Chapter 17 - Wireless Signal and Modulation

amplitude

the height from the top peak to the bottom peak of the signal's waveform; also known as the peak-to-peak amplitude.

band

a contiguous rang of frequencies

Bandwidth

the range of frequencies used by a single channel or a single RF signal.

Carrier signal

The basic, steady RF signal that is used to carry other useful information.

channel

An arbitrary index that points to a specific frequency within a band.

dBd

dB-dipole, the gain of an antenna, measured in dB, as compared to a simple dipole antenna.

dBi

dB-isotropic, the gain of an antenna, measured in dB, as compared to an isotropic reference antenna.

dBm

dB-milliwatt, the power level of a signal measured in dB, as compared to a reference signal power of 1 milliwatt.

decibel (dB)

a logarithmic function that compares one absolute measurement to another.

Demodulation

The receiver's process of interpreting changes in the carrier signal to recover the original information being sent.

direct sequence spread spectrum (DSSS)

a wireless LAN method in which a transmitter uses a single fixed, wide channel to send data.

dynamic rate shift (DRS)

A mechanism used by an 802.11 device to change the modulation coding scheme (MCS) according to dynamic RF signal conditions.

Effective Isotropic Radiated Power (EIRP)

The resulting signal power level, measured in dBm, of the combination of a transmitter, cable, and an antenna, as measured at the antenna.

frequency

the number of times a signal makes one completed up and down cycle in 1 second.

Hertz (Hz)

a unit of frequency equaling one cycle per second.

in phase

the condition when the cycles of two identical signals are in sync with each other.

isotropic antenna

An ideal, theoretical antenna that radiates RF equally in every direction.

link budget

The cumulative sum of gains and losses measured in dB over the complete RF signal path; a transmitter's power level must overcome the link budget so that the signal can reach a receiver effectively.

Maximal Ratio Combining (MRC)

An 802.11n technique that combines multiple copies of a signal, received over multiple antennas, to reconstruct the original signal.

Modulation

the transmitter process of altering the carrier signal according to some other information source.

narrowband

RF signals that uses a very narrow range of frequencies.

Noise Floor

the average power level of noise measured at a specific frequency.

Orthogonal Frequency Division Multiplexing (OFDM)

A data transmission method that sends data bits in parallel over multiple frequencies within a single 20 MHz wide channel. Each frequency represents a single subcarrier.

out of phase

The condition when the cycles of one signal are shifted in time in relation to another signal.

phase

a measure of shift in time relative to the start of a cycle; ranges between 0 and 360 degrees.

Quadrature Amplitude Modulation (QAM)

A modulation method that combines QPSK phase shifting with multiple amplitude levels to produce a greater number of unique changes to the carrier signal. The number preceding the QAM name designates how many carrier signal changes are possible.

radio frequency

the portion of the frequency spectrum between 3 kHz and 300 GHz.

Received Signal Strength Indicator (RSSI)

The relative measure of signal strength (0 to 255), as seen by the receiver.

Sensitivity level

The RSSI threshold (in dBm) that divides unintelligible RF signals from useful ones.

Signal-to-noise ratio (SNR)

A measure of received signal quality, calculated as the difference between the signal's RSSI and the noise floor. A higher SNR is preferred.

spatial multiplexing

distributing streams of data across multiple radio chains with spatial diversity.

Spatial stream

An independent stream of data that is sent over a radio chain through free space. One spatial stream is separate from others due to the unique path it travels through space.

spread spectrum

RF signals that spread the information being sent over a wide range of frequencies.

Transmit Beamforming (TxBF)

A method that allows a MIMO transmitter using multiple antennas to adjust the phase and amplitude of the outgoing transmission in a coordinated method.

Wavelength

the physical distance that a wave travels over one complete cycle.