Introduction to IEEE 802.15.4LR-WPANs/ZigBee

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OutlineIntroductionGeneral Description

Network topologiesPHY SublayerMAC SublayerSuperframe StructureFrame Structure

PHY Specification2450 MHz Mode868/915 MHz Mode

Introduction

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The IEEE 802 Family802.1 => Spanning Tree Bridge802.2 => Logical Link Control (LLC) Protocol802.3 => CSMA/CD Networks (Ethernet) MAC Protocol802.4 => Token Bus Networks MAC Protocol802.5 => Token Ring Networks MAC Protocol802.6 => Metropolitan Area Networks (MAN)802.11 => WLAN (wireless local area network)

802.11b => 2.4GHz Band; 11 Mbps; direct-sequence 802.11a => 5.0GHz Band; 54 Mbps; OFDM802.11g => 2.4GHz Band; 54 Mbps; OFDM

802.15 => WPAN (wireless personal area network)802.15.3 UWB (Ultra Wide Band)802.15.4 LR-WPAN (low rate wireless PAN)

802.16 => WLL (wireless local loop)

LAN

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OverviewLR-WPANs stands for low-rate wireless personal area networks.Wireless personal area networks (WPANs) are used to convey information over relatively short distance.Unlike wireless local area networks (WLANs), connections effected via WPANs involve little or no infrastructure. This feature allows small, power-efficient, inexpensive solutions to be implemented for a wide range of devices.Typically operating in the personal operating space(POS) of 10m.

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ZigBee & IEEE 802.15.4

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ZigBee MembershipZigBee Alliance grows to over 90 members (August 16, 2004)

PromoterEmberHoneywellInvensysMitsubishi ElectricMotorolaPhilipsSamsung

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Traffic TypesPeriodic data

Sensors

Intermittent dataLight switch

Repetitive, low-latency dataMouse

The raw data rate will be high enough (maximum of 250 kb/s) to satisfy a set of simple needs such as interactive toys, but scalable down to the needs of sensor and automation needs (20 kb/s or below) for wireless communications.

General Description

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General DescriptionA LR-WPAN is a simple, low-cost communication network that allows wireless connectivity in applications with limited power and relaxed throughput requirements.Some of the characteristics of an LR-WPAN are:

Over-the-air data rates of 250 kb/s, 40 kb/s, and 20 kb/s.Star or peer-to-peer operationAllocated 16 bit short or 64 bit extended addressesAllocation of guaranteed time slots (GTSs)Carrier sense multiple access with collision avoidance(CSMA-CA) channel accessFully acknowledged protocol for transfer reliability

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General DescriptionLow power consumptionEnergy detection (ED)Link quality indication (LQI)16 channels in the 2450 MHz band, 10 channels in the 915 MHz band, and 1 channel in the 868 MHz band

Two different device types can participate in an LR-WPAN network:

Full-function device (FFD)Can talk to RFDs or other FFDs.

Reduced-function device (RFD)Can only talk to an FFD.Intended for applications that are extremely simple.

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Components of the IEEE 802.15.4 WPAN

The most basic component in the IEEE 802.15.4 WPAN is the device.A device can be an RFD or an FFD.Two or more devices within a POS communicating on the same physical channel constitute a WPAN.A network shall include at least one FFD, operating as the PAN coordinator.An IEEE 802.15.4 network is part of the WPAN family of standards.

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Network TopologiesDepending on the application requirements, the LR-WPAN may operate in either of two topologies: the star topology or the peer-to-peer topology.Each independent PAN will select a unique identifier.

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Star TopologyThe communication is established between devices and a single central controller, called the PAN coordinator.A PAN coordinator is the primary controller of the PAN.The PAN coordinator may be mains powered, while the devices will most likely be battery powered.Applications that benefit from a star topology include home automation, personal computer (PC) peripherals, toys and games, and personal health care.

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Star Network FormationAfter an FFD is activated for the first time, it may establish its own network and become the PAN coordinator.All star networks operate independently from all other star networks currently in operation. This is achieved by choosing a PAN identifier, which is not currently used by other network within the radio sphere of influence.Once the PAN identifier is chosen, the PAN coordinator can allow other devices to join its network; both FFDsand RFDs may join the network.

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Peer-to-Peer TopologyThe peer-to-peer topology also has a PAN coordinator.Any device can communicate with any other device as long as they are in range of one another.Allows more complex network formations to be implemented, such as mesh networking topology.Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory tracking, intelligent agriculture, and security would benefit from such a network topology.Can be ad hoc, self-organizing and self-healing.Allow multiple hops to route messages from any device to any other device on the network.

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Peer-to-peer Network FormationEach device is capable of communicating with any other device within its radio sphere of influence.One device will be nominated as the PAN coordinator, for instance, by virtue of being the first device to communicate on the channel.An example of the use of the peer-to-peer communications topology is the cluster-tree.

The cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs.An RFD may connect to a cluster tree network as a leave node at the end of a branch, because it may only associate with one FFD at a time.

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Topology Models

Full function deviceReduced function device

PAN Coordinator

Cluster treeStar Mesh

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LR-WPAN Device ArchitectureThe upper layers consist of

a network layer, which provides network configuration, manipulation, and message routing.an application layer provides the intended function of the device.

LLC: logical link control.

SSCS: service specific convergence sublayer.

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PHY SublayerThe PHY provides two services

The PHY data serviceThe PHY management service interfacing to the physical layer management entity (PLME).

The PHY data service enables the transmission and reception of PHY protocol data units (PPDUs) across the physical radio channel.The features of the PHY are activation and deactivation of the radio transceiver, ED, LQI, channel selection, clear channel assessment (CCA), and transmitting as well as receiving packets across the physical medium.

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ZigBee Operating Bands

868MHz / 915MHz PHY

2.4 GHz

868.3 MHz

Channel 0 Channels 1-10

Channels 11-26

2.4835 GHz

928 MHz902 MHz

5 MHz

2 MHz

2.4 GHz PHY

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Frequency Band and Data Rate

BPSK-92 dbm1040 kbpsAmericasISM915 MHz

BPSK-92 dbm120 kbpsEurope868 MHz

O_QPSK-85 dbm16250 kbpsWorldwideISM2.4 GHz

ModulationRx Sensitivity

# of ChannelsDataCoverageBandFrequency

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MAC SublayerThe MAC sublayer provides two services:

The MAC data serviceThe MAC management service interfacing to the MAC sublayer management entity (MLME) service access point(SAP).

The MAC data service enables the transmission and reception of MAC protocol data units (MPDUs) across the PHY data service.The features of the MAC sublayer are beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association, and disassociation.

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Superframe StructureThe LR-WPAN standard allows the optional use of a superframestructure.The format of the superframe is defined by the coordinator.The superframe is bounded by network beacons, is sent by the coordinator, and is divided into 16 equally sized slots.The beacon frame is transmitted in the first slot of each superframe.If a coordinator does not wish to use a superframe structure, it may turn off the beacon transmissions.The beacons are used to synchronize the attached devices, to identify the PAN, and to describe the structure of the superframes.

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Superframe Structure without GTSs

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Superframe Structure with GTSs

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Frame StructureThe LR-WPAN defines four frame structures

A beacon frame, used by a coordinator to transmit beaconsA data frame, used for all transfers of dataAn acknowledgement frame, used for confirming successful frame receptionA MAC command frame, used for handling all MAC peer entity control transfers

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Schematic View of the Beacon Frame

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Schematic View of the Data Frame

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Schematic View of the Acknowledgement Frame

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Schematic View of the MAC Command Frame

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Concept of Primitives

PHY Specification

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IntroductionThe PHY is responsible for the following tasks:

Activation and deactivation of the radio transceiverEnergy detection (ED) within the current channelLQI for received packetsCCA for CSMA-CAChannel frequency selectionData transmission and reception

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Operating Frequency RangeFrequency bands and data rates

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Channel Assignments and NumberingA total of 27 channels, numbered 0 to 26, are available across the three frequency bands.

Sixteen channels in the 2450 MHz band.Ten channels in the 915 MHz band.One channels in the 868 MHz band.

The center frequency of these channels is defined as follows:

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Receiver Sensitivity Definition

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General Packet FormatEach PPDU packet consists of the following basic components:

A SHR (synchronization header), which allows a receiving device to synchronize and lock onto the bit stream.A PHR (PHY header), which contains frame length information.A variable length payload, which carriers the MAC sublayerframe.

General packet format

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Packet FieldsPreamble field

Used by the transceiver to obtain chip and symbol synchronization with an incoming message.Composed of 32 binary zeros.

SFD (start-of-frame delimiter) fieldAn 8 bit field indicating the end of the synchronization (preamble) field and the start of the packet data.Format of the SFD field

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Packet FieldsFrame length field

7 bits in length and specifies the total number of octets contained in the PSDU.

PSDU fieldHas a variable length and carries the data of the PHY packet.For all packet types of length five octets or greater than sevenoctets, the PSDU contains the MAC sublayer frame (i.e., MPDU).

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PHY Constants

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PHY PIB AttributesPIB: PAN information base.

