Introduction to IEEE802.22

Presented by Somayeh Mahmoodi

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formation of the IEEE 802.22 WG

• Cognitive Radios are seen as the solution to the current low usage of the radio spectrum

• CRs have the potential to utilize the large amount of unused spectrum in an intelligent way while not interfering with other incumbent devices in frequency bands already licensed for specific uses

• formation of the IEEE 802.22 WG (or simply, 802.22) for WRANs in November/2004

• This WG has been chartered with the specific task of developing an air interface (i.e., PHY and MAC) based on CRs for unlicensed operation in the TV broadcast bands,

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• the 802.22 WG has approved its baseline document and is working on drafts

• But in this presentation, we provide an overview of the status of the work in 802.22 WG– the requirements for incumbent service detection and protection

– The techniques employed for sensing and detecting such

incumbents,

– coexistence issues,

– the air interface,

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IEEE 802.22 Status as of November 2006

• June 2005: Draft standard version 0.1 is out

• January 2007: Expected to release draft version 0.2

• March 2007: Go to WG ballot

• July 2007: Go to Sponsor ballot– Point where Draft 1.0 is expected to be available

• January 2008: IEEE 802.22 standard approved by the IEEE Standards Association

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WRAN Hierarchy

Public IP Network

Service Provider IP Network

HAAAA

SD

ACR ACR

WRANBS

CPE

집

집 집

집

•AAA : Authentication, Authorization and Account Server • ACR : Access Control Router HA : Home Agent

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PAN< 10 m

802.15.1 (Bluetooth) – 1 Mbps802.15.3 > 20 Mbps

802.15.3a (UWB) < 480 Mbps802.15.4 (Zigbee) < 250 kbps

LAN< 150 m

11 – 54 Mbps

802.11a/b/e/gHiperLAN/2

802.11n (proposed) > 100 Mbps

MAN< 5 km

802.16a/d/e - 70 MbpsLMDS - 38 Mbps

WAN< 15 km

802.20 (proposed)GSM, GPRS, CDMA, 2.5G, 3G – 10

kbps to 2.4 Mbps

RAN< 100 km

802.22 (proposed) - 18 to 24 Mbps

Application Overview

• Frequency: – Operates in TV bands from 54 to

862 MHz– 6 MHz, 7 MHz and 8 MHz channel

bandwidth

• Rates:– From 18 Mbps to 24 Mbps– 1.5 Mbps upstream, 300 kbps

downstream

• Transmit power:– 4W CPE transmit power

• Service Range:– Nominally 33 km, up to 100 km– Propagation delays exceed 300 µs

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Protection of Incumbents

• Within 802.22, the sensing mechanism is designed to offer

protection to two types of incumbents, – the TV service

– wireless microphone• wireless microphones are licensed secondary users of the spectrum, and

are allowed by FCC to operate on vacant TV channels on a non-interfering basis

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Topology, Entities and Relationships

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DFS Timing Requirements

• Two key parameters are – the Channel Detection Time (CDT)

• The CDT defines the time during which an incumbent operation can withstand interference before the 802.22 system detects it. It dictates how quickly an 802.22 system must be able to detect an incumbent signal exceeding the IDT

– the Incumbent Detection Threshold (IDT).

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The Cognitive PHY

• OFDMA both in uplink and downlink– Since delay spread in the order of 25μs up to 50μs are expected, the use of a

cyclic prefix of about 40μs is needed.– in order to reduce the impact of the overhead due to cyclic prefix, a 2K FFT size

per TV channel has been selected as the normative mode

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Orthogonal frequency division multiple access

• OFDMA consists of assigning one or several sub-carrier frequencies to each user (terminal station) with the constraint that the sub-carrier spacing is equal to the OFDM frequency spacing 1/Ts

• The signal of user k, k = 0, 1, . . . , K − 1, where K = Nc, has the form

• with

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The Cognitive PHY

• The 802.22 PHY also provides high flexibility in terms of modulation and coding– The BS is capable of dynamically adjusting the bandwidth, modulation and

coding on, at least, a per CPE basis

– Modulation schemes are QPSK, 16-QAM, 64QAM with convolution coding schemes of rate 1/2, 3/4, and 2/3 ---> a few Kbps per sub-channel up to 19 Mbps per TV channel

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channel bonding

• One of the key features supported by the PHY layer is dynamic channel bonding, which allows the 802.22 network to take advantage of multiple vacant TV channels based on availability.

