Abstract:
The present invention relates to a system and method for selecting one of plurality of PRBS generators for use with a modem. The modem includes a measuring device adapted to measure an operating environment of the modem; and a storage device adapted to store a list of PRBS generator definitions. The modem selects one of a plurality of PRBS generators based on the measurement of the operating environment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     [Not Applicable] 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     [Not Applicable] 
       BACKGROUND OF THE INVENTION  
       [0003]     One embodiment of the present invention relates to modems and more specifically to MEDLEY symbols in ADSL modems.  
         [0004]     Pseudo-random bit sequence (hereinafter referred to as “PRBS”) generators or scramblers have various uses in ADSL modems that conform to ITU-T Recommendations G.992.1 and G.992.2, both of which are hereby incorporated herein by reference. It should be appreciated that the term PRBS generator as used herein to identify either a pure PRBS generator or a scrambler based on a PRBS generator. Specifically, two different PRBS generators define the MEDLEY signal used for upstream and downstream training. A different PRBS sequence defines the form of the REVERB signal used in training while user data are scrambled during SHOWTIME using still another different PRBS generator.  
         [0005]     Such a plurality of uses of PRBS generators in ADSL training demonstrates a need for means in an ADSL modem to select one of a set of different PRBS generators appropriate to observations made during training or to parameters that may not be known in advance.  
         [0006]     Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     One embodiment of the present invention relates to a system and method for selecting one of a plurality of PRBS generators for use with a modem.  
         [0008]     Another embodiment of the present invention provides a modem adapted to select one of a plurality of PRBS generators. The modem includes a device adapted to measure an operating environment of the modem and memory adapted to store a list of PRBS generator definitions.  
         [0009]     Yet another embodiment of the present invention provides a communication node adapted to establish a connection with a remote communication node. The node includes circuitry adapted to determine an operating environment of the communication node, and a management information node adapted to control the communication device according a list of stored PRBS generator definitions. In one embodiment, the list of PRBS generator definitions is stored on the management information node. In another embodiment, the circuitry includes a memory communicating with at least the management information node and adapted to store the list of PRBS generator definitions.  
         [0010]     Yet another embodiment of the present invention comprises a method for selecting one of a plurality of PRBS generators. The method includes measuring an operating environment of a modem and selecting a stored PRBS generator from the plurality of generators based on the measured environment.  
         [0011]     Still another embodiment of the invention comprises a method of communicating where a determination is made of the number of carriers to be used by a communication node. The number of carriers determined is then compared to a threshold. If the number of carriers determined is less than the threshold, then a first PRBS generator is selected. If the number of carriers determined is greater than the threshold, then a second PRBS generator is selected. The first PRBS generator may be longer than the second PRBS generator, for example.  
         [0012]     Other aspects, advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings, wherein like numerals refer to like parts.  
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates a block diagram of a generic communication system that may be employed in connection with one embodiment of the present invention.  
         [0014]      FIG. 2  illustrates a method of selecting a PRBS generator in accordance with one embodiment of the present invention.  
         [0015]      FIG. 3  illustrates a generic shift register based PRBS generator of the type employed in the present invention.  
         [0016]      FIG. 4  illustrates another embodiment of a shift register based PRBS generator in accordance with one embodiment of the present invention.  
         [0017]      FIG. 5  illustrates yet another embodiment of a shift register based PRBS generator in accordance with one embodiment of the present invention.  
         [0018]      FIG. 6  is a flow diagram of one embodiment of the method of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     FIG. 1  illustrates a block diagram of a generic communication system that may be employed in connection with one embodiment of the present invention. The system comprises a first communication node  101 , a second communication node  111 , and a channel  109  that communicatively couples the nodes  101  and  111 . In one embodiment, the communication nodes are ADSL modems for example, although any other type of transceiver device that transmits or receives data over a channel is contemplated.  
         [0020]     The first communication node  101  comprises a transmitter  105 , a receiver  103  and a processor  106 . The processor  106  may comprise, for example, a microprocessor. The first communication node  101  is communicatively coupled to a user  100  (e.g., a computer) via communication link  110 , and to the channel  109  via communication links  107  and  108 .  
         [0021]     Similarly, the second communication node  111  comprises a transmitter  115 , a receiver  114  and a processor  118 . The processor  118 , like processor  106 , may comprise, for example, a microprocessor. The second communication node  111  is likewise communicatively coupled to a user  120  (again a computer, for example) via communication link  121 , and to the channel  109  via communication links  112  and  113 .  
         [0022]     During operation, the user  100  can communicate information to the user  120  using the first communication node  101 , the channel  109  and the second communication node  111 . Specifically, the user  100  communicates the information to the first communication node  101  via communication link  110 . The information is transformed in the transmitter  105  to match the restrictions imposed by the channel  109 .  
         [0023]     The transmitter  105  then communicates the information to the channel  109  via communication link  107 . The receiver  114  of the second communication node  111  next receives, via communication link  113 , the information from the channel  109  and transforms it into a form usable by the user  120 . Finally, the information is communicated from the second communication node  111  to the user  120  via the communication link  121 . Communication of information from user  120  to user  100  may also be achieved in a similar manner. In either case, the information transmitted/received may also be processed using the processors  106 / 118 .  
         [0024]     One embodiment of the present invention provides a PRBS MEDLEY generator for use in ADSL modems, i.e., where ADSL modems are used, for example, as communication nodes  101  and  111  of the generic communication system. In one embodiment the ADSL modem selects one of a set of different PRBS generators appropriate to observations made during training or to parameters that may not be known in advance. An infinite number of PRBS generators are available for use in ADSL training. Specifications for some of these PRBS generators are provided in Table 1 below.  
