Abstract:
Proposed is a method for reducing interference in a frequency hopping wireless communications system. In one embodiment of the present invention, a base station and a terminal station each using an orthogonal frequency division multiplexing (OFDM) technique to simultaneously measure an interference level for each system frequency and to enable high speed frequency hop pattern changes which can follow changes in desired and interfering signal levels due to changes in co-channel interference or shadow fading. The terminal station interference level measurement values are then transmitted to the base station. Next, the base station identifies each frequency hop pattern currently in use by each terminal station communicating with that base station. The base station then uses both the base station interference level measurements and the terminal station interference level measurements to identify each frequency hop pattern in which at least one of the current system frequencies should be replaced with a system frequency having a lower interference level. Next, the base station replaces no more than a predetermined number of the current system frequencies within the identified frequency hop pattern(s). The above steps are executed at each base station within the system while ensuring that nearby interfering base stations do not replace frequencies at the same time.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 09/312,279, filed May 14, 1999, which issued as U.S. Pat. No. 6,549,784 and which claims the benefit of U.S. Provisional Application Ser. No. 60/114,080, filed Dec. 28, 1998, each of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present invention is directed to frequency hopping wireless communications systems. More specifically, the present invention reduces interference levels and increases capacity in frequency hopping wireless communications systems by dynamically replacing system frequencies in use within selected frequency hop patterns with system frequencies having lower interference levels and by precluding nearby interfering system components (for example, base stations) from simultaneously making frequency replacements using the same available system frequencies. 
     The demand for wireless communications services continues to grow at an astonishing rate. For example, each day a greater percentage of the public elects untethered access to a telephone system using cellular telephones. Unlike traditional telephones with attached cords which limit the user&#39;s movement, cellular telephones allow users to make telephone calls while in transit between locations. In addition to wireless voice communication services, the public is discovering numerous instances where wireless data communication simplifies their lives. For example, an employee who has traveled on business away from a local area network (LAN) in the home office may use a laptop computer having a radio transceiver to establish a wireless connection to the LAN from within a hotel room. Once the wireless connection is established, the employee may check electronic mail or access other files on the LAN in the same manner these tasks might be performed from within the home office using a desktop computer wired to the LAN. 
     Unfortunately, the number of frequencies available to support the public&#39;s growing appetite for wireless communications services is limited. Thus, service providers must make the most efficient use of these frequencies to meet the growing demand. One method for increasing the efficiency of a wireless communication system entails avoiding the use of frequencies with high interference levels which might otherwise require that data be retransmitted thereby consuming additional system resources. Some of the current wireless communication systems implement some type of frequency hopping technique to reduce the system-wide impact of frequencies which are experiencing high interference levels. As explained below, however, the current methods for implementing frequency hopping techniques leave room for improvement. 
     Understanding the current methods for implementing frequency hopping techniques requires a basic understanding of how typical wireless communication systems transmit data and the major sources of interference within theses systems. In a typical wireless communication system, a transmitter modulates a carrier frequency with voice or data information and transmits the modulated carrier frequency through the air to a receiver. The receiver then demodulates the carrier frequency to obtain the included voice or data information. In some wireless systems, the receiver sends the transmitter a message which indicates whether the transmitted data was successfully received. Co-channel interference, multipath fading, and shadow fading are among the types of interference which may prevent the receiver from successfully receiving transmitted data. 
     Co-channel interference may result when two transmitters within range of each other attempt to transmit data to their respective receivers using the same carrier frequency at the same time. The greater the level of co-channel interference, the greater the chance the transmitted data will become too distorted for the receiver to process. System resources required to retransmit this data are unavailable to transmit newly arriving data. As a result, the flow of data through the system is slowed. As the number of users in a wireless system using the limited available number of frequencies continues to increase, the possibility that two or more transmitters may be located within range of each other and transmit data using the same frequency at the same time also increases. Co-channel interference is particularly relevant to the design and deployment of cellular wireless systems. 
     Multipath fading occurs when a transmitted signal is reflected by objects in the path between the transmitter and receiver. As a result of one or more reflections, multiple versions of the transmitted signal may arrive at the receiver at different times. The division of the transmitted signal into these multiple versions may cause the amplitude of the transmitted signal to fade at the receiver. If the level of fading is great enough, the strength of the signal arriving at the receiver may be too low for proper receiver processing and the signal may need to be retransmitted. 
     Shadow fading is caused by vehicles moving in and out from behind buildings, hills, and other obstructions. Shadow fading changes at a rate of about once per second. 
