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Understanding RF ComponentsRF is, by many, still considered black magic. In reality it is no more magic than the electrical current that travels down the wires in your house to light a lamp or sound a doorbell. The one difference is how the energy is moved from one location to another. To efficiently and properly use RF to perform useful work, you need to be aware of various aspects and factors surrounding RF technologies. In most cases, these characteristics are interrelated, and understanding how one characteristic affects another can help in selection of the proper technology and proper installation techniques. Frequency is a characteristic that relates to the physical relationship of the transmitted signal and time, whereas modulation deals with how information is carried on that signal. Signal strength and RF power are parameters that determine the energy level that is being received or transmitted. FrequencyBack in 1864, the Scottish theoretician James Clerk Maxwell first developed the idea that electromagnetic waves arose as an electric current and changed direction. In the 1880s, Heinrich Hertz used this idea to develop the first RF device that sent and then received electromagnetic waves over the air. This radio was capable of increasing the number or frequency of waves produced in a given period of time and how fast they changed. Based on this discovery, his name became a common unit of measure for frequency, where 1 hertz (Hz) means one complete oscillation, or cycle, per second. In radio, kilohertz (kHz) means thousands of these waves per second, megahertz (MHz) means millions of waves per second, and so on through gigahertz (GHz). amplitude, frequency, and phase. The method in which these properties carry the information on a sine wave is known as modulation, which significantly affects data output and other key RF attributes. Figure 2-1. Sine Wave Throughout the years, technology has pushed frequencies higher and higher. The initial WLANs were designed using radios that were converted from voice-type radios (land mobile walkie-talkies) that utilized the 450-MHz range. With the need for unlicensed spectrum and higher data rates, there was a move to 900 MHz in the late 1980s. Within a few years, the WLAN industry had moved to 2.4 GHz, and in 2000, this was moved even higher with the release of WLAN products in the 5-GHz range. You learn more about the frequency spectrum in Chapter 3, "Regulating the Use of 802.11 WLANs." ModulationModulation is a process by which information signalsanalog or digitalare transformed into waveforms suitable for transmission across some medium or channel. For WLANs, the medium or channel is the RF carrier, which has embedded digital information. The RF carrier will have a particular set of frequencies, with some minimum and maximum range. The overall amount of frequency spectrum used by a channel is known as the RF bandwidth. Modulation and RF bandwidth are fundamental components of a digital communication system. Modulation can be accomplished by changing the amplitude, frequency, or phase of the carrier in accordance with the incoming bits. These techniques are called amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM), and orthogonal frequency division modulation (OFDM). In some cases, as the modulation is increased (more information is placed on the RF carrier), the RF bandwidth will also increase, consuming more RF spectrum. Figure 2-2 illustrates this phenomenon. Figure 2-2. Increasing Bandwidthcompression) become more complex and data rates go up, immunity to noise decreases, and coverage goes down (see Figure 2-3). Figure 2-3. Compressing Data Reduces Range Amplitude ModulationAmplitude modulation occurs when the output power of the transmitter is varied while the frequency and phase of the sine wave remains constant (see Figure 2-4). Figure 2-4. Amplitude Modulation Frequency ModulationFrequency modulation occurs when the output power and phase remain constant while the frequency is varied over a small range (see Figure 2-5). Figure 2-5. Frequency Modulation Phase ModulationPhase modulation occurs when the amplitude and frequency remain constant but the phase within the carrier frequency changes over a small range (see Figure 2-6). A change in phase (polarity or direction of wave travel) is related directly to the digital information comprising the transmitted information. Figure 2-6. Phase Modulation Continuous wave (telegraphy) | |
DSSS | Direct-sequence spread spectrum |
FHSS | Frequency-hopping spread spectrum |
BPSK | Bipolar Phase Shift Keying |
QPSK | Quadrature Phase Shift Keying |
CCK | Complementary Code Keying |
QAM | Quadrature amplitude modulation |
PBCC | Packet Binary Convolutional Coding |
Radios following the 2.4-GHz IEEE 802.11b standard use two different types of modulation, depending upon the data ratesBPSK and QPSK. There are two types of codings: Barker code and CCK. Some people count the CCK as a third type of modulation, but it is, in fact, a particular type of coding (converting desired information into a particular digital algorithm), which is applied to a QPSK modulated signal.
The difference between BPSK and QPSK enables twice the information from the same number of cycles (or sine waves), keeping the RF bandwidth identical for twice the overall data rate transmitted.
Binary Phase Shift Keying
BPSK uses one phase to represent a binary 1 and another phase to represent a binary 0 for a total of 2 bits of binary data (see Figure 2-7). BPSK is used to transmit data at 1 Mbps.
Figure 2-7. BPSK Modulation
Quadrature Phase Shift Keying
With QPSK, the carrier can have four changes in phase or overall direction of the sine wave movement (increasing positive, decreasing positive, increasing negative, or decreasing negative). When compared to the overall 360 degrees of a circle, this is comparable to the 4 quadrants of 090 degrees, 90180 degrees, 180270 degrees, or 270360 degrees. These 4 separate portions of the signal represent 4 binary bits of data (see Figure 2-8). QPSK is used to transmit data at 2 Mbps.
