Patent Publication Number: US-2015071364-A1

Title: Power Line Communication using Padding to Overcome Interleaver Failings

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. 119(e) 
     The present application claims priority to and incorporates by reference U.S. Provisional Application No. 61/877,100, (attorney docket TI-74317PS) filed Sep. 12, 2013, entitled “Padding to Overcome Interleaver Failings.” 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to reliable communication between devices, and in particular to communication over power lines. 
     BACKGROUND OF THE INVENTION 
     Power Line Communication (PLC) is one of the technologies used for automatic meter reading, for example. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history because utility companies have an interest in obtaining fresh data from all metered points in order to better control and operate the utility grid. PLC is one of the technologies being used in Advanced Metering Infrastructure (AMI) systems. 
     A PLC carrier repeating station is a facility at which a PLC signal on a power line is refreshed. The signal is filtered out from the power line, demodulated and modulated, and then re-injected onto the power line again. Since PLC signals can carry long distances (several 100 kilometers), such facilities typically exist on very long power lines using PLC equipment. 
     In a one-way system, readings “bubble up” from end devices (such as meters), through the communication infrastructure, to a “master station” which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change. 
     In a two-way system, both outbound and inbound traffic is supported. Commands can be broadcast from a master station (outbound) to end devices, such as meters, that may be used for control and reconfiguration of the network, to obtain readings, to convey messages, etc. The device at the end of the network may then respond (inbound) with a message that carries the desired value. Outbound messages injected at a utility substation will propagate to all points downstream. This type of broadcast allows the communication system to simultaneously reach many thousands of devices. Control functions may include monitoring health of the system and commanding power shedding to nodes that have been previously identified as candidates for load shed. PLC also may be a component of a Smart Grid. 
     The power line channel is very hostile. Channel characteristics and parameters vary with frequency, location, time and the type of equipment connected to it. The lower frequency regions from 10 kHz to 200 kHz are especially susceptible to interference. Furthermore, the power line is a very frequency selective channel. Besides background noise, it is subject to impulsive noise often occurring at 50/60 Hz, and narrowband interference and group delays up to several hundred microseconds. 
     OFDM is a modulation technique that can efficiently utilize this limited low frequency bandwidth, and thereby allows the use of advanced channel coding techniques. This combination facilitates a very robust communication over a power line channel. 
     On Sep. 30, 2010, the IEEE&#39;s 1901 Broadband Powerline Standard was approved and HomePlug AV, as baseline technology for the FFT-OFDM PHY within the standard, is now ratified and validated as an international standard. The HomePlug Powerline Alliance is a certifying body for IEEE 1901 products. The three major specifications published by HomePlug (HomePlug AV, HomePlug Green PHY and HomePlug AV2) are interoperable and compliant. 
     Another set of open standards has been developed for power line communication (PLC) at the request of Electricité Réseau Distribution France (ERDF), a wholly owned subsidiary of the EDF (Electricité de France) Group. The set of standards include “PLC G3 Physical Layer Specification,” and “PLC G3 MAC Layer Specification.” These standards are intended to facilitate the implementation of an automatic meter-management (AMM) infrastructure in France; however, PLC using these standards or similar technology may be used by power utilities worldwide. 
     The G3 standards promote Interoperability and coexists with IEC 61334, IEEE® P1901, and ITU G.hn systems. 10 kHz to 490 kHz operation complies with FCC, CENELEC, and ARIB. CENELEC is the European Committee for Electrotechnical Standardization and is responsible for standardization in the electro technical engineering field. ARIB is a Japanese standards organization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Particular embodiments in accordance with the invention will now be described, by way of example only, and with reference to the accompanying drawings: 
         FIG. 1  is a conceptual diagram of a PLC system that uses packet padding; 
         FIG. 2  is a block diagram of an example PLC device or modem for use in the PLC system of  FIG. 1 ; 
         FIG. 3  is a block diagram of an example PLC gateway for use in the PLC system of  FIG. 1 ; 
         FIG. 4  is a block diagram of an example PLC data concentrator for use in the PLC system of  FIG. 1 ; 
         FIG. 5  is a plot illustrating power spectral density for an example PLC channel 
         FIG. 6  is an architectural diagram of a power line communication system, illustrating the PHY and MAC layers; 
         FIG. 7  is a flow chart illustrating a method for padding a frame of data to ensure successful transmission on frequency selective PLC channels; and 
         FIG. 8  illustrates a block diagram of an exemplary low cost, low power G3 and 1901.2 compatible device. 
     
