This month's "Essential Wi-Fi" addresses the issue of increasing range by increasing output power. The specific example given is of switching to higher gain antennas. In that article, we discuss how simply increasing output power may not result in the increase in usable range that you expect. It turns out that the usable range of an AP is dependent on several factors. In this column, we examine those factors and discuss the best way to increase the range of an AP in an indoor environment.
We all know of vendors who sell access points with extremely high power output. For example, one vendor sells an AP that outputs a full watt of power (the FCC maximum) with no external amplifiers. Another vendor sells a PCMCIA card that outputs 250 mW with no external amplification (most PCMCIA cards are around 30 mW). Clients using these devices and expecting dramatic range increases may be disappointed. An 802.11 link is two-way; it doesn't do any good for the one device to be able to blast a signal a long way if the other device can't get a signal back. For example, a 100 mW AP might be able to blast its Beacon packets through several walls. A regular PCMCIA card with 30 mW of output power could receive those Beacons and the user would see the AP in his or her list of available wireless networks. But when the user tried to connect to the AP, the user's card wouldn't have enough power to get back to the AP, and the connection would always fail.
This example points out a fundamental limitation of high-output APs. It might seem at first like a high-powered AP is a great, cost-effective way of increasing the range of your network. An casual site survey, which only measured receive signal strength, would even confirm that the AP's coverage had increased (if all you intend to do is receive packets, then the site survey is right). But as soon as clients actually tried to connect to the network, the flaw would become obvious. In general, there's not much point in having an AP that is much more powerful than the clients that it will serve. All of the AP's extra transmit power will go into pushing signal into areas where the clients can't get a signal back to the AP, and that essentially wastes that coverage, and all the money you spent on a high-powered AP. Typical wireless client cards have an output power between 15 and 30 mW, so we at Connect802 usually design networks with APs that have approximately this output power.
Increased output power is useful if you know that all of the devices in the network will be using the same power output. For example, if you've got two wireless bridges at the end of a point-to-point link, you could increase the usable range of the link by putting an equal amplifier on both of the bridges. This circumstance is difficult to guarantee in the more typical one-AP, many clients scenario, so it's usually better to design that type of network with APs that only put out as much power as the lowest-powered client that you expect to use the network. Another option is to design the network so that clients using maximum power (say, 30 mW) will get maximum data rates (say, 54 Mbps) and clients using less power will still be able to connect, but only at lower data rates.
Increased power output in the AP doesn't result in increased range if clients are not able to get a signal back to the AP, but there's a second parameter in this equation that we have so far neglected: receive sensitivity. Receive sensitivity determines the weakest signal that a device can reliably receive. If an AP combines increased power output with a proportionally better receive sensitivity, then it can not only transmit a more powerful signal to the client, but it can also receive the client's weaker signal. For example, consider a client that transmits at 15 mW and has a receive sensitivity of -72 dBm. Now consider an AP that has a transmit power of 30 mW. The AP is transmitting 3 dB "louder" than the client, so its receive sensitivity must be 3 dB better than the client's (-72 dBm minus 3 dB = -75 dBm) to offset the client's weaker transmit power. If this relationship holds true, then the combination of increased transmit power and increased receive sensitivity will result in better range. To put it simply, just making the AP talk louder doesn't work, but by increasing the AP's receive sensitivity, we've made it both talk louder and listen harder.
Unlike transmit power, the receive sensitivity of an radio is not directly adjustable; it's a fundamental property of the engineering of the radio. But the receive sensitivity of an 802.11 radio differs depending on the data rate that the radio is using, with higher data rates requiring more signal power. Typically, an 802.11 network is designed towards a certain minimum desired data rate, and then radios are purchased with an output power and a receive sensitivity that allows them to achieve that data rate in the required coverage area.
At this point, we have discussed the relationship between usable range, transmit power, and receive sensitivity. Essentially, we are considering two link budgets: one from the client to the AP and one from the AP to the client. These link budgets give a maximum range from the AP to the client and from the client to the AP. The usable range of the AP is limited by the smaller of these two ranges. If the AP-to-client range is already greater than or equal to the client-to-AP range, then increasing the AP's transmit power won't result in more usable range because increasing transmit power only increases the AP-to-client side of the link. Increasing transmit power with a corresponding increase in receive sensitivity will result in more usable range because increasing receive sensitivity also increases the client-to-AP side of the link.
Increasing transmit power can increase usable range in another way that is somewhat independent of receive sensitivity. Each 802.11 data rate requires a certain minimum signal-to-noise ratio. If the signal is too weak to achieve the required signal-to-noise ratio for a given data rate, increased transmit power is the only answer. Increasing receive sensitivity or antenna gain doesn't work because both of those options will amplify the noise as well as the incoming signal, leaving the signal-to-noise ratio the same. In summary, if range is limited by interference, you must at least increase transmit power to the point where the clients receive the minimum signal-to-noise ratio for the desired data rate. At that point, range may still be limited by the AP's receive sensitivity or the client's transmit power or receive sensitivity.
Increasing antenna gain is another way of increasing range. Increasing antenna gain differs from increasing transmit power because of the principle of antenna reciprocity. Put one way, antenna reciprocity states that the same qualities that increase an antenna's gain when it transmits also increase its gain when it receives. This means that an increase in antenna gain on either the client or the AP increases BOTH the client-to-AP side of the link AND the AP-to-client side of the link.
Putting a higher-gain antenna on the access point can be a cheap way of increasing usable range for all clients of that AP, regardless of the clients' transmit powers and receive sensitivities. The tradeoff is that increasing antenna gain changes the antenna's coverage pattern, which may limit the maximum antenna gain that can be achieved. More than about 6-10 dB of antenna gain usually results in coverage area that is small enough to preclude any link except one with fixed endpoints at pre-determined locations.
What conclusions can we draw from this analysis? Putting a higher-gain antenna on the AP increases performance for all clients of the AP, but may not offer enough extra power to significantly increase coverage, especially in indoor environments. Increasing the AP's power output might seem like a cheap way of increasing range, but in reality, a complex link budget relationship exists between transmit power and receive sensitivity on the client and AP that dictates the usable range for each client. Simply dropping a 1000 mW into the center of a building is unlikely to provide the results that the AP's vendor would promise or that naive WLAN administrator might expect. Buying an AP with a very good receive sensitivity (for example, an 802.11g AP with receive sensitivity of -75 dBm or lower at 54 Mbps, -95 dBm or better at 6 Mbps) is a cost-effective way of maximizing the effectiveness of the clients' power output.
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