Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
3-10
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
RF Basics
RF Power Terminology
Terms such as dB, dBi, and dBm are used to describe the amount of change in power measured at points
in a system, as perceived by the radio or compared to a reference power level, respectively. The
following sections cover their differences and provide a rule of thumb for their use, in addition to
providing an explanation of effective isotropic radiated power (EIRP).
in a system, as perceived by the radio or compared to a reference power level, respectively. The
following sections cover their differences and provide a rule of thumb for their use, in addition to
providing an explanation of effective isotropic radiated power (EIRP).
dB
The term decibel (dB) is mainly used for attenuation or amplification of the power level. dB is a
logarithmic ratio of a signal to another standardized value. For example, dBm is where the value is being
compared to 1 milliWatt, and dBw is where the value is being compared to 1 Watt.
logarithmic ratio of a signal to another standardized value. For example, dBm is where the value is being
compared to 1 milliWatt, and dBw is where the value is being compared to 1 Watt.
The math is as follows:
Power (in dB) = 10 * log10 (signal/reference)
Plugging in some numbers (signal 100mW, reference 1mW) gives a value in dB of 20 (100 = 10 squared;
taking the exponent 2 and multiplying by 10 gives you 20).
taking the exponent 2 and multiplying by 10 gives you 20).
Remember that it is logarithmic (meaning that it increases or decreases exponentially and not linearly),
and it is a ratio of some value to a reference. Also, remember that it is multiplied by 10.
and it is a ratio of some value to a reference. Also, remember that it is multiplied by 10.
Given that it is logarithmic, there are some general rules of thumb. An increase or decrease of 3 dB
means that the signal doubled (double the power) or halved, respectively. An increase or decrease of
10dB means that the signal went up by 10 times or down to 1/10
means that the signal doubled (double the power) or halved, respectively. An increase or decrease of
10dB means that the signal went up by 10 times or down to 1/10
th
the original value.
Indoor WLAN and outdoor WLAN deployments both offer separate challenges in RF deployments, and
need to be analyzed separately. However, there are some rules of thumb for indoor use. For every
increase of 9dB, the indoor coverage area should double. For every decrease of 9dB, the indoor coverage
area should be cut in half.
need to be analyzed separately. However, there are some rules of thumb for indoor use. For every
increase of 9dB, the indoor coverage area should double. For every decrease of 9dB, the indoor coverage
area should be cut in half.
dBi
The term dBi is used to describe the power gain rating of antennas. The real antennas are compared to
an isotropic antenna (a theoretical or imaginary antenna) that sends the same power density in all
directions, thus the use of dBi.
an isotropic antenna (a theoretical or imaginary antenna) that sends the same power density in all
directions, thus the use of dBi.
Antennas are compared to this ideal measurement, and all FCC calculations use this measurement (dBi).
For example, a Cisco omni-directional AIR-ANT4941 has a gain of 2.2 dBi, meaning that the maximum
energy density of the antenna is 2.2 dB greater than an isotropic antenna.
For example, a Cisco omni-directional AIR-ANT4941 has a gain of 2.2 dBi, meaning that the maximum
energy density of the antenna is 2.2 dB greater than an isotropic antenna.
dBm
The term dBm uses the same calculation as described in the dB section, but has a reference value of 1
milliWatt.
milliWatt.
So, taking into consideration the example previously given in the dB section, if the power jumped from
1 mW to 100mW at the radio, the power level would jump from 0 dBm to 20 dBm.
1 mW to 100mW at the radio, the power level would jump from 0 dBm to 20 dBm.
Besides describing transmitter power, dBm can also describe receiver sensitivity. Receiver sensitivity is
in minus dBm (-dBm), because the signal reduces in value from its point of transmission. The sensitivity
indicates the lowest power the receiver can receive before it considers the signal unintelligible.
in minus dBm (-dBm), because the signal reduces in value from its point of transmission. The sensitivity
indicates the lowest power the receiver can receive before it considers the signal unintelligible.