Bluetooth 2
- Bluetooth enhanced data rate ( EDR )
Bluetooth is now well established as a wireless technology.
It has found a very significant number of applications, particularly in areas
such as connecting mobile or cell phones to hands-free headsets. One of the
disadvantages of the original version of Bluetooth in some applications was that
the data rate was not sufficiently high, especially when compared to other
wireless technologies such as 802.11. In November 2004, a new version of
Bluetooth, known as Bluetooth 2 was ratified. This not only gives an enhanced
data rate but also offers other improvements as well.
Of all the features included in Bluetooth 2, it is the
enhanced data rate (EDR), facility that is giving rise to the most comment. In
the new specification the maximum data rate is able to reach 3 Mbps, a
significant increase on what was available in the previous Bluetooth
specifications.
Why is Bluetooth 2 needed?
As proved particularly by the computer industry, there is always a need for
increased data rates, and ever increasing capacity. With this in mind and the
fact that the previous version of Bluetooth, version 1.2 allowed a maximum data
rate of 1 Mbps which reflected in a real throughput of 723 kbps, the next
specification should allow many new applications to be run. In turn this will
open up the market for Bluetooth even more and allow further application areas
to be addressed.
While speed on its own opens up more opportunities, the
strategy behind Bluetooth 2 with its enhanced data rate is more deep rooted.
When the Bluetooth 2 specification was released there were no applications that
were in immediate need of the new enhanced data rate. For example even a high
quality stereo audio stream required a maximum of only 345 kbps.
The reason is that as Bluetooth use increases, and the number
of applications increase, that users will need to run several links
concurrently. Not only may Bluetooth need to be used for streaming audio, but
other applications such as running computer peripherals will increase. The
reason becomes clearer when looking at real situations when interference is
present. Typically it is found that a good margin is required to allow for
re-sends and other data. Under Bluetooth 1.2, high quality stereo audio can be
sent on its own within the available bandwidth and with sufficient margin.
However when other applications are added there is not sufficient margin to
allow for the system to operate satisfactorily. Bluetooth 2 solves this problem
and enables sufficient bandwidth for a variety of links to be operated
simultaneously, while still allowing for sufficient bandwidth margin within the
system.
There are other advantages to running Bluetooth 2. One of the
major elements is in terms of power consumption. Although the transmitter and
receiver and logic need to be able to handle data at a higher speed which
normally requires a higher current consumption, this is more than outweighed by
the fact that they need only to remain fully active for about a third of the
time. This brings significant advantages in terms of battery life, a feature
that is of particular important in many of the Bluetooth applications.
Compatibility is a major requirement when any system is
upgraded. The same is true for Bluetooth, and this has been a major requirement
and concern when developing the Bluetooth 2 standard. The new standard is
completely backward compatible and allows networks to contain a mixture of EDR
(enhanced data rate) devices as well as the standard devices. A key element of
this is that the new modulation schemes that have been incorporated into
Bluetooth 2 are compatible in their nature with the standard rate specification.
In this way the new standard will be able to operate with any mixture of devices
from whatever standard.
How it works
One of the main reasons why Bluetooth 2 is able to support a much higher data
throughput is that it utilises a different modulation scheme for the payload
data. However this is implemented in a manner in which compatibility with
previous revisions of the Bluetooth standard is still retained.
Bluetooth data is transmitted as packets that are made up
from a standard format. This consists of four elements which are: (a) The Access
Code which is used by the receiving device to recognise the incoming
transmission; (b) The Header which describes the packet type and its length; (c)
The Payload which is the data that is required to be carried; and finally (d)
The Inter-Packet Guard Band which is required between transmissions to ensure
that transmissions from two sources do not collide, and to enable the receiver
to re-tune.
In previous versions of the Bluetooth standard, all three
elements of the transmission, i.e. Access Code, Header and Payload were
transmitted using Gaussian Frequency Shift Keying (GFSK) where the carrier is
shifted by +/- 160 kHz indicating a one or a zero, and in this way one bit is
encoded per symbol.
The Bluetooth 2.0 specification uses a variety of forms of
modulation. GFSK is still used for transmitting the Access Code and Header and
in this way compatibility is maintained. However other forms of modulation can
be used for the Payload. There are two additional forms of modulation that have
been introduced. One of these is mandatory, while the other is optional.
A further small change is the addition of a small guard band
between the Header and the payload. In addition to this a short synchronisation
word is inserted at the beginning of the payload.
Mandatory modulation format
The first of the new modulation formats which must be included on any Bluetooth
2 device gives a two fold improvement in the data rate and thereby allows a
maximum speed of 2 Mbps. This is achieved by using pi/4 differential quaternary
phase shift keying (pi/4 DQPSK). This form of modulation is significantly
different to the GFSK that was used on previous Bluetooth standards in that the
new standard uses a form of phase modulation, whereas the previous ones used on
frequency modulation.
Using quaternary phase shift modulation means that there are
four possible phase positions for each symbol. Accordingly this means that two
bits can be encoded per symbol, and this provides the two fold data increase
over the frequency shift keying used for the previous versions of Bluetooth.
Higher speed modulation
To enable the full three fold increase in data rate to be achieved a further
form of modulation is used. Eight phase differential phase shift keying (8DPSK)
enables eight positions to be defined with 45 degrees between each of them. By
using this form of modulation eight positions are possible and three bits can be
encoded per symbol. This enables the data rate of 3 Mbps to be achieved.
As the separation between the different phase positions is
much smaller than it was with the QPSK used to provide the two fold increase in
speed, the noise immunity has been reduced in favour of the increased speed.
Accordingly this optional form of modulation is only used when a link is
sufficiently robust.
Packet formats
The Bluetooth 2 specification defines ten new packet formats for use with the
higher data rate modulation schemes, five each for each of the enhanced data
rate schemes. Three of these are for the 1, 3 and 5 slot asynchronous packets
used for transferring data. The remaining two are used for 3 and 5 slot extended
Synchronous Connection Orientated (eSCO) packets. These use bandwidth that is
normally reserved for voice communications.
The new format for these packets does not incorporate FEC. If
this is required then the system switches back automatically to the standard
rate packets. However many of the links are over a very short range where the
signal level is high and the link quality good.
It is necessary for the packet type to be identified so that
the receiver can decode them correctly, knowing also the type of modulation
being used. An identifier is therefore included in the header which is sent
using GFSK. This packet header used for the previous version of Bluetooth only
used 4 bits. This gave sufficient capability for the original system. However
there was insufficient space for the additional information that needed to be
sent for Bluetooth 2.
It was not possible to change the header format because
backward compatibility would not be possible. Instead different link modes are
defined. When two Bluetooth 2 or EDR devices communicate the messages are used
in a slightly different way, indicating the Bluetooth 2 or EDR modes. In this
way compatibility is retained while still being able to carry the required
information.
Summary
Bluetooth 2 / EDR is a significant improvement to Bluetooth and will enable it
to retain its position in the market place. Its introduction, as the Bluetooth
has become more widely accepted and used will enable it to build on its position
within the market place.
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