|Publication number||USRE40564 E1|
|Application number||US 09/747,273|
|Publication date||Nov 4, 2008|
|Filing date||Dec 22, 2000|
|Priority date||Sep 17, 1992|
|Also published as||CN1065402C, CN1127056A, US5621786, US5627879, US5642405, US5644622, US5657374, US5852651, USRE43964, USRE45321, WO1994028690A1|
|Publication number||09747273, 747273, US RE40564 E1, US RE40564E1, US-E1-RE40564, USRE40564 E1, USRE40564E1|
|Inventors||Larry G. Fischer, David S. Russell, Philip M. Wala, Charles R. Ratliff, Jeffrey O. Brennan|
|Original Assignee||Adc Telecommunications, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (75), Non-Patent Citations (25), Referenced by (22), Classifications (17), Legal Events (9)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This is a division of application Ser. No. 08/204,660, filed Mar. 2, 1994 U.S. Pat. No. 5,627,879, which is a continuation-in-part of U.S. application Ser. No. 08/183, 221, filed Jan. 14, 1994, now abandoned, which is a continuation-in-part of U.S. application Ser. No. 08/068, 389, filed May 28, 1993, now abandoned, which is a continuation-in-part of U.S. application Ser. Nos. 07/946, 402, 07/946,964, 07/946,931, and 07/946,584, all filed Sep. 17, 1992, all of which are now abandoned. More than one reissue application has been filed for U.S. Pat. No. 5,852,651. Specifically, Reissue application Ser. No. 11/937,255 was filed Nov. 8, 2007 as a continuation of the present application Ser. No. 09/747,273.
This invention relates generally to high capacity mobile communications systems, and more particularly to a digital microcellular communication system.
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
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
A conventional base station 12 is illustrated in
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 each “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
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 antenna's 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 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.
The present invention provides improved coverage and increased capacity by assignment of reusable channel sets throughout the microcell system, without the need to deploy independent, conventional base stations in each microcell area. It also provides good dynamic range over extended distances as compared to analog systems such as the AT&T system described above.
According to one exemplary embodiment of the present invention, there is provided a microcell system wherein a plurality of commonly located microcell base station units communicate with a corresponding plurality of microcell antenna units deployed in respective microcell areas. Each base station unit includes conventional RF base station transmitter and receiver pairs, one for each channel assigned to the microcell. Additional receivers are also provided to receive diversity channels. The RF signal outputs from the transmitters are combined and applied to a broadband analog-to-digital converter. The digitized signal is transmitted over optical fiber to a microcell unit. Each microcell unit receives a digitized RF signal and reconstructs the analog RF signal using a digital-to-analog converter. The reconstructed RF signal is applied to a power amplifier, the output of which is fed to an antenna for broadcast into the microcell area.
The antenna units include both a main and a diversity antenna. The antennas each independently receive RF signals from the mobile units. The RF signal from the main antenna is filtered through a first set of filters, one for each channel assigned to the microcell, and the combined filtered main signal applied to an analog-to-digital converter. A second set of filters receives the diversity signal from the diversity antenna. The diversity signal is also applied to an analog-to-digital converter. The digitized main signal and diversity signal are multiplexed and transmitted over the optical fiber back to the microcell base station. The base station in turn includes a pair of digital-to-analog converters which reconstruct the main and diversity analog RF signals for application to the receivers. The strongest signal is selected for use in accordance with conventional diversity technology. Conventional circuitry interfaces the transmitters and receivers to the MTSO.
Thus, the exemplary embodiment outlined above contemplates that the microcell base station/antenna unit pairs are arranged to provide a reusable pattern of channels (as in conventional cellular technology) in the microcell system. The microcell base station units do not normally includes an antenna, and can be located in a convenient and preferably low cost location, which may be outside of the microcell system territory if desired.
According to another exemplary embodiment, the invention may be deployed to extend the coverage in a conventional cell. In this embodiment, the base station may include an antenna for transmission and reception of analog RF directly from the transmitters and receivers, while at the same time transmitting and receiving from a microcell antenna unit using the digital carrier over a fiber as described with respect to the first exemplary embodiment.
According to another exemplary embodiment of the invention, the digitized microcell traffic is carried in a frame format to and from that antenna units. Each frame includes a plurality of bits assigned to carry a sample of the digitized microcell traffic, with other bits employed for control and monitoring of equipment, error detection and correction, and end-to-end point-to-point voice traffic between the base station and the antenna unit. Alternate services such as personal communications network traffic, paging services and mobile data services may also be carried using the framing format.
According to yet another exemplary embodiment of the invention, the fiber carrier may be replaced with cable or other carrier medium.
According to still anther exemplary embodiment, the invention can be deployed to distribute a single set of channels to a plurality of microcell areas. In this embodiment, a single base station unit sends the same set of digitized channels to a plurality of microcell antenna units, which in turn return the same set of channel signals to the microcell base station.
