Distributed antenna system for MIMO signals

[0001] Embodiments of the invention are directed to wireless communication systems, and specifically directed to a distributed antenna system for a wireless MIMO communications.

[0002] A contemporary wireless communication system, such as distributed antenna system 10, is shown in FIG. 1, and includes a number of remote units 12 distributed to provide coverage within a service area of the system 10. In particular, each remote antenna unit 12 typically includes an antenna 14 and suitable electronics. Each remote unit is coupled to a master unit 16. Each master unit 16 is, in turn, coupled to a RF combination network 18 that combines the signals from at least one single-input- and single-output (“SISO”) base transceiver station (“BTS,” or more simply, “base station”) 20 (hereinafter, “SISO BTS” 20). The system 10 may further include a system controller 22 to control the operation of each master unit 16. As illustrated in FIG. 1, the system 10 may include a plurality of master units 16 and a plurality of SISO BTSs 20, each master unit 16 configured to provide a combination of the signals from at least two SISO BTSs 20 to its respective remote units 12.

[0003] As illustrated in FIG. 1, each remote unit 12 may broadcast a wireless signal 24 that, in turn, may be received by a wireless device 26 that may be a mobile device, such as a telephone device or a computing device. In particular, and as discussed above, the wireless signal 24 from each remote unit 12 may be a combination of signals from the at least two SISO BTSs 20. Thus, the wireless device 26 may communicate with the system 10 through any of the wireless signals 24 from the remote units 12.

[0004] To improve wireless communications, such as communications from a base station to mobile devices, Multiple-Input/Multiple-Output (“MIMO”) technology might be utilized to provide advanced solutions for performance enhancement and broadband wireless communication systems. Through various information series studies, it has been shown that substantial improvements may be realized utilizing a MIMO technique with respect to the traditional SISO systems. MIMO systems have capabilities that allow them to fully exploit the multi-path richness of a wireless channel. This is in contrast with traditional techniques that try to counteract multi-path effects rather than embrace them. MIMO systems generally rely upon multi-element antennas at both of the ends of the communication links, such as in the base station and also in the mobile device. In addition to desirable beam-forming and diversity characteristics, MIMO systems also may provide multiplexing gain, which allows multi data streams to be transmitted over spatially-independent parallel sub-channels. This may lead to a significant increase in the system capacity. Generally, the systems illustrated in FIG. 1 cannot take advantage of MIMO technology.

[0005] For example, the wireless device 26 of FIG. 1 communicates with only one of the remote units 12, though it may be in the range of a plurality of remote units 12. The wireless signals 24 from each remote unit are typically at the same frequency and carry the same data, and communication between a plurality of remote units 12 and the wireless device 26 simultaneously may result in signal degradation and collisions. Moreover, data bandwidth from the wireless device 26 is constricted to the speed of reception and processing of data from one remote unit 12.

[0006] It is therefore, desirable to take advantage of MIMO signals within a wireless system, such as distributed antenna system, without requiring an entirely new system to be installed for handling MIMO signals.

[0007] Embodiments of the invention provide a distributed antenna system (“DAS”) and methods of use that can be used to provide a single-input and single-output (“SISO”) mode of operation and a multiple-input and multiple-output (“MIMO”) mode of operation. In particular, some embodiments include a MIMO base station configured to output at least a first signal and a second signal and a hybrid coupler coupled to the MIMO base station. The coupler is configured to receive the first signal and the second signal from the MIMO base station on respective first and second ports and provide an output signal on at least one output port. The output signal includes at least a portion of the first signal and at least a portion of the second signal. The system further includes at least one master unit communicating with the coupler and configured to receive at least the output signal, and at least one remote unit communicating with the master unit and configured to communicate at least the output signal to a device, such as a customer's wireless device. In those embodiments, the system may be selectively operated to dynamically re-configure the distributed antenna system from operating in a SISO mode of operation to a MIMO mode of operation.

[0019] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of embodiments of the invention. The specific design features of the system and/or sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments may have been enlarged, distorted or otherwise rendered differently relative to others to facilitate visualization and clear understanding.

[0020] FIG. 2A illustrates a schematic view of one possible implementation of a MIMO system, wherein a MIMO base station is incorporated with a distributed antenna system, such as that shown in FIG. 1. With respect to FIG. 1, like reference numerals are utilized in FIG. 2A where applicable. As illustrated, two SISO base stations 20 (each SISO base station 20 a “SISO BTS” 20) are coupled with each of the remote units. Additionally, a MIMO base station 30, including antennas 31 and 32, are coupled with the remote units 12. Antenna 1 of the MIMO BTS 30 is coupled with remote units 12a and 12d through a first master unit 16 (MASTER UNIT 1). Antenna 2 of the MIMO BTS 30 is coupled with remote units 12b and 12c through as second master unit 16 (MASTER UNIT 2). As such, as illustrated by the wireless signals 24 produced at each remote unit, each master unit will transmit signals from the MIMO BTS 30, in addition to a combined signal from the combination of signals output by the SISO BTSs 20. However, because each antenna is not coupled to all the remote units, each remote unit will only transmit one of the two available MIMO signals as shown. The respective wave fronts are illustrated corresponding to the feed or connection lines from each of the appropriate antennas 31, 32 in FIG. 2A.

