1. 802.11 Architecture and Overview

無線通訊協定

無線通訊協定

Outline

1. 802.11 Architecture and Overview

2. Baseband Infrared (IR) Physical Layer Specification

3. Direct Sequence Spread Spectrum (DSSS) Physical Layer Specification

4. Orthogonal Frequency Division Multiplexing (OFDM) Physical Layer Specification

5. IEEE 802.11g Extended Rate PHY (ERP) Specification

6. Frequency Hopping Spread Spectrum PHY of the 802.11 Wireless LAN Standard

7. IEEE 802.11 Wireless LAN MAC Standard

1. 802.11 Architecture

and Overview

Technology Tree for Wireless LAN

HomeRF HomeRF Bluetooth Bluetooth

What is unique about wireless?

• • Difficult media Difficult media

– interference and noise

– quality varies over space and time

– shared with Unwanted 802.11 devices

– shared with non-802 devices (unlicensed

spectrum: microwave ovens, bluetooth microwave ovens, bluetooth, etc., , etc.,)

• Full connectivity cannot be assumed – Hidden node problem Hidden node problem

• Multiple international regulatory requirements

Medium Variations

Uniqueness of Wireless (continued)

• Mobility

– variation in link reliability

– battery usage: requires power managementpower management – want seamless connections

• Security

– no physical boundaries – overlapping LANs

Requirements

• Single MAC to support multiple PHYs.

– Support single and multiple channel PHYs.

– PHYs with different medium sense characteristics.

• Should allow overlap of multiple networks in the same area and channel space.

• Need to be Robust for Interference?

– ISM band (Industry, Science and Medicine)

» 13.56 MHz, 27.55 MHz, 303 MHz, 315 MHz, 404 MHz, 433 MHz, 868 MHz (Europe), 915 MHz (North America), 2.45 GHz, 5.2 GHz (North America), 5.3 GHz, and 5.7 GHz (North America)

» Microwave, other non-802.11 interferers.

» Co-channel interference.

• Need mechanisms to deal with Hidden Nodes?

• Need provisions for Time Bounded Services.

Architecture Overview

• One MAC supporting multiple PHYs –– Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum –– Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum –– InfraredInfrared

–– Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing

• Two configurations

–– Independent (ad hocIndependent ad hoc) and InfrastructureInfrastructure –– Hybrid configuration has being studiedHybrid configuration has being studied

•• CSMA/CA (collision avoidance)CSMA/CA (collision avoidance) with optional Point Coordination Function (PCF)

802.11 Protocol Entities

LLC LLC SAP SAP

MAC MAC Sublayer Sublayer

PLCP

PLCP Sublayer Sublayer

PMD PMD Sublayer Sublayer

MAC Layer MAC Layer Management Management

PHY Layer PHY Layer Management Management

Station Station Management Management MAC MAC

PHY PHY

802.11 Protocol Architecture

• MAC Entity

– basic access mechanism

– fragmentation/defragmentation – encryption/decryption

• MAC Layer Management Entity

– synchronization

– power management – roaming

– MAC MIB

• Physical Layer Convergence Protocol (PLCP)

– PHY-specific, supports common PHY SAP

– provides Clear Channel Assessment signal (carrier sense)

802.11 Protocol Architecture ^(cont.)

• Physical Medium Dependent Sublayer (PMD)

– modulation and encoding

• PHY Layer Management

– channel tuning (channel switching delay : 224us224us in 802.11b)

– PHY MIB

• Station Management

– interacts with both MAC Management and PHY Management

802.11 Configurations - Independent

• Independent

– one Basic Service Set (BSS) –– Ad HocAd Hoc network

– direct communication – limited coverage area

• Current research topics

– Multi-Hop Routing (IETF MANET) – Multicasting

– Multi-channel Access – Security

– QoS ...

Station AH3

Station Station

AH1 AH2

Ad Hoc Network Ad Hoc Network

Mobile Station : STA

Commercial Products : WLAN Cards

• One piece

• Two pieces

802.11 Configurations - Infrastructure

Station

Station Station

Station A1

A2 B1

B2 BSS-A

BSS-B

APA AP

B

Server

DISTRIBUTION SYSTEM (DS) Ethernet or Internet

•• InfrastructureInfrastructure

– Access Points (AP) and stations (STA)

• Distribution System interconnects Multiple Cells via Access Points to form a single Network.

– extends wireless coverage area

Commercial Products : AP

Wireless Bridging

AP

B APA

Station Station B2

B1

Station B3

Ethernet

Station A1

Station A3

Ethernet

A2 Station

private lease line

Data Rate Data Rate

Cost ? Cost ? Legal ? Legal ? Security ? Security ?

