We propose a new coding scheme, called the delayed coding (DC) scheme, for channels with insertion, deletion, and substitution (IDS) errors. The proposed scheme employs delayed encoding and non-iterative detection and decoding strategies to manage the transmission of multiple codewords in a linear code. In the DC scheme, a channel input sequence consists of subblocks of multiple codewords from the previous to current time instances. At the receiver side, the maximum a posteriori detection applies to the received sequences that contain information of the codeword at the current time instance, where priorly decoded codewords aid the detection. The channel code decoding is then performed, and extrinsic messages are exploited for the codeword estimations of the following time instances. We show that the rate achievable with the DC scheme over the IDS channel approaches the symmetric information rate of the channel. Moreover, we show the excellent asymptotic and finite-length performances of the DC scheme in conjunction with low-density parity-check codes.

Over the past two decades, irregular low-density parity-check (LDPC) codes have been hardly able to decode information corrupted by insertion and deletion (ID) errors without markers. Surprisingly, in this paper, we bring to light the existence of irregular LDPC codes that approach the theoretical limit of the channel with ID errors even without markers. These codes contain high fractions of low-degree check nodes that do not appear in irregular codes for other channels. This motivates us to investigate the contribution of low-degree check nodes to correcting ID errors. The investigation provides the following interesting result: degree-2 check nodes are critical to approaching the theoretical limit even without markers, codes with only degree-3, 4, or more check nodes provide moderate decoding performance, and codes with only degree-5 or more check nodes can hardly correct ID errors. Finally, we present simulation results that confirm the excellent decoding performance of the irregular codes without markers.

In this paper, the structured concatenated coding of protograph LDPC codes and markers for insertion and deletion channels are proposed, where the joint detector/decoder can deal with the differences in estimation accuracy of these errors. Conventional concatenated coding of LDPC codes and markers does not address the differences. The proposed approach with optimized protographs can exploit those differences and thus improve the performance of the joint detection/decoding. Iterative decoding thresholds and simulation results are provided to support the advantages of the proposed concatenated coding scheme.

In this study, we developed spatially coupled LDPC (SC-LDPC) codes on the two-dimensional array erasure (2DAE) channel. We propose a method of generating new SC-LDPC codes with the restriction on the check node constraint. We evaluate by density evolution analysis that the improvement of the threshold for the proposed two-dimensional SC-LDPC codes over the one-dimensional SC-LDPC codes. Moreover we verify that the BP threshold of the proposed codes can approach the corresponding maximum a posteriori (MAP) threshold of the original residual graph on the 2DAE channel.

This paper proposes a novel two-dimensional wavelength hopping/time spreading (WH/TS) optical code division multiplexing (2-D WH/TS OCDM) system using prime sequences and a heterodyne detection receiver for broadband optical multiple access networks (OMAN). The performance of the proposed system is theoretically analyzed taking into account various kinds of noise and interference, including multiple access interference (MAI), optical beating interference (OBI), crosstalk and receiver noise. The prime sequences are used as the signature codes for both wavelength hopping and time spreading. Both symmetric and over-colored asymmetric systems are considered. Analytical results show that the proposed system offers much better performance compared with that of a conventional 2-D WH/TS system using direct-detection. As a matter of fact, the OBI is well suppressed in the proposed system resulting in a considerable increase in the number of supportable users. Moreover, the system's sensitivity is significantly improved enabling the proposed system to be a promising candidate for the broadband OMAN deployed using the passive optical network (PON) configuration.

We theoretically analyze the performance of coherent ultrashort light pulse code-division multiple-access (CDMA) communication systems with nonlinear optical thresholders. The coherent ultrashort light pulse CDMA is one of the promising systems for optical local area networks due to the advantages of asynchronous transmission, high information security, multiple access capability, and optical processing. The nonlinear optical thresholder is based on frequency chirping induced by self-phase modulation (SPM) in optical fibers, and discriminates the ultrashort pulse from multiple access interference (MAI) having picosecond duration. Our results show that the nonlinear optical thresholder can discriminate 400-fs ultrashort pulse from /spl ap/100-ps MAI even if the response time of the photodetector is limited to 100-ps, and that the system performance does not always become better as the total number of users becomes smaller because of thermal noise, which is a unique characteristic of this system.

We propose a new technique for rejection of narrow-band interference (NBI) based on multiple-symbol detection of coherent or differential phase-shift keying (DPSK). We first show that the direct use of multiple-symbol detection offers poor performance when NBI is dominant. Our proposed technique employs a special signaling or coding scheme which is shown to be robust against NBI. The evaluation of bit-error rate (BER) shows significant performance improvement in NBI vis-a-vis direct multiple-symbol detection. When viewed as a coding scheme, the proposed signaling scheme is significantly simpler for achieving the same coding gain than conventional error correction codes.

This paper proposes chip coding using convolutional codes for fast frequency-hopping spread-spectrum multiple-access (FFH-SSMA) systems. Its performance is evaluated and compared with that of repetition coding which is most commonly used in FFH systems. In our proposal, an information symbol is encoded by a convolutional code of rate 1/n and n chips are transmitted through a frequency hopper. We consider three models when perfect side information (PSI) is available, no side information (NSI) is available, and the ratio threshold test (RTT) is used. The probability density functions (PDFs) of the received signal are derived for asynchronous hopping systems. The performances of convolutional coding with threshold decoding and Viterbi decoding are presented under the constraint of fixed total bandwidth and information bit rate. In comparison with repetition codes, it is found that convolutional codes drastically enhance the system performance. It is identified that Viterbi decoding with RTT offers the most significant performance improvement.

A carrier slot setting plan is proposed to reduce intermodulation (IM) effects for satellite single-channel-per-carrier (SCPC) systems. Carrier slots are divided into blocks and slight frequency gaps are set among blocks. This setting introduces frequency offset between the center frequencies of carrier slots and the center frequencies of the IM components. A method of deriving optimum division in order to derive the upper bound of improvement of the C/IM ratio in the worst channel is developed. The results show that the setting plan achieves significant improvement of IM effects at a cost of only slight bandwidth expansion, even in fully loaded SCPC systems.>