         [0025]     It should be appreciated that an important aspect of a PRBS generator is the initial state of the shift register. By specifying one of 2 N −1 possible non-zero initial states, one of 2 N −1 possible PRBS sequences can be selected for a given PRBS generator.  
         [0026]     Further, a PRBS generator closely relates to a corresponding scrambler with its input clamped to one. Specifically, the output of such a scrambler when its initial state is not all ones corresponds exactly to the output of the corresponding PRBS generator when its initial state is not all zeros, except that zeros and ones in the PRBS generator output sequence are replaced, respectively, by ones and zeros in the scrambler output sequence.  
         [0027]     In one embodiment, the MEDLEY signal is used in ADSL modems to estimate the channel unit-sample response and/or the signal-to-noise ratio (SNR) profile of the channel, so that the pair of modems may negotiate a set of operating parameters that will best utilize the capacity available on the channel. In order to accomplish this objective, the MEDLEY signal should be pseudo-random. That is, it appears to be random to the channel, but because the transmitter and the remote receiver share knowledge of the exact sequence used, the modems are able to remove the effect of the randomness in executing their signal processing algorithms.  
         [0028]     In one embodiment, specific PRBS generators are employed in ADSL modems that conform to the incorporated ITU-T Recommendations G.992.1 and G.992.2. One such PRBS generator generates repeated bit patterns that repeat every 511 bits in the downstream (i.e. central office-to-remote, denoted D/S) direction; another generates repeated bit patterns that repeat every 63 bits in the upstream (i.e. remote-to-central office, denoted U/S) direction.  
         [0029]     While the 511-bit period of the downstream PRBS generator is sufficiently random for purposes of modem training, the 63-bit period of the upstream PRBS generator is too short. The result is that the estimates of the channel unit-sample response and the channel SNR are insufficiently accurate to predict the performance of the upstream direction of data transmission during SHOWTIME. Consequently, the modems must either add extra SNR margin to compensate for this missing channel knowledge or select a higher data rate that, necessarily, has lower SNR margin. It should be appreciated that either approach results in degraded performance when compared to the ideal situation.  
         [0030]     More particularly, the SNR estimation phase of the G.992.1 and G.992.2 modems is performed during the R-MEDLEY and C-MEDLEY sequences, for the U/S and D/S channels, respectively. In one embodiment, the transmitter sends pseudo-random QPSK data patterns across the DMT tones, enabling the receiver to estimate the per-bin noise power. The per-bin noise power is generally made up of two main contributors, namely, a line noise term, including receiver and transmitter front-end noise and various cross-talk components, for example, plus an Inter-Symbol and Channel Interference (hereinafter referred to as “ISCI”) term.  
         [0031]     The ISCI term results from imperfect channel equalization, which causes the end-to-end channel impulse response (hereinafter referred to as “CIR”) to be longer than the length of the Cyclic Prefix (hereinafter referred to as “CP”). The tails of the CIR outside the CP cause interference from the previous and next DMT symbols (ISI=Inter-Symbol Interference) and among DMT tones (ICI=Inter-Channel Interference).  
         [0032]     It should be appreciated that the ISCI term is by nature a deterministic interference. For a given DMT symbol and tone, the ISCI term&#39;s instantaneous power depends on the particular QAM symbol sets that have been transmitted across the tones in current, previous and next DMT symbols. In one embodiment, it has been found that an ideal estimate of the average power of the ISCI term requires averaging the instantaneous power over all the possible combinations of the interfering QAM symbols. For example, if the ICI term dominates over the ISI, and 20 U/S bins are active, during R-MEDLEY one would need 4 19 ≈2.7×10 11  different DMT symbols. In practice, if a maximum error of 0.5 dB is acceptable, much shorter sequences are enough.  
         [0033]     However, the sequence used for R-MEDLEY of the G.992.1 and G.992.2 modems uses only 63 different QPSK symbols (it is periodic, with a period of 63 DMT symbols). It has been found that this is insufficient and indeed may result in large estimation errors, up to 2˜3 dB, for channels dominated by ISCI. If for MEDLEY the SNR is overestimated during SHOWTIME, it has been found that the bit error rate would degrade below the target value for a given margin. In particular, if the target margin is set below 2˜3 dB, it has been found that the bit error rate in SHOWTIME would be higher than 10 −7 , an unacceptable situation.  
         [0034]     It should be appreciated that, for G.lite.bis and G.dmt.bis modems, a PRBS generator having a period much longer than 63 DMT symbols for the sequence (i.e., around 500 symbols) should be used during the U/S SNR estimation phase initialization.  
         [0035]     One embodiment of the present invention includes selecting one of a family or a plurality of PRBS generators for use in ADSL training is illustrated in  FIG. 2 . ADSL modems operate by making extensive measurements on their operating environment during an elaborate training procedure that is executed when a pair of modems initially establishes a connection, as indicated by  FIG. 2 . According to current standards, the PRBS generator employed at each stage of training is defined a priori.  
         [0036]     According to one embodiment of the present invention, a list of PRBS generator definitions is stored in the memory of the modem or in the management information base (MIB) that control modem operation. Then, depending upon the results of measurements or observations made during training (block  201 ), the modem (or MIB) selects the generator most appropriate to the situation determined by the measurements or observations (as illustrated by block  203 ).  
         [0037]     Once a specific PRBS is chosen, the modem informs the remote modem of its choice as indicated by block  205 . One method for accomplishing this step is similar to those outlined in the commonly assigned in U.S. patent application Ser. No.09/882,100 titled “Method of Intelligently Restricting Symbol Size in ADSL Modems,” which application is hereby incorporated herein by reference.  
         [0038]     A family of PRBS generators is specified that solves the problem outlined above. As discussed above, any PRBS generator with a period greater than about 500 achieves the desired performance. The following Table 1 provides a partial list of PRBS generators that meet the criteria.  
                                                                                   TABLE 1                                       PRBS Generators of length 9, 10, . . . , 23                Basic PRBS       Inverse PRBS                            9   4           9   5                   10   3           10   7           11   2           11   9           12   6   4   1   12   11   8   6           13   4   3   1   13   12   10   9           14   5   3   1   14   13   11   9           15   1           15   14           16   5   3   2   16   14   13   11           17   3           17   14           18   5   2   1   18   17   16   13           19   5   2   1   19   18   17   14           20   3           20   17           21   2           21   19           22   1           22   21           23   5           23   18                      
 
