1runcom technologies ltd. submission eli sofer, runcom january 2005 doc.: ieee802.22-05-0005r1 slide...
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1Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 1
Tutorial on Multi Access OFDM (OFDMA) TechnologyIEEE P802.22 Wireless RANs Date: 2005-01-04
Name Company Address Phone email Eli Sofer Runcom
Technologies 2 Hachoma St., 75655 Rishon Lezion, Israel
+972 3 9528440 [email protected]
Yossi Segal Runcom Technologies
2, achoma St. 75655 Rishon Lezion, Israel
+972 3 952 8440 [email protected]
Authors:
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2Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 2
Abstract
The contribution presents a tutorial on Multi Access OFDM (OFDMA) technology which has been endorsed in leading standards such as- ETSI DVB-RCT and IEEE802.16a,d and 16e. Essential parameters of UpLink and DownLink and simulation results are presented. System capabilities and advantages are also discussed. The tutorial could offer an insight and understanding of OFDMA technology to be considered as a candidate for WRAN system
3Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 3
Tutorial onMulti Access OFDM (OFDMA)
Technology
Eli Sofer
Runcom Technologies Ltd
4Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 4
Contents
• OFDMA System Architecture
• Illustrated Example
• OFDMA System Properties
• Coverage and Capacity
5Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 5
OFDMA System Architecture
6Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 6
• Duplexing Technique FDD/TDD
• Multiple Access Method TDMA/OFDMA
OFDM Symbols allocated by TDMA Sub-Carriers within an OFDM Symbol allocated by OFDMA
• DiversityFrequency, Time, Code (CPE and BS), Space Time Coding, Antenna Array
7Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 7
FDD (Frequency Division Duplexing ) Uses One Frequency for the DownLink, and a Second Frequency for the UpLink.
TDD (time Division Duplexing) Uses the same frequency for the Downlink and the Uplink.
In any configuration the access method is OFDMA/TDMA .
F2 - Frequency band
UpLink
F1 - Frequency band
DownLink
FDD
F1 - Frequency band
UpLink
F1 - Frequency band
DownLink
TDD
Duplexing - Principles
8Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 8
Using OFDMA/TDMA, Sub Channels are allocated in the Frequency Domain, and OFDM Symbols allocated in the Time Domain.
OFDMA-TDMA Principles
TDMA
TDMA\OFDMA
t
N
m
9Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 9
Combining TDMA and FDMA• TDMA/FDMA operation = OFDMA• Frequency sub-channels are composed of multiple,
non-adjacent carriers
TX#4TX#3
TX#2
TX#6
TX#5
TX#1
Time
Fre
quen
cy b
in
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
10Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 10
Multipath – The challenge• The RF signal travels through multiple paths• The RF channel is characterized by a delay profile• Problematic when delay spread is comparable to symbol duration
11Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 11
Multipath – The solution• OFDM modulates in parallel multiple narrow band
sub-carriers• Multipath duration becomes short relative to symbol
duration• Pilot and guard sub-carriers are also inserted
Data carriers
Total Frequency band
Pilot Carriers
Frequency guard Band Frequency Guard Band
12Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 12
Multipath – The solution• The sub-carriers are converted by IFFT to a time domain
signal• A guard interval (cyclic prefix) is added to collect
multipath• A long guard interval (GI) reduces efficiency but enhances
multipath handling capability• OFDM/OFDMA enable handling peak delay spread as
large as the GI
GI
t
OFDM symbol
13Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 13
DownLink OFDMA Symbol
Total Frequency BandGuard Band Guard Band
Symbol PilotsSub-Channel Data Carriers
14Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 14
• Burst Structure is defined from one Sub-channel in the Frequency domain and n OFDMA time symbols in the time domain, each burst consists of N data modulated carriers.
• Adaptive Modulation and Coding per Sub-Channel in the Down-Link
• Forward APC controlling (+6dB) – (-6dB) digital gain on the transmitted Sub-Channel
• Supporting optional Space Time Coding employing Alamouti STC.
• Supporting optional Adaptive Array.
DownLink Specification
15Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 15
• FFT size : 2048• Guard Intervals : ¼, 1/8, 1/16, 1/32• Coding Mandatory: concatenated RS GF(256) and
Convolutional coding (k=7,G1=171,G2=133, keeping overall coding rate to = ½, ¾
• Coding Optional: Convolutional Turbo Code (CTC), Turbo Product Code (TPC) with coding rates close to = ½, ¾
• QPSK, 16QAM, 64QAM modulation• Modulo 4, Pilot based Symbol Structure.• 32 Sub-Channels of 48 data carriers each
Example- DownLink Specification
16Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 16
Downlink Pilot and Data Carriers Allocation Scheme
0 12
symbolindex
n
n+1
L=0
L=2
carrier index
Variable Location Pilot Fixed-location Pilot DataAllocation Key:
n+2 L=1
n+3 L=3
n+4 L=0
time
24 Nused -1
0 6 18 Nused -1
0 3 15 Nused -1
0 9 21 Nused -1
0 Nused -1
30
27
12 24
17Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 17
Space Time Coding
Txdiversityencoder
IFFT DACFilter RF
IFFT DACFilter RF
Subcarrier modulation
IFFT input packing
Tx
Rx
RF DAC Filter FFT Diversity Combiner
Sub-channel demod.
