An ultrafast microring modulator (MRM) is fabricated and presented with ${\mathit{V}}_{\pi }·\mathit{L}$ of $0.825\mathrm{V}·\mathrm{cm}$. A 240 Gb/s PAM-8 signal transmission over 2 km standard single-mode fiber (SSMF) is experimentally demonstrated. PN junction doping concentration is optimized, and the overall performance of the MRM is improved. Optical peaking is introduced to further extend the EO bandwidth from 52 to 110 GHz by detuning the input wavelength. A titanium nitride heater with 0.1 nm/mW tuning efficiency is implemented above the MRM to adjust the resonant wavelength. High bit rate modulations based on the high-performance and compact MRM are carried out. By adopting off-line signal processing in the transmitter and receiver side, 120 Gb/s NRZ, 220 Gb/s PAM-4, and 240 Gb/s PAM-8 are measured with the back-to-back bit error ratio (BER) of $5.5\times {10}^{-4}$, $1.5\times {10}^{-2}$, and $1.4\times {10}^{-2}$, respectively. A BER with different received optical power and 2 km SSMF transmission is also investigated. The BER for 220 Gb/s PAM-4 and 240 Gb/s PAM-8 after 2 km SSMF transmission is calculated to be $1.7\times {10}^{-2}$ and $1.5\times {10}^{-2}$, which meet with the threshold of soft-decision forward-error correction, respectively.