Patent Publication Number: US-8995560-B2

Title: Power detection of individual carriers of a multiple-carrier wideband signal

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a communication system, and more particular to transmit power control of individual channels in a multiple-carrier transmission. 
     2. Description of the Related Art 
     Transmit power control is a necessary feature of communications systems to enable a receiver to reliably receive a signal that is not degraded by noise or interference, while ensuring that the signal is not transmitted at such an unnecessarily high power to cause interference. A transmitter can perform certain open-loop control techniques for transmit power control at the transmitter without the benefit of feedback from the receiver. 
     Increasingly, communication systems are employing multiple-carrier signals in order to carry greater amounts of information simultaneously. For example, a wireless or wired data packet communication can be performed using a plurality of channels separated in frequency in a Radio Frequency (RF) wideband signal. While averaging the overall transmit power for the RF wideband signal for closed-loop control can be readily performed, such RF wideband signals can comprise a large number of channels transmitted at a high data rate. Cable TV systems also use multi carrier systems where the individual carriers can have different power settings. This is needed due to various reasons such as different losses in the combiner systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description of the illustrative embodiments is to be read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  provides a schematic block diagram of a channel measurement module of an example transmitter integrated circuit (IC) for a multiple-carrier transmitter, according to one embodiment; 
         FIG. 2  provides a schematic block diagram of a communication device having a channel measurement module for a multiple-carrier transmitter according to one embodiment; 
         FIG. 3  provides a flow diagram of a method for measuring power of a channel of a RF wideband signal utilizing the channel measurement module, in accordance with an exemplary embodiment; 
         FIG. 4  provides a flow diagram of another method for measuring power of a channel of a RF wideband signal utilizing the channel measurement module, in accordance with one embodiment; 
         FIG. 5A  provides a graphical depiction of an RF wideband signal and an undersampled version for a first simulation of the example method of  FIG. 3 ; 
         FIG. 5B  provides a graphical depiction of power versus channel number for a first simulation of the example method of  FIG. 3 ; and 
         FIG. 6  provides a hybrid schematic block diagram of a modulator apparatus including processing states of a channel measurement module according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A measurement component or measurement module of a modulator apparatus measures Radio Frequency (RF) transmit power for a multiple channel signal by receiving, from a multiple-carrier transmitter, a selected baseband signal of a plurality of baseband signals and a corresponding assigned carrier frequency of a plurality of carrier frequencies. The measurement module also receives, from the multiple-carrier transmitter, RF wideband signal comprising the plurality of baseband signals upconverted to the plurality of carrier frequencies. The measurement module undersamples the RF wideband signal for a sampling interval to generate a plurality of measured aliased samples. The measurement module determines a plurality of determined aliased samples based on the selected baseband signal upconverted to the corresponding assigned carrier frequency. The measurement module correlates the plurality of measured aliased samples with the plurality of determined aliased samples to produce a correlation result. Based on the correlation result, the measurement module determines a transmitted power for a selected channel of the RF wideband signal corresponding to the selected baseband signal upconverted to the corresponding assigned carrier frequency. 
     In the following detailed description of exemplary embodiments of the innovation, specific exemplary embodiments in which the innovation may be practiced are described in sufficient detail to enable those skilled in the art to practice the innovation, and it is to be understood that other embodiments may be utilized and that logical, architectural, programmatic, mechanical, electrical and other changes may be made without departing from the spirit or scope of the present innovation. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present innovation is defined by the appended claims and equivalents thereof. 
     Within the descriptions of the figures, similar elements are provided similar names and reference numerals as those of the previous figure(s). Where a later figure utilizes the element in a different context or with different functionality, the element is provided a different leading numeral representative of the figure number. The specific numerals assigned to the elements are provided solely to aid in the description and not meant to imply any limitations (structural or functional or otherwise) on the described embodiment. 
     It is understood that the use of specific component, device and/or parameter names (such as those of the executing utility/logic described herein) are for example only and not meant to imply any limitations on the described embodiments. The presented embodiments may thus be implemented with different nomenclature/terminology utilized to describe the components/devices/parameters herein, without limitation. Each term utilized herein is to be given its broadest interpretation given the context in which that terms is utilized. 
     As further described below, implementation of the functional features of the innovation is provided within processing devices/structures and involves use of a combination of hardware, firmware, as well as several software-level constructs (e.g., program code). The presented figures illustrate both hardware components and software components within the example signal processing. 
