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Bluetooth-WLAN Coexistence

Due to the ubiquitous deployment of wireless networks and devices on the unlicensed 2.4 GHz Industrial, Scientific, and Medical (ISM) frequency band, multiple homogenous and heterogeneous networks (Bluetooth®, Wi-Fi®, and ZigBee®) operating in this band are likely to coexist in a physical scenario. The wireless personal area network (WPAN) represented by the Bluetooth [1] and wireless local area network (WLAN) represented by IEEE® 802.11 standard both operate in the 2.4 GHz ISM frequency band. Bluetooth and WLAN radios often operate in the same physical scenario and some times in the same device. In these cases, Bluetooth and WLAN transmissions can interfere with each other, impacting the performance and reliability of both networks.

IEEE 802.15.2 Task Group [3] considers proposals for mechanisms to improve the level of coexistence between Bluetooth and WLAN devices and publishes the recommended practices derived from these.

Bluetooth and IEEE 802.11 WLAN Specifications

Bluetooth technology uses low-power radio frequency to enable short-range communication. Bluetooth is equated with the implementation specified by the Bluetooth Core Specification [2] group of standards maintained by the Bluetooth Special Interest Group (SIG) industry consortium. The Communications Toolbox™ Library for the Bluetooth Protocol enables you to model Bluetooth low energy (BLE), BLE mesh, and Bluetooth basic rate/enhanced data rate (BR/EDR) communications system links, as specified in the Bluetooth Core Specification. Bluetooth BR/EDR and BLE devices operate in the unlicensed 2.4 GHz ISM frequency band.

The Bluetooth BR mode is mandatory, whereas EDR mode is optional. The Bluetooth BR/EDR radio implements a 1600 hops/sec frequency-hopping spread spectrum (FHSS) technique. The radio hops in a pseudo-random way on 79 designated Bluetooth channels. Each Bluetooth channel has a bandwidth of 1 MHz. Each frequency is located at (2402 + k) MHz, where k = 0,1,…, 78. The modulation technique for BR and EDR mode is Gaussian frequency shift-keying (GFSK) and differential phase shift-keying (DPSK), respectively. The baud rate is 1 Msymbols/s. The Bluetooth BR/EDR radio uses the time division duplex (TDD) topology in which data transmission occurs in one direction at one time. The transmission alternates in two directions, one after the other.

In BLE, the operating band is divided into 40 channels, k = 0, 1, …, 39, with a channel bandwidth of 2 MHz. The range of RF center frequencies is [2402, 2480] MHz. The user data packets are transmitted using channels in the range [0, 36]. The advertising data packets are transmitted on channels 37, 38, and 39. BLE also implements GFSK modulation. The BLE physical layer (PHY) uses FHSS to reduce interference and to counter the impact of fading channels. The time between frequency hops can vary from 7.5 ms to 4 s and is set at the connection establishment for each Slave with the Master. The Master device provides the synchronization reference. The Slave device synchronizes to the clock and frequency-hopping pattern of the Master device. The support for the data rate at 1 Mbps is mandatory for specification version 4.x compliant devices. At a data rate of 1 Mbps, the data transmission is uncoded. Optionally, devices compliant with the Bluetooth Core Specification 5.x support these additional data rates:

  • Coded transmission at bit rates of 500 kbps or 125 kbps

  • Uncoded transmission at a bit rate of 2 Mbps

To explore the Bluetooth BR/EDR and BLE protocol stack, see Bluetooth Protocol Stack. For information about different packet structures implemented in Bluetooth BR/EDR and BLE transmissions, see Bluetooth Packet Structure. To study the fundamentals of Bluetooth mesh networking and its applications, see Bluetooth Mesh Networking.

