Field of the invention

The invention relates to a signal splitter arrangement for a reverse channel in a cable television network. Background of the invention

Cable television networks may be implemented with various techniques and network topologies, but currently most cable television networks are implemented as so-called HFC networks (Hybrid Fiber Coax), i.e. as combinations of a fibre network and a coaxial cable network. Figure 1 shows the general structure of a typical HFC network. Program services are introduced from the main amplifier 1 00 (a so-called headend) of the network via an optical fibre network 1 02 to a fibre node 1 04, which converts the optical signal to an electric signal to be relayed further in a coaxial cable network 1 06. Depending on the length, branching, topology, etc. of the coaxial cable network, this coaxial cable segment typically comprises one or more broadband amplifiers 1 08, 1 1 0 for amplifying program service signals in a heavily attenuating coaxial media. From the amplifier the program service signals are introduced to a cable network 1 1 2 of a smaller area, such as a master antenna network of an apartment building, which are typically implemented as coaxial tree or star networks comprising signal splitters for distributing the program service signals to each customer. From the signal splitters the signal is further relayed either via a cable modem 1 1 4 to a television receiver 1 1 6 or a computer 1 1 8, or via a so-called set-top box 1 20 to a television receiver 1 22.

Typically the HFC network also provides its users a reverse channel, via which it is possible, for example, to send data signal from a cable modem via the HFC network further to the Internet or to use interactive services provided by a cable television operator, such as electronic trading, different games and video-on-demand services. The upstream signals using the reverse channel are transported using a different frequency range than the program services (i.e. downstream signals) originating from the network provider. In CATV networks the upstream signals are typically modulated to the frequency range of 5 - 65 MHz and the downstream signals are typically modulated to the frequency range of 85 - 862 MHz. The upstream signals from each customer are returned through the signal splitters via the fibre node e.g. to the headend.

Transmission quality of reverse signals in the coaxial media suffers because of noise and so-called RF-ingress interference. They decrease the available bandwidth of the reverse direction. Ingress is especially harmful for the reverse channel services, because it may overload return path lasers used in the fibre nodes and cause operational challenges for the network operators.

With the presumption that ingress mainly originates from the same source, the ingress signal components thus being in the same phase, it has been proposed to reduce ingress in the signal splitters by introducing a phase inverter into every second output of the signal splitter. Thus, the ingress signal components resulting in opposite phases would cancel each other out, when combined. Such a signal splitter is disclosed, for example, in EP 1 7531 32.

However, in practice the ingress signal components arriving at the outputs of a signal splitter travel through coaxial cables of different length. The phase difference of the signals vary as a function of the length of the coaxial cables, and therefore the phase of the ingress signal components arriving at the outputs of a signal splitter are totally random in real networks. Thus, depending on the length of the coaxial cables, the signal splitter of prior art may even increase ingress, when the ingress signal components being in a random phase are combined in the signal splitter.

Brief summary of the invention

Now, an improved arrangement has been developed to reduce the above-mentioned problems. As an aspect of the invention, we present a signal splitter adaptable to a cable television network, which is characterized in what will be presented in the independent claim. The dependent claims disclose advantageous embodiments of the invention.

According to an aspect of the invention, there is provided a bi- directional signal splitter for use in a cable television network, the splitter comprising an input connectable to a node of cable television network, and plurality of outputs, each output being connectable to a different item of subscriber equipment, the splitter being operable to combine, in a summing unit, a plurality of upstream signals at the outputs for upstream transmission from the input; alternate ones of said outputs are connected to a tunable phase shifter; and a feedback control circuit is provided between the tunable phase shifter and said input, wherein a control signal for adjusting the tunable phase shifter is provided on a basis of noise components in the combined upstream signals at said input.

According to an embodiment, the control signal is arranged to adjust the tunable phase shifter such that the noise components from the alternate outputs are in substantially opposite phases.

According to an embodiment, the feedback control circuit comprises means for obtaining samples and a memory, the means for obtaining samples being arranged to gather samples from the noise component of the combined upstream signals at said input and to supply the samples to the memory.

According to an embodiment, the memory is arranged to cumulate said samples from a predefined period of time. According to an embodiment, in response to cumulative sample values in said memory reaching a predefined threshold during the predefined period of time, a control signal respective the cumulative value of the sample values is arranged to be supplied to the tunable phase shifter. According to an embodiment, the memory is arranged to cumulate said samples from a continuous sampling signal obtained from said noise components; and the control signal is arranged to be generated as a continuously changing control signal on the basis of the cumulative sample values presently in the memory.

According to an embodiment, the memory is implemented as a capacitor arranged to be charged by voltages of the noise components.

According to an embodiment, a continuously changing control signal is generated on the basis of the capacitance of the capacitor at a given moment.

According to an embodiment, in response to capacitance of the capacitor reaching a predefined threshold, a control signal is supplied to the tunable phase shifter. According to an embodiment, the tunable phase shifter, the means for obtaining samples and/or the memory are integrated with signal splitter.

The other aspects, embodiments and advantages will be presented later in the detailed description of the invention.

