Abstract:
Techniques are provided herein to optimize up-link transmission power from a wireless terminal to a base transceiver station in a multicarrier system. A pathloss between the wireless terminal and the base transceiver station is determined. A determination is made if the pathloss is less than or larger than a predetermined value. One or more sub-carriers are assigned to the wireless terminal based on whether the pathloss between the wireless terminal and the base transceiver station is less than or larger than the predetermined value. A power cap command signal is sent from the base transceiver station to the wireless terminal for limiting a maximum allowable power transmitted by the wireless terminal to a predetermined level based on proximity of the one or more sub-carriers to an edge of a frequency band used for up-link transmissions made by the wireless terminal.

Description:
CROSS REFERENCE 
     This application is related to, and claims the benefit of U.S. Provisional Application Ser. No. 60/717,487, which was filed on Sep. 14, 2005 entitled “METHOD FOR OPTIMIZING UP-LINK TRANSMIT POWER FOR A WIRELESS BROADBAND TERMINAL IN A MULTI-CARRIER SYSTEM”. 
    
    
     BACKGROUND 
     The present disclosure generally relates to a wireless telecommunications, and more particularly to a method for optimizing up-link transmission power of a wireless terminal in a multi-carrier system. 
     One of the features of new generation wireless devices is the faster data transfer rate. For example, in 3G systems, wireless devices are required to have a data transfer rate up to 10 Mb/s, and in future 4G systems, wireless devices may be required to have a data transfer rate up to 1,000 Mb/s. In order to support such a fast rate of data transfer, these wireless communication devices are often designed with broadband capabilities. 
     As wireless broadband technology advances, many restrictions and limitations are put in place to regulate the use of frequencies. In the United States, the Federal Communications Commission (FCC) puts in place certain regulations that regulate how signals may be transmitted over a spectrum of frequency. For example, FCC has regulations to control the out-of-band spurious emission power in license bands, such as the multipoint multi-channel distribution system (MMDS) and the wireless communication service (WCS) bands. The out-of-band spurious emissions are unwanted frequencies that are outside a designated bandwidth. The out-of-band spurious emissions near a band edge are commonly caused by the inter-modulation distortions from a transmitter. The out-of-band emissions away from the band edge are commonly caused by the noise floor of the transmitter or the combination of the noise floor and the inter-modulation distortions of the transmitter. These regulations limit the allowable output power of the transmitter to a maximum level, in order to ensure the interoperability of various systems in neighboring bands. 
     There are several conventional solutions for wireless system operators to meet the FCC regulations. The first conventional solution uses a high power linear amplifier to minimize inter-modulation distortions. Advantages of this solution include a higher transmitting power and a lower system link budget. However, the disadvantages of the solution include higher costs, higher power consumption, and a larger size of equipment for sinking heat. This conventional solution is particularly not suitable for a wireless terminal that requires a small size and low manufacturing cost. 
     The second conventional solution is to use a channel filter for a wireless system to filter out inter-modulation distortions, and to reduce the out-of-band noise floor. This allows the system to have lower out-of-band spurious emissions, but could lead to problems such as high costs, low transmitter power, and fixed frequency channels. In addition, the system may not be able to reduce the out-of-band spurious emissions near the band edge. Thus, this conventional solution is not suitable for a terminal that requires the ability to communicate with various base transceiver stations (BTS) using different frequency channels. 
     The third conventional solution is to add an extra guard band to the band edge. This allows the system to have a high transmitting power. However, this solution leads to an inefficient use of frequency spectrum, reduction in signal capacity, and an increase in overall system costs. 
     As such, what is needed in the art of wireless telecommunications technology is a method for optimizing the transmission power for a wireless terminal in a multi-carrier system. 
     SUMMARY 
     Described herein are a method for optimizing up-link transmission power from a wireless terminal to a base transceiver station in a multi-carrier system. In one embodiment, the method includes steps of: determining a pathloss between the wireless terminal and the base transceiver station; assigning at least one sub-carrier to the wireless terminal based on the pathloss between the wireless terminal and the base transceiver station; and sending a power cap command signal from the base transceiver station to the wireless terminal for limiting a maximum allowable power transmitted by the wireless terminal to a predetermined level based on the proximity of the sub-carrier to an edge of a frequency band, over which the wireless terminal transmits and receives signals. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a down-link signal comprised of a plurality of sub-carriers in a multi-carrier system in accordance with one embodiment. 
         FIG. 1B  illustrates various up-link signals, each of which is comprised of one or more sub-carriers, in a multi-carrier system in accordance with one embodiment. 
