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of circumstances and application environments.

LDPC信道编码——LDPC channel coding

LDPC codes were originally invented by Gallager in early 1960's [[xix]] as an error correcting code. After the invention LPDC codes were largely forgotten and was rediscovered by Mackay in 1999 [[xx]]. Since then they have experienced a remarkable return in the last few years. As a result of the significant development of the LDPC codes, it found an application as an optional channel coding technique to be used in the WiMAX standard [[xxi]]. LDPC codes are considered as capacity-approaching codes, which mean that the codes allow transmission at rates close to the theoretical maximum, as defined by the Shannon limit, for a symmetric memory-less channels over a very large code length. For instance, the performance of the LDPC code is only 0.0045 dB below the theoretical maximum, for a code length of one million bits.

LDPC codes in WiMAX is based on a set of (one or more) fundamental coding rates: 1/2, 2/3A, 2/3B, 3/4A, 3/4B, and 5/6. Each LDPC code in the standard is defined by a parity check matrix H of size m × n, where m refers to the number of parity bits and n refers to the length of output packet. The number of systematic bits (information bits) of the code is k = n - m. 2/3A, 2/3B and 3/4A, 3/4B has the same coding rates but different parity-check matrices H. Parity check matrix H is obtained by expanding the generator base matrix by replacing the entries with a square matrix (z × z), where the matrix dimension is equal to the block size. The base matrices defined in the IEEE 802.16e standard [69] for the six fundamental code rates are given in Appendix B.


全球微波存取互通介面——Mobile WiMAX (Worldwide Interoperability for Microwave Access)

Mobile WiMAX is an emerging telecommunications technology for which provides mobile wireless internet access. The high data rate and Quality of Service (QoS) provided by WiMAX technology make it attractive to multimedia applications, such as video telephony, video gaming, and video broadcasting. IEEE 802.16e-2005 standard [75] [[xxii]] defines the formal specificities of Mobile WiMAX. Mobile WiMAX standard was developed by the WiMAX forum and is an amendment to IEEE 802.16d-2004, (Fixed WiMAX standard) to introduce support for mobility. The technology promises high data rates, up to 70 Mbps, and wide coverage, coverage radius of up to 50 km, at a lower cost. Consequently, Mobile WiMAX has gain the most commercial attention to date and is being successfully deployed in many countries. To accomplish high efficiency, throughput and reliability, several techniques are built into the MAC and Physical layers of the mobile WiMAX standard. In addition, security and quality of service (QoS) mechanisms are also incorporated. Packet structure of mobile WiMAX is well suited for non line of sight communication, which is the typical mobile WiMAX user experience. Fading distribution of non line of sight communication is closely correlated to Rayleigh distribution, because it processes the statistical time varying properties of non line of sight communication link [[xxiii]][[xxiv]][[xxv]]. Channel coding for error correction is necessary for reliable communication due to channel noise, fading and other transmission impairments. For this purpose the IEEE 802.16e-2005 standard suggests the use of following coding methods [[x本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。