Last month, I covered the applications and architecture of Bluetooth. This month, we go over the Bluetooth radio and baseband services.
By
Mike Rodbell
The physical aspects of the Bluetooth interfaces are a straightforward use of the RF transmission spectrum. The logical link, or baseband aspects of the interface protocol, are considerably more involved, including multi-user access over frequency-hopped transmissions.
Figure 1
shows the overall Bluetooth protocol stack.
The RF Interface
One issue that always presents itself to systems implementers or designers wishing to build wireless communication systems is the selection and authorization to use frequency in the wireless RF spectrum. Limiting factors include the frequencies (the designer doesn’t want to interfere with other transmissions) and the amount of power that can be transmitted over the selected frequencies (how far it can go without interfering with other transmissions).
The
spectrum allocated to Bluetooth is known as the Industrial, Scientific, and Medical (ISM) band. The ISM frequency band is generally available for transmission by several types of equipment, making it suitable for the Bluetooth application. Recognizing that other equipment can also use this band, Bluetooth gets around interference problems by employing a frequency hopping transmission algorithm.
The actual ISM frequency range can varies according to location (see
Table 1
).
Recently, however, there has been some additional convergence in the allocated bands in different countries. There are also ranges of guard bands that need to be observed to ensure that the Bluetooth signals stay out of the way of other transmissions (and vice versa). Fortunately, the differences in the frequency allocations are minor, with some countries limiting access to the edges of the allocated spectrum. As a result, universal Bluetooth devices can be developed and configured for acceptable operation in
different parts of the world. To interoperate, the devices need to be configured to use the same set of frequencies. There are no standards for negotiating the spectrum utilization, and so the Bluetooth link/RF interface is frequency hopped. When operating in countries that permit only a subset of the overall spectrum, equipment merely needs to hop through the approved portions of the spectrum.
| Table 1: International Bluetooth Frequency Allocations
|
| Geography
| Regulatory Range
| RF Channels
|
| US, Europe, and most other countries
| 2.400 to 2.4835 GHz
| f=2.402 + k
MHz, k=0,...,78
|
| Spain
| 2.445 to 2.475 GHz
| f=2.449 + k MHz, k=0,...,22
|
| France
| 2.4465 to 2.4835 GHz
| f=2.454 + k MHz, k=0,...,33
|
Once the frequency bands are setup, the power and actual bandwidth of each individual
transmission must be considered. As a frequency-hopping protocol, the Bluetooth air interface divides the transmission spectrum into a set of 1-MHz wide bands. Output power is another matter, with three classes of equipment that differ from one another on the basis of their power capabilities. The power classes are defined to include:
- Class 1.
Outputs 100mW (+20 dBm) for maximum range. In this class, power control is mandatory ranging from 4 to 20
dBm. This mode provides the greatest distance.
- Class 2.
Outputs 2.5mW (+4 dBm) at maximum. Minimum power 0.25mW (-6 dBm). Power control can be implemented, but is not necessary.
- Class 3.
Lowest power. Nominal output is 1mW (0 dBm).
Power control is only required in Class 1 devices to keep the devices from emitting any more than the necessary RF power. In the case of Class 2 and Class 3
devices, power control is optional, but may be useful when implemented in low-power applications. This power control algorithm is based on interaction between different nodes in a piconet (a small network of Bluetooth devices that directly communicate with one another). This interaction is based on use of the Bluetooth Link Management Protocol (LMP).
Data symbols are sent using a fairly simple modulation scheme called Gaussian frequency-shift keyed (GFSK) modulation. Here, a binary 1 is represented as
a positive frequency deviation and a binary 0 is represented as a negative frequency deviation. By combining this RF scheme with the frequency-hopping algorithms specified in the baseband scheme, Bluetooth is able to provide the needed data transmission services. The signaling rate is 1 million symbols/second.
IF Filters
The term “IFfilter” can represent many things.
The IF filter plays a special role in radio and TV systems, where the input frequency is variable across the set of channels. The heterodyne receiver greatly simplifies radio receiver implementation. Rather than carefully tuning a high-performance receiver filter to the desired channel, the heterodyne receiver relies on a single frequency high-performance filter. Being single frequency provides the design with more stability, more reliability, and very high performance, something difficult to accomplish for
tunable filters at RF frequencies. Instead, the channel frequency is adjusted to a fixed IF frequency by varying the LO frequency, sweeping a different channel through the IF filter as the mixer stimulus varies. When this technique is used, a single high-performance filter can be implemented to provide the signal quality filtering.
Example filters that fill this function are crystal filters (extremely narrowband filters at RF frequencies), and surface acoustic wave (SAW) filters. SAW filters are
known particularly for their impressive shape factor capabilities. That is, the filter characteristics can closely approximate the ideal “brick wall” filter. This technique allows many more channels to share the spectrum, since large guard bands to avoid interference from adjacent channels are no longer necessary.
