1. January 2005 Doc.: IEEE802.22-05-0005r0
Tutorial on Multi Access OFDM (OFDMA) Technology
IEEE P802.22 Wireless RANs Date: 2005-01-04
Authors:
Name Company Address Phone email
Eli Sofer Runcom 2 Hachoma St., 75655 +972 3 9528440 elisofer@runcom.co.il
Technologies Rishon Lezion, Israel
Yossi Segal Runcom 2, achoma St. 75655 +972 3 952 8440 yossis@runcom.co.il
Technologies Rishon Lezion, Israel
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Submission Slide 1 Eli Sofer, Runcom
Runcom Technologies Ltd. 1
2. January 2005 Doc.: IEEE802.22-05-0005r0
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
Submission Slide 2 Eli Sofer, Runcom
Runcom Technologies Ltd. 2
3. January 2005 Doc.: IEEE802.22-05-0005r0
Tutorial on
Multi Access OFDM (OFDMA)
Technology
Eli Sofer
Runcom Technologies Ltd
Submission Slide 3 Eli Sofer, Runcom
Runcom Technologies Ltd. 3
4. January 2005 Doc.: IEEE802.22-05-0005r0
Contents
• OFDMA System Architecture
• Illustrated Example
• OFDMA System Properties
• Coverage and Capacity
Submission Slide 4 Eli Sofer, Runcom
Runcom Technologies Ltd. 4
5. January 2005 Doc.: IEEE802.22-05-0005r0
OFDMA System
Architecture
Submission Slide 5 Eli Sofer, Runcom
Runcom Technologies Ltd. 5
6. January 2005 Doc.: IEEE802.22-05-0005r0
• Duplexing Technique
FDD/TDD
• Multiple Access Method
TDMA/OFDMA
OFDM Symbols allocated by TDMA
Sub-Carriers within an OFDM Symbol allocated by OFDMA
• Diversity
Frequency, Time, Code (CPE and BS), Space Time
Coding, Antenna Array
Submission Slide 6 Eli Sofer, Runcom
Runcom Technologies Ltd. 6
7. January 2005 Doc.: IEEE802.22-05-0005r0
Duplexing - Principles
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 .
DownLink UpLink
FDD
F1 - Frequency band F2 - Frequency band
DownLink UpLink
TDD
F1 - Frequency band F1 - Frequency band
Submission Slide 7 Eli Sofer, Runcom
Runcom Technologies Ltd. 7
8. January 2005 Doc.: IEEE802.22-05-0005r0
OFDMA-TDMA Principles
Using OFDMA/TDMA, Sub Channels are allocated in
the Frequency Domain, and OFDM Symbols allocated
in the Time Domain.
t TDMA
TDMAOFDMA
m
N
Submission Slide 8 Eli Sofer, Runcom
Runcom Technologies Ltd. 8
9. January 2005 Doc.: IEEE802.22-05-0005r0
DownLink OFDMA Symbol
Sub-Channel Data Carriers
Symbol Pilots
Total Frequency Band
Guard Band Guard Band
Submission Slide 9 Eli Sofer, Runcom
Runcom Technologies Ltd. 9
10. January 2005 Doc.: IEEE802.22-05-0005r0
DownLink Specification
• 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.
Submission Slide 10 Eli Sofer, Runcom
Runcom Technologies Ltd. 10
11. January 2005 Doc.: IEEE802.22-05-0005r0
Example- DownLink Specification
• 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
Submission Slide 11 Eli Sofer, Runcom
Runcom Technologies Ltd. 11
12. January 2005 Doc.: IEEE802.22-05-0005r0
Downlink Pilot and Data Carriers Allocation Scheme
carrier index
symbol
index
n L=0
0 12 24 N u s ed -1
n+1 L=2
0 6 18 30 N u s ed -1
n+2 L=1
0 3 15 27 N u s ed -1
n+3 L=3
0 9 21 N u s ed -1
n+4 L=0
0 12 24 N u s ed -1
time
Allocation Key: Variable Location Pilot Fixed-location Pilot Data
Submission Slide 12 Eli Sofer, Runcom
Runcom Technologies Ltd. 12
14. January 2005 Doc.: IEEE802.22-05-0005r0
UpLink OFDMA Symbol
Pilots Carriers Data Carriers Pilots Carriers Data Carriers
Sub-Channel #1 Sub-Channel #1 Sub-Channel #1 Sub-Channel #x
Total Frequency Band
Guard Band Guard Band
Submission Slide 14 Eli Sofer, Runcom
Runcom Technologies Ltd. 14
15. January 2005 Doc.: IEEE802.22-05-0005r0
Example of UpLink Specification
• 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.
