Abstract:
A wireless telecommunications system architecture is disclosed that is capable of supporting a centralized block radio architecture and of distributing the information-bearing signals from the block radio to geographically-dispersed radio heads using relatively low-cost cable (e.g., twisted-pair, etc.). In accordance with the illustrative embodiment of the present invention, a radio multiplexes, modulates, and channel codes one or more information-bearing signals and upconverts them, not to radio frequency, but to an intermediate frequency that can be transmitted over a low-cost cable. Co-located with each radio head is an upconverter that upconverts the intermediate frequency signal to radio frequency in preparation for radiation by an antenna.

Description:
FILED OF THE INVENTION 
     The present invention relates to telecomminications in general, and, more particularly, to an architecture for a wireless telecommunications system. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 depicts a schematic diagram of a portion of a typical wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals  101 - 1  through  101 - 3 ) that are situated within a geographic region. The heart of a typical wireless telecommunications system is Wireless Switching Center (“WSC”)  120 , which may also be known as a Mobile Switching Center (“MSC”) or Mobile Telephone Switching Office (“MTSO”). Typically, Wireless Switching Center  120  is connected to a plurality of base stations (e.g., base stations  103 - 1  through  103 - 5 ) that are dispersed throughout the geographic area serviced by the system and to local and long-distance telephone and data networks (e.g., local-office  130 , local-office  138  and toll-office  140 ). Wireless Switching Center  120  is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal (e.g., wireline terminal  150 ), which is connected to the system via the local and/or long-distance networks. 
     The geographic area serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called “cells.” As depicted in FIG. 1, each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topography of the terrain serviced by the system. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with Wireless Switching Center  120 . 
     For example, when wireless terminal  101 - 1  desires to communicate with wireless terminal  101 - 2 , wireless terminal  101 - 1  transmits the desired information to base station  103 - 1 , which relays the information to Wireless Switching Center  120  via wireline  102 - 1 . Upon receipt of the information, and with the knowledge that it is intended for wireless terminal  101 - 2 , Wireless Switching Center  120  then returns the information back to base station  103 - 1 , again via wireline  102 - 1 , which relays the information, via radio, to wireless terminal  101 - 2 . 
     FIG. 2 depicts a block diagram of the architecture of a typical wireless telecommunications system in the prior art. Typically, each base station is connected to base station controller  201  via a separate and distinct wireline. Base station controller  201  can be, but is not necessarily, co-located with Wireless Switching Center  120 . 
     For example, base station  103 - 1  is connected to base station controller  201  via wireline  102 - 1  and base station  103 - 3  is connected to base station controller  201  via wireline  102 - 3 . Wirelines  102 - 1  and  102 - 3  can be fabricated from inexpensive and easily installed twisted-pair. In accordance with this architecture, each radio is located near the antennas with which it transmits and receives. In contrast, the emergence of another technology suggests removing the radios from the base stations and centralizing their functionality in a single unit known as a block radio. 
     A block radio is a digital signal processor that is programmed to multiplex, modulate, channel code, and upconvert one or more information-bearing signals using digital signal processing techniques. A block radio performs the same functionality as one or more traditional radios, but has several characteristics that are different than traditional radios. First, a traditional radio processes a single information-bearing signal. In contrast, a block radio is generally capable of processing a plurality of information-bearing signals simultaneously. 
     Second, a traditional radio is fabricated from radio-frequency components (e.g., capacitors, inductors, oscillators, etc.) and the processing of the information-bearing signal is performed by, and is largely defined by, the electrical characteristics of the components. In contrast, a block radio principally comprises a digital signal processor and the processing of the information-bearing signals is defined by software and software parameters. 
     Third, a change in the characteristics of a information-bearing signal (e.g., modulation scheme, bandwidth, etc.) can be implemented in a traditional radio by changing one or more of the radio-frequency components. In contrast, a change in the characteristics of a information-bearing signal can be implemented in a block radio by changing software and/or software parameters controlling the block radio. This enables a block radio to be re-defined and upgraded remotely via a telecommunications link. 
     Fourth, a block radio is generally less expensive than multiple traditional radios of comparable quality and processing power. 
