Knowledge Center

Knowledge Center

Spread Spectrum

A secure method of transmitting data wirelessly.
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Description

The wireless arena has had a leg up when it comes to securing data transmission for a long time because of spread spectrum. Securing RF transmissions via spread spectrum has been around since WWI. In fact, the theory was first mentioned in a 1908 book by German radio pioneer Jonathan Zenneck and a primitive form was used by the German military in WWI. However, perhaps the best known and the most celebrated development was when actress Hedy Lamarr and composer George Anthell developed a reasonably complex (for that time) version and received a U.S. patent for it in 1941. However, it really didn’t come to pass until a few decades later when the U.S. Navy implemented that technology in its communications rigs during the Cuban missile crisis. After that, the U.S. military awoke to its phenomenal ability to code transmissions and make them virtually bulletproof. It has been relying on spread spectrum as a primary security scheme for RF communications ever since. In fact it can be called the single most significant development that launched the digital age of communications.

Applying spread spectrum technology to RF transmissions is an extremely effective method of securing both wireless voice and data at the hardware level. It is a very robust TX/RX technology that is inherently immune to inter-symbol interference (ISI), reflections, noise and other environmental factors, as well as jamming. While mainly used by the military in voice radios until the last decade or so, it is how being implemented in various consumer and commercial technologies. All types of radio-based systems, such as Bluetooth, hobby radio control, Wi-Fi, WiMax wireless local area network (WLANs), broadband wireless access (BWA), near-field communications (NFC) and radio-frequency identification (RFID – especially collision avoidance) are all being fitted with spread spectrum hardware.

Types of Spread Spectrum
Spread spectrum comes in two flavors, direct sequence (DSSS) and frequency hopping (FHSS). Both technologies work equally well in benign environments, but at the fringe, i.e., crowded frequencies, cell edges, and areas with la lot of interfering objects, FHSS is more robust and forgiving. The major advantage of DSSS is that it offers better capacity.

DSSS capacity comes at a price, though — environmental sensitivity. DSSS is influenced by many environment factors (the most problematic is reflections) so it isn’t the best choice in dense environments with lots of structures. It works best in point to multipoint for short-distance installations, or point to point in longer distance topologies. In these cases, advantage goes to higher capacity offered by DSSS technology, because reflections, the primary degradation element, can be minimized. As such, typical DSSS applications include indoor office WLANs, building-to-building links, point of presence (PoP) to base station links, and the like.

The robustness of FHSS technology makes it highly immune to the influences of noises, reflections, nearby RF signals and other environment factors. FHSS can support a much higher number of simultaneously active systems in the same geographic area (co-located systems) than DSSS. These metrics make FHSS the technology of choice for large-area cover where a high number of co-located systems are required. The one caveat is that directional antennas must be used to minimize the influence of environment factors. Typical applications for FHSS include cellular deployments for fixed broadband wireless access (BWA), which is expanding at phenomenal rates as small cell deployments come on line.

How the Technology Works
Spread spectrum relies on a handshake between the transmitter and receiver to pass a synchronizing code back and forth. It synchronizes a “key” that only the transmitter(s) and receiver(s) know. Once the key is synchronized and all transmitters and receivers have it, the information can be sent. Only the key holders know the spreading and de-spreading codes that modulated the information. To any other receivers, the information just looks like noise. This is why spread spectrum is such a useful and secure technology for any number of wireless communications links, and an ideal security lock for autonomous IoT objects.

In DSSS, each bit of the original signal is represented by multiple bits in the transmitted signal. A spreading code is injected that spreads the signal across a wider frequency band. The spread is in direct proportion to number of bits used (see Figure 2), with the energy in the information “spread” across a width. The integral value of the power remains the same, just spread across the wider bandwidth.

Mathematically, it can be expressed as: (assuming binary phase-shift keying [BPSK])
sd(t) = A d(t) cos(2π fct)
by c(t) [takes values +1, -1] ∴ s(t) = A d(t)c(t) cos(2π fct)
A = amplitude of signal
fc = carrier frequency
d(t) = discrete function [+1, -1]

At the receiver, the incoming signal multiplied by c(t), since, c(t) x c(t) = 1. This is the original signal, recovered.

For FHSS, the technique is a bit different. Rather than spread the signal over a single wideband frequency, FHSS breaks the signal into multiple “pieces” and spreads them over multiple frequencies (see Figure 3). If the signal is viewed with a spectrum analyzer, is appears as a random series of RF frequencies with a tiny “blip” appearing on each frequency.

That blip is a piece of the information being transmitted. In reality, a pseudorandom code is used. It is not truly random since the receiver must be able to generate the same code as the transmitter, so there must be the ability to synchronize codes prior to the information broadcast. However, many references use the term random code. Since the blips appear randomly on multiple frequencies, it is impossible to collect and decode the bits of information in the original sequence and understand it. Again, it’s a very secure method of transmitting data. Even if there is an attempt to jam the signal, the chances of knocking out more than a few bits is extremely small. As a result, reconstruction of the data is generally very successful, even if jamming is attempted.

Depending upon a set of criterion, a number of channels are allocated to the transmission. How many channels are allocated depends upon a number of variables, but it is related to the bandwidth of input signal. The transmitter sets up the process, starts the hopping sequence, and sets the timing to hop the signals from frequency to frequency at fixed intervals, one channel at a time. The bits are transmitted using a predetermined encoding scheme. At the next interval, a new carrier frequency is selected, and the bits are transmitted on that one. The process repeats until all the data is sent.


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