WO2020002748A1

WO2020002748A1 - An arrangement for catv amplifier control

Info

Publication number: WO2020002748A1
Application number: PCT/FI2018/050509
Authority: WO
Inventors: Matti Heikkilä
Original assignee: Teleste Oyj
Priority date: 2018-06-27
Filing date: 2018-06-27
Publication date: 2020-01-02

Abstract

A network element of a cable television (CATV) network, said network element comprising one or more amplifier units (300) for amplifying downstream signal transmission for digital output into one or more output channels; means (310) for detecting nonlinearity of at least one amplifier unit for all active digital output channels; means (314) for determining a first correction signal based on the detected nonlinearity; and means (314) for adjusting bias current of said at least one amplifier unit (300) according to the first correction signal.

Description

AN ARRANGEMENT FOR CATV AMPLIFIER CONTROL Field of the invention

The invention relates to cable television (CATV) networks, and especially to controlling CATV amplifiers.

Background of the invention

CATV 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 fiber 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 100 (a so-called headend) of the network via an optical fiber network 102 to an optical node 104, which converts the optical signal to an electric signal to be relayed further in a coaxial cable network 106. Depending on the length, branching, topology, etc. of the coaxial cable network, this coaxial cable segment typically comprises one or more broadband amplifiers 108, 1 10 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 12 of a smaller area, such as a distribution 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 a wall outlet the signal is further relayed either via a cable modem 1 14 to a television receiver 1 16 or a computer 1 18, or via a so-called set-top box 120 to a television receiver 122.

Both the optical nodes and amplifiers along the downstream path comprise a plurality of amplifier units/stages for amplifying the downstream signals. The parameters of the components in the amplifier stages within the network elements need to be dimensioned such they can handle the worst-case situation of the whole frequency area being loaded with active channels at maximum output level. On the other hand, in typical real-life use-cases there are a number of unallocated channels in the network and/or the output power level is not even close to maximum. This would allow running the RF amplifier components with smaller bias current to reach the needed RF performance level and thus lowering the power consumption of the device.

There are some traditional analogue amplifiers provided with automatic bias current control, where the output power after the last amplifier stage is measured and tuning the bias current is controlled based on the measured output power. However, the mere output power of RF signals is rather inaccurate in a sense that it provides limited amount of information about the underlying channel configuration. In practice, for adjusting the bias current of an amplifier in analogue channels, the exact channel raster offers better basis for amplifier component bias control.

Brief summary of the invention

Now, an improved arrangement has been developed to reduce the above-mentioned problems. As aspects of the invention, we present a network element of a cable television network, which is characterized in what will be presented in the independent claims.

The dependent claims disclose advantageous embodiments of the invention.

According to an aspect of the invention, there is provided a network element of a cable television (CATV) network, said network element comprising one or more amplifier units for amplifying downstream signal transmission for digital output into one or more output channels; means for detecting nonlinearity of at least one amplifier unit for all active digital output channels; means for determining a first correction signal based on the detected nonlinearity; and means for adjusting bias current of said at least one amplifier unit according to the first correction signal.

According to an embodiment, the at least one amplifier unit is the last amplifier unit of the network element in downstream signal path. According to an embodiment, the network element further comprises means for detecting a signal-to-noise ratio (SNR) value at the output of the amplifier unit for said active digital output channels, wherein the first correction signal is configured to be determined based on the SNR value.

According to an embodiment, said means for determining a first correction signal are configured to measure a dynamic range for each active digital output channel.

According to an embodiment, the network element further comprises means for determining a maximum voltage difference (U max) between normalized input and output voltages of the amplifier unit.

According to an embodiment, said means for determining the maximum voltage difference comprise a differential mixer and a differential amplifier.

According to an embodiment, said means for determining the maximum voltage difference comprise a measuring amplifier.

According to an embodiment, the network element further comprises means for detecting nonlinearity of the maximum voltage difference

(Umax).

