APPARATUS AND METHOD FOR REDUCING THE EFFECTS OF NOISE ON A LIGHT TRANSMITTER

Abstract: According to the present invention, an apparatus and a method that reduce the effects of noise on a light transmitter (140) are disclosed. The current invention is particularly suited to Hybrid Fiber Coax networks. Embodiments of the current invention shape one or more pre-selected, noisy spectra of an electrical signal to be transmitted. The shaping primarily reduces errors due to random noise. The shaped electrical signal is then combined with one or more non-shaped signals and the composite signal is converted to light and transmitted. In addition, the non-shaped signals may be notch filtered around frequencies that have a lot of frequency-specific noise. The embodiments of the current invention essentially limit or reduce power in noisy spectra (which could be noisy because of random or frequency-specific noise), which reduces the chances of the light transmitter (140) being overwhelmed in power. This limits the effects of electrical noise on the light transmitter (140). The signal shaping may be performed in the analog or digital domain. By determining the frequency bands that are more likely to be noisy, the current invention reduces complexity, as only those frequency bands that have the most noise are power limited.

Description: Apparatus and method for reducing the effects of noise on a light transmitter This invention generally relates to mixed electrical and optics networks and more specifically relates to reducing the effects of light-transmitting errors caused by noise. One of the current trends in Cable Television (CATN) is adding the ability for consumers to order movies and other content from their homes. To allow this, many cable and phone companies are updating their current networks with Hybrid Fiber Coax (HFC) networks. In an HFC system, CATN comes into the consumer's home on a fiber optic cable, where it is converted to electrical signals by an electro-optic converter just before entering the customer's home. One or more coaxial cables carry the electrical signal to televisions throughout the home. When the consumer wishes to order a movie, he or she sends a request for a movie from some type of sending unit (generally called a "set-top box") through an electro-optic converter (a transmitter) and to the CATN company, which then sends the movie to the consumer's home through the HFC network. These HFC networks have the added benefit that information other than movies and requests for movies may be transmitted and received by the consumer. Internet services, digital TN, and other broadband services are currently implemented or are planned to be implemented using HFC networks. Additionally, non-coaxial cable information, such as phone service or networks based on twisted-pair cables, are also being added to HFC networks. When going upstream (from the consumers' house to the cable provider), these various signals are added and, if needed, frequency multiplexed before being converted to light. Because the upstream signals originate in and travel some distance in the consumer's house, there are multiple sources of ingress noise into the coaxial cable or twisted pair cable. For instance, microwaves, toasters, radio transmitters (garage door opens, cellular phones, cordless phones, etc.), all can provide ingress noise into the coaxial cable. Additionally, sources outside the consumer's house, such as from CB (Citizen's Band) radios, police and fire transmitters, etc., can also contribute to ingress noise. These ingress noise sources can be further categorized as random or frequency-specific. Any device that causes noise evenly over a frequency band or causes noise at different frequencies each time the device is powered on creates random noises. Electrical switches, toasters, and the like tend to fall into this category. Devices that cause noise at one certain frequency or over a small range of frequencies create frequency-specific noise. CB radio, garage door openers, and cellular phones are examples of devices that generate frequency-specific noise. Both types of ingress noise are particularly damaging in HFC networks, because high amplitude ingress noise can completely overwhelm the laser (or light-emitting diode) in the upstream transmitter. Lasers and diodes used in HFC networks generally operate on a bias direct current (DC) that corresponds to about one milliwatt (mW) of power. This bias current is modulated, usually with an alternating current (AC), with the signal from the consumer's home. The signal from the consumer's home, when converted into power terms, corresponds to about one mW peak-to-peak light transmitter power. When the base signal and the modulating signal are added, the composite signal normally varies between about 0.5 mW and 1.5 mW. Most HFC lasers can handle and produce fairly high positive currents that correspond to high positive powers (up to around ten mW), but generally cannot handle currents below zero. In fact, there needs to be some positive current above a certain value (called a threshold current) for the laser or diode to work. Ingress noise from inside and outside the consumer's home can be as large as, or larger than, ten times higher in power and current than the light transmitter's base signal. When the electo-optic converter uses this noisy signal to modulate current, which is then used to modulate light, the light transmitter may clip the portion of the signal below the threshold current. While the positive signal may not get clipped, the portion of the signal below the threshold current is completely destroyed. Much of the information in the signal is then lost for as long as the light transmitter clips, which is as long as the current in the signal, caused by the ingress noise, to the laser remains below the threshold value. This is referred to as a loss of data transport. Thus, even though HFC networks provide the benefit of sending data upstream, which in turn allows a consumer to access the internet, have a clear phone connection, order movies, and perform other functions, ingress noise from the consumer's home can destroy the very signal trying to be transmitted. Without a method and device for reducing the effects of ingress noise, mixed optical and electrical networks will continue to experience a higher than necessary number of errors due to