Abstract:
The current invention provides a method and apparatus for determining insertion loss in wireline communication systems. The method may be implemented with any modulation protocol but is particularly suited for multi-carrier modulation protocols such as discrete multi-tone (DMT). The method for calculation of the insertion loss uses a received signal and a discrete subset of frequency ranges, e.g. DMT tones, within that signal. For each of the subset of frequency ranges the average received power on each of the selected set of tones is determined. Next the average power on each received subset of tones is converted to log 2  using a simple log 2  estimation circuit that avoids the complexity associated with log tables and other prior art methods of computing logarithms. Then the sum of the log 2  of the average power received in each of the selected subset of tone bins is determined. Next, the known transmitted power for the selected subset of tones is expressed as log 2 , is summed and is then subtracted from the sum of the log 2  of the average power received. Then using a simple electrical length estimation circuit the insertion loss is calculated. A corresponding apparatus is disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of prior filed co-pending Provisional Applications No. 60/209,943 filed on Jun. 6, 2000 entitled “Frequency Dependent Power Backoff”; and No. 60/261,715 filed on Jan. 13, 2001 entitled “Upstream Power Backoff for VDSL” which are incorporated herein by reference in their entirety as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The field of the present invention relates in general to modems and more particularly digital modems. 
     2. Description of the Related Art 
     Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections to high speed back bone networks developed by telecommunications companies. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation. Currently there are over ten discrete XDSL standards, including: G.Lite, ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there are at least two possible line codes, or modulation protocols, discrete multi-tone (DMT) and carrier less AM/PM (CAP). 
     Each new XDSL protocol raises the bandwidth requirements of subscriber lines. As the bandwidth requirements increase so too does the complexity of the modem components. Additionally, because of the enormous variation in loop loss in the individual subscriber lines to which the modem may be coupled the individual components of the modem transmit and receive path must be reconfigurable to match the available bandwidth on a selected subscriber line. For modems which implement the VDSL protocol for example, the length and quality of the subscriber line will determine whether all or a portion of the two upstream and two downstream communications ranges will be available. The sheer length of the line results in significant attenuation of the upper communication ranges rendering their use impractical. Even on shorter loops the presence of bridges and taps can also constrain bandwidth. In each instance it is critical during the line qualification phase to be able to inexpensively and quickly determine insertion power loss into the subscriber line to which the modem is coupled. 
     What is needed is a modem with improved capabilities for determining insertion loss into a subscriber line. 
     SUMMARY OF THE INVENTION 
     The current invention provides a method and apparatus for determining insertion loss in wireline communication systems. The method may be implemented with any modulation protocol but is particularly suited for multi-carrier modulation protocols such as discrete multi-tone (DMT). 
     The method for calculation of the insertion loss uses a received signal and a discrete subset of frequency ranges, e.g. DMT tones, within that signal. For each of the subset of frequency ranges the average received power on each of the selected set of tones is determined. Next the average power on each received subset of tones is converted to log 2  using a simple log 2  estimation circuit that avoids the complexity associated with log tables and other prior art methods of computing logarithms. Then the sum of the log 2  of the average power received in each of the selected subset of tone bins is determined. Next, the known transmitted power for the selected subset of tones is expressed as log 2 , is summed and is then subtracted from the sum of the log 2  of the average power received. Then using a simple electrical length estimation circuit the insertion loss is calculated. A corresponding apparatus is disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
     FIG. 1 is a hardware block diagram showing a receive path of an DSL modem incorporating components for determining the insertion line loss. 
     FIG. 2 is a detailed hardware block diagram of a packet based multi-channel multi-protocol XDSL logical modem which may be used to implement the current invention. 
     FIG. 3 is a process flow diagram showing the processes associated with determining the insertion line loss. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a hardware block diagram showing a receive path of an XDSL modem incorporating insertion loss estimation circuitry. The circuit includes a hybrid front end (HFE)  112  which couples the receive path  180  and transmit paths to subscriber line  110 . The receive path includes analog filter  114 , line amplifier  116 , analog to digital converter (ADC)  118 , digital filter  120 , decimator  122 , cyclic prefix remover  126 , discrete Fourier transform engine (DFT)  128 , the frequency domain equalizer  130 , the decoder  132 , the tone reorderer  134  and the deframer  136 . 
     The components for estimating insertion loss are shown coupled to the output of the DFT  128 . The first of these components is the tone selector  140  which selects a discrete subset of the frequencies, e.g. tones for processing. Those tones obey the relationship where the sum of the square root of the index of each tone equals an integer power of two. Selecting the subsets with these indices greatly simplifies the calcuation of insertion loss, next in power module  142  each of the complex coefficients for each of the selected subset of frequencies is utilized successively to determine the average power in each tone bin. Then in module  144  the log 2  of the average power for each tone bin. The estimator determines the most significant bit location “i” in the average power result for each tone bin. If the adjacent least significant bit equals “1” then the log estimate for the average power of that tone bin equals i+1 else i. Next in summer  146  the sum of the logs of the average power for all the selected subset of tone bins is computed. Then in differencer  148  the log 2  of the transmit power is subtracted and the result is passed to shift register  150 . The shift register shifts the result n+1 places equivalent to dividing in the real number domain by the number of samples in the selected subset. The result at signal line  154  is the log 2  insertion loss. 
     Where power backoff is to be computed the insertion loss may be utilized by multiplying for each transmit tone bin the square root of k the tone index times the log 2  insertion loss output. 
     The hardware blocks shown in this and the following figures may be alternately implemented in software or firmware. The receive path described above and the scalable components thereof may be applied with equal advantage to where a plurality of XDSL channels are multiplexed on a common receive path. The receive path components may be coupled to one another either by packet based transfer of successive packetized portions of a communication channel or by dedicated point-to-point coupling between components. 
     The received power P Rx  is a function of the transmitted power P Tx  and the insertion loss in the line is expressed in Equation 1.                Loss        (   f   )       =         P     Rx        (   f   )           P     Tx        (   f   )           =                   -   d                   α        f              2                 Equation                 1     :                                
     Where d is the distance between transmitter and receiver, f is the frequency of observation, and is a constant that is a function of type of wire used. The type of the wire for most applications is not known a prior; however, distance d can be measured assuming a reference wire type (such as 24 AWG). Here, this distance is referred to as equivalent electrical distance of the line, which is an indication of the loss in the line. Furthermore, assuming that measurements are done on equally spaced grids in frequency domain, the frequency f can be expressed as f=k×Δf. By expressing the electrical distance d as {circumflex over (d)}=da{square root over (Δf)} equation 1 can be simplified into Equation 2.                Loss        (   k   )       =         P     Rx        (   k   )           P     Tx        (   k   )           =                     -     d   ^                       k              2                ←                New                 Equation                   Equation                 2     :                                
     where k is the tone index and {circumflex over (d)} is the electrical length estimator. 
     By estimating the value of {circumflex over (d)} the insertion loss of the line can be characterized. In some applications such as up stream power back off (UPBO) as prescribed by the VDSL standards, this value can be directly used to estimated the insertion loss in the opposite directions, however, in other applications such as line qualification, {circumflex over (d)} must be processed via some intermediate steps such as a lookup table. 
     The received power P Rx(k)  for one tone bin k is expressed in the following Equation 3. 
     
