Abstract:
A band edge amplitude reduction system changes the filtering characteristics of a receiver based on the amplitude of signal(s) adjacent to an edge of the operating band of the receiver and/or of signals not under the power control of the receiver. For example, the receiver measures the power level over a bandwidth at the band edges of the operating band of the receiver. If the signals adjacent to the operating band are strong enough relative to the signal power within the operating band, overload protection circuitry changes the filtering characteristics of the receiver to improve the attenuation of the signal(s) from the adjacent band(s). In certain embodiments, the overload protection circuitry switches in filter(s) with a narrower bandwidth to attenuate the signal(s) from adjacent band(s) at the edge(s) of the operating band of the receiver, thereby preventing interference with or the overload of the receiver by signals from outside the operating band and/or not under the power control of the receiver.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a wireless communications and, more particularly, to a band edge power cancellation scheme to reduce interference in a wireless communications system. 
   2. Description of Related Art 
   The service area of a wireless communication system is partitioned into connected service domains known as cells, where wireless units communicate via radio links with a base station (BS) serving the cell. The base station is coupled to a land network, for example through a Mobile Switching Center (MSC) which is connected to a plurality of base stations dispersed throughout the service area. In the wireless communications industry, a service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an “A” band provider for cellular communications receives frequency channels within the A (825–835 MHz), A′ (845–846.5 MHz) and A″ (824–825 MHz) bands, and the wireless units receive frequency channels within the A (870–880 MHz), A′ (890–891.5 MHz) and A″ (869–870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835–845 MHz) and B′(846.5–849 MHz) frequency bands, and the wireless units receive frequency channels within the B (880–890 MHz) and B′(891.5–894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more PCS bands (1850 MHz–1910 MHz), and the wireless units receive frequency channels on one or more PCS bands (1930–1990 MHz). 
   To improve system performance and increase system capacity, the power levels transmitted by the wireless units and/or the base stations are controlled. Power control is generally done by the receiving unit or station measuring the signal strength from the transmitting station or unit. The receiving unit or station can adjust its transmit power based on the received signal strength, and/or the receiving unit or station can relay power control information to the transmitting unit which adjusts its transmit power level in response to the power control information. The power level transmitted by every wireless unit is typically under the control of the serving base station, and the base station performs power control to reduce the power level that each wireless unit is transmitting while maintaining a good quality reverse link. By decreasing the power level that each wireless unit is transmitting, system-wide interference created by the transmissions of the wireless units is reduced. Such a scenario allows increased capacity for the wireless cellular communications system because as the transmit powers are decreased, the overall signal to interference ratio decreases for all wireless units in the wireless cellular communications system. 
   Since frequency bands of adjacent cells and/or adjacent wireless communications systems are not under the power control of the same base station, there is a possibility that one or more signals/carriers from another cell that are close to the operating frequency band of a base station might be too strong in power and overload the radio receiver circuitry in the base station. For example, in order to reduce system hardware costs, a service provider would want to use common receivers for the simultaneous reception and processing of signals within the non-contiguous frequency bands. Typically, an automatic gain control (AGC) at the front-end of the receiver is effective in protecting the base station from overload but at the expense of any users at the fringe of the cell. 
   However, due to the finite roll-off characteristics of filters in the radio receiver, a signal from an adjacent band may come through the radio receiver at a power level strong enough to saturate the wideband analog to digital (A/D) converter. The A/D converter is the most critical component to protect against overload in a modern cellular radio receiver. The A/D converter does not operate in a soft clipping manner as is the case with amplifiers, mixers and other analog semiconductor devices. Once the A/D converter is saturated (i.e., input signal is above the full scale resolution of the A/D), the digital output code cannot go above the maximum binary number limited by the resolution in bits. The sudden change (or sudden stop/clipping) in binary output pattern from the A/D converter, which digitizes and tracks the analog input signal, is called a discontinuity and results in a massive spurious response in the digital domain (when a Fourier transform is taken of the supposed analog input signal with a sudden clipping of the amplitude). 
   It is necessary to implement some overload protection in order to prevent saturation of the A/D converter. 
