tailieunhanh - Lecture Wireless and mobile computing – Chapter 22: Spread spectrum (Review/Recap)

The following will be discussed in this chapter: Relationship between bandwidth of a signal (before and after and encoding SS), benefits of SS, FHSS, slow and fast FHSS, DSSS, relationship between bit rate of a signal (before and after DSSS encoding), CDMA. | Spread Spectrum Review/Recap Lecture 22 Overview Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal (Before and after DSSS encoding) CDMA Bandwidth Before and After SS Encoding Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum? Definition of Spread Spectrum Spread spectrum is a modulation method applied to digitally modulated signals that increases the transmit signal bandwidth to a value much larger than is needed to transmit the underlying information bits. Spread Spectrum Signal Characteristics They are difficult to intercept for unauthorized person. They are easily hidden, it is difficult to even detect their presence in many cases. They are resistant to jamming. They have an asynchronous multiple-access capability. They provide a measure of immunity to distortion due to multipath propagation. Spread Spectrum Conditions The signal occupies a bandwidth much larger than is needed for the information signal. The spread spectrum modulation is done using a spreading code, which is independent of the data in the signal. Despreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code. Processing Gain : The spread spectrum increases the bandwidth of the message signal by a factor N, called the processing gain where β is the message signal bandwidth, βss is the corresponding SS signal bandwidth. , N > 1 Spread Spectrum Versus Narrowband Technology Narrowband wireless communications can be defined as wireless communications using a single frequency center with no redundancy to communicate information at high power levels chosen to overpower interference in that frequency band. Spread spectrum wireless communications can be defined as wireless communications using a range of frequencies to communicate | Spread Spectrum Review/Recap Lecture 22 Overview Relationship between bandwidth of a signal (before and after and encoding SS) Benefits of SS FHSS Slow and Fast FHSS DSSS Relationship between Bit Rate of a Signal (Before and after DSSS encoding) CDMA Bandwidth Before and After SS Encoding Q:- What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum? Definition of Spread Spectrum Spread spectrum is a modulation method applied to digitally modulated signals that increases the transmit signal bandwidth to a value much larger than is needed to transmit the underlying information bits. Spread Spectrum Signal Characteristics They are difficult to intercept for unauthorized person. They are easily hidden, it is difficult to even detect their presence in many cases. They are resistant to jamming. They have an asynchronous multiple-access capability. They provide a measure of immunity to distortion due to multipath .