noise level and a higher frequency of loss of data transport. According to the present invention, an apparatus and a method that reduce the effects of noise on a light transmitter are disclosed. The current invention is particularly suited to Hybrid Fiber Coax networks. Embodiments of the current invention shape one or more pre-selected, noisy spectra of an electrical signal to be transmitted. The signal shaping primarily reduces errors due to random noise. The shaped electrical signal is then combined with one or more non-shaped signals and the composite signal is converted to light and transmitted. In addition, the non-shaped signals may be notch filtered around frequencies that have a lot of frequency-specific noise. The embodiments of the current invention essentially limit or reduce power in noisy spectra (which could be noisy because of random or frequency-specific noise), which reduces the chances of the light transmitter being overwhelmed in power. This limits the effects of electrical noise on the light transmitter. The signal shaping may be performed in the analog or digital domain. By determining the frequency bands that are more likely to be noisy, the current invention reduces complexity, as only those frequency bands that have the most noise are power limited or power reduced. In addition, because shaping of signals in the manner disclosed herein generally causes non-linear distortion of the signal, shaping of the signal is preferably limited to the portion of the signal containing the relatively noisy frequency band. While there will be distortion caused by shaping, the shaping should only operate when the signal has been corrupted by noise. The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. The preferred embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and: Fig. 1 is a block diagram of an HFC system in accordance with a preferred embodiment of the present invention; Fig. 2 is a flow diagram of a preferred method of determining, and limiting the effect of, noise-prone spectrums in HFC networks; Figs. 3 - 6 are block diagrams of signal shapers and noise effect reducing mechanisms in accordance with preferred embodiments of the present invention; and Fig. 7 is another preferred embodiment of an HFC system. The embodiments of the current invention limit clipping and other effects on a light transmitter that are caused by noise on electrical signals in mixed electrical and optic networks. The invention potentially has its best use in Hybrid Fiber Coax (HFC) networks. The embodiments of the current invention reduce these effects by shaping pre-selected frequency bands (for random noise) and by notch filtering (for frequency-specific noise) the same or other pre-selected frequency bands. The pre-selected frequency bands are preferably chosen by performing a spectral analysis that indicates that these frequency bands are noisy. Ideally, the pre-selected frequency bands that will be shaped will contain at least a majority of the random noise as determined by the spectral analysis. In this manner, the noisiest frequency bands can be power limited or reduced in power, which should reduce clipping by the light transmitter, over-powering the light transmitter, and the loss of other portions of the frequency spectrum due to noise on one portion of the frequency spectrum. The latter benefit is particularly important in HFC networks, because one of the problems associated with these networks is that a portion of the upstream frequency range generally has the vast majority of noise. By limiting or reducing the power in this portion, the optical transmitter for the upstream data should lose data transport and be over-powered less than in prior art systems. As used herein, the terms "noisy frequency band" and "high-noise frequency band" are used to denote a range of frequencies that has been determined to have a relatively high level of random noise. The terms "low-noise frequency band" and "less-noisy frequency band" are used to denote a range of frequencies that has been determined to have a relatively low level of random noise. For frequency-specific noise, specific reference will usually be made to the fact that the noise is frequency specific. Once a spectral analysis has been performed and the noisy frequency band or bands determined, the signal is preferably broken into high-noise frequency bands and a second frequency band (with or without the high-noise frequency bands). The high-noise frequency bands preferably have the highest random noise. The high-noise frequency bands are shaped to reduce the effects of random noise, once the two frequency bands are recombined. The signal shaping may occur in the digital or analog domain. The signal shaping entails reducing the power in the frequency band (e.g., through a reduction in gain) or limiting the power in the frequency band (e.g., through limiting the amplitude of the signal). The second frequency band or bands are allowed to pass through, although notch filtering of noise is permissible on these bands. The notch filtering is preferably designed to remove frequency-specific noise that was found during the spectral analysis. All frequency bands are combined and then coupled to an electro-optic transmitter, which modulates and transmits light. The electro-optic transmitter is termed a "light transmitter" herein. The current invention, thus, minimizes the effects of both random and frequency-specific electrical noise on light transmitters. The current invention is particularly applicable to HFC networks because of the structure of an HFC network and the light transmitters used in the networks. Because of this applicability, the current invention will be discussed mainly in reference to HFC networks, although the invention may benefit other mixed electrical and optical networks. Referring now to Fig. 1, a preferred configuration of an HFC network in accordance with a preferred embodiment of the present invention is shown. The HFC network in Fig. 1 shows a block diagram of links from a number of consumer's houses to the Cable Television (CATN) provider (or other high speed bandwidth provider). Customer interface units 105 through 110 connect customers to trunk 112. The customer interface units can contain any type of electrical connection known to those skilled in the art, but will generally contain coaxial cable and/or twisted-pair