The Bluetooth Radio SystemJaap C. HaartsenEricsson Radio Systems B.V.

IEEE Personal CommunicationFebruary 2000

OutlineAd hoc radio connectivityBluetooth radio system architectureBluetooth standardizationConclusion

Ad Hoc Radio ConnectivityFigure 1. Topologies foracellular radio with squares representing stationary base stationsbconventional ad hoc systemscscatter ad hoc systems

Ad Hoc Radio ConnectivityEnvironmental characteristics have impact on the following fundamental issuesApplied radio spectrumDetermining which units are available to connect toConnection establishmentMultiple access schemeChannel allocationMedium access controlService prioritizationi.e. voice before dataMutualinterferencePower consumption

Bluetooth Radio System ArchitectureRadio spectrumInterference immunityMultiple access schemeThe modulation schemeMedium access controlPacket-based communicationsPhysical link definition

Bluetooth Radio System ArchitectureConnection establishmentHop selection mechanismError correctionPower managementSecurityInterpiconet communications

Radio SpectrumThe IndustrialScientificMedicalISMbandaround 2.45GHzThe regulations in different parts of the world differThe regulations generally specify the spreading of transmitted signal energy and maximum allowable transmit power

Interference ImmunityIn addition to interference from external sourcesmicrowave ovensco-user interference must be taken into accountInterference immunity can be abtained by interference suppression or avoidance

Interference ImmunitySuppression can be obtained by coding or direct-sequence spreadingInterference avoidance in frequency is more attractive and more practical

Multiple Access SchemeFDMA is attractive but dont fulfill the spreading requirements set in the ISM bandTDMA needs a common timing referencecumbersomeCDMA offers the best propertiesDirect sequenceDS-CDMA is less attractiveFrequency-hoppingFH-CDMA is the best choices

Multiple Access SchemeBluetooth is based on FH-CDMAIn the 2.45 GHz ISM banda set of 79 hop carriers have been defined at a 1 MHz spacingThe channel is a hopping channel with a nominal hop dwell time of 625 usThe hopping sequence is determined by the master that controls the FH channel

Multiple Access SchemeAll other participants on the hopping channel are slavesFull-duplex communication is achieved by applying time-division duplexTDDThis means that a unit alternatively transmits and receives

Multiple Access SchemeFigure 2. An illustration of the FH/TDD channel applied in Bluetooth

The Modulation SchemeA binary modulation scheme was chosenBluetooth uses Gaussian-shaped frequency shift keyingFSKmodulation with a nominal modulation index of k = 0.3The modulation scheme allows the implementation of low-cost radio units

Medium Access ControlBluetooth allows a large number of uncoordinated communications to take place in the same areaAn FH Bluetooth channel is associated with a piconetThe number of units that can participate on a common channel is limited to eightone master and seven slaves

Medium Access ControlMaster controls the traffic on the piconet and takes care of access controlIn order to prevent collisions on the channel due to multiple slave transmissionsthe master applies a polling techniqueIndependent collocated piconets may interferethen ALOHA is applied

Packet-Based CommunicationsFigure 3. The format of packets applied in Bluetooth

Packet-Based CommunicationsAccess codeIncludes the identity of the piconet masterOnly if the access code matches the access code corresponding to the piconet master will the packet be accepted by the recipientThis prevents packets sent in one piconet falsely being accepted by units of another piconet that happens to land on the same hop carrier

Packet-Based CommunicationsPacket headerContains link control information3-bit slave address1-bit ACK/NACK for ARQ scheme4-bit packet type code to define 16 different payload types8-bit header error checkHEC

Packet-Based CommunicationsBluetooth defines four control packetsThe ID packetThe NULL packetThe POLL packetThe FHS packetThe remaining 12 packets is divided into synchronous and asynchronous services

Packet-Based CommunicationsFigure 4. The frequency and timing characteristics of single-slotthree-slotand five-slot packets

Physical Link DefinitionTwo physical link types defineSynchronous connection-orientedSCOlinkfor voice trafficpoint-to-pointAsynchronous connectionlessACLlinkfor bursty data trafficpoint-to-multipointThe ACL link can use all of the remaining slots on the channel not used for SCO links

