Until recently, with the exception of some ahead-of-its-time research conducted by Hewlett-Packard in the late 1980s, power line networking meant the support of control networks for energy management, meter reading and home automation. Experience garnered in that environment has been combined with more recent modulation technology, namely orthogonal frequency-division multiplexing (OFDM), to push power line communications as one of the key small office/home office (Soho) networking technologies. The other top networking technologies are phone-line networking, wireless, and traditional "new wires" Ethernet LANs.
The appeal of power line communications (PLC) is obvious. The power source that all but portable devices plug into anyway can double as a network connection, a fact that is obvious but often overlooked. The second key benefit of PLC is availability. Virtually every room in which one would want to put an information appliance has one or more outlets close by. Being connected no longer has to mean a PC placed near a phone jack. New networked devices in new locations are suddenly possible with PLC based on OFDM.
Another advantage is PLC's worldwide applicability. Outside of North America, the in-home phone jack distribution rate is very low, typically one jack per dwelling. Additionally, RF regulations and restrictions vary from country to country and make it difficult to design a single RF networking technology that can be implemented worldwide.
The basic idea of OFDM is to divide the available spectrum into many narrowband, low-data-rate carriers, or subcarriers. To obtain high spectral efficiency, the frequency responses of the subcarriers are overlapping and orthogonal, hence the name orthogonal frequency-division multiplexing. Each narrowband subcarrier can be modulated using various modulation formats in which binary or quadrature phase shift keying (BPSK or QPSK) and quadrature amplitude modulation (QAM) are commonly used.
Since the modulation rate on each subcarrier is very low, each subcarrier experiences flat fading in a multipath environment and is easy to equalize. The need for equalization can be eliminated by using differential QPSK (DQPSK) modulation, wherein the data is encoded as the difference in phase between the present and previous symbol in time on the same subcarrier. Differential modulation improves performance in environments where rapid changes in phase are possible, as is the case in power line communications. OFDM can be implemented equally well, with coherent (non-differential) modulation and demodulation to maximize the signal-to-noise ratio performance of the system. This approach is preferred for performance-oriented systems, as are likely to be found in future wireless networking systems operating at greater than 20-Mbit/second for distributing digital video in the home.
OFDM modulation is generated using a fast Fourier transform (FFT) processor. First, data is encoded in the frequency domain onto individual subcarriers. For example, with DQPSK modulation, 2 bits are encoded onto each subcarrier. The number of subcarriers is one of the design parameters for the system. An inverse FFT (IFFT) is performed on the set of frequency subcarriers, converting to the time domain and producing a single OFDM symbol. The length of time for the OFDM symbol is equal to the reciprocal of the subcarrier spacing (the second critical design parameter) and is generally a very long time compared with the data rate. The final signal-processing step is to create a cyclic prefix, which is a copy of the last part of the symbol that is added to the beginning of the symbol before transmission.
One feature that makes OFDM ideal for power line communications is its ability to put no data on some carriers-that is, turn them off completely-at frequencies where the attenuation is too deep to carry information. OFDM signals are demodulated by the reverse process of the transmitter. The cyclic prefix is removed from the time-domain signal, and an FFT is performed on the symbol to convert to the frequency domain. In the case of DQPSK, data is decoded by examining the phase difference of subcarriers between adjacent OFDM symbols.
Additional requirements for the receiver are frequency and time synchronization. That is, the receiver must know with some precision where the beginning and end of the symbol are so that the FFT will indeed recreate the correct frequency bins. For OFDM PLC systems, a cyclic prefix is added to the symbol to maintain complete orthogonality in a time-dispersive channel. That "smearing" of the signal in time is caused by multipath reflections. The long OFDM symbol time-generally many microseconds-combined with the cyclic prefix, which is usually a small percentage of the OFDM symbol time, are key factors that enable performance in a time-dispersive channel.
Conventional modulation suffers from interference caused by time-delayed reflections of the original signal overlapping successive symbols, a phenomenon called intersymbol interference (ISI). Consider the transmission of conventional QPSK symbols at a rate of 1 MHz, yielding 2 Mbits/s, in a time-dispersive channel with reflections delayed as much as 1 microseconds.
Reflections of 1 microseconds or greater are easily observable in a power line network. In this example, the symbol time is 1/(1 MHz) = 1 microseconds. When multiple copies of the original signal are delayed up to 1 microseconds in arriving at the receiver, the symbol being decoded-which is likely to be the signal from the most direct path-is interfered with, and corrupted by, the delayed signals. The delayed signals contain the previous symbol transmitted 1 microseconds earlier.
That distortion can be corrected with equalization but becomes much more difficult as the symbol rate is increased. For example, to speed the single carrier system to 10 Mbits/s, the symbol rate becomes five times greater, the symbols are five times closer together, and for the same 1-microseconds of delayed reflection, up to five preceding symbols interfere with the symbol currently being decoded.
With OFDM, many hundreds of bits are transmitted for each symbol, making the duration of the symbol many microseconds long. Selecting a cyclic prefix of 1 microseconds for the example above, all received copies of the original signal would contain the same OFDM symbol, although shifted in time, and there would be no ISI.
Communication system design of this nature-to apply OFDM to power line communication at 11 Mbits/s-is the art of combining theory and known building blocks to match a very deep understanding of the communication channel. Understanding the power line as a communication channel-that is, creating a channel model-is arguably the greatest challenge. Very little literature exists that addresses the multipath, noise, attenuation and impedance characteristics of power lines at the frequencies of interest for today's systems. The design team must develop that data on its own through field measurement. "Field measurement" is a euphemism for going into people's homes with bulky test instruments to characterize dozens of outlets.
The other common practice, especially for home network designers, is the collection of "torture chamber" devices that are known to plug into, radiate in, clip, attenuate and otherwise harm the desired medium. For phone line networkers, it is the collection of countless phones, modems, and answering machines. For power line networkers, it is the collection of dimmers, brush motors, halogen lamps, PCs and anything else that turns up as problematic.
These tools, combined with a high-level simulation system, let the R&D team develop and verify the signal-processing algorithms. In parallel, the system engineers and chip designers stitch together the OFDM chip's building blocks, comprised of the FFT engine, forward error correction (FEC) block, modulators, demodulators, time and frequency synchronization block, protocol processor, buffers,and host interface.
That system is simulated in software in conjunction with the previously developed channel model. More important, the system can be verified in the field by using the simulation system to drive an arbitrary waveform generator and post-processing the captured waveforms at the receiver. That process gives the designers a very good understanding of the chip's performance before the chip is reduced to silicon. An ancillary benefit is that radiated emission measurements can be made to ensure compliance with the specialized rules for PLC found in FCC Part 15.
The remaining chip design and fabrication process follows standard practice. The particular instance discussed here, an 11-Mbits/s OFDM power line carrier chip, is yielded in a 0.25-micron CMOS process on the order of complexity of the second-generation phone line networking chips (HomePNA 2.0). The power line communication networks that result create a new opportunity for broadband Internet access sharing, gaming, digital audio distribution and even mundane office work.