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.