Frequency Hopping Spread Spectrum (FHSS) - presentation. Basic Concepts of Spread Spectrum Systems
SPREAD SPECTRUM SYSTEMS
The term spread spectrum has been used in numerous military and commercial communications systems. In spread spectrum systems, each message carrier signal requires significantly more radio frequency bandwidth than a conventional modulated signal. A wider frequency band allows you to obtain some useful properties and characteristics that are difficult to achieve by other means.
Spread spectrum is a method of generating a spread spectrum signal by using an additional modulation stage to not only broaden the spectrum of the signal, but also reduce its influence on other signals. Additional modulation has nothing to do with the transmitted message.
Broadband systems are used due to the following potential advantages:
Increased noise immunity;
Possibility of providing code division of channels for multiple access based on it in systems using CDMA technology;
Energy secrecy due to the low level of spectral density;
High resolution when measuring distances;
Communication security;
Ability to withstand the effects of intentional interference;
Increased capacity and spectral efficiency in some cellular personal communications systems;
A gradual decrease in communication quality with an increase in the number of users simultaneously occupying the same HF channel;
Low cost of sale;
Availability of modern element base (integrated circuits).
Figure 6.1 – Structure of a direct spread spectrum system
According to the architecture and types of modulation used, spread spectrum systems can be divided into the following main groups.
Pseudo-random sequence (PRS)-based direct spread spectrum, including CDMA systems,
Frequency agility (frequency hopping), including CDMA systems with slow and fast frequency agility,
Carrier Sense Multiple Access (CSMA),
With restructuring of the time position of signals (“jumping” time),
With linear frequency modulation of signals (chip modulation),
With mixed methods of spreading the spectrum.
Direct spectrum expansion using pseudorandom sequences
Figure 6.1 shows a conceptual diagram of a direct spread spectrum system based on pseudo-random sequences (a - PSK signal transmitter with subsequent spectrum, b - baseband spread spectrum transmitter, c - receiver). The first modulator performs phase shift keying (PSK) of the intermediate frequency signal with a binary digital signal of the transmitted message d(t) in non-return to zero (NRZ) format with a symbol frequency f b = 1/T b .
Within one cell of a mobile radio communication system, as a rule, there are several subscribers using the communication simultaneously, each of them using the same carrier frequency RF and occupying the same frequency band RF.
The process of generating spread spectrum signals in multiple access systems occurs in two stages: modulation and spread spectrum (or secondary modulation via PSP). Secondary modulation is carried out using the ideal multiplication operation g(t)s(t). With this multiplication, an amplitude-modulated two-way signal with a suppressed carrier is formed. The first and second modulators can be swapped without changing the potential characteristics of the system.
The spread spectrum signal g(t)s(t) is upconverted to the desired radio frequency. Although up- and down-frequency conversion is an almost necessary process for most systems, it is not a critical step. Therefore, in the future we will assume that the signal g(t)s(t) is transmitted and received at an intermediate frequency, excluding the up and down frequency conversion subsystem from consideration.
Thus, the receiver input receives a sum of M independent spread spectrum signals occupying the same RF band.
The concept of spread spectrum systems through software tuning of the operating frequency is in many ways similar to the concept of direct spread spectrum systems. Here, the binary PSP generator controls the frequency synthesizer, with the help of which a transition (“jump”) is made from one frequency to another of the many available frequencies. Thus, here the effect of spectrum expansion is achieved through pseudo-random tuning of the carrier frequency, the value of which is selected from the available frequencies f1,...,fN, where N can reach values of several thousand or more. If the rate of message tuning (the rate of frequency change) exceeds the message transmission rate, then we have a system with fast frequency tuning. If the tuning rate is less than the message transmission rate, so that several bits are transmitted in the tuning interval, then we have a system with slow frequency tuning.
If an ensemble of uncorrelated PSP signals is selected, then after the spectrum compression operation only the modulated useful signal is retained. All other signals, being uncorrelated, retain broadband and have a spectral width exceeding the cutoff bandwidth of the demodulator filter. Figure 6.2 shows simplified timing and spectral diagrams that qualitatively illustrate the processes of expansion and compression of the signal spectrum. In particular, they lack a carrier signal.
