OFDM spread spectrum communications using lapped transforms and interference excision

Two systems that use orthogonal frequency division multiplexing (OFDM) as a spread spectrum technique are described and their performance is evaluated in the presence of tone and pulsed Gaussian interference. One system obtains frequency diversity by sending the same data bit simultaneously on all the carriers while a second system obtains both time and frequency diversity by sequentially sending a given data bit on different carriers until it is sent on all possible carriers. Interference excision, in which some of the transform bins are removed in the detection process, is used in the receivers. Simulated BER performance in AWGN and interference is presented for systems that use a number of different block transforms and the modulated lapped transform (MLT) and it is shown that the MLT provides much better performance due to its ability to confine the interference energy to a few transform bins.