Spread spectrum

The traditional radio broadcasting technique uses a very narrow frequency band in transmission to obtain the best reception.  Reducing the bandwidth of the transmitted signal, using a high signal-to-noise ratio (SNR), enables the receiver to receive the signal easily.

This transmission technique has various drawbacks.  On the one hand, it is sensitive to interference, whether accidental or intentional, by any other signal that is transmitted on the same frequency or very close to it.  On the other hand, the signal can be received by any receiver that is tuned in at the same frequency, and therefore it is vulnerable to interception, so that it is not a suitable technique to transmit confidential information.  In addition, signals with a reduced bandwidth must be transmitted with enough power to avoid distortion with the thermal noise and thus facilitate the receiver’s recovery of the transmitted signal.

Since the nineties, a new technology has been emerging on the telecommunications market; it is known as spread spectrum, and it does not have the above-mentioned drawbacks:  it is very difficult to intercept or block, it is not easily jammed, and it transmits at low power levels.  As the radio spectrum is more congested, there is greater interest in this modulation technique because it enables various users to share frequencies.  The technique is not new, as it was developed and patented more than 60 years ago, during the Second World War, and the military has been using it for 30 years.  To apply it commercially, highly complex digital technology that was not available before had to be developed.

The spread spectrum uses a method that is radically different from the traditional one for transmission.  Instead of using a narrow frequency band in the signal with high power to differentiate it from background noise, the bandwidth of the transmitted signal is considerably broadened and is transmitted at low power, at the thermal noise level.  This is how the maximum rate of data transfer is kept, as explained below.

The Shannon theorem (see note 7) relates the maximum rate of data transfer of channel C, in bps; bandwidth A, in Hz; and signal-to-noise ratio SNR, without units [1]. Using the formula, 

C = A log₂ (1+SNR)

The following table can be built, keeping the bandwidth constant and varying the SNR.

Fig. 1.23 – Effect of the SNR on the data transfer capacity C

We observe that the bandwidth A can be increased and the signal power, that is SNR, reduced to obtain the same maximum rate of data transfer C, as indicated in Fig. 1.23. This is the concept of the spread spectrum represented in Fig. 1.24.

In short, communications that use the spread spectrum are differentiated from the conventional wireless communications in various aspects:

§      The transmitted signal occupies a bandwidth that is much greater than the one required for the information that is going to be transmitted.  It permits the simultaneous use of the band for transmissions by various users and avoids signal jamming and blocking.

§      The power of the signal is very low, at the thermal noise level.  For a conventional radio receiver, the signals blend with the background noise.

Fig. 1.24 – Concept of spread spectrum 

This feature, along with the use of a code to encode and decode signals, which acts as a private security key, ensures very high privacy levels of communications with spread spectrum, and this does not hold true for conventional wireless transmissions.

PN Code

The spread spectrum is a digital modulation technique with security included.  All spread spectrum systems use a pseudo-random [2] code called the pseudo-random noise code (PN), on the basis of which the signal is encoded and decoded and which is used to determine the frequency spectrum that the transmitted signal shall occupy.  The PN code determines and controls how the spectrum is used in the transmission.

Methods to spread the spectrum

Spread spectrum systems (SS) use various techniques to spread the spectrum:

§      Frequency-Hopping (FH)

§      Direct Sequence (DS)

§      Time-Hopping (TH)

§      Multi-Carrier CDMA (MC-CDMA)

§      Combinations of the above

The most common are frequency hopping (FH) and the direct sequence (DS), which are described below.

Jorge Villalobos A.,
Director Centro de Estudios de Telemática
Escuela Colombiana de Ingeniería