connections. Trunk 112 contains all of the frequency multiplexers, if necessary, to interleave the customers' data into one upstream frequency band. The customer interface units communicate, through trunk 112 and coupling 111, with HFC node 160. HFC node 160 then communicates with head end 150, which is usually at the CATN provider. HFC node 160 comprises a diplex filter 120, a noise effect reducing mechanism 190, a transmitter 140, and a receiver 145. Noise effect reducing mechanism 190 comprises a spectral splitter 125, a notch filter 130, a signal shaper 135, and a coupler 148, and is a device that reduces the effects of noise on a light transmitter by limiting or reducing the power of noisy frequency ranges or notch filtering small frequency bands. Receiver 145 receives light transmissions from head end 150, through a fiber optic cable, and converts the light to an electrical signal 113. This electrical signal 113 is then coupled to diplex filter 120. Diplex filter 120, as is known in the art, essentially assigns the downstream data stream 113 and the upstream data stream on coupling 111 two different, non-overlapping frequency bands. By "non-overlapping," there will be some overlap, as is known in the art, but this amount of overlap is held to a reasonable level. Both frequency bands are joined into one large frequency band. Thus, the upstream and downstream data share coupling 111 and the connections to the customer interface units. The upstream data stream from trunk 112 is split by diplex filter 120 into data stream 114, while the downstream data stream 113 is placed onto coupling 111. Light transmitter 140 takes the electrical upstream signal 131 from coupler 148, uses this signal to modify a Direct Current (DC) bias current, converts the modulated signal to light, and transmits the modulated light through fiber optic network 141 to head end 150. Light transmitter 140 is an electro-optic device that includes a laser diode, laser, or other light-emitting device. In current HFC nodes, the output 114 of diplex filter 120 is coupled to the input 131 of light transmitter 140. Output 114, therefore, should be of a certain power such that coupling it to the input 131 of light transmitter 140 will not over-drive light transmitter 140 or cause the light transmitter to clip. Without noise, this would generally be the case, as the customer interface units 105 - 110 should be designed to send signals that are beneath a certain power level. However, the customer interface units usually pass and amplify noise from various sources in and around the customers' homes. Therefore, in prior art systems, the power of the noise associated with each customer interface unit can cause various effects for the light source in light transmitter 140. When the transmitter attempts to modulate signal 131, the often-powerful noise on the signal can cause clipping of the light transmitter when the power of the noise (which corresponds to the modulating current added to the DC bias current for the light transmitter) drives the light output below the threshold current of the light transmitter. When the signal is clipped, all of the information in all of the frequency ranges is essentially lost. This occurs because, while there still may be modulation on the modulating signal, the modulating signal drives the current so low that any modulation may still be below the threshold current. Additionally, over-driving of the light transmitter can occur when noise causes the modulating current to rise to a very high level. While over-driving the light transmitter does not have as deleterious an effect as clipping by the light transmitter, over-driving the transmitter can cause other errors. Because the light transmitter can be a non-linear device, particularly when producing high power, the transmitter can cause distortion. One characteristic of the ingress noise from many consumer's homes is that most of the ingress noise caused by random noise exists in one frequency band. HFC node 160, as shown in Fig. 1, has been modified in accordance with the present invention to make use of this important realization. In accordance with the present invention, a spectral analysis was performed that determined that the major source of random ingress noise was the frequency band from 5 - 15 MegaHertz (MHz). The other portion of the frequency spectrum, the 15 - 65 MHz frequency band, on upstream data stream 114 has much less random ingress noise. Accordingly, noise effect reducing mechanism 160 uses a spectral splitter 125 that splits the spectrum into these two bands. The spectral splitter may comprise a low-pass filter and a high-pass filter to create two outputs, or comprise any number of low-pass, bandpass, or high-pass filters that each create an output for its respective frequency range. Additionally, the spectral splitter may simply let the entire spectrum pass in a second frequency band, then use a filter to filter out the relatively non-noisy spectrum (allowing the relatively noisy spectrum to pass) to produce a first frequency band. This would allow spectral shaping on the first frequency band, which can then be subtracted from the second frequency band. Particular embodiments of the noise effect reducing mechanism 160, spectral splitter 125, and signal shaper 139 will be discussed in FIGS. 3 through 6. In the preferred embodiment of Fig. 1, the 5 - 15 MHz frequency band of signal 134 is coupled to the signal shaper 135, while the 15 - 65 MHz frequency band of signal 132 is applied to notch filter 130. The filtered signal 134 and shaped signal 139 are joined again at coupler 148 to create a composite signal 131 that the light transmitter 140 uses to modulate a light source internal to the transmitter. Coupler 148 is a device that can add the filtered and shaped signals or can subtract one signal from the other. Notch filter 130 is an optional device that beneficially filters the low-noise frequency spectrum to remove frequency-specific noise that may exist in the upper frequency band. Notch filter 130 could have a multitude of notch frequencies, like a comb filter, or could have only one notch frequency. The implementation of notch filter 130 depends on the frequency being notched and how well the notch filter can remove only this frequency. Notch filters are well known in the art. Notch filter 130 is designed to filter out the specific