Carrier frequency offset estimation for OFDM systems using subcarriers

Carrier frequency offset (CFO) in orthogonal frequency-division multiplexing(OFDM) systems, which can induce the loss of orthogonality among subcarriersand result in significant performance degradation, is critical to be estimatedand compensated for. In this paper, a suboptimal scheme is proposed to estimatethe CFO using subcarriers. In the proposed scheme, the CFO is dividedinto the fractional part and the integer part, with the fractional CFO estimatedfollowed by the integer CFO estimation. Compared with previous work using subcarriers and exhaustive search, the implementation complexity of theproposed scheme is significantly lower due to the fact that the main componentrequired is a simple correlator. Furthermore, with only one trainingOFDM symbol and proper subcarrier allocation, the estimation range ofthe proposed scheme can be the inverse of the sampling duration. In contrast,conventional schemes with the same estimation range and complexity require two OFDM symbols. To achieve good performance,it will be shown that the subcarrier allocation should be based uponsome specific binary sequences such as the proposed extended m-sequences. It willalso be shown by simulations that the same subcarrier allocation criterionshould be employed for optimal maximum-likelihood CFO estimation.     

基于OFDM的DF多跳通信系统的功率分配

研究了DF多跳OFDM通信系统的功率分配优化.在总发射功率受限的条件下,提出了一种双向优化功率分配算法(D-OPA):先优化单个子载波上各中继节点的功率分配,使该子载波上的信道容量最大;再优化各个子载波间的功率分配,使单跳的信道容量最大,从而最大化系统的信道容量.仿真结果表明:提出的算法与自适应功率分配算法、平均功率分配算法相比,信道容量明显提高.     

Blind channel equalization and carrier frequency offset estimation for MC-CDMA systems using guard interval redundancy and excess codes

It is well known that multipath channel and carrier frequency offset (CFO) destroy the orthogonality between subcarriers in the downlink of MC-CDMA and degrade the system performance. In this paper, a new low complexity algorithm is proposed to iteratively estimate the CFO and the channel equalizer using only the guard interval (GI) redundancy, or excess codes (EC) or both GI and EC without additional pilots. Simulations illustrate and compare the performance efficiency of these schemes in different scenarios.     

Resource allocation scheme in two-way multi-relay OFDM system

A joint optimization scheme for power allocation and subcarrier pairing under high SNR in two-way multi-relay OFDM system was proposed.Unlike those schemes in which relays use subcarriers separately,all the relays were allowed to forward signal on each subcarrier pair for providing much space diversity.With the constraint of total system power,the proposed scheme firstly allocated each relay power with Cauchy inequality with the assuming that the total relay power was fixed.Then the dichotomy was used to calculate the power allocation between the source node and the relay node by maximizing the equivalent channel gain for different subcarrier pairs.Lastly,the power of different subcarrier pairs was allocated by convex programming,and the subcarriers were paired by Hungarian algorithm to obtain the maximum system capacity.There was no optimal power allocation method with low complexity because of the complexity of the power allocation algorithm in two-way multi-relay networks.This algorithm greatly reduces the complexity of power allocation and simulation results show that the proposed scheme outperforms the relay selection scheme and the relays use subcarriers separately scheme.     

交织OFDMA上行系统CFO估计算法的研究

文中对OFDMA系统中的子载波分配方法进行了研究,介绍了两种子载波分配方法的原理,对两种子载波分配方法进行了仿真,结果显示在频率选择性衰落信道中交织分配的OFDMA系统要比块分配的OFDMA系统获得更多的频率分集.给出了交织OFDMA系统上行链路的信号结构,由于发射机和接收机之间存在的载波频率偏移(CFO)将会影响子载波间的正交性,对多用户的CFO估计提出了一种估计算法,仿真的结果显示所提出的算法具有较好的性能.     

