Spread Spectrum Communication System

SPREAD SPECTRUM COMMUNICATION SYSTEM

The two most important design factors in any digital communication system are:

1) effective channel bandwidth utilization and

2) transmission power conservation.

Some of the most serious issues that arise with specialized communication systems are:

1) Combating or reducing the harmful effects of jamming, interference from other channel users, and self-interference owing to multipath propagation are some of the key issues encountered in certain communication systems.

2) Hiding a signal by broadcasting it with low power and making it difficult to detect by an unwanted listener.

3) Maintaining message privacy when speaking in front of others.

Spread-spectrum modulation is a technology that may be used to successfully overcome these challenges.

SPREAD SPECTRUM COMMUNICATION SYSTEM

If a system meets the following criteria, it is classified as a spread spectrum communication system.

1) The signal uses a lot more bandwidth than the minimum bandwidth required to convey the data.

2) Spreading is performed via a data-independent spreading signal, generally known as 'a code signal'. 

3) At the receiver, despreading (recovering the original data) is accomplished by comparing the received spread signal to a synchronized replica of the spreading signal that was used to spread the data.

The Bandpass modulator circuit is used in the transmitter of a digital communication system to produce such frequency spreading of the signal.

MODEL OF SPREAD SPECTRUM DIGITAL COMMUNICATION SYSTEM

The basic elements of a spread spectrum digital communication system are shown in the figure.

                          Figure:  Model of spread spectrum digital communication system

The modulator/demodulator and the channel encoder/decoder are the fundamental elements of a digital communication system. This also has two identical pseudorandom pattern generators. At the transmitting end, one interface with the modulator. The second interacts with the receiver's demodulator. The pseudo-noise (PN) binary-valued sequence is impressed on the transmitted signal at the modulator and removed from the received signal at the demodulator by these pseudorandom pattern generators.

BENEFICIAL ATTRIBUTES OF SPREAD SPECTRUM SYSTEMS

Spread-spectrum modulation was developed for military applications, where jamming (interference) resistance is a key challenge. However, the unique characteristics of spread spectrum modulation have civilian applications as well. The major advantages of spread spectrum systems are listed below: 

1) Interference suppression benefits:

(i) To avoid intentional interference (jamming), the transmitter introduces an element of unpredictability or randomness (pseudorandomness) to each of the transmitted coded signal waveforms. Only the intended receiver is informed of this (not the jammer). As a result, jamming-related interference is prevented.

(ii) Self-interference can be seen as resolvable multipath components caused by temporal dispersive propagation over a channel. Incorporating a pseudorandom pattern into the broadcast signal can also help to remove this form of interference.

2) Multiple Access

Spread spectrum techniques can be used as a multiple access approach to distributing a communication resource among several users in a coordinated fashion. In multiple access communication systems, when several users share a shared channel bandwidth, interference from other users occurs. By superimposing a separate pseudorandom pattern, also known as a code, in each transmitted signal, the transmitted signals in this shared channel spectrum may be identified from one another. By understanding the code or key used by the associated transmitter, a specific receiver can recover the sent information intended for it. Code Division Several Access (CDMA) is a communication technology that allows multiple users to share a common channel for data transmission at the same time (CDMA).

3) Energy Density Reduction

By spreading a message's bandwidth using coding and delivering the resulting signal at low average power, a message can be hidden in the background noise. The transmitted signal is referred to be "covert" due to its low power level. It has a low chance of being intercepted (detected) by an untrained ear. As a result, it's also known as a signal with a Low Probability of Intercept (LPI).

A radiometer is a basic power meter that may be used to detect the presence of spread-spectrum signals within a certain bandwidth (B).

4) Fine Time Resolution

In radar and navigation, spread spectrum signals are used to accurately determine the range (time delay) and range rate (velocity). The time delay of a pulse as it travels through a channel can be used to calculate distance.

5) Message Privacy

By superimposing a pseudorandom pattern on a transmitted message, communication privacy can be achieved. The message can be demodulated by the intended receivers who know the pseudorandom pattern or key used at the transmitter but not by any other receivers who don't know the exact key.

