How Can I Determine the Number of APs, Antenna Positions, and Channels?
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:
- 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
- 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.
- Determine the coverage areas (indoor coverage 50 m x 13 m).
- Dormitories are major coverage areas.
- Washrooms, toilets, and corridors are minor coverage areas.
- 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.
- 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.
