Abstract:
A system and method of implementing an on-demand change of the RF power class of multi-carrier power amplifiers at a base station radio with minimal disruption of user service is disclosed. The power amplifiers providing signal diversity at said cell site and being able to operate at various RF power class levels. A first multi-carrier power amplifier with multiple RF Power Classes provides a main RF beam transmission at one sector of said cell site. A second multi-carrier power amplifier with multiple RF Power Classes provides RF beam transmission diversity to the main RF beam transmission. A switch operates to disable either the first or second multi-carrier power amplifier when an RF Power class change is required.

Description:
FIELD OF THE INVENTION 
       [0001]    The present application relates generally to radio base stations with signal diversity, more specifically, to radio base stations having radios able to operate on multiple RF power classes. 
       BACKGROUND OF THE INVENTION 
       [0002]    A cellular base station typically consists of a radio equipment controller (REC) connected to one or more radio equipment (RE) units. 
         [0003]    Generally these radios operate at the maximum transmit power levels. This is because RF multi carrier power amplifiers (PA) are designed to be most efficient at the maximum output power. When the PA is operated at lower transmitter output to save base station power, then the efficiency drops rapidly and the output becomes less linear. 
         [0004]    Base station radio&#39;s power amplifiers typically use a Doherty design which is roughly 55%-65% efficient at the amplifier&#39;s optimal output. As the PA output is reduced, the overall power savings is much less than expected because of this reduced efficiency. 
         [0005]    Base station radios can be designed to operate in one of multiple RF power classes. Power Class is a set of radio calibration parameters designed for an anticipated output power; such that when the radio is operating in that output power level this set of parameters do warrant a maximal power efficiency. Therefore each radio contains an array of power class to cover the expected operating range of the radio. This makes it possible to operate in low or high RF power output modes depending on the network coverage requirements. Low power modes are useful during off hour operation or in dense urban areas where cell site is small. High power modes are required for operation in rural areas or to support high bandwidth applications. 
         [0006]    The base station radio&#39;s can be designed to operate in multiple RF output power classes for example 5, 10, 20, 40, and 80 watts. To achieve this, the radio&#39;s power amplifiers have to be re-configured to switch over the settings associated with each RF power class. 
         [0007]    Currently base station radios switch power mode class by shutting off all the RF paths simultaneously as the RF power amplifiers are re-configured to operate at the new power class. This cut over induces a temporary outage thus is carefully managed especially during system operation. A power class change requires altering the power amplifiers&#39; voltage bias to operate at the new transmit power level. The mobiles in this sector would see an abrupt outage and would start hunting for a new radio signal. This outage could last a number of seconds while the radio&#39;s power amplifier ramps up and the pre-distortion compensation is re-computed for the new output level. 
         [0008]    Thus, on the one hand if a power amplifier is to remain in operation while reducing its power, the efficiency suffers drastically and the spurious RF emissions are increased. On the other hand, the amplifier&#39;s power class can be set lower to increase efficiency but this requires that the amplifier be shut off. 
         [0009]    For these reasons, traditional power reduction implementations have limited capabilities in situations as described above. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to alleviating the problems of the prior art. 
         [0011]    The present invention overcomes the problems of the prior art by providing a base station radio having a number of multi-carrier power amplifiers to provide signal diversity at a cell site. The base station radio is operable to provide on-demand RF Power class change. A first multi-carrier power amplifier with multiple RF Power Classes provides a main RF beam transmission at one sector of the cell site. A second multi-carrier power amplifier with multiple RF Power Classes provides RF beam transmission diversity to the main RF beam transmission. A switch is operable to disable either the first or second multi-carrier power amplifier when an RF Power class change is required. 
