Network apparatus and method to provide compressed digital video over mini-fiber nodes

A communication network and method is provided to communicate between a central office/head end and a plurality of end-units (EUs). A first transmission medium is connected between the central office and an intermediate node. A plurality of second transmission mediums are connected between the central office and a plurality of mini-fiber nodes. The intermediate node is also associated with each of the mini-fiber nodes such that an analog broadcast service may be sent over the first transmission medium to each of the mini-fiber nodes. Further, switched digital services and digital broadcast services are also sent over the second transmission mediums to each of the mini-fiber nodes. The mini-fiber nodes combine the signals and send the combined signals to a corresponding subset of EUs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to providing video services to hybrid fiber 
optic/coaxial cable (HFC) networks and, more particularly, to providing 
multi-channel compressed digital video to mini-fiber node (mFN) HFC 
networks. 
2. Background of Related Art 
Conventional CATV systems provide downstream broadcast information from a 
central office (CO) to end-units (EUs) for multiple CATV channels (AM-VSB) 
using analog broadcast signals from 55 MHz to 350 MHz, 550 MHz or even 750 
MHz. Cable operators have incentives to increase the channel capacity of 
their coaxial cable systems to thereby provide additional services such as 
premium and pay-per view channels, which increase revenue. However, 
upgrading conventional coaxial cable or hybrid fiber optic/cable (HFC) 
systems to 750 MHz (or from 350 MHz to 550 MHz) requires re-engineering 
the entire cable plant including at least amplifier replacement (upgrade) 
and associated amplifier spacing. Further, many conventional system 
operators also want to provide broadcast digital signals, as well as 
broadcast analog signals over a single transmission line. However, this is 
difficult, as impulse noise caused by the analog signals can cause errors 
in the digital signals. See, for example, Lu et al., Clipping Induced 
Impulse Noise and Its Effects on Bit-Error Performance in AM-VSB/QAM 
Hybrid Lightwave Systems, PTL July 94, pp. 866-868, which is herein 
incorporated by reference. The expense of such cable plant upgrades 
explains why the majority of all CATV plants in the U.S. have not been 
conventionally upgraded to 750 MHz. 
U.S. patent application Ser. No. 08/526,736 filed Sep. 12, 1995, the 
subject matter of which is incorporated herein by reference, provides an 
alternative mFN upgrade to an HFC network. The pre-existing HFC network 
provides a first access path from the CO to the EUs. In the resulting 
mFN-HFC networks, the mFNs receive signals from a central office (CO) 
through a second access path separate from the preexisting HFC network for 
transmission to EUs. Further, the mFNs can receive upstream signals from 
the EUs for transmission back to the CO over the second access path. In 
addition, conventional wisdom, as exemplified by Stoneback et al., 
Designing the Return System for Full Digital Services, Society of 
Telecommunications Engineers, Jan. 10, 1996, pages 269-277, the subject 
matter of which is incorporated herein by reference, suggests a constant 
power/Hz as the preferred allocation of power/Hz when many different 
signal types including various modulation schemes of differing bandwidth 
are carried. However, problems including mFN-HFC network inefficiencies 
result by not allocating power/Hz based on the services provided by each 
signal type and the performance requirements of each provided service. 
Thus, an efficient, cost-effective apparatus and method is needed to 
upgrade existing CATV systems to provide compressed digital video (CDV) 
for broadcast television channels and improve power allocation over a 
mFN-HFC network. 
SUMMARY OF THE INVENTION 
A communications network is provided that includes a central office and a 
plurality of first transmission mediums, for connecting the central office 
with at least one of a plurality of end-units. The central office 
transmits a first broadcast signal along each of the plurality of first 
transmission mediums and an allocated signal along one of the plurality of 
first transmission mediums to be received by at least one specified 
end-unit. 
Another communications network is provided that includes a first 
transmission medium and a plurality of second transmission mediums, which 
are separate from the first transmission medium, connected to a central 
office. The network further includes a plurality of intermediate nodes and 
a plurality of passive transmission mediums. The central office transmits 
a first broadcast service over the first transmission medium and a second 
broadcast service over the plurality of second transmission mediums. Each 
one of the plurality of intermediate nodes connects to a separate one of 
the plurality of second transmission mediums and connects to the first 
transmission medium to combine the first broadcast service and the second 
broadcast service. Each of the plurality of passive transmission mediums 
connects to one of the intermediate nodes for carrying the combined 
services to be received by a corresponding subset of a plurality of 
end-units. 
