Increasing the bandwidth efficiency of OFDM-MFSK

Abstract

Noncoherent detection is very attractive when transmitting over fast time-variant frequency selective fading channels, since it is difficult or even impossible to obtain reliable channel state information. OFDM-MFSK and its multitone variant are such noncoherent transmission schemes. However, the disadvantage of OFDM-MFSK is its low bandwidth efficiency.

This dissertation is about how to increase the bandwidth efficiency of OFDM-MFSK in a smart way and without a substantial increase in complexity of the underlying system.

After a brief introduction to OFDM-MFSK and its multitone variant as well as noncoherent communication based on subspaces, known from the MIMO context, we first show that OFDM-MFSK and its multitone variant are a special case of subspace based transmission schemes.

Based on that, we develop a new transmission scheme with subspaces of different dimensions, which is called combined OFDM-N/MFSK. This allows us to increase the bandwidth efficiency of OFDM-MFSK and to approach the upper bound of 1 bit/s/Hz, without a substantial increase in complexity of the underlying system. We derive the ML detection rule for different channel models for a transmission with subspaces of different dimensions and discuss the distance criterion.

Finally, we address the topic of extended mapping for the new transmission scheme with subspaces of different dimensions and develop the idea of non-symmetric extended mapping. In comparison to symmetric extended mapping, non-symmetric extended mapping results in a remarkable gain in power efficiency.