Digital Communication system elements

Digital Communication System Elements

• The purpose of a digital communication system is to transmit information from a source to a destination.
• The transmitter converts a signal to a waveform suitable for transmission over a channel.
• A voice bandwidth is 4kHz, which cannot be transmitted over the channel due to its bandlimited nature.
• The transmitter translates the signal's frequency band for suitable transmission.
• Transmitter processes include signal formatting, sampling, quantization, source encoding, encryption, channel encoding, multiplexing, pulses shaping, modulation, amplification, filtering, as well as frequency translation.
• A channel is a physical medium used to send a signal from a transmitter to a receiver.
• Guided wireline channels are fiber optics, twisted pair, and coaxial cable.
• Unguided wireless channels are optical, radio, and acoustic.
• Twisted Pair(TP) cables contain two insulated copper wires.
• Category 3 cables are traditional phone wires, 10 Mbps Ethernet.
• Category 8 cables are 25Gbps Ethernet.
• TP cables can be shielded(STP) or unshielded(UTP).
• Coaxial cables contain two concentric copper conductors and are bidirectional.
• Coaxial cables are either baseband(single channel on cable or legacy Ethernet) or broadband for multiple channels on cable like HFC(Hybrid Fiber Coax).
• Fiber optic cables enable high-speed operation, point-to-point transmission with 10s to 100s Gbps, have a low error rate and are immune to electromagnetic noise.
• Radio link types include terrestrial microwave(90 Mbps channels e.g.), LAN WiFi(11-600Mbps), wide-area cellular(~40Mbps-10Gbps 5G), and satellite(27-50MHz typical bandwidth).
• Satellite geosync has a 270msec end-end delay to orbit.
• Licensed bands are obtained by operators who pay for a license such bands include 800 MHz, 1.8 GHz, and 2.1 GHz.
• Unlicensed bands are used without a license as long as regulatory requirements are met such as 900 MHz, 2.4 GHz, 5 GHz, and 60 GHz ISM.
• Unlicensed bands may vary in different countries.
• Channels cause signal distortion.
• Additive distortion is where the output of the channel contains other signals plus the input signal
• Non-additive distortion includes spectral modification, filtering, and non-linear distortion.
• In analog communication, the signal is amplified which amplifies the noise combined with the original signal.
• In digital communication, the receiver tries to determine if the received signal is 0 or 1 and then replaces it with the corresponding signal.
• The receiver recovers the original signal.
• Receiver processes include frequency translation, amplification, filtering, demodulation, demultiplexing, decoding and error correction, deciphering, reconstruction, and formatting.

Layered Network Architecture

• The Physical(PHY) layer deals with the transmission of bits over the medium and includes modulation, coding, and signal processing.
• The PHY layers design is based on the specific constraints and needs of systems like WLAN(OFDM for short-range high-speed data) and LTE/5G(broader coverage and high-speed data transmission with more complex schemes).
• The Data Link layer handles data framing, error detection, and Medium Access Control(MAC).
• The MAC manages device access and communication medium sharing such as airwaves in wireless systems.
• The Data Link layer is implemented on the network for example WLAN uses CSMA/CA for collision avoidance and LTE/5G uses algorithms for better utilization efficiencies.
• Both PHY and Data Link layers are customized in systems such as WLAN, LTE, and 5G to optimize performance for bandwidth, latency, and efficiency.

Wireless vs. Wired

• Wireless networks have scarce resources especially bandwidth WLAN has 1 Gbps vs LAN with 1Gbps.
• Wireless communication is less reliable due to fading, shadowing, background noise, and interferences.
• Interferences include inter-symbol interference(ISI), intercell interference, and interference from other systems.
• Wireless networks enable user mobility like handoff and location management.
• Channel reliability is affected by Doppler shift(coherence time).
• Wireless channel gain measures how the wireless channel alters a transmitted signal's power taking into account attenuation, amplification, and distortion, can be real or complex.
• Factors influencing channel gain are Path Loss(signal attenuation with distance), Shadowing(obstructions causing additional signal loss), Multipath Fading(signal reflections causing constructive/destructive interference, and Doppler Shift(frequency shifts due to motion).
• Types of channel gain include Large-scale gain(accounts for path loss and shadowing) and Small-scale gain(accounts for rapid fluctuations due to multipath and motion.
• Channel gain plays a critical role in designing wireless systems, directly impacting signal-to-noise ratio(SNR), data rate, and error performance and is used in link adaptation, beamforming, and power control.
• Wireless networks have a time-varying environment, time-varying interferers, and location-dependent errors.
• Wireless networks have the broadcast nature and multiple access for sharing the medium creating less-secure environments.
• Wireless nodes can be static/fixed or mobile, and mobile nodes can use a wireline network.
• Wireless & Mobile Networks (WMN) include nomadic systems and mobile systems.
• Nomadic systems are used typically when the node is stationary in WLAN, WPAN
• Mobile systems are used when the node is moving fast, like 3G/4G systems.
• Nomadic systems can provide a faster link.