     And fifth, because a block radio processes a plurality of information-bearing signals, it is capable of performing inter-information-bearing signal processing (e.g., diversity combining, beamforming, adjacent channel interference reduction, etc.) that a traditional radio, which sees only one information-bearing signal, is incapable of performing. Therefore, a block radio is more flexible, more powerful, less expensive and more-easily upgraded than the traditional radios that is capable of replacing. 
     FIG. 3 depicts a block diagram of a typical wireless telecommunications architecture in the prior art that incorporates block radio technology and that comprises: wireless switching center  120 , baseband unit  301 , radio heads  303 - 1  and  303 - 2 , and wirelines  304 - 1  and  304 - 2 . In accordance with this architecture, each geographically-dispersed base station of FIG. 2 is replaced with a radio head and baseband unit  301 , which comprises block radio  302 , is interposed between wireless switching center  120  and radio heads  303 - 1  and  303 - 2 . Furthermore, in accordance with this architecture, block radio  302  interfaces with base station controller  201  and provides the functionality provided by the distributed traditional radios in the architecture of FIG.  2 . Each of radio heads  303 - 1  and  303 - 2  comprises an amplifier and associated antenna. 
     In addition to the advantages provided by block radio  302 , this architecture is advantageous because of the simple, uniform, and inexpensive design of the radio heads that it affords. The architecture in FIG. 3 is disadvantageous over the predecessor architecture in FIG. 2 in that the information-bearing signals transmitted between baseband unit  301  and radio heads  303 - 1  and  303 - 2  are at RF frequencies, which requires that wirelines  304 - 1  and  304 - 2  be fabricated from expensive and difficult-to-install coaxial cables. Furthermore, the fact that the signals transmitted between baseband unit  301  and radio heads  303 - 1  and  303 - 2  are at radio frequency considerably restricts the distance that baseband unit  301  can be from radio heads  303 - 1  and  303 - 2 . 
     Therefore, the need exists for a wireless telecommunications system architecture that exhibits the advantages of block radio technology without the expense, distance limitation, and implementation difficulty associated with wirelines that are capable of transmitting signals at RF frequencies. 
     SUMMARY OF THE INVENTION 
     The present invention is a wireless telecommunications system that has some of the advantages of block radio technology without some of the disadvantages associated with block radio architectures in the prior art. In particular, some embodiments of the present invention comprise a centralized baseband unit, with a block radio, that is connected to multiple, geographically-dispersed radio heads via low-cost, easily-installed wireline (e.g., twisted-pair, etc.). This is advantageous because it provides an architecture that is more powerful, more flexible, more easily-upgraded, and less-expensive than some architectures in the prior art. 
     In accordance with the illustrative embodiment of the present invention, a block radio multiplexes, modulates, and channel codes one or more information-bearing signals in accordance with the particular air-interface standard (e.g., GSM, IS-54 TDMA, IS-95 CDMA, etc.), in well-known fashion, and upconverts them, not to radio frequency, but to an intermediate frequency that can be transmitted over a low-cost wireline to a radio head. Each radio head then receives one or more intermediate-frequency signals and upconvert them to radio frequency in preparation for radiation by the antenna. This is advantageous because the additional cost of the upconverter in each radio head, which makes transmission to the radio heads over inexpensive wireline possible, is negligible in comparison to the savings gained by using low-cost wirelines. 
     Furthermore, some embodiments of the present invention allow signals to be distributed from a block radio to multiple antennas via a single wireline, which can create further cost savings in some circumstances. 
     And still furthermore, because the signals are transmitted from the baseband unit to the respective radio heads at intermediate frequencies, the baseband unit can be located considerably farther away from the radio heads than if the signals are transmitted at radio frequencies. This is advantageous because it provides additional flexibility in implementation. 
     An illustrative embodiment of the present invention comprises: means for upconverting a first information-bearing signal to a first intermediate frequency and a second information-bearing signal to a second intermediate frequency; means for transmitting the first information-bearing signal at the first intermediate frequency over a first wireline and for transmitting the second information-bearing signal at the first intermediate frequency over a second wireline; a first upconverter for receiving the first information signal at the first intermediate frequency from the first wireline and for upconverting the first information-bearing signal to a first radio frequency; and a second upconverter for receiving the second information signal at the first intermediate frequency from the second wireline and for upconverting the second information-bearing signal to a second radio frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a schematic diagram of a wireless telecommunications system in the prior art. 
