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This paper introduces a novel pseudo-orthogonal carrier interferometry spreading code to orthogonal frequency-division multiplexing (OFDM) systems which spreads each parallel data stream over all the OFDM carriers. Pseudo-orthogonal carrier interferometry (PO-CI) spreading codes are carefully selected to introduce the following benefits to OFDM: up to 2N parallel data streams can be coded onto N carriers, with little degradation in performance; when rate 1/2 channel coding is applied in addition to PO-CI spreading codes, resulting binary phase-shift keying OFDM systems demonstrate the performance of coded OFDM and the throughput of uncoded OFDM; and PO-CI codes are carefully selected to spread in a manner which eliminates the peak-to-average power ratio problems characteristic of traditional OFDM. Index Terms--Carrier interferometry (CI), multicarrier, orthogonal frequency-division multiplexing (OFDM), spread spectrum.