     Both co-channel interference and multipath fading are frequency dependent. For example, two in-range transmitters may transmit at the same time without interfering with each other if each transmitter uses a different frequency. With respect to multipath fading, some ranges of frequencies are more susceptible to fading than others when transmitted along the same path. Thus, some wireless communications systems constantly “hop” from one available carrier frequency to another available carrier frequency while transmitting data to avoid the prolonged use of a frequency which might be experiencing high interference levels. Current frequency hopping systems select frequencies at the time a call is initiated. Prior to hopping from one frequency to another, the transmitting device will usually send a message to the receiving device so the receiving device will anticipate receiving data on the new frequency. Depending on the wireless system, the pattern the transmitter follows while hopping among available frequency be preplanned, random, pseudo-random, or based upon dynamic frequency interference level measurements. Further, when a receiver switches from communicating with one transmitter to communicating with another transmitter, the frequency hop patterns will likely change. 
     Some frequency hopping wireless systems continually measure interference levels for selected system frequencies during system operation. These “dynamic” interference level measurements may be used to substitute frequencies experiencing high interference levels with frequencies having lower interference levels. For example, U.S. Pat. No. 5,323,447 to Mark E. Gillis et al describes a frequency hopping method in which a cordless telephone handset measures interference levels among a first group of frequencies within a frequency hop pattern while using the first group of frequencies to communicate with a base unit. When interference is detected on one of the frequencies in the first group, the base station replaces that frequency with a frequency (from a second group of frequencies) having a lower interference level. In another example, U.S. Pat. No. 5,394,433 to David F. Bantz et al, discloses a frequency hopping method in which the entire frequency hop pattern currently in use by a base station and a mobile station is replaced with a new frequency hop pattern from a predetermined set of patterns upon detecting frequencies with an unacceptable interference level within the current frequency hop pattern. 
     Unfortunately, current dynamic frequency hop management methods measure each system frequency sequentially. Due to the rate at which interference levels may be sequentially measured for each system frequency, these current frequency hop management methods also do not contemplate measuring all system frequencies at a rate near the rate at which the power of a received frequency signal fades while propagating through the transmission medium or the rate at which co-channel interference charges. The medium through which a frequency signal is transmitted influences the strength of the signal at the receiver. The Rayleigh fading rate is typically used to describe the statistical time varying nature of frequency signals transmitted; through the air. Although the Rayleigh fading rate covers a range of rates, a fading rate of 100 Hz (which translates to a period of approximately 10 ms) is typically used to describe the rate at which the power of a received frequency signal varies while propagating through the air. Current frequency hop management methods are only able to measure interference levels for a small portion of the total number of frequencies available to a typical wireless system within a period during which channel and interference changes occur. Thus, current frequency management methods make replacement decisions by selecting high quality frequencies from among fewer than the total number of frequencies available to the system and make replacement decisions with insufficient knowledge of both the propagation medium and interference behavior. The ability to measure interference levels for all available system frequencies at a rate faster than once per second enables a frequency management method to take full advantage of the potential benefits of frequency hopping techniques by selecting best quality frequencies from among all system frequencies when attempting to reduce the impact of both propagation medium and interference influences on the quality of system frequencies. 
     Additionally, current frequency hop management methods do not eliminate the possibility that two interfering transmitters within the same system may respond to measured interference levels by simultaneously switching to the same high quality frequencies and again interfering with each other&#39;s transmissions. 
     In view of the above, it can be appreciated that there is a need for a method and apparatus that solves the above mentioned problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for reducing interference in a frequency hopping wireless communication system. According to an embodiment of the present invention, a wideband transceiver and an orthogonal frequency division multiplexing (OFDM) technique are used to simultaneously measure an interference level for each system frequency. After simultaneously measuring interference levels for each system frequency using a base station and a terminal station communicating with the base station, the base station identifies a frequency hop pattern currently in use for each base station/terminal station communication link. The measured frequency interference levels are then used to identify each frequency hop pattern in which at least one of the current system frequencies should be replaced with a system frequency having a lower interference level. The base station then replaces no more than a predetermined number of the current system frequencies within the identified frequency hop pattern(s) with system frequencies having lower interference levels. The above steps are executed independently for uplink and downlink frequency hop patterns at each base station within the wireless system while ensuring that nearby mutually interfering base stations do not replace frequency hop pattern frequencies at the same time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a system suitable for practicing an embodiment of the present invention. 
     FIG. 2 illustrates an example of a frequency hop pattern composed of six frequency dwells. 
