Direct-Sequence Code Division Multiple Access

Description

Direct-Sequence Code Division Multiple Access (DS-CDMA) is the core multiple access scheme for the 3G Universal Mobile Telecommunications System (UMTS) as standardized by 3GPP. It operates by spreading the user's data signal across a wide bandwidth using a unique, high-rate pseudo-random noise (PN) code, known as a spreading code. Each user's data bits are combined (modulo-2 added) with this spreading code, which has a much higher chip rate (e.g., 3.84 Mcps for UMTS) than the original data symbol rate. This process transforms the narrowband data signal into a wideband signal with a noise-like appearance. At the receiver, the same unique code is used to correlate and 'de-spread' the desired signal, collapsing it back to its original bandwidth while spreading any interfering signals (from other users or noise) further, which are then filtered out. This provides processing gain and resistance to narrowband interference.

The architecture of a DS-CDMA system in UMTS involves several key physical layer components. The spreading process is performed by channelization codes (Orthogonal Variable Spreading Factor codes) which separate physical channels from the same source, and scrambling codes (Gold codes or long PN codes) which distinguish different user equipment or NodeBs. The wideband CDMA (WCDMA) air interface uses two 5 MHz paired bands for Frequency Division Duplex (FDD) mode. Key physical channels include the Dedicated Physical Channel (DPCH) for user data and control, the Common Pilot Channel (CPICH) for channel estimation, and synchronization channels. The system employs power control, notably fast closed-loop power control at 1500 Hz, to combat the near-far problem inherent in CDMA, where a strong signal from a nearby user can drown out weaker signals from distant users.

DS-CDMA's role in the 3GPP network is foundational for the UTRAN (UMTS Terrestrial Radio Access Network). It provides the physical layer for both circuit-switched and packet-switched services, supporting variable data rates through the use of different Spreading Factors (SF). The technique inherently supports soft handover, where a UE can communicate with multiple NodeBs simultaneously, improving handover reliability. Its spread spectrum nature also offers inherent security through the obscurity of the spreading codes and provides frequency diversity, making the link robust against multipath fading when combined with a Rake receiver that combines signals from different propagation paths.

Purpose & Motivation

DS-CDMA was developed to fulfill the International Telecommunication Union's (ITU) IMT-2000 requirements for third-generation (3G) mobile systems, which demanded higher data rates, improved spectral efficiency, and support for multimedia services compared to 2G systems like GSM (which used TDMA/FDMA). The primary problem it solved was enabling multiple users to share the same wide frequency band simultaneously without strict time or frequency partitioning, thereby increasing capacity and facilitating a more flexible allocation of bandwidth. Its creation was motivated by the need for a robust air interface that could provide higher capacity in a cellular environment, support variable bit rate services, and enable seamless handovers.

Compared to the TDMA approach of 2G GSM, DS-CDMA offered several theoretical advantages. It provided inherent frequency diversity, making it more resistant to multipath fading and narrowband interference. The use of spreading codes allowed for the soft capacity limit, where adding more users gradually degrades quality for all, rather than creating a hard block. This was crucial for supporting the bursty nature of packet data. Furthermore, it simplified frequency planning as the same frequency can be reused in every cell (frequency reuse factor of 1), unlike GSM which required careful planning to avoid co-channel interference. DS-CDMA formed the basis for the WCDMA FDD mode, which became the globally dominant 3G technology.

Key Features

• Wideband spread spectrum transmission (e.g., 5 MHz bandwidth)
• Unique spreading and scrambling codes for user/channel separation
• Fast power control (1500 Hz) to mitigate the near-far problem
• Support for soft and softer handover
• Variable data rates through adjustable Spreading Factors
• Robustness against multipath fading using Rake receivers

Evolution Across Releases

Defining Specifications