     With reference now to the figures, in  FIG. 1  transmit signal modulator apparatus  100  comprises a measurement module  102  that measures RF transmit power for a multiple channel signal. The transmit signal modulator apparatus  100  modulates a number of baseband signals for distribution as an RF wideband signal. The transmit signal modulator apparatus  100 , for another example, can be an apparatus that receives a number of television cable channels as baseband signals and that has a defined set of carrier frequencies or channels for carrying each of the television cable channels. The transmit signal modulator apparatus  100  produces an RF wideband signal that is distributed over a wired or wireless network or broadcast. Measurement module  102  comprises a plurality of components. An input interface  104  receives, from a multiple-carrier transmitter  106  of the modulator apparatus  100 , a selected baseband signal  108  of a plurality of baseband signals  110  and a corresponding assigned carrier frequency  112  of a plurality of carrier frequencies  114 . The input interface  104  also receives, from the multiple-carrier transmitter  106 , an RF wideband signal  116  comprising the plurality of baseband signals  110  upconverted to the plurality of carrier frequencies  114 . An undersampling component  118  of the measurement module  102  undersamples the RF wideband signal  116  for a sampling interval to generate a plurality of measured aliased samples. A determination component  120  of the measurement module  102  determines a plurality of determined aliased samples based on the selected baseband signal upconverted to the corresponding assigned carrier frequency. A correlator  122  of the measurement module  100  correlates the plurality of measured aliased samples with the plurality of determined aliased samples to produce a correlation result. The reference signals, which comprise baseband signals together with the known carrier frequencies, can also be undersampled. Thus, the reference signals and the RF wideband signals are both undersampled at the same sampling rate. A transmit power component  124  of the measurement module  100  determines a transmitted power  126  of the correlation result for a selected channel of the RF wideband signal corresponding to the selected baseband signal upconverted to the corresponding assigned carrier frequency. A transmit power control  128  can utilize the transmitted power  126  to perform closed-loop control of transmit power for individual channels. 
     Referring now to  FIG. 2 , which illustrates a communications device or apparatus  200  within which the above described measurement module  100  ( FIG. 1 ) can be implemented, according to one embodiment. As illustrated, measurement module  202  comprises processor integrated circuit (IC)  203 , which is connected to memory  204 . Processor IC  203  may include one or more programmable microprocessors, depicted as a data processor  206 . Processor IC  203  can also include a Field Programmable Gate Array (FPGA) or digital signal processor (DSP)  208 . Processor IC  203  controls the communication and other functions and/or operations of communications device  200 , including the operations of measurement module  202 . These functions and/or operations include, but are not limited to, data processing and signal processing to perform transmit signal power measurement via a channel measurement component  210  that is resident in memory  204 . 
     In this embodiment, the channel measurement component  210  comprises software-implemented versions of input interface  212 , determination component  214 , correlator  216 , undersampling component  217 , and transmit power component  218  that perform the corresponding functions described by the similarly named components in  FIG. 1 . Alternatively, in other embodiments, portions or the entirety of the channel measurement component  210  can be implemented in firmware or other forms of dedicated circuitry. Measurement module  202  can include a network interface  220  to convert to a compatible digital form one or more baseband signals  222 , one or more carrier frequencies  224  and RF wideband (WB) signal  226  from a multiple-carrier transmitter  228  (such as the multiple-carrier transmitter  106  of  FIG. 1 ). The multiple-carrier transmitter  228  can require a sampling component  230  at the network interface  220  for a portion of the RF wideband signal  226  to reduce the amount of data received by the measurement module  202 . The network interface  220  can further communicate a value for transmitted power  232  to the multiple-carrier transmitter  228 . 
     In this example, the channel measurement module  202  may be implemented as part of the data processing functionality of communications apparatus  200 . Communications apparatus  200  comprises a bus architecture or other form of inter-component connectivity that links together various circuits including one or more processors, represented generally by the processor IC  203 , and computer-readable storage media, represented generally by the memory  204 . The processor IC  203  is responsible for general processing, including the execution of software stored on the memory  204 . The software, when executed by the processor IC  203 , causes the measurement module  202  to perform various functions, as described herein. 
     The communication apparatus  200  can comprise input devices, of which keypad  238 , pointing device  240  such as a touch screen or touch pad, and microphone  242  are illustrated, connected to processor IC  203 . Additionally, communication apparatus  234  comprises output devices, such as speaker  244  and display  246 , which are each connected to processor IC  203 . 
     Referring to  FIG. 3 , in an exemplary embodiment, a methodology  300  is depicted for measuring Radio Frequency (RF) transmit power for a multiple channel signal. A module receives from a multiple-carrier transmitter, a first undersampled version of one or more carriers of a plurality of carriers (block  302 ). The module receives, from the multiple-carrier transmitter, a second undersampled version of an RF wideband signal comprising a plurality of baseband signals upconverted to the plurality of carriers (block  304 ). The module correlates the first undersampled version with the second undersampled version to produce a correlation result (block  306 ). The module determines a transmitted power of the correlation result for a selected channel of the RF wideband signal corresponding to the one or more carriers (block  308 ). 