Figure 2-8. QPSK Modulation
Complementary Code Keying
CCK modulation uses a complex set of functions known as complementary codes to send more data. CCK is based on in-phase (I) and quadrature (Q) architecture using complex codes, and replaces the Barker code used at the lower data rates. This provides for higher data rate while maintaining the same required RF bandwidth, as well as providing a path for interoperability with existing IEEE 802.11 lower data rate systems (by maintaining the same RF bandwidth and incorporating the existing physical [PHY] layer structure).
Quadrature Amplitude Modulation
Instead of using the CCK modulation type as specified by 802.11b for higher data rates, 802.11a and 802.11g specify QAM, which encodes via both changes in phase (as is the case with BPSK and QPSK) and changes in amplitude (see Figure 2-9). When encoding a single bit, two possible messages or symbols are possible (0 or 1). When encoding 2 bits, 4 symbols are possible. Working the exponential progression of this, when encoding 4 bits, 16 symbols are possible and when encoding 6 bits, 64 symbols are possible. A 16-QAM encodes 4 bits and provides for either 24-Mbps or 36-Mbps data rates, depending upon the rate of encoding. A 64-QAM encodes 6 bits and provides for either 48-Mbps or 64-Mbps data rates, depending on the rate of encoding. As is the case with 802.11b, increases in data rate are achieved by modulating an increasingly larger number of bits, not by increasing bandwidth. As a greater number of bits are encoded (particularly a greater number of bits than are encoded by 11-Mbps data rate) you can see that the "price" paid for the higher data rates provided by 802.11a and 802.11g is figured in terms of range.
Figure 2-9. QAM Modulation
Orthogonal Frequency Division Multiplexing
OFDM is one of the key factors in 802.11a and 802.11g standards. This section briefly describes the key advantages and how it works. In FDM, the available bandwidth is divided into multiple data carriers. The data to be transmitted is then divided between these subcarriers. Because each carrier is treated independently of the others, a frequency guard band (an area of frequency between each of the carriers) must be placed around it (see Figure 2-10). This guard band lowers the bandwidth efficiency because frequency is not used to carry any useful information. In some FDM systems, up to 50 percent of the available bandwidth is wasted. In most FDM systems, individual users are segmented to a particular subcarrier; therefore, their burst rate cannot exceed the capacity of that subcarrier. If some subcarriers are idle, their bandwidth cannot be shared with other subcarriers.
Figure 2-10. FDM Discrete Carriers
spectral efficiency (more information in the same amount of frequency spectrum) and helps in fighting a signal's delay spread of a signal, which can limit the data rate.
Combined, these features result in OFDM systems providing better tolerance to noise, interference, and multipath situations, which in turn provides improved range and overall performance (when compared to other modulation schemes for the same frequencies).
Modulation Methods for 802.11 Technologies
The 5-GHz IEEE 802.11a specification and the 2.4-GHz 802.11g specification provide for a variety of data rates (see Table 2-2).
Data Rate in Mbps | Modulation Type | Number of Bits Encoded |
|---|---|---|
6 | BPSK | 1 |
9 | BPSK | 1 |
12 | QPSK | 2 |
18 | QPSK | 2 |
24 | 16-QAM | 4 |
36 | 16-QAM | 4 |
48 | 64-QAM | 6 |
54 | 64-QAM | 6 |
Note that with 802.11a and 802.11g, the BPSK and QPSK modulation types used for 802.11 are again employed, encoding 1 and 2 bits respectively. Note, however, that with 802.11a and 802.11g, when using BPSK modulation, the data rate achieved is not 1 Mbps, as is the case with 802.11b, but rather 6 Mbps or 9 Mbps (depending upon the rate at which the encoding takes place). The difference between a 1-Mbps and 6-Mbps data rate is attributed to the greater efficiency of OFDM relative to DSSS. Similarly, with 802.11a and 802.11g, QPSK modulation yields not the 2-Mbps data rate, as is the case with 802.11b, but rather 12 Mbps or 18 Mbps when transmitting via OFDM, as specified by 802.11a and 802.11g.
Signal Strength
Another characteristic that needs to be discussed is the signal strength of an RF signal. Signal strength can be thought of as the volume of a signal. As an RF signal travels, it interacts with its surroundings (air molecules, walls, moisture, and so on) and loses some of its energy. The receiving device has a lower limit, called a receive threshold, that defines the amount of energy needed to receive the signal and be able to read the information that it contains.
If a signal strength is lower than the receive threshold, the information contained in the signal cannot be properly decoded and is useless. Maintaining a certain level of signal strength above the receiver threshold is desirable. The actual amount of signal strength recommended for a good communication link is discussed in Chapter 12, "Installing WLAN Products," and depends on frequency, modulation schemes, and data rates.
When deciding on a WLAN product, it is a good idea to also review the receiver performance as well as the transmitter. Sensitivity, adjacent-channel rejection, and spread delay are a few of the parameters that vary among receivers. A good receiver can improve coverage by a significant amount.
