Abstract:
At least two passive antenna arrays of an existing multicarrier base station transceiver system are supplemented by an auxiliary antenna array to provide a system and method of enhancing reverse link sensitivity and reducing the effects of multipath fading in a wireless communication system without the need for modifications to the mobile station. The auxiliary antenna array provides a complementary transmit diversity path for the two passive antennas such that a predetermined time delay is implemented to a composite transmit signal and the delayed composite transmit signal is fed directly to the auxiliary antenna array. The delayed signal and the signals from the passive antenna arrays provide a reduced noise figure and time delay transmit diversity for the multicarrier BTS operation.

Description:
RELATED APPLICATION 
   This application is related to and claims priority from Provisional U.S. Application No. 60/330,505 filed Oct. 23, 2001. 

   BACKGROUND OF THE INVENTION 
   Reservation of Copyright 
   This patent document contains information subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent, as it appears in the U.S. Patent and Trademark Office files or records, but otherwise reserves all copyright rights whatsoever. 
   BACKGROUND 
   The invention generally relates to the field of cellular communications. More specifically, the present invention relates to time-delay transmit diversity enhancement of the forward link of a cellular communications system. 
   Wireless communications systems generally employ a plurality of base stations (BSs) which communicate with mobile stations (MSs) within a cell. The BSs are dispersed across a geographic service area and include at least one antenna and a base station transceiver system (BTS) to provide wireless service within the cell. The BTSs are coupled to base station controllers (BSCs) which may serve a plurality of BTSs. The BSC may also be coupled to a mobile switching center (MSC), capable of interfacing to the Public Switched Telephone Network (PSTN) and other BSCs. 
   As a MS moves around, transmitted signals on associated wireless channels are influenced by time-varying phenomena. Well-known communications phenomena such as shadowing, fading, Doppler shifting, and polarization mismatches may affect the communications link performance between a MS and a corresponding BS. 
   Digital wireless systems, which employ Code Division Multiple Access (CDMA), for example, may implement diversity transmission techniques to alleviate the effects of fading on a communications link between MSs and BSs. With diversity transmission, multiple replicas of the transmitted information are received at the receiving end. Each of the multiple replicas has an independent level of fading. By employing various receiver detection schemes (e.g., rake receiver) and exploiting the independent levels of fading, it is possible to recover a significant amount of any lost bit error-rate (BER) performance and improve overall system performance. 
   There are several diversity techniques that may be utilized in wireless CDMA systems. Such techniques include delay diversity, space diversity and polarization diversity schemes. Delay diversity relies on the property of minimum correlation between replicas of a direct-sequence (DS) spread-spectrum signal, delayed with respect to each other by more than the chip duration. A rake receiver recovers the delayed replicas of the signal to enhance the effective SNR into the detector. 
   CDMA systems are interference-limited. The number of users that can use the same spectrum and still have acceptable performance is determined by the total interference power of all users. Thus, the number of users that may be supported by each BTS is limited. In an effort to increase the capacity of CDMA systems, additional BSs may be added to increase the number of cells within the service area. However, because user traffic loads are often concentrated within small geographic areas, even with the addition of BSs, there may still be some cells that remain overloaded while neighboring cells are under-loaded. To alleviate such overcrowding in CDMA systems, multiple carriers may be assigned within a single service area to service the overlaying cells. With overlaying frequency coverage, some MSs are serviced by using one of the carrier frequencies while other MSs are serviced by relying on other carrier frequencies. 
   Generally, for such multicarrier operations, the BTS generates two or more carriers, which are then simultaneously transmitted by the BS. BSs that support multicarrier operations typically use two passive antennas per sector for transmission. Of the two passive antennas, one antenna has transmit and receive capabilities, while the other has only receive capabilities. In doing so, such a configuration allows receive diversity. Multicarrier BSs are limited in their ability to mitigate other factors that compromise communications link performance between MSs and BSs. 
