Parallel Interference Cancellation

Description

Parallel Interference Cancellation (PIC) is an advanced receiver algorithm employed in the physical layer of 3GPP radio access networks, notably in Wideband Code Division Multiple Access (WCDMA) systems as specified in UMTS. It is a multi-user detection (MUD) technique designed to combat the near-far problem and intra-cell interference inherent in CDMA systems. In CDMA, multiple users transmit simultaneously on the same frequency band, differentiated by unique spreading codes. At the receiver, these signals interfere with each other, limiting capacity and performance. PIC aims to mitigate this interference.

The algorithm operates in an iterative, multi-stage process. In the first stage, conventional matched filter detectors (e.g., RAKE receivers) provide initial estimates of the data symbols for all users. These estimates are then used to reconstruct the interference each user causes to others. This reconstruction involves re-spreading the estimated symbols with the respective user's spreading code, applying the estimated channel impulse response (including fading and delay), and scaling by the estimated amplitude. The reconstructed interference signals are then subtracted from the received composite signal in parallel for all users, hence the name 'Parallel Interference Cancellation'. This generates a set of 'cleaned' signals for each user, which are then fed into a second detection stage for improved symbol estimation.

Key components of a PIC receiver include the initial detection stage (often a RAKE receiver), the interference reconstruction unit, the parallel cancellation unit, and subsequent re-detection stages. The process can be repeated over multiple iterations to refine estimates further, with each iteration potentially improving the bit error rate (BER) performance. However, performance gains depend on the accuracy of the initial estimates; errors in estimation can lead to incorrect cancellation and potential error propagation. Therefore, PIC often incorporates techniques like partial cancellation (subtracting only a fraction of the estimated interference) to mitigate error propagation effects.

PIC's role in the network is primarily within the Node B (base station) uplink receiver, where it enhances the demodulation of signals from multiple mobile stations. By effectively reducing multi-access interference (MAI), PIC increases the uplink capacity and coverage of the cell, allowing more users to be served simultaneously or improving the data rates for existing users. It is a computationally intensive technique but offers a significant performance advantage over simpler receivers, especially in high-load scenarios. Its specifications and performance requirements are detailed across various 3GPP technical specifications covering physical layer procedures and radio requirements.

Purpose & Motivation

PIC was developed to address a fundamental limitation of CDMA technology: multi-user interference. In early CDMA systems like IS-95, receivers used simple correlators (RAKE receivers) that treated signals from other users as noise. This approach suffered from the near-far problem, where a strong signal from a nearby user could overwhelm weaker signals from distant users, severely limiting capacity. Multi-user detection techniques, including PIC, were introduced to actively cancel this interference, thereby increasing system capacity and robustness.

The motivation for PIC in 3GPP UMTS (WCDMA) was to meet the demanding performance requirements for third-generation mobile services, which promised higher data rates and better spectral efficiency. The original 3GPP specifications from the R99 timeframe included support for advanced receiver techniques like PIC to be implemented in Node Bs. It solved the problem of intra-cell interference, which becomes a dominant performance bottleneck as cell load increases. By cancelling interference, PIC allows the network to operate closer to its theoretical capacity limits.

Compared to simpler successive interference cancellation (SIC), which processes users sequentially, PIC processes all users in parallel within each iteration, potentially offering faster convergence and better performance in symmetric power scenarios. Its development was driven by the need for practical, implementable MUD algorithms that could be deployed in base stations to enhance uplink performance without requiring changes to user equipment. It addressed the limitations of previous single-user detection approaches, enabling UMTS networks to better support capacity-intensive data services.

Key Features

• Multi-stage iterative interference estimation and cancellation
• Processes interference from all users in parallel per iteration
• Improves uplink capacity and mitigates the near-far problem in CDMA
• Can be implemented with partial cancellation factors to control error propagation
• Enhances signal-to-interference-plus-noise ratio (SINR) for detected users
• Specified for UMTS/WCDMA Node B advanced receiver capabilities

Evolution Across Releases

Defining Specifications