Abstract:
The invention is directed to a method and system for supporting MIMO technologies which can require the transport of multiple spatial streams on a traditional Distributed Antenna System (DAS). According to the invention, at one end of the DAS, each spatial stream is shifted in frequency to a pre-assigned band (such as a band at a frequency lower than the native frequency) that does not overlap the band assigned to other spatial streams (or the band of any other services being carried by the DAS). Each of the spatial streams can be combined and transmitted as a combined signal over a common coaxial cable. At the other “end” of the DAS, the different streams are shifted back to their original (overlapping) frequencies but retain their individual “identities” by being radiated through physically separate antenna elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims any and all benefits as provided by law of U.S. Provisional Application No. 60/870,739 filed Dec. 19, 2006, which are hereby incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND 
     Technical Field Of The Invention 
     The present invention is directed to Distributed Antenna Systems and more particularly, to methods and systems for transmitting multiple signals or spatial streams over the same RF frequencies using a Distributed Antenna System (“DAS”). 
     The present invention is directed to a DAS intended to support wireless services employing MIMO technologies, such as a WiMax network. Traditionally, a base station connected to a DAS transmits a single signal (one or more RF carriers) within a frequency band. In the case of a MIMO-enabled base station, multiple signals, often referred to as spatial streams, are transmitted on the same RF frequencies. In order for a DAS to adequately support the distribution of this service, it needs to carry the multiple spatial streams to each radiating point, and at each radiating point radiate (and receive) the different streams on separate antenna elements. 
     One challenge for a traditional DAS architecture in addressing these requirements is that a traditional DAS carries signals at their native RF frequency. Therefore carrying multiple signals at the same frequency (namely the multiple spatial streams) may require the deployment of parallel systems. 
     SUMMARY OF THE INVENTION 
     In referring to the signal flows in DAS systems, the term Downlink signal refers to the signal being transmitted by the source transmitter (e.g. cellular base station) through an antenna to the terminals and the term Uplink signal refers to the signals being transmitted by the terminals which are received by an antenna and flow to the source receiver. Many wireless services have both an uplink and a downlink, but some have only a downlink (e.g. a mobile video broadcast service) or only an uplink (e.g. certain types of medical telemetry). 
     In accordance with the invention, multiple spatial streams are transported on a traditional DAS architecture whereby, at the input end, each spatial stream is shifted in frequency to a pre-assigned band (such as a band at a frequency lower than the native frequency) that does not overlap the band assigned to other spatial stream (or the band of any other services being carried by the DAS). At the other “end” of the DAS, the different streams are shifted back to their original (overlapping) frequencies but retain their individual “identities” by being radiated through physically separate antenna elements. In one embodiment, frequency shifting can be implemented using frequency mixers. 
     Most wireless services of interest in this context are bidirectional, meaning they have both a Downlink (signals transmitted from Base station to terminals) and an Uplink (signal transmitted from terminal to Base station). Some wireless technologies operate in FDD (Frequency division duplexing) mode, meaning Downlink (DL) and Uplink (UL) operate simultaneously on different frequencies, while others operate in TDD (Time division duplexing) mode, meaning DL and UL alternate in time using the same frequency bands. 
     The cabling technologies used in a DAS can differ in the way they transfer DL and UL on the same medium (e.g., cable or fiber). Fiber links can use a separate fiber strand (or wavelength in WDM systems) for UL and DL. Therefore, Fiber links can easily support both FDD and TDD modes. 
     Coax links usually use a single cable for both DL and UL. For FDD services, this does not present a problem since the DL and UL signals can use different frequencies. For TDD services, two different embodiments can be used. In one embodiment, a separate frequency for DL and UL can be used (meaning one or both of the DL and UL need to be shifted from their native, overlapping frequencies to non-overlapping frequencies). In an alternative embodiment, a switching mechanism can be used to alternate the DL and the UL transmission on the same frequency. This embodiment has the advantage of using less spectrum resources, allowing other services (at other frequencies) to run on the same cable. 
     These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of an embodiment of a distributed antenna system according to the invention; 
         FIG. 2  is a block diagram of an alternate embodiment of a distributed antenna system according to the invention; and 
         FIG. 3  is block diagram of an alternative embodiment of a distributed antenna system according to the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In accordance with the invention, a method and system can be implemented in a DAS architecture which uses both fiber links and coax links, for a MIMO service using  2  or more spatial streams and operating in TDD mode. Other configurations, such as those supporting  3  or more special streams, would require simple variations on the scheme presented below. 
