MXPA97005532A - Method for attenuating signals asimetrically in a transmis system - Google Patents

Abstract

The present invention relates to a method for asymmetrically attenuating the signals in a transmission system. the input of noise in a (10) hybrid coaxial fiber transmission system can be reduced by using asymmetric taps (301 - 308 ') between a pair of line extenders (28-28) to attenuate downstream and upstream information for different weights. The downstream steps of the branches are selected to achieve a substantially constant energy level in each branch for the individual subscribers (14 -14). The upstream attenuation of each branch is selected to allow an almost constant level of information generated by the subscriber to be received on the outside of the line, while the upstream information generated by the subscriber is supplied to the branches at a constant value which is maximized to reduce the income effect of the rui

Description

METHOD FOR ASYMMETRALLY ATTENUATING SIGNALS IN A TRANSMISSION SYSTEM Technical Field This invention relates to a technique for attenuating downstream and upstream signals in a hybrid coaxial fiber system by different weights to reduce noise ingress.

BACKGROUND OF THE INVENTION In today's hybrid coaxial fiber transmission systems, downstream information intended for individual subscribers originates at one head end. From the head end, the information downstream is commonly formatted optically for transmission over a fiber optic link to a fiber node in which the information is then converted to an electrical signal. A network (plant) of coaxial cable transmits the electrical signal to the individual subscribers and carries signals upstream of the subscribers to the fiber node for its final supply to the head end. In the coaxial cable based plant, the downstream electrical signal is normally amplified by one or more amplifiers of the main line and one or more line extenders before distribution via leads to the subscriber's premises. (The upstream signals are similarly amplified by amplifiers and extenders of the main line upstream.) The energy of the downstream signal received at the main REP: 24744 upstream.) The energy of the downstream signal received at the facilities is reduces by the loss in the cable, also as the division of inherent energy in each derivation. For example, a referral that serves two households will divide the energy 2: 1 while a derivation that serves four households will divide the energy 4: 1. To achieve an adequate energy level in each installation, the weight of the shunt (that is, the level of attenuation provided by each shunt) is selected to obtain approximately the same loss in all households. From here, the derivation weights should decrease according to the distance that the downstream signal travels from the amplifiers and extenders of the downstream line due to the increased cable losses plus the decreased energy level caused by the energy dissipated in the the previous derivations. Equal losses and consequently equal energy levels received for each home ensure a signal high enough to overcome any noise that may be present in the home. In today's hybrid coaxial fiber systems, shunts have symmetrical losses. In other words, the downstream signals that pass to the subscriber's premises are attenuated by each branch to the same degree (except for small variations due to their different frequency) as upstream signals received in each branch of the subscriber's premises. As it is desirable to achieve a uniform signal level for the downstream signals at the subscriber's premises, it is also desirable to achieve a uniform level for the upstream signals received at each upstream amplifier in the cable plant. Since the weights of the leads are fixed, the levels of the signals emanating from the equipment of the customer premises (CPE) at the subscriber's premises must be varied to ensure that the upstream received signals have all approximately the same level in a common upstream amplifier. Hence, the subscriber's installations closest to the upstream amplifier must have the highest CPE output level to compensate for the highest derivation value. Conversely, subscriber facilities farthest from the upstream amplifier will generally have the lowest signal level. In practice, derivation weights vary in 3 dB increments. Thus, signal levels Actual received at the various facilities of the subscriber may vary somewhat from a uniform desired level. The use of today's symmetrical branches incurs a difficulty associated with the noise input of each subscriber installation and its associated cable drop through which the facilities are connected to the cable plant. The noise that enters the most distant branch of the upstream amplifier has a much greater effect on the operation due to the low loss associated with this remote branch. At the same time, the level of the CPE output signal generated by the subscriber entering the most distant lead is lower than the nearest leads, which makes this ratio lower signal to noise. Thus, there is a need for a technique to reduce the input of noise in a hybrid coaxial fiber system.