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2450 MHz PHY SpecificationsData rate: 250 kb/s.Modulation and spreading

Employs a 16-ary quasi-orthogonal modulation technique.During each data symbol period, four information bits are used to select one of 16 nearly orthogonal pseudo-random noise (PN) sequences to be transmitted.The PN sequences for successive data symbols are concatenated.The aggregate chip sequence is modulated onto the carrier using offset quadrature phase-shift keying (O-QPSK)

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2450 MHz PHY SpecificationsReference modulator diagram

Reference transmitter diagram

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Symbol to Chip Mapping

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2450 MHz PHY SpecificationsO-QPSK modulation

The chip sequences representing each data symbol are modulated onto the carrier using O-QPSK with half-sine pulse shaping.

Pulse shape

( )sin 0 2

20

cc

t t Tp t T

otherwise

π⎧ ⎛ ⎞

≤ ≤⎪ ⎜ ⎟= ⎨ ⎝ ⎠⎪⎩

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2450 MHz PHY SpecificationsSample baseband chip sequences with pulse shaping

Symbol rateThe 2450 MHz PHY symbol rate shall be 62.5 ksymbol/s.

Receiver sensitivityA compliant device shall be capable of achieving a sensitivity of -85 dBm or better.

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868/915 MHz PHY Specifications868/915 MHz band data rates

868 MHz: 20 kb/s.915 MHz: 40 kb/s.

Modulation and SpreadingThe 868/915 MHz PHY shall employ direct sequence spread spectrum (DSSS).The binary phase-shift keying (BPSK) is used for chip modulation.Differential encoding is used for data symbol encoding.

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868/915 MHz PHY SpecificationsReference modulator diagram

Differential encodingDifferential encoding is the modulo-2 addition (exclusive or) of a raw data bit.

1

1

is the raw data bit being encoded, is the corresponding differentially encoded bit,

is the previous differentially encoded bit.

n n n

n

n

n

E R EREE

−

−

= ⊕

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868/915 MHz PHY SpecificationsFor each packet transmitted, R1 is the first raw bit to be encoded and E0 is assumed to be zero.Conversely, the decoding process, as performed at the receiver, can be described by:

For each packet received, E1 is the first bit to be decoded, and E0 is assumed to be zero.

Bit-to-chip mappingEach input bit shall be mapped into a 15-chip PN sequence

1n n nR E E −= ⊕

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868/915 MHz PHY SpecificationsBPSK modulation

The chip sequences are modulated onto the carrier using BPSK with raised cosine pulse shaping (roll-off factor = 1).The chip rate is 300 kchip/s for the 868 MHz band and 600 kchip/s in the 915 MHz band.

Pulse shapeThe raised cosine pulse shape (roll-off factor = 1) used to represent each baseband chip is described by

( ) ( ) ( )( )2 2

sin / cos // 1 4 /

c

c

t T t Tp t

t T t Tπ π

π=

−

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868/915 MHz PHY SpecificationsSymbol rate

868 MHz: 20 ksymbol/s915 MHz: 40 ksymbol/s

Receiver sensitivityA compliant device shall be capable of achieving a sensitivity of -92 dBm or better.

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Receiver Architecture

Half-sine

Matched Filter

A/D

Packet

Detection

Fine Syn.and/or

Start of Data

DownSampling

to Chip Rate

RF

Over-SamplingRate (n‧chip rate)

CoarseSynchronization

Despreadingto

(Sym. Rate)

OQPSKDemodulation(Sym. Rate)

Detection

(Sym. Rate)Data Stream

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Despreading and Demodulation

CI1 CI2 CI3 CI4 CI5 CI6 CI16

CQ1 CQ2 CQ3 CQ4 CQ5 CQ6 CQ16

Find

Maximum

+

-

+

+

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CSMA/CA AlgorithmThe CSMA/CA algorithm shall be used before the transmission of data or MAC command framestransmitted within the CAP, and shall not be used for the transmission of beacon frames, acknowledgment frames or data frames transmitted in the CFP.NB is the number of times the CSMA/CA algorithm was required to backoff.CW defines the number of backoff periods that need to be clear of channel activity.BE is related to how many backoff periods a device shall wait before assess a channel.

＊backoff = 20 symbols

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Slotted

BE=lesser of (2,macMinBE)

BE=macMinBE

Delay for random unit backoff periods

Locate backoff period boundary

(2 1)BE −

Battery life extension?

NB=0,CW=2

Performance CCA on backoff period boundary

Channel idle?

CW=2,NB=NB+1,BE=min(BE+1,aMaxBE)

CW=CW-1

NB>macMaxCSMABackoff? CW=O?

CSMA-CA

Failure Success

N

Y

Y

N

N

N

Y Y

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Unslotted

NB=0,BE=macMinBE

Delay for random unit backoff periods

(2 1)BE −

Perform CCA

Channel idle?

NB=NB+1,BE=min(BE+1,aMaxBE)

NB>macMaxCSMABackoffs?

Failure Success

N

N

Y

Y

CSMA-CA