DD OFDM (Direct Detection OFDM)

Introduction:

Orthogonal Frequency Division Multiplexing (OFDM) is a popular modulation scheme used in modern communication systems such as Wi-Fi, Digital TV, 4G and 5G cellular networks. OFDM is a multi-carrier modulation technique that divides the available frequency band into a large number of subcarriers, each of which carries a small portion of the total information.

OFDM has been known to be highly susceptible to impairments caused by channel frequency selective fading. The Direct Detection OFDM (DD OFDM) is a new approach that has been proposed to overcome the limitations of traditional OFDM.

DD OFDM is a promising solution that combines the advantages of OFDM with the advantages of direct detection. In this article, we will discuss DD OFDM and its key features, including the modulation and demodulation process, channel estimation, and equalization.

Modulation and Demodulation:

In DD OFDM, the input data is first modulated using a standard OFDM modulation technique. The modulated signal is then passed through a low-pass filter, which eliminates the high-frequency components and leaves only the low-frequency components.

After that, the low-frequency signal is directly detected, which means that no coherent reference signal is required. The detected signal is then demodulated using a standard OFDM demodulator.

Channel Estimation:

One of the key challenges of DD OFDM is the accurate estimation of the channel impulse response, which is necessary for equalization. In traditional OFDM, the channel estimation is performed by sending a pilot signal, which is a known signal transmitted along with the data.

However, in DD OFDM, the pilot signal cannot be used because it would introduce high-frequency components that would interfere with the direct detection process. Therefore, a new method for channel estimation is required.

One approach is to use the time-reversal technique, which is based on the fact that the channel impulse response is symmetric in time. In this technique, the transmitter sends a signal that is the time-reversal of the data signal. The signal is transmitted through the channel, and the received signal is time-reversed again at the receiver.

The time-reversed signal is then correlated with the received signal to estimate the channel impulse response. This technique has been shown to be effective in simulations and experiments.

Equalization:

Once the channel impulse response is estimated, it is used for equalization, which is necessary to compensate for the frequency-selective fading of the channel. In DD OFDM, the equalization can be performed using standard OFDM techniques.

One approach is to use the zero-forcing equalizer, which is a linear equalizer that aims to eliminate the effect of the channel impulse response. The zero-forcing equalizer works by dividing the received signal by the estimated channel impulse response.

Another approach is to use the minimum mean square error (MMSE) equalizer, which is a non-linear equalizer that aims to minimize the mean square error between the equalized signal and the transmitted signal. The MMSE equalizer takes into account the noise and the channel impulse response to calculate the equalization coefficients.

Conclusion:

DD OFDM is a promising approach that combines the advantages of OFDM with the advantages of direct detection. DD OFDM does not require a coherent reference signal, which simplifies the system and reduces the cost. DD OFDM is also less susceptible to phase noise and timing errors than traditional OFDM.

However, DD OFDM also has some challenges, such as the accurate estimation of the channel impulse response and the efficient implementation of the time-reversal technique. Future research is needed to address these challenges and to evaluate the performance of DD OFDM in different scenarios.