Ofdma tutorial

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


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



Tutorial on
Multi Access OFDM (OFDMA)
Technology
Eli Sofer
Runcom Technologies Ltd



Contents

• OFDMA System Architecture
• Illustrated Example
• OFDMA System Properties
• Coverage and Capacity



OFDMA System
Architecture



• 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



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



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



DownLink OFDMA Symbol

Sub-Channel Data Carriers
Symbol Pilots

Total Frequency Band
Guard Band Guard Band



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.


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


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



Space Time Coding

IFFT Filter DAC RF
Tx
Tx
Subcarrier IFFT input
diversity
modulation packing
encoder
IFFT Filter DAC RF

Rx

Sub- Log-
RF DAC Filter FFT Diversity
channel Likelihood Decoder
Combiner
demod. ratios



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



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.



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


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



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



• 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

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




• 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 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



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


Illustrated Example



Example
• Subscriber Units at the Current OFDMA Symbol = 3
• Sub-Channels Allocated to Subscriber-Unit #1 = 12
• Number Of New Subscriber-Units Requesting Services = 3

All Subscriber-Units Suffer Different Multi-Paths and
different Attenuation's



Example
• Constellation at the Base Station



Example
• Users Separation



Example - Results
• User Estimation
Constellation to Estiamte Estimated vec

2 2

1.5 1.5

1 1

0.5 0.5

0 0

-0.5 -0.5

-1 -1

-1.5 -1.5

-2 -2
-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



Results
• User Estimation

2 2

1.5 1.5

1 1

0.5 0.5

0 0

-0.5 -0.5

-1 -1

-1.5 -1.5

-2 -2
-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



Results
• User Estimation
2 2

1.5 1.5

1 1

0.5 0.5

0 0

-0.5 -0.5

-1 -1

-1.5 -1.5

-2 -2
-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



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



OFDMA System -
Properties



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



PAPR Reduction

• Using shaping on the signal peaks
• Limiting the PAPR to a constant value by
vector reduction



Spectrum Properties
• Rectangular Spectrum Shape (Brick Wall)
• Small Frequency Guard band
dB

OFDM
Single Carrier
Scheme

-80

Frequency
(MHz)
4 MHz



Spectrum Properties



Group Delay
In OFDM, channel impairment are solved in the same
way Group Delays are solved, by Channel
estimation



Phase Noise Effects

Phase Noise Effect on Phase Noise Effect on
OFDM S.C



• 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


System Coverage and
Capacity



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



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



Coverage
OFDM Cells OFDMA Cell
(64 mode) (2k mode)

64QAM users
16QAM users
QPSK users



Coverage - Simulations




Multi Sector Coverage, 3 Sectors, 3 Frequencies, achieves
2.8Bits/s/Hz/Cell, 22.5Mbps/Sector



Multi Sector Coverage, 6 Sectors, 6 Frequencies, achieves
2.8Bits/s/Hz/Cell, 22.5Mbps/Sector


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.



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.



• 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.



• 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.


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