METHOD AND APPARATUS FOR IMPROVING RADIO LINK BUDGET FOR A CELLULAR BASE STATION

Abstract: A method and apparatus for improving the radio link budget for cellular base stations is disclosed. A transmitter module generates N in-phase downlink signals, wherein each of the in-phase downlink signals are at the same frequency and have a predetermined power level, and a combiner, coupled to the transmitter module, receives and combines the N in-phase downlink signals to produce an output signal at an output power level greater than the power level of any of the N in-phase downlink signals. The transmitter module includes N channel units and each of the N channel units includes a modulator, an upconverter and an amplifier. The system further includes a baseband unit for generating a baseband signal for each of the N channel units for processing. The system further includes a phase shifter for receiving each of the baseband signals and a power detector disposed at the output of the combiner for measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase shifter.

Description: METHOD AND APPARATUS FOR IMPROVING RADIO LINK BUDGET FOR A CELLULAR BASE STATION BACKGROUND OF THE INVENTION 1. Field of the Invention. This invention relates in general to base stations, and more particularly to a method and apparatus for improving the radio link budget for cellular base stations. 2. Description of Related Art. Cellular communication systems are experiencing tremendous growth in the global communication market place. This growth is fueling many research programs and expanding the technology opportunities for all manufacturers of cellular equipment. To attract customers and obtain a larger market share, cellular providers are pushing features enabled by the system's purely digital nature, such as Caller ID and short messaging, a paging equivalent. To further obtain a larger market share, cellular providers have turned to reducing the cost of air time. Cutting prices can take many forms. For example, vendors can give more free minutes per month, charge less for additional minutes, charge nothing for the first minute of incoming calls, reduce monthly fees, eliminate long-term contracts - all tactics available to the established cellular vendors as well. For a provider, this leads to less revenue per customer, but more incentive for increasing the market share. Thus, any reduction in customer traffic due to network problems has a tremendous impact on revenues and profits. Traditionally separate boosters have been used to increase the output power from a base station to enhance the downlink coverage. Boosters are additional amplifiers that are added after the transmitter and power amplifier in a normal channel unit. While such an arrangement provides the required output power, several disadvantages results from such a design. For example, an additional booster unit must be designed, which takes much time and needed resources. Further, an additional booster unit has to be placed in the base-station cabinet. Because the booster units have to deal with very high RF powers, their reliability often posses a problem. While providing a booster unit improves the uplink from the base transceiver station to the mobile station, higher output power from the base transceiver station does not improve the base transceiver stations receiver sensitivity. The radio link is not any more in balance than if the booster unit was not used. Finally, if a booster unit fails, the whole service on that frequency is lost. Since the booster unit works at high higher power levels, bypassing the failed booster unit is very difficult and redundancy is not typically built into the channel units. It can be seen then that there is a need for a method and apparatus for improving the radio link budget for cellular base stations without relying upon a separate booster unit. SUMMARY OF THE INVENTION To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for improving the radio link budget for cellular base stations. The present invention solves the above-described problems by combining in-phase downlink signals to produce an output signal at an output power level greater than the power level of any single downlink signal. A system in accordance with the principles of the present invention includes a transmitter module for generating N in-phase downlink signals, each of the in-phase downlink signals being at the same frequency and having a predetermined power level and a combiner, coupled to the transmitter module, for receiving and combining the N in-phase downlink signals to produce an output signal at an output power level greater than the power level of any of the N in- phase downlink signals. Other embodiments of a system or method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the transmitter module includes N channel units. Another aspect of the present invention is that each of the N channel units includes a modulator, an upconverter and an amplifier. Another aspect of the present invention is that the system further includes a baseband unit for generating a baseband signal for each of the N channel units