ISDB-C
-Cable Television
Transmission for Digital
Broadcasting in Japan-
|
(Courtesy
Asia-Pacific Broadcasting
Union.) |
Kimiyuki Oyamada, Takuya
Kurakake and Hiroshi Miyazawa*,
(Digital Broadcasting Networks)
*Currently
Reception Engineering &
Service Center
Integrated Services
Digital Broadcasting
(ISDB) is an emerging
digital broadcasting
concept. With ISDB,
everything is handled
digitally. The three
kinds of systems,
ISDB-S (Satellite),
ISDB-T (Terrestrial)
and ISDB-C (Cable)
were developed in
Japan to provide flexibility,
expandability and
commonality for the
multimedia broadcasting
services using each
network.
Since digital satellite
broadcasting began
service using ISDB-S
in December of last
year in Japan, we
would like to introduce
the three ISDB systems
mentioned above. The
ISDB-C system is explained
in this article, as
a final part in a
series of three. This
article is a reprint
from ABU Technical
Review No. 189. |
Transmission of Integrated
Services Digital Broadcasting
services (ISDB) over cable
television networks is described.
Once ISDB for Satellite
(ISDB-S) begins on December
1 2000 and ISDB for Terrestrial
(ISDB-T) begins in 2003,
cable subscribers should
be able to enjoy ISDB for
Cable (ISDB-C) services
which include them. As re-transmission
schemes for the digital
broadcasting over cable,
the transmodulation and
pass-through system have
been developed. Transmodulation
converts the modulation
system into 64-QAM. Pass-through
transmits digital broadcasting
services over cable without
changing their modulation
schemes. Using these methods,
digital broadcasting services
will be implemented by a
number of cable television
operators in December 2000.
Cable television in
Japan has recently developed
toward multi-channel and
bi-directional transmission.
The number of subscribers
reached 17.62 million
or 37% of all households
as of March 2000. As broadcasting
satellite (BS) digital
broadcasting and terrestrial
digital broadcasting are
close at hand, technological
development of cable digital
broadcasting (ISDB-C)
is rapidly advancing starting
from the retransmission
of these cable television
systems.
A digital transmission
system in cable televisions
that uses the 64-QAM scheme
was first established
in 1996. This system was
standardized for the re-transmission
of communication satellite
(CS) digital broadcasting.
This system transmits
single MPEG-2 TS (hereafter
single-TS transmission
system). It uses 64-QAM
of 6 MHz bandwidth and
the transmitted information
rate is approximately
29 Mbps. However, since
CS digital broadcasting
serves multiple channels
of SDTV, a transmission
system that converted
digital programs into
analog programs was sufficient
for retransmission. For
this reason, this digital
transmission system was
not adopted very much.
On the contrary, BS digital
broadcasting combines
digital HDTV and data
broadcasting. It is not
possible to offer services
equivalent to those available
in direct BS reception
over cable because digital
programs are converted
into analog programs for
retransmission. Retransmission
of BS digital broadcasting
is an attractive service
for cable television.
As such, retransmission
by a digital transmission
system is an essential
service. For this reason,
development and standardization
of a new digital transmission
system, the multiple-TS
transmission system, has
been carried out. The
main feature of this system
is that it transmits multiple
transport streams (TSs)
with a single 64-QAM carrier.
As methods of retransmission
for digital broadcasting
in cable television systems,
the transmodulation and
pass-through systems have
been developed. The transmodulation
system converts a modulation
scheme into the 64-QAM
scheme. The 64-QAM scheme
suitable for the transmission
of cable televisions.
The first standardized
system using 64-QAM was
the single-TS transmission
system. The multiple-TS
transmission system has
been expanded based on
this single-TS system
and both will be explained
later.
The pass-through system
transmits BS or terrestrial
digital broadcasts to
cable television without
changing their modulation
schemes.
2.