[0021] While the embodiment illustrated in FIG. 2A may be utilized to provide the availability of MIMO signals within a distributed antenna system, such a system may not realize all of the desired performance improvements associated with a MIMO system. For example, even if wireless device 26 receives all the MIMO signals from a combination of at least two of the remote units, there may be a received RF power imbalance because the wireless device 26 might be located much closer to one remote unit 12 than to another. Furthermore, accordingly to wireless standards that support MIMO features, there are some signaling parameters, such as the WiMAX Frame Preamble or the LTE Primary Synchronization Signal (“P-SS”), which are, or actually can be, transmitted by only one of the MIMO BTS antennas 31, 32. Therefore, in a MIMO system, as in FIG. 2A, wherein these signals are not transmitted by all of the remote units, the system may not be sufficiently reliable unless there is a very high level of coverage redundancy/overlap between the remote units. In operation, dynamic switching between SISO and MIMO operating modes may present performance problems.

[0022] FIG. 2B illustrates a schematic view of another possible implementation of a distributed antenna system 40 that incorporates MIMO features consistent with embodiments of the invention. System 40 includes at least one remote unit 42 to provide coverage in a service area. In particular, system 40 includes a plurality of remote units 42a-h. Each remote unit 42a-h includes at least one coverage antenna 44 and is coupled to a master unit 46a-b. Each master unit 46a-b, in turn, is coupled to a respective summing circuit 48a-b that may be configured to combine at least two inputs. In particular, each summing circuit 48a-b combines a signal from an RF combination network 50 coupled to SISO BTSs 54a and 54b with a signal from the MIMO BTS 58. The signals 68, 70 from the MIMO base station are presented through a hybrid coupler 52 coupled with the signals from the antennas 31 and 32 of the MIMO BTS 58. The RF combination network 50 is coupled to a plurality of SISO BTSs 54a-b and outputs at least one combined SISO BTS signal as at 56a-b.

[0023] In one aspect of the present invention, a hybrid coupler 52 is coupled to the MIMO BTS 58 to cross-couple all MIMO signals (in the example illustrated that is two MIMO signals) to each of the remote units 42. Therefore, each of the remote units 42 transmits all of the MIMO BTS 58 data streams, as well as the combined data streams from the SISO BTSs 54. The hybrid coupler 52 is configured to receive at least two MIMO signals from the respective antennas 31 and 32 on respective first and second ports (Ports 1 and 2 as illustrated in FIGS. 2A-B), and provide an output signal on at least one output port (Ports 3 and 4 as illustrated in FIGS. 2A-B). In the illustrated embodiment, combined signals from the two MIMO BTS antennas 31 and 32 are provided at the output ports 3 and 4. Each output signal includes at least a portion of the first signal from antenna 31, and at least a portion of the second signal from antenna 32. In the hybrid coupler circuit, the portion of the first signal (e.g. Antenna 1) or the portion of the second signal (e.g. Antenna 2) presented at one of the input ports 1, 2 is phase shifted with respect to the first signal and/or second signal received at the respective other of first and second coupler ports 1, 2. In particular, the hybrid coupler 52 is disposed between a MIMO BTS 58 and master stations 46a-b such that the hybrid coupler 52 is configured to receive the first and second signals from the MIMO BTS 58, including a first signal at 62 from a first MIMO antenna 31, and a second signal at 64 from a second MIMO antenna. In turn, the hybrid coupler 52 combines a portion of the first signal 62 with a phase shifted portion of the second signal 64 and outputs that first output signal at 68 on a first output port (e.g., output port 3). Coupler 52 also combines a portion of the second signal 64 with a phase shifted portion of the first signal 62 and outputs that second output signal at 70 on a second output port (e.g., output port 4). In one exemplary embodiment, the hybrid coupler 52 is a 90° 3 dB coupler (also referred to as a “quadrature” coupler).

[0024] Furthermore, it will be appreciated that, in some embodiments, the first and second signals from the MIMO BTS 58 may be separately provided to respective summing circuits 48a-b and/or master units 46a-b rather than passing through hybrid coupler 58, such as the embodiment shown in FIG. 2A.

[0025] In some embodiments, the summing circuit 48a is configured to provide a first master unit signal 72 that is a combination of the combined SISO BTS signal 56a and the first combined MIMO signal 68. Summing circuit 48b is configured to provide a second master unit signal 74 that is a combination of the combined SISO BTS signal 56b and the second combined MIMO signal 70. The master units 46a-b and remote units 42a-h, in turn, may be controlled by a system controller 76, which may provide overall supervision and control of the master units 46a-b and remote units 42a-h, as well as alarm forwarding.