According to an embodiment, the network element further comprises means for pre-di storting an input signal of the amplifier unit; means for determining a second correction signal based on the detected nonlinearity of the maximum voltage difference (Umax); wherein said means for pre-di storting an input signal of the amplifier unit is configured to be controlled according to said second correction signal.

According to an embodiment, the network element further comprises means for detecting bit error rate (BER) of all active digital output channels, wherein an adjustment of the bias current provided by the first correction signal is configured to be monitored based on at least the BER value.

According to an embodiment, the digital output channels are modulated according to Single-Carrier Quadrature Amplitude Modulation (SC- QAM) or Orthogonal Frequency-Division Multiplexing (OFDM).

According to an embodiment, said amplifier units comprise one or more of the following: a mid-stage amplifier unit, a gain control amplifier unit, a slope control amplifier unit, an output hybrid amplifier unit.

These and 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 HFC network;

Fig. 2 shows an example of shoulder formation in response to increasing the input power in an amplifier; and

Fig. 3 shows a simplified block chart of a network element according to an embodiment of the invention.

Detailed description of the embodiments

Data Over Cable Service Interface Specification (DOCSIS) is a CATV standard providing specifications for high-bandwidth data transfer in an existing CATV system. DOCSIS may be employed to provide Internet access over existing hybrid fiber-coaxial (HFC) infrastructure of cable television operators. DOCSIS has been evolved through versions 1.0, 1.1 , 2.0 and 3.0 to the latest version of 3.1. DOCSIS provides a lucrative option for cable network providers to maximize both the downstream and upstream data throughput using the existing cable TV network, but without making expensive changes to the HFC network infrastructure.

When implementing the HFC network of Figure 1 according to DOCSIS, the headend 100 of the CATV network comprises inputs for signals, such as TV signals and IP signals, a television signal modulator and a cable modem termination system (CMTS). The CMTS provides high-speed data services to customers thorough cable modems (CM; 1 14) locating in homes. The CMTS forms the interface to the IP-based network over the Internet. It modulates the data from the Internet for downstream transmission to homes and receives the upstream data from homes. The CMTS additionally manages the load balancing, error correction parameters and the class of service (CoS).

Signals from the headend 100 are distributed optically (fiber network 102) to the vicinity of individual homes, where the optical signals are converted to electrical signals at the terminating points 104. The electrical signals are then distributed to the various homes via the existing 75 ohm coaxial cables 106. The maximum data transfer of the coaxial cables is limited due to strong frequency-based attenuation. Therefore, the electrical signals transmitted over coaxial cables must be amplified. The amplifiers 108, 1 10 used for this purpose are suited to a specific frequency range. In addition, the upstream and downstream must occur over the same physical connection. The last part 1 12 of the coaxial connection between the CMTS and the CMs branches off in a star or a tree structure. A CMTS transmits the same data to all CMs located along the same section of cable (one-to-many communications). A request/grant mechanism exists between the CMTS and the CMs, meaning that a CM needing to transmit data must first send a request to the CMTS, after which it can transmit at the time assigned to it.

Depending on the version of DOCSIS used in the CATV network, there is a great variety in options available for configuring the network. For the downstream channel width, all versions of DOCSIS earlier than 3.1 use either 6 MHz channels (e.g. North America) or 8 MHz channels (so- called "EuroDOCSIS"). However, the upstream channel width may vary between 200 kHz and 3.2 MHz (versions 1.0/1.1 ), and even to 6.4 MHz (version 2.0). 64-QAM or 256-QAM modulation is used for downstream data in all versions, but upstream data uses QPSK or 16-level QAM (16- QAM) for DOCSIS 1 .x, while QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM and 128-QAM are used for DOCSIS 2.0 & 3.0.

