Abstract:
Methods and systems capable of improving the transmission of data along an upstream path of a Hybrid Fiber-Coaxial Cable Network, from a transmitter in a node to a receiver in a Cable Modem Termination System.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. 119(e) of priority of U.S. Provisional Application No. 61/862,670, as filed on Aug. 6, 2013. 
    
    
     BACKGROUND 
     The present disclosure relates to systems and methods that provide video and data over a cable transmission network. 
     Referring to  FIG. 1 , cable TV (CATV) systems were initially deployed as video delivery systems. In its most basic form the system received video signals at the cable head end, processed these for transmission and broadcast them to homes via a tree and branch coaxial cable network. In order to deliver multiple TV channels concurrently, early CATV systems assigned 6 MHz blocks of frequency to each channel and Frequency Division Multiplexed (FDM) the channels onto the coaxial cable RF signals. Amplifiers were inserted along the path as required to boost the signal and splitters and taps were deployed to enable the signals to reach the individual homes. Thus all homes received the same broadcast signals. 
     As the reach of the systems increased, the signal distortion and operational cost associated with long chains of amplifiers became problematic and segments of the coaxial cable were replaced with fiber optic cables to create a Hybrid Fiber Coax (HFC) network to deliver the RF broadcast content to the coaxial neighborhood transmission network. Optical nodes in the network acted as optical to electrical converters to provide the fiber-to-coax interfaces. 
     As the cable network evolved, broadcast digital video signals were added to the multiplexed channels. The existing 6 MHz spacing for channels was retained but with the evolving technology, each 6 MHz block could now contain multiple programs. Up to this point, each home received the same set of signals broadcast from the head end so that the amount of spectrum required was purely a function of the total channel count in the program line-up. 
     The next major phase in CATV evolution was the addition of high speed data service, which is an IP packet-based service, but appears on the HFC network as another 6 MHz channel block (or given data service growth, more likely as multiple 6 MHz blocks). These blocks use FDM to share the spectrum along with video services. Unlike broadcast video, each IP stream is unique. Thus the amount of spectrum required for data services is a function of the number of data users and the amount of content they are downloading. With the rise of the Internet video, this spectrum is growing at 50% compound annual growth rate and putting significant pressure on the available bandwidth. Unlike broadcast video, data services require a two-way connection. Thus, the cable plant had to provide a functional return path. Pressure on the available bandwidth has been further increased with the advent of narrowcast video services such as video-on-demand (VOD), which changes the broadcast video model as users can select an individual program to watch and use VCR-like controls to start, stop, and fast-forward. In this case, as with data service, each user requires an individual program stream. 
     Thus, the HFC network is currently delivering a mix of broadcast video, narrowcast video, and high speed data services. Additional bandwidth is needed both for new high definition broadcast channels and for the narrowcast video and data services. The original HFC network has been successfully updated to deliver new services, but the pressure of HD and narrowcast requires further change. The HFC network is naturally split into the serving areas served from the individual fiber nodes. The broadcast content needs to be delivered to all fiber nodes, but the narrowcast services need only be delivered to the fiber node serving the specific user. Thus, there is a need to deliver different service sets to each fiber node and also to reduce the number of subscribers served from each node (i.e. to subdivide existing serving areas and thus increase the amount of narrowcast bandwidth available per user). 
       FIG. 1  is a generalized representation of part of the cable TV infrastructure, which includes the cable head end; the Hybrid Fiber Coax (HFC) transmission network, and the home. The CATV head end receives incoming data and video signals from various sources (e.g., fiber optic links, CDN&#39;s, DBS satellites, local stations, etc.). The video signals are processed (reformatting, encryption, advertising insertion etc.) and packaged to create the program line up for local distribution. This set of video programs is combined with data services and other system management signals and prepared for transmission over the HFC to the home. All information (video, data, and management) is delivered from the head end over the HFC network to the home as RF signals. In the current practice, systems in the head end process the signals, modulate them to create independent RF signals, combine these into a single broadband multiplex, and transmit this multiplex to the home. The signals (different video channels and one or more data and management channels) are transmitted concurrently over the plant at different FDM frequencies. In the home, a cable receiver decodes the incoming signal and routes it to TV sets or computers as required. 
