Microsoft PowerPoint - ZigBee

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1 Wireless Information Transmission System Lab. Introduction to IEEE Low-Rate Wireless Personal Area Networks (LR-WPANs) 國立中山大學通訊工程研究所李志鵬 Mobile: National Sun Yat-sen University

2 Introduction General Description Outline Components of the IEEE WPAN Network topologies Architecture: PHY and MAC Superframe Structure Data transfer models Frame Structures CSMA-CA mechanism Concepts of primitives 2

3 Outline PHY Specification 2450 MHz Mode 868/915 MHz Mode Baseband Signal Processing 2450 MHz Mode 868/915 MHz Mode 3

4 Wireless Information Transmission System Lab. Introduction National Sun Yat-sen University

5 The IEEE 802 Family => Spanning Tree Bridge => Logical Link Control (LLC) Protocol => CSMA/CD Networks (Ethernet) MAC Protocol => Token Bus Networks MAC Protocol => Token Ring Networks MAC Protocol => Metropolitan Area Networks (MAN) 5

6 The IEEE 802 Family => WLAN (wireless local area network) b => 2.4GHz Band; 11 Mbps; direct-sequence a => 5.0GHz Band; 54 Mbps; OFDM g => 2.4GHz Band; 54 Mbps; OFDM n => OFDM+MIMO => WPAN (wireless personal area network) Bluetooth UWB (Ultra Wide Band) LR-WPAN (Low-Rate Wireless PAN) => Fixed Broadband Wireless Access Systems => Mobile Broadband Wireless Access Systems 6

7 Overview LR-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. 7

8 ZigBee & IEEE

9 ZigBee Membership ZigBee Alliance grows to over 90 members (August 16, 2004) Promoter Ember Honeywell Invensys Mitsubishi Electric Motorola Philips Samsung 9

10 Periodic data Sensors Intermittent data Light switch Repetitive, low-latency data Mouse Traffic Types 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. 10

11 Wireless Information Transmission System Lab. General Description National Sun Yat-sen University

12 General Description A LR-WPAN is a simple, low-cost communication network that allows wireless connectivity in applications with limited power and relaxed throughput requirements. The main objectives of an LR-WPAN are ease of installation, reliable data transfer, short-range operation, extremely low cost, and a reasonable battery life, while maintaining a simple an flexible protocol. 12

13 General Description 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 operation Allocated 16 bit short or 64 bit extended addresses Allocation of guaranteed time slots (GTSs) Carrier sense multiple access with collision avoidance (CSMA- CA) channel access Fully acknowledged protocol for transfer reliability Low power consumption Energy 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 13

14 General Description Two different device types can participate in an LR- WPAN network: Full-function device (FFD) Can operate in three modes: a PAN coordinator, a coordinator, or a device. Can talk to RFDs or other FFDs. Reduced-function device (RFD) Can only talk to an FFD. Intended for applications that are extremely simple. Do not have the need to send large amounts of data. May only associate with a single FFD at a time. 14

15 Components of the IEEE WPAN The most basic component in the IEEE 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. A well-defined coverage area does not exist for wireless media because propagation characteristics are dynamic and uncertain. 15

16 Network Topologies Depending 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. 16

17 Network Topologies All devices operating on a network of either topology shall have unique 64 bit extended addresses. This address can be used for direct communication within the PAN, or it can be exchanged for a short address allocated by the PAN coordinator when the device associates. Each independent PAN will select a unique identifier. This PAN identifier allows communication between devices within a network using short addresses and enables transmissions between devices across independent networks. 17

18 Star Topology The communication is established between devices and a single central controller, called the PAN coordinator. A device is either the initiation point or the termination point for network communications. A PAN coordinator can be used to initiate, terminate, or route communication around the network. 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. 18

19 Star Network Formation After 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 FFDs and RFDs may join the network. 19

20 Peer-to-Peer Topology The 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. 20

21 Peer-to-peer Network Formation Each 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. 21

22 Cluster Tree Network 22

23 Architecture The LR-WPAN architecture is defined in terms of a number of blocks in order to simplify the standard. These blocks are called layers. Each layer is responsible for one part of the standard and offers services to the higher layers. The layout of the blocks is based on the open systems interconnection (OSI) seven-layer model. The interfaces between the layers serve to define the logical links that are described in the standard. 23

