What Is Code Division Multiplexing?

Code division multiplexing (CDM) is a channel-sharing technique in which multiple signals are transmitted simultaneously over the same frequency band by assigning each signal a unique spreading code, allowing receivers to separate individual transmissions that overlap in both time and frequency. Each transmitter multiplies its data signal by a spreading code with a chip rate much higher than the data rate, spreading the signal across a wide bandwidth; the intended receiver multiplies the incoming composite signal by the same code and uses correlation to recover the original data, while contributions from other users appear as broadband noise. Code division multiplexing draws from information theory, spread spectrum communications, and digital signal processing, and it is foundational to the code division multiple access (CDMA) systems deployed in second- and third-generation cellular networks and in the Global Positioning System.

Spread Spectrum Fundamentals

Spread spectrum techniques predate commercial cellular use, originating in military communications in the 1940s and 1950s as a means of making transmissions resistant to jamming and interception. Direct-sequence spread spectrum (DSSS), the basis of most CDM implementations, multiplies a narrowband data signal by a pseudo-random noise (PN) code, expanding its bandwidth by a factor equal to the processing gain. Processing gain, expressed in decibels, quantifies how much the correlation receiver can suppress interference from other users and from multipath reflections. IEEE Xplore publications on CDMA spread spectrum techniques document how narrowband interference suppression through spread spectrum makes CDM particularly effective in the multipath-rich propagation environments of urban cellular systems, where time-division and frequency-division alternatives suffer greater performance degradation.

Code Assignment and Multiple Access

When CDM is applied as a multiple access technique, each user in a cell is assigned a distinct spreading code drawn from a set of orthogonal or near-orthogonal codes. Walsh-Hadamard codes, used in cdmaOne and CDMA2000 systems, provide exact orthogonality for synchronous transmissions on the downlink; the uplink uses long PN codes whose partial correlation properties separate asynchronous users with finite but acceptable cross-correlation. The capacity of a CDMA system is interference-limited rather than bandwidth-limited: as more users share the same spectrum, the aggregate interference floor rises and the signal-to-noise ratio at each receiver degrades. Power control, which adjusts each user's transmit power to maintain a target received signal strength, is the central mechanism for managing this tradeoff. CDMA and spread spectrum techniques compared to FDMA and TDMA in IEEE conference proceedings demonstrates that CDMA's graceful interference-limited capacity degradation and natural soft handoff between base stations differentiated it from earlier multiple access architectures.

Multicarrier Code Division Multiple Access

Multicarrier code division multiple access (MC-CDMA) combines the frequency diversity of orthogonal frequency-division multiplexing (OFDM) with the user-separation properties of spreading codes. Each data symbol is spread across multiple subcarriers simultaneously, exploiting frequency diversity to combat selective fading while retaining the code-based user separation of single-carrier CDMA. MC-CDMA and its variant multicarrier CDMA with time-domain spreading were extensively studied for third- and fourth-generation cellular standards and remain relevant in software radio platforms, where flexible waveform synthesis allows the same hardware to implement multiple access methods adaptively. The ScienceDirect overview of code division multiple access surveys the spectrum of CDM-derived waveforms from single-carrier DSSS through multicarrier variants and their performance tradeoffs in fading channels.

Applications

Code division multiplexing has applications across a range of wireless communication and sensing systems, including:

• Cellular telephone networks using cdmaOne, CDMA2000, and WCDMA (3G UMTS) air interfaces
• Global Navigation Satellite Systems, including GPS, Galileo, and GLONASS
• Indoor wireless LAN systems based on DSSS physical layers in early 802.11 standards
• Satellite communication links requiring interference-resistant wideband transmission
• Software-defined radio platforms reconfiguring CDM waveforms for tactical military communications