     Cable receivers, including those integrated into set-top boxes and other such devices, typically receive this information from the head end via coaxial transmission cables. The RF signal that is delivered can simultaneously provide a wide variety of content, e.g. high speed data service and up to several hundred television channels, together with ancillary data such as programming guide information, ticker feeds, score guides, etc. Through the cable receiver&#39;s output connection to the home network, the content is delivered to television sets, computers, and other devices. The head end will typically deliver CATV content to many thousands of individual households, each equipped with a compatible receiver. 
     Cable receivers are broadly available in many different hardware configurations. For example, an external cable receiver is often configured as a small box having one port connectable to a wall outlet delivering an RF signal, and one or more other ports connectable to appliances such as computers, televisions, and wireless routers or other network connections (e.g., 10/100/1,000 Mbps Ethernet). Other cable receivers are configured as circuit cards that may be inserted internally in a computer to similarly receive the signals from an RF wall outlet and deliver those signals to a computer, a television, or a network, etc. Still other cable receivers may be integrated into set-top boxes, such as the Motorola DCX3400 HD/DVR, M-Card Set-Top, which receives an input signal via an RF cable, decodes the RF signal to separate it into distinct channels or frequency bands providing individual content, and provides such content to a television or other audio or audiovisual device in a manner that permits users to each select among available content using the set top box. 
     As previously mentioned, the CATV transmission architecture has been modified to permit data to flow in both directions, i.e. data may flow not only from the head end to the viewer, but also from the viewer to the head end. To achieve this functionality, cable operators dedicate one spectrum of frequencies to deliver forward path signals from the head end to the viewer, and another (typically much smaller) spectrum of frequencies to deliver return path signals from the viewer to the head end. The components in the cable network have been modified so that they are capable of separating the forward path signals from the return path signals, and separately amplifying the signals from each respective direction in their associated frequency range. 
     The Hybrid/Fiber Coax (HFC) cable network architecture broadly depicted in  FIG. 1  includes a head end system  10  having multiple devices for delivery of video and data services including EdgeQAMS (EQAMs) for video, cable modem termination systems (CMTS) for data, and other processing devices for control and management. These systems are connected to multiple fiber optic cables  12  that go to various neighborhood locations that each serve a smaller community. A fiber optic neighborhood or multi-neighborhood node  14  is located between each fiber optic cable  12  and a corresponding trunk cable  16 , which in turn is interconnected to the homes  20  through drop cables  18  and feeder cables (not shown). Because the trunk cable  16 , as well as the branch networks and feeder cables  18 , each propagate RF signals using coaxial cable, the nodes  14  convert the optical signals to electrical signals that can be transmitted through a coaxial medium, i.e. copper wire. Similarly, when electrical signals from the home reach the node  14  over the coaxial medium, those signals are converted to optical signals and transmitted across the fiber optic cables  12  back to the systems at the head end  10 . The trunk cables  16  and/or feeder cables  18  may include amplifiers  17 . Connected to each trunk cable  16  is a branch network that connects to feeder cables (or taps) that each enter individual homes to connect to a respective cable receiver. 
     Hybrid fiber/coax networks generally have a bandwidth of approximately 750 MHz or more. Each television channel or other distinct content item transmitted along the forward path from the head end to a user may be assigned a separate frequency band, which as noted earlier has a typical spectral width of 6 MHz. Similarly, distinct content delivered along the return path from a user to the head end may similarly be assigned a separate frequency band, such as one having a spectral width of 6.4 MHz. In North America, the hybrid fiber/coax networks assign the frequency spectrum between 5 MHz and 42 MHz to propagate signals along the return path, and assign the frequency spectrum between 50 MHz and 750 MHz or more to propagate signals along the forward path. 
