Tobias Breddermann

EXIT Functions for Parallel Concatenated Insertion Convolutional Codes

Abstract-Insertion convolutional codes and the underlying principles attract more and more attention, e.g., in iterative decoding, joint channel coding and cryptography or even in channel estimation. Known bits (dummy bits) are inserted into the information bit sequence before convolutional (turbo) encoding. These bits support the decoding of the information bits resulting in an improved decoding quality. Although this concept is widely employed, there does not exist any theoretical evaluation of these codes. We provide a basis for such an evaluation by means of their EXIT charts. We will analytically derive their EXIT charts as a function of the EXIT charts associated with the underlying convolutional code for a transmission over a BEC as well as a binary input/continuous output AWGN channel. These results can be adopted to all applications where perfect information is exploited by a SISO decoder and provide an excellent basis for the analytical prediction of the expected gains.

Hybrid ARQ scheme for UMTS LTE based on insertion convolutional turbo codes

Mobile communication systems as, e.g, UMTS LTE perform rate matching and Hybrid Automatic Repeat reQuest (HARQ) to provide adequate decoding performance over a wide range of channel conditions. In UMTS LTE this is realized by means of a fixed rate convolutional Turbo code and a rate matching scheme based on bit puncturing and bit repetition. A promising alternative to convolutional Turbo codes with simple bit repetition are Rate-Compatible Insertion Convolutional (RCIC) Turbo codes which have proven considerable performance gains in systems without HARQ. Since modern systems conventionally employ HARQ, a novel HARQ scheme for UMTS LTE based on RCIC Turbo codes is presented in this paper. Extensive simulations have demonstrated that the conventional UMTS LTE HARQ scheme is outperformed by our novel HARQ scheme in terms of system throughput.

On the Design of Hybrid Automatic Repeat Request Schemes with Unreliable Feedback

\gls{HARQ} schemes are employed in mobile communication systems to reduce the residual Frame Error Rate (FER) after channel decoding. If decoding fails, the receiver requests the transmission of additional redundancy about these frames to improve the decoding success. Signaling is carried out over a feedback link from the receiver back to the transmitter. However, errors on the feedback link severely influence the behavior of the HARQ system and may degrade the overall system performance. In this contribution, the impact of HARQ signaling errors on the FER, the average number of HARQ transmissions per frame, and the system throughput is investigated. We derive analytical expressions for all performance measures based on FER simulations of the HARQ system with reliable feedback. The theoretical results are verified by the example of the LTE physical layer, however, they are also applicable to other communication systems. Moreover, target requirements for the maximum tolerable feedback error probabilities are derived which are used to develop design guidelines for HARQ feedback transceivers.

Rate-Compatible Insertion Convolutional Turbo Codes: Analysis and Application to LTE

Rate-compatible channel codes are commonly used in mobile communication systems to adapt the protection against transmission errors to the varying channel conditions. In the LTE radio system, e.g., the code rate adaptation is carried out based on a fixed-rate mother Turbo code by means of bit puncturing and bit repetition. However, lower code rates can alternatively be provided by \gls{IC} Turbo codes. In RCIC Turbo codes, known bits (dummy bits) are inserted into the information bit sequence during Turbo encoding, providing perfect \apri knowledge to the mother decoder. In this contribution, a detailed analysis of RCIC Turbo codes is provided. It will be shown that these codes outperform rate-compatible Turbo codes using bit repetition in terms of frame error rate and convergence speed. Furthermore, their EXIT charts will be derived from the EXIT chart of the mother Turbo code. This EXIT chart analysis enables a novel semi-analytical optimization tool for RCIC Turbo codes and the efficient EXIT chart comparison of competing codes, avoiding complex simulations. Finally, a HARQ scheme for LTE, based on RCIC Turbo codes, is presented which achieves a considerably higher system throughput compared to the standard LTE solution.

EXIT Chart Optimized Rate Matching for Wireless Communication Systems

In modern wireless communication systems different services with different bit rates and individual error protection requirements are supported by the same network. Rate matching by repetition is often employed after convolutional turbo encoding in order to achieve flexibly the required decrease of the code rate. This ensures near error-free transmission down to a specific target channel quality. We propose the optimization of the rate matching using EXIT charts. The rate for random repetition required for error-free decoding is determined semi-analytically avoiding extensive simulations. This includes the analytical optimization of the random repetition scheme and the derivation of the overall EXIT chart taking into account the convolutional turbo encoding and rate matching.

Rate-compatible LDPC codes using optimized dummy bit insertion

Much attention has been paid to Low-Density Parity-Check (LDPC) codes since their rediscovery by MacKay. They belong to the most powerful channel coding techniques known today and have a broad range of applications. In wireless communication systems it is desirable to be able to adjust the code rate of the employed channel coding scheme (rate-matching) to allow for a flexible strength of error protection for different services and to be able to adapt to the varying quality of the wireless transmission channel. Many of the current systems that employ LDPC codes like, e.g., WiMAX or WLAN specify separate codes for each supported code rate. This paper, in contrast, addresses the problem of using only one mother code and matching (almost) arbitrary code rates that are lower than the mother code rate by inserting known (dummy) bits into the information bit sequence before encoding (also known as pruning or code shortening). We present a novel rule of determining (heuristically) optimized positions of dummy bits within the information bit sequence suitable for LDPC codes. Simulation results show that the frame error rate performance can be improved by the novel approach of dummy bit insertion especially in the error floor region.

Influence of HARQ with Unreliable Feedback on the Throughput of UMTS LTE

In this paper, the impact of HARQ signaling errors caused by unreliable feedback on the throughput of communication systems like UMTS LTE is investigated. We derive analytical expressions for the calculation of the average number of transmissions per data frame as well as the system throughput based on a general feedback channel model. In combination with the residual frame error rates (FERs) after channel decoding, measured for a system with error-free feedback, a semi-analytical evaluation of the influence of unreliable feedback information on the system throughput can be carried out for any feedback channel condition. Consequently, time-consuming simulations of the respective system with unreliable feedback can be avoided. The theoretical results are verified by extensive simulations of the UMTS LTE physical layer. They are also applicable to other communication systems.

Bad Parameter Indication for Error Concealment in Wireless Multimedia Communication

Wireless communication of multimedia signals is often based on the transmission of source codec parameters which are protected against channel noise by channel coding. Commonly, a cyclic redundancy check (CRC) is introduced in order to check for erroneous frames after error correction. If the CRC fails, a bad frame indication (BFI) flag is set which controls a codec-specific frame error concealment. This results in the substitution of complete multimedia frames by their concealed versions even if only a few parameters may be corrupted. The proposed system is based on iterative source-channel decoding. A low complexity bad parameter indication (BPI) scheme is introduced which identifies faulty decoded parameters and enables more effective parameter-individual concealment strategies. This concept may be applied to any multimedia codec (speech, audio, video, images).