Contents
What causes frequency selective fading?
Frequency selective fading occurs when the symbol length is shorter than the delay spread, or equivalently when signal bandwidth is larger than the channel bandwidth.
Why frequency of signal is different in spread spectrum?
The Frequency Hopping Spread Spectrum or FHSS signals are highly resistant to narrowband interference because the signal hops to a different frequency band. FHSS signals add minimal interference to narrowband communications, and vice versa. It provides a very large bandwidth.
How does a spread spectrum reduce interference?
Figure 1: In a spread-spectrum system, signals are spread across a wide bandwidth, making them difficult to intercept, demodulate, and intercept. The spread of energy over a wide band, or lower spectral power density, also makes spread-spectrum signals less likely to interfere with narrowband communications.
What are the advantages of spread spectrum technology?
The main advantage of spread spectrum communication technique is to prevent “interference” whether it is intentional or unintentional. The signals modulated with these techniques are hard to interfere and cannot be jammed. An intruder with no official access is never allowed to crack them.
Is frequency hopping used in WIFI?
Wi-Fi uses Direct Sequence Spread Spectrum (DSSS) instead of FHSS. Its carrier does not hop or change frequency and remains centered on one channel that is 22 MHz-wide. While there is room for 11 overlapping channels in this 83 MHz-wide band, there is only room for three non-overlapping channels.
How does the spread spectrum affect a signal?
A spread spectrum signal does not have a clearly distinguishable peak in the spectrum. This makes the signal more difficult to distinguish from noise and therefore more difficult to jam or intercept. This concept is illustrated in Figure 3.
How is direct sequence spread spectrum used in the industry?
We will focus on Direct Sequence Spread Spectrum technique since it is the mostly used in the industry (CDMA, UMTS, 802.11, GPS). Direct sequence spread spectrum (DSSS) introduces rapid phase transition to the data making it larger in bandwidth.
How is the rate of a frequency selective channel determined?
As above, the information bits u k ( k = 1, …, N) are encoded into coded bits x k, p ( k = 1, …, K and p = 1 …, P for a convolutional code of rate r = 1 ∕ P ). These coded bits are interleaved and split into n T sub-blocks, where n T is the number of antennas at the transmitter.
How is the power density of spread spectrum similar to the noise floor?
In fact, the power density amplitude of the spread spectrum output signal is similar to the noise floor. The signal is “hidden” under the noise. To get the signal back, the exact same high bandwidth signal is needed.