     In an exemplary aspect, the module receives the first undersampled version by receiving a selected baseband signal of a plurality of baseband signals and a corresponding assigned carrier frequency of a plurality of carrier frequencies. The module determines a plurality of determined aliased samples based on the selected baseband signal upconverted to the corresponding assigned carrier frequency. The modules receives, from the multiple-carrier transmitter, the second undersampled version of the RF wideband signal by receiving, from the multiple-carrier transmitter, an RF wideband signal comprising the plurality of baseband signals upconverted to the plurality of carrier frequencies and undersampling the RF wideband signal for a sampling interval to generate a plurality of measured aliased samples. The module correlates the first undersampled version with the second undersampled version by correlating the plurality of measured aliased samples with the plurality of determined aliased samples to produce the correlation result. 
     Referring now to  FIG. 4 , which depicts another methodology  400  for measuring transmit power for a multiple channel signal, according to an exemplary embodiment. A measurement module receives, from a multiple-carrier transmitter, a selected baseband signal of a plurality of baseband signals and also receives a corresponding assigned carrier frequency of a plurality of carrier frequencies (block  402 ). The measurement module also receives, from the multiple-carrier transmitter, an RF wideband signal comprising the plurality of baseband signals upconverted to the plurality of carrier frequencies to produce the RF wideband signal (block  404 ). In one embodiment, the RF wideband signal is in an analog form for transmission or broadcast that requires sampling by the measurement module. Alternatively, the RF wideband signal can be in a digital stream that can be undersampled by selecting aliased samples at successive intervals. 
     The measurement module undersamples the RF wideband signal for a sampling interval to generate a plurality of measured aliased samples (block  406 ). The measurement module determines a plurality of determined aliased samples based on the selected baseband signal upconverted to the corresponding assigned carrier frequency (block  408 ). The measurement module correlates the plurality of measured aliased samples with the plurality of determined aliased samples to produce a correlation result (block  410 ). The measurement module determines a transmitted power of the correlation result for a selected channel of the RF wideband signal corresponding to the selected baseband signal upconverted to the corresponding assigned carrier frequency (block  412 ). In one embodiment, the multiple-carrier transmitter performs transmit power control for a channel that corresponds to the selected baseband signal at the corresponding assigned carrier frequency in response to receiving the transmitted power communicated from the measurement module (block  414 ). 
     In a particular embodiment, the multiple-carrier transmitter generates a cable television signal with multiple television channels. Due to variations in calibration or gain variation over frequency in performance, the multiple-carrier transmitter can inadvertently introduce a variation in transmit power for a plurality of channels of RF wideband signal during the upconverting, amplifying and filtering of the plurality of baseband signals to generate the RF wideband signal. Transmit power control maintains each channel within a specified range of transmit power. To that end, the measurement module can, over time, measure all of the channels for transmit power. 
     The measurement module can receive, from the multiple-carrier transmitter, the plurality of baseband signal and the plurality of carrier frequencies. For each subset of the plurality of baseband signals, the measurement module determines a plurality of determined aliased samples based on at least one selected baseband signal upconverted to the corresponding at least one assigned carrier frequency. For each subset of the plurality of baseband signals, the measurement module also correlates the plurality of measured aliased samples with the plurality of determined aliased samples to produce a correlation result. The measurement module determines at least one transmitted power of the correlation result for at least one channel of the RF wideband signal corresponding to each subset of the plurality of baseband signals upconverted to the corresponding at least one assigned carrier frequency. 
     Returning to  FIG. 4 , the next baseband signal and/or subset of baseband signals and corresponding carrier frequency or carrier frequencies is/are selected (block  416 ). Then the afore-mentioned blocks  402 - 410  can be repeated, with the process repeated for each subset of baseband signals. 
     In an exemplary implementation of the present innovation, power detection is simulated of individual carriers in a cable television system spanning 1+GHz using a low sampling rate (e.g., 6-7 MHz) Analog-to-Digital Converter (ADC). In summary, undersampling an RF signal with multiple carriers results in the same carriers spectrally folding on top of each other. Aspects of the described embodiments are implemented based on a determination that if the individual carriers are independent, the individual carriers will remain independent even if undersampled. Additionally, the embodiments are also based on a determination that since a modulator apparatus that comprises a multiple-carrier transmitter knows what is transmitted at each individual carrier, the RF signal can be correlated with an undersampled version of the RF carrier. In particular, because the value of correlation is determined to be proportional to system gain, any change in power level can be readily measured by correlating the undersampled RF signal with a selected carrier. Further, using an undersampled version of the RF signal enables use of inexpensive Digital-to-Analog Converters (DACs). 
     Within the analysis leading to the development of the embodiments, the principal idea is that correlation between two signals does not depend on a sampling rate of two signals. For example, Let R x,y (T) be the true cross correlation between two variables x t  and y t . Let the 
                 R     x   ,   y   ,   N   ,   T       ⁡     (   τ   )       =         lim     n   →   ∞       ⁢       1   N     ⁢       ∑     n   =   1     N     ⁢       y   ⁡     (     n   ⁢           ⁢   T     )       ⁢       x   ⁡     (       n   ⁢           ⁢   T     +   τ     )       _             →       R     x   ,   y       ⁡     (   τ   )               
estimated cross correlation as a function of the number of samples N and the sampling time T be R x,y,N,T (T). The cross correlation does not depend on the sampling rate T. Thus, the correlation, is independent of T.
 
