What is Coded-OFDM?

Coded Orthogonal Frequency Division Multiplexing or Coded-OFDM, is an enhancement to the OFDM modulation scheme, where forward error correction (FEC) coding is applied before the signal transmission to compensate for symbol or bit errors due to various channel impairments such as channel noise, multipath fading, Doppler shifts, frequency selective fading or other propagation effects. Like OFDM, the coded-OFDM technique also separates the available spectrum into multiple sub-carriers, with each sub-carrier contains a low-rate data stream. Each sub-carrier over the bandwidth is orthogonal to every other sub-carrier and thus, the signals will not interfere with each other, thereby preventing cross-talk. A typical OFDM signal is shown below.

A Typical OFDM (applies to Coded-OFDM) Signal, Image Credit: OFDM Notes, NTU Taiwan

Although OFDM performs significantly better compared to traditional single-carrier modulation systems under channel impairments such as multipath reflections and frequency selective fading, some sub-carrier signals are inevitably more prone to get affected due to unreliable multipath reflections and frequency selective fading compared to other sub-carriers. As a result, a portion of the data stream will be corrupted, thereby resulting in bit errors. These bit errors are largely due to inter-symbol interference (ISI) from multipath reflections and frequency selective fading. Therefore, the entire signal cannot be decoded correctly due to the increased ISI and bit error rate (BER). The effective throughput that is achievable would also be unacceptably low.

Thus, an uncoded OFDM signal will not perform well in frequency selective channels, and especially when the sub-carrier signals are deeply affected or faded due to interference.

Coded-OFDM technique uses forward error correction (FEC) coding methods to overcome the inevitable symbol or bit errors occurring in OFDM. It uses convolutional coding, a popular FEC technique that is widely used in digital wireless communication systems.

Instead of processing a block of bits as in OFDM, the output coded bits in coded-OFDM are calculated by logical operations of the current bit and a small number of previous bits in a data stream. By performing these operations, FEC technique transforms a certain number of bits into a larger number of bits (with increased bit length) that contains redundancy in information. Therefore, the new output bits will contain both the original bit information and redundant bits (that do not represent the data). Consequently, the symbol length also becomes longer compared to OFDM signal.

The redundant information bits are appropriately processed and carried over the channel such that even if the sub-carrier signals suffer from multipath and frequency selective fading effects, the original data stream can still be decoded correctly by applying the FEC techniques at the receiver. A comparative BER performance between a traditional OFDM and a coded-OFDM is illustrated in the figure shown below.

OFDM vs. Coded-OFDM, Image Credit: A Study and Survey of OFDM Versus COFDM, International Journal of Science and Modern Engineering

In addition to this, coded-OFDM also incorporates channel impairment mitigating techniques present in OFDM. Coded-OFDM inserts guard time interval between symbols such that the ISI effects can be reduced, while a further reduction in ISI is supported by the use of FEC coding techniques, resulting in better resistance against multipath reflections.

FEC coding incorporated in OFDM signals also ensures that the sub-carrier signals can be decoded even when certain sub-carrier signals experience frequency selective fading where different frequency components face different amounts of attenuation or fading.

Therefore, coded-OFDM improves the BER performance compared to the traditional multi-carrier OFDM technique, resulting in an increased throughput followed by reduced BER, and ISI.

Furthermore, by utilizing FEC coding, it is also possible to correct bit errors to a certain extent when the signals undergo Doppler shifts. OFDM signals are sensitive to Doppler shifts and lose orthogonality between sub-carriers to a certain degree determined by the channel conditions. Very powerful FEC coding techniques will help mitigate the bit errors and improve performance under Doppler shift conditions compared to OFDM.

There are other coding techniques available to improve the performance of coded-OFDM also exist. They include:

1. Interleaving in time or frequency domain with FEC coding in the other domain
2. Adapting sub-carrier bit rate to the channel conditions
3. Spreading of signals over multiple sub-carriers. One such method is based on linear matrix operation

Coded-OFDM, owing to its improved characteristics and resistance to various channel impairments, is ideal for terrestrial broadcasting channels in radio and television systems, digital television programming, mobile television viewing applications. The technique can also be relatively useful for other 4G/LTE and 5G mobile communication systems.

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