       
           P   Rx(k)   =|e   −{circumflex over (d)}{square root over (k)} | 2   P   TX ( k )←New Equation  Equation 3: 
       
     
     The tone indexes k are chosen in a manner which simplifies the electrical line length estimator {circumflex over (d)}. Tone indexes are selected which are distributed across the bandwidth of interest with the additional qualification that the sum of the square roots of the selected tone indexes may be expressed as an integer power of 2 as set forth in the following Equation 4;                  Tone                   Indexes        (   k   )         ≡       ∑     i   =   0     M            k   i           =       2   n     ←                New                 Equation                 Equation                 4     :                                
     where k i  is index of the i th  tone used by the apparatus and there are a total of M tones used. A qualifying set of tones for a frequency space covered by 128 tones right be: k=4,9,16,25,64,100. Each tone bin has integer square root values: 2,3,4,5,8,10 respectively, a sum of 32 which corresponds with 2 5  or n=5. As shown later, this property will enable a simpler hardware block to replace a cumbersome division operation. 
     Once the qualifying tones have been selected their instantaneous (or average power) for each tone bin is computed as expressed in the following Equation 5. 
     
       
           P   Rx(k)   =r   k   2   +i   k   2   Equation 5: 
       
     
     where r and i are the real and imaginary parts of the amplitude of the received signal within each of the tone bins “k” of the set of K tone bins. 
     Next we take the log 2  of the instantaneous or more accurately the average received power for each tone bin, as expressed in the following Equation 6. 
     
