OFDM/OFDMA has several benefits over other transmission schemes:
1. High spectral efficiency: due to the orthogonality between subcarriers it is possible to pack them closely together (15kHz subcarrier spacing)
2. Little interference between subcarriers due to the IFFT/FFT processing (Interference can be introduced by frequency offsets generated by either Doppler or Local Oscillator frequency innacuracies).
3. Robustness in multi-path environments thanks to the cyclic prefix as mentioned before.
4. Straightforward support of the operation in different spectrum allocations with different bandwidths just by varying the number of OFDM subcarriers used for transmission.
5.Simpler receiver design to support high data rate communications. Detecting a rectangular pulse with cyclic prefix requires less hardware. Free capacity can be used then to implement other performance optimization techniques.
6. Easy MIMO techniques implementation
Drawbacks of OFDMA:-
1. The main disadvantage of OFDM/OFDMA is that the signal has a relatively large peak-to-average power ratio (PAPR). This is due to the nature of OFMA where modulated symbols are transmitted in parallel, each one containing a part of the transmission. The power at a certain point in time is the sum of the powers of all the transmitted symbols for a certain connection, which explains that the differences between peak and average powers can be high.
2. This issue reduces the power efficiency of the RF amplifier. Expensive transmission amplifiers are needed, especially on the mobile side, in order to work on a wide range of powers; otherwise the non-linear amplification reduces the orthogonality of the OFDM signal. This is a reason why OFDMA is not optimal for use with mobile or battery-power devices.
3. Other issue of OFDM/OFDMA systems is that tight spacing of subcarriers may lead to loss of orthogonality due to frequency errors. Doppler may cause inter carrier interference (ICI) and the consequent lost of orthogonality. To cope with the problems caused by close subcarrier spacing, LTE has adopted 15 kHz spacing (mobile WiMAX uses 10KHz spacing).
1. High spectral efficiency: due to the orthogonality between subcarriers it is possible to pack them closely together (15kHz subcarrier spacing)
2. Little interference between subcarriers due to the IFFT/FFT processing (Interference can be introduced by frequency offsets generated by either Doppler or Local Oscillator frequency innacuracies).
3. Robustness in multi-path environments thanks to the cyclic prefix as mentioned before.
4. Straightforward support of the operation in different spectrum allocations with different bandwidths just by varying the number of OFDM subcarriers used for transmission.
5.Simpler receiver design to support high data rate communications. Detecting a rectangular pulse with cyclic prefix requires less hardware. Free capacity can be used then to implement other performance optimization techniques.
6. Easy MIMO techniques implementation
Drawbacks of OFDMA:-
1. The main disadvantage of OFDM/OFDMA is that the signal has a relatively large peak-to-average power ratio (PAPR). This is due to the nature of OFMA where modulated symbols are transmitted in parallel, each one containing a part of the transmission. The power at a certain point in time is the sum of the powers of all the transmitted symbols for a certain connection, which explains that the differences between peak and average powers can be high.
2. This issue reduces the power efficiency of the RF amplifier. Expensive transmission amplifiers are needed, especially on the mobile side, in order to work on a wide range of powers; otherwise the non-linear amplification reduces the orthogonality of the OFDM signal. This is a reason why OFDMA is not optimal for use with mobile or battery-power devices.
3. Other issue of OFDM/OFDMA systems is that tight spacing of subcarriers may lead to loss of orthogonality due to frequency errors. Doppler may cause inter carrier interference (ICI) and the consequent lost of orthogonality. To cope with the problems caused by close subcarrier spacing, LTE has adopted 15 kHz spacing (mobile WiMAX uses 10KHz spacing).
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