     The transmitted cable television signal consists of several, e.g., up to 150, carriers and can be written as 
               y   t     =     Real   ⁢     {       ∑     m   =   1     M     ⁢         x   m     ⁡     (   t   )       ⁢     exp   ⁡     (     ⅈ   ⁢           ⁢     ω   m     ⁢   t     )           }             
where x m (t) is the complex baseband signal formed by the I/Q data going through a raised cosine filter, although x m (t) can be any I/Q signal. The center frequency of the m&#39;th carrier is ω m .
 
     A sampled version of this signal is 
     
       
         
           
             
               
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     An assumption is made that the complex baseband signals x n (t) are all uncorrelated with each other and also that the real and imaginary parts of the complex baseband signals x n (t) are uncorrelated. 
     One benefit appreciated by implementation of the methods of the present disclosure is that any two signals that are at different channels, i.e., signals that have no overlap in their spectra, are always uncorrelated. This is true even if the signals are modulated by the same baseband signal. Note that in this case the signals are not in the statistical sense of the word “independent”. Even if the wideband signal consisted of signals that were modulated by the same baseband signal so that each carrier had exactly the same content, the two signals would still appear as uncorrelated signals in the undersampled signal as long as the two signals do not fold exactly onto each other. For instance, using a sampling frequency that is half the channel spacing should be avoided. However, even with the use of such a “bad” sampling frequency, for all practical multiple-carrier systems, for example cable television, the individual carriers are statistically independent, in which case the sampling rate does not matter. 
     Evaluating the cross correlation between y t =kT and a single carrier x p (kT)exp(iω p kT) results in 
                 R     y   ,   x   ,   N   ,   T       ⁡     (   τ   )       =       1   N     ⁢       ∑     n   =   1     N     ⁢     Real   ⁢     {       ∑     m   =   1     M     ⁢         x   m     ⁡     (     k   ⁢           ⁢   T     )       ⁢     exp   ⁡     (     ⅈ   ⁢           ⁢     ω   m     ⁢   k   ⁢           ⁢   T     )           }     ⁢   Real   ⁢     {         x   p     ⁡     (       k   ⁢           ⁢   T     +   τ     )       ⁢     exp   ⁡     (       ⅈ   ⁢           ⁢     ω   p     ⁢   k   ⁢           ⁢   T     +     ⅈ   ⁢           ⁢     ω   p     ⁢   τ       )         }                 
taking the means and recognizing which parts are correlated and using x n =I n +i·Q n , yields the following
 