     Referring to  FIG. 2 , a cable modem termination system (CMTS)  30  may be installed at the head end, which instructs each of the cable modems when to transmit return path signals, such as Internet protocol (IP) based signals, and which frequency bands to use for return path transmissions. The CMTS  30  demodulates the return path signals, translates them back into (IP) packets, and redirects them to a central switch  32 . The central switch  32  redirects the IP packets to an IP router  34  for transmission across the Internet  36 , and to the CMTS which modulates forward path signals for transmission across the hybrid fiber coax cables to the user&#39;s cable modem. The central switch  32  also sends information to, and receives information from, information servers  38  such as video servers. The central switch  32  also sends information to, and receives information from, a telephone switch  40  which is interconnected to the telephone network  42 . In general, cable modems are designed to only receive from, and send signals to, the CMTS  30 , and may not communicate directly with other cable modems networked through the head end. 
       FIG. 3  shows an exemplary architecture for delivering CATV content between a head end  10  to a node  14 . The head end  10  may in some instances include a plurality of direct modulation EdgeQAM units  50  which each receive digitally encoded video signals, audio signals, and/or IP signals, and each directly outputs a spectrum of amplitude-modulated analog signal at a defined frequency or set of frequencies to an RF combining network  52 , which in turn combines the received signals. An optical transmitter  54  then sends the entire spectrum of the multiplexed signals as an analog transmission through an optical fiber network  56  along a forward path to the node  14 . The fiber optic network, as will be explained in more detail later, is also capable of conveying optical signals from the node  14  to the head end  10  via an optical path between a transmitter  58  in the node  14  and a receiver  60  in the head end. In the specification, the drawings, and the claims, the terms “forward path” and “downstream” may be interchangeably used to refer to a path from a head end to a node, a node to an end-user, or a head end to an end user. Conversely, the terms “return path”, “reverse path” and “upstream” may be interchangeably used to refer to a path from an end user to a node, a node to a head end, or an end user to a head end. Also, it should be understood that, unless stated otherwise, the term “head end” will also encompass a “hub,” which is a smaller signal generation unit downstream from a head end, often used for community access channel insertion and other purposes, that generally mimics the functionality of a head end, but may typically not include equipment such as satellite dishes and telephone units. Hubs are commonly known to those skilled in the art of the present disclosure. It should be understood that although  FIG. 3  illustrates a head end  10  that utilizes direct modulation EdgeQAMs, other architectures may employ other modulators, such as an analog EdgeQAM modulator or a Converged Cable Access Platform (CCAP) modulation system. 
     Directly-modulated EdgeQAM units have become increasingly sophisticated, offering successively higher densities, which in turn means that each EdgeQAM unit can process more channels of CATV data. For example, modern EdgeQAM modulation products can now simultaneously generate 32 or more channels on a single output port. With more channels being modulated per output port, the amount of combining required by the RF combining network  52  is reduced, with a corresponding simplification in the circuitry at the head end. The term ‘QAM’ is often used to interchangeably represent either: (1) a single channel typically 6 MHz wide that is Quadrature Amplitude Modulated (thus a “32 QAM system” is shorthand for a system with 32 Quadrature Amplitude Modulated channels; or (2) the depth of modulation used by the Quadrature Amplitude Modulation on a particular channel, e.g. 256 QAM means the signal is modulated to carry 8 bits per symbol while 4096 QAM means the signal is modulated to carry 12 bits per symbol. A higher QAM channel count or a higher QAM modulation means that a higher number of content “channels” can be delivered over a transmission network at a given standard of quality for audio, video, data, etc. QAM channels are constructed to be 6 MHz in bandwidth in North America, to be compatible with legacy analog TV channels and other existing CATV signals. However, more than one video program or cable modem system data stream may be digitally encoded within a single QAM channel. The term channel is unfortunately often used interchangeably, even though a QAM channel and a video program are not often the same entity—multiple video programs can be and usually are encoded within a single 6 MHz QAM channel. In this case, the modern EdgeQAM modulation products generate multiple instances of the 6 MHz bandwidth QAM channels. This simplifies the head end structure since some subset of the RF combining is now performed within the EdgeQAM units rather than in the external RF combining network. Packaging multiple QAM generators within a single package also offers some economic value. 