Building B Building A

Outdoor Application

Outdoor Application - Antenna

Outdoor Application

Long Distances

• Security Issue :

– The transmission distance can be up to 25Miles

25Miles

– If the AP is distanced from the street or on a

high floor of a building, users will be safe from

network trespassers.

Distribution System

• Used to interconnect wireless cells

– multiple BSS connected together form an ESS (Extended Service Set)

– Allows mobile stations to access fixed resources

• Not part of 802.11 standard

– could be bridged IEEE LANs, wireless, other networks –– Only Distribution System Services are definedOnly Distribution System Services are defined

BSS vs ESS

Collocated Coverage Areas

Complete Architecture

DSS : Distribution System Service

Access Points

• Stations select an AP and Associate with it

• • Support Support roaming roaming

–– IAPP (Inter Access Point Protocol) IAPP (Inter Access Point Protocol) IEEE 802.11fIEEE 802.11f –– Mobile IP Mobile IP

• Provide other functions

–– time synchronizationtime synchronization (beaconing) –– power managementpower management support (if any)

–– point coordination functionpoint coordination function (PCF) (if any)

• Traffic typically (but not always) flows through AP

– direct communication possible

Access Points

• In an Infrastructure BSS, all mobile stations communicate with the AP

– quoted from “IEEE 802.11 Handbook”IEEE 802.11 Handbook”, Bob OBob O’’Hara and Hara and AI AI PetrickPetrick

– Disadvantage :

» bandwidth is consumed twicetwice than directional communication between STAs

» more contentions and more collisions – Advantage :

» easily solve hidden terminal problem

» provide power saving function

» meet the AAA (authentication, authorized, accounting) architecture

» provide per flow bandwidth control, QoS guarantee (in the near future)

802.11 Defines the Airwaves IF

• The airwaves interface between stations (including that between station and AP) is standardized

– PHY and MAC

• No exposed MAC/PHY interface specified

• No exposed interface to Distribution System

– only required DS services are defined

• Internals of Distribution System not defined

MAC Services

• Asynchronous MSDU Data Delivery

– provided to LLCLLC (2304 octets maximum)

• Time Bounded Services

– optional point coordination function (PCF) – Existing in commercial products ??

» Bandwidth is not enough for supporting real-time service

» Not necessary, CSMA/CA works well (likes Ethernet history)

» Digitalocean Corp. “Starfish II” AP.

» IEEE 802.11e draft enhances QoS

• Security Services

– confidentiality, authentication, access control

• Management Services

– scanning, joining, roaming, power management

MAC Functionality

• Independent and Infrastructure configuration support

– Each BSS has a unique 4848 bit address – Each ESS has a variablevariable length address

• CSMA with collision avoidance (CSMA/CA)

– MAC level acknowledgment (positive acknowledgement)acknowledgment (positive acknowledgement) – allows for RTSRTS/CTSCTS exchanges

»» hidden node protectionhidden node protection

»» virtual carrier sensevirtual carrier sense

»» bandwidth savingbandwidth saving – MSDU fragmentation

– Point Coordination Function option

» AP polling

MAC Functionality (continued)

• Roaming support within an ESS

– station scansscans for APs, associationassociation handshakes

• Power management support

– stations may power themselves down

–– AP buffering, distributed approach for IBSSAP buffering

• Authentication and privacy

– Optional support of Wired Equivalent Privacy (WEPWEP) – Key exchange

– Authentication handshakes defined

– IEEE 802.1x spec. enhances authentication control – IEEE 802.11i draft enhances security

PHY Layer Services

• PHY_DATA transfers

–– multiple rates (1, 2, 5.5, 11Mbps)multiple rates (1, 2, 5.5, 11Mbps)

–– extended rates (22, 33 or 6, 9, 12, 19, 24, 36, 48, extended rates (22, 33 or 6, 9, 12, 19, 24, 36, 48, 54Mbps)

54Mbps)

–– The algorithm for performing rate switching is beyond The algorithm for performing rate switching is beyond the scope of the standard. (p6, 802.11b)

the scope of the standard. (p6, 802.11b)

»» Question : how to decide the proper data rate ?Question : how to decide the proper data rate ?

