  With the largest dish Five-hundred-meter Aperture Spherical radio Telescope
(FAST), both the mean and single pulses of PSR B2016$+$28, especially including
the single-pulse structure, are investigated in detail in this study. The mean
pulse profiles at different frequencies can be well fitted in a conal model,
and the peak separation of intensity-dependent pulse profiles increases with
intensity. The integrated pulses are obviously frequency dependent (pulse width
decreases by $\sim\,20\%$ as frequency increases from 300 MHz to 750 MHz), but
the structure of single pulses changes slightly (the corresponding correlation
scale decreases by only $\sim\,1\%$). This disparity between mean and single
pulses provides independent evidence for the existence of the RS-type vacuum
inner gap, indicating a strong bond between particles on the pulsar surface.
Diffused drifting sub-pulses are analyzed. The results show that the modulation
period along pulse series ($P_3$) is positively correlated to the separation
between two adjacent sub-pulses ($P_2$). This correlation may hint a rough
surface on the pulsar, eventually resulting in the irregular drift of sparks.
All the observational results may have significant implications in the dynamics
of pulsar magnetosphere and are discussed extensively in this paper.
