System and Method for OFDMA Power Equalization

Abstract

A device, system, and method for determining an adjustment used by an upstream transmitter to set the transmission power of at least one upstream OFDMA channel, the device, system, and method determining a constant in a continuous equation of power as a function of frequency using a calibration value and then using the function to determine the adjustment and making the adjustment.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 63 / 338,849, filed on May 5, 2022.

Background Art

[0002] The subject matter of this application generally relates to systems and methods for communicating data via an access network that propagates data signals between a subscriber's provider facility having an "edge device" at the boundary of a wide - area network such as the Internet, satellite network, etc. Examples of such access networks are cable television (CATV) networks. Historically, CATV networks have provided content to large groups of subscribers from a central distribution unit called a "headend" that distributes channels of content from the headend to subscribers via a branched network that includes a number of intermediate nodes. Historically, the headend has received multiple independent programming contents, multiplexed the contents together, and on the other hand, modulated the contents according to a quadrature amplitude modulation (QAM) scheme that maps the contents to individual frequencies or "channels" that a receiver can demodulate and display the desired content.

[0003] However, modern CATV service networks not only provide media content such as television channels and music channels to customers, but also host digital communication services such as Internet services, video - on - demand, and telephone services such as VoIP. These digital communication services not only require downstream communication from the headend through intermediate nodes to the subscriber, but also require upstream communication from the subscriber through the branched network to the content provider.

[0004] For this purpose, these CATV headends include a separate cable modem termination system (CMTS) that is used to provide high-speed data services such as video, cable Internet, and voice over Internet protocol to cable subscribers. Typically, the CMTS will include both an Ethernet interface (or other more conventional high-speed data interfaces) and an RF interface, whereby traffic coming from the Internet can be routed (or bridged) through the Ethernet interface, through the CMTS, and then onto an optical RF interface connected to the cable company's fiber coaxial hybrid (HFC) system. Downstream traffic is sent from the CMTS to the cable modem in the subscriber's home, while upstream traffic is sent from the cable modem in the subscriber's home to the CMTS. Many of the latest CATV systems combine the functions of the CMTS and the video distribution system (EdgeQAM) into a single platform called a Converged Cable Access Platform (CCAP). Additionally, other latest CATV systems called Remote PHY (or R-PHY) relocate the physical layer (PHY) of the conventional CCAP by pushing it to the fiber nodes of the network. Thus, while the CCAP core performs upper layer processing, the R-PHY device within the node converts the downstream data transmitted by the core from digital to analog and transmits it at radio frequency as a QAM signal, and converts the upstream RF data transmitted by the cable modem from analog to digital format and transmits it optically to the core. Other latest systems push other elements and functions located within the conventional headend, such as MAC layer functions (R-MACPHY), into the network.

[0005] Conventionally, a CATV system divides the available bandwidth between upstream and downstream transmissions, i.e., data is transmitted in only one direction over any portion of the spectrum. For example, in the initial iterations of the specified Data Over Cable Service Interface Specification (DOCSIS), the upstream transmission was assigned to the frequency spectrum of 5 MHz to 42 MHz, and the downstream transmission was assigned to the frequency spectrum of 50 MHz to 750 MHz. In later iterations of the DOCSIS standard, the width of the reserved spectrum for each of the upstream and downstream transmission paths was extended, but the spectra assigned to each respective direction did not overlap.

[0006] In one of the recent developments, Orthogonal Frequency Division Multiplexing (OFDM) is used to utilize the bandwidth more efficiently. In the downstream direction, OFDM propagates multiple signals within a single OFDM "channel" at individual subcarrier frequencies that are spaced apart from each other by a distance calculated to nullify interference between subcarriers, thereby eliminating the need for guard bands between subcarriers within an individual OFDM channel and thereby allowing for more efficient use of the spectrum. In the upstream direction, a similar system is employed, but the cable modem shares subcarriers in a time-division manner. This upstream version is called Orthogonal Frequency Division Multiple Access (OFDMA).

