Digital Communication System Elements

Digital Communication System Elements

• The purpose of digital communication is to send information from source to sink.
• The signal coming from transducer is a baseband, lowpass, and usually bandlimited signal.
• Voice bandwidth is 4kHz and is bandlimited, so it cannot be transmitted over the channel.
• The transmitter (Tx) translates a signal's frequency band to another frequency suitable for transmission.
• Transmitter process includes signal formatting, sampling, encoding, encryption, multiplexing, modulation, amplification, and frequency translation.
• A channel serves as a physical medium for sending signals from the transmitter (Tx) to the receiver (Rx).
• Wireline channels are guided and use fiber optics, twisted pair, or coaxial cable.
• Wireless channels are unguided and use optical, radio, and acoustic methods.
• Fiber optic cables operate at high speeds, support point-to-point transmissions, and have low error rates.
• A TP Category 3 is for traditional phone wires, 10 Mbps Ethernet, and Category 8 is for 25Gbps Ethernet.
• Radio link types include terrestrial microwave, LAN (e.g., Wifi), wide-area (e.g., cellular), and satellite.
• LAN WiFi speeds are 11Mbps, 54 Mbps, 600 Mbps, and 5G cellular is ~40 Mbps-10Gbps.
• Satellite bandwidth is typically 27-50MHz with geosynchronous vs low altitude.
• Geosync has 270msec end-end delay to orbit, and the lower end end-end delay is 270 msec.
• Operators get licensed bands by paying money, such as 800 MHz, 1.8 GHz, and 2.1 GHz.
• Unlicensed bands include 900 MHz, 2.4 GHz, 5 GHz, 60 GHz ISM (industrial, scientific, and medical) bands.
• The common property is a signal is distorted over the channel, either additive or no additive.
• Additive distortion means the output contains other signals plus the input.
• Non-additive distortion involves spectral modification, filtering, and non-linear distortion.
• Modern communication is in the digital realm instead of analog.
• Analog means the signal is amplified, which amplifies noise with the original signal.
• Digital means Rx tries to answer if the received signal is 0 or 1 then the signal is adjusted.
• The receiver recovers the original signal that passed to the transmitter.
• The receiver process includes frequency translation, amplification, demodulation, deciphering, and formatting.

Layered Network Architecture

• The Physical (PHY) Layer deals with the actual transmission of bits including modulation, coding, and signal processing.
• The PHY Layer's design is tailored for the system's constraints, i.e. WLAN employs OFDM for short-range high speed data.
• The Data Link Layer handles data framing, error detection, and Medium Access Control (MAC).
• MAC manages how devices access and share the communication medium like airwaves.
• Data Link Layer implementation is based on the type of network, i.e. WLAN uses CSMA/CA for collision avoidance, and LTE/5G uses advanced scheduling algorithms for efficient spectrum utilization.
• Both the PHY and Data Link layers are optimized for systems like WLAN, LTE, or 5G.

Wireless vs. Wired

• Wireless has scarce resources in terms of bandwidth, such as 1 Gbps WLAN compared to 1 Gbps LAN.
• Wireless communication is less reliable because of fading, shadowing and interferences.
• Wireless has Interferences such as inter-symbol interference (ISI) or inter-cell interference with other systems.
• Characteristics of wireless communication include user mobility with handoff, location management, and channel reliability is affected by Doppler shift.
• Wireless Channel Gain measures how the wireless channel alters a transmitted signal's power, including attenuation, amplification, and distortion.
• Channel gain may be a real numbers for amplitude gain, or a complex number for amplitude and phase gain.
• Path Loss is signal attenuation due to the distance between transmitter and receiver.
• Shadowing is obstructions that cause additional signal loss and multipath fading is signal reflections.
• Doppler Shift is frequency shifts due to relative motion between transmitter and receiver.
• Large-scale gain accounts for path loss and shadowing and changes slowly over distance.
• Small-scale gain accounts for rapid fluctuations due to multipath and motion.
• Channel gain is critical for designing wireless systems because it affects the signal-to-noise ratio (SNR), data rate, and error performance.
• Channel gain is used in link adaptation, beamforming, and power control.
• Wireless has a time-varying channel, time varying interferers and location-dependent errors.
• Broadcast nature channels have multiple access for sharing the medium making the environment less secure.
• A wireless node can be static and fixed, such as a fixed wireless local loop (WLL) or IEEE 802.16 Broadband wireless access (BWA).
• A mobile node network such as e.g., laptop can use an Ethernet link.