frequencies that have been determined in a spectral analysis to be relatively noisy. By performing a spectral analysis and determining the most likely areas of frequency-specific noise, the current invention can better reduce effects due to locally generated frequency- specific noise. Alternatively or in addition to the latter, well known sources of frequency- specific noise, such as that generated by certain highly used Citizen's Band (CB) radio channels, cellular or cordless phones, garage door openers, etc., may be filtered by notch filter 130. Because the noisier frequency spectrum in signal 137 has been determined to have a majority of the random noise in the upstream data stream 114, this spectrum is applied to a signal shaper 135. Signal shaper 135 is a device that, in combination with coupler 148, limits or reduces the power in the lower frequency band. Signal shaper 135 shapes the frequency spectrum on signal 137 such that the shaped frequency signal 139, in combination with signal 134 (notch-filtered or not notch-filtered), will reduce the excessive random noise power in the frequency spectrum. In one embodiment, the signal shaper limits power in noisy frequency spectrum of signal 137, but limiting the amplitude of the signal or by reducing the overall power of the signal. In another embodiment, the signal shaper may also include one or more matched filters, other receiver mechanisms, and transmission mechanisms that allow the shaped signal to be received and then transmitted. In yet another embodiment, the signal shaper can pass high power frequency ranges in the noisy frequency band of signal 137, to be then subtracted from the less noisy frequency band of signal 134 having all frequency components. The signal shaper may perform its functions in the analog or digital domain. Specific embodiments of signal shaper 135 and noise effect reducing mechanism 190 will be discussed in reference to Figs. 3 through 6, but any mechanism capable of limiting or reducing the power in particular frequency bands or spots in a frequency spectrum such that the optical transmitter does not clip the signal resulting from the power-limited frequency bands or spots is applicable to the current invention. The signal shaper 135, thus, prevents noise in the noisy frequency band of signal 37 from creating clipping errors and over-driving errors in transmitter 140. The signal shaper 135, spectral splitter 125, and coupler 148 are designed such that the power in the first frequency band is reduced or limited if the power is greater than a predetermined limit. While the second frequency band of signal 132 still may have random noise on it, the spectral analysis did not indicate a high enough level of random noise on the second frequency band of signal 132 such that the benefit of adding the additional complexity of the signal shaper would outweigh the cost of and non-linearity caused by the signal shaper. It should be noted that shaping the signal 137 that carries the first frequency band does lead to distortion — which could be severe — of this frequency band, and, thus, to information loss in this band. Under the present invention, this information loss only occurs in the first frequency band of signal 137 instead of in the entire frequency spectrum. Because signal 114 should always be beneath a certain power or amplitude, the signal shaper should be active only when there is a burst of noise in this frequency band that has already destroyed the data in the band. Therefore, the inclusion of the non-linearity and distortion caused by shaping the signal 137 introduces minimal or no additional errors because the data in the frequency band was already destroyed by ingress noise. The current invention parlays the fact that, in many HFC systems, there is one or a few frequency ranges that contain a vast majority of the random noise that causes clipping and data transport loss. By using this property of HFC systems, the current invention limits power in random noise frequency ranges while letting the generally non-random-noise frequency ranges through. Although Fig. 1 separates the upstream frequency spectrum into first and second frequency ranges, the upstream frequency spectrum could be separated into multiple frequency ranges, some of which are passed (or put through a notch filter) while other bands are coupled to one or more signal shapers. Additionally, the choice of which frequency ranges of the system are power limited by the signal shapers is preferably determined by the spectral analysis performed in accordance with Fig. 2. Turning now to Fig. 2, Fig. 2 illustrates a preferred method 200 used to determine noisy frequency bands and to shape or notch-filter the noisy frequency bands. A design engineer can use method 200 to create an apparatus similar to or exactly like the HFC node of Fig. 1 to limit noise-induced data transport loss and clipping. Method 200 is preferably split into two parts, parts 210 and 220. Part 210 is a portion of the method that preferably occurs during the design stages of a project. Part 220 is a portion of the method that preferably is performed by an apparatus similar to that shown in Fig. 1. During design part 210, the noise analysis of the spectra and the design of the apparatus used to shape or notch filter the spectra based on the noise analysis are performed. Note, however, that portion 220 and apparatuses based on this portion can be used or placed into service without performing portion 210, as the noise associated with HFC networks is likely very similar in many areas where HFC networks are installed. Portion 210 does provide a more in-depth analysis and will yield a better, and generally cheaper, solution to the noise problem. Method 200 begins when a designer performs a spectral analysis on an HFC network (step 230). This spectral analysis entails determining the frequency ranges that have the highest power (step 240) by statistically examining the frequencies that appear in an HFC network in one or more customers' homes over a relatively long time period. Because the power for all frequency ranges should be beneath a certain limit so that the light transmitter does not clip, the frequency ranges with powers higher than the limit should be caused by ingress noise. The spectral analysis is preferably performed on-site (in one or more customer's homes) and over an extended