DSSS basics     In North America 11 frequencies are used ranging from 2.412 to 2.462 GHz. In DSSS, each user has a chipping sequence (or called code, key, etc) that determines how data is encoded and decoded. In Figure 2, the narrowband signal and the spread-spectrum signal both use the same amount of transmit power and carry the same information. A station wishing to join the network can listen to the beacon and synchronize its hop pattern as part of the network association procedure. The CRC field contains the results of a calculated frame check sequence from the sending station. If the bandwidth of the DSSS, direct sequence spread spectrum signal is W and the input data bit length or period 1/R then the DSSS spreading gain can be defined: It is found that the larger the spreading gain of the direct sequence spread spectrum signal, the more effective the performance of the system is. DSSS transmitters use a spreading PN sequence to modulate the baseband signal before transmission. Data bits are grouped into symbols of two bits, so each symbol can have one of four levels. To obtain the sequence is a necessary and essential part to complete the signal system classification and blind despreading. In the transmitter, the input data stream is XORed with the PN signal before transmission. All information bits transmitted by the DSSS PMD are scrambled using a self-synchronizing 7-bit polynomial. By the way, according to Wikipedia, the first patent on DSSS was granted in 1942 to a Swiss inventor, Gustav Guanella, and … Japan allows the use of all 14 channels. The dwell on each frequency is a system-determined parameter, but the recommended dwell time is 20 ms, giving a hop rate of 50 hops per second. After a receiver has correlated the received signal with its particular PN pattern, the interference from the other PN sequences is therefore low. Table 21.5. DSSS is a transmission technology used in LAWN transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. While overshadowed by the DSSS PHY, acquaintance with the FHSS option in 802.11 may still be of interest. DSSS adds redundant bits of data known as chips to the data to represent binary 0s or 1s. In order to visualise how the direct sequence spread spectrum process operates, the easiest method is to show an example of how the system actually operates in terms of data bits, and how the data is recovered from the DSSS, direct sequence spread spectrum signal. 3. A DSSS signal looks like noise if you don’t know the patterns to look for. Figure 21.9. 3.1 Direct Sequence Spread Spectrum Figure (2) depicts transmitter, channel and receiver of DSSS system. In this case, the synthesizer is being sequentially programmed with different frequencies by the PN sequence to modulate the desired signal over a much wider frequency band. The DS-CDMA has been used in commercial applications of mobile communications. Peng Zhang, in Advanced Industrial Control Technology, 2010. Over short intervals, then, the SCF for most DSSS BPSK signals is identical to that for a general BPSK signal. Each of the physical layers is unique in terms of the modulation type, designed to coexist with each other and operate with the MAC. Often the data is multiplied using the XOR (exclusive OR) function. ▶︎ Electronics Notes Bookshop. communication signals, known as direct sequence spread spectrum (DSSS). well [1–3]. For everything from distribution to test equipment, components and more, our directory covers it. Frequency synthesizers are unable to maintain frequency coherency over such wide bandwidths. Each DSSS PHY channel occupies 22 MHz of bandwidth and allows for three noninterfering channels spaced 25 MHz apart in the 2.4 GHz frequency band (see Figure 21.9). Each bit of the PN sequence is called a chip, and the clock rate of the PN generator is called the chip rate. Superhet radio     DSSS technology breaks down the transmitted stream of data into small pieces across a frequency channel. All three support a 1 to 2 Mbps data rate. DSSS technology uses more bandwidth than FHSS, but is considered more reliable and resists interference. For example, two wireless stations operating on channels 1 and 3 would not associate, but carrier sensing may prevent them from transmitting packets simultaneously. Vijay K. Garg, in Wireless Communications & Networking, 2007. This bandwidth holds for both 1 Mbps and 2 Mbps data rates. Introduction. Take as an example that the data to be transmitted is 1001, and the chip or spreading code is 0010. direct sequence spread spectrum (DSSS) or direct sequence code division multiple access (DS-CDMA): Direct sequence spread spectrum, also known as direct sequence code division multiple access (DS-CDMA), is one of two approaches to spread spectrum modulation for digital signal transmission over the airwaves. In practical systems the chip rate is a large integer multiple L of the databit rate. The information-bearing baseband signal is denoted as v(t) and is expressed as ( ) { 1, }v t a g t nT where a nn T b n( ) n ∞ = − =± −∞< <∞∑ =−∞ and g T (t) is a rectangular pulse of duration T b This signal is multiplied by the signal … ▶︎ HDMI Products. In DSSS, the PN sequence spreads the spectrum of the signal by the chipping rate, resulting in the instantaneous widening of the spectrum. In telecommunications, direct-sequence spread spectrum (DSSS) is a spread-spectrum modulation technique primarily used to reduce overall signal interference. If any bits are damaged during transmission, the original data can be recovered due to the redundancy of transmission. Since wireless networking