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64 Vol. 32 No. 1 - March 2010 Issue 5Vol. 32 No. 1 - March 2010 Issue

DVB-C2: RevolutionisingP. Hasse, D. Jaeger, J. Robert, (Institut fuer Nachrichtentechnik, technische

Universitaet Braunschweig), C. Schaaf (Kabel Deutschland)

This article focuses on the impacts which new OFDM and Data Slice

techniques have on the RF transmission in a cable network and,

consequently, on the digital capacity provided.

ANGA ANGA

Introduction The DVB-C2 specification defines a new sophisticated

physical-layer technology and low-layer protocols for an

efficient and flexible transmission of digital information through

cable networks. The specification was approved by DVB in

April 2009 and, at the time of publication of this article, will

have passed the ETSI ratification procedure through National

Standardisation Organisations.

A description of a preliminary version of DVB-C2 is publicly

available as a draft standard EN 302 769 [1] or the DVB

BlueBook A138 [2]. Complementary literature has been

prepared, DVBs Implementation Guidelines for DVB-C2, and

various presentations and publications prepared by members

of the ReDeSign project [3]. It is assumed that the guidelines

document will be approved and published by ETSI in the same

timeframe as above. A number of ReDeSign publications and

project deliverables on the subject can be found now on the

projects web site.

RF Bandwidths Utilization in Cable

The transmission techniques utilised by DVB-C2 are state

of the art. Some of them constitute real innovation in cable

downstream transmission such as the OFDM and Data Slice

concepts. This article focuses on the impacts which these new

techniques have on the RF transmission in a cable network

and, consequently, on the digital capacity provided. This

article does not explain DVB-C2 technologies themselves. If

necessary, it refers to existing literature on the matter which is

widely available.

Important features of DVB-C2 are the flexible use of the

technology in existing cable topologies and its advanced

spectral efficiency. Both features can be optimally configured

to fit in with the dedicated RF characteristic provided by a

network. For instance, DVB-C2 supports the 6MHz channel

pattern implemented in the USA as well as the 8MHz pattern

preferred in Europe. In fact, the systems flexibility to allocate

bandwidth may make a fixed channel pattern obsolete

in the future. This feature also has an impact on DVB-C2

implementation scenarios as explained overleaf.

RF bandwidth adaptionBasic arrangements

The basic factor for DVB-C2 RF system design is the so-called

Elementary Period. The value of this period needs to be

adapted to the RF channel bandwidth used in a cable network.

All remaining DVB-C2 transmission parameters are then

automatically aligned. The Elementary Period (EP) is equal to

7/64s in 8MHz systems and 7/48s in systems with a 6MHz

spacing. The ratio between the two values is directly reciprocal

to the ratio of the related channel bandwidths:

All other parameters relevant for the RF transmission can be

scaled accordingly. Although the following description refers

to the European 8MHz system, equivalent figures for 6MHz

systems can be easily derived using the scaling factor .

Variable bandwidth extension

A specific RF characteristic in DVB-C2 is the dedicated sub-

carrier spacing of 2,232Hz (see also Table 1) which is fixed

for all modes and independent of the signal bandwidth

implemented. The 3,407 sub-carriers used (plus an additionalEdge Pilot for an 8MHz channel) are applied to an L1 signalling

block and carry information that the receiver requires to access

the data streams transmitted. The bandwidth of an L1 block,

therefore, is constant and has the value of 7.61MHz. In cases

where DVB-C2 signals with larger bandwidths need to be

transmitted, the number of sub-carriers has to be increased

until the desired signal bandwidth is reached. Since at least

one complete L1 signalling block needs to be transmitted,

the L1 block bandwidth of 7.61MHz constitutes the smallest

bandwidth of a DVB-C2 signal in an 8MHz cable system.

Bandwidth extensions beyond 7.61MHz are possible up to

a maximum bandwidth of 450MHz. This maximum value is

limited by the respective signalling parameter. However, it is

expected that, in practice, such large bandwidths will not beimplemented by means of a single DVB-C2 signal since its

creation at the transmitting end would require the application

of a 256k FFT algorithm. The following table gives an overview

of relevant RF transmission parameters of DVB-C2.

