3.1 Wireless Transmission

 Wireless Transmission: Frequency for radio transmission, Signals, Antennas, Signal propagation, Multiplexing, Modulation, Spread spectrum, Cellular systems.

FREQUENCY FOR RADIO TRANSMISSION

WaveLength(lambda) : distance travelled by an EM wave during the time of one cycle

Frequency : No of cycles of waveform per second. expressed in Hertz(Hz).

since EM waves travel at speed of light

c = lambda*f => lambda = c/f

In radio Communication system, the frequencies range from few KHz to many GHz all used for different purpose.

The spectrum of frequencies available for data transmission encompasses a wide range, each with its own characteristics and applications:

1. Very Low Frequency (VLF) and Low Frequency (LF):

   • Used for long-range communication, particularly for submarines and radio stations. Frequencies range from several kHz to several hundred kHz.

2. Medium Frequency (MF) and High Frequency (HF):

   • Utilized for AM radio, shortwave (SW) transmissions, and FM radio. AM ranges from 520 kHz to 1605.5 kHz, SW ranges from 5.9 MHz to 26.1 MHz, and FM ranges from 87.5 MHz to 108 MHz.

3. Very High Frequency (VHF) and Ultra High Frequency (UHF):

   • Employed for TV broadcasting, digital audio broadcasting (DAB), digital TV, mobile phones (analog and digital), GSM, DECT, and 3G cellular systems.
   • VHF: 174–230 MHz, UHF: 470–862 MHz.

4. Super High Frequency (SHF):

   • Used for directed microwave links (2–40 GHz) and fixed satellite services in various bands such as C-band, Ku-band, and Ka-band.

5. Extremely High Frequency (EHF):

   • Approaches infrared frequencies and is utilized in some planned systems.

6. Infrared (IR):

   • Used for directed links, IrDA technology, and connecting devices like laptops and PDAs. Wavelengths approximately 850–900 nm.

7. Visible Light:

   • Historical use for wireless transmission, although not very reliable due to interference. Utilizes built-in human receivers.

REGULATIONS FOR USE OF FREQUNCY SPECTRUM

The management of radio frequencies is complex due to competing national interests and the need for global coordination. 

The International Telecommunications Union (ITU), a UN organization based in Geneva, oversees worldwide telecommunications activities, including frequency planning through its Radio-communication sector (ITU-R). 

To facilitate coordination, the world is divided into three regions, each with its own regulatory framework and national agencies responsible for implementing regulations.

Region 1: Encompasses Europe, the Middle East, countries of the former Soviet Union, and Africa.

Region 2: Includes Greenland, North and South America.

Region 3: Comprises the Far East, Australia, and New Zealand.

National agencies within these regions further regulate frequency usage.

Harmonization efforts are made through the ITU-R's World Radio Conference (WRC), where frequency allocations are discussed and decided upon periodically.

However, achieving harmonization is challenging, especially when nations have already invested significantly in specific technologies. Nonetheless, it is essential for satellite communication systems, which require global frequency availability. 

Harmonization ensures that satellites can operate worldwide without interference and supports global usage with a single device.

SIGNALS

Signals, in the context of telecommunications and electronics, refer to physical phenomena that convey information. These can be electrical, electromagnetic, or optical in nature, carrying encoded data or messages

Signals are the physical representation of data. Users of a communication system can only exchange data through the transmission of signals. Layer 1 of the ISO/OSI basic reference model is responsible for the conversion of data, i.e., bits, into signals and vice versa.

Signals are functions of time and location. Signal parameters represent the data values.

Types of signals for radio transmission are periodic signals, especially sine waves as carriers.

A periodic signal, such as a sine wave, is a signal that repeats its pattern over time at regular intervals. In the case of a sine wave, the waveform resembles a smooth, repetitive oscillation characterized by its amplitude, frequency, and phase.

Amplitude: The maximum displacement of the waveform from its equilibrium position, representing the strength or intensity of the signal. It is usually denoted by the symbol "A."

Frequency: The number of cycles (or complete oscillations) of the waveform that occur in one second. It is measured in Hertz (Hz) and represents how rapidly the waveform repeats. The frequency of a sine wave is denoted by the symbol "f."

Period: The time taken for one complete cycle of the waveform to occur. It is the reciprocal of the frequency and is measured in seconds. The period of a sine wave is denoted by the symbol "T."

