A new frequency plan and power deployment rules

Deliverable D14

A new frequency plan and power

deployment rules

October 2009

ReDeSign – 217014

Research for Development of Future Interactive Generations

of Hybrid Fibre Coax Networks

Version: 1.0

Information for Publication: Status: Public (PU)

Contents

Management Summary ... 5

1 Introduction ... 8

1.1 The future HFC capacity challenge ... 8

1.2 Operational and business restrictions ...11

1.3 Scope of the ReDeSign studies ...15

2 Upstream capacity ...16

2.1 The technical issues ...16

2.2 Currently available solutions ...19

2.3 Allocation of an additional return band spectrum ...22

2.4 Summary and conclusion ...27

3 Downstream capacity ...29

3.1 The DVB-C2 signal level ...29

3.2 Performance simulations ...33

3.3 Network and network load scenario’s ...37

3.4 Network signal quality parameters ...40

3.5 Results ...41

3.6 Summary and conclusion ...46

Management Summary

According to the early concept of the cable distribution network, the customer should be able to connect his terrestrial receivers directly with the cable network. This completely legitimate and logical requirement has shaped the HFC frequency plan. With the advent of digital transmission technologies, this linkage of the cable frequency plan to the terrestrial fre-quency plan has become obsolete; however, most of the transmission systems still respect the historical HFC frequency plan. A major disadvantage of the current frequency plan is the upstream band which is limited to 65 MHz. This frequency limitation is associated with the conservation with the FM radio band (87,5 – 108 MHz).

In this study we have reconsidered the use of the HFC spectrum assuming that in the all digi-tal era all historical restrictions can be abolished thus allowing an operator to redefine the frequency plan according to his needs, with an appropriate balance between up and down stream spectrum.

Considering the outcome of the studies, we have to conclude that operators are still strongly bonded to the existing frequency design of the cable networks and to the terrestrial use of the ether. Therefore, a complete redefinition of the frequency plan appears not possible. Transmission capacity is determined not only by the available frequency spectrum, but by the applicable carrier signal and distortion signal level as well. Therefore, we have analyzed the possibilities to expand the upstream and downstream capacity from the integral viewpoint of available spectrum, the possible signal level and the distortion signal level.

In principle, the frequency plan should be defined to maximize the total upstream and down-stream network transmission capacity in a balanced manner, as demanded by the market. However, as elaborated in the report, a complete abandonment of the historically defined frequency plan appears impossible. The two primary reasons to conserve the existing fre-quency plan are:

• A majority of the cable operators foresees delivery of FM radio signals for at least a full decade

• There are many options to expand the downstream transmission capacity or to im-plement capacity saving solutions. Because of this an expansion of the downstream band beyond 865 MHz is rated the least.

Combining both observations fixates the current frequency plan almost completely.

Irrespective of this fixation of the frequency plan, we have studied the options to expand or to maximize the upstream and downstream capacity.

Regarding the upstream capacity, the operator response on earlier ReDeSign network ques-tionnaire reveals that in most cable networks the upstream band is not used efficiently. In many networks the upstream band is yet not extended up to 65 MHz whereas ingress noise levels prohibit the use of high (64 QAM) modulation schemes. In the report we provide a re-view of the solutions to reduce the ingress noise. Operators should first resolve this problem of ingress noise, possibly in combination with the extension of the upstream band up to 65 MHz, to maximize the upstream capacity and to warrant economical use of EuroDOCSIS equipment. Having thus upgraded the upstream channel, they can keep track with the cus-tomer capacity demand by adding more EuroDOCSIS channels and/or splitting the upstream segments.

Next, once the above capacity expansion solution is exhausted, operators will face the chal-lenge to expand the capacity beyond this level. The capacity of the 30 – 65 MHz band is fully used, and operators are forced to find new spectrum for the upstream band, which requires a substantial network upgrade. Basically, there are two options, extension of the 30 – 65 MHz band to higher frequencies, or the creation of a new frequency band in the UHF band,

be-yond 865 MHz. The first option requires a solution to deliver FM radio to those customers that use this service. In addition, the EuroDOCSIS technology has to be adapted. In the DOCSIS specification, the upstream band is already extended to 85 MHz, so technically there is no serious problem; however, operators do depend on the willingness of the manu-facturers. Likely, the definition of a UHF return band is more promising. By using VHF-UHF frequency converters placed in the network, investments and network adaptations can be limited. This solution is shown in the figure below. In this option, the customer equipment still transmits in the 30 – 65 MHz band and in the lower part of the coaxial network upstream nals are conveyed at these frequencies, but higher up in the network the 30 – 65 MHz sig-nals are converted to a frequency beyond 950 MHz. This way, the upstream capacity can be boosted by a factor of 10 or more, whereas it requires limited network adaptations and no adaptation of the EuroDOCSIS equipment.

As pointed out above, operators are not specifically inclined to extend the downstream band egde beyond 865 MHz because of the numerous ways to make a more efficient use of the spectrum from 85 up to 865 MHz. At the level of the network layer, the replacement of analogue signals by digital carriers and the deployment of DVB-C2 are the basic elements to implement this approach. In the ReDeSign studies we have addressed a crucial issue of this approach: the capability of the existing European HFC networks to support the DVB-C2 4096 QAM modulation mode. Application of this mode requires a high DVB-C2 signal level, and the question is whether a sufficiently high signal level can be deployed without degradation of the analogue TV, DVB-C and DVB-C2 signals by the distortion products (intermodulation products) associated with the non-linear nature of the active components.

To warrant a realistic result, a number of operators provided data from their networks. Four networks scenarios were studied with cascades of 2, 4, 5 and 15 amplifiers respectively. The coaxial parts and the amplifiers were completely specified, including noise and non-linear behavior of the latter. Three network loads included:

• a mixed analogue, DVB-C and DVB-C2 scenario (20 PAL, 30 DVB-C and 43 DVB-C2), • an all digital DVB-C and DVB-C2 scenario (15 DVB-C and 78 DVB-C2).

For these scenario’s, we calculated the signal quality parameters like SNR for the digital car-riers and CNR and CINR for the PAL signals as a function of the DVB-C2 signal level. These calculations showed that in case of the mixed load scenario DVB-C2 4096 QAM mod-ulation can de used in all four networks. In case of the all digital scenario, three out of the four networks support the use of DVB-C2 4096 QAM modulation. This result suggests that DVB-C2 4096 QAM modulation can be applied in many European HFC networks; however, not in all networks.

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1 Introduction

Mastering radio communications via the ether has been one of the major accomplishments of the 20th _{century. Amongst these, analogue terrestrial TV appeared very successful. In} re-sponse of the market demand for more TV channels, cable networks and cable technology was developed. These networks were designed for the distribution of TV services based on the terrestrial analogue TV transmission technology so that the customer could use the stan-dard TV and FM radio sets. Therefore the design reflects the radio technologies and the spectrum allocations of the terrestrial broadcast services: Band I (TV, 47 – 68 MHz), Band II (FM Radio, 87,5 – 108 MHz), Band III (TV, 174-230 MHz) and Band IV/V (TV, 470 – 862 MHz). Up to today, cable or HFC networks reflect this inherited design.

