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III. 4. Radio Link Power Budget

Transmission in uplink and downlink of cellular systems is asymmetric, since the BS transmitter typically uses much higher power than the MS transmitter. However, the transmission quality in uplink and downlink should be equal, especially near the cell edge. Both speech and data services in cellular systems are dimensioned for equal transmission quality in both directions. Transmission quality in uplink and downlink can be determined from the link budget. The terms in the link budget are defined with aid of the block diagram in the adjoining figure.

Where

PBS-tx is the output power level of the base station

Ac is the base station transmitter combing filter loss (dB),

LBS-f1 is the Base station antenna feeder loss (dB),

GBS is the base station antenna gain (dBi, relative to an isotropic radiator),

LC is the radio path loss between isotropic antennas (dB),

GMS is the mobile station gain (dBi),

LMS-f is the mobile station antenna feeder loss (dB),

Ldpx-rx is the mobile station duplex filter loss in the downlink direction (dB).

Ldpx-tx is the duplex filter loss in the uplink direction (dB),

Gdiv is the base station diversity gain (dB),

PMS-rx is the received power level in the mobile station input terminal (dBm),

PBS-rx is the received power level in the base station input terminal (dBm).

The following Figure shows a pictorial example of a radio link power budget. In addition to the link budget of the transmitted signal, it also illustrates noise at the receiver. The noise has power NoB at the input of the receiver, and it is amplified by the noise figure (NF) of the receiver.

The abbreviation FM stands for fading margin for slow fading. Hence, the signal-to-noise ratio(SNR)in the figure corresponds to the SNR that is encountered, when the receiver is shadowed by an obstruction. The average SNR is better that this value. The SNR during a fade should still be larger than the minimum acceptable Eb / No defined for the system. The maximum acceptable path loss can be computed as

Lc =EIRPmax- Prec, minWhere

EIRP is Effective Isotropic Radiated Power

Prec is Received power with (hypothetical) isotropic antenna

For downlink:

EIRPBTS, max= PBS-tx maximum BTS power (mean power over burst)

- Ac combiner and filter loss

- LBS-f1 antenna cable loss

+ GBS antenna gain

PMS, rec, min = PMS-rx MS reference sensitivity (for MS class i)

+ LMS-f antenna cable loss

- GMS antenna gain

+ Ldpx-rx duplex filter loss

For uplink:

EIRPMS, max = PMS-tx maximum transmission power of MS (mean power

over burst for MS class i)

- Ldpx-tx duplex filter loss

- LMS-f antenna cable loss

+ GMS antenna gain

PBTS, rec, min = PBS-rx BTS reference sensitivity

+ LBS-f1 antenna cable loss

- GBS antenna gain

- Gdiv diversity gain (if existing)

A balanced power budget is achieved if and only if

L c,uplink = Lc, downlink

Which is equivalent to

EIRPBTS,max - PMS,rec,min = EIRPMS,max - PBTS,rec,min

In reality, balanced power budget is only achieved with an accuracy of 5 dB, depending primarily on the employed mobile phone class and the initial network design goals. Therefore, uplink and downlink may have different power ranges which may lead to significant performance differences on the cell boundary or at indoor locations.

As an example of a balanced link budget, we can look a link budget for a GSM handheld MS with 2 W peak power, including margin for interference:

Downlink:

BTS transmitter

PBS-tx

38 dBm maximum BTS power (6 W BTS)

-Ac

3 dB

combiner and filter loss

-LBS-fi

4 dB

antenna cable loss (120 m 2dB/100 m +

1.6 dB connector loss)

+GBS

12 dBi antenna gain

= EIRPBTS,max

43 dBm peak EIRP

MS receiver

PMS-rx

-102 dBm

MS reference sensitivity (based on

Eb/No ratio)

+

3 dB

interference margin

+ LMS-f

0dB

antenna cable loss

- GMS

0dBi

antenna gain

+ Ldpx-rx

0dB

duplex filter loss

= PMs,rec,min

-99dBm

received power for isotropic antenna

Acceptable downlink path loss:

EIRPBTS,max

43 dBm

peak EIRP

-PMS,rec,min

-99 dBm

received power with isotropic

antenna

-

3 dB

antenna/body loss at MS

=Lc,d

139 dB

path loss between isotropic antennas

Uplink:

MS transmitter:

PMS-tx

33 dBm

maximum MS power (2 W MS)

-Ldpx-tx

0 dB

duplex filter loss

-LMS-f

0 dB

antenna cable loss

+GMS

0 dBi

antenna gain

= EIRPMS,max

33 dBm

peak EIRP

BTS receiver:

PBS-rx

-104 dBm

BTS reference sensitivity (based on

Eb/No ratio)

+

3dB

interference margin

+ LBS-fi

4dB

antenna cable loss (120 m 2dB/100

m +1.6 dB connector loss )

- GBS

12dBi

antenna gain

- Gdiv

0dB

diversity gain

= PBTS,rec min

-109dBm

received power for isotropic antenna

Acceptable uplink path loss:

EIRPMS,max

33dBm

peak EIRP

- PBTS,rec,min

-109dBm

received power with isotropic

antenna

-

3dB

antenna / body loss at MS

= Lc,u

139dB

path loss between isotropic antennas

The reference sensitivity levels PMS-rx and PBS-rx are the minimum levels that guarantee acceptable BER performance. It should be noted that the reference sensitivity levels in the GSM system are based on BER simulation over a fading radio channel model, thus they already include the effect of fast fading. Therefore a separate margin for fast fading is not necessary in the link budget. However, a margin for slow fading is often included, even though it is omitted in this example.