     
       
         
           
             
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     It should be noted that if the sampling time T is long when using a slow sampling rate, there exists the possibility that 2ω p kT equals a multiple of 2π. This will be the case if the center frequency ω p  is a multiple of the sampling rate 1/T. In the case when this term does not average to zero, then 
                     E   ⁢     {       R     y   ,   x   ,   N   ,   T       ⁡     (   τ   )       }       =       ⁢         1   N     ⁢       ∑     n   =   1     N     ⁢     E   ⁢     {         I   p     ⁡     (     k   ⁢           ⁢   T     )       ⁢       I   p     ⁡     (     k   ⁡     [     T   +   τ     ]       )       ⁢       cos   ⁡     (       ω   p     ⁢           ⁢   τ     )       2       }           +                     ⁢       1   N     ⁢       ∑     n   =   1     N     ⁢     E   ⁢     {         Q   p     ⁡     (     k   ⁢           ⁢   T     )       ⁢       Q   p     ⁡     (     k   ⁡     [     T   +   τ     ]       )       ⁢       cos   ⁡     (       ω   p     ⁢           ⁢   τ     )       2       }                       =       ⁢         R       x   m     ,     x   m         ⁡     (   τ   )       ⁢       cos   ⁡     (       ω   p     ⁢   τ     )       2                   
which does not depend on the actual sampling time T. In the other case
 
     
       
         
           
             
               
                 
                   
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     Assuming that the power in the real and imaginary parts of x m  are the same, then 
                 R       I   m     ,     I   m         ⁡     (   τ   )       =           R       x   m     ,     x   m         ⁡     (   τ   )       2     =         R       x   m     ,     x   m         ⁡     (   τ   )       ⁢       cos   ⁡     (       ω   m     ⁢   τ     )         2       ω   m     =   0                   
so that this scenario does not cause any problems as long as the power in the real and imaginary channels are the same.
 
     A very small delay τin the envelope will have very little effect on Rx m, x m (τ), as long as the delay is small compared to the inverse of the bandwidth of x m . However, if the actual carrier frequency ω m  is large then cos(ω m τ) might take any value between ±1. There is a risk that the estimate of the correlation is much less than that of the true correlation or that the correlation is even negative. The latter result can be a highly erroneous estimate of the power. For the effect on the delay to be small, the value of cos(ω m τ) should be close to one. Since cos(ωτ)≈1+(ωτ) 2 /2, the maximum delay one can have for a 0.1 dB error is 
     
       
         
           
             
               
                 
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     If the highest carrier frequency is 1 GHz, the maximum delay is 34 ps. This is equivalent to matching trace lengths to 3.4% of the wavelength, roughly. An alternate embodiment provides an alternative way of actually estimating cos(ω p τ), to correlate with a phase shifted version of x p (kT)exp(iω p kT+iφ) given the following: 
     
       
         
           
             
