Advantages of OFDM data modulation | Disadvantages of OFDM

This page covers advantages and disadvantages of OFDM data modulation technique. It mentions benefits or advantages of OFDM and drawbacks or disadvantages of OFDM (Orthogonal Frequency Division Multiplexing).

What is OFDM ?

OFDM stands for Orthogonal Frequency Division Multiplexing. It is variation of FDM technique in which orthogonal subcarriers are closely spaced to have efficient utilization of bandwidth. OFDM subcarriers carry different data in parallel simultaneously to achieve high data rate. It is also known as digital multi-carrier modulation scheme.

OFDM transmitter with spectrum

OFDM transmitter uses complex modulation techniques to further increase the data carrying capacity of a OFDM symbol. Each subcarriers of OFDM carry data as per modulation level. For example, 16-QAM modulation maps 6 bits to a complex symbol. This complex symbol is carried by a single subcarrier of a OFDM symbol. In OFDM transmission, multiple such subcarriers are transmitted with each carrying different data symbols to increase the data rate and consecutively the throughput. This is shown in the figure-1 above.

OFDM transmitter and receiver chain

IFFT is used at OFDM transmitter and FFT is used at OFDM receiver as shown in the figure-2. Digital to Analog converter (DAC) is needed to convert digital signal to analog signal in the transmitter to couple IFFT output with analog RF front end (viz. RF transmitter, PA, antenna) before channel. OFDM signal travels through RF front end (LNA, RF receiver) at receiver before it is given to ADC in order to convert analog signal to the digital form for baseband processing. Initially all the equalization algorithms are employed for time, frequency and channel correction at the receiver.
Refer OFDM vs Single Carrier>> , OFDM vs OFDMA>> and OFDM vs FDM>> for more information.

There are various variants of OFDM which include f-OFDM (Flash OFDM), Hybrid-OFDM, OFDMA, COFDM, VOFDM, WOFDM etc.

Benefits or advantages of OFDM data modulation

Following are the benefits or advantages of OFDM data modulation:
➨The OFDM spectrum is composed of overlapped narrow subcarriers. This makes efficient usage of frequency spectrum compare to traditional FDM method.
➨In OFDM broadband channel is divided into smaller narrowband subchannels. This makes OFDM resistive to frequency selective fading. Moreover OFDM transmit/receive chain uses channel encoder/decoder and interleaver/deinterlaver which help in recovering lost OFDM symbols due to fading.
➨OFDM makes use of cyclic prefix to eliminate ISI (Inter Symbol Interference) found in the multipath channel environment. Hence it is robust to multipath fading.
➨Channel estimation and equalization has been carried out using known pattern (i.e. preamble) and embedded pilot carriers in a symbol. This is more simpler and efficient compare to channel equalization used in SC (Single Carrier) system.
➨Time offset estimation and correction algorithms are very easy due to correlation technique.
➨It is possible to allocate bandwidth as per resource requirements. Hence OFDM is bandwidth scalable technique.
➨OFDM is used to implement data modulation and demodulation by using computationally efficient FFT techniques.
➨OFDM is less sensitive to sampling time offset impairments compare to SC system.
➨OFDM is robust against narrow band co-channel interference.
➨OFDM receiver does not require tuned sub-channel filters unlike FDM.
➨OFDM facilitates SFNs (Single Frequency Networks) i.e. transmit macro diversity.

Drawbacks or disadvantages of OFDM data modulation

Following are the drawbacks or disadvantages of OFDM data modulation:
➨OFDM signal has high PAPR ( Peak to Average Power Ratio) due to its noise like amplitude having large dynamic range. Due to this, OFDM based transmission system requires RF PA (Power Amplifier) with higher PAPR.
➨OFDM is more sensitive to CFO (carrier frequency offset) than SC system. It has higher CFO due to different LOs (Local Oscillators) and DFT leakage. This requires complex frequency offset correction algorithms at the OFDM receiver.
➨It is prone to ISI (Inter Symbol Interference) and ICI (Inter Carrier Interference). This requires time offset and frequency offset correction algorithms.
➨As OFDM spectrum travels through multiple paths which require guard band to avoid ISI errors due to timing offsets. Use of cyclic prefix lead to loss of efficiency.
➨OFDM is sensitive to doppler shift.
➨OFDM requires linear transmitter circuitry, which suffers from poor power efficiency.

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Useful Links to OFDM MATLAB codes

What is Difference between

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