uses radio-wave transmission, radio-wave frequency techniques are particularly important to the wireless networks’ performance. Direct-sequence acquisition and tracking A direct-sequence-spread-spectrum receiver must despread the received signal to recover the transmitted data. Data produced by other dispreading codes would appear as noise and can be discarded as it would be lower in value. FHSS implementation may at one time also have been less expensive. At the intended receiver, despreading is accomplished by cross-correlation of the received spread signal with a synchronized replica of the same code signal used to spread the data. Because of the chipping code, data can still be recovered without retransmission of the signal, even in the case of damaged data bits. This is because the wanted signal becomes larger. Chad M. Spooner, Richard B. Nicholls, in Cognitive Radio Technology (Second Edition), 2009. In the DSSS PHY, data transmission over the media is controlled by the PMD sublayer as directed by the PLCP sublayer. The non-conjugate CFs are harmonics of the code-repetition rate, which is equal to the data rate (bit rate) when the code is repeated for each bit (as it is in 802.11b DSSS BPSK). The direct-sequence modulation makes the transmitted signal wider in bandwidth than the information bandwidth. By using this, the transmission signal of each user can be identified by the separate code when it receives the signal. The signal field indicates which modulation scheme should be used to receive the incoming MPDU. With DS-CDMA, adjacent microcells share the same frequencies whereas with FDMA/TDMA it is not feasible for adjacent microcells to share the same frequencies because of interference. Table 21.6. The result in the frequency domain is a signal that is spread over a wide bandwidth at a reduced RF power level. In signal processing, a signal is a function that conveys information about a phenomenon. On the other hand, in FHSS, the PN sequence is used to drive the synthesizer and pseudo-randomly hop the signal bandwidth across a much wider band. Receiver selectivity     Generally, the longer the chipping code, the more likely it is that the original transmitted data will be properly received. . Direct Sequence Spread Spectrum with Barker code and QPSK International organization of Scientific Research 66 | P a g e Figure 10 depicts transmitter, channel and receiver of DSSS system. The first part of the process is to generate the DSSS signal. The specifications for IEEE 802.11 meet the RF emissions guidelines of FCC, ETSI, and the Ministry of Telecommunications. An 11-bit Barker code (1, −1, 1, 1, −1, 1, 1, 1, −1, −1, −1) is used for spreading. However, the same principles apply: The redundant bit pattern in the spreading sequence increases the signals resistance to interference and also distributes the power over a 22-MHz-wide channel that looks essentially like low-power, wideband noise. Part A – Generating a DSSS signal using a simple message As DSSS is basically just DSBSC with a pulse train for the carrier instead of a simple sinusoid, it can be generated by implementing the mathematical model for DSBSC. In order for FHSS network stations to be synchronized, they must all use the same pseudo-random sequence of frequencies, and their synthesizers must be in step, that is, they must all be tuned to the same frequency channel at the same time. DSSS channels for different parts of the world. The MAC layer uses this field to determine the end of a PPDU frame. A 500 kHz Gaussian filter smoothes the four-level 1 megasymbols per second at the input to the FSK modulator. There is an obvious analogy between this process and stream ciphering (Section 14.8) but with the crucial difference that in DSSS the PN sequence is at a much greater clock frequency than the data stream. The spreading code used in this example was only four bits long. The DSSS signal gives LPI (a low probability of intercept) because the total signal power is spread over a wide bandwidth and the signal is noise-like, making it hard to detect. The service field is reserved for future use. Figure 25.9. The spreading sequence or chip sequence has the same data rate as the final output from the spreading multiplier. The spreading code spreads the signal across a wider frequency band in … Since the DSSS channel occupies a 22-MHz band, only the nonoverlapping channels are guaranteed to be free from contention with each other. In the example shown above, the spreading gain is four, as seen by the fact that four "1"s are generated for each required data bit. The signal which is spread spectrum, can be demodulated only by a code used for transmission. To apply a spread spectrum technique, the relevant spread spectrum code must be applied somewhere in the transmitting chain, before the antenna or receiver. The baseband data stream is then modulated onto a carrier and in this way the overall the overall signal is spread over a much wider bandwidth than if the data had been simply modulated onto the carrier. At the physical layer, IEEE 802.11 defines three physical characteristics for WLANs: diffused infrared (baseband), DSSS, and FHSS. Bluetooth has some of the characteristics of 802.11 FHSS but has advanced well beyond the capabilities of the earlier standard. For each data bit, the complete spreading code is used to multiple the data, and in this way, for each data bits, the spread or expanded signal consists of four bits. 4. 