Figure 6.2 - Diagrams for spectrum expansion
In spread spectrum systems, by tuning the operating frequency, the latter remains constant during each tuning interval, but changes abruptly from interval to interval. Transmission frequencies are generated by a digital frequency synthesizer controlled by a code (“words”), arriving in serial or parallel form and containing m binary symbols (bits). Each m-bit word or part thereof corresponds to one of M = 2m frequencies. Although there are M = 2m, m = 2, 3 frequencies available for frequency tuning, not all of them are necessarily used in a particular system. Systems with spectrum expansion by software tuning of the operating frequency are divided into systems with slow, fast and medium tuning speeds.
In systems with slow tuning, the tuning rate fh is less than the message transmission rate fb. Thus, in the tuning interval, two message bits or more (in some systems over 1000 in some systems) can be transmitted before changing to another frequency. In systems with medium tuning speed, the tuning speed is equal to the transmission speed. The most widely used systems are systems with fast and slow adjustment of the operating frequency.
To synchronize receivers when receiving spread spectrum signals, three synchronization devices may be required:
Carrier phase synchronization (carrier recovery);
Symbolic synchronization (clock frequency recovery);
Time synchronization of generators that generate code or pseudo-random sequences.
Time synchronization is provided in two stages, during which the following is performed:
Search (initial, coarse synchronization);
Tracking (precise synchronization).
Figure 6.3 shows block diagrams of the transmitting and receiving parts of the system with frequency tuning.
Figure 6.3 - System with software frequency tuning
The GSM standard uses spectral-efficient Gaussian minimum shift keying (GMSK). The manipulation is called Gaussian because the sequence of information bits before the modulator passes through a low-pass filter (LPF) with a Gaussian characteristic, which results in a significant reduction in the frequency band of the emitted radio signal. The formation of a GMSK radio signal is carried out in such a way that in the interval of one information bit the carrier phase changes by 90°. This is the smallest possible phase change detectable with a given type of modulation. Continuously changing the phase of a sinusoidal signal results in frequency modulation with a discrete change in frequency. The use of a Gaussian filter makes it possible to obtain “smooth transitions” with a discrete change in frequency. The GSM standard uses GMSK modulation with a normalized bandwidth VT = 0.3, where IN- filter bandwidth at level -3 dB, T- duration of 1 bit of digital message. The functional diagram of the modulator is shown in Figure 6.4.
Figure 6.4 - Functional diagram of the modulator
The basis of the GMSK signal shaper is a quadrature (1/Q) modulator. The circuit consists of two multipliers and one adder. The purpose of this circuit is to provide continuous accurate phase modulation. One multiplier changes the amplitude of a sinusoidal oscillation, and the second – a cosine oscillation. The input signal before the multiplier is divided into two quadrature components. The decomposition occurs in two blocks designated "sin" and "cos".
Diagrams illustrating the formation of a GMSK signal are shown in Figure 4.9.
GMSK modulation has the following properties that are preferable for mobile communications:
The envelope is constant in level, which allows the use of efficient transmitting devices with power amplifiers in class C mode;
Compact spectrum at the output of the power amplifier of the transmitting device, which ensures a low level of out-of-band radiation;
Good noise immunity characteristics of the communication channel.
Figure 6.5 - GMSK signal generation
Speech processing. Speech processing in the GSM standard is carried out in order to ensure high quality of transmitted messages and implement additional service capabilities. Speech processing is carried out within the framework of the adopted system of intermittent transmission of speech (Discontinuous Transmission - DTX), which ensures that the transmitter is turned on when the user begins a conversation, and turns it off during pauses and at the end of the conversation. DTX is driven by a Voice Activity Detector (VAD), which detects and separates speech with noise and noise without speech, even when the noise level is comparable to the speech level. The intermittent speech transmission system also includes a device for generating comfortable noise, which is turned on and listened to during pauses in speech when the transmitter is turned off. It has been experimentally proven that turning off background noise at the receiver output in pauses when the transmitter is turned off irritates the subscriber and reduces speech intelligibility, so the use of comfortable noise in pauses is considered necessary. The DTX process in the receiver involves interpolating speech fragments lost due to errors in the channel.
Most modern digital cameras offer users the ability to choose between using the native ISO range and an extended ISO mode.
Experienced photographers understand well which camera functions are really useful and which are practically not used in their work and were added by the manufacturer as a marketing ploy. Beginners, when choosing a camera, can easily get confused by all the variety of options, for example, what is ISO and how to choose the right working ISO range.
