OFDM Variants | COFDM, OFDMA, f-OFDM, VOFDM, WOFDM, flash OFDM
This page covers OFDM Variants which include COFDM, OFDMA, f-OFDM, VOFDM, WOFDM, OFCDM and flash OFDM. It mentions advantages and disadvantages of these OFDM variants or OFDM types including their applications.
What is OFDM ?
The term OFDM is the short form of Orthogonal Frequency Division Multiplexing introduced in IEEE 802.11a WLAN standard to increase the bit rate of the wifi system in comparison to Single Carrier (SC) system. It composed of densely packed subcarriers to save bandwidth and to make the system spectral efficient.
The OFDM is implemented using IFFT at the OFDM transmitter side and FFT at the OFDM receiver side. OFDM is used for data modulation as shown. The figure depicts both frequency domain and time domain views of OFDM signal before and after IFFT. OFDM is used after data modulation module which can have QPSK, 16QAM or 64QAM etc. Based on modulation technique, each of the subcarriers carry more number of bits. For example, when QPSK is used, OFDM carry 2 bits. For 16QAM, it carriers 4 bits and so on. Let us understand different variants or types of OFDM. All these use basic form of OFDM technique with each subcarriers carry low data rate data signals. These subcarriers are placed very close. At the receiver, all these data signals are combined to provide complete signal.
COFDM | Coded OFDM
In this OFDM type, error correction coding is added in the OFDM signal itself. It uses FEC (Forward Error Correction) to transform given bits into larger number of bits which utilizes redundancy to solve frequency selective fading issue observed in OFDM. It is used in DVB-T.
It is short form of Orthogonal Frequency Division Multiple Access.
In OFDM, entire bandwidth of OFDM subcarriers is used by single user.
In OFDMA, entire bandwidth is shared by multiple users based on subchannel allocations to them.
A subchannel consists of multiple subcarriers. These subcarriers are contiguous or distributed as per permutation.
Refer OFDM vs OFDMA >> for more information and difference between OFDMA and OFDM variants.
Advantages : It is more robust to fading as well as interference than OFDM.
Refer advantages and disadvantages of OFDMA.
f-OFDM | Filtered OFDM
In this f-OFDM type, filtering module is applied after IFFT/CP insert module.
It helps in improving out-of-band radiation of sub band signal.
Filters such as Soft-Truncated Sinc Filters either using Hann window or RRC window and
Equi-ripple filter using Remez exchange algorithm are used.
Refer OFDM vs f-OFDM >> for more information.
Advantages : It helps in meeting OOBE requirement of the spectrum in addition to leveraging benefits of OFDM. This makes it more spectrum efficient. Moreover it offers throughput gain in comparison to conventional OFDM. It offers significant reductions on guard band usage.
VOFDM | Vector OFDM
This OFDM type uses MIMO to leverage its benefits. It is developed by Cisco Systems. MIMO helps in achieving higher data rate during transmission. Moreover multiple antennas at transmitter and receiver utilize multi-path effects and helps in enhancing signal reception.
WOFDM | Wideband OFDM
In this OFDM variant, spacing between channels is kept larger. Hence frequency offset between transmitter and receiver do not affect the performance of the system. It is applied in WiFi systems.
OFCDM | Orthogonal Frequency Code Division Multiplexing
It is combination of OFDM and CDMA techniques. In this OFDM variant, spreading and despreading
modules are applied to the OFDM transmitter and receiver respectively. It is also known as hybrid-OFDM.
Refer MATLAB source code of OFCDM transmut and receive chain.
This OFDM variant uses multiple carriers or tones and fast hopping technique to spread the signal over the band. It is developed by Flarion.
OFDM RELATED LINKS
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Difference between 802.11 standards viz.11-a,11-b,11-g and 11-n
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