Patent Publication Number: US-2003228854-A1

Title: Method and system for increasing the output power of a wireless signal

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to wireless transceivers, and more particularly to a method and system for increasing transmitter output power of a wireless signal.  
       BACKGROUND OF THE INVENTION  
       [0002] Cellular providers are in the process of changing or have completely changed their systems from analog to digital. This change to digital allows the cellular providers to provide more capacity, reduce costs and provide more services.  
       [0003] One digital cellular system is the next generation GSM/EDGE system. The evolved system offers many advantages and features that are not available to a user of the current system, be it analog or digital. One advantage is that data can be transmitted and received at a much higher data rate. Because the new system operates at a higher rate, higher power is required at the mobile terminal in order to obtain comparable coverage as a older generation systems. This requirement means that there must be a corresponding higher power output at the base station. Therefore, to implement a next generation system, it may be necessary for the service provider to transmit information from the base station at increased power levels. The output power of the signal from the base station has a direct impact on, and determines the maximum size of, the cell that the base station can support and hence the coverage area that can be provided by the base station.  
       [0004] As 3G wireless systems start to be introduced commercially, output power is going to be even more critical as these higher data rate systems require higher power at the mobile receiver to achieve optimum data throughput rates, in part, because the link may not necessarily be balanced for systems such as (E)GPRS since it may be downlink limited. In addition, operators are going to want to maintain the same cell sizes as they have in their currently installed systems. As a result, there is going to be a need for higher output power from base station transmitters.  
       [0005] Traditionally, separate booster amplifiers have been used to increase the output power of a base station to enhance the downlink coverage. Booster amplifiers are added after the transmitter and power amplifier in a normal transmitter chain unit. While such an arrangement provides the required output power, several disadvantages result from such a design. Additional booster amplifier units have to be located in the base station cabinet. Because the booster units operate at very high RF powers, they are less reliable. Finally, if a booster amplifier unit fails, service on that frequency is lost. Since the booster amplifier unit works at higher power levels, bypassing the failed booster unit is very difficult.  
       [0006] PCT Patent Publication No. WO 00/64072 discloses a method and apparatus for improving the radio link budget of cellular based stations. From the same base station, in-phase downlink signals that have the same frequency are combined to produce an output signal having increased power. The downlink signals are fed to a combiner which provides an output signal at a power level that is greater than the power level of any one of the in-phase down link signals. To insure that the signals from the channel units are always in phase and always at the same frequency, the channel units get their frequency reference from a common source. A phase control provides correction for phase differences caused by different lengths of cables which connect the channel units to the power combiner.  
       [0007] U.S. Pat. No. 5,886,573 discloses a device for high power linear amplification of an amplitude and/or phase modulated signal using multiple amplifiers driven by an appropriate set of switched and/or phase modulated constant amplitude signals derived from the input signal.  
       SUMMARY OF THE INVENTION  
       [0008] The present invention provides a method and apparatus for improving the radio link between a cellular base station and a mobile terminal. In-phase down link signals from at least two transceivers of a base station are combined to produce a downlink output signal having an output power level greater than the power level of any single downlink signal.  
       [0009] A power combiner combines the downlink signal from two or more transceivers where the downlink signals are at the same frequency and each has a predetermined power level. Detector means, which constantly samples the downlink signals to identify a phase difference, or samples the signal from the combiner, is coupled to control a phase controller which adjusts the phase of at least one of the downlink signals to be in phase with the other downlink signals. The phase controller maintains the downlink signals in phase by shifting the phase of at least one of the downlink signals relative to the other. Because one or more of the downlink signals are shifted, if necessary, to be in phase with other downlink signals, the signals may be combined constructively to provide a single, higher output power signal that can provide greater base station range and therefore greater coverage area.  
       [0010] The present invention is an intelligent, closed loop feedback, power combining system which requires minimal modifications to a standard base station installation. Because the system minimizes the power consumption of the power booster, there is no requirement for high power consumption power boosters with their associated higher capital, service and operating costs. The reduced power consumption of the system should also provide an improvement in reliability. Additionally, only one antenna is required, although multiple antennae can also be used.  