DOCSIS 3.1 specifications support capacities of at least 10 Gbit/s downstream and 1 Gbit/s upstream using 4096 QAM. DOCSIS 3.1 rejects the 6 or 8 MHz wide channel spacing and uses narrower orthogonal frequency-division multiplexing (OFDM) subcarriers being 20 kHz to 50 kHz wide, which sub-carriers can be combined within a block spectrum of about 200 MHz wide.

DOCSIS 3.1 further provides the concept of Distributed CCAP Architecture (DCA). Converged Cable Access Platform (CCAP) may be defined as an access-side networking element or set of elements that combines the functionality of a CMTS with that of an Edge QAM (i.e. the modulation), providing high-density services to cable subscribers. Conventionally, the CCAP functionalities have been implemented in the headend/hub, such as the headend 100 in Figure 1. In a DCA, some features of the CCAP are distributed from headend/hub to the network elements closer to the customers, for example to the optical nodes 104 in Figure 1. DOCSIS 3.1 specifies at least two network element concepts, i.e. a Remote PHY Device (RPD) and a Remote-MACPHY Device (RMD), to which some functionalities of the headend can be distributed. A recent version of DOCSIS 3.1 specification also provided Annex F introducing a Full Duplex DOCSIS 3.1 technology, where a new distributed access node called Full Duplex (FDX) Node is determined.

The data transmission between the distributed parts of the CCAP is typically carried out through a fiber connection. This may provide both scale advantages and flexible deployment options by maximizing the channel capacity and simplifying many operations via the usage of digital fiber and Ethernet transport. The amplifiers and optical nodes in a HFC network (e.g. 104, 108, 1 10 in Fig. 1 ) are specified for some frequency range, today typically up to 1.2GHz. The parameters of the components in the amplifier stages within the devices need to be dimensioned such they can handle the worst-case situation of the whole frequency area being loaded with active channels at maximum output level. On the other hand, in typical real-life use-cases there are a number of unallocated channels in the network and/or the output RF power level is not even close to maximum. This would allow running the amplifiers with smaller bias current and thus lowering the power consumption of the device.

In some existing optical node and amplifier products, there is a setting to lower the bias current for the output hybrid (i.e. the last amplifier stage before the output). Reducing the bias current decreases the performance of the output hybrid, but the output hybrid may be operated even with a partial load, if the reduction of the bias current is performed appropriately. Also, the bias current of the other amplifier stages may be reduced, even though the benefits in lowering the power consumption of the device are less obvious. This setting needs to be manually set in the user interface of the device. There are also traditional analogue devices with automatic bias current control, where the output power after the last amplifier stage is measured and tuning the bias current is controlled based on the measured output power. However, the mere output power of RF signals is rather inaccurate in a sense that it provides very little information about the underlying channel configuration. Moreover, measuring overall power gives rather poor results especially in sloped amplifiers.

Consequently, an improved arrangement is presented herein for adjusting the bias current of amplifier units in network elements.

According to an aspect, a network element of a cable television (CATV) network is now introduced, said network element comprising one or more amplifier units for amplifying downstream signal transmission for digital output into one or more output channels; means for detecting nonlinearity of at least one amplifier unit for all active digital output channels; means for determining a first correction signal based on the detected nonlinearity; and means for adjusting bias current of said at least one amplifier unit according to the first correction signal.

Upon aiming to reduce the bias current of an amplifier unit, the nonlinear nature of the typical amplifier unit is utilized herein. When increasing the input control voltage of an amplifier unit, the output voltage first increases linearly, but at a certain point, the increase of the output voltage slows down and finally the output voltage reaches a saturation value, even if the input control voltage is further increased. The nonlinear operation point of the amplifier is dependent on the used bias current. A first correction signal can now be determined based on the detected nonlinearity, and the bias current of the amplifier unit can be adjusted according to the first correction signal.

It is noted that while adjusting the bias current in most cases refers to reducing the bias current, it may also refer to increasing the bias current, for example in a situation where a previously made reduction of the bias current was too strong and drove the amplifier unit to operate in a non- optimal operating region.

