1. Tutorial on Multi Access OFDM (OFDMA) Technology
IEEE P Wireless RANs Date:
Authors:
Notice: This document has been prepared to assist IEEE It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE
Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair
Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE Working Group. If you have questions, contact the IEEE Patent Committee Administrator at
>

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

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

4. Contents OFDMA System Architecture Illustrated Example
OFDMA System Properties
Coverage and Capacity

5. OFDMA System Architecture

6. Multiple Access Method
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

7. 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 .

8. OFDMA-TDMA Principles
Using OFDMA/TDMA, Sub Channels are allocated in the Frequency Domain, and OFDM Symbols allocated in the Time Domain.

9. DownLink OFDMA Symbol

10. 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.

11. 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

12. Downlink Pilot and Data Carriers Allocation Scheme

13. Space Time Coding Tx diversity encoder IFFT DAC Filter RF Subcarrier
modulation
IFFT input
packing Rx
FFT
Diversity Combiner
Sub-
channel
demod.
Log-
Likelihood
ratios
Decoder

14. UpLink OFDMA Symbol

15. 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.

16. 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

17. Example for UpLink Sub-Channel Pilot and Data Carriers Allocation Scheme

18. 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.

19. 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

20. 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.

21. 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

22. DVB-RCT MAC Performance
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

23. Illustrated Example

24. 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

25. Constellation at the Base Station
Example
Constellation at the Base Station

26. Example
Users Separation

27. Example - Results
User Estimation

28. Results
User Estimation

29. Results
User Estimation

30. Results
Finding New Subscriber-Units Requesting Services, Using the Ranging Pilots (CDMA/OFDM Techniques)

31. OFDMA System - Properties

32. Interference Rejection/Avoidance
Narrowband Interference Rejection
Easy to Avoid/Reject Narrowband Dominant Interference .
Less Interfered Part of the Carrier Can Still Be Used .

33. Using shaping on the signal peaks
PAPR Reduction
Using shaping on the signal peaks
Limiting the PAPR to a constant value by vector reduction

34. Spectrum Properties
Rectangular Spectrum Shape (Brick Wall)
Small Frequency Guard band

35. Spectrum Properties

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

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

38. Frequency Sensitivity
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

39. System Coverage and Capacity

40. 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)

41. 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

42. Coverage

43. Coverage - Simulations

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

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

46. 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.

47. 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.

48. 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.

49. 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.