Patent Publication Number: US-9843357-B2

Title: Method for transmitting a signal via a power line network, transmitter, receiver, power line communication modem and power line communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/292,023, filed May 30, 2014, now U.S. Pat. No. 9,191,068, which is a continuation of U.S. application Ser. No. 13/601,772, filed Aug. 31, 2012, now U.S. Pat. No. 8,743,975, which is a continuation of U.S. application Ser. No. 13/412,279, filed Mar. 5, 2012, now U.S. Pat. No. 8,442,127, which is a continuation of U.S. application Ser. No. 12/595,265, filed Oct. 9, 2009, now U.S. Pat. No. 8,160,162. Further, U.S. application Ser. No. 12/595,265 is a National Stage of PCT/EP08/06212, filed Jul. 28, 2008, and is based upon and claims the benefit of priority from prior European Patent Application No. 07016489.2, filed Aug. 22, 2007. The entire content of each of the foregoing applications is incorporated herein by reference. 
    
    
     DESCRIPTION 
     The invention relates to a method for transmitting signals via a power line network, a transmitter and a receiver. The invention relates as well to a power line communication modem, and a power line communication system. 
     BACKGROUND 
     Power line communication (PLC), also called mains communication, power line transmission (PLT), broadband power line (BPL), power band or power line networking (PLN), is a term describing several different systems for using power distribution wires for simultaneous distribution of data. A carrier can communicate voice and data by superimposing an analogue signal over the standard 50 Hz or 60 Hz alternating current (AC). For indoor applications PLC equipment can use household electrical power wiring as a transmission medium. 
     In order to increase the bandwidth of PLC systems it has been proposed to use multiple-input-multiple-output schemes (MIMO) which are known from wireless communication systems. 
     It is an object of the invention to further increase the bandwidth of PLC systems. 
     The object is solved by a method for transmitting a signal, a transmitter, a receiver, a power line communication modem and a power line communication. 
     Further embodiments are defined in the dependent claims. 
     Further details of the invention will become apparent from a consideration of the drawings and ensuing description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows steps of one embodiment of the invention, 
         FIG. 2 a    shows a block diagram of a transmitter according to a further embodiment of the invention, 
         FIG. 2 b    shows a block diagram of a receiver according to a further embodiment of the invention, 
         FIG. 3  shows a block diagram of a power line communication system according to a further embodiment of the invention, 
         FIG. 4  shows a block diagram of a conventional power line communication system, 
         FIG. 5  shows a power line communication system according to a further embodiment of the invention, 
         FIG. 6  shows steps of a further embodiment of the invention, 
         FIG. 7  shows steps of a further embodiment of the invention, 
         FIG. 8  shows a schematic block diagram to explain the function of a transmitter according to a further embodiment of the invention, 
         FIG. 9A  shows a circuit diagram for impedance modulating devices. 
         FIG. 9B  shows a schematic diagram of the time-dependence of the voltage, when impedance modulating devices are present, 
         FIG. 9C  shows a schematic diagram of a voltage-time relation with parts of similar channel capacities to explain a further embodiment of the invention, 
         FIG. 10  shows steps of a further embodiment of the invention, and 
         FIG. 11  shows a block diagram of a power line communication system according to a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, embodiments of the invention are described. It is important to note that all described embodiments in the following and their properties and technical features may be combined in any way, i.e. there is no limitation that certain described embodiments, properties and technical features may not be combined with others. 
     In  FIG. 1  in a step S 100  a channel characteristic is determined and in a power line network a transmitter and at least one receiver communicate via at least two channels, each of said channels having a respective feeding port of said at least one transmitter and a respective port of said at least one transmitter and said transmitter having at least two feeding ports. A corresponding power line network is depicted schematically in  FIG. 3  that will be explained below. 
     The channel characteristics may be derived from a channel estimation and describe the channel by, for instance, bit-error-rate (BER) or signal-to-noise-ratio (SNR). Other channel characteristics may be the power or the energy of the received signal on said channel. 
     In a step S 102  a feeding port selection criterion is applied based on the channel characteristic determined in step S 100 . While applying the feeding port selection criteria the channel characteristics of different channels are compared in order to decide, which feeding port or feeding ports would be used, since the best reception is ensured while using these feeding ports. 
     In a step S 104  an excluded feeding port is selected among the at least two feeding ports based on the feeding port selection criteria, wherein the excluded feeding port is not used during further communication. 
     According to Kirchhoff s Rule in PLC systems in presence of three wires or conductors there are only two independent feeding possibilities. 
     In step S 104  the feeding port is selected based on the feeding port selection criterion, thereby identifying the worst channel characteristics. Since the channel is quasi-static for PLC systems, the selection of the feeding port remains stable until there is a dedicated change in the PLC network topology (for instance a light has been switched on or a device has been plugged or unplugged). 
     A channel capacity C of a channel might be calculated as 
             C   =       B   ·     1   N       ⁢       ∑     i   =   1     N     ⁢           ⁢       log   2     ⁡     (     det   ⁡     (       I     N   R       +       1     n   T       ·   SNR   ·     H   i     ·     H   i   H         )       )                 
with: B being the bandwidth of the channel, N being the number of OFDM sub-carriers, n R  being the number of receive ports, I NR  being the n R ×n R  identity matrix, n r  being the number of transmit ports, SNR being the signal-to-noise ratio, H being the n R ×n R  channel matrix.
 