   SUMMARY 
   Systems and methods are presented to overcome some of the limitations indicated above. For example, systems and methods are provided that enhance reverse link sensitivity and reduce the effects of multipath fading in wireless systems by implementing transmit diversity to a multicarrier base station system (BS). Such features may be incorporated in an existing multicarrier CDMA mobile network without the need for modifications to the mobile station. 
   In one illustrative embodiment, a multicarrier base station transceiver system (BTS) employs a plurality of antennas per sector, for example, three antennas per sector. In such an implementation, a supplemental antenna array supplements the two passive antennas of typical multicarrier BTS operations. The supplemental antenna array provides a transmit diversity path for the two passive antennas such that signals transmitted from the multicarrier BTS are sampled, combined, delayed and fed directly to the supplemental antenna array. With this configuration, the delayed signal from the supplemental antenna array as well as the signals from the two passive antennas achieve a reduced noise figure and provide a time delay transmit diversity for the multicarrier BTS operation, which improves the overall quality of the forward and reverse links of the wireless communication system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The inventions presented and claimed herein are further described in the detailed description which follows, with reference to the drawings, and by way of a non-limiting exemplary embodiment of the present invention, wherein like reference numerals represent similar parts of the present invention throughout the several views and wherein: 
       FIG. 1  is a schematic diagram of a portion of a multicarrier base station system  100 ; 
       FIG. 2A  is a schematic diagram of a portion of a base station  200  that supports multicarrier operation; 
       FIG. 2B  is a functional block diagram of a tower-top low noise amplifier (TT LNA) configuration  250 . 
       FIG. 3A  is a block diagram of a base station  300  that supports multicarrier operation; 
       FIG. 3B  depicts an active antenna array arrangement  350 ; and 
       FIG. 3C  shows an active antenna arrangement  375 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram of a portion of a multicarrier base station system  100 . BSs  100  equipped with BTSs  110  capable of multicarrier operations may utilize two passive antennas  106  and  108  to perform multicarrier transmissions as well as achieve reverse link or receive diversity. BS  100  may arrange passive antennas  106  and  108  to achieve receive (Rx) space diversity reception. 
   BTS  110  generates two or more carriers, which are then simultaneously transmitted from BS  100 . The generated carriers are e.g., combined in two groups of non-adjacent carriers in accordance with a minimum-loss combining scheme. The groups of non-adjacent carriers are illustrated in  FIG. 1  as Carriers 1+3 and Carriers 2+4. Each of the non-adjacent groups is forwarded via duplexers,  102  and  104 , which enables simultaneous transmission and reception through each of the two passive antennas. 
   BS  100  lacks forward link or transmit delay diversity and, thus, does little to reduce fading effects on the forward link. Moreover, the reverse link component chain in BS  100  is susceptible to signal noise contributions inherent in such configurations. 
   As will be described in greater detail below, a BS supporting MC operations may be configured with a supplemental antenna and a delay mechanism to provide transmit delay diversity on the forward link via a 3-antenna configuration. This 3-antenna configuration may also exploit shared components to achieve greater reverse link sensitivity, resulting in e.g., reduced MS transmit power in some cells. 
     FIG. 2A  is a schematic diagram of a portion of a multi carrier base station (MC BS)  200  that supports multicarrier operation in accordance with an embodiment of the inventions. BS  200  incorporates an auxiliary antenna arrangement  220  in addition to two conventional passive antennas  218 ,  222 . Auxiliary antenna arrangement  220  may be configured as a passive antenna arrangement, but it could otherwise be configured. Thus, BS  200  may be implemented in an existing conventional MC BS system having two passive antennas  218 ,  222 . 
   The auxiliary passive antenna array  220  may be positioned so as to employ space diversity with respect to each of the two passive antennas  218 ,  222 . The distance between each of passive antennas  218 ,  222  and passive antenna  220  depends on the frequency of operation and can be determined using methods known in the art. For example, the distance required for space diversity for a large cell may be  10  wavelengths of the transmitted signal. Auxiliary passive antenna array  220  transmits a delayed signal version for all the channels and, thus, also provides transmit diversity for the two passive antennas  218 ,  222  and their respective channels. 