[0007] Unfortunately, signal attenuation increases on the transmission medium as the frequency increases, meaning that higher frequency signals must be transmitted from the source at a higher power level than lower frequency signals if the receiver is to receive them at the same power level. Conventionally, a CMTS and / or cable modem can command a transmitter to determine the received power level in each channel, which covers a small frequency range, and adjust the transmitted power to "equalize" the received power across the spectrum, so this "slope" can be corrected. Because the channel widths involved were so low, any variation in power across the spectrum encompassed by a single channel could be ignored, and each channel was adequately represented by a single measurement of the power level within that channel, and then measurements of multiple such channels could be equalized by the CMTS as described above.

[0008] However, in OFDMA, each channel can be up to 96 MHz wide, accommodating a number of individual signals, and thus the power level of a 96 MHz wide channel varies considerably within that channel. This causes problems at the upstream node that combines signals from different sources, such as a cable modem, transmitting signals within a very wide OFDMA channel. The upstream transmitter needs to be commanded to transmit at each respective power level equalized at the input of the upstream node, but a single measurement of the power within the OFDMA channel does not represent the power of all the signals within the channel.

[0009] Accordingly, a system and method for calculating appropriate transmit power for upstream ODFMA transmission are desired.

Brief Description of the Drawings

[0010] To better understand the present invention and to show how the present invention may be implemented, reference is now made, by way of example, to the following accompanying drawings.

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0012] During the creation of the CableLabs DOCSIS 3.1 specification, orthogonal frequency division multiplexing (OFDM) technology was introduced as a cable data transmission modulation technology. DOCSIS (Data Over Cable Service Interface Specification) is a set of standards for the transmission of high-speed data services over cable systems. Cable facility services are mainly of two types: digital video programming services and high-speed data services. Digital video programming services have a predefined data capacity that utilizes lower-dimensional QAM modulation, and these signals are less affected by the leakage of external spectral energy. Conversely, considering the increasing demand for high-speed data services in both the upstream and downstream directions, higher-dimensional modulation is required to increase the channel capacity for these services. As the dimension of modulation increases, more bits per hertz of the spectrum become possible, but a better signal-to-noise environment is also required to operate. Leakage noise can prevent these systems from operating at the peak bits-per-hertz data rate.

[0013] The DOCSIS 3.1 specification introduced OFDM (Orthogonal Frequency Division Multiplexing) as a way to provide a larger transmission bandwidth for high-speed data services. The OFDM technology is defined for direct use in the downstream direction and adapted for multiple access (Orthogonal Frequency Division Multiple Access - OFDMA) for use in the upstream direction. As will be explained in more detail below, in each direction (upstream / downstream), a relatively wide channel is subdivided into many small subcarriers. Specifically, in OFDM, the downstream data channel can be defined up to 196 MHz for the carriage of data services, and this 196 MHz band is further divided into 50 kHz subcarriers such that there are up to 3,880 subcarriers within a full 196 MHz OFDM channel. Each of the subcarriers is effectively an independent transmission channel in that each subcarrier can utilize its own radio modulation (within the available standard modulations available) according to the signal-to-noise ratio within its 50 kHz channel. From a more familiar perspective, a 50 kHz bandwidth is five times the bandwidth available from today's AM radio. A single 196 MHz OFDM channel could hold 15,000 AM radio stations. In the downstream direction, each of these subcarriers can use its own Quadrature Amplitude Modulation (QAM) level, which is equal to a different bit capacity per QAM symbol of the subcarrier. In the upstream direction, groups of subcarriers are combined to create tiny units of upstream bandwidth allocation known as "mini-slots" when time multiplexed. In the upstream direction, all subcarriers of a mini-slot are assigned the same QAM level and thus all subcarriers of a mini-slot have the same bit capacity per QAM symbol.

[0014] Orthogonal frequency division multiplexing (OFDM) extends the FDM technique by using multiple subcarriers within each channel. Instead of transmitting a high-rate data stream on a single subcarrier, OFDM utilizes a large number of orthogonal subcarriers that are arranged at close intervals and transmitted in parallel. Each subcarrier is modulated at a low symbol rate using a conventional digital modulation scheme (e.g., QPSK, 16QAM, etc.). However, many combinations of subcarriers enable a data rate similar to that of a conventional single-carrier modulation scheme within the same bandwidth.