24 LR-WPAN Device Architecture 24

25 Architecture An LR-WPAN device comprises PHY: contains the radio frequency (RF) transceiver along with its low-level control mechanism. MAC: provides access to the physical channel for all types of transfer. The 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. The logical link control (LLC) can access the MAC sublayer through the service specific convergence sublayer (SSCS), defined in Annex A. 25

26 PHY Sublayer The PHY provides two services The PHY data service The 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. 26

27 ZigBee Operating Bands 2.4 GHz PHY Channels MHz 2.4 GHz GHz 868MHz / 915MHz PHY Channel 0 Channels MHz MHz 902 MHz 928 MHz 27

28 Frequency Band and Data Rate Frequency Band Coverage Data # of Channels Rx Sensitivity Modulation 2.4 GHz ISM Worldwide 250 kbps dbm O_QPSK 868 MHz Europe 20 kbps 1-92 dbm BPSK 915 MHz ISM Americas 40 kbps dbm BPSK 28

29 MAC Sublayer The MAC sublayer provides two services: The MAC data service The 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. 29

30 Superframe Structure The LR-WPAN standard allows the optional use of a superframe structure. 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. Any device wishing to communicate during the contention access period (CAP) between two beacons shall compete with other devices using a slotted CSMA-CA mechanism. 30

31 Superframe Structure without GTSs 31

32 Superframe Structure with GTSs For low-latency applications or applications requiring specific data bandwidth, the PAN coordinator may dedicate portions of the active superframe to that application. These portions are called guaranteed time slots (GTSs). The GTSs form the contention-free period (CFP), which always appears at the end of the active superframe starting at a slot boundary immediately following the CAP. The coordinator may allocate up to seven of these GTSs, and a GTS may occupy more than one slot period. 32

33 Superframe Structure with GTSs 33

34 Data Transfer Model Three types of data transfer transactions exist: Data transfer to a coordinator in which a device transmits the data. Data transfer from a coordinator in which the device receives the data. Data transfer between two peer devices. In star topology only two of these transactions are used, because data may be exchanged only between the coordinator and a device. In peer-to-peer topology, all three types may be used. 34

35 Data transfer to a coordinator Communication to a coordinator in a beacon-enabled network Slotted CSMA-CA 35

36 Data transfer to a coordinator Communication to a coordinator in a nonbeacon-enabled network Unslotted CSMA-CA 36

37 Data transfer from a coordinator Communication from a coordinator in a beacon-enabled network Slotted CSMA-CA Slotted CSMA-CA 37

38 Data transfer from a coordinator Communication from a coordinator in a nonbeacon-enabled network Unslotted CSMA-CA Unslotted CSMA-CA 38

39 Peer-to-peer data transfers In a peer-to-peer PAN, every device may communicate with every other device in its radio sphere of influence. In order to do this effectively, the devices wishing to communicate will need to Receive constantly Transmit data using unslotted CSMA-CA. Synchronize with each other Other measures need to be taken in order to achieve synchronization. 39

40 Frame Structure The LR-WPAN defines four frame structures A beacon frame, used by a coordinator to transmit beacons A data frame, used for all transfers of data An acknowledgement frame, used for confirming successful frame reception A MAC command frame, used for handling all MAC peer entity control transfers 40

41 Schematic View of the Beacon Frame 41

42 Schematic View of the Data Frame 42

43 Schematic View of the Acknowledgement Frame 43

44 Schematic View of the MAC Command Frame 44

45 CSMA-CA Mechanism The CSMA-CA algorithm shall be used before the transmission of data or MAC command frames transmitted within the CAP. The CSMA-CA algorithm shall not be used for the transmission of beacon frames, acknowledgement frames or data frames transmitted in the CFP. 45

46 CSMA-CA Mechanism NB is the number of times the CSMA-CA algorithm was required to backoff while attempting the current transmission; this value shall be initialized to 0 before each new transmission attempt. CW is the contention window length, defining the number of backoff periods that need to be clear of channel activity before the transmission can commence; this value shall be initialized to 2 before each transmission attempt and reset to 2 each time the channel is assessed to be busy. The CW variable is only used for slotted CSMA-CA. BE is the backoff exponent which is related to how many backoff periods a device shall wait before attempting to assess a channel. 46

