Understanding RF Signals
An RF signal is an electromagnetic wave that communications systems use to transport information through air from one point to another. RF signals have been in use for many years. They provide the means for carrying music to FM radios and video to televisions. In fact, RF signals are the most common means for carrying data over a wireless network.
RF Signal Attributes
The RF signal propagates between the sending and receiving stations' antennae. As shown in Figure 3-2, the signal that feeds the antenna has an amplitude, frequency, and phase. These attributes vary in time in order to represent information.
Figure 3-2. Amplitude, Frequency, and Phase Are Basic Elements of RF Signal
The amplitude indicates the strength of the RF signal. The measure for amplitude is generally power, which is analogous to the amount of effort a person needs to exert to ride a bicycle over a specific distance. Power, in terms of electromagnetic signals, represents the amount of energy necessary to push the signal over a particular distance. As the power increases, so does the range.
As a radio signal propagates through the air, it experiences a loss in amplitude. If the range between the sender and receiver increases, the signal amplitude declines exponentially. In an open environment, one clear of obstacles, the RF signals experience what engineers call free-space loss, which is a form of attenuation. The atmosphere causes the modulated signal to attenuate exponentially as the signal propagates farther away from the antenna. Therefore, the signal must have enough power to reach the desired distance at a signal level acceptable that the receiver needs.
The ability of the receiver to make sense of the signal, however, depends on the presence of other nearby RF signals. For illustration, imagine two people, Eric and Sierra, whom are 20 feet apart and trying to carry on a conversation. Sierra, acting as the transmitter, is speaking just loud enough for Eric, the receiver, to hear every word. If their baby, Madison, is crying loudly, Eric might miss a few words. In this case, the interference of the baby has made it impossible to effectively support communications. Either Eric and Sierra need to move closer together, or Sierra needs to speak louder. This is no different than the transmitters and receivers in wireless systems using RF signals for communications.
The frequency describes how many times per second that the signal repeats itself. The unit for frequency is Hertz (Hz), which is the number of cycles occurring each second. For example, an 802.11b wireless LAN operates at a frequency of 2.4 GHz, which means that the signal includes 2,400,000,000 cycles per second.
The phase corresponds to how far the signal is offset from a reference point. As a convention, each cycle of the signal spans 360 degrees. For example, a signal might have a phase shift of 90 degrees, which means that the offset amount is one quarter (90/360 = 1/4) of the signal. A variation in phase is often useful for conveying information. For example, a signal can represent a binary 1 as a phase shift of 30 degrees and a binary 0 with a shift of 60 degrees. A strong advantage of representing data as phase shifts is that impairments resulting from the propagation of the signal through the air don't have much impact. Impairments generally affect amplitude, not the signal phase.
RF Signal Pros and Cons
As compared to using light signals, RF signals have the characteristics defined in Table 3-1.
These pros make the use of RF signals effective for the bulk of wireless network applications. Most wireless network standards, such as 802.11 and Bluetooth, specify the use of RF signals.
RF Signal Impairments
RF signals encounter impairments, such as interference and multipath propagation. This impacts communications between the sender and receiver, often causing lower performance and unhappy users.
Interference occurs when the two signals are present at the receiving station at the same time, assuming that they have the same frequency and phase. This is similar to one person trying to listen to two others talking at the same time. In this situation, wireless NIC receivers make errors when decoding the meaning of the information being sent.
The Federal Communications Commission (FCC) regulates the use of most frequency bands and modulation types to avoid the possibility of signal interference between systems. However, radio interference can still occur, especially with systems operating in license-free bands. Users are free to install and utilize license-free equipment such as wireless LANs without coordinating usage and interference.
Figure 3-3 illustrates various forms of interference. Inward interference is where external signals interfere with the radio signal propagation of a wireless network. This interference can cause errors to occur in the information bits being sent. The receiver eventually discovers the errors, which invokes retransmissions and results in delays to the users. Significant inward interference might occur if another radio system is operating nearby with the same frequency and modulation type, such as two radio LANs operating in the license-free bands within close proximity.
Figure 3-3. Radio Signal Interference Can Be Inward or Outward
Other sources of inward interference are cordless phones, microwave ovens, and Bluetooth devices. When these types of RF devices are in use, the performance of a wireless network can significantly decrease because of retransmissions and competition on the network for use of the medium. This requires careful planning and consideration of other radio devices that might interfere with the wireless network.
One of the best ways to combat RF interference is to eliminate the sources of interference. For example, a company could set a policy for not using cordless phones that fall within the same frequency band as the wireless network. The problem, however, is that it is often impossible to completely restrict the usage of potential interferers, such as Bluetooth devices. If interference is going to be a big issue, consider choosing a wireless network that operates in a frequency band that doesn't conflict.
Outward interference happens when the signals from the radio signal system interfere with other systems. As with inward interference, significant outward interference can occur if a wireless network is in close proximity with another system. Because wireless network transmit power is relatively low, outward interference rarely causes significant problems.
Multipath propagation occurs when portions of an RF signal take different paths when propagating from a source?such as a radio NIC?to a destination node, such as an access point. (See Figure 3-4.) A portion of the signal might go directly to the destination; and another part might bounce from a desk to the ceiling, and then to the destination. As a result, some of the signal encounters delay and travel longer paths to the receiver.
Figure 3-4. Obstacles Cause the Signal to Bounce in Different Directions
Multipath delays cause the information symbols represented in the radio signal to smear. (See Figure 3-5.) Because the shape of the signal conveys the information being transmitted, the receiver makes mistakes when demodulating the signal's information. If the delays are great enough, bit errors in the packet occur, especially when data rates are high. The receiver won't be able to distinguish the symbols and interpret the corresponding bits correctly. When multipath strikes in this way, the receiving station detects the errors through an error-checking process. In response to bit errors, the sending station eventually retransmits the data frame.
Figure 3-5. Smearing of the Signals Because of Multipath Causes Confusion and Bit Errors in Receivers
Because of retransmissions, users encounter lower performance when multipath is significant. As examples, 802.11 signals in homes and offices might encounter 50 nanoseconds (ns) multipath delay while a manufacturing plant could be as high as 300 ns. Based on these values, multipath isn't too much of a problem in homes and offices. Metal machinery and racks in a plant, however, provide a lot of reflective surfaces that cause RF signals to bounce around and take erratic paths. As a result, be wary of multipath problems in warehouses, processing plants, and other areas full of irregular, metal obstacles.
What can you do if multipath is causing problems? Aside from clearing desks and chairs from your building, diversity seems to be the best solution to combat the perils of multipath. Diversity is the use of two antennae for each radio NIC to increase the odds of receiving a better signal on either of the antennae.
Diversity antennae have physical separation from the radio to ensure that one will encounter fewer multipath propagation affects than the other. In other words, the composite signal that one antenna receives might be closer to the original than what's found at the other antenna. The receiver uses signal-filtering and decision-making software to choose the better signal for demodulation. In fact, the reverse is also true: The transmitter chooses the better antenna for transmitting in the opposite direction.