Once upon a time, AC wiring only carried power throughout a home. After a while, someone thought of using the power wiring to carry intercom voice signals around the home using what was known as â€˜current carrier’ technology that employed very simple AM or FM analog modulation schemes.
Then, the new digital world was born and low-speed digital gadgets became available that we could add to our home AC infrastructure to control lighting, and other line-powered devices, from a small control panel. The digital explosion led us to personal computers and the Internet.
Digital communications began to take on many forms from cordless phones to cell phones and from wired to wireless network solutions. Wireless digital communications standards (IEEE 802.11 Wi-Fi and IEEE 802.16 WiMAX), and other wireless solutions, liberated us from the entanglements and short-leashing of wires as home networking needs increased and the desire for mobility grew.
Now, with a plethora of wireless gadgets in our laps, pockets and purses, we find ourselves once again viewing the AC power wiring in our homes as a medium over which high-speed data communications can take place and we are once again willing to accept the fixed immovable nature of wires. How did that happen?
Technologies Mature, Applications Increase and Needs expand
Technologies mature, applications increase and the needs of people expand. We are now living in a time when consumer electronics devices (cell phone, TV, radio, entertainment center, DVR) are converging with computing devices (PDAs, notebook PCs, desktop PCs), meaning that entertainment and computing devices are merging to emerge as networked devices. What used to be separate entertainment appliances and computing devices are now converged as cross-communicating, media-sharing, networked devices, each with some amount of computing power built in.
Along with the convergence of entertainment and computing has come the need for a reliable broadband whole-house network that is robust enough to allow the home-network devices to share entertainment media and the Internet. The human need that has emerged with this new technology can be simply stated as â€˜entertainment and information access on demand’. That means access to all media, at any time, from any place in the home without physical or technical restrictions.
Service providers everywhere are moving quickly to an â€˜on demand’ business model, the most obvious of which is video on demand (VoD) and Internet protocol TV (IPTV). Both standard- and high-definition (SD and HD) video content are arriving at a single point of entry in our homes begging for a means to flow to all points of desired use.
With the realization that even the newest wireless technologies are not up to the task (rapid loss of data rate with distance, signal attenuation from walls and materials, multi-path signal interference and quality of service concerns), one can only turn back to a wired solution for whole-house coverage. The wired infrastructures in a home include telephone wiring, TV cabling and the AC wiring. Obviously, the AC wiring goes everywhere in the home as opposed to only a few jacks for TV and phone.
Robust Standard-based Technology for Powerline Communications
So that brings us back to the AC power wiring in our homes. Not so fast! Is there a technology that can really provide high-speed communications over the AC wiring and reliably deliver entertainment and Internet content to every corner of our homes? The answer is yes, it’s referred to as HomePlug Powerline Communications (HP PLC). HomePlug AV standard-based technology is capable of flooding our household AC wiring with nearly 200 million digital bits every second (200 Mbps). That’s enough to carry several HD video streams at one time.
So, â€œHow did that happen?â€ How did we end up back at the wires, specifically power wires? It happened because technology advanced, needs increased and the AC power wiring is everywhere in our homes just like we want our entertainment and Internet content to be. The HomePlug AV standard is fostering the PLC technology that is taking us back to the wires, offering a robust home network backbone to carry it all.
HomePlugÂ® AV standard-based technology is ushering in a whole new era of high-performance powerline home networking and enabling service providers to deliver data, voice and video to every corner of the customer’s home. It is also enabling home owners to easily create whole-house networks on their own.
Residential house wiring is a very harsh and electrically noisy medium over which to attempt high-data-rate digital communications. Even so, HomePlug powerline communication coexists on the household power wiring along with 120 or 240 volts AC, appliance-generated line noise and other signals, such as security and lighting control signals and induced interference.
What’s more, HomePlug AV is designed to provide robust high-speed powerline communications using very low transmit levels that satisfy regulations around the world. Despite the low transmit power in the presence of high voltage and intermittent noise and invading signals, HomePlug AV technology finds its own space to ride and work above and around would-be interference. How? Let’s take a look and see.
