Universal

Wireless signal strength decreases during transmission because of free-space loss, penetration loss, and device and connection loss. You need to consider these link budgets when calculating the signal strength.

• Free-space loss model

  The free-space loss model is used to calculate the link budget of indoor DAS APs and indoor APs. The following formulas are used:

  • 20logf + 20logd - 28 (f: MHz; d: m)
  • 20logf + 20logd + 32.4 (f: MHz; d: km)
  • 20logf + 20logd + 92.4 (f: GHz; d: km)
• COST231-Hata model

  The COST231-Hata model is used to calculate the link budget of outdoor APs and applies to 2000 MHz or lower frequency bands. To calculate the link budget on the 2.4 GHz frequency band, a correction parameter Cm is used: PL = 46.3 + 33.9lg(f) - 13.82lg(hb)-a(hm) + (44.9-6.55lg(hb))lg(d) + Cm

  The Cm value depends on the environment:

  • Dense Urban: -3
  • Urban: -6
  • Suburban: -12
  • Rural: -20
  • In the formula, hb indicates the height of base station antenna (in meters), and hm indicates the height of mobile station antenna (in meters).
  • f indicates the antenna working frequency (in MHz), and d indicates the transmission distance (in km).
  • a is a function, which also depends on the environment:
• Dense urban and urban: a(Hr) = 3.2log2(11.75 Hr) - 4.97
• Suburban and rural: a(Hr) = (1.1log(f) - 0.7) Hr – (1.56log(f) – 0.8)
• Penetration loss

  APs' coverage area is restricted by the multipath effect. Penetration and diffraction capabilities of wireless signals are weak; therefore, wireless signals attenuate greatly when blocked by obstacles. The following are penetration loss values of 2.4 GHz radios when penetrating various materials:

  • 8 mm board: 1-1.8 dB
  • 38 mm board: 1.5-3 dB
  • 40 mm wooden door: 2-3 dB
  • 12 mm glass: 2-3 dB
  • 250 mm concrete wall: 20-30 dB
  • Brick wall: 15 dB
  • Inter-floor penetration: 30 dB
  • Elevator: 20-40 dB
• Device and connection losses

  Radio frequency (RF) devices, such as cable connectors, splitters, couplers, combiners, and AC filters, have insertion losses.

  • The insertion loss of a cable connector ranges from 0.1 dB to 0.2 dB.
  • The insertion loss of a combiner is 0.5 dB.
  • For the insertion losses of passive devices, see corresponding product manuals. Table 4-2 lists transmission losses of various cables.
    Table 15-10 Cable transmission losses

    Name	Transmission loss in 900 m dB/100 m	Transmission loss in 2100 m dB/100 m	Transmission loss in 2400 m dB/100 m
    1/2-inch feed line	7.04	9.91	12.5
    7/8-inch feed line	4.02	5.48	6.8

• Link budget calculation method
  • Power budget

    Transmit power + Tx gain - path loss + Rx gain = Signal strength

  • AP transmit power

    An AP's transmit power depends on its specifications.

  • AP Tx antenna gain and STA Rx antenna gain

    The antenna gain is determined by antenna specifications. Generally, the value is 2 dBi.

  • Path loss

    Path losses include free-space loss, penetration loss, and loss on cables.

    The penetration loss cannot be calculated accurately because it depends on wall materials and signal transmission angle. Generally, the penetration loss count as 25 dB.

Obstacles in the coverage area of an indoor or outdoor AP can cause an obvious loss of signals. The following table lists the path losses in 2.4 GHz and 5 GHz frequency bands caused by various obstacles.

If there are metal objects, load-bearing columns, or beams in the target coverage area, ensure that wireless signals are not blocked by them because they cause a large penetration loss.

The penetration loss is minimum when signals penetrate a wall vertically, and the penetration loss is much larger when wireless signals penetrate a wall obliquely. Therefore, when you install APs, try to reduce the incidence angle of signals.

Circular polarization has not been applied to WLAN networks. Cross polarization (+45/-45 degree or 0/90 degree) is mainly applied to outdoor directional antennas. Cross-polarized antennas provide wireless signals at 2.4 GHz and 5 GHz frequency bands.

Huawei APs support automatic channel selection. However, in large-scale AP deployment, channels are selected before deployment. Changing one AP's channel will cause channel switching on other APs, affecting wireless services on the entire network. Therefore, automatic channel selection is not recommended.

The SNR value obtained using a signal measurement tool is not the actual SNR on a network adapter. The tool obtains the SNR value by comparing the detected signal strength with a predefined noise value (-96 dBm for example). A high SNR value does not indicate a good signal quality because the high SNR may be caused by interference signals. The signal quality should be evaluated by the SNR and signal to interference ratio (SIR).

