Chapter 5(Wireless Communication)

What is direct sequence spread spectrum?

For DSSS, each bit in the digital signal is represented by mu;tiple bits in the transmitted signal

What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum?

The bandwidth is wider after the signal has been encoded using spread spectrum

What information is available from radiation pattern?

A radiation pattern is a graphical representation of the radiation properties of an antenna as a function of space coordinates.

What two key elements comprise error control?

Detection of errors and retransmission of frames that are received in error

What is frequency hopping spread spectrum?

With FHSS, the signal is broadcast over random series of radio frequencies, hopping from frwquency to frequency at fixed intervals

Indicate three major advantages of digital transmission over analog transmission

1.Digital signals do not get corrupted by noise 2.Useless bandwidth 3.Digital can be encrypted so that only the intended receiver can decoded it

List three benefits of spread spectrum

1.Spread spectrum signals are highly resistant to the jamming. 2.Many users can share a signal band with no interference 3.Unauthorized listening is prevented

What is CRC?

A cyclic redundancy check (CRC) is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.

What is a parity bit?

An extra code element used to detect errors in received data.

What is an isotropic antenna?

An isotropic antenna is a point in space that radiates power in all directions equally.

How are binary value represented in amplitude shift keying?

Binary values 0 and 1 are represented as low and high voltages.When the value is 1, the switch is closed and signal pass through and when value is 0, the switch is closed

What is CDMA?

Code Division Multiple Access is multiplexing technique used with spread spectrum to share the wireless medium

What is the difference between diffraction and scattering?

Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave. The edge in effect become a source and waves radiate in different directions from the edge, allowing a beam to bend around an obstacle. If the size of an obstacle is on the order of the wavelength of the signal or less, scattering occurs. An incoming signal is scattered into several weaker outgoing signals in unpredictable directions.

What is the difference between fast and slow fading?

Fast fading refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a small distance of about one-half a wavelength. Slow fading refers to changes in signal strength between a transmitter and receiver as the distance between the two changes by a larger distance, well in excess of a wavelength.

What is the difference between flat and selective fading?

Flat fading, or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously. Selective fading affects unequally the different spectral components of a radio signal.

Explain how Go-back-N ARQ works

In Go-back-N ARQ , the frames are numbered sequantially, as Go-back-N ARQ sends the multiple frames at the time that requires the numbering approach to distinguish the frame from another frame

Briefly define OFDM, OFDMA, and SC-FDMA

Orthogonal frequency division multiplexing splits a signal into manu lower bit rate streams that are transmitted over precisely spased frequencies. Orthogonal frequency division multiple access uses a combination of FDMA and TDMA by allowing different users to use a subset of the subcarriers at different times. Single-carrier FDMA is relatively recently developed multiple access technique which has similar structure and performance to OFDMA

What is QAM?

Quadrature Amplitude Modulation is a popular analog signaling technique that is used in some wireless standards.

Why would oyu expect a CRC to detect more errors than a parity bit?

The CRC has more bits and therefore provides more redundancy.It provides more information that can be used to detect erros

What is fading?

The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s).