BRIEF DESCRIPTION OF THE INVENTION Briefly, in accordance with the invention, a technique is provided for transmitting downstream signals through a cable distribution network to a plurality of subscribers and for transmitting upstream signals from the subscribers through the network of distribution with reduced noise input. The downstream signals are distributed to the individual subscribers by means of leads which are arranged in a cascaded manner along at least one cable in the network with each successive branch which generally has a smaller downstream attenuation weight. than its neighbor upstream to attenuate the downstream signals. The upstream signals are generated by the subscribers at a substantially equal power or energy level and pass via the leads to the distribution network. The taps, in the aggregate, attenuate the upstream signal received by a different amount than the aggregate attenuation provided by the taps for the downstream signals. The attenuation provided by each of the derivations is adjusted in such a way that the level of the upstream signals received in each amplifier upstream in the distribution network are substantially equal, to thereby minimize the effects of noise ingress. This method results in highly uniform transmission levels for all subscribers and increased attenuation to the noise income of the distant subscribers.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a hybrid coaxial fiber transmission system according to the prior art; Figure 2 shows a portion of the transmission system of Figure 1, showing the downstream attenuation weights of each of a set of leads within the system; Figure 3 shows the same portion of the transmission system as Figure 2, which indicates the upstream attenuation weights for the leads; Figure 4 is a block diagram of an asymmetric shunt according to the invention; Figure 5 is a schematic block diagram of an asymmetric termination shunt according to the invention; Figure 6 shows a modification of the transmission system of figure 1, according to a first embodiment of the invention, wherein the asymmetric branches have been replaced by the symmetrical branches of figure 1; and Figure 7 shows a modification of the transmission system of Figure 1, according to a second embodiment of the invention, wherein the asymmetric branches have been replaced by the symmetrical branches of Figure 1.

