Fundamentals of Wireless Technology

Windows 10

Ministry of Transport of the Russian Federation
State educational institution
Higher professional education
Volga State Academy of Water Transport

Department of Informatics, Control Systems and Telecommunications

Coursework on the topic:
Spread spectrum modulation. Direct spread spectrum "

Completed
student of group Р-312
Aminov A.R.

Checked
Preobrazhensky A.V.

N.Novgorod
2009

Spread spectrum modulation.
Widespread distribution wireless networks, the development of hotspot infrastructure, the emergence of mobile technologies with an integrated wireless solution (Intel Centrino) has led to the fact that end users (not to mention corporate clients) began to pay more and more attention to wireless solutions. Such solutions are considered, first of all, as a means of deploying mobile and fixed wireless local area networks and a means of online access to the Internet. However, the end user who is not a network administrator is generally not well versed in network technologiestherefore it is difficult for him to make a choice when buying a wireless solution, especially given the variety of products on offer today.
Rapid development of technology wireless led to the fact that users, not having time to get used to one standard, are forced to switch to another, offering even higher transfer rates. We are, of course, talking about a family of wireless communication protocols known as IEEE 802.11, which includes the following protocols: 802.11, 802.11b, 802.11b +, 802.11a, 802.11g. Recently, they began to talk about the expansion of the 802.11g protocol.
Different types of wireless networks differ from each other both in range, and supported connection speeds, and data encoding technology. So, the IEEE 802.11b standard provides for a maximum connection speed of 11 Mbit / s, the IEEE 802.11b + standard - 22 Mbit / s, the IEEE 802.11g and 802.11a standards - 54 Mbit / s.
The future of the 802.11a standard is uncertain. Surely in Russia and Europe, this standard will not be widely used, and in the United States, where it is now used, most likely, in the near future there will be a transition to alternative standards. But the new 802.11g standard has significant chances to gain recognition around the world. Another advantage of the new 802.11g standard is that it is fully compatible with the 802.11b and 802.11b + standards, that is, any device that supports the 802.11g standard will work (albeit at lower connection speeds) in 802.11b / b + networks. , and a device supporting the 802.11b / b + standard is used in 802.11g networks, albeit at a lower connection speed.
The compatibility of the 802.11g and 802.11b / b + standards is due, firstly, to the fact that they assume the use of the same frequency range, and secondly, that all modes provided in the 802.11b / b + protocols are implemented in the 802.11 standard g. Therefore, the 802.11b / b + standard can be viewed as a subset of the 802.11g standard.
Physical layer of the 802.11 protocol
It is advisable to start the review of the protocols of the 802.11b / g family with the 802.11 protocol, which, although not found in its pure form, is at the same time the progenitor of all other protocols. The 802.11 standard, like all other standards of this family, provides for the use of the frequency range from 2400 to 2483.5 MHz, that is, the frequency range of 83.5 MHz, which, as will be shown below, is divided into several frequency subchannels.
Spread spectrum technology
All 802.11 wireless protocols are based on Spread Spectrum (SS) technology. This technology implies that the initially narrow-band (in terms of spectrum width) useful information signal during transmission is transformed in such a way that its spectrum is much wider than the spectrum of the original signal. That is, the signal spectrum is "smeared" over the frequency range. Simultaneously with the broadening of the signal spectrum, a redistribution of the spectral energy density of the signal occurs - the signal energy is also "smeared" over the spectrum. As a result, the maximum power of the converted signal is significantly lower than the power of the original signal. In this case, the level of the useful information signal can literally be compared with the level of natural noise. As a result, the signal becomes, in a sense, "invisible" - it is simply lost at the level of natural noise.
Actually, it is precisely in the change in the spectral energy density of the signal that the idea of \u200b\u200bspectrum broadening lies. The fact is that if we approach the problem of data transmission in the traditional way, that is, as it is done in radio broadcasting, where each radio station is assigned its own broadcasting range, then we will inevitably face the problem that in the limited radio range intended for shared use it is impossible "Fit" everyone. Therefore, it is necessary to find a way of transmitting information in which users could coexist in the same frequency range and not interfere with each other. This is exactly what the spectrum broadening technology solves.
Benefits of Spread Spectrum Systems
- High noise immunity. With a limited bandwidth of the spectral density of the interference, the signal-to-noise ratio increases by G p \u003d Pw / P times, where P is the original signal bandwidth, P w is the signal bandwidth after spectrum spreading, G p is the spectrum spreading factor. If the interference spectrum is uniform (white noise), the signal-to-noise ratio does not improve.
- Communication confidentiality. The message cannot be read without knowing the spread spectrum algorithm.
- Possibility of simultaneous transmission of many messages on a single carrier frequency in a code division multiplexing system ( CDMA (English Code Division Multiple Access) - multiple access with code division.