The following technologies can be used against multipath interference: smart antenna, multiple-input and multiple-output (MIMO) beamforming, MIMO space-time block coding (STBC), and MIMO maximal ratio combing (MRC).

Measure the signal loss caused by an obstacle as follows:

1. Place and start a signal source. Place the signal source at a proper position. Ensure that no obstacle exists between the signal source and obstacle to be measured, and keep a certain distance from the signal source to the obstacle. If the signal source is close to the obstacle to be measured, the field strength near the signal source fluctuates greatly, leading to inaccuracy of measured values.
2. Use a signal scanning tool to measure field strengths on both sides of the obstacle. The difference between the measured values is the signal loss.

As shown in Figure 15-4, configure the 2.4 GHz and 5 GHz radios of a Fat AP. The field strengths of 2.4 GHz and 5 GHz signals measured at test point 1 are both -50 dB, and the field strengths of 2.4 GHz and 5 GHz signals measured at test point 2 are -60 dB and -65 dB, respectively. The loss of 2.4 GHz signals caused by the obstacle is 10 dB, while the loss of 5 GHz signals is 15 dB.

Figure 15-4 Measuring the signal loss caused by an obstacle

Standard of good radio signals: The signal strength is no lower than -65 dBm in indoor areas and no lower than -70 dBm in outdoor areas.

To meet users' normal roaming requirements, the recommended signal overlapping area of neighboring APs is between 10% and 20%.

The interval between APs changes according to the field strength and network capacity. In office scenarios, the required per-user bandwidth is 1 Mbit/s to 2 Mbit/s, and the concurrency rate is around 30% to 50%. When the per-user coverage area is 3 square meters, the recommended interval between APs is 15 m to 20 m.

As shown in the following figure, the Fresnel zone is an ellipsoid. Obstacles in the zone will adversely affect signal transmission. If no obstacle exists in the zone, radio signals can travel in an approximately free space.

Figure 15-5 Fresnel zone

Fresnel zones differ depending on the Fresnel zone radius. In a free space, radio signals are mainly transmitted between antennas in the first Fresnel zone. In normal cases, the Fresnel zone refers to the first Fresnel zone.

The radius of the first Fresnel zone is calculated as follows:

• r: indicates the radius of the first Fresnel zone, in meters.
• d: indicates the distance between two antennas, in km.
• f: indicates the signal frequency, in GHz.

With 5 GHz signals as an example, Table 15-12 lists the radius of the first Fresnel zone in different backhaul distances.

When factors such as interference and circuit loss are not considered, the formula for calculating the received signal strength indicator (RSSI) is as follows:

RSSI = Transmit power + Antenna gain at the transmit end – Path loss – Obstacle attenuation + Antenna gain at the receive end

The relationship between the path loss and distance is as follows: (L: Path loss, in dB; f: Working frequency, in MHz; d: Distance, in m)

• Indoor semi-open scenario

  2.4 GHz: L = 46 + 25lg(d)

  5 GHz: L = 53 + 30lg(d)

  The following table lists the typical path loss values at different distances.

  Table 15-13 Relationship between the pass loss and distance in indoor semi-open scenarios

  Distance (m)	2.4 GHz Path Loss (dB)	5 GHz Path Loss (dB)
  1	46	53
  2	53.5	62
  5	63.5	74
  10	71	83
  15	75.4	88.3
  20	78.5	92
  40	86	101

• Outdoor coverage scenario

  2.4 GHz and 5 GHz: L = 42.6 + 26lg(d) + 20lg(f)

  The following table lists the typical path loss values at different distances.

  Table 15-14 Relationship between the pass loss and distance in outdoor coverage scenarios

  Distance (m)	2.4 GHz Path Loss (dB)	5 GHz Path Loss (dB)
  50	76.4	84
  100	84.2	91.9
  200	92	99.7
  300	96.6	104.2
  500	102.4	110
  800	107.7	115.4
  1000	110.2	117.9

• Backhaul scenario

  5 GHz: L = 32.4 + 26lg(d) + 20lg(f)

  The following table lists the typical path loss values at different distances.

  Table 15-15 Backhaul scenario

  Distance (km)	5 GHz Path Loss (dB)
  0.5	100
  1	108
  2	115.8
  3	120.4
  5	126.1
  8	131.4
  10	134

Assume that the transmit power of an indoor AP is 20 dBm and the 5 GHz antenna gain is 6 dBi. In an indoor half-open scenario, the 5 GHz path loss at a point 20 m away from the AP is 92 dB. If the receive terminal is a mobile phone (usually with the antenna gain of 0) and there is no obstacle between the mobile phone and AP, the 5 GHz RSSI of the mobile phone is –66 dBm (20 + 6 – 92 – 0 + 0).