What is spread spectrum communication?Application of spread spectrum technology in CDMA and TDMA
As spread spectrum technology spreads, many electronic engineers outside the field also want to learn about the technology. Many books and websites have been written on this topic. Some articles simply explain complex concepts that are difficult for people to accept, while others discuss several aspects of technology in detail but ignore others (for example: direct spread spectrum). The focus is on the generation of PRN codes).
This article will discuss all aspects of spread spectrum technology as comprehensively as possible.
The idea of spread spectrum communication technology was proposed in 1941 by Hollywood actress Hedy Lamarr and pianist George Antheil. They applied for US patent #2.292.387 based on the idea of secure wireless communication for torpedo control. Unfortunately, the technology did not attract the attention of the U.S. military at that time, and it was not until the 1880s that attention was paid to its use in wireless communication systems in hostile environments.
Typical applications in short-range data transceivers are satellite positioning systems (GPS), 3G mobile communications, WLAN (IEEE 802.11a, IEEE 802.11b, IEEE 802.11g) and Bluetooth (Bluetooth) technologies. Spread spectrum technology also helps improve the utilization of radio frequencies (the radio spectrum is limited and therefore an expensive resource).
The basis of spread spectrum theory
There are many solutions for the specific implementation of spread spectrum technology, but the idea is the same: adding an index (also called a code or sequence) to the communication channel, and the way of inserting the code exactly defines the spread spectrum technology in question. The term "spread spectrum" refers to extending the signal bandwidth by several orders of magnitude. Spread spectrum is achieved by adding an index to the channel.
A more precise definition of spread spectrum technology is: a radio frequency communication system that expands the baseband signal to a wider frequency band by injecting a higher frequency signal (Figure 1), that is, the energy of the transmitted signal is expanded to a wider frequency band, Make it look like noise. The ratio of the expanded bandwidth to the original signal is called the processing gain (dB). Typical spread spectrum processing gains can range from 10dB to 60dB.
Using spread spectrum technology, that is, simply introducing the corresponding spreading code somewhere in the transmission link before the antenna (this process is called spread spectrum processing), the result is that the information is spread into a wider frequency band. Conversely, removing the spreading code before data recovery in the receive link is called despreading. Despreading reconstructs the information at the original bandwidth of the signal. Obviously, the spreading code needs to be known in advance at both ends of the information transmission path (in some cases, it should only be known by both parties transmitting information).
Figure 1. Spread spectrum communication system, bandwidth of spread spectrum processing
Spread spectrum processing bandwidth
Figure 2 provides an evaluation of signal bandwidth in a communication link.
Figure 2. Spread spectrum operation spreads signal energy over a wider frequency band
Spread spectrum modulation is used in front-end or direct conversion of general-purpose modulators such as BPSK. Signals that do not receive a spreading code remain unchanged and are not spread.Despread processing bandwidth
Similarly, the deexpansion process is shown in Figure 3:
Figure 3. The despreading operation restores the original signal
Despreading is usually performed before demodulation. Signals added during transmission (such as interference or blocking) are spread during the despreading process.
Bandwidth wasted due to spread spectrum is compensated by multiple users
The direct result of spreading is that it occupies a wider frequency band (the spreading factor is related to the "processing gain" mentioned earlier), thus wasting limited frequency resources. However, the occupied frequency band can be compensated by multiple users sharing the same extended frequency band (Figure 4).
Figure 4. Spread spectrum technology allows multiple users to share the same frequency band
Spread spectrum is a broadband technology
The spread spectrum process is a broadband technology compared to narrowband technology. For example, W-CDMA and UMTS are broadband technologies that require wider frequency bands (as opposed to narrowband radio).
Benefits of spread spectrum
Anti-interference and anti-blocking performance
Spread spectrum technology will bring many benefits, and anti-interference characteristics are the most important advantage. Because interfering and blocking signals do not have spreading factors, they are suppressed. After the despreading process, only the desired signal containing the spreading factor will appear in the receiver. As shown in Figure 5.
Figure 5. Spread spectrum communication system. When the data signal is despread in the receive link, the energy of the interfering signal is expanded.
If the interfering signal (narrowband or wideband) does not include a spreading factor, its effect can be ignored after despreading. This suppression capability also acts on other spread spectrum signals that do not have the correct spreading factor. It is because of this that spread spectrum communication allows different users to share the same frequency band (such as CDMA). Note that spread spectrum is a broadband technology, but broadband technology is not spread spectrum. Broadband technology does not necessarily include spread spectrum technology.