Cable networks are fully owned and managed by private companies, and by their nature these networks hardly interfere with terrestrial communication services. Thus, in principle, the operator has a high level of freedom to choose the radio design of its networks. Currently, we are witnessing a period of fast change of the customer service demand. At the same time, many new technologies and solutions are developed to address the market demand. For these new services new user equipment is needed that not necessarily requires the historical cable spectrum design. Therefore, it appears logical to reconsider the spectrum design of cable networks as well.

In this report, we present the results of our analysis of the cable radio design and spectrum use. The point of departure is given by the future capacity and services demand, the existing networks and the existing and known forthcoming technologies. Like in all media, the trans-mission capacity of an cable network is limited by i) the available frequency spectrum, ii) the signal level and iii) the distortion signal levels. Operators will face a dramatic change of the exploitation of these “resources”, spectrum, signal level and distortion signal levels.

Here we present such an analysis. First, in the sections 1.1 and 1.2 we will consider the forthcoming network capacity demand in reference to the known network capabilities and the practical and operational limitations of changing the frequency plan respectively. This first analysis shows that operators are still strongly bonded to the existing frequency design of the cable networks and to the terrestrial use of the ether. Because of these limitations, we have adapted the scope of our studies to warrant a good alignment of the ReDeSign studies and the operator’s interests. This narrowed scope is presented in section 1.3. In the next Chap-ters 2 and 3, we discuss the possibilities and the limitations of expanding the upstream and downstream transmission capacity of the network.

1.1 The future HFC capacity challenge

In the forthcoming years, the market demand of digital broadband services and media will challenge the HFC network capacity limits of the cable operators. Although the HFC net-works have a scalable architecture allowing incremental capacity expansions, every network has a maximum capacity limit. Considering the large variation between the European HFC networks and regional market conditions, some operators face this capacity limit in the more near future whereas others have sufficient capacity and may serve the markets for a decade or more.

The network and business questionnaires of the ReDeSign WP2 and WP3 studies included questions regarding the network capabilities and the expected services demand. Analysis of these data provided a more quantitative picture of the network capacity challenge, as briefly

summarized in the following. A more extensive discussion can be found in the March 2009 issue of the Broadband Journal.1

Figure 1 Potential network capacity per fiber node (nodes of 600 HP and of 200 HP) for a networks with 550 MHz and 862 MHz downstream band edge versus future (average) capacity needs assuming capacity saving solutions are timely implemented. The data is based on input of European MSOs. In Figure 1 we match the expected downstream capacity demand versus the HFC capacity limit in Gbps per network node2

Figure 1

. In this analysis, all currently known capacity saving solu-tions, like the use of H.264 video compression and the use of switched TV are taken into account. In the forthcoming years, operators will gradually replace part of the analogue TV channels by DVB-C2 channels with a larger capacity per 8 MHz channel, which adds to the expansion of the maximum capacity as shown by the dashed curves in . The figure shows the network capacity limit for networks which can be segmented in small nodes of 200 homes passed (HP) and large nodes of 600 HP, for networks with a frequency edge of 550 MHz and 862 MHz. These data show that networks with 862 MHz frequency edge that can be segmented into small nodes of 200 HP can support the market service demand for more than a decade; however, networks with larger nodes and/or a frequency edge of less than 862 MHz will face shortages within a decade.

Figure 2 Future upstream capacity demand for segments of 600 HP and 200 HP, in case of an

asymmetrical service demand.

1 _{Jan de Nijs,, Tim Gyselings and Carsten Engelke, “Securing Europe’s Cable Future”, Broadband,}

Vol. 31, No 1, p. 72, March 2009.

2 _{An analogue TV channel is represented by a ‘virtual’ capacity of 52 Mbps.}

DS Capacity (Nodes 200 HP) 0 1 2 3 4 5 6 7 8

Today 1-2 years 2-5 years 5-10 years

Time C ap aci ty [ G bp s] Internet VOD HD VOD SD Switched HDTV Switched SDTV HDTV SDTV PAL/SECAM 550 MHz 862 MHz upgrade toward DVB-C2 DS Capacity (Nodes 600 HP) 0 1 2 3 4 5 6 7 8

Today 1-2 years 2-5 years 5-10 years

Time C apaci ty [ G bps] 550 MHz 862 MHz upgrade toward DVB-C2 DS Capacity (Nodes 200 HP) 0 1 2 3 4 5 6 7 8

Today 1-2 years 2-5 years 5-10 years

Time C ap aci ty [ G bp s] Internet VOD HD VOD SD Switched HDTV Switched SDTV HDTV SDTV PAL/SECAM 550 MHz 862 MHz upgrade toward DVB-C2 DS Capacity (Nodes 600 HP) 0 1 2 3 4 5 6 7 8

Today 1-2 years 2-5 years 5-10 years

Time C apaci ty [ G bps] 550 MHz 862 MHz upgrade toward DVB-C2

In Figure 2 and Figure 3 we show the capacity analysis for the upstream channel for state-of-the-art HFC networks with a 65 MHz return channel and with a low ingress noise level, again for nodes of 600 and 200 homes passed. In such networks, operators can allocate four 6.4 MHz upstream channels with 16 QAM modulation, with a total capacity of 80 Mbps. Often, it is impossible to deploy 64 QAM modulation or a fifth 6.4 MHz channel. Therefore, 80 Mbps can be considered as the maximum upstream capacity per network segment in case of state-of-the-art networks.

Figure 2 shows the data for the case that the current asymmetric traffic profile is conserved whereas in Figure 3 it is assumed that the market will require a more symmetric traffic profile because of the success of new services like videophone, camera surveillance of private homes, network back up services and P2P services. The analysis shows, that when the fu-ture market will not develop a demand for symmetric services, state-of-the-art HFC networks will be able to serve the market for the next decade; however, when the market will require symmetric services, capacity shortages will develop in time, as shown in Figure 3. The 80 Mbps for the state-of-the-art networks will not be sufficient, and operators will have to find ways to further expand the upstream capacity. In the latter figure, also the maximum up-stream capacity limit of 180 Mbps is indicated. This limit is associated with the deployment of 6 upstream channels of 6.4 MHz and 64 QAM modulation. Hence, operators will be chal-lenged to master the deployment of 6.4 MHz @ 64 QAM channels.

Figure 3 Future upstream capacity demand for segments of 600 HP and 200 HP, in case of an asymmetrical service demand.

In this ReDeSign study, we give an in-depth analysis of the possibilities to tackle the down-stream and updown-stream capacity challenges. Since the updown-stream and downdown-stream paths have a completely different electric nature, the capacity challenges will be discussed in separate chapters.