    
    
     Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency. In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the 
     DESCRIPTION 
     Power line communication (PLC) devices operate in the presence of harsh channel and noise environments, such as frequency-selective channels, narrowband interference, and impulsive noise. Traditionally, repetition coding is used to improve the robustness of PLC devices in these harsh environments. However, repetition coding comes at the expense of decreased data rates. As an example, IEEE 1901.2, ITU-G.9903 (G3-PLC) and ITU G.9904 (PRIME) all have a mode, called ROBO or Robust mode, where the information bit is repeated 4 times. This repetition coding helps to ensure that a PLC device can connect with other PLC devices even in the presence of the harsh environments. 
     G3-PLC device can operate in several bands: CENELEC A (35.9375-90.625 kHz), CENELEC B (98.4375-121.875 kHz), ARIB (154.6875-403.125 kHz) and FCC (154.6875-487.5 kHz) bands. In each of these bands, the critical network messages, such as network formation, routing, management, etc., are sent using either ROBO (repetition by 4) or Super ROBO (repetition by 6) mode. 
     Within a band of interest, such as CENELEC A: 35.9375-90.625 kHz, for example, the higher 24 tones (54.6875-90.625 kHz) may experience deep fading, which essentially ensures that no information can be received on them. Thus, typically only the lower 12 tones (35.9375-53.125 kHz) may be used to communicate data. This is equivalent to puncturing the higher 24 tones for each OFDM symbol. Note that the information bits, including the repeated bits, are actually transmitted on all 36 tones in ROBO mode. After the channel, the received data stream may look like 12 “received bits” followed by 24 “erased bits” (low signal values, essentially zeros) for each OFDM symbol. 
     At the receiver, the received data stream is equalized, sliced (calculate soft information), de-interleaved, de-spread (integrating the soft information for the repeated bits), as specified by the G3-PLC or IEEE 190 standard. After these operations, the resulting soft information is sent to a Viterbi decoder. More details about the G3-PLC interleaver/de-interleaver and spreading can be found in the ITU G.9903 February 2014 standard (and future versions), which is incorporated by reference herein. 
     For certain frame lengths (or equivalently, payload bytes), it turns out that the interleaver/de-interleaver specified by the above mentioned standards for use in PLC communication is poorly designed. For these cases, the output of the de-interleaver may contain long stretches of erasures or very unreliable soft information, which is then de-spread and sent to the Viterbi decoder. The long stretches of erasures or unreliable soft information makes it difficult for the Viterbi decoder to recover/determine the transmitted information sequence. In fact, if the length of consecutive erasures is (nearly) equal or longer than the d free  of the code (for the K=7, R=½ code used by G3-PLC, the d free =10), then the Viterbi decoder may not be able to select the correct path. In fact, it will very likely choose the wrong path. This happens even if the SNR (signal to noise ratio) is very high. Thus, the problem is due to the frequency-selective nature of the channel and has less to do with the noise power. Embodiments of the invention may use MAC level frame padding to assure that optimum length frames are transmitted in order to improve the operation of the Viterbi decoder in PLC communication, as will be described in more detail below. 
     Power line communication using G3 standards reduces infrastructure costs by allowing transmission on medium voltage lines, for example, 12kV, for distances of 6 km or more and across transformers with fewer repeaters. Robust operation over noisy channels is provided by an orthogonal frequency division multiplexing (OFDM)-based PHY (physical) layer. The G3 Mac specification is based on the IEEE 802.15.4-2006 “Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs)” which is suitable for lower data rates. Two layers of forward error correction and cyber security features are provided. A 6LoWPAN adaptation layer supports IPv6 packets. An AES-128 cryptographic engine may be included in G3 PLC nodes. Adaptive tone mapping maximizes bandwidth utilization and channel estimation optimizes modulation between neighboring nodes. A mesh routing protocol selects best path between remote nodes. At the MAC layer, a data or command frame may include up to 400 bytes of data. At the PHY layer, a frame may be segmented and transmitted in smaller chunks of data. 
     IEEE standard 1901.2 specifies communications for low frequency (less than 500 kHz) narrowband power line devices via alternating current and direct current electric power lines. This standard supports indoor and outdoor communications in the following environments: a.) low voltage lines (less than 1000 v), such as the line between a utility transformer and meter; b) through transformer low-voltage to medium-voltage (1000 V up to 72 kV); and c) through transformer medium-voltage to low-voltage power lines in both urban and in long distance (multi-kilometer) rural communications. The standard uses transmission frequencies less than 500 kHz. Data rates will be scalable to 500 kbps depending on the application requirements. This standard addresses grid to utility meter, electric vehicle to charging station, and within home area networking communications scenarios. Lighting and solar panel power line communications are also potential uses of this communications standard. This standard focuses on the balanced and efficient use of the power line communications channel by all classes of low frequency narrow band (LF NB) devices, defining detailed mechanisms for coexistence between different LF NB standards developing organizations (SDO) technologies, assuring that desired bandwidth may be delivered. This standard assures coexistence with broadband power line (BPL) devices by minimizing out-of-band emissions in frequencies greater than 500 kHz. The standard addresses the necessary security requirements that assure communication privacy and allow use for security sensitive services. This standard defines the physical layer (PHY) and the medium access (MAC) sublayer of the data link layer, as defined by the International Organization for Standardization (ISO) Open Systems Interconnection (OSI) Basic Reference Model. 
