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Spread Spectrum Communications: Fundamentals, Applications, and Products
6. Important Terms
1. Spread Spectrum: Spread spectrum is a technique that spreads the bandwidth of the desired signals over a very wide band of frequencies by modulating the data. At the receiving end, the original data is recovered by de-spreading the modulated signals.
2. Baseband: The band of frequencies that the original data occupies.
3. Carrier: The frequency of a modulating signal or a local oscillator that is used to convert the base band signals to a higher frequency to transmit over a medium, such as air or cable.
4. Data rate: The rate of transmission of data bits. This is typically measured in bits/sec, or Kbps/sec or Mbps/Sec, or Gbps/Sec.
5. Chip rate: The rate of transmission of code bits. The term "chip" is used to distinguish the code bits from that of data bits.
6. PN Sequence: As the name implies, this is a pseudo code that satisfies a given set of mathematical requirements such as auto-correlation, cross-correlation properties.
7. PN Generator: A mechanism that generates a PN sequence.
8. DS-SS: Short of Direct Sequence Spread Spectrum (also represented as DSSS).
9. FH-SS: Short for Frequency Hopping Spread Spectrum (also represented as FHSS).
10. TH-SS: Short for Time Hopping Spread Spectrum (also represented as THSS)
11. Auto Correlation: It is the correlation of a received signal with itself. Normally, one component of the spread spectrum signal is received from a transmitter, and compared with a locally generated spread signal. For faithful reproduction of received spread spectrum signals, autocorrelation must meet the minimum threshold requirements.
12. Cross Correlation: Imagine more than one spread spectrum transmitter sending SS modulated signals. Obviously, they can't use the same code sequence. They need to use distinct (or unique) code sequences for faithful reproduction at respective receivers. Cross correlation is a measure of dissimilarity (or similarity) between two distinct SS signals using different PN sequences. Obviously, cross-correlation should be minimum between any two different PN sequences. A good cross-correlation is achieved by using orthogonal PN sequences.
13. Acquisition: This is the process of acquiring the received PN sequence. During this process, the locally generated PN sequence is time shifted to match the timing of the received PN sequence.
14. Tracking: This is the process of tracking the received PN sequence signals. Normally, even after acquisition of PN signals, the receiver may loose lock due to drift in the chip timing. The tracking loop generates a tracking voltage (also called error voltage) to make any corrections to the chip rate so that the locally generated chips are in phase with the received signals.
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