Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


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Spring 2009

   
Essential Wi-Fi
For those who are new to Wi-Fi networking...
Technology and Engineering
For the engineer and Wi-Fi network administrator...

Essential Wi-Fi

Trade-Offs in Range, Power and Sensitivity

In fact, at Connect802, we believe that having an AP with good receive sensitivity is more important than having one with high transmit power in terms of increasing range. Assuming a two-way radio link is desired (as it always is in WLAN transmission), the range of a link is limited by the shortest range of the two transmitters. Therefore, if we had an AP with a high transmit power, it might be able to transmit packets to a PCMCIA card several miles away, but the PCMICA card's weak signal probably couldn't push a signal more than a few hundred feet. The effective range of the link is only a few hundred feet. One option might be to increase the transmit power of the PCMCIA cards, but this solution is undesirable for two reasons: first, you have to increase the power of ALL of your cards, which is expensive, and second, increasing the power of a device might require external amplifiers, etc..., which defeats the purpose of having a portable wireless station. On the other hand, if we have an AP with a very good receive sensitivity and only a moderate transmit power, the AP would be able to BOTH push a strong signal out to the PCMCIA card AND receive the PCMCIA card's weak response. This would increase the effective range of the link.


 Technology and Engineering

The Law of Antenna Reciprocity states that if a high gain antenna is used at one end of a wireless link to transmit the signal, then, assuming that both ends of the link have the same output power, it will also be able to receive the signal. This concept can be summed up as, "If you can hear me, then I can hear you.”. This is entirely true, but hearing is not the same as understanding. Because of signal to noise ratio, the statement “If you can understand me, I can understand you” does not necessarily apply.

A final note: we said in the above paragraph that increased antenna gain doesn’t increase SNR, but that’s not entirely true. Remember that antenna gain is accomplished by focusing the antenna’s coverage into a smaller and smaller area. As an antenna’s gain increases and its coverage gets more and more focused, it can happen that an interfering device ends up being excluded from the antenna’s coverage area. If this occurs, the noise floor that is perceived by the receiver could actually go down, and SNR could improve. All of these factors play into choosing an antenna for a wireless system.