什么是OFDM?

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  OFDM(Orthogonal frequency Division Multiplexing 正交频分复用)是一种调制技术,它用大量的正交子载波以并行方式发送符号块。数据被分成多个块,在各子载波上以并行方式发送。这样能增加符号周期,减小延迟扩散效应。

        频分多路复用(FDM )的一种,设计目标为提高频率使用效率,用于地面数字电视播放,现也用于 IEEE 802.11a 规范的无线局域网和电力线调制解调器等。

  • OFDM有时候也被称为DMT(分离多音调制), 是一种基于FDM的传输技术,它把数据分割成独立的符号并通过不同的频率传送它们。
  • OFDM采用采用高速的FFT/IFFT功能
  • OFDM需要附加的补偿电路以解决多径、多普勒以及衰落造成的干扰。
  OFDM是一种多载波传输技术,N个子载波把整个信道分割成N个子信道,N个子信道并行传输信息。OFDM系统有许多非常引人注目的优点。
        第一,OFDM具有非常高的频谱利用率。普通的FDM系统为了分离开各子信道的信号,需要在相邻的信道间设置一定的保护间隔(频带),以便接收端能用带通滤波器分离出相应子信道的信号,造成了频谱资源的浪费。OFDM系统各子信道间不但没有保护频带,而且相邻信道间信号的频谱的主瓣还相互重叠但各子信道信号的频谱在频域上是相互正交的,各子载波在时域上是正交的,OFDM系统的各子信道信号的分离(解调)是靠这种正交性来完成的。另外,OFDM的个子信道上还可以采用多进制调制(如频谱效率很高的QAM),进一步提高了OFDM系统的频谱效率。        第二,实现比较简单。当子信道上采用QAM或MPSK调制方式时,调制过程可以用IFFT完成,解调过程可以用FFT完成,既不用多组振荡源,又不用带通滤波器组分离信号。        第三,抗多径干扰能力强,抗衰落能力强。由于一般的OFDM系统均采用循环前缀(Cyclic Prefix,CP)方式,使得它在一定条件下可以完全消除信号的多径传播造成的码间干扰,完全消除多径传播对载波间正交性的破坏,因此OFDM系统具有很好的抗多径干扰能力。OFDM的子载波把整个信道划分成许多窄信道,尽管整个信道是有可能是极不平坦的衰落信道,但在各子信道上的衰落却是近似平坦的,这使得OFDM系统子信道的均衡特别简单,往往只需一个抽头的均衡器即可。

特点:

  OFDM系统是一种特殊的多载波传输技术,其特点是各子载波相互正交,扩频调制后的频谱可相互重叠,不但减少了子载波间的相互干扰,还大大提高了频谱利用率,OFDM的主要技术特点如下:

(1)可有效对抗信号波形间的干扰,适用于多径环境和衰落信道中的高速数据传输;

(2)通过各子载波的联合编码,具有很强的抗衰落能力;

(3)各子信道的正交调制和解调可通过离散傅利叶反变换IDFT和离散傅利叶变换DFT实现;

(4)OFDM较易与其它多种接入方式结合,构成MC-CDMA和OFDM-TDMA等。

OFDM的基本原理是:

  高速信息数据流通过串并变换,分配到速率相对较低的若干子信道中传输,每个子信道中的符号周期相对增加,这样可减少因无线信道多径时延扩展所产生的时间弥散性对系统造成的码间干扰。另外,由于引入保护间隔,在保护间隔大于最大多径时延扩展的情况下,可以最大限度地消除多径带来的符号间干扰。如果用循环前缀作为保护间隔,还可避免多径带来的信道间干扰。OFDM的基带传输系统如图所示。

 

  在过去的频分复用(FDM)系统中,整个带宽分成N个子频带,子频带之间不重叠,为了避免子频带间相互干扰,频带间通常加保护带宽,但这会使频谱利用率下降。为了克服这个缺点,OFDM采用N个重叠的子频带,子频带间正交,因而在接收端无需分离频谱就可将信号接收下来。OFDM系统的一个主要优点是正交的子载波可以利用快速傅利叶变换(FFT/IFFT)实现调制和解调。对于N点的IFFT运算,需要实施N2 次复数乘法,而采用常见的基于2的IFFT算法,其复数乘法仅为(N/2)log2N,可显著降低运算复杂度。