1. DS-CDMA radios can, therefore, accommodate more mobile users than FDMA/TDMA radios on the same bandwidth. The output of the modulo-2 adder results in a signal with a data rate that is 10 times higher than the information rate. Comparison of multiple access methods. There are two types of FHSS systems, namely slow frequency hopping and fast frequency hopping. OSI model for IEEE 802.11 WLAN. In electronics and telecommunications, it refers to any time varying voltage, current, or electromagnetic wave that carries information. By continuing you agree to the use of cookies. Although development of Wi-Fi for significantly increased data rates has based on DSSS, FHSS does have some advantageous features. Signal bandwidth is increased once this key is attached, creating the “spread spectrum” that gives the method its name. DSSS Using BPSK Multiply BPSK signal, s d (t) = A d(t) cos(2π f c t) by c(t) [takes values +1, -1] to get s(t) = A d(t)c(t) cos(2π f c t) •A = amplitude of signal •f c = carrier frequency •d(t) = discrete function [+1, -1] At receiver, incoming signal multiplied by c(t) oSince, c(t) x c(t) = 1, incoming signal is recovered Direct sequence spread spectrum is a form of transmission that looks very similar to white noise over the bandwidth of the transmission. Further capacity gains for FDMA, TDMA, and CDMA can also result from antenna technology advancement by using directional antennas that allow the microcell area to be divided into sectors. For longer intervals, there are many non-conjugate and conjugate CFs for DSSS BPSK. The spreading … One such example is Bluetooth where frequency hopping is utilized to spread the spectrum as shown in Figure 3.34 and Figure 3.35. The SFD specified is common for all IEEE 802.11 DSSS radios. Two modulation schemes, differential binary phase shift keying (DBPSK) — for 1 Mbps — and differential quadrature phase shift keying (DQPSK) — for 2 Mbps — are available. Radio receiver types     The resulting modulated signal is called DSSS signal. As we will see later, only three independent DSSS networks can be collocated. Figure 21.7. Figure 3.34. In this scenario, one or more data symbols are transmitted each hop. The spectral correlation of DSSS signals is well understood, because over short intervals (relative to the code-repetition interval), the signal is well modeled by PSK or QAM with a symbol rate equal to the DSSS chip rate, and over long intervals, the signal is well modeled as a PSK or QAM signal with a symbol rate equal to the code-repetition rate and a pulse function equal to the repeated chipping sequence. Nominal deviations of the four levels are ± 72 and ± 216 kHz. Uses DSSS Technology U.S. allows the use of channels 1 thru. various options of the Wide-Band Direct Sequence Spread Spectrum (WB-DSSS) scheme. Electronics Notes offers a host of products are very good prices from our shopping pages (in association with Amazon). This is multiplied with the spreading code to regenerate the original data. The code signal is independent of the data and is of a much higher chip rate than the data signal. In DSSS, the narrowband input signal is spread by modulation with a pseudo‐noise (PN) chip sequence. The bandwidth of the spread spectrum signal will be much wider than the original data stream. This is, of course, why its first applications were military. Frequency modulation     Minimum required receiver sensitivity is − 75 dBm. Transmit and receive DSSS PPDU. Table 6.1 provides a summary of access technologies used for various wireless systems. In order to keep the same transmitted bandwidth with a data rate of 2 Mbps, four-level frequency shift-keying is employed. Figure 21.8 shows the PPDU frame, which consists of a PLCP preamble, PLCP header, and MAC protocol data unit (MPDU). A particularly effective and low-cost strategy is to jointly detect several of the cycle frequencies using CF detection. Morse Telegraph Key History & Development. The resulting data stream has a higher data rate than the data itself. Here is the protocol on both the sender and receiver side. However, the implication of such large bandwidths is phase incoherency. More Essential Radio Topics: In the transmitter, the input data stream is XORed with the PN signal before transmission. In the transmitter stage, the baseband data signal m(t) is spread using PN-Sequence c(t).Then, the resultant spread signal s(t) is applied to (BPSK) modulator. The PMD sublayer takes the binary information bits from the PLCP protocol data unit (PPDU) and converts them into RF signals by using modulation and DSSS techniques (see Figure 21.7). In case of direct sequence spread spectrum (DSSS), each bit in the original signal is denoted by multiple bits in the transmitted signal using a spreading code. Fourteen frequency channels are defined for operation across the 2.4 GHz frequency band. Commonly spreading codes may be 64 bits, or even 128 bits long to provide the required performance. From: Telecommunications Engineer's Reference Book, 1993, Ido Dubrawsky, in Eleventh Hour Security+, 2010. In order to visualise how the direct sequence spread spectrum process operates, the easiest method is to show an example of how the system actually operates in terms of data bits, and how the data is recovered from the DSSS, direct sequence spread spectrum signal. In the receiver the received signal is XORed with the PN stream to recover the original data stream; this is equivalent to correlation with the known PN sequence. RF circulator     Unlike DS systems, FH systems can cover a spectrum of several GHz, which is challenging for DS systems still in today's technology. DSSS is a form of CDMA (see Chapter 4). Normal DSSS decoding compares the signal to