DETAILED DESCRIPTION Figure 1 shows a hybrid coaxial fiber transmission system 10 according to the prior art. The system 10 includes a head end 12 for generating information, for example video, audio, multimedia, data and / or text ("downstream information") for transmission to the individual subscribers 14-14. The leading end 12 also receives information ("upstream information"); for example, video, audio, multimedia, data and / or texts; generated by one or more of the subscribers 14-14. In practice, a combination of optical fibers and coaxial cables carry the information downstream from the head end 12 to the subscribers 14-14 and carry the information upstream from the subscribers to the head end. As seen in Figure 1, a fiber optic link 16, consisting of upstream and downstream fibers (not shown), carries the upstream and downstream information in an optical format between the head end 12 and a node 18 fiber. The fiber node 18 converts the downstream information with optical format received from the head end 12 into electrical signals for distribution via a coaxial cable distribution network 20 to the individual subscribers 14-14. further, the fiber node 18 converts the upstream information received from the subscribers 14-14 via the coaxial cable distribution network 20 to an optical format signal for transmission to the head end 12. Note that the upstream and downstream signals could pass electrically between the head end 12 and the network 20 via the coaxial cable, instead of obtaining an optical format for its passage via the fiber optic link 16. The cable distribution network 20 has a tree and branch architecture and commonly includes at least one and typically four coaxial main line cables 22-22 (only one is shown). Each of the main line wires 22-22 commonly has a plurality of amplifiers 24-24 of the main line branched in cascade along its length to amplify upstream information and downstream information. (In practice, each of the branched main line amplifiers consists of individual amplification elements and diplexers (not shown) that separately amplify the upstream information and the downstream information.) Each amplifier 24 of the branched main line feeds to one or more distribution cables 26-26. Each distribution cable 26 commonly has one or more line extenders 28-28 cascaded along its length to amplify the upstream and downstream information carried by each distribution cable. Each distribution cable 26 contains a plurality of leads 31? -30a arranged in cascade fashion between pairs of line extenders 28-28 (only leads 30? -308 are shown in Fig. 1.) It should be understood that a number Major or minor derivations may be present. The leads couple the distribution cable 26 to a plurality of subscribers 14-14 via individual coaxial drop cables 32-32 connecting a subscriber to a branch via a separate unit of the network interface units (NIU) 34-34 . Figure 2 shows a portion of a distribution cable 26 of the network 10, showing a series of cascade branches 301 -308 successively attenuating the signal generated by a first pair of downstream line extenders 28-28 (only shows one). The weights (this is the attenuation) of leads 30? -308 generally decrease in succession according to the distance of the branch from the line extender 28 in FIG. 2. Thus, the branch 30? which is the closest to the line extender 28, has a weight significantly greater than the weight of the branch 308 which is furthest from the line extender. The weights of the shunt are selected or chosen to decrease successively as a function of the distance of the shunt from the line extender 28 to ensure a substantially uniform output level in each shunt. There are several factors that influence the selection of derivation weights. The branched distribution cable 26 along which the branches 30, -308 of Figure 8 are arranged in cascade has a certain loss per unit length. Thus, to ensure approximately the same level received in the upstream line extender of each of the leads 30? -308, it follows that lead 308 must have a lower weight than lead 30? Also, each of the derivations 30? -308 has a certain loss associated with the passage of signals through it. Since the derivations 30? -308 are arranged in cascade, the loss of aggregate derivation, as measured in derivation 308, will be the sum of the signal losses through the preceding derivations. For this reason also, the weight of the branch 308 must be less than the weight of the branch 30-? . In the illustrated embodiment of FIG. 3, the loss of distribution cable 26 at 750 MHz is assumed to be -1.5 dB between successive pairs of leads 30? -308 which are supposed to be equally spaced. further, are derivations 30 supposed? -308 have losses as indicated by the values shown in Fig. 2. Under these conditions, a derivation output level of approximately 18 ± .1 dB for each subscriber 14-14 can be achieved by an output value of the extender 28 of line of +45 dB by selecting the derivation weights as indicated in Table 1. TABLE 1 Derivative No. Weight of Derivation 30? 26 dB 302 23 dB 303 20 dB 30 17 dB 305 14 dB 306 11 dB 307 8 dB 308 4 dB Derivations of the prior art 30? -308 are symmetric in terms of the attenuation that each provides to the upstream and downstream signals. In other words, each branch attenuates the upstream and downstream signals by the same weight, since the branches are symmetric in terms of their attenuation, reaching a substantially constant level for the upstream signals received in the line extender 28. that the level of the upstream signals provided by the subscribers are successively smaller for the remote leads. This can be understood by reference to Figure 3, which shows the weights upstream of the leads 30? -308 cascaded Assuming 30 derivations? -308 of Figure 3 have upstream weights as given in Table I, then, in order to achieve a substantially constant upstream signal level in the line extender 28 for the cable and the bypass losses previously with respect to to table I, the CPE level of the signal input to each of the branches 30? -30 & it must be chosen as indicated in table II.

Table II Number of the derivation Level of entries CPE 30! 45 dB 302 42.8 dB 303 40.6 dB 304 38.4 dB 305 36.2 dB 306 34.0 dB 307 31.8 dB 30ß 28.6 dB As can be seen from Table II, the CPE level input to the more distant branch 308 is lower, due to its relatively low weight, compared to the level of CPE required in branch 30? which is closest to the line extender 28.

The current method of using symmetrical taps 30 308 incurs a difficulty with respect to the noise input in the facilities of each subscriber 14 (FIG. 1) also as the associated drop cable 32 (FIG. 1). The noise entering a remote branch, such as branch 30-308 of FIGS. 2 and 3, will have a much greater influence on the operation of the overall system because the weight of the remote branch is relatively small. Conversely, the entry level of the CPE to such distant derivation is low. Thus, the input noise will have a greater impact for this reason as well.

According to the invention, the input noise problem can be reduced by making each asymmetric tap, such that the weight of each tap is different for the upstream and downstream signals. As will be discussed in more detail below, making each asymmetric derivation allows the CPE values introduced to the leads to be held at a relatively high constant value, thereby minimizing the effects of noise input.

