Page 470 - From GMS to LTE

456 From GSM to LTE-Advanced Pro and 5G

723 kbit/s
kbit/s

390 kbit/s

260 kbit/s

57 kbit/s

Figure 7.1 Three examples of achievable Bluetooth datarates depending on the number of users and
their activity.

As Bluetooth has to share the 2.4 GHz ISM frequency band with other wireless tech-
nologies like Wireless Local Area Network (WLAN), the system does not use a fixed
carrier frequency. Instead, the frequency is changed after each packet. A packet has a
length of either one, three or five slots. This method is called Frequency‐Hopping
Spread Spectrum (FHSS). This way, it is possible to minimize interference with other
users of the ISM band. If some interference is encountered during the transmission of a
packet despite FHSS, the packet is automatically retransmitted. For single‐slot packets
(625 microseconds), the hopping frequency is thus 1600 Hz. If five‐slot packets are
used, the hopping frequency is 320 Hz.
A Bluetooth network in which several devices communicate with each other is called
a piconet. In order for several Bluetooth piconets to coexist in the same area, each
piconet uses its own hopping sequence. In the ISM band, 79 channels are available.
Thus, it is possible for several WLAN networks and many Bluetooth piconets to coexist
in the same area as shown in [3].
The interference created by WLAN and Bluetooth remains low and hardly noticeable
as long as the load in both the WLAN and the Bluetooth piconet(s) is low. As has been
shown in Chapter 4, a WLAN network sends only short beacon frames while no user
data is transmitted. If a WLAN network, however, is highly loaded, it blocks a 25 MHz
frequency band for most of the time. Therefore, almost a third of the available channels
for Bluetooth are constantly busy. In this case, the mutual interference of the two systems
is high, which leads to a high number of corrupted packets. To prevent this, Bluetooth
1.2 introduces a method called Adaptive Frequency Hopping (AFH). If all devices in a
piconet are Bluetooth 1.2‐compatible, the master device (see Section 7.3) performs a
channel assessment to measure the interference encountered on each of the 79 channels.
The link manager (see Section 7.4.3) uses this information to create a channel bitmap
and marks each channel that is not to be used for the frequency‐hopping sequence of the