     FIG. 2 depicts a block diagram of one wireless telecommunications system architecture in the prior art. 
     FIG. 3 depicts a block diagram of a second wireless telecommunications system architecture in the prior art that utilizes block radio technology. 
     FIG. 4 depicts a schematic diagram of a wireless telecommunications system architecture in accordance with the illustrative embodiment of the present invention. 
     FIG. 5 depicts a block diagram of a wireless telecommunications system architecture in accordance with the illustrative embodiment of the present invention. 
     FIG. 6 depicts a graph of power as a function of frequency for a plurality of intermediate-frequency information-bearing signals, control signals, and a synchronization signal that are frequency-division multiplexed on a shared wireline. 
     FIG. 7 depicts a block diagram of a radio head in accordance with the illustrative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 depicts a schematic diagram of the architecture of wireless telecommunications system  400 , which provides wireless telecommunications service to one or more wireless terminals (not shown). It will be clear to those skilled in the art that only those aspects of a wireless telecommunications system architecture concerning the “downlink” or “forward” channels are described in this disclosure. Those aspects of the architecture concerning the “uplink” or “reverse” channel are taught in commonly-assigned co-pending U.S. pat. application Ser. No. 09/023,469 entitled “A Wireless Telecommunications System Architecture Supporting Receive Diversity,” filed Feb. 13, 1998, which is incorporated by reference. Wireless telecommunications system  400  advantageously comprises a plurality of radio heads (e.g.,  401 - 1  through  401 - 6 ) connected to baseband unit  403  via one of wirelines  402 - 1 ,  402 - 2 , and  402 - 3 . 
     Baseband unit  403  advantageously receives information-bearing traffic signals and control signaling, in well-known fashion, and modulates and channel codes the information-bearing signals for radio transmission via one or more of radio heads  401 - 1  through  401 - 6 . The details of baseband unit  403  will be discussed below. Each of radio heads  401 - 1  through  401 - 6  advantageously comprises an amplifier and an antenna for radiating the respective information-bearing signals. The details of each of radio heads  401 - 1  through  401 - 6  will be discussed below. 
     Advantageously, the radio heads are distributed throughout the geographic area serviced by wireless telecommunications system  400  so that the coverage areas overlap and so that there are no interstices or gaps in coverage. For example, as shown in FIG. 4, the coverage area of radio head  401 - 1  overlaps the coverage areas of radio heads  401 - 2  and  401 - 3 . It will be clear to those skilled in the art that radio heads  401 - 1  through  401 - 6  can mounted indoors, outdoors, or a combination of indoors and outdoors. Furthermore, it will be clear to those skilled in the art that a radio head can be mounted on the top of a mast with the baseband unit at the base of the mast. 
     In accordance with the illustrative embodiment, a radio head can be directly connected to baseband unit  403  via a proprietary wireline or, alternatively, two or more radio heads can be connected to baseband unit  403  via a shared wireline. Furthermore, in accordance with the illustrative embodiment, each wireline is fabricated from twisted-pair or any other cabling that is capable of transmitting a intermediate-frequency signal containing one or more information-bearing signals, control signals, and synchronization signals. 
     The illustrative embodiment is advantageously capable of communicating with a wireless terminal in either: (1) Simulcast Mode or (2) Hand-Off Mode. In Simulcast Mode, all of the radio heads in the system radiate the same information-bearing signals at the same frequencies at the same time. There are three principal advantages to Simulcast Mode. First, its simplicity engenders a low-cost installation because there is no RF engineering that has to be done other than ensuring that the respective radio heads are positioned so that there are no gaps in coverage. Second, because each radio head performs the identical function, each radio head can be configured identically, which lowers their cost of manufacture and installation. Third, in Simulcast Mode all of the traffic channels are available at each location, which enables a larger number of clustered wireless terminals to be serviced than could be serviced in Hand-Off Mode, in which only a percentage of the traffic channels are available at each location. And fourth, because there is no hand-off of wireless terminals in Simulcast Mode, baseband controller  403  can be simpler than when hand-off is supported. 