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Channel Bonding Structure

• 6K FFT over 3 TV channels– 2K per TV channel– Null out the outer carriers for 1 or

2 TV channels

• Fixed inter-carrier spacing– Several implementation

possibilities

DataSub-carrier

PilotSub-carrier

Guard/NullSub-carrier

6 MHz

18 MHz

12 MHz

DC

DC

DC

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Fractional Bandwidth Usage

• Considering the location, bandwidth and type of the narrowband incumbent users, we can use the fractionally vacant bandwidth of a single channel

• The number of used sub-carriers is proportional to the fractional bandwidth

• The fractional BW mode is identified by using a Preamble• Example:

6 MHz Unused(6 MHz)6 MHz

f

Incumbent or other CR user(except microphone user)

TV incumbent user Microphone userFractional useof TV channel

GuardBand

Other CR user or non-microphone incumbent (regulations permitting)

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Spectrum Sensing

• the objective of the 802.22 group is to define requirements for sensing that have to be met by all manufacturers. This specification of sensing requirements is ongoing

• The principal metrics for characterizing a sensing algorithm are

“Probability of Detection (PD)” and “Probability of False Alarm (PFA)”, where both these quantities are functions of the received

SNR and threshold.– PFA value of 0.01 – 0.1

– PD of 0.9 – 0.95

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Spectrum Sensing

• There are two main approaches to spectrum sensing– energy detection and feature detection

• Energy detection is used to determine presence of signal energy in the band of interest, and is followed by feature detection to determine if indeed the signal energy is due to the presence of an incumbent.

• Since 802.22 will be implemented in the TV bands, the digital incumbent signals could be either ATSC (North America), DVB-T (Europe), or ISDB (Japan)

• The ATSC signal has a number of features that could be exploited for feature detection algorithms .for more detail please refer to web page

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The Cognitive MAC

• The 802.22 MAC is very flexible, and can provide an adequate

service to secondary users while enforcing the necessary incumbent

protection mechanisms.

• To make an effective use of the radio spectrum, the MAC regulates DS medium access by Time Division Multiplexing, while the US is managed by using a Demand Assigned TDMA scheme.

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Superframe Structure

Superframe n-1 Superframe n Superframe n+1 ...Time

...

Preamble SCH frame 0 frame 1 frame m...

TV Channelt-1

TV Channelt

TV Channelt+1

Time

Preamble SCH

Preamble SCH

Fre

qu

en

cy

Preamble SCHFrame

0Frame

1

Framem-2

(Quiet)...

... Frame0

Frame1

Preamble SCH

Preamble SCH

Occupied by Incumbent

Occupied by Incumbent

Framen

Occupied by Incumbent

Framem

Framem-1

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frame Structure

frame n-1 frame n frame n+1 ...Time

...

DS PHY PDU

Preamble FCH DS burst 1 DS burst 2 DS burst x...

BcastMsgs

MACPDUs

MAC PDU 1 ... MAC PDU y Pad

MACHeader

MAC Payload CRC

DS subframe

Initializationslots

BW requestslots

US PHY PDU(CPE m)

US PHY PDU(CPE p)

...

US subframe

Preamble US burst

MAC PDU 1 ... MAC PDU k Pad

MACHeader

MAC Payload CRC

Sliding self-coexistence

slots

Can appear ineither DS or US

BCH

UCSNotification

Slots

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Frame Element Definitions

• Preamble – synchronization, channel estimation– Long training sequence and optional short training sequence

– There’s also an upstream burst preamble

• FCH – frame control header, info on size of DS- and US- MAP and channel descriptors (PHY characteristics)

• MAPs – resource scheduling info for user bursts• Ranging – timing offset, power adjustment• UCS – urgent coexistence situation, incumbent detection report• BW Request – self-explanatory• SSS - sliding self-coexistence slots – used by coexistence beacon

to improve coexistence with neighbors • BCH – burst control header for upstream – ID information

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Coexistence

• Effective coexistence is one of the key responsibilities of a CR– coexistence with incumbents

– self-coexistence

• coexistence with incumbents deals mechanisms for a reliable, efficient, and timely detection of primary services.

• self-coexistence addresses collocated 802.22 cells..– Self-coexistence in 802.22 is a critical issue given its unlicensed

operation and its very large coverage range

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Measurement and Spectrum Management

• The 802.22 draft standard includes a comprehensive measurement and spectrum management ingredient that provides the necessary flexibility and efficiency. – Here, the BS instructs associated CPEs to perform periodic

measurement activities,

– measurement reports are generated and communicated (e.g., to the BS in case of 802.22), decisions are made as to which channels to use, when, and for how long

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THANKSTHANKS