Thus, the acceptable path loss (without interference margin) for any cellular system can be computed separately for uplink and downlink as:

Example: power balance / unbalance in a GSM system with following parameter values:

PBS-tx = 10 W (40 dBm)

PMS-tx = 1 W (30 dBm, handheld telephone)

PMS-tx = 5 W (37 dBm, car mounted telephone)

SBS = -104 dBm

SMS = -102 dBm (handheld telephone)

SMS = -104 dBm (car mounted telephone)

Ldpx-tx = Ldpx-rx Gdiv = 7 dB

Ac = 3 dB

The power unbalance of the handheld telephone:

L =Lc,d Lc,u = [PBS-tx Ac LBS-fi + GBS]

[PMS-rx + LMS-f GMS + Ldpx-rx]

[PMS-tx Ldpx-tx - LMS-f + GMS]

+ [PBS-rx + LBS-fi - GBS Gdiv]

= PBS-tx -Ac -PMS-rx -PMS-tx+ PBS-rx - Gdiv = 40 3 + 102 30 104 7 = -2 dB

The uplink direction is thus 2 dB better. This difference can be neglected in practical network design.

For the car mounted telephone the power unbalance is:

FORMULA

L = PBS-tx -Ac -PMS-rx -PMS-tx+ PBS-rx - Gdiv = 40 3 + 104 37 7 104 = -7 dB

The uplink direction is now 7 dB better. From the operators point of view it would be better if the power unbalance were in favor of the downlink. This would guarantee better network control. However the real time power control used in GSM can easily rectify situation and produce almost perfect power balance.

Next, we take a closer look to some important terms of radio link budget.

III.4.1. Antenna Feeder loss

Both the base station and mobile station antenna feeders are coaxial cables. In addition to the antenna cables there are connectors and jumpers, which also contribute to the total feeder loss. The feeder cable length is typically larger the antenna height above ground. It depends on the location of the electronic equipment and includes often also horizontal sections.

The antenna cable loss can be calculated as

Lf = 1+ Ljump + Lconn

where

is the characteristic loss of the used cable (dB/m),

l is the length of the feeder cable (m),

Ljump is total loss of the jumpers (dB),

Lconn is the sum of connector losses (dB).

Example:l = hB + horizontal feeder sections = 70 + 15 = 85 m

= 4.03 dB/100m (7/8 plastic foam isolation cable 900 MHz)

Ljump = 3.0 dB,

Lconn =1.0 dB

Lf = 4.03 0.85 + 3.0 + 1.0 = 7.4 dB

The characteristic loss of a cable depends on the carrier frequency: the higher the frequency the higher the loss. The following Table shows representative characteristic losses of plastic foam isolation 50 coaxial cables at 900 MHz and at 1800 MHz.

CableAttenuation dB/100 m

900 MHz1800MHz

1/2"7.2210.3

7/84.035.87

1-1/42.984.31

1-5/82.523.72

III.4.2. Antenna Gain

In base stations either (horizontally) omnidirectional antennas or sectored antennas are used. A sectored antenna usually illuminates a 120 sector (for 3 sectors/cell), but also 90 or 60 sector antennas have been used (for 4 or 6 sectors/cell, respectively). The gain of an omnidirectional antenna depends on the vertical lobe width. With an ideal vertical lobe the gain should be

Gid,omni = 10 log (/)

where is the width of the vertical lobe (in radians).

Thus, a 20 vertical lobe width would then give 9.5 dB gain. The gain of an ideal sectored antenna can be calculated correspondingly. An ideal 120 sector antenna with a 20 vertical lobe would give 9.5 + 4.7 = 14.2 dB gain. Practical antenna gains are always lower than these ideal values.

The following Figures illustrate antenna patterns of two directional antennas. The gain of the first antenna is 12.2 dBi, and the gain of the second antenna is 13.0 dBi.

It should be noted that antenna patterns, which are specified by the manufacture of the antenna, do not necessarily correspond to antenna patterns at the installation at the antenna mast. The antenna mast itself and other nearby objects affect the antenna pattern. This effect is most critical for (nominally) highly directional antennas.

In mobile stations /4-antennas are often used. Typically the antenna gain of handheld telephones are in the order of 0 dBi in free filed conditions and they are almost omnidirectional but the when they are in real use the head will distort the radiation diagram and absorb part of the radiation. This gives lower practical gain values.

III.5. Coverage Planning Example

We examine here the same network as in the capacity planning example (see Section II.3). The Figure of the following page shows the division of the service area into subregions of certain environment types, so that within each subregion traffic demand is uniform.