Still another communications network is provided that includes a central 
office, a transmission medium for connecting the central office to a 
plurality of end-units and a power control device. The central office 
transmits a plurality of digital services along the transmission medium 
and the power control device controls digital service signals on analog 
subcarriers transmitted on the transmission medium based on a power per 
channel determined by bit-error-rate performance requirements of the 
digital services provided. 
A HFC bi-directional communication system (network) is provided using 
mini-fiber nodes (mFNs) and central office interface units to broadcast 
signals over the mFN access path. By using the same video format as direct 
broadcast satellite (DBS) signals the mFN-HFC network employs a developed 
technology. Obviously, other formats could be used. In addition, this 
implementation results in cost-effective connectivity that provides the 
compressed digital video (CDV) signals and broadcast video services in 
mFN-HFC systems. 
Further, by allocating the power/Hz of the transmitted signal based on the 
required bit-error-rate at the end-unit, the mFN-HFC network can transmit 
digital broadcast television services (i.e., CDV) as well as switched 
services. The required bit-error-rate (BER) at the end-unit may be 
determined at least by the service provided, the modulation format and the 
error correction technique, if any. 
In addition, the mFN-HFC network can be implemented as an upgrade to 
conventional HFC networks which surpasses the current state-of-the art 
CATV systems by simultaneously providing bi-directional capabilities and 
additional multichannel broadcast digital video without re-engineering 
existing cable plants or disrupting existing services. 
Other objects, advantages and salient features of the invention will become 
apparent from the detailed description taken in conjunction with the 
annexed drawings, which illustrate preferred embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
As shown in FIG. 1, hybrid fiber/coax (HFC) bi-directional communication 
network 100 will be described using mini-fiber nodes (mFNs) to transmit 
compressed digital video (CDV) signals according to an embodiment of the 
present invention. First, the basic mFN-HFC architecture will be 
described, then additions to provide broadcast signals will be presented. 
Finally we will present apparatus and methods for improving the capacity 
of the optical links. 
FIG. 7 shows a generic schematic of the FIG. 1 embodiment. A transmitter 
142 transmits an analog broadcast signal over optical fiber 101 to fiber 
node (FN) 120. From FN 120, a plurality of coax cables 125 connect 
splitter 124 to respective amplifier/mFN modules 160.sub.1 -160.sub.n. For 
simplicity, only one amplifier/mFN module 160.sub.1 -160.sub.n is shown 
connected to each coax cable, but it is understood that multiple 
amplifier/mFN modules 160.sub.1 -160.sub.n can be connected to each of the 
coax cables 125. Further, CO 110 outputs digital broadcast video signals 
from each central office interface unit (COIU) 150.sub.1, 150.sub.2 and 
150.sub.n along optical fibers 102.sub.1, 102.sub.2 and 102.sub.n, 
respectively. Each COIU 150.sub.1 -150.sub.n may be connected to a single 
or multiple amplifier/mFN modules 160.sub.1 -160.sub.n. 
Each of the COIUs 150.sub.1 -150.sub.n may be appropriately designed to 
also transmit digital switched service signals along each of the optical 
fibers 102.sub.1 -102.sub.n. The optical fibers are further connected to 
amplifier/mFN modules 160.sub.1 -160.sub.n. Accordingly, the amplifier/mFN 
modules 160.sub.1 -160.sub.n each receive the analog broadcast signal that 
was transmitted along fiber 101 and also receive the digital broadcast 
signal sent over fibers 102.sub.1 -102.sub.n. Each amplifier/mFN module 
160.sub.1 -160.sub.n is likewise connected to a respective coax cable 
180.sub.1 -180.sub.n and a corresponding subset of end-units (EUs) along 
each of the cables 180.sub.1 -180.sub.n. Each of the COIUs 150.sub.1 
-150.sub.n can receive return signals, which are not shown in FIG. 7, 
transmitted upstream from the EUs to the CO 110. 
When a specific one of the EUs 184, for example 184.sub.x, along cable 
180.sub.n desires a switched (or allocated) service, then the COIU 
150.sub.n outputs the respective switched service signal along fiber 
102.sub.n to amplifier/mFN module 160.sub.n and finally to cable 
180.sub.n. The specific EU 184.sub.x then receives the switched service, 
preferably in an encrypted format. 