       FIG. 1  is a conceptual diagram of a PLC system in which an electric power distribution system is depicted. Medium voltage (MV) power lines  103  from substation  101  typically carry voltage in the tens of kilovolts range. Transformer  104  steps the MV power down to low voltage (LV) power on LV lines  105 , carrying voltage in the range of 100-240 VAC. Transformer  104  is typically designed to operate at very low frequencies in the range of 50-60 Hz. Transformer  104  does not typically allow high frequencies, such as signals greater than 100 KHz, to pass between LV lines  105  and MV lines  103 . LV lines  105  feed power to customers via meters  106   a - n , which are typically mounted on the outside of residences  102   a - n . Although referred to as “residences,” premises  102   a - n  may include any type of building, facility, or location where electric power is received and/or consumed. A breaker panel, such as panel  107 , provides an interface between meter  106   n  and electrical wires  108  within residence  102   n . Electrical wires  108  deliver power to outlets  110 , switches  111 , and other electric devices within residence  102   n.    
     The power line topology illustrated in  FIG. 1  may be used to deliver high-speed communications to residences  102   a - n . In some implementations, power line communications modems or gateways  112   a - n  may be coupled to LV power lines  105  at meter  106   a - n . PLC modems/gateways  112   a - n  may be used to transmit and receive data signals over MV/LV lines  103 / 105 . Such data signals may be used to support metering and power delivery applications (e.g., smart grid applications), communication systems, high speed Internet, telephony, video conferencing, and video delivery, to name a few. By transporting telecommunications and/or data signals over a power transmission network, there is no need to install new cabling to each subscriber  102   a - n . Thus, by using existing electricity distribution systems to carry data signals, significant cost savings are possible. 
     An illustrative method for transmitting data over power lines may use, for example, a carrier signal having a frequency different from that of the power signal. The carrier signal may be modulated by the data, for example, using an orthogonal frequency division multiplexing (OFDM) scheme or the like. The examples described below are based on IEEE 1901.2 or G3-PLC, however, other embodiments may use other OFDM based protocols now known or later developed. 
     PLC modems or gateways  112   a - n  at residences  102   a - n  use the MV/LV power grid to carry data signals to and from PLC data concentrator  114  without requiring additional wiring. Concentrator  114  may be coupled to either MV line  103  or LV line  105 . Modems or gateways  112   a - n  may support applications such as high-speed broadband Internet links, narrowband control applications, low bandwidth data collection applications, or the like. In a home environment, for example, modems or gateways  112   a - n  may further enable home and building automation in heat and air conditioning, lighting, and security. Also, PLC modems or gateways  112   a - n  may enable AC or DC charging of electric vehicles and other appliances. An example of an AC or DC charger is illustrated as PLC device  113 . Outside the premises, power line communication networks may provide street lighting control and remote power meter data collection. 
     One or more data concentrators  114  may be coupled to control center  130  (e.g., a utility company) via network  120 . Network  120  may include, for example, an IP-based network, the Internet, a cellular network, a WiFi network, a WiMax network, or the like. As such, control center  130  may be configured to collect power consumption and other types of relevant information from gateway(s)  112  and/or device(s)  113  through concentrator(s)  114 . Additionally or alternatively, control center  130  may be configured to implement smart grid policies and other regulatory or commercial rules by communicating such rules to each gateway(s)  112  and/or device(s)  113  through concentrator(s)  114 . 
     In some embodiments, each concentrator  114  may be seen as a base node for a PLC domain, each such domain comprising downstream PLC devices that communicate with control center  130  through a respective concentrator  114 . For example, in  FIG. 1 , device  106   a - n ,  112   a - n , and  113  may all be considered part of the PLC domain that has data concentrator  114  as its base node; although in other scenarios other devices may be used as the base node of a PLC domain. In a typical situation, multiple nodes may be deployed in a given PLC network, and at least a subset of those nodes may be tied to a common clock through a backbone (e.g., Ethernet, digital subscriber loop (DSL), etc.). Further, each PLC domain may be coupled to MV line  103  through its own distinct transformer similar to transformer  104 . 