47 CSMA-CA Mechanism Beacon-enabled networks use a slotted CSMA-CA channel access mechanism, where the backoff slots are aligned with the start of the beacon transmission. Each time a device wishes to transmit data frames during the CAP, it shall locate the boundary of the next backoff slot and then wait for a random number of backoff slots. If the channel is busy, following this random backoff, the device shall wait for another random number of backoff slots before trying to access the channel again. If the channel is idle, the device can begin transmitting on the next available backoff slot boundary. Acknowledgment and beacon frames shall be sent without using a CSMA-CA mechanism. 47

48 Slotted CSMA-CA 48

49 CSMA-CA Mechanism Nonbeacon-enabled networks use an unslotted CSMA- CA channel access mechanism. Each time a device wishes to transmit data frames or MAC commands, it shall wait for a random period. If the channel is found to be idle, following the random backoff, the device shall transmit its data. If the channel is found to be busy, following the random backoff, the device shall wait for another random period before trying to access the channel again. Acknowledgment frames shall be sent without using a CSMA- CA mechanism. 49

50 Unslotted CSMA-CA 50

51 Concept of Primitives 51

52 Concept of Primitives Request: The request primitive is passed from the N-user to the N- layer to request that a service is initiated. Indication: The indication primitive is passed from the N-layer to the N-user to indicate an internal N-layer event that is significant to the N-user. This event may be logically related to a remote service request, or it may be caused by an N-layer internal event. Response: The response primitive is passed from the N-user to the N-layer to complete a procedure previously invoked by an indication primitive. Confirm: The confirm primitive is passed from the N-layer to the N-user to convey the results of one or more associated previous service requests. 52

53 Wireless Information Transmission System Lab. PHY Specification National Sun Yat-sen University

54 Introduction The PHY is responsible for the following tasks: Activation and deactivation of the radio transceiver Energy detection (ED) within the current channel LQI for received packets CCA for CSMA-CA Channel frequency selection Data transmission and reception 54

55 Operating Frequency Range Frequency bands and data rates 55

56 Channel Assignments and Numbering A 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: 56

57 Receiver Sensitivity Definition 57

58 PHY Service Specifications The PHY provides an interface between the MAC sublayer and the physical radio channel, via the RF firmware and RF hardware. The PHY conceptually includes a management entity called the PLME. This entity provides the layer management service interfaces through which layer management functions may be invoked. The PLME is also responsible for maintaining a database of managed objects pertaining to the PHY. This database is referred to as the PHY PAN information base (PIB). 58

59 The PHY Reference Model The PHY provides two services, accessed through two SAPs: the PHY data service, accessed through the PHY data SAP (PD-SAP), and the PHY management service, accessed through the PLME s SAP (PLMESAP). 59

60 General Packet Format Each 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 sublayer frame. General packet format 60

61 Preamble field Packet Fields Used by the transceiver to obtain chip and symbol synchronization with an incoming message. Composed of 32 binary zeros. SFD (start-of-frame delimiter) field An 8 bit field indicating the end of the synchronization (preamble) field and the start of the packet data. Format of the SFD field 61

62 Frame length field Packet Fields 7 bits in length and specifies the total number of octets contained in the PSDU. PSDU field Has a variable length and carries the data of the PHY packet. For all packet types of length five octets or greater than seven octets, the PSDU contains the MAC sublayer frame (i.e., MPDU). 62

63 PHY Constants 63

64 PIB: PAN information base. PHY PIB Attributes 64

65 Data rate: 250 kb/s MHz PHY Specifications 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) 65

66 2450 MHz PHY Specifications Reference modulator diagram Reference transmitter diagram 66

67 Symbol to Chip Mapping 67

68 2450 MHz PHY Specifications O-QPSK modulation The chip sequences representing each data symbol are modulated onto the carrier using O-QPSK with half-sine pulse shaping. Pulse shape () p t t sin π 0 t 2Tc = 2Tc 0 otherwise 68

69 2450 MHz PHY Specifications Sample baseband chip sequences with pulse shaping Symbol rate The 2450 MHz PHY symbol rate shall be 62.5 ksymbol/s. Receiver sensitivity A compliant device shall be capable of achieving a sensitivity of -85 dbm or better. 69

70 868/915 MHz PHY Specifications 868/915 MHz band data rates 868 MHz: 20 kb/s. 915 MHz: 40 kb/s. Modulation and Spreading The 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. 70