       FIG. 1  shows an example of a DAS  100  in accordance with the invention. The DAS can include a Radio Interface Unit (RIU)  110 , a Base Unit (BU)  120 , a Remote Unit (RU)  130  and an Antenna Unit (AU)  150 . 
     The RIU  110  provides the interface to the Base station (BTS, not shown). In this embodiment, the RIU has two DL connections from the BTS and two UL connections to the BTS, however a single DL/UL connection or more than two DL and UL connections can be carried by the system. The RIU  110  can include a mixer  112  on each DL connection and a mixer  112  on each UL connection. The RIU  110  can implement the frequency shifting (“down-converting”) for the multiple DL spatial stream signals, mapping each to a different non-overlapping frequency band. For example the DL signals can be down-converted from the WiMAX 2.5 GHz-2.7 GHz frequency bands to the 100 MHz-300 MHz frequency band or the 320 MHz-520 MHz frequency band. It implements the opposite for the UL signals. The mixers  112  can change the signal frequency on each DL connection to a different non-overlapping frequency band so that all the signals can be carried on the same cable without interference. The duplexer  114   a  combines the DL connections (which use different frequency bands) onto a common cable and can output the signals to the BU  120 . 
     Similarly, the UL signals received from the BU  120  can be input into a de-duplexer  114   b , which separates the UL into separate connections. Each of UL connections can be input to a mixer  112  and converted back to their original or native frequency bands for transmission back to the BTS. For example, the UL signals can be up-converted from the 100 MHz-300 MHz frequency band or the 320 MHz-520 MHz frequency band to the WiMAX 2.5 GHz-2.7 GHz frequency. In an alternative embodiment the same frequencies can be shared for DL and UL and the same circuits and mixers can be used for both DL &amp; UL, alternating in time. In accordance with the invention, where the same frequencies are shared by the DL and UL, the same circuits and mixers can be used for both the DL and UL signal paths, alternating in time using, for example, time division multiplexing. 
     The BU  120  can convert the DL RF signal to an optical signal and split that signal into multiple optical links  122  which can be connected to multiple Remote Units RUs  150 . The BU  120  implements the opposite for UL signals. The BU  120  allows the signals to be distributed, for example, to multiple buildings of campus wide network or multiple floors of a building. The BU  120  can be a dual point to multi-point device that converts an input RF DL signal in to multiple optical output signals, for example to transmit the signals over a fiber-optic link  122  and receives multiple optical input signals and combines them onto a single RF UL signal. One example of a BU  120 , is a MobileAccess Base Unit above from MobileAccess Networks, Inc., of Vienna, Va. 
     The RIU  110  and BU  120  can be co-located and, optionally, can be combined into a single physical element or component. Where the RIU  110  and the Bu  120  are co-located, coaxial cable or twisted pair copper wire can be used to interconnect the units. 
     The RUs  130  can be located in wiring closets in different areas (e.g. floors) of a building. The RU  130  can include a media converting component  132 ,  134  for converting optical signals to electronic signals (DL connection) and electronic signals to optical signals (UL connection), amplifiers  136   a ,  136   b  for amplifying the signals as necessary, a time division duplexing (TDD) switching mechanism  137  for combining the DL and UL signals on a common transmission medium, and a multiplexer  138  for splitting the signal for transmission to multiple antennae and receiving signals from multiple antennae. For the DL connection, the RU  130  can transform the signals from optical to RF, be processed by the TDD switching mechanism  137 , and using the multiplexer  138 , split the signals onto multiple coaxial cables  140  going to multiple Antenna Units  150 . The RU  130  implements the opposite for UL signals. In addition the RU can provide powering over the coax cables to the antenna units. 
     On the DL connection, the RU  130  can include a photo diode based system  132  for converting the optical signal to an RF signal. An amplifier  136   a  can be provided to adjust the amplitude of the signal before it is input into a time division duplexing (TDD) switch  137 . The TDD switch  137  can be connected to a multiplexer  138  which can connect the DL connection to multiple Antenna Units AU  150  over a cable  140 , such as a coaxial cable. 