     Still referring to  FIG. 1 , meter  106 , gateways  112 , PLC device  113 , and data concentrator  114  may each be coupled to or otherwise include a PLC modem or the like. The PLC modem may include transmitter and/or receiver circuitry to facilitate the device&#39;s connection to power lines  103 ,  105 , and/or  108 . 
       FIG. 2  is a block diagram of PLC device or modem  113  that may include an embodiment of the MAC level packet length padding protocol described herein. As illustrated, AC interface  201  may be coupled to electrical wires  108   a  and  108   b  inside of premises  112   n  in a manner that allows PLC device  113  to switch the connection between wires  108   a  and  108   b  off using a switching circuit or the like. In other embodiments, however, AC interface  201  may be connected to a single wire  108  (i.e., without breaking wire  108  into wires  108   a  and  108   b ) and without providing such switching capabilities. In operation, AC interface  201  may allow PLC engine  202  to receive and transmit PLC signals over wires  108   a - b . As noted above, in some cases, PLC device  113  may be a PLC modem. Additionally or alternatively, PLC device  113  may be a part of a smart grid device (e.g., an AC or DC charger, a meter, etc.), an appliance, or a control module for other electrical elements located inside or outside of premises  112   n  (e.g., street lighting, etc.). 
     PLC engine  202  may be configured to transmit and/or receive PLC signals over wires  108   a  and/or  108   b  via AC interface  201  using a particular channel or frequency band. In some embodiments, PLC engine  202  may be configured to transmit OFDM signals, although other types of modulation schemes may be used. As such, PLC engine  202  may include or otherwise be configured to communicate with metrology or monitoring circuits (not shown) that are in turn configured to measure power consumption characteristics of certain devices or appliances via wires  108 ,  108   a , and/or  108   b . PLC engine  202  may receive such power consumption information, encode it as one or more PLC signals, and transmit it over wires  108 ,  108   a , and/or  108   b  to higher-level PLC devices (e.g., PLC gateways  112   n , data concentrators  114 , etc.) for further processing. Conversely, PLC engine  202  may receive instructions and/or other information from such higher-level PLC devices encoded in PLC signals, for example, to allow PLC engine  202  to select a particular frequency band in which to operate. PLC engine  202  may be implemented using a digital signal processor (DSP), or another type of microprocessor, that is executing control software instructions stored in memory that is coupled to the microprocessor, for example, to perform various applications for power line device  113 . 
       FIG. 3  is a block diagram of PLC gateway  112  that may include an embodiment of the MAC level packet length padding protocol described herein. As illustrated in this example, gateway engine  301  is coupled to meter interface  302 , local communication interface  303 , and frequency band usage database  304 . Meter interface  302  is coupled to meter  106 , and local communication interface  304  is coupled to one or more of a variety of PLC devices such as, for example, PLC device  113 . Local communication interface  304  may provide a variety of communication protocols such as, for example, ZIGBEE, BLUETOOTH, WI-FI, WI-MAX, ETHERNET, etc., which may enable gateway  112  to communicate with a wide variety of different devices and appliances. In operation, gateway engine  301  may be configured to collect communications from PLC device  113  and/or other devices, as well as meter  106 , and serve as an interface between these various devices and PLC data concentrator  114 . Gateway engine  301  may also be configured to allocate frequency bands to specific devices and/or to provide information to such devices that enable them to self-assign their own operating frequencies. 
     In some embodiments, PLC gateway  112  may be disposed within or near premises  102   n  and serve as a gateway to all PLC communications to and/or from premises  102   n . In other embodiments, however, PLC gateway  112  may be absent and PLC devices  113  (as well as meter  106   n  and/or other appliances) may communicate directly with PLC data concentrator  114 . When PLC gateway  112  is present, it may include database  304  with records of frequency bands currently used, for example, by various PLC devices  113  within premises  102   n . An example of such a record may include, for instance, device identification information (e.g., serial number, device ID, etc.), application profile, device class, and/or currently allocated frequency band. As such, gateway engine  301  may use database  304  in assigning, allocating, or otherwise managing frequency bands assigned to its various PLC devices. PLC gateway engine  301  may be implemented using a digital signal processor (DSP), or another type of microprocessor, that is executing control software instructions stored in memory that is coupled to the microprocessor, for example, to perform various applications for gateway device  112 . 
