Describe the operation of a direct-sequence spread spectrum (DSSS) system.
1 Answer
A Direct-Sequence Spread Spectrum (DSSS) system is a digital communication technique that involves spreading the bandwidth of a transmitted signal over a wider frequency range than its original bandwidth. This is achieved by multiplying the data signal with a spreading code, which is a pseudo-random binary sequence. The purpose of this spreading is to enhance the signal's resistance to various forms of interference, such as narrowband interference, multipath fading, and jamming.
Here's how a DSSS system operates:
Spreading Code Generation: The first step involves generating a spreading code at both the transmitter and the receiver. This spreading code is a binary sequence with a much higher bit rate than the actual data signal. The code is typically a pseudo-random sequence that appears random but is actually deterministic and known to both the transmitter and the receiver.
Data Modulation: The original data signal, which carries the information to be transmitted, is typically a digital bit stream. Each bit of the data signal is then multiplied (modulated) with a corresponding bit from the spreading code. This operation is often referred to as "chipping" since it involves multiplying the data bit with a chip from the spreading code.
Signal Spreading: The result of the modulation is a signal where each data bit is spread across multiple chips according to the spreading code. This spreads the energy of the signal across a wider frequency range, effectively increasing the bandwidth.
Transmitter Operation: The spread data signal is then transmitted over the communication channel. Because the signal is spread across a larger bandwidth, it appears as noise-like interference to unintended receivers or jammers that do not know the spreading code. This makes it difficult for such unauthorized receivers or jammers to extract the original data.
Receiver Operation: At the receiver end, the received spread signal is correlated with the same spreading code used at the transmitter. This correlation process effectively extracts the original data bits from the received signal. The receiver knows the correct spreading code, allowing it to "despread" the signal and recover the original data.
Demodulation: After the despreading process, the recovered data signal is demodulated using a process that is the inverse of the modulation process at the transmitter. This recovers the original digital data stream.
DSSS offers several advantages:
Interference Resistance: The spread signal looks like noise to unintended receivers, making it resilient against narrowband interference and jamming.
Multipath Fading Mitigation: The spreading effect can help combat multipath fading by distributing the signal energy over a wider frequency range.
Security: Since the spreading code is known only to the authorized parties, DSSS communication is relatively secure against eavesdropping.
DSSS systems are commonly used in applications where robustness against interference and security are crucial, such as military communications, wireless LANs (Wi-Fi), and certain satellite communication systems.
Here's how a DSSS system operates:
Spreading Code Generation: The first step involves generating a spreading code at both the transmitter and the receiver. This spreading code is a binary sequence with a much higher bit rate than the actual data signal. The code is typically a pseudo-random sequence that appears random but is actually deterministic and known to both the transmitter and the receiver.
Data Modulation: The original data signal, which carries the information to be transmitted, is typically a digital bit stream. Each bit of the data signal is then multiplied (modulated) with a corresponding bit from the spreading code. This operation is often referred to as "chipping" since it involves multiplying the data bit with a chip from the spreading code.
Signal Spreading: The result of the modulation is a signal where each data bit is spread across multiple chips according to the spreading code. This spreads the energy of the signal across a wider frequency range, effectively increasing the bandwidth.
Transmitter Operation: The spread data signal is then transmitted over the communication channel. Because the signal is spread across a larger bandwidth, it appears as noise-like interference to unintended receivers or jammers that do not know the spreading code. This makes it difficult for such unauthorized receivers or jammers to extract the original data.
Receiver Operation: At the receiver end, the received spread signal is correlated with the same spreading code used at the transmitter. This correlation process effectively extracts the original data bits from the received signal. The receiver knows the correct spreading code, allowing it to "despread" the signal and recover the original data.
Demodulation: After the despreading process, the recovered data signal is demodulated using a process that is the inverse of the modulation process at the transmitter. This recovers the original digital data stream.
DSSS offers several advantages:
Interference Resistance: The spread signal looks like noise to unintended receivers, making it resilient against narrowband interference and jamming.
Multipath Fading Mitigation: The spreading effect can help combat multipath fading by distributing the signal energy over a wider frequency range.
Security: Since the spreading code is known only to the authorized parties, DSSS communication is relatively secure against eavesdropping.
DSSS systems are commonly used in applications where robustness against interference and security are crucial, such as military communications, wireless LANs (Wi-Fi), and certain satellite communication systems.