Xuedan Zhang

Impulse Response Modeling for Underwater Wireless Optical Communication Links

In underwater wireless optical communication (UWOC) links, multiple scattering may cause temporal spread of beam pulse characterized by the impulse response, which therefore results in inter-symbol interference (ISI) and degrades system error performance. The impulse response of UWOC links has been investigated both theoretically and experimentally by researchers but has not been derived in simple closed-form to the best of our knowledge. In this paper, we analyze the optical characteristics of seawater and present a closed-form expression of double Gamma functions to model the channel impulse response. The double Gamma functions model fits well with Monte Carlo simulation results in turbid seawater such as coastal and harbor water. The bit-error-rate (BER) and channel bandwidth are further evaluated based on this model for various link ranges. Numerical results suggest that the temporal pulse spread strongly degrades the BER performance for high data rate UWOC systems with on-off keying (OOK) modulation and limits the channel bandwidth in turbid underwater environments. The zero-forcing (ZF) equalization designed based on our channel model has been adopted to overcome ISI and improve the system performance. It is plausible and convenient to utilize this impulse response model for performance analysis and system design of UWOC systems.

On Link Misalignment for Underwater Wireless Optical Communications

Absorption and scattering processes respectively cause intensity attenuation and direction deviation of photons during their propagation through underwater environment and therefore affect the spatial and temporal distribution of light intensity. In this paper, we consider the link misalignment caused by the light source properties such as the divergence and elevation angles and evaluate their impacts on the spatial-temporal distribution of light intensity. Numerical examples in turbid water such as coastal and harbor water reveal the relationship among divergence angle, elevation angle, water type, link range and light intensity. Simulation results suggest that the received intensity decreases as the divergence and/or elevation angle increase.

Adaptive energy-harvesting aware clustering routing protocol for Wireless Sensor Networks

Wireless Sensor Networks (WSNs) are energy-constrained. However, recent advances in ambient energy harvesting technology have made it possible for sensor nodes to harvest energy from ambient environment and serve as a supplement or the only energy source. And most of current routing protocols are specially designed for battery-powered WSNs, may not be suitable for energy harvesting WSNs (EH-WSNs), especially for those entirely powered by harvesting energy. In this paper, we propose an Adaptive Energy Harvesting Aware Clustering (AEHAC) routing protocol for EH-WSNs, which takes node energy state into cluster head election algorithm and can adjust its parameter according to the network deploying environment. We analyze and evaluate the routing performance in terms of two metrics available node number and network throughput. Simulation results show that AEHAC maintains available nodes about 15% and networks throughput 19% more than traditional LEACH.

Effect of Random Sea Surface on Downlink Underwater Wireless Optical Communications

Most previous works on underwater wireless optical communications (UWOC) assume an ideal horizontal link geometry or a vertical through-the-surface link geometry. The performance of vertical buoy-based surface-to-bottom (downlink) UWOC systems where the source is located on the sea surface has not been studied yet to the best of our knowledge. In practice, the performance of buoy-based downlink UWOC systems will be affected by both the random sea surface slopes caused by wind and scattering property of seawater. In this letter, we take these two factors into account and investigate the bit-error-rate (BER) performance of this downlink UWOC system. We develop a theoretical model to numerically characterize the relationship among transmit power, link range, wind speed, BER and water types for downlink UWOC system. Numerical results suggest the system BER performance degraded by wind speed can be compensated as the turbidity of seawater increases.

On the capacity of underwater wireless optical channels

We consider the channel capacity of underwater wireless optical communication (UWOC) links with inter-symbol interference (ISI). We present a UWOC system model with the channel characterized by double Gamma functions and the receiver modeled by the ideal photon-counting detector and additive Poisson noise. We then apply this system model to numerically evaluate the channel capacity in terms of mutual information in the presence of ISI. Numerical results reveal the relationship among water types, symbol rate, transmit power and channel capacity. They further suggest that the UWOC channel capacity decays as the symbol rate increases and that this degradation can be decreased by increasing the transmit power for a given water type.

Receiver design for underwater wireless optical communication link based on APD

In this paper, we present a gain control scheme of receiver based on avalanche photodiodes (APDs) for underwater wireless optical communication links. We first derive the approximate expression of optimal gain of APD. Then based on beam-spread function, a closed-form expression of relationship of optimal gain of APD, link range and receiver offset distance is derived. Then a self-adaptive gain control scheme is designed to keep the gain of receiver as optimal gain when the receiver deviates unpredictably. We also evaluate the dynamic range of receiver gain. Our results are beneficial for receiver design and link reliability improvement.

Adaptive modulation schemes for underwater wireless optical communication systems

Optical communication has been widely used in underwater systems. However, different modulation schemes may result in different performances [1-5] for specific link conditions and requirements. For instance, the change of probabilities for source bits 0 and 1 will lead a big difference in the performance of the communication with the special modulation. Therefore, for a special underwater optical communication environment and performance requirements, the research of adaptive modulation is necessary.

Polarization differential pulse position modulation

In underwater wireless optical communication links, pulse position modulation (PPM) and differential pulse position modulation (DPPM) are two typical modulation schemes. In this paper, we present an alternative modulation scheme to improve the communication performance compared with the traditional methods. As an improvement of DPPM, this new modulation scheme, which named as polarization differential pulse position modulation (PDPPM), combines the polarization technology with DPPM scheme. Simulation results suggest that the channel bandwidth efficiency and bit-error-rate (BER) performance are enhanced obviously based on our new method.