     FIG. 3 illustrates a terminal station and a base station suitable for practicing an embodiment of the present invention. 
     FIG. 4 illustrates the terminal station and base station of FIG. 3 modified to implement OFDM processing in hardware in accordance with one embodiment of the present invention. 
     FIG. 5 illustrates an example of an OFDM block, in accordance with an embodiment of the present invention. 
     FIG. 6 illustrates the terminal station and base station of FIG. 3 modified to implement OFDM processing in software in accordance with one embodiment of the present invention. 
     FIG. 7 illustrates a superframe in accordance with an embodiment of the present invention. 
     FIG. 8 is a flow chart illustrating an example of the steps for performing a method in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention reduces interference levels within a frequency hopping wireless communication system by dynamically replacing system frequencies in use within selected frequency hopping patterns with system frequencies having lower interference levels and by precluding nearby mutually interfering system components (such as base stations) from simultaneously making frequency replacements using the same available system frequencies. FIG. 1 is a diagram of a system suitable for practicing an embodiment of the present invention. In FIG. 1, a geographic area served by a frequency hopping wireless communications system is divided into a plurality of cells  110 . In this embodiment three hexagonal cells  110  are shown. The system can have more or less than three cells and the cells need not be hexagonal in shape. Each cell  110  includes a base station  102  and one or more terminal stations  104 . Terminal stations  104  may be fixed or mobile. Each base station  102  and terminal station  104  is adapted to transmit and receive voice and/or data information using radio frequency signals. 
     Each base station  102  is adapted to be connected to a mobile switching center (MSC)  106 . MSC  106  is adapted to be connected to a fixed network  108 . Fixed network  108  may be, for example, a Public Switched Telephone Network (PSTN). 
     Each base station  102  may select from the entire set of radio frequencies available to the communications system for use in two-way communication with terminal stations  104  located within the geographic area of the cell  110  in which each base station  102  is located. Two-way communication between a base station  102  and a particular terminal station  104  within the same cell  110  is accomplished by sequentially modulating a set of system radio frequencies with voice and/or data information. The chronological sequence in which each frequency within the set is modulated with voice and/or data information is known as a frequency hop pattern. Each radio frequency within a given frequency hop pattern is modulated with voice and/or data information for a duration of time known as a frequency dwell. 
     FIG. 2 illustrates an example of a frequency hop pattern composed of six frequency dwells. In FIG. 2, time is incremented in milliseconds (ms) along the horizontal axis and frequency is incremented in megahertz (MHz) along the vertical axis. The frequency hop pattern of FIG. 2 repeats each 60 ms. Each frequency dwell within this frequency hop pattern has a duration of 10 ms. The system frequency in use during each frequency dwell of this frequency hop pattern may be determined using FIG.  2 . For example, the 820 MHz system frequency is modulated with voice and/or data information during the first frequency dwell of this frequency hop pattern. 
     Each base station  102  in FIG. 1 controls which of the system radio frequencies are allocated to the frequency hop pattern used to communicate with each terminal station  104  within that base station&#39;s cell  110 . First, the base station  102  selects the frequencies which will be used to communicate with a particular terminal station  104 . The base station then informs the terminal station  104  of the selected frequencies by, for example, transmitting a message to that terminal station using predetermined designated control frequencies. Similarly, to preclude terminal stations  104  within the same cell from simultaneously transmitting voice and/or data information using the same frequency, each base station  102  controls the sequence of frequencies (i.e., which frequency is used during each frequency dwell) within the frequency hop patterns used by terminal stations  104  within that base station&#39;s cell  110 . 
     FIG. 3 illustrates a terminal station and a base station suitable for practicing an embodiment of the present invention. Terminal station  302  is a known device, such as a cellular telephone, modified in accordance with the present invention. As illustrated in FIG. 3, terminal station  302  comprises a processor  306  adapted to be connected to a transceiver  308  and a computer readable memory  310 . Transceiver  308  is adapted to be connected to an antenna  314 . 
     Computer readable memory  310  stores computer program code segments which, when executed by processor  306  implement the main functionality for this embodiment of the invention. These computer program code segments are included within a quality measurement module  312  and a frequency hopping module  328 . Although in this embodiment of the invention, the computer program code segments are shown in two modules, it can be appreciated that these modules can be further separated into more modules or combined into one module, and still fall within the scope of the invention. 
     Base station  304  is a known device modified in accordance with an embodiment of the present invention. As illustrated in FIG. 3, base station  304  comprises a processor  320  adapted to be connected to a computer readable memory  322  and a transceiver  318 . Transceiver  318  is adapted to be connected to an antenna  316 . 