Phase: The position of the waveform relative to a reference point at a given time. It represents the horizontal shift of the waveform along the time axis. Phase is often measured in radians or degrees.

Mathematically, a sinusoidal (sine) wave can be represented by the equation:

y(t)=A⋅sin(2πft+ϕ)

(In equations, ω is frequently used instead of 2πf.)

y(t) represents the instantaneous value of the signal at time t,

A is the amplitude of the sine wave,

f is the frequency of the sine wave (in Hz),

t is the time variable (in seconds),

ϕ is the phase angle (in radians).

The Fourier series demonstrates that any periodic signal can be decomposed into an infinite sum of sine and cosine functions. 

Signals can be represented in different domains:

Time Domain: In this domain, the amplitude of the signal is plotted against time. This representation is commonly seen on oscilloscopes. Phase shifts can also be depicted in this domain.

Frequency Domain: Here, the amplitude of different frequency components of the signal is plotted against frequency. A spectrum analyzer is used to display the frequency content of a signal. The Fourier transform translates signals from the time domain to the frequency domain and vice versa.

Phase Domain: Also known as the phase state or signal constellation diagram, this representation shows the amplitude and phase of the signal in polar coordinates. It visualizes the phase shift of the signal along with its magnitude. The x-axis represents the in-phase component (I), while the y-axis represents the quadrature component (Q).

ANTENNAS
Antennas play a crucial role in wireless communication systems by transmitting and receiving electromagnetic signals. They serve as the interface between the electronic circuitry of devices and the electromagnetic waves propagating through the air

Transmission: Antennas convert electrical signals into electromagnetic waves for transmission.

Reception: They capture incoming electromagnetic waves and convert them into electrical signals for processing.

Isotropic Radiator

The isotropic radiator is a theoretical antenna that radiates power equally in all directions, represented by a sphere with the antenna at its center

The radiation pattern is symmetric in all directions

such an antenna does not exist in reality.

Real Antennas/simple dipole

Real antennas exhibit directive effects, meaning their radiation intensity varies in different directions.

1.Hertzian Dipole

The simplest real antenna is a thin, center-fed dipole, also called Hertzian dipole,

The dipole consists of two collinear conductors of equal length, separated by a small feeding gap. 

The length of the dipole is not arbitrary, but, for example, half the wavelength λ of the signal to transmit results in a very efficient radiation of the energy.

A λ/2 dipole has a uniform or omni-directional radiation pattern in one plane

This type of antenna can only overcome environmental challenges by boosting the power level of the signal. Challenges could be mountains, valleys, buildings etc.

If an antenna is positioned, e.g., in a valley or between buildings, an omni- directional radiation pattern is not very useful.

2. Marconi Antenna

If mounted on the roof of a car, the length of λ/4 is efficient. 

Directional

Directional antennas focus their radiation in specific directions, such as towards a base station or satellite dish, providing improved signal strength and coverage.

Directed antennas are typically applied in cellular systems

Sectorized

Several directed antennas can be combined on a single pole to construct a sectorized antenna.

Multiple antennas can be combined to mitigate the effects of multi-path propagation and improve reception quality.

Diversity Antenna : this schemes like switched diversity and diversity combining enhance signal reliability and quality by selecting the best antenna element or combining the power of multiple signals.

Smart antennas utilize signal processing techniques to adapt their radiation patterns dynamically, optimizing performance based on changing signal conditions and propagation effects.

Antenna arrays can be used for beamforming, directing signals towards specific users or locations, enabling technologies like space division multiplexing and improving signal efficiency and coverage.

SIGNAL PROPAGATION

Signal is propagated through

Transmission Range : Within a certain radius of the sender transmission is possible, i.e., a receiver receives the signals with an error rate low enough to be able to communicate and can also act as sender.

Detection Range :Within a larger radius, known as the detection range, the transmitted signal can still be detected, but with a higher error rate. The signal's power is sufficient to distinguish it from background noise, but not strong enough for reliable communication.

Interference Range :Beyond the detection range lies the interference range, where the transmitted signal may interfere with other transmissions. While receivers within this range may not detect the signal, its presence adds to the background noise, potentially disrupting other signals.

The propagation of radio signals involves various effects and phenomena that significantly influence signal strength and quality.