     There are two principal disadvantages to Simulcast Mode. First, Simulcast Mode might not efficiently re-use the available RF spectrum, which lowers the overall traffic capacity of the system, and second, all of the information-bearing signals radiated from each radio head in Simulcast Mode must precisely synchronized, in well-known fashion, which requires a mechanism for synchronizing the respective radio heads. 
     In Hand-Off Mode, one or more radio heads transmits one or more different information-bearing signals at different radio frequencies and the system has the capability to hand-off a wireless terminal from one radio head to another radio head, as in a conventional wireless telecommunications system. The principal advantage of Hand-Off Mode is that it allows the efficient re-use of the available RF spectrum, which raises the overall traffic capacity of the system, for a fixed number of frequencies. 
     There are, however, two principal disadvantages to operation in Hand-Off Mode. First, because Hand-Off Mode requires that different radio channels be assigned to different radio heads (so as to reduce co-channel and adjacent-channel interference), Hand-Off Mode requires more extensive, and thus more expensive, RF engineering when the system is installed. Second, if multiple radio heads are networked on a shared wireline, each radio head must contain circuitry to enable it to distinguish those signals from baseband unit  403  that are intended for it from signals that are intended for other radio heads. 
     FIG. 5 depicts a block diagram of baseband unit  403  in accordance with the illustrative embodiment of the present invention. Baseband unit  403  advantageously comprises: radio  501 , digital-to-analog converters  502 - 1  through  502 - 3 , and transmitters  503 - 1  through  503 - 3 , which interface with wirelines  402 - 1  through  402 - 3 , respectively. In accordance with the illustrative embodiment, baseband unit  403  is capable of operation in either Simulcast Mode or Hand-Off Mode and of interfacing with each radio head via a proprietary wireline, a shared wireline, or a combination of the two. 
     The input to baseband unit  403  is one or more information-bearing signals (i.e., traffic channels) and associated control signaling, which can be, but are not necessarily, multiplexed onto lead  404 , in well-known fashion. The source of the baseband information-bearing signals need not be a base station controller, but can be a conventional central office (“CO”) switch or private branch exchange (“PBX”) that is not specifically designed to handle wireless telecommunications. 
     Radio  501  is advantageously, but not necessarily, a block radio that comprises a digital signal processor and memory and that is capable of being re-programmed and controlled via lead  404 , in well-known fashion. Block radio  501  is advantageously programmed to be capable of receiving one or more information-bearing signals and control signals from lead  404  and of multiplexing, modulating, and channel coding in accordance with an air-interface standard (e.g. GSM, IS-54 TDMA, IS-95 CDMA, etc.) and upconverting the information-bearing signals to an intermediate frequency, using well-known digital signal processing techniques. 
     Alternatively, radio  501  could comprise one or more traditional radios made substantially of radio frequency components, in well-known fashion. In such case, each traditional radio is capable of receiving one or more information-bearing signals and control signals from lead  404  and of multiplexing, modulating, and channel coding in accordance with an air-interface standard (e.g., GSM, IS-54 TDMA, IS-95 CDMA, etc.) and upconverting the information-bearing signals to an intermediate frequency, in well-known fashion. There are two techniques that the radio can employ to upconvert an information-bearing signal to an intermediate frequency. First, the radio can simply use the information-bearing signal to modulate an intermediate-frequency carrier. Alternatively, the radio can use the information-bearing signal to modulate a baseband carrier and then upconvert the modulated baseband carrier to an intermediate frequency using, for example, a mixer and local oscillator. It will be clear to those skilled in the art how to make and use embodiments of the present invention in which radio  501  comprises one or more traditional radios. 
     Radio  501  is also advantageously programmed to support either Simulcast Mode or Hand-Off Mode. For example, when the illustrative embodiment operates in Simulcast Mode, each of the information-bearing signals is upconverted to the same intermediate frequency and is transmitted over wirelines  402 - 1  through  402 - 3  for radiation by each radio head. 