       [0011] Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] In the drawings, wherein like reference numerals delineate similar elements throughout the several views:  
     [0013]FIG. 1 is a block diagram of structure for coupling transceiver equipment located within two transceivers;  
     [0014]FIG. 2 is a simplified block diagram of downlink related elements of a transmitter unit of a base transceiver station;  
     [0015]FIG. 3 is a block diagram of an embodiment of a base transceiver station of the present invention where downlink signals from two transceivers are combined to increase the output power level of the downlink signal from one transceiver;  
     [0016]FIG. 4 is a block diagram of an embodiment of a base transceiver station of the present invention where downlink signals from three transceivers are combined to increase the output power level of the downlink signal from one transceiver;  
     [0017]FIG. 5 is a block diagram of an embodiment of a base transceiver station of the present invention where downlink signals from four transceivers are combined to increase the output power level of the downlink signal from one transceiver;  
     [0018]FIG. 6 is a block diagram of an embodiment of the power combiner of the present invention;  
     [0019]FIG. 7 is a block diagram of another embodiment of the power combiner of the present invention; and  
     [0020]FIG. 8 is a block diagram of still another embodiment of the power combiner of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS  
     [0021]FIG. 1 illustrates a mobile communication system  100  in accordance with an embodiment of the present invention. The system  100  comprises a plurality of base stations  102  connected to base station controllers  101 , and a plurality of mobile terminals  106 . The service area of the mobile communication system  100  for each base station  102  is divided into a plurality of cells  110 ,  112 ,  113 ,  116 ,  118 ,  120 . The mobile switching center (MSC)  130  is connected with another mobile communication system or fixed network  132  and coordinates the setting up of calls to mobile terminals  106 . The mobile terminals  106  can move within a service area which is formed by the plurality of base stations  102  for communication through a channel allocated to a base station  102 .  
     [0022] The base stations  102  are network elements that interface the mobile terminal  106  to the network via the air interface. The primary function of the base stations  102  is to maintain the air interface, or medium, for communication to any mobile terminal within its cell. Other functions of the base stations  102  are call processing, signaling, maintenance, and diagnostics. The base stations  102  each include transceivers  140 ,  142 ,  144 . The transceivers  140 ,  142 ,  144 , which represent at least one receiver and one transmitter, provide coverage to the cells  110 ,  112 ,  114  covered by the respective base station  102 . The transceivers  140 ,  142 ,  144  also receive calling signals sent from the mobile terminal  106  while they are located within a corresponding cell, and detect up-link carrier wave power of the received signal.  
     [0023]FIG. 2 illustrates a simplified block diagram of a transmitter unit  200 . A modulator  210  modulates a signal which is then upconverted using a first local oscillator  212  and mixer  214 . The upconverted signal  216  is then amplified by amplifier  220  and filtered by band pass filter  222 . The filtered signal  224  is then upconverted using a second local oscillator  232  and mixer  234  to the desired frequency to produce a radio frequency (RF) signal  230 . The RF signal  230  is then passed through a power amplifier stage  240  that includes, for example, high power amplifiers  252 ,  254 ,  256 . To provide the required RF output power from the transmitter unit  200 , a booster unit may be provided after the last amplifier  256 . The booster unit includes a booster amplifier which is a high output-power amplifier.  
