Low Earth Orbit (LEO) satellites in the nonterrestrial networks (NTNs) are challenged by high satellite moving speeds and dynamic link conditions, which lead to frequent and suboptimal handovers. A handover mechanism based on a composite quality indicator (CQI) that balances the trade-off between the quality and longevity of a link is proposed. To capture the nonlinear behavior of the links between the LEO satellites and the user terminal (UT), two logistic functions (LFs) are employed to dynamically adjust the weight to generate the CQI and the value of the handover threshold, respectively. By using the Two-Line Element (TLE) data, a genetic algorithm (GA) is developed to find the optimal parameter values to facilitate the two LFs by searching a multidimensional solution space. Simulation results demonstrate that the proposed handover mechanism not only enhances link quality and longevity but also reduces the number of handovers compared to the elevation-based and max remaining visibility time (RVT)-based methods.

Two-way full-duplex relaying (TW-FDR) can feature more spectral efficiency than two-way half-duplex relaying since only one channel usage is required for signals exchange when the end nodes also adopt full-duplex transceivers. We propose a new transceiver design of TW-FDR based on time-and spectrum- and antenna-sharing, network coding and self-interference elimination. A beamformer is designed at the relay to null the self-interference by spatial-domain projection, i.e., received and transmitted beams are arranged in orthogonal subspaces, or in minimum eigen-modes for overlapping subspaces. Thus, no iteration is needed in the proposed scheme and the computational complexity can be greatly reduced for implementation. Simulation results evaluate the improved performance of the proposed scheme over existing half-duplex and full-duplex relaying schemes in terms of achievable rate.

This paper addresses the spectral efficiency in dynamic femtocell networks, where users leave and/or enter their serving femtocells frequently. The overall system bandwidth is divided into several subchannels. The spectral efficiency would be decreased if no appropriate resource allocation designed to manage the topology variation due to user mobility. We thus propose a three-step dynamic frequency allocation algorithm to manage the co-channel interference more effectively. First, a weighted clustering of femtocells is centrally utilized to avoid the co-channel interference among clusters. Secondly, each FBS assigns subchannels to its served users. Thirdly, if there is a new user and no available subchannel for assignment, spectrum stealing is processed by reusing a subchannel, which belongs to another cluster while satisfying the interference constraints. Numerical results demonstrate that the proposed algorithm can not only enhance the system throughput but also decrease the outage probability of user accesses.

In this paper, we study the application of physical layer network coding (PNC) and interference alignment scheme to the joint design of uplink and downlink transmissions, where the base station (BS), the mobile stations (MSs) and the relays all have multiple antennas. Leveraging from the interference alignment and physical-layer network coding, a joint signal and interference alignment scheme is proposed, where uplink and downlink communications can be accomplished within two time slots. Since here two relays share the same spectrum to assist the data transmission, co- channel interference (CI) would be induced and the communication reliability would be deteriorated. To address this challenge, precoding matrices at the BS, the MS's, and relays have been carefully designed to ensure the removal of CI. Additionally, the received signals from BS and MSs are mixed and broadcasted to BS and MSs at the next time duration. Simulation results also demonstrate the performance of the proposed joint signal and interference alignment scheme.

Molecular communication has been considered an emerging technology for both scientific and engineering interests, due to molecular biology and the need for nano-scale communication and computation systems. Molecular communication systems are widely analogue to electrical communication systems involving emitting molecules, diffused propagation of molecules, and reception of molecules, under stochastic characteristics of relatively slow propagation speed and short transmission range. To facilitate large-scale molecular information systems for the purpose of communication and computation, state-of-the-art explorations are generally assumed perfect alignment of timing among molecular devices and sub-systems. To reach synchronous condition in electronics, phase locked loop (PLL) is known to align timing or phase of waveforms. We extend PLL into molecular PLL (MPLL) consisting of basic elements such as molecular phase detector, molecular loop filter, and molecular voltage controlled oscillator. Due to stochastic nature of molecular diffusion, we further analyze MPLL in terms of the diffusion jitter, displacement, and the (particle or molecular) counting noise. Simulations verify the MPLL concept model, tracking performance, and robustness in operation.

The issues of frequency offset and channel estimation are considered for the downlink of coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade the system performance. Here, we adopt a parallel interference cancelation strategy to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by a Hadamard product and a Taylor series expansion, to eliminate the inter-carrier interference. Our scheme is significantly less complex than existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably well to these conventional approaches.

We consider the issues of frequency offset and channel estimation in downlink coordinated multiple-input multiple-output orthogonal frequency-division multiplexing systems. Multiple carrier frequency offsets exist in this coordinated system. Without implementing an appropriate compensation for these offsets, both inter-carrier and inter-cell interference will degrade system performance. In response, we adopt parallel interference cancelation to iteratively mitigate inter-cell interference, and propose a frequency offset estimator, approximated by Hadamard product and Taylor expansion, to eliminate the inter-carrier interference. Our scheme is significantly less computational complex than the existing methods. Furthermore, the proposed channel estimator is robust to frequency offsets and performs comparably to conventional approaches.