Essential Wi-Fi

Double The Power, Double The Distance? Not Necessarily!

At Connect802, we like to say, "There's no such thing as a free lunch in the physics department!" What we mean is that RF signal propagation is governed by certain fundamental principles and physical laws that put an upper limit on the performance of a radio link. An understanding of these principles and physical laws provides a framework for understanding why radio links act the way they do, but it's important to remember that real environments usually exhibit more complex behavior than a purely mathematical analysis could capture. You don't always get the results that you want. For example, you'd think that increasing signal power by using a higher-gain antenna or a more powerful radio would increase the range of your signal proportionally to the power increase. Not necessarily... We've discussed the "no free lunch" issues in past newsletters, but we'll recap here in case you didn't read those articles. The primary performance characteristic of an RF link is its throughput--that is, the amount of data that it can carry in a certain amount of time. Throughput is usually measured in kbps or Mbps. Latency--that is, the amount of time data takes to get from one end of the link to the other--is another performance characteristic of an RF link, but, at least as far as WLANs are concerned, latency is often secondary to throughput. The latency of WLAN links is usually lower than that required by the applications that use those links, whereas applications often could use more throughput than the link can provide. In the interest of brevity, therefore, we'll focus just on throughput (at least this month). The throughput of an RF link is a function of two main parameters: the signal-to-noise ratio and the complexity of the signal's encoding and modulation. More complex encoding and modulation generally results in higher data rates, but also more susceptibility to corruption, which means that a higher signal-to-noise ratio is necessary to correctly receive the signal. Signal-to-noise ratio is directly dependent on the output power of the signal. In other words, all other things being equal, in order to get more throughput, you need more power. As you get farther from the transmitter, of course, the power that you receive from the transmitter decreases, so range can be considered as a third parameter in the above equation. More power equals either more throughput or more range or some proportional combination of the two. The floorplan shown to the right depicts a single access point and the coverage pattern resulting from progressively increasing the effective output power. The increase could be from configuring the access point to use a higher transmit power, or by adding a higher gain antenna to the unit. Consider the range of the coverage pattern and notice that increased power output doesn't directly translate into proportionally increased coverage. The discussion that follows explains the situation. In the simplest scenarios, such as two antennas in outer space with nothing between them and nothing around them, the increase in range is directly proportional to the increase in power. In other words, double the power and you get double the range. Manufacturers of RF equipment base the sale of high-gain antennas and high-powered radios on this principle. For example, Linksys sells a 5 dBi antenna that can replace the 2 dBi antennas that come with its access points. Theoretically, the extra 3 dB of gain will double the output power and range of the antenna. But real-world scenarios aren't this simple. Consider a hypothetical wall consisting of gypsum board (drywall) over wood studs. Gypsum board causes approximately 3 dB of attenuation per sheet at 2.4 GHz, which means that a typical interior wall would cause approximately 6 dB of attenuation, since it has drywall on both sides. Imagine now that a weak signal from an AP hits this wall and the first sheet of drywall "soaks up" the last of the signal energy so that a client on the other side of the wall can't see the AP at all. If you were to swap out the regular 2 dBi antennas on the AP with higher gain 5 dBi antennas, would the problem be fixed? The extra 3 dBi of gain would allow the signal to push through the first sheet of drywall, but it would still not have enough energy to push through the second sheet. The client would be in the exact same situation, and you would have wasted the $60 you spent on the higher-gain antenna. The output power of the device has increased, but it hasn't resulted in an increase in usable range. The example is, in some ways, as over-simplified as the "two antennas in outer space" example, but it reveals a very real phenomenon that isn't taken into account by simple mathematical propagation models. In indoor environments, we find that absorption from walls is the major limitation to signal range. In order to get additional usable range, the power of the device must be increased at least enough to "push through" additional walls. This doesn't mean that small increases in signal power have no effect on usable range in indoor environments, just that they don't have the pronounced effect that they would have in an unobstructed environment. For example, in an open field, with the AP mounted on a pole so that there were no obstructions between the AP and the clients, a 3 dB increase in output power probably would result in a near-doubling of usable range, as the mathematics predict. In indoor environments, this isn't the case. In our experience, in a typical indoor residential or office building with drywall construction, an increase in power of roughly 6 to 9 dB is required before you see a significant increase in coverage. For example, we recently did a model based on Cisco AP1200 access points. These APs have adjustable output power and can be set to 1 mW, 5 mW, 10 mW, 20 mW, or 30 mW. In order to optimize the coverage of the APs, we tested several different power levels for each AP. We found that there wasn't much difference between output powers that differed less than about 6 dB. For example, there was a significant difference between 1 mW and 5 mW (about a 7 dB difference), but to get significantly more coverage than that given by 5 mW, we had to go from 5 mW to 20 mW. Going from 5 mW to 10 mW is only a 3 dB difference, but 5 mW to 20 mW is 6 dB.
		At 1 mW of output power, the AP doesn’t reach to the upper-left corner and doesn’t completely fill the room to its lower-right. It doesn’t reach at all into the two rooms to its lower-left.
		At 5 mW (7 dB greater than 1 mW), the AP still doesn’t fill the upper-right corner, because of the acute angle between it and the wall that occludes the upper-right corner. It now fully covers the room to the lower-right, and barely reaches into the two rooms to the lower-left. Notice that if you were in those rooms, you might wonder why the increase in output power hadn’t resulted in much better coverage. There was barely enough power to get to the walls at 1 mW; now there’s barely enough to get through them.
		At 10 mW (3 dB greater than 5 mW), the coverage area is a little more filled-out, but basically the same size. The only major improvements over 5 mW are that one of the two rooms to the AP’s lower-left is now filled in. The AP’s coverage extends more to the right and there is a small spike of coverage to the lower-right. This is because the signal is passing through open space (doorways and windows and open floor space) in these areas, so there is a more proportional relationship between range and output power. In the areas where coverage goes through a wall, it takes a lot more output power to get a little more range
		Here’s 20 mW. The coverage through the open areas to the right is much bigger, but coverage in the walled areas is only a little bigger.
		Here’s 30 mW. Finally,enough output power is present to fully cover the upper-right corner of the building. Notice that the power had to be raised from 1 mW to 30 mW before the signal could push through the walls in the upper-left of the floor to get full coverage in that corner.

