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8/16/2019 Basic RF Cellular Principles
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Basic RF Cellular Principles
1.1 Decibels
1.2 Antenna Theory
1.3 Diversity1.4 Fresnel Zone
1.5 Fading
1.6 Delay Spread
1. Fre!"ency #e"se
1.$ #F %ropagation %rediction
1.& Signal Strength and %ath 'alance
1.1( )den *od"lation
1.11 )den Architect"re
1.12 +andover Algorith,
1.13 -overage *eas"re,ent
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1.1 Decibels
1.1.1 The decibel (dB) provides a means of representing large power ratios asmanageable small numbers and allows the overall gains and losses to be
calculated by addition and subtraction. The notation is also used to express signal
level, but the value is meaningless unless a reference is considered. The usualreference in performance engineering is dBm, meaning dB above one milliwatt
(mW) a negative number of dBm means decibels below one mW. !ther typical
references are dB above the gain of a 1"#$ dipole antenna dBd and gainreferenced to an isotropic radiator dBi.
1.1.# % circuit having either amplification or attenuation is said to have a power gaincalculated as follows&
dB ' 1 log (ower out"ower in)
*xample& % properly terminated transmission line is driven with #+watts. The terminated end measures 1 watts. The power gain of the
transmission line is&
dB ' 1 log (1"#+) ' -1. dB
*xample& %n /0 exciter delivers watts to a power amplifier with a rated
output of watts. The power gain of the amplifier is&
dB ' 1 log (") ' 11.2 dB
1.1.3 The formula for dBm is&
dBm ' 1 log (ower"1 mW)
*xample& %n amplifier with a power gain of 2 watts is said to have a power gain in dBm of&
dBm ' 1 log (2w"1 mW) ' 24dBm
1.1.2 5f the dB value is 6nown, then the power ratio (pr) may be calculated by solving
the e7uations above bac6wards.
pr ' 1 (dB"1)
*xample& %n amplifier has a specification for 3dB gain. The pr for theamplifier is&
pr ' 1(3"1) ' 1
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1.1. The output power level (o) can be found by multiplying the input power level
(i) by the pr&
i x pr ' o
*xample& %n amplifier with a 1mW input level has a pr of 1. Theoutput power level is&
1mW x 1 ' 1 watt
1.1.4 This can be chec6ed by use of the first e7uation given&
dB ' 1 log (1W"1mW) ' 3 dB
1.1. The power ratio of two voltage or current measurements must be measured in the
same value of impedance and is calculated as follows. The result is an expressionof power, not an expression of voltage.
dB ' # log (81"8#) 9:
dB ' # log (51"5#) 94:
*xample& Two carriers are measured with e7uipment calibrated in
microvolts (u8). ;arrier % measures 12u8 and ;arrier B measures 3+u8.
The power ratio of the two measurements is&
dB ' # log (3+u8"12u8) ' +. dB
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1.2 Antenna Theory
1.#.1 The following discussion is a very brief view of antenna theory. 8olumes have been printed on the subraus and
?aport. @ertA was the first to record the use of an antenna in his communicationexperiments of 1++2. ome 114 years later, the basic theory remains the same but
the application of the theory continues to expand.
1.#.# /adio communication is accomplished by the transmission and reception of
electromagnetic waves that are propagated between two locations by the
phenomenon of electromagnetic radiation. The transmitting antenna is made upof a system of conductors that converts /0 energy developed by a transmitter to a
wave field that is radiated into surrounding space (often assigned an impedance of
3 ohms). The waves originate as a disturbance at the antenna and are
propagated by detached electromagnetic fields that travel through the air at the
speed of light. The velocity of the wave is often given as 1+4,2 miles persecond. The receive antenna absorbs (intercepts) energy from the passing wave
field and directs the energy to the receive circuitry.
1.#.3 The transmitting antenna emits one wave for each period, or a total number of
waves per second e7ual to the fre7uency. The wavelength of the wave is&
$ ' velocity"fre7uency
*xample& The transmitter fre7uency is specified as +42. =hA. Thewavelength in feet per second is calculated as&
$ ' (1+4,2 mi"sec x #+ ft"mi)"+42. million cycles"sec
' 1.12 ft"cycle
1.#.2 5sotropic radiators and simple wire antennas are often described in detail to
develop a mathematical understanding of the CantennaD. pherical coordinate
systems, current and voltage distribution and field e7uations are provided leadingto descriptions of practical antenna systems. We will assume that the reader can
independently refresh to the level of desired detail as we charge into the practical
antennas employed in cellular systems.