Traffic channels with this method of dividing the medium are created by assigning each user a separate numerical code that spreads across the entire bandwidth. There is no time division, all subscribers constantly use the entire channel bandwidth. The frequency band of one channel is very wide, the broadcasting of subscribers is superimposed on each other, but since their codes are different, they can be differentiated.
Code division multiple access technology has been known for a long time. In the USSR, the first work on this topic was published back in 1935 by D.V. Ageev.)
- Low-power signal transmission capability. The signal energy is kept high by increasing the signal duration. Energy secrecy of communication is provided. The signal is not detected but is perceived as noise.
- High time resolution (the wider the spectrum, the steeper the front of the signal). The moment of the beginning of the signal is determined very accurately, which is important for distance measurement systems by signal transit time and for synchronizing the transmitter and receiver.
Most common spread spectrum techniques
- Direct spread spectrum (direct sequencing) using a binary pseudo-random sequence (PRS) modulating the signal. The spectrum width is limited by the minimum technically feasible duration of an elementary PSP symbol. The spectrum expands to tens of megahertz.
- Carrier hopping (frequency hopping).Usually M-ary FSK is used. M symbols correspond to M frequencies spaced from each other by an intervalD f. The center frequency f 0 of this range changes in hops under the control of the PSP in the hopping band several times during the transmission of one message symbol (fast hopping) or with an interval equal to the duration of several symbols (slow hopping). Frequency hopping makes it difficult to maintain signal coherence. Therefore, demodulation is usually incoherent. To ensure orthogonality of signals, the distance between frequencies must satisfy the conditionD f \u003d m / T s, m is an integer. The spectrum can expand to several gigahertz: the spectrum spreading factor is higher than with direct spreading.
Direct spread spectrum
In potential encoding, information bits - logical zeros and ones - are transmitted by rectangular voltage pulses. A rectangular pulse of duration T has a spectrum whose width is inversely proportional to the pulse duration. Therefore, the shorter the duration of the information bit, the larger the spectrum is occupied by such a signal.
In order to deliberately broaden the spectrum of an initially narrowband signal in DSSS technology, a sequence of so-called chips is literally embedded in each transmitted information bit (logical 0 or 1). If the information bits are logical zeros or ones, when potential information coding can be represented as a sequence rectangular pulses, then each individual chip is also a rectangular pulse, but its duration is several times less than the duration of the information bit. The sequence of chips is a sequence of rectangular pulses, that is, zeros and ones, but these zeros and ones are not informational. Since the duration of one chip is n times less than the duration of the information bit, then the spectrum width of the converted signal will be n times larger than the spectrum width of the original signal. In this case, the amplitude of the transmitted signal will decrease n times.
Chip sequences embedded in information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and difficult to distinguish from natural noise.
How to broaden the signal spectrum and make it indistinguishable from natural noise is understandable. For this, in principle, you can use an arbitrary (random) chip sequence. However, the question arises: how to receive such a signal? After all, if it becomes noise-like, then it is not so easy, if not impossible, to extract a useful information signal from it. It turns out that it is possible, but for this it is necessary to select the chip sequence accordingly. Chip sequences used to spread the signal spectrum must satisfy certain autocorrelation requirements. The term autocorrelation in mathematics means the degree of similarity of a function to itself at different points in time. If we choose such a chip sequence for which the autocorrelation function will have a pronounced peak only for one moment in time, then such an information signal can be isolated at the noise level. For this, the received signal is multiplied by the same chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal becomes narrowband again, so it is filtered in a narrow frequency band and any interference that falls into the band of the original wideband signal, after multiplying by the chip sequence, on the contrary, becomes wideband and is cut off by filters, and only part of the interference falls into the narrow information band, by the power is significantly less than the interference acting at the input of the receiver.
Basic requirements for PSP
- The unpredictability of the appearance of signs 1 and 0, due to which the signal spectrum becomes uniform, and the determination of the algorithm for the formation of the bandwidth by its section of limited length is impossible.
- Availability of a large set of different bandwidths of the same length for building code division systems.
- Good correlation properties of PSP, described by the functions of autocorrelation (ACF) and cross-correlation (PCF), periodic and aperiodic.
Characteristics of pseudo-random sequences (PSP)
The characteristics of the PSP are the functions of autocorrelation (ACF) and cross-correlation (PCF), periodic and aperiodic. FAC and FVK are calculated by counting the difference in the number of coinciding and non-coinciding bits of the compared PSP at shifts of one of them.
Periodic FAK and FVK
etc.................

