Wireless access network design

Wireless access network design
1. Network design principles


1. Advancement
Adopt advanced design ideas and select advanced network equipment to keep the network technologically advanced for a certain period in the future.

2. Openness
Network design and network equipment modeling comply with international standards and industrial standards, making the network open and compatible.

3. Scalability
While fully considering the current situation, the network design must take into account the needs of business development for a longer period of time in the future, leaving sufficient possibilities for upgrading and expansion.

4. Security
The design of the network system must implement security principles to prevent various damages from inside and outside the network.

5. Reliability
The design of the network system must implement the principle of reliability, so that the network system has a high availability.

6. Manageability
The network system should have good manageability, so that network managers can easily and timely grasp information such as network topology, network performance statistics, and network faults, and can easily configure and adjust the network to ensure that the network is working in a good state.


Second, wireless link calculation


Before the wireless network project is constructed, the entire link must be calculated. The link calculation is carried out on the basis of ensuring the quality of link communication according to the results of field environmental surveys.

The link calculation should include the following points:

(1) Antenna specifications
Calculate according to the distance between the links, the frequency band used, the transmission power of the equipment used, the receiving sensitivity, the specifications of the antenna feeder system, and the length. Calculated as follows:

Pr = Pt + Gta + Gra – Ltl– Ltm– Lrl

Where Ltm = 32.5 + 20 logf +20 logd

Requirements Pr ≥Sr

Pr = receiving power Sr = receiving sensitivity of the device
Gta = gain of the transmitting antenna Gra = gain of the receiving antenna
Ltl = transmission line attenuation at the transmitting end Lrl = transmission line attenuation at the receiving end
Ltm = transmission space attenuation Pt = transmission power of the device
f = frequency of use d = distance between two stations
The reserved degree of Pr ≥ Sr should be determined according to the complexity of the electromagnetic environment on the ground, the physical environment between the links and the communication distance. Generally, in the case of close range, at least 3dBm should be reserved. The farther the transmission distance is, the larger the reserved gain should be. At a long distance, the reserved gain should be around 20dBm.

(2) Antenna polarization
The polarization of the antenna has a great relationship with the electromagnetic environment on the ground. It should be as far as possible from the polarization direction of other local antennas in the same frequency band to minimize external interference. The link of this network should also be analyzed, and the interference of its own internal link should also be considered when it is avoided as much as possible. When placing multiple antennas in the same place at the same time, try not to install antennas with the same polarization in the same direction, and the antennas should be isolated. The size of the separation distance can be calculated according to the specifications of the antenna used and the frequency used.

(3) Antenna installation height
The installation height of the antenna should ensure that the two connected sites are fully visible. Based on field surveys and related map measurements, the optimal height of the antenna installation can be calculated. In the calculation, attention should be paid to the Fresnel zone (Fresnel zone is an area around the signal centerline). There must be no obstacles in the Fresnel zone. If there are obstacles in the Fresnel zone, it will cause diffraction and attenuation of the signal, reducing the signal strength. Special considerations can also be made if the terrain conditions are special, such as cross-sections between links.

(4) Antenna azimuth
Calculating the theoretical angle of the antenna is conducive to the design of the link of the entire network, avoiding interference and reducing the construction difficulty of the project. When calculating the position of the equipment and the angle of the antenna, you should refer to the on-site physical map. The results calculated in this way are not necessarily accurate, and field tests are required.


3. System capacity


The system capacity of each central base station can be determined according to the planned number of sectors and the configured number of central base station equipment.

If SWR5800 broadband wireless access equipment is used, the total code rate of each device can reach 11Mbps, and the payload code rate (throughput payload) is 5Mbps (average speed). If the central base station is configured according to a set of central base station equipment (not multiplexed) in 8 sectors, the total code rate of the central base station is 88Mbps and the payload code rate (average speed) is 40Mbps.

If SWR5800 OFDM high-speed broadband wireless access equipment is used, the total code rate of each set of equipment is 54Mbps, and the payload code rate (throughput payload) is 30Mbps (average speed). If the central base station is configured according to a set of central base station equipment (not multiplexed) in 8 sectors, the total code rate of the central base station is 432Mbps and the payload code rate (average speed) is 240Mbps.


4. Equipment selection


After preliminary analysis, the central base station can use 8 SWR5800 APs with a power of 0.2W, and each is connected with a 16dBi flat antenna for coverage. Using a 45 ° sector antenna, the central base station can allow multi-user access within a 360 ° range. Guaranteed network bandwidth.

The equipment uses direct sequence spread spectrum technology (DSSS), with indoor / outdoor units and 10 / 100BaseT interface, plug and play. Using microwave direct sequence spread spectrum technology, the data transmission rate can reach 11Mbps.

Or use OFDM (Orthogonal Frequency Division Multiplexing) technology to ensure high-speed data rate, high spectrum utilization, and excellent anti-interference and multi-path effects. The data transmission rate can reach 54Mbps.

SWR5800 AP is determined according to the distance: <= 10km, use 0.1W SWR5800 AP;

The antenna uses a 16dBi flat sector antenna.

<= 20km with 0.2W SWR5800 AP;

The antenna uses 22dBi flat sector antenna.

> = 20km with 0.5W SWR5800 AP;

The antenna uses a 26dbi parabolic directional antenna.

If the two central base stations (HE) are more than 20 kilometers apart, relaying is required.

The IP telephone service can make more convenient contact between enterprises and institutions, and the cost of calling within the network is zero. The built-in QOS of the VOIP voice gateway can guarantee the quality of IP voice calls with very little bandwidth. Considering the scalability of the system in the future, it is recommended to use Sunet's voice gateway, which can be connected to the PBX. It is simple to install and easy to operate.

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