According to the authors, IMT-2000 and 802.11 radios will interfere with each other unless they are located approximately 8 meters apart for 2.6 GHz and 16m apart at 2.3GHz.
By Dr. Roberto Aiello and Siddharth Shetty, Staccato Communications
Measurements
A set of measurements were performed in Staccato Communications' lab to quantify the interference effect. Figure 5 shows the measurement set ups.
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.
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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)