Method frequency hopping spectrum spreading (FHSS - Frequency Hopping Spread Spectrum) is based on constant carrier hopping over a wide frequency range.

The carrier frequency F1, ..., FN changes randomly after a certain period of time, called cut-off period (chip) , in accordance with the selected algorithm for generating a pseudo-random sequence. Modulation (FSK or PSK) is applied at each frequency. Transmission on one frequency is carried out for a fixed time interval during which a certain portion of data (Data) is transmitted. At the beginning of each transmission period, sync bits are used to synchronize the receiver with the transmitter, which reduce the useful transmission rate.

There are 2 spread spectrum modes depending on the carrier change rate:

· Slow spreading of the spectrum - several bits are transmitted in one cutoff period;

· Fast spreading of the spectrum - one bit is transmitted over several cut-off periods, that is, it is repeated several times.

In the first case data transfer periodless chip transmission period, in the second - more.

Fast spectrum spreading provides more reliable data transmission in the presence of interference due to multiple repetitions of the value of the same bit at different frequencies, but is more difficult to implement than slow spreading.

Forward sequential spreading

Direct Sequence Spread Spectrum (DSSS) is as follows.

Each "one" bit in the transmitted data is replaced by a binary sequence from Nbit called expanding sequence and the "zero" bit is encoded by the inverse value of the spreading sequence. In this case, the transmission clock rate increases by Ntimes, therefore, the signal spectrum also expands by Ntime.

Knowing dedicated to wireless transmission (communication lines) frequency range, you can appropriately select the baud rate and value Nto make the signal spectrum fill the entire range.

The main goal of DSSS coding, like FHSS, is to improve noise immunity.

Chip speedIs the transmission rate of the resulting code.

Expansion ratio- number of bits Nin an expanding sequence. Usually Nis in the range from 10 to 100. The more N, the larger the spectrum of the transmitted signal.

DSSS is less immune to interference than fast spread spectrum.

Code division multiple access

Spectrum spreading methods are widely used in cellular networks, in particular, when implementing the CDMA access method (Code Division Multiple Access) - code division multiple access ... CDMA can be used in conjunction with FHSS, but more often with DSSS in wireless networks.

Each node in the network uses its own spreading sequence, which is chosen so that the receiving node can extract data from the summed signal.

The advantage of CDMA lies in the increased security and secrecy of data transmission: without knowing the spreading sequence, it is impossible to receive a signal, and sometimes even detect its presence.

WiFi technology. WiMax technology. Wireless personal networks. Bluetooth technology. ZigBee technology. Wireless sensor networks. Comparison of wireless technologies.

WiFi technology

Wireless LAN (WLAN) technology is defined by the IEEE 802.11 protocol stack, which describes a physical layer and a data link layer with two sub-layers: MAC and LLC.

At the physical layer, several specification options are defined that differ:

· Used frequency range;

· Coding method;

· Data transfer rate.

Options for building wireless LANs of the 802.11 standard, called WiFi.

IEEE 802.11 (Option 1):

· Transmission medium - infrared radiation;

· Transmission in line of sight;

3 variants of radiation propagation are used:

Omni-directional antenna;

Reflection from the ceiling;

Focal directional radiation ("point-to-point").

IEEE 802.11 (Option 2):

Coding method - FHSS: up to 79 frequency bands wide

1 MHz, the duration of each of which is 400 ms (Figure 3.49);

· At 2 states of the signal, the throughput of the transmission medium is 1 Mbit / s, at 4 - 2 Mbit / s.

IEEE 802.11 Option 3:

Transmission medium - microwave range 2.4 GHz;

· Coding method - DSSS with 11-bit code as a spreading sequence: 10110111000.

IEEE 802.11a:

1) frequency range - 5 GHz;

2) transmission rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbit / s;

3) coding method - OFDM.

Disadvantages:

· Too expensive equipment;

· Frequencies in this range are subject to licensing in some countries.

IEEE 802.11b:

1) frequency range - 2.4 GHz;

2) transmission rate: up to 11 Mbps;

3) coding method - modernized DSSS.

IEEE 802.11g:

1) frequency range - 2.4 GHz;

2) maximum speed transmission: up to 54 Mbps;

3) coding method - OFDM.

In September 2009, the IEEE 802.11n standard was approved. Its application will increase the data transfer rate by almost four times compared to devices of the 802.11g standards. In theory, 802.11n is capable of delivering data rates of up to 600 Mbps. The range of IEEE 802.11 wireless networks is up to 100 meters.