       
         log 2 ( P   Rx(k) )−log 2( P   Tx(k) )=2(log 2   e )(− {circumflex over (d)}{square root over (k)} )  Equation 6 
       
     
     and the results are summed for all K tone bins to yield the log of the instantaneous or average received power for all of the k sampled tone bins. The purpose of this step is to mitigate the effect of noise and bridge tabs in the estimation process. The sum is effectively equivalent to averaging operation across measured frequency bins. The following Equation 7 expresses this result for the set of tones discussed above with indexes k=4,9,16,25,64,100.                            log   2          (     P     Rx        (   4   )         )       -       log   2          (     P     Tx        (   4   )         )         =     2        (       log   2        e     )          (       -     d   ^            4       )                         log   2          (     P     Rx        (   9   )         )       -       log   2          (     P     Tx        (   9   )         )         =     2        (       log   2        e     )          (       -     d   ^            9       )                         log   2          (     P     Rx        (   16   )         )       -       log   2          (     P     Tx        (   16   )         )         =     2        (       log   2        e     )          (       -     d   ^            16       )                         log   2          (     P     Rx        (   25   )         )       -       log   2          (     P     Tx        (   25   )         )         =     2        (       log   2        e     )          (       -     d   ^            25       )                         log   2          (     P     Rx        (   64   )         )       -       log   2          (     P     Tx        (   64   )         )         =     2        (       log   2        e     )          (       -     d   ^            64       )                         log   2          (     P     Rx        (   100   )         )       -       log   2          (     P     Tx        (   100   )         )         =     2        (       log   2        e     )          (       -     d   ^            100       )                       ∑     i   =   0     M            log   2          (     P     Rx        (     k   i     )         )         -         ∑     i   =   0     M            log   2          (     P     Tx        (     k   i     )         )                known                 a                 prior           =       -   2          (       log   2                   e     )          (   32   )          d   ^                   Equation                 7     :                                
     In the log domain the insertion loss is expressed in terms of transmit and received power as shown in the following Equation 7. Since the transmit power is known by communication between the remote and the local sites or has been defined a prior, Equation 8 is log based 2 of loss easily estimated. 
     
       
         log 2 (Loss)=log 2 ( P   Rx(k) )−log 2 ( P   Tx(k) )=2(log 2 ( e ))(− {circumflex over (d)}{square root over (k)} )−log 2 ( P   Tx(k) )  Equation 8: 
       
     
     Using Equation 8, the electrical distance “d” can be estimated, however, in order to reduce the inaccuracies due to measurement and line noise, an averaged version of Equation 8 as shown in Equation 9 is used.                  ∑     i   =   0     M          2        (       Log   2          (   e   )       )          (       -     d   ^              k   i         )         =         ∑     i   =   0     M            Log   2          (     P     Rx        (     k   i     )         )         -       ∑     i   =   0     M            Log   2          (     P     Tx        (     k   i     )         )                     Equation                 9     :                                
     Equation 10 shows the final stage of estimating −{circumflex over (d)}log 2  e. As shown, this operation requires a division operation, which is very costly both in Hardware as well as Software implementations.                  -     d   ^                       log   2        e     =           ∑     i   =   0       M   -   1              log   2          (     P     Rx        (     k   i     )         )         -       ∑     i   =   0       M   -   1              log   2          (     P     Tx        (     k   i     )         )             2     n   +   1                   Equation                 10     :                                
     The division is easily avoided and replace with a shift right operation by n+1 since the denominator is 2 to the power of n+1. 
     For UBPO applications, the estimated value −{circumflex over (d)}log 2  e is used to determined the scaling coefficients of the Tx power in the opposite direction where Tx power for tone k is given as expressed in the following Equation 11:                  log   2          (     P     Tx        (   k   )         )       =           log   2          (     PSDR     Rx        (   k   )         )              Known                 or                 Calculated                 from                 Received                 Info         -           log   2          (     Loss        (   k   )       )              =       -   2          d   ^                     log   2        e        k                             where the                   Equation                 11     :                                
     PSDRR Rx(k)  is the desired power spectral density at the receiver of the opposite side for tone k and the loss(k). The Amplitude required to achieve the transmit power is defined by the square roots of the power, however, the square root in the log domain is equivalent to division by two operation, therefore the log base  2  of the required amplitude is defined as set forth in the following Equation 12:                  log   2          (     A     Tx        (   k   )         )       =         1   2            log   2          (     PSDR     Rx        (   k   )         )         -       d   ^                     log   2        e        k                   Equation                 12     :                                
     after the estimation of log base  2  of (A TX(k) ), the proper gains can be easily applied to the tones in the transmit direction. 
     For other applications, such as line qualification, the corresponding information such as the distance or possible throughputs can be extracted from estimated, −{circumflex over (d)}log 2  e, via a lookup table or similar apparatus. 
     The method described above can be easily extended to more complicated of the insertion loss of the line such as shown in the following Equation 13: 
     