• Clear Channel Assessment (CCA)

– carrier sense

– detect start frame delimiter

• PHY Management

– channel tuning

Data Rate vs. Range

Four PHYs

•• Frequency Hopping Spread Spectrum (FHSS)Frequency Hopping Spread Spectrum (FHSS) –– 2.4 GHz2.4 GHz band, 11 and 22 Mbps transmission

» 2GFSK, 4GFSK

» 2.5 hops/sec over 79 1MHz channels (North America)

•• Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS) –– 2.4 GHz2.4 GHz band, 11 and 22 Mbps transmission

» 11 chip Barker sequence

» DBPSK, DQPSK (Differential Binary/Quadrature Phase Shift Keying)

–– 2.4 GHz2.4 GHz band, 5.55.5 and 1111 Mbps transmission

» CCK (Complementary Code Keying), PBCC (Packet Binary Convolutional Code)

» CCK : DQPSK(5.5Mbps, 11Mbps)

» PBCC : BPSK(5.5Mbps), QPSK(11Mbps) (optional)

» Sep. 1999 (802.11b)

–– 2.4 GHz2.4 GHz band, 2222 and 3333 Mbps transmission

» PBCC-22, PBCC-33

Four PHYs

•• BasebandBaseband IR (Infrared)IR (Infrared) – Diffuse infrared

–– 1 and 1 22 Mbps transmission, 16-PPM and 4-PPM

» PPM : Pulse Position Modulation

•• Orthogonal Frequency Division Multiplexing (OFDM)Orthogonal Frequency Division Multiplexing (OFDM) –– 2.4 GHz2.4 GHz band (IEEE 802.11g D2.1 DSSS-OFDM, OFDM) –– 5 GHz5 GHz band (IEEE 802.11a)

» Similar ETSI HIPERLAN/II PHY Spec.

–– 6, 96 9, 1212, 1818, 2424, 3636, 4848 and 5454 Mbps

» BPSK(6,9Mbps), QPSK(12,18Mbps), 16-QAM(24,36Mbps), 64- QAM(48,54Mbps)

» Convolutional Code with coding rates ½,2/3,¾.

» 20MHz/64 subcarriers per channel

• 52 subcarriers occupy 16.6MHz

• 12 additional subcarriers are used to normalized the average power of OFDM symbol

» Mandatory : 6, 12, 24 Mbps

» Extended (turbo mode 5-UP protocol): 72/108Mbps (proposed by

Unlicensed Operation RF Bands

• 902MHz

– 26MHz BW (902-928MHz)

– Crowded and Worldwide limited

– IEEE 802.11 WLAN, IEEE 802.15.4 LR-WPAN, coreless phone, .etc.,

• 2.4GHz

– 83.5MHz BW (2400-2483.5MHz) – Available worldwide

– IEEE 802.11(b/g) WLAN, Bluetooth, IEEE 802.15.4 LR-WPAN and HomeRF, etc.,

• 5.1GHz

– 300MHz (three 100MHz segments) – Unlicensed NII

– 802.11a WLAN

» OFDM / 6,12,18,24,36,48,54Mbps / BPSK,QPSK,16-QAM, 64-QAM – HiperLAN I and HiperLAN II

» 23.5Mbps/GMSK and 6-54Mbps/BPSK,QPSK,16-QAM, 64-QAM

ps. 27MHz

2. Baseband Infrared (IR)

Physical Layer Specification

PPM Modulation

• OOKPPM :

– Reduce the optical power

3. Direct Sequence Spread

Spectrum (DSSS) Physical

Layer Specification

What is DSSS?

• Signal symbol is spread with a sequence

• Wider Bandwidth

• Less power density

+1

-1

+1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1 +1

-1

+1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1

Power Power

Power Power

11 chip BARKER sequence

• Good autocorrelation properties

• Minimal sequence allowed by FCC

• Coding gain 10.4 dB

+11

+1

-1

+11

+1

-1

timetime

elationelation

Received chip stream at time (t-1)

Received chip stream at time (t)

Received chip stream at time (t+1)

DSSS benefits

• 10 dB coding gain:

– Robust against interferers and noise (10 dB suppression)

• Robust against time delay spread

– Resolution of echoes

echo

echo peak

echo

echo peak

autocorrelationautocorrelation

timetime

DSSS hardware block diagram

DBPSK DBPSK DQPSK DQPSK

IEEE 802.11 DSSS PHY characteristics

• • 2.4 GHz 2.4 GHz ISM band (FCC 15.247)

• 1 and 2 Mb/s datarate

– DBPSK and DQPSK modulation

– Chipping rate 11 MHz11 MHz with 11 chip Barker sequence11 chip Barker sequence

• 5.5 and 11Mbps (802.11b)

