Power Line Communication - Technical considerations for implementation (Part II)

For implementing PLC, it is necessary to consider next Technical Considerations regarding;
Resource Management and Allocation Techniques:

(1)    Network planning resources, which is related to grid topology;

(2)    PLC resources, which is related to PHYsical (PHY) layer modem parameters

(3)    Traffic resources, which is related to Media Access Control (MAC) layer.

(1) Network Planning
It is necessary to consider the elements in the electric network; taps, branching, splits, bridges, capacitor banks (Reactive Power compensation), recloser, sectionalizers and transformers (implementation of coupling systems) and with Internet Service Provider (ISP), considering at the same time the coexistence with other communication technologies (e.g. , RF, wireless etc).

Bss, as basic element in PLC network, the relays and WANAP's (ISP) must to be quantified properly, PLC modems (or router) are depending of users connected to LV, then it is difficult to be optimized. Then it is necessary to be considered:
(1) Bss (and BM) placement and quantity optimization problem,

(2) relay placement and quantity optimization problem, and

(3) WANAP (Wide Area Network Access Point) choice optimization problem.

Figure 1_ Example of equivalent Electric and PLC scheme of one typical AMI infrastructure. The connection to internet (WANAP) is directly from secondary of the trafo in the secondary substation. Coupling techniques would be installed in Trafos for transmit data from primary to secondary of the trafo or vice versa, depending of the way of communication. Relay is used to switch one station or other station, and one router or routers to other routers (depending of analog or digital meter). Repeater is used to amplify the signal from down link routers, in case of digital meter, mostly are enclosing repeters and of course, router.

(2) PLC Resources

It is possible to define PLC Resource Allocation (PRA) considering Frequency Division Multiple Access (FDMA: channel access method used in multiple-access protocol as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands or channel, it is used in satellite communication. FDMA, like other Multiple Access systems, coordinates acess between multiple users).

PRA gives great improvements in the system performance with collection of techniques and tools; time-frequency assignment, subcarrier assignment, and power- or bit- rate assignment as function of traffic load, channel condition, channel information availability, and Quality of Service (QoS)  requirements. Considering PHY layer management, by channel profile attenuation, additive noise, and access impedance, are summarized as: 
(1) spectrum allocation and 
(2) power allocation.

(2.1)Spectrum Allocation
OFDM is used by different subsets of the subcarriers for physical layer implementation with frequency diversity and multiuser diversity (when it is possible):

 - Frequency diversity refers to different subcarriers having different channel gains due to the frequency-selective nature of the channel.

- Multiuser diversity refers to different users experiencing different channel conditions due to their distinct locations in the network.

Being necessary to allocate to each user a subset of subcarriers that experience good channel conditions, because one subcarrier may be is good for one user but not for the rest of users.

Spectrum allocation OFDM suggest there types:

(1) OFDM, which determines the set of subcarriers for the user and time slot for the user for down link and up link communications, respectively;

(2) OFDMA, which determines the set of subcarriers allocated to each user for down link and up link communications; and

(3) Clustered-OFDM/OFDMA, which determines the allocations of users in each cluster as well as their set of subcarriers for down link and up link communications.

(2.2) Power Allocation

It is necessary to implement optimum approach  to allocate different power in each subcarrier, being frequency selective in PLC channel, and being classified by three categories: 
 (1) Rate Maximization (RM) issue; using the system bandwidth, BS can transmit with the maximum aggregate data rate to serve PLC modems, considering power limit constraints, knowing the sum-rate capacity maximization.

(2) Margin Adaptive (MA) issue, with minimum QoS requirement for all modems with the lowest possible power.

(3) Rate Adaptive (RA) issue, power allocation in each subcarrier, being total power constant.

(3) Traffic Resources

Cross-layer: PHY and MAC, gives great benefits considering delay and capacity.

The classification of traffic resources allocations and management techniques;

(1) Admission control. If resources are available, the session is admitted. A mechanism to estimate QoS level when new user session is needed having enough resources for this service.

(2) Queue management. Packet classification by placed in queues, identifying and classifying traffic flow.

(3) Traffic scheduling. Traffic flow transmission with necessary QoS parameters which gives the possibility to determine the order users transmissions in terms of slots and subcarriers allowing transmission traffic in time and location.

(3.1) Resource Management and Allocation Techniques

Optimization problems can be solved as Single-Objective Optimization (SOO) problems or as Multi-Objective Optimization (MOO*) problems and may be solved either with exact optimization techniques by heuristic algorithms.

*MOO: is an area of multiple criteria decision making, with mathematical optimization problems involving more than one objective function to be optimized simultaneously, applied in engineering, where optimal decisions need to be taken in the presence of trade-off between two or more conflicting objectives, loosing in one of this objectives in benefit of the opposite.

