Molecular communications emerges as a promising scheme for communications between nanoscale devices. In diffusion-based molecular communications, molecules as information symbols diffusing in the fluid environments suffer from molecule crossovers, i.e., the arriving order of molecules is different from their transmission order, leading to intersymbol interference (ISI). In this paper, we introduce a new family of channel codes, called ISI-free codes, which improve the communication reliability while keeping the decoding complexity fairly low in the diffusion environment modeled by the Brownian motion. We propose general encoding/decoding schemes for the ISI-free codes, working upon the modulation schemes of transmitting a fixed number of identical molecules at a time. In addition, the bit error rate (BER) approximation function of the ISI-free codes is derived mathematically as an analytical tool to decide key factors in the BER performance. Compared with the uncoded systems, the proposed ISI-free codes offer good performance with reasonably low complexity for diffusion-based molecular communication systems.

Molecular communications emerges as a promising scheme for communication between nanoscale devices. In diffusion-based molecular communications, molecules as information symbols are released by transmitters and diffuse in the fluid or air environments to transmit messages. Under the diffusion channel modeled by Brownian motion, information sequences suffer from molecule crossovers, i.e., molecules released at an earlier time may arrive later, causing intersymbol interference (ISI). In this paper, we investigate practical channel codes for combating ISI. An ISI-free coding scheme is proposed to increase the communication reliability while keeping the encoding/decoding complexity reasonably low. We exemplify an ISI-free code and theoretically approximate its bit error rate (BER) performance. In addition, repetition codes are revisited out of the complexity concern and proved to be desirable. The BER approximations of the repetition code family are given as well. Compared with convolutional codes, the proposed ISI-free code and repetition codes offer comparable performance with much lower complexity for diffusion-based molecular communication systems.

Molecular communications emerge as a promising paradigm for the nano-scale communication in nanotechnology. Though still at an early stage, some research efforts have been made and various molecular communication systems have been proposed. However, existing works mainly focus on the synchronous communication schemes where the synchronous assumption is made for simplifying analyses. In this paper, we propose an asynchronous communication scheme for molecular communications. For the proposed scheme, we make extensive analyses and develop an approximation for channel capacity as a general performance measure which also applies to synchronous systems. Beyond the theoretical analysis, we also examine a specific asynchronous system and compare its performance with synchronous systems to confirm the feasibility of asynchronous molecular communications.