In many HFC networks the upstream capacity is rather limited because of the limited fre-quency spectrum (up to 65 MHz) and the high return band ingress noise levels. As such, there is a serious concern whether this band may provide the capacity demand of the future market. This issue and the conceivable remedies are treated in Chapter 2.

In the downstream channel, analogue TV currently requires half of the available spectrum. Typically about 50% of the frequency spectrum is used for analogue services. Operators are considering a substantial reduction of the analogue service packages and the deployment of DVB-C2 transmission systems to create the required capacity for digital services. To ac-commodate this change of network load, a revision of the RF planning of the HFC networks

is needed. Operators have to reconsider the use of the available frequency spectrum and they have to define the appropriate power levels for the DVB-C2 transmission systems. In Chapter 3 we will discuss the network issues related to DVB-C2 deployment

In the following section 1.2 of this introduction, we will first elaborate the network capacity issue from the operator and network management viewpoint. This analysis reveals some preferences and restriction regarding the redesign of the HFC frequency spectrum use. Based on these preferences and restrictions, we have narrowed the scope of the ReDeSign studies in section 1.3.

1.2 Operational and business restrictions

The network transmission capacity is basically limited by the available power budget and by the available frequency spectrum. Expansion of the network capacity thus can be accom-plished by expansion of the power budget and/or by expansion of the frequency spectrum, whereby often a larger frequency spectrum will require a larger power budget to provide the signal power for the additional carriers. The reduction of the analogue service packages is one option to create signal power for digital services. From the practical, operational and business perspectives, however, there are several objections against a too radical re-allocation of power budget and frequency spectrum. Moreover, on the short term, today’s operational and business requirements may exceed the long term importance of the suffi-cient up and downstream network capacity. Therefore, any long-term solution must be prop-erly aligned with these more short-term operational and business restrictions. In the WP3 questionnaire, operators were consulted regarded some of the restrictions3

1.2.1 Operator valuation of evolutionary upgrading technologies

. In this subsec-tion we present a high-level review of the operasubsec-tional and business aspects interfere with the expansion of the network transmission capacity.

Currently, the market is flooded by a multitude of innovative cable technologies. Economic solutions for expanding the network capacity are developed like advanced node splitting so-lutions. Furthermore, technologies are developed to use the existing capacity in a more effi-

cient manner, for example improved modulation codes (DVB-C2), switched digital TV and advanced video codecs like H.264. In parallel to the develop-ment and roll out of these innovative technologies, the market conditions are gradually changing. For example customer acceptance of digital TV services is increasing, thus diminishing the inconveniences of reducing the analogue packages.

In the ReDeSign network and services survey, we have asked the operators which evolutionary solu-tions or techniques for expanding the network ca-pacity or for using the existent caca-pacity in a more efficient manner they appreciate most3_{. The} re-sponse is summarized in Table 1. The result con-firms the preference for network segmentation as the most appropriate solution to expand the network capacity. Next, a number of new (digital) technologies and the switch off of analogue

3 _{“Service Requirements Report”, ReDeSign D10, June 2008.}

Table 1 Average rating (Scale 1 -10) evolutionary upgrades technologies

Upgrade option Rating Network segmentation 8,1 Statistical multiplexing 5,6 Analogue switch off 5,3 Better modulation codes 5,0

QAM sharing 4,9

Switched digital TV 4,5 Extension up to 1 GHz 3,2 Extension beyond 1 GHz 2,9

vices are more or less equally preferred with a rating of 4.5 up to 5.6. Finally, the extension of the frequency range beyond 862 MHz is considered as the least favourable solution. In the opinion of the operators, extension of the frequency range is a last solution. Clearly, when considering a new frequency plan, this opinion must be taken into account.

1.2.2 Analogue TV

As indicated earlier, analogue TV consumes about 50% of the frequency spectrum. Reduc-tion of the analogue TV package thus releases spectrum and signal power. The reducReduc-tion of analogue TV paves the way to deploy DVB-C2 modulation which requires a sufficiently strong signal level for the 4K QAM modulation mode.

Today the analogue TV offer comprises about 40 channels3_{. The increasing market} accep-tance of digital TV makes it possible to gradually reduce the analogue packages. However, analogue TV is considered as a unique selling point since the signals are distributed throughout the house in the most convenient manner, allowing customers to view television in all rooms of the home without the need of additional equipment like STBs. Therefore, ca-ble operators foresee an analogue package of 20 TV channels over a time period of 5 – 10 years from now.

1.2.3 Deployment of higher order modulation techniques

Reduction of the analogue service packages results in a release of spectrum resources that can be reused for other (digital) services. The current DVB-C technology dates from more than a decade ago. It is the result of the status of the technology at that time and of the (less demanding) market conditions. Modern technology, and in particular modern error protection algorithms, permits a more efficient use of the frequency spectrum and the market demands for higher capacity. DVB-C2 has been developed to address both issues. It provides a higher throughput per 8 MHz channel in a more efficient manner than DVB-C. In particular DVB-C2 supports 4096 QAM modulation providing a bitrate of 84 Mbps per channel, albeit at an ex-pected signal level higher than that needed for DVB-C 256 QAM modulation.

1.2.4 FM radio services

FM radio is transmitted in the frequency band from 87 MHz up to 107.5 MHz. In most net-works, the FM band demarcates the lower edge of the downstream band. As such, the FM band is the main constraint of the upstream band. To extend the return band to frequencies above 65 MHz, operators have to sacrifice the FM radio band.

In their feedback on the ReDeSign questionnaire, the cable operators expressed the busi-ness relevance of the FM radio services. Currently, all operators, apart from an incidental exception, do offer FM radio services as part of the basic analogue subscription. Over period of 10 years from now, 60% of the operators still will deliver FM radio services. This response indicates that FM radio is considered a crucial service that cannot be simply terminated with-out a negative business impact. Only with the availability of a convenient and economic al-ternative, operators could consider the clearance of the FM band.

1.2.5 Downstream band frequency extensions

To expand the network capacity, the extension of the downstream frequency band toward higher frequencies may appear a most straightforward solution. However, in practice, such an extension appears rather complex. Still many European HFC networks have a

down-stream band edge of less than 862 MHz. Therefore, below we briefly consider the extension up to 862 MHz, up to 1 GHz and beyond 1 GHz.

1.2.5.1

Today’s HFC networks often have an upper frequency edge of 862 MHz. This frequency cor-responds with the frequency edge of the terrestrial analogue TV band (Band V). Consumer equipment like TV sets and many EuroDOCSIS cable modems are not designed for the re-ception of signals with a higher frequency. Because of the limited availability of such equip-ment, 862 MHz is a logical and practical upper edge for today’s HFC networks.