In summary, the present invention allows EUs, for example a specific EU 
184.sub.x to receive analog broadcast signals from transmitter 142 and 
digital signals including broadcast signals and switched service signals 
from one of the COIUs 150.sub.1 -150.sub.n. These signals are 
appropriately combined in the respective amplifier/mFN module and 
transmitted over the respective cable to the physically and logically 
connected subset of end-units. Further, only a specific end-unit 
requesting the switched service is able to receive and properly decode the 
switched service signal. 
As shown in FIG. 1, central office (CO) 110 connects via optical fiber 101 
to a remote signal distribution unit, referred to hereinafter as FN 120. 
Alternatively, the optical fiber 101 can be a coaxial cable. CO 110 
transmits analog broadcast information, such as multiple CATV channels 
(AM-VSB) using high quality laser transmitter 142 and optical fiber 101. 
In mFN-HFC network 100, AM-VSB signals are broadcast by transmitter 142 to 
a plurality of approximately a thousand EUs 184.sub.1 -184.sub.1000 (not 
shown) (hereafter the EUs connected to CO 110 will be referred to as EUs 
184). The broadcast information is transmitted by the transmitter 142 as 
analog information on analog subcarriers. 
At FN 120, optical signals with the broadcast information are received and 
converted to electrical signals by a receiver 122. FN 120 serves a 
plurality of coaxial cables 125 through splitter 124. 
As shown in FIG. 2, the downstream broadcast information on coax cables 125 
includes the analog signals to provide CATV service (AM-VSB). In the 
exemplary system, the analog CATV service occupies a frequency band from 
55 MHz to 550 MHz. The coaxial cables 125 connect the FN 120 to a 
corresponding plurality of amplifier/mFN modules 160.sub.1 -160.sub.n. A 
representative configuration of one of the amplifier/mFN modules 160.sub.1 
-160.sub.n is shown in the amplifier/mFN module 160.sub.n. The 
configuration of amplifier/mFN modules 160.sub.2 -160.sub.n would be 
similar, and thus are not shown in FIG. 1 for clarity. 
From the amplifier/mFN module 160.sub.1, the coaxial cables 180.sub.1 
distribute signals to and receives signals from a physically connected 
subset of EUs 184. Each of EUs 184 can include a network interface unit 
190, which can be connected to a telephone unit 192, a television unit, 
which can include a set-top box 194, and a modem or personal computing 
system 196. An exemplary end-unit is shown in FIG. 1 as EU 184.sub.1. 
As shown in FIG. 1, amplifier/mFN module 160.sub.1 connects mFN 166.sub.1, 
and uni-directional amplifier 162.sub.1, through a diplexer 164.sub.1 to a 
subset of approximately fifty of the EUs 184, of which only EU 184.sub.1 
and EU 184.sub.2 are shown. That is, each amplifier/mFN module 160.sub.1 
-160.sub.n is associated with a subset of EUs 184. The mFN 166.sub.1 
includes an optical receiver 168.sub.1, a laser transmitter 167.sub.1, and 
a diplexer 169.sub.1. An optical fiber 102.sub.1 connects transmitter 
148.sub.1 in COIU 150.sub.1 to mFN 166.sub.1. Similarly, optical fiber 
103.sub.1 connects receiver 151.sub.1 in COIU 150.sub.1 to the mFN 
166.sub.1. Alternatively, a single fiber solution could be implemented 
with optical transceivers or optical couplers between the COIU 150.sub.1 
-150.sub.n and the amplifier/mFN modules 160.sub.1 -160.sub.n. Also 
optical splitters and combiners, possibly using wavelength-division 
multiplexing (WDM), could be used to connect multiple mFNs 166.sub.1 
-166.sub.n to the CO 110 (e.g., one pair of transmitters 148.sub.1 
-148.sub.n and receivers 151.sub.1 -151.sub.n) to reduce the fiber 
required. 
The diplexer 164.sub.1 combines signals transmitted from the FN 120 (via 
the amplifier 162.sub.1) and the mFN 166.sub.1 onto coaxial cable 
180.sub.1. Diplexer 164.sub.1 also directs upstream signals from the 
subset of EUs 184 connected to amplifier/mFN module 160.sub.1 to the CO 
110. Diplexer 164.sub.1 -164.sub.n crossover can be dynamically arranged 
such that the bandwidth of services delivered to the EUs 184 using COIUs 
150.sub.1 -150.sub.n and services delivered using the transmitter 142 can 
be dynamically allocated. The bandwidth allocation to the transmitter 142 
can be limited within the capabilities of the amplifiers 162.sub.1 
-162.sub.n. 