Frequency Separation on the AC Power Wiring
Figure 1 illustrates how the occupants of the AC power wiring coexist in the frequency domain. At the low end of the spectrum is the 50/60 Hz, 120/240 VAC followed by legacy security and control system signals such as X10. HomePlug AV spectrum ranges from approximately 2 to 30 MHz, above power and other carrier signals. Frequency separation allows the use of passive filtering to successfully isolate low-band signals from the HomePlug AV powerline communications spectrum. Figure 1 does not show the random wideband appliance-generated noise or any induced electromagnetic interference (EMI) or radio-frequency interference (RFI) that challenges powerline communications.
Figure 1: AC Line Spectrum â€“ HomePlug AV Separated in Frequency
The HomePlug AV spectrum is easily separated from low-band signals using passive filtering. However, high-pass filtering does not prevent in-band broad-spectrum noise, as generated by home appliances, or electromagnetic interference (EMI) and radio-frequency interference (RFI). Clever techniques must be employed to mitigate the challenge that in-band noise presents. The first step is to use a robust and flexible broadband modulation scheme known as Orthogonal Frequency Division Multiplexing (OFDM).
Windowed OFDM for Broadband Capability
OFDM is a physical-medium transmission technique composed of a large number of evenly spaced carriers (frequency division), each of which can be orthogonally modulated using Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). The robustness and bit efficiency of this technique is emphasized when one considers that it is also used in DSL phone line based systems (tone modulation) and in both Wi-Fi and WiMAX wireless technologies. Figure 2 illustrates how OFDM is employed for HomePlug AV powerline communications – 1155 individual carriers are spaced at approximately 24.4 kHz.
Figure 2: HomePlug AV OFDM Spectrum
HomePlug AV technology utilizes â€˜windowed’ OFDM, which means that selected carriers can be turned off (dropped) to remove interference to and from wireless services that occupy the same spectrum. Carriers, both used and not used, comprise what is known as the â€˜tone map’, each carrier referred to as a tone. A firmware-enabled tone mask is used to mute carriers that would interfere with legacy services such as the Amateur Radio bands. In addition to masking, the HomePlug AV standard allows for individual tones (carriers) to be programmed over a range of amplitude, which allows for line equalization associated with various applications.
In practice, the HomePlug AV tone map consists of 917 active carriers comprising the channel. Channel efficiency is optimized using dynamic channel adaptation in which the channel is frequently analyzed for interference and other line conditions to establish an optimal tone map in which each carrier (tone) is intelligently loaded with data bits before data transfer begins.
Carrier Bit Loading
*Important Note: Like all digital communications technologies, the advertised bit rate is the highest bit rate that the technology can produce over the physical medium (channel) under the best of conditions. Also, it is important to understand that included in the total number of bits that are transmitted every second there are many overhead bits that are necessary for addressing, error correction, quality of service, control and more. That means that the actual throughput rate of the data (payload) is much less than the total channel rate. In general terms, the actual throughput rate of the raw data is usually in the range of one forth to one half of the advertised channel rate, depending on the protocols used and the channel conditions.
Each carrier is modulated (bit loaded) using either Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 8-Quadrature Amplitude Modulation (QAM), 16-QAM, 64-QAM, 256-QAM, or 1024-QAM modulation, depending on channel conditions. 1024-QAM provides the highest bit rate per carrier because each of the 1024 unique analog phase/amplitude symbols represents 10 digital bits.
With 917 carriers active in the HomePlug AV tone map and each modulated with 1024 QAM (10-bit symbol), the channel bit rate is 10 X 917 = 9170 bits per symbol period. The symbol period is the inverse of the carrier spacing (1/24400 Hz = ~41 Âµs) plus a small guard period of approximately 5.5 Âµs. Thus, the total symbol period during which 9170 bits are conveyed is 41 Âµs + 5.5 Âµs = 46.5 Âµs. Using the 46.5 Âµs period, the channel bit rate is 9170 bits/46.5 Âµs = ~197 Mbps. Thus, HomePlug AV offers a maximum physical layer channel rate of approximately *200 Mbps. If all 1155 carriers were used, there would be 11550 bits per symbol period and the maximum channel rate would be approximately *248 Mbps.