Extension up to 862 MHz

Nevertheless, many HFC networks have an upper frequency edge below 862 MHz. Some networks have a frequency edge of 550 MHz, or even below. These band edges are no hard limitations, but the frequencies above can not be used in an efficient manner. From a RF planning viewpoint, the use of the higher frequencies is less attractive because of the in-creasing attenuation of the coaxial cables. A channel at a higher frequency will consume a relatively larger part of the available signal power. Because of this, operators have a prefer-ence to allocate the channels at the lowest frequencies.

In practice, an operator will make optimum use of the available power budget provided by the amplifiers; he will maximize its network load in terms of the number of analogue and digital channels. As such, in a properly designed and operated network, the downstream band edge and the available power budget are properly balanced and it will not be possible to add a substantial number of additional channels. To add a substantial number of additional chan-nels, the operator will have i) to add frequency spectrum by shifting the down stream band edge to a higher frequency and ii) he will have to increase the power budget. In practise, he will have to replace the network amplifiers by ones with a higher output power. Often, the spacing between the amplifiers is rather large, and operators have not only to replace the amplifiers, but they have to add and re-space amplifiers in the cascades to shorten the dis-tance between the consecutive amplifiers, which, of course, is most costly. In the current market there is a growing demand for digital services whereas customers are more willing to accept a reduction of the analogue service package. Therefore, an operator has to make a trade-off between i) further segmentation of the network, ii) replacement of analogue chan-nels by digital ones, iii) the replacement and possible re-spacing of the amplifiers in order to raise the network power budget and to extend the downstream frequency band.

1.2.5.2

For networks with a frequency edge of 862 MHz, extension beyond this frequency up to 1 GHz may appear an attractive solution, though the general problem of enhanced signal at-tenuation at higher frequencies will require a re-planning of the signals, possibly in combina-tion with the replacement and/or a re-spacing of the amplifiers.

Frequency extensions up to 1 GHz

Using the band from 862 MHz up to 1 GHz, however, will invoke some currently unknown problems associated with the terrestrial use of the radio spectrum by others. Most of the ter-restrial spectrum between 87 MHz and 862 MHz is allocated to radio and TV broadcast ser-vices4

4 _{87 – 107,5 MHz FM radio band; 174 – 230 MHz (Band III) digital radio and digital TV broad casting;}

470-862 (Band IV/V) analogue and digital TV.

. Operators are hindered by terrestrial radio and TV broadcasting transmitters; how-ever, the problems are limited to homes with inferior in-home coaxial networks in the vicinity of the transmitter. In contrast, the spectrum ranging from 862 up to 1 GHz is not allocated to

terrestrial broadcast services but, amongst others, to mobile communication systems (GSM). GSM-900 uses 890–915 MHz for uplink transmissions and 935–960 MHz for the downlink. In addition, spectrum is allocated for mobile communications for railways (GSM-R) 876–915 MHz (uplink) and 921–960 MHz (downlink). Because of the different radio services in the 862 MHz – 1 GHz band, mobile communications instead of terrestrial broadcasting, a completely different the interference scenario will occur. In case of mobile communications, customers will use their mobile terminals in home. The isolation of today’s state-off-the art cable equip-ment like STBs and modems is insufficient to avoid interference between the cable RF sig-nals and the transmit sigsig-nals of the mobile termisig-nals. In addition, in case of a low quality in-home network, the coaxial cables will act as an antenna that receives the mobile transmit signals, thus aggravating the interference problem. Because of these ingress problems, Eu-ropean operators are not eager to use the band from 862 MHz up to 1 GHz, as confirmed by the ReDeSign questionnaire.

1.2.5.3

Beyond 1 GHz frequencies, operators have to anticipate further operational problems. Signal attenuation will become even more pronounced than at the frequencies below 1 GHz, in par-ticular in case of “bamboo” coaxial cables that have specific bands of enhanced attenuation at these frequencies. In addition, the operation of network passives will fail because these are not designed for frequencies above 1 GHz. The isolation of the passives will drop dra-matically, thus contributing to further signal loss. Furthermore, at higher frequencies, the iso-lation of the coaxial cables will drop whereas at the output of the amplifiers a higher signal levels is needed to compensate the larger coaxial attenuation. Therefore, conceivably, ex-cessive egress problems may arise beyond the regulatory EMC limits of cable networks.

Frequency extensions beyond 1 GHz

1.2.6 Summary and conclusion

In the above section we have provided a brief overview of the business perspective and technical limitations of changing the frequency plan. In summary it is argued that:

• There are many solutions to expand the network capacity and to make a more effi-cient use of the existing capacity. Taking this situation into account, operators indi-cate that extension of the downstream band beyond 862 MHz is the least favourite solution,

• The analogue packages will be reduced, thus creating room to increase the digital services. However, over a decade operators will still offer some 20 analogue chan-nels,

• FM radio services are considered as a crucial part of the cable services portfolio that cannot be switched off unless a good and convenient alternative transmission solu-tion is offered. Customers should not be disturbed with difficult solusolu-tions,

• To fully use the potential network capacity created upon the reduction of analogue TV services, operators must deploy 1024 QAM or higher order modulation scheme, • The use of frequencies above 862 MHz is for different technical disadvantages very

1.3 Scope of the ReDeSign studies

In this treatise, we are considering the possibilities to expand the downstream and upstream network transmission capacity. From the fundamental viewpoint there are three approaches to create more capacity:

• Reduction of noise and distortion signal levels in combination with the use of higher order modulation schemes,

• Raising the carrier signal levels in combination with the use of higher order modula-tion schemes,

• Extension of the frequency bands.

In the following parts of this report, we will study the above approaches in more detail for both the downstream and the upstream frequency band.

At the very beginning of this chapter we have argued that, considering the historical roots of the current radio design of cable networks and the forthcoming market demands, a complete reconsideration of the RF spectrum use was appropriate. However, taking the overall busi-ness and technical perspective into account, some a priori restrictions of the scope of the revision of the radio design appear appropriate:

• To expand the downstream network capacity, extension of the frequency band beyond 862 MHz appears a less attractive solution than the deployment of a trans-mission technology that supports higher order modulation schemes in combination with all other capacity saving solutions. Therefore, the ReDeSign studies should re-spect this 862 MHz as the upper band edge limit,

• To expand the upstream network capacity, we cannot a priori indicate a preferential solution (higher signal levels, reduction of noise level or extension of the upstream band), though extension of the frequency band beyond 65 MHz should not be consi-dered as a first solution because it requires the termination of FM Radio services in the 87,5 – 107 MHZ band. Therefore, the scope of the ReDeSign studies must in-clude all three approaches to expand the upstream capacity, taking in to account that extension beyond 65 MHz is the least favorable.