  在OFDM系统的发射端加入保护间隔,主要是为了消除多径所造成的ISI(子载波之间的正交性遭到破坏而产生不同子载波之间的干扰)。其方法是在OFDM符号保护间隔内填入循环前缀,以保证在FFT周期内OFDM符号的时延副本内包含的波形周期个数也是整数。这样,时延小于保护间隔的信号就不会在解调过程中产生ISI。

1,峰值平均功率(PAPR)

  由于OFDM信号在时域上为N个正交子载波信号的叠加,当这N个信号恰好都以峰值出现并将相加时,OFDM信号也产生最大峰值,该峰值功率是平均功率的N倍。这样,为了不失真地传输这些高峰均值比的OFDM信号,对发送端和接收端的功率放大器和A/D变换器的线性度要求较高,且发送效率较低。解决方法一般有下述三种途径:

(1)信号失真技术采用峰值修剪技术和峰值窗口去除技术,使峰值振幅值简单地非线性去除;

(2)采用编码方法将峰值功率控制和信道编码结合起来,选用合适的编码和解码方法,以避免出现较大的峰值信号;

(3)扰码技术采用扰码技术,对所产生OFDM信号的相位重新设置,使互相关性为0,这样可以减少OFDM的PAPR。这里所采用的典型方法为PTS和SLM。

2,同步

  与其它数字通信系统一样,OFDM系统需要可靠的同步技术,包括定时同步、频率同步和相位同步,其中频率同步对系统的影响最大。移动无线信道存在时变性,在传输过程中会出现无线信号的频率偏移,这会使OFDM系统子载波间的正交性遭到破坏,使子信道间的信号相互干扰,因此频率同步是OFDM 系统的一个重要问题。为了不破坏子载波间的正交性,在接收端进行FFT变换前,必须对频率偏差进行估计和补偿。

  可采用循环前缀方法对频率进行估计,即通过在时域内把OFDM 符号的后面部分插入到该符号的开始部分,形成循环前缀。利用这一特性,可将信号延迟后与原信号进行相关运算,这样循环前缀的相关输出就可以用来估计频率偏差。

3,信道编码和交织

  为了对抗无线衰落信道中的随机错误和突发错误,通常采用信道编码和交织技术。OFDM系统本身具有利用信道分集特性的能力,一般的信道特性信息已被OFDM 调整方式本身所利用,可以在子载波间进行编码,形成编码的OFDM COFDM即把OFDM 技术与信道编码、频率时间交织结合起来,提高系统的性能,其编码可以采用各种码(如分组码和卷积码)。

  现状及其发展方向:目前,OFDM 技术良好的性能使其在很多领域得到了广泛的应用,如:HDSL 、ADSL 、VDSL 、DAB 和DVB ,无线局域网IEEE802. 11a 和HIPERLAN2 ,以及无线城域网IEEE802. 16 等系统中。而在4G中,一方面带宽作为移动通信中非常希缺的资源,另一方面未来的移动通信对服务质量、服务的多样性及传输速率要求越来越高,使得OFDM 将得到更广泛的应用。

Comparison to FDM

In FDM, multiple signals are sent out at the same time, but on different 频率. Most people are familiar with FDM from 无线电 and 电视: normally, each station broadcasts on a particular frequency band (range of frequencies) or channel.

  • OFDM takes this concept further: In OFDM, a single transmitter transmits on many different orthogonal (independent) frequencies (typically dozens to thousands). (Because the frequencies are so closely spaced, each one only has room for a 窄带信号).
  • This modulation technique coupled with the use of advanced 调制 techniques on each component, results in a signal with high resistance to interference.

Coupling with "Channel Coding"

OFDM is almost always used in conjunction with 信道编码—an 纠错 technique—to create coded orthogonal FDM or COFDM. It is a complex technology to implement, but it is now widely used in 数字 电信 systems to make it easier to encode and decode such signals. The system has found use in 广播 as well as certain types of 计算机网络ing technology. This is particularly due to the fact that such signals show good resistance to multipath fading, best known as the source of "ghosting" on 模拟电视 broadcasts.

特性

An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with 基带 data on each sub-carrier being independently modulated commonly using some type of QAMor PSK. This composite baseband signal is typically used to modulate a main RF carrier.

好处

The benefits of using OFDM are many, including high spectrum efficiency, resistance against multipath interference (particularly in wireless communications), and ease of filtering out noise (if a particular range of frequencies suffers from interference, the carriers within that range can be disabled or made to run slower). Also, the upstream and downstream speeds can be varied by allocating either more or fewer carriers for each purpose. Some forms of Rate Adaptive DSL use this feature in real time, so that bandwidth is allocated to whichever stream needs it most.

