CEOFDM (Constant envelope OFDM)

Introduction:

Orthogonal frequency-division multiplexing (OFDM) is a well-established digital communication technique used in various applications, such as broadcasting, wireless LANs, and cellular systems. In the traditional OFDM system, the signal transmitted by each subcarrier is a complex-valued symbol, and the amplitudes of these symbols vary with the modulation scheme used. However, this variation in amplitude can cause nonlinear distortion in the transmitter, leading to a reduction in the system's efficiency and performance.

To address this issue, Constant envelope OFDM (CE-OFDM) was introduced, where the amplitude of the signal is constant, and only the phase varies. This approach avoids nonlinear distortion and provides better power efficiency and spectral efficiency. In this article, we will discuss the principle, advantages, and limitations of CE-OFDM.

Principle of CE-OFDM:

In CE-OFDM, the amplitude of the transmitted signal is kept constant, while only the phase of each subcarrier is modulated according to the information to be transmitted. This is achieved by using a phase modulation technique, such as phase shift keying (PSK) or differential phase shift keying (DPSK), to modulate the phase of the subcarriers. The constant envelope property is achieved by using a special set of subcarriers, known as constant amplitude zero autocorrelation (CAZAC) sequences, which have a constant magnitude and zero autocorrelation. These subcarriers are used to transmit pilot symbols, which are used at the receiver to estimate the channel response and compensate for the phase distortion caused by the channel.

The CE-OFDM system can be divided into two stages: the modulation stage and the demodulation stage. In the modulation stage, the input data is first converted into symbols using a modulation scheme such as BPSK, QPSK, 16-QAM, etc. These symbols are then mapped onto the subcarriers using a modulation scheme such as PSK or DPSK, which modulate the phase of the subcarriers. The pilot symbols are inserted into the CE-OFDM signal at regular intervals to facilitate channel estimation at the receiver.

In the demodulation stage, the received signal is first synchronized with the transmitted signal, and the channel response is estimated using the pilot symbols. The phase distortion caused by the channel is then compensated for using the estimated channel response. The symbols are then demodulated by measuring the phase of the subcarriers and mapping it to the corresponding symbol using a demodulation scheme. Finally, the demodulated symbols are decoded to recover the original data.

Advantages of CE-OFDM:

The CE-OFDM system has several advantages over traditional OFDM systems, including:

1. High power efficiency: The constant envelope property of the CE-OFDM signal allows for high power efficiency, as the amplifier used in the transmitter can operate in its linear region, avoiding the nonlinear distortion that can occur in traditional OFDM systems.
2. Improved spectral efficiency: The use of CAZAC sequences allows for a better spectral efficiency, as the pilot symbols can be inserted at regular intervals without interfering with the data symbols.
3. Robustness to nonlinear distortion: The constant envelope property of the CE-OFDM signal makes it more robust to nonlinear distortion caused by the transmitter's amplifier, resulting in better system performance.
4. Reduced complexity: The use of phase modulation techniques for the subcarriers reduces the complexity of the modulation and demodulation process, making the system more efficient.
5. Better performance in high-power scenarios: CE-OFDM is particularly suited for high-power scenarios, where the nonlinear distortion caused by the transmitter's amplifier can be more significant, resulting in improved system performance.

Limitations of CE-OFDM:

While CE-OFDM has several advantages, it also has some limitations that need to be considered, including:

1. Limited modulation options: The use of constant envelope modulation techniques restricts the available modulation options, which can limit the data rate and the ability to support higher-order modulation schemes.
2. Complexity of CAZAC sequences: The use of CAZAC sequences can be complex, requiring careful design and optimization to ensure their constant amplitude and zero autocorrelation properties.
3. Increased sensitivity to phase noise: The use of phase modulation techniques for the subcarriers can make the system more sensitive to phase noise, which can degrade system performance.
4. Reduced spectral efficiency: While the use of CAZAC sequences allows for better spectral efficiency, it also reduces the number of available subcarriers, which can limit the system's ability to support high data rates.

Applications of CE-OFDM:

CE-OFDM has been applied in various communication systems, including:

1. Wireless communications: CE-OFDM is particularly suited for wireless communication systems, where power efficiency and spectral efficiency are critical.
2. Satellite communications: CE-OFDM has been used in satellite communication systems, where power efficiency and robustness to nonlinear distortion are important.
3. Military communications: CE-OFDM has also been applied in military communication systems, where robustness to interference and nonlinear distortion is critical.

Conclusion:

CE-OFDM is a promising digital communication technique that offers several advantages over traditional OFDM systems, including high power efficiency, improved spectral efficiency, robustness to nonlinear distortion, and reduced complexity. However, it also has some limitations that need to be considered, including limited modulation options, complexity of CAZAC sequences, increased sensitivity to phase noise, and reduced spectral efficiency. CE-OFDM has been applied in various communication systems, including wireless communications, satellite communications, and military communications, and is expected to play an increasingly important role in future communication systems.