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.