Interference
With multiple services in use simultaneously in a multimode device (as in the usage scenarios above), a Bluetooth 802.11 AMP may interfere with the operation of the 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, the user experience can be poor. As an illustration, we will consider the use of WiMAX in conjunction with an 802.11 AMP.

In the US, the 2.5 to 2.7GHz range is licensed for use by WiMAX systems. Given its nearby proximity to the 2.4GHz spectrum, WiMAX has little isolation from the out-of-band emissions originating from an 802.11 radio, which could dramatically hinder high-reliability WiMAX operation. Interference to WiMAX is being taken very seriously by designers for reasons which can be understood better from an interference study report published by the European Electronic Communications Committee (ECC) in February 2005. This report named The Protection Requirements of Radiocommunications Systems Below 10.6 GHz from Generic UWB Applications (also known as 'Report 64') includes IMT-2000 services operating between 2.5 and 2.7GHz as one of the 'victim' services of interest.

Importantly, although this study focused on UWB being the interferer, the analysis was based purely on transmit power spectral density (PSD), and it made no assumptions about the transmitter's signal characteristics. Hence, any interfering energy with PSD higher than the protection limit specified in this report was concluded to have the potential to interfere with IMT-2000 client stations.

The protection limit was derived in the following manner. The maximum allowable interferer power level at the receiver of the IMT-2000 subscriber unit (without causing degradation) was found to be -115dBm/MHz. For the use case shown in Figure 3, where a protection distance of 36cm is considered, a free space path of ≈30dB (@2.5GHz) results in transmit PSD protection limit of -115+30 = -85dBm/MHz to provide adequate protection to the IMT-2000 client station.

Click here for Figure 3.
Figure 3: A usage scenario used by the ECC to determine interference protection levels for IMT-2000 services.

Click here for Figure 4.
Figure 4: In the scenario shown in Figure 3, the ECC required a separation distance of 36cm to protect the IMT-2000 services.

Note that in the above example, the ECC determined that a separation distance of 36 cm was appropriate to take into account a foreseen frequent scenario where a UWB device operating may be on a desk in an office environment, not far from a potential victim IMT-2000 mobile station.

This analysis can be validly extended to the coexistence requirement between WiMAX and the Bluetooth AMP. In the US, 802.11 radios are allowed to transmit up to +20dBm in 2.4-2.484GHz, while it can transmit up to -41dBm/MHz in the adjacent bands allocated to IMT-2000 services. This means that to avoid interference at 36cm distance, the 2.4GHz 802.11 AMP device would need to limit the emission levels to -85dBm/MHz in the adjacent WiMAX bands. (Note that the WiMedia UWB AMP will be operating above 6 GHz.)

In the light of the above observations, industry leaders are now 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, then, 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.