       
         Loss ( F )=| e   −d(α{square root over (f)}+εf) | 2   Equation 13: 
       
     
     where ε is another constant determined by the wire type. 
     FIG. 2 is a detailed hardware block diagram of a packet based multi-channel multi-protocol XDSL logical modem which may be used to implement the current invention. 
     FIG. 2 is a detailed hardware block diagram of a packet based multi-channel multi-protocol XDSL logical modem which may be used to implement the current invention. In this architecture a DSP  200  handles processing for a number of channels of upstream and downstream subscriber line communications via a number of analog front ends (AFE&#39;s)  204  and  210 . Each AFE in turn accepts packets associated with one or more of subscriber lines to which each AFE is coupled. AFE  204  is shown coupled via HFE  206  with subscriber line  208 . AFE  210  is shown coupled via HFE  212  with subscriber line  214 . The logical modem shown in FIG. 2 supports packet based processing of data between a DSP and AFE as well as within each DSP and AFE. Packet processing between DSP and AFE modules involves transfer over bus  202  of bus packets  210  each with a header and data portion. The header contains information correlating the data with a specific channel and direction, e.g. upstream or downstream, of communication. The data portion contains for upstream traffic digitized samples of the received data for each channel and for downstream packets digitized symbols for the data to be transmitted on each channel. 
     Packet processing within a DSP may involve device packets  216 . The device packets may include a header, a control portion and a data portion. The header serves to identify the specific channel and direction. The header may contain control information for the channel to be processed. The control portion may also contain control parameters for each specific component along the transmit or receive path to coordinate the processing of the packets. Within the AFE the digitized data generated for the received (upstream data) will be packetized and transmitted to the DSP. For downstream data, the AFE will receive in each packet from the DSP the digitized symbols for each channel which will be modulated in the AFE and transmitted over the corresponding subscriber line. These modules, AFE and DSP, may be found on a single universal line card, such as line card  116  in FIG.  1 . They may alternately be displaced from one another on separate line cards linked by a DSP bus. In still another embodiment they may be found displaced across an ATM network. 
     The DSP  200  includes, a DSP medium access control (MAC)  226  which handles packet transfers to and from the DSP bus  202 . The MAC couples with a packet assembler/disassembler (PAD)  232 . For received DSP bus packets, the PAD handles removal of the DSP bus packet header and insertion of the device header and control header which is part of the device packet  216 . The content of these headers is generated by the core processor  224  using statistics gathered by the de-framer  256 . These statistics may include gain tables, or embedded operations channel communications from the subscriber side. The PAD embeds the required commands generated by the core processor in the header or control portions of the device packet header. Upstream device packets (Receive packets) labeled with the appropriate channel identifier are passed through the time domain equalizer (TEQ)  244  and the cyclic prefix/suffix remover  246  to the discrete Fourier transform engine  248 . The DMT engine fetches packets and processes the data in them in a manner appropriate for the protocol, channel and command instructions, if any, indicated by the header. The processed data is then passed to the frequency domain equalizer (FEQ)  250 , the decoder  252 , the tone reorderer  254  and the deframer  256 . Each module reads the next device packet and processes the data in it in accordance with the instructions or parameters in its header. The processed de-framed data is passed to the ATM pad  222  for wrapping with an ATM header and removal of the device header. The ATM MAC  220  then places the data with an ATM packet on the ATM network. 
     Control of the receive modules, e.g. DFT engine  248 , FEQ  250 , etc. is implemented as follows. The core processor  224  gathers statistical information on each channel including gain tables, or gain table change requests from the subscriber as well as instructions in the embedded operations portion of the channel. Those tables  226  are stored by the core processor in memory  228 . When a change in gain table for a particular channel is called for the core processor sends instructions regarding the change in the header of the device packet for that channel via PAD  230  and writes the new gain table to a memory which can be accessed by the appropriate module in the receive path. This technique of in band signaling with packet headers allows independent scheduling of actions on a channel by channel basis in a manner which does not require the direct control of the core processor. Instead each module in the receive path can execute independently of the other at the appropriate time whatever actions are required of it as dictated by the information in the device header which it reads and executes. 
     This device architecture allows the DSP transmit and receive paths to be fabricated as independent modules or sub modules which respond to packet header control information for processing of successive packets with different XDSL protocols, e.g. a packet with ADSL sample data followed by a packet with VDSL sampled data For example as successive packets from channels implementing G.Lite, ADSL and VDSL pass through the DFT  248  the number of tones will vary from G.lite, ADSL and for VDSL. The framer  232  and de-framer  256  will use protocol specific information associated with each of these channels to look for different frame and super frame boundaries. The measured level of each tone is maintained by processor  224  in memory  228 . This same memory may be utilized for calculating the inverse channel model for each of the channels to determine the amount of pre-compensation to be applied to downstream data on each of the channels. 
