Abstract:
A remote radio head unit (RRU) system for multiple operating frequency bands, multi-channels, driven by a single or more wide band power amplifiers. More specifically, the present invention enables multiple-bands RRU to use fewer power amplifiers in order to reduce size and cost of the multi-band RRU. The present invention is based on the method of using duplexers and/or interference cancellation system technique to increase the isolation between the transmitter signal and receiver signal of the RRU.

Description:
RELATED APPLICATIONS 
     This application is a continuation of: 
     U.S. patent application Ser. No. 12/928,933, filed Dec. 21, 2010, entitled REMOTE RADIO HEAD UNIT SYSTEM WITH WIDEBAND POWER AMPLIFIER AND METHOD, which claims the benefit of U.S. Patent Application Ser. No. 61/288,840, filed Dec. 21, 2009, entitled REMOTE RADIO HEAD UNIT SYSTEM WITH WIDEBAND POWER AMPLIFIER AND METHOD and naming as inventors Chengxun Wang and Shawn Patrick Stapleton. Each of these applications is hereby incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to wireless communication systems using power amplifiers and remote radio head units (RRU or RRH). More specifically, the present invention relates to RRU which are part of a distributed base station in which all radio-related functions are contained in a small single unit that can be deployed in a location remote from the main unit. 
     BACKGROUND OF THE INVENTION 
     Wireless and mobile network operators face the continuing challenge of building networks that effectively manage high data-traffic growth rates. Mobility and an increased level of multimedia content for end users require end-to-end network adaptations that support both new services and the increased demand for broadband and flat-rate Internet access. In addition, network operators must consider the most cost-effective solutions to expand network capacity and evolution towards 4G and beyond. 
     Wireless and mobile technology standards are evolving towards higher bandwidth requirements for both peak rates and cell throughput growth. The latest standards supporting this are HSPA+, WiMAX, TD-SCDMA and LTE. The network upgrades required to deploy networks based on these standards must balance the limited availability of new spectrum, leverage existing spectrum, and ensure operation of all desired standards. This all must take place at the same time during the transition phase, which usually spans many years. 
     Distributed open base station architecture concepts have evolved in parallel with the evolution of the standards to provide a flexible, cheaper, and more scalable modular environment for managing the radio access evolution. For example, the Open Base Station Architecture Initiative (OBSAI), the Common Public Radio Interface (CPRI), and the IR Interface standards introduced standardized interfaces separating the Base Station server and the remote radio head part of a base station by an optical fiber. 
     The RRU concept is a fundamental part of a state-of-the-art base station architecture. 2G/3G/4G base stations are typically connected to RRUs over optical fibers. Either CPRI, OBSAI or IR Interfaces may be used to carry data to the RRH to cover a three-sector cell. The RRU incorporates a large number of digital interfacing and processing functions. Traditionally, a multi-channel RRU means that multiple antennas are used, typically with two power amplifiers for two distinct bands. A duplexer is used to combine the two power amplifier outputs. Switches are used to isolate the transmit signals from the received signals as occurs in a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) modulation. To extent the prior art architecture to multiple bands (i.e., two or more bands) implementation would consist of adding additional channelized power amplifiers in parallel. The output of the additional power amplifiers is typically combined in an N by 1 duplexer and fed to a single antenna. 
     While conventional RRU architecture offers some benefits, RRUs to date are power-inefficient, costly and inflexible. Further, their poor DC-to-RF power conversion ensures that they will have a large mechanical housing. In addition, current RRU designs are inflexible. As standards evolve, there is a need for multi-band RRUs that can accommodate two or more operating channels using a single wideband power amplifier. This creates an isolation problem at the individual receivers because the wideband power amplifier is always turned on. Isolation between the wideband transmitter and receivers is a problem with any modulation standard (HSPA+, WiMAX, LTE, etc.) when multi-band RRUs are developed using a single power amplifier. This is a common problem for all communication systems that utilize a wideband power amplifier in a multi-band scenario. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a high performance and cost effective technique for implementing RRU systems that service multi-frequency bands. Further, the present disclosure enables a RRU to be field-reconfigurable, and supports multi-modulation schemes (modulation agnostic), multi-carriers, multi-frequency bands, and multi-channels. The present invention also serves multi-frequency bands within a single RRU to economize the cost of radio network deployment. In particular, the present invention resolves an isolation issue for a RRU with fewer power amplifiers than the number of operating frequency bands. Multi-mode radios capable of operating according to GSM, HSPA, LTE, TD-SCDMA and WiMAX standards and advanced software configurability are key features in the deployment of more flexible and energy-efficient radio networks. 
