Note: Descriptions are shown in the official language in which they were submitted.
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LOCAL MULTIPOINT DISTRIBUTION SYSTEM
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION
The demand for greater quantities of information and data
transfer to and from residential as well as business users
continues to grow faster than supply can keep up with it. This
information demand is being supplied in a variety of forms
consisting of telephone systems of various forms, cable systems,
hybrid fiber/cable systems, and wireless systems. This invention
relates to a Local Multi-Point Distribution System (LMDS) intended
to provide such services as broadcast video, video-on-demand,
multimedia capability, interactive video, high speed data,
telephony, and computer data links as examples. The system can
provide a wireless interface from, for example, a local TELCO
central office {CO), or a cable Head End office, in all cases, from
a facility in a system defined as a «Head End" facility. Figure I
illustrates such a system. Note that the system consists of three
basic components, i.e., a Head End facility, a system of Base
Stations, and a multitude of system service Subscribers. The
overall system is made up of a geographical structure of non-
overlapping cells, wherein each geographical cell consists of some
several hundred Subscribers all of whom are supported by one Base
Station. Some number of Base Stations are ali interfaced to a
single Head End.
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According to the present invention, there is provided a local multipoint
distribution
system which has a head end coupled to a plurality of base stations with each
base station
constituting a cell and having a plurality of sector beam antennas. Each
sector beam
antenna illuminates a predetermined sector of the cell with RF communication
signals. A
plurality of RF subscriber stations are provided for each sector of a cell,
and each
subscriber station has a high gain antenna with a narrow beam width oriented
toward the
sector beam antenna oriented toward its assigned sector. Time division
multiple access
control means are located at each subscriber station operated such that each
subscriber
transmits at a time different from the other subscriber in its sector so the
subscribers in a
given sector do not interfere with each others transmissions, respectively.
Each
subscriber station includes means to measure the power level from the base
station, and
has means for comparing the power level from the base station with a reference
and
adjusting the power at which the subscriber station transmits to such that all
subscriber
signals arrive at their respective base stations at about the same power
level.
In a specific embodiment of the invention, means control the transmit signal
timing
such that all subscriber signals arrive at their respective base stations at
the exclusively
assigned time thereby minimizing the possibilities of mutual interference, and
means
control the transmit signal frequency such that all subscriber signals operate
at their proper
assigned frequency and are orthogonal to all other carrier frequencies
received by the base
station. Each subscriber station first, in order to initiate operation, is
operated in a receive
mode only to detect a stable downstream frequency from a head end signal and
detect any
received frequency error and adjust its initial frequency of operation in
accordance
therewith.
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As shown in Figure 1, the Head End collects all signals to be.
distributed throughout the system thereby forming a star
configuration. The Head End at the center of the star, the Base
Stations~surround the Fiead End and the Subscribers surround the
Base stations. As examples of signals collected, digital video may
be gathered via satellite links, a telephone system interface may
be provided via Class 5 switches, and high rate digital data
networks may be interfaced via a high rate data switch. The data
to/from the Head End is distributed to the system of local base
stations each assigned to serve its geographical "cell" of
subscribers.
While the description presented herein refers generally to a
"micro cellular" system , which is defined as a system of cells of
km radius or less, the invention is equally applicable to a
"macro cellular" system, herein defined as a system of cells of
radius greater than 5km.
The unique features embodied in the invention described herein
include the following:
1) A micro cellular system of signal distribution for local
multi-point distribution system (LMDS) application.
2) A micro cellular system of signal distribution with 100%
frequency re-use of one with a four sector rectangular array of
cells and with a six sector rectangular array of cells.
3) A micro cellular system of signal distribution with 100%
frequency re-use of one with a four sector rectangular array of
cells with a six sector rectangular array of cells incorporating
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the use of cross polarization isolation between sectors of adjacent
cells operating at the same frequencies.
4) A micro cellular system of signal distribution with
100% frequency re-use of one with a four sector rectangular array of
cells and with a six sector rectangular array of cells
incorporating the use of cross polarization isolation between
sectors of adjacent cells operating at the same frequencies, and
operating with increased sectorization of the cells.
5) A micro cellular system of signal distribution with 1000
frequency re-use of one with a four sector rectangular array of
cells and with a six sector rectangular array of cells
incorporating the use of intelligent frequency management between
sectors of adjacent cells operating at the same frequencies.
6) A system capable of providing, for example, analog video
broadcast, digital video in either broadcast or on-demand modes,
interactive multimedia services, high rate digital data services,
telephony, and in home monitoring systems such as might be employed
for power meter reading or home security alarm systems.
?) A frequency reference technique whereby the Subscribers
equipment is synchronized to the Base Station high stability
sources and thereby minimize a) equipment implementation costs,
b) signal acquisition times, and c) signal bandwidth overhead
requirements to accommodate hardware frequency instability
characteristics.
8) A closed loop Subscriber transmit power control technique
whereby the Subscriber received power levels at the Base Station
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are all received at the same level thereby minimizing the '
possibility of any mutual interference between Subscriber ,
signals, and eliminating the need for any significant AGC
reguirements in the Base Station RF receiving equipment.
9) A closed loop subscriber transmit timing control
technique whereby the Subscriber received signal timing as
received at the Base Station is adjusted by the Base Station in
~crement5 equal to the transmit signal symbol period to ensure
the reception of all signals with a minimum of mutual
interference from other Subscriber signals.
10) The optional use of antenna polarization diversity as a
means of minimizing adjacent cell interference signals and
increasing the total achievable capacity of the system.
11) A TDMA signaling structure which maximizes the transmit
signaling format efficiency
12) A signaling system operating within the ATM system
specifications enabling an efficient utilization of system
signaling capacity, as well as a highly flexible and adaptable
signaling format allowing efficient redistribution of system
bandwidth width in real time as the Subscriber data requirements
change.