As shown in FIG. 2, the CO 110 transmits analog broadcast signals 
downstream in the frequency band from 55 MHz to 550 MHz over the optical 
fiber 101 and through the receiver 122, the splitter 124, the coaxial 
cables 125, amplifiers 162.sub.1 -162.sub.n, diplexers 164.sub.1 
-164.sub.n and coaxial cables 180.sub.1 -180.sub.n to the EUs 184. 
Eventually these analog signals may be replaced with digital signals on 
analog subcarriers. For example, as HDTV (high-definition TV) becomes 
deployed, cable operators may replace some AM-VSB channels with HDTV 
channels. 
Amplifier/mFN modules 160.sub.1 -160.sub.n place the mFNs 166.sub.1 
-166.sub.n adjacent to each distribution amplifier 162.sub.1 -162.sub.n 
along coaxial cables 125. Diplexer 164.sub.1 connects both amplifier 
162.sub.1 and the mFN 166.sub.1 to a subset of the EUs 184 via coaxial 
cable 180.sub.1. Therefore, additional services can be incorporated into 
the mFN-HFC network 100 without affecting the downstream broadcast CATV 
services. 
The provisioning of switched services over a mFN-HFC network is described 
in U.S. patent application Ser. No. 08/526,736. The CO 110 can use, for 
example, the COIU 151.sub.1 to deliver switched services to a subset of 
the EUs 184 in the frequency band 580 MHz to 1 GHz over the optical fibers 
102.sub.1, 103.sub.1, the mFN 166.sub.1, the diplexer 164.sub.1 and the 
coaxial cables 180.sub.1. The switched services can be dynamically 
allocated within the bandwidth of the coaxial cable 180.sub.1 outside the 
bandwidth devoted to the broadcast service transmitted over transmitter 
142 and fiber 101. As shown in FIG. 2, the frequency band of 580 MHz to 1 
GHz may be used by services such as telephony, video telephony, facsimile, 
data services, enhanced-pay-per-view (EPPV), etc. In addition, the 5-40 
MHz bandwidth can be used for upstream signals, to maintain compatibility 
with conventional HFC networks. 
Transmitters 148.sub.1 -148.sub.n and the receivers 151.sub.1 -151.sub.n 
use modems 146.sub.1 -146.sub.n to provide access to switched services 
(hereafter also referred to as allocated services) at CO 110. Service 
providers can connect through the modems 146.sub.1 -146.sub.n to the CO 
110. Transmitters 148.sub.1 -148.sub.n deliver switched services in the 
frequency band outside the transmitter 142 bandwidth (e.g., from 580 MHz 
to 1 GHz) over optical fibers 102.sub.1 -102.sub.n to the mFNs 166.sub.1 
-166.sub.n. The mFNs 166.sub.1 -166.sub.n further transmit the broadcast 
or switched services to EUs 184 using diplexers 164.sub.1 -164.sub.n and 
the coaxial cables 180.sub.1 -180.sub.n. Thus, the upstream and downstream 
switched services are in the system bandwidth above the bandwidth 
limitation of the coaxial amplifiers 162.sub.1 -162.sub.n. By using the 
bandwidth above the bandwidth limitation of the amplifiers 162.sub.1 
-162.sub.n services provided to the EUs 184 using the amplifiers 162.sub.1 
-162.sub.n are not affected. Further, the total available bandwidth to the 
mFN-HFC network 100 is increased. 
Although transmitter 142 can broadcast signals to all EUs 184 within the 
HFC network, broadcast signals can also be transmitted over the mFNs 
166.sub.1 -166.sub.n using transmitters 148.sub.1 -148.sub.n. Transmitters 
148.sub.1 -148.sub.n have the capability to transmit CDV signals, however 
the transmitters 148.sub.1 -148.sub.n might not meet the stringent 
specifications required for transmitting analog AM-VSB signals. 