Log-Likelihood
ratiosDecoder
18Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 18
UpLink OFDMA Symbol
Total Frequency BandGuard Band Guard Band
Data CarriersSub-Channel #1
Pilots CarriersSub-Channel #1
Data CarriersSub-Channel #x
Pilots CarriersSub-Channel #1
19Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 19
• Burst Structure is defined from one Sub-channel in the Frequency domain and 3 OFDMA time symbols in the time domain, each burst consists of 144 data modulated carriers.
• Adaptive Modulation and Coding per User in the UpLink
• User Can be allocated 1 up to 32 Sub-Channels• 2 Sub-Channels are used as the Ranging Sub-Channels
for User Ranging and fast Band-Width Request.
Example of UpLink Specification
20Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 20
• FFT size : 2048• Guard Intervals : ¼, 1/8, 1/16, 1/32• Coding Mandatory: concatenated RS GF(256) and
Convolutional coding (k=7,G1=171,G2=133, keeping overall coding rate to = ½, ¾
• Coding Optional: Convolutional Turbo Code (CTC), Turbo Product Code (TPC) with coding rates close to = ½, ¾
• QPSK, 16QAM, 64QAM modulation• Modulo 13, Pilot based Sub-Channel Structure.• 32 Sub-Channels of 53 carriers each, 5 carriers used as
pilots, 48 carriers used for data
Example of UpLink Specification
21Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 21
Example for UpLink Sub-Channel Pilot and Data Carriers Allocation Scheme
0 13 26 27 40 52
26 28 42150 2 52
symbolindex
n
n+1
L=0
L=2
frequency
Variable Location Pilot Fixed-location Pilot DataAllocation Key:
26 30 44170 4 52
n+2 L=4
26 4936220 9 52
n+11 L=9
26 38 51240 11 52
n+12 L=11
0 13 26 27 40 52
n+13 L=0
time
22Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 22
Frequency band
1 2 3 30 31 32
block 1
12
3each group contains
53 carriers
• All usable carriers are divided into 32 carrier groups named basic group, each basic group contains 53 sub-carriers
Using Special Permutations for carrier allocation
23Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 23
User #1
Total Frequency band
User #2
Guard Band Guard Band
0 5 521 222 10 1
User 1 = 0,5,2,10,4,20,8,17,16,11,9, 22 ,18,21,13,19,3,15,6,7,12,14,1User 2 = 2,10,4,20,8,17,16,11,9,22,18, 21 ,13,19,3,15,6,7,12,14,1,0,5
• Carriers are allocated by a basic series and it’s cyclic permutations for example:
• Basic Series:0,5,2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1
• After two cyclic permutations we get:2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1,0,5
Using Special Permutations for carrier allocation
24Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 24
• The Carriers of each Sub-Channel are spread all over the usable frequency for best frequency diversity
• The allocation by permutation gives an excellent Reuse factor - almost 1.
• The allocation by permutation give an excellent interference spreading and averaging.
Using Special Permutations for carrier allocation
25Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 25
• The CDMA like synchronization is achieved by allocating several of the usable Sub-Channels for the Ranging process, the logic unit they consist is called a Ranging Sub-Channel.
• Onto the Ranging Sub-Channel users modulate a Pseudo Noise (PN) sequence using BPSK modulation
• The Base Station detects the different sequences and uses the CIR that he derives from the sequences for:– Time and power synchronization– Decide on the user modulation and coding
Using CDMA like modulation for Ranging
26Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 26
• Aloha vs. CDMA BW request (32 codes)– CDMA efficiency is better by a factor of six – CDMA latency is better by a factor of four
DVB-RCT MAC Performance
0 1 2 3 4 5 6 7 8 9 100
0.5
1
1.5
2
2.5
Collision expectation value
Su
ce
ssfu
l B
W r
eq
ue
sts
pe
r slo
t
27Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 27
Illustrated Example
28Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 28
• Subscriber Units at the Current OFDMA Symbol = 3• Sub-Channels Allocated to Subscriber-Unit #1 = 12• Sub-Channels Allocated to Subscriber-Unit #2 = 9• Sub-Channels Allocated to Subscriber-Unit #3 = 6• Number Of New Subscriber-Units Requesting Services = 3
All Subscriber-Units Suffer Different Multi-Paths and different Attenuation's
Example
29Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 29
• Constellation at the Base Station Example
30Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 30
• Users Separation Example
31Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 31
• User Estimation
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Constellation to Estiamte
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Estimated vec
Example - Results
32Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 32
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Constellation to Estiamte
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Estimated vec
• User Estimation
Results
33Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 33
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Constellation to Estiamte
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Estimated vec
• User Estimation
Results
34Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 34
0 20 40 60 80 100 120 1400
50
100
150
200
250
300Despreading on All Users
• Finding New Subscriber-Units Requesting Services, Using the Ranging Pilots (CDMA/OFDM Techniques)
Results
35Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 35
OFDMA System - Properties
36Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 36
InterferenceUser SubCarriers
Allocation
SubCarriers
Interference
SubCarriers
NulledSubCarriers
Total Frequency band
• Narrowband Interference Rejection
– Easy to Avoid/Reject Narrowband Dominant Interference .
