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OFDM vs FBMC: A Detailed Comparison

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This article compares and contrasts OFDM (Orthogonal Frequency Division Multiplexing) and FBMC (Filter Bank Multi-Carrier) modulation techniques, both of which are multicarrier modulation methods designed for achieving high data rates. We will explore their similarities, differences, and the pros and cons of FBMC.

OFDM Modulation

OFDM is a multicarrier modulation scheme where densely packed, low-frequency subcarriers are modulated with complex data symbols before being upconverted to radio frequency (RF). Each subcarrier carries a portion of the data.

A key aspect of OFDM is its ability to demodulate each subcarrier successfully in the absence of Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). This is accomplished through the use of a cyclic prefix (CP). However, the addition of the CP reduces spectral efficiency, making it less desirable for multicarrier uplink scenarios and cognitive radio systems.

FBMC Modulation

FBMC modulation employs a set of specifically designed synthesis and analysis filters. These filters help achieve desired bandwidth efficiency and spectral efficiency. In FBMC, filters (typically low-pass) are uniformly spaced and highly selective to minimize crosstalk. Modulation is then applied using several subcarrier center frequencies to generate the synthesis and analysis filters. Subcarriers are arranged in either an odd or even stacked configuration. Notably, no center frequency is used.

OFDM vs FBMC Image

OFDM vs FBMC-Difference between OFDM and FBMC

Complexity Comparison

FBMC-based systems are generally more complex than OFDM systems. This complexity arises from the replacement of the Fast Fourier Transform (FFT) / Inverse Fast Fourier Transform (IFFT) modules with filter banks. A major comparison factor between these two techniques is the number of real multiplications per complex symbol.

In OFDM implementation, using a Split-radix algorithm, the number of real multiplications for FFT/IFFT is approximated as:

CFFT/IFFT=M(log⁡2(M)−3)+4C_{FFT/IFFT} = M (\log_2(M) - 3) + 4

In FBMC, the number of real multiplications per complex symbol is calculated as follows:

CSFB=log⁡2(M/2)−3+4∗KC_{SFB} = \log_2(M/2) - 3 + 4*K CAFB=2(log⁡2(M)−3)+4∗KC_{AFB} = 2(\log_2(M) - 3) + 4*K

Where:

  • SFB is the synthesis filter bank
  • AFB is the analysis filter bank

Spectral Selectivity

OFDM exhibits poorer spectral selectivity compared to FBMC, as illustrated in the figure above.

FBMC Merits (Advantages)

  • Provides a spectrum-efficient and more selective system.
  • Cyclic Prefix (CP) is not required.
  • Offers robustness against narrowband jammers.

FBMC Demerits/Challenges

  • The development of MIMO-based FBMC systems is limited and complex.
  • Designing wider bandwidth and higher dynamic range systems poses challenges in achieving RF performance.
  • More complex compared to OFDM, introducing overhead due to overlapping symbols in the filter bank (in the time domain).

Applications of FBMC

  • Cognitive Radio Communications
  • Multiple Access Networks
  • Access to Television White Space (TVWS)
  • Power Line Communication
  • MIMO Communication