Broadcast digital services including broadcast CDV signals can be provided 
using a single broadcast module 144 in the CO 110 and providing a CDV 
decoder module (not shown) in each of the EUs 184. The CDV decoder module 
can be incorporated as a separate unit from a television unit or 
incorporated within the television unit 194. The broadcast module 144 is 
connected to each of the transmitters 148.sub.1 -148.sub.n through 
corresponding combiners 147.sub.1 -147.sub.n. This configuration allows 
broadcast digital services to be provided by the CO 110 over the optical 
cables 102.sub.1 -102.sub.n to all of the EUs 184 connected to the 
amplifier/mFN modules 160.sub.1 -160.sub.n. 
The mFNs 166.sub.1 -166.sub.n are analog optical transceivers and carry 
digital information on analog subcarriers. The amplifiers 162.sub.1 
-162.sub.n in amplifier/mFN modules 160.sub.1 -160.sub.n maintain the 
desired signal levels on coaxial cables 125 and 180.sub.1 -180.sub.n. 
Accordingly, coaxial cable components are passive along cables 180.sub.1 
-180.sub.n from amplifier/mFN modules 160.sub.1 -160.sub.n to each of the 
connected EUs 184. The passive transmission medium including diplexers 
164.sub.1 -164.sub.n have a usable bandwidth of 1 GHz, in contrast to 
active coaxial systems, which are limited to 750 MHz by conventional 
bi-directional and uni-directional amplifiers. As shown in FIG. 2, the 
mFN-HFC network 100 in FIG. 1 advantageously uses bandwidth from 55 MHz to 
1 GHz on the passive coaxial cable components. In addition, since the 
connection to the home is passive, this bandwidth can be flexibly 
allocated between upstream and downstream traffic simply by placing 
appropriate filters in mFNs 166.sub.1 -166.sub.n and the home. 
As discussed above, in one embodiment, mFNs 166.sub.1 -166.sub.n transmit 
digital information on analog subcarriers. Because these signals do not 
require the high performance required by analog AM-VSB signals, lower-cost 
lasers and lower-power electronics can be used for transmitters 148.sub.1 
-148.sub.n, the receivers 151.sub.1 -151.sub.n, receivers 168.sub.1 
-168.sub.n and transmitters 167.sub.1 -167.sub.n. Additionally, because 
COIUs 150.sub.1 -150.sub.n do not carry AM-VSB signals, which have 
stringent SNR and linearity requirements, the high performance laser 142 
is not necessary. Further, the modularity of the mFN-HFC network 100 
provides advantageous connectivity to any pre-existing coaxial cable 
system. However, when transmitting CDV with various types of services 
concurrently, prior-art techniques of power allocation to the transmitted 
signals are insufficient. 
In accordance with one embodiment, up to seventy channels, for example, of 
broadcast digital video services can be provided through the broadcast 
module 144. As shown in FIG. 1, the CO 110 uses compressed digital video 
(CDV) to transmit digital broadcast video to EUs 184. One type of CDV 
technology, which could appropriately be incorporated in the mFN-HFC 
network 100 has been developed for direct-broadcast-satellite (DBS) 
transmission. Accordingly, one of ordinary skill in the art would 
understand how to modify DBS CDV technology to encode the video 
information to be transmitted as digital information on an analog 
subcarrier in mFN-HFC network 100. Using a video compression standard such 
as the MPEG video compression standard, video signals can be compressed, 
for example, to an average bit rate of approximately 4 Mbps. Further, 
error-correction coding such as Reed-Solomon and convolutional 
error-correction codes can be used. In one embodiment, the 
error-correction encoding doubles the necessary transmission bit rate, but 
the signal-to-noise ratio (SNR) requirement is reduced. A signal-to-noise 
ratio as low as 6 dB can be used to transmit CDV using DBS techniques. 
Accordingly, the performance capabilities of the transmitters 148.sub.1 
-148.sub.n are sufficient for transmitting broadcast digital services 
including broadcast digital CDV signals. Broadcast module 144 supplies the 
CDV signals to transmitters 148.sub.1 -148.sub.n via combiners 147.sub.1 
-147.sub.n. 
