Parallel Concatenated Convolutional Code

Description

Parallel Concatenated Convolutional Code (PCCC), universally recognized as the foundation of Turbo Coding, is a sophisticated channel coding scheme. Its architecture is built around the parallel concatenation of two identical, simple recursive systematic convolutional (RSC) encoders. The key innovation is an interleaver (Π) placed before the second encoder. The input data sequence (block of k bits) is fed directly to the first RSC encoder and also to the interleaver, which pseudo-randomly reorders the bits. The scrambled sequence is then input to the second RSC encoder.

This process produces three output sequences: the original systematic bits (unchanged input bits) and two sets of parity bits—one from each encoder. These are then multiplexed and transmitted. The decoder employs an iterative feedback mechanism, typically using the BCJR (Bahl, Cocke, Jelinek, Raviv) or MAP (Maximum A Posteriori) algorithm implemented in a structure with two soft-input soft-output (SISO) decoders. Each SISO decoder corresponds to one of the constituent encoders. They exchange probabilistic information (extrinsic information) about each bit in an iterative loop. The first decoder uses the received systematic bits and its parity bits, along with initial estimates, to produce soft outputs. These outputs, after subtracting the input information (to form extrinsic info), are interleaved and fed as a priori information to the second decoder. The second decoder processes the interleaved systematic bits and the second set of parity bits, along with this new information, to produce refined soft outputs. This extrinsic information is then de-interleaved and fed back to the first decoder for the next iteration. This iterative exchange allows the decoders to progressively correct each other's uncertainties, converging on a highly reliable estimate of the original data after several iterations.

Its role in the network is fundamental at the physical layer of the air interface. In 3G UMTS, it is the primary channel coding scheme for high-speed data channels (e.g., DCH, HS-DSCH). In LTE, it is used for the Physical Downlink Shared Channel (PDSCH), Physical Uplink Shared Channel (PUSCH), and the Physical Broadcast Channel (PBCH). By providing extremely robust error correction, PCCC allows the system to operate at higher code rates (less redundancy) for a given target error rate, or to maintain reliable communication at much lower signal-to-noise ratios. This directly translates to higher data throughput, greater cell coverage, and improved performance at cell edges, which are critical for delivering consistent user experience.

Purpose & Motivation

PCCC was developed to solve the fundamental challenge of approaching the Shannon limit—the theoretical maximum capacity of a noisy channel—with practical computational complexity. Before its invention in 1993, convolutional codes and Reed-Solomon codes were standard, but their performance fell short of the Shannon limit by several decibels. Closing this gap required either excessively complex codes or lower spectral efficiency. PCCC/Turbo coding provided a breakthrough, achieving performance within 0.5 dB of the limit with manageable iterative decoding, making high-efficiency digital communication over very noisy channels feasible.

The primary problem it addresses is the reliable transmission of high-speed digital data over the inherently unreliable and interference-prone wireless radio channel. For 3G UMTS, which promised multimedia and internet services, traditional coding was insufficient to deliver the required quality of service for packet data. PCCC enabled the Wideband CDMA (W-CDMA) air interface to support variable and high data rates with robust link performance. Its adoption was motivated by the need for a coding scheme that could adapt to rapidly changing channel conditions (fast fading) while maintaining a good bit error rate (BER) without requiring excessive transmit power, which would cause more interference.

Furthermore, PCCC provided the necessary performance margin to support advanced radio features like Hybrid ARQ (HARQ). In HARQ, incremental redundancy is transmitted if the first decoding fails. The iterative nature of Turbo decoding synergizes perfectly with HARQ, as each retransmission provides additional parity bits that can be seamlessly incorporated into the iterative decoding process, leading to powerful joint combining gains. This combination became a cornerstone for high-speed downlink and uplink packet access (HSDPA/HSUPA) in 3G and was carried forward as a fundamental technology in LTE and 5G NR, enabling the high spectral efficiency that defines modern mobile broadband.

Key Features

• Parallel concatenation of two recursive systematic convolutional (RSC) encoders
• Use of a pseudo-random interleaver to scramble data between encoders
• Iterative decoding using two soft-input soft-output (SISO) decoders
• Exchange of extrinsic information between decoders to refine bit estimates
• Performance near the Shannon limit with practical complexity
• Generates systematic output plus two streams of parity bits for transmission

Evolution Across Releases

Defining Specifications