Dr. Dobb's | Bluetooth/Wi-Fi Co-existence

Bluetooth/Wi-Fi Co-existence

Adopting highly integrated solutions with proprietary techniques speeds design time.

January 09, 2007
URL:http://drdobbs.com/mobile/bluetoothwi-fi-co-existence/196802355

Bluetooth and 802.11 b/g Wi-Fi have both cemented themselves as essential wireless technologies, leading to the natural convergence of both into devices such as laptops, PDAs, personal multimedia players and cell phones.

Certain devices such as wireless VoIP phones and multi-standard cell phones even demand simultaneous operation of both Bluetooth and Wi-Fi, putting heavy demands on chip design. Therefore, the co-existence of these two technologies can no longer be achieved through limited usage models or simply by creating a distance between the radios.

Without care during development, embedding both Bluetooth and Wi-Fi into a device can cause interference issues that affect user experience.

Both Bluetooth and Wi-Fi operate in the unlicensed 2.4 GHz industrial, scientific and medical (ISM) band and send data in packet form. Although Bluetooth and Wi-Fi use the spectrum differently, interference still occurs when a Wi-Fi receiver senses a Bluetooth signal at the same time as a Wi-Fi signal is being received.

The same applies to a Bluetooth receiver. In addition to the challenges presented by coexistence with other wireless standards, Bluetooth communication links may also be disrupted by other household devices such as microwave ovens, which radiate RF energy as a by-product of their operation and can only be limited to a certain level due to cost and engineering constraints.

In spite of this ambient RF interference, Bluetooth and Wi-Fi have gained increasing popularity with consumers, especially over the past six years where Bluetooth products and wireless LAN networks have appeared in more homes. As both technologies are placed in close physical proximity, coexistence is a priority and a number of detailed mechanisms have been introduced to counteract any interference.

In order to limit the amount of power transmitted in any single area of the ISM band, spread-spectrum techniques of data transmission are mandatory for both Bluetooth and Wi-Fi. Bluetooth employs Frequency-Hopping Spread Spectrum (FHSS) to transmit data packets across a comparatively narrow bandwidth of 1 MHz.

The frequency of the narrow band signal is then changed at a rate of 1600 hops per second within the 79 channels available within the range. By hopping frequently around the spectrum, the signal power is spread across the band.

When normal interference occurs, reception of part of a transmitted packet of data may be interrupted due to an overlap in the Bluetooth and 802.11 b/g signals, resulting in packet errors. Closely located antennae can cause front-end overload interference on the second system running. However, this interference requires a stronger interfering signal and is therefore a less common problem than normal interference.

As the Bluetooth specification has developed, new techniques have been added which allow Bluetooth to coexist easily with Wi-Fi and other potential sources of interference. A number of measures which have been implemented to this end are explained below.

Adaptive Frequency Hopping (AFH)
Adaptive Frequency Hopping (AFH) was introduced in the v1.2 Bluetooth specification developed by the Bluetooth Special Interest Group (SIG) and provides an effective way for a Bluetooth radio to counteract normal interference. AFH identifies "bad" channels where there are either other wireless devices interfering with the Bluetooth signal or where the Bluetooth signal is interfering with another device.

The AFH-enabled Bluetooth device will then communicate with other devices within its piconet, to share details of any identified bad channels. The devices then switch to alternative available "good" channels, away from the areas of interference, thus having no impact on the bandwidth used. For AFH to work, the classification of the bad channels must be accurate and 'normal' interference should be the only form of interference. Figure 1 demonstrates AFH working effectively.

Click here for Figure 1
Figure 1: Proper operation of adaptive frequency hopping (AFH).

Default settings for CSR's BlueCore Bluetooth silicon adapt to interference coming from a new source within approximately 4 seconds.

Channel skipping offers some of the benefits of AFH to Bluetooth v1.1-qualified devices although there is some sacrifice of Bluetooth bandwidth necessary to minimize disruption to Wi-Fi signals. Time-critical media applications such as stereo audio streaming and mono audio headsets are typically not effected as far as the user is concerned when the AFH is switched on. Time Division Multiplexing (TDM)
Time Division Multiplexing (TDM) is one tool used against the front end overload-type interference which cannot be managed by AFH. This approach was originally introduced to protect 802.11b/g transmissions from Bluetooth interference rather than vice versa and works by shutting down all Bluetooth transmissions except those which are high priority when the 802.11b/g radio is active on the ISM band.

This approach, like channel skipping, sacrifices part of the Bluetooth bandwidth but the amount of bandwidth sacrificed is proportionate to the 802.11b/g duty cycle. Therefore, if the 802.11b/g is idle, the link maintenance traffic may lead to a small 2 to 3% bandwidth degradation, which is impossible for a user to detect.

Click here for Figure 2
Figure 2: Time Division Multiplexing (TM).

In order to enhance the effectiveness of TDM, it is necessary to have accurate information regarding the activity of the 802.11b/g radio. One wireless technology company, CSR, uses a WLAN_Active hardware signal to ensure that the b/g signal is protected when the radio becomes active.

However, there are times when degradation of the Bluetooth signal is to be protected from 802.11b/g interference, and so CSR developed BT_Priority, an optional signal which indicates when an important Bluetooth packet is being transmitted or received.

This signal can be used to protect SCO audio using HV3 packets, the most common form of streaming audio to and from a mono headset.

Channel Quality-Driven Data Rate (CQDDR)
Two forms of data packets exist, DH and DM, which use high and medium bandwidth respectively. DH packets can transmit more data within the packets but if a part of the packet is corrupted the entire packet must be retransmitted to recover the data.