for processing. Another aspect of the present invention is that the system further includes an adjustable delay on an adjustable phase shifter for baseband or RF signals and a power detector disposed at the output of the combiner for measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase shifter or adjustable delay. Another aspect of the present invention is that the transmitter module includes a modulating stage, an upconversion stage and an amplification stage. Another aspect of the present invention is that the system further includes a baseband unit for generating a baseband signal, the baseband signal being provided to the modulating stage of the transmitter module for producing a modulated signal. Another aspect of the present invention is that the modulating stage provides the modulated signal to the upconverting stage for upconversion to a predetermined RF frequency. Another aspect of the present invention is that the upconverting stage includes N upconverters, the modulated signal being split into N modulated signals, each N modulated signal being provided to one of the N upconverters to produce N RF signals at the predetermined RF frequency. Another aspect of the present invention is that the amplification stage includes N amplifiers for the N RF signals for generating the N in-phase downlink signals. Another aspect of the present invention is that the amplification stage includes N amplifiers, the upconverting stage providing N RF signals at the predetermined RF frequency, the N amplifiers receiving the N RF signals and generating the N in-phase up-link signals using the N RF signals. Another aspect of the present invention is that the system further includes a phase shifter or delay element and a power detector, the phase shifter being disposed between a baseband unit for producing a baseband signal and the transmitter module, wherein the transmitter module receives the baseband signal, the power detector being disposed at the output of the combiner for measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase shifter. Another aspect of the present invention is that the output power level of the output signal is increased by three decibels when N is two. Another aspect of the present invention is that the output power level of the output signal is increased by six decibels when N is four. Another aspect of the present invention is that the output power level of the output signal is increased by nine decibels when N is eight. Another aspect of the present invention is that each channel unit includes at least one receiver. Another aspect of the present invention is that received signals are combined to achieve uplink diversity gain. Another aspect of the present invention is that combining the signals from a plurality of receivers improve receiver sensitivity. These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings in which like reference numbers represent corresponding parts throughout: Fig. 1 illustrates a mobile communication system according to an embodiment of the present invention; Fig. 2 illustrates a simplified block diagram of downlink related elements of a channel unit of a base transceiver station, including a booster unit; Fig. 3 illustrates a block diagram of one embodiment of a base transceiver station wherein in-phase signals are combined to increase the output power level; Fig. 4 illustrates a block diagram of a second embodiment of a base transceiver station wherein in-phase signals are combined to increase the output power level; Fig. 5 illustrates a block diagram of a third embodiment of a base transceiver station wherein in-phase signals are combined to increase the output power level; Fig. 6 a simplified block diagram of a channel unit with a transmitter (downlink) and two receivers (uplink); Fig. 7 illustrates N-way combinations of channel units according the present invention; and Fig. 8 illustrates a block diagram of a base transceiver station downlink part according to the present invention having phase control functionality to maximize the output power level. DETAILED DESCRIPTION OF THE INVENTION In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. The present invention provides a method and apparatus for improving the radio link budget for cellular base stations. In-phase downlink signals are combined to produce an output signal at an output power level greater than the power level of any single downlink signal. Uplink signals are also combined to achieve diversity gain for the receivers. Fig. 1 illustrates a mobile communication system 100 according to an embodiment of the present invention. The system 100 is comprised of a plurality of base stations 102 connected to system controllers 101 , and the plurality of mobile terminals 103. A service area of the mobile communication system 100 is divided into a plurality of cells 110-120. The mobile switching center 130 is connected with another mobile communication system or fixed network 132 and coordinates the setting up of calls to the mobile terminals 103. The mobile terminal 103 can move within a service area which is formed by a plurality of base stations 