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Description  |
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FIELD OF THE INVENTION
This invention relates generally to high capacity mobile communications
systems, and more particularly to a digital microcellular communication
system.
BACKGROUND
A conventional cellular phone system 5 is shown in FIG. 1A. Such systems
are currently in widespread use in the United States. As illustrated in
FIG. 1A, system 5 has a fixed number of channel sets distributed among the
base stations 12, 13 serving a plurality of cells 11, 16 arranged in a
predetermined reusable pattern. Typical cell areas range from 1 to 300
square miles. The larger cells typically cover rural areas and smaller
cells cover urban areas. Cell antenna sites utilizing the same channel
sets are spaced by a sufficient distance to assure that co-channel
interference is held to an acceptably low level.
A mobile unit 10 in a cell 11 has radio telephone transceiver equipment
which communicates with similar equipment in base station sites 12, 13 as
the unit moves from cell to cell. Each base station 12, 13 relays
telephone signals between mobile units 10 and a mobile telecommunications
switching office (MTSO) 17 by way of communication lines 18. The lines 18
between a cell site and the MTSO 17, typically T1 lines, carry separate
voice grade circuits for each radio channel equipped at the cell site, and
data circuits for switching and other control functions. The MTSO 17 is
also connected through paths 19 to a switched telephone network 15
including fixed subscriber telephone stations as well as various telephone
switching offices.
MTSO 17 in FIG. 1A includes a switching network for establishing call
connections between the public switched telephone network 15 and mobile
units 10 located in cell sites 11, 16, and for switching call connections
from one cell site to another. In addition, the MTSO 17 includes a dual
access feeder for use in switching a call connection from one cell site to
another. Various handoff criteria are known in the art and utilize
features such as phase ranging to indicate the distance of a mobile unit
from a receiving cell site, triangulation, and received signal strength to
indicate the potential desirability of a handoff. Also included in the
MTSO 17 is a central processing unit for processing data received from the
cell sites and supervisory signals obtained from the network 15 to control
the operation of setting up and taking down call connections.
A conventional base station 12 is illustrated in FIG. 1B. A radio
controller unit 22 provides the interface between the T1 lines from the
MTSO and the base station radio equipment. Transmitters 23, one for each
channel serviced by the base station, are driven by circuit 22, which
supplies each transmitter with an analog voice signal. Next, the signals
are passed to a separate nonlinear power amplifier for each channel, or
the signals may be combined and applied to a single linear power amplifier
24 as shown in FIG. 1B. The output of power amplifier 24 is applied
through duplexer 25 to antenna 26, to be broadcast into the cellular area
serviced by the base station.
Signals received in antenna 26 are applied through duplexer 25 to filter
27. Filter 27 isolates the entire cellular band signal from adjacent bands
and applies it to receivers 28, one for each channel. The analog voice
signal outputs of receivers 28 are applied to circuit 22. Base station 20
may optionally include a diversity antenna 26' and corresponding diversity
filter 27' and a plurality of diversity receivers 28', one for each
associated main receiver 28. Where implemented, the outputs of diversity
receivers 28' are applied to circuit 22, which would thus include
circuitry for selecting the strongest signal as between corresponding
receivers 28 and 28' using known techniques.
In densely populated urban areas, the capacity of a conventional system 5
is severely limited by the relatively small number of channels available
in each cell 11, 16. Moreover, the coverage of urban cellular phone
systems is limited by blockage, attenuation and shadowing of the RF
signals by high rises and other structures. This can also be a problem
with respect to suburban office buildings and complexes.
To increase capacity and coverage, a cell area can be subdivided and
assigned frequencies reused in closer proximities at lower power levels.
Subdivision can be accomplished by dividing the geographic territory of a
cell, or for example by assigning cells to buildings or floors within a
building. While such "microcell" systems are a viable solution to capacity
and coverage problems, it can be difficult to find space at a reasonable
cost to install conventional base station equipment in each microcell,
especially in densely populated urban areas. Furthermore, maintaining a
large number of base stations spread throughout a densely populated urban
area can be time consuming and uneconomical.
