GNSS systems rely on direct sequence spread spectrum (DSSS) transmissions to achieve high receiver sensitivity. Typically, GNSS user equipment compares the signal received from a satellite with an internally generated replica of its corresponding code until the maximum correlation for a given delay is achieved. This provides an indirect measurement of the satellite-receiver range.
Spread Spectrum refers to a system originally developed for military applications, to provide secure communications by spreading the signal over a large frequency band.Figure 1 represents a narrow band signal in the frequency domain. These narrowband signals are easily jammed by any other signal in the same band. Likewise, the signal can also be intercepted since the frequency band is fixed and narrow (i.e. easy to detect).Figure 1: Narrow band signal, relatively easy to jam or intercepted.The idea behind spread spectrum is to use more bandwidth than the original message while maintaining the same signal power. A spread spectrum signal does not have a clearly distinguishable peak in the spectrum. This makes the signal more difficult to distinguish from noise and therefore more difficult to jam or intercept. This concept is illustrated in Figure 3. This document will explore basics concepts of spread spectrum for the remaining of the introduction and then it will explore the supporting concepts of the most used technique in spread spectrum systems. The last section will give the reader some insight of more advance topics but will not deeply explore them. We encourage the reader to seek the references for advance knowledge of spread spectrum systems.
Spread Spectrum Systems For Gnss And Wireless Communications Pdf Download
If we perform the following operation:= 15 which is multiply each value by itself and add them all ().Now take ,Performing the same operation:= -1 = This is the autocorrelation for each shift point. If we take them all and plot them so that there are 15 points before 0 and 15 after:a)b)Figure 5: Correlation of a) example sequence and b) other sequence with polynomial created with LabVIEW and MathScriptAs seen, only if the end user having the exact sequence is able to demodulate the message when the sequence is synchronized (peak at correlation = 1). Other users will have very little amplitude of the original signal. This is the principle of Code Division Multiple Access (CDMA) cellular systems, in other words, share the same frequency and time with multiple users with different codes.SpreadingThe block diagram of the DSSS communication system for QPSK is presented in Figure 6. Notice that the PN sequence is introduced here to both in-phase (I) and quadrature (Q) components.Figure 6: Block diagram of the spread spectrum QPSK modulatorThe sequence should be long enough (with respect to the message signal) to have the noise-like spectrum. This is the relation between spreading sequence rate and message rate :In practical systems, is an integer number and it is the number of phase shift of the PN sequence for each message bit. For example, for GPS systems N = 1024.Figure 7: Spreading the message, each bit of the message will contain the entire PN sequenceThe new message has now and therefore The output combined baseband sequence is: Where is the sent baseband waveform, is the PN waveform and is the bit sequence.DespreadingReceived baseband waveform is the combination of the transmitted waveform and noise in the channel.Figure 8: Simple additive white Gaussian noise (AWDG) channel model. LabVIEW Vi The received signal will be combined again with the spreading sequence. Notice that the noise is also going to be processed on the same procedure but correlation properties will not increase the noise power.The received signal will be the combination of the transmitted signal plus noise: We can substitute the sent waveform by the combination of the PN sequence and the bit sequence.The modulator will multiply it by the PN sequence:If is synchronized, and is like multiplying noise times noise which gives other kind of noise (similar in amplitude).Consider the ideal example from Figure 9.
We have now explored different sequences that can spread the signal like noise would behave. Spread spectrum systems expand beyond this point to many different paths: modulation schemes, performance under fading, under interference, capacity in CDMA systems, etc.To finalize, we would like to point out the advantages and disadvantages of using spread spectrum:Advantages:
At present, M code and Gold code generated by linear feedback shift register (LFSR) sequences are the most widely used ranging codes in existing satellite navigation systems, but both of codes have some common disadvantages such as limited number of available code groups and poor anti-decryption ability. With the development of decryption technology to pseudo-noise sequence codes, the above codes are facing the danger of being cracked [9]. In recent 10 years, chaotic sequences have made it possible for ranging as a result of the gradual maturity in theory and application of chaos, which have the following advantages: high sensitivity to initial values, large number of code groups, high linear complexity, and excellent confidentiality. The application of chaotic sequences opens up a new perspective for spread spectrum communication [10], but chaotic sequences also have their shortcomings especially for poor balance. For example, the balance coefficient is larger than 0.02 when sequence length is relatively short, which cannot satisfy the requirement of high-precision positioning systems [11, 12].
Authentication of signals is a critical aspect of secure communication. Most mechanisms of authentication (e.g. digital signatures and certificates) exist above the physical layer, though some (e.g. spread-spectrum communications) exist at the physical layer often with an additional cost in bandwidth. The use of SDRs at physical layer has provided low-cost authentication solutions using various software techniques like fingerprint embedding where a low-power secret modulation is superimposed over the message waveform which serves as an authentication tag [56, 57].
Techniques and technologies aiming to prevent these types of signal manipulation have been used in the military and government sectors and are now moving to commercial satellite systems. They include techniques such as spread-spectrum and frequency hopping, which are applied at the physical layer to make signals appear noisy or switch their frequency usage with pseudo-random sequences [154].
CDMA systems use a signal fast chip rate for spreading the spectrum. It has a high time resolution, due to which it receives a different signal from each path separately. The RAKE receiver prevents signal degradation by summing all the signals.
Direct-Sequence Spread Spectrum (DSSS) is a modulation technique used in telecommunications to reduce interference in signals during transmission. It is a spread spectrum technique which means that the frequency of the signal generated with a particular bandwidth is deliberately increased (spreading) resulting in a signal with a wider bandwidth. Transmitting signals with a spread bandwidth have less unintentional/intentional interference and loss/corruption of data. The bandwidth of the original signal before spreading of frequency (DSSS) occurs is called the information bandwidth. At the receiver end, the signal undergoes despreading or removal of DSSS modulation and the information bandwidth is restored. 2ff7e9595c
コメント