Method of Error Floor Mitigation in Low-Density Parity-Check Codes

A digital communication decoding method for low-density parity-check coded messages. The decoding method decodes the low-density parity-check coded messages within a bipartite graph having check nodes and variable nodes. Messages from check nodes are partially hard limited, so that every message which would otherwise have a magnitude at or above a certain level is re-assigned to a maximum magnitude

Compute-and-Forward Relay Networks with Asynchronous, Mobile, and Delay-Sensitive Users

We consider a wireless network consisting of multiple source nodes, a set of relays and a destination node. Suppose the sources transmit their messages simultaneously to the relays and the destination aims to decode all the messages. At the physical layer, a conventional approach would be for the relay to decode the individual message one at a time while treating rest of the messages as interference. Compute-and-forward is a novel strategy which attempts to turn the situation around by treating the interference as a constructive phenomenon. In compute-and-forward, each relay attempts to directly compute a combination of the transmitted messages and then forwards it to the destination. Upon receiving the combinations of messages from the relays, the destination can recover all the messages by solving the received equations. When identical lattice codes are employed at the sources, error correction to integer combination of messages is a viable option by exploiting the algebraic structure of lattice codes. Therefore, compute-and-forward with lattice codes enables the relay to manage interference and perform error correction concurrently. It is shown that compute-and-forward exhibits substantial improvement in the achievable rate compared with other state-of-the-art schemes for medium to high signal-to-noise ratio regime. Despite several results that show the excellent performance of compute-and-forward, there are still important challenges to overcome before we can utilize compute-and- forward in practice. Some important challenges include the assumptions of \perfect timing synchronization "and \quasi-static fading", since these assumptions rarely hold in realistic wireless channels. So far, there are no conclusive answers to whether compute-and-forward can still provide substantial gains even when these assumptions are removed. When lattice codewords are misaligned and mixed up, decoding integer combination of messages is not straightforward since the linearity of lattice codes is generally not invariant to time shift. When channel exhibits time selectivity, it brings challenges to compute-and-forward since the linearity of lattice codes does not suit the time varying nature of the channel. Another challenge comes from the emerging technologies for future 5G communication, e.g., autonomous driving and virtual reality, where low-latency communication with high reliability is necessary. In this regard, powerful short channel codes with reasonable encoding/decoding complexity are indispensable. Although there are fruitful results on designing short channel codes for point-to-point communication, studies on short code design specifically for compute-and-forward are rarely found. The objective of this dissertation is threefold. First, we study compute-and-forward with timing-asynchronous users. Second, we consider the problem of compute-and- forward over block-fading channels. Finally, the problem of compute-and-forward for low-latency communication is studied. Throughout the dissertation, the research methods and proposed remedies will center around the design of lattice codes in order to facilitate the use of compute-and-forward in the presence of these challenges

A study on an improving method of launch vehicle telemetry system based on link analysis

