4-PAM for High-Speed Short-Range Optical Communications

Abstract

Abstract-In this work, we compare 4-pulse amplitude modulation and on-off keying modulation formats at high speed for short-range optical communication systems. The transmission system comprised a directly modulated verticalcavity surface-emitting laser operating at a wavelength of 850 nm, an OM3+ multimode fiber link, and a photodetector detecting the intensity at the receiver end. The modulation formats were compared both at the same bit-rate and at the same symbol rate. The maximum bit-rate used was 25 Gbps. Propagation distances up to 600 m were investigated at 12.5 Gbps. All measurements were done in real time and without any equalization.

Intersymbol Interference Penalties for OOK and 4-PAM in Short-range Optical Communications

“…which is twice as large as for OOK at the same symbol rate [4]. This approximation comes from the middle eye of 4-PAM eye diagram and does not account for the fact that the top and bottom eye closures are asymmetric, thus in reality the penalties should be slightly higher.…”

“…where I avg is the average photodetector current and σ is the root mean square noise current [4]. If natural labeling is used instead of Gray labeling, the BER is slightly higher in the high BER regime, but it is well approximated by Eq.…”

“…If natural labeling is used instead of Gray labeling, the BER is slightly higher in the high BER regime, but it is well approximated by Eq. 1 in the low BER regime [4]. The theoretical power penalty for using M-PAM instead of OOK at the same bit rate is P pb = 10 log 10 ((M − 1)/( √ log 2 (M))), which in the case of 4-PAM means 3.3 dB more optical power required to reach the same BER as OOK [4].…”

“…1 in the low BER regime [4]. The theoretical power penalty for using M-PAM instead of OOK at the same bit rate is P pb = 10 log 10 ((M − 1)/( √ log 2 (M))), which in the case of 4-PAM means 3.3 dB more optical power required to reach the same BER as OOK [4]. This does not include the ISI penalty for OOK, however.…”

“…Results of an investigation of the ISI penalties of 4-PAM and OOK with fixed bit and symbol rates and varying fiber lengths were presented in [4]. In this work we investigate the effects of the ISI penalties when the frequency response is fixed (with −3 dB bandwidth equal to 8.5 GHz), and the bit rate is increased.…”