Therefore, the invention eliminates the problems associated with analog AM (or FM) systems, such as that illustrated in the above-mentioned AT&T application, by using a digital transport resulting in better signal quality and for greater range between a base station and a microcell antenna unit. As employed in one exemplary embodiment, the invention greatly increases system capacity over existing mobile telephone systems without the requirement of deploying conventional base station equipment in each microcell area, and allows for provision of alternative services such as paging systems, mobile data services or persona communication networks. The present invention also improves the dynamic range of the signal and extends the distance signals may be reliably transported from the base stations to the antenna units. In another exemplary embodiment, the invention provides readily for the transmission of control and monitoring information to and from the microcell antenna unit.
To provide additional advantages, an exemplary all-digital embodiment of a microcell system is also provided wherein a plurality of commonly located digital microcell base station units communicate with a corresponding plurality of microcell antenna units deployed in respective microcell areas. According to this all digital embodiment, the base stations are fully digital and synthesize a digital signal directly from the T1 carrier received from the MTSO. The digital signal is transmitted over optical fiber to the microcell units. The microcell units receive the digital signal, and construct an analog RF signal using a digital-to-analog converter. The RF signal is applied to a power amplifier, the output of which is fed to an antenna for broadcast into the microcell area. The antenna units receive RF signals from the mobile units. The RF signal is filtered through a set of filters, one for each channel assigned to the microcell, and the filtered signal applied to an analog-to-digital converter. The digitized signal is transmitted over the optical fiber back to the digital microcell base station. The base station in turn directly synthesizes the digital signal onto the T1 carrier back to the MTSO. Conventional circuitry interfaces the transmitters and receivers to the MTSO. Thus, this exemplary embodiment contemplates that the microcell base station units are fully digital and eliminate the need for RF equipment at the base station as well as for analog-to-digital and digital-to-analog converters, thus providing the opportunity to reduce both the cost and volume of equipment required at the base station site, and to reduce maintenance needs on inherently less reliable analog equipment. The digital microcell base station units can be located in a convenient and preferably low cost location, which may be outside of the microcell system territory if desired.
A method which allows for the rapid deployment of a system of the type using analog-type base stations while permitting the easy upgrade of such base stations to all digital technology is also provided. The method's first stage calls for deploying a plurality of microcell base station units as described above, each including conventional RF base station transmitters and receivers, one for each channel assigned to the microcell.
In the second stage of deployment, the analog base stations are replaced with all-digital base stations wherein the base stations are fully digital and synthesize a digital signal directly from the T1 carrier received from the MTSO. The digital signal is transmitted over optical fiber to the microcell antenna units installed in the first stage of deployment. The microcell antenna units receive the digital signal, and construct an analog RF signal using a digital-to-analog converter. The RF signal is applied to a power amplifier, the output of which is fed to an antenna for broadcast into the microcell area. The antenna units also receive RF signals from the mobile units. The RF signal is filtered through a set of filters, one for each channel assigned to the microcell, and the filtered signal applied to an analog-to-digital converter. The digitized signal is transmitted over the optical fiber back to the digital microcell base station. The base station in turn directly synthesizes the digital signal onto the T1 carrier fact to the MTSO.
Thus, the exemplary embodiment outlined above contemplates that the antenna units installed in the first stage do not need alteration or replacement when the analog microcell base station units are replaced with all digital microcell base stations. The method thus allows the full benefit of the all-digital base station to be accomplish without the expense of modifying existing installed microcell antenna units.
According to yet another alternate, exemplary embodiment, the digitized RF signal, carrying either microcell or PCN traffic, is framed for transmission over a switched telephone network. In this embodiment, a limited number of digitized microcell or PCN channels are grouped together, in a standard framing format for transmission using a standard DS-3, OC-1, or other protocol.
In yet another alternate, exemplary embodiment, digitized microcell or PCN RF signals are transmitted over the installed fiber infrastructure of a cable system from the head end to the optical nodes, in an amplified modulated (AM) format.
A still further exemplary embodiment contemplates the transmission of the microcell or PCN traffic in digital form over the cable system feeder lines, using QAM modulation or other digital modulation formats.
Thus, according to these embodiments, microcell or PCN channels may be transmitted over an established switched network or using established cable system infrastructure.
According to still another embodiment of the invention, there is provided a passive handoff system using digital signal analysis to rapidly switch transmitters and receivers among different antenna units in different microcell zones of a cell.
According to yet still another embodiment of the invention, there is provided decimation filters for digitally filtering out a selected number of channels from the digital stream output from the analog-to-digital converter, and multiplexing the selected channels onto one or more lower speed carriers, such as a T1 line or SONET carrier.