[0026] In some embodiments, each remote unit 42a-h may be connected to their respective master units 46a-b via high speed digital transport mediums or links 80a-b, 82, 84a-b and/or 86a-b. Alternatively, an analog transport medium/link might be used for connecting the remote units with respective master units. Also, the transport links might be implemented as optical links using optical fiber as discussed below. With such fiber, the traffic between the remote units and master units might be implemented using a radio-over-fiber (RoF) format. In this manner, the first master unit signal 72 and/or the second master unit signal 74 are provided to at least a portion of the remote units 42a-h in a digital format, which may assist in preventing at least some degradation due to transmission line effects. It will be appreciated by one having ordinary skill in the art that filtering may also be used to allow and/or prevent the distribution of specific signals. As such, and in some embodiments, each of the links 80a-b, 82, 84a-b and/or 86a-b may be a wideband digitally modulated optical interface, such as fiber optic cable. Thus, each master unit 46a and/or 46b may be configured to digitize their respective master unit signals 72 and/or 74 and output those digital signals for their respective remote units 42a-42d and/or 42e-h. These digital output signals may, in some embodiments, be time division multiplexed into frames and converted into a serial stream. The remote units 42a-42d and/or 42e-h, in turn, may be configured to receive the digital output signals from their respective master units 46a and/or 46b, convert the digital output signals into electrical signals, if necessary de-frame various time slots and/or de-serialize the electrical signals, and transmit the electrical signals via their local antenna 44 to at least one wireless unit 90.

[0027] The remote units 42a-h are configured to send and/or receive digital RF voice and/or data signals to and/or from a wireless unit 90 via their local antennas 44. As discussed below, depending on how the remote units are coupled to the master units, the remote units 42b, 42d and/or 42f may also be configured to receive a digital signal from remote units 42a, 42c and/or 42e, respectively, which precede it in a chain. This digital signal between remote units may contain signals from the wireless unit 90 received by the preceding remote units 42a, 42c and/or 42e. The digital signal may then be combined with another signal received by the remote units 42a, 42c and/or 42e. As such, digital wireless signals from wireless units 90 may be combined and/or transmitted back to a respective master unit 46a and/or 46b. The master units 46a and/or 46b may then convert a signal from its respective remote units 42a-d and/or 42e-h from an optical signal to an electrical signal and send the electrical signal to the SISO BTSs 54a-b and MIMO BTS 58, which may be configured to detect and receive their respective portions thereof. Alternatively, the master units 46a and/or 46b may then convert a signal from its respective remote units 42a-d and/or 42e-h from an optical signal to an electrical signal, separate the electrical signal into a plurality of electrical signals in a plurality of bands corresponding to those utilized by the SISO BTSs 54a-b and MIMO BTS 58, convert the plurality of electrical signals into a plurality of analog signals, and send the plurality of analog signals to the SISO BTSs 54a-b and/or MIMO BTS 58.

[0028] As illustrated in FIG. 2B, by way of example, a master unit 46a-b may be selectively connected to respective remote units 42a-h in a number of ways. For example, master unit 46a is illustrated as connected to remote units 42a-b through half-duplex link 80a for uplink to the remote units 42a-b and half-duplex link 80b for downlink. However, master unit 46a is illustrated as connected to remote units 42c-d through full-duplex link 82. Similarly, master unit 46b is illustrated as connected to remote units 42e-f through half-duplex link 84a for uplink to the remote units 42e-f and half-duplex link 84b for downlink. However, master unit 46b is illustrated as connected to remote unit 42g through full-duplex link 86a and connected to remote unit 42h through full-duplex link 86b. As such, in a full-duplex link, the uplink signals and downlink signals are carried on different wavelengths and a wavelength division multiplexer (“WDM”) is employed to combine and/or split the two optical signals at the master units 46a-b and remote units 42a-h. Alternatively, the master units 46a-b and remote units 42a-h may communicate through a different transceiver for high data rate media such as coax cable, twisted pair copper wires, free space RF or optics, or shared networks such as Ethernet, SONET, SDH, ATM and/or PDH, among others. As will be appreciated, one or more of the exemplary links, as illustrated in FIG. 2B, might be selected for coupling all of the remote units to the master units.

[0029] In some embodiments, the system 40 illustrated in FIG. 2B may be selectively and dynamically utilized as a SISO system and/or a MIMO system. For example, if the hybrid coupler 52 is not activated, the signals from the SISO BTSs 54a-b may be transmitted to at least a portion of the remote units 42a-h and the system may be utilized similarly to a SISO system. In this manner, each of the remote units 42a-h communicates through at least two wireless frequencies that correspond to those used by the SISO BTSs 54a-b. However, when the hybrid coupler 52 is selectively activated, the signals from the SISO BTSs 54a-b may be combined with the combined MIMO output signals 68, 70 such that each remote unit 42a-h communicates the signals from the SISO BTSs 54a-b through at least two wireless frequencies that correspond to those used by the SISO BTSs 54a-b and communicates both or all of the MIMO signals. Thus, selective activation of the hybrid coupler 52 results in dynamically reconfiguring the system 40 from a SISO mode of operation to a MIMO mode of operation. As such, the system 40 may be used as an indoor MIMO system that is configured to handle a WiMAX Frame Preamble and/or LTE P-SS (Primary Synchronization Signal) that either are, or optionally can be, transmitted by only one of the MIMO BTS antennas.

[0030] Thus, portions of the first and second signals 62 and 64 from the MIMO BTS 58 may be cross-coupled and combined and sent to all the remote units 42a-h without affecting the MIMO operation thereof. For example, each remote unit 42a-h of the system 40 may be configured to transmit both (or all) data streams from the MIMO BTS 58 and its antennas 31, 32 (e.g., the output signal 68 or the output signal 70) along with the combined SISO BTS signals 56a-b.