AT&T has proposed a system to solve the problem of coverage in urban areas
without having to deploy a large number of conventional base stations. The
system is shown and described with respect to FIG. 1 of AT&T's European
Patent Application No. 0 391 597, published on Oct. 10, 1990. In that
system a grid of antenna sites 40 is placed throughout the microcellular
system. An optical fiber network 42 interconnects the antennas with the
base station 44. Optical wavelength carriers are analog modulated with RF
mobile radio channels for transmission through the optical fiber network
26 to the antennas sites 22. A detector circuit 27 is provided for each
antenna site 22 to receive the modulated carrier and reconstruct an RF
signal to be applied to the antenna sites 22, for transmission into the
microcell area 21. RF signals received at antenna sites 22 from mobile
units are likewise modulated onto a fiber and transmitted back through
optical fiber network 26 to base station 25. All of the channels
transmitted from base station 25 are distributed to all antenna sites 22.
Also, all the channels transmitted from the base station 25 can be
received from the mobile units in any microcell 21 and transmitted via
optical fiber to base station 25.
The above-described AT&T system has certain limitations. The ability to
analog modulate and demodulate light, the limitations imposed by line
reflections, and path loss on the fiber all introduce significant
distortion and errors into an analog modulated signal and therefore limit
the dynamic range of the signals which can be effectively carried via an
analog system, especially in the uplink direction. These factors limit the
distance from the base station to the antenna sites.
Moreover, in AM systems an out-of-band signal is required to transmit
control and alarm information to and from the antenna sites, again adding
to the expense of the modulation and demodulation equipment. Moreover,
provision of other services such as paging systems, personal
communications networks (PCN's) or mobile data services are not easily
added to analog AM systems such as that shown in AT&T's European
application.
Furthermore, the AT&T system teaches the use of dedicated fiber lines
installed for each remote antenna site. It would be desirable if
preexisting transmission lines or fiber paths could be utilized so that
installation of new fibers could be avoided.
Another approach to increasing coverage is disclosed in U.S. Pat. No.
4,932,049 to Lee. The Lee patent describes a "passive handoff" system
wherein a cell is subdivided into several zones, with a directional
antenna oriented to cover each zone. All the antennas in the cell are
serviced by the same set of transmitters and receivers. A zone switch is
used to selectively connect the transmitters and receivers to the antenna
units. In operation, the antenna best able to service a mobile unit on a
given channel is connected to the transmitter/receiver pair assigned to
the mobile unit by the MTSO, and the other antennas are disconnected from
that transmitter/receiver pair. To control the switching of transmitters
and receivers to the antennas, a scanning receiver continuously polls the
strength of signals received at the antenna units on all active channels
in the cell. The zone having the best receiver signal strength is selected
as the active zone for the associated channel. The system disclosed in the
Lee patent thus allows for improving communications with mobile units
while at the same time reducing interference with other cells by
directionalizing and limiting overall signal strength in a cell.
SUMMARY OF THE INVENTION
A passive handoff system is provided using digital signal analysis to
rapidly switch transmitters and receivers among different antenna units in
different microcell zones of a cell. The passive handoff communication
system includes a cell divided into a plurality of zones and wherein a
plurality of telephone signals are transmitted and received between a base
station and mobile units in the zones using RF transmission. A base
station is connected by transmission means to a plurality of antenna units
with at least one located in each zone. Each antenna unit having an
antenna located for broadcast and reception of signals in the associated
zone. The base station includes analog RF signal generation circuitry for
generating a plurality of RF signals on different channels for carrying
telephone signals to mobile units in the cell and a first switch
responsive to a first control signal for switching and combining the RF
signals to form a composite RF signal for each zone. The composite signal
for each zone contains selected channels. Analog-to-digital conversion
circuitry converts the composite signal for each zone to a corresponding
digitized stream of samples and applies the samples to the transmission
means for transmission to each corresponding antenna unit. Each antenna
unit includes digital-to-analog conversion circuitry for receiving the
digitized stream of samples from the base station, reconstructing a
corresponding composite analog RF signal, and applying an amplified signal
to the antenna so that it is broadcast into an area associated with the
cell. Analog-to-digital circuitry of the antenna unit receives RF signal
received at the antenna, converts the received RF signal to a digitized
stream of samples and applies the samples to the transmission means for
transmission to the base station. Each base station unit further includes
digital-to-analog converter circuitry for receiving the digitized streams
of samples from each antenna unit and reconstructing an analog RF signal
for each antenna unit. The base stations further include a second switch
and combining means responsive to a second control signal for selectively
combining the RF signals from the antenna units to form a plurality of
composite signals for application to the receivers of the base station.