Path Loss in Free Space:

In free space, radio signals propagate in a manner similar to light, following a straight line.

The received power decreases with the square of the distance between the sender and receiver (inverse square law).

Even in vacuum, signals experience path loss.

Factors such as wavelength and antenna gain also affect received power.

Propagation Behavior Depending on Frequency:

Radio waves exhibit different propagation behaviors based on their frequency:

Ground Wave (<2 MHz): Follows the earth's surface and can travel long distances.

Sky Wave (2–30 MHz): Utilized in international broadcasts and amateur radio, these waves are reflected at the ionosphere, allowing global communication.

Line-of-Sight (>30 MHz): Higher frequencies enable direct communication along a straight line of sight, suitable for mobile phone systems and satellite communication

Blocking or Shadowing: Large obstacles like buildings can block or attenuate signals, particularly at higher frequencies.

Reflection and Refraction: Changes in the density of the medium (e.g., atmosphere) can cause waves to bend, affecting signal paths.

Scattering and Diffraction: Objects smaller than the wavelength can scatter or diffract waves, leading to signal weakening or redirection.

Multi-Path Propagation:

Signals may take multiple paths to reach the receiver due to reflection, scattering, and diffraction.

Different path lengths cause delays in signal arrival times, resulting in delay spread.

Delay spread leads to effects like intersymbol interference (ISI), where symbols overlap and cause transmission errors.

Mobile transceivers or changing environments exacerbate these effects, leading to short-term and long-term fading, where received signal power fluctuates over time.

Doppler Shift:

Movement of senders or receivers introduces frequency shifts due to the Doppler effect, particularly relevant for fast-moving transceivers like satellite

MULTIPLEXING

1. Space Division Multiplexing (SDM):

   • SDM involves dividing the available communication channel into multiple physical paths, typically achieved through the use of spatially separated antennas or fibers.
   • Each path serves as an independent communication channel, allowing multiple users or signals to transmit simultaneously without interfering with each other.
   • Common examples include multiple antennas in MIMO (Multiple Input Multiple Output) systems or multiple optical fibers in fiber optic communication systems.

2. Frequency Division Multiplexing (FDM):

   • FDM involves dividing the available frequency bandwidth of a communication channel into multiple non-overlapping frequency bands.
   • Each user or signal is allocated a specific frequency band within the overall bandwidth.
   • Different users can transmit simultaneously by utilizing different frequency bands, and these signals are combined at the receiving end.
   • Commonly used in radio broadcasting, cable television, and some legacy telephone systems.

3. Time Division Multiplexing (TDM):

   • TDM involves dividing the available time duration of a communication channel into multiple time slots.
   • Each user or signal is allocated a specific time slot within the overall time duration.
   • Users take turns transmitting during their assigned time slots, and these time-slotted signals are interleaved at the receiving end.
   • Commonly used in digital telephone systems, where each conversation is assigned a time slot within a frame.

4. Code Division Multiplexing (CDM):

   • CDM, also known as Code Division Multiple Access (CDMA), involves assigning a unique code or sequence to each user or signal.
   • Users can transmit simultaneously using the same frequency band and time duration.
   • Each user's signal is spread across the entire bandwidth using their unique code.
   • At the receiving end, the receiver correlates the received signal with the appropriate code to extract the desired information.
   • CDMA is commonly used in modern cellular networks, such as 3G and 4G LTE, due to its robustness against interference and ability to support multiple users.

MODULATION
Modulation is the process of modifying one or more properties of a carrier signal, such as its amplitude, frequency, or phase, in order to embed information for transmission. The carrier signal typically has a higher frequency than the signal containing the information to be transmitted. Modulation allows us to convey information over a communication channel by altering certain characteristics of the carrier signal in accordance with the variations in the input signal.

1. Amplitude Shift Keying (ASK):

   • In ASK, digital data is represented by varying the amplitude of the carrier signal.
   • Different amplitudes represent binary values (e.g., 1 and 0).
   • ASK is simple and requires low bandwidth, but it is susceptible to interference.
   • While ASK is not commonly used for wireless transmission due to its susceptibility to amplitude variations in wireless environments, it is used in optical transmission, where light pulses represent binary values.

