CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Application No. 63/313,493 filed February 24, 2022, the content of which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] The subject matter of this application generally relates to the creation and assignment of bit loading profiles to cable modems in a DOCSIS transmission architecture.

[0003] Orthogonal Frequency Division Multiplexing (OFDM) technology was introduced as a cable data transmission modulation technique during the creation of the CableLabs DOCSIS 3.1 specification. OFDM technology was defined for use directly in the downstream direction and was adapted for multiple access (Orthogonal Frequency Division with Multiple Access - OFDMA) for use in the upstream direction. In each direction, the relatively wide channel is subdivided into many small subcarriers. In the dow nstream direction, each of these subcarriers may use its own Quadrature Amplitude Modulation (QAM) level, which equates to a different bit capacity per subcarrier QAM symbol. In the upstream direction, groups of subcarriers are combined and, when time multiplexed, create the atomic unit of upstream bandwidth assignment known as a “minislot.” In the upstream direction, all subcarriers of a minislot are assigned the same QAM level and thus all subcarriers of a minislot have the same bit capacity per QAM symbol.

[0004] The purpose of OFDM/OFDMA technology is to maximize the efficiency of data transmissions across a cable data network by optimizing the QAM modulation level used for each subcarrier of RF frequency bandwidth. Ideally, each cable modem would be assigned its own vector of per-subcarrier QAM modulation levels, i.e. a. bit loading vector, that is uniquely optimized for that cable modem. For cost reasons, however, the DOCSIS 3.1 specification defines a compromise where groups of cable modems having similar RF characteristics can be assigned the same bit loading vector, if that vector is constructed such that that all cable modems assigned that vector could use it. In this manner, the needed number of bit loading vectors could be reduced to a cost-manageable set of “bit loading profiles” that could each be assigned to multiple cable modems at once For example, the current generation of DOCSIS allows head ends that communicate with cable modems to utilize up to sixteen bit loading profiles per channel in the downstream direction and up to seven bit loading profiles per channel in the upstream direction. Similarly, the current generation of DOCSIS permits each cable modem to be assigned up to five profiles per channel in the downstream direction and up to two profiles per channel in the upstream direction.

[0005] However, because each cable modem is no longer assigned a bit loading profile uniquely optimized for that cable modem, transmissions over the network are more prone to errors. What is desired, therefore, is an improved method of determining a plurality of bit loading vectors that are assigned among cable modems in a DOCSIS network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

[0007] FIG. 1 illustrates an Orthogonal Frequency Division Multiplexing technique.

[0008] FIG. 2 illustrates a Quadrature Amplitude Modulation technique.

[0009] FIG. 3 shows a DOCSIS network transmission architecture for delivering content to a plurality of cable modems, organized into groups so as to reduce transmission errors, and where the cable modems each use one or more bit loading profiles.

[0010] FIG. 4 shows and exemplary noise-centric system for assigning bit loading profiles to cable modems based on measured capacity rather than measured noise.

DETAILED DESCRIPTION

[0011] OFDM is based on the well-known technique of Frequency Division Multiplexing (FDM). In FDM different streams of information are mapped onto separate parallel frequency channels. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.

[0012] Orthogonal Frequency Division Multiplexing (OFDM) extends the FDM technique by using multiple subcarriers within each channel. Rather than transmit a high-rate stream of data with a single subcarrier, OFDM makes use of a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. Each subcarrier is modulated with a conventional digital modulation scheme (e.g. QPSK, 16QAM, etc.) at low symbol rate. However, the combination of many subcarriers enables data rates similar to conventional single-carrier modulation schemes within equivalent bandwidths.

[0013] Referring for example to FIG. 1, in the frequency domain, adjacent orthogonal tones or subcarriers 1 and 2 may be each independently modulated with complex data. Though only two subcarn ers are illustrated in FIG. 1, those of ordinary skill in the art will appreciate that a ty pical OFDM transmission will include a large number of orthogonal subcarriers. As just note noted, 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 comprising a sine function having a center frequency 3 with sidebands having peaks and nulls at regular intervals. These sidebands overlap those of subcarrier 2, but each of the spectral peaks of subcarrier 1 align with the nulls of subcarrier 2. Accordingly, the overlap of spectral energy does not interfere with the system’s ability to recover the original signal; the receiver multiplies (i.e., correlates) the incoming signal by the known set of sinusoids to recover the original set of bits sent.

