Link Budget Calculation Example for UMTS

Tuesday, July 10, 2012

Link Budget Calculation Example for UMTS

 Three sectored base station site

 Uplink is designed for 64 kbit/s service with 3dB noise rise ( 50% cell loading)

 Downlink for 128 kbit/s with 6 dB average noise rise (75% cell loading) Uplink Dense Urban Link Budget for Indoor 64 kbit/s Coverage

The link budgets parameters are used to calculate the maximum allowable pathloss for the specified conditions at the edge of the cell. Maximum isotropic pathloss,

 Maximum Uplink Isotropic Pathloss

Max = - NthW + Tottal effective Gain - Mean Noise Rise - average Eb/No +

Processing Gain + Soft Handoff Gain + Power for UE DL connection - Shadow Fade Margin

so, maximum uplink isotropic pathloss = 127.8 dBm  Maximum Downlink Isotropic Pathloss

Max = - NthW + Tottal effective Gain - Mean Noise Rise - average Eb/No +

Processing Gain + Soft Handoff Gain + Power for UE DL connection - Shadow Fade Margin

so, maximum downlink isotropic pathloss = 129.8 dBm

Propagation Models

Propagation models provide a mathematical formula that can be solved to provide the cell range. Due to the deterministic nature of the models they are generally only valid over a certain range and therefore cannot be generally applied to all situations. For UMTS modelling there are two main models.

 Modified Hata COST231 for cell ranges over greater than 1km.  COST 231 Walfish-Ikegami for cell ranges less than 1km

Modified Hata COST231 Propagation Model ( > 1 km)

The basic propagation model used for UMTS is the COST 231 Hata model for frequencies above 1500MHz. This model is detailed in the ETSI GSM specification TR 101 362 V6.0.1 (1987-07). While Hata’s standard equations are for use up to 1000MHz, COST 231 modifies Hata’s equations to cover propagation losses for systems operating between 1500 and 2000 MHz. Also, strictly speaking, Hata’s model is basically for cell ranges greater than 1km and therefore, an alternative propagation model is included for cell ranges below 1km.

Definitions for Equations

• f = frequency (MHz)

• Hm = mobile station antenna height (m)

• Hb = base station antenna height (m)

• d = distance (km)

a. Urban Area : COST 231 – Hata Model

Propagation loss,

Lp = 46.3 +33.9 Log (f) – 13.82 Log (Hb) – A (Hm) + {44.9 – 6.55Log (Hb)} * Log (d) + Cm

Correction factor, Cm = 0dB (for medium sized city areas) = 3dB (for high density urban areas)

Mobile correction factor, A (Hm) = {1.1Log (f) – 0.7} * Hm - 1.56 Log (f) – 0.8

b. Suburban Area: COST 231 – Hata Model

Pathloss is Lp – Lps, where

Lps = - 2Log2 { f / 28 } – 5.4

c. Rural Area: COST 231 – Hata Model

Pathloss is Lp – Lpr, where

Lpr = - 4.78Log2 (f) + 18.33Log(f) – 35.94

d. Open Rural Area: COST 231 – Hata Model

Pathloss is, Lp – Lpo, where

Lpo = - 4.78Log2 (f) + 18.33Log(f) – 40.94

Hata COST231 Propagation Model Example

The following example is based on using a macro base station. The assumptions made for the macro base station are provided:

Uplink frequency of 1940MHz,

Downlink frequency 2140MHz,

1.5m mobile antenna height,

25m base station antenna height for all environments. Using these numbers, the above equation for Lp yields the following simple equations for each environment:Error: Reference source not found

Lp refers to the propagation loss between the base station antenna and the mobile station antenna and can be calculated from the link budget. ’d’ is the distance between the UE and Node B antennae. Thus once the propagation loss is calculated for a certain environment, the variable ‘d’ can be obtained to

establish the maximum range of the cell.

The propagation loss Lp must be derived for each specific case using a standard link budget calculation. These equations may be used for macro cells only and not micro or pico cells. In the propagation loss table, the COST 231 Hata equation for Dense urban environments is:

Lp = 138.5 + 35.7 * Log (d)

Substituting Lp for the Uplink Pathloss of 127.8dB from Table 3: Uplink Cell Range (d) = 0.50 km

Therefore, in a dense urban environment, for 64 kbit/s service, the maximum indoor range of a tri-sectored cell with 50% mean loading is 0.50 km.

COST 231 Walfish-Ikegami Propagation Model

This model provides a method of estimating small cell ranges (<1km), in dense metropolitan urban areas, where there is no line of sight between the Node B and UE. This model has been simplified slightly for the purpose of this document, an explanation of which will be given as the equations are defined.

COST 231 Walfish-Ikegami Propagation Model Pathloss, Lb = Lo + Lrts + Lmsd

Free space loss, Lo = 32.4 + 20 * log( d ) + 20 * log( f ) Roof top to street diffraction and scatter loss;

Lrts = -16.9 – 10 * log( w ) + 10 * log( f ) + 20 * log ( Hr – Hm ) + Lcri Lcri = -10 + 0.354 * Phi { 0 <= phi < 35 deg)

Lcri = 2.5 + 0.075 * ( Phi -35) { 35 <= phi < 55 deg) Lcri = 4.0 - 0.114 * ( Phi – 55 ) { 55 <= phi < 90 deg) Multiscreen diffraction loss;

Lmsd = Lbsh + 54 +18 * log( d ) – 2.34 * log( f ) – 9 * log( b ) Lbsh = -18 * log( 1 + Hb – Hr )

Where;

d = cell range (to be calculated)

f = frequency (UL = 1940MHz, DL = 2150MHz) w = width of road (assume 20m)

Hr = height of roof (assume 20m) Hm = height of mobile (assume 1.5m)

level)

Phi = road orientation with respect to direct radio path (assume 45 deg) b = building separation (assume 30m)

The model has only been considered for cases where the Node B antenna is above the average building height to simplify the examples shown. All the terms in the equation for Lmsd have variable values depending upon antenna height relative to roof height.

Therefore, for Uplink dense urban environment;

Lo = 32.4 + 20 * log (1940) + 20 * log (d) = 98.2 + 20 * log (d)

Lrts = -16.9 – 10 * log (20) + 10 * log( 1940 ) + 20 * log( 18.5 ) + Lcri Lcri = 2.5 + 0.75 = 3.25

Therefore, Lrts = 31.55

Lmsd = -18*log(11)+54+18*log(d) – 2.34 * log (1940) – 9 * log (30) = 14.27 + 18 * log (d)

Therefore, Lb = Lo + Lrts + Lmsd

Uplink Pathloss, Lb = 98.2 + 20 * log (d) + 31.55 + 14.27 + 18 * log (d) = 144.0 + 38 * log (d)

As before, with the Hata Model, the pathloss calculated from the link budget can be substituted

into this equation in order to estimate the maximum cell range. Uplink Dense Urban Pathloss = 127.8dB

Therefore, 127.8 = 144.0 + 38 * log( d ) Uplink cell range, d = 0.37 km

This alternative (COST 231 Walfish-Ikegami) model has produced a cell range of 0.37km compared to the range of 0.50km obtained with the COST 231 Hata model. It is recommended that the Hata model is used only for Suburban and rural environments where cell ranges greater than 1km are predicted.