Techniques are provided to estimate a distance of one received optical subchannel to one or both of its neighbor (adjacent) subchannels. An optical field comprised of a plurality of subchannels of optical signals at respective wavelengths is received on an optical fiber. Using coherent optical reception in conjunction with analog-to-digital conversion, the received optical field is converted to digital complex valued data. The digital complex valued data is transformed to the frequency domain to produce spectrum data. Using either a peak method or a gap method, a distance or spacing is computed between a subchannel of interest among the plurality of subchannels and at least one neighbor subchannel based on the spectrum data.