   SUMMARY OF THE INVENTION 
   The present invention is a band edge amplitude reduction system which changes the filtering characteristics of a receiver based on the amplitude of signal(s) adjacent to an edge of the operating band of the receiver and/or of signals not under the power control of the receiver. For example, the receiver measures the power level over a bandwidth at the band edges of the operating band of the receiver. If the signals adjacent to the operating band are strong enough relative to the signal power within the operating band, overload protection circuitry changes the filtering characteristics of the receiver to improve the attenuation of the signal(s) from the adjacent band(s). In certain embodiments, the overload protection circuitry switches in filter(s) with a narrower bandwidth to attenuate the signal(s) from adjacent band(s) at the edge(s) of the operating band of the receiver, thereby preventing interference with or the overload of the receiver by signals from outside the operating band and/or not under the power control of the receiver. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and advantages of the present invention may become apparent upon reading the following detailed description and upon reference to the drawings in which: 
       FIG. 1  shows a general block diagram of a receiver using the band edge amplitude reduction circuitry according to the principles of the present invention; and 
       FIG. 2  shows a block diagram of an alternative embodiment of a receiver using the band edge amplitude reduction circuitry according to the principles of the present invention. 
   

   DETAILED DESCRIPTION 
   Illustrative embodiments of a receiver system using the band edge power reduction circuitry according to the principles of the present invention is described below. With particular reference to  FIG. 1 , a receiver  10  includes an antenna(s)  12  which receives radio frequency (RF) analog signals which are filtered by a RF bandpass filter  14  to produce an operating frequency band for the receiver  10 . The output of the RF filter  14  is provided to an amplifier  16 , such as a low noise amplifier for amplification. The radio front-end is comprised of at least the antenna  12 , the RF filter  14  and the amplifier  16 . A replica of the operating frequency band is provided to a band edge detection path  18  for detection by band edge power detector circuitry  20 . The band edge power detection circuitry  20  measures at least one amplitude, such as power level, for a bandwidth(s) adjacent to or at the edge(s) relative to the operating frequency band and/or from signals not under the control of the receiver  10 . For example the power level(s) of a narrow band(s) of frequencies adjacent to or at the edge of the frequency band of operation, such as a narrow band from 30 KHz to 1 MHz wide using a SAW, lumped-element or other type of filter. As a function of the rogue, unwanted or interfering amplitude(s), processing circuitry  22  provides control signals to change the filtering characteristics of a variable filter  24  to attenuate the amplitude, for example the interference, at the band edge(s) of the frequency band of operation. Band edge can refer to a bandwidth that is adjacent to and/or encompasses an edge of the frequency band of operation. 
   Depending on the embodiment, the processing circuitry  22  can make a comparison between the amplitude(s) at the band edge(s) and an amplitude, such as a power level, provided by an analog amplitude indicator  26   a  or a digital amplitude indicator  26   b  for the frequency band of operation or a portion thereof. Based on the comparison, the processing circuitry  22  can provide control signals to the variable filter  24  to change the filtering characteristics of the variable filter  24 . For example, the control signals control switches  28  and  30  which switch the received signals between filters  32   a–b  in a filter bank  34 . As a function of the amplitude(s) at band edge(s) adjacent to the frequency band of operation, the processing circuitry  22  can cause the switching between filters  32   a–b  to narrow the band (for example, from 15 to 13 MHz or from 5 MHz to 3 MHz) passed through the variable filter  24  at either or both band edges to attenuate the amplitude, such as power level or interference at the band edge(s), such as power from adjacent frequency bands from wireless units (not shown) and/or not under the power control of the receiver  10 . 
   In alternative embodiments, the processing circuitry  22 , such as a micro-controller and A/D converter, can provide control signal(s) to control the variable filter  24  as a function of the amplitude of signals not under the control of the receiver  10 , for example by comparing the amplitude of signals under the control of the receiver  10  and the total amplitude of signals within the frequency band of operation. Alternatively, the amplitude of signals not under the control of the receiver  10  can be assumed and cause a change in the filtering characteristics of the receiver  10  if the processing circuitry  22  receives an indication of overload or saturation, for example by receiving an overload signal from the A/D  35 . Furthermore, alternative embodiments for the variable filter  24  are possible for which the filtering characteristics can change using control signal(s) from the processing circuitry  22 . For example, a programmable filter, a programmable array of filter elements, a varactor-tuned filter and/or a tunable cavity filter could be used. 