2 Upstream capacity

During the first meeting of the ReDeSign Operator Forum, some operators have expressed their concerns concerning upstream capacity shortages. According to their viewpoint, the forthcoming demand for upstream channel capacity will create the first major bottle neck. In this chapter we study the possibilities to expand the upstream capacity in detail. The scope of this study has been limited to solutions that conserve the current frequency division duplex architecture, taking into account the business limitations of paragraph 1.2. As pointed out earlier, we will analyze the possibilities of i) expanding the frequency spectrum, ii) applying higher signal levels and iii) reducing noise levels. In the following we provide a technical overview of the issue (paragraph 2.1), a treatment of the solutions compliant to the current HFC radio design (paragraph 2.2) and options that require a reconsideration of the HFC ra-dio design (paragraph 2.3). We will end this chapter with a brief summary and conclusion of the analysis.

2.1 The technical issues

2.1.1 Spectrum availability and quality

In the current state-off-the art HFC networks, the upstream channel is located in 5 – 65 MHz frequency band. However, before the advent of two-way services, Band I (47 – 68 MHz) was used for analogue TV broadcasting. Because of this historical use, in many cable networks Band I still is used for broadcast services, thus limiting the upstream band. In the ReDeSign network questionnaire, operators were asked to provide the upstream band edge. The re-sponse is summarized in Table 2. This results shows that although many operators have upgraded their networks up to a 65 MHz band edge, still about half of the operators have not upgraded their networks, or only partially

Table 2 Upstream band edge as obtained from the ReDeSign questionnaire. The most common band edge in the range is indicated between brackets.

Band Edge Occurrence

60 - 65 MHz (65MHz) 53%

50 - 60 MHz (52MHz) 12%

< 50 MHz (42 MHz) 35%

Next to this limited upstream band edge, the background noise levels are the principle cause of the limited capacity. In the mid and late nineties of the past century, the noise characteris-tics of the upstream channel has been extensively studied for reasons of the development of the upstream transmission technology.5,6,7

5 _{P.J. Snijders and C-J L. Van Driel, “Channel modelling of the return channel in a broadband communication}

CATV network”, 28th _{European Microwave Conference, Amsterdam 1998.}

the customer’s in-home networks. The use of inferior quality cables and splitters are a source for ingress of all types of unwanted signals. Such noise emanating from all homes connected to a cable node is aggregated at the hub or head end. From a practical viewpoint, a cable node can be considered as a vast distributed antenna with hundreds up to a thousand an-tennas, receiving and aggregating all electro magnetic signals from the homes. Moreover, today’s homes have various coaxial branches, thus forming dipoles. Such dipoles further enhance the sensitivity of the in-home networks, thus aggravating the ingress problem. An interesting treaty can be found in reference 8

The distortion signals picked up in the home environment can be distinguished in three kinds of different nature and origin:

.

• Universal broadband noise: a roughly +15-20 dB increase of the noise floor below 25 MHz

• Narrowband interferers: short wave radio transmitters (world radio etc)

• Impulse and burst noise: caused by human activity in the home (switching on/off equipment etc) which creates wideband impulses and bursts of several µs with a sig-nal level of hundreds of mV (up to volts).

The activity of many of these sources shows a periodic cycle during the day.

Figure 4 Continuous ingress noise

tak-en from refertak-ence 9 8

. Similar figures can be found in reference .

6 _{C. Eldering et al. “CATV return path characterisation for reliable communications”, IEEE Communications}

Magazine, 62 – 68, August 1995.

7 _{K. Haelvoet et al. “Procedure for measurement and statistical processing of upstream channel noise in HFC}

networks”, IEEE MTT 0S Digest, 1998

8 _{K. Mothersdal, “Ingress Safe, one small step for man, one giant step for your return path”,}

Broad-band, Volume 30, April 1, 2008.

9 _{S.Pfletschinger, “Multicarrier modulation for broadband return channels in cable TV systems”, PhD}

In contrast to the noise signals, upstream signals are transmitted rather incidentally. Moreo-ver, only one cable modem per RF channels is allowed to transmit at the time, thus excluding concurrent transmissions in the same upstream channel.

Table 3 Optical node topology data taken from the ReDeSign reference architectures

Reference Network topology

Tree & Branch Hybrid Star

Average homes passed per

fiber node 400/2000 500/1000/2000 700

Number coaxial branches per

fiber node 1/2/3 1/2/3 2/4

2.1.2 The cable networks

Networks do exhibit a large variety in network parameters and noise levels. The ReDeSign questionnaire provided network figures from 21 European operators. After analyses of these data, the architecture of the European networks was captured in a limited number of refer-ence architectures.10 _{In Table 3 we have listed the relevant topology data. Because of the} large variation in the European networks, unique topology figures could not be established, but instead different optional values were specified, as shown in Table 3. In this table, the average number of homes passed per fiber node is presented showing the large range of 400 up to 2000 homes passed that found. Operators have reported the maximum node size as well, yielding even twice as large nodes. Thus in reality, optical nodes of 4000 homes passed do occur regularly. Next to the node size, the number of coaxial branches emanating from the optical node is included in the reference architecture, showing that each node serves one up to four coaxial subnetworks.

Table 4 Operator US channel parameters

Bandwidth (MHz) 1,6 3,2 6,4 M odul ati on schem e QPSK 3% 17% 6% 16 QAM 6% 30% 13% 64 QAM 3% 13% 8%

To assess the quality and performance of the upstream band in European cable networks, the ReDeSign network questionnaire encompassed some questions regarding the upstream

transmission profiles that are used. Operators will use the transmission profile with the larg-est bit rate that can be deployed in their networks. Hence, this information reflects the quality of the upstream path. This information is shown in Table 4. This table shows the occurrence of the combinations of the bandwidth and modulation mode that is used. The figures reveal that the most attractive profile (6,4 MHz @ 64 QAM) corresponding with a bitrate of 30 Mbps can be used in few networks only. In case of the majority of the networks, the transmission profile is limited to ones with a bitrate of 10 Mbps or less.

2.2 Currently available solutions

The data of Table 2, Table 3 and Table 4 reveal that in many cable networks not all currently available technology to maximize the upstream capacity are currently deployed. In these net-works, substantial capacity gains can be obtained by implementation of today’s technology:

• Further segmentation of the network

• Extension of the upstream band edge up to 65 MHz • Higher return band signal levels

• Reduction of the noise level

In this section we give an overview of the conceivable options. First we will address the op-tions to make maximum use of the current cable architecture and available transmission technology (DOCSIS). In this case, the capacity is limited by the DOCSIS technology that supports upstream channels up to 65 MHz and a spectral efficiency of about 5 bits/Hz. In general, only the band from about 30 MHz up to 60 MHz can be used, thus limiting the max-imum capacity to 150 Mbps per cable segment.

In the further sections, we discuss new options that require new technologies, but that can surpass this 150 Mbps limitation.

2.2.1 Segmentation of the return path

Today, many proven solutions for segmentation of the return path are commercially availa-ble. These solutions are based on the use of wavelength division multiplexing of the up-stream frequency band, thus providing a scalable solution that allows an operator to split the nodes when needed.

Splitting a node yields segments with approximately half of the number of homes. Thus, the aggregated ingress noise of the homes is halved as well, thus allowing the operator to apply a higher mode modulation scheme.