     On the downstream side (Transmit path) the same architecture applies. ATM data is wrapped by PAD  222  with a device header the contents of which are again dictated by the core processor  224 . That processor embeds control information related to each channel in the packets corresponding to that channel. The Framer  232 , tone orderer  234 , encoder  236 , gain scaler  238  and inverse discrete Fourier transform (IDFT) engine  240  process these packets according to the information contained in their header or control portions of each device packet. From the IDFT  240  each updated device packet with a digitized symbol(s) for a corresponding channel is sent to PAD  230  where the device header is removed. The DSP PAD places the DSP packet  210  with an appropriate header to DSP MAC  226  for placement onto the DSP bus  202  for transmission to the appropriate AFE and the appropriate channel and subscriber line within the AFE. 
     During the calibration phase the core CPU  224  generates a calibration sequence which is injected into the tone bins of the IDFT  240  and passes via packets  210  to the corresponding AFE. During the operational phase a local gain table with pre-compensation factors for each tone bin of the IDFT  240  is maintained in memory  228  and utilized to pre-compensate each tone bin for each downstream channel to equalize the frequency response on the transmit path. Separate pre-compensation tables (i.e. local gain tables) are maintained for each channel. 
     Because the data flow in the AFE allows a more linear treatment of each channel of information an out of band control process is utilized within the AFE. In contrast to the DSP device packets which are used to coordinate various independent modules within the DSP the AFE accomplishes channel and protocol changeovers with a slightly different control method. 
     A packet on the bus  202  directed to AFE  210  is detected by AFE MAC  258  on the basis of information contained in the packet header. The packet is passed to PAD  260  which removes the header  270  and sends it to the core processor  262 . The packet&#39;s header information including channel ID is stored in the core processor&#39;s memory  266 . The information is contained in a table  264 . The raw data  272  is passed to interpolator  274 . On the transmit path, the interpolator  274  reads a fixed amount of data from each channel. The amount of data read varies for each channel depending on the bandwidth of the channel. The amount of data read during each bus interval is governed by entries in the control table for each channel which is established during channel setup and is stored in memory  266 . The interpolator up samples the data and passes it to low pass filters  276  to reduce the noise introduced by the DSP. Implementing interpolation in the AFE as opposed to the DSP has the advantage of lowering the bandwidth requirements of the DSP bus  202 . From the interpolator data is passed to the digital-to-analog converter (DAC)  278 . The DAC converts the digitized symbol for each of the input signals on each of the input signal lines/channels to corresponding analog signals. These analog signals are introduced to the amplification stage  280 , from which they are passed to analog filter  282  and then via an associated HFE, e.g. HFE  212  to a corresponding subscriber line e.g. subscriber line  214 . 
     A switch  284  is present in the final stages of the analog portion of the transmit path. That switch during the calibration phase couples the transmit to the receive path thereby providing the feedback for the calibration of the local gain table as discussed above in connection with FIG.  1 . 
     On the upstream path, the receive path, individual subscriber lines couple to the receive path. Subscriber line  214  couples through HFE  212  to the analog filer  286 . The analog filter provides input through switch  288  to the corresponding line amplifier  290 . During the calibration phase the switch  288  couples the transmit to the receive path for feedback of the calibration signal corresponding with the output on the transmit path. From the line amplifier the received analog data is digitized in the analog to digital converter (ADC)  292 . The digitized output is passed through the digital filter  294  and decimator  296  to the pad  260 . The PAD wraps the raw data in a DSP header with channel ID and other information which allows the receiving DSP to properly process it. From the PAD it is passed to the AFE MAC  258  for wrapping in a bus packet  210  and delivery to the DSP  200 . 
     During the calibration phase a calibration sequence is injected into the tone bins of the IDFT  240  and modulated onto the transmit path. Feedback of the resultant analog output signal is provided along the receive path via switches  284 , 288 . On the receive path the ADC  292  performs analog conversion, the DFT  248  conducts spectral analysis on each tone bin. The core CPU normalized the spectral components and generates the local gain table with the precompensation coefficients and stores these in memory  228 . During the operational phase the gain scaler  238  scales each complex number input to each tone bin of the IDFT  240  by both a pre-compensation scale factor from the local gain table and by a gain scale from the remote gain table received from the subscriber side. These correspond to the local gain table  122  and the remote gain table  124  shown in FIG.  1 . 
     FIG. 3 is a process flow diagram showing the processes associated with determining the insertion line loss. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.