Figure 4 illustrates an asymmetric derivation 30 according to the invention for substitution in place of the bypass 30? of Figure 1. (Another asymmetric through-lead, each of a construction similar to lead 30?, would be substituted for leads 30? -308 of Figures 1-3). Is reference made to the asymmetric derivation 30? of Figure 4 as a through-shunt because it functions to attenuate the signals passing between a pair of gates 32 and 34, respectively, in contrast to a determination shunt, as further described with respect to Figure 5, which terminates the signs With reference to Figure 4, the high frequency signals originating at the head end 12 of Figure 1, enter the lead 30? ' in gate 32 and exit via gate 34, while low frequency signals from a downstream bypass enter via gate 34 and exit via gate 32. One pair of AC induction coils 35 (alternating current) and 36 are coupled in series between the gates 32 and 34 of the branch respectively to filter the low frequency energy signals that share the distribution cable 26. A coupler 38 is interposed between the induction coils 35 and 36 and serves to remove or remove a small portion of the high frequency downstream signals entering the bypass via the gate 32 to provide such signals to one or more of the installations 14-14 of the subscriber. In addition, the coupler 38 also serves to inject low frequency signals into the cable 26 for its passage to the head end 12 of Figure 1. The coupler 38 is generally directional, such that upstream signals entering the coupler pass through. to the distribution cable 26 but they are attenuated substantially in the direction away from the head end.

A coupling loss element 40 couples the coupler 38 to a filter assembly 42 consisting of filters 44? and 442 diplexers upstream and downstream the diplexer filters 44? and 442 serve to separate the high-frequency downstream signals (50-750 MHz) along a high-frequency path (H) and the low-frequency upstream signals (5-45 MHz) along a path low frequency (L). The low frequency path of the diplexer filter 441 is coupled via an upstream loss element 46 to the low frequency path of the diplexer filter 442. In contrast, the high frequency trajectories of the filter 44? and 442 diplexer are linked by a conductor 48 of substantially less loss.

The high frequency downstream signals extracted by the coupler 38 of the distribution cable 26 pass via the coupling loss element 40 to the filter 44? upstream diplexer. The high frequency downstream signals are separated by the filter 44? diplexer and pass along its high frequency path to filter 442 downstream diplexer via conductor 48. The high frequency downstream signals received in downstream diplexer filter 442 are likewise divided by a divider 50 for distribution to one or more subscribers in the scheduling gates 52-52 of the subscriber. As can be seen, the high frequency downstream signals extracted from the distribution cable 26 and emitted in the bypass gates 52-52 are attenuated according to the weight (impedance) of the coupling loss value 40 (also as by any parasitic loss on coupler 38 and diplexer filters 44? and 442).