SPREAD SPECTRUM APPROACHES (HISTORICAL BACKGROUND)

Transmitted Reference (TR) and Stored Reference (SR) are two spread-spectrum methods.

(i)   In a TR system, the transmitter transmits two copies of a random spreading signal (wideband carrier), one modulated with data and the other unmodulated. For despreading (correlating) the data modulated carrier, the receiver used the unmodulated carrier as the reference signal.

(ii) The spreading code signal is generated independently at both the transmitter and the receiver in an SR system. The code sequence must be predictable, even if it should look random to unauthorized listeners because the identical code must be created independently at two sites. Pseudonoise (PN) or pseudorandom signals are deterministic signals that look random.

The Stored Reference (SR) technique, which employs a Pseudo Noise (PN) or pseudorandom code signal, is used in modern spread spectrum systems.

Baseband Transmission System

BASEBAND TRANSMISSION SYSTEM

In digital communication, formatting is the first and most important signal processing step. Formatting ensures that the message or source signal may be processed digitally. The method of transforming source data to digital symbols is known as transmit formatting. Source coding is the term used when data compression and formatting are used together. 

The binary ones and zeros logical structure is used to represent digital communications. Baseband processors convert these messages into baseband waveforms via pulse modulation. Waveforms of this nature can then be sent through a cable.

BASEBAND SYSTEMS

The Figure represents the functional diagram of baseband signal formatting and transmission (Baseband Systems).

Figure: Formatting and Transmission of baseband signals (Baseband Systems)

 

Formatting transforms source data into bits, ensuring that the data and signal processing processes of a digital communication system are compatible. Up to the pulse modulation block, the information is in the form of a bit stream.

Analog and discrete information sources are available. As a result, a data source's output might be digital, textual, or analogue. The formatting function would not be used if the data was already in digital format. With the help of a coder, textual data is converted to binary digits. If the data is in the form of alphanumeric text, it will be encoded using one of many common formats, including ASCII, EBCDIC, Baudot, and Hollerith.

The three independent procedures of sampling, quantization, and coding are used to format analogue data. The formatting phase produces a series of binary numbers for all sorts of information sources.

The bit stream is converted into a series of pulse waveforms by the pulse modulator. The digits being transmitted correspond to the features of this series of pulses. These pulse waveforms are then sent across a baseband channel, which might be a pair of wires or a coaxial cable.

The pulse waveforms are recovered (demodulated) and detected after transmission over the channel to generate an approximation of the transmitted digits. The reverse formatting is the final stage, which retrieves an estimate of the source data.

Types of Digital Modulation Techniques

Digital Modulation Techniques

In Baseband pulse transmission, the input data in baseband pulse transmission is represented by a discrete PAM signal (Line codes). At low frequencies, the baseband signals have high power. As a result, they can be transmitted through a pair of wires or coaxial cables.

Due to the impracticality of using huge antennas, baseband signals cannot be sent across radio connections or satellites. As a result, the communication signal's spectrum has to be moved to higher frequencies. This is performed by modulating a high-frequency sinusoidal carrier with the baseband digital signal. The modulated signals are transmitted through a bandpass channel, such as a microwave radio link, a satellite link, or an optical fiber link. This process is termed Digital carrier modulation, or (digital passband communication).

DIGITAL MODULATION:

The mapping of a sequence of input binary digits into a series of corresponding high-frequency signal waveforms is known as digital modulation. These modulated waveforms can change in amplitude, frequency, phase, or a combination of these signal properties (Amplitude and phase or frequency and phase).

Digital Modulation Techniques

There are mainly two types of digital modulation techniques. They are :

1. Coherent digital modulation techniques

2. Non-Coherent digital modulation techniques

1. Coherent Digital Modulation Techniques

Coherent detection is used in coherent digital modulation techniques. The local carrier generated at the receiver is phase synchronized with the carrier at the transmitter in coherent detection. As a result, detection is achieved by comparing the received noisy signal to the locally generated carrier. Synchronous detection is a coherent detection. Coherent detection techniques are more complex, but they can provide better performance than non-coherent detection.

2. Non-Coherent Digital Modulation Techniques

Non-Coherent detection is used in these techniques. The receiver carrier does not need to be phase synchronized with the transmitter carrier for the detection process.   As compared to coherent detection, Non-Coherent detection techniques are easy to implement. However, compared to Coherent detection, the probability of error is high in non-coherent detection.

