OFDM几种多址接入技术的分析

OFDM（正交频分复用）和多址技术的结合能够允许多个用户同时共享有限的无线频谱，从而获得较高的系统容量。首先介绍了正交频分多址技术中的OFDM-FDMA和OFDM-TDMA多址接入方案，然后叉重点讨论了OFDM和CDMA的3种结合方案，包括系统设计、系统性能分析以及系统参数指标，最后提出了未来的研究方向。     

多用户OFDM系统

通过对多用户情况下正交频分复用技术(OFDM)的多载波传输系统(包括OFDM-FDMA,OFDM-TDMA和OFDM-CDMA)的分析,提出了在传统多用户情况下的OFDM-FDMA系统上对子载波进行优化的分配方案,并与各种多载波技术频率选择性衰落信道的环境下的传输性能进行了研究比较,仿真结果表明该方案性能要优于其他的多载波方案.     

多用户OFDM系统

通过对多用户情况下正交频分复用技术(OFDM)的多载波传输系统(包括OFDM-FDMA,OFDM-TDMA和OFDM-CDMA)的分析,提出了在传统多用户情况下的OFDM-FDMA系统上对子载波进行优化的分配方案,并与各种多载波技术频率选择性衰落信道的环境下的传输性能进行了研究比较,仿真结果表明该方案性能要优于其他的多载波方案.     

LTE系统中上行多址接入技术PAPR的研究

LTE（Long Term Evolution）系统下行链路中采用基于OFDM技术的OFDMA多址方案。但是OFDMA多址方案存在一个重大缺陷：PAPR（峰值平均功率比）过高的问题。而SC-FDMA（单载波频分多址）是一种使用单载波调制和频域均衡的技术,具有低PAPR这一显著特性。本文介绍比较了SC-FDMA和0FDMA两种多址方式,分析了LTE系统中SC-FDMA信号实现过程,并且仿真LTE系统中SC-FDMA的PAPR。仿真结果表明,随着系统带宽的增加,发射信号的PAPR越大,同时对SC-FDMA和OFDMA两种多址接入方式下的PAPR进行比较,得出SC-FDMA具有较低PAPR。     

基于OFDMA的LTE下行多址接入方案

文章介绍了基于正交频分复用多址(OFDMA)的长期演进(LTE)链路多址接入方案,阐述了OFDM技术的基本原理,分析了OFDM技术的优势,从而引出OFDMA多址方案,并简要介绍3GPP建议的其他下行多址接入方案。下行     

LTE上行多址接入系统DFT—S—GMC和DFT—S—OFDM性能研究

本文在分析两种LTE上行多址接入系统信号产生的基础上,对DFT—S—GMC和DFT—S—OFDM系统信号的峰均比性能和同步性能进行了仿真分析。从仿真结果分析得出两种系统信号的峰均比性能相当,DFT—S—OFDM-D同步效果相对较好的结论。研究结果可为下一代移动通信系统选择上行多址方案提供参考。     

基于OFDM系统的随机信道接入方案

双忙音多址信道接入(DBTMA)协议是一种信道利用率较高的无线局域网信道随机接入控制方案。但是, 该协议的硬件实现复杂度较高。该文在对DBTMA协议进行简单修改的基础上,结合节能意识多址接入(PAMAS) 协议提出了一种基于OFDM系统的DBTMA信道接入系统,并对此系统做了简要分析。分析结果显示,该文提出的随机信道接入系统的性能良好且实现方式简单。     

一种低峰均比SCMA多址方案

稀疏码多址接入(Sparse Code Multiple Access,SCMA)作为一种新型非正交多址接入技术,具备一定的过载能力,满足5G无线通信网络海量连接等需求。SCMA系统通常将用户码字映射到OFDM子载波上,具有与OFDM信号相似的高峰均比(Peak to Average Power Ratio,PAPR)特性。针对复用OFDM子载波SCMA系统具有高PAPR的特点,提出了一种低峰均比的SCMA多址方案,采用SC-FDMA技术,并进一步利用峰均比抑制技术对PAPR进行降低。理论和仿真结果表明,该算法能在PAPR性能、BER性能和系统复杂度间获得较好的权衡。     