OFDM modulation and demodulation are typically (as of 2001) implemented using 数字滤波器 banks generally using the 快速弗利叶变换 (FFT).

Although highly complex, COFDM has high performance under even very challenging channel conditions.

By combining the OFDM technique with error-correcting codes, adaptive equalization and reconfigurable modulation, COFDM has the following properties:

  • resistance against link dispersion
  • resistance against slowly changing 相位失真 and fading
  • resistance against 多径 using guard interval and cyclic prefix
  • resistance against frequency response nulls and constant frequency interference
  • resistance against 突发噪声

COFDM also generally has a nearly 'white' spectrum, giving it benign electromagnetic interference properties with respect to other signals.

Some COFDM systems use some of the sub-carriers to carry pilot signals, which are used for frequency synchronization. (Loss of synchronization causes errors in the decoded data).

In wide area broadcasting, receivers can benefit from receiving signals from several spatially dispersed transmitters simultaneously, since transmitters will only destructively interfere with each other on a limited number of subcarriers, whereas in general they will actually reinforce coverage over a wide area. This is very beneficial in many countries, as it permits the operation of national single frequency networks, and avoids the replication of program content on different carrier frequencies which is necessary with FM or other forms of radio broadcasting. Also, because effectively the bit rate is slowed down on each sub-carrier, the effects of "ghosting" are much reduced. Such 单频网络 utilise the available spectrum more effectively than existing analogue radio networks.

Disadvantages of OFDM

However, OFDM suffers from time-variations in the channel, or presence of a carrier frequency offset. This is due to the fact that the OFDM subcarriers are spaced closely in frequency. Imperfect frequency synchronization causes a loss in subcarrier orthogonality which severely degrades performance.

Because the signal is the sum of a large number of subcarriers, it tends to have a high peak-to-average power ratio (PAPR). Also, it is necessary to minimise intermodulation between the subcarriers, which would effectively raise the noise floor both in-channel and out of channel. For this reason circuitry must be very linear. This is demanding, especially in relation to high power RF circuitry, which also needs to be efficient in order to minimise power consumption.

OFDM feature abstract

  • No intercarrier guard bands
  • Maximum spectral efficiency (Nyquist rate)
  • Easy implementation by FFTs
  • Controlled overlapping of bands
  • Very sensitive time-freq. synchronization

Usage

OFDM is used in many communications systems such us: ADSL, 无限局域网, 数字声广播, DVB, UWB and PLC.

ADSL

OFDM is used in ADSL connections that follow the G.DMT (ITU G.922.1) standard. (Annex A refers to ADSL-over-POTS).

The fact that COFDM does not interfere easily with other signals is the main reason it is frequently used in applications such as ADSL 调制解调器 in which existing copper wires are used to achieve high-speed data connections. The lack of interference means no wires need to be replaced (otherwise it would be cheaper to replace them with fiber). However, DSL cannot be used on every copper pair, interference may become significant if more than 25% of phone lines coming into a Central Office are used for DSL.

Wireless LAN

OFDM is also now being used in some wireless LAN applications, including WiMAX and IEEE 802.11a/g (and the defunct European alternative HIPERLAN/2). For 业余无线电 applications, experimental users have even hooked up commercial off-the-shelf ADSL equipment to radio transceivers which simply shift the bands used to the radio frequencies the user has licensed.

IEEE 802.11a, operating in the 5 GHz band, specifies data rates ranging from 6 to 54 Mbit/s. Below contains a listing of the eight specified PHY data rates. Four different modulation schemes are used: BPSK, 4-QAM, 16-QAM, and 64-QAM. Each higher performing modulation scheme requires better channel condition for accurate transmission. These modulation schemes are coupled with the various forward error correction convolutional encoding schemes to give a multitude of Number of data bits per symbol (Ndbps) performance.

数字无线通信和电视

DVB-T's implementation of COFDM

COFDM is also now widely used in Europe and elsewhere where the Eureka 147 数字声广播 (DAB) standard has been adopted for digital radio broadcasting, and also for terrestrial digital TV in the DVB-T standard. One of the major benefits provided by COFDM is that it renders radio broadcasts relatively immune to 多径 distortion, and signal fading due to atmospheric conditions, or passing aircraft. The United States has rejected several proposals to adopt COFDM for its digital television services, and has instead opted for 8VSB (残留边带调制) operation. The question of the relative technical merits of COFDM versus 8VSB has been a subject of some controversy.