Choose between native and extended ISO range
When changing the ISO value on a digital camera, the user adjusts the signal strength, thereby changing the ratio of forced gain to the light-receiving ability of the sensor. There are certain minimum and maximum ISO gain values - this range is called standard. Once the standard values are reduced or exceeded, the camera sensors will not be able to adequately read the data.
Until some time, the upper threshold of the photosensitivity value was considered unshakable, but the rapid development of the hardware and software of modern cameras has allowed us to reach incredible heights. The same applies to the lower value of the ISO range - modern technology can significantly reduce it. In essence, taking photos using an extended ISO range is similar to post-processing a photo on a computer, only this process takes place directly in the camera itself.
How an increased ISO range can affect your photos
Cameras with a large ISO range use sensors with standard light sensitivity, the same as those in conventional cameras. Extended ISO ranges such as ISO 12800, ISO 25600, ISO 51200, ISO 102400 are achieved by using conventional sensors and electronic circuits whose light sensitivity is enhanced using software. It follows that the extended ISO range is nothing more than a marketing ploy.
Claims that a camera can shoot up to ISO 102400 are impressive for budding photographers, but that doesn't mean that when they buy a camera they're buying a sensor that has such high light sensitivity. In fact, these values are achieved thanks to software, and often manifest themselves in low-quality images with a lot of digital noise.
Photos taken at extremely high ISOs will only look good in black and white, negating this benefit of cameras with extended ISO ranges.
An attentive user will definitely notice that the camera in the extended ISO range takes frames in JPEG format, but not in RAW. This is due to the fact that when shooting in RAW mode, a digital negative is formed with minimal processing, as this expands the possibilities for post-processing frames using photo editors. (It is worth mentioning, however, that some manufacturers allow the possibility of using an extended ISO range when shooting in RAW format.)
There may be some benefit to using a larger ISO range for JPEG photographers who do not post-process the images. It is still necessary to take into account that you will have to close your eyes to quality.
Spread Spectrum Methods
Initially, spread spectrum methods (PC or SS - Spread-Spectrum) were used in the development of military control and communications 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 over noisy channels and complicating their interception.
The communication system is a spread spectrum system in the following cases:
The frequency band used during transmission is much wider than the minimum required for transmitting current information. In this case, the energy of the information signal expands over the entire frequency band with a low signal-to-noise ratio, making the signal difficult to detect, intercept, or interfere with its transmission by introducing interference. Although the total signal power can be large, the signal-to-noise ratio in any frequency range is small, making the spread spectrum signal difficult to detect over radio communications and, in the context of information hiding by steganographic techniques, difficult to discern by humans.
Spreading the spectrum 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 ranges 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 attacks on a communications system can remove signal energy from some portions of the spectrum, but because the energy has been spread across the entire spectrum, there is still enough data in other bands to recover the information. As a result, if, of course, you do not disclose the key that was used to generate the code signal, the likelihood of unauthorized persons extracting information is significantly reduced.
Reconstruction of the primary information (that is, “narrowing of the spectrum”) is carried out by comparing the received signal and a synchronized copy of the code signal.
There are three main methods of spectrum extension used in radio communications:
Using direct PSP (RSPP);
Using frequency hopping;
Using compression using linear frequency modulation (LFM).
When spreading the spectrum by direct sequence, the information signal is modulated by a function that takes pseudo-random values within established limits and is multiplied by a time constant - the frequency (speed) of repetition of elementary parcels (signal elements). This pseudo-random signal contains components at all frequencies, which, when expanded, modulate the signal energy over a wide range.
In the frequency hopping spread spectrum method, the transmitter instantly changes one frequency of the carrier signal to another. The secret key in this case is the pseudo-random law of frequency changes.
With chirp compression, the signal is modulated by a function whose frequency varies over time.
It is obvious that any of these methods can be extended to use in the spatial domain when constructing steganographic systems.
Let's consider one of the options for implementing the RSPP method, 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 basis function, dimension, multiplied, depending on the value of the bit (1 or 0), by +1 or -1:
The modulated message received in this case is summed pixel-by-pixel with a container image, which is a halftone image of size . The result is a stegan image, with .
Initially, the spread spectrum method was created for intelligence and military purposes. The main idea of the method is to distribute the information signal over a wide radio band, which ultimately makes it much more difficult to suppress or intercept the signal. The first spread spectrum scheme developed is known as frequency hopping technique. A more modern spread spectrum scheme is the direct serial spread method. Both methods are used in various wireless standards and products.