2. Frequency Shift Keying (FSK):

   • FSK assigns different frequencies to binary values.
   • In binary FSK (BFSK), one frequency represents binary 1 and another frequency represents binary 0.
   • FSK modulation can be implemented by switching between oscillators with different frequencies.
   • Demodulation of FSK signals can be achieved using bandpass filters and comparators.
   • FSK requires a larger bandwidth compared to ASK but is less susceptible to errors.

3. Phase Shift Keying (PSK):

   • PSK represents digital data by shifting the phase of the carrier signal.
   • In binary PSK (BPSK), a phase shift of 180° (or π radians) represents a change in the binary value.
   • BPSK modulation can be implemented by multiplying the carrier frequency by +1 for binary 1 and by -1 for binary 0.
   • PSK modulation is more resistant to interference compared to FSK but requires more complex receiver and transmitter designs.
   • Synchronization between the transmitter and receiver in terms of frequency and phase is necessary for accurate signal reception, often achieved using a phase lock loop (PLL).

SPREAD SPRECTRUM

Spread spectrum techniques involve spreading the bandwidth needed to transmit data over a larger frequency range. This process offers several advantages, primarily resistance to narrowband interference. Here's how it works:

1. Resistance to Interference: By spreading the signal over a larger frequency range, the energy needed to transmit the signal remains the same, but it becomes more resistant to narrowband interference. This interference can be effectively masked by the spread signal, making it difficult to distinguish the user signal from background noise.

2. Despreading at Receiver: At the receiver end, the spread signal is despread to convert it back into a narrowband signal. This process effectively separates the user signal from interference, allowing for the reconstruction of the original data.

3. Spread Spectrum for Multiple Channels: Spread spectrum techniques can be applied to multiple channels, each using frequency division multiplexing (FDM) initially. However, with spread spectrum, frequency planning is no longer required, and all channels can use the same frequency band. Code division multiplexing (CDM) is then used to separate the different channels at the receiver end.

4. Security and Military Applications: Spread spectrum combined with CDM offers high resistance to interference and relative security, especially when secret codes are used for spreading. This makes it attractive for military applications where multiple signals need to coexist without coordination.

5. Everyday Applications: Spread spectrum and CDM are also becoming more attractive for everyday applications, particularly in wireless communication systems where frequencies are limited. Spread spectrum allows for the overlay of new transmission technologies on existing narrowband systems, expanding the use of limited frequency bands.

However, spread spectrum technologies also have drawbacks, including the increased complexity of receivers due to the need for despreading and the potential for increased background noise levels, which may interfere with other transmissions if not managed properly.

STEPS

an idealized narrowband signal from a sender of user data is sent

The sender now spreads the signal in step ii), i.e., converts the narrowband signal into a broadband signal.

During transmission, narrowband and broadband interference add to the signal in step iii). The sum of interference and user signal is received.

The receiver now knows how to despread the signal, converting the spread user signal into a narrowband signal again, while spreading the narrowband interference and leaving the broadband interference.

In step v) the receiver applies a bandpass filter to cut off frequencies left and right of the narrowband signal. Finally, the receiver can reconstruct the original data because the power level of the user signal is high enough,

Frequency Hopping Spread Spectrum (FHSS) systems divide the available bandwidth into multiple channels with guard spaces between them. Transmitter and receiver switch between these channels over time, implementing Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). Here's a simplified breakdown:

1. Channel Division: The total available bandwidth is split into channels, each with smaller bandwidth, and guard spaces between them.

2. Hopping Sequence: The pattern of channel usage is called the hopping sequence, and the time spent on each channel is known as the dwell time.

3. Slow vs. Fast Hopping:

   • Slow Hopping: The transmitter uses one frequency for several bit periods before hopping to the next frequency. It's typically cheaper and has relaxed tolerances but is less immune to narrowband interference.
   • Fast Hopping: The transmitter changes frequency several times during the transmission of a single bit. It's more complex to implement but better at overcoming interference and frequency selective fading.

4. FHSS Implementation:

   • Transmitter: The user data is modulated using digital-to-analog modulation schemes like FSK or BPSK. Then, frequency hopping is performed based on a hopping sequence, and the signal is modulated again to generate a spread signal.
   • Receiver: It needs to know the hopping sequence and stay synchronized. The receiver performs inverse operations of modulation to reconstruct user data.