71 868/915 MHz PHY Specifications Reference modulator diagram Differential encoding Differential encoding is the modulo-2 addition (exclusive or) of a raw data bit. E = R E R E E n n n 1 n n n 1 is the raw data bit being encoded, is the corresponding differentially encoded bit, is the previous differentially encoded bit. 71

72 868/915 MHz PHY Specifications For each packet transmitted, R 1 is the first raw bit to be encoded and E 0 is assumed to be zero. Conversely, the decoding process, as performed at the receiver, can be described by: Rn = En En 1 For each packet received, E 1 is the first bit to be decoded, and E 0 is assumed to be zero. Bit-to-chip mapping Each input bit shall be mapped into a 15-chip PN sequence 72

73 868/915 MHz PHY Specifications BPSK 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 shape The raised cosine pulse shape (roll-off factor = 1) used to represent each baseband chip is described by () p t = ( πt T ) ( πt T) sin / c cos / π t/ T 1 4 t / T 2 2 ( c ) 73

74 Symbol rate 868/915 MHz PHY Specifications 868 MHz: 20 ksymbol/s 915 MHz: 40 ksymbol/s Receiver sensitivity A compliant device shall be capable of achieving a sensitivity of -92 dbm or better. 74

75 Wireless Information Transmission System Lab. IEEE 低速率無線近身網路中 868/915MHz 模式之基頻訊號處理 黃偉傑 National Sun Yat-sen University

76 Outline 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 76

77 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 77

78 Format of the PPDU Phy layer Protocol Data Unit (PPDU) consists of Preamble (32 bits) synchronization Start of Frame Delimiter (8 bits) PHY Header (8 bits) PSDU length PSDU (0 to 1016 bits) Data field Preamble Start of Frame Delimiter PHY Header PHY Service Data Unit (PSDU) 78

79 Preamble field The preamble field is used by the transceiver to obtain chip and symbol synchronization with an incoming message. The preamble field shall be composed of 32 binary zeros. Preamble Start of Frame Delimiter PHY Header PHY Service Data Unit (PSDU) 79

80 Start of frame delimiter (SFD) field The SFD is an 8 bit field indicating the end of the synchronization (preamble) field and the start of the packet data. Preamble Start of Frame Delimiter PHY Header PHY Service Data Unit (PSDU) 80

81 PHY Header field The frame length field is 7 bits in length and specifies the total number of octets contained in the PSDU (i.e.,phy payload). Preamble Start of Frame Delimiter PHY Header PHY Service Data Unit (PSDU) 81

82 PPDU packet structure Format of the PPDU Octets: variable Preamble Start of frame delimiter Frame length (7 bits) Reserved (1 bit) PSDU Synchronization header PHY header PHY payload Preamble:32 binary zeros Start of frame delimiter:sfd[0:7] =

83 Spreading and modulation The 868/915 MHz PHY shall employ direct sequence spread spectrum (DSSS) with BPSK used for chip modulation and differential encoding used for data symbol encoding. 83

84 Differential encoding This is performed by the transmitter and can be described by Equation: En = Rn En 1 where R n is the raw data bit being encoded, E n is the corresponding differentially encoded bit, E n 1 is the previous differentially encoded bit.(e 0 =0) 84

85 Bit to chip mapping Each input bit shall be mapped into a 15-chip PN sequence. Binary data From PPDU Differential Encoder Bit-to- Chip BPSK Modulator Modulated signal 85

86 Pulse shape The raised cosine pulse shape (roll-off factor = 1) used to represent each baseband chip is described by equation: sin( π t / Tc ) cos( π t / Tc ) p( t) = 2 2 π t / T 1 (4t / T ) c c Binary data From PPDU Differential Encoder Bit-to- Chip BPSK Modulator Modulated signal 86

87 Raised cosine wave 87

88 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 88

89 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 89

90 Transmitter structure 90

91 Raised cosine wave (oversample=4) Sampled value=[ ] 91

92 Finite Impulse Response (FIR) filter 92

93 Transmitted signal

94 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 94

95 DPSK demodulator T b 0 dt cos(2 π f t) c R( n) 90 o phase shifter sin(2 π f t) c T b 0 dt 95

96 Received signal 96

97 Receiver structure 97

98 Synchronization Synchronization Time synchronization Packet detection Packet detector Chip synchronization Energy detector Bit synchronization Timing synchronization Frequency synchronization Frequency offset estimation 98