基于OFDM-MSFH系统的全部载波接入方法的研究

本文根据MB-OFDM-UWB系统的特点,提出了将OFDM技术和MSFH(多级跳频)技术结合的全部载波多址接入方案.该多址方案有二级构成,其中前一级使用OFDM,设计时参考了Multi-band OFDM Physical Proposal for IEEE 802.15 TaskGroup3a中的物理层建议,后一级采用跳频模块,并将基于余数域的多级跳频图案应用到该多址方案中,不仅大大减小了多用户碰撞的概率,而且有利于超宽带信号的生成.理论分析和仿真结果表明,基于OFDM和多级跳频结构的全部载波多址接入方案在系统容量、误比特率性能、抗干扰等方面具有诸多的优势.在8用户情况下,当系统误比特率为10~(-2)时,基于该结构的全载波方案比TH-PPM多址方案改善信噪比5dB左右.     

多用户OFDM系统

通过对多用户情况下正交频分复用技术 (OFDM)的多载波传输系统 (包括OFDM -FDMA ,OFDM-TDMA和OFDM -CDMA)的分析 ,提出了在传统多用户情况下的OFDM -FDMA系统上对子载波进行优化的分配方案 ,并与各种多载波技术频率选择性衰落信道的环境下的传输性能进行了研究比较 ,仿真结果表明该方案性能要优于其他的多载波方案     

An OFDM-TDMA/SA MAC Protocol with QoS Constraints for Broadband Wireless LANs

Orthogonal frequency division multiplexing (OFDM) is an important technique to support high speed transmission of broadband traffic in wireless networks, especially broadband wireless local area networks (LANs). Based on OFDM, a new multiple access scheme, called OFDM-TDMA with subcarrier allocation (OFDM-TDMA/SA), is proposed in this paper. It provides more flexibility in resource allocation than other multiple access schemes such as OFDM-TDMA, OFDM-frequency division multiple access (OFDM-FDMA), and orthogonal frequency division multiple access (OFDMA). With OFDM-TDMA/SA, a medium access control (MAC) is designed for broadband wireless LANs. It optimizes bit allocation in subcarriers so that maximum bits are transmitted in each OFDM symbol under a frequency selective fading environment. The OFDM-TDMA/SA MAC protocol also supports three classes of traffic such as guaranteed, controlled-load, and best effort services. Based on the optimum subcarrier bit-allocation algorithm and considering heterogeneous QoS constraints of multimedia traffic, a hierarchical scheduling scheme is proposed to determine the subcarriers and time slots in which a mobile terminal can transmit packets. In such a way, the OFDM-TDMA/SA MAC protocol significantly increases system throughput in a frequency selective fading environment and guarantees QoS of multimedia traffic. Computer simulation is carried out to evaluate the performance of the OFDM-TDMA/SA MAC protocol. Results show that the new MAC protocol outperforms other MAC protocols for OFDM-based wireless LANs. This work was supported by the State of Georgia Yamacraw Project (E21-105).     

Adaptive Modulation and Multiple Access for the OFDM Transmission Technique

For the transmission of flexible andhigh data rates in future mobile communicationsystems, the multicarrier transmission technique(Orthogonal Frequency Division Multiplexing, OFDM) canbe regarded as a possible alternative tosingle-carrier transmission that is used in today'ssystems. One of the advantages of OFDM is theflexibility concerning modulation and multiple accesstechniques. Since in OFDM the total bandwidth isdivided into a large number of subcarriers, it can beflexibly shared among all the users. In addition, themodulation scheme can be individually chosen for eachsubcarrier, thus granting the possibility of anadaptation to the radio channel characteristics. Inthis paper, the combination of two different multipleaccess schemes, OFDM-FDMA and OFDM-TDMA, and anappropriate adaptive modulation technique isconsidered for the downlink of a communication system.Different degrees of adaptivity are analysed andcompared to show the benefit of an ``intelligent'multiple access and modulation strategy. For thegiven parameters and assumptions, OFDM-TDMA withadaptive modulation yields an improvement of 6 dB insignal-to-noise ratio (SNR) at a bit error rate (BER)of 10^-2. When OFDM-FDMA with adaptive subcarrierallocation is applied, the SNR gain amounts to 12.5 dB(BER of 10^-2).     

