Important Wireless Facts To Keep In Mind

March 25, 2010
This sidebar helps a wireless designer select an operating frequency, calculate path loss and recieved power in a wireless network.

When selecting and applying wireless modules in a wireless sensor and actuator network (WSAN), keep some of these facts in mind:

• The range of each module is a function of transmitted power, antenna gain, and wavelength (λ). The greater the transmit power (Pt), transmit and receive antenna gains (Gt and Gr), and wavelength (λ), the higher the received power (Pr) for a given distance (d) or range. This is summed up in the basic Friis formula:

Pr = PtGtGrλ2 /16π2λ2

The key takeaway is that the range is greater at longer wavelengths or low frequencies.

• Path loss in dB can be estimated with the modified expression:

dB = (1/GrGt)(4πd/λ)2

Another path loss estimator is:

dB = 32.4 + 20log(f) + 20log(d) where d is in km and f is in MHz.

• For the popular 2.4-GHz band, the path loss can be estimated with:

dB = 40.2 + 20log(D) with d in meters and ≤ 8 m

dB = 58.3 + 33log(d/8) for d ≥ 8 m

For all these path loss calculations, assume a clear line-of-sight (LOS) path.

• Obstacles like walls, floors, and trees add from 3 to 18 dB to the path losses, depending upon the wall and floor composition and the frequency of operation. There is less loss at the lower frequencies. Brick and concrete have a higher attenuation than wood frame and sheet rock.

• At higher frequencies (2.4 GHz and beyond), reflections and multipath become a problem if there are many nearby objects.

• Lower frequencies are generally preferred but require longer antennas or less efficient shorter antennas.

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