主题：有谁知道SPREAD SPECTRUM技术吗？ -- 远航

有一些不解。

因为spreading gain的缘故，扩频通信可以获得更好的信噪比，这不是扩频通信最基本的用途么？

难道他们又和好如初了？

应该不可以随意加大的。

希望可以说清楚罢。首先呢，这个信噪比是一个很模糊的说，signal-to-noise ratio，and signal-to-noise-and-interference ratio，有很多时候是需要区分开的。前者的noise一般指的AWGN，是white，是宽带的，与接收端front end的带宽无关。而后者的interference不是white的，一般是窄带的。

SS包括spreading和despreading两个过程。以direct sequence为例，在发射端，信号和一高频PN sequence相乘，实现spreading；在接收端，同样的PN sequence被再乘一遍，实现despreading。

既定信号带宽为B1，功率为P1，功率谱密度为P1/B1，AWGN的功率谱密度为N，那对于一般的窄带通信系统而眼，signal-to-noise ratio为P1/(N*B1)。

如果SS，经过spreading，B1-->B2，功率谱密度变为P1/B2。现象上就是功率谱密度降低，但是频谱变宽。在接收端，despreading，B2-->B1，但是因为AWGN is "white"，且一般的假设是noise bandwidth is much larger than B2，所以despreading不会改变AWGN的功率谱密度，which is still N。Then, signal-to-noise ratio again is P1/(N*B1)。

如果在信号带宽内有一interference，功率为I，then the narrow-band (no spreading) communication system will provide SINR=P1/(I+N*B1)。

但是如果SS，因为interference只是经过despreading，（发送端不会自己给自己送一interference的），简单计算可以得出，signal-to-interference-and-noise-ratio = P1/(I*B1/B2+N*B1)，interference signal变弱乐，只有以前的B1/B2。

考虑一个极端的例子，如果根本没有noise，或者I远大于N*B1，那末without SS，SINR=P1/I，with SS，SINR=P1/I*B2/B1，这也就是为什么B2/B1 is usually defined as "spreading gain"。

侧重点不一样罢了。

过去强调circuit switch，所以无线通讯里也更侧重带宽资源公平的dedicated alloctaion。ss的思想和这方向比较吻合，抗噪以外，ss（cdma和fh）在同一信道内通过码分来支持多用户。其实ss本身并没有多严重的信道效率问题，问题在如果用户少，闲置码不容易得到充分利用，用户很难得到更多的闲置信道资源。新一代无线通讯系统更多侧重在data centric，并不那么强调每个用户得到自己独用的资源，资源分配多由高层来做时分，更加灵活。不再要求每个客户获得独占的带宽资源，这是ofdm又可以重新获得青眯的必要条件。

ofdm走红的充分条件则是无线通讯宽频化和高频化。如果不做fdm，随着信道越来越宽，那么接收端的equalization就越来越难做。fdm下每个子信道都不宽，没有这个问题。高频化的结果是无线电波传输更像光，直接los和反射传输为主，低频无线的直线nlos传输越来越少。这样multipath fading越来越严重。如果symbol rate越高，对延迟fading抵抗力越弱，不利multipath fading环境。所以分信道，把每个子信道的symbol rate减下来。

fdm是必然趋势，ofdm则是各种fdm中频率使用效率最高的分法。

ofdm技术在二战时就有雏形，不过真正产生要到fft发明之后，这个也有三四十年了。在无线应用之前，dsl也早就用了。所以我说“重获青眯”。

画，输出jpg，然后贴上来。

from the other 小五

注意，这应用只是普通的遥控模型，也不可能有很多用户同时使用。厂家说

最多也不过百。

AM, FM, PCM and now DSM. Spektrum's DSM (Digital Spectrum Modulation) system advances RC radio technology to the next generation. Based on an optimized version of Direct Sequencing Spread Spectrum, DSM offers pure digital control, providing an impenetrable radio link that's immune to all types of interference. And with 4096 bit resolution and 5.6ms response time, the DSM system offers seemingly infinite accuracy and instantaneous response for a totally connected RC experience.