[0015] For example, referring to FIG. 1, in the frequency domain, adjacent orthogonal tones or subcarriers 1 and 2 can each be independently modulated with complex data. Although only two subcarriers are illustrated in FIG. 1, those skilled in the art will understand that a typical OFDM transmission will include a large number of orthogonal subcarriers. As just mentioned, subcarriers 1 and 2 (as well as all other subcarriers) are orthogonal to each other. Specifically, as can be seen in FIG. 1, subcarrier 1 has spectral energy that includes a sinc function with a center frequency 3 having sidebands with peaks and nulls at regular intervals. These sidebands overlap those of subcarrier 2, but each of the spectral peaks of subcarrier 1 aligns with the nulls of subcarrier 2. Thus, the spectral energy overlap does not interfere with the ability of a system to recover the original signal. The receiver multiplies (i.e., correlates) the input signal with a known set of sine waves to recover the original set of transmitted bits.

[0016] In the time domain, all the frequency sub-carriers 1, 2, etc. of a single OFDM channel are combined at each symbol interval 4 by performing an inverse fast Fourier transform (IFFT) on the individual sub-carriers within the frequency domain. The guard band 5 is preferably inserted between each of the symbol intervals 4 to prevent inter-symbol interference caused by multi-path delay spread within the radio channel. In this way, a plurality of symbols included in each sub-carrier can be concatenated to generate the final OFDM burst signal. To recover the signal at the receiver, a fast Fourier transform (FFT) can be performed to recover the original data bits.

[0017] As also mentioned above, each sub-carrier of OFDM transmission can be independently modulated with complex data from among a plurality of predefined amplitudes and phases. FIG. 2 shows, for example, a quadrature amplitude modulation (QAM) technique in which a sub-carrier can be modulated between a selective one of 16 different phase / amplitude combinations (16QAM). Thus, for example, sub-carrier 1 in FIG. 1 can transmit symbol 0000 by having an amplitude of 25% and a phase of 45° at the first symbol interval, and can transmit symbol 1011 by having an amplitude of 75% and a phase of 135° at the second symbol interval. Similarly, sub-carrier 2 can transmit a selected one of a plurality of different symbols.

[0018] Figure 2 shows 16QAM modulation technology, but the latest DOCSIS transmission architecture enables modulation up to 16384QAM. Further, each of the subcarriers such as subcarriers 1, 2 shown in Figure 1 may operate with its own independent QAM modulation, i.e., subcarrier 1 may transmit 256QAM symbols, while subcarrier 2 may transmit 2048QAM symbols. Therefore, in order for the receiver and transmitter to communicate properly, the bitloading profile is a vector that specifies for each subcarrier the modulation order (16QAM, 256QAM, etc.) used by the subcarrier during the symbol interval. The current DOCSIS 3.1 specification allows each cable modem to be assigned up to five different bitloading profiles in the downstream direction and up to two different bitloading profiles in the upstream direction. The bitloading profile used for a given symbol interval is communicated between the cable modem and the headend, so that the transmitted information can be decoded properly.

[0019] Figure 3 shows an exemplary architecture 10 that includes a HFC network 12 that includes a headend 14 that delivers content to a subscriber device 24 at a subscriber premise shown in the figure as a cable modem, but one of ordinary skill in the art will understand that the subscriber device can include a set-top box, a gateway, a wireless phone, a computer, and the like. The HFC network 12 includes a headend 14, a plurality of hubs 20, each hub being associated with a plurality of nodes 22, and a plurality of subscriber devices 24 such as cable modems. The headend 14 typically includes a cable modem termination system (CMTS) 18 and a plurality of video EQAM units 16. Each of the nodes 22 has one or more corresponding access points, and each subscriber may have one or more corresponding network elements 24 shown in Figure 1 as a cable modem.

[0020] In the HFC architecture 10, video is modulated onto the RF network by the VEQ 16, which receives single and multi-program transport streams (SPTS and MPTS) encapsulated in Internet Protocol (IP) via the content delivery network 26 from various sources such as content providers. The content delivery network is typically a switching network in which packetized IP data is routed from one address to another.

[0021] FIG. 3 shows a centralized architecture in which the EdgeQAM unit 16 and the CMTS 18 are located at the head end 14, as described above. However, other distributed architectures move most of the functions of the EQAM / CMTS, such as physical layer and / or MAC layer processing, to the nodes of the network of distributed architectures such as R-PHY, R-MACPHY. Those skilled in the art will understand that the disclosed systems and methods can be used in either the centralized architecture shown in FIG. 1 or within any of such remote devices of distributed architectures.

