Digital Modulation and Multiple Access

Digital modulation is the process of representing digital data, a stream of bits, as variations of a radio carrier wave so it can be transmitted. The carrier has three properties that can be varied: its amplitude, its frequency and its phase. Amplitude-shift keying (ASK) varies the carrier's strength, frequency-shift keying (FSK) switches it between frequencies, and phase-shift keying changes its phase, with quadrature phase-shift keying (QPSK) encoding two bits per symbol. Quadrature amplitude modulation (QAM) combines amplitude and phase changes to carry several bits per symbol, increasing the data rate within a given bandwidth.

Before modulation, data is often shaped by a line code that determines how bits are represented as electrical levels. Schemes such as non-return-to-zero (NRZ), Manchester coding and alternate mark inversion (AMI) balance simplicity, the ability to recover timing from the signal, and the avoidance of a steady direct-current component on the line. The choice of line code affects how reliably a receiver can stay synchronised with the incoming bit stream.

When many users must share one communication medium, a multiple-access method divides the available capacity among them. Frequency-division multiple access (FDMA) gives each user a separate frequency channel. Time-division multiple access (TDMA) lets users share a frequency by taking turns in assigned time slots. Code-division multiple access (CDMA) allows users to transmit at the same time and frequency, separating them by unique spreading codes; it can use direct-sequence spreading (DS-CDMA) or frequency hopping (FH-CDMA). The related multiplexing methods FDM, TDM and wavelength-division multiplexing (WDM) combine several signals onto one shared path.

Frequently asked questions

What are the main types of digital modulation?

ASK varies the carrier's amplitude, FSK shifts its frequency and phase-shift keying changes its phase; QAM combines amplitude and phase to carry more bits per symbol.

How do FDMA, TDMA and CDMA differ?

FDMA assigns each user a separate frequency, TDMA gives users time slots on a shared frequency, and CDMA lets users share both, distinguished by unique codes.

Why is a line code needed?

A line code sets how bits become electrical levels, helping the receiver recover timing and avoid a steady DC component that would hinder reliable detection.