Abstract:
A method of operating a communication system is disclosed. The method includes forming a data frame having plural orthogonal frequency division multiplex (OFDM) symbols. A first set of preamble subcarriers is allocated to at least one of the OFDM symbols. A second set of data subcarriers is allocated to said at least one of the OFDM symbols.

Description:
This application claims the benefit under 35 U.S.C. §119(e) of Provisional Appl. No. 62/133,537, filed Mar. 16, 2015 , which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Embodiments of the present invention relate to power line communication (PLC) and, more particularly, to an optimized narrowband orthogonal frequency division multiplex (OFDM) based physical (PHY) frame structure. 
     Powerline communications (PLC) include systems for communicating data over the same medium that is used to transmit electric power to residences, buildings, and other premises. Once deployed, PLC systems may enable a wide array of applications, including, for example, automatic meter reading and load control for utility-type applications, automotive uses such as charging electric cars, home automation for controlling appliances and lights, and computer networking for internet of things (IoT). 
     Various PLC standardizing efforts are currently being undertaken around the world, each with its own unique characteristics. Examples of competing PLC standards include the IEEE 1901.2, HomePlug AV, and ITU-T G.hn (e.g., G.9960 and G.9961) specifications. Generally speaking, PLC systems may be implemented differently depending upon local regulations and characteristics of local power grids. For example, the U.S. FCC implementation of IEEE 1901.2 uses OFDM subcarriers from 10 kHz to 490 kHz. CENELEC, the European standard, has various implementations using OFDM subcarriers from 3 kHz to 148.5 kHz. ARIB, the Japanese standard, uses OFDM subcarriers from 10 kHz to 450 kHz. Another standardization effort includes, for example, the Powerline-Related Intelligent Metering Evolution (PRIME) standard designed for OFDM-based (Orthogonal Frequency-Division Multiplexing) communications. The current or existing PRIME standard is the Draft Standard prepared by the PRIME Alliance Technical Working Group (PRIME R1.3E) and earlier versions thereof. 
     Current and next generation narrowband PLC standards are directed to multi-carrier based systems, such as orthogonal frequency division multiplexing (OFDM) in order to get higher network throughput. OFDM uses multiple orthogonal subcarriers to transmit data over frequency selective channels. A conventional OFDM structure for a data frame includes a preamble, followed by a physical layer (PHY) header, a media access control (MAC) header, followed by a data payload. However, PLC channels are highly challenging environments for digital communication because they suffer from periodic bursts of impulse noise, and the channel impulse response also varies over time. 
     A conventional synchronization preamble structure for a narrowband OFDM PLC standard, such as IEEE 1901.2 (G3), includes 8 SYNCP symbols followed by 1.5 SYNCM symbols. The synchronization symbols are typically transmitted at a higher (3 dB) rms voltage than the data payload, and there is no cyclic prefix between adjacent symbols. Each SYNCP symbol is a known preamble sequence of different subcarriers phase shifted by a multiple of π/8. Subcarriers of the SYNCM symbol are phase shifted by π with respect to SYNCP so that SYNCM=−SYNCP. For example, a SYNCP symbol may be a chirp-like sequence of a specific binary sequence of 1 s and −1 s or a constant amplitude, zero autocorrelation (CAZAC) sequence. The definition of the SYNCP symbol for the FCC band in IEEE P1901.2 is defined in section 6.6 for specific subcarriers or tones. 
     The preamble serves several purposes including: 1) indicating to other nodes in the PLC network that a transmission is in progress; 2) determining the frame boundary between the preamble and the PHY header, and between the PHY header and the data payload; 3) determining accurate channel estimates; and 4) for frequency offset compensation. SYNCM symbols help determine the frame boundary and indicate the end of the preamble sequence. The repetitive SYNCP symbols also assist in preamble detection as receiver nodes are looking for the repetitive sequence of symbols in the PLC channel to determine whether or not a frame is on the powerline. Multiple SYNCP symbols also help in obtaining more accurate channel estimates by averaging the channel estimates across multiple symbols to reduce noise. Improved channel estimates also help in improving the header decoding performance when the header is coherently modulated with respect to the SYNCP preamble. 
