Experimental study demonstrates superior data rates for 4-PAM over on-off keying in multimode optical links, indicating improved bandwidth efficiency for short-range communications.
We present results of experimental and theoretical investigations of intersymbol interference in 4-PAM transmission in short-range optical communications links based on the power penalty. A test link comprised of a directly modulated 850 nm VCSEL with up to 200 m of multimode fiber and direct detection was used. The link bandwidth was below 10 GHz and the maximum achieved data rate with 4-PAM was 44 Gbps over 100 m of fiber. In the same case and at similar sensitivity, only 32 Gbps could be achieved with OOK. If typical forward error correction could be applied, the sensitivity of the 4-PAM system was improved by up to 4 dB, reaching –10 dBm at 25 Gbps.
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Szczerba et al. (2013) studied this question.
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