Listing of various types of digital modulation methods:

Based on the mapping techniques, we can broadly classify the digital modulation methods.

I. Binary Scheme / M-ary Scheme:

During each signaling interval of duration Tb, we send one of the two possible signals in a binary scheme. The examples for the binary scheme are:

1. Amplitude Shift Keying (ASK), 2. Frequency Shift Keying (FSK) and 3. Phase Shift Keying (PSK)

M-ary Scheme:

During each signaling period 'Tb' in the M-ary system, we can send any of the M possible signals. The examples are::

1. M-ary ASK

2. M-ary FSK

3. M-ary PSK

4. Minimum shift keying (MSK) is a type of phase frequency shift keying in which the minimum shift is used (CPFSK).

5. M-ary PSK with M=4 is represented by quadriphase shift keying (QPSK). Quadrature carrier multiplexing systems are of two types:  MSK and QPSK.

6. Amplitude Modulation in M-ary Quadrature (M-ary QAM)

M-ary Amplitude-Phase Keying is the result of combining discrete changes in the amplitude and phase of a carrier (APK). M-ary QAM is a special form of this hybrid modulation.

II. Based on the performance of the modulation scheme and properties of a modulated signal.

1. Power efficient scheme / Bandwidth efficient scheme

2. Continuous phase (CP) modulation / In phase Quadrature phase (IQ) modulation

3. Constant envelope modulation / Non-Constant envelope modulation

4. Linear modulation / Non-linear modulation

5. Modulation scheme with memory/modulation scheme without memory.

Design Goals of Digital Communication System

A digital communication system designer has a variety of modulation/detection techniques. Each design has its own set of trade-offs. The use of available primary communication resources, transmission power, and channel bandwidth determine which modulation/detection system is used. The choice depends on achieving as many of the design goals as possible.

1. Maximum data rate

2. Minimum possibility of symbol error

3. Minimum transmitted power

4. Minimum channel bandwidth

5. Maximum resistance to interfering signals

6. Minimum circuit complexity.

Gram-Schmidt Orthogonalization Procedure

The process of converting an incoming message mi into a modulated wave Si(t) may be broken down into discrete-time and continuous-time procedures. The Gram-Schmidt orthogonalization process allows any collection of M energy signals, S(t), to be represented as linear combinations of N orthonormal basis functions. As a result, the provided collection of real-valued energy signals S1(t), S2(t),... Sm(t), each with a period of T seconds, may be represented in the form

The real-valued basis functions φ₁(t), φ₂(t), …… φ_N(t) are orthonormal. Hence we have

In the first condition, each basis function must be normalized to have Unit energy. The second condition specifies that throughout the interval 0≤𝑡≤𝑇, the basis functions φ1(t), φ2(t), …… φN(t) are orthogonal to each other.

In equation (1), the coefficients of the expansion can be defined as:

 

Modulator Design:

Assume that the set of coefficients {Sij}, j = 1, 2, … N is operational. Then, as shown in Figure, we can apply the equation to create the signal Si(t), where I = 1, 2,... M, from equation (1)

Figure - Scheme for generating the signal S_i(t)

It is composed of a bank of N multipliers, each with its basic function, followed by a 'summer'. This method functions similarly to a modulator in a transmitter.

Detector Design:

Figure - Scheme for generating the set of coefficients {Sij}

Assume that the collection of signals {𝑆(𝑡)},𝑖=1,2,… 𝑀, is operating as input. To calculate the set of coefficients {𝑆𝑖𝑗}, j = 1, 2, ….N as per equation, we may utilize the scheme shown in figure. A bank of N product integrators or correlators with a common input makes up this method. Every multiplier has its basis function. This method is comparable to the receiver's detector.