         [0039]     The rows in Table 1 provide a list of coefficients for shift register based PRBS generators, generally designated  300 , similar to that illustrated in  FIG. 3  for example. The set of coefficients defining such a PRBS generator has a general form as follows: 
        {c N ,c N-1 , . . . ,c 1 }, 
 
 and each row of Table 1 contains an entry i if c i =1, and no entry if c i =0 for the generator corresponding to that row. Such a PRBS generator of length N produces an output sequence that repeats with a period of 2 N -1, so the repetition period exceeds 500 whenever N exceeds 9. 
       
 
         [0041]      FIG. 4  illustrates another embodiment of a PRBS generator, generally designated  400 . In this embodiment, for example, the first row in Table 1 corresponds to a set of coefficients as follows: 
        {c 9 ,c 8 , . . . ,c 1 }
 
 with c 9 =c 4 =1 and c i =0 for other values of i based on the primitive polynomial1+x −4 +x −9 .  FIG. 5  illustrates a shift register based PRBS generator, generally designated  500 , taken from the last row of Table 1, the (23, 18) PRBS generator, based on the primitive polynomial, 1+x −18 +x 31 23 . 
         
         [0043]     It should be appreciated that each PRBS generator has an inverse that is, itself, a PRBS generator; the inverse PRBS generators. occupy the right side of Table 1.  
         [0044]      FIG. 6  is a flow diagram of one embodiment of the method of the present invention, in which a PRBS generator is selected based on the number of carriers used in communication. Specifically, the system makes, during training, a determination of the number of carriers to be used in communication (block  601 ). The determination may be made by one of the processors  106  and  118  of  FIG. 1 , for example. Once the number of carriers is determined, the number is compared to a threshold (block  603 ). If it is less than the threshold, a first PRBS generator is selected for use (block  605 ). If instead it is more than the threshold, a second PRBS generator is selected for use (block  607 ). In either case, the remote modem is informed of the selection (block  609 ).  
         [0045]     Referring to  FIG. 6 , in general, the second PRBS generator is longer than the first PRBS generator. The first PRBS generator may be, for example, that shown in  FIG. 4 , which has a 511-bit period. The second PRBS generator may be, for example, that shown in  FIG. 5 , which has a period of approximately 8 million bits. The threshold used may be 32, for example. Of course any number of different PRBS generators and thresholds may be used and are contemplated by the present invention. In addition, multiple thresholding may be used, so that more than two PRBS generators may be available for selection given channel conditions.  
         [0046]     Many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as described hereinabove.