–– CCKCCK (QPSK, DQPSK modulations – mandatory) –– PBCCPBCC (BPSK, QPSK modulations – optional)

• 22 and 33Mbps ( 802.11g 802.11g ) )

–– PBCCPBCC-22-22, PBCCPBCC--3333 modulation (TI proposal (TI proposal –– optional)

• Multiple channels in 2.4 to 2.4835 GHz band

DSSS Channels

CHNL_ID Frequencies

FCC Channel Frequencies

ETSI Channel Frequencies

Japan Frequency

(MKK)

Japan Frequency (New MKK) 1 2412 MHz X X - X 2 2417 MHz X X - X 3 2422 MHz X X - X 4 2427 MHz X X - X 5 2432 MHz X X - X 6 2437 MHz X X - X 7 2442 MHz X X - X 8 2447 MHz X X - X 9 2452 MHz X X - X 10 2457 MHz X X - X 11 2462 MHz X X - X 12 2467 MHz - X - X 13 2472 MHz - X - X

14 2484 MHz - - X X

Table 1, DSSS PHY Frequency Channel Plan

• FCC(US), IC(Canada) and ETSI(Europe) : 2.4GHz - 2.4835GHz

• Japan : 2.471GHz - 2.497GHz (MKK : channel 14; new MKK : channels 1-14)

• France : 2.4465GHz - 2.4835GHz (channels 10, 11, 12, 13)

• Spain : 2.445GHz - 2.475GHz (channels 10, 11)

• Adjacent cells using different channels : ≥ 30MHz (25MHz in 802.11b)

IEEE 802.11 PHY Terminology in Spec.(s)

• • 1 Mbps : Basic Rate (BR) 1 Mbps : Basic Rate (BR)

• • 2 Mbps : Extended Rate (ER) 2 Mbps : Extended Rate (ER)

• • 5.5/11 Mbps : High Rate (HR) 5.5/11 Mbps : High Rate (HR)

• • 22~33/6~54 Mbps : Extended Rate PHY (ERP) 22~33/6~54 Mbps : Extended Rate PHY (ERP)

PLCP Frame Formats in IEEE 802.11b

• Two different preamble and header formats

–– Long PLCP PPDU formatLong PLCP PPDU format (Mandatory in 802.11b)Mandatory in 802.11b

» 144-bit preamble : 1Mbps DBPSK

» 48-bit header : 1Mbps DBPSK

» Spend 192us

» PSDU : 1, 2, 5.5, 11Mbps

» Compatible with 1 and 2 Mbps

–– Short PLCP PPDU formatShort PLCP PPDU format (Optional in 802.11bOptional in 802.11b)

» Minimize overhead, maximize data throughput

»» 72-bit preamble : 1Mbps DBPSK72

» 48-bit header : 2Mbps DQPSK

» Spend 96us

» PSDU : 2, 5.5, 11 Mbps

PLCP (PHY Convergence) Sublayer

LLC LLC SAP SAP

MAC MAC Sublayer Sublayer

PLCP

PLCP Sublayer Sublayer

PMD PMD Sublayer Sublayer

MAC Layer MAC Layer Management Management

PHY Layer PHY Layer Management Management

Station Station Management Management MAC MAC

PHY PHY

Long PLCP Frame Format

Preamble and Header always at 1Mb/s DBPSK Barker

1Mbps DBPSK Barker 2Mbps DQPSK Barker 5.5, 11Mbps DQPSK CCK 1Mbps DBPSK

192us

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

• Mandatory in 802.11b

DBPSK Modulation

I Q

I Q

Bit Input Phase Change (+jω)

0 0

1 π

Table 1, 1 Mb/s DBPSK Encoding Table.

DQPSK Modulation

I Q

Dibit pattern (d0,d1)

d0 is first in time Phase Change (+jω)

00 0

01 π/2

11 π

10 3π/2 (-π/2)

Table 1, 2 Mb/s DQPSK Encoding Table

PLCP synchronization

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

• 128 one bits (‘1’)

• scrambled by scrambler

• Used for receiver to clock on to the signal and to correlate to the PN code

Start Frame Delimiter

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

• 16 bit field (hF3A0)

• used for

– bit synchronization

Signal Field

• 8 bits

• Rate indication

– h0A 1Mb/s DBPSK – h14 2Mb/s DQPSK

– h37 5.5Mb/s CCK or PBCC – h6E 11Mbps CCK or PBCC

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

Service Field

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

• Reserved for future use – Bit 2 : locked clock bit

» Indicate transmit freq. (mixer) & symbol clocks (baseband) derived from same oscillator