WMN: Infrastructure vs. Ad hoc

• Infrastructure-based wireless networks use an access point(AP) as the interface between wireless and wireline backbone in a star topology with handoff support, cell planning with reuse, and cellular and 802.11 WLANs with APs.
• Ad-hoc networks use wireless multi-hop transmission in peer-to-peer topology like 802.11 ad hoc mode, and Bluetooth.
• Wireless mesh networks consist of wireless infrastructure.
• Wireless networks connect and allow devices to communicate without physical connections using radio or infrared signals enabling users to move freely within a network area.
• Elements of a wireless network are wireless hosts, network infrastructure and wireless links.
• Wireless hosts run on laptops and smartphones and can be stationary or mobile.
• Base stations are connected to a wired network and relay packets between the wired network and the wireless hosts in their area like cell towers that manage mobile connection to a wired network(infrastructure) and handoffs between base stations.
• In ad hoc mode there are no base stations, nodes can only transmit to other nodes within link coverage and nodes organize themselves into a network routing among themselves.
• Single-hop infrastructures are used for hosts connected to the base stations of WiFi, WiMAX, and cellular with connections to the larger internet.
• Single-hop topologies without infrastructure include Bluetooth and ad hoc networks.
• Multi-hop topologies with infrastructure are used by hosts that have to relay through several wireless nodes to connect to larger Internet mesh nets.
• Multi-hop topologies without infrastructure are MANET and VANETs.

Frequency, Wavelength, Amplitude, and Phase

• Coding and modulation transfers or mapping digital information to signals so a receiver may use a decoder and demodulator.
• Coding and modulation plays a direct role on the the capacity and data rate of communication systems.
• Continual development is being done in coding and modulation to meet the increasing mobile data requirements
• Signals waveforms are fundamental carriers of data in communication systems and waves act similar to electromagnetism in wireless networking.
• Electromagnetic waves are generated and received using electronic circuits.
• A sine wave, denoted A sin(2π ft + θ), is the simplest form of a wave with A denotes amplitude, f is frequency, θ is phase, and t current time.
• Sine waves are cyclic and the completion of one pattern is described as a cycle.
• Frequency is measured in Hertz (Hz) which is number of cycles per second
• A wave period (T) is how much time wave takes for one cycle to pass with T=1/f represented in units of time.
• As the wave speeds up the wave period decreases.
• Amplitude is the height of the wave from zero to its maximum.
• Phase is the zero-amplitude shift from a reference point and the maximum phase shift is 360° for a full cycle returning to zero.
• Phase is usually measured in radians, where 360° = 2ㅠ radians and in example a 45° phase shift equals π/4 radians.
• A sine wave with a 45° phase can be written as the sum of two parts: sin(2π ft + π/4) = (1/√2)*sin(2π ft) + (1/√2)*cos(2π ft) .
• The first part is called the In-phase component(I).
• The second part is called the Quadrature component (Q).
• Waves propagate through space covering varying distances over its periods of transmission
• Wavelength is the distance is the length of the distance per cycle.
• In air or space, all electromagnetic waves travel at the speed of light (300 m/µs) =3x108 m/s.
• The period (T) is the time taken to complete one cycle with T=1/f.
• Wavelength is inversely proportional to frequency and can be represented using the equation λ=cT= c/f for the speed of light c.
• For acoustic (sound) communications, C represents the speed of sound (343 m/s in dry air at 20° Celsius).
• The speed of light for IEEE802.11b for one second is f = 2.4 GHz = 2,400,000,000 cycles, wavelength(λ) = c / f which gives us 12.5 cm.
• Antennas tend to work well with a size of 1, ½, or ¼ of wavelegth.

Time and Frequency Domains

• Waves can be represented in both time and frequency domains and can be converted back and forth.
• A sine wave with frequency 1 (f) and amplitude 1 (A) will be converted to a frequency domain to a pulse at f=1 with a height of A=1.
• A second sine wave that has a frequency of 3f and amplitude of 1/2A in the frequency domain will have a pulse at f= 3 with a height A = 0.5.
• The Fourier transform transforms time to the frequency domain whereas the inverse Fourier transform transforms from frequency to time.
• Fast algorithms are Fast Fourier Transform (FFT) and Inverse FFT (IFFT).

Electromagnetic Spectrum

• Wireless communications use electromagnetic waves that can propagate through air or a vacuum and are generated through TVs, power supplies, remote controls, microwave ovens, and wireless routers,.
• Electromagnetic waves has a wide frequency spectrum that is found from 10 Hz to 300 THz.
• Authorities such as the FCC in the US and ACMA in Australia regulate spectrum use.
• The government controls large spectrums to be used for government use such as radar, and military communications.
• Remaining spectrum are is allocated to network operators with exclusive use.
• Spectrum allocation principles and authorities follow principles of maximizing utilization to improve on new technologies ensuring they are satisfy market competition and fairness.
• Historically, wireless communications have exploited the spectrum between 100 and to 6 GHz
• The industry is looking towards using spectrums beyond 6 GHz to achieve mobile data demands such as the 60 GHz used for Wi-Fi standards and 5G planning targeted past 60 GHz.
• Some spectrums are unlicensed which enables free use such as e.g., 2.4 GHz for Wi-Fi/
• The uses are subject to rules such as the Wi-Fi transmission being constrained to approximately 100 mW.
• The 433 MHz license-exempt spectrum has use cases such as keyless entry..
• The 900 MHz license-exempt spectrum has use cases such as amateur radio or IoT devices.
• The 2.4 GHz license-exempt spectrum has use cases such as WiFi and microwave ovens use this spectrum range.
• The 5.2 GHz/5.3 GHz/5.8 GHz license-exempt spectrum has use cases in WiFi and cordless phones.