13) An order wire channel capability and signalling format
which enable the entry and exit of Subscribers easily and
efficiently in real time as the Subscriber needs and Subscriber
population changes.
14) An order wire channel capability and signalling format
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which easily and efficiently accommodates real time Subscriber
requests for changes in the services required, additions to the
services required, and the execution of control functions to the
services.being provided, e.g., "VCR" like controls to a video
being viewed such as the "pause" function.
15) A frequency plan which on the Down Stream provides
orthogonal OC-1 channels spaced at F frequency steps where F is
the transmission data rate of the Down Stream channel, and on the
Up Stream provides 1/2 OC-1 channels spaced at F' Where 2F'= F.
16) The optional use of a two-way satellite link interface
front the Head End to the Base Station of a cell which is
geographically remote from the central cell system.
17) The incorporation of maximally efficient burst modem
techniques thereby enabling reception of multiple mutually
asynchronous time multiplexed signals each emanating from
different transmitting sources by a single receiver thereby
simplifying the design and cost of both the transmitting and
receiving equipment.
18) The use of asynchronous transfer mode over a wireless
medium for high speed data delivery incorporating the
implementation of a highly efficient transmit data frame format
which partitions the frame into five different types of data and
containing the following characteristics and attrbutes:
a) A medium access control method employing time
division multiplexing on the down stream and time-
division multiple access on the up stream and where the
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up stream frame timing is synchronized to the down '
stream frame timing; .
b) Use of a frame start ATM cell on the down stream as
a synchronization mechanism for the up stream frame
timing;
c) Use of a region of the up stream frame consisting of
a group of contiguous net entry slots for asynchronous
access by new subscribers entering the network after
power on;
d) Use of a region of the up stream frame consisting of
a group of contiguous slots for random access
transmissions of subscriber service requests;
e) Use of a portion of the up stream frame for polling
of Subscribers for the purpose of gathering power meter
data, home security alarm monitoring, and health,
status data on the operational system hardware;
f) Use of a remainder of the frame for the
communication of data in the ATM mode.
DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the
invention will become more apparent when considered with the
following description taken in conjunction with the accompanying
drawings, wherein:
Figure 1 is a schematic block diagram of a Local Multipoint
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Distribution System (LMDS) incorporating the invention,
Figure 2 is a diagrammatic representation of a hexagonal,
three sector cell pattern incorporating the invention,
Figure 3(a) is Table 1 of typical parameters, and Figure
3(b) is Table 2 of typical Baseline Communication Link
parameters,
Figure 4(a) is a diagrammatical representation of a typical
hexagonal, single, three sector cell pattern; and Figure 4(b) is
a typical hexagonal seven cell, three sector pattern,
Figure 5 is a typical hexagonal, seven cell, six sector,
cellular system configuration incorporating the invention,
Figure 6(a) is an illustration of a typical rectangular
single, eight sector cellular system configuration, and Figure
6(b) is a n illustration of a typical rectangular, nine cell,
eight sector cellular system configuration incorporating the
invention,
Figure 7 illustrates one embodiment of an optimal hexagonal,
three sector, cellular system with 1000 frequency reuse,
Figure 8 Is an embodiment of an optimal rectangular, four
sector, cellular system configuration with 100a frequency reuse,
Figure 9 illustrates one embodiment of an optimal
rectangular, eight sector, cellular system configuration with
200 capacity,
Figure 10 illustrates an embodiment of an optimal hexagonal,
six sector, cellular system configuration with 200 frequency
reuse,
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Figure 11 is a local multipoint distribution system with a
satellite link to geographically remote subscribers,
Figure 12(a) is a typical LMDS frequency plan of the
downstream frequency allocation, 1 in 4 frequency reuse, 6
frequency channels per sector, and Figure 12(b) is a typical LMDS
frequency allocation, 2 channels per sector,
Figure 13(a) is a typical signalling frame structure for
down stream frame and Figure 13(b) is a typical signalling frame
structure for the upstream frame,
Figure 14 is a typical upstream frame format,
Figure 15 is Table 3 which is a chart of typical control and
data messages,
Figures 16(a) to 16(d) illustrate typical message formats to
be used in Up Stream and Down Stream communications,
Figure 1? is a block diagram illustrating the Subscriber
frequency adjustment command generation at the Base Stations,
Figure 18 is a block diagram illustrating Subscriber power
level adjustment command generation at the Base Station,
Figure 19 is a block diagram illustrating Subscriber timing
signal adjustment command generation at the Base Stations,
Figure 20 is a block diagram illustrating Subscriber
frequency tracking and correction operations at Subscriber
terminals, and '
Figure 21 is a block diagram illustrating Subscriber
transmit power level adjustment operations at the Subscriber
terminal.
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DETAILED DESCRIPTION OF THE INVENTION
A typical LMDS system configuration was shown in Figure 1, wherein a
Head End facility HEF is shown collecting signals and data from three
exemplary
sources in this case. These exemplary sources are one from a satellite down
link
input, one from a two-way public telephone system network interface, and one
from a two-way data network interface. The system forms a star network
communicating signal to/from the head end to a system of base stations, each
of
which services a geographical cell of subscribers communicating and receiving
a
host of services which may include such data as:
1 ) Analog video broadcast signalso
2) Digital video with Distributon operating in either broadcast or
on-demand modes with remote control capability of the video display as
when controlling a VCR when operating in the on-demand mode.
3) Delivery of MPEG-2 encoded digital video consisting of either
cable television programming or programming from a video server
delivered on an on-demand basis.
4) High rate digital data services including interactive multimedia
service, World Wide Web access, file transfer protocol, and electronic
mail.
?0 5) Desk top and fill screen video conferencing.
6) Two way telephony including plain-old telephone service
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(POTS) with single or mufti-line capability, T1 access, and
basic rate and primary rate ISDN services. ,
7) Interactive multimedia and games using both computers and
game players such as Sega or Nintendo.
9) Remote in-home monitoring services such as might be
employed for power meter reading or home security alarm
services), and operational system hardware health and status
monitoring.
