This paper studies the performance of multi-hop and mesh networks composed of millimeter wave-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in cases with and without hybrid automatic repeat request (HARQ). Using the central limit theorem as well as other state-of-the-art approximation schemes, we derive closed-form expressions for the networks’ outage probability and ergodic achievable rates. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we determine the minimum number of the transmit antennas in the RF link such that the same rate is supported in the RF- and the FSO-based hops. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, HARQ can effectively improve the outage probability/energy efficiency, and compensate for the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability of 10⁻⁴ and code rate of 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to cases with open-loop communication, by 13 and 17 dB, respectively.

Compared to legacy wavelength division multiplexing networks, elastic optical networks (EONs) have added flexibility to network deployment and management. EONs can include previously available functionality, such as signal regeneration and wavelength conversion, as well as new features such as finer-granularity spectrum assignment and modulation conversion. Yet each added feature adds to the cost of the network. In order to quantify the potential benefit of each functionality, we present a link-based mixed-integer linear programming (MILP) formulation to solve the optimal resource allocation problem. We then propose a recursive model in order to either augment existing network deployments (spectrum and regenerators) or speed up the resource allocation computation time for larger networks with higher traffic demand requirements than can be solved using an MILP. We show through simulation that systems equipped with signal regenerators or wavelength converters require a notably smaller total bandwidth, depending on the topology of the network. We also show that the suboptimal recursive solution speeds up the calculation and makes the running time more predictable, compared to the optimal MILP.

Visible light communications (VLC), a short-range optical wireless communication scheme using light-emitting diodes (LEDs) as transmitters, has been proposed to support multiple users, predominantly in indoor environments. This paper explores using orthogonal frequency division multiple access (OFDMA) for VLC due to its high spectrum utilization efficiency and potentially high transmission data rate. The proposed optical OFDMA uses adjacent subcarriers as a set to support each user. Doing so allows some users to sample at a lower sampling rate and employ a smaller fast Fourier transform (FFT) to lower the computational complexity and hardware cost. A multi-cell structure is naturally obtained in VLC since more than one LED lamp is usually installed in an indoor environment. In this paper, users located in different cells can reuse the same subcarrier sets, potentially causing multiple access interference. Analysis and simulation results are used to predict the subcarrier-set reuse probability over the entire indoor space. When the OFDM modulation index is optimized considering the peak-power-limit of the LEDs, the average transmission data rate is twice that of using OFDM that does not reuse the spectral resources, for the scenarios tested.

Resource allocation for transport optical networks has traditionally used a transmission reach constraint to estimate physical impairments. For flexible-grid networks, nonlinear impairments are not sufficiently well characterized by the transmission reach since various modulations and bandwidths can coexist on the same link. We propose a resource allocation algorithm for a single flexible-grid fiber link based on a nonlinear signal distortion model, as a first step toward whole-network design. An optimization problem is formulated to provide a close-to-optimal resource allocation that guarantees the quality of transmission for every channel. Compared with a simpler algorithm based on transmission reach, our proposed algorithm shows the potential of bandwidth savings. It is demonstrated to be insensitive to channel ordering.