The telemetry system for the launch operation is one of wireless communication system which acquires the data about the operational status and flight information of launch vehicle, and receives, processes, and distributes telemetry data in real time to determine the progress of launch mission and confirm the satellite separation. In NARO Space Center, four telemetry ground stations including two stations with large 11 m parabola antenna are in operation in order to ensure stable reception and acquisition of telemetry signal, and total five ground stations including additional ground station with 7 m antenna in south pacific, PALAU, will be operated to launch the NURI(KSLV-II) which scheduled to launch 2021. When designing a launch telemetry system, the antenna with large reflector over 7 m in a ground station is usually considered in order to ensure sufficient link margin. However, it should be required such supporting equipment, site for installation, and operating personnel because of that. If the size of antenna can be reduced significantly, the ground station can be operated in the form of VAN vehicle with small mobile antenna without the site and building for a large antenna, power generator and auxiliary facilities. This will makes it very easy to deploy ground station for various launch mission, and can help to reduce the cost of maintaining large ground stations especially in case of not frequent launch. The most widely used modulation method in aerospace telemetry since the 1970s is pulse code modulation/frequency modulation(PCM/ FM), which is still widely used in spite of the disadvantage of low spectrum efficiency. Forward error correction codes are also used in limited, even though they are now standard in most communications applications. These are the same for all the launch vehicles launched from the Naro Space Center so far, including NARO and NURI. Also, in case of on-board Tx. antenna, two antennas are mounted symmetrically on the surface of launch vehicle fuselage to have a omni directional pattern, and this causes large nulls at the overlapping section in the antenna pattern, especially in the forward and backward axes, and lots of loss in link budget. In this dissertation, it is analyzed that the margin required to perform the launch mission through link analysis based on the telemetry data which was acquired during the launch mission of the NURI Test Launch Vehicle, and verified the margin that can be acquired when designing a communication link using digital communication method and forward error correction code recommended by IRIG-106 which is the aerospace telemetry standard. On the basis of this, I propose a method to improve the on-board Tx. antenna to secure additional margin, and to make the size of antenna of the ground station as small as possible. In order to verify the validity of the proposed method, the actual received signal during the launch mission of NURI Test Launch Vehicle was used for analysis, and verification was performed through simulation.|발사체 텔레메트리 시스템은 발사체에 대한 각종 동작상태 및 비행정보 등에 관한 제반 자료를 획득하는 무선통신 시스템으로 발사임무진행의 판단 및 위성궤도 진입 상태를 파악하기 위해 실시간으로 자료를 수신, 처리하고 분배한다. 나로우주센터에는 발사체 발사로부터 위성분리 시점까지 텔레메트리 신호의 안정적인 수신 및 획득을 위해 11 m 급 대형 안테나를 가진 지상국 2기를 포함하여 4기의 텔레메트리 지상국을 운영 중이며, 2021년 발사 예정인 누리호(KSLV-II) 발사를 위해 남태평양 팔라우에 7 m 급 안테나를 가진 지상국 1기를 추가하여 총 5기의 지상국을 운영 예정이다. 일반적으로 발사체 텔레메트리 시스템 설계시 충분한 링크 마진 확보를 위해 7 m 급 이상의 대형 반사판을 가진 안테나를 고려하고 지상국에 설치하여 운용하게 되는데, 이를 위해 여러 지원 장비와 설치 부지 및 운용 인력이 필수적이다. 만약 지상국 안테나를 비약적으로 소형화 시킬 수 있다면 대형 안테나 설치를 위한 부지와 건물, 대형 발전기 및 부대시설 없이 차량 견인이 가능한 형태로 지상국을 운영할 수 있어 다양한 발사 임무에 따른 지상국 배치가 매우 용이해 지고, 발사가 빈번하지 않은 경우 대형 지상국 유지에 따른 비용 감소에도 크게 도움이 될 수 있다. 1970년대 이후로 우주항공 텔레메트리 분야에서 가장 폭 넓게 사용된 변조 방식은 펄스 부호 변조/주파수 변조(PCM/FM) 방식으로 대역폭 효율이 좋지 않은 단점에도 불구하고 오늘날에도 많이 사용되고 있다. 순방향 오류정정 부호의 경우도 현재 대부분의 통신 분야에서 표준이 된 기술임에도 제한적으로 사용되고 있으며, 이는 나로호와 누리호를 포함하여 나로우주센터에서 지금까지 발사한 모든 발사체가 동일하다. 발사체 탑재 송신 안테나의 경우 발사체 동체 표면에 두 개의 안테나를 대칭으로 배치하여 전방향성 패턴을 가지도록 하였는데, 이로 인해 패턴 중첩구간, 특히 전방과 후방 진행 축에서 큰 널이 발생하여 링크버짓상 많은 손실을 가지게 한다. 본 연구에서는 누리호 시험발사체 발사시 획득한 데이터를 바탕으로 링크 분석을 통해 발사임무 수행에 필요한 마진을 분석하고, 항공우주분야 텔레메트리 표준인 IRIG-106에서 권고하는 디지털 통신 방식과 순방향 오류정정 부호를 적용해 통신링크를 설계할 경우 확보가능한 마진을 계산한다. 이를 바탕으로 탑재 송신 안테나를 개선해 추가 마진을 확보하여 지상국의 안테나를 최대한 소형화 시킬 수 있는 방안을 제안한다. 제안된 방법의 타당성 입증을 위해 누리호 시험발사체 발사임무에서 실 수신된 신호를 분석에 사용하였으며 시뮬레이션을 통해 검증을 수행하였다.제 1 장 서론 1.1 연구 배경 1 1.2 연구 목적 4 1.3 논문 구성 5 제 2 장 통신링크 분석 2.1 텔레메트리 지상국 개요 6 2.2 누리호 시험발사체 발사시 지상국 운용 8 2.3 정적 링크분석 12 2.4 동적 링크분석 20 제 3 장 링크 마진 개선 방안 3.1 디지털 변조 - SOQPSK 33 3.2 채널 부호화 기법 - 저밀도 패리티 검사 부호(LDPC) 43 3.3 탑재 송신 안테나 개선 49 3.3.1 원통형 배열 이론 50 3.3.2 안테나 설계 및 시뮬레이션 56 제 4 장 지상국 안테나 최적화 분석 83 제 5 장 결론 87 참고문헌 90Docto