» optional in 802.11b and mandatory in 802.11g

– Bit 3 : modulation selection

» 0 : CCK / 1 : PBCC

– Bit 7 : length extension bit (in the case datarate > 8Mbps)

Length Field

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU PSDU/MPDU 1, 2, 5.5, 11 Mbps 1, 2, 5.5, 11 Mbps

• Indicates number of micosceonds to be transmitted in PSDU/MPDU

– Decided by Length and datarate (in TXvector)

• Used for

– End of frame detection

– Perform Virtual Carrier Sense (for those with lower datarate)

CRC field

PPDU

SYNC 128 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144 bits in 1 Mbps

Long PLCP Header 48 bits in 1 Mbps

PSDU/MPDU 1, 2, 5.5, 11 Mbps

• CCITT CRC-16

• Protects Signal, Service and Length Field

CRC Implementation

Short PLCP Frame Format in 802.11b

2Mbps DQBSK 5.5/11Mbps CCK 1Mbps DBPSK

96us

PPDU

SYNC 56 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Short PLCP Preamble 72 bits in 1 Mbps

Short PLCP Header 48 bits in 2 Mbps

PSDU/MPDU 2, 5.5, 11 Mbps

• Optional in 802.11b

2Mbps DQPSK

PLCP synchronization

PPDU

SYNC 56 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Short PLCP Preamble 72 bits in 1 Mbps

Short PLCP Header 48 bits in 2 Mbps

PSDU/MPDU 2, 5.5, 11 Mbps

• 56 zero bits (‘0’)

• scrambled by scrambler with seed ‘0011011’

• Used for receiver to clock on to the signal and to correlate to the PN code

Start Frame Delimiter

PPDU

SYNC 56 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Short PLCP Preamble 72 bits in 1 Mbps

Short PLCP Header 48 bits in 2 Mbps

PSDU/MPDU 2, 5.5, 11 Mbps

• 16 bit field (h05CF)

• used for

– bit synchronization

Signal Field

PPDU

SYNC 56 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Short PLCP Preamble 72 bits in 1 Mbps

Short PLCP Header 48 bits in 2 Mbps

PSDU/MPDU 2, 5.5, 11 Mbps

• Rate indication

– h14 2Mb/s DQPSK

– h37 5.5Mb/s CCK or PBCC – h6E 11Mbps CCK or PBCC

• Other values reserved for future use (100 kb/s quantities)

Data Scrambler/Descrambler

• ALL bits transmitted/received by the DSSS PHY

are scrambled/descrambled

PLCP Transmit Procedure

PLCP Receive Procedure

Complementary Code Keying (CCK)

• HR/DSSS adopts 8 8 - - chip chip CCK as the modulation scheme with 11MHz 11MHz chipping rate chipping rate

• It provides a path for interoperability with existing

1,2 Mbps Spec.

Complementary Code Keying (CCK)

• Spreading code length = 8, c={c0-c7} and

where ϕ

is added to all code chips,

ϕ

is added to all odd code chips,

ϕ

is added to all odd pairs of code chips, and ϕ

is added to all odd quads of code chips.

Cover code

Cover code : c4 and c7 chips are rotated 180° (with -) by a cover

sequence to optimize the sequence correlation properties and minimize dc offsets in the codes.

Complementary Code Keying (CCK) 5.5Mbps

• At 5.5Mbps CCK, 4 data bits (d0,d1,d2,d3) are transmitted per symbol

• (d0,d1) is DQPSK modulated to yield ϕ1, which the

information is bear on the “phase change” between two adjacent symbols

• (11/8)*(4 data bits per symbol)*1Mbps = 5.5Mbps

Complementary Code Keying (CCK) 5.5Mbps

• (d2,d3) encodes the basic symbol, where

Complementary Code Keying (CCK) 11Mbps

• At 11Mbps CCK, 8 data bits (d0-d7) are transmitted per symbol

• (d0,d1) is DQPSK modulated to yield ϕ1, which the

information is bear on the “phase change” between two adjacent symbols

• (d2,d3),(d4,d5),(d6,d7) encode ϕ2, ϕ3, ϕ4, respectively, based on QPSK

• (11/8)*(8 data bits per symbol)*1Mbps = 11Mbps

Complementary Code Keying (CCK)

Packet Binary Convolutional Code (PBCC)

• 64-state BCC

QPSK : 11Mbps BPSK : 5.5Mbps

Packet Binary Convolutional Code (PBCC)

• PBCC convolutional encoder

– Provide encoder the “known state”