   During multicarrier operation of BS  200 , carriers generated by BTS  204  are combined in the forward link or transmit direction. This may be achieved by combining, for example, carriers  1  and  3  into the transmit chain terminating with passive antenna  218 , and combining carriers  2  and  4  into the transmit chain terminating with passive antenna  222 . The transmit and receive signals of MC BTS  204  pass through duplexers  206 ,  208  which may be located internally within the MC BTS  204 , but do not have to be so located. Generally, transmit signals emanating from duplexers  206 ,  208  are relatively high-powered and are sampled and combined by a directional coupling and combining mechanism  210  to produce a composite transmit signal. The composite transmit signal is delayed a predetermined amount by TD unit  212  and then amplified by high power amplification device  214  to produce a delayed composite transmit signal. 
   It will be appreciated that a TT LNA may be used, for example, to enhance reverse link reception quality by improving the effective noise figure (NF) of the receive paths of passive antennas  218  and  222 . As such, each of the passive antennas  218  and  222  used for receiving may be equipped with tower-top low-noise amplifiers (TT LNAs)  224  and  226  which may be commercial-off-the-shelf (COTS) items. 
     FIG. 2B  illustrates a functional block diagram of TT LNA configuration  250  that may be used in the embodiment of  FIG. 2A . As indicated in  FIG. 2B , LNA  265  is isolated from the high-power transmit signal by filters  255 ,  260 , while the transmit signal reaches the antenna unaffected by the LNA  265 . Tx filter  270 , typically a bandpass filter, provides a low loss path in the transmit direction. In situations when it is necessary to block reverse injected strong or nearby interference, the TT LNAs of the existing passive antennas  218  and  222  may equipped with an optional isolator  275 . 
   Returning to  FIG. 2A , high power amplifier  214  may be located next to the MC BTS  204 , possibly as an indoor unit, or high power amplifier unit  214  may be placed at the tower base or tower top next to the TT LNA units. 
   As described above, the time delay transmit diversity scheme of the present invention may be applied to an existing multicarrier BTS to enhance both forward and reverse link signal quality in a tower-top passive antenna configuration, as illustrated in  FIG. 2A , or for an active antenna configuration, as illustrated in  FIG. 3A . 
     FIG. 3A  illustrates a MC BS  300  in accordance with another embodiment of the inventions. Much like the first embodiment of MC BS  200 , BS  300 , capable of MC operations, may be configured with a supplemental antenna and a delay mechanism to provide transmit delay diversity on the forward link via the 3-antenna configuration. The 3-antenna configuration may also exploit shared components to achieve greater reverse link sensitivity. The factors concerning the location of the active antenna array  320  with respect to passive antennas  322 ,  318  are similar to those discussed above with regard to placement of the passive antenna arrangement  220  with respect to passive antennas  218  and  222 . 
   As indicated in  FIG. 3A , BS  300  incorporates a supplemental antenna arrangement such as active antenna array  320  in addition to the two conventional passive antennas  318 ,  322 . Supplemental antenna arrangement  320  may be configured as an active antenna array, as shown, but does not necessarily have to be so configured. Active antenna array  320  receives time-delayed replicas of the original transmit signals generated by MC BTS  302 . Also, passive antennas  318 ,  322  radiate the original transmit signals. Thus, active antenna array  320  provides a transmit diversity path for passive antennas  318  and  322 . Further, the combination of the time-delayed and original transmit signals due to the use of active antenna array  320  enhances the forward link at the MSs for all the carriers of the BTS, and enhances the reverse link by providing pre-amplification to all the antennas with additional delay processing performed at ICU  332  that effectively modifies the receive path for three-branch diversity reception. 