[0031] FIG. 3 illustrates a schematic view of a wireless communication system 100 similar to the system 40 of FIG. 2B, but that includes a plurality of remote units 42a-d used in an indoor environment 104. In particular, FIG. 3 illustrates that remote units 42a-b coupled to the master unit 46a may be positioned about the indoor environment 104 such that their signals are not substantially overlapping. Remote units 42c-d may be similarly positioned. As such, a wireless device 90 in a portion of the environment 104 may be able to receive signals from two remote units (as illustrated in FIG. 3, wireless device 90 receives signals from remote units 42a and 42d). As such, MIMO spatial multiplexing may be exploited, as the wireless device 90 is capable of receiving two non-identical signals from two remote units 42a and 42d fed by two different master units 46a and 46b, respectively.

[0032] Referring to FIG. 3, by incorporating a hybrid coupler in the distributed antenna system in accordance with the invention, all of the MIMO signals (in this case, both MIMO signals) can be cross-coupled and sent to all of the remote units without affecting the MIMO operation. Each remote unit can transmit both the MIMO parallel data streams without producing inter-stream interference because there are 90° of phase shifting between them. That is, a distributed MIMO concept is split in two parallel distributed MIMO systems. The first one is “in phase”, while the second one is “90° phase shifted”. Of course, in order to exploit the MIMO spatial multiplexing, it is necessary that the wireless device 90 receive substantial power contributions from at least two of the remote units that are fed by different master units. Therefore, as such, it is desirable that the wireless device 90 receives power from more than one of the two remote units, for example 42a, 42d, that are coupled with different master units, in order to maintain MIMO spatial multiplexing.

[0033] One benefit of the invention is that it solves problems noted above wherein the remote units transmit signals associated with only one of the MIMO base station antennas. Similarly, performance impairments between transmitted parallel data streams that may affect the wireless unit 90 when located closer to a specific remote unit 42a-d may be addressed, as received power levels for two signals from two remote units 42a-d are typically similar for most locations in the indoor environment 104, thus increasing data throughput. This issue is often referred to as the “near-far problem” affecting a distributed MIMO system with remote units transmitting only a single data stream. This issue is addressed as discussed herein below using a 3 dB 90° Hybrid coupler.

[0034] Another particular benefit of the present invention is the ability to provide deployment of a MIMO system within an existing distributed antenna infrastructure that is originally implemented for a SISO system. The present invention may also operate with a selective coupling or dynamic switching between a SISO and a MIMO operation mode that is performed by a MIMO base station. Furthermore, when the MIMO base station operates in downlink spatial multiplexing mode, the invention provides the performance equalization related to the transmitted parallel data streams. That is, as noted, the 90° 3 dB hybrid coupler is used in order to solve the “near-far problem”. The inter-stream cross-coupling performed through the Hybrid Coupler acts similarly to or as a substitute for the MIMO pre-coding as specified by the 3GPP LTE standard in order to address the potential mismatch in performance between the two data streams. That is, the pre-coding provided by the invention is intended to equalize the performance (like bit error rate, error vector magnitude, etc.) of two data streams experiencing different channel conditions. In case of the “near-far problem” the two streams experience different channel path-losses. Furthermore for a proper operation of the LTE standard, it is mandatory that the pre-coding coding scheme is orthogonal so that the original symbols can be recovered at the receiver avoiding inter-stream interference. This condition is met by the 90° Hybrid Coupler input-output transfer function as discussed below in accordance with one aspect of the invention.

[0035] With reference to FIG. 8, a 90° 3 dB hybrid coupler is shown and acts as a “hardware” MIMO pre-coding circuit to compensate for possible performance impairment between the data streams (code-words) due to the “near-far problem”. The equation shown in FIG. 8 illustrates the input and output port relationship and the transfer function matrix of a 90° 3 dB hybrid coupler, respectively. As such, in accordance with an aspect of the invention, the transfer function matrix reflected in FIG. 8 can also be regarded as the MIMO pre-coding matrix of the 90° 3 dB hybrid coupler. FIG. 9 illustrates the location of the MIMO pre-coding block in a typical LTE physical channel processing stream. In accordance with an aspect of the present invention, the exploitation of a 90° 3 dB hybrid coupler into the LTE MIMO distributed antenna system as disclosed herein locates the performance of the pre-coding at the BTS antenna ports rather than in the BTS physical channel processing. Therefore the invention also represents a hardware improvement to the MIMO BTS scheduler circuitry which is responsible for pre-coding selection on the User Equipment's feedback basis.

[0036] In accordance with another aspect of the present invention, the hybrid coupler that is utilized in embodiments of the invention makes input signals orthogonal to each other. The device's reciprocity between the input ports 1, 2 and the output ports 3, 4 provides that the resulting transfer function matrix remains the same, even exchanging the input and output ports. This provides the invention with the ability to combine MIMO signals without affecting their capability to support spatial multiplexing.

[0037] FIGS. 4-6 illustrates an exemplary distributed antenna system for implementing embodiments of the invention. Focusing now on a master unit 46, FIG. 4 contains a detailed block diagram of the master unit 46. Each master unit 46 may contain from one to six radio channels (hereinafter referred to as a “path”) 110, from one to four digitally modulated optical channels 112, a controller 114, a clock generator 116, and an Ethernet switch 118.

