多载波调制在通信系统的应用

多载波调制在通信系统的应用摘要：首先简单解释多载波调制ＭＣＭ，多路副载波调制ＭＳＭ的由来和在高速率通信系统的可能利用。

其后着重地提出多路副载波调制适合于光无线通信的应用。

最后说明大气光通信对多路副载波调制的影响。

１、多载波调制ＭＣＭＭＳＭ的由来近年来，高速率通信系统发展很快，可以看到很多不同的通信系统采用“多载波调制ＭＣＭ（ＭｕｌｔｉｐｌｅＣａｒｒｉｅｒＭｏｄｕｌａｔｉｏｎ）”或“多路副载波调制ＭＳＭ（ＭｕｌｔｉｐｌｅＳｕｂｃａｒｒｉｅｒＭｏｄｕｌａｔｉｏｎ）”的技术方案。

不论电通信或光通信，也不论有线通信或无线通信，都有实际使用ＭＣＭ？ＭＳＭ的情况。

具体地说，无线电射频通信、光纤通信以至光无线通信等通信系统，凡是传输较高数字速率的系统就有可能利用ＭＣＭＭＳＭ。

所谓多载波调制是指传送线路上每次传送信号时利用多个不同的但依次排列的载波，每路载波各自受到数字信号的调制。

这些载荷数字信息信号的多路载波一同沿线路向接收端传送。

如传送线路有不良的频率选择性和多途径通路等情况，用了多载波调制可以有效地减小“符号间干扰ＩＳＩ（Ｉｎｔｅｒ－ｓｙｍｂｏｌＩｎｔｅｒｆｅｒｅｎｃｅ）”。

因此，ＭＣＭ对于实现高速数字通信非常有利。

有类似的情况：例如在射频无线电通信，曾经使用“正交频分多路ＯＦＤＭ（ＯｒｔｈｏｇｏｎａｌＦｒｅｑｕｅｎｃｙＤｉｖｉｓｉｏｎＭｕｌｔｉｐｌｅｘ）”，证明确有成效，因而很多情况曾经或正在实际使用ＯＦＤＭ，包括无线移动通信蜂窝网基台以及数字电视、不对称数字用户线（ＡＤＳＬ）和有些局域网（ＬＡＮ），都曾经使用ＯＦＤＭ，收到一定效果。

另一些多路用途，包括城市的“有线电视ＣＡＴＶＣａｂｌｅＴｅｌｅｖｉｓｉｏｎ？”，它在光纤线路上利用“副载波多路ＳＣＭ（ＳｕｂｃａｒｒｉｅｒＭｕｌｔｉｐｌｅｘ）”，同时传送多路模拟电视信号。

在有些“局域网”，利用ＳＣＭ传送多路数字光信号，容许直接检测，可以降低成本。

２、多载波调制在光无线通信的应用在光无线通信，多路副载波调制ＭＳＭ受到优先考虑，因为利用了ＭＳＭ，可以实现高速数字通信而不需要均衡。

过去大多使用单路载波调制，包括开关键控ＯＯＫ和Ｍ进制脉冲位置调制。

现在ＭＳＭ是让多路副载波载荷的电信号对一个光载波进行调制。

这可以用信号强度调制ＩＭ，或用调频ＦＭ、调相ＰＭ。

最简单的ＭＳＭ系统是利用直接检测的强度调制（ＩＭＤＤ），设备简单。

这样利用多路宽带副载波，可以减少多途径信号的码间干扰，也可以防止其它的噪音干扰。

但是，在这样ＩＭＤＤ的ＭＳＭ系统，光的平均功率效率较差，这是它的主要缺点。

这是因为多路副载波的电信号是受调制的正弦波的总和，既有正值又有负值，而光强度瞬时功率必须不是负的。

因此对于多路副载波的电信号必须加直流偏置，才能对光载波强度进行调制。

在利用ＭＳＭ的光无线系统，需要使用分组码藉以减小直流偏置并改善系统的功率效率。

对此，可考虑使用“副载波信号点序列ＳＳＰＳ？ＳｕｂｃａｒｒｉｅｒＳｉｇｎａｌＰｏｉｎｔＳｅｑｕｅｎｃｅ？”，每一符号由多路副载波传送，就是说，每一符号在几个副载波上有信号点。