     FIG. 3B  depicts an example of an active antenna array arrangement. As depicted in  FIG. 3B , antenna array  350  comprises a combination of two active transmit antenna elements  355 A,  355 B and two active receive antenna elements  360 A,  360 B, arranged in a single vertical (columnar) array. The two active transmit antenna elements  355 A,  355 B and two active receive antenna elements  360 A,  360 B, are preferably printed elemental radiators having a multi-layer configuration &amp; sealed by an epoxy-fiberglass radome. By incorporating separate transmit antenna elements  355 A,  355 B and receive antenna elements  360 A,  360 B within a single array, the BS is capable of achieving full transmission and reception functionality for wireless operations while eliminating the need for independent transmission and reception antenna arrays. In doing so, antenna array  350  achieves full BS antenna array functionality. 
   Moreover, the transmit and receive elements are spatially separated to avoid intermodulation effects as well as allowing for flexibility in BS transmission and reception optimization schemes, such as, for example, independent gain control and beam-shaping. 
     FIG. 3B  further illustrates that, within the vertical arrangement, the antenna elements are disposed in an alternating fashion such that a first transmit antenna element  355 A is followed by a first receive antenna element  360 A and a second transmit antenna element  355 B is followed by a second receive antenna element  360 B. The interleaving of the transmit  355 A,  355 B and receive antenna elements  360 A,  360 B within the array enables the optimal vertical separation distance S to be established. Optimal vertical separation distance S is the vertical distance between like antenna elements which, for a given frequency, maximizes the main lobe gain of a signal while minimizing the contribution of minor lobes. The optimal vertical separation distance S can vary. For example, in PCS arrays, S may be from 0.70λ to 0.95λ. 
   The specific arrangement of antenna array  350  may be modified to provide redundancy or otherwise enhance the attributes and characteristics of the array configuration. For example, antenna array  350  may be augmented by stacking combinations of the array elements to achieve antenna elements arranged in an 8×1, 12×1, or 16×1 array configuration, as illustrated in  FIG. 3C . 
   Returning to  FIG. 3A , MC BS  300  combines the carriers generated by BTS  302  in the forward link or transmit direction. This may be achieved by combining, for example, carriers  1  and  3  into the transmit chain terminating with passive antenna  322 , and carriers  2  and  4  into the transmit chain terminating with passive antenna  318 . Each of the two combined transmit signals, Tx 13  and Tx 24 , may pass through internal duplexers  304 ,  306  or external duplexers  312 ,  314  respectively, with one of the two Rx signals. Thus, the BTS  302  has two input/output (I/ 0 ) ports  308 ,  310  with each supporting Tx/Rx functionality. 
   The signal from ports  308  and  310  may generally be a high-power MC composite signal that is passed through an external duplexer  312 ,  314  to separate the transmit and receive signals. External duplexers  312  and  314  may optionally be eliminated if Internal duplexers  304  and  306  are not utilized. The transmit signals from ports  308 ,  310  are sampled via a directional coupling and combining mechanism  316  and combined at low power to yield a composite multicarrier low-power signal. The directional coupling and combining mechanism  316  may have a value, for example, of 30 dB. The high-power original transmit signals from the external duplexers  312  and  314  are routed to passive antennas  318  and  322 . Prior to entering passive antennas  318  and  322 , the high-power original transmit signals pass through a pair of tower top diplexers  324  and  326  respectively. Tower top diplexers  324  and  326  separate the transmit and receive signals per passive antenna  318  and  322 . 
   A transmit isolator  328  and  330  may optionally be included as a part of the transmit signal flow from external duplexers  312  and  314 . Passive antennas  318  and  322  may suffer reverse injection from a nearby strong interference that may couple into the antenna and flow backwards through the tower top diplexers  326  and  324 , the long transmit cabling, and the directional coupling mechanism  316  (through the directivity defining the isolation between its output and the coupled port) that joins the coupled transmit signal from the multicarrier BTS and the duplexers  312  and  314 . 