     Alternatively, in an adaptive OFDM-(orthogonal frequency division multiplexing)-system, a channel equalizer within the receiver provides information about the signal-to-noise-ratio (SNR) for each sub-carrier of the OFDM system. Depending on the SNR condition on each sub-carrier, a suited constellation size is selected. The less SNR is available, the more robust the constellation has to be. As an example, for quadrature amplitude modulation (QAM), different constellations with a different SNR requirement exist 
     
       
         
           
             constellation 
             ∈ 
             
               { 
               
                 
                   
                     BPSK 
                   
                   
                     
                       1 
                       ⁢ 
                       bit 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     QPSK 
                   
                   
                     
                       2 
                       ⁢ 
                       bits 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     
                       16 
                       - 
                       QAM 
                     
                   
                   
                     
                       4 
                       ⁢ 
                       bits 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     
                       64 
                       - 
                       QAM 
                     
                   
                   
                     
                       6 
                       ⁢ 
                       bits 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     
                       256 
                       - 
                       QAM 
                     
                   
                   
                     
                       8 
                       ⁢ 
                       bits 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     
                       1024 
                       - 
                       QAM 
                     
                   
                   
                     
                       10 
                       ⁢ 
                       bits 
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       symbol 
                     
                   
                 
                 
                   
                     … 
                   
                   
                     … 
                   
                 
               
             
           
         
       
     
     Finally, the overall bit loading of one OFDM symbol can be taken as qualitative capacity criteria. The higher the total number of bits for one OFDM symbol (as a sum over all N sub-carriers), the higher the capacity C: 
     
       
         
           
             C 
             ∝ 
             
               
                 ∑ 
                 
                   i 
                   = 
                   1 
                 
                 N 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 constellation 
                 i 
               
             
           
         
       
     
     In case of MIMO, there are equalizers for all different receiving ports M. In this case, the overall sum of all channel equalizers can be taken as feeding port selection criterion: 
     
       
         
           
             C 
             ∝ 
             
               
                 ∑ 
                 
                   m 
                   = 
                   1 
                 
                 M 
               
               ⁢ 
               
                   
               
               ⁢ 
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   N 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   constellation 
                   
                     m 
                     , 
                     i 
                   
                 
               
             
           
         
       