WiMax technology

The technology of wireless broadband access with high bandwidth WiMax is represented by the IEEE 802.16 group of standards and was originally intended for building long (up to 50 km) wireless networks belonging to the class of regional or metropolitan networks.

The IEEE 802.16 or IEEE 802.16-2001 (December 2001) standard, which is the first point-to-multipoint standard, was focused on working in the 10 to 66 GHz spectrum and, as a result, required the transmitter and receiver to be in line of sight, which is a significant disadvantage, especially in urban conditions. According to the described specifications, the 802.16 network could serve up to 60 clients at a T-1 channel speed (1.554 Mbps).

Later, the standards IEEE 802.16a, IEEE 802.16-2004 and IEEE 802.16e (mobile WiMax) appeared, in which the requirement of line of sight between the transmitter and the receiver was removed.

The main parameters of the listed standards of the WiMax technology.

Consider the main technology differencesWiMax from WiFi.

1. Low mobility.The standard was originally developed for long-range fixed wireless communications and allowed for user mobility within a building. Only in 2005 was the IEEE 802.16e standard developed, focused on mobile users... Currently, the development of new specifications 802.16f and 802.16h for access networks supporting the operation of mobile (mobile) clients at a speed of up to 300 km / h.

2. Using better quality radios and transmittersleads to higher network construction costs. 3. Long distancefor data transmission, a number of specific problems need to be solved: the formation of signals of different power, the use of several modulation schemes, information security problems.

4. Large number of usersin one cell.

5. Higher throughputprovided to the user.

6. High quality of service for multimedia traffic.

It was originally believed that the IEEE 802.11 – mobile Ethernet analog, 802.16 - wireless stationary analog cable TV ... However, the emergence and development of WiMax (IEEE 802.16e) technology to support mobile users makes this claim controversial.

Spread spectrum techniques

Initially, spread spectrum methods (PC or SS - Spread-Spectrum) were used in the development of military control and communication systems. During World War II, spread spectrum was used in radar to combat intentional interference. In recent years, the development of this technology is explained by the desire to create effective radio communication systems to ensure high noise immunity when transmitting narrow-band signals through noisy channels and complicating their interception.

A communication system is a spread spectrum system in the following cases:

The frequency band used for transmission is much wider than the minimum necessary for the transmission of current information. In this case, the energy of the information signal expands over the entire bandwidth with a low signal-to-noise ratio, as a result of which the signal is difficult to detect, intercept or prevent its transmission by introducing interference. Although the total signal power can be large, the signal-to-noise ratio in any frequency range is low, which makes the spread spectrum signal difficult to detect in radio communications and, in the context of steganographic information hiding, difficult to distinguish by humans.

Spreading is performed using a so-called spreading (or code) signal, which is independent of the information being transmitted. The presence of signal energy in all frequency bands makes the spread spectrum radio signal resistant to interference, and the information embedded in the container using the spread spectrum method is resistant to its elimination or removal from the container. Compression and other types of attacks on the communications system can remove signal energy from some portions of the spectrum, but since the latter was spread over the entire range, there is enough data in other bands to recover information. As a result, if, of course, you do not disclose the key that was used to generate the code signal, the probability of information retrieval by unauthorized persons is significantly reduced.

Recovery of the primary information (that is, "spectrum narrowing") is carried out by comparing the received signal and a synchronized copy of the code signal.

There are three main methods of spreading spectrum in radio communications:

Using direct PSP (RSPP);

Using frequency hopping;

By means of compression using chirp.

When spreading the spectrum with a direct sequence, the information signal is modulated by a function that takes pseudo-random values \u200b\u200bwithin the specified limits, and multiplied by a time constant - the frequency (rate) of chips (chips). This pseudo-random signal contains components at all frequencies, which, when spread, modulate the signal energy over a wide range.

In frequency hopping spread spectrum, the transmitter instantly changes one carrier frequency to another. The secret key is the pseudo-random frequency variation law.

In chirp compression, the signal is modulated by a function whose frequency changes over time.

Obviously, any of these methods can be extended to use in the spatial domain when constructing steganographic systems.

Let's consider one of the variants of the RSPP method implementation, the authors of which are J.R. Smith and V.O. Comiskey. The modulation algorithm is as follows: each bit of the message is represented by some basic function, the dimension multiplied, depending on the bit value (1 or 0), by +1 or -1:

(11.7)

The modulated message received in this case is pixel by pixel added to the container image, which is used as a grayscale image in size. The result is a stegano image, when .

In order to send a high-power radio signal in the microwave range, an expensive transmitter with an amplifier and an expensive large-diameter antenna are needed. To receive a low power signal without interference, you also need an expensive large antenna and an expensive receiver with an amplifier.