102 for communication through a channel allocated to the neighboring base station 102. The base station 102 is a network element that interfaces the mobile terminal 103 to the network via the air interface. The primary function of the base station 102 is to maintain the air interface, or medium, for communication to any mobile terminal 103 within its cell. Other functions of the base station 102 are call processing, signaling, maintenance, and diagnostics. The base station 102 includes transceivers 140, 142, 144. The transceivers 140, 142, 144, which represent at least one receiver and one transmitter, provide coverage to cells 110, 112, 114 respectively, wherein each transmitter/receiver pair 140, 142, 144 comprises a channel unit. The transceivers 140, 142, 144 also receive calling signals sent from the mobile terminal 103 moving in the corresponding cell, and detect up-link carrier wave power of the received signal. Fig. 2 illustrates a simplified block diagram of a transmitter unit 200. In Fig. 2, 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 using a bandpass filter 222. The filtered signal 224 is then upconverted to the desired frequency to produce an RF signal 230 using a second local oscillator 232 and mixer 234. The RF signal 230 is then passed through a power amplifier stage 240 that includes, for example, power amplifiers 252, 254, 256. To provide the required RF output power from the transmitter unit 200, a booster unit 260 is provided after the last amplifier 256. The booster unit 260 includes a booster amplifier which is a high output-power amplifier. Fig. 3 illustrates a transceiver downlink portion 300 according to the present invention. The base station transceiver downlink portion 300 includes a base-band unit 310 which provides identical signals to a transmitting module 312, which includes a channel unit 1 320 and channel unit 2 322. The output of the channel units 320, 322 are provided to a wide-band combiner 330 to provided the desired output signal 340. The wide-band combiner 330 is used to combine two signals from the two separate channel units 320, 322. The two signals from the two channel units 320, 322 are controlled such that they are in- phase. By combining the two signals from the channel units 320, 322 in-phase provides ideally 3 dB more output power at the output 340 of the combiner 330. Typically, the combiner 330 includes two 90 degree long microstrips 350, 352 which, according to the Wilkinson principle are combined in-phase to double the output power at the output 340. Traditionally wide-band combiners have been used in base station transmitters to combine two signals that are at different frequencies in which have no correlation in-phase. In such a scenario, each subsignal experiences a 3 dB attenuation. In contrast to the present invention as illustrated in Fig. 3, the two signals from the two channel units 320, 322 are identical in terms of frequency and are at the same phase. In Fig. 3, if one channel unit 320 or 322 fails, the output power at the output 340 will decrease by 6 dB. This decrease in output power at the output 340 occurs because there is no signal at the other port resulting in a 3 dB attenuation for the inserted signal and an additional 3 dB of attenuation occurs simply from the fact that there are not two signals that are being combined together at the output 340. Those skilled in the art will recognize that typical combiners/dividers of the type described herein include about 0.2-0.4 dB loss. Accordingly, this loss has to be taken into account throughout the system. Those skilled in the art will recognize that other embodiments of the present invention may be utilized other than the configuration illustrated in Fig. 3. As described above, Fig. 3 illustrates separate signals being fed to two channel units 320, 322 wherein the signals are modulated upconverted and amplified separately and combined in combiner 330 thereafter. However, those skilled in the art will recognize that in keeping with the principles of the present invention the system as illustrated in Fig. 4 may also be utilized. In Fig. 4, base station 400 includes a baseband unit 410 which provides a single signal to the modulating stage 418 which includes a modulator 420. Modulator 420 provides two modulated output signals 422, 424 to amplifier and upconverter stages 430. The amplifier and upconverter stages 430 provide channels 432, i.e., the amplifier and upconverter stages 430 provides a separate amplifier and upconverter for each of the channels 432 for signals 422 and 424. The outputs of the amplifier and upconverter stages 430 are then to provide to the combiner 440 to provide the combined output 450, which is similar as illustrated above to reference to Fig. 3. Fig. 5 illustrates another embodiment of the present invention. The base station 500 in Fig. 5, includes a baseband unit 510. Baseband unit 510 provides a single signal to the modulator and upconverter stages 520. After the baseband signal is modulated and upconverted to the desired RF frequency, the signal is split into separate signals 522 and 524. Upconverter signal 522 