WORLD'S MOST SECURE RADIO LINKS

For the highest security levels of radio communications, NASA, the U.S. military and government agencies like the CIA and FBI rely on Spread Spectrum technology. Whether guiding unmanned reconnaissance aircraft over enemy territory, controlling satellites in space or communicating critical mission orders to the battlefield, the impenetrable secure link that Spread Spectrum coding provides makes it ideal for these decisive security applications.

And now Spektrum offers this technology for RC cars, park flyers and micro helis.

SPEKTRUM'S DSM TECHNOLOGY

In development for over four years, Paul Beard, a leading engineer in the Spread Spectrum industry, designed Spektrum's DSM 2.4GHz Spread Spectrum Technology. The DSM 2.4GHz Spread Spectrum Technology utilizes Direct Sequencing Spread Spectrum technology that has been optimized for RC use. Unlike current narrow band 27 and 75MHz systems, Direct Sequencing Spread Spectrum or DSSS generates a wide signal on a single frequency, and information is encoded with its own Globally Unique Identification number (called GUID) such that the receiver only recognizes the information from its specific transmitter. And with over 4.2 billion available GUID codes, it is virtually impossible for a receiver to be controlled by anything other than its mating transmitter. Spektrum calls this optimized form of modulation DSM-Digital Spectrum Modulation.

FHSS VS. DSSS

There are two primary types of Spread Spectrum technology- Frequency Hopping (FHSS) and Direct Sequencing (DSSS). FHSS systems transmit a narrow band signal and rapidly jump from one frequency to the next spending a few milliseconds on each frequency. DSSS systems transmit on a single selected frequency but on a very wide band. Only a small portion of that band is used for specially encoded information. Originally, Spektrum engineers started their development with FHSS-based systems because they were relatively easy and inexpensive to develop. However, it was soon discovered that FHSS had several limitations that would prevent it from being the optimal solution for RC.

While more difficult and costly to develop, our engineers began experimenting with Direct Sequencing Spread Spectrum and optimized the modulation scheme to overcome critical response and re-link issues. In addition, DSSS offered 18dB increase of processing gain for significant improvements in range. With years of development and testing the DSSS modulation scheme was optimized for RC car use and Spektrum's DSM 2.4GHz Spread Spectrum Technology was born.

HOW DSM WORKS

Collision avoidance eliminates the possibility of more than one Direct Sequencing Spread Spectrum system from transmitting on the same frequency. Here's how it works: When the transmitter is turned on, the system scans the 2.4GHz band looking for an open channel. 79 channels are available. When an open channel is found, the system locks in and transmits on that channel as seen below.

This process takes about 2 seconds. In the unlikely event an open channel is unavailable, the transmitter continues to scan without emitting a signal until an open channel becomes available. The transmitter will remain on that selected channel until it's turned off. Each individual module is factory programmed with its own unique serial code called GUID (Globally Unique Identification code). Once a receiver is programmed to a specific module (called binding) the receiver will only recognize that module ignoring signals from any other sources. And with over 4 billion possible GUID codes, it's virtually impossible for a receiver to listen to anything other than its bound transmitter.

When the receiver is turned on, it scans the 2.4GHz band and searches for its specific transmitter's encoded signal. When found, it locks in on that channel. If the signal is lost, the receiver goes into a hold mode, positioning the servo to a preset fail-safe position until the signal is reacquired. If the receiver is turned on before the transmitter, it will continuously scan the band until the encoded transmitter signal is present. During this period, the receiver drives the servos to the preset fail-safe position. All Direct Sequencing Spread Spectrum systems are required by the FCC to incorporate active collision avoidance, making it impossible and illegal for more than one transmission on a single frequency.

BINDING

Each module has it own unique code (called GUID). The receiver must be programmed to a specific module so that the receiver will only recognize that module, ignoring signals from any other sources. This process called binding is push-button-easy and takes only about 30 seconds. During the binding process the servo fail-safe positions are also set. It's necessary to bind the receiver to the module during first installation and is recommended when the receiver is moved from one car to another. Multiple receivers can be bound to a single transmitter module, common when using one transmitter to operate several models.