The IEEE 802.11 (Wi-Fi) standard is a wireless technology that connects devices and an infrastructure in a WLAN. WLAN is compliant with various IEEE 802.11 standards. Some of the prominent and widely implemented standards are 802.11 a/b/g/n/ac/ax. The 802.11a standard uses the 5 GHz unlicensed national information infrastructure (U-NII) band and provides at least 23 nonoverlapping 20 MHz wide channels instead of three nonoverlapping 20 MHz-wide channels offered by the 2.4 GHz band. The 802.11ac standard also operates in only the 5 GHz frequency band. As per Part 15 of the U.S. Federal Communications Commission (FCC) Rules and Regulations, 802.11b, 802.11g, and 802.11n standards use the 2.4 GHz. Devices that use these standards suffer interference in the 2.4 GHz band from Bluetooth devices. To mitigate this interference, devices that use 802.11b, 802.11g, or 802.11n standards implement direct-sequence spread spectrum (DSSS), Orthogonal Frequency-Division Multiplexing (OFDM), and multiple-input, multiple-output (MIMO) OFDM signaling techniques, respectively. Devices that use the 802.11n or 802.11ax (Wi-Fi-6) standard operate in dual-band at 2.4 GHz and 5 GHz. The 802.11ax standard enhances the existing 802.11 a/b/g/n/ac standards even if they are not fully upgraded to 802.11ax. The OFDM-based channel access technique of 802.11ax standard is completely backward-compatible with traditional enhanced distributed channel access/carrier-sense multiple access (EDCA/CSMA). IEEE 802.11ax provides maximum compatibility, coexisting efficiently with 802.11a/n/ac devices.

For more information about WLAN radio frequency channels, see WLAN Radio Frequency Channels (WLAN Toolbox). For more information about WLAN packet structures, see WLAN PPDU Structure (WLAN Toolbox) and Packet Size and Duration Dependencies (WLAN Toolbox).

Spread Spectrum Techniques

Bluetooth and WLAN technologies operate using the spread spectrum signal structuring. This signal structuring technique enables a narrowband signal such as a stream of 1s and 0s, to spread across a given frequency spectrum and transform into a wideband signal. Bluetooth devices implement the basic FHSS technique defined in the Bluetooth Core Specification [2]. This basic frequency-hopping technique is modified into an adaptive frequency hopping (AFH) technique to mitigate interference. WLAN devices use the DSSS technique.

FHSS

The basic Bluetooth frequency-hopping technique or the FHSS spreads the narrowband signal by hopping across different channels on the 2.4 GHz frequency spectrum. This figure shows how the FHSS transmits a Bluetooth signal on different frequencies at specific intervals to spread the signal across a relatively wide operating band.

Frequency-hopping spread spectrum in Bluetooth

The transmitting and receiving Bluetooth devices adhere to a specific hopping sequence during a particular session so that the receiving device can anticipate the frequency of the next transmission. In this case, Bluetooth makes full use of the 2.4 GHz frequency spectrum.

DSSS

With the DSSS, the narrowband data signal is divided and simultaneously transmitted on multiple frequencies within a specific frequency band. This figure shows how the DSSS continually transmits the data signal across different channels.

Direct-sequence spread spectrum in Bluetooth

The DSSS adds redundant data bits known as chips, to the data signal to denote 1s and 0s. The ratio of chips to data is called the spreading ratio. Increasing the ratio increases the immunity of the WLAN signal to interference. This is because if part of the transmission is corrupted, the data can still be recovered from the remaining part of the chipping code. The DSSS technique provides greater transmission rates than the FHSS. The DSSS also protects against data loss through redundant simultaneous data transmission. However, because DSSS floods the channel with redundant transmissions, it is more vulnerable to interference from Bluetooth devices operating on the same frequency band.

Orthogonal Frequency-Division Multiplexing

OFDM is a flexible, multicarrier modulation technique implemented by IEEE standards 802.11g/n/ac/ax. OFDM partitions the channel bandwidth into multiple narrow-band orthogonal subcarriers to carry the information. This partitioning enables the removal of guard bands. However, because the orthogonal subcarriers are unrelated, they can overlap each other. Therefore, OFDM is bandwidth efficient. This figure shows the frequency domain representation of the orthogonal subcarriers in an OFDM waveform.