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     It should be appreciated with the benefit of the present disclosure that one can perform the correlation with different values of φ to find the maximum of cos(ω p τ−φ). For a 0.1 dB error, φ should have resolution of 0.214 radians out of 6.14 radians for a maximum of 30 possible angles. In practice, negative values can be excluded so that more than fifteen (15) different values should not be needed. In addition, a gradient descent method could converge fairly quickly, requiring less than fifteen (15) steps and result in a higher precision. Once the optimal phase shift has been found, there should be no need to constantly search for this maximum. Since any drift can be assumed to be slow, any tracking would be straightforward and would not add any significant processing overhead. Delays that are introduced by different lengths of integrated circuit traces that propagate signals are fixed, and a delay introduced when transmitting through a wired or over-the-air link is generally static. Thus, if the difference between the trace lengths is well known and the delay in the RF chain is constant; then the delay, specifically phase shift ω p τ, need only be characterized once. 
       FIGS. 5A-5B  provide several graphical outputs that are generated during one or more simulations of the described embodiments. In an illustrative first simulation, a representative RF signal is generated and then down sampled.  FIG. 5A  is a graphical depiction  500  of the spectra of an actual RF signal  502 , referred to as the “original” spectra and the undersampled or “down selected” version  504 . It should be noted that the individual carriers are folded on top of each other in the undersampled signal. 
     After the undersampling, the various signals fold on top of each other. In this simulation, ten Quadrature Amplitude Modulation (QAM) modulated signals were put at ten different carrier frequencies. A second signal was formed by undersampling this signal by a factor of 100. Thus,  FIG. 5B  is a graphical depiction  510  of power per carrier measured by correlating the received signal with the different carriers with the actual data  512  (fs=1 Ghz) and undersampled data  514  (fs=10 MHz). The estimate based on the undersampled signal is noisier due to the shorter data record of 1/110 th  of the actual data. 
     The simulations show the method to work as expected. Applying observations from the testing and simulations to a practical cable television scenario, around 100 k samples are needed for a 0.2 dB precision. Less samples would be needed when fewer carrier are present. For a 6 MHz signal, a sampling time spanning around 0.02 seconds at a 6 MHz sampling rate would be required. If choosing to evaluate one channel at a time, the power of all channels can be estimate within one (1) to two (2) seconds. 
     A 4 GHz Digital-to-Analog (DAC) in a laboratory setting was used to generate ten (10) 6 MHz channels that constitute the spectrum. Between each channel is an empty channel. Given that the signal needs to be created for a very high data rate, the length of the modulating signal or QAM data for each channel is somewhat short. Only 2048 QAM symbols per channel were generated and then interpolated to get a sampling rate of 4 GHz. This signal was captured on an oscilloscope that acted as a high-speed DAC. The same signal was sampled at 10 MHz and 1 GHz. The spectrum of the undersampled signal was evaluated for a sampling frequency (fs) of 10, 50 and 100 MHz. Often the signals fell onto each other, while sometimes only partially overlapping. When the sampling rate was 10 MHz, all of the signals overlapped. 
     In one implementation, the captured data was then correlated with twenty different carriers that are the carriers used to make the signals as well as the carriers that should have been between the active channels, i.e., the carriers that should have been in the gaps in the spectrum. Correlating of the carriers with non-existing carriers was done mainly to verify that the correlation is working satisfactorily as well as to obtain an indication about the noise level or resolution that is inherent in the process. 
       FIG. 6  illustrates a functionally-descriptive version of a measurement module  602  that can be implemented in conjunction with a modulation apparatus  600 . The modulation apparatus  600  receives more than one baseband signal  604  {x 1 (t)+x 2 (t)+ . . . +x m (t)}. More than one carrier frequencies  606  are assigned by a channel frequency assignment component  608  to each of the baseband signals  604 . A multiple-channel digital upconverter (DUC)  610  upconverts the baseband signals  604  to the corresponding assigned carrier frequency  606 , forming a multiple-carrier, RF wideband signal  612  comprised of a number of channels {y(t)=real(x 1 (t)exp(iω1t)+x 2 (t)exp(iω2t)+x 3 (t)exp(iω3t)+ . . . }. In an exemplary aspect, this unamplified RF wideband signal requires amplification for transmission to users  614 , which amplification is depicted as being completed via Digital-to-Analog Converter (DAC)  616  in series with RF amplifiers and filters circuitry  618 . As depicted at  620 , the resulting RF wideband signal  622  {y(t)=real(g1×1(t)exp(iω1t)+g2×2(t)exp(iω2t)+g3×3(t)exp(iω3t)+ . . . } can have impairments since each carrier or channel undergoes different gains/attenuations. In an exemplary aspect, multi-channel digital up converter (DUC)  624  provides a power level for each carrier or channel. The power level is maintained between an upper power threshold P 1  and a lower power threshold P 2 . 