Alloys of Aluminium and its Uses

Alloys of Aluminium and its Properties and Uses

Aluminium is a very malleable and ductile metal with a soft white colour. It's melting and boiling points are 655°C and 2057°C, respectively. At 20 degrees Celsius, it has a density of 2.7 gmm / square centimetre. Pure aluminium has a thermal conductivity of 0.503 cal/cm sec/°C. At 20 degrees Celsius, it has a temperature coefficient of 0.0035 per °C. 2.8 x 10^-8 ohm-m is its resistivity. It has a high corrosion resistance as well as a high resistance to touch. Its tensile strength ranges from 0.95 to 1.57 tonnes per square centimetre and it cannot be soldered. When exposed to air, an oxide layer forms, with a tensile strength of 9 kg / sq.mm. Aluminium alloys include the following:

 

(a) Aldrey (b) Duraluminium (c) Hindalium (d) Magnelium

 

1. Aldrey :

Aldrey is a magnesium alloy with 0.3 to 0.5 % magnesium, 0.4 to 0.7 % silicon, and 0.2 to 0.3 % iron. It has high conductivity and is mechanically strong. This alloy is utilised in the construction of small and medium-distance overhead transmission lines. The following are the alloy's physical properties:

 

Tensile strength 32 to 37 kg / mm²

Specific gravity 2.7 gm / cm²

Melting point 1100°C

 

2. Duraluminium:

 

It is made up of 4.22 % copper, 0.65 % manganese, 0.54 % magnesium, 0.22 % silicon, 0.42 % iron, and the balance is aluminium. The alloy is castable and is used to make the conductors and end rings of a squirrel cage induction motor's rotor. Because it is non-magnetic, light, and machining-friendly, it is also utilised for bus-bar castings in a switch gear. Bow Pantograph assembly is made of an aluminium alloy with 2.5 % to 6% copper because of its low inertia, good electrical conductivity, and mechanical strength. This can also be done with Aldrey.

3. Hindalium :

It's a brand name for an alloy including Al, Mg, Mn, Cr, and Si, etc. It is primarily produced as a 16 gauge rolled product for anodized utensil manufacturers.

4. Magnelium : 

It's made up of 0 to % Cu, 1 to % Mg, 0-1.2 % Ni, 0.3 % Sn, 0.9 % Fe, 0 to 0.03 % Mn, 0.2 to 0.6 Si, and 85 to 95% Al. It's a brittle, lightweight alloy with poor castability. Welding and machining are both possible with this material. It is primarily employed in the aerospace and automobile industries for vehicle door handles, the largest racks, and gearbox housing, among other applications.

 

ACSR CONDUCTOR :

The ACSR Conductor is explained in the figure. ACSR stands for Aluminium Conductor Steel Reinforced. It has a galvanised steel wire central core and one or more layers of stranded aluminium wires on the outside. Such a conductor has a much higher ultimate tensile strength than analogous copper conductors while yet weighing around 25% less. Due to its lighter weight and strong tensile strength, this conductor has less sag. It's utilised for transmission lines with a span of above 100 metres.

 

Physical Properties of A.C.S.R.Conductors:

 

	x 1 : 6	x 1 : 4
Specific gravity	3.45	3.7
Young's Modulus in kgf/mm2	7.5	8.3
Ultimate strength in kgf/mm2	120	120

 

x is the ratio of the cross-section area of iron to aluminium.

Synchronous and Asynchronous Transmission in Data Communication

 

Synchronous and Asynchronous Transmission in Data Communication

Serial transmission may be divided into two categories. Depending on how the timing and framing information is communicated, they are classified as asynchronous or synchronous. Bit synchronization is a function that determines when the data transmission starts and ends.

Asynchronous transmission in Data Communication

The transmitter sends data with its timing clock that is unknown to the receiver in asynchronous transmission.

Only one character is sent at a time with the asynchronous transmission. The character can be an alphabet letter, a number, or a control character. It transfers data one byte at a time.

The start bit and stop bit are used to synchronize bits between two devices, as shown in Figure.

The transmit and receive clocks in an asynchronous system are free-funning and set at about the same speed. At the start of each character, a 'Start' bit is broadcast. The start bit informs the receiver that a new group of bits has arrived. Each byte begins with a start bit, which is normally '0.'

After the character, a 'stop' bit, generally '1', is transmitted. The stop bit signifies the end of data, i.e., it informs the receiver that the byte is complete. The end of the byte is added with one or more extra '1' bits.