Cellular System Configurations
Many different cellular system configurations can be used with
the system of this invention. The two configurations discussed
herein are rectangular arrays of cells and hexagonal arrays of
cells. It will be shown that a 100% frequency re-use of one can
be achieved in several ways with both types of arrays. It will
also be shown that the capacity of the system can be easily
doubled by proper selection of frequency assignment plans within
the array, combined with increased sectorization of the cells of
the array. In addition, it will also be shown that in some
disadvantaged cell arrays the 1000 frequency re-use of one can
still be achieved by incorporating polarization diversity in the
signals transmitted by adjacent cells.
A typical cellular system configuration is illustrated in '
Figure 2. A simple single hexagonal cell is shown in bold outline.
The cell has three~120 degree sectors labeled A, B, and C. The cell
Base Station is located at the center of the cell. In this case
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the Base Station is supplied with three 120 degree sector beam
antennas AA, AB, and AC, each sector beam antenna illuminating one
of the sectors in the direction indicated by the arrows pointing
out from the center away from the Base Stations and into sectors A,
B, and C. Note that sector A antenna AA does not illuminate sectors
B nor C. Similarly sector B antenna AB does not illuminate sectors
A nor C, and sector C antenna AC does not illuminate sectors A nor
B. Each Base Station supports a system of Subscribers all of whom
will typically lie within its sector of service. Two such
subscribers are shown for sector A as X1, and X2. Subscribers are
typically provided with relatively high gain antennas with narrow
beam widths. A typical set of overall system parameters are
identified in Table 1 (Fig. 3a). A set of baseline communication
link parameters is shown in Table 2 (Fig. 3b). The Subscriber's
narrow beam antennas are painted directly at the Base Station as
indicated by the arrows emanating from XI and X2 arid aimed at the
center of the cell where the Base Station is located. The narrow
beam antennas provide the effect of almost eliminating the
possibility of multipath interference.
The direction of transmission from the Base Station to the
Subscribers is referred to herein as the "down stream" direction,
whereas the direction of transmission from the Subscribers to the
Base Station is herein referred to as the "up stream" direction.
Note from Table I (Fig. 3) that, as defined by the Federal
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Communications Commission (FCC) there are 850 MHz allocated for
the down stream traffic, and 150 MHz allocated for the up stream
traffic. This total allocated bandwidth is to be divided equally
among the various sectors, however many there are for the
particular cell structure chosen, in this example three. Thus,
sectors A, B, and C operate using different and equal portions
(1/3 each) non-overlapping portions of the total allocated system
bandwidth.
Figure 4 shows a system of seven cells. The single cell pattern
as shown in Figure 2 is repeated in Figure 4(a). The seven cell
pattern repeats the single cell configuration identically in
every cell. The cells are differentiated only by the numerical
labels placed on them here wherein all the sectors in cell 1 are
identified by a 1 attached to its label.
Similarly a 2 is attached to its label if it is in cell 2, a 3
is attached to its label if it is in cell 3, and so on for all
the cells. Note however that all sectors labeled A operate on
the same assigned frequency bands, all the sectors labeled B
operate on their same assigned frequency bands, and all the
sectors labeled C operate on their same assigned frequency bands.
This suggests that mutual interference may occur between Base
Stations and Subscribers in sectors of adjacent cells operating
in the same frequency bands. This problem is eliminated as
described below.
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Subscriber Transmit Power Control and Adjacent Cell Interference
Signals
In one embodiment, all transmission will be communicated in a
TDMA format. The down stream transmitted power level for all
signals from the Base Station will be adjusted to provide the
proper receive signal level to the Subscribers at the maximum
range. The up stream transmitted power levels originate from all
the many Subscribers in the cell. These Subscribers will transmit
at different assigned times so as to not interfere with each others
transmissions. The transmitted power level from all Subscribers
will be maintained at a level such that all Subscriber signals
arrive at their respective Base Stations at approximately the same
operating level. This further minimizes the possibility of mutual
interference between Subscribers in the same cell.
Subscribers and Base Stations in one cell will generate signals
in the same frequency band and at the same assigned time slots of
Subscribers and Base Stations of sectors of adjacent cells assigned
to use those same frequencies and time slots. Thus, there are two
primary possible sources of interference in the system. The
Subscribers of one cell can transmit interference signals into the
Base Station of an adjacent cell operating at the same frequency,
or a Base Station can transmit interference into the Subscribers of
an adjacent cell operating in the same frequency band. For example
the Subscribers of sector A2 in cell 2 can illuminate the Base
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Station of sector A in cell 1 or cell 7. The Base Station of sector
AI in cell 1 can illuminate the Subscribers in sector A of cells 2
and 3.
Consider the first situation. The Subscribers in sector A2 of
cell 2 have their narrow beam antennas all pointed at the Base
Station of sector A2 cell 2. This being the case, and considering
the fact that the Subscriber antenna 3 dB beam width is only 3.8
degrees, its antenna cannot be radiating into the A sector Base
Stations of cells 1 or 7 unless the Subscribers lie within the
3.8°
segment centered along the double lines drawn through sector A2 and
A7, and through sector A2 and A1. In addition a 3.8° segment of
the cell 2 Subscribers also radiate into sector A6. Subscribers in
sector A2 are three cell radii away from the Base Stations of
sectors A7 and A1, and are four radii away than the Base Station of
sector A2 such that their interference signals, will be
significantly attenuated by the time they arrive at the sector A1,
A7, or A6 Base Station antenna. Moreover, at these increased
operating ranges it is expected that the propagation range loss is
no longer a function of 201og(range), but rather a function of
401og ( range ) , or 501og ( range ) so that the total attenuation is very
significant and the received level of interference is negligible.