This is the case when using a conventional "narrowband" radio signal, when transmission occurs at one specific frequency, or rather, in a narrow band of the radio spectrum surrounding this frequency (frequency channel). The picture is further complicated by the various mutual interference between high-power narrowband signals transmitted close to each other or at close frequencies. In particular, a narrowband signal can simply be jammed (accidentally or intentionally) by a transmitter of sufficient power tuned to the same frequency.

It was this vulnerability to conventional radio signal interference that gave rise to the development, first for military applications, of a completely different principle of radio transmission, called broadband signal technology, or noise-like signal (both variants of the term correspond to the abbreviation Spread Spectrum). After many years of successful defense use, this technology has also found civilian use, and it is in this capacity that it will be discussed here.

It was found that, in addition to their characteristic properties (intrinsic noise immunity and low level of generated interference), this technology proved to be relatively cheap when mass produced. Economics comes from the fact that all the complexity of broadband technology is programmed into several microelectronic components ("chips"), and the cost of microelectronics in mass production is very low. As for the rest of the components of broadband devices - microwave electronics, antennas - they are cheaper and simpler than in the usual "narrowband" case, due to the extremely low power of the radio signals used.

The idea behind Spread Spectrum is that a much wider bandwidth is used to transmit information than is required for conventional (in a narrow frequency channel) transmission. Two fundamentally different methods of using such a wide frequency band have been developed - the Direct Sequence Spread Spectrum (DSSS) method and the Frequency Hopping Spread Spectrum (FHSS) method. Both of these methods are covered by the 802.11 (Radio-Ethernet) standard.

State of the art wireless communication is determined by the situation with the IEEE 802.11 standard. The standard is developed and improved by working group over wireless LAN (Working Group for Wireless Local Area Networks) of the Institute of Electrical and Electronic Engineers (IEEE) standards committee chaired by Vic Hayes of Lucent Technologies. The group has about a hundred members with a casting vote and about fifty with an advisory vote; they represent virtually all OEMs as well as research centers and universities. The group meets in plenary four times a year and makes decisions on improving the standard.

The standard defines one type of MAC layer media access protocol and three different protocols for physical (PHY) channels.

The MAC layer defines the basic components of the network architecture and the list of services provided by this layer. There are two typical wireless network architectures:

Independent “ad-hoc” configuration where stations can communicate directly with each other. The network area and functionality are limited.

An infrastructure configuration in which stations communicate through an access point, either autonomous or connected to a wired network. The standard defines the radio channel interface between stations and the access point. Access points can be connected to each other using radio bridges or segments of the cable network.

The standard fixes the protocol for using a single transmission medium, called Carrier Sense Multiple Access Collision Avoidance (CSMA / CA). The likelihood of conflicts for wireless nodes is minimized by first sending all nodes a short message (ready to send, RTS) about the destination and the duration of the upcoming transmission. Nodes delay transmission for a time equal to the advertised message duration. The receiving station replies to the RTS with a send (CTS), by which the transmitting node knows if the medium is free and if the node is ready to receive. Upon receipt of the data packet, the node transmits an error-free acknowledgment (ACK). If no ACK is received, the data packet will be retransmitted.

The specification provided by the standard prescribes the division of data into packets provided with control and address information. This information, which takes about 30 bytes, is followed by an information block of up to 2048 bytes. This is followed by a 4-byte information block CRC. The standard recommends using packets of 400 bytes for a physical channel such as FHSS and 1500 or 2048 for a DSSS channel.

The standard provides for data security, including authentication (to verify that a node entering the network is authorized in it) and data encryption using the RC4 algorithm with a 40-bit key. For laptop computers the standard provides for a power-saving mode: transferring the device to a “dormant” mode and bringing it out of this state for a short time, necessary to receive a service signal from network nodes that start transmission. There is also a roaming mode that allows mobile subscriber move between access points without losing connection.

Spread spectrum

At the physical level, the standard allows the use of one of two types of radio channels and one type of infrared channel. Both types of radio channels use spread spectrum technology, which results in a decrease in the average signal power spectral density due to the distribution of energy in a frequency band wider than necessary to achieve a given transmission rate. This technology reduces the level of generated interference and provides improved reception immunity.