   As an example of the use of transmit isolators  328  and  330  in an embodiment of the present invention, assume −20 dBm interference at passive antenna ports  322  and  318 , attenuated backwards through the tower-top duplexers  326  and  324  and cabling by 10 dB, and assume a directivity of 40 dB. This provides an interference level of −70 dBm at the transmit coupled port where the desired transmit signal may appear as 0 dBm (or higher). Thus, in this example, the undesired interference will be 70 dBm below the desired transmit signal, which may be well below the regulatory limits for spurious transmit signals. Nevertheless, as a precaution, the inclusion of an isolator in each transmit path that leads into passive antennas  318 ,  322 , further attenuates any uncontrolled strong interference e.g., from a BS belonging to a different mobile network that may appear as a reverse injection signal into the transmit path. 
   The combined low-power composite multicarrier transmit signal from the directional coupling mechanism  316  is passed into an interface and control unit (ICU)  332 . The ICU  332  delays the composite multicarrier transmit signal by, for example, approximately 2 microseconds. The delayed signal is then pre-amplified by a transmit active bias t-connector (TxABT) board contained within the ICU  332 . ICU  332  may include, for example, slots labeled RD, R and TD. RD includes, for example, two receive active bias t-connectors (RxABTs), a receive surface acoustic wave (Rx SAW) delay, and a combiner. TD includes, for example, a TxABT and a transmit surface acoustic wave delay (Tx SAW). From the ICU  332  TD slot, the delayed and pre-amplified transmit signal is fed to active antenna array  320 . Thus, the signal at active antenna array  320  is transmitted as a delayed version of the transmit signals transmitted from each of passive antennas  318  and  322 . 
   For the Rx signals, each of the two passive antenna array ports may be fed through tower-top diplexers  326  and  324 . The two Rx signals from passive antennas  318  and  322  may be fed into the active antenna array  320  auxiliary Rx inputs, and are pre-amplified by dedicated low-noise amplifiers (LNAs) that are a part of the active antenna array  320  configuration. This greatly enhances the effective noise-figure (NF) of the Rx paths from passive antennas  318 ,  322 , and allows the use of thin, low-cost RF cabling from active antenna array  320  down to ICU  332  and the MC BTS  302 . In addition to the two Rx signals from passive antennas  318  and  322 , active antenna array  320  provides an additional Rx signal that is also pre-amplified by a LNA. Thus, there are three independent Rx signals fed from the two passive  322 ,  318  and one active antenna  320  arrays. Each of the three Rx signals are pre-amplified and fed into the ICU  332 , where they are conditioned by a receive active bias t-connector (RxABT) board, employing a total of 3 slots. 
   As indicated in  FIG. 3A , the three signals fed into ICU  332  enter slots RD and R. Two of the three Rx signals are combined. A predetermined delay of, for example, approximately 2 microseconds, is introduced to one of the two combined Rx signals prior to being combined with the second Rx signal. Thus, the RD slot contains, for example, the combination of one Rx Delay unit, two RxABT units, and a 2:1 Rx combiner unit. 
   As a result of this implementation of the present embodiment, ICU  332  outputs two Rx signals to the two ports of the MC BTS, thereby creating 3-branch diversity reception. The Rx output signals from ICU  332  are first fed to duplexers  312 ,  314  prior to passing into the MC BTS  302 . MC BTS  302  serves those two Rx signals and performs optimal Rx diversity processing for all carriers involved. 
   The embodiment of the present invention as illustrated in  FIG. 3A  enhances the overall system performance of both the forward and reverse links while utilizing the high-power transmit function of the MC BTS  302  without removing or deactivating any parts of the two existing passive antennas  318 ,  322  supported by the MC BTS  302 . 
   While the invention has been described with reference to the certain illustrated embodiments, the words that have been used herein are words of description, rather than words or limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and spirit of the invention in its aspects. Although the invention has been described herein with reference particular structures, acts, and materials, the invention is not to be limited to the particulars disclosed, but rather extends to all equivalent structures, acts, and materials, such as are within the scope of the appended claims.