Brief description of the drawings

The invention will now be described in more detail in connection with preferred embodiments with reference to the appended drawings, in which:

Fig. 1 shows the general structure of a typical H FC network;

Fig. 2 shows a prior art signal splitter in a simplified block chart;

Fig. 3 shows a table illustrating the effect of the prior art signal splitter on ingress as a function of the frequency and the length of coaxial cables; shows a signal splitter according to an exemplified embodiment; and Fig. 5 shows an exemplary embodiment for carrying out a feedback control circuit of the signal splitter.

Detailed description of the embodiments

Figure 2 illustrates the basic operational principle of the prior art signal splitter. The signal splitter 200 comprises an input 202 and a plurality of outputs, of which only a first output 204 and a second output 206 are shown for the purpose of clarity. The function of these signal splitters 200 is to act as an interface between a network element of the coaxial cable network, such as an amplifier, and a number of customers. Therein, the splitter input 202 is connected to the network element and each customer is connected to one output of the splitter 200. It is also possible to connect the several splitters in cascade, wherein an input of at least one signal splitter is connected to an output of another signal splitter.

The program services (i.e. downstream signals, arrow 208) are transmitted from the service provider to the input 202 of the splitter where the signal is divided to be transmitted further to the customers. The divided program service signals 208a and 208b are transmitted via the first and second outputs 204, 206 of the splitter 200.

In the opposite direction, data signals (i.e. upstream signals, arrows 21 0a, 21 0b) from a first and a second subscriber are received in the first and second outputs 204, 206 of the splitter. The splitter 200 sums the data signals from all subscribers connected to the splitter and applies the summed signal 21 0 to the input 202 of the splitter to be transmitted further to the service provider. The upstream data signals 21 0a, 21 0b comprise, in addition to payload signals, also unwanted noise components, i.e. ingress. The splitter 200 not only sums the wanted payload signal but also sums the noise components and applies them to the input 202 of the splitter.

The signal splitter of prior art further comprises a phase inverter connected to every second output of the splitter. In Figure 2, this is illustrated by the phase inverter 21 2 connected to the first output 204 of the signal splitter 200. Regarding the operation of the prior art signal splitter, it is presumed that the signal characteristics of the noise components are very similar because they arise for the most part from the same source, i.e. radio frequency electromagnetic radiation picked up by the subscribers' equipment and the cables connecting the outputs of the splitter to the equipment of the subscribers. Therefore, it is further presumed that the noise components will have much the same frequency, amplitude and phase. Then hypothetical^, by applying a 1 80 degree phase inverter to alternate ones of the plurality of outputs, the noise components of every second output were in antiphase with the noise components of the remaining outputs. When summing these noise components, they would cancel each other out so that the noise components of the summed signals applied to the input of the splitter would be much reduced.

However, in practice the noise components arriving at the outputs of a signal splitter travel through coaxial cables of different length. The phase of the signal varies as a function of the length of the coaxial cable, and therefore the phase of the noise components arriving at the outputs of a signal splitter are totally random.

Figure 3 shows a table illustrating the effect of the prior art signal splitter on ingress as a function of the frequency and the corresponding wavelength and the length difference of coaxial cables. In the table of Figure 3, it is for exemplary purposes presumed that the coaxial cables connected to the outputs of the signal splitter have the length difference of three meters. Consequently, when upstream signals having a frequency of 20 MHz arrive at the prior art signal splitter through (a first group of) coaxial cables at the wavelength of 1 0,1 meters and through (a second group of) coaxial cables including the phase inverter and having a length difference of three meters, the summed noise component (i.e. the ingress towards the CATV network) results in 1 ,3 times increase in the noise signal power.

Similar effects can be seen at other frequencies and the corresponding wavelength used on the reverse channel with coincidental length of (the first group of) coaxial cables and (the second group of) coaxial cables having similar length difference. For example, on the frequencies of 30 - 35 MHz with coincidental length of coaxial cables, the noise signal power may even be nearly doubled. Thus, depending on the length difference of the coaxial cables, the signal splitter of prior art may in many cases increase ingress, when the ingress signal components being in a random phase are combined in the signal splitter.

When installing signal splitters into a CATV network, it is practically impossible to ensure that the noise components will have much the same frequency, amplitude and phase, for example by adjusting the length of the coaxial cables. Therefore, an improved signal splitter is needed in order to achieve at least some decrease in ingress.

Figure 4 shows the basic operational principle of a signal splitter according to an embodiment. Similarly to the prior art signal splitter, the signal splitter 400 comprises an input 402 and a plurality of outputs, of which only a first output 404 and a second output 406 are shown for the purpose of clarity. However, instead of applying a 1 80 degree phase inverter to every second output, a tunable phase shifter 408 is applied to alternative outputs, i.e. to output 404 in Figure 4.

