Abstract:
The present invention relates to a first transceiver unit, acting as a receiver unit, and to a second transceiver unit, acting as a transmitter unit. The first transceiver unit measures the signal to noise ratio for each tone, and determine whether a tone shall be shut off, thereby reducing interference on neighboring lines and power consumption. If so, the first transceiver unit keeps on measuring the noise level over that tone. If the ratio of the initially measured signal level to the newly measured noise level exceeds a pre-determined threshold, then the first transceiver unit requests the second transceiver unit to re-activate that tone. A new initialization sequence is transmitted over that tone for initializing the frequency domain equalizer, for measuring the signal to noise ratio, and finally for agreeing on a bit loading.

Description:
BACKGROUND 
     1. Field 
     The present invention relates to a first transceiver unit, acting as a receiver unit with respect to a direction of communication, and a second transceiver unit, acting as a transmitter unit with respect to the same direction of communication, 
     said first transceiver unit comprising: 
     
         
         
           
             a receiver adapted to receive from a physical channel a signal modulated over at least one carrier, 
             a channel analyzer coupled to said receiver, and adapted, while an initialization sequence is being transmitted over said at least one carrier, to determine a signal component and a noise component within a frequency interval enclosing one carrier out of said at least one carrier, 
             a first communication means adapted to establish a communication link with said second transceiver unit, 
             a power remote control unit coupled to said channel analyzer and to said first communication means, and adapted to request said second transceiver unit to shut off said carrier if the ratio of said signal component to said noise component is lower than a first pre-determined threshold,
 
said second transceiver unit comprising:
 
             a transmitter adapted to transmit over said physical channel a signal modulated over said at least one carrier, 
             a second communication means adapted to establish a communication link with said first transceiver unit, 
             a power control unit coupled to said transmitter and to said second communication means, and adapted to shut off said carrier upon a first request from said first transceiver unit. 
           
         
       
    
     Such a pair of transceiver units is already known in the art, e.g. from the recommendation entitled “ Asymmetric Digital Subscriber Line  ( ADSL )  Transceivers -2 ( ADSL 2)”, ref. G.992.3, published by the International Telecommunication Union (ITU) in July 2002. 
     2. Related Art 
       FIG. 1  depicts a pair of transceiver units with a first Digital Subscriber Line (DSL) transceiver unit TU_C, housed in a Digital Subscriber Line Access Multiplexer (DSLAM) at a central office CO, and a second DSL transceiver unit TU_R, sited at customer premises CP, the transceiver unit TU_C being coupled to the transceiver unit TU_R via a twisted pair of copper wires L. 
     With respect to the direction of communication from the central office CO to the customer premises CP or downstream direction, the transceiver unit TU_C is a transmitter unit and the transceiver unit TU_R is a receiver unit. With respect to the direction of communication from the customer premises CP to the central office CO or upstream direction, the transceiver unit TU_R is a transmitter unit and the transceiver unit TU_C is a receiver unit. 
     Transceiver initialization is required in order for a physically connected pair of transceiver units to establish a communication link via a physical channel. 
     In order to maximize the throughput and reliability of this communication link, a transceiver unit shall determine certain relevant attributes of the physical channel and establish transmission and processing characteristics suitable to that channel. 
     Each receiver determines the relevant attributes of the channel by means of the transceiver training and channel analysis steps. Determination of channel attribute values and establishment of transmission characteristics requires that each transceiver produces, and appropriately responds to, a specific set of precisely-timed signals. 
     During a further data exchange step, each receiver shares with its peer transmitter certain transmission settings that it expects to see. Specifically, each receiver communicates the number of bits and relative power level to be used on each carrier. 
     Upon completion of the initialization procedure, the carriers are sorted as follows (see §8.6.4, p. 76-78):
         Loaded carriers: these are the carriers for which the bit loading is greater than or equal to 1, that is to say the carriers used for communication.   Monitored carriers: these are the carriers for which the bit loading is set to 0, yet which keep on being transmitted. The purpose is to track the SNR and, in case the SNR improves, to use these carriers for communication.   Shut-off carriers: these are the carriers that are never used for communication, and for which the bit loading and the relative gain are both set to 0.       