[0022] As described above, a communication system that uses OFDMA to transmit upstream data can be configured with a much wider channel (up to 96 MHz of active spectrum) compared to a conventional upstream SCF-QAM channel. Since attenuation at higher frequencies is greater than at lower frequencies, the power levels can vary quite significantly across the bandwidth of an OFDMA subcarrier. CMTS 18 is configured to instruct the cable modem to equalize the power transmission levels among the subcarriers within a given OFDMA channel, but there is no easy way to determine the target or desired overall or average power level at which the OFDMA channel itself should be set. As a result, different upstream transmissions from different cable modems are equalized at the input of an upstream receiver such as a node. This can be a problem. For example, an upstream receiver such as node 22 may be configured to receive multiple upstream signals at different inputs and cannot tolerate an excessive mismatch in power among those inputs, or else it cannot distinguish the signals from noise. Further, even at a single input of node 22, the power loss as a function of frequency reduces the available bandwidth at higher frequencies as the signal strength degrades.

[0023] However, CMTS 18 has access to calibration measurements that specify an average power adjustment to be made over a specified frequency band across the entire upstream spectrum, in either a centralized or distributed architecture. The devices, systems, and methods disclosed in this application use these individual measurements to determine a function of the continuous power across the entire upstream spectrum and, from that determined function, calculate the power adjustment to be made at selected individual frequencies within a single upstream OFDMA channel. In a preferred embodiment, this selected individual frequency can be the center frequency of the OFDMA channel.

[0024] For example, in one embodiment, it may be estimated that the power attenuation (and thus the compensating power adjustment PA) varies as a quadratic polynomial function of frequency generalized by the following equation.

[0025] [Equation 1] PA = aF 2 +bF+c (Equation 1) Where PA is the desired power adjustment, F is the center frequency at which the power adjustment is performed, and a, b, and c are three constants that need to be calculated.

[0026] Preferably, the disclosed devices, systems, and methods receive individual power adjustments calculated across several OFDMA channels. One of ordinary skill in the art will understand that devices using the disclosed technology will vary depending on the architecture. For example, in a centralized architecture such as that disclosed in FIG. 3, the disclosed technology may be employed by the CMTS 18. In other architectures, particularly distributed access architectures, the disclosed technology may be employed by remote MACPHY devices (RMDs), remote physical devices (RMDs), etc.

[0027] For example, in one embodiment, the CMTS 18 may be configured to measure the received power at high, medium, and low split points supported by various DOCSIS iterations and instruct the cable modem to perform compensation adjustments accordingly. For example, assume that the CMTS 18 calculates appropriate power adjustments of 0.00 at the low split point of 45 MHz, 0.21 at the medium split point of 115 MHz, and 0.45 at the high split point of 185 MHz. For these values, the following equations hold.

[0028] [Equation 2] a45 2 +b45+c=0 a115 2 +b115+c=0.21 a185 2 +b185B+c=0.45.

[0029] Considering these equations, a continuous curve passing through all three points can be calculated as follows.

[0030] [Equation 3] PA = 3.06×10-06 F 2 + 2.51×10 -03 F - 1.19×10 -01

[0031] Figure 4 shows this curve.

[0032] Next, this equation can be used by CMTS 18 to instruct the cable modem 24 to transmit the upstream OFDMA channel at an average power level set to the value calculated for the center frequency, as just described. It should be understood that the foregoing method is fully compatible with existing techniques in which CMTS 18 equalizes the power levels of subcarriers within OFDMA transmissions by any given cable modem 24.

[0033] Those skilled in the art will understand that the foregoing examples have been described for illustrative purposes only, and that many variations of these examples can be readily adopted. For example, the generalized polynomial of Equation 1 can be extended to a cubic or higher-order polynomial equation, particularly when more sample point power levels are available and it is desired that they be used. Other implementations may assume other generalized functions, including linear functions, exponential functions, and the like. Further, more or fewer than three calibration values may be collected and used to determine any constants in the generalized equation exemplified by Equation 1.

