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# FAQ-How Can I Determine the Number of APs, Antenna Positions, and Channels

Publication Date:  2015-06-19 Views:  254 Downloads:  0
Issue Description
How Can I Determine the Number of APs, Antenna Positions, and Channels?
Solution
The following uses an example to describe the method to determine the number of APs, antenna positions, and channels.

A dormitory building has seven floors, with 20 rooms on each floor. Each room has four users, and each user requires 2 Mbit/s bandwidth. Assuming that maximum number of concurrent users is 30% of the total number of users, plan the indoor DAS system as follows:

1, Determine the required bandwidth.

Bandwidth required on each floor = 20 x 4 x 2 x 0.3 = 48 Mbit/s

Number of APs required on each floor = 48/20 = 2.4

2, Determine the number of APs.

According to the preceding calculation result, each floor needs 3 APs, so a total of 21 APs are required in the seven-floor dormitory building.

A 24-port PoE switch can be used as the aggregation switch and provides power to APs.

Determine the number of ACs based on the number of APs. ACs can be connected to a BAS device in branched networking mode.

3, Determine the coverage areas (indoor coverage 50 m x 13 m).

Dormitories are major coverage areas.

Washrooms, toilets, and corridors are minor coverage areas.

4, Determine antenna installation positions, coverage radius, and interval between antennas based on signal coverage requirements.

Deploy an antenna between each four rooms so that radio signals need to penetrate only one wall. The interval between antennas is 7.5 m.

An additional antenna is deployed for the last dormitory at the end of the corridor to ensure a good signal coverage. Washrooms, toilets, and corridors do not need dedicated antennas.

Determine the indoor DAS system layout.

If no 2G/3G indoor DAS system is deployed in the dormitory system, establish a WLAN network.

Install three APs on each floor. Each AP has two antennas, which are connected to the AP through a two-way splitter.

The coverage distance is 50 m, so 1/2-inch cables can be used to connect the APs and antennas.

Calculate the length of each cable.

APs can be wall-mounted or installed in a cabinet. Choose the installation positions that need the shortest cable length.

5, Calculate the power of each antenna interface.

The expected coverage radius of each antenna is 6.5 m.

Use the free-space loss model to calculate signal loss. The loss in a 6.5 m radius is about 56.5 dB.

Signals need to penetrate a wall, which brings approximate 25 dB loss.

The required field strength in the major coverage is -65 dBm.

The antenna gain is 2 dBi.

Therefore, the transmit power on each antenna interface should be: 65 + 25 + 56.5 - 2 dBi = 14.5 dBm.

NOTE:
Antenna gains of STAs are not considered in the preceding formula. If the antenna gain of STAs is 2 dBi, a transmit power of 12.5 dBm is enough. Actually, antenna gains of STAs are not taken into account because the antenna gains cannot be determined.

6, Calculate the loss of the antenna system and set APs' transmit power.

Each AP has two antennas. Output power of the two antennas may be different due to loss on the cables. The transmit power of each antenna interface is calculated using the power calculator as follows:

Set the transmit power of AP1 to 20 dBm.

Output power of ant1 is 15.4 dBm.

Output power of ant2 is 14.5 dBm.

Set the transmit power of AP2 to 22 dBm.

Output power of ant3 is 15.5 dBm.

Output power of ant4 is 13.7 dBm.

Set the transmit power of AP3 to 24 dBm.

Output power of ant5 is 14.4 dBm.

Output power of ant6 is 13.5 dBm.

7, Select radio channels.

Use the channel distribution with the lowest co-channel interference.

If the AP's channels conflict with channels of users' Wi-Fi devices, adjust the channel distribution.

If channel conflicts cannot be avoided by adjusting APs' channel distribution, discuss with the owners of the Wi-Fi devices to re-distribute the channels.

END