Physical Link DefinitionFigure 5. An example of mixing synchronous SCO links and asynchronous ACL links on a single piconet channel

Connection EstablishmentThree elements have been defined to support connection establishmentscanpageand inquiryWhen a Bluetooth unit wakes up to scanit opens its sliding correlator which is matched to the access code derived from its own identityThe scan window is a little longer than 10msEvery time the unit wakes upit scans at a different hop carrier

Connection EstablishmentPaging units help the unit that wants to connect setup up the connectionThe paging unit transmits the access code repeatedly at different frequencies every 1.25msthe paging unit transmits two access codes and listens twice for a response

Connection EstablishmentFigure 6. Frequency and timing behavior for a Bluetooth paging unit

Connection EstablishmentThe maximum access delay therefore amounts to twice the sleep timeThe paging unit becomes the master using its identity and clock to define the FH channeland the idle unit becomes the slaveThe above-described paging process assumes that the paging unit has no knowledge at all of the idle units clock

Connection EstablishmentIf the units have met beforethe paging unit can have an estimate of the clock and the phase in the idle unitIn this casethe average response time is reduced to half the sleep time

Connection EstablishmentTo establish a connectiona unit may broadcast an inquiry message that induces recipients to return their address and clock informationFor the return of the FHS packeta random backoff mechanism is used to prevent multiple recipients transmitting simultaneously

Hop Selection MechanismFigure 7. The basic concept of hop selection in Bluetooth

Hop Selection MechanismThe mechanism satisfies the following requirementsThe sequence is selected by the unit identitythe phase by the unit clockThe sequence cycle covers about 23 hours32 consecutive hops span about 64 MHz of spectrumOn averageall frequencies are visited with equal probabilityThe number of hop sequences is very largeBy changing the clock and/or identitythe selected hop changes instantaneously

Hop Selection MechanismFigure 8. The hop selection mechanismthe dashed line for the more significant clock part is used in connection mode only

Error CorrectionBluetooth includes both FEC and packet retransmission schemesFor FECa 1/3-rate code and 2/3-rate FEC code are supported1/3-rate code uses a 3-bit repeat coding applied on packet headers and the payload of the synchronous packets2/3-rate FEC cod uses a shortened Hamming code applied on both the payload of the synchronous and asynchronous packets

Error CorrectionBluetooth implemented a fast-ARQ schemeFigure 9. An example of retransmission operation in Bluetooth

Error CorrectionFigure 10. ARQ mechanisms where received ACK/NAK information decides on retransmission and received payload determines transmitted ACK/NAK information

Power ManagementIn the idle modethe unit only scans a little over 10ms every Ts where Ts can range from 1.28 to 3.84sPARK mode where the duty cycle can be reduced even more can only be applied after the piconet establishedIn the SNIFF mode during connectionthe slave does not scan at every master-to-slave slotbut has a large interval between scans

Power ManagementIf no useful information needs to be exchangedno transmission takes placeSince power control cannot be coordinated among different systemsit cannot be prevented that certain systems always try to overpower their contendersand the strongest transmitter will prevail

SecurityAt connection establishmenta conventional challenge-response is carried out

Figure 11. The Bluetooth authentication procedure

SecurityBefore the transmission of each packetthe LFSR is initialized by a combination of EN_RANDthe master identityan encryption keythe slot numberThe 128-bit link key is a secret key residing in the Bluetooth hardware and is generated during an initialization phaseBluetooth provides a limited number of security elements at the lower level

Interpiconet CommunicationTens of piconets operate in the same area without noticeable performance degradationAt any instant in time a unit can communicate in one piconet onlyThe unit can jump from one piconet to another by adjusting the piconet channel parametersi.e. the master identity and master clock

Interpiconet CommunicationHOLD mode has been introduced to allow a unit to temporary leave on piconet and visit anotherTraffic scheduling and routing in a scatternet with interpiconet communications is a challenge and still a subject for future study

Bluetooth StandardizationFigure 12. The Bluetooth protocol stack

ConclusionA robust radio system which provides a universal wireless interface to a large range of low-costportable devicesDescribe the motivation of the various design choices