       FIG. 4  is a block diagram of a PLC data concentrator that may include an embodiment of the MAC level packet length padding protocol described herein. Gateway interface  401  is coupled to data concentrator engine  402  and may be configured to communicate with one or more PLC gateways  112   a - n . Network interface  403  is also coupled to data concentrator engine  402  and may be configured to communicate with network  120 . In operation, data concentrator engine  402  may be used to collect information and data from multiple gateways  112   a - n  before forwarding the data to control center  130 . In cases where PLC gateways  112   a - n  are absent, gateway interface  401  may be replaced with a meter and/or device interface (now shown) configured to communicate directly with meters  116   a - n , PLC devices  113 , and/or other appliances. Further, if PLC gateways  112   a - n  are absent, frequency usage database  404  may be configured to store records similar to those described above with respect to database  304 . 
       FIG. 5  is a plot illustrating power spectral density for an example PLC channel and is representative of the type of frequency-selective channel that appears on a power line. In the CENELEC A band, the information bits (including the repeated bits) are transmitted using a differential BPSK (DBPSK) modulation scheme on an OFDM system having 36 tones (tone spacing=1.5625 kHz). The repetition coding helps to ensure that packets can be transmitted reliably even on frequency-selective channels, such as the PLC channel illustrated in  FIG. 5 . 
     As mentioned above, within the band of interest, such as CENELEC A: 35.9375-90.625 kHz, the higher 24 tones (54.6875-90.625 kHz) experience deep fading, which essentially ensures that no information can be received on them, i.e., only the lower 12 tones (35.9375-53.125 kHz) may be used to communicate data. This is equivalent to puncturing the higher 24 tones for each OFDM symbol. Note that the information bits, including the repeated bits, are actually transmitted on all 36 tones in ROBO mode. After the channel, the received data stream will look like 12 “received bits” followed by 24 “erased bits” (low signal values, essentially zeros) for each OFDM symbol. At the receiver, the received data stream is equalized, sliced (calculate soft information), de-interleaved, de-spread (integrating the soft information for the repeated bits). After these operations, the resulting soft information is sent to the Viterbi decoder. 
     As mentioned above, for certain frame lengths, it turns out that the interleaver/de-interleaver is poorly designed. For these cases, the output of the de-interleaver may contain long stretches of erasures or very unreliable soft information, which is then de-spread and sent to the Viterbi decoder. The long stretches of erasures or unreliable soft information makes it difficult for the Viterbi decoder to recover/determine the transmitted information sequence. In fact, if the length of consecutive erasures is (nearly) equal or longer than the d free  of the code (for the K=7, R=½ code used by G3-PLC, the d free =10), then Viterbi decoder may not be able to select the correct path. In fact, it will very likely choose the wrong path. This happens even if the SNR (signal to noise ratio) is very high. Thus, the problem is due to the frequency-selective nature of the channel and has less to do with the noise power. 
     This problem results in the G3-PLC modem failing for certain frame lengths over frequency-selective channels. Even though, this problem has been discussed in the context of the CENELEC A band, it turns that this problem also exists for all bands (CENELEC NB, ARIB, and FCC) with the G3-PLC standard. In addition, this problem also exists for all bands (CENELEC NB, ARIB, and FCC) with the IEEE 1901.2 standard. The details of IEEE 1901.2 interleaver/de-interleaver and spreading can be found in the IEEE 1901.2 standard, which is incorporated by reference herein. 
     Padding a Frame of Data 
     Fortunately, this problem does not occur for all frame lengths. For example, in the CENELEC band the maximum length packet (133 bytes) may be transmitted successfully using ROBO mode across a frequency-selective channel, such as the one described by the channel in  FIG. 6 . Therefore, a simple and counterintuitive solution is to pad the payload with zero bytes such that the frame length is always the maximum length packer of 133 bytes. This may be easily done at the MAC layer after receiving data from the data service access point and creating the ROBO packets. 
     Note that the padding does not have to be zero bytes, but could be any set of bytes. As an example, if the application layer passes down a payload of 90 bytes, then the MAC layer may pad the payload to 133 bytes and transmit the result frame using the PLC physical layer (PHY). The reason this solution is counterintuitive is because typically longer packets have a greater chance of failure; however, in this case the longer packets are in fact the more reliable packets because of the limitations imposed by a poorly designed interleaver/de-interleaver that is specified by the G3-PLC standard and also by the IEEE 1901.2 standard. 
     From a capacity point-of-view, it may not make sense to always pad up to the maximum frame length. For example, when the majority of the packets have a small frame length, the padding may result in a lot of overhead that increases transmission times on the power line, thereby, reducing the overall capacity of the channel/network. While in some cases small packets may be combined to create larger packets, Table 1 provides an example of several types of packets that are defined by the IEEE 1901.2 standard to have a specific length. These examples convey information regarding network joining. In these cases, there is no data from a next packet that can be combined to form a larger packet. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Examples of defined packets with a short packet length 
               
