Molecular Communication (MC) is a promising micro-scale technology that enables wireless connectivity in electromagnetically challenged conditions. The signal processing approaches in MC are different from conventional wireless communications as molecular signals suffer from severe inter-symbol interference (ISI) and signal-dependent counting noise due to the stochastic diffusion process of the information molecules. One of the main challenges in MC is the high computational complexity of the existing time-domain ISI mitigation schemes that display a third-order polynomial or even exponential growth with the ISI length, which is further exasperated under the high symbol rate case. For the first time, we develop a frequency-domain equalization (FDE) with lower complexity, capable of achieving independence from the ISI effects. This innovation is grounded in our characterization of the channel frequency response of diffusion signals, facilitating the design of receiver sampling strategies. However, the perfect counting noise power is unavailable in the optimal minimum mean square error (MMSE) equalizer. We address this issue by exploiting the statistical information of the transmit signal and decision feedback for noise power estimation, designing novel MMSE equalizers with low complexity. The FDE for MC is successfully developed with its immunity to ISI effects, and its signal processing cost has only a logarithmic growth with symbol length in each block.

A generalized molecular-shift keying (GMoSK) modulation scheme is proposed for supporting diffusive molecular communication (DMC). Instead of activating one type of molecules in the traditional molecule shift keying (MoSK) modulation, GMoSK simultaneously activates several types of molecules, with the objective to increase data rate and the potential to further mitigate inter-symbol interference (ISI) beyond MoSK. For signal detection in the GMoSK-modulated DMC (GMoSK-DMC) systems, we first derive a symbol-based idealized maximum likelihood (IML) detector, from which we then deduce two practical detection schemes, namely, the zero-level decision feedback maximum likelihood (ZDF/ML) detector and the one-level decision feedback maximum likelihood (1DF/ML) detector. The error performance of the GMoSK-DMC systems with the IML-and ZDF/ML-detectors is mathematically analyzed. Furthermore, the error performance of the GMoSK-DMC systems with our proposed detection schemes is investigated and compared, which is also compared with that of the DMC systems employing the legacy MoSK modulation, the binary concentration shift keying (BCSK) modulation, the depleted MoSK (D-MoSK) modulation, and the Molecule-as-a-Frame (MaaF) modulation. Our studies and performance results demonstrate that GMoSK has the potential to achieve the performance beyond the above four modulation schemes, and employs the merit of high-flexibility for attaining relatively high data rate and also for ISI mitigation.

There is a recent surge of research interest in the study of performance-enhancing techniques for orthogonal frequency division multiplexing (OFDM)-based relay systems. Among those, subcarrier mapping has been verified to be an effective one for boosting the system capacity and improving the error performance. However, it has to be performed at the relay, which subsequently conveys the subcarrier permutation information to the destination. The existing signaling scheme occupies a portion of subcarriers to this end, leading to a loss of spectral efficiency. In this paper, we propose a novel signaling scheme to eliminate this overhead by transferring the subcarrier permutation to the mode permutation that can be implicitly conveyed without consuming additional spectrum resources. We adopt phase rotation for mode design considering both non-adaptive and adaptive modulation, and illustrate the proposed scheme by taking the dual-hop OFDM relaying with semi-blind amplify-and-forward protocol as an example. An asymptotically tight upper bound on the bit error rate (BER) of the proposed scheme is derived in closed-form over Rayleigh fading channels. BER simulation results validate the analysis and show that the proposed scheme asymptotically approaches the ideal case that assumes perfect knowledge of subcarrier permutation information at the destination and significantly outperforms the existing scheme in the asymptotic signal-to-noise ratio region at the same spectral efficiency.

Communication between nanomachines is still an important topic in the construction of the Internet of Bio-Nano Things (IoBNT). Currently, molecular communication (MC) is expected to be a promising technology to realize IoBNT. To effectively serve the IoBNT composed of multiple nanomachine clusters, it is imperative to study multiple-access MC. In this article, based on the molecular division multiple access technology, we propose a novel multiuser MC system, where information molecules with different diffusion coefficients are first employed. Aiming at the user fairness in the considered system, we investigate the optimization of molecular resource allocation, including the assignment of the types of molecules and the number of molecules of a type. Specifically, three performance metrics are considered, namely, min–max fairness for error probability, max–min fairness for achievable rate, and weighted sum-rate maximization. Moreover, we propose two assignment strategies for types of molecules, i.e., best-to-best (BTB) and best-to-worst (BTW). Subsequently, for a two-user scenario, we analytically derive the optimal allocation for the number of molecules when types of molecules are fixed for all users. In contrast, for a three-user scenario, we prove that the BTB and BTW schemes with the optimal allocation for the number of molecules can provide the lower and upper bounds on system performance, respectively. Finally, numerical results show that the combination of BTW and the optimal allocation for the number of molecules yields better performance than the benchmarks.

Molecular communication (MC) via diffusion is envisioned to be a new paradigm for information exchange in the future nanonetworks. However, the strong inter-symbol interference (ISI) caused by the diffusion channel significantly deteriorates the performance of MC systems. To this end, we propose a novel modulation technique to reduce the ISI effect, termed as molecular type permutation shift keying (MTPSK), which encodes information on the permutations of multiple types of molecules. We design a Genie-aided maximum-likelihood detector and a conventional maximum-likelihood detector, and analyze their performance in terms of bit error rate (BER). Aiming at lower computational complexity, we further design a low-complexity maximum-likelihood detector using a Viterbi-like algorithm with compromised error performance. BER simulation results corroborate that the proposed MTPSK can outperform the prevailing modulation schemes for MC, including molecular shift keying (MoSK), concentration shift keying, depleted MoSK, and pulse position modulation.

Unmanned aerial vehicles (UAVs) serve as flexible aerial platforms, enriching air-ground communication networks in various ways. To support massive connectivity within limited time-frequency blocks, non-orthogonal multiple access (NOMA) is proposed to be integrated into UAV networks. However, a common issue associated with almost all NOMA schemes is the susceptibility to inter-user interference (IUI). Therefore, in this paper, we propose a multi-UAV cooperative system aided by NOMA with index modulation (IM), termed MCU-NOMA-IM, to improve the performance of air-ground networks by mitigating IUI and also avoiding the successive interference cancellation (SIC) decoding method that is prone to error floors. With MCU-NOMA-IM, the information bits pertaining to multiple UAVs are mapped into multiple dimensions, including the modulated symbols, subcarrier indices, and energy allocation patterns. To fully investigate the performance of MCU-NOMA-IM on air-ground networks, we consider scenarios in the presence of three and four UAVs and derive upper-bounds for the bit error rates (BERs). In addition, we propose a multi-clustered-UAV cooperative system aided by NOMA with IM (MCCU-NOMA-IM), which groups closely located UAVs into several clusters to reduce the requirement for time resources. Simulation results demonstrate that both MCU-NOMA-IM and MCCU-NOMA-IM greatly outperform cooperative NOMA and non-cooperative NOMA-IM schemes, especially for distant UAVs when the signal-to-noise ratio is sufficiently high. Also, we show that the derived BER upper bounds are asymptotically tight.