[0014] In the time domain, all frequency subcarriers 1, 2 etc. are combined in respective symbol intervals 4 by performing an Inverse Fast Fourier Transform (IFFT) on the individual subcarriers in the frequency domain. Guard bands 5 may preferably be inserted between each of the symbol intervals 4 to prevent inter-symbol interference caused by multi-path delay spread in the radio channel. In this manner, multiple symbols contained in the respective subcarriers can be concatenated to create a final OFDM burst signal. To recover the signal at a receiver, a Fast Fourier Transform (FFT) may be performed to recover the ongmal data bits. [0015] As also noted previously, each subcarrier in an OFDM transmission may be independently modulated with complex data among a plurality of predefined amplitudes and phases. FIG. 2, for example, illustrates a Quadrature Amplitude Modulation (QAM) technique where a subcarrier may be modulated among a selective one of sixteen different phase/amplitude combinations (16QAM). Thus, for example, subcarrier 1 of FIG. 1 may in a first symbol interval transmit the symbol 0000 by having an amplitude of 25% and a phase of 45° and may in a second symbol interval transmit the symbol 1011 by having an amplitude of 75% and a phase of 135°. Similarly, the subcarrier 2 may transmit a selected one of a plurality of different symbols.

[0016] FIG. 2 illustrates a 16QAM modulation technique, but modem DOCSIS transmission architectures allow for modulations of up to 4096QAM. Moreover, each of the subcarriers 1, 2, etc. shown in FIG. 1 may operate with its own independent QAM modulation, i.e. subcarrier 1 may transmit a 256QAM symbol while subcarrier 2 may transmit a 2048QAM symbol. Thus, in order for a receiver and a transmitter to properly communicate, a bit loading profile is a vector that specifies, for each subcarrier, the modulation order (16QAM, 256QAM, etc) used by the subcarrier during a symbol interval. The current DOCSIS 3.1 specification allows each cable modem to be assigned up to five different bit loading profiles that it is allowed to use in the downstream direction, and up to two different bit loading profiles that it is allowed to use in the upstream direction. The particular bit loading profile used for a given symbol interval is communicated between the cable modem and a head end, so that transmitted information can be properly decoded.

[0017] FIG. 3 illustrates a system that uses bit loading profiles to communicate data in a DOCSIS architecture. Specifically, a system 10 may include a Cable Modem Termination Service (CMTS) 12 typically found within a head end of a video content and/or data service provider. The CMTS 12 communicates with a plurality of cable modems 16 at its customers' premises via a network through one or more nodes 14. Typically, the network may be a hybrid fiber-coaxial network where the majority of the transmission distance comprises optical fiber, except for trunk lines to Optical Network Units (not shown) at the customers' premises and cabling from the ONUs to the cable modems 16, which are coaxial. More recent architectures, e.g. Fiber-to-the Premises (FTTP) however, have replaced the entire line from the ONUs to the upstream node with optical fiber.

[0018] As already mentioned, ideally each cable modem 16 would be assigned a bit loading profile specifically tailored to the performance characteristics of that cable modem. For example, higher nodulation orders can be assigned to subcarriers experiencing higher a SNR characteristic over a channel used by a cable modem, and lower modulation orders may be best for subcarriers with a low SNR characteristic. In this manner, the bandwidth efficiency of transmissions to and from a cable modem are high when if the cable modem’s ideal bit loading vector closely follows the bit loading profile in use by the cable modem. However, because the DOCSIS standard restricts the number of available profiles that can be used by cable modems, a Cable Modem Termination Service (CMTS) must communicate with multiple cable modems 16 with different SNR profiles using the same bit loading profile. For example, as FIG. 3 shows an example where cable modems 16 are segmented into groups 18, 20, and 22 where the cable modems 1 in each group are assigned a common bit loading profile by the CMTS 12. This virtually guarantees that not all cable modems will use a bit loading profile that closely follows its optimum bit loading vector.

[0019] Thus, in order to most efficiently use the limited number of available bit loading profiles, the CMTS 12 preferably divides cable modems 16 into groups that each have similar performance characteristics. To this end, the CMTS 12 may periodically include in the downstream transmission known pilot tones that together span the entire OFDM downstream bandwidth. Each cable modem 16 then uses these pilots to measure its error for received downstream transmissions at each subcarrier frequency, where the error at a particular modulation frequency is measured based on the vector in the I-Q plane (shown in FIG. 2) between the ideal constellation point at that modulation order and the actual constellation point received by the receiver. Such error measurements may comprise any of several available forms, including the actual error vector, the Euclidian distance between these two points, or the Modulation Error Ratio (MER) calculated from the error vector. Alternatively, in some embodiments, the error measurement may be expressed as a maximum QAM value that a cable modem may reliably use at a given subcarrier, given the measured error. For example, the DOCSIS 3.1 PHY specification contains tables that map modulations orders to the minimum carrier-to-noise ratios (approximated by MER) required to carry them, as shown in the following exemplary table in the downstream direction:

Constellation CNR (1 GHz) CNR (1.2 GHz)

4096 41 41.5

2048 37 37.5

1024 34 34.

512 30.5 30.5

256 27 27

128 24 24

64 21 21

16 15 15

In this exemplary table, “CNR” or Carrier Boise Ration is defined as the total signal power in an occupied bandwidth divided by the total noise in that occupied bandwidth, and ideally is the equivalent of equalized MER.

