An optical receiver apparatus and methods for mitigating intersymbol interference in a differentially-encoded modulation transmission system by controlling constructive and destructive transfer functions. The LPGs are illuminated by three wavelength modulated, narrow bandwidth optical signals, each having a different wavelength and modulation frequency. A portable device forms another part of the system for receiving the data signal relating to the operational parameter for use by the portable device. A controller may be configured to control linear reciprocation of the tray to have one of a piecewise constant velocity profile and piecewise constant acceleration profile in which the number of piecewise constants does not exceed 24.