            
           
           
               
               
               
            
               
                   
                   
                 Packet Size 
               
               
                   
                 Packet Type 
                 (output bytes/symbols) 
               
               
                   
                   
               
               
                   
                 RREQ Packet size 
                 19 bytes/36 symbols 
               
               
                   
                 Discover SN-&gt;DC Beacon Request 
                 18 bytes/36 symbols 
               
               
                   
                 Discover DC-&gt;SN Beacon Request 
                 18 bytes/36 symbols 
               
               
                   
                 Joining No Security SN-&gt;DC Request 
                 40 bytes/76 symbols 
               
               
                   
                 Joining No Accepted DC-&gt;SN 
                 36 bytes/54 symbols 
               
               
                   
                   
               
            
           
         
       
     
     An alternative solution is to determine a set of frame lengths L={L 1 , L 2 , . . . , L N } that work even over the most severe frequency-selective channels and then pad the payload to the next highest frame length from that set L. Note that if the payload length is in the set L, then the frame can be transmitted as is because it is known that the frame will be transmitted successfully, i.e., padding is not required. For example, if the set L={20, 50, 80, 133} bytes and the payload is 44 bytes, then the payload may be padded so that the frame length would be equal to 50 bytes, a frame length that is known to work on frequency-selective channels. Using this approach, the amount of padding added to each frame may be minimized, thereby preserving the channel/network capacity. Some examples of alternative sets are L={33, 100, 141} bytes. Another example is L={47, 67, 94, 135} bytes. 
     A set L of payload lengths may be determined in several different manners. One way is to simply transmit a set of packets that have different lengths from a transmission node to receiver node across the frequency selective PLC network and then examine the received packets. For example, a set of packets ranging in length from 10 bytes to the maximum 133 bytes may be transmitted. The test may be repeated multiple times with different data loads to determine any data sensitivities. Those packets that are correctly received may indicate a packet length that may be successfully transmitted on the frequency selective channel. A set L may then be selected from all of the packet lengths that were correctly received. 
     Another way to determine a set L of payload packet lengths is to simulate operation of a receiver node using simulated OFDM packets in which various higher tones of each OFDM symbol are punctured to simulate the operation of the target frequency selective PLC channel. A simulator that mimics the operation of the equalization, slicing, de-interleaving, de-spreading, and Viterbi decoding may be used. The general operation of simulators is well known and need not be described in more detail herein. For example, reception of a set of packets ranging in length from 10 bytes to the maximum 133 bytes may be simulated. The simulation may be repeated multiple times with different data loads to determine any data sensitivities. Those simulated packets that are correctly received may indicate a packet length that may be successfully transmitted on the frequency selective channel. A set L may then be selected from all of the packet lengths that were correctly received. 
     The frame lengths contained in the set L may depend upon the frequency band, i.e., a set of frame lengths that work for the CENELEC A band maybe different than a set of frame lengths that work for the ARIB band, for example. 
     To maximize the channel/network capacity, set L should be as large as possible, which implies that the padding added to a frame will be minimized. Of course, this comes at the expense of complexity. The set L needs to be stored in memory at each modem device. It is possible to trade-off the channel/network capacity vs. complexity by reducing the size of set L. 
       FIG. 6  is an architectural diagram of a power line communication system, illustrating the PHY (physical) and MAC (media access control) layers used for data packet management as defined by the G3-PLC and IEEE 1901 standards. The Higher Layer Entities (HLEs) above the H1 (Host) Interface may be bridges, applications or servers that provide off-chip services to clients below the H1 Interface. The Data Service Access Point (SAP) accepts Ethernet format packets, so all IP based protocols are easily handled. The PLC communicating standards define two planes as shown in  FIG. 6 . The data plane provides the traditional layered approach with the M1 interface between the Convergence Layer (CL) and the MAC, and the PHY interface between the MAC and the PHY. In the control plane, the MAC is a monolith without conventional layering. In  FIG. 6  it is labeled as the Connection Manager (CM) since that is its primary function. The approach adopted for the control plane was chosen to provide more efficient processing and to provide implementers greater flexibility for innovation. Although part of the control plane may be in all stations, the Central Coordinator (CCo) entity may be active in only one station in a single PLC network. 
     In order to better understand embodiments of the invention, an overview of IEEE 1901.2 will now be described. Additional details may be found in various IEEE documents. A more detailed overview is provided in “An Overview, History, and Formation of IEEE 1901.2 for Narrowband OFDM PLC”, Jul. 2, 2013, which is incorporated by reference herein. 
     Details on PHY building blocks have been presented in various IEEE publications. The ultimate result is now a universal PHY structure for NB PLC. The fundamental PHY elements in the transceiver start with the scrambler. The scrambler&#39;s function is to randomize the incoming data. Both G3-PLC and PRIME utilize the same generator polynomial, as illustrated in equation 1. 
         s ( x )= x   7   +x   4 +1  Eq 1
 