Prevent signal interception
Anti-signal interception is the second advantage gained through spread spectrum. Because unauthorized users do not know the spreading factor that spreads the original signal, they cannot decode it. Without the correct spreading factor, the spread spectrum signal is equivalent to noise or interference (of course, if the spreading factor is very short, scanning methods can be used to crack it). Thankfully, spread spectrum communications allow signal power to be below the noise floor because the spread spectrum process reduces the spectral density, see Figure 6 (the total energy is the same but spread across the entire frequency domain). In this way, information can be hidden. This effect is a significant feature of direct sequence spread spectrum (DSSS) (direct sequence spread spectrum will be introduced in detail later). Other receivers cannot resolve this emission, and the effect on them is only a slight increase in the total noise power!
Figure 6. The spread spectrum signal is buried below the noise floor. Without the correct spreading factor, the receiver cannot "interpret" the transmission.
Fading suppression (multipath effects)
Wireless channels usually have multipath propagation effects, with more than one path from the transmitter to the receiver (Figure 7). These paths are created due to reflection or refraction in the air and reflections from the ground or objects such as buildings etc.
Figure 7. Signal travels through multiple paths to the receiver
The interference caused by the reflected path (R) to the direct path (D) is called fading. Because the despreading process is synchronized with signal D, even if signal R contains the same spreading factor, it will also be suppressed. The signal in the reflected path can be despread and its rms value added to the main signal.
Application of spread spectrum technology in CDMA
It is important to note that spread spectrum is not a modulation method and should not be confused with other types of modulation. For example, we can use spread spectrum technology to transmit an FSK or BPSK modulated signal. From the perspective of basic coding theory, spread spectrum can also be used as a method to achieve multi-access communication (multiple communication links coexist on the same physical medium at the same time). So far, there are three main ways.
FDMA—Frequency Division Multiple Access
FDMA assigns a specific carrier frequency to each communication channel, and the number of users is limited by the number of frequency bands of the spectrum (Figure 8). Among the three multiple access implementation methods, FDMA has the lowest frequency band utilization. Typical applications include wireless broadcast, TV, AMPS and TETRAPOLE.
Figure 8. Carrier frequencies allocated to different users in the FDMA system
TDMA—Time Division Multiple Access
In TDMA, communication between different users is based on allocated time slots (Figure 9). In this way, different communication channels can be established on one carrier frequency. TDMA is used in GSM, DECT, TETRA and IS-136.
Figure 9. Time slots allocated to different users in the TDMA system
CDMA—Code Division Multiple Access
CDMA's spatial access depends on the spreading factor or code (Figure 10). From a certain perspective, spread spectrum is a method of CDMA. The transmitter and receiver need to know the defined spreading code in advance. Typical applications include IS-95 (DS), IS-98, Bluetooth technology and WLAN.
Figure 10. CDMA systems use different spreading factors or codes using the same frequency band
In practical applications, the above multiple access methods can be comprehensively utilized. For example, GSM combines TDMA and FDMA, uses different carrier frequencies to define topological areas (cells), and sets time slots in each cell.
Spreading and codec factors
The main feature of spread spectrum is that the transmitter and receiver must know a preset spreading code or spreading factor in advance. In modern communications, spreading codes must be long enough to approximate a random number sequence similar to noise. However, in any case, they must remain recoverable. Otherwise, the receiver will not be able to extract the transmitted information. Therefore, this sequence is approximately random. Spreading codes are often called pseudo-random codes (PRN) or pseudo-random sequences. A feedback shift register is usually used to generate pseudo-random codes.
Figure 11 gives an example of a pseudo-random code. The shift register contains 8 data flip-flops (FF), and the contents of the shift register are shifted left bit by bit on the rising edge of the clock. The data moved into FF1 depends on the feedback information from FF8 and FF7. The pseudo-random code PRN is read from FF8. The contents of the flip-flop are reset at the beginning of each sequence.
Figure 11. Block diagram of a simple PRN generator
Many books have been written about the generation and characteristics of PRN, but these basic instructions have not kept pace with its development. Neither the generation nor the selection of a suitable sequence (or set of sequences) is simply done directly. To ensure effective spread spectrum communication, PRN sequences must consider several criteria, such as length, autocorrelation, cross-correlation, orthogonality and bit equalization. The more commonly used PRN sequences are: Barker, M-Sequence, Gold, Hadamard–Walsh, etc. The more complex the sequence set used by a spread spectrum communication link, the higher its reliability. However, the price paid is that the electronics required for the despreading operation will also be more complex (both in terms of speed and performance). Digital despreading chips can contain millions of equivalent 2-input NAND gates, switching at tens of megahertz.
