I. PLC Technical Elements 1. Power Line Network Unit (PNU) It controls the power line network and integrates traffic from the distribution network. Through a suitable telecommunications trunk interface, the PNU then transmits the traffic to the feeder network. Depending on the medium used in the feeder network, the PNU can also convert data traffic from the low-voltage distribution network. 2. Power Line Network Terminal (PNT) It provides a suitable interface, such as Ethernet or USB, for end-user PCs or other users. To reduce costs, this standalone device can be integrated with PCs or other devices. 3. Coupling Unit It transmits signals to the line and filters noise. Currently, it is a relatively standalone device that plugs into an electrical outlet; in the future, it may be integrated with a PLC modem. The combination of a PLC modem and a coupling unit within a PC will one day allow the PC to run directly on the network. A distribution network is a shared medium, meaning all connected users share the same "cable." In a typical urban configuration, this translates to approximately 100 to 200 users connected to a single transformer. The PLC system can support 80 users at an optimal transmission rate of 1Mbps, which is sufficient. Customers supported by PLC technology require a Media Access Control (MAC) layer with strong bandwidth allocation capabilities. This enables the power line network to not only support data exchange between 80 Internet users but also flexibly adapt to uplink and downlink data transmission at different rates. II. Data Signal Transmission Technology 1. Digital Spread Spectrum Technology (SST) In current practical applications, to realize communication and control networks for home or economical products, more reliable PLC communication technology for multi-user environments is needed, leading to the development of spread spectrum carrier communication technology. Spread spectrum communication has certain technical advantages over narrowband communication, mainly in terms of interference resistance. Because the bandwidth of spread spectrum carrier signals is usually large (tens to hundreds of kHz), the proportion of frequencies affected by interference is relatively reduced. In other words, various noises can only affect a small portion of the transmitted signal, while most signals can reach their destination completely and correctly. Therefore, it has strong resistance to various types of interference. For the most common type of impulse noise, although receivers in narrowband communication have a narrow passband, allowing only a small portion of noise to enter, the high quality factor of the filters in these receivers means that instantaneous impulse noise can cause self-interference, leading to malfunctions in the transmitted signal. Conversely, using filters with low quality factors increases the passband bandwidth, allowing more noise to enter the receiver. Therefore, narrowband communication has poor resistance to impulse noise. However, using spread spectrum technology, when a high-energy noise signal is received, the receiver automatically stops operating when the high-energy portion of the noise arrives. Thus, the receiver only needs to perform error correction and decoding on a small portion of the affected signal. Furthermore, the filters used in spread spectrum receivers have lower quality factors, thus avoiding system self-interference. Therefore, spread spectrum technology has strong noise immunity. Generally speaking, there are three main approaches to implementing spread spectrum: direct sequence modulation, frequency hopping carrier, and carrier frequency scanning using chirps. 1) Direct-Sequence Modulation: This technique distributes the signal energy evenly across the entire frequency band and spreads the signal by multiplying the data stream using a pseudo-random sequence. This sequence has a symbol rate several times higher than the bit rate of the transmitted signal's binary data. 2) Frequency-Hopping: The spread spectrum signal represents one, several, or a portion of a data bit by extending at a specific frequency for a certain period. When the signal is interfered with at a certain frequency, it can switch to other frequencies within the spread spectrum bandwidth, thus greatly reducing its susceptibility to interference. This method has strong resistance to CW interference. 3) Chirps as Carrier: This method is often used in CSMA networks similar to Ethernet. It uses a series of short, self-synchronizing chirps as carriers. Each chirp typically lasts 100 µs and represents the most basic communication symbol time (UST). These chirps cover a frequency band of 100-400 kHz, always starting at 200-400 kHz and ending at 100-200 kHz. Because the linear scanning bandwidth of the chirps signal is much larger than the signal bandwidth, its linear acceleration is high, while the frequency acceleration of CW interference is generally stable. Therefore, by designing the filter to only allow signals with specific angular acceleration, CW interference can be excluded. Furthermore, this type of chirps waveform also has strong autocorrelation characteristics. This fuzzy logic correlation ensures that all devices connected to the network can simultaneously identify this unique waveform emitted from any device on the network, without requiring synchronization between the transmitting and receiving devices. Power line digital spread spectrum (SST) technology can fully utilize the transmission bandwidth to achieve broadband high-speed data transmission. Spread spectrum communication can overcome the effects of narrowband noise and multipath propagation, making it very suitable for power line communication environments. SST technology is easy to implement, automatically selecting high signal-to-noise ratio frequency bands to resist transient interference; however, inter-symbol interference is severe, requiring a nonlinear equalizer. 2. Orthogonal Frequency Division Multiplexing (OFDM) Technology: OFDM technology uses multiple narrowband orthogonal subcarriers to transmit multiple data streams simultaneously. Each signal has a relatively long symbol time, which avoids inter-symbol interference. By dynamically selecting available subcarriers, this technology can reduce narrowband interference and the impact of frequency valleys. The application of OFDM technology can be traced back to the 1960s, mainly in military high-frequency communication systems. However, the structure of an OFDM system is very complex, thus limiting its further promotion. Until the 1970s, it was proposed to use Discrete Fourier Transform (DFT) to modulate multiple carriers, implementing complex OFDM processing in software, simplifying the system structure and making OFDM technology more practical. In recent years, due to the rapid development of Digital Signal Processing (DSP) technology, OFDM, as a high-speed transmission technology that can effectively combat inter-waveform interference, has been widely used in civilian communication systems. OFDM technology has been applied to broadband data transmission in high-speed modems and wireless FM channels. Fourth-generation (4G) mobile communication will employ OFDM technology, enabling data transmission rates up to 10 Mbit/s. This technology is already used in wireless local area networks (WLANs). Digital terrestrial television broadcasting, currently under development, and the high-speed wireless LAN "IEEE 802.11a" are both slated to utilize this new technology. Orthogonal frequency division multiplexing (OFDM) technology can improve the transmission quality of power line networks. Even under severe interference in the distribution network, OFDM can provide high bandwidth and ensure bandwidth transmission efficiency, and appropriate error correction techniques can ensure reliable data transmission. In an OFDM system, the carriers of each sub-channel are orthogonal, resulting in overlapping spectra. This reduces inter-carrier interference, improves spectral efficiency, and resists equal-amplitude wave interference. However, OFDM receivers are complex, costly, require linear amplification with a large dynamic range, and are sensitive to transient interference. III. Compared with other access technologies, powerline broadband access networks have the following advantages: 1) They fully utilize existing low-voltage power distribution network infrastructure, requiring no cabling whatsoever, making them a "No New Wires" technology that saves resources. The elimination of trenching and wall drilling avoids damage to buildings and public facilities, while also saving manpower. 2) They can provide users with high-speed internet access and voice services, thus increasing users' choices for internet access and phone calls, which benefits other telecommunications service providers by allowing them to improve their services and lower prices. 3) They support home networking, enabling people to enjoy home audio and video networks, multiplayer games, and other entertainment brought by PLC technology. 4) They are a vital force in home automation, connecting smart home appliances through wall sockets throughout the rooms, allowing users to enjoy the comfort and convenience of a digital home in advance. 5) Utilizing the permanent online connection of PLCs, security monitoring systems for fire prevention, theft prevention, and prevention of toxic gas leaks can be built, giving working professionals peace of mind; medical emergency systems can provide reassurance to families with elderly people, children, or patients. PLCs can also be used to provide independent digital community services and e-commerce, enabling home office and remote appliance control. 6) Remote automatic reading of water, electricity, and gas meter data saves utility companies significant costs and provides convenience for users.