Recent advancements in technology, increasing growth in machine-to-machine connectivity, and the need for better resource management, are giving narrowband power line communications the attention it deserves as the most universally-available network medium.
Communications over the power grid is a notoriously difficult concept, due to harsh noise and equipment and standards that can vary widely. New approaches to power line communications are required in order to both reliably operate in this challenging environment and to successfully interoperate with previously installed equipment. Semitech’s earlier article addressed the difficulties of communicating over noisy power lines and the use of OFDMA modulation to overcome these difficulties (see Enhancing reliability in medium-voltage power line communications).
However, there is more than just noise to worry about. Engineers using power line communications must optimize their designs to achieve the best combination of distance, data throughput, error rates (packet retries) and overall reliability. This article addresses how engineers can configure power line communications devices to provide optimal performance for their PLC systems. To meet PLC systems requirements, engineers must configure modulation, select modulation frequencies, determine level of packet redundancy, and select appropriate signal levels.
Power Line Communications Applications
The most prevalent use of narrowband power line communications (NB-PLC) today is in connecting consumer to utilities for Automatic Meter Reading (AMR) and load control - Smart Grid. Such systems have long been a favorite at many utilities because it allows them to move data over an infrastructure that they control. Other rapidly emerging applications include Street Light Control (SLC) and Smart Appliances. In addition NB-PLC starts finding its way into a wide range of applications involving electrically connected devices requiring monitoring and control, such as vending machines, solar panels, electrical vehicle charging and other data gathering and control systems.
These varying applications have different requirements for data throughput, power, cost, communications distances and communications reliability. Additionally, the power line channels vary in noise, attenuation, phase shift and maximum allowable signal levels. Different trade offs are required to achieve best power line performance. Implementing these tradeoffs requires a flexible, configurable-yet low cost-power line communications architecture.
PLC Modulation Approaches
Power line communications devices are beginning to use advanced modulation approaches like OFDM and OFDMA modulation. Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a noisy channel, such as the power grid. The technology works by splitting the signal into multiple smaller sub-signals that are then transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some sort of PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) i.e. BPSK, QPSK. Besides its high spectral efficiency, an OFDM system reduces the amount of crosstalk in signal transmissions and can efficiently overcome interference and frequency-selective fading caused by multipath.
While OFDM addresses communications in noisy smart grid environments, it is still insufficient to achieve reliable communications in the very harsh conditions. To further improve reliability, the OFDM method can be combined with a multiple access scheme. The approach is called OFDMA.
Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of the OFDM scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual data streams. This allows simultaneous transmission of several individual data streams. OFDMA further improves OFDM robustness to fading and interference, but more importantly the individual data streams can be used either to communicate with multiple nodes (power meters) simultaneously or for redundancy, thus greatly improving the reliability of the system.
PLC OFDMA Architectures
The following shows the architecture of a typical OFDMA PLC device:
Semitech SM2200 Power Line Communications Modem
Signals from the power line are converted to digital, filtered, demodulated, buffered and transferred to the external processor to implement data links and point-to-point, star or ad hoc networks. Data from the MAC is buffered, serialized, modulated and converted to analog for transmission of the power line.
What determines optimal configuration for cost, data throughput, communications distance, minimizing packet error rates and communications reliability is how the device is configured. The ability to configure modulation per channel, carrier frequencies for each channel, data redundancy approach, signal gain, and signal attenuation by channel is critical. The MAC software determines in real time the appropriate configuration to achieve the design objectives. The following figure is an example of configuration options that leading PLC controllers offer (the Semitech SM2200 as an example):
During development, engineers configure various options to maximize performance. In operation, MAC software can configure these options to maximize performance- even in changing power line characteristics.
Configuring PLC to Optimize Performance:
The right choice of modulation per channel, carrier frequencies for each channel, data redundancy approach, signal gain, and signal attenuation by channel determines the tradeoffs between data throughput, transmission range, packet error rates and overall communications reliability.
The choice of modulation used by each of the independent OFDMA channels is critical. The most simple, most robust, though lowest data throughput approach is single carrier BPSK. A spectral image of single carrier BPSK is shown below. Here the Semitech SM2200 uses a single carrier frequency for each of the 18 independent data channels.
18 Channels of Single Carrier BPSK
With 3 times the data throughput at the cost of noise sensitivity 3-BPSK or 3-QPSK can be configured as shown below. Each independent channel has three selectable frequencies. Each frequency then is modulated with either 2 phase states (BPSK) or 4 phase states (QPSK). Though data throughput is increased with QPSK, so is the error rate as a result of phase shift on the power line.
18 Channels of Triple BPSK/QPSK
For greatest reliability, a smaller number of single carrier channels can be used. Here the noise immunity is decreased with fewer frequencies that can be interfered with.
Example of 4 Single Carrier Channels
Selecting Modulation Frequencies
After selecting modulation, selecting carrier frequencies is critical. One chooses frequencies that avoid noise on the power line. The SM2200 allows the designer (and in real time, MAC software) to select each of the 18 sub-channels to be used. Each sub channel can be located anywhere in the channel band. Determining each desired frequency numbers enables the user to put more spacing in noisy regions and pack more densely in cleaner regions.
Selecting Redundancy Approach
The next configuration option is redundancy. Redundancy specifies how many channels carry the same data simultaneously. By definition, the most robust mode is redundancy 18, where the same data is repeated on all 18 channels. The receiver automatically discards identical packets therefore when the packet is received on any channel, it completes the communication. This is a tradeoff between reliability (using frequency diversity) and throughput. For best reliability – use redundancy mode of 18, for highest throughput on low error rate channels use redundancy 1.
These options are shown below.
Determining Gain and Signal Attenuation
Countries regulate maximum allowable signals on the power line channel differently. These regulations specify maximum average signal levels across the channel band (55 to 500 khz). To increase the range and increase noise immunity, one wants the largest amplitude that does not saturate the DAC or ADC, and meets the maximum average amplitude requirements. By specifying attenuation for each channel independently, one can optimize these tradeoffs.
There are large variations in noise, phase distortion and attenuation over different power lines. In addition, different applications require different tradeoffs to require the best combination of data throughput, reliability and distance. To achieve these requirements over varying power line conditions requires a flexible, configurable power line communications device. Having frequency flexibility through adding multiple access channels by assigning subsets of subcarriers to individual data streams results in improved communications reliability. Further, enabling modulation choice is critical. Finally, channel redundancy allows for optimal choice of robustness level vs. data throughput.
Sidebar: The Semitech SM2200
SM2200 is a next generation OFDMA (Orthogonal Frequency Division Multiple Access) power line communication transceiver designed for networking applications. The SM2200 contains a complete packet data modem with a simple physical layer protocol. When combined with a microcontroller (MCU), it provides a cost effective solution for data links and point-to-point, star or ad hoc networks. Interface to the MCU is a three/four wire serial peripheral interface (SPI) and interrupt request outputs used for control and data transfer. The MCU and software can scale to application for the most cost effective and versatile OFDMA PLC product for AMR and HA applications.
- Next generation OFDMA transceiver
- High tonal and impulse noise immunity
- 175kbps maximum data rate
- Super-robust mode of up to 125 kbps
- 54 carriers grouped into 18 independent OFDMA channels allowing frequency division
- BPSK/QPSK selectable modulation for each channel
- Selectable 55kHz-500kHz frequency range for each carrier
- Channel specific in-band noise estimation
- Programmable band in use (BIU) threshold for noise avoidance
- Narrowband emulation mode
- Transmit power regulation
- Error correction