     As noted previously, modern CATV delivery systems over an HFC network provides content that requires communication along both a forward path and a return path, and over time, the quantity and quality of data transmission along each of these paths has increased drastically, which can be seen for example in the evolution of the DOCSIS standard from its original 1.0 release to the impending 3.1 release. 
     DOCSIS (Data Over Cable Service Interface Specifications) was developed by a consortium of companies, including Cable Labs, ARRIS, Cisco, Motorola, Netgear, and Texas Instruments, among others. The first specification, version 1.x, was initially released in March 1997 and called for a downstream throughput of approximately 43 Mbps and an upstream throughput of approximately 10 Mbps along a minimum of one channel. DOCSIS 2.0, released in late 2001 increased the maximum upstream throughput to approximately 31 Mbps, again for a minimum of one channel. DOCSIS 3.0, released in 2006 required that hardware be able to support the DOCSIS 2.0 throughput standards of 43 Mbps and 31 Mbps, respectively, along minimum of four channels in each direction. The DOCSIS 3.1 platform is aiming to support capacities of at least 10 Gbps downstream and 1 Gbps upstream using 4096 QAM. The new specification aims to replace the 6 MHz downstream and 6.4 MHz upstream wide channel spacing with smaller 25 kHz to 50 kHz orthogonal frequency division multiplexing (OFDM) subcarriers, which can be bonded inside a block spectrum that could end up being about 192 MHz wide for downstream and 96 MHz for upstream. 
     Providing increasing throughput along the upstream path has been particularly problematical the presence of upstream impairments including ingress. Ingress is radio frequency (RF) energy that has varying bandwidth and RF levels and can enter the CATV upstream plant via cable network defects. CATV network defects may include loose or corroded connectors, unterminated ports, and damaged cables, for example. Thus, to continue to meet the evolving needs of delivering CATV content, improved techniques for transmission along the upstream path, and particularly in the presence of upstream impairments. are desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows an exemplary Hybrid/Fiber Coax CATV network including a head end that delivers CATV content to a plurality of homes. 
         FIG. 2  shows an exemplary architecture of a head end, such as the ones shown in  FIG. 1 . 
         FIG. 3  shows an exemplary EdgeQAM architecture for a head end to communicate with a node along a forward path to deliver CATV content over a network. 
         FIG. 4  shows an example of an upstream transmission system according to one aspect of the present disclosure. 
         FIG. 5  shows a band-limited signal capable of being sampled at a rate of less than twice its upper cutoff frequency while still preserving all its information. 
         FIG. 6  shows a brassboard DRR output spectrum using existing 2×85 Digital Return Transmitter (DRT) and Digital Return Receiver (DRR) systems. 
         FIG. 7  shows an exemplary architecture including the system shown in  FIG. 1 . 
         FIGS. 8 and 9  each show respective examples of DOCSIS 3.0 capacity aggregation in a node. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  generally illustrates an improved system for transmitting data along a return path from a Digital Return Transmitter (DRT)  107 , in a node for example, to a Digital Return Receiver (DRR)  109  in a head end, for example. Specifically,  FIG. 4  shows a DRT  107  receiving two signals  102  and  104  for transmission to the DRR  109 . Future DOCSIS channels may occupy a maximum bandwidth of 96 MHz, approximately 15 times larger than the largest upstream DOCSIS 3.0 channel, where each of the signals  102  and  104  can generate a digital signal rate anywhere in the range of 800 Mbps to 2.25 Gbps depending on the signal band, sampling rate and the number of bits used to sample each signal. Even when digitizing only two of these signals, and transmitting them both together using time division multiplexing of the digital signals, the aggregated digital signal rate may exceed the capabilities of existing digital and optical transport platforms. 