         [0012]    In a further embodiment, the present invention provides a method of implementing an on-demand change of the RF power class of multi-carrier power amplifiers at a base station radio operating at a cell site. An RF power class change request is sent to the base station radio. To enable a change to a new RF power class for the first power amplifier, power is switched off at the first power amplifier. When the first power amplifier is set at the new RF power class, power is then switched on. Power is then switched off at a second power amplifier providing transmission diversity with the first power amplifier to enable a change to the new RF power class. Power is then switched back on at the second power amplifier when set at the new RF power class. 
         [0013]    Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram illustrating of a typical radio base station providing signal diversity; 
           [0015]      FIG. 2  is a block diagram illustrating the basic cell site configuration for a base station such as shown in  FIG. 1 ; 
           [0016]      FIG. 3  is a diagram illustrating the effects of reducing input power of Power Amplifiers to cell site signal coverage; 
           [0017]      FIG. 4  is a block diagram of a radio base station according to the present invention; and 
           [0018]      FIG. 5  illustrates the signal coverage at a cell site when the power amplifier of a diverse branch is turned off to adjust the RF power class. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    In order to lighten the following description, the following acronyms will be used: 
       ADC Analog-to-digital Converter 
     CPRI Common Public Radio Interface 
     DAC Digital-to-analog Converter 
     LNA Low Noise Amplifier 
     PA Power Amplifier 
     REC Radio Equipment Control 
     RE Radio Equipment 
     RX Receive 
     TX Transmit 
       [0020]    With reference to  FIG. 1 , we have shown a block diagram illustrating a typical wireless base station radio offering signal diversity. 
         [0021]    Cellular network base station radios are designed to be either cabinet mounted (RE) with the Radio Equipment  100  Controller (REC)  101  or at a remote site (RRE). The REC  101  includes a modem block (not shown) which modulates and demodulates the wireless network data into a form termed baseband data. A CPRI link  102  between the REC  101  and the RE  100  carries this baseband data in the CPRI&#39;s User Plane. Information at this baseband data layer is encapsulated inside antenna carrier (A×C) blocks inside the CPRI frame. 
         [0022]    The radio has a digital board  103  which on the transmit path has a channelizer  104  to multiplex the downlink AxCs onto the assigned carriers. This digital signal is then pre-distorted  105  to compensate for the RF power amplifier&#39;s non-linearity. The digital signal is converted to analog (DAC)  106  before RF synthesizing  107  and amplifying it  108 . The multi carrier power amplifier (PA)  106  typically accounts for 50-75% of the radio&#39;s power consumption. 
         [0023]    On the receive path the low noise amplifiers (LNA)  109  boosts the received signal before converting it to a digital signal (ADC)  110 . The radio board&#39;s channelizer  104  then de-multiplexes the received signals into the AxCs sent back to the REC  101  on the uplink via the COM interface  111 . 
         [0024]    The RE  102  amplifies the baseband signal from the REC  101  and transmits it over the antenna system  112 . When the radio supports transmit diversity, the same modulated carrier is transmitted over two or more separate antenna systems  113   a  and  113   b  and whereas with received diversity, the radios receive over two or more separate antenna systems  114   a  and  114   b . Multiple antennas provide a more robust link to the mobiles. When one path is experiencing a deep fade the other path could be operating at normal signal strength. These antennas usually have the same characteristics and are physically separated from one another by a planned distance as a function of carrier wavelength. 
         [0025]    The Power Amplifier unit makes the final amplification of the down link analog signal from the radio&#39;s digital section. To optimize on power efficiency and bandwidth an asymmetric Doherty design is typically used in the base station radio using Multi-Carrier Power Amplifiers. 
         [0026]    The asymmetric Doherty PA design improves the power efficiency over a conventional Class AB amplifier. Both the main and peak RF signals are amplified in three cascaded amplifier stages. The two paths are not identical in the power amplifier; the final stage in the peak path consists of a parallel stage, while the final stage in the main path has one single transistor. The two paths are combined after the final stage. A directional coupler detects the amplified signal which is passed to the Transmitter Observer Receiver (TOR). A single circulator module at the output will protect the PA from excessive reflected power. What should be noted is that PA efficiency drops off rapidly as the output power is reduced below the optimal operating point when an asymmetrical Doherty amplifier is used. 
