Understanding Spread Spectrum Modulation in RF Systems
This technical brief discusses the characteristics and benefits of spread-spectrum modulation.
This technical brief discusses the characteristics and benefits of spread-spectrum modulation as used in RF systems.
There’s a good chance you have seen the term “spread spectrum”, or at least the abbreviation DSSS, which stands for “direct-sequence spread spectrum”. If you’ve ever wondered what this means, or why people are talking about it, read on.
First, we need to understand that the term “spread spectrum” can be used in the context of both digital electronics and RF systems. This article focuses on spread spectrum in relation to RF, and we’ll address the digital side of spread spectrum in a separate article.
The Narrowband Signal
A typical RF transmission involves a sinusoidal carrier wave at a specified frequency. Information is transferred by modulating the amplitude, frequency, or phase of the carrier. The frequency-domain representation of the transmitted signal looks something like this:
This is referred to as a “narrowband” signal. The band of frequencies covered by the signal is . . . well, narrow.
What would happen if we took this transmitted signal and modified it in a way that increased the bandwidth while maintaining the same total transmitted power? We would have something like this:
As you can see, the original signal has been “spread” in the sense that the bandwidth is much wider, but the average amplitude is much lower. We now have a spread-spectrum signal.
Inverting and Hopping
One way to spread the spectrum is to multiply the original signal by a sequence composed of ones and negative ones; this is equivalent to inverting the signal wherever it is multiplied by negative one. Furthermore, the transitions in this multiplying signal occur at a frequency that is higher (in practice, much higher) than the original frequency of the transmitted signal. You can visualize this as follows:
The two inverted sections of the sinusoid indicate multiplication by negative one.
These higher-frequency inversions introduce higher-frequency spectral components into the signal—in other words, the bandwidth has increased, but we’ve done nothing to increase the total power delivered to the antenna. This technique is referred to as direct-sequence spread spectrum (DSSS), and the multiplying sequence is known as a pseudo-noise (PN) code.
You can achieve similar advantages by repeatedly changing the carrier frequency. This technique is referred to as frequency hopping; however, it’s a little bit confusing to include frequency hopping in the same category as DSSS because it doesn’t spread out the spectrum in quite the same way. Frequency hopping sequentially expands the spectrum; the transmit power is distributed over a wider band only when averaged over time.
Spread-spectrum techniques are far from convenient, but in some applications the benefits justify the increased system complexity.
The bottom line is that spreading the spectrum allows for a more reliable and robust RF communication system. Think about the frequency-domain plots shown above: If there is another transmitter operating at the same frequency as the original narrowband signal, you’ll have serious interference. But that same interfering signal will be much less problematic for the spread-spectrum signal because a large portion of the transmitted energy is not affected by this narrowband interference.
The same idea applies to frequency hopping: an interfering signal will cause problems initially, but communication will be restored as soon as the transmitter and receiver switch to the new carrier frequency.
This interference scenario can be extended to other situations: spreading the spectrum (or changing the carrier frequency) makes the RF link resistant to jamming (which is just intentional interference) and interception. Thus, it’s no surprise that spread-spectrum techniques are valuable for military applications, though they are also used in commercial protocols, including Bluetooth and IEEE 802.11.