According to an embodiment, the at least one amplifier unit is the last amplifier unit of the network element in downstream signal path. The last forward amplifier unit (i.e. the output hybrid) typically draws a significant part of the total power in CATV amplifiers. Thus, the most significant benefits in terms of power reduction are obtained by adjusting the bias current of the last amplifier unit. Therefore, in many of the following examples the discussion concerns the output hybrid. It is, nevertheless, noted that the same principles of detecting the nonlinearity at the output of an amplifier unit and adjusting the bias current of said amplifier unit may be applied to any of the amplifier units.

After the reduction of the bias current of the output hybrid, the nonlinear operation point of the amplifier has probably changed. If the performance of the output hybrid in the linear operating region is“good enough”, i.e. still allowable, this may imply a further reduction of the bias current via re-determining the first correction signal based on the detected nonlinearity. This process may be iteratively repeated by moving along the gradient toward a minimum bias current, thereby also achieving a minimum power loss in the output hybrid while at the time still providing a required signal quality.

According to an embodiment, the network element further comprises means for detecting a signal-to-noise ratio (SNR) value at the output of the amplifier unit for said active digital output channels, wherein the first correction signal is configured to be determined based on the SNR value.

Thus, the network element may comprise a detector circuit or a component for detecting the SNR values of the active channels. Herein, existing components of a typical CATV amplifier may used for detecting the SNR values. For example, a forward path tuner may detect the SNR values for each active channel of the output hybrid. From the SNR values of each channel, the lowest SNR value may be selected as a basis for determining the first correction signal.

Driving an amplifier near a distortion (i.e. nonlinear) region with too small current may cause so-called shoulders at certain frequencies in the output power curve. Figure 2 shows an example of shoulder formation in response to increasing the input power. The shoulders degrade the quality of the signal, wherein the dynamic range (DR) may be determined, as shown in Figure 2, as difference between the maximum signal power and the shoulder power.

Hence, by measuring the power at certain point in a shoulder, a minimum bias current that fulfils the required signal quality can be determined. If, for example, the forward path tuner is used, the tuner may be configured to measure a maximum signal power and a shoulder power of a channel and calculate the dynamic range for said channel. The same measurement is repeated for all channels in the forward path and the lowest SNR among the measured channels may be used as a basis for the first correction signal.

Herein, if the network element operates according to a DOCSIS standard, instead of or in addition to the forward path tuner, a DOCSIS receiver may be used for detecting both the BER value and the SNR values for each active channel of the output hybrid. From the SNR values provided by the DOCSIS receiver, the lowest values of SNR are used as a basis for the first correction signal. While the BER values do not directly indicate the nonlinearity of the signal, the adjustment of the bias current provided by the first correction signal may be monitored such that the BER values after the correction are detected, and if the BER values have been improved, it may indicate that the correction made on the basis of SNR values was correct.

According to an embodiment, the network element further comprises means for pre-di storting an input signal of the amplifier unit; means for determining a second correction signal based on a detected nonlinearity of the maximum voltage difference (Umax) between input voltage and output voltage of the last amplifier unit; wherein said means for pre distorting an input signal of the amplifier unit is configured to be controlled according to said second correction signal.

Thus, in addition to first correction signal obtained by detecting the nonlinearity in the output channel parameters, a pre-distortion unit may be applied before the output hybrid to further enhance the accuracy of bias current reduction toward a minimum bias current and a minimum power loss in the output hybrid while at the time still providing a required signal quality. For controlling the operation of the pre-distortion unit, a second correction signal may be determined based on a detected nonlinearity of the maximum voltage difference (Umax) between input voltage and output voltage of the output hybrid.

