Overlaid Code Division Multiple Access

Description

Overlaid CDMA (O-CDMA) is a multiplexing scheme introduced in 3GPP Release 13 for GERAN (GSM/EDGE Radio Access Network) to support the massive number of devices required for Machine-Type Communications (MTC). Unlike traditional CDMA, which spreads signals across a wide bandwidth, O-CDMA operates within the same narrowband physical channel (e.g., a timeslot in a GSM frame) and separates up to four simultaneous uplink transmissions through the use of orthogonal codes. These codes are assigned to individual mobile stations, which then spread their data symbols before modulation. The resulting signal is the sum of the spread signals from the multiplexed users.

The primary application of O-CDMA is in Extended Coverage (EC) operation, a feature that provides up to 20 dB of additional link budget for MTC devices in harsh radio conditions (e.g., basements, rural areas). To achieve this, devices use repetition and enhanced power control, which increases the time and resource required for each transmission. O-CDMA compensates for this by allowing multiple users to share the same physical resources, thereby improving spectral efficiency and enabling a larger number of devices per cell.

The orthogonal codes for O-CDMA are defined in 3GPP TS 45.002, with the physical layer performance specified in TS 45.005 and base station conformance in TS 51.021. The codes have a length of four, and the scheme effectively creates four logical subchannels (Overlaid CDMA subchannels) on a single physical channel. Each mobile station is assigned one of these codes (or subchannels) by the network. The receiver (base station) then applies the corresponding code to extract the individual signals from the composite received signal.

O-CDMA is tightly integrated with the GERAN EC architecture, which includes features like repeated transmission, reduced modulation order (e.g., GMSK), and extended uplink power control. When two or more users are multiplexed, the network must coordinate their transmissions to ensure orthogonality is maintained. This requires tight synchronization and power control to prevent near-far effects that could degrade code orthogonality. The scheme is particularly beneficial for periodic small data transmissions typical of MTC.

Since its introduction in Release 13, Overlaid CDMA has been carried forward through subsequent releases up to Release 19 without fundamental changes to its definition or operation. It remains an optional feature for GERAN networks supporting EC-MTC, and its usage depends on operator deployment and the need to support high device densities in challenging coverage scenarios.

Purpose & Motivation

Overlaid CDMA was developed to address the capacity and coverage challenges posed by the rapid growth of Machine-Type Communications (MTC) in GSM/EDGE networks. Prior to Release 13, the standard GSM uplink allowed only one mobile station per timeslot per carrier. With the introduction of Extended Coverage (EC) operation for MTC, devices in poor coverage needed to repeat transmissions many times, which consumed significant resources and limited the number of devices that could be supported. A solution was needed to multiplex multiple devices on the same physical resources while maintaining the robustness required for EC.

The key motivation was to enable a single physical channel to serve up to four devices simultaneously without introducing significant interference. Traditional multiple-access techniques like FDMA or TDMA could not achieve this within the narrowband GSM channel without additional hardware or bandwidth. O-CDMA leverages the processing gain of orthogonal codes to separate users in the code domain, allowing them to share the same time and frequency resources. This increases the effective capacity of the network for MTC by a factor of up to four, directly improving the support for massive numbers of low-cost, low-data-rate devices.

Furthermore, O-CDMA complements other EC techniques such as repetition and power boosting. By multiplexing users, the network can schedule more devices in the same resources, reducing the overall time required to serve a large device population. This is crucial for applications like smart metering, where thousands of devices may need to report infrequently but simultaneously. The feature was standardized in Release 13 as part of the GERAN EC enhancements and has since been maintained to ensure backward compatibility and continued support for legacy MTC deployments.