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CMT/FBMC (Cosine modulated multitone based filter bank multicarrier)

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Introduction:

The Cosine Modulated Multitone (CMT) and Filter Bank Multicarrier (FBMC) are two popular techniques used in modern communication systems to transmit digital data over wireless channels. These techniques are considered to be efficient alternatives to the traditional Orthogonal Frequency Division Multiplexing (OFDM) technique because they have better spectral properties and can support higher data rates with lower power consumption. In this article, we will provide a detailed explanation of the CMT/FBMC techniques, their advantages, and disadvantages.

What is CMT/FBMC?

CMT/FBMC is a digital modulation technique that uses a bank of bandpass filters to split the input signal into multiple subcarriers. These subcarriers are then modulated using complex sinusoidal waveforms that are designed to minimize the inter-symbol interference (ISI) caused by the frequency-selective fading of the wireless channel.

CMT/FBMC vs OFDM:

The main advantage of CMT/FBMC over OFDM is that it provides better spectral efficiency by using non-orthogonal subcarriers. In OFDM, the subcarriers are orthogonal to each other, which simplifies the receiver design, but also reduces the spectral efficiency. This is because the guard bands between the subcarriers, which are necessary to prevent inter-carrier interference (ICI), waste a significant portion of the available bandwidth.

On the other hand, CMT/FBMC uses non-orthogonal subcarriers that overlap with each other, allowing for more efficient use of the available bandwidth. This is because the spectral overlap between adjacent subcarriers helps to reduce the bandwidth requirements of the system, thereby increasing the spectral efficiency.

Another advantage of CMT/FBMC over OFDM is that it provides better resistance to frequency-selective fading. In OFDM, the subcarriers are affected differently by the frequency-selective fading, leading to inter-symbol interference (ISI) at the receiver. This ISI can significantly degrade the system performance, especially when the channel conditions are poor.

CMT/FBMC overcomes this problem by using complex sinusoidal waveforms that are specifically designed to minimize the ISI caused by frequency-selective fading. These waveforms, known as Cosine Modulated Waveforms (CMWs), have better time-frequency localization than the rectangular waveforms used in OFDM, which helps to reduce the ISI caused by the frequency-selective fading.

FBMC Types:

There are two types of FBMC: filter bank based FBMC and offset-QAM based FBMC.

Filter Bank Based FBMC:

Filter bank based FBMC is the original FBMC technique, which uses a bank of bandpass filters to split the input signal into multiple subcarriers. The subcarriers are then modulated using complex sinusoidal waveforms that are designed to minimize the ISI caused by the frequency-selective fading.

The main advantage of filter bank based FBMC is its simplicity and flexibility. It can be easily implemented using standard digital signal processing (DSP) techniques and can support a wide range of applications, including multi-user access, adaptive modulation, and diversity techniques.

Offset-QAM Based FBMC:

Offset-QAM based FBMC is a more recent development that uses a combination of QAM and phase-shift keying (PSK) to modulate the subcarriers. The main advantage of offset-QAM based FBMC is that it provides better spectral efficiency than filter bank based FBMC by reducing the spectral overlap between adjacent subcarriers.

Offset-QAM based FBMC achieves this by using a combination of two different types of subcarriers: offset-QAM subcarriers and phase-shift keying (PSK) subcarriers. The offset-QAM subcarriers are used to transmit the data bits, while thePSK subcarriers are used to provide a reference phase that helps to eliminate the spectral overlap between adjacent subcarriers. This allows for more efficient use of the available bandwidth and improves the system performance.

Advantages of CMT/FBMC:

  1. Better spectral efficiency: CMT/FBMC provides better spectral efficiency than OFDM by using non-orthogonal subcarriers that overlap with each other, allowing for more efficient use of the available bandwidth.
  2. Better resistance to frequency-selective fading: CMT/FBMC uses complex sinusoidal waveforms that are specifically designed to minimize the ISI caused by frequency-selective fading, which improves the system performance in poor channel conditions.
  3. Flexibility: FBMC can be easily implemented using standard DSP techniques and can support a wide range of applications, including multi-user access, adaptive modulation, and diversity techniques.
  4. Lower power consumption: CMT/FBMC can achieve higher data rates with lower power consumption than OFDM, making it a more energy-efficient solution.

Disadvantages of CMT/FBMC:

  1. More complex receiver design: CMT/FBMC requires a more complex receiver design than OFDM due to the non-orthogonal subcarriers and complex waveforms used.
  2. Higher computational complexity: CMT/FBMC has higher computational complexity than OFDM due to the use of complex waveforms and filter banks, which may require more processing power and memory.

Applications of CMT/FBMC:

CMT/FBMC is used in various wireless communication systems, including:

  1. 5G and beyond: CMT/FBMC is considered as a potential candidate for future 5G and beyond wireless communication systems due to its improved spectral efficiency and better resistance to frequency-selective fading.
  2. Digital broadcasting: CMT/FBMC can be used for digital broadcasting applications such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and Terrestrial Digital Multimedia Broadcasting (T-DMB).
  3. Wireless LANs: CMT/FBMC can be used for wireless LAN applications such as IEEE 802.11, which is the standard for Wi-Fi.

Conclusion:

CMT/FBMC is a digital modulation technique that uses a bank of bandpass filters to split the input signal into multiple subcarriers. These subcarriers are then modulated using complex sinusoidal waveforms that are designed to minimize the ISI caused by the frequency-selective fading of the wireless channel. CMT/FBMC provides better spectral efficiency and better resistance to frequency-selective fading than OFDM, making it a more efficient solution for modern wireless communication systems. However, it also has some disadvantages, such as a more complex receiver design and higher computational complexity, which must be taken into consideration when implementing this technique.