The baseband specification
The broad range of
techniques and services covered by the baseband operation is described in the easy-to-understand RF portion of the Bluetooth specification. Data encoding, transmitter management, frequency hopping, security, error detection and correction, and framing are all covered by the link services specified in the baseband specifications.
One of the basic assumptions of the Bluetooth communication protocols is that only small sets of stations actively participate in a communication session at any particular time. These
small groups of machines are organized into piconets. Each one can contain one master station, and up to seven slave stations. In networks where more than eight machines need to communicate, a group of piconets can be gathered into a scatternet. In the case of a scatternet, any of the stations within a single piconet can also communicate in a second piconet, provided that they act as a master for only one piconet at a time.
Frequency hopping
The Bluetooth air interface achieves its transmission goals through frequency hopping. The basic concepts of the Bluetooth hopping scheme include:
- Time synchronization.
Transmissions must be on frequencies known to both the transmitter and receivers, and at time intervals that are also synchronized.
- Information delivery.
Bluetooth
information is sent in packet formats, with a packet being contained between one and five frequency hops.
- Frequency selection.
A shared algorithm is required for the frequency selection. In the case of Bluetooth, the master station’s device address and clock determine the frequencies. For implementations in the US and Europe, there are seventy-nine channels from which to chose. In other parts of the world the spectrum is limited to twenty-three channels.
- Hop frequency.
The hop rate is 1,600 frequencies/second (hop time length of 625 ms).
With a combination of several frequencies to select from and a wide range of hopping sequences, Bluetooth transmissions can be reliable and can avoid collisions with transmissions on physically adjacent piconets.
Time-slot division communications
The master station in the piconet manages the time slot transmission arrangement. As each station receives information from the master, it then determines which slots to use for transmission. The master station sends all of its information in the even numbered time slots, and the slave station(s) transmit information in the alternating odd time slots.
Data transmission
Two general types of logical links can be established between stations:
Synchronous connection oriented (SCO) links that allocate fixed circuit-switched bandwidth between a point-to-point connection involving the master station and a single slave station.
Asynchronous connection less (ACL) links that are used to pass framed data between stations in the piconet.
With these two data formats, the Bluetooth
system is able to handle all of the voice and data applications that we reviewed in last month’s column. Up to three SCO connections can be transported in a single piconet, and the ACL traffic is limited primarily by the transmission quality and data rate of the underlying RF transport. The ACL traffic flow is managed by the master station, with slave stations speaking only when spoken to.
Data framing
Like any information transport service, the link control protocol is responsible for the framing and delivery of the basic data elements. All packet information is transmitted in the following frame format:
Access code.
This is a 72-bit field that contains a 4-bit preamble, 64 bits of synchronization information, and 4 bits of trailer. There are three types of access codes in the specification:
Channel access codes
Device access codes
Inquiry access codes
Header.
This is a 54-bit field that is used to identify the frame type. The header field contains six fields, which are:
AM_ADDR: 3- bit active member address
TYPE: 4-bit type code
FLOW: 1-bit flow control
ARQN: 1-bit acknowledge indication
SEQN: 1-bit sequence number
HEC: 8-bit header error check
Payload.
0 to 2,745 bits of application data. This is the portion of the frame that contains the application-specific information being transported by the Bluetooth link layer services.
With this collection of information, the Bluetooth link control protocol can be used to manage the
associations and delivery of information between the various stations within a piconet or scatternet. This format is used for both synchronous (voice) and asynchronous (data) modes of operation, with specific formats being specified for the voice transport mode. Several frame types are available to assist in the sequencing of control and data information between the master and slave stations.
As in the case of any wireless interface, there are issues associated with the reliable delivery of
information. Factors such as noise and interference from other ISM band transmissions must be addressed. Bluetooth provides two general mechanism types to ensure the accurate delivery of information, forward error correction (FEC) and automatic repeat request (ARQ). FEC is typically applied to the SCO traffic, where the timeliness of the delivery takes precedence over the accuracy; late voice traffic is no better than undelivered traffic. ARQ mechanisms are employed for data applications.
Data multiplexing
The link data frames are designed to transport a range of data types. There are five logical data channels that can be transported in the data link payload:
- Link control (LC).
Used to manage the flow of packets over the link interface.
- Link manager (LM).
Transports link
management information between participating stations.
- User asynchronous (UA)/user isochronous (UI).
Carries user data.
- User synchronous (US)
. Carries synchronous (SCO) data.
All of these frame types are carried over the air interface.
This month, we’ve covered some general information that should help you get a feel for the nature of the Bluetooth RF/baseband interface operation. There is a
considerable amount of detailed reference information in the Bluetooth specification, that is available over the internet at the Bluetooth forum web site (
www.bluetooth.com
).
Mike Rodbell
is director of embedded software development for CIENA Communications, Inc. He has developed voice and data communication systems for a wide range of commercial and military systems.
He holds a BSCS from Trinity College in Hartford, CT, and MSEE from Loyola College of Baltimore, MD. He can be reached at
mrodbell@ciena.com
or
http://www.ciena.com.
.
Puzzled by a network processing design issue?