time period. These preferable considerations allow the test to be performed under real-world situations, and should allow a better analysis of ingress noise. Note also that the spectral analysis can be performed on a very fine frequency spectrum basis, if desired. In other words, the finer the gradations into which the entire frequency spectrum is split, the finer the determination of ingress noise will be. Performing the spectral analysis thusly allows particular small ranges of noisy frequencies to be notch filtered, if desired. Using the results of this analysis, a designer can design an HFC node that splits the frequency ranges into frequency ranges having high random noise (called "high- noise" frequency ranges herein) and frequency ranges having a low amount of random noise (called "low-noise" frequency ranges herein). Additionally, a designer can choose to notch filter specific noisy frequencies in the low-noise frequency ranges. The step of designing a node may be added as step 245 to method 200, if desired. A designer takes the entire frequency spectrum for the upstream HFC signal and splits it into single or multiple low- and high-noise frequency spectra (step 250). Note that this step is also performed real-time, in that an apparatus similar to that used in Fig. 1 would also be used to split the entire frequency spectrum into single or multiple low- and high-noise frequency spectra. The power is then determined in all of the high noise spectra (step 260). Determining the power for a frequency spectrum (and a signal) is well known to those skilled in the art. The power density may be determined by determining the amplitude of a signal at a frequency (or over some small frequency interval), or, if using digital techniques, through a number of well known algorithms. As an example of determining power, a detector diode could be used to determine the power in a frequency range. Finally, the power is limited in the high noise spectrum (step 270). Various mechanisms for limiting power are shown in Figs. 3 - 6, but there are additional devices that may be used. For example, automatic gain control could be used to reduce the power in the frequency band that has been determined to be relatively noisy. However, because the frequency band would likely be corrupted with noise when an automatic gain control would reduce the overall power in this frequency band, the automatic gain control would be effectively passing the noise-corrupted signal to the light transmitter. While this would limit clipping and over-driving of the light transmitter because the power in the signal would be correct for modulation, there may not be very much gain in performance of the system over the mechanisms used in Figs. 3 - 6. This is because it is unlikely that a receiver at the head end (shown in Fig. 1 as element 150) would be able to retrieve the data that was sent because of the high level of noise in the signal. Although it is unlikely that the receiver at the head end can recoup the data, it is possible. Because of this possibility, once the signal has been split into two frequency bands (noisy and less noisy), the bands may each be given a different emphasis (gain). In this manner, the Signal-to-Noise Ratio (SNR) of the composite signal, after both emphasized frequency bands have been recombined, could be adjusted. This would allow the less noisy frequency bands to be increased in power at the expense of the noisier frequency bands. The original signal's Carrier-to-Noise Ratio (CNR) could be, thus, optimally adjusted to the light transmitter's CNR. In general, the power will be determined (step 260) and limited (step 270) at the same time and by the same device. For instance, a hard clipper essentially limits the amplitude of the signal. The hard clipper, therefore, determines that there is high power at the same time that it limits the power. Similarly, devices that automatically adjust gain determine the power at about same time that they limit power. This does not always have to be the case, however. For example, if using digital signal processing techniques, a Fast Fourier Transform (TFT) could be run that determines the amplitudes of various frequency components. Another processing step could then reduce the amplitudes of frequency components to a predetermined level such that the light transmitter will not clip. What is important in steps 260 and 270 is that the power transferred to the light transmitter after the steps are performed be of a value that limits clipping of modulated light by the light transmitter. If performed correctly, the steps may also eliminate clipping due to noise on frequency bands that are being power limited. However, if the light transmitter can "soft clip," it may be beneficial to allow some small amount of distortion if there is a possibility of retrieving the data in the light stream when it is received at the head end. Because the noise generally destroys the data, steps 260 and 270 will usually be designed to prevent clipping of the light transmitter. Furthermore, simple gain control that decreases the gain of the high noise frequency band while increasing the gain of the low noise frequency band may not prevent clipping of the light transmitter, but should reduce clipping of the light transmitter. Even automatic gain control may not completely eliminate clipping of the light transmitter, but should reduce clipping of the light transmitter. Method 200 may also be applied to specific small frequency bands of high noise concentration. In this embodiment, the relatively low noise spectrum (or spectra) may still have these small pockets of noise. Consequently, notch filters that are designed to reduce the pockets of noise may be added using the method of Fig. 2 to the low noise spectrum to further improve the noise immunity of the system. Even without adding notch filters to the low noise frequency ranges, nodes designed with the current invention should still yield better noise immunity because the majority of the random noise should be in the predetermined, relatively high-noise spectra that are than shaped to reduce the effects of random noise. Additionally, because the majority of the noise is in one spectrum or a few spectra, the main noise contributions to loss of data transport and clipping should be reduced. This is particularly true for most HFC networks, as the majority