     Two levels of error correction follow, starting with a Reed-Solomon (RS) encoder where typically data from the scrambler is encoded by shortened systematic Reed-Solomon (RS) codes using Galois Field (GF). The second level of error correction, employed by both G3-PLC and PRIME, uses a ½ rate convolutional encoder with constraint rate K=7. The convolutional encoder is followed by a two-dimensional (time and frequency) interleaver. Together these blocks significantly improve robustness and overall system performance in the presence of noise. 
     Following the FEC (forward error correction) block is the OFDM modulator. The modulation technique of PRIME and G3-PLC was selected to be used in IEEE 1901.2. The defined modulator describes the modulation (BPSK, QPSK, 8PSK, etc.); the constellation mapping; the number of repetitions (4, 6, etc.); the type of modulation (differential, coherent); the frequency domain pre-emphasis; OFDM generation (IFFT, with cyclic prefix); and windowing. 
     Structure of the physical frames is defined according to the fundamental system parameters, including the number of FFT points and overlapped samples, the size of cyclic prefixes, the number of symbols in the preamble, and the sampling frequency. The physical layer supports two types of frames: the data frame and the ACK/NACK frame. Each frame starts with a preamble used for synchronization and detection, as well as automatic gain control (AGC) adaptation. The preamble is followed by data symbols allocated to the frame control header (FCH) with the number of symbols depending on the number of carriers used by the OFDM modulation. 
     The FCH is a data structure transmitted at the beginning of each data frame. It contains information regarding modulation and the length of the current frame in symbols. The FCH also includes a frame control checksum (CRC, or cyclic redundancy check), which is used for error detection. The size of the CRC depends on the frequency band being utilized. 
     The PHY layer includes an adaptive tone mapping (ATM) feature to optimize maximum robustness. The added ATM feature is implemented first by estimating the SNR of the received signal subcarriers (tones), and then adaptively selecting the usable tones and the optimum modulation and coding type to ensure reliable communication over the powerline channel. Tone mapping also specifies the power level for the remote transmitter and the gain values to be applied for the various sections of the spectrum. The per-carrier quality measurement enables the system to adaptively avoid transmitting data on subcarriers with poor quality. Using a tone map indexing system, the receiver understands which tones are used by the transmitter to send data and which tones are filled with dummy data to be ignored. The goal of the ATM is to achieve the greatest possible throughput under the given channel conditions between the transmitter and the receiver. 
     A transmission protocol between the MAC and the PHY layer includes different data primitives accessible between the MAC and PHY layers. Several primitives may be provided. A PD-DATA.request primitive is generated by a local MAC sublayer entity and issued to its PHY entity to request the transmission of a PHY service data unit (PSDU). A PD-DATA.confirm primitive confirms the end of the transmission of a PSDU from the local PHY entity to a peer PHY entity. A PD-DATA.indication primitive indicates the transfer of a PSDU from the PHY to the local MAC sublayer entity. The PHY layer may include a management entity called the PLME (physical layer management entity). The PLME provides layer-management service interfaces functions. It is also responsible for maintaining the PHY information base. 
     The MAC layer is an interface between the logical link control (LLC) layer and the PHY layer. The MAC layer regulates access to the medium by using CSMA/CA (carrier sense, multiple access with collision avoidance). It provides feedback to upper layers in the form of positive and negative acknowledgements (ACK or NACK) and also performs packet fragmentation and reassembly. Packet encryption/decryption is carried out by the MAC layer as well. 
     A tone map response MAC command may be provided to utilize adaptive tone mapping. The MAC sublayer generates a tone map response command if the tone map request (TMR) bit of a received packet segment control field is set. This means that a packet originator has requested tone map information from a destination device. The destination device must estimate this particular communication link between two points and report the optimal PHY parameters. The tone map information includes the index associated with PHY parameters: the number of used tones and allocation (tone map), the modulation mode, the TX power control parameters, and the link quality indicator (LQI). 
     The Physical Layer (PHY) may operate in the frequency range of less than 500 kHz and provide up to 500 kbps PHY channel rate. An IEEE 1901.2 device may send MAC data frames that are greater in length then 400 bytes, while a G3 device is limited to a maximum of 400 bytes. 
       FIG. 7  is a flow chart illustrating a method for padding a frame of data to ensure successful transmission on frequency selective PLC channels. This method may be implemented in either software or hardware or a combination of software and hardware. 
     Initially, a set L of packet lengths that result in successful transmission of packets over a frequency selective PLC channel is determined  702 , as described in more detail above. Set L may be determined by performing physical experiments or by simulation, for example. As discussed above, a particular set L may depend on which frequency band is being used for the PLC communication. 
     Set L may be determined and then stored in a transmitter device when the device is manufactured, or when it is installed in the field and its software is configured, for example. The set L may be periodically updated in the field, for example. 
     In some embodiments, a network central coordinator may occasionally or periodically initiate a testing phase to validate set L. In this case, the central coordinator may distribute an updated set L for use in a current network configuration. 
     During normal network operation, as data packets are received  704  at the MAC layer of a transmitter node, the MAC layer may determine  706  if the payload length of the packet is one of the lengths contained in set L. If not, then the MAC layer may pad  708  the packet to a larger frame length that is contained in set L. As discussed above in more detail, set L may contain several lengths or in some cases it may contain only a single packet length. 
     Each packet that is padded may be padded  708  with data bytes that have a zero value, or the padding bytes may have non-zero values. 
     For data packets that are determined  706  to have a length that is in set L, no padding is needed. 
     Each packet is then forwarded  710  to the PHY layer where it is encoded into OFDM symbols and then transmitted across the frequency selective communication channel. 
     This method may be applied to all frequency bands using the G3-PLC standard. This method may also be applied to all frequency bands using the IEEE 1901.2, for example. 
       FIG. 8  is a block diagram of an exemplary low cost, low power G3 and 1901.2 compatible device  800  illustrating an OFDM transmitter  810  and receiver  820  for use in a power line communication node for PLC over a power line  802 . As discussed above, the power line channel is very hostile. Channel characteristics and parameters vary with frequency, location, time and the type of equipment connected to it. The lower frequency regions from 10 kHz to 200 kHz used in G3 PLC and in IEEE 1901.2 are especially susceptible to interference. Furthermore, the power line is a very frequency selective channel. Besides background noise, it is subject to impulsive noise often occurring at 50/60 Hz, and narrowband interference and group delays up to several hundred microseconds. 