Now consider the radiation from the Base Station into the
Subscriber antennas. Again recall that the Subscribers have their
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antennas pointed directly at their respective Base Stations, This
being the case, again only a small number receive any interfering
signal, i.e., a 3.8° degree segment in sector A2, will receive
interference from the Base Stations in A1, A7, and A6. This
segment is that segment of Subscribers whose antenna pattern
encompasses both the Base Station in sector 2 as well as the Base
Station in sector Al, A7, or A6. Once again this interference
signal is significantly attenuated, first of all because it is
three or four times farther away than its own Base Station and
secondly because at this range the propagation loss is increasing
much more rapidly as already explained and the received signal
level will be insignificant.
Improved Hexagonal Pattern
Even though the interference signal levels may be attenuated
sufficiently in most cases, it is desirable to be able to have
improved cellular system designs for improved adjacent cell
isolation if necessary. This can be done by use of the cell
arrangement illustrated in Figure 5.
Figure 5 again illustrates a hexagonal seven cell pattern with
what appears to be a three sector pattern. In this case each of
the original three sectors has been divided in half, as shown for
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the A sector such that there are now effectively six sectors.
These half sectors are shown for the A sectors as Ai and Aj.
Note that the placement of the Ai and Aj is the same for all
cells along the diagonal following the arrow shown above the
cellular pattern. The same pattern is followed in the third
diagonal row, i.e., two rows below the first_ However, in the
second row the order of the Ass is reversed. By this technique
the system is now in fact a six sector pattern which eliminates
any interference radiation from any Ai or AJ cell into another Ai
or Aj. When any radiation does arrive from the previously
offending adjacent sectors, it arrives at an angle which is
significantly outside the 3 dB beam width of the receiving
antenna.
Rectangular Cellular Structures
An example of a single rectangular cellular system structures
is given in Figure 6(a), and expanded into a nine cell structure
in Figure 6(b). Recalling the description provided for the
hexagonal cell structure shown in Figure 2, it can be said that
the description is the same, i.e_, the center of the cell is the
location of the Base Station. The Base Station transmits away
from the center. The Subscribers in a given sector, e.g., sector -
A1 aim their antennas directly at the location of the Base
Station. Each of the eight sectors operate on different portions
of the FCC allocated frequency bands. Note, however, that there
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are two of each letter designator. The reason tar this is shown
in Figure 6(b). Note that the A's are reversed in their position
in adjacent cells as was done in Figure 5 with the hexagonal cell
structure. The cells are identified by a number in the lower
center of the cell. Note that in moving horizontally from cell 2
to 3 to 4, the sequence of Ass reverses each time. Similarly in
moving vertically from cell 2 to 9 to 8 the sequence of Ass
reverses. The reason for this is to provide the same conditions
as in the hexagonal pattern, i.e., the Subscribers do not radiate
directly into the 3 dB beam width of the Base Stations of
immediately adjacent cells. Similarly Base Stations do not
radiate into Subscriber antennas located in immediately adjacent
cells. This minimizes the likelihood of adjacent cell ,
interference until the range is at least three times larger than
the normal range for operation within a cell.
Polarization Diversity Between Cells
Isolation between adjacent cells is of utmost importance so
that transmissions intended for one set of subscribers, is not
received by another set of subscribers. As noted by the
preceding discussion, a significant degree of isolation can be
provided by the appropriate design of the cell structure whether
it be hexagonal or square. The use of antenna polarization
diversity can help increase the isolation between adjacent cells
and in addition it may increase the effective capacity of the
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total system. Considering the cellular pattern in Figures 4(a) -
and 4(b), if polarization diversity were used between adjacent
diagonal rows of cells, a frequency reuse of one is achievable
throughout the hexagonal system. Similarly, considering the
cellular pattern in Figure 6(b) and even assuming a four sector
pattern rather than eight sector pattern, if polarization
diversity were used between adjacent diagonal rows of cells, a
frequency reuse of one is again achievable, in this case,
throughout the rectangular system.
The factors limiting the usefulness of polarization diversity
are, 1) the amount of cross polarization isolation which can be
achieved by an antenna, 2) the degree to which rain may
depolarize a signal, or the degree to which a cross polarized
component may be created due to rain effects, and 3) the degree
to which multipath may exist in the system and degrade the cross
polarization isolation. It is known, for example, that horn
antennas of the approximate gain required for the system
Subscribers can provide at least 25 dB of cross polarization
isolation. It is believed that cost effective designs can be
achieved for the system in this respect. It can also be shown
that at the system operating frequency band there will not be
sufficient depolarization of the signal to degrade the system
significantly, i.e., the resulting signal to interference ratio
will not degrade below approximately 25 dB for 99.9 ~ of the time
in the rainiest areas of the United States, for example. Finally
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- the multipath problem is circumvented by the fact that the
Subscriber antennas have very narrow beam widths and can be
expected to receive insignificant interference in the form of
multipath signals. Similarly the Subscriber narrow beam
transmission will generate minimal multipath signaling to the
Base Station.
Increased Cell Sectorization and Use of Polarization Diversity
Achieve 100% Frequency re-use of 1
The baseline system assumes a Subscriber population density of
1000 subscribers per 1 km radius cell. As the population density
increases, the system must be designed to be able to accommodate
the growth. It was shown that increased cell sectorization, e.g.,
from 4 to 8 for rectangular cells, and from 3 to 6 for hexagonal
cells, improves the conditions of adjacent cell interference.
Similarly it was shown that by use of polarization diversity a
frequency re-use of 1 is achievable for both hexagonal and
rectangular cell structures. Finally it was shown that by
appropriate selection and assignment of the operating frequency
bands, e.g., as shown by the use of the two bands for the A sectors
labeled as Ai and Aj in Figure 5 and Figure 6(b), the adjacent cell
interference was again reduced. All three techniques can be
employed in the event of increased population density to achieve an
increase in frequency re-use and thereby in effective total system
subscriber capacity for these array configurations.
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The above described techniques achieve the maximum capacity for
the array configurations assumed. It is possible that such .
configurations may have to be used for reasons of topography or
some other consideration. The array configurations described are
not he optimal configurations for use in this system. Optimal
configurations permit achievement of at least 2000 system capacity
and are described below.