这就可能减小直流偏置，从而提高系统的功率效率。

高速数字通信系统原来使用“通断键控ＯＯＫ（ｏｎ－ｏｆｆＫｅｙｉｎｇ）”和“Ｍ进脉位调制ＭＰＰＭ（Ｍ－ａｒｙＰｕｌｓｅＰｏｓｉｔｉｏｎＭｏｄｕｌａｔｉｏｎ）”等制式，都是属于单载波调制系统。

这样的系统传送宽频带信号如符号率超过１０Ｍｂａｕｄ，则在传送过程中如遇到多径分散的情况必将发生“符号间干扰ＩＳＩ（Ｉｎｔｅｒ－ｓｙｍｂｏｌＩｎｔｅｒｆｅｒｅｎｃｅ）”。

而在改用多路副载波调制系统以后，在同样高速率传送，每一路副载波的符号率将低于原来的单载波系统载波的符号率，因此受到畸变很小，可以不需要均衡。

另外，多路副载波系统需用的器件，可以是微波所用的器件，比光器件容易制作，并且微波振荡器的稳定性和微波滤波器的频率选择性都优于相应的光器件特性。

而且，射频振荡器的相位噪声低，它用来制作相干检测要比光相干检测容易实现。

多路副载波调制的传输系统使用的发信机和收信机，可以采取四进制移相键控的制式ＱＰＳＫ（ＱｕａｔｅｒｎａｒｙＰｈａｓｅＳｈｉｆｔＫｅｙｉｎｇ）。

发信机用Ｎ路副载波ωｎ(ｎ＝１，２，…Ｎ)。

在每一符号的持续时间Ｔ以内，发信机发送Ｋ个信息比特。

编码器把Ｋ个信息比特变换为一个具有符号幅度ａｍ的矢量。

因为ＭＳＭ信号ｓ（ｔ）可能有正的或负的，所以发信机增加一个基带偏置信号ｂ(ｔ)使ＭＳＭ复数没有负的，即ｘ(ｔ)＝Ａ[ｓ(ｔ)＋ｂ(ｔ)]，其中Ａ是正的因数。

这样，平均光功率将是ρ＝ＡＥ[ｓ(ｔ)]＋ＡＥ[ｂ(ｔ)]。

一般地，这个基带偏置信号ｂ(ｔ)是一个常数与一个基带脉幅调制（ＰＡＭ）信号之和，如采用矩形的发送脉冲，则载波频率ωｎ＝ｎ(２π/Ｔ)，ｎ＝１，２，…Ｍ，Ｅ[ｘ(ｔ)]＝０，不管怎样的信号集ａｍ。

