Abstract:
A base station and receiver system for use in a base station which achieves enhanced dependability by logically separating the diversity reception paths into different failure paths. In one embodiment, the receiver system includes a first diversity reception path for receiving a first radio signal and a second diversity reception path for receiving a second radio signal. The first and second radio signals may be amplitude and phase shifted versions of the same information signal according to well known principles of diversity reception. At least one demodulator diversity combines the first and second radio signals and demodulates the diversity combined first and second radio signals. Additionally, the first and second diversity paths are logically separated into different failure paths.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to wireless receivers and receive paths in a base station. More specifically, the present invention relates to an improved receiver system architecture for wireless base stations which achieves enhanced dependability by separating diversity reception paths. 
     II. Description of the Related Art 
     In the field of wireless telecommunications, such as various cellular, Personal Communication Services (PCS), and Wireless Local Loop (WLL) communication systems, many different communication standards exist. For example, Code-Division Multiple Access (CDMA) digital communications may be governed by either Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) Interim Standard IS-95 (series) for cellular systems, or by ANSI J-STD-008 for PCS systems. Additionally, Time-Division Multiple Access (TDMA) digital communications may be governed by the TIA/EIA IS-54, or by the European standard Global System for Mobile Communications (GSM). Furthermore, analog FM-based communications systems may be governed by the Advanced Mobile Phone System (AMPS) standard or a related standard such as N-AMPS. Other wireless communication standards also exist for both digital and analog modulation. 
     According to any one of the above standards, wireless base stations communicate signals to one or more wireless mobile stations, such as cellular phones, PCS phones, or WLL phones. The wireless base stations primarily serve as the wireless “gateway” to the telephone system. In general, the wireless base station will be in communication with many mobile stations at one time. 
     The ability of the base station to operate when an internal software, hardware or other failure occurs is inherent to the base station architecture. The ability of the base station to continue to operate, either through “switching-in” additional backup or properly working components or by operating in a “reduced capacity” mode, is a measure of how well the base station architecture was designed. 
     For wireless communication systems, the system designer strives to design a base station architecture which is both cost-effective and highly reliable. One aspect of this is when a failure occurs at the base station, it does not result in loss of communications with the many mobile stations it may be serving. As a result, the system designer strives to connect the various base station components: front ends, receivers, demodulators, etc., in a manner which provides the best system reliability while still maintaining good performance, low cost, small size, low complexity, high degree of modularity, etc. 
     Wireless service providers who purchase and operate the base stations often specify a Mean Time Between Failure (MTBF) which represents the average amount of “downtime” that is tolerable. Often, this MTBF will be expressed as a total allowable downtime per year. “Downtime” is frequently defined as when the base station is unable to communicate at all with any mobile stations. Most service providers are keenly aware of this downtime because it results in a complete loss of revenues from that base station for the duration of the outage. As a result, a service provider will generally prefer that if a base station subsystem or component fails, that failure should affect the operation of the base station in the least significant way. Thus, reduced capacity modes of operation or partial degradations in service are strongly preferred over total loss of service. 
     A common base station architecture  100  which does not have optimum redundancy is shown in FIG.  1 . In FIG. 1, a pair of antennas  102 A,  102 B capture RF signals and provide them to RF front end  104 . Antennas  102 A,  102 B may be used for diversity reception, a well-known receiving technique in which the signal of interest is better received and processed by virtue of having two antennas receiving signals which can be compared and/or combined. 
     RF front end  104  typically comprises various bandpass filters and low-noise amplifiers which perform some initial frequency selection and signal amplification. RF front end  104  outputs two amplified signals  106 A,  106 B which correspond to antennas  102 A and  102 B, respectively. Receiver  108  receives, downconverts, and performs intermediate-frequency (IF) processing on the amplified signals  106 A,  106 B, and generates received signals  110 A and  110 B which correspond to antennas  102 A and  102 B, respectively. Demodulators  112 A- 112 N demodulate and perform IF and/or baseband processing on the signals  110 A,  110 B, thereby recovering the signal of interest from the RF signals received by antennas  102 A,  102 B. The architecture of FIG. 1 may be generalized to multiple receive paths, one for each sector being served by the base station. 
     In the architecture of FIG. 1, the RF front end  104  and the receiver  108  are single points of failure. That is to say that when either RF front end  104  or receiver  108  fails for any reason, it breaks the receive path from antennas  102 A,  102 B to demodulators  112 A- 112 N. Thus, any failure of RF front end  104  or receiver  108  will result in total loss of service for the base station employing the architecture  100  of FIG. 1. A single failure path defined by RF front end  104  and receiver  108  exists whereby failure of any unit in the failure path will result in failure of the entire reception path. Namely, RF front end  104  and receiver  108  are both in the same diversity reception path and also in the same failure path. 
