OFDM,SC-FDMA

OFDM:-Orthogonal Frequency division multiplexing

Theory:- In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that  between the sub-channels is eliminated and inter-carrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional FDM, a separate filter for each sub-channel is not required.

Advantages:-

• Can easily adapt to severe channel conditions without complex equalization.
• Robust against narrow-band co-channel interference.
• Robust against intersymbol interference (ISI) and fading caused by multipath propagation.
• High spectral efficiency as compared to conventional modulation schemes, spread spectrum, etc.

Disadvantages:-

• Sensitive to Doppler shift.
• Sensitive to frequency synchronization problems.
• High peak-to-average-power ratio (PAPR),

Application of OFDM:-

􀁁 DAB:- Digital Audio Broadcasting

􀁁 HDTV

􀁁Wireless LAN Networks

􀁁ADSL:- The modulation technique DMT is OFDM based

􀁁8.4 IEEE 802.16 Broadband Wireless Access System and Wimax, wireless MAN OFDM

Single carrier FDMA

In SC-FDMA, multiple access among users is made possible by assigning different users, different sets of non-overlapping fourier-coefficients (sub-carriers). This is achieved at the transmitter by inserting (prior to IFFT) silent fourier-coefficients (at positions assigned to other users), and removing them on the receiver side after the FFT.

Transmitter and Receiver Structure of LP-OFDMA/SC-FDMA

• DFT: Discrete Fourier Transform IDFT: Inverse Discrete Fourier Transform ,CP: Cyclic Prefix ,PS: Pulse Shaping ,DAC: Digital-to-Analog Conversion ,RF: Radio Frequency ,ADC: Analog-to-Digital Conversion

 

Applications of SC FDMA

Single Carrier Frequency Division Multiple Access (SC-FDMA) is a novel method of radio transmission under consideration for deployment in future cellular systems; specifically, in 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) systems. SC-FDMA has drawn great attention from the communications industry as an attractive alternative to Orthogonal Frequency Division Multiple Access (OFDMA).