Table 1 shows that the absolute bandwidth of the frequency

guard bands implemented at the edge of each signal

spectrum is constant and has a value of almost 200kHz. This

phenomenon differs from traditional single-carrier modulation

schemes such as DVB-C where Nyquist shaping with a slope

of 15% of the entire signal bandwidth is used, independent

of the signal bandwidth. While DVB-C (and correspondingly

DOCSIS downstream) efficiency of modulation would not

increase with increasing signal bandwidths (the latter by the

way is not specified in the DVB-C standard), the efficiency of

DVB-C2 increases as indicated in Figure 1. The overheads,

caused by the OFDM pilots and the temporal Guard Interval

(GI) applied using GI = 1/128 of the symbol period, were taken

into account for the efficiency calculations.

The DVB-C2 L1 block and number of sub-carriers used are

essential elements for characterising the RF band of DVB-C2

signals in cable networks. In contrast to the channel number,

both parameters have a direct proportional relationship with the

signal bandwidth consumed. It is, therefore, worth considering

whether a future RF grid numbering should be based on one

of these parameters.

No of L1 blocks No of sub-carriers Signal BW Channel BW Comment

1 3,408 7.61MHz 8MHZ Smallest BW

2 6,815 15.21MHz 15.6MHz 2 L1 blocks

2.05 6,992 15.61MHz 16MHz 2 channels

8.36 28,497 63,61MHz 64MHz 8 channels

Table 1: RF transmission parameters of DVB-C2

Figure 1: Efficiency of modulation as a function of signalbandwidth


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ANGA ANGA

The method of Direct OFDM Generation, as defined for

DVB-C2, allows a definition of a dedicated numbering of

the sub-carriers in the RF band. Assuming a single DVB-

C2 OFDM signal is generated with a bandwidth equal to

the entire cable frequency band starting at DC (frequency

of 0MHz) and ending at the frequency 862MHz, for instance

the corresponding sub-carrier index k could be defined

accordingly: starting with 0 at 0MHz and ending with 386,176

at 862MHz. The correspondence between k and frequency is

depicted in Figure 2.

With these parameters, an exact definition of each single sub-

carrier of any DVB-C2 signal transmitted in a cable network

could be determined. The first sub-carrier which could be used

in cable networks in practice is the one transmitted directly

above the FM radio frequency band. It would carry a number

in the order of 49,000. The spectral band occupied by a single

DVB-C2 signal could, thus, be unambiguously defined by the

maximum and the minimal sub-carrier indexes.

Optimized frequency utilization

As mentioned previously, the bandwidth of a single DVB-C2

signal used in practice will not consume the entire frequency

band of a cable network at least, not for many years to come.

There is, however, another approach which could be applied to

create an ultra wide-band DVB-C2 signal of an almost unlimited

bandwidth. This is called Optimized Frequency Utilization; its

implementation works as follows.

Two DVB-C2 signals are transmitted in frequency bands

closely aligned to each other. We assume that the highest

frequency sub-carrier of the DVB-C2 signal transmitted in

the lower frequency band carries the sub-carrier index k. The

DVB-C2 signal transmitted in the higher frequency band has

to be converted to RF in such a way that its lowest index sub-

carrier is transmitted at the frequency position of index k+1. In

addition, a strict temporal synchronization needs to be applied

to the two signals. If these two requirements are fulfilled, the

orthogonality conditions between the two signals are met and

the two signals can be separated without interference.

Such a composition of signals has the same physical behaviour

as a single wide-band signal of corresponding bandwidth.

Although the implementation of the signalling information

differs, the gain in spectral efficiency is identical to the one

reached by a wide-band signal and corresponds to the curve

depicted in Figure 1. The power spectral density (PSD) of both

cases, Optimized Frequency Utilization (OFU) and side-by-

side transmission in separate channels, is depicted in Figure

3 by means of the simple example of two DVB-C2 signals of

7.61MHz bandwidth each.

RF capacity advancementsThe increased spectral efficiency which DVB-C2 provides, in

comparison with cable transmission systems rolled out today,

is described in various publications (e.g. [4], [6], [7], [8], [9]). In

the following analysis of the RF capacity optimisation, the most

important performance factors are summarised below:

1. DVB-C2 specific processing at the receiving end leads

to bit errors, generated by impairments which are typical

for cable networks, having a quasi random occurrence

probability. Modules such as the FFT and the time and

frequency de-interleavers ensure that any deterministic

error pattern is dispersed. For instance, impulse noiseand single-frequency interference will generate error

patterns with stochastic characteristics and quasi random

distributions.

