Hassan Moradi

Thresholding-Based Optimal Detection of Wireless Optical Signals

The receiver design for a high-speed free-space (wireless) optics (FSO) signal is necessarily highly complex when channel state information (CSI) is not available. Currently, although most approaches provide high detection performance in terms of bit error, receiver design is difficult to implement. This paper proposes two practical thresholding-based detection schemes, which offer significant improvement to receiver throughput on a computational load basis when CSI is not available. The first is based on a simple maximum likelihood (ML) function where the bit error rate (BER) is the same as conventional symbol-by-symbol detection. This method, however, causes a loss of BER performance. The second uses the aid of pilot-symbol-assisted modulation (PSAM) to modify the ML function when channel coefficients are temporally correlated. While numerical analysis based on this method shows that the BER performance in a log-normally distributed fading channel is very close to detection achieved with perfect CSI, the receiver suffers from increased complexity. If random processes for fading and noise are assumed as stationary and given that the detection threshold is quickly calculated and applied during a given period, such complexity of PSAM-based and symbol-by-symbol detection methods can be reduced.

BER analysis of optical wireless signals through lognormal fading channels with perfect CSI

Due to inconsistent atmospheric conditions, scattering and scintillation of free space optical (FSO) signal can occur, thus negatively influencing the received signal intensity. The channel is usually modeled as a normalized fading coefficient with additive Gaussian noise. Optimal detection of the received signal is designed based on a decision rule, e.g., Maximum Likelihood (ML), assuming the receiver knows the noise statistics and fading correlation of the channel. This paper briefly deals with analysis on bit error rate (BER) of a wireless optical signal passing through a lognormally distributed fading channel, when perfect knowledge of channel state information (CSI) at the receiver side is available. Two approaches will be presented to provide closed-form expressions for BER. One uses Gauss-Hermite quadrature approximation and the other one is based on power series. While numerical analysis shows a very small approximation error when the Gauss-Hermite approach is considered, the power series approach does not uses any approximation.

Availability Modeling of FSO/RF Mesh Networks through Turbulence-Induced Fading Channels

Even if a line-of-sight condition of Free Space Optics (FSO) is satisfied, atmospheric-induced fading, scattering, and attenuation may severely deteriorate the availability of the communication link. This argument is true except in reconfigurable FSO ad hoc networks, a path reconfiguration scheme replaces a severed FSO link with an operational one. Reconfigurability, as a property of our hybrid FSO/RF (Radio Frequency) work in progress, provides connection reliability and network throughput. The traffic can be directed to a different FSO link or even an RF link as backup. Hence, node failure and outage probabilities due to link failure will be reduced, thus resulting in a higher availability of nodes. Mathematical investigation and statistical consideration of availability and capacity of FSO/RF ad hoc mesh network is the focus of this paper. We assume normalized scintillation fading channels in our analysis. We apply different availability cases of channel state information to derive closed-form expressions for system capacity.

Circular MIMO FSO Nodes With Transmit Selection and Receive Generalized Selection Diversity

Mobile free space optical (FSO) nodes have been characterized as a way to resolve movement and misalignment incompatibilities that are inherent in optical nodes. By introducing multibranch angular diversity mobility to FSO nodes, connection reliability is enhanced. In this paper, receiving branches are assumed to be circularly placed on a single plane, and transmitting branches are positioned on a different platform. A coverage of 360 in this unbalanced branch configuration is achieved with the deployment of diversity combining, wherein the transmitter follows a transmit selection diversity rule. Channel turbulence is modeled as weak lognormal with spatially correlated fading samples. On the receiver side, selection combining (SC), equal gain combining (EGC), maximum ratio combining (MRC), and threshold generalized SC (T-GSC) schemes serve as candidates for combining purposes. Statistical discussion relative to branch signals and combiner output is presented, and bit error performance and outage probability are numerically evaluated. When compared with MRC, T-GSC was found to have lower estimation error. The T-GSC scheme additionally demonstrated overall superior performance over SC and EGC for mobile FSO.

On the Capacity of Hybrid FSO/RF Links

Hybrid Free Space Optics (FSO)/Radio Frequency (RF) communication systems have emerged as a way to improve network performance by providing enhanced availability and reliability. In an effort to mitigate individual drawbacks in the optical link during adverse weather conditions, network traffic flows simultaneously between channels. Based on the Shannon-Hartley theorem, channel capacity is dependent, among others, on both the signal to noise ratio (SNR) and channel bandwidth. As such, combined link throughput may be affected by channel state conditions. Because atmosphere turbulence can be modeled as a time-varying fading channel, capacity analysis can be investigated. In this paper, authors apply various availability scenarios of channel state information (CSI) on the optical link to derive closed-form expressions for combined link capacity. Numerical simulation provides a comparison to the results.