According to yet still another embodiment of the invention, a passive switching method is described for use in a cellular phone system having a plurality of macrocells including a first macrocell, each macrocell sharing a common set of channels, the method comprising the steps of providing a plurality of primary and secondary microcell antenna units; dividing the first macrocell into a plurality of primary microcells, wherein the step of dividing includes placing the primary microcell antenna units so as to provide coverage over the first macrocell; providing a plurality of secondary microcell antenna units; placing the secondary microcell antenna units to provide macrocell coverage overlapping the primary microcells; at a base station, generating a digitized representation of a telephone signal received from a mobile telephone switching office, selecting a microcell from said plurality of primary and secondary microcells and transmitting the digitized representation to the microcell antenna unit of the selected microcell; receiving, at the selected microcell, the digitized representation, generating a corresponding RF signal by digital-to-analog conversion, and broadcasting the RF signal in the selected microcell; receiving RF signals in each of the plurality of primary and secondary microcells for the set of channels, and converting the RF signals received for corresponding digitized RF signal representations for transmission back to the base station; at the base receiving the digitized RF signal representations from the primary and secondary microcells; and monitoring the digitized RF signal representations from each of the primary and secondary microcells and based on the energy level of each channel in each zone, selectively controlling the channels broadcast into each of the primary and secondary microcells and selectively choosing the microcell from the plurality of primary and secondary microcell in which a received channel is received so that passive switching may be accomplished.
According to yet still another embodiment of the invention, a method of sectorizing coverage over a particular cellular communications area is described, the method comprising the steps of providing a remote unit having a plurality of microcell antenna units, including a first and a second microcell antenna unit, wherein each microcell antenna unit comprises an antenna configured to cover a particular sector and a channel filter unit used to filter channels assigned to the particular sector; connecting the remote unit to a sectorized base station unit, wherein the step of connecting comprises providing a unique sector frequency associated with each antenna unit sector; connecting the sectorized base station unit to a mobile telecommunications switching office; generating, at the sectorized base station unit, a digitized representation of a telephone signal received from the mobile telephone switching office; transmitting the digitized representation to the microcell antenna unit for a particular sector; receiving, at the first microcell antenna unit, a first RF signal, digitizing the first RF signal and converting the digitized first RF signal to a first sector frequency; receiving, at the second microcell antenna unit, a second RF signal, digitizing the second RF signal and converting the digitized second RF signal to a second sector frequency; and multiplexing the digitized first RF signal at the first sector frequency and the digitized second RF signal at the second sector frequency and transmitting the multiplexed signal to the sectorized base station.
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:
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
Referring now to
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 signal 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
Referring now to
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.
To achieve synchronization with the parallel transfer word, the frame signal shown in
Referring now to
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
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 descried 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 steam 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
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.
Referring now to
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 transmitter 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 132 for application to frame generator/multiplexer 214. Synthesis may be accomplished, for instance, by electronic or software simulation of the generation of the analog telephone signal and the modulation of the transmittal signal therewith. The simulated signal transmitter output signal can then be directly represented in digital form that can be processed to emulate the output of the A/D converter.
An alternate embodiment of the system of
Yet another alternate embodiment of the all digital base station is shown in FIG. 11C. In
Thus, all-digital base station 210 synthesizes the effect of digitizing the transmitter data stream, providing for an all-digital conversion from circuit 202 to the data stream applied to fiber 104A. The synthesized signal is received at the remote antenna unit 102, which constructs the radio frequency signal, using digital-to-analog converter 164, thus eliminating the need for transmitters 23. Similarly, digital demodulator or receiver circuit 224 eliminates the need for receivers 28, by converting the demultiplexed digitized RF data stream directly into digital phone channels for application to circuit 202 and transport to the MTSO.
Yet another exemplary alternative embodiment of the invention is shown in FIG. 12. The alternate embodiment shown in
Referring now to
Yet another alternate exemplary embodiment of the invention is shown in FIG. 15. In
For ease of implementation of the present all-digital embodiment, two-stage deployment is contemplated. In the first stage, microcell base station units 106, of the design shown in
Thus, as described above, the present invention provides not only improved coverage, but also for increased capacity by assignment of reusable channel sets throughout the microcell system, without the need to deploy independent, conventional base stations in each microcell area. Also, by virtue of digital transmission, it also provides good dynamic range over extended distances as compared to analog systems.
The exemplary configuration illustrated with respect to base station 106 and remote antenna unit 102 provides control/alarm/monitoring and two-way point-to-point voice channels to be readily multiplexed on the digital carrier, providing advantages over analog systems such as that disclosed by AT&T. Furthermore, a diversity channel can also be multiplexed into the data stream to provide the diversity function without the need for additional fiber paths.
The invention also permits ready adaptation to carry alternate services such as PCN, mobile data and paging services together with microcellular traffic.
Another advantage of the invention is its ready adaptation to all digital base station technology, wherein microcell traffic data received from an MTSO in digital form can be digitally converted to a synthesized stream of data samples for application to the digital-to-analog converter in the remote antenna unit 102.
It shall be understood that other control or monitoring type channels between the base station and antenna units are also possible, and that the invention is not limited to the particular channels illustrated in the exemplary embodiments.
A conventional cable system is illustrated in FIG. 16. System 300 includes one or more satellite dishes 304 receiving satellite television signals from satellite 302. In addition, the head end may receive video feeds from local sources or over other media such as fiber, coaxial cable or line of sight microwave link. Video unit 308 provides video signal splitting, and provides a video signal to AM transmitters 310, which apply an amplitude modulated signal, typically down-converted prior to transmission, for application to a fiber feeder. The fiber optic feeder transmits the video signal to a optical node 312, which processes the received signal for delivery to a plurality of homes 314, typically over copper coax cable, or in state of the art installations, over a fiber link. In a typical suburban installation of the type most adaptable to the benefits of this exemplary embodiment, an optical node 312 preferably provides service to approximately 250 homes, covering a geographic area of approximately 1-2 square miles.
