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802.22 based on CRs
802.22 based on Cognitive Radios
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General Information
802.22 based on Cognitive Radios
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General Information
802.22 is a new 802 LAN/MAN standardsWireless Regional Area Network (WRAN)White spaces in TV frequency spectrum
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General Information
Deployment Scenario
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General Information▣Why CRs
Solution to low usage of the radio spectrumFlexible, efficient, reliable spectrum usePotential to utilize the large amount of unused spectrum
=> Required real-time measurement dissemination
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General Information
Example of TV band occupancy over time and frequency
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General Information
Diagram of the channel bonding scheme illustrating
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General Information▣ Used frequencies
54-862 MHz in North America region41-910 MHz, ongoing debateStandard shall accommodates various
international TV Ch. Bw. such as 6, 7, 8 MHz
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General Information▣Target market
Wireless broadband access in rural/remote areas ( less concentrated area)
e.g. single-family residential, multi-dwelling units, small office/home office (SOHO), small businesses, multi-tenant buildings, and public and private campuses
South America, Africa and Asia
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General Information▣ Service Capacity
Spectral efficiencies from 0.5 up to 5 bit/(sec/Hz)
If average of 3 bits/sec/Hzthen Total PHY data rate of
18 Mbps in a 6 MHz TV channel
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General Information▣ Service Coverage
Go up to 100 Km (power isn’t an issue)Current coverage range
- 33 Km at 4 Watts CPE EIRP
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General Information▣ Service Coverage
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Physical Layer Standard
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Background
Standard body – IEEE 802.22 WRAN Working groupAll documentation not available to general public (standardization ongoing). Some documents found at http://www.ieee802.org/22/Major players – Samsung, Philips, Huawei, Nanotron, ETRI and others
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Requirements
High performance, low complexitySupport for variable number of usersEfficient usage of available frequencyDynamic changes in bandwidth, modulation and coding
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Block Diagram – CR System
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PHY Block Diagram -Transmitter
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PHY Block Diagram - Receiver
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OFDMA
Used as a Multi-access Broadcast schemeSimultaneous transmission from usersAdaptive user-to-subcarrier assignmentRobust against fast fading and co-channel interferenceData rate and error probability for each user individually controlled
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Pulse Shape
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Coding Schemes
Convolutional Coding, specifically turbo codesa. Close to Shannon limit of capacityb. Used in low-power applicationsc. High decoding complexityd. High latency applications 1/2, 3/4, 2/3 coding rates
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Modulation Schemes
a. QPSK
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b. 16-QAM
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c. 64-QAM
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Physical Layer
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About Proposed PHY...
Which company proposed it?
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What they have proposed...Adaptive OFDMA- Adaptively scalable to spectrum availability
(1,2,3,4,6,7,8MHz bandwidth)
- New frame structure for CR-enabled operation
- Enhanced PHY features-> Cyclic prefix and cyclic postfix-> Adaptive pilot insertion-> Enhanced channel coding, e.g., LDPC
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Adaptive OFDMA
Flexible Bandwidth Allocation using FFT- Channel bonding- Fractional bandwidth usage
Adaptive resource allocation according to user environments- Channel selectivity
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OFDMA Symbol Time Structure
Type I: Conventional
Mode 2K 4K 8K
Cyclic Prefix Time
56 us 112 us 224 us
Cyclic Prefix
• Type II: Hybrid– 4K & 8K Modes – 2K Mode
Cyclic PostfixCyclic Prefix
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Frame Structure : TDD
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Frame Structure : FDD
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Advantages of Adaptive OFDMAFlexible Bandwidth Allocation◦ To use the partial bandwidth (1,2,3,4,6,5,7,8MHz) adaptively,
depending on the channel state information (availability)◦ To fully utilize available bandwidth under a unified PHY
framework
Single Sampling Frequency◦ Sampling frequency is the same, i.e., 64/7MHz, for all FFT modes.
Constant Subcarrier Spacing◦ The subcarrier spacing is constant for all different channel
bandwidths Robust to the frequency offset
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MAC Layer
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802.22 MAC
• Based on the specifications for 802.16d (WirelessMAN), with some modifications.
• Issue with Cognitive Radio Systems: Situations in which the IU is detected by just one of the BS or the CPE.
• Proposed solution “Implicit signal based co‐operative sensing”.
• Features: Minimal Changes, Channel Management, Scanning and Radio Resource Management
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Frame Structure
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Channel Maintenance
• Definitions: Active Set 1, Active Set 2, Candidate set, Occupied Set, Disallowed Set, Null Set.