   In the embodiment shown in  FIG. 1 , RF analog signals are received by the antenna  12  and the radio front end onto a main signal path  40 . A coupler  42  provides a replica of the RF analog signals on the main signal path  40  directly from the radio front end at radio frequency (RF) onto the band edge detection path  18  for the band edge detection circuitry  20 . The band edge detection circuitry  20  receives the RF spectrum from the band edge detection path  18 , and an amplifier  44  amplifies the RF analog signals on the path  18 . The RF analog signals are then filtered in a desired manner by a bandpass filter  46 , for example to include frequencies adjacent to the frequency band of operation. A splitter  48  divides the RF analog signals on the band edge detection path  18  and provides a replica of the RF spectrum on an upper edge detection path  50  and a lower edge detection path  52 . On the upper edge detection path  50 , a mixer  54  mixes the RF analog signals with a signal from a local oscillator (LO)  56  to frequency convert the RF spectrum, for example to a lower frequency, for improved filtering and detection. The analog spectrum from the mixer  54  is provided to a bandpass filter  58  which passes an upper band edge bandwidth of frequencies from the upper band edge relative to the frequency band of operation and attenuates other frequencies, for example a surface acoustic wave (SAW) filter. The upper band edge bandwidth is provided to a power detector  60  which produces a signal indicating or representing the power level of the upper band edge bandwidth. A low pass filter  62  filters the signal from the power detector  60  to produce for the processing circuitry  22  a slower changing amplitude signal which represents the power level of the upper edge bandwidth. 
   On the lower edge detection path  52 , a mixer  64  mixes the RF analog signals with a signal from a local oscillator (LO)  66  to frequency convert the RF spectrum, for example to a lower frequency, for improved filtering and detection. The analog spectrum from the mixer  64  is provided to a bandpass filter  68 , such as a SAW filter, which passes a lower band edge bandwidth of frequencies relative to the frequency band of operation and attenuates other frequencies. The lower band edge bandwidth is provided to a power detector  70  which produces a signal indicating or representing the power level of the lower band edge bandwidth. A low pass filter  72  filters the signal from the power detector  70  to produce for the processing circuitry  22  a slower changing amplitude signal which represents the power level of the lower edge bandwidth. Thus, in this embodiment, the processing circuitry  22  receives signals indicating or representing the power levels of the upper and lower band edge bandwidths. Depending on the embodiment, additional band edge detection paths can be used to detect additional band edges if the receiver  10  operates within a changing frequency band of operation or operates within a fragmented or non-contiguous frequency band of operation between different cells and/or wireless communications systems. Additionally, depending on the frequency band of operation, the processing circuitry  22  can provide control signal to the LOs  56  and  66  to tune the LOs to different frequencies such that the band edge bandwidth can be changed. 
   On the main signal path  40  after the coupler  42 , the RF analog signals are downconverted to an intermediate frequency (IF) by providing the spectrum to a mixer  80  which also receives a signal from a local oscillator (LO)  82 . After passing through any additional IF stage  84 , the spectrum of interest is further downconverted by providing the spectrum to a mixer  86  which also receives a signal from a local oscillator (LO)  88 . The spectrum is then provided to the variable filter  34  which filters the spectrum as a function of the amplitude(s) at the band edge(s) adjacent to the frequency band of operation. The filtered signal is amplified by an amplifier  90 , and in this embodiment, a bandpass filter  92  further filters the spectrum. In this embodiment, the analog spectrum is provided to the analog to digital (A/D) converter  35  for conversion into the digital domain. The analog signals are sampled and digital sample values are produced (from which a digital representation of the analog spectrum can be obtained) onto a bus  96  to processing circuitry  98  which can include digital downconverters (DDCs) and digital signal processors (DSPs) (as well as the processing circuitry  22 ). If the power level at the band-edge(s) relative to the frequency band of operation for the receiver  10  is above a certain level, for example from RF signal sources operating in adjacent frequencies and/or not under the control of the receiver  10 , the power level at the band edges could saturate the A/D converter  35 . 