     Computer readable memory  322  stores computer program code segments which, when executed by processor  320  implement the main functionality for this embodiment of the invention. These computer program code segments are included within three modules: a quality measurement module  324 , a frequency hop pattern adaptation module  326 , and a frequency hopping module  330 . Although in this embodiment of the invention, the computer program code segments are shown in three modules, it can be appreciated that these module can be further separated into more modules or combined into one module, and still fall within the scope of the invention. 
     By simultaneously (rather than sequentially) measuring an interference level for each system frequency, the method of the present invention obtains frequency interference level measurement values faster than current methods. In one embodiment of the present invention, simultaneous system frequency interference level measurements are rapidly obtained using a wideband transceiver and an OFDM technique. FIG. 4 illustrates the terminal station and base station of FIG. 3 modified to implement OFDM processing in hardware, in accordance with one embodiment of the present invention. Terminal station  402  includes a wideband transceiver  408  and an OFDM block  432  (described in greater detail below) in addition to the components described above with reference to terminal station  302  of FIG.  3 . Similarly, Base station  404  includes a wideband transceiver  418  and an OFDM block  434  (described in greater detail below) in addition to the components described above with reference to terminal station  304  of FIG.  3 . 
     FIG. 5 illustrates an example of an OFDM block, in accordance with an embodiment of the present invention. In FIG. 5, an OFDM block  502  comprises a serial to parallel conversion device  504  adapted to receive an input signal from a wideband transceiver, such as wideband transceiver  408  or wideband transceiver  418  of FIG. 4. A fast Fourier transform (FFT) processing device  506  is adapted to receive “N” input signals from serial to parallel conversion device  504 . A parallel to serial conversion device  508  is adapted to receiver “N” input signals from FFT processing device  506 . Parallel to serial conversion device  508  is also adapted to send an output signal to a processor such as processor  406  or processor  420  of FIG.  4 . OFDM block  502  also comprises a serial to parallel conversion device  514  adapted to receive an input signal from a processor such as processor  406  or processor  420  of FIG.  4 . An inverse fast fourier transform (IFFT) device  512  is adapted to receive “N” input signals from serial to parallel conversion device  514 . A parallel to serial conversion device  510  is adapted to receive “N” input signals from IFFT processing device  512 . Parallel to serial conversion device  510  is also adapted to send an output signal to a wideband transceiver such as wideband transceiver  408  or wideband transceiver  418  of FIG.  4 . 
     As mentioned previously terminal stations and base stations of the present invention are adapted to transmit and receive data. Thus, in one embodiment of the present invention, an OFDM block similar to that illustrated in FIG. 5 is included within each terminal station and each base station of the wireless communication system. To transmit data from terminal station  402  to base station  404 , terminal station  402  provides a high bit rate data stream to an OFDM block within terminal station  402  such as OFDM block  502  illustrated in FIG.  5 . Serial to parallel conversion device  514  receives the high bit rate data stream and uses this data stream to generate “N” parallel low bit rate data streams (where “N” is an integer value). The value of “N” is determined by the number of frequencies available to the wireless communication system. Serial to parallel conversion device  514  then sends these “N” parallel low bit rate data streams to IFFT processing device  512 . IFFT processing device  512  uses each of the “N” parallel low bit rate data streams to modulate “N” different carrier frequencies and then converts each of these “N” frequency domain signals to “N” corresponding time domain signals. IFFT processing device  512  sends these “N” time domain signals to parallel to serial conversion device  510 . Parallel to serial conversion device  510  uses the “N” time domain signals to generate a single signal comprised of a high bit rate serial stream of data and sends this high bit rate serial data stream signal to a wideband transceiver or a modulating device where the data stream is used to modulate a range of carrier frequencies available to the wireless communications system. The modulated signal is then transmitted through the air to base station  404 . 