[0038] In one embodiment, each path, such as 110a, may be configured to handle a signal to and from SISO BTSs 54a-b and/or MIMO BTS 58, for example. For a FDD air interface, the paths 110a employ a combiner and a duplexer 120 to handle the uplink signal and the downlink signal. An RF downconverter 122 may amplify the received signal from the combiner/duplexer 120 to ensure that an A/D converter 124 is fully loaded. The RF downconverter 122 sets a center frequency of a band within the A/D converter pass band. The wideband A/D 124 digitizes the entire downlink band of the air interface to ensure all downlink channels are digitized. A resampler 126 converts the signal to a complex format, digitally downconverts the frequency band in some cases, decimates and filters the signal, and resamples it. This reduces the amount of data associated with a downlink signal, such as 128a, that has to be transferred over the optical lines and synchronizes the rate of the digitized data to the optical network bit rate.

[0039] The uplink section of the radio channel 110a sums 120 the uplink signals, such as signals 129a-d, for its assigned band from remote units 42 coupled to the master unit 46 after they are converted to an electrical signal. The summation 130 is resampled, interpolated to change to a different data rate in some cases, and upconverted by the resampler 132 and then converted to an analog form by the D/A converter 134. The RF upconverter 136 translates the center frequency of the analog signal to the appropriate frequency for the air interface and amplifies it. The amplified signal is applied to the combiner/duplexer 120 and is routed back to the SISO BTSs 54a-b and/or MIMO BTS 58.

[0040] In embodiments utilizing TDD air interfaces, the combiner and duplexer are replaced by a switching function 138 shown in FIG. 4 for example in radio channel 110b and detailed in FIG. 5. While the master unit 46 is receiving the downlink signal, a RF amplifier in the RF upconverter is disabled and a shunt switch in the switching function 138 may shunt the RF amplifier to ground to further reduce leakage. During intervals when the master unit 46 is sending the uplink signal to the base station 42, the RF amplifier is enabled, the shunt switch is opened and a series switch in the switching function 138 may be opened to protect the RF downconverter from damage due to high power levels. The switch control timing 144 is determined by a master unit controller 114 from the downlink signal 128b. Additionally, a formatter 146 may apply a data compression to reduce the redundant digital information included in the serial data stream before it is sent to the transmitter in an electro-optical transceiver 148. The compression may allow for saving bandwidth or for using a less costly transceiver with lower bit rate. The compressed serial data may be converted into an uncompressed data stream after being received on the opposite ends in the optical received of 148 by the receiver side formatter 146.

[0041] Each digitally modulated optical channel 112a-b is composed of a formatter 146 and an electro-optical transceiver 148. On the outgoing side, the formatter 146 blocks, into time division multiplexed frames, the digitized downlink signal 128a-b along with a customer Ethernet in Reduced Media Independent Interface (“RMII”) format 150a-b, operation and maintenance (“O&M”) data 152a-c and synchronization information. In other embodiments, other interfaces such as MII, RMII, GMII, SGMII, XGMII, among others may be used in place of the RMII interface. The framed data may be randomized by exclusive or'ing (XOR) it with the output of a linear feedback shift register to remove long strings of logic ones or zeros. Other known coding formats such as 8 bit/10 bit or 64 bit/66 bit coding may also be used, but may result in a decrease in efficiency in the use of the digital serial link. This digital data is then converted to a serial stream which is used to modulate an optical transmitter within the electro-optical transceiver 148. In a single fiber implementation, a wavelength division multiplexer (“WDM”) 149 may be employed to combine or split the two optical signals.

[0042] For incoming signals from the remote units 44, the electro-optical transceiver 148 converts the optical signal to an electrical signal. The formatter 146 phaselocks to the incoming bit stream and generates a bit clock that is phaselocked to the data rate and aligned with the serial data stream. The formatter 146 then converts the serial stream to a parallel digital data stream, de-randomizes it and performs frame synchronization. It then breaks out the digitized uplink signal for each band, buffers each band and routes the bands to the appropriate radio channel 110a, 110b, if necessary. Finally, the formatter 146 breaks out the buffers and O&M Ethernet data 152a-c and the user Ethernet data 150a-b and routes them to the controller 114 and the Ethernet switch 118, respectively.

[0043] The master unit controller 114 uses locally stored information and information from the O&M Ethernet data to configure and control the other blocks in the master unit 46. It also passes this information to the remote units 42 and reports status of the remote units 42 and the master unit 46 to the system controller 76. When a radio channel, such as 110b, is assigned to a TDD air interface, the master unit controller 114 also uses the corresponding downlink signal 128b to derive TDD switch control timing 144.

[0044] The system controller 76 generally has overall system control. The master unit controller 114 functions to configure individual modules as well as supervise individual modules. As part of the configuration and supervision functions, the master unit controller 114 is operable to determine the uplink/downlink switch timing in TDD systems by decoding the downlink signaling or acquiring it from a different source such as the time variant UL RSSI, or some base station clock signal provided from an external source. The downlink frame clock in TDMA systems may be determined and distributed by decoding the downlink signaling to allow time slot based functions such as uplink or downlink muting, uplink or downlink Received Signal Strength Indication (“RSSI”) measurements within time slots, uplink and downlink traffic analysis, etc. The master unit controller 114 may detect active channels in the RF spectrum to assist in or automatically configure the filter configuration in the resampler 126, 132. Optimal leveling of the individual signals in the resampler may also be determined by measurement of the RSSI of the various signals in the downlink RF band. A remote unit controller may perform similar tasks in the uplink of the remote unit 42.