   Depending on the embodiment, the processing circuitry  22  can provide control signals to the variable or tunable filter  34  as a function of the amplitude(s) at the band edge(s), of adjacent frequencies and/or of signals not under the control of the receiver  10  to attenuate the amplitude, such as the power, at the band edge(s). In addition to the power at the band edge(s), the processing circuitry  22  can change the filtering characteristics as a function of the amplitude, such as power level, of the frequency band of operation or a portion thereof. For example, the processing circuitry  22  can provide control signals to the variable filter  34  based on a comparison of the power level of band edge(s) and a power level of the frequency band of operation or a portion thereof. Depending on the embodiment, the analog power indicator  26   a  can include a coupler  100  which provides a replica of the frequency band of operation onto a analog power detection path  102  to a power detector  104 . The power detector  104  provides a signal to the processing circuitry  22  which indicates or represents a power level for the frequency band of operation. Alternatively, the digital power level indicator  26   b  can include a portion of the processing circuitry  98  which provides a signal or value representing the power level of the frequency band of operation or a portion thereof to the processing circuitry  22  (which could be part of the processing circuitry  98 ). Thus, the band edge power reduction system can prevent the overload of the A/D converter  35  interference within the operating bandwidth caused by signals from wireless units or RF signal sources operating in adjacent bands and/or not under the power control of the receiver  10 . 
     FIG. 2  shows an alternative embodiment of a receiver  120  which uses an embodiment of the band edge reduction system which searches for overloading band edge signals after the last intermediate (IF) frequency stage. The receiver  120  receives radio frequency (RF) analog signals at the antenna  122  and the radio front end  124  onto a main signal path  126 . The RF analog signals are down-converted to an intermediate frequency (IF) by providing the RF analog signals to a mixer  128  which also receives a signal from a local oscillator (LO)  130 . After passing through a possible IF stage  132 , the spectrum of interest is further down-converted by providing the spectrum to a mixer  134  which also receives a signal from a local oscillator (LO)  136 . After passing through a possible additional IF stage  140 , a coupler  142  provides a replica of the analog signal spectrum on the main signal path  126  onto a band edge detection path  144  for band edge detection circuitry  146 . 
   The band edge detection circuitry  146  receives the analog spectrum from the band edge detection path  144 , and an amplifier  148  amplifies the analog signals on the path  144 . The analog signals are then filtered in a desired manner by a bandpass filter  152 . A splitter  154  divides the analog signals on the band edge detection path  144  and provides a replica of the spectrum on an upper edge detection path  160  and a lower edge detection path  162 . On the upper edge detection path  160 , the analog spectrum is provided to a bandpass filter  164 , such as a SAW filter, which passes an upper band edge bandwidth of frequencies from the upper band edge relative to the frequency band of operation and attenuates other frequencies. The upper band edge bandwidth is provided to a power detector  166  which produces a signal indicating or representing the power level of the upper band edge bandwidth. A low pass filter  168  filters the signal from the power detector  166  to produce for processing circuitry  170 , such as a micro-controller including an A/D converter, a slower changing amplitude signal which represents the power level of the upper edge bandwidth. 
   On the lower edge detection path  162 , the analog spectrum is provided to a bandpass filter  172 , such as a SAW filter, which passes a lower band edge bandwidth of frequencies from the lower band edge relative to the frequency band of operation and attenuates other frequencies. The lower band edge bandwidth is provided to a power detector  176  which produces a signal indicating or representing the power level of the lower band edge bandwidth. A low pass filter  178  filters the signal from the power detector  176  to produce for the processing circuitry  170  a slower changing amplitude signal which represents the power level of the lower edge bandwidth. Thus, in this embodiment, the processing circuitry  170  receives signals indicating or representing the power levels of the upper and lower band edge bandwidths, and as a function of the amplitude at the band edge(s), the processing circuitry provides control signals to change the filtering characteristics of a variable filter  180 , for example to attenuate the power at the band edge(s). Depending on the embodiment, additional band edge detection paths can be used to detect additional band edges, for example if the receiver  120  operates within a changing frequency band of operation or operates within a fragmented or non-contiguous frequency band of operation between different cells and/or wireless communications systems. 