     Alternatively, when the illustrative embodiment operates in Hand-Off Mode, each of the information-bearing signals is transmitted to only those radio heads through which the information-bearing signals are to be radiated. If two or more radio heads share a wireline, the information-bearing signals for those radio heads are frequency-division multiplexed over the shared wireline. In such case, each radio head on the shared wireline must be capable of receiving and isolating (i.e., demultiplexing) the information-bearing signals intended for it. The details of a radio head will be discussed below. 
     Radio  501  also advantageously generates one or more intermediate-frequency control signals that are capable of: (1) controlling which intermediate-frequency information-bearing signals each radio head selects from the wireline, and (2) controlling the radio frequency to which each information-bearing signal is upconverted by each radio head. These intermediate-frequency control signals are advantageously frequency-division multiplexed along with the intermediate-frequency information-bearing signals on each wireline. 
     Radio  501  also advantageously generates an intermediate-frequency synchronization signal that is transmitted to all of the radio heads so that they can synchronize their respective output signals. The synchronization signal is advantageously frequency-division multiplexed along with the intermediate-frequency information-bearing signals and the intermediate-frequency control signals on each wireline. 
     Radio  501  forwards the output (e.g., the information-bearing signals, control signals, and synchronization signal) for wireline  402 -i, to digital-to-analog converter  502 -i, which converts the digitized intermediate-frequency output to one or more analog signals. The output of digital-to-analog converter  502 -i is output to transmitter  503 -i, which amplifies and prepares the signals for transmission via wireline  402 -i, in well-known fashion. 
     FIG. 6 depicts a graph of power as a function of frequency for an illustrative plurality of intermediate-frequency information-bearing signals, control signals and a synchronization signal that are frequency-division multiplexed over a shared wireline to two radio heads. Information-bearing signals  601 - 1  through  601 - 4  are intended for one radio head as are control signals  601 - 1  through  601 - 4  and synchronization signal  603 . Control signal  601 -i is associated with information-bearing signal  601 -i. Information-bearing signals  604 - 1  through  604 - 4  are intended for a second radio head as are control signals  605 - 1  through  605 - 4  and synchronization signal  603 . Control signal  605 -i is associated with information-bearing signal  604 -i. Although each information-bearing signal is depicted as being associated with a unique control signal, it will be clear to those skilled in the art how to make and use embodiments of the present invention in which two or more information-bearing signals are associated with one control signal. 
     FIG. 7 depicts a block diagram of radio head  401 -i, which advantageously comprises: filter  701 , upconverter  702 , amplifier  703 , antenna  704 , and controller  705 . Filter  701  is advantageously a bandpass filter that is tunable by controller  705  for isolating and capturing (i.e., frequency-division demultiplexing) only those intermediate-frequency information-bearing signals intended for radiation via antenna  704 . 
     Upconverter  702  advantageously comprises: (1) a mixer for upconverting the intermediate-frequency information-bearing signals from filter  701  to radio frequency under the control of controller  705 , and (2) a filter for filtering out the spurious frequency components created by the mixer. In an alternative embodiment, upconverter comprises a plurality of mixer/filter pairs and is capable of upconverting each intermediate-frequency information-bearing signal by a different amount, again under the control of controller  705 . 
     Amplifier  703  amplifies the radio-frequency information-bearing signals, in well-known fashion, in preparation for radiation via antenna  704 , also in well-known fashion. 
     Controller  705  advantageously comprises control circuitry that is capable of isolating and capturing the synchronization signal and only those intermediate-frequency control signals associated with the intermediate-frequency information-bearing signals intended for radiation via antenna  704 . Based on those control signals, controller  705  is advantageously capable of: (1) controlling filter  701  to isolate and capture only those intermediate-frequency information-bearing signals intended for radiation via antenna  704 , (2) controlling the radio frequency to which the information-signals are upconverted by upconverter  702 , and (3) synchronizing the radio frequency to which the information-signals are upconverted by upconverter  702 . It will be clear to those skilled in the art how to make and use radio head  401 -i. 
     It is to be understood that the above-described embodiments are merely illustrative of the invention and that many variations may be devised by those skilled in the art without departing from the scope of the invention. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.