Abstract
A modified version of pulse position modulation (PPM), asynchronous digital pulse interval modulation (PIM), is examined. The modulation scheme strengthens the viability of using LEDs as sources on a single mode fiber system, by allowing an increase in the peak power coupled into the fiber. First, an overview of the PIM code is provided. This is then followed by a detailed description of the 3B17P PIM code, which was implemented in an experimental 90-Mb/s communication system. Experimental results of the system's bit error rate (BER) versus the average received power were obtained for both the PIM and NRZ code over a coaxial cable link. The results are compared and discussed. The PIM code has a power advantage of up to 3.5 dB over the NRZ binary code. At a constant peak received power, the effect of the threshold level on the BER is similar for both codes at a high SNR. At a low SNR, there appears to be an optimal threshold level that depends on the duty cycle of the received code. >
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