     Upon receiving the modulated high bit rate serial data stream signal, base station  404  may use either wideband transceiver  418  or another demodulating device to demodulate the high bit rate serial data stream signal from the carrier frequency. This high bit rate serial data stream signal is then sent to an OFDM block within base station  404  such as the OFDM block  502  illustrated in FIG.  5 . Serial to parallel conversion device  504  receives the high bit rate serial data stream and converts this data stream to “N” parallel low bit rate data stream signals. These “N” parallel low bit rate data stream signals are then sent to FFT processing device  506 . FFT processing device  506  uses the “N” parallel low bit rate data stream signals to generate “N” output signals, where “N” is the number of frequencies available to the communications system. Each system frequency is simultaneously represented by one output from FFT processing device  506 . As described below, signals output from FFT processing device  506  are used to simultaneously determine the quality of each system frequency. In one embodiment, the quality of each system frequency is determined by comparing the relative amplitudes of each signal output from FFT processing device  506 . If measurements are taken during a period in time when no terminal station is transmitting data using a particular system frequency, the output signal from FFT processing device  506  which corresponds to this particular signal may represent the value of interference at that frequency. Thus, the higher the amplitude of that FFT processing device  506  signal, the higher the interference level experienced at that particular frequency. Other methods for representing the quality of each system frequency such as a ratio of signal to noise may be obtained in a similar manner. 
     The parallel signals output from FFT processing device  506  are sent to parallel to serial conversion device  508 . Parallel to serial processing device  508  uses these parallel signals to regenerate the high bit rate serial data stream which terminal station  402  sent to base station  404 &#39;s OFDM block. Parallel to serial processing device  508  then sends this high bit rate serial data stream to processor  420  for further processing by base station  404 . This further processing may entail sending the data stream to a PSTN through an MSC as illustrated in FIG.  1 . 
     OFDM processing may be implemented in hardware, as described above or in software. FIG. 6 illustrates the terminal station and base station of FIG. 3 modified to implement OFDM processing in software, in accordance with one embodiment of the present invention. Terminal station  602  includes a wideband transceiver  608 , an analog to digital/digital to analog processing device  636 , and an OFDM module  632  (described in greater detail below) in addition to the components described above with reference to terminal station  302  of FIG.  3 . Similarly, base station  604  includes a wideband transceiver  618 , an analog to digital/digital to analog processing device  638 , and an OFDM module  634  in addition to the components described above with reference to base station  304  of FIG.  3 . Both analog to digital/digital to analog processing devices  636  and  638  include components used to convert a received analog signal to a digital output signal (and vice versa) as appropriate. Analog to digital/digital to analog processing devices  636  and  638  transmit signals to and receive signals from OFDM modules  632  and  634  respectively via processors  606  and  620  respectively. OFDM modules  632  and  634  include computer program code segments (as described above with reference to FIG. 3) which implement OFDM signal processing in a manner similar to that described above with reference to FIG.  5 . 
     For example, upon receiving a modulated signal from terminal station  602 , base station  604  first demodulates the signal from the carrier frequency using wideband transceiver  618  or another demodulation device. The demodulated signal is then converted from analog form to a corresponding digital representation using analog to digital/digital to analog conversion device  638 . The digital signal is then processed as described above with reference to FIG. 5 by using processor  620  to execute the computer program code segments within OFDM module  634 . Similar steps are followed in reverse by base station  604  when transmitting data to terminal station  602 . 
     An example of the method of the present invention will now be described with reference to FIG. 1, FIG.  4  and FIG.  5 . To initialize the system, each base station  102  is time synchronized using a system such as the Global Positioning System. (GPS). Once each base station  102  is time synchronized, a repeating time segment (referred to herein as a “superframe”) is divided into three time segments known as frames and each base station  102  is assigned to a frame within the superframe. FIG. 7 illustrates a superframe in accordance with an embodiment of the present invention. The repeating superframe of FIG. 7 is divided into three frames. 
     Once base stations  102  are time synchronized, the following steps (explained below with reference to FIG. 4) are performed at each of the three base stations  102  of FIG.  1 . For the purpose of this example, assume a plurality of terminal stations such as terminal station  402  illustrated in FIG. 4 are physically located within the geographic area of base station  404 &#39;s cell. First, base station  404  uses quality measurement module  424  to obtain quality measurement values for each system frequency. These quality measurements may be obtained, for example, through processor  420  from a device such as OFDM block  434 . More particularly, these measurements may be obtained from a device such as FFT processing device  506  (illustrated in FIG. 5) included within OFDM block  434 . Each frequency is represented by an FFT output. In one embodiment, if a frequency was not being used for transmission during a time period when measurements are obtained from FFT module  506 , all the energy observed at the output corresponding to that frequency represents the value of interference at that frequency. Thus, the higher the amplitude of the value received from FFT module  506 , the higher the interference level for that frequency. In another embodiment, each output of FFT module  506  represents a ratio of the strength of the frequency signal to the noise level experienced by that frequency signal: 
     Either on a continuous basis or upon receiving a request from base station  404  each of the plurality of terminal stations  402  obtain quality measurements for all system frequencies available to the communications system. These measurements are accomplished in a manner similar to that described above. The plurality of terminal stations  402  then send their frequency quality measurements to base station  404 . Using both the frequency quality measurements obtained by base station  404  and the frequency quality measurements received from the plurality of terminal stations  402 , quality measurement module  424  determines a quality value for each system frequency and assigns a rank number to each system frequency based upon the determined quality values. The rank numbers associated with each system frequency increase as the quality value of each frequency decreases. This measurement and ranking is accomplished independently for uplink and downlink frequency hop patterns. 
