The RF signal that carries data

Although the wireless transmission of data is largely taken for granted, when one stops to consider how it actually happens, it does seem magical. There is, of course, a scientific explanation that can be understood by a non-WLAN professional. That explanation is the subject of this blog post.

The first thing to understand is that all wireless transmission is accomplished through waves. Consider the example of when a rock is thrown into a body of water producing circular waves emanating from the point of impact. The waves produced in the medium of water is a repeating pattern comprised of higher densities of molecules followed by lower densities, followed by higher densities and so on. When we hear a sound, we experience the impact of a wave being transmitted through the air. So just like on water (except now in three dimensions) the source of the sound emanates a force which produces higher densities of air molecules, followed by lower densities, followed by higher densities and so on. These spherical sound waves then come into contact with the eardrum to produce a vibration which the brain then translates into the experience or perception of sound.

In the case of Wi-Fi, the waves are produced on the electromagnetic spectrum. The electromagnetic spectrum consists of many different types of waves that make up the spectrum. The smallest type of waves is the gamma-ray, followed by X-rays. Further down the spectrum is the visible spectrum of light. Further, still are micro-waves. Finally, the largest waves found at the other end of the spectrum are radio waves. Radio waves are the waves upon which data are transmitted.

All types of waves can be measured by their amplitude and frequency. The amplitude of a wave measures how tall a wave is from its midpoint to its top or bottom. The frequency of a wave measures how fast a wave is traveling. This also translates into how compressed the wave is or how many crests and troughs can be found per unit of distance. Radio waves can be naturally generated through electric pulses. Both the amplitude and the frequency of a wave can be modulated.

Amplitude Modulation and Frequency Modulation

Amplitude Modulation (which gives the name to AM radio) is a process by which the amplitude (i.e., the height) of a radio wave can be modulated by a sound wave. Essentially, sound waves create vibrations in an electrical current flowing through a microphone or other device. This current can then be transmitted out through an antenna in the form of a modulated radio waves. These sound modulated radio waves emanate spherically from the source and can then be picked up by a device designed to receive these radio waves and convert them back into sound waves.

Frequency Modulation (which gives the name to FM radio) is the modulation of the frequency of a radio wave in order to transmit data. Essentially, the frequency can be modulated (i.e., sped up and slowed down) into patterns. A fast pattern (for example) could be made to represent a "one" and a slower pattern could be made to represent a "zero." These can then be transmitted as a binary code in various combinations which is the foundation of all data processed by computers. This data is then processed by the wireless device receiving the data into the programming for whatever function the device is accessing wirelessly.

QAM Modulation

Of course, the preceding descriptions of amplitude modulation and frequency modulation were purposefully simplistic in order to provide a conceptual understanding of the process by which it is possible to transmit data wirelessly. Certainly, the process by which the ever-increasing amount of data can be transmitted at the ever-increasing speeds of transmission makes the actual process more complicated. To better illustrate this complexity let's take a look at Quadrature Amplitude Modulation.

Quadrature Amplitude Modulation ("QAM") is a process of modulation that facilitates the transmission of digital information from one point to another. QAM efficiently transmits an analog signal which carries digital information. It does this by modulating the amplitude of two radio waves simultaneously out of phase with each other. This allows a wireless network operator to transmit more data at a faster speed, thus augmenting the bandwidth. Discussing this in any further detail is beyond the scope of this blog post. As said, the reason for including a description of QAM Modulation is simply to provide a glimpse as to how complicated this process can get and will continue to get as scientists and engineers develop new methods of transmitting more data at faster speeds as required by the current market of Wi-Fi consumers.

Transmitting Data Wirelessly is Simple and Complex

In short, the transmission of data wirelessly is made possible by the manipulation of radio waves. These waves are generated naturally by generating pulses of electricity. These radio waves can then be modified by their amplitude or frequency in order to transmit sound or data. This process can also be improved in order to increase the amount of data transmitted as well as the speed by which the data can be transmitted. As is illustrated by this blog post, the fundamentals of this process are relatively simple but can quickly become complex as the process is explored in greater detail.

Wi-Fi is all about data communication, the transferring of information between two or more components. There are three basic requirements for successful communications:

• Two or more devices that want to communicate
• A medium, a means, or a method for them to use to communicate
• A set of rules

Many components contribute to the successful transmission and reception of RF signals but I will focus on the key components.

• First, there is a transmitter that begins the RF communication. The transmitter takes the initial data and modifies the signal using a modulation technique to encode the data into the signal. The transmitter is also responsible for determining the power level of the wave, which is ultimately regulated by local domain authorities (such as the FCC in the United States).
• Next, an antenna collects the signal that it receives from the transmitter and directs the RF waves away from the antenna. As the RF waves move away from the transmitting antenna they move towards another antenna attached to the receiver, which is the final component in the wireless medium. The receiver takes the signal that it received from the antenna and translates the modulated signals and passes them on to be processed.

The signal is often altered during transmission between the two antennas due to interference and other RF behavior.

So the ultimate question is, will the RF communication work between all the main components? Although I won’t go into in-depth RF calculations, it’s important to highlight a few practical uses of RF measurement. 

Many Wi-Fi vendors define signal quality with a term called the signal-to-noise ratio (SNR). The SNR is the difference between the received signal and the background noise level. Data transmissions can become corrupted with a very low SNR which means your communication or data transfer via Wi-Fi will not work very well.

The power level of an RF signal required to be successfully received by the receiver radio is called the receive sensitivity. The lower the power level that the receiver can successfully process, the better the receive sensitivity. All of these radio frequency components and measurements determine the Wi-Fi experience you have when you connect your mobile device to a wireless access point.

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.

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

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.

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

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.

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.

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.