Code Division Multiple Access

Description

Code Division Multiple Access (CDMA) is a spread-spectrum multiple access technique fundamental to 2G cdmaOne (IS-95) and 3G CDMA2000 standards. It operates by allowing multiple transmitters to send information simultaneously over a single communication channel. This is achieved by spreading each user's signal across a wider bandwidth using a unique, pseudo-random spreading code. At the receiver, the desired signal is recovered by correlating the received composite signal with the same specific code, while signals from other users appear as low-level noise due to the orthogonality or low cross-correlation of the codes. This process is known as despreading.

Architecturally, a CDMA-based network comprises Mobile Stations (MS), Base Transceiver Stations (BTS), Base Station Controllers (BSC), and a Mobile Switching Center (MSC). The air interface is characterized by its use of direct-sequence spread spectrum (DSSS). Key physical layer components include Walsh codes for channelization (providing orthogonality among users in the forward link) and long pseudo-noise (PN) sequences for scrambling and user identification. Power control is a critical subsystem, constantly adjusting mobile station transmit power to ensure all signals arrive at the base station with nearly equal power, mitigating the near-far problem and maximizing capacity.

In the network, CDMA's role is to provide the foundational radio link for voice and data services. Its inherent characteristics, such as frequency reuse factor of 1 (every cell uses the same frequency), soft handoff (where a mobile communicates with multiple cells during a transition), and resistance to multipath fading using RAKE receivers, directly contribute to enhanced spectral efficiency, call quality, and network capacity. These technical attributes made it a competitive and robust alternative to the prevailing GSM/Time Division Multiple Access (TDMA) technology path in the 1990s and early 2000s.

Purpose & Motivation

CDMA was developed to address the capacity and quality limitations of first-generation analog (e.g., AMPS) and early digital cellular systems like GSM which used FDMA and TDMA. The primary motivation was to achieve higher spectral efficiency, allowing more users to be supported within the same allocated radio spectrum. Previous approaches allocated distinct frequency slices or time slots to users, which imposed hard limits on capacity and required careful frequency planning to avoid co-channel interference between cells.

CDMA solved these problems by enabling all users to occupy the entire frequency band all the time, differentiated only by codes. This provided statistical multiplexing gains, inherent privacy due to the pseudo-random codes, and improved performance in multipath environments. Historically, its development was driven by military spread-spectrum communications research, which was adapted for commercial use by Qualcomm. The technology's ability to handle soft capacity limits (where quality gradually degrades as more users are added, rather than a hard block) and its elegant support for soft handoff were key advantages that motivated its standardization and widespread deployment, particularly in North America and parts of Asia, leading to the cdmaOne and CDMA2000 family of standards.