Spread-spectrum communications is a technique that spreads the signal over a bandwidth larger than the minimum required to transmit the information. This technique provides interference attenuation and is used in various applications, including antijam systems, code division multiple access (CDMA), and systems combating multipath. The paper provides a tutorial on the theory of spread-spectrum communications, discussing its applications, properties of spreading sequences, and techniques for acquisition and tracking. The technique involves spreading the signal using a code, which is independent of the data. The signal is then despread at the receiver using the same code. The key concept is the processing gain, which is the ratio of the dimensionality of the signal space to the dimensionality of the data signal. This gain allows the desired signal to have an effective power advantage over interference. The paper discusses different spreading techniques, including direct-sequence (DS), frequency-hopping (FH), and time-hopping (TH). It also covers the use of pseudorandom sequences for spreading, which have properties such as randomness, long periods, and difficulty in reconstruction. These sequences are generated using linear feedback shift registers (LFSRs) or other methods, and they are crucial for the performance of spread-spectrum systems. The paper also addresses the challenges of jamming in spread-spectrum systems. It explains how different types of jamming, such as broad-band noise and single-frequency tones, affect system performance. It discusses the importance of error-correction coding in mitigating the effects of jamming, especially in frequency-hopping systems. In the context of CDMA, the paper highlights the use of codes that are approximately orthogonal to minimize cross-correlation between users. The design of these codes is crucial for system performance, and the paper discusses the parameters that influence this design, such as cross-correlation and partial-correlation functions. The paper concludes by emphasizing the importance of spread-spectrum techniques in various applications, including military communications, mobile radio networks, and timing systems. It also notes the challenges of jamming and the need for robust coding and modulation techniques to ensure reliable communication in the presence of interference.Spread-spectrum communications is a technique that spreads the signal over a bandwidth larger than the minimum required to transmit the information. This technique provides interference attenuation and is used in various applications, including antijam systems, code division multiple access (CDMA), and systems combating multipath. The paper provides a tutorial on the theory of spread-spectrum communications, discussing its applications, properties of spreading sequences, and techniques for acquisition and tracking. The technique involves spreading the signal using a code, which is independent of the data. The signal is then despread at the receiver using the same code. The key concept is the processing gain, which is the ratio of the dimensionality of the signal space to the dimensionality of the data signal. This gain allows the desired signal to have an effective power advantage over interference. The paper discusses different spreading techniques, including direct-sequence (DS), frequency-hopping (FH), and time-hopping (TH). It also covers the use of pseudorandom sequences for spreading, which have properties such as randomness, long periods, and difficulty in reconstruction. These sequences are generated using linear feedback shift registers (LFSRs) or other methods, and they are crucial for the performance of spread-spectrum systems. The paper also addresses the challenges of jamming in spread-spectrum systems. It explains how different types of jamming, such as broad-band noise and single-frequency tones, affect system performance. It discusses the importance of error-correction coding in mitigating the effects of jamming, especially in frequency-hopping systems. In the context of CDMA, the paper highlights the use of codes that are approximately orthogonal to minimize cross-correlation between users. The design of these codes is crucial for system performance, and the paper discusses the parameters that influence this design, such as cross-correlation and partial-correlation functions. The paper concludes by emphasizing the importance of spread-spectrum techniques in various applications, including military communications, mobile radio networks, and timing systems. It also notes the challenges of jamming and the need for robust coding and modulation techniques to ensure reliable communication in the presence of interference.