Accordingly, mFN-HFC network 100 shown in FIG. 1 provides broadcast analog 
services, switched digital services and broadcast digital services to the 
plurality of EUs 184 from the CO 110. The broadcast analog services are 
provided using the transmitter 142 while the digital services (both 
switched/allocated services and broadcast services) are provided using 
transmitters 148.sub.1 -148.sub.n. In summary, each of the COIUs 150.sub.1 
-150.sub.n includes transmitters 148.sub.1 -148.sub.n for transmitting 
switched services and broadcast services over one of optical fibers 
102.sub.1 -102.sub.n to one of amplifier/mFN modules 160.sub.1 -160.sub.n, 
associated with a subset of EUs 184. In addition, each of COIUs 150.sub.1 
-150.sub.n can service a different optical fibers 102.sub.1 -102.sub.n, 
103.sub.1 -103.sub.n, a different amplifier/mFN module 160.sub.1 
-160.sub.n or set of amplifier/mFN modules 160.sub.1 -160.sub.n. Also, 
each of COIUs 150.sub.1 -150.sub.n can correspond to a set of optical 
fibers 102.sub.1 -102.sub.n, 103.sub.1 -103.sub.n. 
A RF spectrum of signals transmitted by the CO 110 to the EUs 184 can be 
allocated with respect to the predetermined or dynamically requested 
services by EUs 184. As the mFN-HFC network 100 is configured to provide 
digital broadcast services in addition to analog broadcast services, the 
RF spectrum can be allocated between the analog transmitter 142 and the 
transmitters 148.sub.1 -148.sub.n in the COIUs 150.sub.1 -150.sub.n to 
most closely resemble the requests of the EUs 184. The broadcast services 
requested by EUs 184 can include basic television services, radio 
services, premium channel services transmitted in the form of broadcast 
pay-per-view (PPV) or premium channels. With respect to the PPV or premium 
channels broadcast digital services, each end-unit selecting the service 
is preferably equipped with a decoder device. 
The digital switched services can include telecommuting, multimedia, data 
transmission, audio and video telephony and Internet services. 
In contrast to the broadcast services, the switched digital services 
including switched digital video are transmitted to the EUs 184 upon a 
specific one of the associated EUs 184 initiating a request or 
acknowledging a call. The switched services are then transmitted only to 
the amplifier/mFN module 160.sub.1 -160.sub.n Upon receipt of the 
transmitted RF spectrum, each of EUs 184 decodes a portion of the digital 
switched signal carrying the selected service intended for that one of the 
EUs 184. That is, only one of the EUs 184 that requested or specified a 
switched service may be able to "decode" the transmitted switched service. 
FIG. 3 shows a further embodiment in which the EUs 184 in a mFN-HFC network 
are dynamically allocated into broadcast groups. The granularity of the 
broadcast group could be as low as the number of EUs 184 physically 
connected to a mFN such as mFNs 166.sub.1 -166.sub.n. As each of the mFNs 
166.sub.1 -166.sub.n is deeper in mFN-HFC network 100 relative to FN 120, 
the granularity of the broadcast group from the COIUs 150.sub.1 -150.sub.n 
is greater than the FN 120. Each of the EUs 184 in a broadcast group would 
receive the same digital broadcast services. One of the CDV channel 
selectors 310 is associated with each broadcast group. The broadcast 
digital channels to be transmitted are selected by the one of the CDV 
channel selectors 310 associated with that broadcast group. As shown in 
FIG. 1, a plurality of CDV channel selectors 310 are located in CO 110. 
However, each of the CDV channel selectors 310 can serve multiple 
broadcast groups or multiple COs 110. Further, the CDV channel selectors 
310 can be located at the CO 110 or at a location remote from the CO 110. 
The CO 110 permits the mFN-HFC network operator to configure the broadcast 
group using demographics or geographic location parameters of the 
subscribing EUs 184. 
Video signals are digitized and compressed using CDV encoders. CDV encoders 
are preferentially located at video source 320, so that separate broadcast 
groups using the same channel do not each need to encode that channel. 
Alternatively, CDV encoders can be located in the channel selectors 310. 
RF modems (not shown) are used to place the CDV signals on analog 
subcarriers, and frequency converters (not shown) are used to convert 
these subcarriers to the appropriate broadcast frequencies. The RF modems 
may be placed at the video source 320, in the channel selectors 310, or in 
a plurality of broadcast modules 144 shown in FIG. 3. The frequency 
converters may be placed in either the channel selectors 310 or the 
broadcast modules 144. The channel selectors 310 and video source 320 may 
be located at the CO 110, though they need not be. The video source 320 
may be distributed over many locations. 