of ingress noise occurs in the 5 to 15 MHz band. By limiting the power of this band, a large portion of the loss of data transport and clipping should be reduced. Thus, what has been shown so far is a method and apparatus for reducing errors in HFC networks due to ingress noise on electrical signals that primarily are generated in consumer's homes. The following Figs, depict several preferred embodiments for the noise effect reducing mechanism 190, and in particular signal shaper 135, of Fig. 1, although any mechanism that can reduce power in a spectrum may be used to meet the objectives of the current invention. Referring now to Fig. 3, one preferred embodiment of a signal shaper 135 is shown. Signal shaper 135 takes the noisier frequency spectrum of signal 137 and passes this spectrum through a power limiter 310. Power limiter 310 could be a hard-clipper or a soft- clipper, of a type known in the art. Additionally, as described above, power limiter 310 could adjust the gain of the noisier frequency spectrum to a reasonable level. Adjusting the gain provides the benefits of creating less distortion. However, the reaction time of gain control circuitry is generally slower than the reaction time of clippers. After passing through power limiter 310, the spectrum is preferably filtered again by filter 320 to reduce any potential frequency effects or distortion that are caused by limiting the power. Filter 320 should be around the same bandwidth as the frequency bandwidth to which signal 137 is limited. The resultant signal 139 is then added, through coupler 148, to the second frequency band of signal 134 to create composite signal 131. Power limiter 310 thus determines the power in signal 137 and then limits this power. It is important to note that the gain values applied to each frequency band and the predetermined value that is used as a clipping level for the noisy frequency band are calculated to limit the effects of noise on a light transmitter. In particular, clipping of the modulated light by the light transmitter is the main effect that should be reduced by the current invention, although other effects, such as over-driving the transmitter, will be reduced. These predetermined values depend on the specifics of the light transmitter's design that are not addressed here, but those skilled in the art can adapt the current invention for their particular light transmitter and its internal circuitry. What is important is that clipping of the modulated light transmitted by the light transmitters reduced. Referring to Fig. 1 in addition to Fig. 3, the 15 - 65 MHz frequency band of signal 132 is optionally notch filtered to become a second frequency band on signal 134. Power limiter 310 and filter 320 shape the 5 - 15 MHz frequency band corresponding to signal 137 into a shaped frequency band corresponding to signal 139. This frequency band is added, through coupler 148, to the 15 - 65 MHz frequency band corresponding to signal 132, and coupled to light transmitter 140 through composite signal 131. Referring now to Fig. 4, a preferred embodiment of a noise effect reduction mechanism 190 is shown. Noise effect reduction mechanism 190 in this embodiment still comprises spectral splitter 125 and signal shaper 135. However, in this embodiment, spectral splitter 125 comprises a splitter 405 that splits signal 114 into two signals 132 and 407, each of which carries the entire frequency spectrum carried by signal 114. The splitter may also have pre-amplification, before splitting, or post-amplification, after splitting, to reduce the power lost by the splitter. Filter 410 is a band-pass, low-pass, or high-pass filter that passes the noisier frequency spectrum (preferably as determined by method 200 of FIG. 2) of signal 137. In the example of Fig. 1, this relatively noisy spectrum is 5 to 15 MHz. Signal shaper 135 comprises a threshold device 420 and a filter 430. Threshold device 420 examines the signal for power levels above a certain threshold. Those frequencies that have magnitudes above a threshold are allowed to pass. The resultant signal is filtered again in filter 430 (which preferably passes the same or similar frequencies as filter 410 passes) to reduce or eliminate undesired frequency effects caused by threshold device 420. Resultant signal 139 is then subtracted from signal 132 by coupler 148. Threshold device 420, thus, shapes the high noise spectrum, carried by signal 407, by only allowing high amplitude frequencies to pass, and the noise effect reduction mechanism 190 limits the power in this spectrum by subtracting the powerful noise from the full spectrum. One type of threshold device that may be used is generally used as a noise suppression device, as the circuit allows only signal powers greater than the noise level to be passed. This device could be composed of two diodes in a parallel and opposed configuration. In other words, the incoming signal is connected to the P-type region of one diode and the N-type region of the second diode, and the outgoing signal is connected to the N-type region of the first diode and to the P-type region of the second diode. In the current invention, the circuit would be designed to pass noise that is greater than the signal strength should be. If desired, the diodes may be selected, or other elements added, to pass noise that is a certain level above what the signal strength should be. This would provide additional headroom, should the signal actually be out-of-limits for whatever reason. If desired, signal 132 can be notch filtered in the less-noisy frequency ranges. Also, delay elements may be added to signal 132 so that signal 132 and 139 reach coupler 148 approximately simultaneously. Referring now to Fig. 5, this Fig. shows a preferred embodiment of signal shaper 135. Signal shaper 135 comprises an analog-to-digital (A/D) converter 510, a signal processing mechanism 520, and a digital-to-analog converter 530. Signal processing mechanism 520 is a Digital Signal Processor (DSP) or other device able to mathematically determine the power in the high noise spectrum of signal 137 and to limit the power in the spectrum. Although a DSP is a relatively inexpensive mechanism for signal processing, any mechanism, including generic processors, application specific integrated circuits, programmable logic devices, etc., that is able to digitally limit power is