     A common improvement made to the base station architecture of FIG. 1 is to provide a separate, redundant receive path which can be switched-in when the primary receive path fails. This is implemented by providing duplicate components such as a duplicate receiver  109  coupled by bypass switches  107 ,  111  which connect RF front end  104  and demodulators  112 A- 112 N to the duplicate receiver  109  when the primary receiver  108  fails. This is often referred to as providing “N+1 redundancy” where there are N primary operating components and 1 duplicate component in standby that can be switched in to take the place of any one of the N primary operating components when there is a failure. Note also that bypass switch  107  could be placed before the RF front end  104 , and a redundant RF front end (not shown) could also be switched in. 
     In addition to the increased cost, size and complexity of providing duplicate components for the N+1 redundancy, the bypass switches  107 ,  111  introduced in the receive path can introduce further undesirable signal level losses, thereby degrading the receive path performance. For example, a typical signal level loss incurred when introducing a switch matrix into the receive path is approximately 0.2 dB to 0.5 dB. This can be very significant when the receive path noise figure is typically in the 3 dB to 6 dB range. In addition, the control circuitry hardware and software (not shown) needed to detect a failure and control the switches also adds complexity, cost, size, and power dissipation to the base station. One can also call into question the reliability of the switches themselves. 
     What is needed is a base station architecture which improves the overall base station reliability without adding significant complexity or cost. 
     SUMMARY OF THE INVENTION 
     The present invention is a novel and improved base station and receiver system for use in a base station which achieves enhanced dependability by logically separating the diversity reception paths into different failure paths. In one embodiment, the receiver system includes a first diversity reception path for receiving a first radio signal and a second diversity reception path for receiving a second radio signal. The first and second radio signals may be amplitude and phase shifted versions of the same information signal according to well-known principles of diversity reception. At least one demodulator compares and/or combines the first and second radio signals in a diversity reception manner. But the first and second diversity paths are logically separated into different failure paths. The receiver system may further comprise a distribution bus which provides the received first radio signal and the received second radio signal to the demodulator. 
     In this embodiment, the first and second diversity reception paths may each comprise first and second diversity antennas and first and second diversity receivers. The first diversity receiver is coupled to an output of the first diversity antenna and the second diversity receiver is coupled to an output of the second diversity antenna. Furthermore, a first RF front end circuit may be coupled to an output of the first diversity antenna, and a second RF front end circuit may be coupled to an output of the second diversity antenna. The first and second RF front end circuits filter and amplify signals received by the first and second diversity antennas. 
     In an exemplary embodiment, the first and second receivers generate in-phase and quadrature samples of signals received by the first and second diversity antennas. Other embodiments generate other received signal formats. 
     The receiver system described above is useful for application in a wireless base station. In particular, the receiver system described above, having a first diversity antenna and a second diversity antenna, may be used in a base station having a single or plurality of sectors with each sector supporting a single or plurality of frequency assignments. The base station may also have many first and second diversity receivers, with the first diversity receivers coupled to an output of each of the first diversity antennas and the second diversity receivers coupled to an output of each of the second diversity antennas, i.e. many diversity receivers sharing one or more common diversity specific antennas. Again, the first and second diversity receivers are logically separated into different failure paths. Additionally, each of the first and second diversity receivers may handle many frequency assignments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: 
     FIG. 1 is a functional block diagram of a prior art base station wherein the diversity reception paths are logically combined in the same failure path; and 
     FIG. 2 is a functional block diagram of a first embodiment of the base station of the present invention in which diversity reception paths are logically separated into separate failure paths. 
     FIG. 3 is a functional block diagram of a second embodiment of the base station of the present invention in which diversity reception paths are logically separated into separate failure paths. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various embodiments of the present invention are applicable to all wireless base stations which utilize diversity reception techniques. For clarity and simplicity, the embodiments of the present invention will be disclosed with reference to an exemplary Code-Division Multiple Access (CDMA) wireless communication system. However, it is understood that the present invention is equally applicable to wireless communication systems using other modulation techniques whether digital or analog in nature. The present invention pertains to optimally designing the architecture of a base station that employs receive diversity antennas to allow enhanced redundancy on the receive path without additional hardware cost and without direct impact on the normal receive path operating performance. 
     The term “soft redundancy” implies that when a failure does occur in a base station architecture, the failure results in reduced capacity or reduced coverage and not the complete loss of communication with mobile stations in the affected sector. The present invention achieves soft redundancy on the receive path without adversely affecting the receiver path gain and noise figure and without adding the duplicate hardware, software, and control circuitry needed to effect N+1 redundancy. 
