Code-division multiple access
Code-division multiple access (CDMA) is a channel access method for wireless communication systems that enables multiple users to share the same physical channel simultaneously by assigning each a unique pseudorandom code sequence to distinguish their signals.[1] This technique relies on spread-spectrum modulation, particularly direct-sequence spread spectrum (DS-SS), where the data signal is multiplied by a high-rate spreading code, expanding its bandwidth far beyond the original information bandwidth to allow overlapping transmissions without interference.[1] At the receiver, the intended signal is recovered by correlating the received composite waveform with the matching code, while signals from other users, appearing as pseudonoise, are suppressed due to low cross-correlation properties of the codes.[1] Originating from military spread-spectrum applications in the 1940s and 1950s—with parallel developments in the Soviet Union during the mid-20th century—for secure, jam-resistant communications, CDMA was adapted for commercial cellular use in the late 1980s by Qualcomm engineers, including Irwin Jacobs and Andrew Viterbi.[2] A pivotal 1991 paper by Qualcomm researchers demonstrated that CDMA could achieve significantly higher spectral efficiency in cellular networks through universal frequency reuse and power control, outperforming traditional frequency-division multiple access (FDMA) and time-division multiple access (TDMA) systems.[3] This led to the development of the IS-95 standard, approved by the Telecommunications Industry Association in 1993, which became the basis for the 2G cdmaOne networks deployed commercially starting in 1995 in Hong Kong and later in the United States by carriers like Sprint and Verizon.[2][4] CDMA's key advantages include enhanced capacity via soft capacity limits, seamless soft handoffs between base stations, and inherent resistance to multipath fading and interference, making it suitable for dense urban environments.[3] It evolved into 3G standards such as cdma2000 (using 1.25 MHz channels with 1.22 Mcps chip rates) and wideband CDMA (WCDMA) in UMTS (using 5 MHz channels with 3.84 Mcps chip rates), supporting data rates up to several Mbps for voice, video, and internet services.[1] Although CDMA networks have been largely phased out in favor of orthogonal frequency-division multiple access (OFDMA) in 4G LTE and 5G, with major carrier shutdowns occurring in the early 2020s, CDMA's foundational principles of code-based multiplexing continue to inform advanced multiple-access schemes in modern wireless systems, including satellite communications and IoT networks.[4][5]Fundamentals
Definition and Core Principles
Code-division multiple access (CDMA) is a channel access method that enables multiple users to share the same frequency band simultaneously by assigning each user a unique spreading code to encode their data signal.[6] This technique relies on spread-spectrum signaling, where the bandwidth of the original data signal is deliberately expanded to allow coexistence of multiple signals with minimal interference.[7] The primary mechanism in CDMA is direct-sequence spread spectrum (DSSS), in which a pseudo-noise (PN) code—a sequence of bits with noise-like properties—is multiplied by the data signal to spread its spectrum across a wider bandwidth.[8] PN codes, generated from deterministic algorithms, appear random and have low autocorrelation, enabling the signal to be distinguished from noise or other signals. The chip rate of the PN code, which determines the spreading, is significantly higher than the original data rate, typically by a factor of 100 or more.[9] A core principle of CDMA is code orthogonality, which ensures that the spreading codes assigned to different users are mutually orthogonal, thereby minimizing cross-interference when signals are superimposed.[10] Examples include Walsh codes, derived from Hadamard matrices and providing perfect orthogonality for synchronous systems, and Gold codes, which offer good autocorrelation properties for asynchronous scenarios.[11] This orthogonality allows the receiver to separate user signals effectively within the shared spectrum.[12] The spreading process can be expressed mathematically as the transmitted signal s(t) = d(t) \cdot c(t), where d(t) is the baseband data signal and c(t) is the spreading code with values typically ±1.[12] The processing gain G_p, which quantifies the interference rejection capability, is defined as G_p = \frac{R_c}{R_d}, the ratio of the chip rate R_c to the data rate R_d; higher values of G_p enhance the system's ability to support more users.[13] At the receiver, despreading recovers the original data by correlating the received signal with the user's specific spreading code, which collapses the spread spectrum back to the narrowband data signal while treating other users' signals as uncorrelated noise.[8] This process rejects interference from non-matching codes, as their correlation yields near-zero output, allowing the desired signal to emerge with amplified power relative to noise.[14]Comparison to Other Multiple Access Methods
Code-division multiple access (CDMA) differs fundamentally from other multiple access methods such as frequency-division multiple access (FDMA), time-division multiple access (TDMA), and orthogonal frequency-division multiple access (OFDMA) in how it allocates shared communication resources to multiple users.[15] FDMA divides the available spectrum into non-overlapping frequency bands, assigning each user a dedicated sub-band to transmit continuously, which requires guard bands to prevent adjacent channel interference.[16] TDMA, in contrast, allocates the entire bandwidth to users in non-overlapping time slots, allowing sequential transmissions within a frame, often combined with FDMA for hybrid systems like GSM.[15] OFDMA extends this by dividing the spectrum into orthogonal subcarriers and assigning subsets to users, enabling flexible allocation and high spectral efficiency in broadband systems like LTE. Unlike these orthogonal methods, CDMA permits all users to transmit simultaneously over the same frequency band and time using unique spreading codes, relying on code orthogonality to distinguish signals at the receiver.[16] A primary distinction lies in interference management and resource utilization. FDMA and TDMA minimize intra-system interference through physical separation of resources—frequency guards in FDMA and time guards in TDMA—avoiding the multi-user interference inherent in CDMA.[15] CDMA's shared medium introduces the near-far problem, where a strong nearby signal can overwhelm weaker distant ones, necessitating sophisticated power control to maintain signal-to-interference ratios.[15] OFDMA reduces interference via subcarrier orthogonality but can suffer from inter-carrier interference in multipath environments without proper cyclic prefixes. While FDMA and TDMA provide predictable resource division, CDMA's code-based approach enhances spectrum reuse in cellular systems by allowing overlapping transmissions, though it demands higher receiver complexity for despreading and multiuser detection.[16]| Parameter | FDMA | TDMA | CDMA | OFDMA |
|---|---|---|---|---|
| Resource Allocation | Frequency bands | Time slots | Spreading codes | Subcarriers |
| Spectrum Efficiency | Low (due to guard bands) | Moderate (slot overhead) | High (code reuse) | High (orthogonal subcarriers) |
| Complexity | Low (simple filtering) | Moderate (timing sync) | High (code correlation) | Moderate (FFT processing) |
| Susceptibility to Fading | High (narrowband fading) | High (burst errors in slots) | Low (rake receiver diversity) | Low (frequency diversity) |
| Interference Handling | Guard bands | Time guards | Power control for near-far | Cyclic prefix for multipath |