Page 468 - From GMS to LTE

454 From GSM to LTE-Advanced Pro and 5G

Table 7.1 lists the different Bluetooth protocol versions. In general, a new version is
always downward compatible with all previous versions. This means that a Bluetooth
2.1 device is still able to communicate with a Bluetooth 4.0 device. Functionality which
has been introduced with a newer version of the standard cannot of course be used with
a device that supports only a previous version of the standard.

7.2 Physical Properties

Up to version 1.2 of the standard, the maximum datarate of a Bluetooth transmission
channel was 780 kbit/s. All devices that communicate directly with each other have to
share this datarate. The maximum datarate for a single user thus depends on the follow-
ing factors:
the number of devices exchanging data with each other at the same time
●
the activity of the other devices.
●
The highest transmission speed can be achieved if only two devices communicate
with each other and only one of them has a large amount of data to transmit. In this
case, the highest datarate that can be achieved is 723 kbit/s. After removing the over-
head, the resulting datarate is about 650 kbit/s. The bandwidth remaining for the other
device to send data in the reverse direction is about 57 kbit/s. This scenario occurs quite
often, for example, when a file is transferred. In this case, one of the two devices sends
the bulk of the data while the other device sends only small amounts of acknowledge-
ment data. The left‐hand side of Figure 7.1 shows the achievable speeds for this scenario.
If both ends of the connection need to send data as quickly as possible, the speed that
can be achieved at each side is about 390 kbit/s. The middle section of Figure 7.1 shows
this scenario.
If more than two devices want to communicate with each other simultaneously, the
maximum datarate per device is further reduced. The right‐hand side of Figure 7.1
depicts this scenario.
In 2004, the Bluetooth 2.0 + EDR (Enhanced Datarate) standard [2] was released. This
enables datarates of up to 2178 kbit/s by using additional modulation techniques. This
is discussed in more detail in Section 7.4.1.
To reach these transmission speeds, Bluetooth uses a channel in the 2.4 GHz ISM
(Industrial, Scientific and Medical) band with a bandwidth of 1 MHz. Gaussian
Frequency Shift Keying (GFSK) is used as modulation up to Bluetooth 1.2, while
Differential Quadrature Phase Shift Keying (DQPSK) and eight‐phase differential phase
shift keying (8DPSK) are used for EDR packets. Compared to a 22 MHz channel required
for wireless LAN, the bandwidth requirements of Bluetooth are quite modest.
For bidirectional data transmission, the channel is divided into timeslots of 625
microseconds. All devices that exchange data with each other thus use the same chan-
nel and are assigned timeslots at different times. This is the reason for the variable data-
rates shown in Figure 7.1. If a device has a large amount of data to send, up to five
consecutive timeslots can be used before the channel is given to another device. If a
device has only a small amount of data to send, only a single timeslot is used. This way,
all devices that exchange data with each other at the same time can dynamically adapt
their use of the channel based on their data buffer occupancy.