[0020] The collection of the errors for a cable modem, across all subcarrier frequencies, produces the modulation error vector for that cable modem 16, which is transmitted back to the CMTS 12. For upstream transmissions, the process is generally reversed; the CMTS 12 commands each cable modem to send known pilot tones to the CMTS together spanning the entire OFDM upstream bandwidth in a single upstream probing signal for each particular cable modem 16. The CMTS 12 uses these received probing signals to estimate the upstream modulation error vectors for each of the cable modems.

[0021] Once the CMTS 12 has assembled the modulation error vectors for all cable modems that it serves, it preferably uses these vectors to organize the cable modems into “N” groups of cable modems, where “N” is at most the number of profiles available to the collection of cable modems. For example, in a DOCSIS 3.1 environment, cable modems could be arranged in up to sixteen groups for receiving signals in the downstream direction and up to seven groups for receiving signals in the upstream direction.

[0022] Once all cable modems are finally assigned into the desired plurality of groups, a set of available bit loading profiles may be generated for the head end to assign to the population of cable modems serviced by the head end in each of the upstream and downstream directions, and subsequently the cable modems in each group may be assigned profiles from this set For example, some methods may select an initial or starting set of bit loading profiles, each representing a different tier or quality of service, where the bit loading profiles are subsequently adjusted based upon the groupings of cable modems as appropriate where, say a small change in a profile can bring several cable modems into that tier or to guarantee a tier of service to a specific cable modem. Such methods are disclosed in prior U.S. Patent No. 9,647,786 which is hereby incorporated in its entirety into this disclosure. Preferably in some embodiments, for each cable modem in a group, the nearest five bit loading profiles, ordered by vector distance, should be assigned to the cable modem in the downstream direction and the nearest two bit loading profiles, ordered by vector distance should be assigned to the cable modem in the upstream direction.

[0023] As just described, RF Quadrature Amplitude Modulation (QAM) techniques trade noise immunity for channel capacity for channels within the same amount of bandwidth. In other words, the assignment of higher orders of channel modulation can increase the information capacity of a channel occupying a region of RF spectrum but this will also reduce the amount of tolerated interference on the channel.

[0024] Current RF networking devices (including, but not limited to, DOCSIS networking devices with MAC Domain Controllers, such as CMTS, CCAP Core, RMD, etc.) may assign modulation levels (a.k.a. bitloading) to entire channels or perhaps to spectral regions of channels to maximize channel capacity in the presence of noise. In order to do this, a budget is created for the maximum amount of interference that is expected be present in each region on the channel. The network device will then assign a modulation order which is appropriate for this expected noise level. As a result, a calculated channel capacity can then be assumed for customer traffic. The device will attempt to assign traffic up to a significant fraction of this calculated channel capacity.

[0025] This approach is capacity-centric, rather than robust-centric (noise immunity). If an unforeseen event causes interference in excess of the interference budget, the unexpected interference may cause communications to fail, even when the current level of modulation provides excess capacity for the demanded traffic. To guard against this failure, some existing systems poll subscriber devices for metrics of modulation error ratios (MER). These metrics are collected periodically and the bitloading of certain channels may be adjusted to fit the MER measurements. This method also attempts to maximize the channel throughput and therefore offers a minimum of channel RF robustness at each interval. This method can adjust to noise conditions only at a rate set by the metrics collection polling interval, however.

[0026] Rather than maximizing (perhaps unused) channel capacity based upon an assumed interference noise budget, this invention seeks to maximize noise robustness for the current traffic demand. In this case, the idea is to find the lowest modulation scheme (with the most noise-immunity) that will support the current traffic demand.

[0027] In the interest of supporting bursts and perhaps a gradually growing traffic demand, a “cushion” of extra bandwidth may be configured that could be added to each of multiple thresholds, described as follows. A series of increasing thresholds of traffic demand could be configured to allow the system to automatically adjust (thus increasing the modulation order) to an increasing traffic demand. In this case, the system would adjust the modulation of the channels to allow the capacity of the next higher threshold, plus the cushion for all but the highest threshold. Similarly, a series of decreasing thresholds (not necessarily exactly matching the series of increasing thresholds) could be configured to allow the noise robustness to increase by decreasing the overall capacity to meet the new (lower) traffic demand, plus the cushion.

[0028] These increasing and decreasing thresholds could be configured for individual channels, pieces of channels, or for the entire set of channels that reach a subscriber device. FIG. 4 shows such an implementation where traffic demand over a communication network fluctuates over time between a plurality of different traffic demand levels. A number of thresholds are set, each associated with a particular level of traffic demand. In the example shown in FIG. 4, the thresholds are preferably set to include a small cushion of bandwidth to accommodate bursts.

[0029] When implemented, this invention would provide a good trade-off between noise immunity and traffic demand. Note that, if desired, the uppermost increasing threshold could be configured to provide the same noise budget noise immunity and capacity of the methods currently used while providing noise immunity benefits at lower traffic levels. This invention provides noise immunity benefit over the PMA solution in all cases except when the system is running at maximum capacity Tn addition, this invention can adjust to traffic demand much faster than the expected PMA solution metrics polling interval.

[0030] It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word "comprise" or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.