99 Packet detector 99

100 Packet detection W: Complex AWGN with zero mean and variance S: Transmitted signal with zero mean and unit variance R: Received signal (S and W are uncorrelated) Case 1: R = W => E R = E W = var ( W ) 2 2 ( ) ( ) 2 σ W Case 2: 2 2 ( ) ( ) R = S + W => E R = E S+ W = var( S + W) = var( S) + var( W) 100

101 Packet detection L L ( R ) 1 L L ( R ) L 101

102 Frequency offset estimation 102

103 Frequency offset In 868M/915MHz mode, max frequency offset is ±40ppm. 868MHz: 2πΔft = 2π 868M 80ppm / 20k = 6.944π 1250 /bit 0.463π 83 /chip 0.116π 21 /sample 915MHz 2πΔft = 2π 915M 80ppm/40k = 3.66π 659 /bit = 0.244π 44 /chip 0.061π 11 /sample 103

104 Frequency offset estimation r c * ( ) L 1 c= r r k = 0 L 1 k = 0 L 1 k = 0 * k k+ D = r r e k = r e k * j 2πε D k 2 j 2πε D 104

105 Received signal 105

106 Phase shift π + θ θ θ π + θ 106

107 Energy detector 107

108 Energy detection 108

109 Timing synchronization 109

110 Timing synchronization Matched filter in Synchronization 110

111 Simulation result 80 The output of matched filter Amplitude Chip index 111

112 Despreading & Differential Decoder 112

113 Despreading & Differential Decoder I Tb y Decision device y>0 bit 1 y<0 bit 0 Q Tb Despreading Differential Decoder 113

114 1. Introduction 2. System design Transmitter Receiver 3. Simulation results 114

115 Channel: AWGN Parameters Packet length: 168 bits(psdu: 160bits+PHY header: 8bits) Number of packets: Idle time: random between 1000 to 1500 samples Initial phase: random Frequency offset : ±40ppm 115

116 Packet detect fail Packet detect fail decision False alarm: Packet detect timing before Preamble. Packet lose: Packet detect timing after Preamble s 3rd zero. 116

117 Packet fail rate (different threshold in PD) 117

118 PER (Different A/D converter) 10-1 PER on the condition of frequency offset 80ppm 3 bits ADC 4 bits ADC 10-2 PER SNR 118

119 PER (Different frequency offset) 119

120 Wireless Information Transmission System Lab. IEEE 低速率無線近身網路 (ZigBee) 中 2.45GHz 模式之實體層設計 王森弘 National Sun Yat-sen University

121 Outline What s ZigBee 2.45GHz mode ZigBee transmitter 2.45GHz mode ZigBee receiver Conclusion and future work 121

122 Addresses unique Why do we need ZigBee? We need more addresses to maintain most devices that be the network applications. Enable broad Low cost Low power Lifetime Enable monitoring applications to run for years on inexpensive primary batteries. 122

123 Low power and low cost Features Low duty cycle BPSK s and OQPSK s DSSS:can resist interference, then disuse channel filter Low transmit power :-3dBm Low sensitivity :868/915MHz is -92dBm ; 2.45GHz is -85dBm Varied Network topology Star, Peer-to-Peer Cluster, Mesh, Cluster tree 123

124 ZigBee Operating bands 2.4 GHz PHY Channels MHz 2.4 GHz GHz 868MHz / 915MHz PHY Channel 0 Channels MHz MHz 902 MHz 928 MHz 124

125 Frequency Band and data rate Frequency Band Coverage Data # of Channels Rx Sensitivity Modulation 2.4 GHz ISM Worldwide 250 kbps dbm O_QPSK 868 MHz Europe 20 kbps 1-92 dbm BPSK 915 MHz ISM Americas 40 kbps dbm BPSK 125

126 2.45GHz mode ZigBee transmitter 126

127 Transmitter block diagram 127

128 Symbol to Chip Spreading 128

129 Modulation and pulse shaping Chip data I T c Q T c TXos = 4 129

130 Hardware design of transmitter Pulse_shape_Q reg3 reg2 reg1 Data_out 130

131 2.45GHz mode ZigBee receiver 131

132 Noncoherent detection Noncoherent receiver In phase local oscillator = cos(2 π ft c + φ) in phase axis Quadrature local oscillator = sin(2 π ft c + φ) Quadrature axis Phase error = φ In phase axis LPF output At ()cos( φ) + Bt ()sin( φ) Quadrature axis LPF output Bt ()cos( φ) At ()sin( φ) 132