Frequency domain representation of orthogonal subcarriers in an OFDM waveform

The use of narrow-band subchannels (compared to a single wideband channel) helps mitigate channel fading. As each subchannel operates at a low data rate, OFDM is very resilient to intersymbol interference and interframe interference. As data is transmitted simultaneously on multiple orthogonal subcarriers, OFDM can provide very high throughput. To further maximize the throughput, you can use OFDM with MIMO, extended rate physical (ERP), and multiuser (MU) technologies.

Bluetooth-WLAN Coexistence Problem

As Bluetooth and WLAN devices operate in the same 2.4 GHz frequency band, a mutual interference exist between the two wireless networks. This interference results in performance degradation. For example, consider the scenario shown in this figure. The scenario consists of two Bluetooth piconets collocated with a WLAN.

Bluetooth and WLAN coexistence scenario

If the transmission in piconet 1 is overlapped in time and frequency by transmissions from piconet 2 and/or the WLAN, a Bluetooth packet can be lost. This figure shows how the Bluetooth and WLAN devices share the 2.4 GHz frequency spectrum.

Bluetooth and WLAN coexistence on 2.4 GHz frequency band

If the Bluetooth packets transmitted through the FHSS hops to the portion of the frequency spectrum occupied by the DSSS WLAN transmitter, then mutual interference occurs. This interference results in packet collisions. Factors such as the distance between WLAN and Bluetooth devices, the data traffic present in these two networks, power levels of the devices, and data rate of the WLAN network impact the level of interference. Additionally, different types of data traffic have different levels of sensitivity to the interference. For example, voice traffic can be more sensitive to interference than data traffic.

Bluetooth in Presence of 802.11b WLAN Interferer

A transmission that uses one spread spectrum technique interferes with a receiver that uses different spread spectrum technique. 802.11b WLAN devices operate in 22 MHz bandwidth. In Bluetooth, 22 of the 79 hopping channels are subject to interference. A frequency-hopping system like Bluetooth is vulnerable to interference from the adjacent channels as well. This vulnerability increases the total number of interference channels from 22 to 24. Based on these assumptions, the results shown in [3] quantify the packet error rate (PER) in Bluetooth transmissions with a 802.11b WLAN interferer. The results show that the network throughput decreases and network delay increases for Bluetooth in the presence of 802.11b interference.

To study the impact of WLAN interference on BLE transmission, see BLE Coexistence Model with WLAN Signal Interference and Statistical Modeling of WLAN Interference on BLE Network examples.

802.11b WLAN in Presence of Bluetooth Interferer

When a Bluetooth device hops into the 802.11b passband, a packet collision can occur with the WLAN device. This collision occurs because 22 of the 79 Bluetooth channels fall within the WLAN passband. As 802.11b devices support four data rates (1, 2, 5, and 11 Mbps), the transmission time of the WLAN packets may vary significantly for packets carrying the exact same data. Increasing the duration of the WLAN packet increases the likelihood that the packet collides with an interfering Bluetooth packet. If automatic data rate scaling is implemented and enabled in the WLAN device, the Bluetooth interference can cause the WLAN device to scale to a lower data rate. Lower data rate increase the temporal duration of the WLAN packets. This increase in packet duration can lead to frequent packet collisions with the interfering Bluetooth packets. In some implementations, the frequent packet collisions can result in WLAN scaling down its data rate to 1 Mbps. In this case, to ensure reliable packet delivery, the IEEE 802.11 medium access (MAC) layer incorporates an acknowledgement (ACK) and retransmission mechanism.

Coexistence Mechanisms

Interference between Bluetooth and WLAN can be addressed by two coexistence mechanisms – noncollaborative and collaborative.

Noncollaborative Coexistence

Noncollaborative mechanisms do not exchange information between two wireless networks. These coexistence mechanisms are applicable only after a WLAN or Bluetooth piconet is established and the data is to be transmitted. These coexistence mechanisms do not help in the process of establishing a WLAN or Bluetooth piconet. As per the recommended practices mentioned in [3], these noncollaborative coexistence mechanisms are used to mitigate interference between Bluetooth and WLAN.