Each of the composite signals comprises one or more of the RF signals from
the antenna units. A controller monitors the digitized streams of samples
from all of the antenna units and digitally analyzes the energy level of
each channel in each zone, and in response thereto generates the first and
second control signals to selectively control the zones in which each
channel is broadcast and the zone from which each channel is received.
In another embodiment, the base stations are all digital base stations.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the invention and its various features,
objects and advantages may be obtained from a consideration of the
following detailed description, the appended claims, and the attached
drawings in which:
FIG. 1A is a functional block diagram of a first prior art mobile
communications system;
FIG. 1B is a functional block diagram of a prior art base station;
FIG. 1C is a functional block diagram of a prior art microcell mobile
communications system;
FIG. 2 is a simplified block diagram of an exemplary embodiment of the
microcell communications system of the present invention;
FIG. 3 is a more detailed block diagram of the base station embodiment
shown in FIG. 2;
FIG. 4 is a more detailed block diagram of the base station shown in FIG.
3;
FIG. 5 is a more detailed block diagram of the frame generator/multiplexer
134 shown in FIG. 4;
FIG. 6 is a simplified diagram of the structure of one exemplary data
frame;
FIG. 7 is a diagram of the structure of another exemplary data frame;
FIG. 8 is a functional block diagram of a microcell antenna unit according
to the exemplary embodiment shown in FIG. 2;
FIG. 9 is a functional block diagram of the demultiplexer 142 and
associated interfaces of FIG. 4;
FIG. 10 is a functional block diagram of an all-digital exemplary
embodiment of the invention;
FIG. 11A is a more detailed block diagram of the system illustrated in FIG.
10;
FIG. 11B is an alternative embodiment of the system illustrated in FIG.
11A;
FIG. 11C is yet another alternate embodiment of the system illustrated in
FIG. 11A;
FIG. 11D is still another alternative embodiment of the system illustrated
in FIG. 11A;
FIG. 12 is a simplified illustration of an alternate embodiment of the
microcell communication system according to the present invention;
FIG. 13 is a functional block design of the alternate embodiment 106' of
the system of FIG. 12;
FIG. 14 is another alternate exemplary embodiment of the microcell
communication system of the present invention;
FIG. 15 illustrates yet another alternate exemplary embodiment of the
invention wherein alternate services, such as personal communication
network (PCN) traffic and paging traffic is multiplexed with cellular
system traffic;
FIG. 16 is a simplified illustration of a prior art cable television system
infrastructure;
FIG. 17 is a simplified block diagram of an alternate exemplary embodiment
of the invention, wherein cable system infrastructure is used to transmit
digitized RF to and from a microcell location;
FIG. 18 is a block diagram of a base station unit of the exemplary
embodiment of FIG. 17;
FIG. 19 illustrates the head end unit located at the head end of the cable
system of the exemplary embodiment of FIG. 17;
FIG. 20 is a more detailed block diagram of the AM modulator/demodulator,
located in the head end of the cable system of the exemplary embodiment of
FIG. 17;
FIG. 21A is a more detailed block diagram of analog-to-digital converter
132, as used throughout the various embodiments in the invention;
FIG. 21B is a more detailed block diagram of digital-to-analog converter
144 as used throughout the various embodiments of the invention;
FIG. 22 is an alternate preferred framing structure for the embodiment of
FIG. 2 of the present invention;
FIG. 23 is yet another alternate preferred framing structure for the
embodiment of FIG. 2 of the present invention;
FIG. 24 is a more detailed block diagram of the microcell remote unit to be
positioned at the optical node in the cable system embodiment of FIG. 17;
FIG. 25 is an illustration of the amplitude modulator as used in the
embodiment of FIG. 17;
FIG. 26 is a more detailed illustration of the amplitude demodulator, as
used in the embodiment of FIG. 17;
FIG. 27A is an illustration of a base station of an alternate exemplary
embodiment of the system illustrated in FIG. 17, wherein the RF microcell
or PCN signal is digitally modulated;
FIG. 27B is an illustration of an alternate embodiment of the system
illustrated in FIG. 17, wherein the RF microcell or PCN signal is
digitally modulated;
FIG. 28 is a further illustration of the alternate embodiment using digital
modulation;
FIG. 29 further illustrates the construction of the optical node in the
digital modulation embodiment;
FIG. 30 is an overview diagram of yet another exemplary embodiment wherein
digitized microcell or PCN RF traffic is framed and transmitted over a
switched telephone network;
FIG. 31A is a more detailed block diagram of the base station units of the
embodiment of FIG. 30;
FIG. 31B is an alternate exemplary embodiment of the base station units of
the embodiment of FIG. 30;
FIG. 32A is a more detailed block diagram of the analog-to-digital
converter and framing circuits of the base station units illustrated in
FIG. 31A;
FIG. 32B is a more detailed block diagram of the analog-to-digital
converter and framing circuits of an alternate exemplary embodiment of the
base station units illustrated in FIG. 31B;
FIG. 33A is a more detailed block diagram of the remote antenna units of
the system illustrated in FIG. 30;
FIG. 33B is a more detailed block diagram of an alternate exemplary
embodiment of the remote antenna units of the system illustrated in FIG.