[0045] The clock generator 116 may use a stable temperature compensated voltage controlled crystal (“TCVXO”) to generate stable clocks and reference signals 154 for master unit 46 functional blocks. Although, one of ordinary skill in the art will appreciate that other devices or crystals may also be used to generate clocking signals as long as they are capable of producing the stable clocks required by the system.

[0046] Focusing now on a remote unit 42, FIG. 5A and FIG. 5B contain a detailed block diagram of a remote unit 42 consistent with embodiments of the invention. Each unit 44 may contain from one to six radio channels 160, one or two DMOCs 162, a remote unit controller 164 and an Ethernet switch 166.

[0047] The DMOCs 162 may be designated as the downstream 168 and upstream channels 170. The downstream channel 168 is connected to a remote unit 42 that precedes this remote unit 42 in a daisy chain, if so configured. The upstream channel 170 is connected to a master unit 46 or another remote unit 42. The DMOC 162 functional blocks are similar to those in the master unit 46. Both consist of a formatter 172 and electro-optical transceiver 174. Outgoing data is buffered, formatted into frames, randomized, parallel to serial converted and used to modulate an optical transmitter in the electro-optical transceiver 174. Incoming data is converted from an optical to electrical format, bit synchronized, de-randomized, frame synchronized and converted to a parallel format. The various data types are then broken out buffered and distributed to other function blocks within the remote unit 42. In some embodiments, formatter 172 may implement compression and decompression schemes to reduce bandwidth over the digital optical link.

[0048] Radio channels in the remote unit 42 are functionally similar to those in the master unit 46. Each radio channel is configured to handle a single RF band. Unlike the master unit 46 radio channels 110, the remote unit 42 radio channels 160 are connected via a cross band coupler 176 to its antenna 44. For FDD air interfaces, the radio channels, such as radio channel 160a, employ a duplexer 178 to split the uplink and the downlink signal. Duplexers, cross-band combiners and couplers may be optional for some embodiments of either the master unit 46 or remote units 42. In these embodiments, additional antennas may replace the duplexer 178 and cross-coupler 176 in the remote units 42. Extra cables would be required in the master unit 46. A RF downconverter 180 amplifies the received uplink signal from the antenna 44 to ensure an A/D converter 182 is fully loaded and sets the center frequency of the band within the A/D converter pass band. The wideband A/D 182 digitizes the entire uplink band of the air interface to ensure all uplink channels are digitized. A resampler 184 converts the uplink signal to a complex format, digitally downconverts the signal in some cases, decimates and filters the signal, and resamples it with a multi-rate filter bank. This reduces the amount of data that has to be transferred over the optical links and synchronizes the rate of the digitized data to the optical network bit rate. The output of the resampler 184 is added to the uplink signals 186a from the downstream remote units 42 in summer 187. The summed uplink signal 188a for each band is then sent to a formatter 172 in the upstream channel 170 in the DMOC 162.

[0049] The downlink signal 190 for each band (190a, 190b) is interpolated and frequency shifted in the resampler 192. The group delay of individual spectral components can be adjusted via filters or delay elements in the resampler 192. The signal is then converted to an analog form by the D/A converter 194. The RF upconverter 196 translates the center frequency of the analog downlink band to the appropriate frequency for the air interface and amplifies it. The amplified signal is then applied to the antenna 44 and transmitted to a wireless unit 90.

[0050] For TDD air interfaces, the duplexer 178 is replaced by the switching function 138 shown in radio channel 160b and FIG. 5A. While the remote unit 42 is receiving the uplink, the RF power amplifier in the RF upconverter 196 is disabled and a shunt switch in the switching function 138 shunts the RF power amplifier to ground to further reduce leakage. When the remote unit 42 is transmitting the downlink signal, the RF power amplifier is enabled, the shunt switch is opened to permit the downlink signal to reach the antenna 44 and a series switch in the switching function 138 is opened to protect the RF downconverter 180 from damage due to high power levels. As with the master unit 46, the switch control timing 144 is determined by the controller 164 from the downlink signal 190a, 190b.

[0051] The clock generator 198 includes a voltage-controlled crystal oscillator (“VCXO”) that is phaselocked to the incoming serial data stream bit rate via a narrowband phaselocked loop (“PLL”). The VCXO output is split and is used as the frequency reference 200 for the local oscillators in each radio channel 160a-b, the sampling clocks for the A/D 182 and D/A 194 converters, and a clock for the other blocks in the remote unit 42. One of ordinary skill in the art will realize that the long term frequency accuracy should be good to ensure the local oscillators are on frequency and that the short term jitter levels should also be low to ensure that the jitter does not corrupt the A/D and D/A conversion processes. By phaselocking to the data rate of the optical link, which is derived from the stable TCVCXO in the master unit 46, the remote unit 42 does not require an expensive oven compensated oscillator or a GPS disciplining scheme to maintain long term frequency accuracy, thereby, making the more numerous remote units 42 less expensive. The use of a narrow band PLL and a crystal controlled oscillator may assist in reducing short term jitter for the A/D and D/A converter clocks. Using the recovered, jitter reduced clocks 202 to re-clock the transmit data in the optical links at each remote unit 42 reduces jitter accumulation which may assist in improving A/D and D/A converter clocks in the downstream remote units 42 and may assist in reducing the bit error rate (“BER”) of the optical communication channels 162.

