Modulation is the process by which the bits comprising the transmitted data are converted to the voltage levels that will be mixed with the carrier, amplified, and sent to the antenna. The variations of the signals are based on changes in some parameter, such as a time or frequency difference, or changes in amplitude or position. Modulation is a conversion and mixing of raw data with a carrier wave that is oscillating at the channel center frequency. As a result of this modulation, sidebands (or subcarriers) are generated and are used to convey useful data across the channel spectrum.
Modulation should not be confused with encoding. Encoding converts individual bits into symbols, which are essentially another collection of bits. This is done to facilitate spreading, as in spread spectrum; or to make data secure, as in CCMP (counter mode with cipher block chaining message authentication code protocol); or to perform forward error correction, a technique used in orthogonal frequency division multiplexing (OFDM). Modulation is the process of converting these individual bits into a form that can be put onto the physical layer. This form is a periodic variation in the amplitude, frequency, phase, or position.
The most basic modulation is the keying of the carrier, called CW (continuous wave). This was the first use of wireless, utilizing Morse code to send information over great distances.
Amplitude modulation, or AM, varies the amplitude of the carrier in proportion to the impressed frequency of the input data. AM has a very low bandwidth and is not typically used for data communication.
Another familiar method is frequency modulation, or FM. FM varies the frequency of the carrier in proportion to the frequency of the frequency of the input data. FM has a higher bandwidth and is used for high-quality audio transmission, such as stereo radio broadcasting. It is also used for video signals because of the wider bandwidth requirement.
These techniques have served the entertainment industry well, but frequency spectrum usage has been very inefficient. Very early in the development of data communication systems, it was recognized that the nature of digital communication was such that it would lend itself to multiplexing of several distinct data streams onto a shared medium. Multiplexing is a technique that takes multiple data streams and combines them into one stream, which is modulated and placed onto the shared medium; this greatly increases the spectral efficiency of the medium, allowing it to carry much more information than would be possible if limited to a single stream.
Two ways to effectively divide the transmitting channel efficiently are by using frequency and by using time division. Two techniques widely used are frequency division multiplexing (FDM) and time division multiplexing (TDM). In the first, separate frequencies—usually subcarriers of the channel center frequency—are individually modulated by another technique to allow multiple bit streams to be transmitted independently. TDM uses time slots to transmit individual channels, similar to, but not strictly analogous to frequency hopping; in TDM, information is transmitted during a specific time period of the carrier wave cycle. TDM requires fine synchronization of the transmitter and receiver in order to work properly. Other types of multiplexing techniques are code division multiplexing (CDM), which is widely used in cellular communication as code division multiple access (CDMA). CDM is a technique by which several information channels share the same frequency spectrum; you may recognize this as spread spectrum.
Diving deeper, how does an individual bit modulate the carrier? Regardless of the method of multiplexing, the carrier voltage level still needs to be manipulated in such a fashion as to change its characteristics before application to the antenna. Two common methods used in Wi-Fi are frequency shift keying (FSK) and phase shift keying (PSK). In FSK, the frequency is modulated to represent a 0 or a 1. In PSK, the phase is altered to represent the two digital values. Note that these techniques use analog values to represent the two digital states. Also note that this conversion takes place at the physical layer.
FDM has issues and is not efficient in most cases. PSK is more efficient in coding and spectral usage. Two forms of PSK are used: binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Consider the sine wave as an alternating quantity that revolves through 360 deg of values; each method uses this variation as a means of encoding. In BPSK, two binary values can be encoded per cycle. In QPSK (also called 4-QAM), four values can be encoded per cycle. The underlying principle of PSK is the abrupt shifting of the phase of the sine wave. The wave will be shifted N degrees to represent a 0, and it will be shifted another value to represent a 1.
PSK tops out at eight phase changes per cycle, after which error rates become too high for practical use. To increase spectral efficiency and speed, a technique called quadrature amplitude modulation (QAM) is used. Quadrature is a measurement technique that relies on four positions. A sine wave travels through 360 deg; 0-180 deg being the positive half, while 180-360 is the negative. Each positive and negative excursion peaks at 90 deg, traveling to and from a zero point through many intermediate values of phase and amplitude; this is the essence of all forms of QAM. A "constellation diagram" for a clearer visualization of the process represents these values. QAM is a hybrid of analog and digital modulation techniques. QAM transmits two carriers, 90 deg out of phase, called quadrature carriers, which are summed, and the resulting waveform is a phase and amplitude shifted value.
As can be seen by the constellation diagrams, as the number of possible data points increases, so does the ability to transmit increasing amounts of data during one cycle. The improvements in throughput should be obvious. As one would expect, as more information is stuffed into the available space, there is a real possibility of symbols running into each other and corrupting the data. This is why a transmitter and receiver negotiate a favorable modulation and coding scheme (MCS). Channel conditions, traffic priority, and device characteristics and capabilities all factor into what type of modulation can be used to most effectively transmit error-free data at the highest possible speed.
The advantage of using QAM is that the technique allows more bits per symbol and higher spectral efficiency: BPSK has a theoretical bandwidth efficiency of 1 bit/second/Hz; 256QAM has a theoretical efficiency of 8 bits/second/Hz. Higher orders of QAM translate into a higher data rate. The following table gives a quick summary of the bit rates of the various modulation techniques described above.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). He can be reached at email@example.com. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
This blog also appears in the January 2015 print edition of Control Engineering.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Characteristics of 802.11n and 802.11ac
Understanding modulation and coding schemes
OFDM: Orthogonal frequency division multiplexing
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.