In practice, not all carriers can be loaded with the maximum number of bits (10). In a process called â€˜channel estimation’, bit loading for each channel is established when the communications path between two HomePlug AV devices is initialized and is continuously refreshed to adapt to changing channel conditions to achieve the highest throughput rates. Each carrier is dynamically loaded with the maximum number of bits that prevailing line conditions allow.
The individual carrier bit loading capability enables interesting flexibility. Under the very worst of line conditions, all carriers could be modulated with the same data using a low-bit rate loading, BPSK for example. This would ensure that the information gets through on one or more carriers. At the other extreme, each carrier can contain different data, each with the highest bit loading. This would yield the highest possible channel bit rate and information flow.
AC Line Synchronization
While windowed OFDM allows channel frequency-domain adaptation and individual-carrier bit loading, AC line synchronization is also employed to adapt to time domain characteristics. AC cycle synchronization allows the system to identify and work around periodic and intermittent appliance-generated line noise in relation to the AC waveform. Synchronization to the AC cycles allows the system to optimize channel capacity by adjusting bit loading during periodic noise events of significant amplitude.
Line noise, in general, tends to fluctuate during a line cycle period. Impulse noise tends to be synchronous and occurs in limited periods of the line cycle. HomePlug AV line synchronization capability utilizes multiple time slots that are synchronized with the AC cycle and evaluated for noise during channel estimation periods. This allows HomePlug AV devices to minimize the effects of the noise by optimizing carrier bit loading according to noise conditions. The carrier bit loading is part of the tone map that is generated with each channel estimation period, allowing the communications to adapt to the line and impulse noise in any network path.
Powerful Forward Error Correction Technology
HomePlug AV technology employs powerful advanced coding technology to accomplish Forward Error Correction (FEC). This technology achieves exceptional performance and realizes greater throughputs in the presence of noise, allowing HomePlug AV home networks to operate at a lower signal-to-noise ratio. This advanced FEC coding has power efficiencies approaching (within 1 dB of) the theoretical Shannon limit and is well proven, having been widely adopted in harsh communications environments such as cellular telecommunications (CDMA2000 and W-CDMA), satellite (DVB-RCS, CCSD), and broadband wireless (802.16, WiMAX).
Back to Wires? Great!
With the truly robust 200-Mbps HomePlug AV standard-based PLC technology, home networks are established instantly with plug-and-play simplicity. It is the reason why service providers worldwide are having this proven technology embedded into their gateways, PVRs and set-top boxes. Consumer electronics manufacturers are now in the process of rolling out audio and video â€˜cable extenders’ that allow the central home entertainment center to serve up stored content to every corner of the house. Even wireless technology is benefiting from a robust whole-house powerline backbone because wireless access points can easily be set at any locations throughout the home to create mobile cells. So, going back to wires, especially household AC wiring is a great idea. AC outlets are everywhere and that’s the way we want to access all of our entertainment and information content â€“ everywhere on demand.
About Intellon Corporation
Intellon is the world leader in powerline communications, providing HomePlugÂ® compliant and other powerline integrated circuits for home networking, networked entertainment, BPL access and commercial applications. Intellon created and patented the baseline technology for HomePlug 1.0, and is a major contributor to the baseline technology for the new 200-Mbps HomePlug AV powerline standard. With over three million HomePlug ICs sold, Intellon holds the dominant market share of the rapidly growing HomePlug market. The company was founded in 1989 and is headquartered in Ocala, Florida, with offices in San Jose and Toronto. For additional information, visit www.intellon.com.
Intellon and No New Wires are registered trademarks of Intellon Corporation. HomePlug is a registered trademark of the HomePlug Powerline Alliance. All other trademarks are the property of their respective owners. www.intellon.com