Bluetooth 5 PHY and interference
Bluetooth devices operate in the 2.4000 to 2.4835 GHz industrial, scientific, and medical (ISM) unlicensed frequency band. As mentioned earlier in this chapter, this particular unlicensed area is congested with a number of other wireless media, such as 802.11 Wi-Fi. To alleviate interference, Bluetooth supports frequency-hopping spread spectrum (FHSS).
Adaptive Frequency Hopping (AFH) was introduced in Bluetooth 1.2. AFH uses two types of channels: used and unused. Used channels are in-play as part of the hopping sequence. Unused channels are replaced in the hopping sequence by used channels when needed in a random replacement method. BR/EDR mode has 79 channels and BLE has 40 channels. With 79 channels, BR/EDR mode has less than a 1.5 percent chance of interfering with another channel. This is what allows an office setting to have hundreds of headphones, peripherals, and devices all in the same range contending for frequency space (for example, fixed and continuous use sources of interference).
AFH allows a slave device to report channel classification information to a master to assist in configuring the channel hopping. In situations where there is interference with 802.11 Wi-Fi, AFH is used with a combination of proprietary techniques to prioritize traffic between the two networks. For example, if the hopping sequence regularly collides on channel 11, the master and slaves within the piconet will simply negotiate and hop over channel 11 in the future.
In BR/EDR mode, the physical channel is divided into slots. Data is positioned for transmission in precise slots and consecutive slots can be used if needed. By using this technique, Bluetooth achieves the effect of full-duplex communication through Time Division Duplexing (TDD). BR uses Gaussian Frequency-Shift Keying (GFSK) modulation to achieve its 1 Mbps rate, while EDR uses Differential Quaternary Phase Shift Keying (DQPSK) modulation to 2 Mbps and 8-phase differential phase-shift keying (8DPSK) at 3 Mbps.
LE mode, on the other hand, uses Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) access schemes. With 40 channels rather than 79 for BR/EDR and each channel separated by 2 MHz, the system will divide the 40 channels into three for advertising and the remaining 37 for secondary advertising and data. Bluetooth channels are chosen pseudorandomly and are switched at a rate of 1600 hops/second. The following figure illustrates BLE frequency distribution and partitioning in the ISM 2.4 GHz space.
TDMA is used to orchestrate communication by requiring one device to transmit a packet at a predetermined time and the receiving device to respond at another predetermined time. The physical channels are subdivided into time units for particular LE events such as advertising, periodic advertising, extended advertising, and connecting. In LE, a master can form a link between multiple slaves. Likewise, a slave can have multiple physical links to more than one master and a device can be both a master and slave simultaneously. Role changes from master to slave or vice versa are not permitted.
Other forms of interference occur with mobile wireless standards in the 2.4 GHz space. Here, a technique called train nudging was introduced in Bluetooth 4.1.
For BLE modes, SAM is not available. However, an overlooked mechanism in Bluetooth called Channel Selection Algorithm 2 (CSA2) can help with frequency hopping in noisy environments susceptible to multipath fading. CSA2 was introduced in Bluetooth 4.1 and is a very complex channel mapping and hopping algorithm. It improves the interference tolerance of the radio and allows the radio to limit the number of RF channels it can use in high interference locations. A side effect of limiting channels with CSA2 is it allows the transmit power to increase to +20dBm. As mentioned, there are limits to transmitting power imposed by governing regulatory bodies since BLE advertising channels and connected channels are so few. CSA2 allows for more channels to be available in Bluetooth 5 than in previous versions, which may open up regulatory restrictions.