Why Need Link Budget Analysis In Gsm 526m3c

Why link budget analysis in GSM When we talk about GSM Rf planning then first thing into mind is Link Budget,here i write about why link Budget analysis in GSM Link budget analysis provides:

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Coverage design thresholds

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EIRP needed to balance the path

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Maximum allowable path loss

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It is important that the uplink and downlink paths be balanced, otherwise not enough signal will survive the transmission process to achieve the required signal to noise ratio (SNR) or the bit-error-rate (BER).

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Path imbalance results from the facts that the gains and losses in the uplink and downlink paths are not the same.

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The calculations have to be done separately on the uplink and the downlink.

The RF Path

INPUTS

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Base station and Mobile receiver Sensitivity Parameters

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Minimum acceptable Signal to Noise ratio

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Environmental / Thermal Noise

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Receiver Noise figure

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Antenna gain at the base station and mobile station.

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Hardware Losses (Cable , Connectors, Combiners, Duplexers etc)

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Target Coverage reliability.

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Fade margins.

OUTPUTS

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Base station ERP

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Maximum allowable path loss

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Cell size estimates

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Cell count estimates

Effect of Antenna Gain and Diversity Gain in GSM RF Gain is most important thing in wireless communication and if its ive then it will be very useful here I write about antenna gain and diversity gain definition and its effect. Antenna Gains Mobile Station Antenna

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Portable mobile phones antennas have typically gain of 0 to 1 dBd.

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Car mounted antenna has a typical gain of 1 to 3 dBd.

Base Station Antenna

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Omni directional antennas typically have a gain of 0-9dBd.

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Directional antennas typically have a gain of 9 to 14 dBd.

Diversity Gain

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Diversity is used on the uplink to overcome deep fades due to multipath by combining multiple uncorrelated signals.

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Diversity antenna systems are used mostly at the BTS on the uplink.

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Diversity antenna system can be realised by physically separating two receive antenna in space or by using polarization diversity.

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Diversity gain should be considered in Link Budget Analysis whenever it is used.

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Typically a gain of 3dB is considered whenever diversity is used in the Uplink calculation.

Value of Cable, Connector and Combiner Loss When talk about Practical RF Planning then its also needs to take care about losses and physical losses like Cable,Connector and Combiner at BTS is more important. Cable Loss

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Two types of cables are used, main cable and jumper cable.

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Cable losses are given in per 100feet.

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Jumper cable has more loss than main cable.

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Cable loss is also dependant on frequency

Connector Loss

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Connectors used to connect RF components have a typical loss of 0.1dB each.

Combiner Loss

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A combiner is a device that enables several transmitters of different frequencies to transmit from the same antenna.

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Two types of combiners are available.

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Hybrid combiners combine two inputs to one output.

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Hybrid combiners have a typical insertion loss of 3dB.

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Cavity combiners combine more input to one output ( typically 5 inputs)

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Cavity combiners have around 3dB loss.

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Cavity combiners cannot be used in cells where synthesizer frequency hopping is used.

Losses by Duplexer, Body And Penetration In GSM System Losses is different type here i write about Losses by Duplexer, Body And Penetration Duplexer Loss

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A duplexer enables simultaneous transmission and reception of signals on the same antenna.

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It provides isolation between the transmitted and received signal.

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Duplexers typically have a insertion loss of 0.5 to 1 dB

Body Loss

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For all receiving environments a loss associated with the effect of s body on propagation has to be used I.e. proximity of the with the mobile.

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This effect is in the form of few dB losses in both the uplink and downlink directions.

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Body loss is typically taken as 2 dB.

Penetration losses

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Penetration losses depend on the location of the subscriber with respect to the site.

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Generally 3 types of scenarios are taken into consideration viz. In-building, In-car and on street.

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Body loss is also a type of penetration loss.

Fade Margin Calculation in GSM As previously I write about fading effect in GSM here I write about Fade Margin Calculation in GSM.

