This document provides an overview of multicarrier modulation and OFDM. It describes how multicarrier modulation converts frequency selective fading channels into flat fading channels to make signal detection easier. OFDM is a special case of multicarrier modulation that divides a system bandwidth into overlapping orthogonal subbands. This allows for densely packed and spectrally efficient transmission. However, OFDM also has disadvantages like high peak-to-average power ratio and sensitivity to frequency and timing offsets. Synchronization methods are discussed to help address some of these issues.

2. CONTENTS
 Multicarrier Modulation
 OFDM
 OFDM vs. FDM
 OFDM Advantages
 Block diagram
 Cyclic prefix
 OFDM Disadvantages
 Conclusion

3. MULTICARRIER MODULATION
 Multipath propagation, time domain spreading of a signal, ISI occurs
 Frequency selective fading, distortion, signal detection becomes
difficult
 MCM converts a frequency selective fading into flat fading channel
 Flat fading makes signal detection easier
 MCM easily removes ISI
 MCM transmits data over several carrier frequencies simultaneously
 Each sub-stream modulates different carrier frequency
 Parallel conversion lowers the data rate & bandwidth in each stream

4. OPERATION OF MCM
 PSK or QAM modulation scheme is used
 Signals from each data stream are summed together to form the
transmitted signal
 No change in original data rate & system bandwidth, but ISI is
eliminated
 MCM needs larger bandwidth, spectrally inefficient
 Multiple carriers and multiple oscillators increases the BW
 OFDM is a special case of MCM, which addresses all the above issues

5. OFDM CONCEPT
 System bandwidth is divided into a set of parallel overlapping, yet
orthogonal sub-bands independent to each other
 Data is first split into independent streams, which modulate different
sub-carriers
 Then are multiplexed to create OFDM signal
 OFDM is a special case of FDM
 Significantly improves spectral efficiency
 Avoid the need for steep band pass filters
 Avoids the need of a bank of oscillators, since can be implanted
digitally

8. OFDM ADVANTAGES
 Permits densely packed & overlapping sub-carriers
 Offers spectrally efficient transmission scheme
 Can be digitally implemented using, fast & efficient signal processing
 Permits flexible use of spectrum
 Supports different modulation schemes based on channel conditions
 Almost completely avoids the need for an equalizer

9. OPERATION OF OFDM
Typically PSK or QAM modulations schemes are used
IFFT performs the transformation efficiently and ensures orthogonality
of the sub carriers
Output of IFFT is
 Number of computations are significantly reduced by IFFT
 IFFT needs that the number of sub-carriers be an integer to the power
of 2
 Unused sub-carriers are set to zero
 Complexity of OFDM system is largely determined by IFFT points
 More IFFT points demands more power, but enhances resolution

11. CYCLIC PREFIX
 Guard time between adjacent symbols is inserted to eliminate ISI
 No ISI will occurs, if guard time is larger than delay spread
 Guard time is a pure system overhead, contains no information
 CP is inserted in order to preserve orthogonality
 CP provides multipath immunity & synchronization tolerance
 CP increases required transmission bandwidth, hence lowers spectral
efficiency
 Transmit power associated with CP is a waste

13. OFDM DISADVANTAGES
HIGH PAPR
 A number of independently modulated sub-carriers result in HIGH PAPR
 Non-linear power amplifier, efficient, but cannot be used
 Causing performance degradation due to high distortion & out of band
radiations
 High PAPR also increases complexity of ADC & DAC
FREQUENCY OFFSET
 Sub-carriers are very close and overlapping
 Even a small frequency offset will result in ISI
 Causes of frequency offset:
Frequency mismatch in local oscillators of transmitter & receiver
Doppler shift
Phase noise caused in the channel

14. TIMING OFFSET
 OFDM is slightly more tolerant to time offset, compared to frequency
offset
 Causes lack of precision in symbol boundaries, resulting in ISI
 ISI occur only when time offset differs from CP duration
 Causes phase change & may also result in frequency offset
OFDM demands strict synchronization in frequency & time to
preserve orthogonality

15. CONCLUSION
 Hence synchronization is necessary in OFDM because it minimizes
performance degradation
 Synchronization Methods
Several approaches to estimate jointly or individually, used iteratively
or in one step
1. Data aided method (pilot based)
2. Non-data aided method (blind)
3. Hybrid method

16. REFRENCES
 webe.org - 2GHz BAS Relocation Tech-Fair, COFDM Technology
Basics. 2007-03-02
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OFDM(A) mobile radio systems", Vehicular Technology
Conference, 1999. VTC 1999 - Fall. IEEE VTS. Link
 Haas, R.; Belfiore, J.C. (1997). "A Time-Frequency Well-localized
Pulse for Multiple Carrier Transmission". Wireless Personal
Communications 5 (1): 1–18. doi:10.1023/A:1008859809455..
 Roque, D.; Siclet, C. (2013). "Performances of Weighted Cyclic Prefix
OFDM with Low-Complexity Equalization". IEEE Communications
Letters 17 (3): 439–442. doi:10.1109/LCOMM.2013.011513.121997..
 Jeon, W.G.; Chang, K.H.; Cho, Y.S. (1999). "An equalization
technique for orthogonal frequency-division multiplexing systems
in time-variant multipath channels". IEEE Transactions on
Communications 47 (1): 27–32. doi:10.1109/26.747810..