An exemplary embodiment of the present invention, wherein the cable system 300 is utilized to transmit microcell or PCN traffic to microcell areas will now be explained with reference to FIG. 17. The system of
The head end further includes a plurality of AM modulator/demodulators 338, which are coupled to microcell optical nodes 342 through fibers 340A and 340B. Optical nodes 342 each include an antenna for the transmission and reception of microcell or PCN traffic and are interfaced to a plurality of subscriber homes 343. POTS/data source 336, multiplexer 308 and head end 332 are each connected to the respective AM modulator/demodulators 338, as more fully illustrated in
Base station unit 330 is shown in more detail in FIG. 18. Unit 330 functions identically to unit 106 as described above with reference to FIG. 3. The base station unit 330 may be positioned, as in the embodiment of
As shown in
Referring now to
Referring now to
The AM modulators (338A, 432) and demodulators (338C, 402) are illustrated in more detail in
As shown in
Thus, as described above, the alternate embodiment illustrated generally in
In the above-described system, the digitized RF signal is converted to an analog form prior to being transported to the remote optical node unit 342. According to the alternate exemplary embodiment now to be described, the digitized form of the RF signal may be maintained through to the remote optical node units 342 by use of digital modulation such as QAM modulation. In the alternate exemplary embodiment of
As illustrated in
Referring now to
Thus, as described above, this alternate exemplary embodiment provides a system for maintaining the RF or PCN signal in digital format all the way to the optical node unit 342. It thus can advantageously provide a higher quality signal than might otherwise be obtained with AM modulation schemes.
Yet another alternate exemplary embodiment of the present invention is shown in FIG. 30. In
Each of circuits 614 functions to convert the analog RF (after suitable down-conversion) to a digital signal which is framed and applied to the switched telephone network. In addition, each of circuits 614 receives a signal from the switched network, which it demultiplexes and converts back to a corresponding analog RF signal, for application to a respective receiver group 620 or diversity receiver 622.
In the exemplary embodiment illustrated herein, it is contemplated that approximately ten 30 KHz, PCN or AMPS cellular channels (given current 7 channel spacing requirements) may be digitized into a respective 1.05 or 1.25 MHz bandwidth which may be formatted as a 44.736 Mb/s DS-3 or OC-1 signal for application to the switched telephone network through a T1 line or optical fiber link, with bits available for control and error detection. AMPS, or Advanced Mobile Phone Service, is the original and standard format for cellular service consisting of frequency modulated (FM) channels at 30 KHz spacings. However, the system could carry 15 to 18 time division multiple access (TDMA) signals, or a combination of AMPS and TDMA signals could be carried. As is well known to those of skill in the art, TDMA is an alternative modulation technique for cellular which replaces each AMPS channel with three time-multiplexed digital signals. Hence 5 to 6 AMPS channels are 15 to 18 TDMA channels.
Referring now to
The remote antenna units 602 are illustrated in more detail in FIG. 33A. Antenna units 602 are essentially identical in construction to the remote antenna units 102 as illustrated with respect to FIG. 8. However, in place of the digital optical receiver 160 and digitally modulated laser 174, there are provided network interfaces 640 and 642 for interfacing to the switched network 120.
The same framing structure illustrated above with respect to FIGS. 6,7,22 and 23 are applicable to this exemplary embodiment of the invention, except at lower speeds as necessary. In the case where the diversity function is provided, the return path would include additional DS-3 or OC-1 signals, requiring additional T1 or SONET line capacity on the return path.
Referring now to
Referring now to
Decimation filter 802 is preferably, for example, a decimating digital filter. Part Number HSP 43220, available from Harris Semiconductor, Inc. of Melbourne Fla. Another vendor of such filters may be ESL, a division of TRW, Inc. Referring back to
A frame generator/multiplexer of generally the same design as generator multiplexer 134′, is provided to multiplex the data stream from each digital filter 802 onto one or more T1, SONET or other carriers. For instance, a single channel of 72 KSamples/s, with 12-bit samples, constitutes an 864 Kb/s serial data stream. Adding framing and control bits, as, for example, illustrate in
Frame generator/multiplexer 134′ can thus multiplex the output of one of digital filters 802 into a DS1 format on a T1 carrier with a capacity of 1.55 mb/s, or can combine multiple outputs of digital filters 802 on a 44.736 Mb/s DS-3 or OC-1 signal for application of the switched telephone network.
A filter control circuit 803 is also provided in unit 614′, and has an input to each of digital filters 802. Filter control 803 allows digital filters 802 to be programmed, so that their filtering characteristics (and channel selection) may be selectively chanced, if desired. Filter control 803 further includes an input from radio controller 22, which may provide control input, in order to specify the channels to be extracted from the data stream. A network interface circuit 630′, interface frame generator/multiplexer 134′ to the switched telephone network.