     
     In a further embodiment a period of an alternating current on said power line network is divided at least into a first and a second part. A first channel characteristic is determined for the first part and a second channel characteristic is determined for the second part. Then a first excluded feeding port is selected for said first part based on said feeding port selection criteria and a second excluded feed is selected for said second part based on said feeding port selection criteria. If impedance-modulating devices are present in the power line network the main impedance changes depending on the line cycle duration and depending on the number of impedance modulating devices. When the number of impedance modulating devices is not changing, the impedance changes are periodic with the line cycle duration, e.g. 20 ms for a 50 Hz alternating current. The impedance changes have dramatic influence to data transmission over power line. An impedance change during a data burst results in wrong channel equalization values after the impedance change and causes non-correctable transmission errors. Therefore it is proposed to place the burst in time intervals where the impedance keeps stable. In presence of impedance-modulating devices the feeding selection is performed separately for each impedance condition so that the excluded feeding port change with different impedance settings. The feeding port selection can include an additional port selection criterion. Feeding ports, which are at least faced to impedance modulating behavior, may be determined, since not every feeding point combination is faced to the same level of impedance modulation. 
     According to a further embodiment a respective channel capacity based on the channel characteristics for said channel is determined and an excluded channel with the channel capacity below a predetermined threshold is determined which is not used during farther communication afterwards. 
     Within this embodiment not only the feeding ports are determined but also singular channels may be excluded from further communication. This might be useful in case of impedance modulating devices or in case of impulsive noise on the power line network. 
     In a further embodiment the channel characteristics of the channel is determined by transmitting an OFDM test signal via a plurality of channels simultaneously and determining a respective plurality of channel capacities for said plurality of channels based on the received version of said OFDM test signal. 
     According to a further embodiment a multiple-input-multiple-output coding scheme (MIMO-scheme) is set based on the respective channel capacities. By setting an appropriate MIMO in data throughput and reliability of the PLC system is farther optimized. Depending on the channel characteristics and/or the bandwidth demand of the application, an appropriate MIMO coding scheme is selected. Available MIMO modes are tested sequentially and the best MIMO mode regarding throughput and/or bit error rate is chosen. In further embodiments the data transmission is optimized regarding maximum throughput and/or transmission reliability. For instance, Alamouti MIMO is designed in a way to achieve better bit error rates (BER) performance without increasing the throughput rate (special code rate is one). On the other hand, multiplex MIMO systems like HBLAST (Horizontal Bell Laboratories Layered Space-Time), VBLAST (Vertical Bell Laboratories Layered Space-Time) or Eigenbeamforming-MTMO are designed to maximize the data throughput while BER performance optimization on the physical layer is not the primary focus (special code rate is two). 
     In  FIG. 2 a    a block diagram of a transmitter  200  is depicted. The transmitter  200  comprises two feeding ports  202 ,  204  each of which is configured to feed signals into at least two channels and a processor  206  configured to select an excluded feeding port of said at least two feeding ports  202 ,  204  based on a determination of channel characteristics of said at least two channels, said processor  206  being further configured to not use channels of said at least two channels during communication which are fed by said excluded feeding port ( 202  or  204 ). 
     With respect to the wording “transmitter” and “receiver” it should be emphasized that within this description “transmitter” and “transmitting modem” as well as “receiver” and “receiving modem” are used interchangeably, since a power line communication modem for bidirectional communication comprises a transmitter as well as a receiver. Thus, in a power line system, the communication of payload data between power line communication modems is performed between a transmitting modem (i.e. the transmitter) and a receiving modem (i.e. the receiver). 
     In a further embodiment the processor  206  might be further configured to exclude channels with the channel capacity below the predetermined threshold from further communication and the processor  206  might be configured to set a multiple-input multiple-output coding scheme based on the respective channel capacities. 
     In  FIG. 2 b    a block diagram of a receiver  250  is depicted. The receiver  250  comprises at least one receiving port  252 , which is the receiving end of at least two channels from the power line communication network, the channels being fed by at least two different feeding ports (not depicted). The receiving port  252  is connected to a channel estimation unit  254 , which is configured to determine channel characteristics of said at least two channels. A processor  256  is connected to said channel estimation unit  254  and is configured to select the feeding port, which should be excluded from further communication based on the determination of the channel characteristics from the channel estimation unit  254 . A transmitting unit  258  is connected to the processor  256  for transmitting an information about the excluded feeding port to a transmitter, which afterwards only uses non-excluded feeding ports for the communication with the receiver  250 . 
     Thus, the identification of the excluded feeding port might be performed in the receiver  250  or in the transmitter  200  depending on the information which is fed back to the transmitter. If the channel characteristics are fed back from the receiver  250  to the transmitter  200 , then within the transmitter the excluded feeding port is selected. If the receiver  250  already selects the excluded feeding port, then only an information about the excluded feeding port has to be fed back to the transmitter  200 . 
     In  FIG. 3  a schematic block diagram of a power line communication system  300  is depicted, which comprises a transmitter  302  and a receiver  304 . The transceiver  302  might be part of a power line communication modem  305  and the receiver  304  might be part of a farther power line communication modem  306 . The transceiver transmits signals to the receiver  304  among a plurality of channels  307  wherein each of the plurality of channels  307  has a feeding port FP 1 , FP 2 , or FP 3  and a receiving port RP 1 , RP 2 , RP 3 , or a RP 4 . In the depicted example with three feeding ports FP 1 , FP 2 , FP 3  and four receiving ports RP 1 , RP 2 , RP 3 , RP 4  both possible channels  306  might be used for transmitting a signal from the transmitter  302  to the receiver  304 . 
     In  FIG. 4  the conventional power line communication system.  400  is depicted with a transmitting PLC modem  402  and a receiving PLC modem  404 . The transmitting PLC modem  402  and the receiving PLC modem  404  are connected via power lines P, N, PE and a corresponding power line network  406 . The wires which represent the power line network are a phase line P, a neutral line N and a protective earth line PE, In conventional power line communication schemes only one feeding port is used, i.e. the feeding of signals between the phase P and the neutral line N and also only one receiving port RP 1  is used while receiving the signal, between the phase line P and the neutral line N at the receiver  404 . 
     When using also the protective earth line PE—as it is depicted in  FIG. 4  for a further embodiment of the power line communication system  500 —it is possible for a transmitting PLC modem  502  to transmit a signal to a receiving PLC modem  504  via any combination of the phase line P, the neutral line N and the protective earth line PE. Thus, in total three feeding port possibilities FP 1 , FP 2 , FP 3  are present, namely a first feeding port FP 1  where the transmitted signal is sent via the phase line P and the neutral line N, a second feeding port FP 2  where the signal is sent between the phase line P and the protective earth line PE and a third feeding port FP 3  where the signal is sent between the neutral line N and the protective earth line PE. On the receiver side there are a first receiving port RP 1  evaluating a received signal between, the phase line P and the neutral line N, a second receiving port RP 2  evaluating a signal received between the phase line P and the protective earth PE and a third receiving port RP 3  evaluating a signal received between the neutral line N and the protective earth line PE. A fourth receiving port RP 4  is also available, which describes the reception via a so-called common mode (CM). CM signals are created unintentionally at unbalanced networks. Unbalanced parasitic capacities from installations or devices to ground cause a CM current returning to the source. Due to electro-magnetic coupling between neighbored wires, cross talk arises, i.e. the transmit signal from any feeding port is visible on all four reception ports RP 1 , RP 2 , RP 3 , RP 4 . 
       FIG. 6  shows a message sequence chart for the feeding port selection process. At the beginning the transmitting modem  600  selects in a step S 602  the first (out of three) feeding possibilities and indicates this by a control message in a step S 604  to the receiving modern  606 . Such control messages might be handled in upper layers of any OSI layer system (e.g. a medium access layer (MAC) or even a data link control layer (DLC)). The receiving modem  606  acknowledges this request In a step S 608  and waits for the start of the test transmission. The transmitting modem  600  starts the capacity test of the transmission possibility of the first feeding port  1  in a step S 610  and sends a corresponding test signal in a step S 612 . In case the receiving modem  606  knows the length of a test transmission (e.g. a certain number of data bursts) it starts automatically to calculate the channel capacity as channel characteristic after the test sequence is received in a step S 614 . The result of the capacity calculation is sent back to the transmitter  600  in a step S 616 . 
     The steps are repeated for the other two remaining feeding possibilities. In a step S 620  the transmitting modern  600  selects the second feeding possibility and indicates this by a control message in a step S 622  to the receiving modem  606 . The receiving modem  606  acknowledges this request in a step S 624  and waits for the start of the next test transmission. The transmitting modem  600  starts the capacity test of the second feeding port FP 2  in a step S 626  and sends a corresponding test signal in a step S 628 . The receiving modem  606  calculates the channel capacity for this second feeding possibility in a step S 630  and reports the capacity back to the transmitting modem  600  in a step S 632 . 
     In a step S 634  the transmitting modem  600  selects the third feeding possibility and indicates this by a control message in a step S 636  to the receiving modem  606 . The receiving modem.  606  acknowledges this request in a step S 638  and waits for the start of the next test transmission. The transmitting modem  600  starts the capacity test of the third feeding port FP 3  in a step S 640  and sends a corresponding test signal in a step S 642 . The receiving modem  606  calculates the channel capacity for this second feeding possibility in a step S 644  and reports the capacity back to the transmitting modem  600  in a step S 646 . 
     After all three test transmissions are finished; the transmitting modem  606  starts to send regular data bursts in a step S 650 . 
     In  FIG. 7  an alternative scheme for testing the channels is depicted. In case a fixed length of the test sequences is used, i.e. a receiving modem  706  knows the length of the test transmission from the transmitting modem  700 , the handshaking to signal the start of the test sequence can be omitted. Thus, in a step S 702  the transmitting modem  700  signals to the receiving modem  706  that a signal feed test is requested. In a step S 704  the receiver  706  acknowledge the feed test request to the transmitter  700 . In a step S 708  the transmitter  700  selects the first feeding possibility and starts the capacity test directly for the first feeding port FP 1  in a step S 710 . The test signal is transmitted in a step  712  and the receiving modem  706  calculates the capacity in a step S 714 . The channel capacity is reported back to the transmitter  700  in a step S 716 . 
     These steps are repeated for all feeding possibilities. In a step S 720  the transmitting modem  700  selects the second feeding possibility and starts the capacity test directly for the second feeding port FP 2  in a step S 722 . The test signal is transmitted in a step  724  and the receiving modem  706  calculates the capacity in a step S 726 . The channel capacity is reported back to the transmitting modem  700  in a step S 728 . 
     In a step S 730  the transmitting modem  700  selects the first feeding possibility and starts the capacity test directly for the third feeding port FP 3  in a step S 732 . The test signal is transmitted in a step S 734  and the receiving modem.  706  calculates the capacity in a step S 736 . The channel capacity is reported back to the transmitting modem  700  in a step S 738 . 
     Afterwards the transmitting modem  700  selects the best feeding possibilities and starts the transmission in a step S 740 . 
     In  FIG. 8  the block diagram of the transmitting PLC modem  800  is depicted in order to explain how to switch between the different feeding ports in the transmitter  800 . Depending on the results of the feeding port selection mechanism, two of the available three ports are selected from the two MIMO transmitting paths  802 ,  804  with the help of a switching mechanism  806 . MIMO transmitting path  802  and MIMO transmitting path  2   804  are never set to the same position within the switching mechanism  806 . Within this embodiment the first transmitting path  802  is using P-N as feeding port and the second transmitting path is Using P-PE as feeding port. 
       FIG. 9A  shows a circuit diagram including circuit  910  and  FIG. 9B  is a graph  920  that shows a corresponding time dependence of the voltage UA on a power line, if impedance modulating devices are present. Mobile phone chargers and other charging devices convey in the circuitry that has the following properties:
         If the capacity C charges, HF-signals from mains are shortcut.   If the diode is blocking, the rectifier has high input impedance.       

     So the mains impedance changes at least twice within a line cycle duration. 
     The periodic impedance changes have dramatic influence to data transmission over power line. An impedance change during a data burst results in wrong channel equalization values after the impedance change and causes non-correctable transmission errors. Therefore it is important to place the burst in time intervals where the impedance keeps stable, which is a task for a medium access control (MAC) layer of a power line communication system. 
       FIG. 9C  is a graph  930  showing that depending on the line cycle frequency, different channel conditions result in different feeding port selections and/or different MIMO schemes (in this example: two different channel conditions, but all different channel conditions might be possible as well). The Y-axis represents the voltage UA of an AC line cycle. 
     In  FIG. 10  steps for determining an appropriate MIMO coding scheme is depicted. After the operation has been started in step  1000 , the channel characteristics are determined in a step S 1002 . Afterwards in a step S 1004  it is investigated whether the signal-to-noise-ratio SNR is below a certain threshold. If the answer is yes in a step S 1006  a stable, bit error rate (BER)-optimized MIMO coding is selected, for example, an Alamouti MIMO scheme. If the signal-to-noise-ratio is above a certain threshold it is determined whether significant disturbances are present in the power line network in a step S 1008 . If a significant disturbance is present then in a step S 1006  a stable, bit error rate optimized MIMO coding like Alamouti MIMO is used for the transmission as well. If there are no disturbances in the power line network then in a step S 1010  a throughput optimized MIMO coding, like HBLAST, VRLAST or Eigenbeamforming-MIMO is selected. Afterwards in a step S 1012  the transmitter is informed about the selection which selection should be used afterwards in a regular operation in a step S 1014 . Thus, depending on the channel characteristics, and/or the bandwidth demand of the application, an appropriate MIMO coding is selected. 
     In order to determine the quality of the channel, an initial phase before regular operation is proposed. During this initial phase the power line communication channel is examined for disturbances (impedance modulating or impulsive noise). All available MIMO schemes are tested sequentially. The best MIMO mode regarding throughput and/or bit error rate might be chosen. 
     In  FIG. 11 a    further embodiment for a power line communication system  1100  is depicted. In the power line communication system  1100  a first node  1102  is connected via a first channel  1104  with a second node  1106  and via a second channel  1108  with a third node  1110 . The second node  1106  and the third node  1110  are connected via a third channel  1112 . As an example an impulsive noise source  1114  disturbs the third channel  1112  between the second node  1106  and the third node  1110 . 
     Since the selection of the MIMO mode might be performed for each connection between ail nodes,  1102 ,  1106 ,  1110  in the network  1100 , different connections between different nodes might choose different MIMO modes depending on the connection conditions. In the example depicted in  FIG. 11  the communication between the first node  1102  and the second node  1106  on the first channel  1104  with a short distance has a good signal-to-noise-ratio SNR without any disturbance. Thus, a throughput optimized MIMO can be chosen. On the third channel  1112  between the second node  1106  and the third node  1110  there is a disturbance present, resulting from the impulses of the noise source  1114 , Thus, even if there is only a short distance between the second node  1106  and the third node  1110  the bit error rate optimized MIMO (e.g. Alamouti) is selected. Between the first node  1102  and the third node  1110  there is a long distance on the second channel  1108  but no disturbance is present. A bit error rate optimized MIMO (e.g. Alamouti) might be selected in order to overcome a bad SNR due to the long distance. 
     Due to the quasi-static behavior of power line communication channels the process to determine the optimized MIMO mode might be performed when a new node enters the network (and again if the channel conditions change fundamentally). It is proposed how to select the best possible feeding ports for MIMO communication over power line communication channels. The channel characteristics for different feeding ports are measured for all transmission possibilities and the port with the worst channel characteristics is excluded from further communication. In addition it has been prevented how HHto deal with impedance modulating devices in order to choose the appropriate feeding ports for different parts of an alternating current.