The debate over 8VSB vs COFDM modulation is still ongoing. Proponents of COFDM argue that it resists multipath far better than 8VSB. Early 8VSB DTV (digital television) receivers often had difficulty receiving a signal in urban environments. However, newer 8VSB receivers are far better at dealing with multipath. Moreover, 8VSB modulation requires less power to transmit a signal the same distance. In less-populated areas, 8VSB often pulls ahead of COFDM because of this. In urban areas, however, COFDM still offers better reception than 8VSB.

DRM and Eureka-147's implementation of COFDM

COFDM is also used for other radio standards, most prominently for Digital Radio Mondiale (DRM), the standard for digital broadcasting at 短波 and mediumwave frequencies (below 30 MHz).

  • The USA again uses an alternate standard, a proprietary system developed by iBiquity dubbed "HD Radio" However, it uses COFDM as the underlying broadcast technology to add digital audio to AM (mediumwave) and FM broadcasts.
  • Both Digital Radio Mondiale and HD Radio are classified as in-band on-channel systems, unlike Eureka 147 which uses VHF or UHF broadcasts instead.

超宽带

UWB, or ultra wideband local wireless link technology may also utilise OFDM as multiband OFDM (MB-OFDM). This UWB specification is advocated by the WiMedia Alliance (formerly by both the Multiband OFDM Alliance {MBOA} and the WiMedia Alliance, but the two have now merged), and is one of the competing UWB radio interfaces.

Flash-OFDM

Flash-OFDM is a system that is based on OFDM and specifies also higher protocol layers. It has been developed and is marketed by Flarion. Flash-OFDM has generated interest as a packet-switched cellular bearer, on which area it would compete with GSM and 3G networks. As an example, old 450 MHz frequency bands that were used by NMT (an 1G analog network, now decommissioned) in Europe are being considered to be licenced to Flash-OFDM operators.

BST-OFDM

The BST-OFDM (Band Segmented Transmission - Orthogonal Frequency Division Multiplexing) system proposed for Japan goes one better than COFDM by exploiting the fact that some OFDM carriers may be modulated differently from others within the same multiplex. The 6 MHz television channel may therefore be "segmented", with different segments being modulated differently and used for different services.

It is possible, for example, to send an audio service on a segment that includes a segment comprised of a number of carriers, a data service on another segment and a television service on yet another segment - all within the same 6 MHz television channel. Furthermore, these may be modulated with different parameters so that, for example, the audio and data services could be optimized for mobile reception, while the television service is optimized for stationary reception in a high-multipath environment.

OFDM的历史:

  • 1957: Kineplex, multi-carrier HF modem
  • 1966: Chang, Bell Labs: OFDM paper + patent
  • 1971: Weinstein & Ebert proposed use of FFT and guard interval
  • 1985: Cimini described use of OFDM for mobile communications
  • 1987: Alard & Lasalle: OFDM for digital broadcasting
  • 1995: ETSI DAB standard: first OFDM based standard
  • 1997: ETSI DVB-T standard
  • 1998: Magic WAND project demonstrates OFDM modems for wireless LAN
  • 1999: IEEE 802.11a wireless LAN standard (Wi-Fi)
  • 2000: proprietary fixed wireless access (V-OFDM, Flash-OFDM, etc.)
  • 2002: IEEE 802.11g standard for wireless LAN
  • 2004: IEEE 802.16-2004 standard for wireless MAN (WiMAX)
  • 2004: ETSI DVB-H standard
  • 2004: Candidate for IEEE 802.15.3a standard for wireless PAN (MB-OFDM)
  • 2004: Candidate for IEEE 802.11n standard for next generation wireless LAN
  • 2005: Candidate for 3.75G mobile cellular standards (3GPP & 3GPP2)
  • 2005: Candidate for 4G standards (CJK)

References

  • Chang, R. W. (1966). Synthesis of band-limited orthogonal signals for multi-channel data transmission, Bell Systems Technical Journal (46), 1775-1796.
  • Chang, R. W. & and Gibbey, R. A. (1968). A theoretical study of performance of an orthogonal multiplexing data transmission scheme, IEEE Transactions on Communications Technology (16) (4), 529-540.
  • Saltzberg, B. R. (1967). Performance of an efficient parallel data transmission system, IEEE Transactions on Communications Technology (15) (6), 805-811.
  • "Detailed OFDM Modeling in Network Simulation of Mobile Ad Hoc Networks," G. Yeung, M. Takai, R. Bagrodia, A. Mehrnia, B. Daneshrad. In Proceedings of the 18th Workshop on Parallel and Distributed Simulation (PADS 2004), May 16-19, 2004.