(3.2) Resource Allocation During Network Planning

The key contributors to implement PLC in terms of resource management and allocation techniques are able to organize in three main categories:

 (1) resource allocation for network planning, which is related to grid topology;

(2) PLC resources based on PHY layer; and

(3) traffic resources, which is related to MAC layer.

(3.3) Analysis of the Transmission Line
Transmission line modeling is one serial circuit with: resistance, capacitance and inductance (R, L , C), determinated by the intrinsic characteristics of the cable: material and geometry. By this parameters, or by experimentally measuring, it is possible to deduce the line's Zc and the propagation constant g.

(3.4) Channel Noise

The noise on the power line communication system is in the 2-30+MHz, being necessary to use mathematical modeling for determining the level of noise. Mostly in LV, the noise has significant impulsive component, limited to frequencies less than 10-20 MHz, with synchronous and asynchronous impulses.
Similarly, aperiodic synchronous and asynchronous impulsive noise is caused by; generatosr and motos synchronized to the power system frequency, power electronic systems by swithching power supplies and automotive spark plugs
On the other hand, high-order harmonics noise of the power line in  2–30+ MHz are able to enter in PLC band (consider harmonics are increasing day by day due to the higher implementation of non linear loads in electronic devices). 

At the same time, especially in MV and HV lines, corona and gap noise must to be considered with background noise. Considering background noise in power line channel, the power spectral density (PSD) is decreasing with the frequency increasing. 

By experimental practices is determined that LV presents low level of noise than MV and HV.

At the same time it is necessary to be considered the change in the characteristics of the noise in the power line due to being or not the power line charged or uncharged.

(3.5) Modulation Schemes

PLC mostly uses Orthogonal Frequency Division Modulation (OFCM), due to the very hostile and time-varying channel, with extensive frequency selectivity to prevent interference to other services.

DS2, Intellon, Amperion and other PLC manufacturers are using OFDM in PLC applications to be compliant to the UPA Digital Home Standards (DHS) specified to Media Access Control (MAC) and PHYsical layers (PHY), based in trellis-coded modulation and TDMA: Time Division Multiple Access which allows several users to share the same frequency channel by dividing the signal into different time slots.

(3.6) Channel Capacity

The level of error-free data rate, by given signal power and channel noise must to be considered for the communication performance.

Of course, higher power offers more capacity, but emissions limits on allowable interference to other users like HAM (Amateur Radio), etc., are able to prevent power from being raised without limit..

(3.7) Power Line Communication Systems

The data in PLC systems are modulated onto a single carrier (*PSK, *QAM, etc) or the multiple carriers of a multi-carrier system (e.g.: OFDM) by coupling systems, in the points of the communication, then coupling systems in the transmitter and the receiver.. 

Effectively PLC uses In the US and Europe appears with proper power control, notching out of HAM frequencies between 2–34 MHz band (up to even 80 MHz), but final definition is not yet implemented, waiting for PLC more relevance.

*PSK: Phas-Shift Keying: Digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each patterns of bits forms the symbol that is represented by the particular phase.

*QAM: Quadrature amplitude modulation, is both, an analog and digital modulation scheme. It conveys two analog message signals, or two digital bit stream, by changing (modulating) the amplitudes of two carriers waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carriers waves, usually sinusoidal, are out of phase with each other by 90º (quadrature).

(3.7.1) Coupling systems

Coupling systems are essential in PLC systems, for transmitting and receiving data, in a very small communication level signal (1 mW) in a channel with high power (MW). 

This coupling systems are available in inductive or capacitive (in the range of microfarads and microhenry) with isolation transformer and protection systems. In an inductive coupler, a transformer with a capacitive blocker for the power line frequency is used to select the PLC signal from the signal on the line. The couplers can be installed between two phases or between a phase and ground, requiring a minimal conditions of losses at least of 80 dB for the power system frequency, and low of (< 5 dB) of losses in the communication frequency band. 

Ambient Corporation, which provide solutions for Smart Grid Infrastructure, from metering; voltage & current, Partial Discharge monitoring etc, and support PLC applications has a medium voltage coupler power lines up to 25 kV with maximum of 50 kV. The signals can be coupled between the line and ground (neutral), or between two phases in a differential fashion. 

More recent coupling method has lower attenuation, working in high-frequency coupler for the 50–550 MHz band (less than 5 dB loss in a back-to-back connection). 

Figure 2_ Example of Transformer in Distribution Primary Substation, where large component of impulsive noise (overall in LV grid) does not pass through transformer, being coupling techniques the way for achievement the communication

Bibliography: 

1_ Power Line Communication for Smart Grid, Smart Cars, and Smart Homes - Krzysztof Iniewsky and Tracey Mozel.

2_ IEEE 1901.1 - IEEE Standard for Broadband over Power Line Networks: Medium Access Control and Physical Layer Specifications.