The first type of radio channel is Frequency Hopping Spread Spectrum (FHSS) Radio PHY. The transmission rate is 1 Mbps (optional 2 Mbps). The 1 Mbps version uses two-level Gaussian frequency modulation (2GFSK), while the 2 Mbps version uses four-level frequency modulation (4GFSK). At 1 Mbit / s, the signal frequency changes for a message symbol duration of 1 μs according to the Gaussian law from the nominal value to +170 kHz and returns to the nominal value. To transmit zero, the signal frequency is changed by –170 kHz. For 2 Mbps, there are four levels of frequency offset (+225, +75, –75, –225 kHz), so each chip (character) carries two message bits. The signal spectrum width for this modulation is 1 MHz, regardless of the transmission rate. This makes it possible to use 79 frequency positions for transmission in the range from 2402 to 2480 MHz in 1 MHz steps. To expand the spectrum, the signal frequency changes according to the pseudo-random law at least once every 400 ms.

The second type of radio channel is Direct Sequence Spread Spectrum (DSSS) Radio PHY. This option provides for transmission at speeds of 1 and 2 Mbit / s. At a transmission rate of 1 Mbps, Binary Phase Shift Keying (BPSK) is used. A one bit is represented by an 11-element Barker code of the form 11100010010, and a zero bit is represented by an inverse Barker code. The elementary symbols of the Barker code do not carry information, the bits are transmitted at once by the entire Barker code - direct or inverse. This allows you to give the signal noise properties that provide noise immunity. The spectrum width of such a signal is 22 MHz. For a speed of 2 Mbps, the standard provides for Quadrature Phase Shift Keying (QPSK). In this case, two bits are transmitted for the duration of the message symbol. This requires not two, but four different signals. Therefore, together with the main carrier wave, an additional one is used, which is phase-shifted by 90 °. The phase of each of these oscillations is controlled by the direct or inverse Barker sequence, and both oscillations are added. Thus, for the duration of the symbol, the signal has four degrees of freedom, allowing two bits to be transmitted. This doubles the transmission rate while maintaining the same bandwidth as binary transmission. The DSSS signal uses one of the 14 overlapping frequency bands defined by the standard in a total bandwidth of 83.5 MHz.

For the infrared channel (Infrared PHY), the standard provides for a speed of 1 Mbit / s (optional 2 Mbit / s) with pulse position modulation. This type of channel is not of great interest, since the transmission range provided by the standard does not exceed 20 m.

There are several different spread spectrum technologies, but to further understand the 802.11 protocol, we only need to learn more about Direct Sequence Spread Spectrum (DSSS).

DSSS technology

In potential encoding, information bits - logical zeros and ones - are transmitted by rectangular voltage pulses. A rectangular pulse of duration T has a spectrum whose width is inversely proportional to the pulse duration. Therefore, the shorter the duration of the information bit, the larger the spectrum is occupied by such a signal.

In order to deliberately broaden the spectrum of an initially narrowband signal in DSSS technology, a sequence of so-called chips is literally embedded in each transmitted information bit (logical 0 or 1). If information bits - logical zeros or ones - during potential coding of information can be represented as a sequence of rectangular pulses, then each individual chip is also a rectangular pulse, but its duration is several times less than the duration of the information bit. The sequence of chips is a sequence of rectangular pulses, that is, zeros and ones, but these zeros and ones are not informational. Since the duration of one chip is n times less than the duration of the information bit, then the spectrum width of the converted signal will be n times larger than the spectrum width of the original signal. In this case, the amplitude of the transmitted signal will also decrease n times.

Chip sequences embedded in information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and difficult to distinguish from natural noise.

How to broaden the signal spectrum and make it indistinguishable from natural noise is understandable. For this, in principle, you can use an arbitrary (random) chip sequence. However, the question arises: how to receive such a signal? After all, if it becomes noise-like, then it is not so easy, if not impossible, to extract a useful information signal from it. It turns out that it is possible, but for this it is necessary to select the chip sequence accordingly. Chip sequences used to spread the spectrum of the signal must satisfy certain autocorrelation requirements. The term autocorrelation in mathematics means the degree of similarity of a function to itself at different points in time. If we choose such a chip sequence for which the autocorrelation function will have a pronounced peak only for one moment in time, then such an information signal can be isolated at the noise level. For this, the received signal is multiplied by the same chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal becomes narrowband again, so it is filtered in a narrow frequency band and any interference that falls into the band of the original wideband signal, after multiplying by the chip sequence, on the contrary, becomes wideband and is cut off by filters, and only part of the interference falls into the power is much less than the interference acting at the input of the receiver (Fig. 7.1).

Barker codes

There are quite a lot of chip sequences that meet the specified autocorrelation requirements, but the so-called Barker codes are of particular interest to us, since they are used in the 802.11 protocol.

Barker codes have the best noise-like properties among the known pseudo-random sequences, which led to their widespread use.