» 6 memory elements are needed and

» one octet containing all zeros is appended to the end of the PPDU prior to transmission

•• One more octet than CCKOne more octet than CCK

– For every data bit input, two output bits are generated

Packet Binary Convolutional Code (PBCC)

• For 11Mbps, two output bits (y0,y1) produce one symbol via QPSK

– one data bit per symbol

• For 5.5Mbps, each output bit (y0 or y1) produces two symbols via BPSK

– One-half a bit per symbol

Packet Binary Convolutional Code (PBCC)

• Pseudo-random cover sequence

– use 16-bit seed sequence (0011001110001011)

– to generate 256-bit pseudo-random cover sequence

Transmit Spectrum Mask

fc -11 MHz fc fc -22 MHz

Sinx/x

fc +11 MHz fc +22 Mhz 0 dBr

-30 dBr

-50 dBr Unfiltered

Transmit Spectrum Mask

Clear Channel Assessment

• Five methods

:

– CCA mode 1: Energy above threshold (detect energy) (11b- HR, 11g-ERP)

– CCA mode 2: Carrier sense only (detect DSSS signal) – CCA mode 3: Carrier sense with energy above threshold

(2Mbps)

– CCA mode 4: Carrier sense with timer (11b-HR)

» 3.65ms is the duration of the longest possible 5.5Mbps PSDU

– CCA mode 5: Carrier sense (detect DSSS signal) with energy above threshold (5.5Mbps, 11Mbps) (11b-HR, 11g- ERP)

• Energy detection function of TX power in modes 1 & 3 – Tx power > 100mW: -80 dBm (-76dBm in mode 5)

– Tx power > 50mW : -76 dBm (-73dBm in mode 5) – Tx power <= 50mW: -70 dBm (-70dBm in mode 5)

• Energy detect time : 15 µs

• Correct PLCP header --> CCA busy for full (intended)

DSSS Specification Summary

• Slottime 20 us 20 us

• TX to Rx turnaround time 10 us

• Rx to Tx turnaround time 5 us

• Operating temperature range

» type 1: 0 - 40 ^oC

» type 2: -30 - 70 ^oC

• Tx Power Levels

» 1000 mW USA (FCC 15.274)

» 100 mW Europe (ETS 300-328) (=20dbm)

» 10 mW/MHz Japan (MPT ordinance 49-20)

• Minimum Transmitted Power 1 mW

• Tx power level control required above 100 mW

– four power levels

DSSS Specification Summary (cont)

• Tx Center Frequency Tolerance +/+/-- 25 25 ppmppm

• Chip Clock Frequency Tolerance +/-+/- 25 ppm25 ppm

• Tx Power On Ramp 2 µs

• Tx Power Down Ramp 2 µs

• RF Carrier suppression 15 dB

• Transmit modulation accuracy test procedure

• Rx sensitivity -80 dB (-76dbm)

@ 0.08FER (1024 Bytes)

<@ 0.10FER (1000 Bytes) in 11g

• Rx max input level -4 dB (-10dbm)

• Rx adjacent channel rejection >35 dB

@ > 30(25)30(25)MHz separation between channels

4. Orthogonal Frequency

Division Multiplexing (OFDM)

Physical Layer Specification

IEEE 802.11a PLCP

• TxVector / RxVector

– length 1-4095 octets

– Mandatory data rates : 6, 12, 24 Mbps – 8 power levels

IEEE 802.11a PLCP

IEEE 802.11a PLCP frame format

PLCP Header

Coded/OFDM (BPSK, r =1/2)

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

SERVICE 16 bits

Tail

6 bits Pad Bits

PLCP Preamble PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL

One OFDM Symbol

DATA DATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits PSDU

Coded/OFDM

(RATE is indicated in SIGNAL)

6Mbps

24 bits

PCLP Preamble

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

SERVICE 16 bits

Tail

6 bits Pad Bits

PLCP PreamblePreamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATADATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits PSDU

1. preamble field contains

–– 10 short training sequence10 short training sequence

» used for AGC convergence, diversity selection, timing acquisition, and coarse frequency acquisition in the receiver

–– 2 long training sequence2 long training sequence

» used for channel estimation and fine frequency acquisition in the receiver

– and a guard interval (GI)

PCLP Preamble

PLCP Preamble

PCLP Rate/Length

PPDU

RATERATE 4 bits 4 bits

Length 12 bits

Parity 1 bit

SERVICE 16 bits

Tail

6 bits Pad Bits

PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATADATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits PSDU