Efficient user pairing algorithm for enhancement of spectral efficiency and interference cancelation in downlink NOMA system

A new subcarrier-user allocation algorithm for the downlink non-orthogonal multiple access system is presented in this paper. The proposed algorithm aims to enhance the spectral efficiency of the system and the successive interference cancelation performance by guaranteeing a high difference in channel-gain between the paired users per subcarrier. To enhance the spectral efficiency, the proposed algorithm provides a higher priority to the subcarrier that has a higher best (maximum) channel gain value rather than that has a lower best channel gain value. Also, it pairs the strong user with the second minimum channel-gain user rather than the minimum channel gain user. Besides, the proposed algorithm divides the subcarriers into two groups according to the standard deviation of the channel gain of each subcarrier. Then, it gives the priority to the group with low standard deviation values during subcarrier-user allocation to guarantee a high difference in channel-gain between the paired users per subcarrier. Later, fractional transmit power allocation is applied to distribute the subcarrier power between the paired users. Simulation results prove that the proposed algorithm improves the spectral efficiency of the system, and guarantees a significantly higher difference in channel-gain between the paired users per subcarrier compared to the conventional algorithms.

     

多用户OFDM系统中一种新颖的自适应功率分配方案

针对多用户正交频分复用(OFDM)系统自适应资源分配的问题,提出了一种新的自适应子载波分配方案。子载波分配中首先通过松弛用户速率比例约束条件确定每个用户的子载波数量,然后对总功率在所有子载波间均等分配的前提下,按照最小比例速率用户优先选择子载波的方式实现子载波的分配;在功率分配中提出了一种基于人工蜂群算法和模拟退火算法(ABC-SA)相结合的新功率分配方案,并且通过ABC-SA算法的全局搜索实现了在所有用户之间的功率寻优,同时利用等功率的分配方式在每个用户下进行子载波间的功率分配,最终实现系统容量的最大化。仿真结果表明,与其他方案相比,所提方案在兼顾用户公平性的同时还能有效地提高系统的吞吐量,进而证明了所提方案的有效性。     

QoS-Driven Jointly Optimal Subcarrier Pairing and Power Allocation for Decode-and-Forward OFDM Relay Systems

In this paper, we investigate the quality-of-service (QoS) driven subcarrier pairing and power allocation for two-hop decode-and-forward (DF) OFDM relay systems. By integrating the concept of effective capacity, our goal is to maximize the system throughput subject to a given delay-QoS constraint. Based on whether the destination can receive the signal transmitted by the source, we consider two scenarios, i.e. OFDM DF relay systems without diversity and OFDM DF relay systems with diversity, respectively. For OFDM DF relay systems without diversity, we demonstrate that the jointly optimal subcarrier pairing and power allocation can be implemented with two separate steps. For OFDM DF relay systems with diversity, we propose an iterative algorithm to achieve jointly optimal subcarrier pairing and power allocation. Furthermore, we find that the analytical results show different conclusions for the two types of OFDM relay systems. For OFDM relay systems without diversity, the optimal power allocation depend on not only the channel quality of subcarriers but also the delay QoS constraints, while the optimal subcarrier pairing just depends on the channel quality of subcarriers. For OFDM relay systems with diversity, both the optimal subcarrier pairing and power allocation depend on the channel quality of subcarriers and the delay QoS constraints. Simulation results show that our proposed scheme offers a superior performance over the existing schemes.     

A Low Complexity Resource Allocation Method for OFDMA System Based on Channel Gain

In orthogonal frequency division with multiple access (OFDMA) systems dynamic radio resource allocation improves overall performance by exploiting the multiuser diversity gains. A key issue in OFDMA is the allocation of the OFDM subcarriers and power among users sharing the channel. This paper proposes a new rate adaptive resource allocation scheme in the OFDMA downlink transmission system. Our proposed algorithm is based on the users’ sensitivity to the subcarrier allocation which means how frequency selective is the channel from the user’s perspective. As a result of frequency selectivity of the channel, different subchannels of the same user experience different levels of fade. However, how different they undergo fading could be measured by difference between maximum and minimum channel gain of that user. Our proposed method is based on difference between maximum channel gain and minimum channel gain for each user and uniform distribution of power among subcarriers. Simulation results show that the proposed algorithm achieves higher capacity over fixed TDMA method, and reported suboptimal methods with acceptable rate proportionality.     