According to an embodiment, the digital output channels are modulated according to Single-Carrier Quadrature Amplitude Modulation (SC- QAM) or Orthogonal Frequency-Division Multiplexing (OFDM). Thus, the embodiments described herein are especially applicable to CATV system according to DVB-C and DOCSIS standards, such as DOCSIS 3.1. Because the CATV systems typically involve both analog and digital signals with different modulations, various kinds of measurements are preferably performed for detecting the nonlinearity. Therefore, monitor the SNR values are monitored for analog signals and digital QAM signals, whereas a maximum voltage difference (Umax) between normalized input and output voltages of the amplifier unit is monitored for OFDM signals, as described more in detail further below.

Figure 3 shows an example of a simplified block chart of network element according to an embodiment. Figure 3 shows a simplification of the downstream path within the node; thus, no components relating to upstream path are shown. It is further noted that while Figure 3 shows the implementation with only an output hybrid amplifier unit, the network element typically comprises a plurality of amplifier units along the downstream path, such as one or more mid-stage amplifier units, a gain control amplifier unit, and a slope control amplifier unit before the output hybrid amplifier unit. The same principles of detecting the nonlinearity at the output of an amplifier unit and adjusting the bias current of said amplifier unit may be applied to any of the amplifier units.

The output hybrid amplifier unit 300 (i.e. the last amplifier unit) supplies one or more active digital output channels to a network segment via an output node 302. There between, the output signals of each channel may be sampled e.g. using a tap 304, which may provide the samples to a DOCSIS receiver 306, if a DOCSIS-based network element is used. If e.g. a DVB-C-based network element is used, the samples may be provided to a forward path tuner 308. The DOCSIS receiver 306 may carry out the detection 310 of both BER and SNR values of each channel, while the forward path tuner 308 may carry out the detection 310 of SNR values of each channel as described above (i.e. calculating the dynamic range for said channel on the basis of the difference between the maximum signal power and the shoulder power).

The SNR (and BER) values are provided to a decision engine 312, which may be configured to select the lowest acceptable SNR (and BER) value as a basis for the first correction signal. The bias control circuitry 314 generates the first correction signal and uses it to reduce the bias current of the output hybrid amplifier unit 300.

The network element further comprises a pre-distortion unit 316 for running digital predistortion (DPD) algorithms in said network element. DPD provides means for improving the linearity of radio transmitter amplifiers, wherein non-linearity of the amplifier is cancelled by applying a predistortion (i.e. inverse distortion) into the input of the amplifier. Since the non-linearity of the amplifier typically varies depending on the operation mode, a control signal is used for adjusting the level of DPD.

For producing the control signal, the network element further comprises means for determining a maximum voltage difference (Umax) between normalized input and output voltages of the amplifier unit. Thus, the input and output voltages of the amplifier unit are preferably scaled and normalized in a selected operation point to be comparable to each other.

According to an embodiment, said means for determining the maximum voltage difference comprise a differential mixer and a differential amplifier. The differential mixer 318 and the differential amplifier 320 produce the maximum voltage difference (Umax), from which an AC component is extracted by a bandpass filter 322. Because of the used bandwidth is a fraction of total system bandwidth, measurement of distortion may be carried out in several sub-bands. Non-linearity of the particular frequency sub-band can be found by measuring the distortion values in said sub-band. In order to generate the sub-bands, an agile oscillator 324, for example, may be used for generating local oscillator signals for both mixers of the differential mixer 318 via a wideband phase shifter 326, which adjusts the phase difference substantially to zero. An adjustable attenuator element may be used to cancel gain differences. If the phase function and gain function in both branches of the differential system are adjusted properly (i.e. to be as equal as possible), it is possible to measure small differences between the input signal and output signal with such good balance properties.

It is noted that said means for determining the maximum voltage difference are not limited to a differential mixer and a differential amplifier, but any other suitable circuitry may be used herein. Hence, according to an embodiment, said means for determining the maximum voltage difference may comprise a measuring amplifier, in which the processed input voltage and output voltage are provided. Herein, the input voltage of the output hybrid amplifier unit may be sampled to a delay circuit and the output voltage of the output hybrid amplifier unit may be sampled to an attenuator. The measuring amplifier produces a voltage difference proportional to Umax, from which an AC component may be extracted, for example, by a band pass filter.