The low frequency upstream signals received at the bypass gates 52-52 are separated by the downstream diplexer filter 44 and pass along the low frequency path of the filter via the upstream loss element to the filter 44, diplexer and from that filter to coupler 38 via loss element 46. As can be appreciated, the upstream low frequency signals are thus attenuated by the upstream coupling loss elements 46 and 44, respectively, (also as by any parasitic loss on the coupler 38 and the diplexers 44? And 442) . The upstream attenuation reached by branch 30? it will be at least as large as this, but larger than the downstream loss and can be adjusted independently by varying the value of the upstream loss element 46. If it is desirable that the derivation 30-? ' Asymmetric of Figure 4 will provide a higher downstream attenuation than the upstream attenuation, then a downstream loss element 49 (shown in broken lines) 49 could be replaced by the conductor 48. In some instances, it is desirable that the asymmetric shunt terminate the distribution cable 26, instead of passing the signals along it. Figure 5 shows a schematic block diagram of a shunt 300 * of asymmetric termination according to the invention. The branch 300, 'of asymmetric termination of Figure 5 is similar to the through branch 30?' and similar numbers have been used to identify similar elements. The main difference between the termination branches and the through branches 300, 'and 30?' of Figures 4 and 5 respectively, is that the termination branch has a single gate 32 and a single AC induction coil 35 (alternating current) coupled directly via the coupling loss element 40 to the diplexer filter assembly 42. In this way, the coupling loss element 40 in the branch 300, asymmetric through-hole terminates the distribution cable 26 of FIG. 5. With regard to the construction of the asymmetric through-shunts and termination 30 '? and 300 of Figures 4 and 5, other variants are possible. In the illustrated embodiments of FIGS. 4 and 5, the asymmetric through and termination leads 30 and 300, respectively, are configured as passive elements to reduce cost and space requirements. The asymmetric thru-and-through shunts 30? ' and 300 / could be easily implemented via active circuits to achieve lower losses or increased gain, as well as insulation between the gates. If derivations 30, 'and 300?' Asymmetric traverses and terminators are made up of active or passive elements, the derivations in combination have unequal upstream and downstream weights. Directional couplers could also be used to allow the creation of asymmetric tap losses in the upstream and downstream directions. The capacity of asymmetric through and asymmetric tap-offs 30 / and 300? ' to provide different weights upstream and downstream can be advantageously employed to reduce the noise input. This can be seen by reference to figure 6, which shows a series of asymmetric through branches in cascade 30, '- 308' replaced by the series of symmetrical branches 30? - 308 of figures 2 and 3. The capacity of asymmetric taps 30? - 308 of Figure 6 to provide different weights upstream and downstream allows the series of derivations to have their adjusted upstream weights to allow a CPE level of the constant subscriber (say +45 dB) introduced to each branch and still achieve a signal level upstream substantially constant at the input to the line extender 28. The advantage of adjusting the CPE levels of the upstream information supplied to the leads at a constant level allows the CPE level to be maximized for all subscribers, to greatly reduce the effect of noise ingress, especially for distant subscribers. As briefly indicated, with the symmetric derivations of the prior art the CPE levels of the subscribers must be varied, such that the more distant derivation transmits the lower CPE level. For the modality illustrated in Figure 6, the downstream and current weights above each of the derivations (where / is an integer) will be given by the ratios: Weight of the downstream derivation = At level 28 of the extensor of line - Signal received from desired CPE - Cumulative cable loss (up to the / th derivation) - Cumulative loss of the cumulative derivation (up to / -th - 1 derivation). Weight of the upstream derivation is equal to CPE transmission level - Desired received level - Cumulative cable loss (up to / -th derivation) - Cumulative derivative through loss (up to / - th - 1 derivation).

If the cable loss between the leads is -0.3 dB and the individual lead loss is -0.5 dB, then to achieve a CPE level on the line extender 28 of approximately 19 dB for a CPE transmission level of + 45 dB, the current weights for leads 30, '- 308' will be as indicated in Table III. TABLE III Derivation Loss of Derivation Level of CPE in the Loss of Derivation Current Down Extender 28 line Upstream , '26 dB 18.7 dB 26 dB 302' 23 dB 18.9 dB 25 dB 303 '20 dB 19.1 dB 24 dB 30' 17 dB 19.3 dB 23 dB 305 '14 dB 19.5 dB 22 dB 306' 11 dB 19.7 dB 21 dB 30 / 8 dB 19.9 dB 20 dB 308 '4 dB 19.1 dB 20 dB As compared to leads 30, -308 of Figures 1-3, leads 30? '-30s' of Figure 6 have significantly higher upstream weights, certainly, the upstream weight of each of the leads 30, '-308' of Figure 6 is at least as large (and in most cases greater) than its weight downstream. By providing each of the 30? '- 308' leads with a larger current weight helps reduce noise ingress. The large upstream weight of each shunt allows it to more effectively block the noise ingress than if the shunt had a low upstream weight as with the taps of the prior art 30? -308 of Figures 1-3. Digital signals in a hybrid coaxial fiber environment or environment are adversely affected by input noise and signal reflections attributable to a lower VSWR. The use of the asymmetric taps according to the invention reduces the noise input of the subscribers, while simultaneously reducing the reflections by increasing the VSWR in the 5-45 MHz bandwidth over which the upstream signals are transmitted. usually. The above describes a transmission system which uses asymmetric taps 30, '-308' to attenuate the upstream information by a different amount of the downstream information to allow upstream information to be transmitted at a constant level, to reduce the influence of upstream noise, particularly in distant leads. It will be understood that the embodiments described above are only illustrative of the principles of the invention. Various modifications and changes can be made to it by those skilled in the art which implement the principles of the invention and fall within the spirit and scope thereof.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following