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Cell area probability (CAP ) is the percentage of the cell area that has signal strength greater than the receiver sensitivity.

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CAP is dependent on the radio environment, primarily the standard deviation of the log normal faded signal (s) and the propagation loss constant (n)

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The CAP is calculated using the following equaion

PCA=½ ( 1+ erf (a) + exp (2ab+1/b2)(1 – erf(ab+1/b))) Where: PCA = Cell area probability A = Mfade/s B = 10nLog10(e) / sÖ2 MFADE = Fade margin applied s = Standard deviation of received signal N = Propagation constant Outdoor Fade Margin

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The outdoor fade margin depends on the standard deviation of the lognormal shadowing and the propagation constant

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The propagation constant depends on the environment and the frequency.

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For urban areas propagation constant varies from 2.7 to 5 , with a typical value of 5 for both 850 Mhz and 1900 Mhz.

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Standard deviation also varies on environment and frequency , and may vary slightly with frequency.

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The urban areas have higher standard deviation than rural areas.Typical value ranges from 5-12dB with a typical value of 8dB

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Outdoor fade margin can be calculated using a plot of the CAP equation.

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The next figure shows the CAP plot for a propagation constant of 3.5 and standard deviation of 5, 8 and 12.

From the figure fade margin to be applied to the Link Budget may be selected depending on the standard of the received signal.

Receiver sensitivity and Uplink Losses for GSM

Receiver sensitivity is the ability of the receiver to receive signals in the sense that any signal below the sensitivity is considered as noise and is not usable. Receiver sensitivity is given by S = Antenna Noise (dBm) + Receiver Noise Figure (dB) + C/N (dB) S = the receivers sensitivity C/N = Carrier to noise ration required in the presence to achieve a specified BER. Antenna Noise (dBm) = 10log (kTB) Where k = Boltzmann constant 1.38 X 10-20 milli Joules / Kelvin T = Room temperature in degrees kelvin B = Bandwidth in Hz Uplink Losses

UPLINK

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Mobile Transmit power

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Mobile antenna gain

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Body Loss

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Fade Margin

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Receive antenna gain

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Cable loss(includes jumper and connector loss)

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BTS receiver sensitivity

DOWNLINK

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Transmitter power

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Combiner loss

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Cable loss(includes jumper and connector loss)

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Transmit Antenna gain

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Fade margin

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Body loss

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Mobile antenna gain

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Mobile receiver sensitivity

Hata´S Empirical Formula And Cell Size Estimation Hata Model is applied for GSM Link Budget,here i write about its formula and Cell Size Estimation. Hata´S Empirical Formula PL = 69.55 +26.6log10fc - 13.82log10hb + (44.9 – 6.55log10hb)log10R – a(hm) -CF Where , fc – Frequency in MHZ

hb – Transmitter antenna height hm – Receiver antenna height R – Radius in Km a(hm) is the correction factor for effective mobile antenna height Solving backwards the cell radius is given by log10R = MAPL +CF – 69.55 +26.6log10fc + 13.82log10hb + a(hm) / (44.9 – 6.55log10hb) Cell Size/Count Estimation

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Once the Maximum allowable path loss is known, the achievable cell size can be evaluated.

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Cell radius is calculated using MAPL and Hata’s empirical formula.

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Cell radius is the distance from base station where the path loss equals MAPL. Beyond this radius, the signal is too weak to be acceptable.

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Each area has a different correction factor.

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Also the coverage objectives are usually different for Urban, Suburban and Rural areas.

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Therefore MAPL has to be calculated for each area and then cell size determined separately.

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Once the cell radius is calculated, cell count estimates can be made.

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Once the cell radius for each area is calculated, then the minimum number of cells required to provide coverage can be determined.

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For each area

A = 2.6R2 Where, 1.

R – radius of cell

2.

A – Area of the corresponding hexagon.

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Cell count = Urban Area(Km2) + Suburban area(Km2) + RuralArea(Km2)

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Cell Count = Aurban(Km2) + Asuburban(Km2) + Arural(Km2)