The 802.11 family protocols use the 11-chip Barker code (11100010010).

In order to transmit a signal, a logical one is transmitted by a direct Barker sequence, and a logical zero - by an inverse sequence.

Speed \u200b\u200b1 Mbps

The 802.11 standard provides two speed modes: 1 and 2 Mbps. To encode data at the physical layer, the DSSS method with 11-chip Barker codes is used. At an information rate of 1 Mbit / s, the repetition rate of individual chips in the Barker sequence is 11 × 106 chips / s, and the spectrum width of such a signal is 22 MHz. Considering that the width of the frequency range is 83.5 MHz, we find that in total in this frequency range it is possible to fit 3 non-overlapping frequency channels... The entire frequency range, however, is usually divided into 11 overlapping frequency channels of 22 MHz, spaced 5 MHz apart. For example, the first channel covers the frequency range from 2400 to 2423 MHz and is centered on the frequency of 2412 MHz. The second channel is centered on 2417 MHz, and the last channel, 11, is centered on 2462 MHz. In this view, the first, sixth and 11 channels do not overlap with each other and have a 3 megahertz gap relative to each other. These three channels can be used independently of each other.

A Differential Binary Phase Shift Key (DBPSK) is used to modulate a sinusoidal carrier signal (a process required to fill the carrier signal). In this case, the information is encoded due to the phase shift of the sinusoidal signal with respect to the previous signal state. Binary Phase Modulation provides two possible phase shifts, 0 and π. Then a logical zero can be transmitted in-phase signal (phase shift is 0), and one - a signal that is phase-shifted by π.

Speed \u200b\u200b2 Mbps

The data rate of 1 Mbit / s is mandatory in the IEEE 802.11 standard (Basic Access Rate), but an optional speed of 2 Mbit / s (Enhanced Access Rate) is also possible. To transmit data at this rate, the same DSSS technology is used with 11-chip Barker codes, but to modulate the carrier wave, Differential Quadrature Phase Shiftey is used. With relative quadrature phase modulation, the phase shift can take four different meanings: 0, π / 2, π and 3π / 2. Using four different signal states, it is possible to encode a sequence of two information bits (dibit) in one discrete state and thereby double the information transmission rate. For example, dibit 00 may correspond to a phase shift of 0; dibet 01 - phase shift equal to π / 2; dibit 11 - phase shift equal to π; dibit 10 - phase shift equal to 3π / 2.

In conclusion of the examination of the physical layer of the 802.11 protocol, we note that at an information rate of 2 Mbit / s, the repetition rate of individual chips of the Barker sequence remains the same, that is, 11 × 10 6 chips / s, and therefore the bandwidth of the transmitted signal does not change either.

7.2 7.2 Physical layer of the 802.11b / b + protocol

The IEEE 802.11b protocol, adopted in July 1999, is a kind of extension basic protocol 802.11 and in addition to speeds of 1 and 2 Mbps, it provides speeds of 5.5 and 11 Mbps. For operation at speeds of 1 and 2 Mbit / s, spectrum spreading technology using Barker codes is used, and for rates of 5.5 and 11 Mbit / s, the so-called Complementary Code Keying (CCK) codes are used.

CCK sequences

Complementary codes or CCK-sequences have the property that the sum of their autocorrelation functions for any cyclic shift other than zero is always zero.

The IEEE 802.11b standard deals with complex complementary 8-chip sequences defined on many complex elements.

Here it is worth making a small lyrical digression, so as not to alienate the reader with the complexity of the mathematical apparatus used. Complex number math can evoke a ton of negative memories, associating with something completely abstract. But in this case, everything is quite simple. Complex signal representation is just a convenient mathematical tool for representing a phase modulated signal.

Using a set of complex elements (1, –1, j, –j), it is possible to form eight complex numbers that are identical in modulus, but differing in phase. That is, elements of an 8-chip CCK sequence can take one of the following eight values: 1, –1, j, –j, 1 + j, 1 – j, –1 + j, –1 – j. The main difference between CCK sequences and the previously considered Barker codes is that there is not a strictly specified sequence by means of which it was possible to encode either logical zero or one, but a whole set of sequences. Considering that each element of an 8-sip sequence can take one of eight values \u200b\u200bdepending on the phase value, it is clear that 8 8 \u003d 16777216 sequence variants can be combined, however, not all of them will be complementary. But even taking into account the requirement of complementarity, a sufficiently large number of different CCK sequences can be formed. This circumstance makes it possible to encode several information bits in one transmitted symbol and thereby increase the information transmission rate.