[0052] The remote unit controller (“RUC”) 164 uses locally stored information and information from the O&M Ethernet to configure and control the other blocks in the remote unit 42. Downstream RMII 152d and upstream RMII 152e may also be supplied to the formatter 172. In addition, local O&M data 206 may be configured at a local O&M terminal 204. Remote unit 42 also passes this information to the up and downstream remote units 42 and/or master unit 46. The RUC 164 additionally uses the appropriate downlink signal to derive TDD switch control timing 144 when required.

[0053] In an alternate embodiment of the radio channel 160c utilized in a remote unit 42, the radio channel 160c may also employ digital pre-distortion to linearize the power amplifier. This embodiment of the radio channel 160c in a remote unit 42 is shown in the block diagrams of FIG. 6. In this embodiment, a third signal path may be added to one or more radio channels 160c. The third path couples off the downlink signal after power amplification and digitizes it. The signal from the antenna 44 is received in an RF downconverter 208, which amplifies the received signal to ensure an A/D converter 210 is fully loaded and sets the center frequency of the band within the A/D converter pass band. The wideband A/D 210 digitizes the entire uplink band of the air interface to ensure all uplink channels are digitized. The digitized signal is compared to a delayed version of the downlink signal in the digital pre-distortion unit 212 and the difference is used to adaptively adjust the gain and the phase of the signal prior to D/A conversion to correct for non-linearity in the power amplifier.

[0054] While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, a distributed antenna system consistent with embodiments of the invention may have more or fewer remote units 42, master units 46, summing circuits 48, RF combination networks 50, hybrid couplers 52, SISO BTSs 54, MIMO BTSs 58 and/or system controllers 76 than those illustrated. In particular, each MIMO BTS 58 may include more or fewer output ports 62 and/or 64.

[0055] Additionally, each master unit 46 may be connected to more or fewer remote units 42 than those illustrated. As such, a plurality of remote units 42 may be connected to each master unit 46 through two links and/or along a single link. Alternatively, each remote unit 42 may be connected to a master unit 46 through a dedicated link. In some embodiments, up to six remote units 42 may be connected in series from a master unit 46. As such, remote units 42 may be positioned to optimize coverage within a coverage area.

[0056] Furthermore, system 40 and/or 100 may not include summing circuits 48a-48b. As such, the master unit 46a may combine the combined SISO BTS signal 56a and first output signal 68, while the master unit 46b may combine the combined SISO BTS signal 56b and second output signal 70. Additionally, the system 40 may also not include RF combination network 50. As such, the master unit 46a may combine one or more signals from the SISO BTSs 54 and the first output signal 68, while the master unit 46b may combine one or more signals from the SISO BTSs 54 and the second output signal 70.

[0057] Moreover, and in some embodiments, the master unit controller 114 may measure a pilot signal strength of CDMA or Orthogonal Frequency-Division Multiplexing (“OFDM”) signals to properly set the level of the downlink signals, as the RSSI can vary at different capacity loading. The pilot signals generally remain constant with a configured ratio between pilot level and a maximum composite for full loading, the required headroom for the signals may be maintained. The master unit controller 114 may also measure and supervise the signal quality of the provided downlink channels. In case of signal degradation, an alarm may be set and the operator can focus on a base station (e.g., SISO or MIMO BTS) without having to troubleshoot the entire system 40 and/or 100.

[0058] In some embodiments, the master unit controller 114 determines the amount of channels for a narrowband base station standard such as Global System for Mobile communications (“GSM”). Together with the measurement of the Broadcast Control Channel (“BCCH”), which is constant in power, the proper headroom that is required for a multichannel subband may be determined and overdrive or underdrive conditions may be avoided. In other embodiments, the master unit controller 114 monitors the crest factor of a transmitted spectrum in the presence of multiple channels. The crest factor may provide input to the leveling of the transmit power or the power back-off of particular gain stages of the system. The configured headroom is generally higher than the measured crest factor to avoid signal degradation due to clipping or distortion. In addition, a crest factor reduction mechanism may be employed in the resampler in some of the embodiments to reduce the crest factor and make more efficient use of the RF power amplifier in the remote unit 42 or assist in reducing the number of required bits per sample that need to be transmitted over the link.

[0059] As illustrated in FIGS. 7A and 7B, the invention provides benefits in regard to the Uplink path of a MIMO communication system. Both WiMAX and LTE wireless standards encompass Uplink MIMO features. In particular the “Uplink Collaborative MIMO” is implemented in Mobile WiMAX, while “Uplink Multi-User MIMO” is the term adopted in LTE for indicating the same technique. The peculiarity of this MIMO scheme is to increase the total Uplink sector capacity by reusing time/frequency resources allocated to the different UEs (User Equipments) or mobile devices, rather than to boost the data rate per single user as for Downlink Single-User MIMO (Spatial Multiplexing).

