Comparative modulation: Spread spectrum modulation terms and definitions for wireless networking
We touched briefly on modulation in the last wireless tutorial blog on radio frequency (RF). Modulation is, simply put, a mixing of the carrier wave with the data to be transmitted; analog or digital signals are combined with a carrier producing a hybrid signal, which is then demodulated at the receiver. This is the underlying theory behind all modulation-demodulation schemes. Below see the terms and definitions for wireless networking acronyms you may have seen: FHSS, DSSS, OFDM, FSK, PSK, and QAM.
Until recently, the predominant form of Wi-Fi modulation was the method known as spread spectrum. This technology breaks data into smaller pieces, called "chips," and spreads these chips over multiple subcarriers. This method allows for a much more reliable and interference resistant transmission. Spread spectrum has the advantages of low power density and redundancy. Low power density results from the signal being spread over a wide band, allowing for less power per individual subcarrier, and redundancy refers to the fact that data is spread over several subcarriers and can be recovered in case of error. This also makes the signal more robust and resistant to interference and interception.
Characteristics of spread spectrum modulation
Spread spectrum modulation has the following characteristics:
- The bandwidth of the transmitted signal is greater than that of the original data.
- The bandwidth of the transmitted signal is determined by the data transmitted and the spreading code.
Related terms, acronyms, and definitions for spread spectrum modulation follow.
– Frequency hopping spread spectrum (FHSS) is a form of spread spectrum that has been in use for more than 70 years. Bluetooth uses FHSS; data is spread over 40 subcarriers of 1 MHz each. FHSS is very resistant to narrowband interference and FHSS transmissions are very difficult to intercept. However, data rates top out at about 3 MBPS.
– Direct sequence spread spectrum (DSSS) uses an algorithm that modulates every bit with a sequence of 11 bits called "chips." The process is known as "chipping" and follows a spreading code, typically the Barker Code, to chip data into pseudonoise (PN) symbols. The transmitted signal sounds very much like static.
– Orthogonal frequency division multiplexing (OFDM) uses 52 orthogonally sequenced subcarriers (56 in IEEE 802.11n) to transmit data. While not technically a spread spectrum technology, it is included here because it is a dominant form of modulation in Wi-Fi. Orthogonal means "at right angles"; the center frequency of each subcarrier is 90 deg out of phase with adjoining carrier center frequencies, eliminating crosstalk or interference.
– Frequency shift keying (FSK) is a very old technology dating back to telegraphy. FSK is simply a shifting of frequency between two different frequencies to represent a one or a zero (mark and space). Similarly, phase shift keying (PSK) uses a shift in phase angle to represent a symbol or change in state.
– Quadrature amplitude modulation (QAM) uses a phase map that produces coordinates defined by the amplitude and phase angle of a signal. The phase map has four quadrants, corresponding to the four angular parts of a sine wave. In the most basic form, four symbols can be represented. Common iterations are 8, 16, 32, 64, 128, and 256-QAM. Each number represents the number of symbols capable of being encoded per cycle. As you can see, very high spectrum efficiency can be achieved with the appropriate modulation.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano.
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.