Optimal hexagonal Array
An improved hexagonal array configuration is shown in Figure-
It is referred to here as the 'optimal" array because it provides
frequency re-use of 1 without the need for polarization diversity.
This advantageous because the system capacity can be increased by
cell sectorization alone.
Note that in Figure 7 the sector operating in the same frequency
band but in adjacent cells are placed adjacent each other. Again,
here, as before, the arrows indicate the direction of RF
transmission from the Base Station. There will be no mutual
interference between adjacent A sectors because the antennas of the
Subscriber point to their respective Base Stations away from the
Base Stations of the adjacent cells. Examining the direction of
signal transmissions relative to A1 cell, there is no interferon
until the sector A9 is reached. This is four cell radii away which
guarantees that the received signal strength will be well below any
noticeable interference level at A1.
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Optimal Rectangular array
A similar ~~optimal~~ rectangular array is shown in Figure 8.
Again the optimality is defined here to mean that 1000 frequency
re-use of one is achieved without resorting to use of polarization
diversity. Note here that the transmission arrows are deleted for
clarity and the center of each cell, i.e., the location of the Base
Stations is identified by a black dot. In this case the first
source of interference to sector A1 is not reached until sector A10
which is five cell radii away which again guarantees that the
received signal strength will be below any noticeable interference
level at Al.
200% Capacity Optimal Rectangular Array
As noted previously it is desired that the system be able to
support growth should the population density increase and the
number of Subscribers to be serviced in a given cell exceed
capacity. Increased capacity can be achieved by modification of the
optimal array patterns and increased sectorizatian. Figure 9
illustrates an eight sector rectangular array pattern which
achieves a 2000 system capacity. Note that there are still only
four frequency bands identified by the A< B< C< and D labels, but
they are used twice per cell. This increases the capacity by a
factor of two. Also note that the interference between adjacent
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cells is minimal, e.g., the lower sector A9 illuminating the lower
sector A3. It is also noted that the system capacity can be further
doubled by further sectorization and the use of polarization
diversity.
200 Capacity Hexagonal Array
A similar optimal array can be developed fnr hexagonal arrays
and is shown in Figure 10. As for the rectangular array, the
capacity is doubled because the three frequency bands are used
twice in each cell. For the array shown, the distance to the first
interfering sector is three cell radii away, e.g., sector C2
illuminating sector C1 on the far side of the adjacent cell. This
may well be far enough such that there will be no noticeable
interference. Should there be any interference the problem is
solved by using polarization diversity in successive cells. Again, ,
a further increase in capacity can be achieved by additional
sectorization and the use of polarization diversity if found to be
necessary to maintain interference at or below acceptable levels.
Operation With Satellite Fink Signal Distribution to Remote
Base Stations
It will be the case that some cell locations may be
geographically remote from other regions of well established LMDS
cell distribution sites, When this is the case it may not be
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economically possible to connect to these remote cells by either
fiber, coax, or line of sight communication links and it may be
more economical to provide the remote connection via satellite. In
this event it is planned to provide satellite links from the Head
End to the remote Base Station(s). The satellite link will play
exactly the same role that the Head End to Base Station fiber links
play in the system configuration shown in Figure 1. A system
configuration for this option is illustrated in Figure 11. This
satellite link performs all the functions carried out by the fiber
links to the other cells in the system. It may be the case that the
satellite link will be of reduced capacity depending on economic
considerations and on needs of the remote cell site(s).
Typical System Frequency Plan
[change drawing numbers}
Typical system frequency plans are shown in Figures 12(a) and
12(b). Figure 12(a) shows the frequency plan for the Down Stream
signals, and Figure 12(b) shows the frequency plan for the Up
Stream signals. Recall that there are 850 MHz allocated for the
Down Stream signals. As shown there are 24 Down Stream 67.3 Mbps
QPSK carriers spaced at 33.65 MHz intervals. Recall that the
basic payload data rate is 51.84 Mbps with rate 7/8 convolutional
coding concatenated with (60,54) Reed Solomon coding. In this
case since all signals originate at the Base Station and they are
mutually synchronous, they can be spaced at orthogonal frequency
steps equal to 1/T where T is the transmitted symbol period.
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The Up Stream QPSK signals are spaced at frequency steps of
18.75 MHz. These carry a transmitted payload data rate of 28.8
Mbps. These signals cannot be assumed to be mutually synchronous
and will therefore not be overlapped. In this case the adjacent
bands will lie next to each other as shown in Figure 12(b).
Subscriber Frequency Stability Considerations and Frequency
Control
It is the intent to implement the most economical hardware
configuration while satisfying fully all system performance
requirements. As a part of this the system must be able to be
deployed easily and quickly, and transmitted signals must be
acquired and re-acquired rapidly should loss of signal occur
momentarily. In order that this be possible, system signal
frequency uncertainties must be controlled. The Base Station will
provide high stability precision oscillators whose frequency will
be known to an accuracy of at least lE-9, such that frequency
uncertainty of the carrier is no more than a few Hertz. Such
oscillators are relatively costly and can not be provided for the
Subscribers terminals. What will be provided at the Subscriber
terminals will be oscillators with stability on the order of lE-5
to 1E-6, which provides a frequency uncertainty range of 29,000
Hz to 290,000 Hertz. operation with such large frequency
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uncertainties does not allow rapid acquisition operations.
In order to circumvent this problem the Subscriber equipment
will initiate operations in a receive mode only. It will acquire
the precise and very stable Down Stream signal, track it with a
phase-lock loop, and synchronize alI its signals to it. The
Subscriber equipment will then measure the error it perceives to
exist in the received Head End signal and assume that the error
in fact lies within its own hardware frequency reference. The
transmitted Subscriber signal frequency will be corrected by the
amount measured in the received Head End signal. The Subscriber
will then initiate transmission operations with a transmit signal
which may have a small frequency error but which will be as
stable as the received Head End carrier. If an error of any
significance remains it will be measured by the Head End and a
correction signal transmitted to the Subscriber during the
network entry operations to be described below. Thus, the
possibility of a Subscriber coming on the air with a large
frequency error will not be possible.