     [0024]FIG. 3 illustrates a transceiver downlink portion according to the present invention for constructively (i.e., in-phase) combining the RF output signals from two transceivers to provide a single, higher output power signal that can effectively increase the range (the area of coverage) of a base station. A first transceiver has a base band unit  308  which provides a signal to a transceiver  320 . A second transceiver has a base band unit  310  which provides a signal to a transceiver  322 . One or both of units  308  and  310  may alternatively control base band units. The output of transceivers  320 ,  322  are provided to a power combiner  330  to provide an output signal  340 . The power combiner  330  combines two RF output signals from the transceivers  320 ,  322  from two different transceivers. The phase of at least one of the two RF signals from the two transceivers  320 ,  322  is controlled to be in phase with the other signal. A phase controller  332  is coupled to receive a function signal  334  from the power combiner  330 . The function signal  334  is a sampling of the phase relation of the signal from one transceiver  320  relative to the phase of the other transceiver  322 , or a sampling of the RF power of the output signal  340 . The output signal  336  of the phase controller  332  is directed to one or both of the transceivers  320 ,  322  to adjust the phase of the RF signals of the two transceivers  320 ,  322  so that they are in phase so that the RF signals will combine constructively. Adjustment of the phase of the RF signal of only one of the transceivers  320 ,  322  will obtain a match of the phase of the RF signals of the two transceivers. Alternatively, the RF signals of both of the transceivers  320 ,  322  may be adjusted. In operation, phase controller  332  adjusts the phase of the RF signal of transceiver  322  by means of phase adjustment signal  336  until the RF signal  340  from the power combiner  330  is at a maximum value as determined by the function signal  334 . If the RF signal of the other transceiver  320  is also to be adjusted, a second phase adjustment signal  338  (shown as a dashed line) is transmitted from the phase controller  332  to transceiver  320 .  
     [0025] The system is a closed loop feedback system that continuously samples the phase of the output signals from the transceivers  320 ,  322  of the different transceivers, or the signals after they are combined to provide a function signal  334  that is directed to a phase controller  332  which generates a phase adjustment signal  336 ,  338  to adjust the phase of the RF signal of one or more of the transceivers  320 ,  322  to provide a signal  340  which has maximum output power at the output of the power combiner.  
     [0026]FIG. 4 shows a transceiver downlink portion according to the present invention in which the RF signals of three transceivers  320 ,  322 ,  323  are combined by power combiner  330 . This embodiment is identical to that shown in FIG. 3 except for the additional transceiver  323  and its associated base band unit  311 , which may be a control base band unit. Similarly, the phase of the RF signal output by transceiver  323  is adjusted by a signal  337  generated by the phase controller  332 . Additional transceivers can be added in a similar fashion.  
     [0027]FIG. 5 shows a transceiver downlink portion according to the present invention in which the RF signals of four transceivers  320 ,  322 ,  325 ,  327  (and their respective base band (or control base band) units  308 ,  310 ,  313 ,  315 ) are combined by power combiner  335 . In this embodiment, two transceiver downlink portions as shown in FIG. 3 are combined in parallel, with the outputs of their respective power combiners  330 ,  331  being combined by power combiner  335 . The phase controller  330 ,  331  of each transceiver downlink portion is controlled by a signal from its respective power combiner  330 ,  331 , respectively, and by a signal from power combiner  335 . Although in FIG. 5, only two transceivers are shown for each parallel path, i.e., the signals from transceivers  320 ,  322  being fed to power combiner  330 , alternatively, each parallel path may have more than two transceivers, as is shown in FIG. 4. In addition, although only two parallel transceiver paths with respective power combiners  330 ,  331  feeding their signals to power combiner  335  are shown in FIG. 5, alternatively, there may be more than two parallel paths, with each path feeding its signal to power combiner  335  and their respective phase controller  33  being controlled by a signal from power combiner  335 . Additionally, one of the parallel paths for which the signal is fed to power combiner  335  may comprise only a single base band unit and a single transceiver.  
     [0028] As discussed above, the phase adjusting signal(s)  336 ,  337 ,  338  from the phase controller(s)  332 ,  333  is used to control the phase of the RF signals from the transceivers so that the RF signals are substantially in phase. Such phase adjustment can be accomplished in at least three ways. In an initial carrier phase method, for each RF signal burst, the initial phase of the RF carrier is adjusted by loading the appropriate initial phase into the base band oscillator. In an initial modulation phase method, for each RF signal burst, the initial phase of the modulation is adjusted by using a fixed initial phase on the base band oscillator. In a reference clock phase method, the phase of the transceiver reference clock is adjusted.  