因此平均光功率只是取决于偏置信号：ρ＝ＡＥ[ｂ(ｔ)]。

所以，只要适当选择偏置信号，就可以减小平均光功率。

偏置可以是固定偏置，也可以是随时间变化的偏置。

固定偏置的值可以与ＭＳＭ信号的最小值相同。

随时间变化的偏置可以用符号间最小容许的偏置，它可以使平均功率较小。

３、大气光通信对多载波调制的影响在大气中传送的光无线通信可以考虑采用多路副载波调制ＭＳＭ。

大气光通信的优点是它在设计完善以后不受无线电干扰、城市噪音和电子脉冲冲击等不良影响。

在实际应用，大气光通信可以用作“无线接入线ＷＬＬ（ＷｉｒｅｌｅｓｓＬｏｃａｌＬｏｏｐ）”，使用户连接城市通信网。

当然，在大气中传送的光波一般不是可见光而是红外波。

它们在传输过程中必将遇到大气的分子吸收以及散射和湍流等影响。

在晴朗天气，大气中有些分子引起各种吸收带，相应地引起接收光束的起伏变化。

微热起伏引起折射率变化而发生的湍流也将引起接收光束的起伏。

已经确认，大气光通信系统使用ＭＳＭ，将使大气通路得到良好性能。

在试验大气光通信系统时，曾经使用二进制移相键控ＢＰＳＫ和二进制脉位调制ＢＰＰＭ，互相作比较。

在晴朗天气、各种闪烁情况下，对两种制式的误码率与信噪比关系作比较，结果显示ＢＰＳＫ比ＢＰＰＭ优约３ｄＢ。

这两种系统都不需对接收机门限作估计，而在使用通断键控ＯＯＫ时，都有必要对接收机门限作动态估计。

总的结论认为：大气无线光通信系统确实有必要使用多路副载波调制系统ＭＳＭ，藉以得到较好的通信性能。

Multicarrier modulation in communication system applicationAbstract: the first simple interpretation multicarrier modulation MCM, multiple sub-carrier modulation of the origin and MSM in high speed communication system may use. Subsequently emphatically put forward multiple sub-carrier modulation for the application of optical wireless communication. The last that atmospheric optical communication on the multiple sub-carrier modulation influence.1, the origin of the MCMMSM carrier modulationIn recent years, high speed communication system development soon, you can see a lot of different communication system USES "multicarrier Modulation MCM (Multiple Carrier Modulation)" or "Multiple sub-carrier Modulation MSM (Multiple Subcarrier Modulation) technology solutions. Whether electric communication or optical communication, or wire or wireless communication, have actual use MCM? The MSM. To be specific, radio radio frequency communication, optical fiber communication and optical wireless communication and other communication system, every transmission high rate of digital system it is possible to use MCMMSM.The so-called multicarrier modulation is refers to the transfer line every time when transmitting signals using a number of different but tactic carrier, every road each carrier by digital signal modulation. These load digital information signal multiple carrier along with way to transfer the receiving end. Such as transmission line have bad frequency selective and multi-channel access, etc, with the multicarrier modulation can be effectively reduced "intersymbol Interference (ISI (base, symbol Interference)". Therefore, MCM for high speed digital communication is very favorable.Have a similar situation: such as in radio communication, ever use "Orthogonal Frequency Division multiple OFDM (Orthogonal Frequency Division Multiplex)", have proved effective, so a lot of things have been or are actual use OFDM, including wireless mobile cellular network base station and digital TV, asymmetric digital subscriber line (ADSL) and some local area network (LAN), have been using OFDM, received a certain effect. Other than LuYongTu, including city "cable TV CATVCable Television?" It in optical fiber line by using "subcarrier multiple SCM (Sub carrier Multiplex)", and at the same time transfer multichannel analog television signal. In some "LAN", the use of SCM transfer multiple digital optical signal to allow direct detection, can reduce the cost.2 and carrier modulation in optical wireless communication applicationsIn optical wireless communication, multiple sub-carrier modulation MSM is preferred, because the use of the MSM, can realize high speed digital communication and don't need to balance. In the past most use single way carrier modulation, including on-off keying OOK and M into system pulse position modulation. Now MSM is to make multiple subcarrier load of the electrical signal to a light carrier for modulation. It can use signal intensity modulation IM, or with FM FM, PM phase. The most simple MSM system is the use of direct detection of intensity modulation (IMDD), simple equipment. So using multiple broadband subcarrier, can reduce multi-channel signal intersymbol interference, also can prevent other noise interference. However, in this system, the light of the IMDD MSM average power efficiency is bad, this is its main faults. This is because multiple subcarrier electrical signal is affected by modulation of the sum of sine wave, both positive and negative, and light intensity instantaneous power must not negative. So for multiple subcarrier signal must add the dc bias, can light carrier strength modulation. In the light of the MSM use wireless system, need to use block code in order to reduce the dc bias and improve the system of power efficiency. To this, can consider to use "Subcarrier Signal Point Sequence SSPS? Subcarrier Signal Point