1st Module MCE

7/28/2019 1st Module MCE

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Frequency diversity:

The signal is transmitted using several frequency channels or spread overa wide spectrum that is affected by frequency-selective fading. Middle-late20th century microwave radio relay lines often used severalregularwideband radio channels, and one protection channel for automaticuse by any faded channel. Later examples include:

OFDM modulation in combination with

subcarrierinterleaving and forward error correction

Spread spectrum, for example frequency hopping orDS-CDMA.

Space diversity:

The signal is transmitted over several different propagation paths. In thecase of wired transmission, this can be achieved by transmitting viamultiple wires. In the case of wireless transmission, it can be achievedby antenna diversity using multiple transmitter antennas (transmit diversity)and/or multiple receiving antennas (reception diversity). In the latter case,a diversity combining technique is applied before further signal processingtakes place. If the antennas are far apart, for example at different cellularbase station sites or WLAN access points, this is called macrodiversity orsite diversity. If the antennas are at a distance in the order ofone wavelength, this is called micro diversity. A special case isphased antenna arrays, which also can be used forbeamforming, MIMO channels and spacetime coding (STC).

Fade Margin (fading margin) has the following meanings:

A design allowance that provides forsufficient system gain orsensitivity to accommodate expected fading,for the purpose of ensuring that the required quality of service ismaintained.

The amount by which a received signal level may be reduced withoutcausing system performance to fall below a specified threshold value. Itis mainly used to describe a communication system such as satellite, forexample a system like global staroperates at 25-35 dB Fade margin
http://en.wikipedia.org/w/index.php?title=Frequency_diversity&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Frequency_diversity&action=edit&redlink=1http://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Microwave_radio_relayhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/Interleavinghttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/Frequency_hoppinghttp://en.wikipedia.org/wiki/DS-CDMAhttp://en.wikipedia.org/wiki/Space_diversityhttp://en.wikipedia.org/wiki/Space_diversityhttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Transmit_diversityhttp://en.wikipedia.org/wiki/Reception_diversityhttp://en.wikipedia.org/wiki/Diversity_combininghttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Site_diversityhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/w/index.php?title=Microdiversity&action=edit&redlink=1http://en.wikipedia.org/wiki/Antenna_arrayhttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/MIMOhttp://en.wikipedia.org/wiki/Space%E2%80%93time_codinghttp://en.wikipedia.org/wiki/Space%E2%80%93time_codinghttp://en.wikipedia.org/wiki/Space%E2%80%93time_codinghttp://en.wikipedia.org/wiki/Systemhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Sensitivity_(electronics)http://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Satellitehttp://en.wikipedia.org/wiki/Globalstarhttp://en.wikipedia.org/wiki/Globalstarhttp://en.wikipedia.org/wiki/Satellitehttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Sensitivity_(electronics)http://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Systemhttp://en.wikipedia.org/wiki/Space%E2%80%93time_codinghttp://en.wikipedia.org/wiki/MIMOhttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Beamforminghttp://en.wikipedia.org/wiki/Antenna_arrayhttp://en.wikipedia.org/w/index.php?title=Microdiversity&action=edit&redlink=1http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Site_diversityhttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Macrodiversityhttp://en.wikipedia.org/wiki/Diversity_combininghttp://en.wikipedia.org/wiki/Reception_diversityhttp://en.wikipedia.org/wiki/Transmit_diversityhttp://en.wikipedia.org/wiki/Antenna_diversityhttp://en.wikipedia.org/wiki/Space_diversityhttp://en.wikipedia.org/wiki/DS-CDMAhttp://en.wikipedia.org/wiki/Frequency_hoppinghttp://en.wikipedia.org/wiki/Spread_spectrumhttp://en.wikipedia.org/wiki/Forward_error_correctionhttp://en.wikipedia.org/wiki/Interleavinghttp://en.wikipedia.org/wiki/OFDMhttp://en.wikipedia.org/wiki/Widebandhttp://en.wikipedia.org/wiki/Microwave_radio_relayhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/w/index.php?title=Frequency_diversity&action=edit&redlink=1


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SMALL - SCALE FADING

Small-scale fading refers to the dramatic changes in signal amplitude and phase thatcan be experienced as a result of small changes (as small as half wavelength) in thespatial position between transmitter and receiver.