133 Receiver architecture (noncoherent) cos(2 π ft+ φ) c At ()cos( φ) + Bt ()sin( φ) sin(2 π ft+ φ) c Bt ()cos( φ) At ()sin( φ) 133

134 Problems 封包偵測最佳取樣點偵測頻率同步展頻碼同步框架同步 134

135 封包偵測 Problems ZigBee 是一個封包交換 (Packet Switch) 的系統架構, 傳送端每次傳送一個封包 在接收端的部份, 並不知道傳送端傳的封包何時會收到, 因此必須去偵測無線通道是否有封包在傳送 最佳取樣點偵測 由於收到的訊號會做超取樣 (Over Sampling) 的處理, 在一個位元時間 T b 中, 會取樣 n 次, 在這當中, 只有一個是具有最佳訊雜比的樣本, 其他都較它差 此具有最佳訊雜比的樣本, 會讓系統有最佳的效能 如何找尋到此最佳樣本, 即是一重要課題 135

136 頻率同步 Problems 由於傳送端與接收端的振盪器不同, 無法振出完全相同的頻率, 會有些許的誤差, 此一誤差可能是由於本身振盪器內部元件造成的 環境的溫度及溼度等 此一頻率誤差, 會導致接收到的訊號, 隨著時間會有相位旋轉 在 ZigBee 的規格中, 允許此誤差達 ±40ppm 2450 MHz 80 ppm Δ θ = 2000 kchip / sec =0.098 =35.28 per chip 136

137 Problems 137

138 展頻碼同步 Problems 在接收端, 並不知道目前解調出來的 chip 是在這組展頻碼中的第幾個, 因此需要先找尋展頻碼的第一個 chip 位置 由於展頻碼是 cyclic shift 的, 每四個 chips 一群, 不同組的展頻碼是這些群的位移 若展頻碼同步沒達成, 會造成接收端誤判成其他組的展頻碼, 導致解出來的 symbol 錯誤 138

139 框架同步 Problems 在 Preamble 中, 有重複的 32 個 0, 接收端收到封包後, 並不知道目前解出來的是屬於這裡面的哪個部份, 可能是當中的任意一個 無法確定在封包的哪個位置, 就無法知道從哪個地方開始是屬於 PHY Header, 哪個地方是 PSDU 的部份 Preamble 之後有一段 SFD, 可以利用這部份, 從解調出來的 bit 中找尋 SFD 的部份, 來達到框架同步 139

140 Receiver Block Packet Detection I Q arctan(q/i) LUT and Phase Difference T c Delay 2T c Delay Find the Optimum Sample and Down- Sampling T c Delay 2T c Delay Frequency Offset Estimation and Compensation T c Delay 2T c Delay Noncoherent Demodulator (MSK) Differential Encoder Find the Start of PN Code Despreading Find SFD 140

141 Packet Detection Block Packet Detection Block 主要功能為偵測是否有封包到達, 若無封包到達則不發出置能 (Enable) 訊號使後端電路動作, 以此可以達到較省電的功能 此 Block 使用的偵測方法是看收到訊號的能量來決定是否有封包進來, 所以一般而言應該如下圖所示, 將收到訊號的平方值累加一定的窗格長度, 當電路出來的值超過一個臨界值一段時間後則判定為封包已到達 141

142 Packet Detection Block 我們所使用的架構則是它的簡化版, 也就是我們把圖中的平方換成了絕對值如下圖所示 我們之所以要簡化架構是因為平方電路較為複雜且佔較大的硬體資源 在系統前方的 Matched Filter 主要是為了降低雜訊的干擾, 在這裡我們還多了一個動作那就是我們還把它做平均, 所以整個 Matched Filter 很類似移動平均 (Moving average) 的概念 142

143 Find optimum sample & downsampling Block 因為我們在接收訊號時有使用到超取樣 (Oversampling), 所以會有多筆的相位差被前方的計算電路處理出來, 然而接收端不知道哪個相位差才是 Demodulator 所需要的, 所以我們可以運用 MSK 訊號在最佳取樣點時每隔 Tc 相位會位移 π/2 的特性來找出最有可能的相位差取樣點 如圖所示, 我們可以看出在非最佳取樣點其相位差的絕對值一定小於 π/2, 這是因為 MSK 在 T c 間隔內最大只有 π/2 的相位位移 我們所使用的方法就是每隔 Tc 去累加其絕對值並統計 32Chips 長度, 然後再找出最大值的點即為最佳的相位差取樣點 當我們找到最佳的 Sample 點後, 即可將訊號做 Downsampling 將不必要的取樣點去除 143