30;
FIG. 34 illustrates yet another exemplary embodiment of the invention
wherein digitized RF signals are transmitted over a switched telephone
network and a cable system; and
FIG. 35A is an overview functional block diagram of an exemplary embodiment
of a microcell communications system, having passive handoff capability
according to the present invention;
FIG. 35B is a more detailed block diagram of an exemplary base station unit
114' of the system of 35A according to the present invention;
FIG. 35C is a schematic illustration of the movement of a mobile unit from
one zone to another;
FIG. 36 shows an exemplary embodiment of digital transmitting/receiving
unit 130" of the system of FIG. 35A;
FIG. 37 illustrates an exemplary embodiment of controller 810 of the system
of FIG. 35A;
FIG. 38 is a simplified block diagram of the operation of controller 810 of
the system of FIG. 35A;
FIGS. 39A, 39B, 39C and 39D are still other alternate exemplary embodiments
of passive handoff systems with all-digital base station units;
FIG. 40 is an alternate embodiment of the system of FIG. 35B;
FIGS. 41A, 41B and 41C are exemplary embodiments of redundant microcell
coverage;
FIG. 42 is a simplified block diagram of an exemplary embodiment of a
sectorized microcell communications system according to the present
invention;
FIG. 43 is a more detailed block diagram of the base station embodiment
shown in FIG. 42;
FIG. 44 is a more detailed block diagram of the remote unit embodiment
shown in FIG. 42;
FIG. 45 is a more detailed block diagram of one example of a channel filter
unit which can be used in the remote unit shown in FIG. 44;
FIG. 46 is an alternate embodiment of the base station embodiment shown in
FIG. 42; and
FIG. 47 is an alternate embodiment of the remote unit embodiment shown in
FIG. 42.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following detailed description of exemplary embodiments of the
invention, reference is made to the accompanying drawings which form a
part hereof, in which like numerals refer to like elements throughout the
several views, and which is shown by way of illustration only, specific
embodiments in which the invention may be practiced. It is to be
understood that other embodiments may be utilized and structural changes
may be made without departing from the scope of the present invention.
The general configuration of one exemplary embodiment of the present
invention is shown in FIG. 2. The microcell system includes a plurality of
microcell areas 100. Deployed within each microcell area 100 is a
microcell remote antenna unit 102. Such units may be deployed on the roof
of a building or within a building, or on or in other structures. For
example, a microcell antenna unit 102 may be deployed on each floor of a
building on or adjacent an antenna tower, or along a highway corridor.
Remote antenna units 102 are connected through fiber 104 (or optionally
another high bandwidth carrier) to respective base station units 106. Base
station units 106 are interfaced to MTSO 110 over T1 lines 112. MTSO 110
is interfaced with a switched telephone network 120, as in a conventional
cellular phone system. Microcell base station units 106 are preferably
located in a single location 114. Such location may be inside or outside
of the area serviced by the microcell system, but in any event is
preferably conveniently located for maintenance purposes.