The DM packets include forward error correction (FEC) code which takes up a third of the payload: for every 10 bits of data, a 5-bit forward error correction code is added, allowing the correction of up to two bit errors in each 15-bit data/FEC block.

This data packet format may reduce maximum data rate but is more robust than the DH packets which have no error correction included. It allows a receiving device to negotiate with a transmitter to agree which packet format is used according to ambient interference.

For example, if one device believes it is receiving packets with many errors, it tells the transmitter to send the data in DM packets. If the link clears up, it allows the other side to switch back to DH packets. Figure 3 illustrates this communication interchange.

Click here for Figure 3
Figure 3: Channel Quality-Driven Data Rate (CQDDR).

CQDDR remains an optional addition to a Bluetooth link and is not required by any Bluetooth specification.

Therefore, for example, when a CSR's BlueCore-enabled Bluetooth device sends data to a non-CQDDR-enabled device, CSR has developed an algorithm to estimate the performance of the link and to modify the type of data packets sent in accordance with the ratio of acknowledged packets (ACKs) to not-acknowledged packets (NACKs).

However, when receiving information from a non-CQDDR-enabled device, BlueCore cannot implement any such countermeasures if the data packets are corrupted.

Extended Synchronous Connection Oriented Channels (eSCO)
eSCO are error-checking voice channels that allow the retransmission of corrupted voice data. Each packet has a CRC (Cyclic Redundancy Check) so the receiver can check that packets have been received correctly.

Packets which are received with errors or lost altogether are negatively acknowledged. Retransmission windows allow retransmission of unacknowledged packets. eSCO was introduced with v1.2 of the Bluetooth specification.

Version 1.1 SCO used in earlier versions of Bluetooth only used single slot packets. Extended SCO (eSCO), on the other hand, allows the use of 3 slot packets for synchronous voice or data.

This means it is possible to get >100 kbps connection compared with the fixed 64 kbps from version 1.1. This is possible due to link capacity being lost in the case of single slot packets, to gaps between packets while the radio changes frequencies.

Click here for Figure 4
Figure 4: Extended Synchronous Connection Oriented Channels (eSCO).

At each eSCO instant the master transmits an eSCO packet, the slave responds using the normal SCO rules (the slave is allowed to respond even if it doesn't receive the master's packet). Then the differences from SCO become apparent: there is a retransmission window during which unacknowledged packets can be resent until acknowledged. The spacing of the eSCO instant is negotiable.

With version 1.1 SCO there was a choice of 3 different packet spacings all giving the same 64kb/s. With extended SCO, both packet length and intervals can be negotiated in both directions of the link allowing asymmetric traffic.

Although eSCO channels do not actively handle or avoid interference, the retransmission of corrupt data packets ensures that audio quality is less affected by interference from other radio devices than before. Proprietary techniques
In addition to the above mechanisms, companies have also made enhancements through proprietary techniques. CSR for example, also manufactures an 802.11 b/g hardware solution for embedded applications (UniFi).

Due to its experience in embedded wireless technologies it has been able to develop further optimization measures via priority and channel signaling. CSR has implemented these additional features because even when current protection techniques are utilized there are still some co-existence issues.

Take the example of someone using a Bluetooth headset that is paired with a wireless VoIP phone for voice communication. The synchronous Bluetooth SCO connection can still be disrupted by the packet reception acknowledgements that Wi-Fi is forced to transmit—resulting in bad voice quality for the Bluetooth link.

With TDM and CSR's proprietary measures built into the UniFi device (with UMA-compliant 17 dBm radio frequency output power), synchronous Bluetooth HV3 packets do not cause interference.

In this and other instances, users who deploy both CSR's BlueCore and UniFi single-chip silicon are assured of seamless coexistence in foreseeable operating scenarios thanks to the additional measures that take account of the need for such technologies to work together.

Given the increasingly multimedia nature of state-of-the-art phones, such quality of service is likely to have a significant impact on the user experience in what is clearly unfolding as a huge global market. Using Bluetooth and Wi-Fi silicon from the same company can simplify and speed integration as well, reducing the number of suppliers to deal with for application support

Conclusion
Bluetooth and Wi-Fi technologies have made incredible progress in the years since they were first launched regarding the issues of both interference and power consumption.

Design engineers have made huge advancements in making Bluetooth and Wi-Fi silicon more power-efficient and robust with new approaches to chip architecture, low power modes and software implementations, researched to provide the best interference and power consumption solutions available.

Sophisticated methods and techniques have allowed the two technologies to be embedded, side by side, in the smallest of form factors.

Coexistence systems such as Adaptive Frequency Hopping (AFH), Time Division Multiplexing (TDM), power control and Channel Quality Driven Data Rates (CQDDR) have made the Bluetooth link more robust. However, wireless design does not stop at just employing techniques like AFH and TDM. Effective implementation lies with the ability to have highly integrated solutions with proprietary techniques that decrease the hurdle of designing the two technologies into one device.

The most appropriate choice for device designers is to integrate a combined Bluetooth + Wi-Fi solution that has been developed in conjunction with each other. Designers need ready-engineered coexistence solutions that are designed specifically to communicate between radios in order to intelligently reduce interference.

Solutions like CSR's have enhanced the user experience of Bluetooth as a complementary technology to other popular standards like 802.11b/g Wi-Fi but the real challenge that now lies ahead is combining Bluetooth and Wi-Fi on the same chip.

About the author
Simon Finch is vice president of CSR's Wi-Fi strategic business unit. Simon is responsible for the development of Wi-Fi technology, specifically for UniFi. Prior to joining CSR, Simon was Director of Engineering at Home Wireless Networks, and prior to this, Cambridge Consultants (CCL).