Sequence?" , each symbol consists of multiple subcarrier transfer, that is to say, each symbol in several subcarrier signal point on. This could reduce the dc bias, so as to improve the efficiency of power system.High speed digital communication system for use "on-off Keying OOK (on - off Keying)" and "M into Pulse Position Modulation MPPM (M - ary Pulse Position Modulation)" and so on the system, it is to belong to single carrier Modulation system. Such a system transfer broadband signal such as symbol rate more than ten mbaud in the transfer process such as the multipath dispersion situation will happen "intersymbol interference (ISI (base, symbolInterference)". While in use multiple sub-carrier modulation system later, in the same high rate transmission, every way subcarrier symbol rate will be lower than the original single carrier system carrier of symbol rate, therefore they are distortion is small, can not need to balance. In addition, multiple subcarrier system need device, can be used in microwave devices, than light device is easy to make, and microwave oscillator stability and microwave filter frequency selective are better than the corresponding optical device characteristics. Moreover, radio frequency oscillator phase noise is low, it is used to make coherent detection than light coherent detection easy to implement. Multiple sub-carrier modulation transmission systems use the transmitter and receiver, can take quaternary Phase Shift Keying formats QPSK (Quater nary Phase Shift Keying). Transmitter use N roadsubcarrier ω N (N = 1, 2,... N). In the duration of each symbol within T, transmitter send K information bits. The encoder K information bit transformation as a symbolic amplitude am vector. Because the MSM signal s (t) may be positive or negative, so transmitter add A baseband bias signal b (t) make MSM complex not negative, i.e., x (t) = A/s (t) + b (t)], which is A positive factor. In this way, the average optical power will be ρ = AE [s (t)] + AE [b (t)]. Generally, the baseband bias signal b (t) is a constant and a baseband pulse amplitude modulation (PAM) signal, such as the sum of the rectangular send pulse, the carrier frequency ω n = n (2 π/t), n = 1, 2,... M, E [x (t)] = 0, no matter what kind of signal set am. So the average optical power only depends on the bias signals: ρ = AE [b (t)].So, as long as the appropriate selection bias signal, can reduce the average optical power. Offset can be fixed bias, also can be changing with time offset. Fixed offset value can and MSM signal minimum values are the same. Time varying bias can use intersymbol minimum allowable bias, it can make the average power smaller.3, atmospheric optical communication on the influence of carrier modulationIn the atmosphere of the transmitted light wireless communication can consider to use multiplesub-carrier modulation MSM. The advantages of atmospheric optical communication is that it in design perfect later from radio interference, urban noise and electronic pulse impact and other adverse effects. In the practical application, the atmospheric optical communication can be used as a "Wireless access line WLL (Wireless Local Loop)", the user connecting the city network. Of course, in the atmosphere transmission of light waves are usually not visible but infrared wave. They are in the transmission process will encounter atmospheric molecular absorption and scattering and turbulence effect. In the sunny weather, the atmosphere some molecular cause all sorts of absorption band, accordingly the receiving beam fluctuation of. Micro thermal fluctuation caused by refraction change and produces turbulence will also cause receive beam of ups and downs. Has confirmed that atmospheric optical communication systems use MSM, will make atmospheric path get good performance.In the test, the atmospheric optical communication system, ever use binary phase shift keying BPSK and binary pulse position modulation BPPM, compared with each other. In the sunny weather, all kinds of scintillation cases, the two kinds of system of bit error rate and signal to noise ratio (SNR) is the relationship between, and the results showed that the optimal BPSK BPPM than about 3 db. The twosystems are not need to receiver threshold estimate, but in the use of on-off keying OOK, it is necessary to receiver threshold for dynamic estimation.The conclusion is that atmospheric wireless optical communication system really have the need to use multiple sub-carrier modulation system MSM, in order to get good communication performance.。