is provided to amplifier 530 to produce an amplified signal 534 which is provided to combiner 540. Upconverted signal 524 is provided to amplifier 532 to provide RF power signal 536 to combiner 540. Signals 534, 536 are then combined in combiner 540 to produce output 550 as illustrated above with reference to Fig. 3. Those skilled in the art will recognize that the present invention is not made to be limited by the embodiments illustrated with reference Figs. 3-5, but that other embodiments are possible. The present invention provides in an ideal case a 3 dB increase in output power without designing difficult booster units. The higher power results in an increased network coverage and the elimination of a booster unit design results in faster base station design cycles. Further, redundancy is provided so that if one transmitter fails, the maximum output power decreases by 6 dB, but operation is still possible at the lower power levels. In addition, there is no need to use very high power RF components are required in booster units, which also provides increase reliability. Because the high power amplifier as required in the booster unit is not needed, heat dissipation is facilitated. Accordingly, the present invention provides a flexible modular structure, wherein additional channel units may be included to provide additional increases in output power, e.g. four channel units to provide an increase of 6 dB, eight channels units to provide an increase of 9 dB. Finally, the concept of providing (end of side 1 ) an increase in power by combining in-phase signals may be combined with diversity receiving to keep link balance at all times. For example, if a receiver sensitivity and output power capability are well chosen, the channel unit maintains a link balance. When each channel unit includes two receivers, the channel unit may be used as a general building block for N-way diversity receiver that has also sufficient output power to keep the radio link in balance. Fig. 6 illustrates an embodiment of a channel unit 600 according to the present invention. In Fig. 6, the channel unit 600 includes a receiver 610 and transmitter 620. A receiver provides two-way diversity via receive signal 630 and 632. Transmitter 620 provides transmitter outputs 640. A single channel unit with a 2-way diversity receiver providing a balanced link can provide improvements in link budgets. Fig. 7 illustrates this diversity gain affect and the summed output power affect 700 by combining channel units according to the present invention. A two way combined channel unit 720 provides a +3 dB diversity gain affect 722 and an increase in the summed output power of 3 dB 724. Four channel units may be combined into a four-way system 730 providing a diversity gain affect of +6 dB 732 and an increase in output power of +6 dB 734. Finally, 8 channel units may be combined in an 8-way arrangement 740 to provided +9 dB diversity gain affect 742 and +9 dB output power increase 744. As described above, the combined transmitted signals must be at the phase and on the same frequency. To accomplish this, all channel units must get their frequency reference from a common unit. In addition, phase and frequency detectors in the local oscillators of the channel units must be used to keep frequencies and their phases exactly the same between channel units. The cable length between the in-phase power combiner and channel units have to be in principle of equal length to maintain phase equalization when combining the two signals. However, there are many methods to overcome this problem. For example, at an operation frequency of 900 megahertz, the radio signal has a wave length in free space of 0.3 meters and a wave length within a cable of 0J8 meters, assuming the cable has a propagation constant which is 60% of free space. Accordingly, a 5 millimeter difference in cable length results in a 10 degrees separation in-phase. To ensure maximum output power, the combined signals must be within a few degrees. However, such extreme accuracy is not easily achieved without adaptive phase control. Fig. 8 illustrates a block diagram of one possible circuit 800 for providing phase control. In Fig. 8, signals are provided to phase shifters 810, 812 which in turn are processed by power amplifiers 820, 822 in channel units 1 and 2, respectively. The output signal 860 of power amplifier 820 in channel unit 1 is provided to combiner 870 along with the output signal 862 of power amplifier 822 in channel unit 2 to provide the desired output signal 872. The power level of the output signal 872 is monitored and detected by circuit 880 which generates a phase control signal 882 to control phase shifters 810, 812 to maximize the output power level of output signal 872. By maximizing the output power level of output signal 872, the phase error is minimized. By looping back in phase combined signals in the channel units own receiver, the quality of the transmitted signal can be optimized. Again, some phase shifting element in the transmitter chain is used to optimize the phases. The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.

