Essential Wi-Fi

Trade-Offs in Range, Power and Sensitivity

In the Fall 2008 issue, we explored three of the major factors that affect whether an 802.11 station can receive a signal: the signal's initial transmit power, the distance between the transmitter and the receiver, and the data rate of the signal. As promised, this month, we'll take a more in-depth look at these factors and some other factors that affect signal range.

Radio signal propagation is an area of consideration that has no options. That is, either the radio equipment provides proper connectivity to the user community in the desired coverage area, or it doesn't. Any trade-offs or compromises related to the radio equipment must always meet the requirement that the user community has the proper connectivity in the specified coverage area.

Previously, we said that as signal power increased, range increased, all other things being equal. When considering signal power, it's important to keep in mind the directionality of the antenna. An omnidirectional antenna, such as is typically found on access points and PCMCIA cards, transmits its energy equally in all directions. With an omnidirectional antenna, the range will be the same in all directions, assuming that obstructions (or the lack thereof) are the same in all directions. In reality, there will probably be different obstructions in each direction, so range in each direction will be different. For example, there might be a concrete wall behind the AP and open air in front of the AP. Range behind the AP will be reduced because the concrete wall will absorb more of the signal than the open air.

When range is needed in one specific direction, such as with a point-to-point link between two buildings, a directional antenna is typically used. A directional antenna puts more of the signal's energy in one direction than another, allowing greater range in one direction at the expense of decreased range in another. Depending on the area that needs coverage, this tradeoff may be worth it.

Examples of types of omnidirectional antennas include: dipole and slotted waveguide. Examples of types of directional antennas include: yagi, waveguide, patch, panel, parabolic dish, and sector.

Understanding Receive Sensitivity

Previously we also discussed receive sensitivity. We stated that the better a station's receive sensitivity, the weaker signal the station could receive, and the more range the station would have. Connect802 has performed a survey of the receive sensitivity of many models of access point and bridge on the market, and has concluded that receive sensitivity is one of the major factors that differentiates low-end, consumer-grade access points from high-end, enterprise-grade access points. The more expensive an access point is, the more likely it is to have good receive sensitivity. This might sound fairly obvious, but other qualities, such as transmit power, don't follow this pattern.

 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.

Signal to Noise Ratio

The amount of background RF noise in an environment is known as the noise floor. Background noise can be caused by natural phenomena like sunspots, certain electrical devices and systems, and other wireless devices. The power level of your RF transmission relative to the power level of the background RF noise is called the signal-to-noise ratio or SNR.

Imagine that you are in a totally silent environment like a deep cave, and you want to talk to a person 100 feet away from you. This environment has a very low noise floor. You could probably be heard at a volume not much more than normal talking voice. Now, imagine that you both are in a crowded airport. The sound of loudspeakers, people talking, music playing, etc. is background noise, and there is a significant noise floor. If you were to talk in the same volume, the person would probably not be able to hear you, even though your “output power” was the same as it was in the cave. This is because the energy level of your voice is either at, below, or just too close to the energy level of the noise floor. If you were to use a loudspeaker to amplify your voice, you would be raising the energy level of your voice well above the noise floor. Your signal-to-noise ratio is now high enough that the person you are talking to can hear you.

The key concept here is that signal strength (volume) alone is not enough to guarantee that a received signal will be understood. In addition to having sufficient raw energy, a signal must also be sufficiently stronger than the background noise. If a signal is too weak, it may simply not reach the receiver with enough strength to be heard. But even if a signal is strong enough to be heard, excess background noise may make it unintelligible to the receiver. This is especially relevant to the troubleshooting of RF networks, because the amount of background noise in an environment can, and often does, change over time. You might install a wireless point-to-point link with more than enough output power to make the link work, and then one day the link might stop working due to a new noise source increasing the noise floor and therefore decreasing the SNR of the signal that the receiver gets.

SNR in relation to The Law of Reciprocity and Antenna Gain

The law of antenna reciprocity states that an antenna will amplify signals it receives the same amount as it amplifies signals it sends. One unexpected effect of this is that the gain of the antenna also amplifies the noise floor! This means that your SNR is based upon the gain of the sent signal ONLY. Here is an example:

Consider a case where a signal is transmitted at a certain strength, travels a certain distance, and is received by two separate devices. Just before the signal enters Device A’s antenna, it is at a strength of 65 dBm. The noise floor in Device A’s vicinity is -100 dBm. This means that the SNR of the signal just before it enters Device A’s antenna is 65 dBm – 100 dBm = 35 dB. Assume that Device A has a 10 dBi antenna. That gain will apply to both the received signal and the received noise, meaning that Device A will perceive an incoming signal of -55 dBm and a noise floor of -90 dBm (both parameters 10 dB greater than their environmental value). The resulting SNR perceived by Device A is 55 dBm – 90 dBm, still 35 dB.

Conclusion

In the Fall 2008 article, we discussed that the law of antenna reciprocity meant that you could increase a device’s range, both transmitting and receiving, with a higher-gain antenna. Because of that same principle, higher antenna gain on the receiver doesn’t increase SNR. The most straightforward way of increasing SNR is to increase the output power or antenna gain of the transmitting device. This causes the receiving device to perceive a higher signal strength, which means that the incoming signal is further above the noise floor, therefore, the SNR of the received signal is higher. If this method is used, it may be necessary to also increase the output power of the receiving device, so as to avoid the unbalanced power effect (also discussed last month).