Referring now to
Alternate embodiment of remote unit 602′ further includes a plurality of digital filters 802, which operate in the same manner as digital filters 802 of base station unit 614′ to extract selected microcell digitized channels from the output of the broadband digitized signal from the outputs of broadband analog-to-digital converter 170. Framer/multiplexer 172′, of generally the same design as multiplexer 172, operates in a manner similar to frame generator/multiplexer 134′ to multiplex the extracted channels onto one or more T1, SONET or other carriers, applied to the switched telephone network through network interface 642′.
Referring again to
As illustrated in
In yet another alternate embodiment, the system of
Thus, as described above, this alternate exemplary embodiment of the invention provides that PCN or microcell traffic may be conveniently carried over a switched telephone network. This operation has obvious advantages, permitting rapid installation of additional capacity, rather than the necessity of installing additional transmission lines.
Thus, these alternate exemplary embodiments provide for an ability to transmit radio frequency microcell or PCN traffic through a switched network and through a cable system installation.
Various modifications and alternate configurations of the embodiments of
Referring now to
For the purposes of describing system 800, microcell areas 100 are referred to as “microcell zones,” which zones are labeled for the purposes of one exemplary embodiment as A1-A6, B1-B6 and C1-C6. Each zone includes an independent antenna for transmitting to and receiving from mobile units. Zones A1-A6 collectively comprise “Cell A,” zones B1-B6 collectively comprise “cell B,” and zones C1-C6 collectively comprise “cell C.” Each cell A, B and C has a set of reusable frequencies to be used within the cell, according to conventional cellular system design. Passive handoff system 800 provides that a transmission frequency or channel assigned to a mobile unit in a given cell may be broadcast from the remote unit 182 in any one of microcell zones 100 under the control of a unit 114′ without interaction with or control from MTSO 110. A channel can thus follow a mobile telephone unit from one microcell zone to another within a give cell. Accordingly, multiple microcell zones may be served by the same set of channels (i.e. transmission frequencies) allowing the signal transmission power level within each zone to be minimized, and thereby avoiding undesirable interference with adjoining microcell zones or cells. The system also reduces the switching load on MTSO 110. However, when a mobile unit travels from one cell to another, MTSO 110 switches the unit to a new channel (and corresponding pair of transmit and receive frequencies) in the newly entered cell, in a conventional manner.
Referring now to
The respective outputs of transmitter/receiver digitizing units 130″ carrying the analog microcell traffic, are each applied to matrix switch 808. Matrix switch 808 selectively connects any input to any one of receivers 28-1 to 28-N through respective outputs 806-1 to 806-X, and combining circuits 807-1 to 807-X. A controller 810 controls matrix switch 802 and matrix switch 808 using respective control lines 812 and 814. Controller 810 receives a sample of digitized microcell traffic from each of the digitizatoin units 130′ over sample lines 816.
As described in more detail below, controller 18 continuously processes the digital samples received from units 130″ and in response thereto controls matrix switches 802 and 808 in order to switch each of transmitter units 23 through to one (or more or none) of units 130″ and to connect receivers 28 to one (or more or none) of units 130″. For instance, in one exemplary switching configuration, matrix switch 802 might connect all three transmitters 23-1, 23-2 and 23-N through outputs 803-1 to combiner circuit 804-1, so that all three transmitter frequencies F1, F2, and Fn are combined and applied to unit 130″-1 for digitization and transport to a microcell zone. In another configuration, transmitter 23-1 and might be connected to combiner 804-X through one of outputs 803-X, while transmitter 23-2 is connected to combiner 804-2 through one of outputs 803-2, and transmitter 23-X is connected to combiner 804-1, through one of outputs 803-1. Matrix switch 802 thus allows any one of transmitters 23 to be connected to any one of combiners 804, in any combination.
Switch 802 thus permits a transmission frequency to “follow” a mobile unit from one microcell zone to another. For example, with reference to
Thus, as initially set up, mobile unit 820 transmits and receives on frequencies F1 and F′1, respectively. Controller 810 constantly monitors the signal strength of transmissions from mobile units 820 in all zones in Cell A as received at the antenna units of the remote units 102 positioned in the zones. Signal strength in each zone is detected by sampling the digitized RF microcell traffic returning from remote units 102 to units 130″. While mobile unit 820 is within microcell zone A1, the strength of the received signal F′1 is likely the greatest because of the proximity of mobile unit 820 to the antenna unit of remote unit 102-1 in zone A1. Frequency F′1 might, however, also be received at the antenna of remote unit 102 in zone A2, or in the more distant zone A3. Control unit 810 monitors the strength of received signal F′1 in all of the digitized microcell traffic streams received from all of remote units 102 in the Cell A, and, according to at least one exemplary approach, identifies the remote unit 102 which receives the strongest signal at frequency F′1. Assuming for this example, that the signal F′1 received at the remote unit 102 in zone A1 is the strongest among the zones, controller 810 signals matrix switch 802 to connect transmitter 23-1 to combiner 804-1, which in turn applies its output to digitizing 130″-1. Unit 130″-1 in turn transmits the digitized microcell traffic stream containing the frequency F1 to the remote unit in zone A1, which in turn broadcasts frequency F1 in zone A1 (along with any other frequencies switched into the combiner 804-1). On the return path, controller 810 causes matrix switch 808 to connect the output of digitizing unit 130″-1, as received on line 806-1, to receiver 28-1. Preferably, transmitter 23-1 is connected to no other digitizing units 130″, such that no other remote unit 102 is broadcasting at the frequency F1, except for unit 102-1. Similarily, it is preferable that no other digitizing units 130″ are connected through matrix switch 808 to receiver 28-1. As a result, interference between adjacent microcell zones caused by broadcasting the same frequency is avoided and interference resulting from a receive 28 receiving the same frequency (at different phases and varying distortion) from more than one zone is avoided.
