Johan Hokfelt

Interleaver design for turbo codes based on the performance of iterative decoding

The performance of a turbo code is dependent on two properties of the code: its distance spectrum and its suitability to be iteratively decoded. Both these properties are dependent on the specific interleaver used in the turbo-coding scheme. This paper describes a strategy for interleaver design that includes a criterion for the performance of iterative decoding. This criterion is based upon correlation properties of the extrinsic information, which is used as a priori inputs to the constituent decoders. Simulations comparing interleaver choices indicate an improved turbo code performance for interleavers designed with algorithms including a correlation criterion.

A turbo code interleaver design criterion based on the performance of iterative decoding

The performance of a turbo code is dependent on two code properties: its distance spectrum and its suitability to be iteratively decoded. Both these properties are influenced by the choice of interleaver used in the turbo encoder. This paper presents an interleaver design criterion that focuses on the performance of iterative decoding, based on the correlation properties of the extrinsic inputs. Interleavers designed with the proposed criterion achieve very competitive performances, both in terms of convergence rates and error correcting capabilities.

On the theory and performance of trellis termination methods for turbo codes

The performance of a turbo code can be severely degraded if no trellis termination is employed. This paper investigates the implications of the choice of trellis termination method for turbo codes, and explains the origin of the performance degradation often experienced without trellis termination. An efficient method to derive the distance spectrum of turbo codes for different trellis termination methods is presented. Further, we present interleaver design rules that are tailored to each termination method. Using interleavers designed with these restrictions, we demonstrate that the performance difference between various termination methods is very small, including no trellis termination at all. For example, we demonstrate a turbo code with a 500-bit interleaver that exhibits no sign of an error floor for frame error rates as low as 10/sup -8/, even though no trellis termination is employed.

Turbo codes: correlated extrinsic information and its impact on iterative decoding performance

The performance of a turbo code is dependent on two properties of the code: its distance spectrum and its suitability to be iteratively decoded. The performance of iterative decoding depends on the quality of the extrinsic inputs; badly correlated extrinsic inputs can deteriorate the performance. While most turbo coding literature assumes that the extrinsic information is uncorrelated, we investigate these correlation properties. An iterative decoding suitability measure is presented, intended to serve as an indication on the degree of correlation between extrinsic inputs. The suitability measure can be used as a complement to the weight distribution when ranking interleavers.

A survey on trellis termination alternatives for turbo codes

Numerous strategies and methods for trellis termination of turbo codes have been presented and proposed in the literature. In this paper the most common trellis termination methods are compared and their relative performances are investigated. An important observation is that the performance of a termination method is highly dependent on the particular interleaver choice. Conclusions should thus be drawn for the combination of interleaver and termination method, and not for the termination method alone. Another important conclusion is that the particular choice of termination method is not crucial for the error correcting performance, as long as certain precautions are taken regarding the choice of interleaver.

On the design of low rate turbo codes

Low rate codes can be used to achieve coding gain in spread spectrum applications such as direct-sequence code division multiple access. Super-orthogonal- and maximum free distance convolutional codes as well as super-orthogonal turbo codes have previously been reported to be suitable for such systems. We focus on the design of component codes for low rate turbo coding schemes targeting the frame-error rate at low signal-to-noise ratios. We present code search criteria which yield high performing turbo codes on the additive white Gaussian noise channel. Simulation results show that these codes perform better than other low rate turbo codes previously reported in the literature.

Interleaver structures for turbo codes with reduced storage memory requirement

This paper describes two interleaver structures that reduce the memory requirement for the storage of interleaver rules. The first structure offers memory reductions of more than 50%, compared to storing the entire interleaver vector. The second structure is useful when a range of interleaver sizes are to be stored, offering memory reductions asymptotically approaching 50%. By combining the two structures, a total memory reduction of approximately 75% is achieved. This is obtained without any significant reduction of the error correcting performance of the codes.

A comparison of turbo codes using different trellis terminations

This paper investigates the incidence of trellis termination on the performance of turbo codes and accounts for the performance degradation often experienced in the absence of trellis termination. Analytical upper bounds on the performance for the ensemble of turbo codes using different trellis termination strategies as well as performance results obtained by computer simulation are presented. In the case of uniform interleaving, the performance differences between various termination methods are relatively small, except when using no trellis termination at all.RésuméOn étudie l’influence de la terminaison du treillis sur les performances de turbocodes et on rend compte de la dégradation souvent observée en l’absence de terminaison. On présente des bornes supérieures analytiques de la probabilité d’erreur sur l’ensemble des turbocodes pour différentes manières de terminer le treillis, ainsi que l’évaluation de ces performances par simulation sur ordinateur. Dans le cas où l’entrelacement est uniforme, les performances varient assez peu d’un type de terminaison à un autre, excepté quand aucun des treillis n’est terminé.