     The present invention achieves the above objects using techniques generally based on methods and techniques for maximizing the isolation between the transmitted signal (Tx Signal) and the received signal (Rx Signal). The Tx Signal may comprise noise generated at the output of the power amplifier or it may comprise an unwanted transmitter band leaking into the receiver. With the use of the present invention, conventional RRU&#39;s can be extended to a multi-band and multi-channel configuration. Multi-band means that more than one frequency bands are used in the RRU and multi-channel means that more than one output antenna is used. Various embodiments of the invention are disclosed. 
     An embodiment of the present invention utilizes duplexers, switches and circulators to maximize the isolation between the transmitter and receiver. Another embodiment of the present invention utilizes an interference Cancellation System (ICS) together with duplexers, switches and circulators. 
     Applications of the present invention are suitable for use with all wireless base-stations, remote radio heads, distributed base stations, distributed antenna systems, access points, repeaters, mobile equipment and wireless terminals, portable wireless devices, and other wireless communication systems such as microwave and satellite communications. The present invention is also field upgradeable through a link such as an Ethernet connection to a remote computing center. 
    
    
     
       THE FIGURES 
       Further objects and advantages of the present invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a block diagram of a TD-SCDMA dual-band single PA configuration in a remote radio head unit system in accordance with the present invention. 
         FIG. 2  is a block diagram of the TD-SCDMA dual-band single PA with an Interference Cancellation System (ICS) configuration in a remote radio head unit system in accordance with the present invention. 
         FIG. 3  is a FDD Modulation Agnostic Dual-Band Remote Radio Head with an Interference Cancellation System. 
         FIG. 4  is an Interference Cancellation System using Power Detection. 
         FIG. 5  is a TDD Modulation Agnostic Dual-Band Remote Radio Head with an Interference Cancellation System. 
         FIG. 6  is an Interference Cancellation System using Correlation. 
     
    
    
     GLOSSARY OF TERMS 
     
         
         ACLR Adjacent Channel Leakage Ratio 
         ACPR Adjacent Channel Power Ratio 
         ADC Analog to Digital Converter 
         AQDM Analog Quadrature Demodulator 
         AQM Analog Quadrature Modulator 
         AQDMC Analog Quadrature Demodulator Corrector 
         AQMC Analog Quadrature Modulator Corrector 
         BPF Bandpass Filter 
         CDMA Code Division Multiple Access 
         CFR Crest Factor Reduction 
         DAC Digital to Analog Converter 
         DET Detector 
         DHMPA Digital Hybrid Mode Power Amplifier 
         DDC Digital Down Converter 
         DNC Down Converter 
         DPA Doherty Power Amplifier 
         DQDM Digital Quadrature Demodulator 
         DQM Digital Quadrature Modulator 
         DSP Digital Signal Processing 
         DUC Digital Up Converter 
         EER Envelope Elimination and Restoration 
         EF Envelope Following 
         ET Envelope Tracking 
         EVM Error Vector Magnitude 
         FFLPA Feedforward Linear Power Amplifier 
         FIR Finite Impulse Response 
         FPGA Field-Programmable Gate Array 
         GSM Global System for Mobile communications 
         I-Q In-phase/Quadrature 
         IF Intermediate Frequency 
         LINC Linear Amplification using Nonlinear Components 
         LO Local Oscillator 
         LPF Low Pass Filter 
         MCPA Multi-Carrier Power Amplifier 
         MDS Multi-Directional Search 
         OFDM Orthogonal Frequency Division Multiplexing 
         PA Power Amplifier 
         PAPR Peak-to-Average Power Ratio 
         PD Digital Baseband Predistortion 
         PLL Phase Locked Loop 
         QAM Quadrature Amplitude Modulation 
         QPSK Quadrature Phase Shift Keying 
         RF Radio Frequency 
         RRU Remote Radio Head Unit 
         SAW Surface Acoustic Wave Filter 
         UMTS Universal Mobile Telecommunications System 
         UPC Up Converter 
         WCDMA Wideband Code Division Multiple Access 
         WLAN Wireless Local Area Network 
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a novel RRU system that utilizes a wideband power amplifier. The present invention is a hybrid system of digital and analog modules. The interplay of the digital and analog modules of the hybrid system eliminates interference between the wideband power amplifier output and the receiver&#39;s inputs. The present invention, therefore, achieves higher Transmitter (Tx) to Receiver (Rx) isolation when using wideband power amplifiers with multiple frequency bands. 