applicable to the current invention. Determining the power of digital signals is well known in the art. For example, an FFT could be used to provide amplitude and phase information at various frequency points. The amplitude of these frequencies could be limited to a predetermined value. Other more complex methods may also be used that determine the power spectral density of the signal. There are a number of textbooks that describe power spectral density estimations in detail. Once it is determined that the power in the spectrum (or a sub-spectrum of the high noise spectrum) is above a threshold value, the magnitude in that part of the spectrum may be reduced or the signal filtered with a filter or window having a gain that is predetermined to reduce chances of clipping. Even simpler methods of reducing power may be used. Signal processor 520 could simply reduce power in the noisy frequency band by limiting each sample to a particular value. The benefit of using an FFT or a power spectral analysis is that specific noisy frequencies, rather than particular samples, may be targeted for power reduction. This may improve overall system performance, if the noise is cancelled with little or no destruction of the underlying data. These methods, however, come at an increased cost. In addition, it would be beneficial, after limiting the power in the signal corresponding the frequency band, to post-filter the signal to limit any frequency effects caused by the amplitude limitation. This post-filtering could be performed in the digital or analog band. In addition, blocks 510, 520, and 530 could be applied directly to signal 114 of Fig. 1 instead of signal 137 of Fig. 1. This embodiment would allow the entire frequency band to be examined. If this embodiment is used, because of the non-linear transformation caused by the power limiting operation, it is still recommended that the power be limited in only the relatively noisy frequency spectrum. As in the previous embodiments, the low-noise frequency band (shown as the frequency band of signal 134 in Fig. 5) may be notch filtered, as shown in Fig. 1, to further reduce the effects of noise on the light transmitter. If the signal processor is connected to signal 114, the signal processor can split upstream signal 114 into two frequency bands and perform notch filtering and signal shaping as these are described previously. Referring now to Fig. 6, Fig. 6 shows another preferred embodiment for signal shaper 135. In this embodiment, a power limiter 610 acts on the high-noise frequency spectrum of signal 137, as in Fig. 3. However, receiver 620 then acts to receive (by using matched filters, decision apparatuses, etc.), the signal. Transmitter 630 transmits the received signal again. This embodiment attempts to recover the original information in the signal, then re-send the original information again. If desired, power limiter 610 may be removed from signal shaper 135. The embodiment of the signal shaper that is shown in Fig. 6 is the least preferred embodiment, because this embodiment requires knowledge of the signal being transmitted. In other words, to receive and then re-transmit the signal, the modulation schemes and other necessary design criteria that are needed to be able to perform these functions must be known. In the other embodiments, only a minor amount of information (such as the bandwidth of the signal) need be known, as the power in predetermined frequency ranges are all that is being used to perform these embodiments. Turning now to Fig. 7, this Fig. illustrates another embodiment of an HFC system. In HFC system 700, an A/D converter converts combined signal 131 to a digital signal 720, which is then fed to light transmitter 140. Light transmitter 140 is usually a laser, but can be any light transmitter known to those skilled in the art. Light transmitter 140 uses the digital signal to modulate a light signal on fiber optic network 141. The light signal is usually a series of digital "bits," such that a certain power or level of light indicates a one, and a different power or level of light indicates a zero. HFC system 700 is still susceptible to ingress noise, as the bits of information being produced will be incorrect if the analog input signal (combined signal 131 in this case) is corrupted with noise. As such, the noise effect reduction mechanism 190 may be any of the previously discussed embodiments, and the general embodiment of Fig. 1 is shown. Furthermore, the signal 114 from diplex filter 120 could be coupled directly (not going through noise effect reduction mechanism 190) to A/D converter 710. This would allow digital signal 720 to be put into a digital signal processor, which would perform the power limiting or reducing functions of noise effect reduction mechanism 190. However, performing the power limiting or reducing functions in the analog domain has the benefit of allowing a relatively cheaper A/D converter to be used, as there will be less dynamic range needed. As the previous Figs, illustrate, the current invention provides notch filtering of a frequency spectrum, which is primarily affected by frequency-specific noise, and signal shaping of a frequency spectrum, which is primarily affected by random noise, to minimize the effects of these noises on a light transmitter. By limiting the effects of both frequency- specific and random noise on light transmitters, less data transport loss should occur in mixed electrical and optical networks, and particularly in HFC networks. Even if only the signal containing the noisy frequency spectrum is shaped, the current invention should greatly reduce loss of data transport and clipping errors in a light transmitter due to the realization that much of the noise in an HFC system exists in one or a few frequency bands. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims: CLAIMS: 1. An apparatus for reducing the effects of noise on a light transmitter (140), the apparatus comprising: • a spectral splitter (125) suitable for receiving an electrical signal, the spectral splitter (125) splitting the electrical signal into a first signal (137) having a first frequency band and a second signal (132) having a second frequency band; • a spectral shaper (135), coupled to the first signal (137), that shapes the first signal (137) to produce a shaped signal (139); and • a coupler (148) coupled to the shaped signal (139) and to the second signal (132), the coupler (148) combining the shaped signal (139) and the second signal (132) to produce a composite signal suitable for coupling to a light transmitter (140), wherein the spectral shaper (135), spectral splitter (125), and coupler (148) are designed such that the power in the first frequency band is reduced or limited if the power is greater than a predetermined limit. 