         [0027]    As indicated above, the base station radio has multiple transmit  113   a ,  113   b  and receive antennas  114   a ,  114   b , which provides diversity in both directions. In  FIG. 1 , the radio (RE)  102  is connected to dual antennas transmitting to multiple mobiles or User Equipment (UEs)  115 . 
         [0028]    Typically base station radios are deployed with high gain antennas with advanced directivity to cover a sector. These antennas are either mounted at the top of the tower  201  or bottom mounted  202 , as shown in  FIG. 2 . 
         [0029]    Radio waves obey the inverse square law, so as an example when the transmit power density is halved the maximum operating distance to the receivers is reduced by a factor of four assuming other factors such as ambient noise remains constant. 
         [0030]    When the PA operates below its optimal power level as discussed above, it operates at reduced efficiency. 
         [0031]    Base station radios are designed to receive signals near the thermal noise floor, typically down −120 dBm/MHz. The distance of transmission is limited by the radio noise floor:
       Over the air;   In components inside the radio receiver; and   In the antenna and feeder cable.       
 
         [0035]    As an example, and with reference to  FIG. 3 , if the power input to a PA is reduced by 50%, and because of the efficiency drop, the output is reduced a further 15%. The operating range in this scenario is reduced by a factor of 1/(2+0.15)̂2=0.216 or a 78% drop in coverage range. 
         [0036]    Network service providers can request certain base stations to enter a power reduction mode when certain conditions exist by reducing or lowering the power class mode of the radio. 
         [0037]    Currently base station radios switch power mode class by shutting off all the RF paths simultaneously as the RF power amplifiers are re-configured to operate at the new power class. This cut over induces a temporary outage thus is carefully managed especially during system operation. A power class change request from the network provider requires the radio control software to re-configure the power amplifiers&#39; voltage bias to operate at the new transmit power level. The mobiles in this sector would see an abrupt outage and would start hunting for a new radio signal. This outage could last a number of seconds while the radio&#39;s power amplifier ramps up and the pre-distortion compensation is re-computed for the new output level. 
         [0038]    The technique proposed in this disclosure implements the RF power class switching for radios with multiple transmit branches not all at once but rather by switching over the power class in a sequential way, that is, one transmit radio branch at a time. In this way, service to the mobiles continues to be uninterrupted since one of the radio branches is still operating, either at the old power class level or the newly applied power class level. 
         [0039]    Using this technique, if the radio is being configured to operate at a lower power class then there will be no disruption in service. If it is to be operated at a higher power class, then overall RF output will be reduced as each RF branch is switched over. At least the radio will continue to provide uninterrupted service to affected mobiles. 
         [0040]    The technique proposed in this disclosure implements a change in the RF amplifier power class by sequentially turning off the power amplifier on one of the radio&#39;s transmitter branches, switching its power class level, turning the power amplifier back on and repeating this for another power amplifier providing RF transmission diversity for a sector covered by the affected RF amplifiers. Once the diverse radio branch is operating at the new designated power class, then the main radio branch is switched over to the new power class. The connected mobile may observe a transitional change in received power level but the connection to the base station can be maintained. This minimizes service disruption to the mobiles connected to the base station over this radio link. 
         [0041]    This is illustrated in  FIG. 4 . By changing the power class alongside reducing power to that RF amplifier, the amplifier operates at better operating efficiencies. 
         [0042]    A switch module  400  is configured with the digital board  401  of the Radio Equipment  402 . The switch module  400  operates to turn off power to a power amplifier  403  via a switch  404 . Switch  404  can be implemented in hardware under software control in the switch module  400  and digital board  401 . The switch module  400  and switch  404  can be fully integrated with the existing radios as part of a remote service update conducted by the manufacturer or service provider. This way, a command received from the service provider can be delivered to the switch module  400  via the REC  405  and the communication interface  406 . 
