IEEE 802.11: Modulation Formats

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Modulation Formats

The 802.11 family consists of a series of half-duplex over-the-air modulation techniques that use the same basic protocol.

Quick List of Modulation Types

802.11 Protocol	Modulation Type
-	DSSS, FHSS
a	OFDM
b	OFDM
g	OFDM, DSSS
n	OFDM
ac	OFDM

Direct-Sequence Spread Spectrum (DSSS)

Like other spread spectrum technologies, the bandwidth of the transmitted signal is larger than the information signal that modulates the carrier or broadcast frequency. "Spread Spectrum" indicates that the carrier signals occur over the full bandwidth (spectrum) of the device's transmitting frequency.

DSSS phase-shifts the sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips." The chips have a much shorter duration than the information bit, or in other words every information bit is modulated by a sequence of faster chips. The chip rate is much higher than the information signal bit rate.

DSSS uses a signal structure in which the sequence of chips produced by the transmitter is already known by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Transmission Method

DSSS transmission multiple the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of "1" and "-1" values at a frequency that is much higher than the original signal. The resulting signal resembles white noise, but this signal is used to exactly reconstruct the original data at the receiving end by multiplying it by the same pseudorandom sequence. This process of reconstruction is known as "de-spreading" and it mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver already knows the transmitter is using.

The resulting effect of enhancing signal to noise ratio in the channel is called process gain (or processing gain). This effect can be made larger by employing a longer PN sequence and more chips per bit, but the physical devices used to generate the PN sequence impose practical limits on the attainable processing gain.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process has reduced processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.

A plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

In contract, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, the latter process results in a uniform frequency distribution whose width is determined by the output range of the pseudorandom number generator.

Process gain for DSSS

In a spread spectrum system, the process gain (or processing gain) is the ratio of the spread (or RF) bandwidth to the unspread (or baseband) bandwidth. It is usually expressed in decibels (dB). The process gain does not reduce the effects of the wideband thermal noise. It can be shown that a direct sequence spread spectrum (DSSS) system has exactly the same bit error behavior as a non-spread spectrum system with the same modulation format. Thus, on an additive white Gaussian noise (AWGN) channel without interference, a spread system requires the same transmitter power as an unspread system, all other things being equal.

Unlike a conventional communication system, however, a DSSS system does have a certain resistance against narrowband interference, as the interference is not subject to the process gain of a DSSS signal and hence the signal-to-interference ratio is improved.

In frequency modulation (FM), the processing gain can be expressed as:

<math>G_p = \cfrac{1.5B_n\Delta f^2}{W^3}</math>

where

G_p is the processing gain

B_n is the Noise Bandwidth

Δf is the peak frequency deviation

W is the sinusoidal modulating frequency

Benefits

• Resistance to intended or unintended jamming
• Sharing of a single channel among multiple users
• Reduced signal/background-noise level hampers interception
• Determination of relative timing between transmitter and receiver

Uses

• IEEE 802.11b 2.4 GHz and IEEE 802.11-1999
• IEEE 802.15.4 (used in ZigBee and Wireless HART)
• Radio-controlled automotive vehicles
• United States GPS, European Galileo, and Russian GLOSNASS satellite navigation systems
• Cordless phones
• Automatic meter reading

FHSS

FHSS was only used in the first version of WiFi which is not used regularly today. Remember that 802.11b was the first standard to be widely accepted.

Orthogonal Frequency-Division Multiplexing (OFDM)

OFDM is a method of encoding digital data on multiple carrier frequencies. It is currently a popular scheme for wideband digital communication both wired and wireless. This scheme is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying_ at a low symbol rate, maintaining total data rates similar to the conventional single-carrier modulation schemes in the same bandwidth.

The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example attenuation of high frequencies in a long copper wire or narrowband interference) with complex equalization filters. Channel equalization is simplified because ODFM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate inter-symbol interference (ISI) and utilize echoes and time-spreading to achieve diversity gain such as a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs) where several adjacent transmitters send the same signal simultaneously at the same frequency as the signals from multiple distant transmitters may be combines constructively, rather than interfering as would typically occur in a traditional single-carrier system.

Advantages

• High spectral efficiency as compared to other double sideband modulation schemes, spread spectrum, etc.
• Can easily adapt to sever channel conditions without complex time-domain equalization
• Robust against narrow-band co-channel interference
• Robust against inter-symbol interference (ISI) and fading caused by multi-path propagation
• Efficient implementation using Fast Fourier Transform (FFT)
• Low sensitivity to time syntonization errors
• Tuned sub-channel receiver filers are not required
• Facilitates single frequency networks (SFNs) for example transmitter macro-diversity

Disadvantages

• Sensitive to Doppler shift
• Sensitive to frequency synchronization problems
• High peak-average power ratio (PAPR) requiring linear transmitter circuitry which suffers from poor power efficiency
• Loss of efficiency cause by cyclic prefix/guard interval

Uses

Cable

• ADSL and VDSL broadband access
• DVB-C3, an enhanced version of the DVB-C digital cable TV standard
• Power line communication
• ITU-T G.hn, a standard that provides high-speed local area networking of the existing home wiring (power lines, phone lines, and coaxial cables
• TrailBlazer telephone line modems
• Multimedia over Coax Alliance (MoCA) home networking

Wireless

• Wireless LAN radio interfaces including IEEE 802.11a, g, n, ac, and HIPERLAN/2
• Digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB, and ISDB-TSB
• Terrestrial digital TV systems DVB-T and ISDB-T
• Terrestrial mobile TV systems DVB-H, T-DMB, ISDM-T, and MediaFLO format link
• Wireless personal area network ultra-wideband IEEE 802.15.3a implementation suggested by WiMedia Alliance

Cellular

• Mobility mode of the wireless MAN/broadband wireless access (BWA) standard IEEE 802.16e (for Mobile-WiMAX)
• Mobile broadband wireless access (MBWA) standard IEEE 802.20
• Downlink of the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard