Abstract:
A hybrid balanced code is formed from a low rate (narrow bandwidth) balanced code and a high rate (wide bandwidth) low density code. Data encoded using the hybrid balanced code is transmitted between a first communication network entity and a second communication network entity. The hybrid code enables a system having a hybrid transmitter to transmit either a low rate stream detectable by a low rate receiver or a hybrid stream, from which the low rate data may be detected by a low rate receiver while both the high rate data and the low rate data may be detected by a high rate receiver.

Description:
FIELD OF THE INVENTION 
     The invention relates in general to coding in communication networks and in particular to balanced coding in such communications. 
     BACKGROUND OF THE INVENTION 
     Balanced codes (encoded sequences) have gained a lot of interest, since they can be received using a simple AC (Alternate Current)-coupled interface, which is simpler and more compatible with major interface types than a DC (Direct Current)-coupled interface. A balanced code can be defined in a number of ways: a) a code in which the number of 0s and 1s within a length of any N consecutive bits is close to equal; b) a code in which the number of 1s and 0s within any length of M symbols is identical or close to identical; c) a code constructed such that the frequency spectrum resulting from the transmission of any code word has no DC component; or d) a code that has a finite digital sum variation. The use of “balanced code” in this application should be understood as including any of the definitions above. 
     The superposition of two encoded sequences over one channel has also gained of a lot of interest, as shown in U.S. Pat. Nos. 6,621,427, 6,351,501, 6,198,413, 6,628,213, 5,539,846 and 5,243,628. Usually, each encoded sequence carries different types of information. In such applications, the two superimposed encoded sequences generate a new code which needs to be balanced for reception with an AC-coupled interface. 
     Much of the existing communication network infrastructure, in particular in Ethernet Passive Optical Networks (EPON), uses low rate transmissions, for example 1 Gigabit per second (Gbps). We refer to such low rate transmission networks or communication systems as “old”. With the introduction of new high speed transmission systems such as 10 Gbps EPON, there is a need to continue using and to retrofit “old” systems in the simplest and most inexpensive way to carry the new high rate traffic. The use of known two code superposition schemes cannot answer this need, since such coding schemes are complex, change the original (low rate) coding and do not enable to detect only one of the encoded sequences. 
     SUMMARY OF THE INVENTION 
     The invention discloses a Hybrid Balanced Coding scheme (also referred to simply as “hybrid coding” or “Hybrid Code”) which represents a family of pairs of codes. Each code in the family of Hybrid Codes is generated from two different codes, a high rate code and a low rate code. In some embodiments, the Hybrid Code generation is by an Exclusive Or (“XOR”) operation. The high rate code is superposed on the low rate code, leaving the main characteristics of the low rate code unchanged. The low rate code is defined to be a balanced code, while the high rate code is defined to be a low density code. The resulting Hybrid Code is a balanced code with certain properties. 
     Note that the use of the XOR operation, while preferable, does not exclude the use of other operations for generating a Hybrid Code of the invention. For example, another possible operation to generate the Hybrid Code is a conditional Add or Subtract operation, where the condition is the polarity of the low rate code: if the low rate symbol is “1”, subtract the high rate symbol and if the high rate symbol is “0”, add the high rate symbol. Regardless of the type of operation used to generate it, the resulting Hybrid Code can be separated into low rate balanced code and high rate low density code streams. For simplicity, the invention is explained in detail with reference to the XOR operation, which has an added advantage of being very simple to implement. 
     The information from each of the two codes forming the Hybrid Code can be detected. Specifically, the low rate code can be easily detected by using a simple low pass filter (LPF) which filters the high rate code. The family of pairs of Hybrid Codes can be used to upgrade an “old” low rate communication system to a “new” high rate communication system such that the new, hybrid balanced code, is compatible with the old system (in which the old low rate code can be detected by an “old” low rate receiver), while the new system can operate at a higher rate. In other words, the old low rate system can receive the Hybrid Code as if it were low rate transmission, while the new, high rate system can receive the Hybrid Code and detect both the low rate data and the high rate data. 
     The common 8b 10b coding (in Ethernet and other types of communication systems) is one type of low rate coding. In particular, the Hybrid Balanced Coding scheme of the invention can be used to upgrade “old” 8b10b-based systems to higher rate systems while retaining compatibility with the old systems. Thus, a 1 Gbps EPON (an example of an “old” 8b 10b-based system) can be upgraded to a 10 Gbps EPON. 
     According to the invention there is provided a method for transmitting encoded data including the steps of forming a hybrid balanced code from a low rate balanced code and a high rate low density code and transmitting data encoded using the hybrid balanced code between a first communication network entity and a second communication network entity. 
     In some embodiments of the method, the step of forming a hybrid balanced code includes performing a XOR operation between the low rate balanced code and the high rate low density code. 
     In some embodiments, the method further includes the step of separately detecting data encoded with the low rate balanced code and data encoded with the high rate, low density code. 
     In some embodiments of the method, the first communication network entity is an optical line terminal (OLT) and the second communication network entity is an optical network unit (ONU). 
     In some embodiments of the method, the low rate balanced code is a 8b10b code. 
     In some embodiments of the method, the high rate low density code is a 4-bit data 8-bit symbol low density code. 
     In some embodiments of the method, the low rate includes a 1 Gigabit per second (Gbps) rate and wherein the high rate includes a 10 Gbps rate. 
     According to the invention there is provided a system for transmitting encoded data including a first network unit for transmitting data coded in a hybrid balanced code formed from a low rate balanced code and a high rate low density code and a second network unit operative to receive at least part of the data coded in a hybrid balanced code. 
     In some embodiments of the system, the first communication network entity is an OLT and the second communication network entity is an optical network unit ONU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a general Hybrid Balanced Coding scheme of the invention; 
         FIG. 2  shows a special case of Hybrid Balanced Coding scheme of the invention based on 8b10b-encoded low-rate code 
         FIG. 3  shows an example of a 10 Gbps rate Hybrid Code of the invention based on 8b10b-encoded 1.25 Gbps; 
         FIG. 4  shows a low rate decoding of Hybrid Code using a low pass filter; 
         FIG. 5  shows a first example of an eye-diagram at the output of the LPF (Low Pass Filter); 
         FIG. 6  shows a second example of an eye-diagram at the output of the LPF; 
         FIG. 7  shows a fully digital decoding of the Hybrid Code; 
         FIG. 8  shows a logic separation between Type 1 and Type 2 codes; 
         FIG. 9  shows an old EPON 1 Gbps 8b10b transmission system; 
         FIG. 10  shows a 10 Gbps EPON system compatible with 1 Gbps EPON System. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Hybrid Balanced Coding scheme is a family of pairs of codes that generates a new, hybrid balanced code from a “Type 1” low rate balanced code and a “Type 2” high rate low density code. In some embodiments, the generation is done by a simple XOR operation between the Type 1 and Type 2 codes. Moreover, by using a simple Low Pass Filter (LPR), the high rate code is removed (low pass filtered) and the low rate code can be easily detected. 
       FIG. 1  shows a general circuit  100  which generates a Hybrid Balanced Coding scheme. Circuit  100  includes a simple XOR logic gate  102  which receives Type 1 and Type 2 codes and superimposes them into a single, combined Hybrid Code. The Type 1 code is a low rate balanced code, with equal number of 1s and 0s. The Type 2 code is a high rate, low density code, with more 0s than 1s in each symbol. The two code types are explained next. 
     A Type 1, low rate balanced code, may be defined as any code in which the number of 1s is “equal” to the number of 0s in a consecutive, defined number of bits. “Equal” may mean that the number of 1s is exactly the same as, or is statistically equal to, the number of 0s. A “low rate balanced code” may also be defined as a code in which the number of 1s is not higher or lower than the number of 0s by more than a defined number. Examples for Type 1 codes can be 010101 . . . , which carry no information in it, the common 8b10b code which carries 8 bits of data in each 10 bit symbol, the 4b5b code, any PRBS (Pseudo Random Bit Stream) code and others. 
     A Type 2, high rate low density code is a code which consists of equal length symbols, L bits each. Each L bit symbol represents a data word of K bits (L&gt;K). The number of 1s in each symbol is smaller than the number of 0s. The number of 1s may be equal for all the symbol possibilities or may be defined as smaller than a certain number. Two examples of high rate, low density codes are shown in Table 1 and Table 2. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 3-Bit Data 8-bit Symbol Low Density Code 
               
             
          
           
               
                   
                 Data 
                 Codeword 
               
               
                   
                   
               
               
                   
                 000 
                 10000000 
               
               
                   
                 001 
                 01000000 
               
               
                   
                 010 
                 00100000 
               
               
                   
                 011 
                 00010000 
               
               
                   
                 100 
                 00001000 
               
               
                   
                 101 
                 00000100 
               
               
                   
                 110 
                 00000010 
               
               
                   
                 111 
                 00000001 
               
               
                   
                   
               
             
          
         
       
     
                                   TABLE 2                   4-Bit Data 8-bit Symbol Low Density Code                Data   Codeword                       0000   10000000           0001   01000000           0010   00100000           0011   00010000           0100   00001000           0101   00000100           0110   00000010           0111   00000001           1000   10000001           1001   10000010           1010   01000001           1011   10000100           1100   01000010           1101   00100001           1110   01000100           1111   00100010                        
The duration of each bit of a Type 1 codeword is equal to the duration of each symbol of a Type 2 code. This means that the duration of 1 bit of Type 1 code is equal to L bits of Type 2 code. Also, when performing the XOR operation in  FIG. 1 , each symbol of Type 2 code is aligned to each bit of Type 1 bit, as in the example shown in  FIG. 2 .
 
     A special case of Hybrid Balanced Coding according to the invention is shown in  FIG. 3 . The Type 1 code  302  is of 8b10b type encoded at 1.25 Gbps, and the Type 2 code is a low density code  304  defined in Table 1. Code  302  and code  304  are combined (using the XOR operation) to generate a 10 Gbps rate Hybrid Code  306 . 
       FIG. 4  shows the process of low rate decoding (i.e. of detecting only the low rate code by removing the high rate code) of a Hybrid Code in an old system defined as a narrow band system. The high frequency components are removed using a low pass filter LPF  402  and the low-frequency components are translated to data using a Type 1 decoder  404 . The LPF is an inherent part of an old system because of its limited bandwidth. 
       FIG. 5  shows an eye-diagram of a Hybrid Code using the Type 1 8b10b code and the Type 2 code defined in Table 1, as shown in  FIG. 3 .  FIG. 6  shows an eye-diagram of a Hybrid Code using the Type 1 8b10b code and the Type 2 code defined in Table 1. In both  FIG. 5  and  FIG. 6 , the eye diagram consists of two low rate symbols with amplitudes from 0 to 1 filtered by an LPF. It is seen that the higher the rate of the high rate code (4 bits of information per symbol in Table 2 as opposed to 3 bits of information per symbol in Table 1) the worse the performance (the jitter and noise are higher). 
     A fully digital decoding of a Hybrid Code is shown in  FIG. 7 . The Hybrid Code is the input to a logic unit  702 . The logic unit separates the Type 1 and Type 2 codes by counting the number of high rate 1 bits in each L-bit symbol (after the delineation of  FIG. 8 ). If the number of 1s is higher than the number of 0s, the low rate bit is 1 and the L-bit symbol is inverted. If not, the low rate bit is 0, and the L-bit symbol is not inverted. Each of the codes is respectively decoded in a Type 1 decoder  704  and a Type 2 decoder  706 . Synchronization (delineation) is performed as shown in the flow chart of  FIG. 8 . Note that the scheme in  FIG. 8  assumes no errors. If errors are expected, additional logic is required to the flow chart of  FIG. 8 . 
     The Type 1 and Type 2 decoders depend on the type of codes. In the examples shown in this embodiment, the low rate decoder is the famous 8b10b decoder, while the high rate decoder uses Table 1 and Table 2 to decode each codeword into a data sequence. 
       FIG. 8  shows a possible delineation method, i.e. a way to detect the exact beginning of a low rate bit or the beginning of a high rate symbol (of L bits). The bits in the block diagram of  FIG. 8  are always high rate bits. After a start in step  802 , a COUNTER is reset in step  804 . Next, L consecutive high rate coded bits (D(n+1), . . . , D(n+L), n=1, L+1, 2L+1, . . . ) are read and a difference Diff between the number of 1s and 0s (absolute value) is calculated in step  806 , in which COUNTER is also advanced by 1. If Diff is lower than Max (the maximum possible absolute difference between the number of 1s and 0s) in a check step  808 , the next L high rate bits are advanced (D(n+2), . . . , D(n+L+1), n=1, L+1, 2L+1, . . . ) in step  810 . The COUNTER is checked against a number Num in step  812 . As long as the COUNTER is lower than Num, the process in steps  806 - 812  is repeated i.e. there are Num iterations). When the COUNTER equals Num, a delineation is done in step  814 . 
     Specific Example 
       FIG. 9  shows an old EPON system in which an OLT  902  transmits 1 Gbps 8b10b encoded data to a plurality of ONUs  904 . “ONU 1G” indicates that an ONU operates at 1 Gbps.  FIG. 10  describes exemplarily a 1 Gbps EPON system modified to 10 Gbps. A “hybrid” OLT  1002  can now transmit a Hybrid Code instead of the 8b10 code. This enables an “old” CPE (Customer Premise Equipment) i.e. any ONU 1G, to receive low rate transmissions as before, while new CPEs (i.e. “ONU 10G”, representing ONUs operating at 10 Gbps) can receive both high rate and low rate data. In other words, hybrid OLT  1002  now supports both old ONUs (ONU 1G) and new high rate ONUs (ONU 10G). Both “old” and “new” systems use an optical splitter  904  as part of the point to multi-point transmission in the PON system. 
     All patents mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Those skilled in the art will appreciate that the invention can be embodied by other forms and ways, within the scope of the invention. The embodiments described herein should be considered as illustrative and not restrictive.