Download dB - ANTS

dB, dBm and relative issues in WCDMA
radio network planning
Speaker: Chun Hsu 許君
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Outline
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dB, dBm
Noise rise vs. throughput
Coverage vs. total throughput or system load
GSM Co-planning
Comments
Reference
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dB
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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
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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.
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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:
Definition – Amplitude and voltage
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When referring to measurements of amplitude it is usual to
consider the ratio of the squares of A1 (measured amplitude)
and A0 (reference amplitude).
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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.
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where V1 is the voltage being measured, V0 is a specified reference
voltage, and GdB is the power gain expressed in decibels.
Examples
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To calculate the ratio of 1 kW (one kilowatt, or 1000
watts) to 1 W in decibels, use the formula
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To calculate the ratio of 1 mW (one milliwatt) to 10 W in
decibels, use the formula
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Example question
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A signal traveling one kilometer in a coaxial cable loses onehalf its voltage. Express the,
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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:
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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.
Merits
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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.
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Essentially this is because log(A × B × C × ...) = log(A) +
log(B) + log(C) + ...
dBm
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dBm (sometimes dBmW) is an abbreviation for the power
ratio in decibels (dB) of the measured power referenced to
one milliwatt (mW).
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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
1W
20 dBm
100 mW
0 dBm
1.0 mW
-30 dBm
1.0 µW
dBi
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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
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The noise rise is defined as the ratio of the total received
wideband power to the noise power
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Noise rise = Itotal / PN
Coverage vs. Load (1/8) – path loss
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Path loss (or path attenuation) is the reduction in power
density (attenuation) of an electromagnetic wave as it
propagates through space.
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Path loss is usually expressed in dB.
L=10nlog10(d) + C
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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.
Coverage vs. Load (2/8) – Loss exponent
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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
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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 flatearth 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.
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On the other hand, a tunnel may act as a waveguide, resulting
in a path loss exponent less than 2.
Coverage vs. Load (3/8)
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Coverage vs. Load (4/8)
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In both uplink and downlink the air interface load affects
the coverage but the effect is not exactly the same.
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In the downlink, the coverage depends more on the load than in
the uplink.
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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.
Coverage vs. Load (5/8)
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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
760kbps->720kbps
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Coverage vs. Load (8/8) - Power splitting between
Frequencies
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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.
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The power splitting approach requires that the operator’s
frequency allocation allows the use of two carriers in the base
station.
GSM Co-planning
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The assumptions made and the results of the comparison
of coverage.
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GSM Co-planning
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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,
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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.
Comments (1/2)
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Nortel’s 802.16 BS seems to adopt the power splitting
technique.
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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.
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Beamforming antennas
Uplink link budget improvement
Relay technology
Comments (2/2) – Nortel 802.16j
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reference
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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
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A receiver's sensitivity is a measure of its ability to discern
low-level signals.
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Because receive sensitivity indicates how faint a signal can be
successfully received by the receiver, the lower power level, the
better.
Backup slides - interference margin
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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
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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
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The body loss accounts for the loss when the terminal is
close to the user’s head.
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