     Referring first to  FIG. 1 , an embodiment of some aspects of the invention is shown in block diagram form.  FIG. 1  depicts the analog section of a dual channel RRU. In this embodiment a single wideband power amplifier  404  is used. The two distinct frequency band signals  401 ,  402  are combined in a duplexer  403  and input to the wideband power amplifier  404 . The output of the wideband power amplifier  404  is sent to a diplexer  405  in order to separate the two frequency band signals. This configuration enables the individual transmitter frequency bands to be independently turned-off. The Tx switches  406  and  407  are placed in the signal path after the diplexer  405 . The signals are then passed through circulators  411  and  412  and a duplexer  413  in order to gain further isolation between the Tx signals and the Rx signals. The Rx switches  408  and  410  are placed on the third port of the circulator  411 ,  412 . Alternatively, two or more frequency bands can be combined in one power amplifier using the same architecture as in  FIG. 1 . 
       FIG. 2  illustrates a further alternative embodiment of the dual-band single wideband power amplifier RRU analog section. Although the embodiment in  FIG. 2  shows a dual-band implementation, the invention can also be utilized in single band embodiments. In the embodiment of  FIG. 2 , an interference cancellation system (ICS)  520  is utilized to improve the isolation between the transmitter and receivers. The interference cancellation system  520  generates a replica of the unwanted feedback signal but in anti-phase so as to eliminate the interference. The interference cancellation system  520  comprises five primary blocks: Delay, variable attenuator, variable phase shifter, Down Converter (DNC) and DSP controller, alternative arrangements of which are shown in  FIGS. 4 and 6 , discussed hereinafter. The ICS  520  of  FIG. 2  receives incoming signals through links  506  and  507 . The anti-phase output of the ICS  520  is combined with the signals from switches Rx 1  and Rx 2 , indicated at  510  and  511 , respectively, by the use of adders  551  and  552 , and the resulting signal provides the inputs to the LNA&#39;s  515  and  516 . The ICS  520  is an adaptive control system which continuously adjusts the variable attenuator as well as the variable phase shifter so as to maintain good interference cancellation. Alternatively, an embodiment of the ICS can comprise a fixed attenuator and phase shifter setting, eliminating the need for DSP control, although in at least some cases this results in inferior performance compared to the adaptive ICS system of  FIG. 2 . The remaining elements of  FIG. 2  correspond to those shown in  FIG. 1 , and are indicated by the same numerals except that the most significant digit has been changed from “4” to “5”. 
       FIG. 3  shows another embodiment of the analog section of a dual-band single wideband power amplifier RRU in Frequency Division Duplex (FDD) mode. This embodiment is modulation agnostic for FDD standard systems, and elements  601 - 604  operate analogously to elements  401 - 404  of  FIG. 1 . In  FIG. 3 , the triplexer  608  separates the transmitter bands from the receiver bands. FDD systems use different transmit and receive frequencies for each channel. The function of the triplexer  608  is to pass the output of power amplifier  604  to the antenna while isolating the receivers from the transmitter output. The ICS  609  system is utilized for increasing the isolation between the transmitter output and the receiver inputs as with  FIG. 2 , and in  FIG. 3  receives the output of PA  604  through link  605 . The output of the ICS  609  is combined with the appropriate triplexer outputs through adders  610  and  611 , and the links  616 ,  617  feeding the LNA&#39;s  612  and  613 . 