144 Find optimum sample & downsampling Block T b I Q arctan( I Q ) 0 π 4 π 2 π 4 0 π 4 θ θ Tc delay π 2 0 π

145 Find optimum sample & downsampling Block 145

146 Find optimum sample & downsampling Block I Q 相位運算器 取絕對值 暫存器 ( 長度 = n) 1 累加器 (m 次 ) 1 2 累加器 (m 次 ) 2 串進並出器 找最大值 n 累加器 (m 次 ) n 146

147 Frequency offset estimation & compensation Block 為了使後端 Demodulator 不會判斷錯誤, 我們需要把 frequency offset 所產生的影響補償回來 IEEE 標準規定傳送端頻率合成器所合成的頻率誤差 ±40ppm 而接收端也是 ±40ppm, 所以最大的頻率偏移可達到 80ppm,Sample 每隔 Tc 會有 度的相位位移如何估計出 Frequency offset 呢? 就是在 Frequency offset 為 80ppm 的時候, 此演算法除了原本的固定角度之外, 整體還會多增加或減少一定的相位偏移量, 那即是 Frequency offset 所產生的相位位移 我們用來估計 Frequency offset 的方法如圖所示, 圖中的細線為受到頻率偏移影響的相位差, 而黑粗線為理論上該偏移的相位差, 將其與原本該偏移的角度相減即可獲得 Frequency offset 的值, 但只看一個值會容易受到雜訊的影響, 我們可以取多筆資料來平均即可達到抑制雜訊的功能 147

148 Frequency offset estimation & compensation Block 148

149 Frequency offset estimation & compensation Block I Q 相位運算器 補償完頻率誤差之訊號 延遲 T b 頻率誤差補償器 π 2 一對二解多工器 二對一多工器 累加器 (m 次 ) 1 m π 2 比較器 頻率誤差估測器 149

150 Noncoherent demodulator Block 此 Block 主要是運用 Differential detection 的方法所引伸出來的架構,Differential detection 主要是根據 MSK 的相位軌跡圖所發展出來的 一般的 MSK 調變還隱含著錯誤更正的功能, 簡單來說, 就是當其相位連續每隔 Tc 都是走了 ±π/2, 那隔 2T c 是不是走了 ±π 或者 0 用此原理可延伸出整個錯誤更正的演算法, 所以整個 Noncoherent demodulator block 的原始架構如圖所示, 整個架構中有原本解調的部分與錯誤更正的部份 OQPSK = MSK + Differential Encoder + [ ] mapping 150

151 Noncoherent demodulator Block MSK 相位軌跡圖 151

152 Noncoherent demodulator Block Diff. detector ckt MSK signal T 90 D i T Di 1 edi 1 Correcte d Output 0 D i T Si 1 P i S i 2T Diff. detector error correct ckt 152

153 MSK [5] d k a k b k sin( πt/ 2 T) cos( πt/ 2 T) 153

154 MSK I ( ) = sin( π / 2 ) x t a t T k Q ( ) = cos( π / 2 ) x t b t T k ( ) π ( ) x = x t cos(2 f t) + x t sin(2 π f t) MSK I c Q c dd, 0~10 i i+ 1 i= ˆd 154

155 MSK signal (a) (b) d data wave form 0 2T 4T 6T 8T 0 2T 4T 6T 8T (f) (g) 1 sin θ() t θ( t T) 0 1 Discriminator output π (c) θ () t π 0 (h) θ ( t 2T) 0 π π π (i) θ () t θ( t 2T) 0 π (d) (e) θ ( t T) θ 0 () t θ( t T) π π π /2 0 π /2 (j) (k) () t θ( t T) cos θ 2 Discriminator output

156 No error received signal 0 D i Error correct principle the received data at the i-th decision instant 0 P i the parity at the i-th decision instant P = D D i i i Received signal ( could error ) 0 D = D e i i Di 1 0 P = P e edi, i i Pi e Pi are error variable 156