     The system of  FIG. 4 , however, permits the signals  102  and  104  to be simultaneously transmitted using existing architectures by taking advantage of the principles of bandpass sampling. Ordinarily, the Nyquist sampling theorem dictates that in order to completely preserve the information in a transmitted signal occupying a limited portion of the frequency spectrum, the signal must be sampled at a rate equal to twice the upper frequency limit of the signal. For example, if the maximum frequency of the signal is 100 MHz, the signal must be sampled at 200 million samples per second in order to preserve all the information in the signal. At even modest symbol bit rates of say six to eight bits per sample, the required throughput to sample at the full Nyquist rate can become significant. 
       FIG. 5 , however, illustrates a circumstance in which all information in a transmitted signal may be preserves without sampling at the rate of twice the upper cutoff frequency of the signal. Specifically, a signal  140   a ,  140   b  may be bandlimited to a 20 MHz segment of the frequency spectrum from plus or minus 90 MHz to plus or minus 110 MHz. It will be understood by those skilled in the art that the signal need only be sampled on the positive sideband, as the negative sideband is simply a mirror image of the positive sideband.  FIG. 5  also shows spectral aliases  142  of the signal  140   a  that extend from the baseband to the signal component  140   a . Stated differently, a theoretical repeating signal occupying a single frequency on the spectrum at 100 MHz, thus repeating 100 million times per second, has a spectral alias occurring at every 1 Hz interval. By extension, the signal  140   a  has spectral aliases  142  that begin at baseband and repeat at every integral multiple of the width of the signal  140   a . If signal  140   a  is located on the spectrum at an integral multiple of its width, the spectral aliases do not overlap, and thus the signal  140   a  may be fully sampled by sampling its baseband spectral alias only, i.e. by sampling at a rate of twice the spectral width signal  140   a  rather than twice its upper cutoff frequency. If the signal  140   a  is located on the spectrum at a position that is not an integral multiple of its width, the spectral aliases will overlap, but the signal  140   a  may be fully sampled by sampling at a rate that is greater than twice its spectral width, but still much less than twice its upper cutoff frequency. 
     As noted previously, existing hybrid fiber coax architectures include components that would ordinarily be considered as lacking the capability of processing two or more signals  102  and  104  sequentially positioned on the frequency position, if those signals conformed to anticipated future DOCSIS standards. The present inventors realized, however, that by taking advantage of bandpass sampling principles, existing architectures could be modified without the need to upgrade much of the equipment in existing architectures. 
     Referring again to  FIG. 4 , the disclosed system may preferably sample and transmit plural wideband orthogonal frequency division multiple access (OFDM) channels  102  and  104 , each with up to 96 MHz BW. It should be understood, however, that other embodiments may simultaneously transmit more than two OFDM channels, such as four channels for example, guardband  106  between channels may be included to prevent interference between the channels. In this example, a fully loaded DOCSIS return may require an upper frequency of &gt;200 MH, e.g. a lower OFDM channel  102  from 10-106 MHz and a upper OFDM channel  104  from 126-222 MHz. To digitize the continuous band from 0 to 222 MHz, the sampling rate would need to be &gt;444 Msps (likely ˜510 Msps), well in excess of devices used in current 2×85 digital return. 
