  A two-dimensional signal space for intensity- modulated channels is
presented. Modulation formats using this signal space are designed to maximize
the minimum distance between signal points while satisfying average and peak
power constraints. The uncoded, high-signal-to-noise ratio, power and spectral
efficiencies are compared to those of the best known formats. The new formats
are simpler than existing subcarrier formats, and are superior if the bandwidth
is measured as 90% in-band power. Existing subcarrier formats are better if the
bandwidth is measured as 99% in-band power.