Different modulation methods of spread spectrum technology
Different spread spectrum modulation methods can be obtained depending on the position where the pseudo-random code (PRN) is inserted into the communication channel. Figure 12 is a basic RF front-end principle illustration.
Figure 12. Various spread spectrum techniques at different locations in the transmit link
If pseudo-random sequence codes are directly added to the data, direct sequence spread spectrum (DSSS) can be obtained (in practical applications, the pseudo-random sequence is multiplied by the communication signal to produce data that is completely "scrambled" by the pseudo-random codes) . If a pseudo-random code is applied to the carrier frequency, we get Frequency Hopping Spread Spectrum (FHSS). If the pseudo-random code acts on the local oscillator, the FHSS pseudo-random code forces the carrier to change or jump according to the pseudo-random sequence. If a pseudo-random sequence is used to control the on or off of the transmitted signal, time hopping spread spectrum technology (THSS) can be obtained. This is also a chirp technique, which linearly sweeps the carrier frequency over a period. The above technologies can also be combined to form a hybrid spread spectrum technology, such as DSSS + FHSS. DSSS and FHSS are the two most commonly used technologies today.
Direct Sequence Spread Spectrum (DSSS)
In direct sequence spread spectrum technology, pseudo-random codes are added directly to the data from the carrier modulator. Therefore, the modulator appears to have a higher code rate, related to the chip rate of the pseudo-random sequence. The result of modulating a radio frequency carrier with such a code sequence is to produce a direct sequence modulation spread spectrum centered at the carrier frequency and with a spectrum of ((sin x)/x)². The bandwidth of the main lobe of the spectrum (zero to zero) is twice the modulation code clock rate, and the side lobe bandwidth is equal to the modulation code clock rate. Figure 13 is a typical example of a direct sequence spread spectrum signal. Some changes in the shape of the direct sequence spread spectrum spectrum are related to the actual carrier and data modulation methods used. Below is a Bi-Phase Shift Keying (BPSK) signal, a common type of modulation used in direct sequence spread spectrum systems.
Figure 13. Spectrum analysis diagram of DSSS signal. The original signal (unspread spectrum) only accounts for half of the central main lobe.
Frequency Hopping Spread Spectrum (FHSS)
As the name suggests, the carrier in FHSS jumps from one frequency to another over a wide frequency band according to the definition of pseudo-random code. The hopping rate is determined by the data rate of the original message, and we can identify fast frequency hopping (FFHSS) and slow frequency hopping (LFHSS). The latter (the most versatile) allows several consecutive data bits to modulate the same frequency, FFHSS hops multiple times within each digital bit.
The transmission spectrum of frequency hopping signals is very different from that of direct sequence spread spectrum. The frequency hopping output is flat across the frequency band (as shown in Figure 14), rather than having a ((sin x)/x)² envelope. The bandwidth of the frequency hopping signal is N times the frequency gap, where N is the bandwidth of each hopping channel.
Figure 14. Spectrum analysis diagram of FHSS spread spectrum signal
Time hopping spread spectrum (THSS)
Figure 15. THSS block diagram
Figure 15 shows the time-hopping spread spectrum technology. This technology has no major breakthroughs so far. It uses pseudo-random sequences to control the on/off of the PA.
System implementation and conclusion
A complete spread spectrum communication link requires the use of various advanced technologies and processes: RF antennas, high-power, high-efficiency power amplifiers, low-noise, high-linearity LNAs, highly integrated transceivers, high-resolution ADCs and DAC, high-speed, low-power digital signal processor (DSP), etc. Designers and manufacturers competed and collaborated to make spread spectrum systems possible.
The most difficult circuit to implement is the receiving channel, especially the despreading of DSSS, because the receiving end must be able to recover the original information and achieve real-time synchronization. Code recognition is also called correlation operation, which is implemented in the digital domain and requires fast and large number of binary addition and multiplication operations.
By far the most complex issue in receiver design is synchronization. Compared with other technologies of spread spectrum communication, the development of synchronization technology takes more time, money, and more manpower and material resources. Currently, there are many ways to solve synchronization problems, and most solutions require a large number of discrete components. The emergence of DSP and application-specific integrated circuits (ASIC) has brought major breakthroughs. DSP provides high-speed mathematical operation capabilities to perform analysis, synchronization and decorrelation operations after dividing the spread spectrum signal. Leveraging Very Large Scale Integration (VLSI) technology, ASICs reduce system costs and make them suitable for a variety of applications by creating a basic modular architecture.
Review Editor: Liu Qing
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