     A deficiency of the disclosed pair of transceiver units is that the shut-off carriers cannot be enabled again. This means that if the noise conditions change, the transceiver units have to stick with the carriers that are left. 
     SUMMARY 
     It is an object of the present invention to provide transceiver units with a more flexible design, which enables to re-activate a shut-off carrier if the noise conditions on that carrier improve. 
     According to the invention, this object is achieved due to the fact that said channel analyzer is further adapted, after said carrier has been shut off, to determine a pure-noise component within said frequency interval, 
     and that said power remote control unit is further adapted to request said second transceiver unit to re-activate said carrier if the ratio of said signal component to said pure-noise component is higher than a second predetermined threshold, 
     and due to the fact that said power control unit is further adapted to power up said carrier upon a second request from said first transceiver unit, 
     and that said transmitter is further adapted thereupon to transmit an initialization sequence over said carrier. 
     The basic idea is to shut off the carriers which are not used after the initialization, thereby reducing interference on neighboring lines and power consumption, yet to remember the measured signal level during initialization. 
     The noise can be monitored on those carriers as it is a pure-noise measurement. 
     The noise measurements together with the initially measured signal level can point out if a shut off carrier can be re-used. 
     An initialization sequence shall be triggered over the carriers that have been re-enabled, while user data keep on being transmitted on the loaded carriers. This is necessary for initializing the Frequency domain EQualizer (FEQ), for measuring the Signal to Noise Ratio (SNR), and finally for agreeing on a bit loading over those carriers. 
     The signal and noise components can be characterized by any of the following attributes:
         the average signal and noise amplitude,   the average signal and noise power.       

     The signal and noise components can be determined in the time domain, or in the frequency domain by means of a Fourier expansion. 
     The scope of the present invention is not limited to DSL transceiver units. The present invention is applicable to whatever type of digital transceiver unit receiving or transmitting data over a discrete set of carriers, being by means of Discrete Multi-Tones (DMT) modulation, Single Carrier (SC) modulation, Code Division Multiple Access (CDMA) modulation, etc, and to whatever type of physical transmission medium, being coaxial cables, optical fibers, the atmosphere, the empty space, etc. 
     Further characterizing embodiments of the present invention are mentioned in the appended claims. 
     It is to be noticed that the term ‘comprising’, also used in the claims, should not be interpreted as being restricted to the means listed thereafter. Thus, the scope of the expression ‘a device comprising means A and B’ should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the relevant components of the device are A and B. 
     Similarly, it is to be noticed that the term ‘coupled’, also used in the claims, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression ‘a device A coupled to a device B’ should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a pair of transceiver units. 
         FIG. 2  represents a first transceiver unit TU 1  according to the invention, 
         FIG. 3  represents a second transceiver unit TU 2  according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In a preferred embodiment of the present invention, the transceiver units TU 1  and TU 2  are ADSL transceiver units. 
     The first transceiver unit TU 1  comprises the following functional blocks:
         a transmitter TX 1 ,   a receiver RX 1 ,   a hybrid circuit H 1 ,   a line adaptator T 1 ,   a communication means COM 1 ,   a channel analyzer ANAL,   a power remote control unit PRCU.       

     The transmitter TX 1  and the receiver RX 1  are both coupled to the hybrid circuit H 1 . The hybrid circuit H 1  is coupled to the line adaptator T 1 . The communication means COM 1  is coupled to both the transmitter TX 1  and the receiver RX 1 . The channel analyzer ANAL is coupled to the receiver RX 1 . The power remote control unit PRCU is coupled to both the channel analyzer ANAL and the communication means COM 1 . 
     The second transceiver unit TU 2  comprises the following functional blocks:
         a transmitter TX 2 ,   a receiver RX 2 ,   a hybrid circuit H 2 ,   a line adaptator T 2 ,   a communication means COM 2 ,   a power control unit PCU.       