According to an embodiment, the network element further comprises means for detecting nonlinearity of the maximum voltage difference (U max), which may be implemented e.g. as a peak detector 328. The acquired non-distortion profile gives rules to adjust pre-distortion engine, for example by using a second order or a third order compensator. The amount of nonlinearity is provided to the decision engine 312, which generates the second correction signal and adjusts the level of the pre distortion unit 316 accordingly.

In general, the various embodiments may be implemented in hardware or special purpose circuits or any combination thereof. While various embodiments may be illustrated and described as block diagrams or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

A skilled person appreciates that any of the embodiments described above may be implemented as a combination with one or more of the other embodiments, unless there is explicitly or implicitly stated that certain embodiments are only alternatives to each other.

The various embodiments can be implemented with the help of computer program code that resides in a memory and causes the relevant apparatuses to carry out the invention. Thus, the implementation may include a computer readable storage medium stored with code thereon for use by an apparatus, such as the network element, which when executed by a processor, causes the apparatus to perform the various embodiments or a subset of them. Additionally or alternatively, the implementation may include a computer program embodied on a non- transitory computer readable medium, the computer program comprising instructions causing, when executed on at least one processor, at least one apparatus to apparatus to perform the various embodiments or a subset of them. For example, an apparatus may comprise circuitry and electronics for handling, receiving and transmitting data, computer program code in a memory, and a processor that, when running the computer program code, causes the apparatus to carry out the features of an embodiment.

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.

Claims

Claims:

1. A network element of a cable television (CATV) network, said network element comprising

one or more amplifier units for amplifying downstream signal transmission for digital output into one or more output channels;

means for detecting nonlinearity of at least one amplifier unit for all active digital output channels;

means for determining a first correction signal based on the detected nonlinearity; and

means for adjusting bias current of said at least one amplifier unit according to the first correction signal.

2. The network element according to claim 1 , wherein the at least one amplifier unit is the last amplifier unit of the network element in downstream signal path.

3. The network element according to claim 1 or 2, further comprising

means for detecting a signal-to-noise ratio (SNR) value at the output of the amplifier unit for said active digital output channels, wherein the first correction signal is configured to be determined based on the SNR value.

4. The network element according to any preceding claim, wherein said means for determining a first correction signal are configured to measure a dynamic range for each active digital output channel.

5. The network element according to any preceding claim, further comprising

means for determining a maximum voltage difference (Umax) between normalized input and output voltages of the amplifier unit.

6. The network element according to claim 5, wherein said means for determining the maximum voltage difference comprise a differential mixer and a differential amplifier.

7. The network element according to claim 5, wherein said means for determining the maximum voltage difference comprise a measuring amplifier.

8. The network element according to any of claims 5 - 7, further comprising

means for detecting nonlinearity of the maximum voltage difference (U max)·

9. The network element according to any of claims 5 - 8, further comprising

means for pre-di storting an input signal of the amplifier unit; means for determining a second correction signal based on the detected nonlinearity of the maximum voltage difference (Umax);

wherein said means for pre-di storting an input signal of the amplifier unit is configured to be controlled according to said second correction signal.

10. The network element according to any preceding claim, further comprising

means for detecting bit error rate (BER) of all active digital output channels,

wherein an adjustment of the bias current provided by the first correction signal is configured to be monitored based on at least the BER value.

1 1. The network element according to any preceding claim, wherein the digital output channels are modulated according to Single- Carrier Quadrature Amplitude Modulation (SC-QAM) or Orthogonal Frequency-Division Multiplexing (OFDM).

12. The network element according to any preceding claim, wherein said amplifier units comprise one or more of the following: a mid stage amplifier unit, a gain control amplifier unit, a slope control amplifier unit, an output hybrid amplifier unit.