     The measurement module  602  receives one or more baseband signals  604  at multi-channel DUC  624  and the corresponding assigned one or more carrier frequencies  606  within channel frequency assignments  608 . Each of the baseband signals  604  can be selected over time as represented by a first switch  626 . Each of the carrier frequencies  606  can be selected over time as represented by a second switch  628 . The measurement module  602  determines an undersampled version of the selected baseband signal  604  (e.g., the third baseband signal x 3 (t)) upconverted to the corresponding assigned carrier frequency (channel)  606  (e.g., x 3 (t)exp(iω 3 t) at t=n/fs), resulting in determined aliased samples (block  630 ). 
     The measurement module  602  can include a low-speed ADC  632  that is used to sample the wideband RF signal  620  from the multiple-carrier transmitter  602 . Note that the ADC  632  requires a front end sample and hold circuitry  634  with sufficient bandwidth for undersampling the signal. A correlator  636  correlates the determined aliased sampled with the measured aliased samples, as shown at block  638 . A value for power “g 3 ” measured for the third baseband signal results from the correlation, as depicted at block  640 . The measurement module  602  communicates this value to a channel transmit power control  642  of the multiple-carrier transmitter  600 . 
     It should be appreciated with the present disclosure that, especially when sampling over longer time periods, a signal that is correlated will appear uncorrelated if the clock used for generating the signal, i.e., the DAC clock is different from the ADC clock. The ADC clock, for instance, could be locked to the DAC clock. Also, when sampling rate is low compared to the data rate, e.g., sampling 6 MHz signals at 10 MHz, time offsets that are a fraction of a sample can have a significant impact on the correlation, resulting in roughly about 3 dB-6 dB errors. This fractional delay can be easily estimated and digitally compensated for without any substantial increase in the computational load. Once the delay has been found, the embodiments include verification that the delay is infrequently changed. 
     By virtue of the foregoing, an apparatus and method of the present disclosure provide substantially precise measurement and control of individual carrier levels in a multiple-carrier signal, such as multicarrier cable television signal. The disclosed closed-loop control avoids cumbersome or exacting factory calibrations required with conventional open-loop methods of controlling carrier power. Power detection of an individual carrier in a multicarrier uplink signal is calculated by correlating an undersampled multiple-carrier baseband signal and a baseband signal for the individual carrier. In one embodiment, calculating the correlation is performed by sampling the multicarrier signal at a rate below the bandwidth of the multiple-carrier signal, multiplying baseband signal samples of the individual carrier with the multicarrier signal samples, and averaging the multiplied baseband and multicarrier signal samples. In another embodiment, carrier power levels are controlled in a multicarrier uplink signal by calculating the correlation of the undersampled multicarrier signal and a baseband signal for an individual carrier, comparing the correlation with a predetermined level to generate an error signal, and adjusting the individual carrier level to minimize the error signal. 
     In the flow charts of  FIGS. 3 and 4  described above, one or more of the method processes may be embodied in a computer readable medium containing computer readable code such that a series of steps are performed when the computer readable code is executed on a computing device. In some implementations, certain steps of the methods are combined, performed simultaneously or in a different order, or perhaps omitted, without deviating from the spirit and scope of the innovation. Thus, while the method steps are described and illustrated in a particular sequence, use of a specific sequence of steps is not meant to imply any limitations on the innovation. Changes may be made with regards to the sequence of steps without departing from the spirit or scope of the present innovation. Use of a particular sequence is therefore, not to be taken in a limiting sense, and the scope of the present innovation is defined only by the appended claims. 
     As will be appreciated by one skilled in the art, aspects of the present innovation may be embodied as a system, method or computer program product. Accordingly, aspects of the present innovation may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present innovation may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Computer program code for carrying out operations for aspects of the present innovation may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or assembly level programming or similar programming languages. 
     Aspects of the present innovation are described above with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the innovation. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     While the innovation has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the innovation. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the innovation without departing from the essential scope thereof Therefore, it is intended that the innovation not be limited to the particular embodiments disclosed for carrying out this innovation, but that the innovation will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the innovation. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present innovation has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the innovation in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the innovation. The embodiment was chosen and described in order to best explain the principles of the innovation and the practical application, and to enable others of ordinary skill in the art to understand the innovation for various embodiments with various modifications as are suited to the particular use contemplated.