  • Adaptive frequency hopping (AFH) — Prior to the emergence of AFH, Bluetooth devices implemented the basic FHSS signal structuring scheme. The FHSS scheme often resulted in Bluetooth and WLAN packet transmissions interfering with each other, as shown in this figure.

    Bluetooth and WLAN packet collisions with basic FHSS

    On the contrary, AFH enables Bluetooth to adapt to its environment by identifying fixed sources of WLAN interference and excluding them from the list of available channels. This figure shows the previous scenario with AFH enabled.

    Bluetooth and WLAN coexistence with AFH

    AFH dynamically alters the frequency hopping sequence to avoid the interference observed by the Bluetooth devices. AFH operates through these four processes.

    • AFH capability discovery: This process informs the Master about the Slave(s) that support AFH and the associated parameters.

    • Channel classification: This process classifies the channels as good or bad. Channel classification takes place in the Master and optionally in the Slave(s).

    • Channel classification information exchange: This process uses AFH link manager protocol (LMP) commands to exchange information between the Master and the supporting Slave(s) in the piconet.

    • Adaptive hopping: This process adaptively selects good channels for frequency hopping.

    For more information about how AFH mitigates interference and enables coexistence between Bluetooth and WLAN, see End-to-End Bluetooth BR/EDR PHY Simulation with WLAN Interference and Adaptive Frequency Hopping.

    Note

    For more information about AFH, see Annex B of IEEE 802.15.2 Task Group [3].

  • Adaptive interference suppression — This mechanism is exclusively related to signal processing in the WLAN physical layer (PHY). The adaptive interference suppression mechanism requires a Bluetooth receiver collocated with a WLAN receiver. The WLAN receiver has no prior knowledge of the timing or frequency used by the Bluetooth network. The WLAN receiver uses an adaptive filter to estimate and cancel the interfering signal.

    Note

    For more information about adaptive interference suppression, see Clause 8 of IEEE 802.15.2 Task Group [3].

  • Adaptive packet selection and scheduling — Bluetooth transmissions involve various packet types with different configurations such as packet length and degree of error protection used. By selecting the best packet type according to the channel condition of the upcoming frequency hop, better throughput and network performance can be achieved. Additionally, packet transmissions can be scheduled efficiently so that the Bluetooth devices transmit during hops that are outside of the WLAN frequencies and refrain from transmitting while in-band. This type of packet transmission scheduling minimizes mutual interference and also increases the throughput of Bluetooth networks.

    Note

    For more information about adaptive packet selection and scheduling, see Clause 9 of IEEE 802.15.2 Task Group [3].

  • Packet scheduling for synchronous connection-oriented (SCO) links — Voice applications are among the most sought-after applications for Bluetooth devices but are vulnerable to interference. Interference from an in-band WLAN network degrades the voice quality of the Bluetooth SCO link, making it inaudible to the users. This noncollaborative coexistence mechanism recommends improvements that can significantly improve the quality-of-service (QoS) for SCO links. The fundamental idea is to enable the SCO link the flexibility of selecting hops that are out-of-band with the collocating WLAN spectrum for transmission. The duty cycle of the SCO link does not change.

    Note

    For more information about packet scheduling for SCO links, see Annex A of IEEE 802.15.2 Task Group [3].

  • Packet scheduling for asynchronous connection-oriented logical (ACL) links — This mechanism defines a procedure to minimize the impact of WLAN interference on Bluetooth devices by using these two components.

    • Channel classification: It is performed on every Bluetooth receiver and is based on the measurements conducted per frequency or channel to locate the presence of interference. A channel is considered as good if it can correctly decode a received packet. Otherwise, the channel is considered as bad. Good and bad channels are classified based on different criteria such as the received signal strength indicator (RSSI), PER, or negative ACKs.

    • Master delay policy: It uses the information available in the channel classification table to avoid packet transmission in a bad channel. Because the Master device controls and manages all transmissions in a piconet, the delay rule must be implemented in the Master device only. Also, a Slave transmission must follow each Master transmission. Therefore, the Master checks the receiving frequency of the Slave and its own receiving frequency before choosing to transmit a packet in a given frequency hop.

