Spreading code, also known as spreading sequence or spreading code sequence, is a fundamental concept in spread spectrum communication systems. Spread spectrum techniques are used to enhance the performance and security of wireless communication systems by spreading the signal across a wider frequency band. Spreading codes play a crucial role in achieving this goal.
Here’s how spreading codes work:
- Spread Spectrum Techniques: Spread spectrum communication techniques involve spreading the information signal over a larger bandwidth than is strictly necessary for communication. This makes the signal more resilient to interference, noise, and jamming, and also provides security against eavesdropping.
- Direct Sequence Spread Spectrum (DSSS): In DSSS, the information signal is multiplied (modulated) by a spreading code before transmission. This multiplication causes the signal to spread across a wider frequency band. The receiver then uses the same spreading code to despread the received signal, recovering the original information.
- Frequency Hopping Spread Spectrum (FHSS): In FHSS, the transmitter and receiver hop between different frequency channels according to a predetermined hopping sequence. This hopping pattern is often synchronized using a spreading code. This technique provides robustness against narrowband interference and fading.
- Orthogonal Spreading Codes: Spreading codes are carefully designed to be orthogonal to each other. Orthogonal codes exhibit low cross-correlation, which means that they can be separated efficiently at the receiver even when transmitted simultaneously.
- Pseudorandom Codes: Spreading codes are often generated using pseudorandom number generators (PRNGs). These codes appear random but can be reproduced at the receiver using the same PRNG.
- Code Properties: Desirable properties of spreading codes include good autocorrelation and cross-correlation characteristics. Autocorrelation properties ensure that the signal can be easily detected, while low cross-correlation reduces interference between different users in a multi-user system.
- Chip Rate: The rate at which the spreading code changes is known as the “chip rate.” This is typically much higher than the data rate of the information signal.
- Application: Spreading codes are used in various wireless communication systems, including CDMA (Code Division Multiple Access) cellular networks, satellite communication, and GPS navigation systems.
In summary, spreading codes are essential elements in spread spectrum communication systems. They enable the signal to be spread over a wider bandwidth, providing benefits such as increased resistance to interference, enhanced security, and the ability to support multiple users in the same frequency band.