• Data Rates (determined from TXVECTOR)

– 1101 : 6Mbps (M) – 1111 : 9Mbps

– 0101 : 12Mbps (M) – 0111 : 18Mbps – 1001 : 24Mbps (M) – 1011 : 36Mbps – 0001 : 48Mbps

Rate-dependent Parameters

(for SIGNAL field)

PCLP Tail Subfield

24 bits

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

SERVICE 16 bits

Tail

6 bits Pad Bits

PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATA DATA

Variable Number of OFDM Symbols Reserved

1 bit

TailTail 6 bits

6 bits PSDU

• 6 ‘zero’ bit

•• to make the length of SIGNAL field to be 24 bits to make the length of SIGNAL field to be 24 bits (for the

(for the NN_DBPSDBPS=24=24 in 6Mbps mode)in 6Mbps mode)

• to facilitate a reliable and timely detection of the RATE and LENGTH fields

PCLP Service

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

Tail

6 bits Pad Bits

PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATADATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits SERVICESERVICE PSDU 16 bits

16 bits

For synchronization For synchronization

PCLP PSDU tail

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

TailTail 6 bits

6 bits Pad BitsPad Bits

PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATA DATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits SERVICE PSDU

16 bits

• Append 6 non-scrambled tail bits for PSDU to return the convolutional code to the “zero state”

• Add pad bitspad bits (with “zero” and at least 6 bits) such that the length of DATA field is a multiple of NN_DBPSDBPS

PCLP DATA encoding

PPDU

RATE 4 bits

Length 12 bits

Parity 1 bit

TailTail 6 bits

6 bits Pad BitsPad Bits

PLCP Preamble 12 Training Symbols

SIGNAL SIGNAL One OFDM Symbol

DATA DATA

Variable Number of OFDM Symbols Reserved

1 bit

Tail

6 bits SERVICE PSDUPSDU 16 bits

1.1. encode data string with convolutional encoder (include punctured coding)encode 2.2. divide encoded bit string into groups of divide NN_CBPSCBPS bits

3. within each group, perform data interleavinginterleaving

4. For each of the groups, convert bit string group into a complex number according to convert the modulation tables (see next page)

5. divide the complex number string into groups of 4848complex numbers, each such group will be associated with one OFDM symbolone OFDM symbol

•• map to subcarriersmap to subcarriers ––2626~~--22, 22, --20~20~--8, 8, --6~6~--1, 1~6, 8~20, 22~261, 1~6, 8~20, 22~26

•

• 44sucarrierssucarriers––21, 21, --7, 7, 21 are used for pilot7, 7, 21 are used for pilot

•• subcarriersubcarrier0 is useless0 is useless

6. convert subcarriers to time domain using inverse Fast Fourier transform (IFFT)

Modulation Tables

Convolutional Encoder

• use the industry-standard generator polynomials, – g0 = 133₈ and g1 = 171₈, of rate R = 1/2,

(first output)

(second output)

Punctured Coding

• to omit some of the encoded bits in the transmitter

– thus reducing the number of transmitted bits and increasing the coding rate

– inserting a dummy “zero” metric into the convolutional decoder on the receive side

– decoding by the Viterbi algorithm is recommended.

Timing-related Parameters

• Slot time : 9us

OFDM PHY Characteristics

• OFDM

• Slottime 9 us9 us

•• SIFSSIFS 16 us (6us for decoder)16 us (6us for decoder)

• CCA Time < 4 us

• TX to Rx turnaround time < 10 us

• Rx to Tx turnaround time < 5 us

• Preamble Length 16 us

• PLCP Header Length 4 us

• MPDUmax Length 4095

• aCWmin 15

• aCWmax 1023

Channelization

•• 8 independent channels in 5.15GHz8 independent channels in 5.15GHz--5.35GHz5.35GHz

•• 4 independent channels in 5.725-4 independent channels in 5.725-5.825GHz5.825GHz

PCLP Transmit Procedure

PCLP Receive Procedure

IEEE 802.11a vs IEEE 802.11b (max.)

1500 bytes per frame 1500 bytes per frame

IEEE 802.11a vs IEEE 802.11b (average)

1500 bytes per frame 1500 bytes per frame

IEEE 802.11a vs IEEE 802.11b (average)

for a cell radius of 65 feet

Cell allocation

IEEE 802.11a vs IEEE 802.11b (average)

IEEE 802.11a vs IEEE 802.11b (average)

IEEE 802.11a vs IEEE 802.11b (average)

5. IEEE 802.11g Extended Rate PHY (ERP)

Specification

IEEE 802.11g

•• Extended Rate PHY (ERP) Goal :Extended Rate PHY (ERP) Goal : –– coexists with 802.11b coexists with 802.11b (…(…..?)..?)