       FIG. 4  is a depiction of one embodiment of an Interference Cancellation System (ICS). The function of the ICS is to generate a replicate of the interfering signal and place it in anti-phase to the interference, thereby eliminating the interfering signal. The input to the ICS system is a sample of the power amplifier output. Coupler  605  as illustrated in  FIG. 3  is used to sample the power amplifier output. In  FIG. 4 , the power amplifier&#39;s output is sampled and sent to a diplexer  710 . This separates the two frequencies into distinct sections. The delay block  701  time-aligns the feedback interfering signal with the sampled power amplifier output. The variable attenuator  702  is adjusted to insure that the interfering signal and the sampled signal have equal magnitude. The variable phase shifter  703  is adjusted to insure that the interfering signal and the sampled signal are in anti-phase. A Digital Signal Processor (DSP)  707  or Microprocessor is used to control the attenuator and phase shifter. A power detection based adaptive algorithm in the DSP continuously monitors the signal at the Down Converter (DNC)  708  output and minimizes the level of the interference based on the detected power level. The power level of the interference is measured at the receiver while that band is in the transmit mode of operation. The second band is similarly processed using elements  704 ,  705  and  706 . 
       FIG. 5  shows an embodiment of the analog section of a dual-band single wideband power amplifier RRU in Time Division Duplex (TDD) mode. This embodiment is modulation agnostic for TDD standard systems. The output of wideband power amplifier  804  feeds a circulator  807 . The circulator  807  provides some isolation between the transmitted signals and the receiver inputs. A multi-band filter  820  is placed between the circulator  807  and the output antenna in order to attenuate out-of-band emissions. The third port of the circulator  807  is delivered to a diplexer  808 , which separates the two distinct operating bands. TDD mode requires the transmitter and receiver to operate using the same frequency band at distinct times. In order to provide isolation between the transmitter and receiver, switches  821 ,  822  are used. The switches  821 ,  822  can provide some isolation but additional isolation may be required depending on the system specifications. The ICS  809  can provide additional isolation between the transmitter output and the receiver inputs in the manner described above. 
       FIG. 6  is a depiction of another embodiment of an Interference Cancellation System (ICS). The function of the ICS is to generate a replicate of the interfering signal and place it in anti-phase to the interference, thereby eliminating the interfering signal. The input to the ICS system is a simple of the power amplifier output. The power amplifier&#39;s output is sampled and sent to a diplexer  910 . This separates the two frequencies into distinct sections. The delay block  901  time aligns the feedback interfering signal with the sampled power amplifier output. The variable attenuator  902  is adjusted to insure that the interfering Signal and the sampled signal have equal magnitude. The variable phase shifter  903  is adjusted to insure that the interfering signal and the sampled signal are in anti-phase. A Digital Signal Processor (DSP)  907  or Microprocessor is used to control the attenuator and phase shifter. A correlation-based adaptive algorithm in the DSP is used to minimize the level of interference. The DSP correlates the two signals by controlling the output of switch  911  and the output of switch  912  after the signals have been translated to baseband using the two Downconverters  920  and  909 . The switches  911  and  912  alternate between the two channels. The objective of the algorithm is to minimize the correlation between the sampled power amplifier output and the interference at the receiver. The computed correlation coefficient is used as the error function in an adaptive algorithm such as a Least Mean Squared (LMS) algorithm. 
     From the foregoing teachings, those skilled in the art will appreciate that the RRU system of the present invention enables the use of single wideband power amplifier for multi-band operation, which consequently saves hardware resources and reduces costs. The RRU system is also reconfigurable and field-programmable since the algorithms can be adjusted like software in the digital processor at anytime. 
     Moreover, the RRU system is agnostic to modulation schemes such as QPSK, QAM, OFDM, etc. in CDMA, TD-SCDMA, GSM, WCDMA, CDMA2000, and wireless LAN systems. This means that the RRU system is capable of supporting multi-modulation schemes, multi-frequency bands and multi-channels. 
     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.