     The transmitter TX 2  and the receiver RX 2  are both coupled to the hybrid circuit H 2 . The hybrid circuit H 2  is coupled to the line adaptator T 2 . The communication means COM 2  is coupled to both the transmitter TX 2  and the receiver RX 2 . The power control unit PCU is coupled to both the transmitter TX 2  and the communication means COM 2 . 
     The transmitters TX 1  and TX 2  accommodate the necessary means for encoding user and control data, and for modulating DSL tones with the so-encoded data. 
     The transmitter unit TX 2  further accommodates the necessary means for tuning the transmit power of each tone, upon control from the power control unit PCU, and as initially determined by the power remote control unit PRCU. 
     The receivers RX 1  and RX 2  accommodate the necessary means for demodulating a DMT signal, and for decoding user and control data from the so-demodulated signal. 
     The hybrid circuit H 1  is adapted to couple the transmitter unit TX 1 &#39;s output to the twisted pair L, and the twisted pair L to the receiver unit RX 1 &#39;s input. The hybrid circuit H 1  accommodates an echo cancellation means to avoid the transmitted unit TX 1 &#39;s signal to couple into the receiver unit RX 1 &#39;s input. 
     The hybrid circuit H 2  is adapted to couple the transmitter unit TX 2 &#39;s output to the twisted pair L, and the twisted pair L to the receiver unit RX 2 &#39;s input. The hybrid circuit H 2  accommodates an echo cancellation means to avoid the transmitted unit TX 2 &#39;s signal to couple into the receiver unit RX 2 &#39;s input. 
     The line adaptator T 1  is adapted to isolate the transceiver unit TU 1  from the twisted pair L, and to adapt the input and output impedance of the transceiver unit TU 1  to the line characteristic impedance. 
     The line adaptator T 2  is adapted to isolate the transceiver unit TU 2  from the twisted pair L, and to adapt the input and output impedance of the transceiver unit TU 2  to the line characteristic impedance. 
     The communication means COM 1  and COM 2  provide data exchange capabilities between the transceiver unit TU 1  and the transceiver unit TU 2 , more specifically between the power remote control unit PRCU and the power control unit PCU. The communication means COM 1  and COM 2  accommodate the necessary means for checking and guaranteeing message integrity. 
     The channel analyzer ANAL is adapted, for each tone of the MEDLEY set (see definition of the MEDLEY set §3.23, p. 12 of the cited document):
         to determine a SNR,   to determine therefrom a bit loading and a relative power gain.       

     The channel analyzer ANAL proceeds as follows. 
     Denote the frequency at which the received signal is sampled as 
               F   s     =       1     T   s       .           
Denote the DMT symbol period as T c  (1/4312, 5 seconds for xDSL).
 
     Denote samples of the i th  received DMT symbol as r i (n), and denote its signal and noise components as s i (n) and e i (n) respectively:
 
 r   i ( n )= s   i ( n )+ e   i ( n )  n= 0, 1 , . . . N− 1  (1)
 
 N×T   s   =T   c   (2)
 
     The noise e i  is assumed to be a zero-mean Additive White Gaussian Noise (AWGN). 
     Denote the N-point Discrete Fourier Transform (DFT) of the i th  DMT symbol as R i (k), and denote the N-point DFT of its signal and noise components as S i (k) and E i (k) respectively: 
                         R   i     ⁡     (   k   )       =         ∑     n   =   0       N   -   1       ⁢           r   i     ⁡     (   n   )       ·     e       -   j     ⁢       2   ⁢   π     N     ⁢   kn         ⁢   k       =   0       ,   1   ,   …   ⁢           ,     N   -   1             (   3   )                     S   i     ⁡     (   k   )       =         ∑     n   =   0       N   -   1       ⁢           s   i     ⁡     (   n   )       ·     e       -   j     ⁢       2   ⁢   π     N     ⁢   kn         ⁢   k       =   0       ,   1   ,   …   ⁢           ,     N   -   1             (   4   )                     E   i     ⁡     (   k   )       =         ∑     n   =   0       N   -   1       ⁢           e   i     ⁡     (   n   )       ·     e       -   j     ⁢       2   ⁢   π     N     ⁢   kn         ⁢   k       =   0       ,   1   ,   …   ⁢           ,     N   -   1             (   5   )                 R   I ( k )= S   I ( k )+ E   i ( k )  (6)
 
