By Serghei Druta and Agnès Cassin-Delaurière
Use of the power grid for data transmission is very commercially attractive to utilities as having the lowest operational costs. The research effort focused for years on developing a cheap bi-directional technology that would enable power line communications (PLC) for applications such as remote meter reading, power grid control and maintenance, telemetry and others.
The current S-FSK PLC solutions allow the deployment of AMR systems in an efficient and cost effective way. Looking forward, the growing demand for new applications and increasing data flow may lead to more persistent demand for more robust and faster PLC, such as the narrowband OFDM technique, which is becoming more and more attractive as a PLC solution for AMM systems. The flexibility of the software solutions powered by high performance DSP allows an efficient low cost PLC implementation that integrates both S-FSK and OFDM solutions in the same hardware.
Narrowband PLC began soon after electrical power supply became widespread and is under continuous development. Unfortunately, the electrical distribution circuits were not originally built for communication purposes. Varying levels of impedance and attenuation, considerable noise and time-variant interference from various sources make it a rather hostile medium for data transmission. Nevertheless, the economic efficiency of applications based on PLC is so high that the research effort is continuously focused on the development of new techniques usable on power line channels.
The most widespread PLC techniques on the market are frequency shift keying (FSK) and spread frequency shift keying (S-FSK). Though having well known drawbacks (like low data rate, up to 2.4 kbps, and sensitivity to narrowband noise) the good thing about them is the presence of standards that specify the stack of communication protocols from the physical (IEC 61334-5-1 for S-FSK) up to the application (IEC 62056-53 for COSEM) layer and allow development of interoperable solutions. Such AMR/AMM solutions are now provided by a number of companies and are widely and successfully implemented by utilities.
But, as the process of getting a working communication technique standardised and supported by different manufacturers takes time, solutions using a more advanced technique are already on their way to the market. Such a technique that is to make a revolution in PLC for AMI is narrowband orthogonal frequency division multiplexing (OFDM).
Narrowband OFDM implementation
OFDM is a well known technique that is currently used in xDSL technologies, terrestrial wireless distribution of television signals (DVB-T, DVB-H), and has also been adapted for IEEE’s high rate wireless LAN Standards (802.11a and 802.11g). In less than twenty years, in fact, OFDM has developed into a popular scheme for virtually all new telecommunication standards.
OFDM is a digital multi-carrier modulation scheme that uses a large number of closely spaced orthogonal subcarriers to carry data. Each subcarrier is modulated with a conventional modulation scheme at a low symbol rate, maintaining data rates similar to conventional single carrier modulation schemes in the same bandwidth.
The primary advantage of OFDM over single carrier schemes is its ability to cope with severe channel conditions – for example, attenuation of high frequencies in long power lines, narrowband interference and frequency selective fading due to multipath – without additional complex mechanisms. Channel equalisation is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. Additionally, low symbol rate makes affordable the use of a guard interval between symbols that make it possible to handle time spreading and eliminate inter-symbol interference. In order to further increase the robustness of OFDM a forward error coding (FEC) mechanism is added.
Another advantage of the OFDM technique is higher data rates. The current set of AMM applications is already uncomfortable with the bottleneck of the PLC, but there are more applications to come and there is more data to be transmitted as the infrastructure is built.
ADD GRUP has a long history in using single carrier PLC in its AMM solutions. But, looking into the future, the development of more modern PLC was always an objective of the company. This has resulted with a power line modem that is able to do both – S-FSK and OFDM by simple software reconfiguration.
The core of the modem is a high performance 32 bit Texas Instruments digital signal processor (DSP) (TMS320F28TM) that implements at least the physical and the data link layers for efficient PLC. Depending on the DSP flash resource additional layers of the communication protocols stack may be encapsulated. The whole stack is based on standard or open communication protocols (TCP/IP, DLMS/COSEM).