– Less Interfered Part of the Carrier Can Still Be Used .
Interference Rejection/Avoidance
37Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 37
• Using shaping on the signal peaks
• Limiting the PAPR to a constant value by vector reduction
PAPR Reduction
38Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 38
OFDM
Single CarrierScheme
4 MHz
-80
Frequency(MHz)
dB
• Rectangular Spectrum Shape (Brick Wall)• Small Frequency Guard band
Spectrum Properties
39Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 39
Spectrum Properties
40Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 40
In OFDM, channel impairment are solved in the same way Group Delays are solved, by Channel estimation
Group Delay
41Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 41
Phase Noise Effect on OFDM
Phase Noise Effect on
S.C
Phase Noise Effects
42Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 42
• Timing Sensitivity
Low timing sensitivity is needed, and simple phase and channel estimators solve timing problems.
• Frequency Sensitivity
solved by locking onto the Base-Station transmission and deriving the Subscriber Unit’s clocks from it.
• EqualizationNo Equalizers are needed, channel impairment and timing problems are both solved with simple phase and channel estimators
43Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 43
System Coverage and Capacity
44Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 44
By allocating different Sub-Channels to different sectors we can reach reuse factor of 1 with up to 12 sectors (changing the polarity enhances the performance)
HorizontalSub-hannel
s Set 1F1
VerticalSub-hannel
s Set 1F1 V
ertic
alS
ub-h
anne
ls
Set
2F
1
Hor
izon
tal
Sub
-han
nel
s S
et 2
F1
Using Reuse Factor of 1
45Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 45
Use modulations with various Bit/Hz capabilities as Adaptive N-QAM.
Use Adaptive FEC (Convolutional & Reed-Solomon or Turbo code)
Maximal frequency reuse between cells/sectors (close to 1).
Maximum sectors allocation. The use of statistical Multiplexing and concentration. Adaptive Carrier Allocations. Adaptive Power Control
Capacity
46Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 46
OFDM Cells(64 mode)
OFDMA Cell(2k mode)
64QAM users
16QAM users
QPSK users
Coverage
47Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 47
Coverage - Simulations
48Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 48
Coverage - Simulations
Multi Sector Coverage, 3 Sectors, 3 Frequencies, achieves 2.8Bits/s/Hz/Cell, 22.5Mbps/Sector
49Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 49
Coverage - Simulations
Multi Sector Coverage, 6 Sectors, 6 Frequencies, achieves 2.8Bits/s/Hz/Cell, 22.5Mbps/Sector
50Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 50
• Averaging interference's from neighboring cells, by using different basic carrier permutations between users in different cells.
• Interference’s within the cell are averaged by using allocation with cyclic permutations.
• Enables orthogonality in the uplink by synchronizing users in time and frequency.
• Enables Multipath mitigation without using Equalizers and training sequences.
• Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage.
OFDMA Advantages- Summary (1)
51Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 51
OFDMA Advantages - Summary (2)
• Enables spatial diversity by using antenna diversity at the Base Station and possible at the Subscriber Unit.
• Enables adaptive modulation for every user QPSK, 16QAM, 64QAM and 256QAM.
• Enables adaptive carrier allocation in multiplication of 23 carriers = nX23 carriers up to 1587 carriers (all data carriers).
• Offers Frequency diversity by spreading the carriers all over the used spectrum.
• Offers Time diversity by optional interleaving of carrier groups in time.
52Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 52
OFDMA Advantages - Summary (3)
• Using the cell capacity to the utmost by adaptively using the highest modulation a user can use, this is allowed by the gain added when less carriers are allocated (up to 18dB gain for 23 carrier allocation instead of 1587 carriers), therefore gaining in overall cell capacity.
• The power gain can be translated to distance - 3 times the distance for R4 and 8 time for R2 for LOS conditions.
• Enabling the usage of Indoor Omni Directional antennas for the users.
• MAC complexity is the same as for TDMA systems.
53Runcom Technologies Ltd.Submission Eli Sofer, Runcom
January 2005 Doc.: IEEE802.22-05-0005r1
Slide 53
OFDMA Advantages - Summary (4)
• Allocating carrier by OFDMA/TDMA strategy.• Minimal delay per OFDMA symbol of 300sec.• Using Small burst per user of about 100 symbols for
better statistical multiplexing and smaller jitter.• User symbol is several times longer then for TDMA
systems.• Using the FEC to the outmost by error detection of
disturbed frequencies.