Flexible Hybrid PAM2/4 for Fidelity Optimization in Digital Mobile Fronthaul

Four-level pulse amplitude modulation (PAM4) has been considered as a low-cost modulation format for capacity upgrade in digital mobile fronthaul; however, the bit error ratio increases due to the reduced Euclidean distance. In this letter, a novel scheme is proposed to suppress bit-error-induced error vector magnitude (EVM) distortion upon radio signal in PAM4-based fronthaul. The scheme is realized by pairing one high-order bit and one low-order bit onto one PAM4 symbol and setting a portion of low-order bits to "0." As a result, sample bits with high priority are delivered by PAM2 symbols with small error probability, and sample bits with low priority are transmitted by PAM4 signal with high spectral efficiency. Without any modification of the hardware, the proportion between PAM2 and PAM4 can be flexibly adjusted according to the quality of optical link. Experiments validate that, under respective optimal PAM2/4 proportion, EVM of long term evolution-advanced like radio signal decreases by at least 20 dB, and up to 1024-quadratic-amplitude modulation can be supported.