Theoretically, Kramer-Kronig (KK) receivers are able to fully cancel signal-signal beat interference (SSBI), which results from a field-modulated single-sideband (with carrier) transmitter being used with a direct-detection receiver. However, even for strong carriers, which should meet the minimum-phase (MP) condition, bits are frequently erroneous. In this article, we simulate two methods of reducing error rates for weak carriers. The first is to limit the minimum value of the received signal, so that strong negative-going peaks are not produced by the logarithm function of the KK's 'correction path'. The second is to replace the square-root function in the main signal path with a logarithmic function. This is to allow analog processing using semiconductor diodes. Our numerical simulations show that grossly approximating the square-root function is actually beneficial to the system, as it reduces the power in the signal close to the clipping events. When both the first and second techniques are combined, low error rates can be obtained at 2-dB less carrier power than for conventional processing, and 0.25-dB better receiver sensitivity. When 100-km of standard fiber is included, the carrier can be reduced by 3 dB, and the system has a 0.18 dB better sensitivity, and potentially supports analog processing. This work suggests that there may be other improvements possible, in signal quality and computational load, by moving away from a theoretically perfect implementation of the KK algorithm.
- direct detection
- optical fiber communications