OQAM/FBMC Offset quadrature amplitude modulation based filter bank

Offset Quadrature Amplitude Modulation (OQAM) is a digital modulation scheme that has gained attention in recent years due to its potential advantages in efficient spectrum utilization and robustness against multipath fading. OQAM is often used in combination with Filter Bank Multicarrier (FBMC) techniques to achieve high spectral efficiency and low out-of-band radiation. In this article, we will explore the principles, advantages, and applications of OQAM/FBMC systems.

FBMC is a multicarrier modulation technique that divides the available spectrum into multiple subcarriers. Unlike traditional Orthogonal Frequency Division Multiplexing (OFDM), FBMC uses a filter bank structure that enables individual filtering for each subcarrier. This allows for a more flexible allocation of subcarriers and better spectral containment, resulting in reduced interference and increased spectral efficiency.

In FBMC systems, the data symbols are typically modulated using complex-valued symbols, which contain both amplitude and phase information. This is known as Quadrature Amplitude Modulation (QAM). However, in OQAM/FBMC, each data symbol is split into two real-valued symbols, which are then modulated using offset QAM. The offset refers to the fact that the subcarriers in the frequency domain are shifted by half a subcarrier spacing compared to the conventional FBMC.

To understand the motivation behind OQAM/FBMC, let's consider the time and frequency characteristics of conventional FBMC. In FBMC, each subcarrier is modulated using complex-valued symbols, resulting in the interference between adjacent subcarriers. This interference is known as the Inter-Carrier Interference (ICI) and is a major drawback of FBMC. ICI limits the achievable spectral efficiency and requires complex equalization techniques to mitigate its effects.

OQAM/FBMC overcomes the ICI problem by splitting the complex symbols into real-valued symbols and introducing a time-domain filtering operation. In the time domain, OQAM/FBMC applies real-valued filters with half the subcarrier spacing, which effectively decouples the real and imaginary parts of the symbols. This decoupling eliminates the ICI, allowing for higher spectral efficiency and simplified equalization.

The filtering operation in OQAM/FBMC is performed using a prototype filter, which is a real-valued filter with a specific frequency response. The prototype filter is applied to the real and imaginary parts of the data symbols separately, resulting in two filtered streams. These filtered streams are then combined to reconstruct the modulated signal.

One of the key advantages of OQAM/FBMC is its improved spectral efficiency compared to conventional FBMC and OFDM. Due to the decoupling of real and imaginary parts, OQAM/FBMC achieves better spectral containment, which reduces the interference between subcarriers. This enables tighter packing of subcarriers in the frequency domain, resulting in increased data rates for a given bandwidth.

Moreover, OQAM/FBMC exhibits lower out-of-band radiation compared to OFDM. The real-valued prototype filter used in OQAM/FBMC has a better frequency selectivity, which leads to reduced spectral leakage outside the desired bandwidth. This characteristic is particularly advantageous in scenarios where adjacent frequency bands are allocated to different users or services, as it helps to minimize interference and improve overall system capacity.

Another significant advantage of OQAM/FBMC is its robustness against multipath fading. In traditional OFDM systems, the cyclic prefix (CP) is used to combat inter-symbol interference caused by multipath propagation. However, the CP introduces a loss in spectral efficiency. In OQAM/FBMC, the real-valued prototype filter inherently provides a time-domain pulse shaping property, which reduces inter-symbol interference without the need for a CP. This makes OQAM/FBMC more resilient to multipath fading and a suitable choice for wireless communication systems operating in challenging environments.

The applications of OQAM/FBMC are diverse and span various communication scenarios. One prominent application is in 5G and beyond-5G wireless systems. The high spectral efficiency and low out-of-band radiation of OQAM/FBMC make it an attractive candidate for next-generation wireless networks, where efficient spectrum utilization and coexistence with other services are essential.

OQAM/FBMC is also well-suited for cognitive radio systems, which aim to dynamically allocate spectrum resources to different users or services based on their needs. The robustness of OQAM/FBMC against interference and its flexible subcarrier allocation capabilities make it suitable for spectrum-sharing scenarios, where efficient utilization of the available spectrum is crucial.

Furthermore, OQAM/FBMC has potential applications in underwater communication systems. Underwater channels are characterized by severe multipath propagation and limited bandwidth. The robustness of OQAM/FBMC against multipath fading and its spectral efficiency advantages make it a promising modulation scheme for underwater acoustic communication, enabling improved data rates and reliable communication links.

In conclusion, OQAM/FBMC is a digital modulation scheme that combines the advantages of offset QAM and filter bank multicarrier techniques. It offers improved spectral efficiency, lower out-of-band radiation, and robustness against multipath fading. The decoupling of real and imaginary parts of symbols, achieved through real-valued filtering, eliminates inter-carrier interference and enables efficient spectrum utilization. OQAM/FBMC finds applications in various communication systems, including wireless networks, cognitive radios, and underwater communication. Its unique characteristics make it a promising modulation scheme for future communication technologies.