Digital spectrum modulation- Radio interference problem solved

What is the solution to Radio Interference?
Ans: Digital spectrum modulation or DSM is a new radio technology recently adapted to the RC vehicles world.

A. In summary, DSM technology is an optimized version of Direct Sequence Spread Spectrum, also referred to as FHDSS "frequency hopping digital spread spectrum" technology. This optimized digital, crystal-free two-way communication technology eliminates and is immune to the cross-interference common with traditional radio frequency transmitters and receivers. The response time of DSM controllers and receivers is both impressive and reliable. Now that DSM technology had been incorporated into the RC world, RC enthusiasts can enjoy a safer, more rewarding radio controlled racing experience without the frustration of radio frequency interference.

How Do I Use a Direct Transmitter and Receiver?

OK, now that you have this new piece of radio technology you can't just turn it on and go. You have to perform a few steps to get your controller to lock onto the receiver. The process is called binding. The DSM receiver has to search and recognize the DSM transmitter's GUID code and lock into it. This process has to be done on every module you plan to use with this transmitter or receiver. Once the receiver or transmitter locks in, a special software which helps prevent a collision for the given frequency takes over and further helps eliminate frequency interference. This software, incorporated in both the transmitter and receiver, is required by the FCC and must be installed to help prevent a collision of frequency channels and illegal use of a particular frequency channel at the same time by more than one controller. In other words, the DSM transmitter/receiver and the software do the work for you of setting the proper frequency -- no need to change crystals or find out what frequencies are currently in use at your local RC track.

The reference material below is an Engineering concept behind DSM technology.

What is Direct spread spectrum actually??

In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.

Features

1. It phase-modulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.

Transmission method

Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and −1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.

The resulting signal resembles white noise, like an audio recording of "static". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and −1 × −1 = 1). This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the receiver's assumed sequence.

For de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process. However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems.

The resulting effect of enhancing signal to noise ratio on the channel is called processing gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.

If an undesired transmitter transmits on the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.

As this description suggests, a plot of the transmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission.

In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudo-random number generator.

Benefits

• Resistance to intended or unintended jamming.
• Sharing of a single channel among multiple users.
• Determination of relative timing between transmitter and receiver.

Posted by: Raja Babu Dwivedy.
ID: 4810750499.
Reference: www.wikipedia.com , www.about.com