1 beyond4g.org beyond4g.org Understanding WiMAX PHY BP Tiwari
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beyond4g.org beyond4g.org
Understanding WiMAX PHY
BP Tiwari
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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 wireless communications
Delay Spread)
Frequency selective fading ( Delay Spread)
Multiple copies of signal arrive at the receiver, offset in time
Time Selective Fading ( Doppler )
MU
LT
IPA
TH
FA
DIN
G C
HA
NN
EL
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-symbol
Interference (ISI) by the introduction of the cyclic prefix (CP)
Sensitivity to frequency offset
Higher Peak to average power ratio(PAPR)
In OFDM available bandwidth is divided into N (large number of) orthogonal sub-carriers
Each sub-carrier is individually modulated (QAM) at 1/N total data 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 a
subchannel– The sub carriers may and
may not be adjacent– Distributed allocation
(diversity)– Contiguous allocation
(selectivity)
OFDM
OFDMA
SC
Subcarrier
Subchannels
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OFDMA Advantages
• Users/bursts are scheduled across both frequency (subchannels)and time (symbols)
• Per burst modulation, coding and TX power
• Higher granularity in resource allocation
• More degrees of freedom in scheduling
• Improved fairness and QoS
D D
C C
D D
C C
B B
A A
B B
E E
TIME
SU
BC
HA
NN
EL
•Orthogonality of users are maintained using large symbol duration and cyclic prefix
•Users concentrate transmit power over partial 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 orthogonally between sub-carriers (ICI)
Pilots used for channel estimation and tracking
Interleaving (bits distributed across tones) and error correcting facilitates recovery under frequency selective fading or narrow-band interference
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OFDMA Frame Structure
PREAMBLE
DL-MAP
UL-MAP
DL BURST 2
DL BURST 1
DL BURST 3
DL BURST 4
SUBCHANNELS
S1 S2 SnDOWNLINK
CDMARANGING
PeriodiCRang.
ACKCH’s
CQICHs
UL BURST 1
UL BURST 2
UL BURST 3
UPLINKTTG RTG
Uplink and Downlink is divided into time zones
Different Permutation for downlink and uplink– DL PUSC,AMC etc – UL PUSC,AMC– Activation STC/MIMO
CDMA Codes– Special allocation in
uplink for ranging and BW Request
Fast Feedback/CQICHs– SS measured CINR
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Localized vs Distributed Sub channels
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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
ym
2 S
ym2
Sym
2 S
ym2
Sym
Cluster
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Permutation schemes in UL : PUSC
3 SY
MB
OLS
3
Sym
3
Sym
3
Sym
3
Sym
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 high CINR conditions of uncorrelated signal to achieve full advantages. 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 for two MSs with single transmit antenna Two MS transmit on the same
resource (sub channels) For orthogonal zing
– Orthogonal pilot patterns (pattern A, 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 capacity and improve network quality.
20 to 25% over MIMO
5-6 db in Downlink
DL-Beam Forming
MIMO A in low SINR condition improves channel fading. Switching between MIMO A and B is automatic and controlled by BTS.
5-10*MIMO A
Improves cell radius by 3db in uplink.
Nil 3 db over 2nd
order diversity
4th order Diversity
Average improvement of 30% in full load , customers higher CINR or uncelebrated signals will take advantage of MIMO B
30% in average
Nil MIMO 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 , SM Uplink : 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 HARQ Error correction and HARQ
5 ms , 1/8Frame size and cyclic prefix
TDD , 5 and 10 MHz , 20 (optional) Scalable Channel Bandwidth
Support Feature
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Thanks