• Channel Set Maintenance
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Channel Maintenance
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Radio Resource Management
• Channel grouping for MAP overhead reduction in the multi‐FA system
• Active set update to maximize the average system throughput
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Scanning Operation: Implicit Signaling
• Rendezvous procedure for band switching
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Spectrum Sensing Technologies
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Problems to be solved
Provide spectrum occupancy information to MACIdentify type of incoming signalFast tracking time to improve data throughputFlexible resolution for adaptive and scaling searchingSimple computation for low powerEasy implementation for low cost
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FeaturesSensing clock is separated from transceiverMultiple sensing strategy : Coarse and FineSensing while in communication and not in
communicationCritical computation is performed at analog domain
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FeaturesTransmitter
(RF/IF)
Receiver(RF/IF)
Coarse“MRSS”
Fine“AAC”
Low SpeedADC
PHY(Baseband)
MAC
SW orDuplexer
Sensing Receiver
Directive Antenna
Omni Antenna
Spectrum RecognitionAlgorithm
Proposed sensing scheme
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Sensing techniqueMulti-Resolution Spectrum Sensing (MRSS)
: Coarse sensing, detect existence of signal
Analog Autocorrelation (AAC) : Fine sensing, categorize the signal type
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AdvantagesSensing block is separated from transceiver
- performed without waking-up PHY - performed, while not in using & in using
Multiple sensing strategy- coarse sensing for spectrum occupancy- fine sensing for indentifying incoming signal
this reduces the false detect rate
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Advantages
Critical computation is performed at analog domain- no significant computation in baseband
: FFT nor Correlation -> faster recognition time- drastically reduce power consumption- require very low speed/low resolution ADC
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Coarse Spectrum Sensing
Multi-Resolution Spectrum Sensing (MRSS)
it detects spectrum occupancy
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MRSS implementation
a. MRSS detects spectral components of incoming signal by the Fourier Transform
b. Fourier Transform is performed in analog domainc. MRSS may utilize wavelet transforms as the basis
function of the Fourier Transformd. Bandwidth, resolution and center frequency can be
controlled by wavelet transform
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Spectrum Sensing Technologies
X ∫ ADC
v(t)*fLO(t)
Driver Amp CLK#2
MACTiming Clock
CLK#1
x(t)
w(t)
z(t) y(t)
MRSS Schematics
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MRSS simulation result (example)
The spectrum of the wireless microphone signal
The corresponding signal spectrum detected with the MRSS technique
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 106
-100
-80
-60
-40
-20
0
20
40
Frequency
Pow
er S
pect
rum
Mag
nitu
de (d
B)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 10 6
-120
-110
-100
-90
-80
-70
-60
-50
Frequency (Hz)
PS
D (d
B)
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Fine Spectrum Sensing
Analog Autocorrelation (AAC)
It indentifies incoming signal
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AAC implementationa. An input RF signal x(t) is divided and delayed by a certain
delay value Td
b. The correlation between x(t) and x(t-Td) is performed at analog domain
c. If the resulting integrator output shows sharp pulse, that Td indicates the feature of the incoming signal
d. Since AAC is performed at analog domain, low speed ADC is sufficient
802.22 based on Cognitive Radios
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Spectrum Sensing Technologies
Multiplication Integrate
x(t)
Delay Td
Low SpeedADC
x(t-Td))Decision Making
FIR
Sensing Antenna
AAC Schematics
802.22 based on Cognitive Radios
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Spectrum Sensing TechnologiesRecognize the periodic features of the input signals unique
for each modulation format or frame structureAuto correlation is done at the analog domainAAC can recognize the following input signals :
IS-95, WCDMA, EDGE, GSM, Wi-Fi, Wi-MAX, Zigbee, Bluetooth, Digital TV (ATSC, DVB), etc.
802.22 based on Cognitive Radios
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AAC simulation result (example)
Multiplier Output Waveform FIR Output Waveform
(3)
(3)(1)
(2)
(1)
(2)
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Summary of spectrum sensing block
Specification
ADC resolution6 bits (MRSS)1-3 bits (AAC)
ADC sampling time> 5M sample/sec (MRSS)> 120k sample/sec (AAC)
Sensing time< 1 ms (while in communication)< 4 ms(while not in communication)
Sensing threshold < -110 dB
Baseband processing in PHY No significant computation, such as FFT nor convolution, is required at the base band
Baseband processing in MAC Noise reduction, harmonic suppressions
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Conclusion
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What is the performance improvement
WRAN Functional
RequirementsProposal
Minimum Data Rate
DL: 1.5 Mbps/subscriber
UL: 384 kbps/subscriber
DL:2.8 Mbps/subscriber
UL:400 kbps/subscriber
Service Coverage
Typical: 33 km
Maximum: 100 km
Typical: 42.5 km(2K mode), 60.6 km(4K mode) Maximum: 145.5km(8K mode) (*)
Spectral Efficiency
Minimum: 0.5 bits/s/Hz
Maximum: 5 bits/s/Hz
Minimum: 0.73 bits/s/Hz
Maximum: 5.16 bits/s/Hz
Maximum
Excess Delay
Pre-echo: 3 us
Post-echo: 60 us
Pre-echo: 3.5 us (2K mode) Post-echo: 112 us(4K mode), 224 us (8K mode)
(*) It is calculated from the point of view of TTG time, not from link budget.
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Cognitive radio: an integrated agent architecture for software defined radio
Wireless Internet
Emergency Services
Applications
802.22 based on Cognitive Radios