5. Bluetooth as an Example: Bluetooth is an FHSS system that performs 1,600 hops per second using 79 hop carriers spaced equally with 1 MHz in the 2.4 GHz ISM band.

Direct Sequence Spread Spectrum (DSSS) systems are a type of spread spectrum communication that involves the use of chipping sequences to spread the bandwidth of a signal. Here's a simplified breakdown:

1. Chipping Sequence: DSSS takes a user bit stream and performs an XOR operation with a chipping sequence. This sequence consists of smaller pulses called chips, and if generated properly, it appears as random noise.

2. Spreading Factor: The spreading factor (s) determines the bandwidth of the resulting signal. A higher spreading factor means a wider bandwidth.

3. Modulation: In the transmitter, the user data is spread with the chipping sequence (digital modulation), and then modulated with a radio carrier.

4. Receiver Complexity: The receiver needs to perform the inverse functions of modulation to recover the original data. This involves demodulating the received signal, synchronizing with the transmitter's chipping sequence, and performing correlation to reconstruct the original data.

5. Challenges: DSSS receivers face challenges such as noise, multi-path propagation, and synchronization issues. Rake receivers, which use multiple correlators to handle multi-path signals, are often employed to address these challenges.

CELLULAR SYSTEMS
A cellular system is a type of mobile communication network that divides a geographical area into smaller units called cells. Each cell is served by a base station or transmitter, and these cells collectively cover the entire service area. 

The purpose of cellular systems is to provide wireless communication services to mobile users within the coverage area.

Base Stations: These are radio transmitters/receivers located within each cell. They handle the transmission and reception of signals to and from mobile devices within their respective cells.

Mobile Devices: These are the user devices such as cell phones, tablets, or other wireless devices that communicate with the base stations to make calls, send messages, or access data services.

Cellular systems for mobile communications are designed with small cells, each served by a transmitter or base station. These cells vary in size but are typically smaller in urban areas and larger in rural regions. While the shape of cells is not perfect, they are influenced by factors such as terrain, buildings, and system load.

Advantages of Small Cells:

1. Higher Capacity:

   • Small cells enable frequency reuse, allowing for more users per square kilometer compared to large cells.
   • Frequency reuse is crucial in mobile phone systems, where frequencies are limited but the number of users is high.

2. Less Transmission Power:

   • Mobile stations in small cells require less transmit power, which is beneficial for energy-constrained handheld devices.
   • Large cells would require much higher transmit power for adequate coverage, leading to energy inefficiency.

3. Localized Interference:

   • Small cells reduce interference issues compared to large cells, as interference is primarily local rather than spanning long distances.

4. Robustness:

   • Cellular systems with small cells are decentralized and more resilient to component failures. A failure in one cell affects only a small area.

Disadvantages of Small Cells:

1. Infrastructure Needs:

   • Small cell networks require a complex infrastructure including antennas, switches, and location registers, making the system expensive to deploy and maintain.

2. Handover Requirements:

   • Mobile stations must perform frequent handovers when moving between cells, particularly in densely populated areas with many small cells.

3. Frequency Planning:

• Careful frequency planning is necessary to avoid interference between adjacent cells. Different schemes, such as frequency division multiplexing (FDM) and time division multiplexing (TDM), are employed to minimize interference.

various techniques and strategies are used in cellular systems to minimize interference and optimize frequency allocation, particularly frequency division multiplexing (FDM), time division multiplexing (TDM), and code division multiplexing (CDM).

Frequency Division Multiplexing (FDM): In FDM, different transmitters within each other's interference range use disjointed sets of frequencies to avoid interference. Cells are organized into clusters, where each cell within a cluster uses a unique set of frequencies. This helps minimize interference between neighboring cells. Sectorized antennas may also be employed to further reduce interference, especially for larger cell radii.

Dynamic Channel Allocation (DCA): DCA dynamically allocates frequencies to cells based on their traffic load. Cells with heavier traffic are allotted more frequencies, while cells with lighter traffic may borrow frequencies from neighboring cells. This dynamic allocation helps optimize frequency utilization and mitigate congestion.

Code Division Multiplexing (CDM): CDM separates users not by frequency but by the unique codes they use. This allows for more efficient use of the available spectrum and simplifies frequency planning. CDM cells are said to "breathe" as their coverage area can adjust based on the current load. As more users join a cell, it shrinks to maintain signal quality and reduce interference.