主题：有谁知道SPREAD SPECTRUM技术吗？ -- 远航

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.