SkyPilot Link Budget Calculations
SkyPilot Link Budget Calculations – 2.4 GHz
Link budget defines the amount of power available in a communication link for transmission
loss through the path, whether the loss is through the air or through obstructions like trees
and buildings. With a known link budget, the range of a communication link can be
determined given a fixed path loss and fade margin. Because of the large variation in path
loss models for propagation in real world environments, the link budget becomes a more
easily comparable specification for evaluation of communication systems. It is generally true
that a higher link budget will provide longer range. For this reason, link budget is an
important specification for all RF deployments.
The basic equation for link budget is a fairly simple formula when using units of power in dB.
Link Budget = Transmit Power + Transmit Antenna Gain + Receive Antenna Gain -
Receiver Sensitivity
Transmit Power
“Transmit power” is the power coming out of the radio/power amplifier and into the antenna.
Transmit power is normally measured in dBm. Although many companies highlight peak
power, it is not accurate to use peak power for link budget calculations. Instead, link budget
calculations should always use average power. If you’ve ever used a stereo with a power
output display, you should be able to understand the difference between peak power and
average power. A general rule of thumb is that the average power is about 3 (CCK) to 5 dB
(OFDM) less than the peak power. As an example, the FCC limits the maximum peak power
in the ISM (2.4 to 2.483 GHz) frequency band to 1 W/30 dBm, which would translate to an
average power for link budget calculations around 25-27 dBm.
The 2.4 GHz access point in SkyPilot’s SkyExtender DualBand is FCC rated at an average
power of about 26 dBm and the average power output changes slightly depending on the
modulation. For the lower modulations (BPSK & QPSK), the output can be driven harder so
the average power is 26 dBm. For the higher modulations (QAM), the average power backs
off to between 21 and 24 dBm. This is generally referred to as amplifier back off.
Antenna Gain
“Antenna gain” represents the gain of the antenna relative to a single point antenna radiating
evenly in all directions (sphere). So the gain is actually a measure of how well focused the
antenna is at radiating the signal. The important thing about antennas is that the larger the
antenna the larger the gain and the smaller the focus. Improvements in antenna are
especially beneficial to link budgets because they are counted twice, once for the transmit
antenna and once for the receive antenna. The 2.4 GHz antennas on the SkyExtender
DualBand has 7.4 dBi of antenna gain.
SkyPilot Link Budget Calculations 1 © 2005 SkyPilot Networks, Inc.
SkyPilot Antenna Characteristics
Product SkyExtender DualBand
Antenna Gain 7.4 dBi
Azimuth beam width 360°
Elevation beam width 20°
Antenna down tilt 12°
Receiver Sensitivity
“Receiver sensitivity” is a measure of the minimum signal level that can be received by a
radio. The word “received” has to be defined in terms of the quality of the link. For our
system we use a 10% packet error rate (PER) as the metric for link quality (this is the same
metric used by RF chip companies as well). Testing is done with no external interference
and the signal power is reduced until a 10% PER occurs, at which time the average power is
then measured. Receiver sensitivity changes with modulation and data rates as shown in the
table below:
SkyPilot Measured Receiver Sensitivity
Data Rate Modulation Format Receiver Sensitivity
(802.11b) (all numbers +/- 2
dBm)
1 Mbps DBPSK -97 dBm
2 Mbps DQPSK -96 dBm
5.5 Mbps DQPSK-CCK -95 dBm
11 Mbps DQPSK-CCK -92 dBm
Data Rate Modulation Format Receiver Sensitivity
(802.11g) (all numbers +/- 2
dBm)
6 Mbps BPSK – 1/2 -94 dBm
9 Mbps BPSK – 3/4 -93 dBm
12 Mbps QPSK – 1/2 -91 dBm
18 Mbps QPSK – 3/4 -90 dBm
24 Mbps 16QAM – 1/2 -86 dBm
36 Mbps 16QAM – 3/4 -83 dBm
48 Mbps 64QAM – 1/2 -77 dBm
54 Mbps 64QAM – 3/4 -74 dBm
SkyPilot Link Budget Calculations 2 © 2005 SkyPilot Networks, Inc.
Link Budget Performance Summary
To calculate the link budget, the Wi-Fi client’s transmit power, transmit antenna gain, receive
antenna gain, and receiver sensitivity need to be included. The following table can help
calculate the link budget between SkyPilot’s SkyExtender DualBand and a IEEE 802.11b/g
client over the 2.4 GHz frequency:
Modulation Transmit Transmit Receive Receiver
Rate Power Antenna Gain Antenna Sensitivity
(802.11b) (all +/- 1 dB) Gain (all +/- 2 dB) Link Budget
1 Mbps 26 dBm 7.4 dBi
2 Mbps 26 dBm 7.4 dBi
5.5 Mbps 26 dBm 7.4 dBi
11 Mbps 26 dBm 7.4 dBi
1 Mbps 7.4 dBi -97 dBm
2 Mbps 7.4 dBi -96 dBm
5.5 Mbps 7.4 dBi -95 dBm
11 Mbps 7.4 dBi -92 dBm
Modulation Transmit Transmit Receive Receiver
Rate Power Antenna Gain Antenna Sensitivity
(802.11g) (all +/- 1 dB) Gain (all +/- 2 dB) Link Budget
6 Mbps 26 dBm 7.4 dBi
9 Mbps 26 dBm 7.4 dBi
12 Mbps 26 dBm 7.4 dBi
18 Mbps 26 dBm 7.4 dBi
24 Mbps 26 dBm 7.4 dBi
36 Mbps 24 dBm 7.4 dBi
48 Mbps 22 dBm 7.4 dBi
54 Mbps 21 dBm 7.4 dBi
6 Mbps 7.4 dBi -94 dBm
9 Mbps 7.4 dBi -93 dBm
12 Mbps 7.4 dBi -91 dBm
18 Mbps 7.4 dBi -90 dBm
24 Mbps 7.4 dBi -86 dBm
36 Mbps 7.4 dBi -83 dBm
48 Mbps 7.4 dBi -77 dBm
54 Mbps 7.4 dBi -74 dBm
Document Title 3 © 2005 SkyPilot Networks, Inc.
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© 2005 SkyPilot Networks, Inc. All rights reserved. SkyConnector, SkyControl, SkyExtender, SkyGateway,
SkyPilot, SkyPilot Networks, SkyProvision, the SkyPilot logo, and other designated trademarks, trade names,
logos, and brands are the property of SkyPilot Networks, Inc. or their respective owners. Product specifications are
subject to change without notice. This material is provided for informational purposes only; SkyPilot assumes no
liability related to its use and expressly disclaims any implied warranties of merchantability or fitness for any
particular purpose.
SkyPilot Link Budget Calculations 4 © 2005 SkyPilot Networks, Inc. 9/05
