RF Engineering for 802.11

802.11 has been adopted at a stunning rate. Many network engineers accustomed to signals flowing along well-defined cable paths are now faced with a LAN that runs over a noisy, error-prone, quirky radio link. In data networking, the success of 802.11 has inexorably linked it with RF engineering. A true introduction to RF engineering requires at least one book, and probably several. For the limited purposes I have in mind, the massive topic of RF engineering can be divided into two parts: how to make radio waves and how radio waves move.

RF Components

RF systems complement wired networks by extending them. Different components may be used depending on the frequency and the distance that signals are required to reach, but all systems are fundamentally the same and made from a relatively small number of distinct pieces. Two RF components are of particular interest to 802.11 users: antennas and amplifiers. Antennas are of general interest since they are the most tangible feature of an RF system. Amplifiers complement antennas by allowing the antennas to pump out more power, which may be of interest depending on the type of 802.11 network you are building.

Antennas

Antennas are the most critical component of any RF system because they convert electrical signals on wires into radio waves and vice versa. In block diagrams, antennas are usually represented by a triangular shape, as shown in Figure 10-8.

Figure 10-8. Antenna representations in diagrams

To function at all, an antenna must be made of conducting material. Radio waves hitting an antenna cause electrons to flow in the conductor and create a current. Likewise, applying a current to an antenna creates an electric field around the antenna. As the current to the antenna changes, so does the electric field. A changing electric field causes a magnetic field, and the wave is off.

The size of the antenna you need depends on the frequency: the higher the frequency, the smaller the antenna. The shortest simple antenna you can make at any frequency is 1/2 wavelength long (although antenna engineers can play tricks to reduce antenna size further). This rule of thumb accounts for the huge size of radio broadcast antennas and the small size of mobile phones. An AM station broadcasting at 830 kHz has a wavelength of about 360 meters and a correspondingly large antenna, but an 802.11b network interface operating in the 2.4-GHz band has a wavelength of just 12.5 centimeters. With some engineering tricks, an antenna can be incorporated into a PC Card or around the laptop LCD screen, and a more effective external antenna can easily be carried in a backpack or computer bag.

Antennas can also be designed with directional preference. Many antennas are omnidirectional, which means they send and receive signals from any direction. Some applications may benefit from directional antennas, which radiate and receive on a narrower portion of the field. Figure 10-9 compares the radiated power of omnidirectional and directional antennas.

Figure 10-9. Radiated power for omnidirectional and directional antennas

For a given amount of input power, a directional antenna can reach farther with a clearer signal. They also have much higher sensitivity to radio signals in the dominant direction. When wireless links are used to replace wireline networks, directional antennas are often used. Mobile telephone network operators also use directional antennas when cells are subdivided. 802.11 networks typically use omnidirectional antennas for both ends of the connection, although there are exceptionsparticularly if you want the network to span a longer distance. Also, keep in mind that there is no such thing as a truly omnidirectional antenna. We're accustomed to thinking of vertically mounted antennas as omnidirectional because the signal doesn't vary significantly as you travel around the antenna in a horizontal plane. But if you look at the signal radiated vertically (i.e., up or down) from the antenna, you'll find that it's a different story. And that part of the story can become important if you're building a network for a college or corporate campus and want to locate antennas on the top floors of your buildings.

Of all the components presented in this section, antennas are the most likely to be separated from the rest of the electronics. In this case, you need a transmission line (some kind of cable) between the antenna and the transceiver. Transmission lines usually have an impedance of 50 ohms.

In terms of practical antennas for 802.11 devices in the 2.4-GHz band, the typical wireless PC Card has an antenna built in. Built-in antennas work, but they will never be anything to write home about. At best, the built in antenna in a PC card is mediocre. Larger antennas perform better. Some PC Card 802.11 interfaces have external antenna jacks. With an optional external antenna, the card has better performance, at the cost of aesthetic quality. In response to the need for improved antenna performance without ugly space-consuming external antennas, many laptops now use an antenna built into the frame around the laptop screen.

Amplifiers

Amplifiers make signals bigger. Signal boost, or gain, is measured in decibels (dB). Amplifiers can be broadly classified into three categories: low-noise, high-power, and everything else. Low-noise amplifiers (LNAs) are usually connected to an antenna to boost the received signal to a level that is recognizable by the electronics the RF system is connected to. LNAs are also rated for noise factor, which is the measure of how much extraneous information the amplifier introduces. Smaller noise factors allow the receiver to hear smaller signals and thus allow for a greater range.

High-power amplifiers (HPAs) are used to boost a signal to the maximum power possible before transmission. Output power is measured in dBm, which are related to watts (see the "Decibels and Signal Strength" sidebar earlier in this chapter). Amplifiers are subject to the laws of thermodynamics, so they give off heat in addition to amplifying the signal. The transmitter in an 802.11 PC Card is necessarily low-power because it needs to run off a battery if it's installed in a laptop, but it's possible to install an external amplifier at fixed access points, which can be connected to the power grid where power is more plentiful.

This is where things can get tricky with respect to compliance with regulations. 802.11 devices are limited to one watt of power output and four watts effective radiated power (ERP). ERP multiplies the transmitter's power output by the gain of the antenna minus the loss in the transmission line. So if you have a 1-watt amplifier, an antenna that gives you 8 dB of gain, and 2 dB of transmission line loss, you have an ERP of 4 watts; the total system gain is 6 dB, which multiplies the transmitter's power by a factor of 4.