Down Stream TDMA Signaling Frame Structure
Each time slot on the Down Stream will be equal to one ATM cell.
Each Down Stream channel, or carrier, will have a total payload
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data rate of 52.2 Mbps. With the FEC coding over head this becomes -
66.29 Mbps. The data transmission signals will have a frame
structure with each cell ( or frame slot ) having a capacity equal to
a 64 kbps voice or one DS-0 channel. This translates to 53.2
Mbps/(64 kbps + overhead) - 736 cells per frame. Since each cell
contains 53 bytes (or 424 bits) of the 64 kbps signal, the frame
length will be 6.625 ms. The first cell in each frame will be the
frame indicator, or frame sync cell. The frame structure for the
Down Stream signals is shown in Figure 13(a). Since all Down
Stream data will be handled as ATM signals, the ATM cells from a
given source will be transmitted consecutively, however, they may
appear separated from each other in the transmitted bit stream
intermingled with data from other sources.
Up Stream TDMA Signaling Frame Structure
Each Up Stream channel will have the throughput of one half an
OC-1 channel. Including overhead bits the transmission rate will be
28.8 Mbps. The data transmission signals will have a frame
structure with each cell (or frame slot) having a capacity equal to
a 64 kbps voice or one DS-0 channels This translates to 28.8
Mbps/64 kbps + overhead) - 368 cells per frame. Since each cell
contains 53 data bytes (or 424 bits), the frame length will be
6.625 ms. The first cell in each frame will be the frame indicator,
or frame sync cell. The frames of the transmitted Up Stream
signals will be synchronized to the frames of the received Down
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Stream signals, such that the transmitted Up Stream does not
require a separate frame sync slot. The frame structure for the
Down Stream signals is shown in Figure 13(a). The data transmitted
by the Subscribers will occur in assigned slots in a format to be
described below.
UP Stream Data Frame Format
As noted previously the Up Stream signal transmitted by each
Subscriber. will be synchronized to the Down Stream signal as
received by the Subscriber and will therefore not require a~
separate frame sync slot. The frame format to be used in the Up
Stream signals is shown in Figure 14. The figure shows a single
signal as it will occur on one channel. All channels will have the
same format but will communicate with a different set of
Subscribers, In a preferred embodiment, there are four "regions" in
the overall frame. The regions are identified by the function
performed by the cells comprising the region. The regions are as
follows
1) The net entry region consists of four consecutive cells and
occurs at the beginning of the Up Stream frame.
frame identifier, or frame sync cell_
2) The channel and services assignment region to which
Subscribers are assigned immediately after completing operations
in the net entry region occurs next.
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3 ) The net polling region consisting of fifteen cells shown here
occurring consecutively in the frame.
4 ) The assigned cell region consisting of however many cells are
required to carry the Up Stream data from all the Subscribers
operating on this channel.
The details of the operations are described below.
Net Entry, and Power, Timing and Frequency Control
When a Subscriber is first turned on and attempts to enter the
system the Subscriber s frequency, timing, and power levels will
not have been checked and adjusted. The Subscsiber will measure
the received Base Station signal power level and, knowing what it
expects to receive as a function of range, it adjust its transmit
level accordingly so that its signal will arrive at the Base
Station at the proper power level. When the Subscribers' signal is
received at the Base Station its level is measured and refinements
are made as necessary. This is accomplished by the Base Station as
follows. The Base station has a reference received power level
stored in memory for each subscriber. When it receives the
Subscriber signal it compares the signal level received to that
stored in the memory. The difference is transmitted to the
subscriber as a correction factor initially in the Net Entry region
of the frame and later on in the Polling region of the frame as
described below.
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The Subscriber s frequency must be corrected. This is done in
large part by the Subscriber equipment which locks on to the
received Base Station signal, tracks it with a phase locked loop
and synchronizes its system reference to it.Corrections to the
Subscribers frequency are communicated to the Subscriber in both
the Net Entry and Polling regions of the frame.
The timing of the transmit signal must be set so that it
arrives properly synchronized to the TDMA format to be received
at the Base Station. A Subscriber close to the Base Station will
have essentially zero delay in its return signal assuming that it
initiates signal transmission in perfect synchronism with the
received frame identifier signal received from the Base Station.
A Subscriber at the farthest range, assumed here to be at least 2
km for the baseline microcellular system, but could conceivably
be 10 km for a macrocellular system, will have a delay of
approximately 23.44 microseconds and 61.2 microseconds
respectively in its return signal assuming that it similarly
initiates transmission in perfect synchronism with the received
frame identifier signal from the Base Station. Each cell in the
Up Stream frame is approximately 18 microseconds long. Thus, a
net entry region four cells long provides a time slot 72
microseconds long which is sufficiently long to accommodate the
maximum delays possible when the Subscribers transmit timing has
not been adjusted.
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When a Subscriber s equipment is first turned on it transmits
with no adjustment to its timing, i.e., zero delay, in
transmission. As soon as the Base Station receives the new
Subscriber signal it measures the timing offset with respect to
the first second net entry cell in the frame, it compares the
received signal power level with respect to a stored reference
level, and it measures its frequency offset, if any. The Base
Station immediately transmits correction data to the Subscriber
for all three parameters, power level, frequency, and timing, and
waits to receive a subsequent transmission to verify that the
corrections were properly received and implemented. Once the
corrections are verified the Subscriber is directed to transfer
operations to the channel and services assignment region whose
location is also identified to the Subscriber. All correction
parameters are stored by the subscriber terminal for future use.