Now, upstream data signals (arrows 41 4a, 41 4b) from a first and a second subscriber are received in the first and second outputs 404, 406 of the splitter. The data signal 41 4a from the output 404 travels through the tunable phase shifter 408 to a summing unit 41 0, whereas the data signal 41 4b from the output 406 travels directly to the summing unit 41 0. The summing unit 41 0 sums, besides the payload data signals, also the noise components of the data signals from all subscribers connected to the splitter and applies the summed signal 41 4 to the input 402 of the splitter to be transmitted further to the service provider. The summed signal 41 4 is further used as a control signal for a feedback control circuit 41 2 to adjust the tunable phase shifter 408 to generate an appropriate phase shift to the data signal 41 4a such that the data signals 41 4a, 41 4b, when arriving at the summing unit 41 0, would be in substantially opposite phases. Figure 5 shows an exemplary embodiment for carrying out the feedback control circuit 41 2 of the signal splitter. The data signal 41 4a is subjected to a phase shift in the tunable phase shifter 408, whereafter the data signal 41 4a is supplied to the summing unit 41 0. The data signal 41 4b is supplied directly to the summing unit 41 0. Again, the summing unit 41 0 sums the payload data signals, and also the noise components of the data signals. Now, if the tunable phase shifter 408 is appropriately adjusted, the noise components from the alternate outputs are in substantially opposite phases and cancel each other out. This significant advantage is achieved regardless of the original phase of the noise component.

The appropriate adjustment for the tunable phase shifter is achieved by the feedback control circuit 41 2, which preferably comprises a sampling unit 41 6 and a memory 41 8. The sampling unit 41 6 gathers samples from the noise component of the summed signal and supplies the samples to the memory 41 8. Provided that the tunable phase shifter 408 is appropriately adjusted, there are substantially no noise components in the summed signal and the samples are zeros.

According to an embodiment, the sampling may be implemented as continuous analog sampling, for example on the basis of voltages of the noise component of the summed signal. Therein, a separate sampling unit is not necessarily needed, but the samples (i.e. the voltages) may be conducted directly to the memory 41 8. Thus, a conductor may operate as means for obtaining samples.

However, if a new source of noise, for example, is applied to the data signal, the phase of the noise component may be changed and the tunable phase shifter is no more appropriately adjusted. Thus, the resulting noise components in the summed signal cause non-zero samples to be supplied in the memory 41 8. On the basis of the cumulative sample values in the memory 41 8, a control signal 420 is supplied from the memory to the tunable phase shifter 408 to adjust the phase shift applied to the data signal 41 4a such that corrective measures are taken to re-adjust the tunable phase shifter 408 Naturally, occasional noise components may occur in the data signals arriving from the customers and thus in the summed data signal as well. Therefore, the sample values are preferably gathered for a predefined period of time, whereupon at least some of these occasional noise components, if being in opposite phases, may cancel each other out. Accordingly, if the tunable phase shifter 408 is appropriately adjusted, the sample values cumulating in the memory should accumulate approximately to zero in the long run. According to an embodiment, the cumulative sample values may be monitored according to a sliding time window principle, where the oldest sample within the time window is neglected when a new sample is included in the memory. However, if the cumulative sample values reach a predefined threshold during the time window, this indicates that tunable phase shifter is no more appropriately adjusted. Then, depending on the cumulative value of the sample values, a respective control signal 420 is supplied to the tunable phase shifter 408.

According to another embodiment, if the continuous analog sampling is used, the control signal may be generated as a continuously changing control signal on the basis of the cumulative sample values presently in the memory.

According to an embodiment, the memory 41 8 may be implemented, at its simplest, as a capacitor which is charged by the voltages of the noise components. Such a capacitor serves as a simple control signal generator, wherein a continuously changing control signal is generated on the basis of the capacitance of the capacitor at a given moment.

According to another embodiment, a predefined threshold may be set for the value of the capacitance, whereupon if the capacitance of the capacitor reaches the predefined threshold, a respective control signal is supplied to the tunable phase shifter.

For the operation of the invention, any type of tunable phase shifters may be used. However, since the operation of the phase shifter in this context is rather straightforward and the number of phase shifter required in a typical CATV network is large, it is beneficial to use rather simple and inexpensive analog phase shifters. Thus, for example varactor diodes, nonlinear dielectrics, or ferroelectric materials can be used as electrically controlled analog phase shifters. Alternatively, mechanically-controlled analog phase shifters may be used, wherein the phase of the data signal is adjusted in accordance with the propagation delay of data signal, which is increased by mechanically lengthening the transmission line. The material used in a mechanically- controlled analog phase shifters may have different propagation characteristics than a coaxial cable.

According to an embodiment, the tunable phase shifter 408, the sampling unit 41 6 and/or the memory 41 8 are integrated with signal splitter, thus bringing space savings in the actual implementation. Now, since the phases of the noise components arriving at the outputs of the signal splitter are random, the signal splitter according to the above embodiment may provide significant advantages in terms of decrease in ingress in certain network configurations. Assuming that the noise components of data signals substantially cancel each other out and comparing this to the values in the table of Figure 3, it can be seen in certain circumstances and network configurations an average reduction of 6 dB of the noise components may be achieved by choosing an appropriate type of signal splitter, which in turn can give rise to a significant increase in the data transmission capacity of the upstream signal channel.

It will be obvious for a person skilled in the art that with technological developments, the basic idea of the invention can be implemented in a variety of ways. Thus, the invention and its embodiments are not limited to the above-described examples but they may vary within the scope of the claims.