Extending the example further, consider now that mobile unit 820 moves from zone A1 to microcell zone A2 at a time t2. As mobile unit 820 moves from microcell zone A1 to zone A2, controller 810 continues to sample and detect the received signal strength of transmission frequency F′1 from all the remote units 102 in cell A. Upon movement from microcell zone A1 to A2, controller 810 should detect an increasingly stronger signal at frequency F′1 in microcell area A2, and correspondingly a reduction in signal strength at that frequency in microcell area zone A1. When certain switching criteria are met, controller 810 performs a “passive handoff,” by switching transmitter 23-1 from connection to combiner 804-1 to connection with combiner 804-2, and correspondingly switching receiver 28-1 to receive its input from digitizing 130″-2. As a result, transmission at frequency F1 ceases at remote unit 102 in zone A1, and the signal received at that remote unit 102 in zone A1, and the signal received at that remote unit 102 is no longer applied through switch 808 to receiver 28. Thus, system 800 can passively switch a channel from one zone to another within a cell to follow a mobile unit.
The following example illustrates the operation of system 800 when the mobile unit moves from one cell to another. For example, if mobile unit 820 moves from microcell zone A3 to zone B1 at a time t3, controller 810 again detects a corresponding reduction in signal strength received at the remote unit 102 in zone A3. However, no corresponding increase in signal strength in another zone in cell A is detected to trigger a passive handoff. Rather, the handoff from cell A to cell B is handed by MTSO 110 as MTSO 110 senses the movement of the mobile unit 820 between cell A and B. Prior to leaving the cell, as the signal strength decreases, transmission and reception may be achieved using all zones in the cell. As the unit 820 moves into the B cell, MTSO 110 operates to assign a new channel to the mobile unit, from frequencies assigned to cell B. The base station unit 114′ serving cell B then operates in the same manner as described above to identify the initial zone to transmit and receive from, and to perform passive handoffs within cell B. Accordingly, switching between cells A, B or C is carried out independently of the passive handoff of assigned frequencies between zones in a cell. Cell B could, of course, be of conventional design with a single antenna serving the entire cell.
Thus, as described above, the present invention provides a passive handoff system, wherein a transmission frequency is assigned to a mobile unit, and that frequency tracks or follows the mobile unit from one microcell zone to another under the control of controller 810, and without intervention from or switching of transmission frequencies by the MTSO 110. This mode of operation is particularly advantageous in certain microcell applications, wherein multiple remote units 102 are required to cover an area, but there is not enough traffic density in a given zone within the area to support a conventional cell site installation. For example, a narrowed depression in the terrain, such as a ravine or along a road adjacent to a river bed may require multiple antenna installations to obtain adequate signal coverage, due to blockage from nearby terrain. Another example might be in an underground parking garage, or even in large office buildings where larger than normal signal attenuation results in unacceptable signal levels. Furthermore, cell sites in some cellular systems are not located close enough together, thus resulting in poor coverage areas between the cells. Still another example is along a traffic corridor between population centers. For these situations and others, it is advantageous to use a passive handoff system permitting an expansion of the area covered without assigning separate frequency sets and corresponding transmitters and receivers form each zone within the area.
Preferably, each switch 802 and 808 provides support for at least twenty (20) transmitters and twenty (20) receivers, respectively. In addition, each of switches 802 and 808 preferably permits connection of the transmitters and receivers to up to six digitizing units 130″. Accordingly, matrix switch 802 may be used, for example, to connect up to twenty (20) transmitters (where N=20), through to any one of digitizing units 130″. Similarly, the output from digitizers 130″ may be selectively connected to any one of receivers 28, such that a single one of digitizers 130″ may be connected to all of receivers 28, or all of the digitizing units 130″ may be connected to a single one of receivers 28. However, it shall be understood that switches 802 and 808 may be adapted to handle more or less than twenty (20) receivers or transmitters, or more or less than six (6) units 114′.
Switches 802 and 808 are preferably matrix switches, wherein the combining function is integrated into the switch at the matrix nodes, in the form of Wilkinson combiners using nonreflective pin diode attenuators. Such components are available from Salisbury Engineering, Inc., of Salisbury, Md. The switches are preferably of the attenuator type, allowing linear control of rise and fall time. Switching is preferably make before break.