     The channel analyzer ANAL may use any of the Fast Fourier Transform (FFT) algorithms as known to the skilled person, such as a RADIX-4 FFT algorithm, provided that log 2 (N) is an non-null positive integer (e.g., N=2 9 =512). 
     R is a discrete random process with means m R  and variance σ R   2  given by:
 
 m   R =ε( R )=ε( S )+ε( E )=ε( S )  (7)
 
σ R   2 =ε(| R−m   R | 2 )=ε(( R=m   R )·( R−m   R ))=(| R|   2 )−2ε(R)· m   R   +|m   R | 2 =ε(| R|   2 )−| m   R | 2   (8)
 
where ε denotes the expected operator, and · the scalar product.
 
     We also have:
 
ε(| R|   2 )=ε(( S+E )·( S+E ))=ε(| S|   2 )+2ε( S )·ε( E )+ε(| E|   2 )=ε(| S|   2 )+ε(| E|   2 )=ε(| S|   2 )+σ E   2   (9)
 
     Assuming E, and thus R, are ergodic process, one obtains an non-biased estimate of the average received signal and power by time-averaging over a sufficiently high number I of DMT symbols: 
     
       
         
           
             
               
                 
                   
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     The channel analyzer ANAL determines m R(k)  during the REVERB sequence, wherein each tone is 4-QAM modulated with identical binary symbols, and wherein the signal component S stays at a fixed location, thereby has a constant amplitude. 
     The channel analyzer ANAL then scales and rotates R i (k) such that m R(k)  matches its expected location in the 4-QAM decoding grid: 
                             R   i   ′     ⁡     (   k   )       =       ⁢       A   ⁡     (   k   )       ⁢     e     j   ⁢           ⁢     ϕ   ⁡     (   k   )           ×       R   i     ⁡     (   k   )                     =       ⁢           A   ⁡     (   k   )       ⁢     e     j   ⁢           ⁢   ϕ   ⁢           ⁢     (   k   )         ×       S   i     ⁡     (   k   )           ︸       S   i   ′     ⁡     (   k   )           +         A   ⁡     (   k   )       ⁢     e     j   ⁢           ⁢   ϕ   ⁢           ⁢     (   k   )         ×       E   i     ⁡     (   k   )           ︸       E   i   ′     ⁡     (   k   )                           (   12   )               ε( R′ ( k ))=ε( S′ ( k ))= A ( k ) e   jφ(k)   ×m   R(k)   (13)
 
     The channel analyzer ANAL determines ε(|R′(k)| 2 ) during the MEDLEY sequence, wherein each tone is 4-QAM modulated with pseudo random binary symbols, and wherein the amplitude of the signal component S is constant over the symbol space (the 4 constellation points ‘00’, ‘01’, ‘10’ and ‘11’ are located on a circle of radius A(k)×|m R(k) |). 
     The average signal power for tone k is then given by:
 
ε(| S ′( k )| 2 )= A ( k ) 2   ×|m   R(k) | 2   (14)
 
     The average noise power for tone k is given by:
 
σ E′(k)   2 =ε(| R′ ( k )| 2 )− A ( k ) 2   ×|m   R ( k )| 2   (15)
 
     The SNR for tone k is given by: 
     
       
         
           
             
               
                 
                   
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     The bit loading over tone k is given by: 
                           b   ⁢     (   k   )       =       ⁢     ⌊       log   2     ⁡     (     1   +       SNR   ⁡     (   k   )       Γ       )       ⌋                 =       ⁢     ⌊       log   2     ⁡     (     1   +             A   ⁡     (   k   )       2     ×     |     m     R   ⁡     (   k   )         ⁢     |   2         Γ   ×     (         ɛ   ⁡     (     |       R   ′     ⁡     (   k   )       ⁢     |   2       )       -         A   ⁡     (   k   )       2     ×       |     m     R   ⁡     (   k   )         ⁢     |   2       )           )       ⌋                   (   17   )               
where:
         the SNR-gap is denoted as Γ,   the nearest integer value lower than or equal to x is denoted as └x┘.
 