The start bit controls the framing in asynchronous transmission. Throughout the character's limited duration, the timing is consistently precise. Between the transmissions of distinct data packets, there is a period of idle time. The' gap' is the name given to this idle time.

As seen in Figure, the gap or idle time might be of varying lengths. This technique is dubbed asynchronous because the sender and receiver do not need to be synced at the byte level. However, receivers must be synced with the incoming bitstream within each byte.

Advantages of Asynchronous transmission

• When the data byte to be communicated becomes available, the transmission can begin.

• Signals from different sources with varying bit rates can be sent.

• This data transfer method is simple to set up.

• Data transfer using this approach is less expensive. For example, if the lines are short, asynchronous transmission is preferable since the line cost is cheap and idle time is little.

Disadvantages of asynchronous transmission

• Due to the overhead of additional bits (start and stop) and gap insertion in the bitstream, this approach is less efficient and slower than synchronous transmission.

• Identification of the start bits is essential for successful transmission. These bits might be corrupted or lost.

Applications

• Since asynchronous transmission does not require any local storage at the terminal or computer, it is particularly suited for keyboard type-terminals and paper tape devices.

• Asynchronous transmission is best suited to Internet traffic when data is sent in small bursts.

 •Used for communication between microcomputers, modems employ this form of transmission.

Synchronous transmission in Data Communication

The transmitter and receiver are synced to the same clock frequency in synchronous transmission. Because the start and stop bits are not required, this manner of transmission is more efficient than asynchronous transmission. Data is delivered in blocks, each of which may include several bytes. As given in Figure, there is no gap or idle time between the various bytes in the data stream.

The transmitter and receiver achieve bit synchronization by "timing" the transmission of each bit. Because the various bytes are placed on the connection without any gaps, the receiver must break the bitstream into bytes to recreate the original data. Because synchronous communication uses larger message blocks, the transmitter and receiver clocks must be perfectly synchronized.

Framing

It is important to find the start of a block of data in synchronous communication. The receiver can maintain track of the remaining data without having to use stop and start bits as long as the transmitter and receiver clocks stay synchronized. As a result, synchronous systems are far more efficient than asynchronous ones.

Synchronous Data-Link Protocols

Character-oriented or bit-oriented synchronous communication protocols exist. BISYNC, an IBM product, is an example of a character-oriented protocol. As illustrated in the Figure, these protocols begin with at least two synchronizing (SYN) characters, followed by control and data characters.

For the SYN character, a bit pattern of 00101101 may be retained to indicate the start of the block, i.e. for synchronization. For error control, the Block check character BCC is utilized.

High-level data link control (HDLC), an ISO standard, and synchronous data link control (SDLC), an IBM product, are two examples of bit-oriented protocols. A flag, an eight-bit sequence that signifies the start of a frame, precedes each block of data. The bit pattern 01111110 is used in the flag. The length of the data block might be fixed or variable. A data block in HDLC is shown in the figure.

Frame check bits (FCS) follow the data and can be utilized for error control. If a frame is immediately followed by another, the flag ends the previous frame and starts the next frame.

Advantages of Synchronous transmission

• Synchronous transmission is quicker than asynchronous transmission, and it is more efficient since it eliminates the need for 'start' and 'stop' bits.

• There are no gaps or periods of inactivity between the bytes in the data stream.

• It is appropriate for high-speed computer connection.

Disadvantages of synchronous transmission

• It has more hardware and software complexity, and it needs perfectly synchronized clocks at both the transmitter and receiver. 

• The system must be properly synchronized.

• Assembling data blocks need local buffer storage at both ends of the line.

• It is more expensive than the asynchronous technique.

Applications:

• Synchronous transmission systems are used in mainframe computers to communicate at greater speeds.

• These are also used in the telephone system to transfer digitized analog signals.

Comparison

A comparison of synchronous and asynchronous transmission is shown in the table below.

 

Factor	Asynchronous	Synchronous
1. Data sent at one time.	Usually one byte	Multiple bytes
2. Start and stop bits.	Required	Not required
3. Gap between data units.	Present	Not present
4. Data transmission speed.	Slow	Fast
5. Cost.	Low	High
6. System complexity.	Simple	Complex