Chapter 1: RF Signals and Modulation (Key Terms)

Chip

A bit produced by a coder.

Symbol

A complete group of encoded chips that represents a data bit.

Band

A contiguous range of frequencies.

Decibel (dB)

A logarithmic function that compares one absolute measurement to another.

dBd

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

dBi

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

Amplitude

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

Received Signal Strength Indicator (RSSI)

The measure of signal strength (in dBm) as seen by the receiver. RSSI is normally negative (0 to -100) because the received signal is always a degraded form of the original signal that was sent.

Frequency

The number of times a signal makes one complete up and down cycle in 1 second.

Wavelength

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

Radio Frequency (RF)

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

dBm

The power level of a signal measured in dB, as compared to a reference signal power of

Bandwidth

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

Demodulation

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

Link Budget

The cumulative sum of gains and losses measured in dB over the complete RF

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.

Coder

A function that converts data bits into multiple encoded bits before transmission, to provide resilience against noise and interference.

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.

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.

Differential Binary Phase Shift Keying (DBPSK)

A modulation method that takes 1 bit of encoded data and changes the phase of the carrier signal in one of two ways.

Differential Quadrature Phase Shift Keying (DQPSK)

A modulation method that takes 2 bits of encoded data and changes the phase of the carrier signal in one of four ways.

Hertz (Hz)

A unit of frequency equaling one cycle per second.

Frequency-Hopping Spread Spectrum (FHSS)

A wireless LAN method where a transmitter "hops" between frequencies all across a band.

Direct-Sequence Spread Spectrum (DSSS)

A wireless LAN method where a transmitter uses a single fixed, wide channel to send data.

Barker 11 Code

An 11-bit sequence of encoded bits that represents a single data bit.

Channel

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

Complementary Code Keying (CCK)

An encoding method that takes either 4 or 8 data bits at a time to create a 6-bit or 8-bit symbol, respectively. The symbols are fed into DQPSK to modulate the carrier signal.

Isotropic Antenna

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

Spread Spectrum

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

Narrowband

RF signals that use a very narrow range of frequencies.

Sensitivity Level

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

Noise Floor

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

Carrier Signal

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

Out of Phase

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

In Phase

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

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.

Modulation

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