Modulation

Spread spectrum signal and modulation

A spread spectrum signal is one which consists of a range of frequencies. Starting from a single frequency signal and using spread spectrum modulation, we can distribute the original signal over a range of frequencies (band). As the energy of the signal is spread over a frequency band, its amplitude decreases significantly, making it difficult to detect. As a result, we lower the probability of its interception or jamming, creating signals that are characterized as "low-probability-of-intercept" (LPI).

In video https://youtu.be/3QofQYcL490?feature=shared&t=10, we have an electromagnetic sine wave of a 2.5 MHz (Figure 1). It is produced using a frequency generator. This signal is modulated using carrier waves of different forms, such as rectangular, triangular and "upramp", in order to create a spread spectrum signal.

The optimal spread spectrum signal is expected to be flat on top (flat peak). Such a signal is optimally produced using modulation with an "upramp" carrier wave, as shown in Figure 2. Notice that the amplitude of the peak has been reduced significantly. A signal of much lower amplitude has been created.

Figure 1. Electromagnetic sine wave of a 2.5 MHz (from video) .

Figure 2. Top: "Upramp" signal used for modulation. Bottom: Spread spectrum signal. (From video) .

Spread spectrum using the frequency-hopping scheme (frequency changing)

An example of a spread spectrum signal and modulation is the Bluetooth wireless technology. Instead of transmitting a signal at a single frequency, such as for instance the 2.4 GHz, the transmission signal is spread over the frequency band of 2.402 GHz to 2.48 GHz. This is performed using the spread spectrum modulation technique.

In addition, Bluetooth uses the frequency-hopping scheme, meaning constantly changing frequencies, always within the mentioned band. Specifically, it divides the above mentioned frequency band into a specific number of channels, that is 79 channels, each having a bandwidth of 1 MHz (similarly to the figure). For instance, the first channel is from 2.402 to 2.403, the second from 2.403 to 2.404 etc.

The data to be transmitted are separated into packets. Transmission of each packet starts at a specific channel or frequency. It is then changed to a different channel every 0.6 ms. Data packets appear to be hopping up and down in frequencies, performing 1600 hops per second.

The following video and Figure 3 illustrate the above: https://youtu.be/CkhA7s5GIGc

A specific code for changing frequencies is used e.g. F2, F5, F3, F6, F1, F6, F4. This code is generated by a mathematic equation. The transmitter uses that to create the code, while the receiver must have it registered in its system in order to distinguish the signals that are intended for it.

Additionally, Bluetooth may use adaptive frequency hopping. That means that it will adapt its sequences to avoid channels which overlap with emissions from other devices. For instance, a microwave oven may function at a specific instance emitting at 2.41 GHz. Bluetooth dynamically monitors the frequencies corresponding to its channels and avoids noisy or busy channels when sending packets. 

An example of Bluetooth avoiding a microwave emission is shown in this video: https://www.youtube.com/watch?v=HFZMBMnsjFY.


Figure 3: Bluetooth frequency-hopping (from video).