     While preceding approaches provide improvement and standardization in PLC operation, the present inventors recognize that still further improvements are possible. This is particularly true for high data rate PLC applications. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first embodiment of the present invention, there is disclosed a method of operating a communication system. The method includes forming a data frame having plural orthogonal frequency division multiplex (OFDM) symbols. A first set of preamble subcarriers is allocated to at least one of the OFDM symbols. A second set of data subcarriers is allocated to said at least one of the OFDM symbols. 
     In a second embodiment of the present invention, there is disclosed a method of operating a communication system. The method includes receiving a data frame having plural orthogonal frequency division multiplex (OFDM) symbols. A first set of preamble signals is received from at least one of the OFDM symbols. A second set of data signals is received from said at least one of the OFDM symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagram of a power line communication (PLC) environment of the present invention; 
         FIG. 2  is a block diagram of an IEEE 1901.2 (G3) compatible device of the present invention; 
         FIG. 3  is an IEEE 1901.2 (G3) compatible coherent frame structure of the present invention; 
         FIG. 4  is a diagram of a circuit for preamble symbol generation according to a first embodiment of the present invention; 
         FIG. 5A  is a diagram of sequential preamble symbols as generated by the circuit of  FIG. 4 , for a first logic level of a modulation control scheme (MCS) signal; 
         FIG. 5B  is a diagram of sequential preamble symbols as generated by the circuit of  FIG. 4 , for a second logic level of the modulation control scheme (MCS) signal; 
         FIG. 6  is a diagram of a circuit for preamble symbol generation according to a second embodiment of the present invention; 
         FIG. 7  is a diagram of sequential preamble symbols as generated by the circuit of  FIG. 6 , for respective odd and even preamble symbols; and 
         FIG. 8  is a diagram of throughput gain as a function of payload size according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is an electric power distribution system is depicted according to the present invention. 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 communication (PLC) modems or gateways  112   a - n  may be coupled to LV power lines  105  at meter  106   a - n . PLC 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, communication systems, high speed internet, telephony, video conferencing, and video delivery, to name a few. By transporting telecommunications 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 electrical distribution systems to carry data signals, significant cost savings are possible. 
     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, and 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 , which may be a utility company, via network  120 . Network  120  may include, for example, an internet protocol (IP) based network, a cellular network, a WiFi network, a WiMax network, or the like. As such, control center  130  may be configured to collect power consumption information and other types of relevant information from gateways  112  and devices  113  through concentrator  114 . Additionally, control center  130  may be configured to implement smart grid policies and other regulatory or commercial rules by communicating such rules to each gateway  112  or device  113  through concentrator  114 . 
     In some embodiments, concentrator  114  may be 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 , devices  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. 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 such as Ethernet or digital subscriber loop (DSL). 
     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 receiver circuitry to facilitate the device&#39;s connection to power lines  103 ,  105 , and/or  108 . 
       FIG. 2  is a block diagram of an exemplary low cost, low power IEEE 1901.2 compatible device  200  that may be used in blocks  112   a - n  ( FIG. 1 ) according to the present invention. The diagram illustrates an OFDM transmitter  210  and receiver  220  for use in a power line communication node for PLC over a power line  202 . 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. 
     Preamble circuit  232  produces a preamble to synchronize each transmitted data frame with a receiving device. Preamble sequence allocator circuit  230  determines which tones in an OFDM symbol will be occupied by the preamble. Data  211  and a frame control header (FCH)  212  are provided by an application via a media access layer (MAC) of the communication protocol. Similar to the preamble sequence allocator circuit  230 , data sequence allocator circuit  234  determines which tones in an OFDM symbol will be used by data circuit  211  or FCH circuit  212 . An OFDM signal is generated by performing an inverse fast Fourier transform (IFFT)  215  on the complex valued signal points that are produced by differentially encoded phase modulation from forward error correction encoder  213  using Reed Solomon encoding. Tone mapping  214  is performed to allocate the signal points to individual subcarriers. An OFDM symbol is built by appending a cyclic prefix (CP)  216  to the beginning of each block generated by IFFT  215 . 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  217  and impressed on power line  202  via analog front end (AFE)  218 . AFE  218  provides isolation of transmitter  210  from the 50/60 Hz power line voltage. 