     The present invention physically separates the diversity receive paths for each base station sector on different physical components. By keeping the diversity receive paths physically separate, the present invention obtains soft redundancy without additional hardware, software, size or power dissipation. Physical separation of the diversity receive paths may include physically separate hardware, software, timing signals and other control signals into different failure paths. It further may include distributing some hardware components, software modules and control signals throughout the base station in a manner that still maintains a soft redundancy architecture. 
     Referring now to FIG. 2, a functional block diagram of an exemplary embodiment of the base station architecture  200  of the present invention is shown. In FIG. 2, a three-sectored base station architecture  200  is illustrated. However, the present invention is equally applicable to base stations with more or fewer than three sectors. The present invention is also equally applicable to base stations with one or many frequency assignments. Each sector of the three-sectored base station includes associated diversity antennas and RF front end components. Antennas  202 A,  202 B belong to a first sector, antennas  204 A,  204 B belong to a second sector, and antennas  206 A,  206 B belong to a third sector. Each pair of antennas  202 A,  202 B;  204 A,  204 B; and  206 A,  206 B are arranged in a diversity reception architecture whereby a first antenna in each antenna pair receives a version of the information signal which may be shifted in amplitude and phase from the version received by the second antenna in the same antenna pair due to the physical separation between the first and second antennas. 
     Each antenna is coupled to a respective RF front end. Thus, antenna  202 A is coupled to RF front end  208 A, and antenna  202 B is coupled to RF front end  208 B for the first sector. Antenna  204 A is coupled to RF front end  210 A, and antenna  204 B is coupled to RF front end  210 B for the second sector. Finally, antenna  206 A is coupled to RF front end  212 A, and antenna  206 B is coupled to RF front end  212 B for the third sector. Similar to RF front end  104  of FIG. 1, RF front ends  208 A,  208 B,  210 A,  210 B,  212 A, and  212 B may comprise bandpass filters and low-noise amplifiers as are known in the art. The present invention is not limited by the construction of RF front ends  208 A,  208 B,  210 A,  210 B,  212 A, and  212 B. However, it should be noted that in contrast to the single point of failure RF front end  104  of FIG. 1, the RF front ends  208 A,  208 B,  210 A,  210 B,  212 A, and  212 B of FIG. 2 are each coupled to only one of the respective antennas  202 A,  202 B,  204 A,  204 B,  206 A,  206 B. Specifically, RF front ends  208 A,  208 B,  210 A,  210 B,  212 A, and  212 B are preferably physically separate units, each being separately field-replaceable upon failure without disturbing any of the other RF front ends. Thus, when one of the RF front ends  208 A,  208 B,  210 A,  210 B,  212 A, or  212 B fails, it will have no effect on the continuing operation of the remainder of the RF front ends. Although not depicted in FIG. 2 or  3 , the present invention also contemplates grouping the RF front ends  208 A- 212 B by diversity. This would involve merely routing all diversity A antennas  202 A,  204 A,  206 A to one RF front end, and all diversity B antennas  202 B,  204 B, and  206 B to another RF front end in accordance with the principles described herein. 
     In the illustration of FIG. 2, antennas  202 A,  204 A, and  206 A shall be referred to as the “diversity A” antennas, and antennas  202 B,  204 B, and  206 B shall be referred to as the “diversity B” antennas. The output of each of the diversity A antennas  202 A,  204 A,  206 A is coupled through its respective RF front end  208 A,  210 A,  212 A to a first receiver  214 A which shall be referred to as the “diversity A” receiver. Similarly, the output of each of the diversity B antennas  202 B,  204 B,  206 B is coupled through its respective RF front end  208 B,  210 B,  212 B to a second receiver  214 B which shall be referred to as the “diversity B” receiver. Receivers  214 A and  214 B are similar to receiver  108  of FIG. 1 in that they downconvert and perform IF processing on signals received by the various antennas  202 A,  202 B,  204 A,  204 B,  206 A, and  206 B. However, in contrast to the single point of failure receiver  108  of FIG. 1, the receivers  214 A,  214 B each process only one of the two sets of diversity channels. Specifically, the diversity A receiver  214 A and the diversity B receiver  214 B are preferably physically separate units which are individually field replaceable without disturbing the operation of the other diversity receiver. Thus, when either diversity A receiver  214 A or diversity B receiver  214 B fails, it does not affect the continuing operation of the remaining receiver. 