Frequency Hopping Spread Spectrum (FHSS)
To ensure that radio traffic could not be intercepted or suppressed by narrow-band noise, it was proposed to transmit with a constant change of carrier within a wide frequency range. As a result, the signal power was distributed over the entire range, and listening to a specific frequency produced only a small amount of noise. The sequence of carrier frequencies was pseudo-random, known only to the transmitter and receiver. An attempt to suppress a signal in a certain narrow range also did not degrade the signal too much, since only a small part of the information was suppressed.
The idea of this method is illustrated in Fig. 1.10.
For a fixed period of time, transmission is carried out on a constant carrier frequency. At each carrier frequency, standard modulation methods such as FSK or PSK are used to transmit discrete information. In order for the receiver to synchronize with the transmitter, sync bits are transmitted for a period of time to indicate the start of each transmission period. So the useful speed of this encoding method is less due to the constant synchronization overhead.
Rice. 1.10. Spectrum expansion by frequency hopping
The carrier frequency changes in accordance with the numbers of frequency subchannels generated by the pseudo-random number algorithm. The pseudorandom sequence depends on some parameter called initial number. If the receiver and transmitter know the algorithm and the value of the seed, then they change frequencies in the same sequence, called a pseudo-random frequency hopping sequence.
If the frequency of subchannel changes is lower than the data transmission rate in the channel, then this mode is called slow spectrum expansion(Fig. 1.11a); otherwise we are dealing with rapid spectrum expansion(Fig. 1.11b).
The fast spread spectrum method is more resistant to interference because the narrowband interference that suppresses the signal in a particular subchannel does not result in bit loss because its value is repeated several times in different frequency subchannels. In this mode, the effect of intersymbol interference does not appear, because by the time the signal delayed along one of the paths arrives, the system has time to switch to another frequency.
The slow spectrum spreading method does not have this property, but it is simpler to implement and involves less overhead.
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Rice. 1.11. Relationship between data rate and subchannel change frequency
FHSS methods are used in IEEE 802.11 and Bluetooth wireless technologies.
In FHSS, the approach to using the frequency range is different from other encoding methods - instead of economically using a narrow bandwidth, an attempt is made to occupy the entire available range. At first glance, this does not seem very effective - after all, only one channel is operating in the range at any given time. However, the latter statement is not always true - spread spectrum codes can also be used to multiplex multiple channels over a wide range. In particular, FHSS methods allow you to organize the simultaneous operation of several channels by selecting such pseudo-random sequences for each channel so that at each moment of time each channel operates at its own frequency (of course, this can only be done if the number of channels does not exceed the number of frequency subchannels).
Direct Sequence Spread Spectrum (DSSS)
Direct Sequential Spread Spectrum also uses the entire frequency range allocated to a single wireless link. Unlike the FHSS method, the entire frequency range is occupied not by constant switching from frequency to frequency, but by replacing each bit of information with N-bits, so that the clock speed of signal transmission increases by N times. And this, in turn, means that the signal spectrum also expands N times. It is enough to select the data rate and N value appropriately so that the signal spectrum fills the entire range.
The purpose of DSSS coding is the same as FHSS - to increase immunity to interference. Narrowband interference will distort only certain frequencies of the signal spectrum, so that the receiver is likely to be able to correctly recognize the transmitted information.
The code that replaces the binary unit of the original information is called spreading sequence, and each bit of such a sequence is a chip.
Accordingly, the transmission rate of the resulting code is called chip speed. A binary zero is encoded by the inverse of the spreading sequence. Receivers must know the spreading sequence that the transmitter uses in order to understand the information being transmitted.
The number of bits in the spreading sequence determines the spreading factor of the source code. As with FHSS, any type of modulation, such as BFSK, can be used to encode the bits of the result code.
The larger the spreading factor, the wider the spectrum of the resulting signal and the higher the degree of interference suppression. But at the same time, the spectrum occupied by the channel increases. Typically the expansion factor ranges from 10 to 100.
Spectrum extension
In this lecture we will look at the basic principles of signal spreading technology.
Spread spectrum is a technology, in simple terms, in which a modulated signal is represented as a signal with a bandwidth much greater than the bandwidth of the information signal.
Modern mobile communications are based on spread spectrum technology and are widely used under the name “CDMA”.