 Technology and Engineering

How To Get More Range: Output Power, Antenna Gain, or Receive Sensitivity?

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.


To Infinity... and Beyond!

Will Sprint deploy nationwide WiMAX ?

The word on the web is that Sprint intends to deploy a national, mobile WiMAX network that will compete with the current 3G cellular CDMA network.

Of course, before that can happen an open modem technology must be standardized to allow WiMAX devices from multiple vendors to interoperate. Unfortunately, Sprint lacks the licenses to deploy a national WiMAX network in the 700 MHz band. They're going to have to deal with the FCC on obtaining the appropriate spectrum rights to proceed. Of course, even if every aspect of such an aggressive project had the "green light" today it would be years before an infrastructure and a user-product base could roll out to make fantasy into reality. There is a key point to take away from these conjectures however. The move towards a converged communications infrastructure is proceeding. Little by little we'll continue to see new technologies offering faster data rates and enhanced services. As we say at Connect802, "There's no free lunch in the physics department". The impact of mobility and long range in the wide area broadband environment contrasted with the relatively stationary (pedestrian speeds) and short range in the in-building and HotZone environment mean that there will always be a capacity and performance distinction between the two. As wide area technologies improve, so will in-building and HotZone technologies. As the wide area moves towards 20 Mbps data rates, in-building will approach 100 Mbps. In the years to come, when wide area connectivity pushes 100 Mbps to your handheld device, in-building will be pushing Gigabit rates to your notebook computer. But, at the proverbial end of the day, at Connect802, we've got you covered.

Web Searching: The Connect802 Web Presence

At Connect802 we're your PAGE ONE resource for wireless networking!

Connect802 has the experience, expertise, and resources to help you with your wireless network system. Use your favorite search engine and see what Connect802 is doing. Each month we give you some suggested search terms for you to explore. Here's this month's list. As you look down the search engine results you'll find Connect802 either at the top, or on the first page (true for Google and Excite, unknown for the rest).

• RF Spectrum Analysis for Wi-Fi
• Antenna System Designer
• What Wi-Fi antenna system should I use
• Wi-Fi for RFID backhaul
• Metro Wi-Fi design
• HotSpot Access Controller
• On-Line Wi-Fi Encyclopedia
• WMTS Design Consulting