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May 1, 2005

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

Determining the maximum range for 802.11 communication

Three major qualities determine 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.  These qualities are interrelated, and a change in one has direct effects on the others.  This month, we'll explore how these qualities go together to determine the range and data rate of an 802.11 device.

Like every RF signal, an 802.11 signal is transmitted at some power level and then grows weaker as it travels (propagates) away from the transmitter. This power loss is called "free space path loss," and is abbreviated as "FSPL". Two conditions will cause the signal to become unreceivable. First, all 802.11 radios have a minimum power level, called the receive sensitivity, below which a signal cannot be received. More sensitive cards can receive weaker signals than less sensitive cards. Second, at a certain point, the signal will fade into the background RF radiation, at which point the receiver won't be able to make out the signal, no matter how sensitive the receiver is.

Free space path loss occurs even when the signal is travelling through open air. When the signal has to pass through objects such as walls, it loses additional energy. This loss is known as absorption.

Understanding FSPL and absorption, we can begin to understand the relationship between transmit power and the signal's range. The stronger the signal starts out, the more power it can afford to lose to FSPL before it becomes too weak to be received. The more power the signal can afford to lose, the more distance it can travel and the more obstacles it can push through. Therefore, as transmit power goes up, so does range.

Digital RF signals like 802.11 use various encoding methods to encode 1's and 0's onto an analog RF waveform. More complex methods can squeeze more 1's and 0's into the same space, resulting in a higher data rate, but the more complex the encoding method is, the more susceptible the signal is to corruption. Higher data rates are more easily corrupted, while lower data rates are more corruption-resistant. The effect of this is that higher data rates fade into the background RF radiation sooner than lower data rates. Since the lower data rates remain recognizable longer, they have greater range. Therefore, as data rate goes up, range goes down.

In summary, transmit power, range, and data rate have a reciprocal relationship. If transmit power increases and data rate remains the same, range also increases. If data rate increases and transmit power stays the same, range decreases. If you want to increase data rate while keeping range the same, you'll need to increase transmit power accordingly.

Next month, we'll take a more in-depth look at these three factors and other factors that might affect the range and data rate of a signal.

Technology and Engineering

The 1st Fresnel Zone is a football-shaped valume of space centered around the line of sight between any two RF antennas. The picture depicts not only this Fresnel Zone, but a number of other antenna characteristics that are described fully
on the Antenna System Designer page.