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Restructuring Bluetooth for mobile use

Posted: 17 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:integrate Bluetooth in mobile phone? 2.4GHz? power consumption?

The trend of integrating Bluetooth as a key component in cellphones is nothing new. Most handset makers today list Bluetooth as one of the most product salient points.

More countries are rolling out legislation to enforce drivers to use handsfree kits or headsets. Bluetooth presents the ideal wire-free solution to this. Hence, more and more handsets must include Bluetooth technology. Wearing a Bluetooth headset while walking around town is increasingly acceptablethis is boosting Bluetooth's attach rate in phones. The attach rate continues to increase year on year, with more headsets supplied in the box ("bundling"), and with mobile handsets helping operators and handset vendors differentiate their products.

While the trends of integrating Bluetooth and other technologies (mobile TV, FM and GPS) continue to put phone manufacturers under increasing pressure, certain design pressures remain constant. Users will not tolerate large handsets, compromised performance, and shortened battery life due to power-hungry technologies. These present a tough design challenge for the handset maker. In turn, the handset maker is forced to demand the best from its component suppliers. This article looks at the steps that Bluetooth chip vendors are taking to address these issues.

Environment challenges
Information sent across a Bluetooth wireless link comes in two basic forms: asynchronous data and synchronous voice.

Bluetooth was designed to support both types of air interface: asynchronous connectionless (ACL) packets to support data traffic and synchronous connection-oriented (SCO) packets for voice traffic. SCO is circuit-switched. It has symmetric synchronous services and slot reservation at fixed intervals, making it suitable for time-critical transmissions such as voice. Users will not tolerate parts of their conversation arriving at different times or their speech broken up. Smooth and real-time information (streaming audio) is essential for quality voice connection. The Bluetooth spec supports three simultaneous voice channels (SCO links) per master device.

Voice channels use the continuously variable slope delta (CVSD) voice coding scheme, and SCO defines that packets are not retransmitted. The CVSD codec was chosen for Bluetooth because of its robustness in handling dropped and damaged voice samples. Rising interference levels are experienced with increased background noiseeven at BERs of up to 4 percent, the CVSD coded voice remains acceptable.

Bluetooth ver 1.2 sought to address some of the limitations of SCO: SCO's uplink and downlink bit-rates are fixed at 64Kbps and have no provision for retransmission of packets. Ver 1.2 introduced Extended SCO transport, which provides for retransmission of corrupted packets; this increases the transmission quality of SCO transfers, which is important in connections where transmission quality is not necessarily a "binary issue" (e.g. transmission of audio or video data).

Despite supporting a retransmit function, however, Bluetooth voice packets still continued to be coded with the default CVSD coding system. This leaves room for improvement in the way voice traffic is handled.

Some cellphone users wear the handset clipped to their belts with a Bluetooth headset connected wirelessly (perhaps on the ear at the opposite side of the body). The Bluetooth signal has to cross the body, which is not the best "conductor" of Bluetooth's RF, especially in a noisy environment where the caller's voice may not be very clearly transmitted to the headset.

Hence, we have a growing trend of consumers demanding Bluetooth for voice applications, but with a voice-coding scheme (CVSD) designed to be resistant to bit errors. Moreover, we have the physical challenge of transmitting 2.4GHz through or around the human body while maintaining the highest quality signal and not compromising on power consumption.

Power concerns
CSR has incorporated improvements to the RF (especially to the scan techniques) to improve sensitivity and transmit power. The company has introduced Auristream, which adopts ADPCM as an enhanced voice-coding method. ADPCM performs the same scan at half the rate of CVSD (ADPCM scans at 32Kbps, while CVSD scans at 64Kpbs), saving up to half the power. ADPCM is also more accurate, providing a much higher voice quality.

The company has focused on reducing the power in standby and restructured how Bluetooth devices scan for other devices. It has combined two additional scan modesconditional and casual scanto save up to 86 percent of Bluetooth's total power consumption.

In conditional scan, the Bluetooth system will wake up the minimum parts of the radio to perform a quick scan for any activity in the 2.4GHz band. If the radio discovers activity, it will initiate a normal Bluetooth page scan/inquiry. Otherwise, the radio will remain asleep.

Casual scan mode uses the timing crystal's (VCTCXO) periodic wake-up cycle so that the Bluetooth scan cycle does not wake up the VCTCXO unnecessarily. The Bluetooth silicon often shares a VCTCXO with a mobile phone's host system to minimize the use of additional components and save power on the cellphone system.

- Matthew Phillips
Senior VP of Mobile Handset Connectivity Business Unit
CSR plc




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