2.2.2 Extension of the US band edge up to 65 MHz

In many networks, the US band is extended up to 65 MHz already. This upgrade can be con-sidered as well-known and proven. Extending the US band edge, not only brings new spec-trum, but moreover spectrum of a better quality allowing for a higher modulation scheme.

2.2.3 Higher return band signal levels

As a first remedy to a low signal noise ratio, one could raise the signal level of the upstream signals. However, the upstream signal level is limited to a value of 114 dBµV to avoid harmful egress11

11 _IEC-60728-10

. The maximum transmit signal level of the EuroDOCSIS technology already is set to this maximum value and as such the signal level cannot be further raised.

2.2.4 Reduction of noise level

Table 4 reveals that many operators are troubled by high noise levels in their networks. As such, reduction of the noise levels has to be considered as a serious option to expand the upstream capacity. Below we briefly discuss a number of remedies.

2.2.4.1

Inferior quality of in home components and a bad installation is the major cause of ingress. Conceivable, an operator could reinstall the customer in home networks, however, in practice this is a very difficult approach since each home has to be visited. Operators can encourage the use of high quality components in combination with professional installation practices, for example by creating awareness with the customers or by introducing a component quality hallmark that is supported by manufacturers, vendors and consumer stores.

Full replacement cables and splitters in the home environment

2.2.4.2

The ingress signals from two in-home cable branches are strongly correlated as pointed out in reference

Replacement of in home splitters

8. These distortion signals of different in home coaxial branches are induced by the same in-home events or received from the same external terrestrial radio sources. Be-cause of this common source, the induced distortion signals of two branches are strongly correlated. The currently used splitters simply combine these signals. Since they are in phase, the voltages are summed. In reference 8 it is proposed to replace the current splitters by ones that create a 180 degrees phase shift in one of the branches so that correlated sig-nals arriving from the different branches are cancelled (partially). It is shown that such split-ters allow a reduction of 10 dB or more of the noise level. All distortion signals, impulse and burst noise, narrow band interferers and the universal broadband noise below 25 MHz are effectively reduced.

Such a solution appears attractive since splitters are relatively easy to replace as compared to a full rebuild of the customers in home network, although a visit of a technician is still re-quired and in practice appointments with customers are difficult to make and second and even third visits are required.

In networks with a high noise level, a 10 dB reduction of the noise level allows a twofold ca-pacity expansion of a DOCSIS upstream channel. In addition, it will allow extension of the usable upstream band to lower frequencies.

2.2.4.3

In the late nineties of the past century, the concept of ingress blocking has been developed. Currently commercial systems like that of Proxilliant are on the market. An interesting white-paper can be found on the John Weeks Enterprise site

Deployment of ingress blocking systems

12

An ingress blocking system acts as a control port that generally disconnects a cable up-stream segment, but that connects the segment when an upup-stream DOCSIS packet is transmitted. The blocking system responds to the increased signal level of the preamble of the DOCSIS packet. The system is not channel specific: irrespective of the transmit frequen-cy and bandwidth of the packet, the full upstream band is conveyed. As such, the noise re-duction strongly depends on i) the number of DOCSIS upstream carriers deployed and ii) the number of homes connected to the network segment that is controlled by the blocking sys-tem. The noise reduction Rnoise (that is the ratio of the noise level at the CMTS without and with blocking system) is given by:

Rnoise = Nnode / (Nblock segment x Ncarriers)13

12_{http://www.jw-ent.com/Images/DIBWhitePaperA1.pdf}

13 _{This formula provides an approximation of R}

With:

Nblock segment = number of homes connected to the segment controlled by the ingress blocking system

Nnode = number of homes served by the fibre node Ncarriers = number of DOCSIS upstream carriers

When deployed sufficiently deep in the cable networks, say one blocking system serving a cable segment of 20 homes in a network with fibre nodes of a 1000 homes, the blocking sys-tem provides 10 – 15 dB noise reduction. Such a gain is substantial, and it will allow expand-ing the capacity of an upstream channel by a factor 2 to 4, for example an operator usexpand-ing QPSK modulation and 1.6 MHz upstream channel may upgrade to 16 QAM modulation and a 3.2 MHz channel. Furthermore, the reduced noise level allows the extension of the usable up stream band toward lower frequencies. As such, the solution is attractive for very noise networks.

A disadvantage of the ingress blocking solution is the reduced noise suppression when dep-loying several upstream carriers in the same segment. Because of this, the blocking equip-ment should be installed near the homes thus creating small blocking segequip-ments.

2.2.4.4

Conventionally, the in-home wall outlet is equipped with separate TV and FM sockets, where the FM and TV ports respectively pass on the FM band and the bands 5 – 70 MHz and 120 – 862 MHz. Without replacement of this wall outlet, a customer can connect its in-home net-work to the TV port and connect a cable modem everywhere in the home provided the signal strength is sufficient. In this case, all ingress distortion signals received by the in-home are injected in to the return channel. Alternatively, an operator can replace the wall outlet by one with a dedicated data port for connecting the cable modem. Only this data port allows bi-direction communications. Next the cable modem is connected by a short lead to the data port whereas the in-home network is connected to the TV socket. In such a wall outlet, the upstream band of the TV socket is blocked and thus the in-home network will not contribute to the ingress noise in the cable return path. A major disadvantage of this solution is the need of a separate cable modem and of dedicated in-home data network based such as WiFi or 10/100baseT Ethernet. A major advantage is that the use of a dedicated three port wall outlet resolves the ingress problem at the root. It disconnects the in home network from the 0 – 65 MHz upstream channel.

Deployment of wall outlet with a separate FM radio, TV and EuroDOCSIS outlets

2.2.5 Analysis and summary noise reduction solutions

In this subparagraph we have briefly discussed a large number of currently known solutions to increase the upstream capacity. The information from the ReDeSign questionnaire shows that many networks are not completely upgraded yet, and as such major capacity gains can be realized when properly upgrading the networks. In addition, different improvements of the in-home network are possible and operators can continue with further splitting the upstream segments. The data of Table 4 show that some first operators have mastered the successful deployment of the 6,4 MHz bandwidth / 64 QAM modulation profile. As such, the current in-formation obtained from the operators shows that the operators still are in the process of restructuring and improving their networks. Considering the large number of options to re-duce the ingress noise problems, we can assume that in the end all cable networks will be able to support 6,4 MHz bandwidth / 64 QAM modulation profile. In the end, a capacity of 120 up to 150 Mbps per segment can be obtained.

For completeness we have to emphasis that the implementation of the above improvements is complex and demanding; there is no cheap solution or quick gain. One thus can expect that operators will implement the solutions when the market forces them to do so, and not earlier. Ranking the solutions then installation of a wall outlet with a dedicated EuroDOCSIS

port appears the most effective and future proof solution because it disconnects the in-home network from the 0 – 65 MHz upstream path, thus eliminating the problem at the source.

