Recently, a lot of research on the filter bank multicarrier (FBMC) transmission scheme offset quadrature-amplitude modulated orthogonal frequency division multiplexing (OQAM-OFDM) has been carried out. Compared to the well-established multicarrier technique cyclic prefix OFDM (CP-OFDM), OQAM-OFDM provides important advantages like better side-band suppression and higher spectral efficiency because a cyclic prefix is not needed.

Fig. 1: System model of an Offset QAM-OFDM transmission

Fig. 1 shows the block diagram of an OQAM-OFDM transmission system in which the choice of the impulse response g[i] allows flexible spectral and temporal shaping. In contrast to conventional QAM, in an OQAM-OFDM transmission system, real and imaginary parts of complex-valued quadrature-amplitude modulated symbols s_n,κ of the n-th subcarrier and the κ-th block are transmitted in a real-imaginary interleaved manner. Thus, OQAM-OFDM can be regarded as a special pulse amplitude modulated (PAM) transmission scheme with real-valued symbols α_n,k, where k is the time index at doubled sample rate 2/T. The real-imaginary pattern in OQAM-OFDM is encountered by the factors Θ_n,k, e.g. Θ_n,k = j^mod(n-k,2).

Fig. 2: Real and imaginary parts of complex QAM symbols are transmitted in an interleaved way

After demodulation at the receiver side, the equalized real-valued symbols â_n,k are re-formed to the complex-valued QAM symbols ŝ_n,κ as shown in Fig. 2.

Nowadays, OQAM-OFDM is considered as one physical waveform candidate for future 5G communications. To exploit channel frequency-selectivity, also OQAM OFDM allows for adapting the modulation schemes (and powers) on a subcarrier or subgroup basis.

Based on results obtained from CP-OFDM, we adapted the signaling-free adaptive transmission concept to OQAM-OFDM and analyzed its performance in terms of the automatic modulation classification reliability and the effective bandwidth efficiency.

Fig. 3: Effective bandwidth efficiency at target FER_target = 10^-1 vs. SNR for different transmission setups

Fig. 3 shows the superior performance of the introduced adaptive OQAM-OFDM transmission scheme without signaling (green curve: 3-tap, adaptive) in terms of the effective bandwidth efficiency, also compared to the well-known CP-OFDM transmission.

Currently, we are investigating multiuser MIMO FBMC transmission in application to wireless automation and cognitive radio systems.