PERFORMANCE OF A SPREAD SPECTRUM OFDM SYSTEM IN A DISPERSIVE IFADING CHANNEL WITH INTERFERENCE Gary J. Saulnier Zhong Ye ECSE Dept, Rensselaer Polytechnic Institute Troy, New York 12180-3590 Michael J. Medley Air Force Research Laboratory/IFGC Rome, New York 13441-4505 ABSTRACT This paper investigates the use of frequency domain equal- ization techniques to suppress narrowband interference and combine multiple paths in both direct sequence spread spec- trum (DS-SS) and orthogonal frequency division multiplexing (OFDM) spread spectrum (OFDM-SS) systems. The simi- larities between the two signaling formats are highlighted and it is shown that the same receiver structures can be used for both. Two and four ray channels are considered, including those with delay spreads an excess of the symbol interval, along with tone jamming. The results show that all the equal- izers that were evaluated are able to suppress the tone inter- ference while combining the energy in the multiple paths. Ad- ditionally, in order to maintain good performance, decision feedback and multiple layers of forward taps are necessary when the delay spread is in excess of a symbol interval. 1. INTRODUCTION One of the features of Direct Sequence Spread Spectrum (DS- SS) signaling is that it is resistant to multipath propagation effects and interference. The interference resistance arises from the fact that the interference is spread in the receiver as the DS-SS signal is being despread. The ability to tolerate multipath is a result of the impulse-like autocorrelation func- tion of the spreading sequences which, within limits, allows a correlation receiver to isolate a particular path. The main constraint is that the particular path be resolvable from the other paths, meaning that the other paths must be delayed by at least one chip interval from the path that you want to demodulate. Any paths which are closer will produce ei- ther constructive or destructive interference with the desired path, depending on the relative carrier phases. While a simple correlation receiver is relatively tolerant of multiple received paths, a RAKE receiver can take advan- tage of multipath propagation by isolating and coherently summing the energy in the individual resolvable paths, pro- ducing an improvement in bit-error-rate (BER) performance. The RAKE requires knowledge of the channel response to perform the coherent combining operation and requires that all the paths be resolvable to take full advantage of the addi- tional energy. The equalizers discussed in this paper perform RAKE-like combining of multipath energy. Orthogonal frequency division multiplexing (OFDM) [I, 21 has received considerable attention as a method to efficiently utilize channels with non-flat frequency responses and/or non-white noise. In its most common form, a high rate data stream is divided up among the many carriers in the sys- *This workpartially supported by the Air Force Research Labora- tory, Air Force Materiel Command, USAF, under agreement number F30602-98-1-0054 and contract number F30602-98-C-0018 tem in a manner which optimizes the capacity of the over- all channel. OFDM can also be used as a spread spectrum modulation (OFDM-SS) [3, 4, 51 wherein spectral spreading is accomplished by putting the same data on all the carriers, producing a spreading factor equal to the number of carriers. At the receiver, the energy from all the carriers is coherently combined to produce the decision variable. Multiple users can be supported in the same channel through Code Ilivi- sion Multiple Access (CDMA) [6]. In this case each user has a unique signature sequence which determines the set of carrier phases. To receive a particular signal, the receiver needs to know the signature sequence for that user in order to align the carrier phases for the coherent combining opera- tion. Figure 1 is a block diagram of an OFDM-SS transmit- ter/receiver pair. As shown in the figure, carrier generation is usually performed efficiently using an inverse Fast Fourier Transform (FFT) while demodulation is performed using a forward FFT. Transmitter Lj Add - Inverse Channel Tx Data- - Signature Figure 1. Block Diagram of an OFDM Spread Spectrum System Spread Spectrum OFDM has many of the same proper- ties as DS-SS. In essence, the primary difference between the two systems is that DS-SS uses a binary spreading code, consisting of a sequence of 1’s and -l’s, while the OFDM-SS system uses a spreading waveform, consisting of a series of samples which have non-discrete amplitude values. Indeed, an OFDM-SS modulator can be constructed by storing the spreading waveform and using it to modulate the data in a manner similar to that used with a spreading sequence in DS-SS. The spreading waveform is clearly broadband like the DS-SS spreading sequence and, likewise, has an impulse-like autocorrelation function. Consequently, the OFDM signal is also tolerant of multipath and interference. Many times, OFDM systems employ guard times between 0-7803-4506-1/98/$10.00 0 1998 IEEE. 679