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AvaLAN Wireless Systems Inc.

1020 Corporation Way

Palo Alto, CA 94303

Application Note:

Indoor Path Loss

August 2006





Email: support@avalanwireless.com

AvaLAN_AN001 Page 1 of 5 ©2005 AvaLAN Wireless Inc and MaxStream, Inc.

AvaLAN Wireless Systems Inc.
1020 Corporation Way
Palo Alto, CA 94303

Link Margin

When designing any system, engineers and system architects usually want to know how well the various components of
the system will perform. In a microprocessor, logic or other digital system these questions are usually answered in
terms of clock speeds, instructions per second or data throughput. With wireless communication the most common
performance questions involve range.

Unlike digital systems, trying to quantify the range performance of radio frequency (RF) communication systems can be
difficult due to the large role the environment has on radio frequency signals. Buildings, trees, obstructions and lack of
antenna height can all contribute to a decrease in signal strength at the receiving end. In order to estimate the
communication distance (transmission range) for a system, four factors must be considered:

1. Transmit Power- The power that is broadcast by the transmitter. This is usually measured in Watts or

2. Receive Sensitivity- A measure of the of the minimum signal strength that a receiver can discern.

3. Antenna gain- The amount of signal gain provided by the antennas.

4. Path Loss- The signal decrease that occurs as the radio waves travel through the air or through obstacles.

Antenna Gain

Transmitter Path Loss Receiver

(Transmit Power) (Receive Sensitivity)

Path Loss or attenuation of RF signals occurs naturally with distance. Obstacles between the transmitter and receiver
also attenuate signals. The amount of attenuation varies with the frequency of the RF signal and the obstructing
materials type and density. Generally speaking, the lower the frequency of transmission the better the signal will
travel through the air and through objects. If two radio systems had identical transmit power and receive sensitivity,
but one system was at 900 MHz and the other at 2.4 GHz, then the 900 MHz radio would perform better because it has
less path loss than a 2.4 GHz system. These parameters can be used to estimate the distance a system can

Knowing how strong the communication link is or just how close a system is to failure can be important in some
situations. Link Margin is a parameter that is used to measure how close the link is to failing. Link Margin is the
difference between the system gains and the system losses. Successful communication takes place when the Link
Margin is greater than zero.

Link Margin = Transmit Power – Receiver Sensitivity + Antenna Gain – Path Loss

Comparing an RF communications system to a human voice communication can help illustrate these principles.
Transmit power represents the volume of the person speaking. Receive sensitivity represents the minimum volume
required by the listener to discern the message. Antenna gain is equivalent to the use of a megaphone, and Path Loss is
the attenuation that occurs as the voice travels over a distance or through obstacles.

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AvaLAN Wireless Systems Inc.
1020 Corporation Way
Palo Alto, CA 94303

The transmit power on the indoor AvaLAN AW900i is 125mW (21 dBm). The receive sensitivity on the same unit is -97
dBm. If you were to use these radios in a system with 2.5 dBi gain antennas, there could be 123 dB of signal
attenuation between the transmitter and receiver and still have communication occur.

21 dBm – (-97 dBm) + 2.5 dBi + 2.5 dBi = 123 dB

In line-of-site conditions, every 6dB of link margin will double the transmission range. A radio with a link margin of 111
dB has 12 dB less link margin than the AvaLAN radio, and thus it can only achieve a quarter of the range.

In line-of-site conditions the path loss can be determined by using a mathematical formula (Friis transmission
equation).The path loss for 900 MHz and 2.4 GHz in free space is given for several distances in the table below.

Distance 900 MHz free-space loss 2.4 GHz free-space loss

10 meters 72.5 dB 81 dB

100 meters 92.5 dB 101 dB

1000 meters 112.5 dB 121 dB

Table 1

Figuring out the range for non line-of-site and indoor communication systems is a lot more difficult and can involve a
lot of obstructions and variables. The different obstacles and materials that are found in typical indoor environments
make it difficult to determine the actual path loss in a given situation. In order to know how systems will perform in a
given indoor environment, on sight testing must be performed. If there are known obstructions of a particular material
then estimation of signal losses through the obstruction may aid in determining link margin and antenna placement.

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AvaLAN Wireless Systems Inc.
1020 Corporation Way
Palo Alto, CA 94303

Indoor Communications

For indoor communication, the construction materials that make up the obstructions are the largest attenuators. The
following is a list of common construction materials and their approximate attenuation at 900 MHz (Thicknesses of
materials are given in parenthesis).

Material Attenuation @ 900 MHz

Glass 0.25” (6mm) 0.8 dB

Glass 0.5” (13mm) 2 dB

Lumber 3” (76mm) 2.8 dB

Brick 3.5” (89mm) 3.5 dB

Brick 7” (178mm) 5 dB

Brick 10.5” (267mm) 7 dB

Concrete 4” (102mm) 12 dB

Masonry Block 8” (203mm) 12 dB

Brick faced concrete 7.5 “ (192mm) 14dB

Masonry Block 16” (406mm) 17dB

Concrete 8” (203mm) 23dB

Reinforced Concrete 3.5” (203mm) 27dB

Masonry Block 24” (610mm) 28dB

Concrete 12” (305mm) 35dB

Table 2

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AvaLAN Wireless Systems Inc.
1020 Corporation Way
Palo Alto, CA 94303

Solid metal structures are not listed in the tables and graphs above because radio waves do not propagate through
metal. In practice, metallic masses such as towers, panels, fences or vehicles that are present in the vicinity of radio
transmitting equipment and the radio receiver act as reflectors and scatterers of electro magnetic radiation. The
interference of the direct wave and the reflected waves can produce local maxima (constructive interference) or local
minima (destructive interference) at the radio receiver.

Suppose there exists a situation where it is desired to communicate 100 meters through 4 standard sheetrock walls and
one concrete wall. Table 1 indicates that the free-space loss for 100 meters at 900 MHz is approximately 93 dB. In this
particular indoor situation, it is necessary to also take into account the effect the walls will have on communication.
The office walls consist of two pieces of drywall (-0.8 dB each) and lumber (-2.8 dB) for a total attenuation of about
4.4dB per wall. The concrete wall is 102mm thick and attenuates the signal by 12dB. Using the equation:

Power (TX) - Sensitivity (RX) >= Signal Attenuation

148 dB >= 93 dB + (4 walls * 4.4 dB) + (12 dB)

148 dB >= 122.6 dB

The link margin of 148 dB is greater than the path loss of 122.6 dB, allowing communication to occur. It should be kept
in mind that this is only a theoretical estimation. An on sight test should be performed in order to verify conditions
and assumptions.


When trying to determine just how far any particular radio will transmit indoors, the main difficulty lies in figuring out
just what path the radio signal will take and how many walls and obstacles the signal must transmit through. While
taking into account the different building materials and their thicknesses can be helpful for estimation purposes,
testing in the actual environment is the only sure way to determine whether or not communication will be successful.

The radio transmit power, receive sensitivity and frequency need to be considered in any wireless communication
systems. Antenna or radio placement can help in avoiding some obstacles, but in most situations the system designer
does not have control over what building materials must be transmitted through. That leaves transmit power, receive
sensitivity and antenna gain as the only parameters that are left to the designer’s choosing. If communication in a
particular environment is not robust, choosing a radio set with better link margin can help improve data reliability. The
expectation should be that some data will be lost. Protocols can be developed that allow for graceful recovery from
data corruption and reliable delivery of the information. Whatever the conditions, estimating materials and doing site
testing in the actual environments can aid in setting performance expectations during the design phase of the system.

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