Referring now to
Referring now to
The output of FFT processor 856 is a plurality of 16 bit words in bins, with each bin representing the strength or amplitude of a 30 KHz channel (or channel of a PCS or other service) within the digitized cellular data stream. The output of FFT processor 856 is applied to system 860 over data bus 859, using control line 861. A select circuit 886 receives a control signal 863 from system 860, and selectively generates signals on enable lines 834. Enable lines 834 are used to selectively enable the outputs of buffers 832, so that FFT processor 856 can be selectively filled with digitized microcell traffic samples from a selected source. Microprocessor system 860 is connected to a matrix switch driver 875, which drives matrix switches 802 and 808. The operation of controller 810 as shown in
Referring now to
Routine 914 provides that FFT processor 856 is activated for loading of the digitized microcell traffic stream under the control of microprocessor system 860 using control line 861. A buffer 832 may load, for instance, 1024 samples of the digitized microcell traffic. As microcell traffic data is received from a buffer 832, FFT processor 856 clocks in digitized 12 bit microcell traffic samples or words. The output of FFT processor 856 comprising a series of 16 bit words specifying the signal strength of the respective channels carried in the digitized microcell traffic stream.
Microprocessor system 860 preferably employs an Intel brand “486” type microprocessor or better running at least 33 MHz. At this speed, the time between selectoin of the digitized microcell traffic stream and the receipt of the frequency spectrum analysis from FFT processor 856 can be less than 5 milliseconds. Once microprocessor system 860 has received (916) the frequency spectrum data from FFT circuit 856, which contains the signal amplitude for each frequency in the zone, the data is recorded for immediate or later analysis (routine 918). Optionally, the date and time of the signal measurement is also recorded, together with any other parameters of interest. The polling process continues if all zones in the cell have not yet been measured within the current polling cycle. If polling continues, the digitized microcell traffic stream for the next zone in the cell is selected (routine 924) in the above-described process of data acquisition analysis and storage is repeated.
Once all zones have been measured in a current cycle, microprocessor system 860 determines the channel (i.e., transmitter/receiver) zone assignments based on the signal levels recorded during the cycle. The particular manner in which this determination is made is not essential to the invention, but preferably may take one of the forms described below.
It is contemplated that the switching algorithms for the transmit and receive paths of unit 114′ will be different. In the transmit path, it is contemplated that the method of switching will use the coverage received signal strength in a given zone over a period of ½ second to 3 seconds, with the zone with the greatest strength chosen as the active zone. Alternatively, a zone which is not currently fading, even if at a lower signal strength, may be chosen. If it doesn't matter which zone is used, for example, if signal strengths are comparable, a zone may be chosen which evens out the distribution of channel assignments in the cell. Where the optimum zone cannot be determined, several or all zones can be selected or active, for example, as might occur when a mobile unit is on the edge of a cell.
For switching receivers, instantaneous and average levels are tracked, and fades are tracked so that trends can be predicted and the switching from one zone to another on the receive side can be anticipated. If the received signal strength is below a threshold level, then a receiver may be connected form reception from all zones, for instance where a mobile unit is on the edge of a cell. Switching on the receive side is typically accomplished at a much faster rate of change, than on the transmit side owing to the greater problem of reception and fading from the relative low power transmitters in the mobile units.
Of course, other switching algorithms for both the transmit and receive channels are possible, and certainly those applicable to conventional cellular switching are good candidates.
Once the new channel (transmitter/receiver) assignment has been determined, system 860 switches the transmitters and receivers using switches 802 and 808, through matrix switch drivers 875.
As an alternative to the operation specified for program 900 described above, channel (transmitter/receiver) zone assignments may be determined on a continual basis after each new frequency spectrum measurement is obtained. For instance, program 900 of
A possible side advantage of fast analysis would be to accumulate statistical data on fading that might assist service providers in finding optimum antenna/microcell placement.
As mentioned above, microprocessor system 860 may optionally record the date and time of each measurement of the frequency spectrum of the digitized microcell traffic stream. Accordingly, a history of channel usage and signal strength within any given channel may be readily obtained, and later used for the purpose of reconfiguring the system, for example, by moving antenna units. Accordingly, the present invention further contemplates a method of recording the use of the channels within the zones and the corresponding signal strength, and later using this information to reconfigure the system.
As an alternate exemplary embodiment, the system of
In the alternate embodiment of
On the return path, the digitized sample 816 is taken from the demultiplexed digital data stream returning from the units 106, and supplied to controller 810′. The digital samples are obtained from the demultiplexer 221′ in a like manner as described above with respect to FIG. 36. Controller 810′ in turn uses the sample data as described above with respect to control 810 to control switching. Selector 880 can be used to select the received signal for any desired channel from any one of demodulators 224, for application to T1 interface 202. Alternatively, selector/processor 880 is configured to process two or more of the incoming streams for each channel to create a reduced noise composite stream.