Channel and Services Assignment Region Operations
The size of the channel and services assignment region will be
variable. It will consist of all the cells not needed by all the
other regions but will not be allowed to be smaller than a size
guaranteeing adequate service for the total population of
Subscribers to be serviced by this channel. The exact minimum
size will depend on the size of the Subscriber population.
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When assigned to the channel and services assignment region
the Subscriber s signal parameters have all been adjusted so that
when received at the Base Station its signal will be in timing
synchronism with respect to the Base Station frame structure, its
frequency error, if any, has been corrected and its power level
will be properly set. Once these operations have been completed
the Subscriber is removed from further operation in the net entry
region so as to make it available for other Subscribers entering
the system, if necessary I.
operations in the channel and services assignment region
consist of two types:
1) Establishment of the Subscribers capacity and service
requirements, and assignment of operating cells in the assigned
cell region. Upon conclusion of operations in the channel and
services assignment region the Subscriber becomes an on-line
Subscriber being provided a fixed portion of the channel capacity
and system services as requested.
2) Issuance of additional requests for service by any of the
on-line Subscribers if they cannot receive service from the
polling operations to be described below.
operations in the channel and services assignment region are
in a slotted ALOHA fashion. The Subscribers proceed to access
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the channel as required in a slot they hope is vacant.
Collisions may occur at which time the Subscribers repeat the
transmission with a random length delay.
The function to be accomplished by operations in the channel
and services assignment region are for the Subscriber to define
for the Base Station the services it requires. The Base Station
will then review the operational system conditions and define to
the Subscriber which slots, or cells, it is to occupy in the
assigned cell region of the frame so long as it continues to
utilize the services requested. once the issuance of cell
assignment instruction is completed the Subscriber transfers
operations to the assigned cell region as directed by the Base
Station. Thereafter, operation in the channel and services
assignment region will only be made use of by a Subscriber in
emergency or priority situations which may arise and which cannot
be serviced rapidly enough by the polling operations as
described below.
Assigned Cell Region
The assigned cell region is that portion of the frame carrying
the on-line services data from the Subscriber to the Base
Station. The Subscriber will be assigned a channel capacity,
i.e., a number of cells, sufficient to accommodate the data to be
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communicated in a timely manner. The cells to be occupied are
precisely defined so that a Subscriber has the capacity needed in
an exact location reserved for his,use only until the service is
no longer needed at which time it is relinquished for use by
other Subscribers.
Net Polling Region
The net polling region serves two purposes: it provides a
means of accessing off-line Subscribers and requesting a response
for low rate monitoring and health check purposes, and it
provides a means whereby the Base Station checks to see if a
Subscriber requires servicing at that particular time.
It is expected that a cell will service on the order of 1000
Subscribers. This implies an average of 167 Subscribers to be
serviced by each Up Stream channel. With 15 cells assigned to
perform cyclic polling of all Subscribers, whether on-line or
off-line, this translates to a check of 15 Subscribers every
frame (6.625 ms), or a check of every Subscriber once every 74
ms. This is adequate for purposes of such operations of
monitoring utility power meters, security alarm systems, and
hardware health check operations. It may not be sufficiently
fast to keep up with the stream of service requests which will be
generated be the Subscribers when demanding changes of service
conditions while using a computer connection or controlling
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operating conditions of a television selection being viewed. In '
this event use will be made of the channel and services
assignment region for communicating these service requests.
Control and Data Messages
All functions including MAC functions will be controlled by a
series of control messages which are exchanged between the Head
End and the Subscribers. All control messages flow through the
Base Stations but in general all decisions will be made by the
Head End. The detailed distribution of channel capacity as it
relates to the physical channel occurs at the Base Station.
Some of the message types to be used are illustrated in Figure
Z5.
Upstream Message Content
The Up Stream message contains a number of fields. The exact
content varies depending on the type of message. All messages
must contain certain standard characterization fields as follows:
-Destination address- a unique N (e. g., 48) bit address
identifying the Head End
-Source Address- a unique N (e. g., 48) bit address identifying
the Subscriber
-Message Type- a N (e.g., 12) bit number identifying the
message type.
Down Stream Message Content
The Down Stream Message similarly contains a number of fields.
The exact content varies depending on the type of message. All
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' messages must contain certain stasndard characterization fields
as follows:
-Destination Address- a unique N (e. g., 48) bit address
identifying the Subscriber
-Source Address- a unique N (e. g., 48) bit address identifying
the Head End
-Subscriber ID#- a temporary identification number assigned to
the subscriber for the duration of the presently established
service session. This ID# need only be used if the Head End
capacity is expected to be less than the total number of
Subscriber services to be provided at any one time by the Head
End. If this is never to be the case, the Destination Address
is adequate for all purposes.
-Message Type- a N (e.g., 12) bit number identifying the
message type.
Figurel4 illustrates several typical complete message
structures. The size of the various segments is shown in terms of
bytes.
The simplest shown is Figure 16(a) which illustrates the
basic requirements for an Up Stream message. The configuration
shown is adequate for use as either an Initiation Request message
or a Terminate Request message. It can also be used for executing
' such functions as Request to Execute File Transfer by adding the
details of what is to be transferred in the successive undefined
cells of the message.
The next simplest message format is that shown in Figure 16(b)
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which can be used for a Terminate Command from the Head End
following a Termination request from a Subscriber. It can also be
used for executing such functions as a Command to Prepare to
Receive a File Transfer by adding the details of what is to be
transferred in the successive undefined cells of the message.
An Initiation Response message becomes more complex and can
take the form of Figure 16(d). Note that Figure 16(d) containd
the Subscriber ID# and a parameter adjustment data. This type
adjustment data would be provided to a new Subscriber in order to
provide correct power level, carrier frequency, and signal
transmit timing adjustments to ensure proper system operation and
minimal interference between received Subscriber signals at the
Head End. This message format can also be used for polling
operations as a Status Request and a Parameter Adjustment
Command. The message type would be changed to indicate that two
functions are being performed, i.e., parameter adjustment data is
being provided and a status data is requested from the
Subscriber. Additionally, this general format is to be used as a
Service Request Response by again changing the message type
designator to indicate the appropriate two functions being
performed and by appending to the message a definition of the
services being allocated to the Subscriber.