Dynamic Path Reconfiguration Among Hybrid FSO/RF Nodes

To increase the availability of free space network (FSO) networks, the optical link is provided with a redundant backup network, such as RF, to form a hybrid network. The immediate switching from FSO to RF in hybrid networks leads to suboptimal utilization of FSO network. In this work, a network of hybrid nodes, where each node is equipped with multiple transceivers and has multiple associations with other nodes of the network is proposed. Alternate FSO paths are established and prioritized through these multiple associations via intermediate nodes. On the loss of an active FSO link, path reconfiguration takes place, and the FSO path next in priority is utilized. Thus, communication can be maintained in the optical domain for a longer period, and efficiency is increased. A hardware testbed with four hybrid nodes having multiple IR transceivers to establish optical links was constructed to validate proof of concept. Dynamic path reconfiguration in the optical domain was demonstrated in the testbed by blocking different transceivers of the nodes. We measure network performance in terms of end-to-end delay along alternative FSO paths. Results show how the proposed path reconfiguration algorithm can provide performance improvement in hybrid networks.

Switched diversity approach for multireceiving optical wireless systems

A number of existing spatial diversity schemes have been shown to improve the performance of optical wireless communication systems in diversity-rich environments. Among all, switched diversity has low complexity and is simple to implement. In this paper, an innovative spatial diversity scheme based on switched diversity is proposed. The scheme, namely switch-to-dominant combining, contributes to a higher bit error rate (BER) performance when compared to conventional switched diversity schemes, including switch-and-stay and switch-and-examine diversity. The optical multireceiver wireless system operates in a spatially correlated and lognormally distributed fading channel. Analytical analyses are conducted to demonstrate BER and processing load performance offered by the new scheme and compare them to available schemes, i.e., conventional switched combining and selection combining.

Spatial Diversity for Fiber-Bundled FSO Nodes With Limited Mobility

In spite of exceptional advantages demonstrated by optical wireless signals, alignment distortion from building sway or transceiver displacement, make the development of corresponding high-speed links less attractive. This paper investigates a fiber-bundled design providing limited transceiver mobility. By placing three fibers horizontally behind a convex lens, angular diversity is exploited. A modified scheme of switched diversity is utilized to combine signals from various receiving fibers. Coverage area, combiner processing load, bit error rate, and outage probability served as performance criteria throughout the analytical and simulation assessment. High bit error performance was achieved while maintaining low implementation complexity over a turbulent fading channel.

A PSAM-Based Estimator of Noise and Fading Statistics for Optimum Receivers of Free Space Optics Signals

Incoherent receivers of Free Space Optical (FSO) signals have no knowledge of instantaneous channel state. Thus, the receiver requires some information about the noise and fading statistics for a maximum likelihood (ML)-based optimal detection. Using pilot-aided symbols, we develop a simple multi slot averaging (MSA) estimation technique to approximate the values of parameters required at the incoherent detector. No channel state information (CSI) is available at the receiver side and this work will not be also trying to estimate it. But the estimation of noise and fading statistics will be practically investigated. We evaluate the bit error rate (BER) performance of FSO links with MSA estimation over both Gaussian and lognormal atmospheric turbulence fading (scintillation) channels. Numerical simulation will be completed to evaluate the estimation error of the MSA estimator. We will see that at signal to noise ratio (SNR)=13dB, the performance loss of the Gaussian estimator improves from 3dB to 0.4dB when we increase the number of pilot symbols from 16 to 64. This paper also presents the hardware design of the estimator using Xilinx system generator.

Reconfiguration Modeling of Reconfigurable Hybrid FSO/RF Links

The purpose of devising a hybrid FSO/RF (Free Space Optics/Radio Frequency) system is to allow for reconfiguration, which enables an increased availability and reliability grade of the communication system. In the event of loss of an active FSO link, path reconfiguration ensures utilization of the FSO path queued next in priority. Thus, communication can be maintained in the optical domain with increased efficiency for a longer period. However, for all its benefits, path reconfiguration may inadvertently cause increased system delay and lower throughput, as well as packet loss. Given these advantages and disadvantages, path reconfiguration studies for hybrid systems are of great importance. A mathematical investigation and statistical consideration of path reconfiguration probability is the focus of this theoretical paper. A closed-form expression of the reconfiguration probability will be derived. The first-order Markov chain will provide a mathematically tractable model for atmosphere turbulence fading channels, as it uses only the received SNR of the FSO symbol immediately preceding the current one. Results show that increasing the intensity of turbulence-induced lognormal fading for FSO link will not necessarily increase the reconfiguration probability.