     Similarly, receiver  220  receives OFDM signals from power line  202  via AFE  221  that isolates receiver  220  from the 50/60 HZ power line voltage. OFDM demodulator  222  removes the CP, converts the OFDM signal to the frequency domain using a fast Fourier transform (FFT), and performs demodulation of the differential binary or quadrature phase shift keyed (DBPSK, DQPSK) symbols. FEC decoder  223  performs error correction using Reed Solomon decoding and then descrambles the symbols to produce received data  224 . Frame control header  225  information is also produced by FEC decoder  220 , as defined by the G3 and IEEE 1901.2 PLC standards. Similar to the transmitter  210 , receiver  220  also has a preamble sequence allocator circuit  236  and data sequence allocator circuit  238  to indicate which tones are used for the preamble and which tones are used for data in any OFDM symbol. 
     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 a bad signal to noise ratio (SNR) and not transmit data on them. 
     Transmitter  210  and receiver  220  may be implemented using a digital signal processor (DSP) or another type of microprocessor that is executes control software instructions stored in memory. For example, the processor may perform operations such as FEC encoding, mapping and OFDM modulation, demodulation and FEC decoding in software. In other embodiments, portions or all of the transmitter or receiver may be implemented with hardwired control logic. The analog front ends  218  and  221  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 automatic gain control (AGC) in order to achieve enough gain to compensate for the overall attenuation. 
     Turning now to  FIG. 3 , there is a coherent frame structure for communication between the network of  FIG. 1  and the device of  FIG. 2  according to the present invention. The frame includes a preamble, a frame control header (FCH) and payload data. The preamble includes synchronization symbols such as SYNCP  300  and SYNCM  302  and is terminated by a half SYNCM symbol  304 , which is preferably a repetition of the first half of SYNCM  302 . The preamble is separated from the FCH by an overlap region  306 . The exemplary FCH includes 12 OFDM symbols, but this may vary with different band plans. The FCH is coherently modulated and contains information regarding the current frame such as the type of frame, the tone map index, and the length of the frame. Each FCH symbol is preceded by a respective guard interval (GI). For example, the FCH 1   310  is preceded by respective GI  308 , and each FCH symbol is separated by an overlap region. Symbols S 1  and S 2  are inserted between the FCH and the payload data (DATA). Symbol S 2  is similar to SYNCP  300  except that it includes a cyclic prefix, GI, and overlap regions. Symbol S 1  is an inverted version of S 2  (−S 2 ). The payload data follows symbols S 1  and S 1  and includes data in respective OFDM symbols. The first symbol, for example, includes overlap  312 , GI  314 , and data  316 . 
     Referring next to  FIG. 4 , there is a diagram of a circuit for preamble symbol generation according to a first embodiment of the present invention. The circuit includes IFFT and parallel-to-serial (P/S) converter  215  from  FIG. 2 . Cyclic prefix circuit  216  appends a cyclic prefix from the end of the P/S converter output to the beginning of the serial output to complete the OFDM symbol. As previously discussed, IEEE 1901.2 has specified 128 tones for FCC narrowband PLC. This requires a minimum of N=256 IFFT samples. In this exemplary embodiment, however, only K=72 of these tones are used, and the remaining 56 tones at the ends of the IFFT are unused and set to zero. Preamble tones 1-36 (P 1 -P K/2 ) are applied as inputs to the IFFT circuit. Multiplex circuit  400  is coupled to receive preamble tones  37 - 72  (P K/2 -P K ) and 36 data tones (X 1 -X K/2 ). Multiplex circuit  400  selectively applies the 36 preamble tones or the 36 data tones to the IFFT circuit in response to a modulation control scheme (MCS) signal. 