Single-TS Transmission
System |
|
Figure 1 shows the configuration
of the single-TS digital
transmission system. The
specification of the 64-QAM
transmission system is
summarized in Table 1.
|
Figure
1: Transmission System
of Single-TS Digital
Cable Television System |
Table
1: Specification of
64-QAM Transmission
System |
Input
signal |
MPEG2-TS
packets
|
Frame
synchronization |
Sync
byte inversion for
every 8 packets
|
Randomization |
PRBS
(polynomial 1+X14+X15)
|
FEC |
Reed-Solomon
(204,188)
|
Interleave |
Byte
unit convolutional
(Depth: 12)
|
Modulation |
64QAM
|
Mapping |
Given
in Figure 3
|
Roll-off |
13%
|
Bandwidth |
6
MHz
|
Symbol
rate |
5.274
Mbaud
|
Transmission
rate |
31.644
Mbps
|
Information
rate |
29.162
Mbps
|
2.1 Transmission Frame
Structure
The transmission frame
structure is based on
the MPEG-2 transport packet
structure. A frame consists
of 8 TS packets. The first
byte of a TS packet is
a sync byte (value is
0x47). The sync byte of
the first TS packet in
the frame is bit-wise
inverted for frame synchronization.
To ensure clock recovery,
data other than the sync
bytes is randomized using
pseudo random sequence
(PRBS) generated by the
polynomial shown in Table
1.
2.2 Reed-Solomon Coding
To each TS packet 16 bytes
of Reed-Solomon error
correction information
are added to make 204-byte
RS frame. When 8 error
bytes exist in 1 TS packet,
a receiver can correct
such errors.
2.3 Interleaving
If an error occurs continuously
during transmission and
errors exist in more than
8 bytes of a TS packet,
correction by the Reed-Solomon
coding is no longer effective.
As a measure for this,
information is diffused
in time at the transmission
side and restored to the
original at the reception
side, to diffuse any continuous
errors in time during
transmission. This interleaving
operation can effectively
correct the error. For
RS frames, convolutional
interleaving is carried
out. However, sync bytes
are output at the position
as they are without interleaving.
2.4 Byte-To-Symbol
Mapping
With the 64-QAM modulation
scheme, 6-bit information
can be sent with a single
symbol. As shown in Figure
2, three bytes (24 bits)
are mapped into 4 symbols.
|
Figure
2: Byte/Symbol Conversion |
2.5 Differential Coding
And Mapping
After the two MSBs of
each symbol are differentially
coded, the symbols are
mapped into the 64-QAM
constellation as shown
in Figure 3. In this mapping,
rotation-invariant constellation
is adopted in which four
LSBs become the same values
even when the signal point
is turned 90, 180 or 270
degrees.
|
Figure
3: Constellation chart
for 64 QAM |
2.6 Roll-Off Factor
As seen in Figure 4, bandwidth
is limited with a filter
that has a 13% roll-off
factor. As the symbol
rate is 5.274 Mbaud as
shown in Table 1, the
bandwidth becomes approximately
5.9 MHz after modulation
so that the signal can
be transmitted within
a 6 MHz bandwidth.
|
Figure
4: Baseband filter
characteristics |
2.7. Transmission
Characteristics
During transmission of
digital broadcasts via
cable television, interference
occurs with adjacent channels
in addition to the deterioration
of the CN ratio due to
thermal noise. Because
of this, signal quality
after transmission is
specified as follows.
(a) CN Ratio
The CN ratio is specified
at 31 dB or more (noise
bandwidth of 4 MHz), where
the bit error rate is
10-4 or less without error
correction, and 10-9 or
less with error correction.
(b) Adjacent Channel
Interference
The QAM signals are frequency-multiplexed
together with the analog
VSB-AM signals. As digital
signals generally have
stronger resistance against
interference, they are
transmitted at a lower
power level. If an analog
signal is adjacent to
the lower frequency of
a digital signal, the
digital signal is transmitted
with its relative level
against the analog signal
at -18 to -4 dB. On the
contrary, if an analog
signal is adjacent to
the upper frequency of
a digital signal, the
relative level of the
digital signal must be
set at -20 to -6 dB. This
relationship is illustrated
in Figure 5. The power
level ranges are different
from upper-adjacent and
lower-adjacent signals
because the frequency
spectrum of a VSB-AM signal
is asymmetrical.
|
Figure
5: Carrier level in
adjacent transmission
of NTSC signal and
QAM signal |
3.
Multiple-TS Transmission
System |
|
3.1 Multiplexing Frame
Structure
The information rate of
64-QAM/6 MHz is 29.162
Mbps per channel, as shown
in Table 1, whereas 52.17
Mbps of information will
be transmitted on a BS
digital broadcast single
carrier. Accordingly,
at least 2 cable television
channels must be used
to retransmit information
of a single BS carrier.