Figure 16(c) shows a typical format for a Subscriber Service
Request message. Since more than one type of service might be
requested at any one time, multiple simultaneous service request
may be issued in a single message as shown.
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Control Messages
All functions including MAC functions will be controlled by a series of
control messages which are exchanged between the Head End and the
Subscribers. All control messages flow through the Base Stations but in
general
all decisions will be made by the Head End. The detailed distribution of
channel
capacity as it relates to the physical channel occurs at the Base Station.
Some of the message types to be used include the following:
1 Initialization
) request
2) Initialization
response
3) Termination request
4) terminate command
5) Special commands
Figure 18 and Figure 2I: Power Level Adjustnleni
The Subscriber power level is adjusted in two steps. Figure 18 and Figure 21
indicate the system level details of the operation as it is performed at the
Subscriber and Base Station respectively.
During net entry operations the Subscriber receives the Base Station signal
while
operating in a receive only mode. The Base Station always operates at a fixed
transmit power level. Following Figure 18, the Subscriber receiver system
receives and measures the Base power level. Based on this measurement, and
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knowing the transmit power level of the Base Station, the Subscriber system
can .
estimate the range to the Base Station. Knowing the range to the Base Station,
the Subscriber system can estimate the proper transmit power level at which it
should operate. The Subscriber system sets the transmit power level to this
estimated power level and initiates transmit operations.
Once the Subscriber begins Net Entry transmit operations the Base Station,
following Figure 21 immediately performs a signal level computation on the
received Subscriber signal. It then compares the received power level to a
stored
reference level in a comparator circuit. The comparator provides a power level
error measurement which is provided to a processor circuit (which may take the
form of a microprocessor or a look up table) which provides a correction
signal to
be relayed by the Subscriber. The processor circuit assembles a power level
adjustment command and relays it to the Subscriber. The Base Station
periodically continues to perform a power level measurement and correction
during the polling operations as required so long as the Subscriber is in
transmit
operation.
Figure 20 and Figure 17: Subscriber Frequency Adjustment Opea~alions
The Subscriber carrier frequency is adjusted in two steps. Figure 20 and
Figure
17 indicate the system level details of the operation as it is performed at
the
Subscriber and Base Station respectively.
During net entry operations the Subscriber receives the Base Station signal
while
operating in a receive only mode. The Subscriber receiving system contains a
low cost oscillator whose frequency accuracy will be on the order of lE-5 or
lE-
6. The frequency error can be quite large with this stability oscillator. Both
the
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receive and transmit programmable phase lock loop (PLL) synthesizers are
initial
locked only to this crystal oscillator. The Base Station always operates at a
fixed
and very stable carrier frequency, with a stability on the order of lE-9.
Following
Figure 20 the Subscriber receiving system receives and tracks the Base Station
carrier frequency with a PLL and synchronizes its transmit system frequencies
and transmit programmable PLL synthesizer to the received Base Station carrier
signal. A comparison is made between the received frequency as seen by the
Subscriber receive system and that frequency which is expected. It is assumed
that the total error as seen by the Subscriber system is due to an error in
the
Subscriber crystal oscillator frequency. An error signal is generated on the
basis
of this assumption and a correction signal applied to the transmit
programmable
PLL synthesizers. This correction signal places transmit frequency at the
correct
transmit frequency when correcting for the Subscriber system error. Transmit
operations and Net Entry can now be initiated.
Figure W operations are now initiated. The Base Station receives the
Subscriber
signal as adjusted by the Subscriber system. There may still be an error which
can develop over time due to temperature changes or changes in component
values due to aging.
The receive Subscriber signal is tracked in a PLL and an error signal is
generated
on the basis of the PLL tracking operations. All received frequencies are
compared to the Base Station frequency system as a standard. This measured
frequency error is translated by the Base Station into a correction signal and
transmitted to the Subscriber as a frequency adjustment command. The Base
Station periodically continues to perform a carrier frequency measurement and
correction during the polling operations as required so long as the Subscriber
is in
transmit operation.
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Figure 19: Subscriber Timing Adjustment Operations
The Subscriber signal timing is adjusted by the Base Station. Figure 19
indicates
the system level details of the operation as it is performed by Base Station.
During net entry operations the Subscriber receives the Base Station signal
while
operating in a receive only mode. The Subscriber acquires the Base Station
signal and transmits its response immediately following reception of the Base
Station frame sync signal. The signal delay as seen at the Base Station will
depend on the range to the Base Station from the Subscriber. During Net Entry
operations the Base Station receives the Subscriber signals, compares the
received time of arrival to an expected time of arrival (in this case the
frame sync
time), measures the delay, and derives a timing error measurement Based on
this
delay, or timing error measurement, the Base Station computes a signal timing
correction adjustment command for the Subscriber. This command is
communicated to the Subscriber whose system timing is adjusted on the basis of
this instruction. Additional measurements and corrections are made during the
Net Entry operations as required until the timing is adequately adjusted to
allow
the Subscriber to proceed to the Channel and Services Assignment region. The
Base Station periodically continues to perform a Subscriber signal timing
measurement and correction during the polling operations as required so long
as
the Subscriber is in transmit operation. It should be noted that timing error
measurements at all times after net entry is complete are made on the basis of
cell
timing rather than frame sync since the Subscribers do not transmit frame sync
signals and only transmit cells. The Base Station can compute all received
signals, after initial adjustments are made, to cell timing since all
transmissions
are to be made in synchronization with cell slots.
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Wfiife prcfermd embotlirnents of the invention have been shown and
descrif,ed, it will he appreciated that vuious other embodiments, adaptations
and
modifications corning Within the scope of the invention will he apparent to
those
skilled in the arc.
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