Submission Slide 15 Eli Sofer, Runcom
Runcom Technologies Ltd. 15
16. January 2005 Doc.: IEEE802.22-05-0005r0
Example of UpLink Specification
• 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
Submission Slide 16 Eli Sofer, Runcom
Runcom Technologies Ltd. 16
17. January 2005 Doc.: IEEE802.22-05-0005r0
Example for UpLink Sub-Channel Pilot and
Data Carriers Allocation Scheme
frequency
symbol
index
n L=0
0 13 26 27 40 52
n+1 L=2
0 2 15 26 28 42 52
n+2 L=4
0 4 17 26 30 44 52
n+11 L=9
0 9 22 26 36 49 52
n+12 L=11
0 11 24 26 38 51 52
n+13 L=0
0 13 26 27 40 52
time
Allocation Key: Variable Location Pilot Fixed-location Pilot Data
Submission Slide 17 Eli Sofer, Runcom
Runcom Technologies Ltd. 17
18. January 2005 Doc.: IEEE802.22-05-0005r0
Using Special Permutations for carrier allocation
• All usable carriers are divided into 32 carrier groups
named basic group, each main group contains 53 basic
groups.
block 1
1 2 3 30 31 32
Frequency band
1
each group contains 2
53 carriers 3
Submission Slide 18 Eli Sofer, Runcom
Runcom Technologies Ltd. 18
19. January 2005 Doc.: IEEE802.22-05-0005r0
Using Special Permutations for carrier allocation
• 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
User #1 User #2
0 2 5 10 21 22 1 5
Total Frequency band
Guard Band Guard Band
User 1 = 0,5,2,10,4,20,8,17,16,11,9,
22,18,21,13,19,3,15,6,7,12,14,1
User 2 = 2,10,4,20,8,17,16,11,9,22,18, 1,13,19,3,15,6,7,12,14,1,0,5
2
Submission Slide 19 Eli Sofer, Runcom
Runcom Technologies Ltd. 19
20. January 2005 Doc.: IEEE802.22-05-0005r0
Using Special Permutations for carrier allocation
• 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.
Submission Slide 20 Eli Sofer, Runcom
Runcom Technologies Ltd. 20
21. January 2005 Doc.: IEEE802.22-05-0005r0
Using CDMA like modulation for Ranging
• 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
Submission Slide 21 Eli Sofer, Runcom
Runcom Technologies Ltd. 21
22. January 2005 Doc.: IEEE802.22-05-0005r0
DVB-RCT MAC Performance
2.5
2
Sucessful BW requests per slot
1.5
1
0.5
0
0 1 2 3 4 5 6 7 8 9 10
Collision expectation value
• 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
Submission Slide 22 Eli Sofer, Runcom
Runcom Technologies Ltd. 22
23. January 2005 Doc.: IEEE802.22-05-0005r0
Illustrated Example
Submission Slide 23 Eli Sofer, Runcom
Runcom Technologies Ltd. 23
24. January 2005 Doc.: IEEE802.22-05-0005r0
Example
• 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
Submission Slide 24 Eli Sofer, Runcom
Runcom Technologies Ltd. 24
25. January 2005 Doc.: IEEE802.22-05-0005r0
Example
• Constellation at the Base Station
Submission Slide 25 Eli Sofer, Runcom
Runcom Technologies Ltd. 25
26. January 2005 Doc.: IEEE802.22-05-0005r0
Example
• Users Separation
Submission Slide 26 Eli Sofer, Runcom
Runcom Technologies Ltd. 26
30. January 2005 Doc.: IEEE802.22-05-0005r0
Results
• Finding New Subscriber-Units Requesting Services,
Using the Ranging Pilots (CDMA/OFDM Techniques)
Despreading on All Users
300
250
200
150
100
50
0
0 20 40 60 80 100 120 140
Submission Slide 30 Eli Sofer, Runcom
Runcom Technologies Ltd. 30
31. January 2005 Doc.: IEEE802.22-05-0005r0
OFDMA System -
Properties
Submission Slide 31 Eli Sofer, Runcom
Runcom Technologies Ltd. 31
32. January 2005 Doc.: IEEE802.22-05-0005r0
Interference Rejection/Avoidance
• Narrowband Interference Rejection
– Easy to Avoid/Reject Narrowband Dominant Interference .
– Less Interfered Part of the Carrier Can Still Be Used .