Consider the CDMA IS-95 (cdmaOne) standard as the most widely used at present. Spread spectrum technology was first proposed for mobile communicators in the 1980s, and commercial distribution was first undertaken by Qualcomm Inc, which introduced this standard in the DS-CDMA (Direct Sequence Code Division Multiple Access) format. Commercial use of the IS-95 standard began in 1996 in the USA. The abbreviation IS (interim standard) is used for accounting in TIA, and the number means the serial number. From the full name of the TIA/EIA/IS-95 standard, it is clear that the EIA, which unites seven large US organizations, also took part in its consideration.
Types of multiple access: Multiple access is the problem of numbering users who want to use the same electromagnetic spectrum. It can be solved in several ways:
- Selection with frequency division (signals are distributed only between specific communicators);
- Spatial filtering;
- Frequency Division Multiple Access (FDMA);
- Time Division Multiple Access (TDMA);
- Code Division Multiple Access (CDMA).
TDMA (Time Division Multiple Access - time division multiple access) is a method of using radio frequencies when there are several subscribers in the same frequency interval, different subscribers use different time slots (intervals) for transmission. TDMA gives each user full access to a frequency slot for a short period of time.
FDMA (Frequency Division Multiple Access - frequency division multiple access) - a method of using radio frequencies when there is only one subscriber in the same frequency range, different subscribers use different frequencies within the cell.
CDMA (Code Division Multiple Access - code division multiple access) is a mobile communication technology in which transmission channels have a common frequency band, but different code modulation.
Basically CDMA is used as a term for a system of modulating information into a signal having a wider bandwidth, i.e. spectrum expansion. This expansion is carried out through binary "code", which is usually very long, and for most considerations, random in nature. Of course the code is not random, it is quite predictable, and the term pseudo-random (a confusing term in itself) is often used.
One of the fundamental concepts that determines the noise immunity and efficiency of a CDMA system is the “signal base” (in English literature the term “processing gain” is used). The physical meaning of this concept is an increase in the frequency band of the transmitted signal relative to the original one (measured in decibels). For spread spectrum systems, the signal base is defined as the ratio of the bandwidths of the emitted and source signals. However, more often the value of the signal base (B) is calculated as the product of the spectrum width (F) and the duration of the elementary symbol (T). For broadband signals, the base is significantly greater than 1 (B>>1). It is clear that the wider the frequency band on the air and the lower the speed of the input signal, the larger the signal base and, accordingly, the higher the noise immunity.
However, it is important to understand that the signal base is not a characteristic of the entire CDMA system, but only of its individual channel. Let us explain this with an example. So, with a chip speed of 1.2288 Mchip/s (IS-95) and an information speed of 9.6 kbit/s, the signal base is 21.1 dB (1.2288x103 / 9.6 = 128). The base of a signal is proportional to its transmission speed.
Wideband is a system that transmits a signal that occupies a very wide frequency band, significantly exceeding the minimum bandwidth that is actually required to transmit information. In a wideband system, a source baseband signal (eg a telephone signal) with a bandwidth of only a few kilohertz is distributed over a frequency band that can be several megahertz wide. This is done by double modulating the carrier with a transmitted information signal and a wideband coding signal. The main characteristic of a broadband signal is its base B, defined as the product of the signal spectrum width F and its period T. As a result of multiplying the signal of a pseudorandom noise source with an information signal, the energy of the latter is distributed over a wide frequency band, i.e. its spectrum is expanding.
The technology is optimized for providing high-speed multimedia services such as video, Internet access and video conferencing; provides access speeds of up to 2 Mbit/s over short distances and 384 Kbit/s over long distances with full mobility. Such speed values
Data transmissions require a wide frequency band, so the WCDMA bandwidth is 5 MHz.
The technology can be added to existing GSM and PDC networks, making the WCDMA standard the most promising in terms of network resource utilization and global compatibility.
At the transmitter, the narrowband information signal is multiplied by a pseudo-noise reference N-symbol sequence, and the resulting signal is modulated using BPSK or QPSK (direct operation). The base of the resulting signal is equal to the number of symbols of the pseudo-random sequence (B = N). In this case, the use of noise-like signals with a high clock frequency leads to the fact that the original narrow-band
the signal is “spread out” over a wide band and becomes less than the noise level.
At the receiver, the original signal is reconstructed using a pseudo-random sequence of known structure (inverse operation). Other signals arriving at this receiver are perceived as noise.
In a similar way, powerful narrowband interference from other operating transmitters is suppressed. In the receiver, such interference is also “spread out” over a wide frequency band and, after filtering, only slightly degrades the quality of communication. With further digital processing, the interference can be completely suppressed.