    Note

    For more information about packet scheduling for ACL links, see Clause 10 of IEEE 802.15.2 Task Group [3].

Collaborative Coexistence

In collaborative coexistence mechanisms, two wireless networks collaborate and exchange network-related information. As per the recommended practices stated in [3], the three collaborative coexistence mechanisms are:

  • Alternating wireless medium access (AWMA) — In the AWMA mechanism, a WLAN radio and a Bluetooth radio are collocated in the same physical unit, enabling a wired connection between the two radios. The collaborative coexistence mechanism uses this wired connection to coordinate access to the wireless medium between WLAN and Bluetooth. The AWMA mechanism uses part of the wireless IEEE 802.11 beacon interval for the Bluetooth operations. From a timing perspective, the medium assignment alternates between usage following the IEEE 802.11 procedures and usage following the Bluetooth procedures. Each wireless network limits its transmissions to the appropriate time segment, thus preventing mutual interference between the two networks.

    Note

    For more information about AWMA, see Clause 5 and Annex I of IEEE 802.15.2 Task Group [3].

  • Packet traffic arbitration (PTA) — In the PTA mechanism, the WLAN station and the Bluetooth device are collocated. The PTA control entity provides per-packet authorization of all transmissions. This mechanism can deny permission for transmission if it has chances of collisions. The PTA mechanism dynamically coordinates sharing of the wireless medium based on the traffic load of WLAN and Bluetooth. If a collision occurs, the PTA mechanism prioritizes transmission based on the priorities of different packets. Using the PTA mechanism in case of high variability in the WLAN and Bluetooth traffic load or whenever a Bluetooth SCO link needs to be supported.

    Note

    For more information about PTA, see Clause 6 and Annex J of IEEE 802.15.2 Task Group [3].

  • Deterministic interference suppression — In this mechanism, a null is inserted in the WLAN receiver at the frequency of the Bluetooth signal. Because Bluetooth devices hop to a new frequency for each packet transmission, the WLAN receiver must know the hopping pattern and timing of the Bluetooth device. The hopping pattern and timing is obtained by using a Bluetooth receiver as part of the WLAN receiver. Deterministic interference suppression is a collocated, collaborative coexistence mechanism.

    Note

    For more information about deterministic interference suppression, see Clause 7 and Annex K of IEEE 802.15.2 Task Group [3].

References

[1] Bluetooth Technology Website. “Bluetooth Technology Website | The Official Website of Bluetooth Technology.” Accessed April 17, 2020. https://www.bluetooth.com/.

[2] Bluetooth Special Interest Group (SIG). "Bluetooth Core Specification." Version 5.2. https://www.bluetooth.com/.

[3] P802.15.2/D09 - IEEE Draft Recommended Practice for Information Technology Telecommunications and Information Exchange Between Systems Local and Metropolitan Area Networks Specific Requirements - Part 15.2: Coexistence of Wireless Personal Area Networks With Other Wireless Devices Operating in Unlicensed Frequency Bands. LAN/MAN Standards Committee, IEEE Computer Society, 2003, https://ieeexplore.ieee.org/document/4040972.

[4] Macleod, M D. “14 - Coding.” In Telecommunications Engineer’s Reference Book, edited by Fraidoon Mazda, 14–1. Butterworth-Heinemann, 1993. https://www.sciencedirect.com/science/article/pii/B9780750611626500204.

[5] Xiao, Yang, and Yi Pan. “Coexistence of Bluetooth Piconets and Wireless LAN.” In Emerging Wireless LANs, Wireless PANs, and Wireless MANs: IEEE 802.11, IEEE 802.15, 802.16 Wireless Standard Family, 151–85. Wiley, 2009. https://ieeexplore.ieee.org/abstract/document/8040602.

[6] “IEEE SA - The IEEE Standards Association - Home.” Accessed May 4, 2020. https://standards.ieee.org/.

[7] IEEE P802.11ax/D4.1. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 1: Enhancements for High Efficiency WLAN.” Draft Standard for Information technology — Telecommunications and information exchange between systems. Local and metropolitan area networks — Specific requirements.

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