The SNR-gap Γ defines the gap between a practical coding and modulation scheme and the channel capacity. The SNR-gap Γ depends on the coding and modulation scheme being used, and also on the target probability of error. At theoretical capacity, Γ=0 dB.
       

     The relative power gain for tone k is given by: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The channel analyzer ANAL passes the so-determined bit loading b(k) and relative power gain g(k) to the power remote control unit PRCU (see b(k), g(k) in  FIG. 2 ). 
     The power remote control unit PRCU is adapted to sort the tones of the MEDLEY set as loaded, monitored or shut-off tones. 
     If the bit loading on a tone k is strictly lower than 1, then the tone k is sorted as a monitored tone or a shut-off tone. 
     If the SNR on the tone k is still higher than a first threshold T 1 , the tone k is sorted as a monitored tone, else it is sorted as a shut-off tone. 
     The identity of the shut-off tones is passed to the channel analyzer ANAL (see shuttoff_tone_id_ind(k) in  FIG. 2 ). 
     The power remote control unit PRCU is further adapted to provide the second transceiver unit TU 2  with a bit loading and a relative power gain for each tone of the MEDLEY set (see b(k), g(k) in  FIGS. 2 and 3 ). For a monitored tone, the power remote control unit PRCU sets the bit loading to 0 and the relative power gain to a non-null value. For a shut-off tone, the remote power control unit RPCU sets both the bit loading and the relative power gain to 0. 
     The power control unit PCU is adapted to shut-off a tone, the bit loading and the relative gain of which have been set to 0 by the transceiver unit TU 1 . The power control unit PCU requests the transmitter TX 2  unit to shut-off that tone until further notification (see powerdown_req(k) in  FIG. 3 ). 
     The channel analyzer ANAL is further adapted, for each shut-off tone of the MEDLEY set:
         to keep on measuring the average noise power,   to determine therefrom a virtual SNR,   to compare that virtual SNR to a second threshold T 2  at regular time intervals.       

     The channel analyzer ANAL proceeds as follows. 
     Denote the attributes related to the showtime period with a double quote. 
     The virtual SNR is defined as being the ratio of the initial average signal power to the newly measured average noise power: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     ε(|R″(k)| 2 ) is determined during the showtime period by means of equation (11), while ε(|S′(k)| 2 ) has been initially determined during transceiver training by means of equations (10) and (14). 
     If VSNR(k) is higher than T 2 , then the channel analyzer ANAL triggers the power remote control unit PRCU to re-activate tone k (see T 2 _exceeded_ind(k) in  FIG. 2 ). 
     The power remote control unit PRCU is further adapted to request the transceiver unit TU 2  to re-activate tone k, by means of a newly defined message, or by means of an existing message with additional information elements (see reactivate_req(k) in  FIG. 2  and reactivate_ind(k) in  FIG. 3 ). 
     The power control unit PCU is further adapted, upon trigger from the transceiver unit TU 1 :
         to request the transmitter TX 2  to power up tone k (see powerup_req(k) in  FIG. 3 ),   to trigger the transmission of a new initialization sequence over tone k, restricted to the transceiver training, channel analysis and data exchange steps (see training_req(k) in  FIG. 3 ).       

     The new bit loading and relative power gain values are determined by the channel analyzer ANAL as previously described, and passed to the transceiver unit TU 2  via the communication means COM 1  and COM 2 . 
     The power control unit PCU tunes the transmit power value of tone k accordingly, and starts transmitting user data over that tone, thereby achieving the object of the present invention. 
     In an alternative embodiment of the present invention, the average signal and noise powers are determined in the time domain, e.g. by means of a bank of bandpass digital filters centered over each tone, and the bandwidth of which matches the tone spacing 1/T c , and next by computing the mean square value of each filter&#39;s output. 
     A final remark is that embodiments of the present invention are described above in terms of functional blocks. From the functional description of these blocks, given above, it will be apparent for a person skilled in the art of designing electronic devices how embodiments of these blocks can be manufactured with well-known electronic components. A detailed architecture of the contents of the functional blocks hence is not given. 
     While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention, as defined in the appended claims.