   On the main signal path  126  at the output to the coupler  142 , the spectrum is then provided to the variable filter  180  which filters the spectrum as a function of the amplitude(s) at the band edge(s), adjacent frequency band(s) and/or of signals not under the control of the receiver  120 . In this embodiment, the analog spectrum is provided to an analog to digital (A/D) converter  182  for conversion into the digital domain. The analog signals are sampled and digital sample values are produced (from which a digital representation of the analog spectrum can be obtained) onto a bus  184  to processing circuitry  188  which can include digital downconverters (DDCs) and digital signal processors (DSPs) (as well as the processing circuitry  170 ). If the power level(s) at the band-edge(s) adjacent to the frequency band of operation for the receiver  120  is above a certain level, for example from a wireless unit operating in adjacent frequencies, the power level at the band edge(s) could saturate the A/D converter  182 . 
   The processing circuitry  170  can provide control signals to the variable filter  180  as a function of the amplitude(s) at the band edge(s), at adjacent frequencies and/or of signals not under the control of the receiver  120  to attenuate the power at the band edge(s). In addition to the power at the band edge(s), at adjacent frequencies and/or of signals not under the control of the receiver  120 , the processing circuitry  120  can change the filtering characteristics as a function of the amplitude, such as power level, of the frequency band of operation or a portion thereof. For example, the processing circuitry  170  can provide control signals to the variable filter  180  based on a comparison of the power level of band edge(s) and a power level of the frequency band of operation or a portion thereof. Depending on the embodiment, an analog power indicator  190   a  can include a coupler  192  which provides a replica of the frequency band of operation onto a analog power detection path  194  to a power detector  196 . The power detector  196  provides a signal to the processing circuitry  170  which indicates or represents a power level for the spectrum of operation. Alternatively, the digital power level indicator  190   b  can include a portion of the processing circuitry  188  which provides a signal or value representing the power level of the frequency band of operation or a portion thereof to the processing circuitry  170  (which could be part of the processing circuitry  188 ). Thus, the band edge power reduction system can prevent the overload of the A/D converter  182  caused by signals from wireless units or RF signal sources operating in adjacent frequency bands and/or not under the power control of the receiver  120 . 
   In alternative embodiments, the processing circuitry  170  can provide control signal(s) to control the variable filter  180  as a function of the amplitude of signals not under the control of the receiver  120 , for example by comparing the amplitude of signals under the control of the receiver  120  and the total amplitude of signals within the frequency band of operation. Alternatively, the amplitude of signals not under the control of the of the receiver  120  can be assumed and cause a change in the filtering characteristics of the receiver  120  if the processing circuitry  170  receives an indication of overload or saturation, for example by receiving an overload signal from the A/D  182 . Furthermore, alternative embodiments for the variable filter  180  are possible for which the filtering characteristics can change using control signals from the processing circuitry  170 . 
   In addition to the embodiment described above, alternative configurations of the band edge amplitude reduction system according to the principles of the present invention are possible which omit and/or add components and/or use variations or portions of the described receiver architecture. As would be understood by one of ordinary skill in the art, the various components making up the receiver architecture and their respective operating parameters and characteristics should be properly matched up to provide the proper operation. For example, an embodiment of the receiver system can be used to receive signals from a North American TDMA system, a Global System For Mobile Communication (GSM) system, a code division multiple access (CDMA) system or frequency division multiple access (FDMA) systems. Accordingly, the receiver according to the principles of the present invention can receive analog signals using different frequency bands or schemes. The analog signals can be characterized as wideband, broadband and/or narrowband. Additionally, the embodiments of the receiver according to the principles of the present invention have been described with frequency band(s) associated with base station receive frequencies, but the receiver architecture according to the principles of the present invention can be used in wireless units, such as mobile units, receiving information from other frequency band(s), such as a wireless unit receive band. 
   Furthermore, the receiver system has been described using a particular configuration of distinct components, but it should be understood that the receiver system and portions thereof can be implemented in application specific integrated circuits, software-driven processing circuitry, firmware, programmable logic devices, hardware or other arrangements of discrete components as would be understood by one of ordinary skill in the art with the benefit of this disclosure. Although in the illustrative embodiment is shown with a particular circuitry, the band edge amplitude reduction system can use different components which together perform similar functions when compared to the circuitry shown. What has been described is merely illustrative of the application of the principles of the present invention. Those skilled in the art will readily recognize that these and various other modifications, arrangements and methods can be made to the present invention without strictly following the exemplary applications illustrated and described herein and without departing from the spirit and scope of the present invention.