Distributed antenna system for MIMO technologies

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.

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REFERENCE TO MICROFICHE APPENDIX

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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.

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 using2or more spatial streams and operating in TDD mode. Other configurations, such as those supporting3or more special streams, would require simple variations on the scheme presented below.

FIG. 1shows an example of a DAS100in accordance with the invention. The DAS can include a Radio Interface Unit (RIU)110, a Base Unit (BU)120, a Remote Unit (RU)130and an Antenna Unit (AU)150.

The RIU110provides 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 RIU110can include a mixer112on each DL connection and a mixer112on each UL connection. The RIU110can 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 mixers112can 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 duplexer114acombines the DL connections (which use different frequency bands) onto a common cable and can output the signals to the BU120.

Similarly, the UL signals received from the BU120can be input into a de-duplexer114b, which separates the UL into separate connections. Each of UL connections can be input to a mixer112and 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 & 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 BU120can convert the DL RF signal to an optical signal and split that signal into multiple optical links122which can be connected to multiple Remote Units RUs150. The BU120implements the opposite for UL signals. The BU120allows the signals to be distributed, for example, to multiple buildings of campus wide network or multiple floors of a building. The BU120can 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 link122and receives multiple optical input signals and combines them onto a single RF UL signal. One example of a BU120, is a MobileAccess Base Unit above from MobileAccess Networks, Inc., of Vienna, Va.

The RIU110and BU120can be co-located and, optionally, can be combined into a single physical element or component. Where the RIU110and the Bu120are co-located, coaxial cable or twisted pair copper wire can be used to interconnect the units.

The RUs130can be located in wiring closets in different areas (e.g. floors) of a building. The RU130can include a media converting component132,134for converting optical signals to electronic signals (DL connection) and electronic signals to optical signals (UL connection), amplifiers136a,136bfor amplifying the signals as necessary, a time division duplexing (TDD) switching mechanism137for combining the DL and UL signals on a common transmission medium, and a multiplexer138for splitting the signal for transmission to multiple antennae and receiving signals from multiple antennae. For the DL connection, the RU130can transform the signals from optical to RF, be processed by the TDD switching mechanism137, and using the multiplexer138, split the signals onto multiple coaxial cables140going to multiple Antenna Units150. The RU130implements 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 RU130can include a photo diode based system132for converting the optical signal to an RF signal. An amplifier136acan be provided to adjust the amplitude of the signal before it is input into a time division duplexing (TDD) switch137. The TDD switch137can be connected to a multiplexer138which can connect the DL connection to multiple Antenna Units AU150over a cable140, such as a coaxial cable.

On the UL connection, the RU130receives RF signals from one or more AUs150and inputs each signal into multiplexer138which multiplexes the UL signals onto a single connection. The single UL connection can be fed into the TDD switch137. The TDD switch137separates the UL connection from the DL connection and converts the UL signal to an optical signal. An amplifier136b can be provided to adjust the amplitude of the signal before transmission to the BU120. The RU130can include a laser based optical system134for converting the electrical signals to optical signals.

The Antenna Units (AU)150can be located in the ceilings of the building. For the DL, the AU150implements the TDD mechanism152separating the DL and UL signals (opposite the RU130), 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 AU150implements the opposite for UL signals. The UL signals received from the antenna elements164A,166A are amplified162as necessary and then down-converted by mixers158from their native frequencies to a non-overlapping intermediate frequency and combined onto a single line by duplexer156bfor transmission back to the RU130.

The AU150can include a TDD switch mechanism152for duplexing and deduplexing (combining and separating) the UL connections and the DL connections, an amplifier for the DL connections154aand the UL connections154b, a deduplexer156afor recovering the two DL connections, a duplexer156bfor combining the two UL connections, a mixer158for each DL connection for restoring the RF frequency of the signal for transmission to the antenna164A, a mixer158for each UL connection for converting the RF frequency of each UL connection to different, non-overlapping frequency bands, amplifiers162for each of the DL and UL connection, a TDD switching mechanism164for channel1which connects the RF signal to antenna164A and a TDD switching mechanism for channel2which connects the RF signal to antenna166A.

For the DL, the AU150implements the opposite of the RU130in 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 AU150down-converts the two or more spatial streams to a lower frequency band and duplexes them onto a single cable for transmission to the RU130

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 inFIG. 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 toFIG. 1,FIG. 2shows an embodiment of the present invention combined with other services. The DAS200includes a Radio Interface Unit (RIU)210, a Base Unit (BU)220, a Multiband Remote Unit (RU)230and an Antenna Unit (AU)250.

The RIU210can include two or more spatial stream inputs from BTS (not shown) and any number of other services, for example, Service1, Service2, and Service3. As described above with regard toFIG. 1, mixers212can be used to down-convert the DL connection and up-convert the UL connection, and a duplexer/de-duplexer214can be use can be used to combine the DL streams and separate the UL streams. The RIU210sends the DL signals to the BU220and receives the UL signals from the BU220.

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 Channel1and Channel2provide 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 BU220can be same as described above and shown inFIG. 1. The BU220can 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 inFIG. 2, the Multiband RU230receives the DL optical signals from the BU220and sends UL optical signals to the BU220. The processing block236can include the components ofFIG. 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 block236can 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 block236can also include a multiplexer for splitting the combined DL signal to be transmitted to multiple antenna units250and for combining the individual UL signals received from the multiple antenna units250.

The AU250ofFIG. 2is similar to the AU150ofFIG. 1, in that it includes a TDD switching system252, amplifiers254aand254b, de-duplexer256a, duplexer256b, mixers258, amplifiers262, TDD switching system264, TDD switching system266, antenna264aand antenna266a. In addition, AU250includes duplexer/de-duplexer.268which separates the signals for the other services from DL RF signal and feeds the signals for the other services to passive antenna270and the spatial streams to TDD switching system252. For the UL signals, the duplexer/de-duplexer268combines the signals for the other services with the spatial streams in order to send them to the Multiband RU230.

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 RU130,230and the AU150,250in order to support multiple sectors. This reduces the amount of spectrum available to support other services.

As shown inFIG. 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 RU330can map all sectors to the same frequency band and use a switch335to select the sector to be associated with each of its ports and each port being connected over a separate coax cable to a specific AU350. 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 ofFIG. 3is similar toFIGS. 1 and 2above. The RIU310can be connected to one or more BTS units (not shown). The RIU310can include mixers312and duplexer/de-duplexers314and be coupled to the BU320over a DL connection and an UL connection. The BU320can be the same as BU120and BU220as describe above. Further, each antenna unit AU350can be the same as AU150or AU250as described above.

The RU330can be similar to RU130and RU230, and include a photo diode based system332for converting the DL optical signals to RF signal and a laser based system334for converting the UL RF signals to optical signals, along with amplifiers336a,336bto for adjusting the signal as needed.

For the DL spatial streams, the RU330includes a switch335which selectively connects a particular DL spatial stream to one of set of TDD switching systems337which is associated with a particular sector and uses multiplexer338to connect each sector to one or more antenna units AU350. Each TDD switching system337can 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 AU350can 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 cable340for 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.