2. The apparatus of claim 1 wherein the first frequency band comprises 5 - 15 MegaHertz (MHz) and the second frequency band comprises 15 - 65 MHz, and wherein the apparatus is coupled to a light transmitter (140) in a Hybrid Fiber Coax network. 3. The apparatus of claim 1 wherein the spectral shaper (135) comprises a power limiter (310) and a filter (320), the power limiter (310) limiting power by limiting amplitudes of the first signal (137). 4. The apparatus of claim 1 wherein the spectral splitter (125) further comprises a first filter (410) coupled to the first signal (137) and filtering the first signal (137) to create the first frequency band within the second frequency band, the spectral shaper (135) comprising a threshold device (420) coupled to a second filter (430) and to the first filter (410), the threshold device (420) passing amplitudes of the first signal (137) that are above a second predetermined limit, the second filter (430) passing the first frequency band, the coupler (148) subtracting the shaped signal (139) from the second signal (132). 5. The apparatus of claim 1 wherein the spectral shaper (135) comprises an Analog-to-Digital (A/D) converter (510) coupled to a signal processor (520) that is coupled to a Digital-to- Analog converter (530). 6. The apparatus of claim 5 wherein the A/D converter (510) produces digital samples and wherein the signal processor (520) reduces amplitudes of the digital samples if the digital samples are above a certain value. 7. The apparatus of claim 1 wherein the spectral splitter (125), spectral shaper (135), and coupler (148) are performed by a signal processor (520). 8. The apparatus of claim 1 further comprising a notch filter (130) that is interposed between the spectral splitter (125) and the coupler (148), the notch filter (130) coupled to the second signal (132) and the coupler (148), the notch filter (130) attenuating the second signal (132) at least one frequency. 9. The apparatus of claim 1 wherein the spectral shaper (135) is a gain control that reduces the gain of the first signal (137). 10. The apparatus of claim 9 wherein the gain control reduces the gain of the first signal (137) a predetermined amount and wherein the apparatus further comprises a second gain control interposed between the spectral splitter (125) and the coupler (148), the second gain control coupled to the second signal (132) and the coupler (148), the second gain control increasing the gain of the second signal (132) a predetermined amount. 11. The apparatus of claim 9 wherein the gain control automatically adjusts the gain of the second signal (132) to a predetermined level. 12. A Hybrid Fiber Coax (HFC) node (160) comprising: • a light transmitter (140); • a spectral splitter (125) suitable for receiving an electrical signal, the spectral splitter (125) splitting the electrical signal into a first signal (137) having a first frequency band and a second signal (132) having a second frequency band; • a spectral shaper (135), coupled to the first signal (137), that shapes the first signal (137) to produce a shaped signal (139); and • a coupler (148) coupled to the shaped signal (139) and to the second signal (132), the coupler (148) combining the shaped signal (139) and the second signal (132) to produce a composite signal that is coupled to the light transmitter (140), wherein the spectral shaper (135), spectral splitter (125), and coupler (148) are designed such that the power in the first frequency band is reduced if the power is greater than a predetermined value. 13. The HFC node of claim 12 wherein the predetermined value is designed to reduce clipping of modulated light produced by the light transmitter (140). 14. The HFC node of claim 12 further comprising an analog-to-digital (A/D) converter (710) interposed between the coupler (148) and the light transmitter (140), the A/D converter (710) coupled to the composite signal and converting the composite signal from an analog form to a digital form. 15. A method for reducing the effects of noise on a light transmitter (140), the method comprising the steps of: • splitting a signal into a high random noise frequency band and a low random noise frequency band; and • limiting or reducing the power of the high random noise frequency band if power in the high random noise frequency band surpasses a predetermined value, the predetermined value designed to reduce clipping of modulated light produced by a light transmitter (140). 16. The method of claim 15 further comprising the following steps: • performing a spectral analysis on the signal; and • determining the high random noise frequency band and the low random noise frequency band by determining high and low, respectively, signal strength areas from the spectral analysis. 17. The method of claim 15 wherein the step of limiting or reducing the power in the high random noise frequency band comprises the step of limiting amplitudes of the signal. 18. The method of claim 15 further comprising the step of notch filtering the low random noise frequency band, the notch filtering attenuating the signal at least one frequency. 19. The method of claim 15 further comprising the step of determining the power in the high random noise frequency band. 20. The method of claim 15 wherein the high random noise frequency band has a gain, and wherein the step of limiting the power of the high random noise frequency band comprises the step of reducing the gain of at least a portion of the high random noise frequency band. 21. The method of claim 20 wherein the low random noise frequency band has a gain, and wherein the method further comprises the step of increasing the gain of the low random noise frequency band.

Inventors: JANSEN VAN DOORN ARNOLD K, MUTALIK VENTAKESH, SCHEMMANN MARCEL F C, JANSEN VAN DOORN, ARNOLD, K., MUTALIK, VENTAKESH, SCHEMMANN, MARCEL, F., C.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2001
Main classification: H04B10
IPC codes: H04B10