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Test highlights Bluetooth issues

Posted: 01 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:802.11-based Bluetooth testing? WiMedia? ultrawideband technology? UWB? spectrum analyzer?

The Bluetooth Special Interest Group (SIG) is preparing specifications for the next-generation of Bluetooth known as high-speed Bluetooth, which will support high-speed file transfers and video streaming applications. Initially, the Bluetooth SIG selected WiMedia UWB technology to enable the new protocol. Recently, it announced the option of using 802.11 as an interim solution, piggybacking Bluetooth protocols on existing Wi-Fi radios in portable devices.

This means that WiMedia UWB and 802.11 are candidate alternate MAC/PHYs (AMPs) for the high-speed Bluetooth release. The principle is to allow the existing Bluetooth technology to be used in consumer devices while achieving faster throughput with the use of a secondary radio. However, many are concerned about the interference issues that will result between the 802.11 radio in the Bluetooth device and other IMT-2000 services operating in adjacent frequencies, such as WiMAX, LTE, UMTS and W-CDMA. The worry is that if consumers have a poor user experience with the initial implementation of high-speed Bluetooth using the 802.11 AMP, the long-term risks to the technology's success could outweigh any short-term time-to-market gains. Bluetooth already has a high-profile position in the consumer marketplace, and rushing time-to-market with an interim technology is risky.

WLANs and IMT-2000 do not typically operate simultaneously because they both provide access to the network infrastructure. However, high-speed Bluetooth and IMT-2000 services will support independent applications, and they will often be operating simultaneously. Thus, if the high-speed Bluetooth device is using the 802.11 AMP, it is likely to be running in an environment with IMT-2000 services operating in nearby frequency bands.

Spectrum allocation
One of the initial reasons for concern regarding interference between the Bluetooth 802.11 AMP and IMT-2000 services is that they operate in adjacent parts of the spectrum. The use of 802.11 as a high-speed option for Bluetooth would have severe detrimental effects on other services operating in adjacent licensed frequency bands.

In addition, even though the Bluetooth SIG intends to limit the 802.11 AMP to file transfer applications, once this high-speed radio functionality exists, users may decide to use it for video streaming. Because of their continuous nature, these streaming applications over an 802.11 AMP will have an even higher potential for interference than file transfers.

With multiple services in use simultaneously in a multimode device, a Bluetooth 802.11 AMP may interfere with the operation of other radios, leading to desensitization or even blocking of reception. If any of the blocked services are transmitting time-critical content, such as conversational audio or streaming media, user experience can be poor.

In the United States, the 2.5-2.7GHz range is licensed for use by WiMAX systems. Given its nearby proximity to the 2.4GHz spectrum, WiMAX has little isolation from out-of-band emissions originating from an 802.11 radio, which could hinder high-reliability WiMAX operation.

At present, industry leaders are suggesting a coexistence mechanism be added in 2.4GHz Bluetooth so that it does not transmit during WiMAX operation. If next-generation WPANs use the 802.11 AMP to link desktop peripherals, the results will dramatically exacerbate the interference situation. For example, if a user receives a streaming video WiMAX transmission on a mobile handset, and the nearby desktop connections start transferring a file to an iPod (or a printer), the WiMAX video connection will stop and the user will be staring at a blank screen.

Measurements
Measurements to quantify the interference effect were performed in Staccato Communications' lab using conducted cables (Figure 1). The spectrum analyzer settings were held constant for both measurements to establish a common reference plane. A 15dB attenuator pad was inserted in the 802.11 measurement to drive a lower signal level to the spectrum analyzer front-end and hence get a lower noise floor reading by turning down the input attenuation on the spectrum analyzer.

Spectrum analyzer measurements of in-band and out-of-band emissions for an off-the-shelf 802.11g card with an external antenna connector (red) and a WiMedia UWB radio operating above 6GHz were conducted (green). Figure 2 shows that 802.11 out-of-band emissions in the WiMAX, UMTS and LTE bands exceed the protection limit by 30dB, while UWB emissions are 5dB lower than the protection limit.

Measurements were performed in Staccato Communications' lab to quantify the interference effect from the 802.11 AMP and UWB AMP.

This means that the 802.11-based high-speed Bluetooth will interfere with IMT-2000 services unless they are located about 8m apart for 2.6GHz and 16m apart at 2.3GHz. If they are co-located in a single device, achieving this level of isolation between radios is unrealistic. Thus, the most practical solution in such co-located cases would be to time-synchronize transmission and reception of different radios, i.e. one must be turned off for the other to operate. This could make receiving a WiMAX call while using a high-speed Bluetooth feature impossible to achieve. The UWB emission in the IMT-2000 band is below the protection level and doesn't cause any interference. This means that the WiMedia UWB AMP can be used in conjunction with IMT-2000 services, even co-located in the same device.

Based also on the measurements, the desensitization of a WiMAX receiver increases as the distance between it and an 802.11 AMP decreases. For the setup, the desensitization computations assume -101dBm as the sensitivity for WiMAX MAP messages (critical control messages transmitted in downlink signal).

Recommendations
Real-world measurements show that UWB AMP emission levels are sufficient to protect IMT-2000 services at 2.3/2.6GHz bands as per the protection requirements specified by the European Electronics Communications Committee.

802.11 out-of-band emissions in the WiMAX, UMTS and LTE bands exceed the protection limit by 30dB while UWB emissions are 5dB lower than the protection limit.

Real-world measurements show that 802.11 AMP radios have the potential to interfere with IMT-2000 systems at 2.6GHz even at 8m separation (assuming free space loss and the -115dBm/MHz max allowable interference PSD). For WiMAX, out-of-band emissions from an 802.11 AMP can desense a client station sitting 10m away.

As an industry, we need to take these user experience issues seriously. The success of high-speed Bluetooth relies on the industry understanding and addressing them before deploying them into the market. The appropriate next step would be to conduct adequate coexistence studies between 802.11-based high-speed Bluetooth and licensed services in 2.3/2.5GHz bands. Subsequently, the industry should consider developing mutual interference mitigation mechanisms for high-speed Bluetooth using the 802.11 AMP. Alternatively, the industry should move operation of the 802.11 AMP to 5GHz.

- Roberto Aiello
Co-Founder and Chief Technology Officer

Siddharth Shetty
Communication Systems Engineer

Staccato Communications





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