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The threshold current of multi-quantum well laser diodes increased by about 1.5 mA at a proton fluence of 6x[10.sup.2] p/[cm.sup.2]. It recovered gradually due to forward-bias annealing at [I.sub.bias]= 35 mA and became about 0.8mA less than before irradiation after 86 h of forward bias annealing. At the same time, the optical power at a given bias became greater than before irradiation. There is a fast recovery during the first 100 s after proton irradiation. Increasing the forward bias current during annealing speeds up the annealing process. The degradation of optical power at a proton flux of 1.87x[10.sup.9] p/[cm.sup.2]/s(4.7% at a fluence of 6x[10.sup.12] p/[cm.sup.2]) is less than that at a proton flux of 1.45x[10.sup.10] p/[cm.sup.2]/s (10.5% at a fluence of 6x[10.sup.12] p/[cm.sup.2]) due to the in-situ forward-bias annealing effects.

200 MeV proton damage effects on multi-quantum well GaAs/GaAlAs laser diodes are studied. The threshold current damage factor can vary from 1.73 to 4.60x[10.sup.-15] [cm.sup.2]/p for different bias conditions and from 1.52 to 3.58x[10.sup.-15] [cm.sup.2]/p for different incident directions. A simple model, based on threshold current damage factor, is presented to quantify the degradation of optical power. Preliminary annealing effects are also presented. The annealing effects are greater for devices shorted during irradiation than for those biased at [I.sub.bias] = 35mA.

The availability of Channel State Information (CSI) and the effects of channel memory on the capacities and the achievable rates of free-space optical communication channels are investigated. For memoryless channels, the capacities and achievable rates are computed and compared for both uniform and ldquopositiverdquo Gaussian inputs subject to different assumptions on the CSI availability. For the strong turbulence regime, it is shown that the knowledge of CSI both at the transmitter and the receiver increases the achievable rates for low-to-moderate Signal-to-Noise Ratios (SNRs) in comparison to the cases for which the CSI is known only at the receiver. For the weak turbulence regime however, the availability of CSI at both ends of the link does not provide any improvement over a system with CSI known at the receiver alone, and we find that a simple channel inversion technique suffices. In addition, for low SNRs, Pulse Amplitude Modulation (PAM) with M ges 4 levels outperforms Gaussian-distributed inputs regardless of the knowledge of CSI at the transmitter. For high SNRs, a Gaussian distribution gives superior results, implying the need for new, more efficient positive signal constellations. For channels with memory and without knowledge of CSI, a change in the channel quasifrequency has negligible effects on the capacity for any turbulence regime. [ABSTRACT FROM PUBLISHER]

In this paper, coded multiple-input-multiple-output (MIMO) communication schemes for data transmission over the optical atmospheric turbulence channels are studied. Two strategies are proposed and compared. The first is based on repetition coding, and the second on space-time (ST) coding. Both approaches employ low-density parity-check (LDPC) codes. The LDPC codes are designed using the concept of pairwise balanced design (PBD), balanced incomplete block design (BIBBD), and block-circulant (array) codes. To improve the spectral efficiency, we employ a bit-interleaved (BI) LDPC-coded modulation based on the pulse amplitude modulation (PAM). A better bit error rate (BER) performance is achieved by the iteration of extrinsic information between a demapper and LDPC decoder. The simulations show that the LDPC-coded MIMO schemes can operate under a strong atmospheric turbulence and at the same time provide excellent coding gains compared with the transmission of uncoded data. To verify the efficiency of the proposed coding schemes, achievable information rates are computed when the turbulence is modeled by a gamma-gamma distribution. [ABSTRACT FROM PUBLISHER]

Low-density parity-check (LDPC) coded orthogonal frequency division multiplexing (OFDM) is proposed as an efficient coded modulation technique suitable for optical communication systems. We show that in long-haul fiber optics communications, LDPC-coded OFDM increases spectral efficiency, and reduces the influence of residual chromatic dispersion and the influence of fiber nonlinearities. LDPC-coded OFDM is as also a promising polarization-mode dispersion (PMD) compensation technique. In free space optical communications, LDPC-coded OFDM improves the immunity to the atmospheric turbulence; while in multimode fiber links, it reduces the influence of intermodal dispersion. In plastic optical fiber links it reduces the influence of dispersion and helps to overcome the bandwidth limitations. Several power efficient OFDM schemes and LDPC codes suitable for use in LDPC-coded OFDM optical communication systems are introduced. [ABSTRACT FROM PUBLISHER]

Cognitive radar is a recently proposed approach in which a radar system may adaptively and intelligently interrogate a propagation channel using all available knowledge including previous measurements, task priorities, and external databases. A distinguishing characteristic of cognitive radar is that it operates in a closed loop, which enables constant optimization in response to its changing understanding of the channel. In this paper, we compare two different waveform design techniques for use with active sensors operating in a target recognition application. We also propose the integration of waveform design with a sequential-hypothesis-testing framework that controls when hard decisions may be made with adequate confidence. The result is a system that updates multiple target hypotheses/classes based on measured data, customizes waveforms as the class probabilities change, and draws conclusions when sufficient understanding of the propagation channel is achieved [ABSTRACT FROM PUBLISHER]

Presents a study concerning the damaging effects which 200 Me V proton have caused on the multi-quantum well laser diodes. Variation of the threshold current damage factor; Presentation of preliminary annealing effects; Important role which the multi-quantum well laser diodes play.