Multicast resource allocation with min-rate requirements in OFDM systems

This article addresses the multicast resource allocation problem with min-rate requirement constraints in orthogonal frequency division multiplexing (OFDM) systems. Due to the prohibitively high complexity for nonlinear and combinatorial optimization, the original problem is relaxed and reformulated to form a standard optimization problem. By theoretical derivation according to the Karush-Kuhn-Tucker (KKT) conditions, two propositions are presented as the necessary criteria for optimality. Furthermore, a two-step resource allocation scheme, including subcarrier assignment and power allocation, is proposed on a basis of the propositions for practical implementation. With the min-rate based multicast group order, subcarriers are assigned in a greedy fashion to maximize the capacity. When subcarrier assignment is determined, the proposed power allocation can achieve the optimal performance for the min-rate constrained capacity maximization with an acceptable complexity. Simulation results indicate that the proposed scheme approximates to optimal resource allocation obtained by exhaustive search with a negligible capacity gap, and considerably outperforms equal power distribution. Meanwhile, multicast is remarkably beneficial to resource utilization in OFDM systems.     

一种新的OFDM系统载波频偏估计算法

在正交频分复用(OFDM)系统中,频率偏移会破坏子载波间的正交性,降低整个系统的性能。在研究传统频偏估计算法的基础上,利用简单的实数训练序列,进行OFDM系统的载波频偏估计,只需要一个训练序列,就可以有效地在时域内同时对小数倍和整数倍的频率偏移进行估计。仿真结果表明,基于实数训练序列的载波频偏估计算法不仅方法简单,易于实现,而且可提高频偏估计精度。     

基于FPGA的OFDM调制解调系统设计与实现

正交频分复用(OFDM)是第四代移动通信的核心技术,该技术在高速数据传输中得到了广泛的应用。文章给出了正交频分复用系统的基本模型和该系统所采用的调制与解调方法;根据OFMD的特点,本设计提出了一种基于IFFT/FFT的OFDM调制解调器的低成本FPGA的实现方法。在ISE环境下完成对OFDM无线通信系统中快速傅里叶变换算法的仿真,仿真结果表明快速傅里叶变换在OFDM技术中具有一定的使用价值。实践证明,该方法具有设计简单、快速、高效和实时性等特点。     

基于OFDM/DMT的短长度循环前缀分析

基于正交频分复用系统/离散多音调制系统(OFDM/DMT)的短长度循环前缀,设定相关信号并引入参数完成系统模型和干扰消除.通过分析短长度循环前缀对系统性能的影响,导出载波和符号间干扰能量谱密度计算公式,且得知二者的能量谱密度值相等.该分析以非对称数字环路传输系统为例,其噪声在DC附近集中.     

Beamforming and Synchronization Algorithms Integration for OFDM HAP-Based Communications

High Altitude Platforms (HAPs) are currently experiencing a great interest as potential solutions for the growing demand of wireless broadband applications, but their rapid motion and the characteristics of the stratospheric channel, which may cause strong attenuation, multipath fading and Doppler shift, can strongly impact their performances. The described scenario requires the use of beamforming techniques, able to provide medium to high gains in a mobile environment. Given these premises, this paper discusses the conjugation of OFDM techniques with smart antennas for HAP-based communications, stressing the need for integration of the different functional blocks of an OFDM receiver. In fact, in order to face a time varying scenario, adaptive techniques are needed, and only a careful joint design of time and frequency synchronization with beamforming algorithms can guarantee reliable communication. The detailed and integrated receiver design described in the paper is based on the OFDM version of the IEEE 802.16a transmission standard.

OFDM系统中频域同步技术及FPGA实现

针对OFDM系统频域中的整数倍频率偏移、小数倍频率偏移、采样钟频率偏移和定时偏移等问题,本文提出了相应的解决方案,并采用FPGA对各方法进行硬件电路实现.这些硬件实现方法巧妙,估计精确,能节省大量硬件资源.通过实际的电路时序波形仿真验证,证明了这些方法的实用性.     

基于高阶累积量的OFDM信道估计算法研究

基于高阶累积量与LS准则联合估计信道参数的思想,在数据相关条件下,提出采用加权最小二乘法进行信道估计.算法提升了LS算法信道估计的精度,降低了信道估计算法的复杂度.仿真结果表明,该算法性能接近于传统MMSE和ML算法性能.     

无线局域网的探讨

无线局域网(WLAN)就是采用无线通信技术代替传统电缆,提供传统有线局域网(LAN)功能的网格.它是计算机网络与无线通信技术相结合的产物.但这并不说明无线局域网不需要传输介质,而是使用了人眼无法看到的电磁波.现在的有线局域网技术已经发展得比较完善,然而无线局域网无线的特性具有常规网络无可比拟的优点.分别阐述了无线局域网的特点及其优点、在发展中所遇到的障碍及其发展前景.     

OFDM技术在移动通信系统中的应用

正交频分复用技术是下一代移动通信的核心技术之一.其最突出的优点是频谱利用平高、抗干扰、抗衰落,适合未来高速宽带无线通信的应用.本文对OFDM技术的基本原理进行了简要概述,并着重用述了OFDM技术在移动通信系统中的典型应用,包括与多种多址接入技术的结合,与MIMO技术的结合.     

Spectrally encapsulated OFDM: Vectorized structure with minimal complexity

To efficiently use frequency resources, the next 6th generation mobile communication technology must solve the problem of out-of-band emission (OoBE) of cyclic prefix (CP) orthogonal frequency division multiplexing (OFDM), which is not solved in 5th generation technology. This study describes a new zero insertion technique to replace an existing filtering scheme to solve this internal problem in OFDM signals. In the development of the proposed scheme, a precoder with a two-dimensional structure is first designed by generating a two-dimensional mapper and using the specialty of each matrix. A spectral shaping technique based on zero insertion instead of a long filter is proposed, so it can be applied not only to long OFDM symbols, but also very short ones. The proposed method shows that the transmitted signal is completely blocked at the bandwidth boundaries of signals according to the current standards, and it is confirmed that the proposed scheme is ideal with respect to bit error rate (BER) performance because its BER is the same as that of CP-OFDM. In addition, the proposed scheme can transformed into a real time structure through vectorizing process with minimal complexity.