–– enhances the ability of interference protectionenhances the ability of interference protection

•• ERPERP--DSSS/CCK (Mandatory) DSSS/CCK (Mandatory) (1,2,5.5,11 Mbps)(1,2,5.5,11 Mbps) – short PLCP PPDU is mandatory

– transmit center frequency and symbol clock frequency shall refer the same oscillator (locked oscillator, mandatory)

•• ERP-ERP-OFDM (Mandatory) OFDM (Mandatory) (6,9,12,18,24,36,48,54 Mbps)(6,9,12,18,24,36,48,54 Mbps)

– Optional 9 us slot time when the BSS consists of only ERP devices

•• ER-ER-PBCC (Optional) PBCC (Optional) (5.5,11,22,33 Mbps)(5.5,11,22,33 Mbps) – 256-state binary convolutional code

•• ERPERP--DSSSDSSS--OFDM (Optional) OFDM (Optional) (6,9,12,18,24,36,48,54 Mbps)(6,9,12,18,24,36,48,54 Mbps) – Hybrid modulation

– DSSS : for preamble and headerpreamble and header – OFDM : for data payloaddata payload

IEEE 802.11g PCLP

• • Three Three different mandatory mandatory PLCP PPDU format

– Long Preamble and header (same as 11b) (for DSSS-OFDM and ERP-PBCC)

– Short Preamble and header (same as 11b) (for DSSS-CCK)

» Differences in SERVICE field

• Diff 1 : a bit in SERVICE field is used to indicate DSSS-OFDM

• Diff 2 : two bits in SERVICE field are used to resolve the length ambiguity for PBCC-22 and PBCC-33

– OFDM preamble and header (similar as 11a) (for ERP-OFDM)

b0

b0 B1B1 b2b2 b3b3 b4b4 b5b5 b6b6 b7b7

Modulation Modulation selection selection 0 = Not 0 = Not DSSS-DSSS- OFDM OFDM 1 = DSSS 1 = DSSS-- OFDM OFDM

Reserved

Reserved Locked Locked Clock Bit Clock Bit 0 = not 0 = not locked locked 1 = locked 1 = locked

Modulation Modulation Selection Selection 0 = CCK 0 = CCK 1 = PBCC 1 = PBCC

Reserved

Reserved Length Length Extension Extension BitBit (PBCC) (PBCC)

Length Length Extension Extension Bit Bit (PBCC) (PBCC)

Length Length Extension Extension BitBit

Long/Short PLCP for PBCC-22 and PBCC-33

PPDU

SYNC 128/64 bits

SFD 16 bits

SIGNAL 8 bits

SERVICE 8 bits

LENGTH 16 bits

CRC 16 bits

Long PLCP Preamble 144/72 bits in 1 Mbps

Long PLCP Header 48 bits

PSDU/MPDU 1/ 2, 5.5, 11 Mbps

• Rate indication

– h0A 1Mb/s DBPSK (for long only) – h14 2Mb/s DQPSK

– h37 5.5Mb/s CCK or PBCC – h6E 11Mbps CCK or PBCC – hDC 22Mbps PBCC-22

PBCC-22 in 802.11g

• 256-state binary convolutional code of rate R=2/3

• PBCC-22 convolutional encoder

– Provide encoder the “known state”

» 4 memory elements are needed and

» one octet containing all zeros is appended to the end of the PPDU prior to transmission

•• One more octet than CCKOne more octet than CCK

– For every pair of data bits input, three output bits are generated (R=2/3)

Z^-1 Z^-1 Z^-1 Z^-1

y0

y1 b_2j Z^-1 Z^-1 Z^-1 Z^-1

y0

y1 b_2j

PBCC-22 in 802.11g

• For 22Mbps, three output bits (y0,y1,y2) produce one symbol via 88--PSKPSK

– two data bits per symbol

PBCC-33 in 802.11g

• Upgrade the 802.11b 11Msps (in 20MHz bandwidth) as 11Msps (in 20MHz bandwidth) 16.5Msps

16.5Msps

• by using pulse shaping and adaptive equalization

• enhance 50% data rate

Preamble Encoded Data

11Msps 11Msps

Shift

16.5Msps 16.5Msps Clock

Switch (10 clock cycles10 clock cycles)

ReSync Tail Head

11Msps

11Msps 16.5Msps16.5Msps