In addition to the most commonly used DS-CDMA method, there are other spectrum extension technologies, for example using multiple carriers - MC-CDMA (Multi-Carrier CDMA) or frequency hopping - FHCDMA (Frequency Hopping CDMA). The features of these technologies will be discussed in future issues of the magazine.
Real-time digital signal processing prior to RF transmission. The principle of constructing a transmitter/receiver is the same as with DS-CDMA, only the final modulated signal is supplied to the DAC. The transmitter/receiver uses a special filter called a raised cosine filter, which minimizes intersymbol distortion by representing a portion of the spectrum in its simplest form into a cosine wave that is raised so that it sits on the horizontal axis.
Chipping is any operation by which symbols (bits) are split (chipped) into smaller time intervals. The operations of scrambling, channeling and spreading are the chipping operation.
Scrambling is a reversible transformation of a digital stream without changing the transmission rate using a random sequence. After scrambling, the appearance of “1” and “0” in the output sequence is equally likely. Scrambling is a reversible process, that is, the original message can be restored by applying the reverse algorithm.
Channelization is a reversible transformation of a digital stream by dividing the information signal into chips using a fixed sequence.
Comprehensive presentation.
Note that the complex representation is purely mathematical and is introduced for convenience of notation. In third generation CDMA networks, all three representations are used in an integrated form. Channeling in the Uplink system is carried out by the first presentation method, and in the Downlink system - by the second.
Each user has a unique spreading/channeling code, most likely an orthogonal Walsh code. In downstream signal transmission, the real part of the complex representation of the chipped sequence is taken as a basis and transmitted at the same speed. The transmitted encoded signals will be synchronized. Each mobile station knows the scrambling code of the current base station, and its established (and only) spreading code - from here the transmitted data is recovered.
Logical downlink channels include:
Pilot channel;
Synchronization channel;
Personal call channel;
Direct traffic channel.
In the forward channel (from BS to mobile), signal modulation by Walsh functions (binary phase shift keying) is used to distinguish between different physical channels of a given BS; long PSP modulation (binary phase
manipulation) - for the purpose of encrypting messages; modulation of a short PSP (quadrature phase shift keying of two PSPs of the same period) - to expand the bandwidth and distinguish between signals from different BSs.
Distinction between signals from different stations is ensured by the fact that all BSs use the same pair of short bandwidths, but with a shift of 64 samples between different stations, i.e. There are a total of 511 codes on the network; in this case, all physical channels of one BS have the same sequence phase.
4 types of channels are formed on the BS: pilot signal channel (PI), synchronization channel (SYNC), calling channel (PCN) and traffic channel (TCN).
Signals from different channels are mutually orthogonal, which guarantees the absence of mutual interference between them on the same BS. Intra-system interference mainly arises from transmitters of other BSs operating at the same frequency, but with a different cyclic shift.
The pilot signal is emitted continuously. To transmit it, the zero-order Walsh function (W0) is used. The pilot signal is a carrier signal that is used by the MS to select a working cell (based on the most powerful signal), and also as a reference signal for synchronous detection of information channel signals. Typically, about 20% of the total power is emitted on the pilot signal, which allows the mobile station (MS) to ensure accurate carrier frequency selection and coherent reception of signals.
In the synchronization channel (SYNC), the input stream at a rate of 1.2 kbit/s is re-encoded into a stream transmitted at a rate of 4.8 kbit/s. The synchronization message contains the technological information necessary to establish initial synchronization on the MS: data on the exact system time, the transmission speed in the PCH channel, and the parameters of the short and long code. The transmission speed in the synchronization channel is lower than in the calling (RSN) or schedule channel (TSN), which increases the reliability of its operation. Upon completion of the synchronization procedure, the MS is tuned to the PCH call channel and constantly monitors it. Function W32 is used to encode the sync channel.
In the reverse channel (uplink), an asynchronous version of code division is implemented in combination with incoherent reception of signals at the BS. This eliminates the need for a pilot channel and a synchronization channel. This leaves only two types of uplink logical channels:
Access channel;
- return traffic channel.
The asynchrony of code division makes it irrational to use Walsh functions as channel-forming sequences (signatures) of physical channels, since with relative time shifts they cannot maintain orthogonality and have very unattractive cross-correlation properties.
