dB, dBm and relative issues in WCDMA radio network planning

Speaker: Chun Hsu 許君

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Outline dB, dBm Noise rise vs. throughput Coverage vs. total throughput or system load GSM Co-planning Comments Reference

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dB The decibel (dB) is a logarithmic unit of measurement that

expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level.

The decibel is useful for a wide variety of measurements in science and engineering and other disciplines.

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Definition - Power When referring to measurements of power or intensity, a

ratio can be expressed in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference level. Thus, if L represents the ratio of a power value P1 to another

power value P0, then LdB represents that ratio expressed in decibels and is calculated using the formula:

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Definition – Amplitude and voltage When referring to measurements of amplitude it is usual to

consider the ratio of the squares of A1 (measured amplitude) and A0 (reference amplitude). This is because in most applications power is proportional to the

square of amplitude. Thus the following definition is used:

In electrical circuits, dissipated power is typically proportional to the square of voltage or current when the impedance is held constant.

where V1 is the voltage being measured, V0 is a specified reference voltage, and GdB is the power gain expressed in decibels.

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Examples To calculate the ratio of 1 kW (one kilowatt, or 1000

watts) to 1 W in decibels, use the formula

To calculate the ratio of 1 mW (one milliwatt) to 10 W in decibels, use the formula

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Example question A signal traveling one kilometer in a coaxial cable loses one-

half its voltage. Express the, a.) input-to-output voltage ratio b.) input-to-output power ratio c.) input-to-output voltage ratio in dB d.) input-to-output power ratio in dB.

Ans: a.)2:1; b.) (2)2:(1)2=4:1; c.) 20 log (2/1) = 6 dB; d.) 10 log (4/1) = 6

dB This illustrates one of the primary advantages to expressing gains or

losses in dB. As long as the impedance is constant, it is not necessary to specify

whether a ratio is power or voltage when it is expressed in dB.

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Merits The decibel's logarithmic nature means that a very large

range of ratios can be represented by a convenient number.

The overall decibel gain of a multi-component system can be calculated simply by summing the decibel gains of the individual components, rather than needing to multiply. Essentially this is because log(A × B × C × ...) = log(A) +

log(B) + log(C) + ...

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dBm dBm (sometimes dBmW) is an abbreviation for the power

ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). x = 10log10(P) P = 10(x / 10) with P power in mW and x power ratio in dBm.

Typical dBm level:

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60 dBm 1 kW

30 dBm 1 W

20 dBm 100 mW

0 dBm 1.0 mW

-30 dBm 1.0 µW

dBi dB(isotropic) — the forward gain of an antenna compared

to the hypothetical isotropic antenna, which uniformly distributes energy in all directions.

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Uplink noise rate as a function of uplink data throughput in WCDMA The noise rise is defined as the ratio of the total received

wideband power to the noise power Noise rise = Itotal / PN

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Coverage vs. Load (1/8) – path loss Path loss (or path attenuation) is the reduction in power

density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is usually expressed in dB. L=10nlog10(d) + C

where L is the path loss in decibels n is the path loss exponent d is the distance between the transmitter and the receiver, usually

measured in meters C is a constant which accounts for system losses

The total of all energy lost or wasted on a system due to line loss and other forms of energy loss, unaccounted energy use and theft among other factors is referred to as system loss.

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Coverage vs. Load (2/8) – Loss exponent In the study of wireless communications, path loss can be

represented by the path loss exponent, whose value is normally in the range of 2 to 4 2 is for propagation in free space 4 is for relatively lossy environments and for the case of full

specular reflection from the earth surface -- the so-called flat-earth model).

In some environments, such as buildings, stadiums and other indoor environments, the path loss exponent can reach values in the range of 4 to 6. On the other hand, a tunnel may act as a waveguide, resulting

in a path loss exponent less than 2.

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Coverage vs. Load (3/8)

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Coverage vs. Load (4/8) In both uplink and downlink the air interface load affects

the coverage but the effect is not exactly the same. In the downlink, the coverage depends more on the load than in

the uplink. The reason is that in the downlink the power of 20W is shared

between the downlink users: the more users, the less power per user. Therefore, even with low load in the downlink, the coverage

decreases as a function of the number of users.

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Coverage vs. Load (5/8) The same as figure in last slide for 64-kbps users

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Coverage vs. Load (6/8)

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Coverage vs. Load (7/8) – Power splitting between Frequencies

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760kbps->720kbps

Coverage vs. Load (8/8) - Power splitting between Frequencies

Assume we had 20W downlink transmission power available.

Splitting the downlink power between two frequencies would increase downlink capacity from 760kbps to 2* 720kbps=1440 kbps, i.e. by 90%.

The splitting of the downlink power between two carriers is an efficient approach to increasing the downlink capacity without any extra investment in power amplifiers. The power splitting approach requires that the operator’s

frequency allocation allows the use of two carriers in the base station.

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GSM Co-planning The assumptions made and the results of the comparison

of coverage.

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GSM Co-planning Utilization of existing base station sites is important in

speeding up WCDMA deployment and in sharing sites and transmission costs with the existing 2G system.

From the table, A 144 kbps WCDMA data service can be provided when using

GSM1800 sites, with the same coverage as GSM1800 speech. If GSM900 sites are used for WCDMA and 64 kbps full

coverage is needed, a 3 dB coverage improvement is needed in WCDMA.

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Comments (1/2) Nortel’s 802.16 BS seems to adopt the power splitting

technique. Three sets of antenna are built in one BS (share the power),

each set is configured with different wideband. In most situation, the downlink path loss is greater than

uplink path loss so improving uplink coverage is a important issue. Beamforming antennas Uplink link budget improvement Relay technology

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Comments (2/2) – Nortel 802.16j

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reference Harri Holma and Antti Toskala, “WCDMA for UMTS

Radio Access for Third Generation Mobile Communications,” 3rd, 2004

Pan Yu-Chun, “Nortel WiMAX 802.16j-MMR Solution,” Oct, 2006

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Backup slides - Receiver sensitivity A receiver's sensitivity is a measure of its ability to

discern low-level signals. Because receive sensitivity indicates how faint a signal can be

successfully received by the receiver, the lower power level, the better.

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Backup slides - interference margin The interference margin is needed in the link budget

because the loading of the cell affects the coverage. The more loading is allowed in the system, the larger is

the interference margin needed in the uplink, and the smaller is the coverage area.

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Backup slides - Fast fading margin Some headroom is needed in the mobile station

transmission power for maintaining adequate closed loop fast power control.

This applies especially to slow-moving pedestrian mobiles where fast power control is able to effectively compensate the fast fading.

Typical values for the fast fading margin are 2.0–5.0 dB for slow-moving mobiles.

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Backup slides – body loss The body loss accounts for the loss when the terminal is

close to the user’s head.

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