Introduction to IEEE802.22
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Transcript of Introduction to IEEE802.22
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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