Distributed termination system for two-way hybrid networks

Details Distributed termination system for two-way hybrid networks Distributed termination system for two-way hybrid networks

1999-10-07

2002-12-03

Faile, Andrew (Department: 2611)

Interactive video distribution systems

Video distribution system with upstream communication

Transmission network

C725S127000, C725S148000, C725S149000

Reexamination Certificate

active

06490727

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to RF and fiber-optic architecture of HFC (Hybrid Fiber Coaxial cable) cable television or cable-like network systems and two-way digital communications to cable modem and digital settop units.
BACKGROUND
HFC Networks
FIGS. 1
a
,
1
b
show a generic HFC network.
FIG. 1
a
shows the head-end whereas
FIG. 1
b
shows the entire network.
The Head-end (HE)
8
contains the equipment respectively
10
,
12
and
14
that receives the analog and digital television signals and the digital data signals from multiple local and remote sources (satellites, off-air sources, terrestrial microwave, local tape systems, local video servers, computer servers, IP routers) and conditions these signals for transmission to the home terminals (HT). The home terminals
18
-
1
,
18
-
2
, etc., are analog and digital video “settops” (cable TV set top boxes) and digital cable modems. The head end
8
also receives the reverse (upstream) transmissions from the home terminals and processes them, in coordination with the downstream transmissions and the input/output signals from/to outside digital networks. The equipment to perform this transceiving function at the HE with respect to the home devices and the core data networks connected to the HE is called in this disclosure Interactive Termination System (ITS).
Consider first the downstream transmission of analog video, then the transmission of digital video and data. Analog video is transmitted downstream by FDM (Frequency Division Multiplexing), whereby a composite spectrum consisting of multiple analog channels is generated by RF (radio frequency) combining the output of analog modulators
20
-
1
,
20
-
2
, etc., each of which is driven by a baseband or IF analog video channel. The composite FDM signal is then applied to one or more linear analog laser transmitters
24
-
1
,
24
-
2
,
24
-
3
, etc., and transmitted over a “tree” structure
28
of optical fiber to the fiber nodes
30
-
1
,
30
-
2
, etc., where conversion from optical to RF format occurs at optical receiver
29
.
From the fiber nodes the signals are distributed to the home terminals via a tree and branch structure
40
consisting of coaxial cables with amplifiers
42
-
1
,
42
-
2
, etc., periodically placed to make up for the signal loss, taps and coaxial drops to the subscriber units HT. Each coaxial cable drop terminates in an RF set-top converter HT which bandpass selects a particular analog television channel out of the composite spectrum.
The band from 550 MHz to 750 MHz is used for downstream digital transmission from digital TV sources
12
. Digital QAM (quadrature amplitude modulation) modulators
50
-
1
,
50
-
2
,
50
-
3
, etc., are used to map multiple streams, each of several tens of Mbps into 6 MHz channels. The modulators
50
-
1
, etc., are positioned at the transmit side of a digital link, which runs over an analog linear medium, such as the HFC medium. The digital input to each QAM modulator at the HE is typically an MPEG-2 multiplexed digital signal carrying multiple digital video MPEG-2 programs and/or data channels from multiplexes
52
-
1
,
52
-
2
,
52
-
3
, etc.
The digital video inputs to the multiplexers
52
-
1
,
52
-
2
, etc., are generated by digital video encoders in sources
12
each of which digitizes and compresses an analog video input signal. Alternatively the digital video input signals originate from digital video servers or are received from remote sources via satellite. On the transmit side, an RF summing/splitting matrix
53
combines the RF signals carrying analog TV, digital TV, and data signals; the data signals are provided from the ITS via modulators
55
-
1
,
55
-
2
, etc.
The coaxial cable path is used for return (upstream) as well as for forward (downstream) transmission. The 5 to 42 MHz band (used in the US) and the corresponding range in international cable systems, called here the lowband, is dedicated to the upstream transmission. Home terminals HT such as cable modems and interactive digital video settops, in addition to receiving downstream digital transmissions by means of their QAM demodulators, also have the ability to map their digital return transmissions onto RF waveforms using upstream burst transmitter modulators. Modulation formats such as QPSK or 16-QAM are typically used, however in the return path the transmission is not of a continuous bitstream as in downstream but rather occurs in bursts of short packets of data randomly occurring in time. The data bursts at the home terminals HT are encoded into short sequences of symbols by the QPSK or 16-QAM burst transmitter modulator (called here a burst transmitter). After upstream propagation all the way up to the head-end
8
, these bursts are converted by QPSK or 16-QAM burst receiver demodulator (called here a burst receiver) into the original data packets. This process which is called detection occurs in the front end of the ITS
56
-
1
,
56
-
2
. Several burst receivers may be used with the corresponding number of upstream channels, each receiving packets over a single upstream channel frequency. Each upstream channel frequency may be shared by many home terminals, by TDMA (Time-Division Multiple Access) as arbitrated by the ITS at the head-end.
In the return path, RF return signals from the home terminals HT propagate back towards the head-end
8
, going back up the drop, the tap in tree
40
, and back through the amplifiers
42
-
1
, etc., which have bidirectional capabilities to support the return path. When the return signal reaches the fiber node
30
-
4
it is diplexed by diplexer
59
(i.e., directed on a separate upstream path based on the orthogonality of the upstream and downstream frequency bands) amplified and applied to a return optical transmitter
58
and transmitted back up to the head-end, typically on a separate optical fiber than the one used for downstream transmission.
At the head-end
8
the return signal is photodetected in a return-path-receiver
60
-
1
,
60
-
2
, i.e. converted back to electrical (RF) form, and is then split at RF summing/splitting matrix
68
and fed to analog or digital receivers for the various return service applications, in particular, it is input into the return path demodulators
64
-
1
,
64
-
2
.
High performance broadband HFC networks are essentially broadcast networks. To increase the capacity, a combination of a digital switching network and a multiplicity of smaller scale broadcasting subsystems can be employed, whereby the subscriber population is partitioned into multiple sets, with each set of subscribers being allocated one switched bi-directional digital data stream, such that different streams belonging to different subscriber sets are generally independent. This narrowcasting architecture essentially consists of a master switched system or network, with the switch ports driving smaller scale HFC broadcast, each addressing a serving area of a few tens or hundreds of subscribers called the narrowcasting domain. Domain-specific digital content, namely , two-way interactive data and interactive digital video, namely VOD (Video-On-Demand) is routed or switched to/from each domain via the HE or multiple hubs. One can differentiate between downstream and upstream narrowcast domains.
As subscriber penetration increases it is necessary to develop methods to efficiently concentrate the return path signals from large number of subscribers all the way to the head-end, while maintaining small upstream narrowcast domains. This is done by segmenting the HFC system into a larger number of return path domains, associating a smaller number of subscribers with each return path transmitter at the node. This is beneficial with respect to the ingress noise accumulation but also increases the upstream bandwidth per subscriber.
The relationship between downstream and upstream narrowcast domains is as follows. The relevant factors in determining the optimal ratio of the upstream and downstream domain sizes are digital transport capacity in the downstream and upstream (a function

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