Adaptive subcarrier allocation and bit loading for voice/data transmission in multiuser OFDM systems

A new algorithm of adaptive subcarrier allocation and bit loading (A‐SABL) is proposed for simultaneous voice and data transmission in multiuser OFDM systems. The algorithm takes advantage of the frequency diversity and the voice/data transmission requirements to dynamically assign the number of subcarriers and bits/per symbol on each subcarrier for each user in a single cell. Due to the strict delay requirement of voice service, the subcarriers with low channel gains are assigned for voice transmission with a small number of bits per symbol to guarantee its required bit‐error‐rate (BER) and transmission rate. Based on the remaining subcarriers with high channel gains and the transmission power, the throughput of data transmission is then maximized by loading as many bits as possible on each subcarrier to achieve the required transmission bit rate and BER. Theoretical analysis and simulation on the proposed algorithm show that a better performance is obtained than previously reported schemes. Copyright © 2008 John Wiley & Sons, Ltd.     

一种改进的MC-CDMA系统ORC技术

基于对频率非选择性Rayleigh衰落信道中采用ORC(orthogonality restoring combining)技术的MC-CDMA系统误码率下边界的分析,提出了忽略最弱子载波(信道衰减系数最小的子载波)的改进ORC检测方案。仿真结果表明,该方案比传统ORC在性能上有很大提高,比较接近MMSEC技术;如果忽略过多的弱子载波反而会影响用户间扩频码的正交性,引起系统性能恶化。改进方案具有随子载波数线性增长的运算复杂度,是一种比较实用的MC-CDMA系统检测技术。     

An Efficient Adaptive Modulation Scheme for Wireless OFDM Systems

An adaptive modulation scheme is presented for multiuser orthogonal frequency‐division multiplexing systems. The aim of the scheme is to minimize the total transmit power with a constraint on the transmission rate for users, assuming knowledge of the instantaneous channel gains for all users using a combined bit‐loading and subcarrier allocation algorithm. The subcarrier allocation algorithm identifies the appropriate assignment of subcarriers to the users, while the bit‐loading algorithm determines the number of bits given to each subcarrier. The proposed bit‐loading algorithm is derived from the geometric progression of the additional transmission power required by the subcarriers and the arithmetic‐geometric means inequality. This algorithm has a simple procedure and low computational complexity. A heuristic approach is also used for the subcarrier allocation algorithm, providing a trade‐off between complexity and performance. Numerical results demonstrate that the proposed algorithms provide comparable performance with existing algorithms with low computational cost.     

Worst subcarrier avoiding water-filling subcarrier allocation scheme for OFDM-based CRN

Efficient and reliable subcarrier power joint allocation is served as a promising problem in cognitive OFDM-based Cognitive Radio Networks (CRN). This paper focuses on optimal subcarrier allocation for OFDM-based CRN. We mainly propose subcarrier allocation scheme denoted as Worst Subcarrier Avoiding Water-filling (WSAW), which is based on Rate Adaptive (RA) criterion and three constraints are considered in CRN. The algorithm divides the assignment procedure into two phases. The first phase is an initial subcarrier allocation based on the idea of avoiding selecting the worst subcarrier in order to maximize the transmission rate; while the second phase is an iterative adjustment process which is realized by swapping pairs of subcarriers between arbitrary users. The proposed scheme could assign subcarriers in accordance with channel coherence time. Hence, real time subcarrier allocation could be implemented. Simulation results show that, comparing with the similar existing algorithms, the proposed scheme could achieve larger capacity and a near-optimal BER performance.     

Proportional-fairness resource allocation for uplink OFDM-based cognitive radio systems

This paper has proposed a proportional-fairness resource allocation algorithm, including both subcarrier assignment algorithm and power allocation algorithm, for uplink orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) systems. First, to get a better performance in the proposed system model, the influence factor (a,b,c) was introduced to realize the assignment of the subcarriers. Second, the transmit power of the secondary users (SUs) was allocated to the corresponding subcarriers in order to maximize the uplink capacity of the SUs subject to both power and interference constraints. With the appropriate influence factor in the subcarrier assignment, the loss of transmitted data rate arising from the fairness was minimized. Simulation results showed that the proposed algorithm can achieve a perfect fairness among the SUs while maximizing the system capacity simultaneously, and is of a low computation complexity.     

