Ofdm vs Ofdma
Transcript of Ofdm vs Ofdma
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Understanding WiMAX PHY
BP Tiwari
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Beyond 4GBeyond 4G
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Agenda
OFDMA
WiMAX Frame
Sub channels and Permutations
Smart Antenna Systems
Summary of WiMAX PHY features
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Lets understand fading in wirelesscommunications
Delay Spread)
Frequency selective fading (
Delay Spread)
Multiple copies of signal arrive at
the receiver, offset in timeTime Selective Fading ( Doppler )
M UL T I P AT H
F ADI N G C HANN
E L S
Fading (Delay Spread)
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WiMAX Addresses Fading
Multiple copies of signal arrive at
the receiver, offset in time
Fading (Delay Spread) Frequency selective fading (Delay Spread)
OFDMA
Diversity Schemes
Multiple Antenna Systems
Cyclic Prefix ( ISI )
Forward Error Correction
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Concept of OFDMSubcarrier
OFDM is robust to fading
High spectral efficiency
Eliminate Inter-symbolInterference (ISI) by theintroduction of the cyclicprefix (CP)
Sensitivity to frequencyoffset
Higher Peak to averagepower ratio(PAPR)
In OFDM availablebandwidth is dividedinto N (large numberof) orthogonal sub-carriers
Each sub-carrier isindividually modulated
(QAM) at 1/N totaldata rate
FFT implementation(fast and efficient)
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OFDM vs. OFDMA Subcarrier
– Type: Data, Pilot, null(guard and DC)•
– Number: 128, 512, 1024,2048 (at least one)
Subchannel
– A set of sub carriers forms asubchannel
– The sub carriers may andmay not be adjacent
– Distributed allocation(diversity)
– Contiguous allocation(selectivity)
OFDM
OFDMA
SC
Subcarrier
Subchannels
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OFDMA Advantages
• Users/bursts are scheduledacross both frequency(subchannels)and time (symbols)
• Per burst modulation, codingand TX power
• Higher granularity in resourceallocation
• More degrees of freedom inscheduling
• Improved fairness and QoS
D D
C C
D D
C C
B B
A A
B B
E E
TIME
S U B C H A N N E L
•Orthogonality of users are maintainedusing large symbol duration and cyclicprefix
•Users concentrate transmit power overpartial bandwidth
•High granularity in resource allocation
& power control
•Inherent resistance to multi-path(OFDM)
•Flexible Sub-channelization
•Distributed (pseudo-random)permutations –frequency diversity
•Contiguous (clustered)permutations -> selectivity
•Scalability of Channel Sizes
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OFDM Symbols and Cyclic Prefix
Use Cycle Prefix (CP) as
guard interval Prepend segment of
OFDM symbol –>overhead
CP duration > max delay
spread Eliminates ISI
Retains orthogonallybetween sub-carriers (ICI)
Pilots used for channel
estimation and tracking Interleaving (bits
distributed across tones)and error correctingfacilitates recovery underfrequency selective fading
or narrow-bandinterference
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OFDMA Frame Structure
P
R
E
A
M
BL
E
DL-
MAP
UL-
MAP
DL BURST 2
DL BURST 1
DL BURST 3
DL BURST 4
S
U
B
C
H
A
N
N
E
L
S
S1 S2 SnDOWNLINK
CD
M
A
R
A
N
G
I
N
G
P
e
ri
o
d
i
C
R
a
n
g
.
ACKCH
’s
CQICH
s
UL BURST 1
UL BURST 2
UL BURST 3
UPLINKTTG RTG
Uplink and Downlink isdivided into time zones
Different Permutationfor downlink and uplink
– DL PUSC,AMC etc
– UL PUSC,AMC– Activation STC/MIMO
CDMA Codes
– Special allocation inuplink for ranging andBW Request
Fast Feedback/CQICHs
– SS measured CINR
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Localized vs Distributed Subchannels
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Subchannels in 16e
UL PUSC
– 1 tile = 4 subcarriers x 3 OFDM symbols
– 1 subchannel = 6 tiles (frequency domain)
– 1 slot =1 subchannel x 3 OFDM symbols
– 1 slot = 72 subcarriers
= 48 data subcarriers + 24 pilots
DL/UL AMC2X3
– 1 bin = 9 subcarriers x 1 OFDM symbols
– 1 subchannel = 2 bins (frequency domain)
– 1 slot = 1 subchannel x 3 OFDM symbols
– 1 slot = 54 subcarriers
= 48 data subcarriers + 6 pilots
DL PUSC – 1 cluster = 14 adjacent subcarriers
x 2 OFDM symbolsx 2 OFDM symbols
– 1 subchannel = 28 subcarriers
– 1 slot = 1 subchannel x 2 OFDM symbols
– 1 slot = 56 subcarriers
= 48 data subcarriers + 8 pilots
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Permutation schemes in DL : PUSC
2 S y m
2 S y m
2 S y
m
2 S y m
2 S y m
Cluster
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Permutation schemes in UL : PUSC
3 S Y MB OL S
3 S y m
3 S y m
3 S y m
3 S y m
Pilot
Tile Structure
• 4 Sub carrier X 3 Symbol
• 4 Pilot + 8 Data
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PHY Data Rates
Without MIMO and overheads
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Advanced Antenna Systems : STBC
STC gains vary from 5-10 db depending on channels condition
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MIMO B : Spatial Multiplexing
SM can double throughput in theory by require very highCINR conditions of uncorrelated signal to achieve fulladvantages. 30% sector improvement in loaded conditions.
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Beam forming
Beamforming can provide additional 5-6 db gains in downlink
improving overall network quality. Improves sector capacity by 20 to 25%
4Rx in uplink provides additional 3 db gain
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UL Collaborative MIMO
Collaborative Spatial Multiplexing fortwo MSs with single transmitantenna
Two MS transmit on the sameresource (sub channels)
For orthogonal zing
– Orthogonal pilot patterns (patternA, pattern B)
Multiple data detection
– Using multiple receive antennas
– Interference suppression: MMSE,
ZF– Nonlinear interference
cancellation: SIC, PIC
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MIMO Summary
BF helps in improving capacityand improve network quality.
20 to 25%over MIMO
5-6 db inDownlink
DL-BeamForming
MIMO A in low SINR conditionimproves channel fading.Switching between MIMO A and Bis automatic and controlled byBTS.
5-10*MIMO A
Improves cell radius by 3db in
uplink.
Nil3 db over 2nd
orderdiversity
4th order
Diversity
Average improvement of 30% in
full load , customers higher CINRor uncelebrated signals will takeadvantage of MIMO B
30% in
average
NilMIMO B
CommentsGain in
Capacity
Gain in Link
Budget
Parameters
*STC + 2 Rx in receiver with possibility MRC combining
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Mobile WiMAX PHY Summary
Downlink : 2x2 , STC , SMUplink : 1X2 , Virtual SM
MIMO
AMC or PUSC
UL channel sounding
BeamForming
Downlink :QPSK,16QAM,64QAM
Uplink : QPSK, 16QAM
Modulation
Ranging , CQICH’s , ACKCHUplink Control channels
PUSC,AMC,FUSCSub-carrier allocations
CC, CTC with HARQError correction andHARQ
5 ms , 1/8Frame size and cyclicprefix
TDD , 5 and 10 MHz , 20 (optional)Scalable ChannelBandwidth
SupportFeature
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Thanks