Generally speaking, using CCK codes can encode 8 bits per symbol at 11 Mbps and 4 bits per symbol at 5.5 Mbps. Moreover, in both cases, the symbol rate is 1.385 × 10 6 symbols per second (11/8 \u003d 5.5 / 4 \u003d 1.385), and given that each symbol is specified by an 8-chip sequence, we get that in both cases the repetition rate individual chips is 11 × 10 6 chips per second. Correspondingly, the bandwidth of the signal as at the speed of 11 Mbit / s and 5.5 Mbit / s is 22 MHz.

Considering the possible transmission rates of 5.5 and 11 Mbit / s in the 802.11b protocol, we have so far ignored the question of why the speed of 5.5 Mbit / s is needed if the use of CCK sequences allows data transfer at a speed of 11 Mbit / s ... In theory, this is true, but only if you do not take into account the interference environment. In real conditions, the noise level of the transmission channels and, accordingly, the ratio of noise and signal levels may turn out to be such that transmission at a high information rate, that is, when many information bits are encoded in one symbol, may be impossible due to their erroneous recognition. Without going into mathematical details, we only note that the higher the noise level of communication channels, the lower the information transmission rate. At the same time, it is important that the receiver and transmitter correctly analyze the interference environment and choose an acceptable transmission rate.


Similar information.


direct sequence spread spectrum) - wideband direct spread spectrum modulation, is one of the three main spread spectrum methods used today (see spread spectrum methods). It is a wideband radio signal shaping technique in which the original binary signal is converted into a pseudo-random sequence used to modulate the carrier. Used in IEEE 802.11 and CDMA networks for deliberate spreading of the transmitted pulse spectrum.

The Direct Sequence Method (DSSS) can be thought of as follows. The entire used "wide" frequency band is divided into a certain number of subchannels - according to the 802.11 standard of these subchannels 11. Each transmitted bit of information turns, according to a predetermined algorithm, into a sequence of 11 bits, and these 11 bits are transmitted as if simultaneously and in parallel (physically signals transmitted sequentially) using all 11 subchannels. Upon reception, the received bit sequence is decoded using the same algorithm as when encoding it. Another pair of receiver-transmitter can use a different encoding-decoding algorithm, and there can be a lot of such different algorithms.

The first obvious result of using this method is the protection of transmitted information from eavesdropping (a “foreign” DSSS receiver uses a different algorithm and will not be able to decode information not from its own transmitter).

In this case, the ratio of the level of the transmitted signal to the noise level (that is, accidental or intentional interference) is greatly reduced, so that the transmitted signal is already indistinguishable in the general noise. But thanks to its 11-fold redundancy, the receiving device will still be able to recognize it.

Another extremely useful property DSSS devices are that, due to the very low level of their signal power, they practically do not interfere with conventional radio devices (narrowband high power), since these latter take the broadband signal for noise within the acceptable range. And vice versa - conventional devices do not interfere with broadband ones, since their high-power signals "noise" each only in its own narrow channel and cannot completely drown out the entire broadband signal.

The use of broadband technologies makes it possible to use the same part of the radio spectrum twice - with conventional narrowband devices and “over them” - with broadband devices.

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A sequence of so-called chips is embedded in each transmitted information bit (logical 0 or 1). If information bits - logical zeros or ones - during potential coding of information can be represented as a sequence of rectangular pulses, then each individual chip is also a rectangular pulse, but its duration is several times less than the duration of the information bit. The sequence of chips is a sequence of rectangular pulses, that is, zeros and ones, but these zeros and ones are not informational. Since the duration of one chip is n times less than the duration of the information bit, then the width of the spectrum of the converted signal will be n times larger than the width of the spectrum of the original signal. In this case, the amplitude of the transmitted signal will also decrease n times.

Chip sequences embedded in information bits are called noise-like codes (PN-sequences), which emphasizes the fact that the resulting signal becomes noise-like and difficult to distinguish from natural noise.

Chip sequences used to broaden the signal spectrum must satisfy certain autocorrelation requirements. The term autocorrelation in mathematics means the degree of similarity of a function to itself at different points in time. If we choose such a chip sequence for which the autocorrelation function will have a pronounced peak only for one moment in time, then such an information signal can be isolated at the noise level. For this, the received signal is multiplied by the same chip sequence in the receiver, that is, the autocorrelation function of the signal is calculated. As a result, the signal becomes narrowband again, therefore it is filtered in a narrow frequency band and any interference that falls into the band of the original wideband signal, after multiplying by the chip sequence, on the contrary, becomes wideband and is cut off by filters, and only part of the interference falls into the narrow information band, the power is significantly less than the interference acting at the input of the receiver (if a receiver with Boatswain's algorithm is not used).

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