dB, dBm and relative issues in WCDMA radio network planning Speaker: Chun Hsu 許君 1

Outline  dB, dBm  Noise rise vs. throughput  Coverage vs. total throughput or system load  GSM Co-planning  Comments  Reference 2

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. 3

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: 4

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 V 1 is the voltage being measured, V 0 is a specified reference voltage, and G dB is the power gain expressed in decibels. 5

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 6

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. 7

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)

dBm  dBm (sometimes dBmW) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW).  x = 10log 10 (P)  P = 10 (x / 10)  with P power in mW and x power ratio in dBm.  Typical dBm level: 9 60 dBm1 kW 30 dBm1 W 20 dBm100 mW 0 dBm1.0 mW -30 dBm1.0 µW

dBi  dB(isotropic) — the forward gain of an antenna compared to the hypothetical isotropic antenna, which uniformly distributes energy in all directions. 10

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 = I total / P N 11

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. 12

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. 13

Coverage vs. Load (3/8) 14

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. 15

Coverage vs. Load (5/8)  The same as figure in last slide for 64-kbps users 16

Coverage vs. Load (6/8) 17

Coverage vs. Load (7/8) – Power splitting between Frequencies kbps->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. 19

GSM Co-planning  The assumptions made and the results of the comparison of coverage. 20

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. 21

Comments (1/2)  Nortel’s 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 22

Comments (2/2) – Nortel j 23

reference  Harri Holma and Antti Toskala, “WCDMA for UMTS Radio Access for Third Generation Mobile Communications,” 3 rd, 2004  Pan Yu-Chun, “Nortel WiMAX j-MMR Solution,” Oct,

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. 25

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. 26

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. 27

Backup slides – body loss  The body loss accounts for the loss when the terminal is close to the user’s head. 28