Referring now to FIG. 3 there is shown a simplified diagram of a microcell
base station 106 according to one exemplary embodiment of the present
invention. Base station 106 includes conventional transmitters and
receivers 23 and 28, respectively, and conventional radio controller or
interface circuitry 22 to the MTSO 110. A digital transmitter/receiver
unit 130 receives the combined RF signal from transmitters 23, digitizes
the combined signal and transmits it in digital format over fiber 104A
connected to a remote antenna unit 102. Unit 130 also receives a digitized
RF signal over fiber 104B from a remote antenna unit 102, reconstructs the
corresponding analog RF signal, and applies it to receivers 28.
Accordingly, conventional equipment may be used on the downstream (MTSO)
side of digital transmitting/receiving unit 130.
Referring now to FIG. 4, there is shown digital transmitting/receiving unit
130 in greater detail. Unit 130 includes a broadband digitizer 132
receiving the combined RF signal from transmitters 23. Digitizer 132
provides a digitized microcell traffic stream, consisting of a series of
samples of the incoming analog RF signal. Frame generator/multiplexer 134
frames the digitized microcell traffic data, together with control, voice
and error checking data, and applies it to a digitally modulated laser
136. The voice data channel, also termed the order wire channel,
originates from order wire interface 135, which has an input for a handset
137 or a two-wire phone line. Order wire interface 135 provides for
two-way point-to-point voice grade communications. Typically a handset is
used at the remote site to connect with a handset at the base site.
Control signals originate from control/alarm circuit 131, which generates
control information for the remote antenna unit 102 to monitor error and
alarm information.
The laser signal from digitally modulated laser 136 is applied to fiber
104A for transmission to the corresponding remote antenna unit 102.
According to one possible embodiment, digitizer 132 preferably provides a
24 bit wide word (parallel structure sample) running at 30.72
MegaSamples/second (MSamples/s). The frame generator/multiplexer 134
converts the 30.72 MSamples/s word to a single serial bit stream running
at 819.2 MegaBits/second (Mb/s).
The digitizer 132 conditions the broadband RF signal by providing bandpass
filtering sufficient to eliminate out of band signals, and sufficient gain
adjustment to prevent overloading of the analog-to-digital converter. The
analog-to-digital converter converts the conditioned broadband RF signal
into a parallel bit stream, either by direct sampling at RF, or by
sampling following down-conversion to baseband or to an intermediate
frequency band. In the preferred embodiment, the digitizer is obtained
from Steinbrecher Corporation of Woburn, Mass., with sampling performed on
a 12.5 MHz wide signal down-converted to either the first or second
Nyquist zone, with 12 bit sampling occurring at a rate of 30.72
MSamples/s.
Unit 130 further includes a digital optical receiver 140. Receiver 140
outputs an electronic digital signal, which is applied to demultiplexer
142, which extracts the digitized microcell traffic data generated at the
remote antenna unit 102, as will be explained further below. Demultiplexer
142 further extracts alarm (monitoring) and voice information framed with
the microcell traffic data. The digitized microcell traffic signal is
applied to digital-to-analog converter 144, which reconstructs the analog
RF signal, to be applied to receivers 28.
The digital-to-analog converter 144 operates on the microcell traffic
parallel bit stream extracted by demultiplexer 142, reconstructing a
baseband replica of the broadband RF signal digitized by digitizer 132.
The baseband replica is then up-converted to its original radio frequency
by mixing with a local oscillator and filtering to remove image
frequencies. In the preferred embodiment, the digital-to-analog converter
is obtained from Steinbrecher Corporation of Woburn, Mass., and operates
at the preferred sample rate of 30.72 MSamples/s.
Referring now to FIG. 21A, there is illustrated in more detail the
broadband digitizer or analog-to-digital converter circuit 132 in FIG. 4
and 170 in FIG. 8. Analog-to-digital converter circuit 132 preferably
includes a local oscillator 132A, which applies its output to mixer 132B,
which receives the combined output from the transmitters 23. Mixer 132B
reduces the high frequency microcell signal (approximately 850 MHz in the
case of conventional cellular phone service or approximately 1.8 GHz in
the case of PCN traffic), to an intermediate (or baseband) frequency of
approximately 1 to 15 MHz (such that the 12.5 MHz frequency fits between
these limits) prior to application to analog-to-digital converter 132C.
Illustrated in FIG. 21B is the digital-to-analog converter 144 and 164, of
FIGS. 4 and 8, respectively, which performs the reverse operation of
analog-to-digital converters 132 and 170. Digital-to-analog converter 144
includes a digital-to-analog converter 144A, which outputs an intermediate
frequency signal, which is up-converted with mixer 144B, using the local
oscillator 144C. Up-conversion restores the operating frequency of the RF
to the broadcast frequencies of the cellular or PCN systems.
