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Introduction

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This Java applet allows you to visually experience the basics of direct sequence spread spectrum, a type of spread spectrum communication.
It’s interesting to look at the spectrogram and try changing the various parameters. You can understand the background to the characteristics of the signal.

To run the applet, click the picture.

*Java Runtime Environment version 5.0 or higher is required to run the applet.
Download:http://www.java.com/en/download/index.jsp

An outline of spread spectrum method

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Spread spectrum communication is the modulation method used in the current standard wireless systems such as wireless LAN, Bluetooth, ZigBee and other public-use radio. You may have heard of spread spectrum communication, but wouldn’t you like to know what it actually means?

There are two types of spread spectrum communication, direct sequence spread spectrum (DS) and frequency hopping (FH) spread spectrum. However, as a general term it’s called spread spectrum communication, but the only thing it has in common is the broad spread of the spectrum. Please note that otherwise, from the point of view of the approach and the wireless hardware, it’s completely different.

Spreading the spectrum (dispersing the signal energy over a wide band) gives rise to a variety of characteristics.
Security
With spread spectrum communication, a spread code sequence is applied to the information signal. For this reason it’s difficult for somebody who doesn’t know the spread code sequence to demodulate the information signal. However, although there’s a tendency towards misinterpretation, the spread code sequence of commercial products is known, so it doesn’t offer particularly high security as a radio wave. Security is maintained at the code level. With independently developed radio equipment for military and other uses, the spreading code sequence is unique, making demodulation difficult.

Interference resistance
With systems using narrowband modulation, if there is only slight interference in the transmission channel, errors occur, and in some cases, communication stops. This is a big problem in the current circumstances of wireless use when a variety of radio equipment is operating in the same area.
With spread spectrum communication, if there is only slight interference, errors become frequent and the response is poor, but communication itself can continue.
Also, its resistance to interference means that it’s also resistant to multipath fading.

Wide frequency band
Since the information signal is spread several times wider than the spreading code sequence, the frequency band required for communication is wide. For this reason, it has poor bandwidth utilization efficiency. Employing transmitted power control and so on improves bandwidth utilization efficiency.

Direct sequence spread spectrum

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Direct sequence spread spectrum is called DS (Direct Sequence).
It’s the modulation method used for wireless LAN and ZigBee.。

◇Transmitter
The information signal undergoes primary modulation by PSK, FSK or other narrow band modulation and secondary modulation with spread spectrum modulation. Spread spectra are obtained by multiplying the primary modulated signal and the square wave, called the PN sequence. Contrariwise, as with commercial radio, there are cases where spread modulation is applied to the data first, and narrow band modulation such as PSK or FSK is applied afterwards.
The figure below is an example of spread spectrum modulation and demodulation using PSK for primary modulation.

ss03.gif

◇Receiver
If despreading is applied to the received diffuse wave, it returns to the PSK or FSK modulated wave resulting from primary modulation. Then, as with narrowband demodulation, if the despread wave and local signal are multiplied, and appropriate low pass processing is applied, the information signal is obtained. Despreading involves multiplying the same PN code as that used at the transmitting end for the receiving wave. At this time, it’s necessary to synchronize the receiving wave and PN code.
There are two processing methods on the receiving side, demodulation of the information signal after despreading, and obtaining a positive and negative PN code by multiplying the local signal by the receiving wave and despreading using correlation detection. With the former there is process gain but the problem of synchronization remains. With the latter, the spectrum density of the receiving wave itself is low, and regeneration of the local carrier for performing synchronous detection is a problem. Commercial SS radio equipment uses the latter, but it requires considerable power and has a short communication range.

・Despreading
The signal that enters the antenna of the receiver includes outside interference waves and noise. If this signal is despread, the signal component returns to a narrowband modulated wave and the interference components are diffused, expanding the spectrum infinitely so that its power density falls. Therefore, by inputting the signal with frequency band restricted using a BPF, the interference component power that falls into the demodulation frequency band is reduced. The occurrence of errors is calculated using a stochastic process, so ultimately, using a spread spectrum results in fewer errors, and this is why spread spectrum communication is resistant to interference.

・Demodulation
Demodulation is normal narrowband demodulation. The local signal is regenerated from the receiving wave and after multiplication by the receiving wave, unnecessary components are eliminated with an LPF. Primary modulation uses PSK, so synchronous detection is necessary.


◇PN sequence
The PN sequence is switched at a far faster speed than the symbol rate of the information signal and its spectrum covers a wide band. For this reason, the spectrum of the modulated wave after primary modulation also covers a wide band. We won’t go into detail here, but PN sequences must meet the conditions required for spread spectrum modulation such as the relationship of the numbers 1 and 0.

FH02.gif

*Since propagation loss in space occurs with radio communication, the power of diffused waves is dampened significantly when it reaches the receiver. However, for the sake of convenience, we assume with this applet that there’s no loss.

Frequency hopping

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FH is short for frequency hopping.
The information signal undergoes primary modulation by PSK, FSK or other narrow band modulation and secondary modulation with spread spectrum modulation. This secondary modulation is modulation (frequency conversion) using a frequency synthesizer (hopping synthesizer) that changes the frequency with a random hopping pattern. The spectrum is spread over a certain bandwidth, but in the moment of hopping, modulation is narrowband, and demodulation is also narrowband. In other words, in appearance, the spectrum is spread over the specified bandwidth.

FH01.gif

The receiving side of FH systems synchronize with the hopping pattern from the receiving side to perform demodulation.
The hopping pattern is random, and even if other equipment is operated in the same area, communication is not disrupted despite instantaneous radio wave conflicts (collision). However, there is a high possibility that a radio wave collision will occur somewhere in the hopping pattern, so error correction functions are essential in FH systems.
Security is improved with a longer, more random hopping pattern. However, bearing in mind demodulation and radio wave collisions when simultaneous hops occur, code sequences suited to FH systems are used. Sequences include extended M sequence code and Reed-Solomon code sequence.

FH is used in the Bluetooth system. In a total bandwidth of 79 MHz, the frequency band of a single frequency is 1 MHz. Frequency conversion is performed with a hopping pattern that changes with a speed of 1,600 times per second (625 μs). The primary modulation method is GFSK.


*Since it is difficult to express FH with a Java applet, we have not made one. You can think of it as an extension of narrow band modulation and demodulation.
Spread spectrum modulation applet

Spread spectrum modulation and demodulation

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Click the “Explain” button at the bottom right of the applet for how to use it.

SS_00.gif

Click the image: Spread spectrum modulation and demodulation applet

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