The broadcast groups can be dynamically allocated by implementing a 
switching device 305 between the broadcast modules 144 and COIUs 150.sub.1 
-150.sub.n in the CO 110. Each of the COIUs 150.sub.1 -150.sub.n is 
physically connected via an optical fiber 102.sub.1 -102.sub.n to an 
associated amplifier/mFN module 160.sub.1 -160.sub.n as shown in FIG. 1. 
Through one of the amplifier/mFN modules 160.sub.1 -160.sub.n, each of the 
COIUs 150.sub.1 -150.sub.n is physically connected to a subset of the EUs 
184. That is, each amplifier/mFN module 160.sub.1 -160.sub.n is associated 
with its own subset of EUs 184. Dynamic allocation using the switching 
device 305 allows subsets of EUs 184 who share common interests to be 
grouped together even as the geographic boundary between neighborhoods 
move. If a Spanish speaking neighborhood is expanding, more COIUs may be 
added to the broadcast group that contain Spanish language stations at an 
associated CO. 
The broadcast group can therefore be a selection of a subset of EUs 184 
that request digital video channels that can be collected within a given 
set, for example, of fifty transmitted channels. A plurality of broadcast 
modules 144 are shown in FIG. 3. If all the illustrated COIUs 150.sub.1 
-150.sub.n were connected by the switching device 305 to one of the 
broadcast modules 144, the broadcast digital services provided would be 
similar to FIG. 1. Further, the digital video channels transmitted to the 
broadcast group can then dynamically change through the operation of the 
channel selectors 310 with the preference of the subset of EUs 184 in the 
broadcast group. 
In other words, if a retirement community were adjacent a residential 
community, the financial news network and travel channels instead of 
children's television channels could be allocated to the respective 
broadcast groups by the channel selectors 310. The EUs 184 also receive 
analog broadcast service from the analog broadcast transmitter 142. 
The simultaneous transmission of digital switch services and digital 
broadcast services with their associated modulation formats and error 
correction techniques over a single transmission medium increases the 
complexity of the associated transmitted RF spectrum. Optimal performance 
of the various signals transmitted over the large available bandwidth of 
the mFN access path a mFN-HFC system requires an improved allocation of 
channel power over the transmitted signals bandwidth. An appropriate 
allocation of power/Hz will allow cable operators to efficiently use the 
upstream and downstream channels provided by mFN-HFC networks to provide 
new services (i.e., telephony, Internet services, etc.). Accordingly, the 
allocation of power per Hertz is preferably determined based on the 
services provided by the mFN-HFC network. (The power per Hertz can be 
calculated from the optical modulation depth (OMD) of the RF channel, when 
the channel's bandwidth is known.) Further, as the services requested can 
be dynamically allocated, the power per Hertz could be modified 
accordingly. 
FIG. 4 shows an embodiment in which each type of service to be provided 
within an RF spectrum transmitted by a COIU 450 has an associated 
attenuating device 407, 408, 409. The COIU 450 is similar to COIUs 
150.sub.1 -150.sub.n accordingly, only differences between COIU 450 and 
COIUs 150.sub.1 -150.sub.n will be described hereafter. The RF spectrum 
transmitted by COIU 450 will be received by a subset of the EUs 184. As 
shown in FIG. 4, the broadcast module 144 is connected to attenuator 407. 
Modems 146 providing a switched service are connected to attenuator 408 
and a representative future service module 416 providing a representative 
future service is connected to attenuator 409. Each attenuator 
individually varies the RF power provided by the associated service 
provider to the RF spectrum signal transmitted by COIU 450. 
The attenuators 407, 408, and 409 set the power-per-Hertz (PPH) or 
power-per-channel based on the required BER performance of the services 
provided by the COIU 450. Each attenuator can be individually controlled 
or controlled through an attenuator control device 415 as shown in FIG. 4. 
Further, the attenuators 407, 408 and 409 can optionally be incorporated 
into the broadcast module 144, the modems 146 and the future service 
module 416, respectively, or other associated equipment. In other words, 
the attenuator 407 can be part of the broadcast module 144. 