     As described in more detail above, OFDM is a modulation technique that can efficiently utilize the limited bandwidth specified by CENELEC and IEEE 1901.2, and thereby allows the use of advanced channel coding techniques. This combination facilitates a very robust communication over a power line channel. 
     The CENELEC bandwidth is divided into a number of sub-channels, which can be viewed as many independent PSK modulated carriers with different non-interfering (orthogonal) carrier frequencies. Convolutional and Reed-Solomon coding provide redundancy bits allowing the receiver to recover lost bits caused by background and impulsive noise. A time-frequency interleaving scheme may be used to decrease the correlation of received noise at the input of the decoder, providing diversity. 
     Data  811  and a frame control header  812  are provided by an application via a media access layer (MAC) of the communication protocol. An OFDM signal is generated by performing IFFT (inverse fast Fourier transform)  815  on the complex-valued signal points that are produced by differentially encoded phase modulation from forward error correction encoder  813  using Reed Solomon encoding. Tone mapping  814  is performed to allocate the signal points to individual subcarriers. An OFDM symbol is built by appending a cyclic prefix (CP)  816  to the beginning of each block generated by IFFT  815 . The length of a cyclic prefix is chosen so that a channel group delay will not cause successive OFDM Symbols or adjacent sub-carriers to interfere. The OFDM symbols are then windowed  817  and impressed on power line  802  via analog front end  818 . AFE  818  provides isolation of transmitter  810  from the 50/60 Hz power line voltage. 
     Similarly, receiver  820  receives OFDM signals from power line  802  via AFE  821  that isolates receiver  820  from the 50/60 HZ power line voltage. OFDM demodulator  822  removes the CP, converts the OFDM signal to the time domain using FFT (Fast Fourier Transform), and performs demodulation of the phase shift keyed (DBPSK, DQPSK) symbols. FEC decoder  823  performs error correction using Reed Solomon decoding and then descrambles the symbols to produce received data  824 . Frame control header  825  information is also produced by FEC decoder  820 , as defined by the G3 and IEEE 1901.2 PLC standards. 
     A blind channel estimation technique may be used for link adaptation. Based on the quality of the received signal, the receiver decides on the modulation scheme to be used, as defined in the PLC standards. Moreover, the system may differentiate the subcarriers with bad SNR (signal to noise ratio) and not transmit data on them. 
     Transmitter  810  and receiver  820  may be implemented using a digital signal processor (DSP), or another type of microprocessor, that is executing control software instructions stored in memory that is coupled to the microprocessor, for example, to perform FEP encoding, mapping and OFDM modulation, demodulation and FEP decoding. In other embodiments, portions or all of the transmitter or receiver may be implemented with hardwired control logic, for example. The analog front ends  818 ,  821  require analog logic and isolation transformers that can withstand the voltage levels present on the power line. 
     A G3 and IEEE 1901.2 PLC system is specified to have the ability to communicate in both low voltage (LV) power lines, typically 100-240 VAC, as well as medium voltage (MV) power lines (up to approximately 12 kV, by crossing LV/MV transformers. This means that the receiver on the LV side must be able to detect the transmitted signal after it has been severely attenuated as a result of going through a MV/LV transformer. As the signal goes through the transformer it is expected to experience overall severe attenuation in its power level as well as frequency-dependent attenuation that attenuates higher frequencies. Both transmitter and receiver have mechanisms to compensate for this attenuation. The transmitter has the capability to adjust its overall signal level as well as shape its power spectrum based on tone map information provided by a target receiver, while the receiver has both an analog and digital AGC (Automatic Gain Control) in order to achieve enough gain to compensate for the overall attenuation. 
     Other Embodiments 
     In the description herein, some terminology is used that is specifically defined in the G3 and IEEE 1901.2 standards and/or is well understood by those of ordinary skill in the art in PLC technology. Definitions of these terms are not provided in the interest of brevity. Further, this terminology is used for convenience of explanation and should not be considered as limiting embodiments of the invention to the G3 and IEEE 1901.2 standards. One of ordinary skill in the art will appreciate that different terminology may be used in other encoding standards without departing from the described functionality. 
     Embodiments may be operated on various frequency bands by determining a set of packet lengths that provide successful communication for the particular frequency band, such as CENELEC A, CENELEC B, ARIB or FCC band, for example. 
     While embodiments may perform padding in the MAC layer, other embodiments may perform padding of a data packet as described herein prior to sending the data packet to the MAC layer, for example. 
     Embodiments of the receivers and transmitters and methods described herein may be provided on any of several types of digital systems: digital signal processors (DSPs), general purpose programmable processors, application specific circuits, or systems on a chip (SoC) such as combinations of a DSP and a reduced instruction set (RISC) processor together with various specialized accelerators. A stored program in an onboard or external (flash EEP) ROM or FRAM may be used to implement aspects of the signal processing. 
     The techniques described in this disclosure may be implemented in hardware, software, firmware, or any combination thereof. Various combinations of hardware and/or software state machines may be used. If implemented in software, the software may be executed in one or more processors, such as a microprocessor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), software state machines, or digital signal processor (DSP), for example. The software that executes the techniques may be initially stored in a computer-readable medium such as a flash drive, a compact disc (CD), a diskette, a tape, a file, memory, or any other computer readable storage device and loaded at a manufacturing site for execution in the processor. In some cases, the software may also be sold in a computer program product, which includes the computer-readable medium and packaging materials for the computer-readable medium. In some cases, the software instructions may be distributed via removable computer readable media (e.g., floppy disk, optical disk, flash memory, USB key), via a transmission path from computer readable media on another digital system, etc. 
     Certain terms are used throughout the description and the claims to refer to particular system components. As one skilled in the art will appreciate, components in digital systems may be referred to by different names and/or may be combined in ways not shown herein without departing from the described functionality. This document does not intend to distinguish between components that differ in name but not function. In the discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the term “couple” and derivatives thereof are intended to mean an indirect, direct, optical, and/or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through an optical electrical connection, and/or through a wireless electrical connection. 
     Although method steps may be presented and described herein in a sequential fashion, one or more of the steps shown and described may be omitted, repeated, performed concurrently, and/or performed in a different order than the order shown in the figures and/or described herein. Accordingly, embodiments of the invention should not be considered limited to the specific ordering of steps shown in the figures and/or described herein. 
     It is therefore contemplated that the appended claims will cover any such modifications of the embodiments as fall within the true scope and spirit of the invention.