Mobile handsets are rapidly evolving from simple cellular phones to versatile multi-mode, multi-media devices with a plethora of connective capabilities. This development has the potential to benefit users, carriers, web-services providers and application developers, but it is also fraught with difficulty for handset OEM's due to intractable interference issues between certain radio protocols. Examples include:
- Bluetooth; a standard feature in mid/high-end phones that provides short range connectivity to headsets, laptops (wireless PC modems and/or synchronizing capability), and such peripherals as printers
- WiFi, which enables users to access the Internet and make VoIP calls
- WiMAX, which will soon extend the same capabilities of WiFi through much greater range and robustness
Handset manufacturers realized years ago that frequency proximity between Bluetooth and WiFi (2.4GHz band), and the co-location of their respective antennas--combined with the fact that the two protocols are completely un-coordinated, posed a serious performance challenge to the point of malfunction. Bluetooth and WiFi chipsets vendors, who added coexistence interfaces to products, enabling arbitration over the shared radio frequency medium to prevent contention and signal degradation, resolved the issue.
With the advent of Mobile WiMAX (IEEE802.16e), OEMs face new interference challenges as the new protocol operates over several frequency bands (defined by 'profiles'' in WiMAX terminology), the most common being 2.3-2.4GHz and 2.5-2.7GHz. The frequency separation, although greater than between Bluetooth and WiFi, is still not enough to prevent coexistence problems.
A typical usage scenario where a user is conducting a cellular conversation via a Bluetooth headset, while simultaneously downloading email or browsing the Internet through the phone's WiMAX air link, will necessitate a mechanism to guarantee the coexistence of wireless interfaces. Without such a mechanism, voice quality and packet throughput degradation will result in a poor user experience. Given the prevalence, and popularity with end-users of Bluetooth and WiFi accessories (e.g. Bluetooth headsets, WiFi routers), the optimal solution must operate with the existing installed device base and not assume modifications to it.
WiMAX and Bluetooth interference
The previously described scenario will be used to analyze the interference pattern that emanates from WiMAX to Bluetooth air links, and determine its impact. Figure 1 shows a system comprised of a Bluetooth headset and a WiMAX-enabled mobile handset.
The transmit power of a Bluetooth headset is 0dBm. When the signal is received at the handset antenna its power level is -40dBm. The Bluetooth specification requires the receiver to handle interfering signals of up to -27dBm.
The handset WiMAX transmitter in this example operates in the 2.5-2.7GHz band. The output power of the WiMAX Power Amplifier (PA) may be as high as +25dBm. The WiMAX and Bluetooth transmit antennas are in close proximity to each other, with the user's hand, or the surface on which the handset is placed, typically causing 10dB path-loss between them. This yields a +15dBm signal at the Bluetooth Band Pass Filter (BPF) input. The BPF must pass frequencies of up to 2.48GHz (the highest Bluetooth hopping frequency), hence it is unable to reject more than 3dB of the undesired WiMAX signal and so passes at least +12dBm of interference signal to the Bluetooth Low Noise Amplifier (LNA).
Given the -27dBm Bluetooth rejection capability, it is clear that it is not capable of rejecting the WiMAX signal, and therefore blocking occurs. Moreover, such a strong signal in the Bluetooth LNA input might exceed the LNA's maximum rating and cause severe reliability issues.
Click here for Figure 1.
For the purposes of this discussion, the term "local party" denotes the party using the handset, and the term "remote party" denotes the party on the other side of the conversation. Whenever the handset Bluetooth reception is blocked by a WiMAX transmission, a "click" is heard on the remote party's end.
The impact of the WiMAX blocking on the local party is less severe, due to higher path loss from the handset to the headset, however interference on the local party's end cannot be overlooked completely. Probable occurrence of such "clicks" is surprisingly high. Assuming the following scenario (explained in the next section), the Bluetooth receiver in the handset can be expected to be active ~1/6 of the time, and depending on the WiMAX usage scenario, will be blocked at increased frequency as traffic intensifies. As mentioned previously, there is a negative impact of Bluetooth transmission on WiMAX reception, but it is less severe.
Solving the coexistence challenge
Given the analysis of the scenario in the previous section, it is clear that nothing can be done to eliminate, or even mitigate the interference in the radio or Physical Layer (PHY) since it is inherent to the system. Therefore, the solution must be provided via a higher layer, i.e. the Media Access Control (MAC) layer. In the MAC layer, it is possible to perform synchronization between the different protocols, and ensure that bandwidth over the shared spectrum is allocated in a time multiplexed, non-concurrent, yet fair basis. Such a solution would eliminate any potential conflict and still maintain inherent link performance attributes.
There are various scenarios and use cases that need to be addressed, i.e. varying combinations of WiMAX, Bluetooth and WiFi transmission and reception, each in different link scanning, establishment, and activity modes. For illustrative continuity purposes, the same use case defined in the previous sections will be used to help explain the proposed coexistence solution. Later we will add a WiFi air link to the same use case--which is characterized by the following elements:
- An active WiMAX link between a mobile handset and a WiMAX base station
- An active Bluetooth voice link, operating in SCO/HV3 mode (the standard profile used by commercial Bluetooth headsets)
The first step is to synchronize the protocol time bases. To do that, we must first find the lowest common denominator between the different system clocks and then ensure that they are coordinated. The Bluetooth SCO/HV3 profile's time base is 625us, while the WiMAX time base is based on 5ms frames. This means that 15ms is the lowest common denominator time interval during which three WiMAX frames and twenty-four Bluetooth slots are processed. Once a solution is found to address the 15ms time interval, the repetitive pattern ensures that the solution is applicable to the mode as a whole.
Having identified the repetition pattern, it is necessary to ensure that the two time bases are synchronized and remain so throughout the concurrent operation of the links. Since the WiMAX the WiMAX base station determines time base, it is impossible for the mobile handset to control its phase relative to the Bluetooth time base. The Bluetooth chipset in the mobile handset on the other hand, assuming it is the master over the Bluetooth link, has the ability to control the clock's phase and synchronize it with that of the WiMAX link.