一种基于子载波合并的多播资源调度算法

针对无线OFDM多播系统,该文提出一种基于子载波合并的多播资源调度算法。该算法通过提前将子载波分组,避免了不必要的子载波配对;自适应地选择子载波合并\非合并,在分集和复用两种模式中选取最优方案;同时根据子载波功率分配的特点,将其解耦为配对子载波集合内功率分配和集合间功率分配两个子问题,进一步优化了算法性能。仿真结果表明,与现有方案相比,所提方案能够在复杂度较低的情况下,较好地提升系统性能。     

多用户OFDM系统中子载波比特分配算法

针对多用户OFDM系统中的功率优化问题,提出了一种新的子载波、比特分配算法。利用匈牙利算法对子载波进行初始分配,然后根据信道的状态信息为每个子载波动态地分配比特,最后对分配的结果作进一步调整,直到发射总功率不再减小为止。仿真结果表明,调整后的结果比原有方法有所改善,在满足传输速率和误码率要求的条件下,有效地降低了系统发射总功率。     

Multiuser OFDM with adaptive subcarrier, bit, and power allocation

Multiuser orthogonal frequency division multiplexing (OFDM) with adaptive multiuser subcarrier allocation and adaptive modulation is considered. Assuming knowledge of the instantaneous channel gains for all users, we propose a multiuser OFDM subcarrier, bit, and power allocation algorithm to minimize the total transmit power. This is done by assigning each user a set of subcarriers and by determining the number of bits and the transmit power level for each subcarrier. We obtain the performance of our proposed algorithm in a multiuser frequency selective fading environment for various time delay spread values and various numbers of users. The results show that our proposed algorithm outperforms multiuser OFDM systems with static time-division multiple access (TDMA) or frequency-division multiple access (FDMA) techniques which employ fixed and predetermined time-slot or subcarrier allocation schemes. We have also quantified the improvement in terms of the overall required transmit power, the bit-error rate (BER), or the area of coverage for a given outage probability     

A Fast Subcarrier, Bit, and Power Allocation Algorithm for Multiuser OFDM-Based Systems

In this paper, we propose a real-time subcarrier, bit, and power allocation algorithm for orthogonal frequency-division multiplexing-based multiuser communication systems in downlink transmission. Assuming that base stations know the channel gains of all subcarriers of all users, the proposed loading algorithm tries to minimize the required transmit power while satisfying the rate requirement and data error rate constraint of each user. The novel algorithm simultaneously determines subcarrier, bit, and power allocation by enhancing the suboptimal algorithm by Wong while having the same computational complexity. The proposed scheme offers better performance in terms of transmit power than that of Wong , as demonstrated in the simulation results, whereas the performance of the scheme in Wong was close to that of the optimal solution.     

基于解调数据二阶统计特性的OFDM盲频偏估计方法

OFDM系统对载波频偏(CFO)非常敏感,CFO破坏了子载波正交性从而引起载波间干扰(ICI)导致系统性能下降.本文分析了CFO对解调数据二阶统计特性的影响,提出了一种适用于频率选择性衰落信道下的非数据辅助的频偏估计方法,利用解调数据二阶统计量矩阵的对角能量提取CFO信息,可实现CFO的精确估计.该方法不需要发送导频数据和信道响应先验知识,因而提高了系统传输效率.文中同时给出了估计的闭环求解方法,大大简化了实现复杂度.最后对算法在多径信道环境下进行了仿真,结果表明本文提出的估计方法在低SNR时仍具有较好的估计性能.     

具有时延QoS保证的OFDM中继系统子载波配对与功率分配算法

 本文研究具有直接通信链路的OFDM解码转发(Decode-and-Forward,DF)中继系统的子载波配对与功率分配算法,目标是在满足业务时延QoS要求的前提下最大化系统容量.利用有效容量模型,首先把OFDM DF中继系统的子载波配对与功率分配问题形成为混合整数规划问题,然后把其转化为连续松弛凸规划问题,利用凸优化方法得到原问题的最优解,从而提出了一种联合最优的子载波配对与功率分配迭代算法.理论推导结果和仿真结果表明,最优子载波配对与功率分配不仅取决于子载波的信道增益,还取决于业务的时延QoS要求.与已有算法相比,本文算法获得的有效容量最大.