2. The spectral efficiency supported by DVB-C2 in a noisy

environment (AWGN channel) allows the transmission of

a bit rate of more than 80Mbps per 8MHz channel using

4096-QAM. In the case of larger signal bandwidths or a

transmission complying with the principle of Optimized

Frequency Allocation referred to previously, the spectral

efficiency and thus the bit rate per frequency slot can be

further increased.

3. The transmission of DVB-C2 signals using 4096-

QAM constellations are supported by current network

implementations as explained in the ReDeSign article [5]

published in this Broadband issue.

RF scenarios before, during and afterintroduction of DVB-C2For a first estimation of digital capacity available in cable

networks, data was taken from the results of a survey which

the ReDeSign project conducted among European cable

operators in 2008. From the responses of the cable operators

(representing almost a third of the entire European operators

community, in terms of numbers of operators and subscribers

served by their networks) average scenarios were derived and

used as the basis for the following calculations.

Scenario 1 describes an average service portfolio transmitted

through cable networks today. The number of occupied channels

was chosen to be 95 with 40 channels used for the provision of

analogue TV services. The total digital capacity of 2.41Gpbs is

subdivided into two parts of approximately 1Gbps for STDV and

similar for HDTV and VOD. 11 DOCSIS channels provide High

Speed Internet (HSI) and other IP-based services.

In Scenario 2, the number of analogue TV signals is reduced

to 25. The freed-up spectrum is used either to introduce new

services such as HD and VoD via DVB-C2 or to migrate the

existing ones to the new technology. Assuming that 4096-

QAM could be used, the 120MHz frequency slot makes an

additional digital capacity of approximately 1.25Gbps available.

This capacity allows the entire HD and VOD services from

the 20 DVB-C channels to be converted into the 15 DVB-C2

channels while providing opportunities for service extensions

through the introduction of new services.

In a next step, the 20 DVB-C channels using 256-QAM signals

are upgraded to DVB-C2 using 1024-QAM. This step increases

the digital capacity of the 160MHz band from 1Gbps to some

1.4Gbps and allows a migration of the SDTV service to DVB-

C2. Since the bit rate of the SDTV programs is smaller than

the available bit rate, the SDTV service or another TV service

using DVB-C2 technology could be extended by introducing

new services. Finally, the freed-up spectrum of the 24 SDTV

channels can be used by DOCSIS for instance for the

introduction of an IPTV service while enlarging the DOCSIS

capacity from 0.5Mbps to 1.4Mbps.

The final scenario is similar to Scenario 3 which uses DVB-

C2 as an integrated portion of DOCSIS for downstream

transmission. If the remaining 25 analogue TV channels are

Figure 2: DVB-C2 RF frequency grid based on sub-carrier index k

Figure 3: Principle of OFU and gain in spectral efficiency

Service (technology) Channels, modulation constellation Digital capacity

Analogue TV (PAL, SECAM) 40 n.a.

SDTV (DVB-C) 24 x 64-QAM 0.91Gbps

HDTV & VOD (DVB-C) 20 x 256-QAM 1.0Gbps

HSI/IP (DOCSIS) 11 x 256-QAM 0.5Gbps

Total 95 channels 2.41Gbps

Table 2: Digital capacity estimation for todays channel line-up


Analogue TV (PAL, SECAM) 25 n.a.

SDTV (DVB-C2) 120MHz (15 x) 4096-QAM 1.25Gbps

IPTV (DOCSIS) 24 x 64-QAM 0.91Gbps

HDTV & VOD (DVB-C2) 160MHz (20 x) 1024-QAM 1.41Gbps

HSI/IP (DOCSIS) 11 x 256-QAM 0.5Gbps

Total 95 channels 4.07Gbps

Table 3: Digital capacity of a cable system when DVB-C2 is used for TV and DOCSIS for HIS and IP(TV) services


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ANGA ANGA

replaced by DVB-C2, the entire downstream channel line-up

would be filled with DVB-C2 signals. Assuming that again a

4096-QAM constellation is used in these former analogue TV

channels and a 256-QAM DVB-C2 signal in the former SDTV

channels, the spectrum will be occupied as indicated by Table

4. The entire digital capacity of the cable network reached a

figure of almost 6.9Gbps.

DOCSIS system integration issuesIn the last section, it was explained that the digital capacity

provided by a DOCSIS system can be significantly increased

by utilizing DVB-C2 as an integrated solution for DOCSIS

downstream transmission. The gain in capacity, for instance,

allows a 1Gbps downstream access connection to be

established by a DOCSIS system if twelve 8MHz channels are

combined to a 96MHz downstream signal using DVB-C2 in a

4096-QAM mode.