     [0029]FIG. 6 is a block diagram of a phase detector embodiment of the power combiner  330  of the present invention where the phase relationship between the two RF signals is used to obtain the function signal (e.g., signal  334  in FIG. 3). The RF signal  610  from a first transceiver is directed to an input terminal of RF combiner  614  and sampled by prescaler  616 . The RF signal  612  from a different transceiver is directed to a second input terminal of RF combiner  614  and is sampled by prescaler  618 . The RF signals  610 ,  612  have the same frequency. In RF combiner  614 , the signals input thereto are combined or added to one another to generate output signal  340 . A phase detector  620  receives the output signals from prescalers  616 ,  618 . The prescalers  616 ,  618  are used to frequency divide the input signals  610 ,  612  so that they are at a frequency that can be processed by phase detector  620 . Phase detector  620  compares the two signals  610 ,  612  to determine how one or both of the input signals must be modified to be substantially in phase with the other. The output signal of phase detector  620  is the function signal  334  which provides the appropriate function to modify the phase of the appropriate input signals  610 ,  612 . The function signal  334  is directed to a phase controller (e.g., phase controller  332  in FIG. 3), the output of which is fed to one or more of the transceivers to appropriately adjust the phase of the RF signal from that transceiver to be similar to the phase of the RF signal from the other transceiver.  
     [0030] In the embodiment of the present invention shown in FIG. 7, a power combiner  330  is shown in which the combined RF signals from the transceivers output from the RF combiner  614  is sampled and used to adjust the transceiver outputs. The RF signal  610  from a first transceiver is fed to an input terminal of RF combiner  614 , and the RF signal  612  from a different transceiver is fed to a second input terminal of the RF combiner  614 . The output signal  340  of the RF combiner  614  is sampled by power detector  710  which generates a function signal  334 , the value of which represents the power level of the signal from the RF combiner  614 . The RF power detector  416  samples the composite output signal  340  to determine when the output power is the highest, indicating that the input transceiver signals  610 ,  612  are substantially in phase.  
     [0031]FIG. 8 shows another embodiment of the present invention in which an RF mixer is used to obtain a function signal  334 , the value of which is determined by the phase difference between the two RF signals from different transceivers. The RF signal  610  from a first transceiver is fed to an input terminal of RF combiner  614  and is sampled by limiter/attenuator  810 . The RF signal  612  from a different transceiver is directed to a second input terminal of RF combiner  614  and is sampled by limiter/attenuator  820 . The RF signals  610 ,  612  have substantially the same frequency. RF mixer and low pass filter  830  receives the outputs from limiter/attenuators  810 ,  820 . The output signal of RF mixer  830  is the function signal  334  which is fed to the phase controller  332  to control the phase of at least one of the transceivers. The embodiment of FIG. 8 is similar to the embodiment of FIG. 6 in that it measures the phase difference between the two RF signals from two transceivers of two different transceivers. However, in this embodiment, an RF mixer is used to obtain the phase difference. A sample of each of the RF signals is passed through an RF limiter to remove substantially all amplitude modulation (AM) of the signal that is present. AM is removed because it can create an error in the phase detection process. It does not have to be completely removed, only reduced below an acceptable limit. AM can be removed by using a limiter which clips the level of the signal to a predetermined value which is independent of the input level to remove the AM on the signal. It does this in a controlled manner and does not add a significant amount of phase modulation (i.e., phase error) in the process. Overdriving or saturating the mixers will result in nonlinear operation of the mixer. To prevent this, attenuators are used. If the level of the input signal to the mixer is low, the mixer acts as a simple multiplier. When mixing (multiplying) two sinusoidal signals in unit  830 , the outputs are the sum and difference frequencies. The low pass filter removes the sum (2×frequency) and passes the low frequency component, the difference frequency.  
     [0032] Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices described and illustrated, and in their operation, and of the methods described may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.