[0034] FIG. 6 shows an exemplary method 100 according to the present disclosure. In a preferred embodiment, method 100 can be implemented within a processing device of a head end or remote device within a communication network, such as an RPD, RMD. However, in other embodiments, the method can be implemented in white box hardware locally or remotely connected to a CMTS or similar receiver that receives an upstream signal from a cable modem or other such user equipment. In some embodiments, the processing device can be configured to repeatedly collect measurements and adjust the equalization value upon a change in the equipment conditions, i.e., a change in the constants in the generalized equation exemplified by Equation 1.

[0035] In method 100, at step 102, calibration values are received for individual frequencies across a plurality of OFDMA channels. In some embodiments, this step may include measuring the power level of each upstream signal received from one or more cable modems 24 at several individual frequencies and calculating the power adjustment necessary to equalize the received power at those frequencies. The calibration values are preferably received for individual frequencies across all OFDMA channels.

[0036] At step 102, the calibration values are used to calculate constants in a function PA = G(F) that represents the power adjustment as some continuous function of frequency. The form of the function PA = G(F) may vary according to the embodiment, i.e., some embodiments may use a quadratic (or higher-order) polynomial as this function, and other embodiments may specify a linear function, an exponential function.

[0037] At step 106, once the constants of the function PA = G(F) are calculated, the function can preferably be used to determine a power adjustment value associated with the upstream transmission of the OFDMA channel as a whole, e.g., the power level at the center frequency of the OFDMA channel. Next, an upstream transmitter such as a cable modem can be instructed to transmit the OFDMA channel at a power level that conforms to that determination.

[0038] The present invention is not limited to the specific embodiments described, and may be modified therein in accordance with the principles of superior law, including the doctrine of equivalents or any other principle that extends the scope of enforceable patent claims beyond their literal scope, without departing from the scope of the present invention as defined in the appended claims. It will be understood that any incorporation by reference of the above documents is limited so that no subject matter contrary to the explicit disclosure of this specification is incorporated. Any incorporation by reference of the above documents is further limited so that the claims contained in the document are not incorporated herein by reference. Any incorporation by reference of the above documents is still further limited so that any definition provided in the document is not incorporated herein by reference unless explicitly included herein. If there is a conflicting use between this document and the documents so incorporated by reference, the use in the incorporated reference(s) should be considered supplementary to that of this document. For irreconcilable conflicts, the use in this document shall govern. Unless the context otherwise indicates, a reference to the number of instances of an element in a claim requires at least the recited number of instances of the element, whether it be a reference to one instance or to more than one instance, but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than recited. The word "comprising" or its derivatives as used in the claims is used in a non-exclusive sense, not intending to exclude the presence of other elements or steps in the claimed structure or method.

Claims

1. A method comprising: receiving calibration values determined from a plurality of received upstream signals at respective different frequencies; using the calibration values to determine constants in a continuous function that associates power adjustment with signal frequency; using the continuous function to determine a power adjustment associated with an entire OFDMA channel; and instructing at least one upstream transmitter to transmit an upstream OFDMA signal that conforms to the determined power adjustment.

2. The method of claim 1, wherein the continuous function is a polynomial function.

3. The method of claim 2, wherein the polynomial function is a quadratic function.

4. The method of claim 1, wherein at least three calibration values are received.

5. The method of claim 1, wherein the determined power adjustment is associated with a center frequency of the OFDMA channel.

6. The method of claim 1, implemented in at least one of a CMTS and a remote device in a distributed access architecture.

7. The method of claim 1, wherein the calibration values together span more than one upstream OFDMA channel.

8. A device comprising: a processor configured to receive calibration values determined from a plurality of upstream signals; use the calibration values to determine constants in a continuous function that associates power adjustment with signal frequency; use the continuous function to determine a power adjustment associated with an entire OFDMA channel; and instruct at least one upstream transmitter to transmit an upstream OFDMA signal that conforms to the determined power adjustment.

9. The device of claim 8, wherein the continuous function is a polynomial function.

10. The device of claim 9, wherein the polynomial function is a quadratic function.

11. The device of claim 8, wherein at least three calibration values are received.

12. The device of claim 8, wherein the determined power adjustment is associated with a center frequency of the OFDMA channel.

13. The device of claim 8, comprising at least one of a CMTS and a remote device in a distributed access architecture.

14. The device of claim 13, comprising at least one of an RPD and an RMD.

15. The device of claim 8, wherein the calibration values together span more than one upstream OFDMA channel.

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