In this section, we will develop the small-scale fading component r(t). Analysis proceedson the assumption that the antenna remains within a limited trajectory so that theeffect of large-scale fading m(t) is constant. Assume that the antenna is traveling andthere are multiple scatter paths, each associated with a time-variant propagation delayn(t) and a time variant multiplicative factor n(t). Neglecting noise, the receivedbandpass signal can be written as below

r (t)=nn(t)s(tn(t))(1)

Substituting Equation (1) in the module of Characterizing Mobile-Radio Propagation into

Equation (1) above, we can write the received bandpass signal as follow

r(t)=Re((nn(t)g(tn(t))ej2fc(tn(t)))(2)=Re((nn(t)ej2fcn(t)g(tn(t)))ej2fct

We have the equivalent received bandpass signal is

s(t)=nn(t)ej2fn(t)cg(tn(t))(3)

Consider the transmission of an unmodulated carrier at frequency fc or in other words,for all time, g(t)=1. then the received bandpass signal becomes

s(t)=nn(t)ej2fcn(t)=nn(t)ejn(t)(4)

The baseband signal s(t) consists of a sum of time-variant components havingamplitudes n(t) and phases n(t). Notice that n(t) will change by 2 radianswhenever n changes by 1/fc (very small delay). These multipath components combineeither constructively or destructively, resulting in amplitude variations or fading of s(t).Equation (4) is very important because it tell us that a bandpass signal s(t) is the signalthat experienced the fading effects and gave rise to the received signal r(t), theseeffects can be described by analyzing r(t) at the baseband level.

Small scale manifests itself in two mechanisms - time spreading of signal (or signaldispersion) and time-variant behavior of the channel (Figure 2). It is important todistinguish between two different time references- delay time and transmission time t.Delay time refers to the time spreading effect resulting from the fading channels non-optimum impulse response. The transmission time, however, is related to the motion ofantenna or spatial changes, accounting for propagation path changes that are perceivedas the channels time-variant behavior.


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Multipath Fading

MULTIPATH is simply a term used to describe the multiple paths a radio wave may

follow between transmitter and receiver. Such propagation paths include the ground

wave, ionospheric refraction, reradiating by the ionospheric layers, reflection from

the earths surface or from more than one ionospheric layer, and so on. Figure 1

shows a few of the paths that a signal can travel between two sites in a typical

circuit. One path, XYZ, is the basic ground wave. Another path, XFZ, refracts the wave

at the F layer and passes it on to the receiver at point Z. At point Z, the received

signal is a combination of the ground wave and the sky wave. These

two signals, having traveled different paths, arrive at point Z at different times.

Thus, the arriving waves may or may not be in phase with each other. A similarsituation may resultant point A. Another path, XFZFA, results from a greater

angle of incidence and two refractions from the F layer. A wave traveling

that path and one traveling the XEA path may or may not arrive at

point A in phase. Radio waves that are received in phase reinforce each other

and produce a stronger signal at the receiving site, while those that are


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received out of phase produce a weak or fading signal.

Small alterations in the transmission path may change the phase relationship of

the two signals, causing periodic fading.

Figure 1Multipath transmission.

Multipath fading may be minimized by practices called SPACE DIVERSITY and

FREQUENCY DIVERSITY In space diversity, two or more receiving antennas are

spaced some distance apart. Fading does not occur simultaneously

at both antennas. Therefore, enough output is almost always available from one of

the antennas to provide a useful signal. In frequency diversity, two transmitters and

two receivers are used, each pair tuned to a different frequency, with the same

information being transmitted simultaneously over both frequencies. One of the

two receivers will almost always produce a useful signal.

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1st Module MCE