Referring now to FIG. 5, there is shown in greater detail the frame
generator/multiplexer circuit 134 according to the exemplary embodiment of
the invention shown in FIG. 4. Circuit 134 includes a cyclic redundancy
check (CRC) generator 155, which receives microcell traffic data from
digitizer 132 and outputs a CRC code.
According to one exemplary embodiment, framer/multiplexer 154 multiplexes
the CRC channel, microcell traffic, order wire (voice) channel and control
(alarm) channel into the frame structure illustrated in FIG. 6. Each frame
includes a 12-bit microcell traffic word, a one bit CRC channel, a one bit
control-alarm/order wire channel and a six bit framing word. The
control-alarm and order wire data are multiplexed together in a single
channel.
FIG. 7 shows an alternate frame structure having 12 bits for the main
antenna channel, 12 bits for 12.5 MHz coverage of alternate service or
diversity channel, a one or two bit CRC channel, 1 bit control-alarm
channel and 6 bit frame word. Other possible framing structures could
involve a total of 48 information bits for full band coverage and
diversity capability, or for carrying additional services. It shall be
understood that the present invention is not limited to these or any other
particular framing format, but rather that any format could be used
without departing from the scope of the present invention.
To achieve synchronization with the parallel transfer word, the frame
signal shown in FIGS. 6 and 7 runs at 819.2 Mb/s (i.e.
32.times.25.6.times.10.sup.6 bits/second=819.2.times.10.sup.6 bit/second).
(The bit rate and sampling rate for 40 MHz/48 bit or other frame structure
would change accordingly.) Synchronization is achieved at the receiving
demultiplexer 142 (162 in FIG. 8 described below) by searching for the
frame pattern. Thirty-two individual frames are grouped into a superframe.
One of the 32 frames has a bit sequence different from the other 31
frames. Each frame byte is a balanced code having an equal number of ones
and zeros. The frame search is initiated by the demultiplexer 142 to find
consecutive patterns, followed by a search for the unique bit sequence in
one of the 32 frames. When the frame and superframe are found by the
demultiplexer 142 (or 162), valid traffic pattern or data patterns result.
Framing methods of this type are well known in the telecommunications
arts, and those of skill in the art will recognize that various alternate
framing methodologies may also be used. Preferably, frame
generator/multiplexer 134 includes circuitry for scrambling the outgoing
data to provide for the balanced line code preferred for fiber optic
transmission.
Referring now to FIGS. 22 and 23, there are shown the alternate preferred
framing structures of the embodiment of FIG. 2. As shown in FIG. 22, the
framing structure includes 12 bits of PCN/microcell traffic, one framing
bit, one bit of CRC and an alarm-control/order wire channel, and four
reserve bits. The framing structure in FIG. 23 is identical, except for 13
bits have been allocated to the PCN/microcell traffic. Neither of these
framing structures is designed to accommodate diversity traffic, however,
they could be so expanded. The framing structures of FIGS. 22 and 23
assumes a 12 bit sampling at 30.72 Mb/s. The basic framing structure is 18
bits, which, when run at 30.72 Mb/s, results in a rate of 552.96 Mb/s
serial rate. As shown in FIG. 22, one bit is dedicated to framing. Another
bit is multiplexed between CRC, alarm-control, and the order wire
function. These two bits achieve framing and multiplexing by virtue of the
following sequence:
______________________________________
Framing Bit CRC, Etc.
______________________________________
00 Frame 1
01 Frame 2
10 Frame 3
10 Frame 4
1C Frame 5
1D Frame 6
______________________________________
As illustrated above, the framing structure of this embodiment contemplates
that six frames make up a "super frame." The first four frames of each
super frame include the 00, 01, 10, 10 sequence. In the fifth frame, the
framing bit is a 1, and the other bit represents one bit of CRC code. In
the sixth frame, the framing bit is a 1 and the other bit is an
alarm-control/order wire channel bit.
Preferably, the CRC code is 32 bits wide, so that 32 frames must be
received in order to accumulate the entire CRC code. Accordingly, errors
are checked every 32 words of data. As in the case of the previously
described framing structure, a balanced line code is provided.