Also, with a single-TS
transmission system, just
a single TS can be transmitted
with a single carrier.
However, in BS digital
broadcasting, multiple
TSs are transmitted with
a single carrier. If BS
digital broadcasting is
retransmitted with single
TS transmission system,
a large number of channels
are required as every
TS occupies a cable channel
even if the TS has small
capacity. This results
in inefficient use of
frequency.
To solve this problem,
the single-TS transmission
system has been expanded
to a system that transmits
multiple TSs. Using this
system, information on
a single BS carrier is
divided into two groups
in TS units. The two groups
are then separately transmitted
at 64-QAM/6 MHz.
Multiple-TS transmission
system multiplexes multiple
TSs using the frame structure
shown in Figure 6. The
multiplexing frame consists
of 53 slots with a header
packet as in the first
slot. In the header packet,
information about multiplexing
and demultiplexing, as
shown in Table 2, is stored.
The receiver reads this
information and extracts
a desired TS.
Table
2: Key Items of Frame
Header Information |
Information
|
Description |
Multiplexing
frame header PID |
Packet-ID
to indicate frame
header
|
Slot
layout |
Specifies
method for placing
each TS in the slots
on the multiplex
frame
|
Multiplexing
frame format |
Specifies
frame length and
maximum multiplex
TS number
|
Table
of TS-ID and relative
TS number |
Correspondences
between TS-ID and
relative TS number
|
Receiving
condition |
Information
to indicate BS receiving
condition at head-end
|
Emergency
alarm instruction |
Emergency
alarm signal used
for start control
of a receiver
|
Table
of slot and relative
TS number |
Information
on relative TS number
to which packet
in slot belongs
|
CRC |
Detects
transmission error
|
Since a multiplexed signal
is a TS packet stream,
it is possible to carry
out channel coding identical
to that of the single-TS
transmission system, as
shown in Figure 7. The
technology and standards
developed so far for the
single-TS transmission
system can be applied.
New technology development
could have been minimized
and cost reduction achieved.
With present specification,
15 TSs at maximum can
be multiplexed. Every
TS in ISDB is uniquely
distinguished in the pair
of original network identification
(Original_Network_ID)
and TS identification
(TS_ID). A TS packet in
a slot is identified in
two stages: corresponding
information between the
pair of Original_Network_ID
and TS_ID and relative
TS number, and corresponding
information between relative
TS number and slot number.
With this method, fewer
bits are required for
header information than
the method that identifies
the TS packet in a slot
directly by the pair of
Original_Network_ID and
TS_ID.
In the following, the
key items of multiplexing
frame header information
will be explained.
|
|
|
Figure
6: Frame Structure |
Figure
7: Relationship between
single and multiple
TS transmission systems |
3.2 Multiplexing Frame
Header Information
(a) Multiplexing Frame
Header PID
The first 4 bytes of the
multiplexing frame header
have a structure similar
to the MPEG-2 TS packet
header. The first byte
is a sync byte with a
value of 0x47. Also, value
of the second byte, which
is called a multiplexing
frame header PID, is set
to 0x2f. The header packet
can be identified from
other TS packets in the
slots of a multiplexing
frame as this value of
0x47 is not used as PIDs
of TS packets.
(b) Multiplexing Frame
Header SYNC
A 13-bit pattern (0x1A86)
is used as the multiplexing
frame sync. Whole bits
are reversed for every
two multiplex frames.
The header packet is also
distinguished by this
multiplexing frame sync.
Using both a multiplexing
frame sync and a multiplexing
frame header PID, frame
synchronization is ensured.
(c) Slot Layout
The slot layout identifies
the layout of TS packets
in the slots. Presently,
we only define the case
where TS packets that
belong to a certain TS
are placed in the same
slots in every multiplexing
frame.
(d) Multiplexing Frame
Format
The multiplexing frame
format is used to identify
the multiplexing frame
length and the maximum
number of multiplexed
TSs. At present, only
one format with a multiplexing
frame length of 53 and
15 maximum multiplexed
TSs is specified. The
present format (53, 15)
is chosen for efficient
transmission of TSs broadcasts
for BS.
