OFDMA
Various communications and networking technologies are vying for a role in the movement to a smart electrical grid. Wireless is certainly a popular choice because of its flexibility. But there is a significantly growing interest in power-line communications (PLC). Data is just superimposed on the ac power line, so no new wires are needed. What could be more convenient?
The downside is that the power line is a horribly noisy medium and data reliability over long distances is a major issue. Semitech Semiconductor is working to mitigate that noise problem, and its SM2200 chip brings a new twist to the PLC method, orthogonal frequency-division multiple access (OFDMA).
Most PLC communications chips use either frequency shift keying (FSK) or OFDM because of their inherent resistance to noise. But Semitech’s new OFDMA offering provides significant improvements. Targeting networking applications, the SM2200 power-line transceiver is designed to operate in particularly noise-prone Smart Grid environments.
The SM2200 delivers high reliability in advance metering infrastructure (AMI) and automated meter-reading (AMR) applications that require low cost and high performance. The beauty of the SM2200 is that it can be used in home automation/building automation, in the backhaul to the utility, or within the grid infrastructure to substations. Another interesting new application is streetlight automation and control, as cities attempt to get a handle on energy costs.
Due to harsh noise and variations in equipment and differing standards, communications over the power grid is difficult. The SM2200 uses modulation and signal processing technology that’s adjustable in speed but also “frequency agile” to deliver more reliable communications. The SM2200 adapts to the noise environment by choosing the most effective transmission frequency. It also employs a multi-access scheme to provide additional robustness and enables communication with multiple nodes simultaneously, providing higher throughput.
The SM2200 achieves this with a unique 54-subcarrier OFDMA format. Instead of using all subcarriers for a single transmission, the device divides the 54 subcarriers into 18 three-subcarrier groups. Each group can be used to transmit a stream of data. Or, a single stream of data could be switched from one group to another if one segment of the spectrum is noisy or otherwise compromised. This greatly improves robustness of the connection.
The SM2200 includes a complete packet data modem with a simple physical-layer (PHY) protocol (see the figure). When combined with a microcontroller (MCU), it provides a cost-effective solution for data links and point-to-point, star, or ad hoc networks. It also complies with ANSI standards EIA709.1 and EIA709.2, featuring OFDMA with 54 carriers and up to 18 channels. Other key features include:
- High data-transfer rates in noisy environments with speeds up to 175 kbits/s
- User selectability between binary phase-shift keying (BPSK) modulation for higher carrier voltages and higher noise immunity and quadrature phase-shift keying (QPSK) modulation for higher data rate
- Narrow-band emulation and low-frequency mode that put channels in CENELEC frequency range
- Carrier frequency range from 5 to 500 kHz with selectable frequency range for each carrier
- Received packets that have a received signal strength indicator (RSSI) field
- Independently programmable carrier transmission voltages in 3-dB steps with transmit power regulation
- Error correction with cyclic redundancy code (CRC)
- Serial peripheral interface (SPI) connection to an external microcontroller up to 10 Mbits/s
- A 48-pin low-profile quad flat package (LQFP)
- A temperature range of –40°C to 85°C
The technology behind the SM2200 was tested and successfully deployed in more than 1000 nodes in China using the SM2200’s predecessor, the SM6401. The SM2200 is now sampling.