         FIG. 2A  illustrates a diagram showing up-link transmission power of a wireless terminal at a sub-carrier near a band edge in a multi-carrier system in accordance with one embodiment. 
         FIG. 2B  illustrates a diagram showing up-link transmission power of a wireless terminal at a sub-carrier away from a band edge in a multi-carrier system in accordance with one embodiment. 
         FIG. 3  illustrates a flowchart showing a method for optimizing up-link transmission power from a wireless terminal to a BTS in a multi-carrier system in accordance with one embodiment. 
         FIG. 4  illustrates an operator owned frequency band in accordance with one embodiment. 
         FIG. 5  illustrates a spectral density profile over a frequency band in accordance with one embodiment. 
     
    
    
     DESCRIPTION 
       FIG. 1A  illustrates a down-link signal comprised of a plurality of sub-carriers in a multi-carrier system in accordance with one embodiment. The multi-carrier system includes at least one BTS and at least one wireless terminal, such as customer premise equipment (CPE), a Personal Computer Memory Card International Association (PCMCIA) card, or other wireless devices, for exchanging information there between over a wireless channel. The signal transmitted by the wireless terminal and received by the BTS is referred to as an up-link signal, whereas the signal transmitted by the BTS and received by the wireless terminal is referred to as a downlink signal. In this embodiment, the down-link signal contains ten sub-carriers CO, C 1 , C 2  . . . C 9 , each of which is 0.5 MHz wide. As a result, the total width of the signal is 5 MHz. 
       FIG. 1B  illustrates various up-link signals, each of which is comprised of one or more sub-carriers, from the wireless terminal to the BTS in the multi-carrier system in accordance with the embodiment. Diagrams  102 ,  104 , and  106  show three examples of up-link signals with various numbers of sub-carriers. The diagram  102  shows an up-link signal with one sub-carrier, thereby forming a 0.5 MHz wide signal. In the diagram  104 , the up-link signal contains two sub-carriers, thereby forming a 1 MHz wide signal. In the diagram  106 , the up-link signal includes four 0.5 MHz sub-carriers, thereby forming a 2 MHz wide signal. In the multi-carrier system, the width of the up-link signal can be varied depending on the needs of the wireless terminal, as the bandwidth of a signal transmitted by the wireless terminal can be variable or fixed over a period of time. 
     It is noted that the width of each sub-carrier and the number of sub-carriers each up-link or down-link signal has are not limited to those disclosed in the above embodiment. It is understood that a person skilled in the art can implement a multi-carrier system with a sub-carrier of a different size, and signals with a different number of sub-carriers, without departing from the principles described herein. 
       FIG. 2A  illustrates a diagram  200  showing up-link transmission power of a wireless terminal at a sub-carrier near a band edge in a multi-carrier system in accordance with one embodiment. As shown in  FIG. 2A , there is no room between the output signal and the FCC mask indicated by the black bars at two sides of the signal. The diagram  200  shows that, as an example, the wireless terminal can transmit a +24 dBm signal with four carriers at an edge of a WCS band (2,305-2,360 MHz), thereby meeting the FCC out-of-band spurious emission requirements.  FIG. 2B  illustrates a diagram  202  showing up-link transmission power of a wireless terminal at a sub-carrier away from a band edge in a multi-carrier system in accordance with one embodiment. As shown in  FIG. 2B , there is room between the output signal and the FCC mask indicated by the black bars at two sides of the signal. The diagram  202  shows that, as an example, the wireless terminal can transmit the signal with a 1 MHz or wider guard band from the band edge at +28 dBm and still meet the FCC out-of-band spurious emission requirements. 
     In another embodiment where a MMDS band (2,500-2,686 MHz) is used, the wireless terminal can transmit a +27 dBm signal with four sub-carriers at a band edge in a 5.5 MHz channel, and meet the FCC out-of-band spurious emission requirements. In this embodiment, the wireless terminal can again transmit the same four subscarrier signals with a 1 MHz or wider guard band from the band edge at +30 dBm, and still meet the FCC out-of-band spurious emission requirements. 
       FIG. 3  illustrates a flowchart  300  showing a method for optimizing up-link transmission power from a wireless terminal to a BTS in a multi-carrier system in accordance with one embodiment. In the flowchart  300 , a pathloss between the wireless terminal and the BTS is determined in step  302 . The BTS is designed with forward and reverse power control schemes, such that the pathloss between the wireless terminal and the BTS can be detected. In step  304 , the BTS determines if the pathloss is less than a predetermined value. For example, the predetermined value can be set from 60 to 80 percent of the maximum pathloss allowed for the BTS, or it can be a fixed pathloss from −60 dBm to −80 dBm. If the pathloss is less than the predetermined value, the process flow proceeds to step  306 , whereas if the pathloss is larger than the predetermined value, the process flow proceeds to step  308 . 
