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Ofdma

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Ofdma

  1. 1. Orthogonal Frequency Division Multiple Access (OFDMA) Khaja Mohammad Shazzad
  2. 2. 2 Outline 1. Background  Multiple Access (MA) Methods 1. Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA)  Orthogonality Principle  OFDM  OFDM-FDMA 1. Advantages and Disadvantages of OFDMA 2. Conclusion
  3. 3. Multiple Access (MA)  General wireless cellular systems are multi-users systems  Radio resource are limited – Limited Bandwidth – Limited number of channels  The radio resource must be shared among multiple users  Multiple Access Control (MAC) needed – Contention-based – Non-contention-based
  4. 4. 4 Contention-based Multiple Access(MA)  Contention-based – Each terminal transmits in a decentralized way – No central controller (Base stations or access points) – Example:  ALOHA  Carrier Sensing Multiple Access (CSMA) Standard: – GSM [l] uses the slotted ALOHA in the terminal’s initial access process – IEEE 802.11 uses CSMA/CA based contention access scheme
  5. 5. Non-contention-based Multiple Access (MA)  A logic controller (BS or AP) is needed to coordinate the transmissions of all the terminals  The controller informs each device when and on which channel it can transmit  Collisions can be avoided entirely  Two Subdivisions 1. Non-channelization 2. Channelization
  6. 6. Non-channelization Non-contention-based MA  Terminals transmit sequentially using the same channel  Example: – Polling based medium access  Standard: – IEEE 802.15(WPAN) – IEEE 802.11(WLAN)
  7. 7. Channelization Non-contention-based MA  Terminals transmit simultaneously using different channels  Most commonly used protocols in cellular systems  Example: – 1. Time Division Multiple Access (TDMA) – 2. Code Division Multiple Access( CDMA) – 1. Frequency Division Multiple Access (FDMA)  Standard – 1. GSM (TDMA) – 2. IS-95 (CDMA) – 3. American Mobile Phone System, AMPS (FDMA)
  8. 8. 8 Time Division Multiple Access (TDMA)  GSM – Time slot 0.577 ms – Frame 4.6 ms – 8 time slots per frame – Frequency band 20 KHz
  9. 9. Code Division Multiple Access (CDMA)  IS-95 – Orthogonal Walsh codes – 64 codes (channels) – One pilot channel – Seven paging channels – 55 traffic channels – Each carrier 1.25 MHz
  10. 10. Frequency Division Multiple Access (FDMA)  American Mobile Phone System (AMPS) – Total Bandwidth 25 MHz – Each Channel 30 KHz
  11. 11. Orthogonal Frequency Division Multiplexing (OFDM) Based Multiple Access (OFDMA) Orthogonality Principle OFDM OFDM-FDMA
  12. 12. 12 Orthogonality Principle  Vector space – A, B and C vectors in space are orthogonal to each other – A.B=B.C=C.A=0 – (A+B+C).A=(mod A)^2 – (A+B+C).B=(mod B)^2 – (A+B+C).C=(mod C)^2 A B C
  13. 13. Orthogonality Principle cont..  Real Function space 0)()( )cos()( )sin()( 0)()( )cos()( )sin()( 21 2 1 = = = = = = ∫ ∫ + + dttftf nwtNtf mwtMtf dttftf wtBtf wtAtf n T m n m T τ τ τ τ
  14. 14. Orthogonality Principle cont.. )2sin()sin()( wtwttf = ∫ = ≠∫ = ∈∀ T 0 0s(nwt)dtsin(mwt)co nmwhere T 0 0n(nwt)dtsin(mwt)si Νm.n Here mw and nw are called m-th and n-th harmonics of w respectively
  15. 15. 15 Orthogonal Frequency Division Multiplexing(OFDM)  It is a special kind of FDM  The spacing between carriers are such that they are orthogonal to one another  Therefore no need of guard band between carriers.  One example makes the thing clear
  16. 16. Example of OFDM  Lets we have following information bits – 1, 1, -1, -1, 1, 1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, …  Just converts the serials bits to parallel bits C1 C2 C3 C4 1 1 -1 -1 1 1 1 -1 1 -1 -1 -1 -1 1 -1 -1 -1 1 1 -1 -1 -1 1 1
  17. 17. Example of OFDM cont.. Modulated signal for C1 Modulated signal for C2 Modulated signal for C3 Modulated signal for C4 Modulate each column with corresponding sub-carrier using BPSK
  18. 18. Example of OFDM cont..  Final OFDM Signal = Sum of all signal )2sin()()( 1 0 nttItV N n n π∑ − = = Generated OFDM signal, V(t) V(t)
  19. 19. OFDM-FDMA (OFDMA)  Each terminal occupies a subset of sub-carriers  Subset is called an OFDMA traffic channel  Each traffic channel is assigned exclusively to one user at any time user1 user2 user3 user4
  20. 20. OFDM-FDMA (OFDMA)  The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique – Bandwidth options 1.25, 5, 10, or 20 MHz – Entire bandwidth divided into 128, 512, 1024 or 2048 sub carriers – 20 MHz bandwidth with 2048 sub carriers has 9.8 KHz spacing between sub carriers
  21. 21. OFDM-FDMA (System View)
  22. 22. 22 Advantages of OFDMA  Multi-user Diversity – broadband signals experience frequency selective fading – OFDMA allows different users to transmit over different portions of the broadband spectrum (traffic channel) – Different users perceive different channel qualities, a deep faded channel for one user may still be favorable to others
  23. 23. Advantages of OFDMA cont.. Multi-user Diversity
  24. 24. Advantages of OFDMA cont..  Efficient use of Spectrum 4/3 Hz per symbol 6/5 Hz per symbol
  25. 25. Advantages of OFDMA cont..  Receiver Simplicity – It eliminates the intra-cell interference avoiding CDMA type of multi-user detection – Orthogonality of code destroyed by selective fading – Only FFT processor is required  Bit Error Rate performance is better only in Fading environment
  26. 26. Disadvantages of OFDMA  Peak to average power ratio (PAPR) avg P tx PAPR 2 )( = The large amplitude variation increases in-band noise and increases the BER when the signal has to go through amplifier nonlinearities.
  27. 27. 27 Disadvantages of OFDMA cont..  Synchronization – Tight Synchronization between users are required for FFT in receiver – Pilot signals are used for synchronizations  Co-channel interference – Dealing with this is more complex in OFDM than in CDMA – Dynamic channel allocation with advanced coordination among adjacent base stations
  28. 28. 28 Research issues and Conclusion  Future works – Peak-to-average power reduction in OFDM – Timing and Frequency Synchronization – Efficient digital signal processing Implementation of OFDM – Multiple input/Multiple output (MIMO) OFDM  Conclusion – Different variations of OFDMA are proposed and have different pros and cons
  29. 29. Thank You Questions or Comments?

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