     A DRT  107  may first filter the signals  102  and  104  with a diplex filter  108  that separates the signals  102 ,  104  into a first transmission path  109  for the signal  104  and a second transmission path  111  for the signal  102 . The diplex filter preferably splits the signals  102  and  104  at a split frequency of approximately 116 MHz. The term “approximately in this context means anywhere within a range of 106 MHz to 126 MHz, on the assumption that each of the signals  102  and  104  are about 96 MHz in width and the guardband is about 20 MHz in width. Alternative embodiments may use a split frequency outside of this range, however, depending on the width of the signals  102  and  104 . The signal  102  is preferably filtered by a low pass filter  110  while the signal  104  is preferably filtered by a bandpass filter  112 . Preferably, each signal  102  and  104  may be amplified after being respectively filtered. The bandpass filter  112  is preferably configured to pass a range of frequencies that closely matches the frequency bounds of the signal  104 , and to filter out all other frequencies. After each signal  102 ,  104  is filtered by a respective one of the filters  110  and  112 , the signals  102  and  104  are converted to a digital signal by a dual A/D converter  114 . The dual A/D converter  114  can be operated with independent analog inputs and/or can be used for diversity reception of signals, operating identically on the same carrier but from separate antennae. The output from the dual A/D converter  114  is input to a multiplexer  116 , a device that selects one of several analog or digital input signals and forwards the selected input into a single line. 
     As can be seen in  FIG. 4 , between signal  102  and signal  104  is a guardband  106 , which accounts for the fact that the diplex filter  108  has a significant cross-over region, in which frequencies above the lowpass portion of the diplex filter get passed, and frequencies below the highpass portion of the diplex filter get passed. The guardband, though necessary to protect the integrity of the respective signals  102  and  104 , contains no data, hence the present inventors recognized that there is no reason for sampling frequencies within the guardband. Thus, the dual A/D converter is preferably configured to only sample the respective bands occupied by the signals  102  and  104 , and without sampling the frequencies that occupy the guardband. 
     As can easily be recognized, by using bandpass filter  112  in conjunction with the disclosed dual A/D converter  114 , a signal with bandwidth of 96 MHz can be sampled at &gt;192 Msps (likely ˜232 Msps), close to the current 2×85 sampling rate of 202 Msps. Thus, the use of bandpass sampling transforms the 2×85 Digital Return into a 1×200+ Digital Return. The use of digitized blocks to leverage the DSP concept of bandpass sampling may achieve strict Nyquist compliance, aggregating the signals on the receive end. 
     In some embodiments, a single hardware design can support both 2×85 MHz and 1×200+MHz. Thus, one hardware design can support both 2×85 MHz and 1×200+MHz, avoiding a total re-design of the 2×85 MHz legacy designs, which would be more complex and costly. In the DOCSIS digital return implementation shown in  FIG. 4 , an increase in current digital processing rates, e.g., from 202 to 232 Msps, supports the two (2) 96 MHz bandwidth channels  102  and  104 . For example, one of the current time-division multiplexing (TDM) channels may carry the lower 96 MHz DOCSIS 3.1 channel  102  via conventional lowpass sampling, such as via low pass filter  110 . The second of the TDM channels may carry the upper 96 MHz DOCSIS 3.1 channel  104  via bandpass sampling using bandpass filter  112 . 
     As shown in  FIG. 4 , by bandpass sampling the advanced signaling on a DOCSIS channel, ingress impairments may be filtered out from the optical transmission path, since the filter characteristics are closely matched to the advanced signaling channel, e.g. the 96 MHz orthogonal frequency division multiple access (OFDMA signal) proposed by the DOCSIS 3.1 standard. 
     Preferably, the bandpass filter characteristics closely match the advanced signaling channel (or channels), thereby filtering out ingress impairments from the optical transmission path. Thus, the disclosed techniques may address modern cable upstream issues in a more economical manner than existing architectures. For example, in a Data Over Cable Service Interface Specifications (DOCSIS) implementation, the disclosed techniques may enable DOCSIS over extended splits by using the relationship between the state of the art in upstream digital return products (2×TDM @ 85 MHz split) and newly defined upstream approaches, e.g., using 2×96 MHz bands of OFDMA. 