expected values, using a correlation algorithm that multiplies the two and gives a score. In order to spread the sequence using DSSS, an M-chip-long spreading sequence is selected.The M chips of the spreading sequence are replicated N times, with each of the replications being multiplied by one of the N symbols (see Figure 3.2).The symbols of this spread sequence are referred to as “chips.” Direct Sequence Spread Spectrum, DSSS Includes: ▶︎ Computer Products. Phase locked loops     Also, flexibility in system design and deployment are significantly improved since interference to others is not a problem. RF mixing     The start of frame delimiter (SFD) contains information that marks the start of the PPDU frame. This research developed and validated a generic simulation for a direct sequence spread spectrum (DSSS), using differential phase shift keying (DPSK) and phase shift keying (PSK) modulations, providing the flexibility for assessing intentional interference effect using DSSS quadrature phase shift keying receiver (QPSK) with matched filtering On the other hand, FDMA and TDMA radios must be carefully assigned a frequency or time slot to assure that there is no interference with other similar radios. With the signal obtained and transmitted, it needs to be decoded within the remote receiver: In this way it can be seen that the original data is recovered exactly by using the same spreading or chip code. Modulation types & techniques     This would have appeared as noise in the system. The primary advantage of DS-CDMA is its ability to tolerate a fair amount of interfering signals compared to FDMA and TDMA that typically cannot tolerate any such interference(Figure 6.7). C. Direct Sequence Spread Spectrum System . Direct sequence spread spectrum requires the overall channel (including, where relevant, equalisation in the receiver) to have approximately unity gain, pure delay characteristics over the whole signal bandwidth. The SYNC field is 128 bits (symbols) in length and contains a string of 1s which are scrambled prior to transmission. In fast frequency hopping, on the other hand, each data symbol is transmitted over multiple hops and the hopping rate is an integer multiple of the data symbol rate. RF attenuators     A pseudo-random code is generated, matching the anticipated signal. The patch is included in the CC1310 SDK and can be setup to be imported in the final application by Code Export When there is no DSSS signal present at the input to the demodulator there is none-the-less a noise output. Receiver sensitivity must be better than − 80 dBm for a 3% frame error rate. The output signal of … Direct-sequence spread spectrum (DSSS) is one of two types of spread spectrum radio, the other being frequency-hopping spread spectrum.. DSSS Transmission. Table 21.5 lists the DSSS channels used in different parts of the world. But instead of a pseudorandom binary sequence (PRBS), an 11-bit Barker code is used as the spreading sequence. Note: Electronics Notes receives a small commission on sales at no cost to you. In the DSSS system, the entire bandwidth of the RF carrier is made available to each user. The use of direct sequence spread spectrum is a powerful principle and has many advantages. This helps to increase the signal's resistance to interference. PSD and SCF estimates for captured 802.11b signals. To decode the signal and receive the original data, the CDMA signal is first demodulated from the carrier to reconstitute the high speed data stream. Both DSSS and FHSS use the 2.4 GHz ISM band (2.4–2.4835 GHz). ? In the transmitter stage, the baseband data signal m(t) is spread using PN-Sequence c(t) [6]. applications, but have seen more use in commercial applica-tions in later years [1]. The ITU CRC-16 error detection algorithm is used to protect the signal, service, and length field. After the despreading or removal of the direct-sequence modulation in the receiver, the information bandwidth is restored, while the unintentional and intentional interference is substantially reduced. Why is it said that the signal is recovered from below the noise floor ? The SFD specified is common for all IEEE 802.11 DSSS radios. Maximum allowable transmit power. In anti-jamming (AJ) applications, the transmitter introduces an unpredictable element into the modulation of the signal, known also to the receiver but kept secret from opponents, as in stream ciphering. Direct sequence spread spectrum requires the overall channel (including, where relevant, equalisation in the receiver) to have approximately unity gain, pure delay characteristics over the whole signal bandwidth. We use cookies to help provide and enhance our service and tailor content and ads. The received signal is mixed with a local carrier to recover the spread digital signal. A redundant bit pattern (known as a chipping code) is generated for each bit transmitted. Had another code been used to regenerate the CDMA spread spectrum signal, then it would have resulted in a random sequence after de-spreading. Check out our book shop for essential reading and reference on electronics related topics: OFDM     There are three technical issues are particularly relevant; frequency allocation, frequency roaming, and frequency interference. The DSSS approach is the basis to implementation of the direct sequence code division multiple access (DS-CDMA) technique introduced by Qualcom. However, because DSSS floods the channel it is using, it is also more vulnerable to interference from EM devices operating in the same range.
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