2.3 Allocation of an additional return band spectrum

In the above section we have studied the possibilities to expand the upstream capacity by expanding the upstream band up to 65 MHz and/or by reduction of the ingress noise signals. It is argued that when applying the appropriate measures, an upstream capacity of 120 – 150 Mbps per segment would be feasible. Although some small capacity gains beyond 150 Mbps are conceivable in case of some specific networks, any substantial gains will not be possible. In other words, with an upstream capacity of 150 Mbps or perhaps a slightly larger bitrate, the limits of the current cable network design are fully reached. These solutions are fully compliant with the current HFC radio design. For a further substantial increase of the up-stream capacity, drastic changes in terms of expanding the spectrum for upup-stream signals are inevitable. In this subsection we will discuss the conceivable options.

2.3.1 Extension of the current 5-65 MHz VHF return band

A most evident and logical option to expand the upstream transmission capacity concerns the annexation of the lowest part of the downstream band. Moreover, such an annexation of neighboring spectrum would yield high-quality spectrum. This option is illustrated in Figure 5.

2.3.1.1

As pointed out in the introduction, the extension of the upstream band edge toward higher frequencies requires the sacrifice of the FM radio band. The key question thus appears whether customers will accept the switch-off of FM radio or not. As a rule, customers will ac-cept such a switch-off in case there are sufficient alternatives that are well received in the market. For FM radio, already many alternatives do exist (DAB, DVB-T, DVB-C, internet ra-dio,…); however, on the other hand one should note that in particular FM radio is much ap-preciated for its high-quality audio signal, large number of channels including local channels and good reception in cars. Therefore, although the switch-off of terrestrial FM radio is under consideration, termination of this service within the next decade appears uncertain. As such, if cable operators will decide to stop broadcasting FM radio services, likely they will be the first to so, which may result in public arousal and at a time that alternatives are still insuffi-ciently developed.

Spectrum issues

Figure 5 Architecture of an E2E VHF return band solution. The 120 MHz upper edge is

arbi-trary chosen and intended as an illustration only.

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A possible, but likely not appealing, midway solution could be a partial annexation of the FM band, say an extension of the upstream band edge up to 80 MHz so that the 100MHz – 108 MHz part of the FM band is conserved. In this case, an operator could add 2 6,4 MHz chan-nels, or 60 Mbps per segment.

An alternative solution could be the transmission of the FM signals at a higher frequency using block conversion technology. Next, those customers that appreciate reception of the FM radio signals must be provided with an appropriate frequency down converter. Such a solution can be developed by an individual manufacturer since it doesn’t require cooperation between manufacturers to specify a standard.

If an operator would decide to shift the upstream band up to frequencies beyond 85 MHz, he could as well extend the upstream band well beyond this frequency up to 100 or 120 MHz, which would respectively bring say 6 up to 8 extra upstream channels of 30 Mbps each. In case of networks with modular components (amplifiers, nodes etc) with replaceable diplex filter and upstream amplifiers, the costs of upgrading the network are the least, albeit certain-ly not negligible; however, in case of fulcertain-ly integrated components, an operator faces a large network investment.

2.3.1.2

Apart from the HFC network adaptations needed to extend the upstream band, transmission technology capable of using these higher frequencies is needed. The current EuroDOCSIS technology only supports upstream frequencies up to 65 MHz. However, the newest Ameri-can DOCSIS technology (CM-SP-PHYv3.0-I07-080522) already supports the upstream fre-quencies up to 85 MHz.

Transmission system issues

For the adaptation of the EuroDOCSIS equipment the operators are dependent on the wil-lingness of the vendors to implement this solution in EuroDOCSIS. They will have to discuss this option with this industry.

For frequencies above 85 MHz, there currently is no (Euro)DOCSIS upstream technology. Such technology needs to be developed by the industry. Clearly, for the deployment at neighboring frequencies, no new design of the upstream technology is needed.

In summary we can conclude that an extension of the downstream band beyond 65 MHz will require a substantial network upgrade. Nevertheless, there appear no fundamental obstacles or great technological uncertainties, only many practical problems that certainly should not be underestimated. Therefore we may conclude that operators and manufacturers together should be able to develop the equipment and solutions when the market requires for such higher upstream capacity. Assuming an extension with for example 65 MHz, this will triple the total upstream capacity.

2.3.2 Creation of a new UHF return band > 862 MHz

Apart from extending the existing upstream band, one can consider the creation of a new UHF upstream band above 862 MHz. Such a solution can be based on the use of a frequen-cy up converter and down converter. To convey the signals, the operator will have to install UHF return band amplifiers and diplex filters in the coax network. Both components of this solution can be considered as known technologies that can be developed by a manufacturer. Furthermore, this solution is fully compliant with today’s DOCSIS upstream transmission technology so that there is no need to introduce new transmission technology.

To separate the new upstream band from the downstream band, a diplex filter is needed. Such a diplex filter will consume a substantial frequency band of the spectrum that cannot be used for other services. In Figure 6 an example of such a filter is shown. Simulation of this specific filter show that the -3dB stop band edges are situated at the frequencies of 883 and 965 MHz. Thus at least 80 MHz of spectrum is needed as a guard band. Taking this

mini-mum band into account, an upstream channel could start from the midst of the 900 MHz band.

Figure 6 Example of a UHF diplex filter (9th order elliptical filter).

2.3.2.1 In section

Basic spectrum considerations

1.2 we have argued the inappropriateness of the use of the spectrum above 862 MHz for downstream services. The major disadvantages are i) interference from terrestrial mobile communications systems, ii) doubts regarding the electrical performance of the coaxial cables at frequencies above 1 GHz and iii) a failing performance of the passives in the network. For upstream services, in contrast, some of these disadvantages conceivably are less a handicap.

Regarding the first concern, interference of a nearby mobile transmitter like a GSM cell phone, one should note that the cable downstream signal is very susceptible for such a dis-tortion because of the low signal levels (about 60-70 dBµV). In contrast, the upstream signals will have a 40 dB higher signal level and, moreover, the (Euro)DOCSIS upstream transmitter has been designed for operations in a environment with impulse and burst noise events. Therefore, these frequencies appear more appropriate for upstream than for downstream use. However, interference between cable and terrestrial radio services is a mutual problem. When applying high cable signal levels, there is a risk of distorting the terrestrial mobile communication signals, such as the down stream signal from the mobile base station to the user terminal. In particular if the cable modem is connected via an (inferior) in home network that is installed and/or managed by the customer, there is a serious risk of such harmful in-terference.