For OFDM physics the modem uses up to 48 carriers in the CENELEC A band for data transmission at a data rate up to 76.8 kbps for one mains phase (230.4 kbps for threephase communication). The transmission is done by OFDM with FEC using a differential phase modulation, with bit interleaving, data randomisation, automatic gain control, cyclic prefix as guard interval and 32 bit CRC for error detection. An effective MAC layer with auto-discovery and repetition mechanisms, acknowledged transmission, broadcast service and QoS support, and a standard IEEE 802.2 type 1 LLC are also implemented.
The flexibility of the software solution allows adjustment of a large number of parameters – from subcarriers notching to selection of manual or automatic control of various algorithms, or even using the modem as a spectrum analyser or as a channel sniffer.
For S-FSK the modem implements the IEC 61334-5-1 standard and allows free configuration of the Mark and Space carrier frequencies (within the CENELEC A band). Above the standard a data rate of 2.4 kbps is also implemented.
The high integration of peripherals into the TMS320F28TM series of DSPs allows a dramatic reduction of schematic complexity and solution cost. The receiver makes use of the integrated high accuracy ADC and for the transmitter two PWM channels are used (with a 33-66% duty cycle ratio to reduce harmonics).
The modem is designed as a module (46x22x13 mm) that can be easily integrated into any endpoint (meter) or network (DCU or router) device of an AMM system.
Benefit of TI-based solutions
Providing that the grid infrastructure was not initially designed to handle communications traffic, high performance signal processing is critical to ensure two-way PLC. Based on Texas Instruments’ DSP technology, TMS320F28TM controllers offer a unique combination of 32 bit control optimised performance, system integration and microcontroller (MCU)-like ease of use at an affordable price. Available PLC implementations illustrate the value of combining 32 bit digital signal processing with the design flexibility of an MCU.
The controller’s programmability lets designers integrate on a single chip a complete modem and simultaneously manage other stack layers. All TMS320F28xTM devices are fully hardware and software compatible offering a wide portfolio of 36 devices both tools and software compatible, among which are 12 pin compatible devices allowing engineers to easily switch controllers for optimised features and cost across a family of products. This enables a single PLC modem development (S/W+H/W) addressing both the endpoint and the concentrator, thereby leveraging development investments.
A programmable controller also affords any other additional standards to be integrated with no additional cost. A good example is the Euridis protocol (as defined in IEC 62056-31), which is a popular solution for communication on a twisted pair cables field bus for the remote reading of residential meters. The implementation of the Euridis 2 with a data exchange rate up to 9.6 kbps has been developed and certified on a TMS320F28xTM.
In addition to the digital PLC unit, Texas Instruments offers developers the option to optimise the full AMM bill of materials with a scalable portfolio including the e-meter chip and the ultra-low power MSP430. Devices within the MSP430FE42x series are energy measurement system-on-chips (SOC), providing for distributed energy metering applications.
The e-meter and the PLC modem may communicate through one of the many built-in communication interfaces which makes the complete e-meter system highly integrated and low cost.
As the worldwide stride for energy saving increases year by year, the implementation by utilities of AMR/AMM systems is becoming a must. In different countries this process is at different stages, but the global vector is to update the energetic sector to a modern high-tech level. In this situation the utilities that have a significant number of metering points and that will have an AMM implementation period counted in years are in deep thought about what the communication technology should be for such a system. The biggest concerns of the utilities are that the solution must be working and robust, it must be standardised or the protocols must be open, it must be interoperable and it must be future proof.
The ADD GRUP communication solution, powered by the Texas Instruments digital to analogue metering solutions, was specifically designed to address all these concerns. Implemented on the TMS320F28xTM controller the PLC modem can be easily integrated into currently deployed S-FSK-based standard solutions or it can be configured to deliver a lot more performance to the communication infrastructure by switching to the OFDM technique. It can be seamlessly integrated into TCP/IP-based solutions. It removes the burden of communication problems from the host CPUs that may focus on application tasks.
The future AMI will certainly evolve to PLC using the OFDM technique as has happened with many other communication environments. The solution to make this process quick and easy is already on the market.
For more information on Texas Instruments’ PLC solutions, see www. ti.com/c2000plc; www.ti.com/amr