Such enormous capacity per connection has, of course, many

operational side effects. For instance, it paves the way for the

establishment of an integrated solution for full service provision

through a single IP pipe. Bandwidth could be dynamically

assigned for different service types in an easy manner. On the

other hand, IPTV services could be operated in a traditional

quasi broadcast way. Since the Data Slice concept in

combination with OFDM allows for assigning individual IPTV

streams occurring in dedicated frequency bands [4], simple

IPTV STBs could be deployed with a traditional 8MHz tuner

bandwidth. Many more benefits can be imagined, which will

not be expounded on in this article for space reasons.

Summary and outlookThis article describes an estimation of the digital capacity of

a cable network before, during and after the introduction of

DVB-C2; eventually utilizing the entire RF spectrum. The

digital capacity of a network today was chosen to have an

order of 2.41Gbps, which is a rather optimistic assumption

and not supported by a number of networks in their current

configuration.

In fact, it was assumed that 95 channels were occupied; 55

of them by traditional digital transmission systems i.e. DVB-C

and DOCSIS. The digital capacity could be increased by an

introduction of DVB-C2 in combination with a reduction and

final switch-off of analogue TV channels. The digital capacity

of a full DVB-C2-enabled network was estimated to be

6.87Gbps. These figures indicate that cable networks in the

future could have a significantly higher digital capacity than

todays networks when using DVB-C2 but without having the

need to apply intrinsic network upgrades.


DOCSIS/DVB-C2 (band I) 320MHZ (40 x) 4096-QAM 3.36Gbps

DOCSIS/DVB-C2 (band II) 192MHz (24 x) 256-QAM 1.34Gbps

DOCSIS/DVB-C2 (band III) 248MHz (31 x) 1024-QAM 2.17Gbps

Total 760MHz = 95 channels 6.87Gbps

Table 4: Digital capacity provided by an integrated DVB-C2/DOCSIS system utilized in the entire cable downstream spectrum.

AcknowledgementThe investigations outlined in this article were subject to studies

carried out in the ReDeSign project co-funded by the European

Union through its seventh framework programme. The authors

thank the EU for this co-funding. Thanks go also to the ReDeSign

partners who made available some of their research results

building the basis for the investigations explained above as well as

to colleagues of the authors companies for fruitful discussions.

References[1] Digital Video Broadcasting (DVB); Frame structure channel coding and

modulation for a second generation digital transmission system for

cable systems (DVB-C2), Draft ETSI EN 302 769, April 2009

[2] Digital Video Broadcasting (DVB); Frame structure channel coding and

modulation for a second generation digital transmission system for

cable systems (DVB-C2), DVB Blue book A 138, Geneva, April 2009

(dvb.org/technology/standards)

[3] Jaeger, D.; Brusse, B.: ReDeSign - an introduction to the projects

objectives and intended work results. SCTE Broadband, Volume 30,

No 1, p. 28 - 31, April (www.ict-redesign.eu)

[4] Hasse, P.; Jaeger, D.; Robert, J.: DVB-C2 a new Transmission

System for Hybrid Fibre Coax Networks. ICT-Mobile Summit 2009

Conference Proceedings, Santander, June 2009

[5] De Nijs, J.; Boschma, J.; Popova, M.: DVB-C2 Deployment: 4096

QAM or not. SCTE Broadband, (current issue) May 2010

[6] Hasse, P.; Jaeger, D.; Robert, J.; Schaaf, C.: DVB-C2 - A surprising

specification for optimising the efficiency and flexibility of cable,

Broadband, Vol 31, No. 1, P. 66-71, March 2009

[7] Hasse, P.; Jaeger, D.; Robert, J.: DVB-C2 - The second generation transmission

technology for broadband cable. NCTA Technical Papers. Proceedings of The

Cable Show, pp.159-167, Washington D.C., 1-3 April, 2009

[8] Hasse, P.; Jaeger, D., Robert, J.; Schaaf, C.: DVB-C2 - die zweite

Generation des digitalen Fernsehens. Fernseh- und Kinotechnik (FKT),

Heft 5/2009, S. 210-215, Mai 2009

[9] Hasse, P.; Jaeger, D.; Robert, J.; Schaaf, C.; Stadelmeier, L.: DVB-C2

- Effizienz kombiniert mit Flexibilitt. Cable!vision, Heft 3/2009, S. 49-

53, Mai 2009

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