Claims: WHAT IS CLAIMED IS: 1. A base transceiver station, comprising: a transmitter module for generating N in-phase downlink signals, each of the in-phase downlink signals being at the same frequency and having a predetermined power level; and a combiner, coupled to the transmitter module, for receiving and combining the N in-phase downlink signals to produce an output signal at an output power level greater than the power level of any single in-phase downlink signal. 2. The base transceiver station of claim 1 wherein the transmitter module comprises N channel units. 3. The base transceiver station of claim 2 wherein each of the N channel units comprises a modulator, an upconverter and an amplifier. 4. The base transceiver station of claim 3 further comprising a baseband unit for generating a baseband signal for each of the N channel units for processing. 5. The base transceiver station of claim 4 further comprising a phase shifter disposed between the baseband unit and the transmitter module for receiving each of the baseband signals and a power detector disposed at the output of the combiner for measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase shifter. 6. The base transceiver station of claim 1 wherein the transmitter module comprises a modulating stage, an upconversion stage and an amplification stage. 7. The base transceiver station of claim 6 further comprising a baseband unit for generating a baseband signal, the baseband signal being provided to the modulating stage of the transmitter module for producing a modulated signal. 8. The base transceiver station of claim 7 wherein the modulating stage provides the modulated signal to the upconverting stage for upconversion to a predetermined RF frequency. 9. The base transceiver station of claim 8 wherein the upconverting stage comprises N upconverters, the modulated signal being split into N modulated signals, each N modulated signal being provided to one of the N upconverters to produce N RF signals at the predetermined RF frequency. 10. The base transceiver station of claim 9 wherein the amplification stage comprises N amplifiers for the N RF signals for generating the N in-phase downlink signals. 11. The base transceiver station of claim 8 wherein the amplification stage comprises N amplifiers, the upconverting stage providing N RF signals at the predetermined RF frequency, the N amplifiers receiving the N RF signals and generating the N in-phase up-link signals using the N RF signals. 12. The base transceiver station of claim 1 further comprising a phase shifter and a power detector, the phase shifter being disposed between a baseband unit for producing a baseband signal and the transmitter module, wherein the transmitter module receives the baseband signal, the power detector being disposed at the output of the combiner for measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase shifter. 13. The base transceiver station of claim 1 wherein the output power level of the output signal is increased by three decibels when N is two. 14. The base transceiver station of claim 1 wherein the output power level of the output signal is increased by six decibels when N is four. 15. The base transceiver station of claim 1 wherein the output power level of the output signal is increased by nine decibels when N is eight. 16. The base transceiver station of claim 1 further comprising a receiver. 17. The base transceiver station of claim 1 further comprising a plurality of receivers, wherein uplink diversity gain is obtained by combining signals from each receiver. 18. The base transceiver station of claim 1 wherein the uplink diversity gain provides greater receiver sensitivity. 19. A method for increasing base station output power, comprising: generating N in-phase downlink signals, each of the in-phase downlink signals being at the same frequency and having a predetermined power level; and combining the N in-phase downlink signals to produce an output signal at an output power level greater than the power level of any of the N in-phase downlink signals. 20. The method of claim 19 wherein the generating further comprises generating a baseband signal for use in generating N in-phase up-link signals. 21. The method of claim 20 further comprising receiving the baseband signal and measuring the output power level of the output signal and maximizing the output power level of the output signal by adjusting the phase of the baseband signal. 22. The method of claim 19 wherein the generating further comprises modulating the baseband signal to produce a modulated signal. 23. The method of claim 19 wherein the modulated signal is upconverted to a predetermined RF frequency. 24. The method of claim 23 wherein the modulated signal is split into N modulated signals, each N modulated signal being upconverted to produce N RF signals at the predetermined RF frequency. 25. The method of claim 24 wherein the generating further comprises amplifying the N RF signals to generate the N in-phase downlink signals. 26. The method of claim 19 wherein the combining further comprises increasing the output power level of the output signal by three decibels when N is two. 27. The method of claim 19 wherein the combining further comprises increasing the output power level of the output signal by six decibels when N is four. 28. The method of claim 19 wherein the combining further comprises increasing the output power level of the output signal by nine decibels when N is eight. 29. The method of claim 19 further comprising receiving uplink signals using a receiver. 30. The method of claim 19 further comprising receiving uplink signals using a plurality of receivers, wherein the signals of each receiver are combined, combining signals from each receiver provides uplink diversity gain. 31. The method of claim 30 wherein the uplink diversity gain provides greater receiver sensitivity.

Inventors: KETONEN VELI-PEKKA, KETONEN, VELI-PEKKA
Applicants: KETONEN VELI PEKKA, KETONEN, VELI-PEKKA
Application year: 2000
Main classification: H04B7
IPC codes: H04B1, H04B7