Mingbo Niu

Error rate performance comparison of coherent and subcarrier intensity modulated optical wireless communications

A detailed analysis and comparison is carried out for optical wireless communications (OWCs) with coherent and subcarrier-intensity-modulation-based systems, which are the two major implementations for detection-threshold-free operation without irreducible error floors. Error rate performance is studied for communications with binary phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying over weak-to-strong (gamma-gamma distributed) turbulence conditions. Series-form error rate expressions are also derived for diversity reception schemes, including maximum ratio combining, equal gain combining, and selection combining. Based on our analysis, it is found that coherent OWC systems typically outperform subcarrier intensity modulation systems, with 24-30 dB improvements in sensitivity, mainly due to their elimination of thermal and background noise effects. The performance improvements of coherent systems are confirmed through numerical studies. The findings can offer significant benefits for future OWC systems that are subject to transmitted power limitations.

Error Rate of Subcarrier Intensity Modulations for Wireless Optical Communications

Subcarrier intensity modulations are analyzed for the Gamma-Gamma turbulence channels. We study the error rate of a subcarrier intensity modulated wireless optical communication system employing phase-shift keying, differential phase-shift keying, and noncoherent frequency-shift keying. Closed-form error rate expressions are derived using a series expansion approach. Asymptotic error rate analysis and truncation error analysis are also presented. Our asymptotic analysis shows that the diversity order of the system and the signal-to-noise ratio penalty factors for noncoherent modulations depend only on the smaller channel parameter.

Error Rate Analysis of M-ary Coherent Free-Space Optical Communication Systems with K-Distributed Turbulence

Coherent free-space optical (FSO) communications is analyzed for long-range, high turbulence FSO systems operating under K-distributed turbulence conditions. Exact error rates are presented for M-ary phase-shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) using a closed-form moment generating function. Maximum ratio combining (MRC) reception is employed in the system with MPSK and MQAM to mitigate the turbulence effects, and asymptotic error rate analysis is presented for this MRC reception. The error rate performance degradation caused by phase compensation error is also studied, and it is found that the impact of phase compensation error on the error rate performance can be small when the standard deviation of the phase compensation error is kept below twenty degrees.

Performance analysis of coherent wireless optical communications with atmospheric turbulence

Coherent wireless optical communication systems with heterodyne detection are analyzed for binary phase-shift keying (BPSK), differential PSK (DPSK), and M-ary PSK over Gamma-Gamma turbulence channels. Closed-form error rate expressions are derived using a series expansion approach. It is shown that, in the special case of K-distributed turbulence channel, the DPSK incurs a 3 dB signal-to-noise ratio (SNR) penalty compared to BPSK in the large SNR regime. The outage probability is also obtained, and a detailed outage truncation error analysis is presented and used to assess the accuracy in system performance estimation. It is shown that our series error rate expressions are simple to use and highly accurate for practical system performance estimation.

Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels

Exact error rate performances are studied for coherent free-space optical communication systems under strong turbulence with diversity reception. Equal gain and selection diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate for binary phase-shift keying and outage probability are developed for equal gain diversity. Analytical expressions are obtained for the bit-error rate of differential phase-shift keying and asynchronous frequency-shift keying, as well as for outage probability using selection diversity. Furthermore, we provide the closed-form expressions of diversity order and coding gain with both diversity receptions. The analytical results are verified by computer simulations and are suitable for rapid error rates calculation.

Coherent Wireless Optical Communications With Predetection and Postdetection EGC Over Gamma–Gamma Atmospheric Turbulence Channels

Wireless optical communication systems with coherent detection are analyzed for Gamma-Gamma distributed turbulence channels. In addition to the shot noise, we consider the impacts of both turbulence amplitude fluctuations and phase fluctuations on the error performance. Error rate analyses of predetection and postdetection equal gain combining (EGC) are carried out. We derive the exact error rate expressions for predetection and postdetection EGC using a characteristic function method. In the case of predetection EGC, we also study the impact of phase noise compensation error on the error rate performance. It is shown that the error rate performance of predetection EGC is sensitive to phase noise compensation errors for both weak and strong turbulence conditions. In order to alleviate the impact of phase noise, postdetection EGC with differential phase-shift keying is introduced and analyzed. In addition, postdetection EGC is compared with predetection EGC in the presence of phase noise compensation errors, and it is found to be an effective alternative to predetection EGC with low complexity implementation.

MIMO architecture for coherent optical wireless communication: System design and performance

A coherent multiple-input multiple-output architecture is proposed for optical wireless communications (OWCs) to mitigate atmospheric turbulence effects. Transmitter optical signals operate at distinct carrier frequencies to allow the received optical signals to be separately processed. The accumulated phase noise in each transmission branch can then be independently and electrically compensated. Based on the proposed architecture, several diversity combining techniques are used at the receiver end for system performance evaluation. Three different turbulence models are considered in this paper for different scintillation level ranges, including gamma-gamma turbulence, K-distributed turbulence, and negative exponential turbulence. Closed-form error rate expressions are derived using a series expansion approach. The diversity order in the gamma-gamma turbulence channel is found to depend only on the smaller channel parameter, while the K-distributed and negative exponential turbulence channels are found to have the same diversity order. The presented numerical results demonstrate substantial system performance improvement over single-link coherent OWC.

Terrestrial Coherent Free-Space Optical Communication Systems

In the past three decades, the demand for high-speed communications has increased dramatically, while fiber optical communications has been applied in the majority of data transmission networks. Optical fiber has advantages over existing copper wire in long distance and high demand applications. The ever increasing need for higher bandwidth and higher speed optical data and communications transmission is driving the development of 100 gigabit per second (Gbit/s) communication links. However, infrastructure development within cities is relatively difficult and time-consuming, and fiber-optic systems are complex and expensive. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost.

Performance Analysis of Coherent Free Space Optical Communication Systems with K-Distributed Turbulence

Error performance of a coherent free space optical system with K-distributed turbulence is studied. A closed-form expression of the moment generating function for K-distributed turbulence is derived, and the moment generating function is used to obtain the exact error rates for binary phase shift keying (BPSK), BPSK with spatial diversity, as well as differential phase-shift keying. Based on our analytical moment generating function expression, asymptotic error rates in large signal-to-noise ratio regions are obtained.

Alamouti-Type STBC for Atmospheric Optical Communication Using Coherent Detection

Alamouti-type space-time block coding is studied for optical wireless communication systems using coherent detection over atmospheric turbulence channels. Atmospheric turbulence-induced fading and phase noise are known to impair the performance of coherent optical wireless systems. Two new Alamouti-type space-time coded architectures are proposed to overcome turbulence-induced fading and phase noise from transmitter lasers, receiver local oscillators, and turbulence channels. Their error rate performance is analyzed for a wide range of turbulence conditions. With developed analytical error rate expressions, the usefulness of the proposed Alamouti-type space-time block coding is demonstrated for coherent optical wireless communication systems. Numerical results confirm the performance improvements of the proposed systems over that of single-input-single-output systems.