     In the embodiment of FIG. 2, the output of each receiver  214 A,  214 B may be separately coupled to a distribution bus  216  which distributes the outputs of the receivers  214 A,  214 B to one or more of the demodulators  218 A- 218 N. The construction of distribution bus  216  may be different depending on the nature of the output from the receivers  214 A,  214 B. In the simplest embodiment; distribution bus  216  is merely a backplane-style arrangement of inputs and outputs which each route either one or both of the outputs of receivers  214 A,  214 B to one or more of the demodulators  218 A- 218 N. In another embodiment, distribution bus  216  may combine or multiplex the signals output from both receivers  214 A,  214 B onto a traditional multi-conductor bus for further distribution to the demodulators  218 A- 218 N. Many distribution bus architectures known in the art may be used for distribution bus  216 . The present invention is not limited by the specific construction of distribution bus  216 . 
     In an exemplary embodiment, the receivers  214 A,  214 B each produce in-phase (I) and quadrature (Q) digital samples of the received signals, and output these I/Q samples onto the distribution bus  216 . The I/Q samples may then be processed by one or more of the demodulators  218 A- 218 N in parallel. This provides even further inherent redundancy as many of the separate demodulator units  218 A- 218 N may all “listen” to the same I/Q sample data stream produced by the receivers  214 A,  214 B. Similar to the demodulators  112 A- 112 N of FIG. 1, demodulators  218 A- 218 N of FIG. 2 demodulate and perform baseband processing on the received signals. In an exemplary CDMA base station, the demodulators  218 A- 218 N demodulate I/Q samples produced by the receiver  214 A,  214 B in accordance with well-known principles. However, in other embodiments, receivers  214 A,  214 B may generate raw IF digital samples which have not been separated into I/Q components, or even raw IF analog waveforms which are then demodulated by the demodulators  218 A- 218 N. 
     As will be evident to one of ordinary skill in the art, the loss of any single antenna  202 A,  202 B,  204 A,  204 B,  206 A, or  206 B will result in merely the loss of one of the diversity channels for one of the three sectors. The remaining antennas will continue to operate properly and be unaffected by the failure. Likewise, the failure of any single RF front end  208 A,  208 B,  210 A,  210 B,  212 A, or  212 B will result in merely the loss of one of the diversity channels for one of the three sectors. 
     Additionally, the failure of either diversity receiver  214 A or  214 B will result merely in the loss of one of the diversity paths from each of the three sectors, leaving the other diversity path from each of the three sectors intact. As a result, the advantages of diversity reception (i.e., improved processing gain) will be lost for the affected sectors. However, the remaining diversity receiver will continue to operate properly, allowing a somewhat degraded mode of operation without total loss of service for any sector. 
     Thus, by keeping the diversity receive paths defined by the diversity “A” RF front ends  208 A,  210 A,  212 A and receiver  214 A physically separate from the diversity receive paths defined by diversity “B” RF front ends  208 B,  210 B,  212 B and receiver  214 B, the present invention obtains soft redundancy without additional hardware, software, size, power dissipation or other problems associated with the traditional N+1 redundancy approach. This is because the diversity “A” and diversity “B” reception paths are logically separated into separate failure paths. During a failure of one of the diversity paths (A or B), the base station  200  will continue to operate in each physical sector, but with only one diversity antenna working. In an exemplary CDMA system, the reduction in base station call capacity incurred by using only one of the two diversity antennas in each sector depends on how well the two diversity receive paths were originally balanced. For example, if each of the two antennas in each sector were receiving 50% of the total signal energy for that sector (i.e., equally balanced), then loss of one of the two diversity receive paths would result in approximately a 50% loss in total signal energy for the affected sectors. 
     FIG. 3 illustrates an alternate embodiment of the present invention in which a distribution bus  216  (of FIG. 2) is not used. In the embodiment of FIG. 3, receivers  214 A,  214 B are directly coupled to the plurality of demodulators  218 A- 218 N. This embodiment is useful, for example, when the receivers  214 A,  214 B have dedicated demodulation resources, or when the bussing function is inherent to the demodulators  218 A- 218 N themselves. In other respects, the elements of FIG. 3 function similarly to their correspondingly numbered counterparts of FIG.  2 . 
     Thus, the present invention gains enhanced redundancy in a base station using diversity reception techniques by logically aligning the possible single points of failure for the receivers on a per-diversity-path basis rather than on a per-base station or per-sector basis. Furthermore, the present invention actually reduces the modular complexity of a three-sector diversity reception base station because instead of having four receivers (one for each sector, plus one for redundancy in standby), the present invention only uses two receivers  214 A and  214 B. 
     The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.