The services provided to the EUs 184 include different modulation formats 
(e.g., quadrature-phase-shift keying and 64 quadrature amplitude 
modulation). Modulation formats for transmitting broadcast or switched 
digital services such as digital video services or telephony services are 
well known to those skilled in the art. Further, the services provided to 
EUs 184 may include different error correction techniques. Error 
correction techniques incur additional costs and signal propagation 
delays. For example, video telephony cannot accommodate significant delay 
and should therefore incorporate fewer error-correction techniques. On the 
other hand, broadcast video is insensitive to delays and therefore can 
incorporate error-correction using techniques that introduce a delay due 
to the signal processing. 
Services delivered to the EUs 184 may have different requirements, as is 
well known to those skilled in the art. The requirements can be described 
at least in terms of delay tolerances or error tolerances. For example, 
transmitted music is extremely tolerant to delay but is very intolerant to 
errors. In contrast, transmitted voice services are tolerant of errors but 
intolerant of delay. 
Thus, setting the PPH of the transmitted RF spectrum of the services 
provided to subsets of the EUs 184 according to the required BER 
performance improves the quality of the overall services received. The 
overall quality is improved because the impact of the modulation 
technique, the error correction technique and the received service 
tolerance are incorporated into the required BER performance. 
FIG. 5 shows experimental data of a 300 MHz to 900 MHz signal that includes 
broadcast digital video services 510, telephony services 520 and data 
transmission services 530. The broadcast digital video services were 
transmitted using 16 DBS derived CDV signals 511 encoded by 
quadrature-phase-shift keying (QPSK), with each 40-Mbps QPSK channel 
carrying five video channels. The QPSK channels were separated by 30 MHz, 
so that each video channel required the same bandwidth as if AM-VSB were 
used, however, a SNR of only about 6 dB is required for good image 
quality. 
Uncooled, unisolated, lasers have been demonstrated to be capable of 
transmitting 2 data channels, and over seventy channels of CDV. In the 
experiment CDV signals were derived from a commercial DBS system and 
frequency shifted to operate in the 320-800 MHz range. A 20-Mbps channel 
was transmitted at 880 MHz using simple on-off keying and envelope 
detection to demonstrate data transmission services 530. A 2-Mbps QPSK 
channel was transmitted to demonstrate that telephony signals could be 
transmitted. Currently, commercial equipment is available to transmit 
telephone service 520 using time-division multiplex telephony signals from 
many homes onto one 2-Mbps QPSK channel. 
As shown in FIG. 5, the RF spectrum of the transmitted signals illustrates 
that the telephony services 520 were transmitted with approximately 8 dB 
higher power than the broadcast digital video (CDV) services 510. Further, 
the data transmission services 530 were transmitted approximately 16 dB 
higher than the CDV services. The power levels of the three types of 
signals were balanced so that the minimal RF drive for acceptable 
performance on all the signals would coincide. 
FIG. 6 shows the BER of the data channels plotted as a function of the RF 
drive level. The image quality was also monitored and deemed unacceptable 
if any errors were visible. Errors appear as either a "blockiness" in the 
picture or as a frozen picture. Data was taken at both room temperature 
(25.degree. C.) and at 85.degree. C. The BER was good (remained below 
10.sup.-9) over a 20-dB range in the RF drive input level from 
approximately -8 dB to -28 dB despite a 60.degree. C. temperature change 
from 25.degree. C. to 85.degree. C. The picture quality was also good over 
this range. The errors at low drive level were due to a poor SNR, with 
relative-intensity noise (RIN) being the dominant noise source. The errors 
at high drive level were due to the laser being driven below threshold, 
which generated impulse noise. 
The onset of clipping (when the laser gets driven below threshold) is 
dependent on the total RF drive to the laser. If the signals had equal 
PPH, then errors due to clipping would occur when the power in the 20 Mbps 
data channel was lower. At lower drive levels the SNR of a particular 
channel is dependent on the PPH of that channel. Since errors due to low 
SNR would occur at the same minimum signal level, and the maximum signal 
level where errors are due to clipping would occur at a lower signal level 
in the 20 Mbps data channel the acceptable range of signal levels would be 
reduced. If constant PPH is used in all channels, then to avoid this 
degradation in the 20 Mbps data channel will require that fewer CDV 
signals be transmitted. 
While the invention has been described in conjunction with the specific 
embodiments outlined above, it is evident that many alternatives, 
modifications and variations will be apparent to those skilled in the art. 
Accordingly, the preferred embodiments of the invention as set forth above 
are intended to be illustrative, not limiting. Various changes may be made 
without departing from the spirit and scope of the invention as defined in 
the following claims.