  In this paper, the design and analysis of a new bandwidth-efficient signaling
method over the bandlimited intensity-modulated direct-detection (IM/DD)
channel is presented. The channel can be modeled as a bandlimited channel with
nonnegative input and additive white Gaussian noise (AWGN). Due to the
nonnegativity constraint, standard methods for coherent bandlimited channels
cannot be applied here. Previously established techniques for the IM/DD channel
require bandwidth twice the required bandwidth over the conventional coherent
channel. We propose a method to transmit without intersymbol interference in a
bandwidth no larger than the bit rate. This is done by combining Nyquist or
root-Nyquist pulses with a constant bias and using higher-order modulation
formats. In fact, we can transmit with a bandwidth equal to that of coherent
transmission. A trade-off between the required average optical power and the
bandwidth is investigated. Depending on the bandwidth required, the most
power-efficient transmission is obtained by the parametric linear pulse, the
so-called "better than Nyquist" pulse, or the root-raised cosine pulse.
