Code Time Division Multiple Access

Description

Code Time Division Multiple Access (CTDMA) is the fundamental multiple access technology specified for the Time Division Duplex (TDD) mode of the Universal Terrestrial Radio Access (UTRA) within 3GPP standards, starting from Release 99. It is a hybrid scheme that merges the principles of Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA). In CTDMA, the radio resource is partitioned in two dimensions: the code domain and the time domain. The system uses a direct-sequence spread spectrum technique where user data is multiplied by a high-rate pseudo-noise spreading code, which expands the signal bandwidth. This spread signal is then transmitted within designated, repeating time slots. A single radio frame is divided into 15 time slots, and within each slot, multiple users can be multiplexed by assigning them different orthogonal variable spreading factor (OVSF) codes. The spreading factor can be varied to support different user data rates, a concept known as multi-rate transmission.

The architecture of a CTDMA system, as defined in 3GPP TS 25.222, involves key physical layer procedures such as channel coding, interleaving, data modulation, spreading, and scrambling. The spreading process uses OVSF codes to separate different physical channels from the same source (e.g., dedicated physical channels - DPCH) within a single time slot. Subsequently, a cell-specific scrambling code is applied to all physical channels in that time slot to differentiate transmissions from different cells. The time slot structure includes fields for the data payload, a midamble used for channel estimation and joint detection, and a guard period. The use of joint detection in the receiver is crucial for mitigating the intra-cell interference caused by the non-perfect orthogonality of codes, especially in the uplink.

CTDMA's operation is intrinsically linked to the TDD duplex method, where uplink and downlink transmissions occur on the same carrier frequency but in different time slots. This allows for asymmetric and dynamic allocation of capacity between the uplink and downlink by assigning a variable number of slots to each direction based on traffic demand. The combination of CDMA's interference averaging and frequency reuse of 1 with TDMA's time-slot structure provides a flexible air interface capable of supporting both circuit-switched and packet-switched services. Its role in the network is to define the radio interface for UMTS TDD, enabling efficient use of unpaired spectrum bands and supporting applications with asymmetric traffic patterns, such as internet browsing.

Purpose & Motivation

CTDMA was created to provide a standardized radio access scheme for 3G systems operating in Time Division Duplex (TDD) mode, addressing the need for efficient utilization of unpaired spectrum allocations. Prior to 3G, GSM utilized pure TDMA, and early CDMA proposals (like IS-95) used Frequency Division Duplex (FDD). The hybrid CTDMA approach was motivated by the desire to combine the benefits of both techniques: the flexible capacity and frequency planning of CDMA with the inherent support for asymmetric traffic and precise interference control offered by TDMA's time-slotted structure. This was particularly important for operators who held unpaired spectrum licenses, for which pure FDD solutions were not suitable.

The technology solved the problem of providing high-capacity, multimedia mobile services in spectrum bands not requiring paired uplink/downlink channels. It addressed limitations of pure TDMA, such as rigid capacity allocation and the need for frequency planning, and limitations of pure CDMA in FDD, which is less adaptable to rapidly changing uplink/downlink traffic ratios. By integrating CDMA into a TDMA frame, CTDMA allowed multiple users to share a single time slot, increasing trunking efficiency and enabling the system to better handle bursty packet data traffic. Its design also facilitated advanced receiver techniques like joint detection, which was necessary to manage the intra-cell interference in the uplink direction of a TDD-CDMA system, thereby improving overall system capacity and performance.

Key Features

• Hybrid multiple access combining CDMA spreading and TDMA time-slots
• Uses Orthogonal Variable Spreading Factor (OVSF) codes for channel separation
• Supports multi-rate transmission via variable spreading factors
• Employs TDD for dynamic asymmetric uplink/downlink capacity allocation
• Utilizes midambles for channel estimation and advanced joint detection
• Operates in unpaired spectrum bands, enabling flexible spectrum use

Evolution Across Releases

Defining Specifications