         [0043]    By switching power off to a PA  403  of a diverse branch and then adjusting its power class, the radio&#39;s power consumption is reduced more efficiently then by reducing output power of each PA individually without a power class change. So instead of reducing the input power equally at each of the PAs, one is removed from service, while its power class is changed and the other PA continues to operate at the optimal output power. 
         [0044]    For each radio to operate at a particular output energy level efficiently, a set of matching parameters are engineered such that when they are applied the radio will operate with maximal energy efficiency and reduce waste. For example when a radio is transmitting 10W with a 10W power class; it can consume up to 7% less energy than transmitting identical 10W but with a 20W power class. As far as a mobile is concerned the received power is identical but the energy saving at the base station can be important. This saving can be significant as the radios account for the majority of the equipment in a deployed network. 
         [0045]    Also matching the power class to the output power level is important to ensure RF integrity. The power class defines the operating boundaries of the radio. When the radio is operating suboptimal either too high or too low, the RF characteristic distortion may be increased due to the RF branch is not operating in its engineered optimal range. 
         [0046]    As illustrated in  FIG. 5 , the radio coverage is reduced when a PA is turned off since the gain from transmitter diversity is reduced. However, this is still more efficient than simply reducing a PA&#39;s output power. With this technique the energy saving impact on the wireless network will be more than the current power saving approach which operates the PAs at a lower non-optimal output. The radio implements sleep mode by turning off the RF output of a diverse radio branch. All carriers in this branch are released and the power to the branch&#39;s PA is turned off. The main RF branch remains operational at its optimal efficiency. Adjacent transmit patterns have minimal effect on one another. The base station radio optimizes power savings, by turning off the diverse PA but keeping the main PA operating at optimal output. There will be an approximate 3 dB drop in signal strength in the coverage area of the disabled beam. Once the diverse PA is operating at the new power class then it is the main radio branch PA&#39;s turn to switch power class. 
         [0047]    In another embodiment, a change to a lower power class can be triggered to provide power savings during off-peak hours. During off-peak hours when there are less active mobiles and hence interference, then the base station radios can radiate at lower transmit powers. In current practice the radio output power is reduced evenly on all RF branches but in this case they no longer operate with optimal drain efficiency. By matching it with a lower power class ensures correct engineering parameters are applied and at the same time reduces power consumption of the base station radios. That translates to direct saving in Operational Expenditure. Before this invention the operator may decide to stay with suboptimal power class to avoid service interruption. This invention allows the operator a scheme to switch over without service interruption. In the following example the radio switches from the 20 watt power class to the 10 watt power class resulting in a power savings of 34%. 
         [0048]    In another embodiment, a sleep mode condition can be triggered to provide high temperature and Power Amplifier failure recovery. In a radio equipment (RE) the PA is known to be one of its major heat generators. As indicated above, matching the power class to the expected output power results in maximal drain efficiency. In this case the energy waste is controlled and consequently reducing waste energy lowers the overall heat generation. This embodiment allows one or more of its PA the freedom to match power class with the output power resulting in lower heat dissipation. 
         [0049]    This satisfies the self-healing aspect of the Self Organizing Network. 
         [0050]    The present invention can be realized in hardware, or a combination of hardware and software. Any kind of computing system, or other apparatus adapted for carrying out the methods described herein, is suited to perform the functions described herein. A typical combination of hardware and software could be a specialized computer system, having one or more processing elements and a computer program stored on a storage medium that, when loaded and executed, controls the computer system such that it carries out the methods described herein. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which, when loaded in a computing system is able to carry out these methods. Storage medium refers to any volatile or non-volatile storage device. 
         [0051]    Computer program or application in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following a) conversion to another language, code or notation; b) reproduction in a different material form. 
         [0052]    Thus, one embodiment is a computer readable medium containing computer readable instruction that, when executed by a processor, cause the processor to perform functions for maintaining clock synchronization between a first and a second radio. 
         [0053]    In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.