The second issue concerns the quality of the coaxial cables for carrying signals above 1 GHz. Dependent on the type and brand, the signal attenuation may increase dramatically for specific frequencies whereas screening effectiveness may reduce as well. As such, the sig-nal balance may change completely. In the distribution part of the coaxial network, however, the output signal of the amplifier is split one or several times to feed different coaxial branches. In contrast, the upstream signals of the different coaxial branches are combined. Thus the signal loss for the upstream signals will be less than that for the downstream sig-nals. Conceivably, this smaller signal attenuation due to combining instead of splitting the signal provides a sufficient margin to compensate for the higher attenuation of the cables and

for lowering the return signal level to control the egress of the cables. In case of the coaxial trunk feeding the coaxial distribution network, however, the signal loss due to splitting the signal power is rather limited, and as such there is less or no margin to absorb the higher attenuation of the cables and for lowering the signal level to reduce the egress.

Next to the above, one should take into account that the coaxial part of the network is com-posed of different types of coaxial cables, which will complicate the identification of the most appropriate frequency band to allocated a new return path.

The passives in the network have not been designed for these high frequencies, and as such one may expect a rapid performance decline for frequencies above 1 GHz. Likely, an opera-tor will have to replace the passives.

From the above, one may conclude that the creation of such a return band, assuming it’s a viable option, concerns a large network upgrade, worth doing only when an operator may gain a fourfold or preferentially eightfold upstream capacity. For such a four or eightfold ca-pacity expansion, respectively some 120 or 240 MHz of spectrum is needed. Per coaxial segment this would yield an upstream capacity of about 500 or 1000 Mbps. If the upstream band starts off from say 950 MHz, then the spectrum up to at least 1070 or 1190 MHz is needed.

Table 3 lists the typical topology parameters of the reference architectures. In the worst case situation nodes of an average size of 2000 homes passed and with 1-3 coaxial branches from the node are found. For the extreme case of a single coaxial branch, 2000 homes have to be served. In case of a market development toward symmetrical services, a capacity of about 3000 Mbps is needed by the end of the next 10 year period, see reference 1 and or . Clearly, the creation of a 240 MHz return band in the UHF band is not sufficient to serve such a demand. However, in the case that the node of 2000 homes is composed of 3 separate coaxial branches, than the new return band will provide this capacity of 3000 Mbps. One should note that this analysis refers to the real worst-case networks and assuming a market development toward symmetrical services. For networks with smaller fibre nodes and larger number of coaxial branches emanating from the node, the maximum capacity per home will proportionally larger. Thus, although this solution will not be applicable for all European net-works, it will nevertheless cover the needs of a vast majority of all European networks. In the above capacity analysis we have not considered the reduction of the noise level asso-ciated with the reduction of the size of the coaxial segments. In principle, a fourfold expan-sion of the upstream spectrum will yield segments of one-fourth of the homes passed or a 6 dB noise level reduction.

2.3.2.2

Conceivably, there exist two options to implement such a new return band: Implementation options

• From the home to the head end: an E2E UHF return band

The frequency up converter is placed in the customer home, conceivably integrated with the EuroDOCSIS cable modem. The down converter is located in the head end, or possibly integrated in the (EuroDOCSIS) upstream receiver. This solution is illu-strated in Figure 7.

• From a concentration point in the network to the head end: hybrid VHF/UHF return band

The frequency up converter is placed in the coaxial part of the HFC network at a net-work aggregation point, as shown in Figure 8. All 30 – 65 MHz return band signals from the homes in the associated coaxial segment are aggregated and up converted to a frequency band above 862 MHz.

Figure 7 Architecture an E2E UHF return band solution

In case of the E2E UHF return band, the frequency up converters can be directly connected to the cable modem, or even integrated in the cable modem. At higher frequencies, the in-home network will be less prone to ingress noise associated with in-in-home human activity. Therefore, this implementation of the up converter directly connected to or integrated with the cable modem offers the advantage of a substantial reduction of this kind ingress noise. How-ever, as said, at these frequencies mobile communication devices may interfere with the ca-ble upstream service and vive versa, the caca-ble UHF upstream signals may degrade the mo-bile downstream signals. Thus, this solution will require dedicated frequency planning to avoid harmful interference with terrestrial services.

As an alternative, the up converter could be integrated in the customer wall outlet or with the multitap serving the coaxial drop lines. In such a solution, the in-home network will carry the conventional 30 – 65 MHz upstream signals, and not the alternative UHF return band sig-nals. Thus, less interference with terrestrial communication systems can be expected. How-ever, the advantage of a reduced ingress in the 30 -65 MHz upstream channel is sacrificed as well.

The second implementation option is based on placing the return band up converter at a concentration point close to the optical node. In such an architecture, the interference with the terrestrial radio services is minimized; however, at the expense of a raised ingress of 30 – 65 MHz distortion signals.

Comparing the practical implementation of both options, than it shows that the option where the upstream band conversion is placed as close as possible to the optical node requires a substantially smaller network upgrade. Only the parts where a new UHF upstream band is needed require an upgrade in terms of placing the diplex filters, upstream amplifiers and like-ly replacing the passives. Moreover, onlike-ly the cables of the coaxial trunk will carry the UHF return band signals, and not all the cables from the home to the node. This spatial limitation of the UHF upstream band thus enlightens the problem of finding suitable spectrum.

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Figure 8 Architecture of a hybrid VHF / UHF return band solution

This solution of a UHF return band has the feature that it pairs frequency stacking of 4 or 8 upstream channels with a deeper segmentation of the HFC network without the need to dep-loy fiber beyond the optical node. This is shown in Figure 8. Assuming that this architecture

allows a further segmentation by another factor of 3, it can boost the upstream capacity by a factor of 10 – 20.

2.3.2.3

Summarizing, the creation of upstream channels above 1 GHz will be challenging, however, this solution allows a vast and scalable expansion of the upstream capacity. Analysis of the spectrum indicates a preference to use the spectrum above 862 MHz for upstream signals, and not for downstream signals; this spectrum is used for terrestrial mobile services and the cable downstream signals will be more prone to interference than cable upstream services.

Summary and conclusion

An attractive solution appears the creation of an UHF upstream band from a concentration point A near the optical node to the head end, with the frequency conversion point located at this point A. From the home up to concentration point A the conventional 30 – 65 MHz band is conserved. This solution minimizes the network upgrade while minimizing interference with terrestrial services.

Nevertheless, one should be aware that here we have presented a conceptual analysis, and no proof or results from trials. Therefore, this architecture must be considered as a technical proposal as input for the development of a solution to expand the upstream capacity.

2.4 Summary and conclusion

In this chapter we have studied the options to expand the upstream capacity of cable net-works. First we have presented an overview of the current status of the netnet-works. This over-view shows that in many networks the upstream path can be classified as far from state of the art; not the full frequency range up to 65 MHz is used, and ingress levels are rather high, inhibiting the use a 6.4 MHz bandwidth and/or 64 QAM modulation. The operators will have to extend the upstream band up to 65 MHz in combination with a reduction of the ingress noise level. Assuming that all the appropriate measures are implemented, a maximum ca-pacity of 120 up to 150 Mbps per segment can be obtained, and not more.

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