Can Bluetooth, Wi-Fi coexist in the future?

Can Bluetooth, Wi-Fi coexist in the future?
Many in the industry see Bluetooth and Wi-Fi fighting for the limelight. Some usage models could be viewed as overlapping, but Bluetooth (WPAN) is a short-range, cable replacement technology and Wi-Fi (WLAN) is an extension of a wired local-area network so they are different enough in power consumption, cost, range, usage environment and applications for both to be successful.

Still, there has been some confusion over RF interference between the two, since they both operate in the same 2.4-GHz frequency band. And there is even some question about whether the FCC would allow both to operate simultaneously in the same device.

To deal with those perceived barriers, WLAN and WPAN working groups have taken on the task of enabling the two technologies to live together and have built the case for their complementary use. Within the IEEE, the 802.15.2 task group is charged with delivering a "recommended practice" that will describe techniques to allow 802.11b (Wi-Fi) and 802.15.1 (Bluetooth) to coexist. It should be noted that these same techniques should be applicable to the recently approved 802.11g proposal. Within the Bluetooth SIG, the Coexistence Working Group is quantifying the interaction between Bluetooth and a variety of interference sources and recommending strategies for enhancing its performance.

To date, the techniques evaluated to improve coexistence can be categorized as:

- Manual switching - Driver-layer switching - Media access controller (MAC) layer switching - Adaptive frequency hopping - System solution (MAC+physical layer +antenna)

With manual switching the user must intervene to enable either the Bluetooth or WLAN connection. This can be quickly and simply implemented, but does not offer transparent connectivity. In addition, a connection to one type of system must be completely disrupted to connect to the other. Needless to say, it is impossible to operate Bluetooth SCO links while connected to the WLAN using manual switching.

Driver-layer switching is used when Bluetooth and WLAN are in the same platform, though not necessarily sharing common silicon. Here, the driver toggles between the two on a periodic or event-driven basis, suspending operation in the other system by going into a temporary suspend state, called DOZE in Wi-Fi and PARK, HOLD or SNIFF in Bluetooth. Since the driver runs under the host operating system, it may not have tightly controlled timing; thus, switching between Bluetooth and WLAN can only be loosely coordinated with data traffic and it would be virtually impossible to coordinate Bluetooth SCO (voice) traffic and WLAN data. Peripherals such as keyboards, joysticks and mice could not use this mode easily because they must be polled; when active, these devices generally are polled at least as fast as the video refresh rate and typically faster. Thus, servicing a number of human interface devices (HID) slaves in a piconet would preclude the driver from servicing them when contending with WLAN traffic, especially if all available slots were used for polling as is typically the case. Nevertheless, companies are working on this type of solution as a quick way to deploy notebook computers that offer at least some level of coexistence.

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MAC layer switching takes this concept a step further by switching between Bluetooth and WLAN in the media access controller. This MAC — in WLAN specifications — is called the link manager in Bluetooth. Since these functions are typically in the digital baseband circuitry an integrated solution is implied, at least on the digital side of the chip set. Because many of these functions are in hardware or embedded software, the timing of packet traffic can be controlled more precisely than with driver switching. As a result, MAC layer switching offers better performance than driver-layer switching, but still has problems maintaining SCO links during WLAN transfers.

Adaptive frequency hopping (AFH) is under extensive study both by the Bluetooth SIG and IEEE. In late 2000, a group of companies petitioned the FCC to amend or reconsider its initial report and order for wideband frequency hopping, also known as ET Docket 99-231, to allow Bluetooth to hop across as few as 15 1-MHz channels in the 2.4-GHz ISM band, where these channels would have to be chosen "intelligently," making the hop set adaptive. This practice would theoretically allow modified Bluetooth devices to operate simultaneously with Wi-Fi devices under many circumstances and is currently permissible under FCC regulations for radios operating under at or below -1.3 dBm of transmit power under Part 15.249 of the rules. However, the regulation must be changed to allow the typical Class 1, 2 and 3 Bluetooth devices to operate in this mode. AFH will provide a viable and important solution to the coexistence of Wi-Fi and Bluetooth in environments where the Bluetooth and Wi-Fi devices are at least a half meter apart.

Adaptive hopping Bluetooth devices will move into that mode based on one or more of several possible interference-detection mechanisms. Among the technical approaches:

- The Bluetooth device gradually adapts its normal operation hop pattern based on observed packet loss.

- The Bluetooth device detects and assesses received signal strength across its wireless environment before commencing operation.

- The Bluetooth device transmits a "test" pattern of packets across the entire spectrum, observes the ratio of lost packets across available channels and locates its adapted piconet in the least active or interference-prone channel.

- If the Bluetooth device is co-located with a Wi-Fi device and can receive the Wi-Fi passband location from the Wi-Fi device, it simply avoids operating within the Wi-Fi passband.

Under the proposed FCC rules, the first three listed items must be performed at least every 30 seconds to adequately respond to changes in the radio environment. This additional complexity will significantly improve Bluetooth voice performance, especially in the presence of WLAN or microwave ovens. Approval of the general rules for AFH by the FCC is expected during 2002 and ratification of a specific AFH technique by the Bluetooth SIG is expected in 2003. Backward compatibility is a requirement and techniques to allow interoperability between Bluetooth 1.1 and Bluetooth AFH devices are being investigated.

The best performance — where both systems can operate at full speed in all modes — requires a system approach, combining advanced antenna designs with RF signal processing to excise interfering signals, use of digital signal processing to maximize signal-to-interference ratios and MAC-level coordination to intelligently manage packet traffic to optimize performance. Using such a system approach, Bluetooth and Wi-Fi can be integrated in the same chip set while still achieving truly simultaneous operation and full compatibility with chip sets from different vendors.

While some in the industry believe interference in the 2.4-GHz ISM band is a serious problem, it simply represents an opportunity to develop innovative, intelligent, robust wireless systems that deliver data reliably. So Bluetooth and Wi-Fi can operate in close proximity and at full performance; both the Bluetooth SIG and IEEE 802.15.2 are crafting specification documents that will allow the wireless industry to deploy robust systems that build on the momentum that both of these standards have developed. In addition, these same techniques can be applied to the emerging 802.11g specification to allow even faster Wi-Fi — up to 54 Mbits/second — and Bluetooth to peacefully coexist. n