OFDM (Orthogonal Frequency Division Multiplexing )
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OFDM is a multicarrier modulation technique that divides the available bandwidth into multiple orthogonal subcarriers. It overcomes the disadvantages of single-carrier modulation by making the subcarriers orthogonal to each other to avoid interference. The document discusses the concepts of OFDM including modulation, carrier signals, and orthogonality. It explains how OFDM works by dividing a high-rate data stream into multiple parallel low-rate streams that are mapped to subcarriers. OFDM provides benefits like high spectral efficiency and resilience to interference. It sees applications in technologies like ADSL, WiFi, and WiMax.
OFDM (Orthogonal Frequency Division Multiplexing )
- 1. OFDM Juan Camilo Sacanamboy
- 2. Content 1. Concepts 2. Basic idea 3. FDM: The “mother” of OFDM 4. OFDM 5. Applications
- 3. Concepts (1) Modulation Modulation is the process of conveying a message signal (modulating signal) inside another signal (carrier signal) that can be physically transmitted. Carrier signal Waveform that is modulated with an input signal for the purpose of conveying information. Reference: Link
- 4. Concepts (2) Broadband Wide bandwidth of a transmission medium. Ability to transport multiple signals and multiple traffic types simultaneously.
- 5. Concepts (3) Subcarrier It is an already-modulated s i g n a l , w h i c h i s t h e n modulated into another signal of higher frequency and bandwidth. Frequency range in a given bandwidth Reference: Link
- 6. Concepts (4) Orthogonality Reference: Link The peak of one subcarrier coincides with the null of an adjacent subcarrier.
- 7. Concepts (5) Intersymbol Interference Reference: Link
- 8. Concepts (6) Selective Fading Reference: Link Is a radio propagation anomaly caused by partial cancellation of a radio signal by itself – the signal arrives at the receiver by two different paths, and at least one of the path is changing (lengthening or shortening).
- 9. Basic idea • Orthogonal Frequency Division Multiplexing • Multicarrier broadband modulation technique for transmitting large amounts of digital data.
- 10. FDM: The “mother” of OFDM • Frequency Division Multiplexing • Signals from multiple transmitters are transmitted simultaneously over multiple frequencies. • Each subcarrier is modulated separately by different data stream and a guard band is placed between subcarriers to avoid signal overlap. Reference: Link
- 11. OFDM (1) • Like FDM, OFDM uses multiple subcarriers BUT: o There are closely spaces to each other without causing interference, removing guard bands. o Its possible because subcarriers are orthogonal. Reference: Link
- 12. OFDM (2) Basic OFDM System • A very high rate data stream is divided into multiple parallel low rate data streams. • Each smaller data stream is then mapped to individual data subcarrier and modulated using some sorts of PSK or QAM. i.e. BPSK, QPSK, 16-QAM, 64-QAM.
- 13. OFDM (3) Basic OFDM System Basic OFDM System (Hanzo, Webb, & Keller, 2000)
- 14. OFDM (4) Characteristics • High Spectral Efficiency: OFDM needs less bandwidth than FDM to carry the same amount of information. • Resilience: More resilient in NLOS environment than FDM. • Fault Tolerance: It can efficiently overcome interference and frequency-selective fading caused by multipath because ecualizing is done on a subset of subcarriers instead of a single broader carrier. • Supress effect of ISI (Inter Symbol Interference): Longer symbol period of the parallel OFDM subcarriers than a single carrier system.
- 15. OFDM (5) Disadvantage The main disadvantage of the OFDM system is the complexity of implement N modulators at the transmitter and N demodulators at the receiver. Solution: This problem can be reduced using the discrete Fourier transform (DFT), implemented as a fast Fourier transform (FFT).
- 16. OFDM (6) QAM-OFDM The basic system has N sub-bands, each separated from its neighbour by a guard band. The available spectrum can be used much more efficiently if the spectra of the individual sub-bands are allowed to overlap. Serial to parallel convertor Detailed OFDM System (Hanzo, Webb, & Keller, 2000)
- 17. OFDM (7) Detailed OFDM System • The input serial data stream is rearranged into a sequence { 푑↓푛 } of N QAM symbol at baseband. • At the 푛th symbol instant, the QAM symbol is represented by an in-phase component 푎(푛) and a quadrature component 푏(푛). ▫ 푑(푛)=푎(푛)+푗푏(푛) • A block of N QAM symbols are applied to a serial-to-parallel convertor and the resulting in-phase symbols are applied to N pairs of balanced modulators.
- 18. OFDM (8) Detailed OFD System • The quadrature components 푎(푛) and 푏(푛) modulate the carriers cos ( 푤↓푛 푡)and sin( 푤↓푛 푡) respectively. • The modulated carriers 푎(푛) cos( 푤↓푛 푡) and 푏(푛)sin ( 푤↓푛 푡) when added together constitute a QAM signal. The 푛th QAM signal is given by: ▫ 푋↓푛 =푎(푛) cos( 푤↓푛 푡) +푏(푛)sin ( 푤↓푛 푡)
- 19. OFDM (9) FDM/QAM signal 퐷(푡)= Σ푛=0↑푁−1▒ 푋↓푛 (푡)
- 20. OFDM (10) Modulation by Discrete Fourier Transform • Large number of sub-channels modems • Taking the DFT of the original block of N QAM symbols and then transmitting the DFT coefficients serially is exactly equivalent to the operations required by the OFDM transmitter of a Detailed OFDM System. • Simplifications can be achieved if the bank of sub-cannel modulators/demodulators is implemented using (IFFT/FFT).
- 21. OFDM (11) Reference: Link FFT and IFFT are linear transformations on signals. One is the reverse of the other one. It doesn’t matter the order if apply IFFT in the transmitter or in the receiver because IFFT and FFT are inverse. The question becomes why use IFFT in the transmitter. That’s because signals need to be modulated by N-QAM of the ortogonal subcarriers. Mathematically, the process can be represented by IFFT.
- 22. Applications • ADSL • HomePlug AV • WiMedia UWB • Wifi (801.11 a/g/ac) • WiMax
- 23. References • Conniq. (s.f.). Introduction to FDM, OFDM, OFDMA, SOFDMA. • Hanzo, L., & Keller, T. OFDM and MC-CDMA. • Hanzo, L., Webb, W., & Keller, T. (2000). Single- and Multi-carrier Quadrature Amplitude Modulation : principles and applications for personal communications, WLANs and broadcasting.