An alternate embodiment of the system of
Yet another two alternate embodiments of the system of
Referring now to
In normal operation, primary microcells 102 provide full coverage over macrocell 103. In case of a failure in one of the microcells 102, however, the two adjacent microcells 105 can provide coverage over the region served by the failed primary microcell 102. In another embodiment, primary microcells 102 provide primary converge to first regions of macrocell 103 and secondary coverage to second regions of macrocell 103 while secondary microcells 105 provide primary coverage to the second regions of macrocell 103 and secondary coverage to the first regions.
A second method of providing redundant coverage is illustrated in two emboiments shown in
Sectorization will be discussed next.
According to yet another aspect of the invention, the microcell system of the present invention may be used to replace the conventional base station transmitter 12 in a conventional cell as for example shown in FIG. 1A. In addition, as can be seen in
The antenna pairs 902 in each macrocell are supported by a remote unit 904 which receives digitized RF for the channels in all three sectors, and converts the digitized RF into analog RF for transmission into the sectors covered by the antenna pairs 902. Remote units 904 further include analog-to-digital converters for digitizing RF received in each sector, and form transmitting the digitized RF to the sectorized base station units 906. Each of the sectorized base station unit 906 is connected to the MTSO 17, which in turn is connected in turn to the switched telephone network 15.
Each sectorized base station unit 906 includes radio frequency transmitters and receivers for each of the channel sets used in each of the sectors of the macrocell, and digital-to-analog and analog-to-digital conversion units for transmitting digitized RF to the remote units and for receiving digitized RF and applying it to the receiver units. Sectorized base station units 906 are preferably connected to remote units 904 over a single fiber optic link 905 using wave division multiplexing as described above, although separate transmit and receive links could be used if desired.
Referring now to
Referring now to
Referring now to
In the preferred embodiment, frequency offset will be minimized by synchronizing remote unit 904 with sectorized base station unit 906. In one such embodiment, sectorized base station unit 906 transports the RF spectrum by downconverting from RF to an IF (in, for example, the 0-30 MHz range), and then digitizing. After being transported to the other end, the IF signal is reconstructed, and then up-converted back to RF.
The down-conversion and up-conversions are implemented by mixing the signal with a local oscillator (LO). In order for the original frequency of the signal to be restored, the signal must be up-converted with an LO that has exactly the same frequency as the LO that was used for down conversion. Any difference in LO frequencies will translate to an equivalent end to end frequency offset. In the embodiment described above, the down conversion and up conversion LO's are at locations remote from one another. Therefore, in one preferred embodiment, frequency coherence between the local and remote LO's is established as follows: at the host end, there is a 552.96 MHz master clock which establishes the bit rate over the fiber. This clock also generates a 30.72 MHz clock (30.72=522.96+18), which serves as a reference to which the host digitizer LO's are locked.
At the remote end, there is another 552.96 MHz clock, which is recovered from the optical bit stream with the help of a phase lock loop. Because this clock is recovered from the bit stream generated at the host, it is frequency coherent with the master clock. A 30.72 MHz clock is then generated to serve as a reference for the remote local oscillators. Because the 552.96 MHz clocks are frequency coherent, so are the 30.72 MHz references, and any LO's locked to them, thus ensuring that host and remote LO's are locked in frequency.
Referring now to
Thus, the sectorized microcell system of the present invention allows for the replacement of the conventional cell site base station in a convention macrocell. In the above described embodiments, the antennas used for each sector are directional, and are all located in the same place. Each directional antenna, one transmit and receive for each sector, is then directed outwardly across the sector service by them. For instance, the sectors may be pie-shaped, with the directional antennas positioned at the center of the pie. Alternatively, nondirectional antennas could be used and positioned at different locations in the cell site. In such a case, the antennas are coupled to the cell site through coaxial cables. In addition, though the above sectorization examples have been described using antenna pairs, it should be obvious to one skilled in the art that sector units having one antenna, or even units having three or more antennae may be used advantageously within such a system. Furthermore, although the examples described entail only the digitization of RF signal generated from the telephone signal received from the MTSO, it should be apparent that the techniques of digital synthesis described in the context of
Finally, although each of the examples above describes the use of an analog RF signal transmitted and received by each remote unit, it should be obvious that the above system and method can be applied advantageously to a digital RF cellular system in a manner well known in the art.
Thus, as described above, the sectorized cell replacement system provides for greater reuse of channels, by dividing conventional cells or even microcells into a plurality of sectors. Furthermore, the system provides all the benefits and advantages of the microcell systems described hereinabove, wherein the transmitters and receivers for all the channels in the cell are centrally located in a convenient and inexpensive location.
Thus, as described above, the present inventions provide a variety of digital systems and methods for transporting cellular traffic to and from antenna units, and for passively switching. Although the invention(s) has been described in its preferred form, those of skill in the art will recognize that many modifications and changes may be made thereto without departing from the spirit and the scope of the claims appended hereto.
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|U.S. Classification||455/561, 455/562.1, 455/447, 455/448, 398/115, 398/41, 455/557|
|International Classification||H04B10/20, H04B1/38, H04M1/00, H04W88/08, H04W16/24, H04W36/18|
|Cooperative Classification||H04W88/085, H04W36/18, H04W16/24|
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