157 Error correct principle Syndrome S = D D P i i i 1 i = D e D e D D e = e e e i Di i 1 Di 1 i i 1 Pi Di Di 1 Pi Si 1 = edi 1 edi 2 epi 1 = e e Di 1 Pi 1 edi 2 has been eliminated in the previous decoding interval 157

158 Error correct principle e e 1 epi epi 1 If only a single error exists among Di, Di, and, e then Di 1 can be determined correctly from the two successive syndromes as e = S is Di 1 i i 1 = ( e e e ) i( e e ) Di Di 1 Pi Di 1 Pi 1 e By adding Di 1 to the T-delayed receive data, the received data will be error corrected. 158

159 Differential Encoder 差分編碼是利用互斥或 (XOR) 的方式, 將先前的資訊位元編碼, 在傳送端編碼的方式如下列方程式所示 : E n = Rn En 1 其中 R n 是要被編碼的資訊位元 E n 是經過差分編碼後的值 E n-1 是前一個經過差分編碼後的位元 而在每個封包要傳送時,R 1 是第一個要被編碼的位元, 且 E 0 是設為 1 159

160 MSK 資料與相位關係圖 160

161 161 Noncoherent demodulator Block Data Mask E X R π/2 π/2 -π/2 -π/2 π/2 -π/2 π/2 π/2 X Δθ π π/2 0 π/2 π π/2 π π/2 0 θ Q I

162 Despreading Block 因為在傳送端我們先做了展頻的動作, 因此在接收端我們必須解展頻確定資料為哪一個 Symbol 因為 Noncoherent demodulator block 是適用於 MSK 的演算法, 所以我們直接轉換了用來解展頻的虛擬雜訊碼而產生新的虛擬雜訊碼, 如此可以直接對所解調出的資料進行解展頻的動作 162

163 [ ] Mapping 163

164 Rx Chip Map 164

165 Find the start of PN code Block 當訊號解展頻之前整個系統還有一個很重要的程序尚未執行, 那就是解展頻端要從哪裡開始解展頻, 因為展頻碼有移位的特性, 所以若將展頻開始的地方設定錯誤將會導致全部都解錯的狀態 其工作原理如圖所示, 將解調出來的 Chips 去比對接收端 Chip Map 中第一組的 PN Code 來找到開始點 由於 PN Code 的長度是 32, 若依序將第一組 PN Code 與解調出的 Chips 連續 32 筆比對了話, 必定有其中一筆是能吻合的 在此 32 筆得到的值中找尋其最大值即為解展頻的開始點 第一組 PN Code 是 { ; ; ; ; ; ; ; } 165

166 Find the start of PN code Block 166

167 Despreading 而解展頻的架構如圖所示, 將資料對虛擬雜訊碼做相關 (Correlation) 的動作, 其值高的即為最有可能的符號 為了在 Noncoherent demodulator Block 的 Differential decoder 有可能使整個資料變號的情形下仍能有高相關值, 所以要加入圖中的絕對值 167

168 Find SFD 從解展頻後得到的資訊位元中, 比對 SFD 的值, 找尋到即可達成框架同步 在 SFD 後面接著的就是 PHY Header 及 PSDU 的部份 到此, 完成接收機的同步 168

169 169 PER

170 Reference [1] IEEE Std , Institute of Electrical and Electronics Engineers, Inc., 1 October [2] Shan, Q., Liu, Y., Prosser, G., Brown, D., Wireless intelligent sensor networks for refrigerated vehicle, Proceedings of the IEEE 6th Circuits and Systems Symposium, Vol.2, pp , 31 May-2 June, [3] IEEE WPANTaskGroup4(TG4),URL: [4] ZigBee Alliance, URL: [5] S. A. Gronemeyer and A. L. Mcbride, MSK and offset QPSK modulation, IEEE Trans. Commun. Vol. COM-24, no. 8, August [6] T. Masamura, S. Samejima, Y. Morihiro and H. Fuketa, Differential detection of MSK with nonredundant error correction, IEEE Trans. Commun. Vol. COM-27, no. 6, June [7] T. Masamura, Intersymbol interference reduction for differential MSK by nonredundant error correction, IEEE Trans. on Vehicular Technology. Vol. 39, no. 1, Feb [8] R. Mehlan, Y-E Chen and H Meyr, A fully digital feedforward MSK demodulator with joint frequency offset and symbol timing estimation for burst mode mobileradio, IEEE Trans. on Vehicular Technology. Vol. 42, No. 4, Nov