This invention relates to modulation techniques for transmitting information over fiber optic links.
In digital communications systems, bandwidth efficient modulation techniques are used to maximize the amount of information, in the form of digital voice, video and data channels, transmitted across a medium such as a fiber optic link. They do so by reducing the spectral bandwidth required for the transmission of each such digital data channel.
Some bandwidth efficient modulation techniques involve encoding two or more data bits in each information symbol transmitted across the digital communications link, instead of the familiar single bit binary code. The number of bits encoded by each symbol, and hence the number of possible bit patterns, determines the number of transmission states, or levels, possible for the symbol to take. A four-level code, where each transmitted symbol represents one of four possible symbol states, effectively encodes each of the four possible patterns for a pair of data bits in each transmitted symbol.
One technique for transmitting a pair of data bits with a four-level code is quadrature phase shift keying (QPSK) modulation, where each pair of bits is encoded during each symbol period as one of four possible phases of a transmitted carrier signal. Where each pair of bits is derived sequentially from the same data stream, the bit period for the data stream is one half the symbol period of the resulting QPSK signal. However, where each pair of bits is derived from two parallel data streams, the bit period for each data stream is equal to the symbol period of the resulting QPSK signal.
A technique related to QPSK modulation is differential QPSK (DQPSK) modulation, where each pair of data bits is encoded as one of four possible changes of phase of a transmitted carrier signal. Often, a differential QPSK demodulator for recovering the data bits from the transmitted carrier signal requires circuitry to perform an inverse tangent operation (tan.sup.-1), a data bit synchronization operation, and a four-level logical decision, all operating at data rates up to and beyond 200 Mb/sec.
In optical fiber communications systems, optical fiber transmission links typically rely on an optical modulator to modulate a laser light source with the information to be transmitted across the fiber. One type of optical modulator phase modulates the laser light source with one or more microwave subcarrier signals, each itself modulated with digital information. An optical heterodyne detector and subcarrier demodulator at the receiver end of the optical link detects the digital information from each of the modulated subcarrier channels.