(e) Table of TS_ID
and Relatives Number
Each TS is indirectly
distinguished using a
relative TS number. This
table shows the correspondence
between a pair of Original_Network_ID
and TS_ID, and a relative
TS number.
(f) Receiving Condition
BS digital broadcasting
system has a mechanism
called hierarchical service,
in which multiple modulation
schemes are applied simultaneously
and a user can choose
data of robust modulation
when reception is poorer
due to heavy rainfall
or other conditions. To
enable hierarchical service
to be retransmitted via
cable television, it is
necessary for the receiver
to detect the condition
of reception at the head-end.
As such, the receiving
condition of each TS is
described in the header
information.
(g) Emergency Alarm
Instruction
Emergency alarm instruction
is used to identify the
start control signal of
the receiver.
(h) Table of Relatives
Number and Slot Number
The relative TS numbers
of the TS packets allocated
into 52 slots, except
the first slot for the
header packet, are identified
with this table.
(i) CRC
Since an error in the
multiplexing frame header
information has significant
influence on the reception,
CRC (cyclic redundancy
check) is added to detect
any errors. As defined
in ITU-T Recommendation
H.222.0, the value of
CRC has zero register
output when 184 bytes
of a multiplexing frame
header excluding first
4 bytes are input into
the register of a decoder.
4.
Pass-Through System
for ISDB-S |
|
One of the simplest systems
for retransmitting BS
digital broadcasts is
to transmit the signals
(1-1.3GHz) of the BS reception
antenna output as is.
However, as a 1GHz-band
signal cannot be transmitted
with cable television,
the signals are converted
into lower frequencies
at the head-end and transmitted
via cable. The signals
are restored to the original
1GHz-band signals at each
subscriber with a frequency-up-converter
(Figure 8). This system
has following characteristics.
- Equipment costs less
and initial installation
cost is inexpensive.
- Same receiver for
direct reception can
be used.
- Compared with multiple-TS
transmission system,
many cable channels
are required.
Approximately 174MHz
bandwidth including the
guard band is required
for the transmission of
4 carriers of BS digital
broadcasts. In the multiple-TS
transmission system, 8
channels are required.
This pass-through system
requires as many as 29
channels, however. Therefore,
this system is suitable
for facilities that have
many unused channels and
retransmission is possible
without much extra cost.
|
Figure
8: Frequency conversion
in "pass-through system" |
5.
Pass-Trough System
for ISDB-T |
|
For terrestrial digital
broadcasts, the Orthogonal
Frequency Division Multiplex
(OFDM) scheme with a 6
MHz bandwidth will be
used. Since the bandwidth
of a cable television
channel is also 6 MHz,
it is possible to transmit
OFDM signals as they are
without changing their
modulation scheme. OFDM
was a new modulation scheme
for cable television.
Thus, the establishment
of new standards was required.
For the transmission of
OFDM signals via cable,
adjacent channel interference
with the signals of other
modulation scheme had
to be avoided. The relationship
at the carrier level for
the adjacent transmission
of a VSB-AM signal (NTSC)
and an OFDM signal is
shown in Figure 9. Also,
the relationship at the
carrier level for the
adjacent transmission
of a 64-QAM signal and
an OFDM signal is shown
in Figure 10. As a result
of experiments, it has
been confirmed that OFDM
signals can be transmitted
without interfering with
other signals by setting
the transmission level
of the OFDM signals at
a proper level in cable
facilities under the current
specification standard.
It is possible to retransmit
terrestrial digital broadcasts
via a transmodulation
system. The number of
required cable channels
is equivalent to the channels
needed for the pass-through
system.
|
Figure
9: Carrier level in
adjacent transmission
of NTSC and OFDM signals |
|
Figure
10: Carrier level
in adjacent transmission
of 64QAM and OFDM
signals |
Development of equipment
that employs the multiple-TS
transmodulation system is
rapidly progressing at manufacturers.
To insure compatibility
among the equipment, Japan
Cable Laboratories has established
standard specifications
for the multiple-TS system.
Manufacturers also have
already commercialized low-cost
equipment that employs the
pass-through system for
BS digital broadcasts. When
BS digital broadcasting
starts in December 2000,
digital broadcasting will
be implemented by a number
of cable television operators.
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