This means that now 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 in the industry are concerned about the known 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 WCDMA. 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 success of the technology could outweigh any short-term time-to-market gains. Bluetooth already has a high-profile position in the consumer marketplace (last year the SIG welcomed its 9000th member ), and rushing time-to-market with an interim technology is risky.
Usage models
While 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. This means that 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. To illustrate this point, consider the following usage models.
Figure 1 shows two multi-radio handsets located close to each other. One is making a voice call over WiMAX, the other is transferring a file to a PC with 802.11-based high speed Bluetooth. The voice over WiMAX call will drop when the other handset transfers a file to the PC even at a distance of several meters. Figure 2 shows a handset making a voice call with WiMAX and at the same time printing using high-speed Bluetooth. The handset needs to wait for the end of the call before printing without dropping the call. Alternatively, it would not be able to receive a call if the print job already started.
Click here for Figure 1.
Figure 1: One handset transfers a file to a laptop using 802-11-based high-speed Bluetooth. The other handset operates a WiMAX voice call. The two handsets will interfere with each other even at 8m separation.
Click here for Figure 2.
Figure 2: A handset with WiMAX and 802.11-based Bluetooth cannot operate both functions at the same time.
In both of these usage models, any interference between the Bluetooth system and the WiMAX or cellular services would be extremely detrimental to the end user's experience. The reality is that consumers expect to be able to use multiple technologies simultaneously, without interference.
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 (see Table 1). The concern is that 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.
Click here for Table 1.
Table 1. Spectrum allocations.
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 (as described by Bluetooth press materials ). Because of their continuous nature, these streaming applications over an 802.11 AMP will have an even higher potential for interference than file transfers.
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.
Click here for Figure 5.
Figure 5: Measurement set up to determine potential interference from the 802.11 AMP (top) and UWB AMP (bottom).
All measurements were performed in a lab environment using conducted cables. The spectrum analyzer (SA) 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 SA front end and hence get a lower noise floor reading by turning down the input attenuation on the SA.
Click here for Figure 6.
Figure 6: In-band and out-of-band emissions for an off-the-shelf 802.11g card. The red slashed lines show where the card's emissions exceed the ECC protection level. The green line shows the performance of a WiMedia UWB radio operating above 6GHz (which is where the UWB AMP will operate).
Figure 6 shows the 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 (green). Results show that 802.11 out-of-band emissions in the WiMAX, UMTS, and LTE bands exceed the protection limit by about 30dB, while UWB emissions are 5 dB lower than the protection limit.
This means that the 802.11-based high speed Bluetooth will interfere with IMT-2000 services unless they are located approximately 8 meters apart for 2.6 GHz and 16m apart at 2.3GHz. If they are co-located in a single device, achieving this level of isolation between radios is unrealistic. Therefore, 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. Figure 6 also shows that the UWB emission in the IMT-2000 bands is below the protection level and it 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.
Figure 7 shows a WiMAX receiver's desensitization caused by 802.11 AMP interference as a function of the distance between the 802.11 transmitter and WiMAX receiver. The desensitization computations assume -101dBm as the sensitivity for WiMAX MAP messages (critical control messages transmitted in downlink signal), and the two curves correspond to desensitization of WiMAX systems in 2.3 and 2.5GHz bands . Out-of-band emission levels of -51 and -60 dBm/MHz from an 802.11 radio are used, according to the measurement results described above, and free space path loss is considered.
Click here for Figure 7.
Figure 7: The desensitization of a WiMAX receiver increases as the distance between it and an 802.11 AMP decreases.
Observations and recommendations
Analysis of real-world measurements shows that UWB AMP emission levels are sufficient to protect IMT-2000 services at 2.3 / 2.6 GHz bands as per the protection requirements specified in ECC Report 64.
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. Such effects can directly impact the capacity and functionality of these systems.
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. We also have the responsibility to protect licensed services from interfering with their proper operation. The appropriate next step would be to conduct adequate coexistence studies between 802.11-based high speed Bluetooth and licensed services in 2.3 / 2.5 GHz bands. Subsequently, the industry should consider developing mutual interference mitigation mechanisms for high-speed Bluetooth using the 802.11 AMP, based on the results of the coexistence study. Alternatively, the industry should exclusively move operation of the 802.11 AMP to 5GHz.
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About the authors
Dr. Roberto Aiello is co-founder and CTO at Staccato Communications. A recognized leader in the UWB community, he is actively involved in regulatory and standards-setting committees and is the author of more than 20 patents on UWB technology.
Siddharth Shetty is a communication systems engineer at Staccato Communications. A graduate of University of Colorado, Boulder, he is an expert in spectrum regulations for Ultra-Wideband and a main contributor to the world-wide UWB standards and regulations.
Footnotes:
i Bluetooth SIG selects WiMedia Alliance UWB technology for high-speed Bluetooth .
ii Bluetooth technology to harness the speed of 802.11
iii History of the SIG.
iv Mobile WiMAX—Part I: A Technical Overview and Performance Evaluation, WiMAX Forum (2006)