OFDM-FDMA Scheme for the Uplink of a Mobile Communication System

The Orthogonal Frequency Division Multiplexing (OFDM) transmission technique can efficiently deal with multi-path propagation effects especially in broadband radio channels. It also has a high degree of system flexibility in multiple access schemes by combining the conventional TDMA, FDMA and CDMA approaches with the OFDM modulation procedure which is particularly important in the uplink of a multi-user system. In OFDM-FDMA schemes carrier synchronization and the resulting subcarrier orthogonality plays an important role to avoid any multiple access interferences (MAI) in the base station receiver. An additional technical challenge in system design is the required amplifier linearity to avoid any non-linear effects caused by a large peak-to-average ratio (PAR) of an OFDM transmission signal. A specific OFDM-FDMA uplink procedure is proposed and will be designed in this paper which can be seen as a combination of a specific subcarrier spreading scheme and subcarrier selection process. The resulting transmit signal consists of a periodic extension and multiple repetition of all modulation symbols and leads therefore to an extremely low computation complexity in the transmitter. Furthermore, the transmit signal shows simultaneously a constant envelope to avoid any non-linear effects in the amplification process. This uplink scheme can be considered as a trade off between low computation complexity and system performance. Prof. Hermann Rohling is with the Technical University Hamburg-Harburg, Germany where he has developed an international reputation in the field of OFDM transmission techniques. Previously Prof. Rohling was with the AEG Research Institute, Ulm as a researcher working in the area of digital signal processing for radar and communications applications. His research interests have included Wideband Mobile Communications especially based on Multicarrier Transmission Techniques (OFDM) for future broadband systems (4G), signal theory, digital radar signal processing, detection, estimation and differential GPS for high precision navigation. Prof. Rohling is a member of Informationstechnische Gesellschaft (ITG), German Institute of Navigation (DGON) and a senior member of IEEE. He is chairman of the August 2005 International OFDM Workshop (InOWo 2005) in Hamburg, Germany. Martin Stemick received the Dipl.-Ing. degree in electrical engineering from the Technical University Hamburg Harburg (TUHH), Germany in 2004. Currently, he is a Ph. D. student and belongs to the scientific staff of the Department of Telecommunications, TUHH, Hamburg, Germany. His research interests include wireless OFDM-systems, with a current emphasis on resource allocation and equalization techniques.     

Effect of Carrier Frequency Offset on the Channel Capacity in Multiuser OFDM-FDMA Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a very robust transmission procedure in multipath and frequency selective radio channels. A Frequency Division Multiple Access (FDMA) resource allocation technique offers the opportunity of a detailed link adaptation scheme. The combination of these transmission- and multiple access techniques in OFDM-FDMA is an ideal and very strong candidate for the downlink of future fourth generation (4G) mobile communication systems. This technical combination offers high cell capacities by exploiting the inherent multiuser diversity effect of the system. To apply OFDM-FDMA in the uplink, the time and carrier synchronization accuracy becomes very important. Non-ideal synchronization of the user signals to the carrier frequency of the base station leads to intercarrier interferences (ICI). In this paper, an analytical model for the ICI consideration in the uplink of a multiuser OFDM-FDMA based system is derived. The impact of the carrier frequency offset (CFO) on the performance of a cellular multiuser system with respect to different subcarrier allocation schemes is analyzed.