[0060] FIG. 7A shows a MIMO BTS 300 in an outdoor scenario, which can coordinate the data reception from two different mobile devices A and B, equipped with a single transmitter (Tx) antenna each, and then allocate the same time/frequency resources to them. The decoding of their respective data streams is performed by the BTS through the same signal processing as for a Single-User MIMO situation. That is, the two data streams belonging to spatially separated users, rather than to a single user with two co-located Tx antennas, are spatially multiplexed. As a consequence the saved time/frequency resources can be allocated to more users in order to increase the total Uplink sector capacity. Finally the MIMO transmission might benefit from the fact that the two transmitters are largely separated leading to a consequent increase of the probability to have uncorrelated radio channels which is one important requirement for successful MIMO operation.

[0061] FIG. 7B highlights such a potential benefit in an indoor system. FIG. 7B illustrates a schematic view of at least a portion of a wireless communication system 220 somewhat similar to the system 100 of FIG. 3, but that does not show the plurality of SISO BTSs 54a-b, the RF combination network 50, and the summing circuits 48a-b. With respect to FIG. 3, like reference numerals are utilized in FIG. 7B where applicable. The system 220 of FIG. 7 includes a plurality of master units 46a-b that provide signals to respective remote units 42a-b with a single Rx antenna each and positioned in respective portions 208a-b of an indoor environment 224. In particular the indoor environment example is illustrated as two rooms 228a-b separated by a wall 230.

[0062] As illustrated in FIG. 7B, the respective portions 228a-b of the indoor environment 224 are somewhat electromagnetically isolated (e.g., low levels of a signal sent from a wireless device 232 (device A or B) in one portion 228 detected by the remote unit 42 of another portion 228). In specific embodiments, the respective portions 228a-b are separated by the partition or wall 230. This illustration shows the case of good Uplink power isolation between the different remote unites 42a-b and the related mobile devices 232a and 232b. Therefore the Uplink multi-user MIMO feature operates desirably because at the BTS antenna ports, the mutual interference of the signals from the two devices 232a-b only depends on the isolation provided by the indoor radio planning. Although isolation will be determined by the deployment of the remote units and the location of the users and mobile devices, indoor scenarios offer good isolation due to the presence of multiple walls and floors. Also, the hybrid coupler of the invention doesn't affect the BTS MIMO decoder since the signals from the mobile devices 232a-b are orthogonally cross-coupled to the BTS antenna ports thus avoiding their mutual interference. Therefore in another aspect of the invention, with two fully isolated groups of users served by two remote units connected to different Master Units, the Uplink Multi-User MIMO feature can achieve a complete reuse of the time/frequency resources of the BTS. As a consequence the number of users manageable in the Uplink path by the MIMO BTS would be increased and possibly doubled.

[0063] It will be appreciated that such an aspect of the invention might be in contrast to the feature discussed herein of maintaining a certain degree of signal coverage overlapping between Remote Units as requested by the Downlink Single-User MIMO when implemented through DAS. Therefore, for realizing both such advantages, a tradeoff would have to be considered and managed to balance the benefits of both the MIMO features. In this context the same 90° 3 dB Hybrid Coupler can be exploited both in Downlink and in Uplink paths of an indoor DAS for MIMO signals.

[0064] Thus, each remote unit 42a-b provides signals to, and receives signals from, respective wireless devices 232a-b that are present within those respective portions 228a-b. One benefit of this arrangement as noted is that uplink collaborative MIMO (for WiMAX) and/or uplink multi-user MIMO (for LTE) may be used to increase the total uplink capacity by reusing the time and/or frequency resources associated with the different wireless devices 232a-b.

[0065] The invention in its broader aspects is not limited to the specific details representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the applicants' general inventive concept. For example, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may be configured with an extension unit (not shown) disposed between a master unit 46 and its corresponding remote units 42. The extension unit may provide additional links for coupling a master unit 46 to additional remote units 42 and/or the extension unit may extend the range of coupling between a master unit 46 and remote units 42. Moreover, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may be configured with more or fewer remote units 12 or 42, master units 16 or 46, SISO BTSs 20 or 54, MIMO BTSs 30 or 58, system controllers 22 or 76, summing circuits 48, RF combination networks 50, and/or hybrid couplers 52, as well as support more or fewer wireless devices 26, 90, and/or 232 consistent with embodiments of the invention. Similarly, the system 10 of FIG. 2A, the system 40 of FIG. 2B, the system 100 of FIG. 3, and/or the system 220 of FIG. 7 may include a MIMO BTS 30 or 58 with more or fewer antennas 31 and/or 32, a hybrid coupler 52 with more or fewer ports, as well as a master unit 16 or 46 configured with more or fewer inputs or outputs consistent with embodiments of the invention.

[0066] Additionally, it will be appreciated that the indoor environments 104 and 224 of FIGS. 3 and 7B, respectively, are merely included to show operation of embodiments of the invention therewith, and that embodiments of the invention may be used with outdoor environments without departing from the scope of the applicants' general inventive concept. Moreover, one of ordinary skill in the art will appreciate that system 220 of FIG. 7B may include SISO BTSs 54a-b as well as the RF combination network 50 and summing circuits 48a-b consistent with alternative embodiments of the invention.

[0067] Furthermore, in some embodiments, the indoor environment 224 of FIG. 7B will be configured in other ways that just including partition 230. As such, the respective wireless devices 232a-b may be isolated in other ways.

[0068] Other modifications will be apparent to one of ordinary skill in the art. Therefore, the invention lies in the claims hereinafter appended.