     In step  306 , the BTS assigns at least one sub-carrier at an edge of a frequency band to the wireless terminal. In this embodiment, the “edge of frequency band” or “band edge” refers to an edge of an operator-owned frequency band. Referring to  FIG. 4 , an operator of a multi-carrier system has the rights to transmit and receive signals over a band of frequency from 2.305 GHz to 2.315 GHz, which contains two WCS bands A and B. The frequency spectrum of WCS band A ranges from 2.305 GHz to 2.310 GHz, and the frequency spectrum of WCS band B ranges from 2.310 GHZ to 2.315 GHz. With the rights to use the consecutive WCS bands A and B, the operator can design a BTS that transmits signals with full power at the border of bands A and B. The band B neighbors a band C, over which the operator has no rights to transmit or receive signals. Thus, the operator needs to design the BTS to transmit signals with lower power at the border of bands B and C. 
     In step  310 , the BTS sends out a power cap command to the wireless terminal for limiting its maximum allowable transmission power to a lower level. The wireless terminals that are closer to the BTS do not need to transmit signals with full power to communicate with the BTS. Thus, the BTS assigns the sub-carriers that are near the owned band edge, and caps the wireless terminal up-link transmission power at a lower maximum allowable power level such as from +24 to +27 dBm. 
     In step  312 , a modulation scheme with a low signal peak-to-mean ratio is assigned for the sub-carriers at the band edge. A signal with a low peak-to-mean ratio drives a power amplifier less intensively, thereby generating less amount of out-of-band emission. In step  314 , the transmission power is limited and the modulation schemes of the wireless terminal are updated, such that the allocation of sub-carriers for the wireless terminals can be optimized based on their pathloss with respect to the BTS. 
     In step  308 , the BTS assigns at least one sub-carrier away from an edge of a frequency band to the wireless terminal, when the pathloss between the wireless terminal and the BTS is larger than the predetermined value. For example, the sub-carrier assigned is away from the edge by at least 20 percent of the width of the operator owned frequency band. 
     In step  316 , the BTS sends a power cap command to the wireless terminal for limiting its maximum allowable transmission power to a higher level. The wireless terminals that are further away from the BTS need to transmit signals with more power than those close to the BTS do. Thus, the BTS assigns the sub-carriers that are away from the band edge, and caps the wireless terminal up-link transmission power at a higher maximum allowable power level such as from +28 to +30 dBm. 
     In step  318 , a modulation scheme with a high signal peak-to-mean ratio is assigned for the sub-carriers away from the band edge. Although a signal with a high peak-to-mean ratio drives a power amplifier, it will not generate a significant amount of out-of-band emission, as the sub-carrier is away from the band edge. The process flow then proceeds to step  314  where the transmission power limits and the modulation scheme of the wireless terminal are updated, such that allocation of sub-carriers for the wireless terminals can be optimized based on their pathloss with respect to the BTS. 
     In this embodiment, the power cap command can indicate a fixed cap or a power spectral density profile.  FIG. 5  illustrates a spectral density profile  500  over a frequency band utilized by a multi-carrier system. The operator has the rights to transmit signals over sub-carriers on the right side of the frequency band, but do not have such rights to sub-carriers on the left side of the band. The height of the profile  500  at a specific frequency represents the maximum allowable transmission power at that frequency. As shown in the figure, the sub-carriers in section  502  have a lower maximum allowable transmission power, starting from the lowest at the left edge, with its value gradually stepping up until the border between sections  502  and  504 . The sub-carriers in the section  504  have a relatively stable allowable power distribution, as they are away from the band edge. The wireless terminal will adjust its signal transmission power based on its assigned sub-carrier and the spectral density profile. 
     The proposed method is able to optimize the up-link transmission power based on a pathloss between a wireless terminal and a BTS in a multi-carrier system. No high power amplifier, channel filter or extra guard band is needed in order for the multi-carrier system to meet the out-of-band emission requirements. As such, the proposed method allows the BTS of the multi-carrier system to be designed in a simple and cost-effective way. 
     The above illustration provides many different embodiments or embodiments for implementing different features for the techniques described herein. Specific embodiments of components and processes are described for clarity. These are, of course, merely embodiments and are not intended to limit the techniques described herein from the broader scope of the claims. 
     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.