Wireless & Mobile Networks (WMN)

• Nomadic networks are communications is typically done while the node is stationary, and it includes WLAN and WPAN.
• Mobile systems can be done while the node is moving fast, such as 3G/4G cellular systems.
• Infrastructure-based wireless networks have an access point as an interface between wireless and wireline backbone using a star topology.
• Ad hoc is a wireless multi-hop transmission using a peer-to-peer topology like 802.11 ad hoc mode, Bluetooth.
• Wireless mesh networks are the network for a type of wireless infrastructure.
• Wireless networks are telecommunications networks that allow devices to connect and communicate without cables, using radio or infrared signals.
• Wireless networks allows devices to move freely within a network area.
• Elements of a wireless network include wireless hosts (laptops, smartphones) that run applications and can be stationary or mobile.
• Elements include a base station that is typically connected to the wired network.
• A base station relays packets and sends packets between the wired network and wireless hosts.
• Wireless links are typically used to connect mobile(s) to base station.
• Infrastructure mode connects the base station to the wired network.
• Handoff helps the mobile change base station offering connection into the wired network.
• Ad hoc mode does not have base stations, the nodes can only transmit to other nodes withina link coverage, creating a network route.
• In a Wireless network: with single hop the connects to a base station, WiFi, WiMAX or cellular) which connects to larger Internet.

Frequency, Wavelength, Amplitude, and Phase

• Coding and modulation map digital information to signals, enabling retrieval by receivers using a decoder and demodulator.
• Coding and modulation directly impact the capacity and data rate of communication systems.
• Innovation: Continuous development of new coding and modulation techniques to meet growing mobile data demands.
• Signal waveforms are carriers of data in communication systems, and electromagnetic waves are waves generated by electronic circuits.
• A sine wave, A sin(2π ft + 0), represents the simplest form of a wave, where "A" is Amplitude, "f" is frequency, 0 is phase, and "t" is current time.
• Sine waves are cyclic and one complete pattern is a cycle.
• Frequency, measured in Hertz (Hz), is the number of cycles per second.
• Wave period (T) is just how much time a wave takes for one cycle to pass; T = 1/f.
• Amplitude is the height of the wave from zero to its maximum value.
• Phase is the shift from a reference point, with zero phase starting at zero amplitude.
• The maximum phase shift is 360°, equating to a full cycle, returning phase to zero.
• A sine wave with a phase of 45° can be written as the summation of two parts which the in-phase component and the quadrature component.
• Wavelength (λ) is the distance between two points of corresponding phase in consecutive cycles.
• Wireless standards, for example, 802.11b, 802.11g, 802.11n and 802.16 can all work at 2.4 GHz.

Time and Frequency Domains

• Waves can be represented in both time and frequency domains.
• Time domain representation can be converted to the frequency domain and vice versa.
• Fourier transform: The transformation from time to the frequency domain
• Inverse Fourier transform: The reverse transformation.
• Fast algorithms include Fast Fourier Transform (FFT) and Inverse FFT (IFFT).

Electromagnetic Spectrum

• Wireless communications use electromagnetic waves that can propagate through air or a vacuum.
• TVs, power supplies, remote controls, microwave ovens, and wireless routers generate or use electromagnetic waves.
• Electromagnetic waves span a wide frequency range, from 10 Hz to 300 THz.
• The spectrum comprises all usable frequency ranges, but its a limited natural resource.
• Spectrum use is regulated by authorities like the FCC in the US and ACMA in Australia.
• Large portions of the spectrum are reserved for government use (e.g., radar, military communications).
• Spectrum-allocation authorities follow principles to maximize utilization and promote new technologies.
• Historical wireless communications have used the spectrum between 100 kHz to 6 GHz.
• Industry is trying to explore over 6 GHZ for mobile data demands such as 60 ghz for 5G.
• Some spectrum is unlicensed and available for free use (e.g., 2.4 GHz band for Wi-Fi), but transmit power is often limited to around 100 mW.
• Examples of unlicensed spectrum include keyless entry at 433 MHz, amateur radio and IoT at 900 MHz, WiFi and microwave ovens at 2.4 GHz, and cordless phones at 5.2/5.3/5.8 GHz.