Sparse graph-based coding schemes for continuous phase modulations

The use of the continuous phase modulation (CPM) is interesting when the channel represents a strong non-linearity and in the case of limited spectral support; particularly for the uplink, where the satellite holds an amplifier per carrier, and for downlinks where the terminal equipment works very close to the saturation region. Numerous studies have been conducted on this issue but the proposed solutions use iterative CPM demodulation/decoding concatenated with convolutional or block error correcting codes. The use of LDPC codes has not yet been introduced. Particularly, no works, to our knowledge, have been done on the optimization of sparse graph-based codes adapted for the context described here. In this study, we propose to perform the asymptotic analysis and the design of turbo-CPM systems based on the optimization of sparse graph-based codes. Moreover, an analysis on the corresponding receiver will be done

VLSI decoding architectures: flexibility, robustness and performance

Stemming from previous studies on flexible LDPC decoders, this thesis work has been mainly focused on the development of flexible turbo and LDPC decoder designs, and on the narrowing of the power, area and speed gap they might present with respect to dedicated solutions. Additional studies have been carried out within the field of increased code performance and of decoder resiliency to hardware errors. The first chapter regroups several main contributions in the design and implementation of flexible channel decoders. The first part concerns the design of a Network-on-Chip (NoC) serving as an interconnection network for a partially parallel LDPC decoder. A best-fit NoC architecture is designed and a complete multi-standard turbo/LDPC decoder is designed and implemented. Every time the code is changed, the decoder must be reconfigured. A number of variables influence the duration of the reconfiguration process, starting from the involved codes down to decoder design choices. These are taken in account in the flexible decoder designed, and novel traffic reduction and optimization methods are then implemented. In the second chapter a study on the early stopping of iterations for LDPC decoders is presented. The energy expenditure of any LDPC decoder is directly linked to the iterative nature of the decoding algorithm. We propose an innovative multi-standard early stopping criterion for LDPC decoders that observes the evolution of simple metrics and relies on on-the-fly threshold computation. Its effectiveness is evaluated against existing techniques both in terms of saved iterations and, after implementation, in terms of actual energy saving. The third chapter portrays a study on the resilience of LDPC decoders under the effect of memory errors. Given that the purpose of channel decoders is to correct errors, LDPC decoders are intrinsically characterized by a certain degree of resistance to hardware faults. This characteristic, together with the soft nature of the stored values, results in LDPC decoders being affected differently according to the meaning of the wrong bits: ad-hoc error protection techniques, like the Unequal Error Protection devised in this chapter, can consequently be applied to different bits according to their significance. In the fourth chapter the serial concatenation of LDPC and turbo codes is presented. The concatenated FEC targets very high error correction capabilities, joining the performance of turbo codes at low SNR with that of LDPC codes at high SNR, and outperforming both current deep-space FEC schemes and concatenation-based FECs. A unified decoder for the concatenated scheme is subsequently propose