     On the UL connection, the RU  130  receives RF signals from one or more AUs  150  and inputs each signal into multiplexer  138  which multiplexes the UL signals onto a single connection. The single UL connection can be fed into the TDD switch  137 . The TDD switch  137  separates the UL connection from the DL connection and converts the UL signal to an optical signal. An amplifier  136 b can be provided to adjust the amplitude of the signal before transmission to the BU  120 . The RU  130  can include a laser based optical system  134  for converting the electrical signals to optical signals. 
     The Antenna Units (AU)  150  can be located in the ceilings of the building. For the DL, the AU  150  implements the TDD mechanism  152  separating the DL and UL signals (opposite the RU  130 ), up-converts the two or more spatial channels to their native frequencies and transmits each on a dedicated antenna element, with appropriate amplification. For the UL connection, the AU  150  implements the opposite for UL signals. The UL signals received from the antenna elements  164 A,  166 A are amplified  162  as necessary and then down-converted by mixers  158  from their native frequencies to a non-overlapping intermediate frequency and combined onto a single line by duplexer  156   b  for transmission back to the RU  130 . 
     The AU  150  can include a TDD switch mechanism  152  for duplexing and deduplexing (combining and separating) the UL connections and the DL connections, an amplifier for the DL connections  154   a  and the UL connections  154   b , a deduplexer  156   a  for recovering the two DL connections, a duplexer  156   b  for combining the two UL connections, a mixer  158  for each DL connection for restoring the RF frequency of the signal for transmission to the antenna  164 A, a mixer  158  for each UL connection for converting the RF frequency of each UL connection to different, non-overlapping frequency bands, amplifiers  162  for each of the DL and UL connection, a TDD switching mechanism  164  for channel  1  which connects the RF signal to antenna  164 A and a TDD switching mechanism for channel  2  which connects the RF signal to antenna  166 A. 
     For the DL, the AU  150  implements the opposite of the RU  130  in that it de-duplexes the signal into two or more spatial stream and up-converts the two or more spatial streams to the native frequency for transmission on a dedicated antenna element, with the appropriate amplification. For the UL, the AU  150  down-converts the two or more spatial streams to a lower frequency band and duplexes them onto a single cable for transmission to the RU  130   
     When the frequencies used for transport through the DAS (the “down-converted” signals) are relatively low, it is possible to use low cost cabling such as Multi-mode fiber and CATV-grade coax (e.g. RG-11 or RG-6). For example, the down-converted signals can be in the 100 MHz-300 MHz and 320 MHz-520 MHz frequency bands. 
     As shown in  FIG. 2 , the present invention can also be used to combine other services, such as non-MIMO services, on the same system, with the same cabling infrastructure. Additional MIMO bands can be handled in the same way, and they would be transported using additional non-overlapping frequency bands with respect to the frequency bands used for the first MIMO service. Non-MIMO bands can be transported at their native frequency and amplified at the RU, using passive antenna elements to radiate them at the AU. 
     In an embodiment similar to  FIG. 1 ,  FIG. 2  shows an embodiment of the present invention combined with other services. The DAS  200  includes a Radio Interface Unit (RIU)  210 , a Base Unit (BU)  220 , a Multiband Remote Unit (RU)  230  and an Antenna Unit (AU)  250 . 
     The RIU  210  can include two or more spatial stream inputs from BTS (not shown) and any number of other services, for example, Service  1 , Service  2 , and Service  3 . As described above with regard to  FIG. 1 , mixers  212  can be used to down-convert the DL connection and up-convert the UL connection, and a duplexer/de-duplexer  214  can be use can be used to combine the DL streams and separate the UL streams. The RIU  210  sends the DL signals to the BU  220  and receives the UL signals from the BU  220 . 
     The other services can include any other service that uses frequency bands that do not interfere with the frequency bands already used by the system. In one embodiment of the invention, the spatial streams on Channel  1  and Channel  2  provide WiMAX network services in the 2.5-2.7 GHz frequency band and the other services can include, for example, CDMA based services (e.g. in the 1.9 GHz PCS band) and iDEN based services (e.g. in the 800 MHz and 900 MHz bands). 
     The BU  220  can be same as described above and shown in  FIG. 1 . The BU  220  can be any device that converts the DL RF signal to an optical signal and splits the signal to feed multiple optical links and combines the UL optical signals received over multiple optical links and converts the UL optical signals into RF signals. 
