellular handsets to all work simultaneously in the same small living room, with most of the boxes stacked in the same cabinet.

Reportedly discussions have begun between vendors, but at a very tentative level. And it’s not even clear yet that all these different radios physically can share the same spectrum successfully with anything short of enforced time-domain multiplexing. That could present huge political problems for the industry, or, alternatively, infuriating and unresolvable problems for consumers.