A quad-mode optical carrier generated by mode-deviated injection-locking a vertical-cavity surface-emitting laser (VCSEL) at a beating frequency of 40 GHz is demonstrated for establishing a long-reach millimeter-wave over fiber (MMWoF) link, in which only one mode of the quad-mode optical carriers is modulated by the 16-quadrature amplitude modulation (16-QAM) orthogonal frequency division multiplexing (OFDM) data stream to achieve an optical single-carrier modulation (OSCM). Such an OSCM operation reduces the long-reach single-mode-fiber (SMF) transmission induced RF power fading effect to significantly suppress the frequency notch in the modulated spectrum of the delivered QAM-OFDM data after remotely heterodyne detection. With destructively interfered beating the quad-mode optical carrier at the remote node, the peak power of the optically heterodyned MMW central carrier can be attenuated by 38.3 dB, which saves the gain of the photodetector and electrical amplifier to efficiently boost the carried QAM-OFDM data. This architecture ensures sufficient power of the 16-QAM OFDM data after optoelectrical conversion and facilitates the quad-mode optical carrier based MMWoF link to achieve 50-km wired SMF and 10-m wireless MMW transmissions with the error vector magnitude of 17.2%, signal-to-noise ratio of 15.28 dB, and bit error rate of 3.5×10-3at a data rate of 12 Gbps.

BACKGROUND: Overlapping genes (OGs) are defined as adjacent genes whose coding sequences overlap partially or entirely. In fact, they are ubiquitous in microbial genomes and more conserved between species than non-overlapping genes. Based on this property, we have previously implemented a web server, named OGtree, that allows the user to reconstruct genome trees of some prokaryotes according to their pairwise OG distances. By analogy to the analyses of gene content and gene order, the OG distance between two genomes we defined was based on a measure of combining OG content (i.e., the normalized number of shared orthologous OG pairs) and OG order (i.e., the normalized OG breakpoint distance) in their whole genomes. A shortcoming of using the concept of breakpoints to define the OG distance is its inability to analyze the OG distance of multi-chromosomal genomes. In addition, the amount of overlapping coding sequences between some distantly related prokaryotic genomes may be limited so that it is hard to find enough OGs to properly evaluate their pairwise OG distances. RESULTS: In this study, we therefore define a new OG order distance that is based on more biologically accurate rearrangements (e.g., reversals, transpositions and translocations) rather than breakpoints and that is applicable to both uni-chromosomal and multi-chromosomal genomes. In addition, we expand the term "gene" to include both its coding sequence and regulatory regions so that two adjacent genes whose coding sequences or regulatory regions overlap with each other are considered as a pair of overlapping genes. This is because overlapping of regulatory regions of distinct genes suggests that the regulation of expression for these genes should be more or less interrelated. Based on these modifications, we have reimplemented our OGtree as a new web server, named OGtree2, and have also evaluated its accuracy of genome tree reconstruction on a testing dataset consisting of 21 Proteobacteria genomes. Our experimental results have finally shown that our current OGtree2 indeed outperforms its previous version OGtree, as well as another similar server, called BPhyOG, significantly in the quality of genome tree reconstruction, because the phylogenetic tree obtained by OGtree2 is greatly congruent with the reference tree that coincides with the taxonomy accepted by biologists for these Proteobacteria. CONCLUSIONS: In this study, we have introduced a new web server OGtree2 at http://bioalgorithm.life.nctu.edu.tw/OGtree2.0/ that can serve as a useful tool for reconstructing more precise and robust genome trees of prokaryotes according to their overlapping genes.