The access channel provides connection between the MS and the BS until the MS is tuned to the reverse traffic channel assigned to it. The access channel selection process is random - the MS randomly selects a channel number from a certain range. The access channel is used to register the MS in the network, transmit a request to establish a connection to the BS, respond to commands transmitted over the call channel, etc. The data transfer rate over the access channel is fixed and amounts to 4.8 kbit/s.
The reverse traffic channel ensures the transmission of voice information and subscriber data, as well as control information from the MS to the BS, when the MS is already occupying the physical channel allocated to it.
Walsh codes.
The CDMA standard uses orthogonal Walsh codes for code division. Walsh codes are formed from the rows of the Walsh matrix:
The peculiarity of this matrix is that each of its rows is orthogonal to any other or row obtained using the logical negation operation. The IS-95 standard uses a 64th order matrix. A digital filter is used to isolate the signal at the receiver output. With orthogonal signals, the filter can be configured so that its output will always be a logic "0" unless the signal to which it is configured is received. Walsh coding is used in the forward channel (from BS to AT) to separate users. In systems using the IS-95 standard, all speakers operate simultaneously in the same frequency band. Matched filters of BS receivers are quasi-optimal in conditions of mutual interference between subscribers of the same cell and are very sensitive to the “far-close” effect. To maximize the subscriber capacity of the system, it is necessary that the terminals of all subscribers emit a signal of such power that would ensure the same level of signals received by the BS. The more precise the power control, the greater the subscriber capacity of the system.
Pseudo-random sequence.
PSP is a deterministic periodic signal that is known to both correspondents. It has all the statistical properties of white noise and to a third party it will appear to be completely random - a pseudonoise signal. In order for the PSP to be a random process, a number of conditions must be met:
- the number of binary ones should not differ from the number of binary zeros by no more than one element;
- The PSP must have good correlation properties, namely, the levels of the ACF side lobes of such a sequence must have a minimum level.
Many sequences satisfy these properties - Walsh, Barker, Gold sequences, M-sequences and many others.
FCSR (Feedback with carry shift register) - shift register, feedback function and carry register. The length of the shift register is the number of bits. When a bit needs to be retrieved, all the bits in the shift register are shifted to the right by one position. The new leftmost bit and the new value of the carry register are determined by the function of the remaining bits of the shift register and the carry register (their bits are added together). The least significant bit of the result becomes the new leftmost bit, and the remaining bits of the result (except the least significant bit) become the new value of the carry register.
Unlike LFSR, there is a delay for FCSR before it goes into cyclic mode, that is, it begins to generate a cyclically repeated sequence. Depending on the selected initial state, 4 different cases are possible:
1. The initial state may be part of the maximum period.
2. The initial state may enter the maximum period sequence after some initial delay.
3. The initial state may, after an initial delay, produce a sequence of zeros.
4. The initial state can, after an initial delay, produce a sequence of ones.
Gold's sequence is a pseudo-random sequence formed by adding modulo 2 two pseudo-random sequences.
Kasami is a type of pseudo-random sequence. Used in CDMA. The significance of these sequences comes from their very low cross-correlation. The Kasami code of length N = 2m − 1, where m is an even integer, can be obtained by taking periodic samples from M-
sequences and performing modulo 2 summation on cyclically shifted sequences. Samples are taken every s = 2m / 2 + 1 elements of the M-sequence to form a periodic sequence and then adding this sequence incrementally to the original M-sequence modulo 2 to form s = 2m / 2 Kasami sequences. The cross-correlation function of two Kasami sequences takes values [-1, -s, s-2].
Orthogonal codes
The ability to adapt the system to different transmission rates is ensured through the use of so-called channelization codes. The principle of their generation can be illustrated (Fig. 1) with a code tree diagram for orthogonal variable-length codes
(Orthogonal Variable Spreading Factor, OVSF).
Each level of this code tree has its own code words, the length of each of which is equal to the spreading factor (SF). The complete code tree contains 8 levels (the last, eighth, corresponds to the coefficient SF=256).
The structure of the code tree is such that at each subsequent level the possible number of channel-forming codes is doubled. So, if at level 2 only 2 codes are generated (SF = 2), then at level 3 4 codewords are generated (SF = 4), etc. The ensemble of OVSF codes is not fixed, but depends on the spreading factor SF, i.e., in fact, on the transmission speed of the channel.
The problem of orthogonality.
Suppose there is a simple system with two users and two signal paths. The two paths have a relative latency of one chip. Orthogonal Walsh codes are used to propagate the data sequence.