User SubCarriers
Interference
Allocation
SubCarriers
Interference Nulled
SubCarriers
SubCarriers
Total Frequency band
Submission Slide 32 Eli Sofer, Runcom
Runcom Technologies Ltd. 32
33. January 2005 Doc.: IEEE802.22-05-0005r0
PAPR Reduction
• Using shaping on the signal peaks
• Limiting the PAPR to a constant value by
vector reduction
Submission Slide 33 Eli Sofer, Runcom
Runcom Technologies Ltd. 33
34. January 2005 Doc.: IEEE802.22-05-0005r0
Spectrum Properties
• Rectangular Spectrum Shape (Brick Wall)
• Small Frequency Guard band
dB
OFDM
Single Carrier
Scheme
-80
Frequency
(MHz)
4 MHz
Submission Slide 34 Eli Sofer, Runcom
Runcom Technologies Ltd. 34
35. January 2005 Doc.: IEEE802.22-05-0005r0
Spectrum Properties
Submission Slide 35 Eli Sofer, Runcom
Runcom Technologies Ltd. 35
36. January 2005 Doc.: IEEE802.22-05-0005r0
Group Delay
In OFDM, channel impairment are solved in the same
way Group Delays are solved, by Channel
estimation
Submission Slide 36 Eli Sofer, Runcom
Runcom Technologies Ltd. 36
37. January 2005 Doc.: IEEE802.22-05-0005r0
Phase Noise Effects
Phase Noise Effect on Phase Noise Effect on
OFDM S.C
Submission Slide 37 Eli Sofer, Runcom
Runcom Technologies Ltd. 37
38. January 2005 Doc.: IEEE802.22-05-0005r0
• 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.
• Equalization
No Equalizers are needed, channel impairment and timing
problems are both solved with simple phase and channel
estimators
Submission Slide 38 Eli Sofer, Runcom
Runcom Technologies Ltd. 38
39. January 2005 Doc.: IEEE802.22-05-0005r0
System Coverage and
Capacity
Submission Slide 39 Eli Sofer, Runcom
Runcom Technologies Ltd. 39
40. January 2005 Doc.: IEEE802.22-05-0005r0
Using Reuse Factor of 1
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)
Sub-hannel
Horizontal
Horizontal
s Set 2
Sub-hannel
F1
s Set 1
F1
Sub-hannel
Vertical
Vertical
s Set 2
Sub-hannel
F1
s Set 1
F1
Submission Slide 40 Eli Sofer, Runcom
Runcom Technologies Ltd. 40
41. January 2005 Doc.: IEEE802.22-05-0005r0
Capacity
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
Submission Slide 41 Eli Sofer, Runcom
Runcom Technologies Ltd. 41
43. January 2005 Doc.: IEEE802.22-05-0005r0
Coverage - Simulations
Submission Slide 43 Eli Sofer, Runcom
Runcom Technologies Ltd. 43
44. January 2005 Doc.: IEEE802.22-05-0005r0
Coverage - Simulations
Multi Sector Coverage, 3 Sectors, 3 Frequencies, achieves
2.8Bits/s/Hz/Cell, 22.5Mbps/Sector
Submission Slide 44 Eli Sofer, Runcom
Runcom Technologies Ltd. 44
45. January 2005 Doc.: IEEE802.22-05-0005r0
Coverage - Simulations
Multi Sector Coverage, 6 Sectors, 6 Frequencies, achieves
2.8Bits/s/Hz/Cell, 22.5Mbps/Sector
Submission Slide 45 Eli Sofer, Runcom
Runcom Technologies Ltd. 45
46. January 2005 Doc.: IEEE802.22-05-0005r0
OFDMA Advantages- Summary (1)
• 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.
Submission Slide 46 Eli Sofer, Runcom
Runcom Technologies Ltd. 46
47. January 2005 Doc.: IEEE802.22-05-0005r0
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.
Submission Slide 47 Eli Sofer, Runcom
Runcom Technologies Ltd. 47
48. January 2005 Doc.: IEEE802.22-05-0005r0
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.
Submission Slide 48 Eli Sofer, Runcom
Runcom Technologies Ltd. 48
49. January 2005 Doc.: IEEE802.22-05-0005r0
OFDMA Advantages - Summary (4)
• Allocating carrier by OFDMA/TDMA strategy.
• Minimal delay per OFDMA symbol of 300 sec.
• 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.
Submission Slide 49 Eli Sofer, Runcom
Runcom Technologies Ltd. 49