     It should be understood that other center frequencies, bandwidths, and aggregation schemes can be chosen to provide desired performance. For example, if a signal band is not completely full, all the dynamic range (sampling rate) can be given to the occupied bandwidth to achieve a better SNR ratio. Similar techniques can be used to protect dynamic range and/or avoid overdrive due to regions of significant interference and ingress that are unoccupied with desired signals, such as the low end of the upstream band, or perhaps the FM band when upstream signal capacity is extended even further beyond DOCSIS 3.1. 
     Also, the disclosed techniques apply to any future extension of upstream bandwidth capacity—the tradeoff between full band digitization, processing, and transport versus aggregated spectrum increments. While the summation of summing spectrum segments to a total bandwidth is linear, the implementation costs of summing spectrum components that match a full single bandwidth approach may not be linear. 
     The multiplexer  116  may combine several input signals into one output signal, which carries several communication channels. The multiplexer  116  may increase the amount of data that can be sent over the channel within a certain amount of time and bandwidth. The multiplexer  116  makes it possible for the bandpass filter and low pass filter to share the Dual A/D  114 . The MUX and Framing FPGA  116  may provide the output to a serializer and Electrical-to-Optical (E20) converter  118 . 
     A Digital Return Receiver (DRR)  109  may included an Optical-to-Electrical (O2E) converter and deserializer component  120  that provides an output to a deframe and demultiplexer FPGA  122 . The demultiplexer  122  may take a single input signal and select one of many data output-lines connected to a single input. The demultiplexer  122  may take a single input signal that carries many channels and separate those over multiple output signals for delivery to a Dual A/D converter  124 , where a demultiplexed first output signal  125  can be delivered to a bandpass filter  126  and a second output signal  127  is provided to a low pass filter  128 . The signals  125  and  127  may be combined by a diplex filter/combiner  130  to re-create the original signals  102  and  104 . 
     The center frequencies and bandwidths described herein are non-limiting examples of trading between full band digitization, processing and transport vs. the disclosed aggregated spectrum increments. For example, other center frequencies and bandwidths and aggregation schemes can be chosen to achieve this result to provide other performance advantages. In particular, for example, if the band is not completely full, all of the dynamic range can be given to the occupied BW to optimize performance (better SNR). This same mechanism also can be used to protect the dynamic range and/or avoid overdrive due to regions of significant interference and ingress that are unoccupied with desired signals, such as the low end of the upstream band or perhaps the FM band when the upstream is extended such as is anticipated by this disclosure. 
     Further, the disclosed concepts of separately digitizing single channels applies to future upstream bandwidth and recognizes the tradeoff between full band digitization, processing, and transport vs. aggregated spectrum increments. While the algebra of summing spectrum segments to a total BW required is linear, the implementation costs of summing spectrum components that match a full single-BW approach may not be linear, in particular when processing is pushing the technology envelope, which has been inherently the situation recently in the competitive environment of broadband evolution. Existing solutions do not make use of bandpass sampling in the area of digital return; in particular, the disclosed techniques for using bandpass sampling, a digital signal processing technique, to enable flexible selection of critical system parameters including bandwidth, and center frequency location, have not been contemplated. Dynamic selection of these critical system parameters enables both efficient transmission of CATV upstream paying services and avoidance of upstream impairments including ingress and laser clipping. 
       FIG. 6  depicts a brassboard DRR output spectrum using existing 2×85 DRT and DRR boards. In this example, 85 MHz analog lowpass filters in DRT and DRR are bypassed in one of the TDM channels. Analog amps currently used have decreasing gain from 85 to 200 MHz and therefore the passband slope and ripple may be further optimized. Sampling and processing rate has not been bumped, in this example, still running at a rate of 202 Msps. Due to keeping rate at 202 Msps, in this embodiment, two ˜80 MHz BW channels are used rather than two (2) 96 MHz channels. In this embodiment, the lower channel is 5-85 MHz and the upper channel is 113-197 MHz. An 80 MHz noise block to simulate lower wideband channel and fourteen 256 QAMs is used to simulate upper wideband channel. Signals are being passed with respectable SNR and distortions but MER at higher frequencies is being limited by phase jitter. 