Claims (13)

1. Claims 1. A method for transmitting information downstream through at least one cable in a distribution network to the individual subscribers and for transmitting information upstream of the subscribers or subscribers through the cable, characterized in that it comprises the steps of: supplying a downstream signal to the individual subscribers via a series of cascaded leads along the cable, each branch has a downstream attenuation weight successively in decrease than a neighboring branch current to attenuate the downstream signals, in such a manner that the downstream signal received in the most distant branch has substantially the same signal strength as the downstream signal received in the nearest branch; and attenuate, in each derivation, the upstream signal by an amount such that the derivations provide an aggregate upstream attenuation different to an aggregate downstream attenuation, the upstream attenuation of each derivation is set in such a way that the level of The information generated by the subscriber received in the distribution network is substantially the same for all subscribers.
2. 2. The method according to claim 1, characterized in that the attenuation upstream provided by each derivation is adjusted by subtracting, from a signal level upstream of the given subscriber, a desired level for the information generated by the subscriber within the distribution system , the loss of cumulative cable associated with the derivation and the losses of the cumulative derivation associated with the derivation.
3. 3. The method according to claim 1, characterized in that the attenuation upstream provided by each branch varies according to the distance of the branch of the distribution system, such that a more distant branch has a lower current attenuation than a branch closest.
4. 4. The method according to claim 1, characterized in that each branch attenuates the downstream information by passing such information through a coupling loss element and thereafter filtering such information downstream from the received upstream information before making pass such information downstream to the individual subscribers.
5. 5. The method according to claim 1, characterized in that each derivation attenuates upstream information received from the subscribers by first filtering such information from the downstream information, then transmitting such information through a loss element upstream and then through of the coupling loss element before transmission to the cable.
6. 6. The method according to claim 1, characterized in that a more distant derivation attenuates the information upstream by an amount greater than the information downstream.
7. 7. The method according to claim 1, characterized in that the nearest branch attenuates the information upstream by an amount approximately equal to the information downstream.
8. 8. A method for establishing an interconnection to the system via a derivation, in a distribution system for transmitting downstream signals to the subscribers and for transmitting signals upstream of the subscribers, the method is characterized in that it includes the attenuation step, via the derivation, the downstream and upstream signals by separate selectable attenuation values.
9. 9. The method according to claim 8, characterized in that it includes the step of separating the upstream and downstream signals in the shunt via a diplexer filter assembly based on the frequency to allow the upstream and downstream signals to be attenuated by Separate selection values.
10. 10, The method according to claim 8, characterized in that it further includes the step of deflecting the low frequency energy signals through the derivation.
11. 11. A method for establishing an interconnection to the system via successive derivations in a distribution system containing cables for transmitting downstream signals to the subscribers and for transmitting signals upstream of the subscribers, the method is characterized in that it includes the attenuation step, via each Bypass, the upstream signals via separate separate attenuation values of the downstream signals, such that the upstream signals received from the vibrations in an upstream amplifier within the distribution system are substantially equal.
12. 12. The method according to claim 11, characterized in that the level of the upstream signals introduced to each branch is substantially equal.
13. 13. The method according to claim 11, characterized in that the attenuation upstream of each branch is adjusted to compensate for the losses of the upstream cable and the branch.