Referring now to FIG. 8, there is shown a block diagram of the remote
antenna unit 102, according to the first exemplary embodiment of the
present invention. A digital optical receiver 160 receives the optical
digital data stream transmitted from the microcell base station on fiber
104A. Receiver 160 converts the optical data stream to a corresponding
series of electrical pulses, which are applied to demultiplexer 162.
Demultiplexer 162 extracts the microcell traffic and applies the 12-bit
(or 13-bit) samples to digital-to-analog converter 164. Converter 164
reconstructs the analog RF signal and applies it to linear power amplifier
24. Converter 164 is preferably the same as digital-to-analog converter
144 described and shown above with respect to FIG. 4. Amplifier 24 is
connected to the main antenna 26 through a duplexer 25. Accordingly, radio
frequency signals originating from transmitters 23 in the microcell base
station are transmitted from main antenna 26. Demultiplexer 162 also
extracts control signals for application to a control/alarm circuit 161.
Order wire data is also extracted and applied to order wire interface 163
to provide two-way, point-to-point voice grade communication.
RF signals received at main antenna 26 are passed through duplexer 25 to
filter 27. Power amplifier 24, duplexer 25, main antenna 26 and filter 27
are conventional base station components, as are described with reference
to FIG. 1B. The output of filter 27 is combined and applied to a broadband
analog-to-digital converter 170 (of the same type as 144 described above
with respect to FIG. 4), which digitizes the analog RF signal and applies
it to a frame generator/multiplexer circuit 172. The output of circuit 172
is applied to digitally modulated laser 174, which applies the
corresponding optical digital stream to fiber 104B. Frame
generator/multiplexer 172 is of substantially the same design as
framer/multiplexer 34. It receives an alarm (or monitoring) signal data
stream from control/alarm circuit 161, and an order wire data stream
signal from order wire interface 163.
Optionally, remote antenna unit 102 may include a diversity antenna system
180. System 180 includes a diversity antenna 26', which applies its output
to filter 27' and in turn to broadband analog-to-digital converter 170',
which operate in the same manner as main antenna 26, filter 27 and
broadband analog-to-digital converter 170, respectively. The output of
analog digital converter 170' is applied to circuit 172, which multiplexes
the digitized RF signal from the diversity antenna into the data stream
applied to fiber 104B. In such a case, the framing scheme includes
diversity traffic capacity.
Referring now to FIG. 9, there is shown in greater detail demultiplexer
circuit 142 (and correspondingly 162) shown in FIG. 4 and FIG. 8. Circuit
142 (162) includes a demultiplexer 190, which receives the digital data
stream from digital optical receiver 140. Demultiplexer 190 extracts the
control/alarm channel, order wire channel, CRC channel and microcell
traffic channel from the digital data stream. Optionally, where the
diversity function is provided, the diversity CRC channel and diversity
microcell channel are also extracted. The main CRC channel and microcell
traffic channel are applied to CRC checking circuit 192, which provides an
error signal to the control/alarm circuit 131. Circuit 131 monitors the
error rate of data and alarms occurring at the remote antenna unit 102.
The order wire channel is applied to order wire interface 163, to provide
two-way point-to-point communication.
Where diversity is optionally included, a second CRC checking circuit 192'
receives the diversity CRC channel and diversity microcell channel and
produces an error signal which is applied to control/alarm circuit 131.
All-Digital Embodiment
Referring now to FIG. 10, there is shown an alternate exemplary embodiment
200 of the present invention. Alternate embodiment 200 includes a remote
antenna unit 102 as described with respect to FIG. 8. Remote antenna unit
102 is connected to an all-digital microcell base station 210 through
fibers 104A and 104B. Microcell base station 210 is connected to an MTSO.
All-digital microcell base station 210 is shown in more detail in FIG. 11A.
Circuit 210 includes a T1 interface 202, which extracts digitized voice
channels carried by a T1 line or other carrier from an MTSO and applies
those channels in digital form to digital synthesizer 212. Digital
synthesizer 212 replaces transmitters 23 and the analog-to-digital
converter 132 of the embodiment shown in FIG. 4. Digital synthesizer 212
constructs, with digital logic or software, an equivalent to the digitized
output of broadband digitizer 13 | | |