     In accordance with the embodiment shown in  FIG. 2 , the Multiband RU  230  receives the DL optical signals from the BU  220  and sends UL optical signals to the BU  220 . The processing block  236  can include the components of  FIG. 1 , including the photo diode based system for converting the DL optical signals back to RF signals and the laser based system for converting the UL RF signals to optical signals and amplifiers for adjusting the signal amplitude as necessary. The processing block  236  can also include duplexer/de-duplexer system for combining the DL RF signals with the signals for the other services and separating the UL RF signals from the signals for other services. The processing block  236  can also include a multiplexer for splitting the combined DL signal to be transmitted to multiple antenna units  250  and for combining the individual UL signals received from the multiple antenna units  250 . 
     The AU  250  of  FIG. 2  is similar to the AU  150  of  FIG. 1 , in that it includes a TDD switching system  252 , amplifiers  254   a  and  254   b , de-duplexer  256   a , duplexer  256   b , mixers  258 , amplifiers  262 , TDD switching system  264 , TDD switching system  266 , antenna  264   a  and antenna  266   a . In addition, AU  250  includes duplexer/de-duplexer.  268  which separates the signals for the other services from DL RF signal and feeds the signals for the other services to passive antenna  270  and the spatial streams to TDD switching system  252 . For the UL signals, the duplexer/de-duplexer  268  combines the signals for the other services with the spatial streams in order to send them to the Multiband RU  230 . 
     In cases where significant capacity is required in a facility covered by a DAS, multiple base-stations (or multiple sectors on a single base station) can be used to “feed” the DAS, where each segment of the DAS can be associated with one of the base stations/sectors. In order to provide additional flexibility in assigning capacity to areas in the facility, it is desirable to be able to independently associate each AU with any one of the base stations/sectors. 
     In accordance with one embodiment of the invention, the RIU can have multiple, separate interfaces for each base station/sector (2 spatial streams from each in the 2-way MIMO example discussed above). The RIU can map each pair of signals from each base station/sector to a different pair of bands, non-overlapping with the bands assigned to other base stations/sectors. The BU and RU can retain the same functionality as above. The AU can have the ability using software to select the specific sector to use, based on tuning to the respective frequency bands. 
     However, one of the disadvantages of the approach described in the previous paragraph is that multiple blocks of spectrum are required on the link between the RU  130 ,  230  and the AU  150 ,  250  in order to support multiple sectors. This reduces the amount of spectrum available to support other services. 
     As shown in  FIG. 3 , in accordance with an alternative embodiment of the invention, the system can maintain the same flexibility of association of sectors to antennas and the functionality of the RIU is as described above (mapping each sector to a different frequency band). The RU  330  can map all sectors to the same frequency band and use a switch  335  to select the sector to be associated with each of its ports and each port being connected over a separate coax cable to a specific AU  350 . In this embodiment, the amount of spectrum consumed on the coax under this scheme is the amount required to support a single sector, regardless of the number of sectors supported in the full system. 
     The embodiment of  FIG. 3  is similar to  FIGS. 1 and 2  above. The RIU  310  can be connected to one or more BTS units (not shown). The RIU  310  can include mixers  312  and duplexer/de-duplexers  314  and be coupled to the BU  320  over a DL connection and an UL connection. The BU  320  can be the same as BU  120  and BU  220  as describe above. Further, each antenna unit AU  350  can be the same as AU  150  or AU  250  as described above. 
     The RU  330  can be similar to RU  130  and RU  230 , and include a photo diode based system  332  for converting the DL optical signals to RF signal and a laser based system  334  for converting the UL RF signals to optical signals, along with amplifiers  336   a ,  336   b  to for adjusting the signal as needed. 
     For the DL spatial streams, the RU  330  includes a switch  335  which selectively connects a particular DL spatial stream to one of set of TDD switching systems  337  which is associated with a particular sector and uses multiplexer  338  to connect each sector to one or more antenna units AU  350 . Each TDD switching system  337  can include a DL mixer for converting the DL spatial stream to a common frequency band and an UL mixer for converting the UL spatial stream from the common frequency band to the initial received frequency band. Each AU  350  can be configured to communicate using the common frequency band. The common frequency band can be selected based on environmental conditions and the distances of the runs of cable  340  for the system. The common frequency can be the same as the most common frequency used the RIU for converting the spatial streams, so no conversion is required for some signals (the most common) thus reducing the power requirements and potential for signal distortion on the most common signals. 
     Other embodiments are within the scope and spirit of the invention. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
     Further, while the description above refers to the invention, the description may include more than one invention.