At initial stage, manufacturers of BPL systems have developed their own proprietary solutions for injectors, repeaters, extractors and couplers. These solutions have been implemented in a variety of system architectures, which feature different modulation techniques and designs at PHY, MAC and LLC layers. Such systems have raised incompatibility issues.
In order to combat channel impairments (noise, multipath, strong channel selectivity, non-linear channel characteristics), a number of different technologies at PHY layer have been employed that range from spread spectrum to orthogonal frequency-division multiplexing (OFDM). OFDM is a type of frequency-division multiplexing system that provides better channel throughput, better spectrum efficiency and robustness against frequency selectivity because all of the underlying sub-carriers are orthogonal to one another.
There are numerous modulation and access design techniques that can be used in association with OFDM. These include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (QAM) and 6-bit, 64-constellation QAM. Various forward error correction codes are also used.
Since BPL access networks operate in a shared transmission medium where subscribers compete to use the same transmission resources, MAC has been designed for point-to-multipoint applications and is based on collision-sense multiple-access with collision avoidance (CSMA/CA). MAC layer also describes how secure communications are delivered, by using secure key exchange during authentication and encryption (using advanced encryption standard or data encryption standard) during data transfer.
In order to streamline the functionality of BPL systems, Open Power line communication European Research Alliance, European Telecommunications Standards Institute, Universal Powerline Association, European Committee for Electrotechnical Standardization, HomePlug Powerline Alliance and Institute of Electrical and Electronics Engineers (IEEE) have developed standards for BPL systems.
The IEEE has constituted IEEE P1675 ‘Standard for Broadband over Power Line Hardware,’ IEEE P1775 ‘Powerline Communication Equipment—Electromagnetic Compatibility (EMC) Requirements—Testing and Measurement Methods’ and IEEE P1901 ‘Draft Standard for Broadband over Power Line Networks: Medium Access Control and Physical Layer Specifications.’
HomePlug Powerline Alliance has developed in-house BPL specifications: HomePlug 1.0 for speeds up to 14 Mbps and HomePlug AV for speeds greater than 100 Mbps.
BPL issues
Interference issues. Most BPL systems are designed to operate in the frequency spectrum from 2 to 30 MHz, but occasionally up to 80 MHz, using MV and LV power distribution network lines. The frequency spectrum from 2 to 30 MHz constitutes a limited natural resource that includes the HF band (3 to 30 MHz), which is being used for many decades by shortwave radio stations, military, aviation agencies, etc. Above the HF band, the frequency spectrum from 30 to 54 MHz is reserved for use by public service and business communications. The spectrum from 54 to 80 MHz hosts television channels (channel numbers 2 to 5) with a small segment used for some other emergency services.
However, because BPL uses some of the radio frequencies used for over-the-air radio systems, mutual interference is a major problem. In order to mitigate interference at some specific frequency, OFDM may be used as it has the ability to notch these specific frequencies.
Moreover, power lines are unshielded and act as radiating antennae for the signals they carry, resulting in interference with communication systems of broadcast radio, military, aviation agencies, etc.
Electromagnetic fields associated with access-BPL system are shown in Fig. 2 and can be categorised as guided mode, coupler fields and radiation mode. Guided mode serves to transport signal energy along the line. Signal energy decays rapidly in directions perpendicular to the line but slowly along its length. Coupler fields are associated with the coupler itself rather than the power line. Radiation fields are considered to be a source of interference because these fields decay relatively slowly.
Interference associated with BPL can be broadly categorised into two groups: near-field (0.62(D3/λ)1/2<r<2D2/λ) radiation and far-field (r>2D2/λ) radiation, where ‘r’ is the distance from the radiator, ‘D’ is the largest linear dimension of the radiator and ‘λ’ is signal wavelength. Near-field effects persist along the entire length of the wire and may cause serious interference issues. Far-field effects can be quite troublesome because ionospheric propagation of HF radiation can travel thousands of kilometres. In order to reduce these radiations, a balanced configuration of two MV wires driven differentially and spaced appropriately can be employed.
Power line noise. MV/LV power lines are inherently very noisy due to changing nature of the load connected to power lines, number and types of line branches, length of line branches, type of power line equipment connected (such as capacitor banks and transformers), and impedance mismatch caused by unterminated stubs and line branches. In addition, on/off switching of capacitor banks used to correct power factor and switching power supplies often introduce noisy harmonics into the line.
Sadly due to numerous complaints by ham operators of interference, the entire BPL concept was canned and abandoned nationwide.
This is bound to happen with anything that uses RF frequencies since are no frequency bands left that aren’t already in use somewhere.
Even the coming 802.11ah wifi solution will likely be met with complaints from hams since it uses the 902-928mhz ham radio band.