     In embodiments, phase jitter and phase noise requirements are determined for the clocks to support 42+dB MER at the upper end of the band, which may dictate hardware design choices to meet the phase jitter requirements. Further, analog amps and filters may be designed to support upper wideband channel and optimize passband response. In embodiments where the sampling rates are bumped from 202 to 232 MHz, the phase jitter and phase noise requirements may be determined. The DRT and DRR boards may be configured to deliver &gt;42 dB MER at high end of band. 
       FIG. 7  depicts an exemplary architecture  150  by which data may be transmitted along a return path between a plurality of end user cable modems  152  and a CMTS  156  in a head end  157 , through a node  154 . In this architecture, a DRT  158  in the node receives respective signals from each of the modems  152  via an array of coaxial cables  160 . The coaxial cables may be connected to modems  152  using two-way splitters used as combiners  162  such that a respective two-way splitter  162  propagates signals onto a coaxial cable  160  from a pair of modems  152 . The transmitter  158  is preferably configured as shown in  FIG. 4 , and propagates a digital signal onto a fiber optic transmission line  164  at two wavelengths, and which is carried to a receiver  166  that is also preferably configured as shown in  FIG. 4 . After converting the incoming digital signals to respective analog RF signals, the receiver  166  may convey them to the CMTS  156  using coaxial cables  168  and two-way splitters  170 . 
     It should be understood that, although  FIG. 4  only shows an architecture for providing data along an upstream path from the cable modems  152  to the CMTS  156 , such an architecture also includes equipment to provide data along the forward path from the CMTS  156  to the node  145  and on to the cable modems  152 . 
       FIG. 8  shows estimates in a system similar to that of  FIG. 6 , but without using the bandpass filter  112  of  FIG. 4 . In this example, data is delivered along an upstream path to the node by a coaxial cable carrying 85 MHz blocks of data. In this example, the throughput or capacity at the node was estimated to range from approximately 307 Mbps to 983 Mbps depending on the number of channels and/or the depth of modulation, e.g. 64 QAM versus 256 QAM. 
       FIG. 9  shows estimates in a system similar to that of  FIG. 6 , but instead using the bandpass filter  112  of  FIG. 4 , under several channel conditions presumed to be consistent with the upcoming DOCSIS 3.1 standard, e.g. up to 1024 QAM while still using 85 MHz blocks. In this example, the throughput or capacity at the node was measured to range from approximately 491 Mbps to 1,536 Mbps depending on the number of channels and/or the depth of modulation, e.g. 256 QAM versus 1024 QAM. As can be seen from a comparison between  FIGS. 8 and 9 , the use of a bandpass filter and bandpass sampling as disclosed in the present application approximately doubles the upstream throughput in a CATV transmission system using lower complexity hardware. 
     In some embodiments, the disclosed techniques may be implemented via an incremental complexity addition to a current digital return product, e.g., an &gt;85 MHz digital return product. Incremental complexity addition to existing products may enable upgrades, e.g., &gt;85 MHz CATV upstream upgrades, at an incremental cost to cable system operators, thereby avoiding the use of more expensive analog-to-digital converters (ADCs) associated with sampling equivalent frequency ranges. In an example digital return device, real estate in the node housing of an SG4 optical hub would enable cable operators to easily aggregate capacities as high as 6 GB per node as needed. 
     The disclosed techniques provide flexibility to support dynamic center frequency locations. The ability to support dynamic center frequency locations may allow cable operators the flexibility to focus optical link transmission on paying services, rather than the traditional approach of transmitting both paying services plus ingress. 
     The terms and expressions that have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the claimed subject matter is defined and limited only by the claims that follow.