In this case, the receiver will extract two different signals from the channel for each user, corresponding to two different paths, the relative delay between them will be one chip.
For each user, the receiver will receive two signals from the channel, the desired signal (the PRP is synchronized with this signal) and a delayed version of it.
The result of narrowing the four received signals in the case of two-channel transmission to two users will be:
B N (bit of interest) from narrowing the desired user signal;
- 0 from narrowing of orthogonal noise-like signals, no interference due to the use of Walsh codes;
- undesirable conditions when the narrowing causes delay of the desired signal and interference.
Multipath.
For a code sequence with ideal correlation properties, the autocorrelation function gives a zero output in the interval , where Tc is the chip time. This means that the wanted signal (main path) and a delayed version of that signal for a time greater than 2Tc are received at the receiver, then, with coherent demodulation/down-spreading conditions, the receiver will identify the delayed signal as interference. In addition, the power level of the delayed signal is less than the useful one due to reflections during multipath, therefore, the delayed signal in the form of interference is “smeared” over the entire bandwidth, and the receiver receives only the useful signal.
The "near - distant" problem.
Despite the high efficiency of CDMA technology, it also has a number of disadvantages. One of them is high sensitivity to power dispersion of mobile stations. The most difficult situation arises due to the “far-near” problem, when a mobile station located near the base station operates at high power, creating an unacceptably high level of interference when receiving other, “far” signals, which leads to a decrease in throughput. capabilities of the system as a whole. This problem exists in all mobile communication systems, but the greatest signal distortion occurs in CDMA systems operating in a common frequency band, which use orthogonal noise-like signals. If these systems lacked power control, they would be significantly inferior in performance to TDMA-based cellular networks. Therefore, the key problem in CDMA systems can be considered individual power control of each station.
Detection.
The receiver has access to a code bank that stores all codes allocated at base stations (BS). For a specific user, the BS knows what code to expect and the code is detected by comparing the received sequence with the expected code. The correlation operation is carried out by narrowing, which can be performed in a matched filter. Before correlation can begin, the recipient must know the exact point in time. Synchronization is achieved by using a pilot signal, which is located before the transmitted information. The pilot signal is the same for all users. When synchronization is completed, the matched filter begins the correlation operation: if the correlation is above a predefined threshold, the matched filter is positive defined by the user.
Multiplying the received signal and the signal from the same pseudorandom noise source (PRN) that was used in the transmitter compresses the spectrum of the useful signal and simultaneously expands the spectrum of background noise and other sources of interference. The resulting gain in signal-to-noise ratio at the receiver output is a function of the ratio of the broadband and baseband signal bandwidths: the greater the spectrum spread, the greater the gain. In the time domain, this is a function of the ratio of the transmission rate of the digital stream in the radio channel to the transmission rate of the basic information signal. For the 1S-95 standard, the ratio is 128 times, or 21 dB. This allows the system to operate at a level of interference interference that exceeds the level of the useful signal by 18 dB, since signal processing at the receiver output requires the signal level to exceed the interference level by only 3 dB. In real conditions, the level of interference is much less. In addition, expanding the signal spectrum (up to 1.23 MHz) can be considered as an application of frequency diversity reception methods. A signal propagating in a radio path is subject to fading due to the multipath nature of propagation. In the frequency domain, this phenomenon can be represented as the effect of a notch filter with a variable notch bandwidth (usually no more than 300 kHz). In the AMPS standard, this corresponds to the suppression of ten channels, and in the CDMA system, only about 25% of the signal spectrum is suppressed, which does not cause any particular difficulties in restoring the signal in the receiver.
Rake receiver.
Digitized samples of the input signals are received from the RF input stages and are represented as quadrature I and Q branches (i.e., complex number format of the low-pass filter at the receiver output). Code generators and a correlator perform compression and summation of user data transmission symbols. The channel device uses the pilot symbols to estimate the channel state, the effect of which will then be compensated by a phase shifter for the received symbols. The delay is compensated by the difference in the arrival time of the symbols in each path. The Rake adder then adds the compensated channel symbols, thereby providing multipath diversity as a means of combating fading.
Also shown is the matched filter used to determine and update the current multipath delay profile of the channel. This measured and possibly averaged multipath delay profile is then used to sum the highest peak Rake receiver path outputs.
In typical implementations, a Rake receiver performing chip-rate processing (correlator, code generator, matched