     Operation of the circuit of  FIG. 4  will now be described with reference to  FIGS. 5A-5B . When the PLC signal-to-noise ratio (SNR) is good, a high data rate MCS may be selected for the frame of  FIG. 3 . In this case, 36 preamble tones are sufficient for synchronization detection at a receiver ( FIG. 2 ). Thus, MCS has a high logic level and multiplex circuit  400  applies 36 data tones to the IFFT circuit. Sequential SYNCP symbols, therefore, have a structure as illustrated in  FIG. 5A , where the vertical axis is subcarrier frequency and the horizontal axis is time. Each SYNCP OFDM symbol includes 36 preamble tones  500  and 36 data tones  502 . The preamble tones are preferably contiguous within the OFDM symbol to provide a better correlation profile. This greatly increases data throughput when the PLC SNR is good. When the PLC signal-to-noise ratio (SNR) is compromised by noise, a lower data rate MCS may be selected for the frame of  FIG. 3 . In this case, 72 preamble tones may be required for synchronization detection. Thus, MCS has a low logic level and multiplex circuit  400  applies 36 additional preamble tones (P K/2 -P K ) to the IFFT circuit. Sequential SYNCP symbols, therefore, have a structure as illustrated in  FIG. 5B . Each SYNCP OFDM symbol includes 72 preamble tones  504  and no data tones. In a first mode of operation, a receiver operates on a static allocation of data for each OFDM preamble symbol. This is preferably a default mode. In a second mode of operation, the receiver operates on a semi-persistent or adaptive allocation as determined by a received data frame. This mode is preferably adapted to the communication system SNR. This embodiment of the present invention advantageously increases data throughput when the PLC SNR is good and reverts to normal data throughput in a high noise environment. 
     Referring now to  FIG. 6 , there is a diagram of a circuit for preamble symbol generation according to a second embodiment of the present invention. The circuit includes IFFT and parallel-to-serial (P/S) converter  215  and cyclic prefix circuit  216 . Multiplex circuit  600  selectively applies the 36 preamble tones (P 1 -P K/2 ) for odd-numbered symbols or 36 data tones (X 1 -X K/2 ) for even-numbered symbols in response to control signal EVEN. Likewise, multiplex circuit  602  selectively applies the 36 data tones for odd-numbered symbols or 36 preamble tones for even-numbered symbols in response to control signal EVEN. 
     Operation of the circuit of  FIG. 6  will now be described with reference to  FIG. 7 . For odd-numbered symbol  700  (Symbol 1), control signal EVEN has a low logic state. Thus, multiplex circuit  600  applies preamble tones P 1 -P K/2  to IFFT circuit  215 , and multiplex circuit  602  applies data tones X 1 -X K/2  to IFFT circuit  215 . Symbol  700 , therefore, includes 36 preamble tones  702  at the upper frequency range and 36 data tones  704  at the lower frequency range. For even-numbered symbol  706  (Symbol 2), control signal EVEN has a high logic state. Thus, multiplex circuit  600  applies data tones X 1 -X K/2  to IFFT circuit  215 , and multiplex circuit  602  applies preamble tones P 1 -P K/2  to IFFT circuit  215 . Symbol  706 , therefore, includes 36 data tones  708  at the upper frequency range and 36 preamble tones  710  at the lower frequency range. In a first mode of operation, a receiver operates on a static allocation of alternating preamble and data subcarriers in each OFDM preamble symbol. This is preferably a default mode. In a second mode of operation, the receiver operates on a semi-persistent or adaptive allocation as determined by a received data frame. This mode is preferably adapted to the communication system SNR and desired throughput. This embodiment of the present invention advantageously improves frequency diversity gain by alternating preamble and data frequencies in adjacent OFDM symbols. 
     Referring to  FIG. 8 , there is a diagram of throughput gain as a function of payload size according to the present invention. In the foregoing embodiment of the present invention, throughput gain increases from 19% to 34% as payload data increases from 30 to 70 bytes. Further increases in payload data size are accompanied by a gradual decrease in throughput gain. This is because additional data in the preamble becomes less significant as payload data (after FCH) in the frame increases. At 480 bytes of payload data, the throughput gain decreases to 14%. 
     Embodiments of the present invention may be readily adapted to other frame structures as one of ordinary skill in the art having access to the instant specification will understand. For other applications, the number of used preamble symbol tones K may be more or less than 72. Likewise, the IFFT samples may be more or less than 256. Although previous embodiments have specifically addressed addition of payload data to SYNCP symbols, payload data may also be added to SYNCM symbols or to frame symbols S 1  and S 2  of the FCH. Moreover, embodiments of  FIGS. 4 and 6  may be combined to selectively include or omit data tones in response to a modulation control scheme, and the included data tones may alternate with preamble tones in alternating OFDM symbols. 
     Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims. Furthermore, embodiments of the present invention may be implemented in software, hardware, or a combination of both. Other combinations will be readily apparent to one of ordinary skill in the art having access to the instant specification.