Design considerations for subscriber multiplexed (SCM) systems distributing multichannel analog/digital signals are reviewed. In particular, video distribution schemes based on lightwave transmission technologies are discussed. The two most important system design considerations in an SCM system are the control of intensity noise and nonlinear distortions. Intensity noise characteristics and degradation caused by optical reflections are discussed. Laser and detector linearity requirements are reviewed. System signal-to-noise ratio analysis is performed. Design considerations for receivers for SCM systems are reviewed, and various system applications are examined. >

Subcarrier multiplexed lightwave system design considerations for subscriber loop applications

A design procedure has been developed to determine the required operating conditions for a pulse-frequency modulation (PFM) system (modulation index, required system bandwidth, and receiver carrier-to-noise ratio (CNR) for a specified receiver SNR (signal-to-noise ratio) and number of clicks per second. The PFM processing gain (=SNR/CNR) was measured to be 40 dB for a modulation index of beta =2.5, which agrees closely with the theory. The theoretical processing gain advantage of PFM over FM is 15.9 dB for a modulation index of beta =2.5 with receiver bandwidth B/sub rx/=50 MHz. >

Video transmission in optical fiber communication systems using pulse frequency modulation

Lightwave-subcarrier-multiplexed (SCM) transmission systems can provide high-capacity and low-cost delivery of both current and future advanced TV signals. Compatibility of SCM technology with existing baseband digital transmission equipment would eliminate the need for a separate analog distribution network and thereby permit a more graceful evolution to a broadband integrated digital services network (B-ISDN). One version of a hybrid system whereby SCM video and baseband digital signals are simultaneously transmitted by a single laser diode, and the technical issues related to such an implementation are considered. An experiment is discussed in which the three color components of a high-definition TV (HDTV) signal and a baseband 622.08-Mb/s SONET STS-12 rate data signal are transmitted simultaneously by a DFB laser diode via frequency-modulated (FM) subcarriers, over 2 km of a single-mode fiber. >

A hybrid lightwave transmission system for subcarrier multiplexed video and digital B-ISDN services in the local loop

Predictions of future satellite communications traffic indicate an increased capacity for satellite communications systems is required. Crosslinks between satellites provide improvements in communications throughput for these systems. Crosslinks also increase system flexibility and remove the dependence of world-wide information flow on relay ground stations located outside the continental United States. Laser crosslinks provide the additional advantage of eliminating susceptibility to space-based or ground-based jammers. Laser terminals are also smaller and require smaller antennas than a radio frequency (RF) terminal. This paper describes the advantages of adding laser crosslinks to RF satellite communications systems. Characteristics of the required RF/optical interfaces on-board the satellites are addressed. Terminal configurations that provide reliable, accurate laser communications at high data rates are described.

Laser Crosslink Configurations for RF Satellite Communications Systems

Pulse frequency modulation (PFM) has been considered by several authors for transmitting wide-band signals. However, the noise performance of PFM has been analyzed only above the SNR threshold. In this correspondence the results on the noise performance of PFM available in the literature are summarized, and a more general expression describing noise performance of PFM, both above and below the SNR threshold, is derived. This expression can be used to evaluate the SNR threshold for channels with both signal-independent noise (e.g., metallic facilities) and signal-dependent noise (e.g., fiber optic facilities). In particular, it is shown that in systems with signal-independent white noise, the SNR threshold is approximately 20 dB.

Noise Performance and SNR Threshold in PFM

A nonlinear analog modulation scheme, Pulse Frequency Modulation (PFM), is highly amenable to fiber optic transmission, attaining performance characteristics which are attractive in the context of fiber optic tether cables between undersea vehicles and their support platforms. PFM, contrasted with traditional fiber optic modulation techniques, is shown to exhibit superior characteristics for video bandwidth, long haul transmission. A specific hardware realization which employs PFM is presented, and its operating characteristics discussed.

S. Cowen

Papers

Fiber Optic Video Transmission System Employing Pulse Frequency Modulation