The cell radii (in km) calculated from the coverage areas obtained in the capacity planning example by using the Approach 2 are also shown (see section II.3). When we require that a circular coverage area completely covers a square cell, the radius of the circle should be r = d / , where d is the length of a side of the square cell; or since d = , then r = . For example, in region B the area A = 9.00 km2, so that r = 2.12 km.

The equipment parameter values (and in some cases their maximum values) are given in the following Table.

Parameters chosen by

the operator Parameters given in the

system specifications Other parameters

Fu = 0.95

HBSmax = 100 m

GBSmax = 12.0 dB

GBSmax = 24.0 dB

(for street)

min 1/2", max 15/8

coaxial feeder at BS

f = 900 MHz

PMS-txmax = 5 W

(car mounted)

PMS-txmax = 1 W

(handheld)

PBS-txmax = 25 W

PCMS-rx = -104 dBm

PHMS-rx = -102 dBm

PBS-rx = -104 dBm

hMS = 1.5 m

1BS = hBS + 5m

Ac = 4.0 dB

Gdiv = 5.0 dB

Ljump = 2.5 dB

Lconn = 1.5 dB

HBSO = 25 m

GBSO = 6dBi

(Min.Value)

GCMS = 3.0 dBi

GHMS = -3.0 dBi

The task of coverage planning is to determine the BS transmitter powers, antenna gains and heights, and feeder dimensions so that the received signal level in the mobile stations will guarantee service with the probability given by the operator in the entire service area. In addition the power balance between uplink and downlink must be checked.

The approach is to first check whether the parameter minimum values will provide the sufficient link quality with the maximum BS transmitter power. If this is not the case the next step is to choose an antenna with higher gain. If this is not sufficient the feeder type is changed to larger diameter types. The last step is to increase antenna heights. If the heights antenna mast still not gives good enough quality the cell radius must be decreased. This will produce changes into the capacity plan. This approach is chosen to be optimal in economical sense, since the adopted order of the modifications tries to minimize the price of site.

SubregionArea typeR (km)

AII2.12

BII2.12

CIII4.10

DII1.83

EI1.41

FIII4.78

GIII5.15

HII2.44

IIV8.85

The required BS transmitter power can obtained from the downlink radio link budget as

PBS-tx = PMS-rx GMS + Lc + Ac + LBS-fl GBSIn this expression the MS antenna gain GMS also contains the feeder and duplex filter losses (LMS-f and Ldpx-rx). The first task is to determine the minimum acceptable received power that is required to fulfil the service probability Fu objective.

When we write the path loss given by the Hata model as a function of distance as

Lu = Lo + 10n log (d),

the path loss exponent n varies from

n = [44.9 6.55 log (hB)]/10 = [44.9 6.55 log(25)]/10 = 3.57

to

n = [44.9 6.55 log (hB)]/10 = [44.9 6.55 log(100)]/10 = 3.18

so that, when the standard deviation of slow fading is 6 dB, the parameter b in the service probability equation is varying on the range

Then we can solve the parameter a from the service probability equation

as being in the range

a = -0.686 (for 25 m) to a = -0.718 (for 100 m).

Next, we can solve the required average received power at the input of MS receiver at cell boundary corresponding to the reference sensitivity 102 or 104 dBm as

We can see that the required received signal level for obtaining the service probability objective depends very little (about 0.3 dB) on antenna height. The value for 100 m antenna height (the more demanding requirement) will thus be used for all macrocell antenna heights. The system losses are calculated as follows:

LBS-f = 1 + Ljump + Lconn = l + 4.0 dB

From the Hata model for typical path loss (in dB) for urban area is computed as

Lc = 69.55 + 26.16 log(f) 13.82 log(hB) a(hM) + 10n log(r)

= 146.83 13.82 log (hB) a(hM) + 35.7 log(r)

Correction factors are added according to terrain type, as shown in the following Table

Terrain Type

I Large citya(1.5)=3.2[log(11.75 x 1.5)]2- 4.97 0.0 dB

II Medium sized citya(1.5)=[1.1 log(900)-0.7] x 1.5-1.56 log(900)+0.8 0.0 dB

III SuburbanCS=2[log (900/28)] 2+5.4=9.9 dB

IV OpenCO=4.78[log(900)]2-18.33 log(900)+40.94=28.5dB

Subregions A and B (type II)

Using the basic equipment parameters the losses are:

LBS-f = (hB +5)+ Ljump + Lconn

=(7.22/100) x (25+5)+4.0 dB=6.2 dB

Lc= 146.83-13.82 log (hB)-a(hM)+35.7 log (r)

=146.83-13.82 log(25)-0.0+35.7 log (2.12)=139.2 dB

The required base station power level for car-mounted MSs can be calculated from the radio link budget as

PBS-tx = PMS-rx + Ac + LBS-fl GBS + Lc GMS

= -97.9 + 4.0 + 6.2 6 +139.2 3.0 = 42.5 dBm = 17.8 W

This value is within the allowed range (