Carrier interferometry

Carrier Interferometry (CI) is a spread spectrum scheme designed to be used in an Orthogonal Frequency-Division Multiplexing (OFDM) communication system for multiplexing and multiple access, enabling the system to support multiple users at the same time over the same frequency band.

In the simplest case, CI code values are coefficients of a discrete Fourier transform (DFT) matrix.

A significant advantage of CI-OFDM over other OFDM techniques is that CI spreading shapes the time-domain characteristics of the transmitted waveform.

Thus, CI-OFDM signals have a much lower peak-to-average-power ratio (PAPR), or crest factor, compared to other types of OFDM.

The advantage is that the receiver can select time-domain or frequency-domain equalization based on how much scattering occurs in the transmission channel.

In radio systems, users share the same subcarriers, but use different orthogonal CI codes to achieve Carrier Interference Multiple Access (CIMA) via spectral interferometry mechanisms.

Many applications of CI principles were published in dozens of subsequent patent filings, conference papers, and journal articles.

Since OFDM's over-sampled DFT is KxN, with K>N, the basic CI spreading matrix performs like a sinc pulse-shaping filter which maps each data symbol to a cyclically shifted and orthogonally positioned pulse formed from a superposition of OFDM subcarriers.