Cross-layer QoS Analysis of Opportunistic OFDM-TDMA and OFDMA Networks

Performance analysis of multiuser orthogonal frequency division multiplexing (OFDM-TDMA) and orthogonal frequency division multiple access (OFDMA) networks in support of multimedia transmission is conducted in this work. We take a cross-layer approach and analyze several quality-of-service (QoS) measures that incilude the bit rate and the bit error rate (BER) in the physical layer, and packet average throughput/delay and packet maximum delay in the link layer. The authors adopt a cross-layer QoS framework similar to that in IEEE 802.16, where service classification, flow control and opportunistic scheduling with different subcarrier/bit allocation schemes are implemented. In the analysis, the Rayleigh fading channel in the link layer is modeled by a finite-state Markov chain, and the channel state information (CSI) is assumed to be available at the base station. With the M/G/1 queueing model and flow control results, the analysis provides important insights into the performance difference of these two multiaccess systems. The derived analytical results are verified by extensive computer simulation. It is demonstrated by analysis and simulation that OFDMA outperforms OFDM-TDMA in QoS metrics of interest. Thus, OFDMA has higher potential than OFDM-TDMA in supporting multimedia services.     

Iterative frequency domain channel estimation for dft-precoded ofdm systems using in-band pilots

We consider two techniques of in-band frequency domain multiplexed (FDM) pilots using interleaved frequency domain multiple access (IFDMA) signal with a Chu sequence for DFT-precoded OFDM (or single-carrier (SC)) systems. One, called frequency domain superimposed pilot technique (FDSPT), superimposes pilot tones onto scaled or deleted data tones, which preserves spectral efficiency at the expense of a slight performance loss. The other, called frequency expanding technique (FET), multiplexes pilot tones by displacing data tones, which slightly reduces spectral efficiency. Using FDM pilots in SC systems facilitates flexible and efficient assignment of signals to available spectrum. We propose an iterative frequency domain decision-directed interference cancellation technique to reduce the intersymbol interference level of SC signals with FDSPT pilots (resulting from the suppression of data tones). Moreover, we propose a low complexity frequency domain iterative decision-directed channel estimation (IDDCE) technique for SC systems using FDM pilots. Using IDDCE, the frame error rate (FER) performance for coded SC systems using FET and FDSPT pilots with interference cancellation is found to be about 0.2 dB and about 0.5 dB, respectively, away from the FER performance with known channel frequency response at FER=10^-2. FDSPT pilots can also be used for OFDM systems with channel coding. It is found that an extra 1 dB of SNR is required at FER=10^-2,compared with that using the conventional FET pilots for OFDM systems.     

Clustered OFDM with channel estimation for high rate wireless data

Clustered OFDM can provide in-band diversity gain for wideband wireless communication systems. It is a promising technique for high rate packet data access over wideband mobile wireless channels. Due to the smaller size of each cluster for clustered OFDM than for classical (nonclustered) OFDM, edge effects can be very large. In this letter, we present new transforms for channel estimators in clustered OFDM systems. The new transforms are independent of the channel delay profiles and can effectively mitigate the edge effects. It is shown by computer simulation that the performance of clustered OFDM with the estimator using the new transforms is very close to the performance with the optimum estimator that is determined by the individual channel delay profile. Furthermore, clustered OFDM with the new estimator is almost as good as classical OFDM with transmit diversity     

OFDMA Uplink Performance for Interactive Wireless Broadcasting

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called OFDMA is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This work focuses on the performance of OFDM when used as a modulation and access technique for the uplink of an interactive wireless broadcasting system. In such an application, we present our investigations of the effects of symbol timing misalignments and peak-to-average power ratio (PAR). Under the condition of the same side information bits, the selective mapping (SLM) approach can be implemented by decimating OFDM samples, which is less complex compared to the ordinary SLM incurring a slight degradation of the PAR performance. Taking into account the effects of clipping operation and symbol timing misalignment, the bit error rate (BER) and throughput performances are investigated for a typical OFDMA uplink scenario.