     Quality measurement module  424  next retrieves the identity of each system frequency used in each frequency dwell of each frequency hop pattern used by each of the plurality of terminal stations  402 . As mentioned previously, base station  404  controls and assigns the frequencies within the frequency hop patterns implemented by terminal stations  402  within base station  404 &#39;s cell. Thus, this information may be retrieved from within computer readable memory  422 . Next, quality measurement module  424  assigns the rank number to each frequency dwell which corresponds to the system frequency modulated during that frequency dwell. 
     Quality measurement module  424  then analyzes each frequency hop pattern using the rank information to identify terminal station frequency hop patterns in which one or more frequencies should be replaced with system frequencies having higher quality values (lower interference levels). This replacement information is then sent to frequency hop pattern adaptation module  426 . Frequency hop pattern adaptation module  426  determines which frequencies should be replaced and informs frequency hopping module  430 . Frequency hopping module  430  makes the appropriate frequency changes and uses processor  420  to transmit a message to frequency hopping module  428 . This message instructs frequency hopping module  428  to make the same frequency changes. Frequency changes within frequency hop patterns also occur independently for uplink and downlink frequency hop patterns. 
     One method for analyzing the frequency hop patterns in use by terminal stations  402  communicating with base station  404  is referred to herein as the “mobile ranking method.” The mobile ranking method entails first assigning a cumulative score to each terminal station. The cumulative score for a terminal station is obtained by summing the rank numbers (or the quality measurement values) assigned to the frequency dwells within that terminal station&#39;s frequency hop pattern. Each terminal station is then ranked according to the individually assigned cumulative scores. The terminal station with the worst (highest) cumulative score receives a new frequency hop pattern composed of the best quality frequencies available for each frequency dwell of that frequency hop pattern. The terminal station with the second worst score receives a new frequency hop pattern composed of the next best quality set of frequencies for each individual frequency dwell. This procedure is repeated until the terminal station with the lowest cumulative score receives a new frequency hop pattern composed of the remaining best quality frequencies. In the preferred embodiment, the total number of available system frequencies exceeds the number of frequencies required to assign one system frequency to each frequency dwell within each frequency hop pattern by such a margin that, during the mobile ranking method, the lowest quality frequencies will not be allocated to any frequency hop pattern. 
     In another embodiment of the present invention, the above frequency hop pattern analysis method is modified by comparing the cumulative score assigned to each terminal station to a predetermined threshold value. This analysis method is referred to herein as the “threshold based mobile ranking method.” No frequency hop pattern reassignments are made for terminal stations having a cumulative score below the threshold value. The frequencies in use by those terminal stations with a cumulative score below the threshold value are not available for reassignment to terminal stations having a cumulative score higher than the threshold value. The terminal stations having cumulative scores above the threshold value are ranked according to their cumulative scores. The terminal station with the worst (highest) cumulative score receives a new frequency hop pattern composed of the remaining available best quality frequencies for each frequency dwell of that frequency hop pattern. The terminal station with the second worst score receives a new frequency hop pattern composed of the next remaining best quality set of frequencies. This procedure is repeated until the terminal station (from among the pool of terminal stations having a cumulative score above the threshold value) with the lowest cumulative score receives a new frequency hop pattern composed of the remaining best quality frequencies. In the preferred embodiment, the total number of available system frequencies exceeds the number of frequencies required to assign one system frequency to each frequency dwell within each frequency hop pattern by such a margin that, during the threshold based mobile ranking method, the lowest quality frequencies will not be allocated to any frequency hop pattern. 
     In another embodiment of the present invention, the frequency hop patterns in use by terminal stations  402  communicating with base station  404  are analyzed by identifying each frequency dwell which includes a frequency having a rank number in the higher end of the range of rank numbers. In accordance with this method, referred to herein as the “frequency dwell ranking method,” frequencies with higher rank numbers are systematically replaced with frequencies having lower rank numbers. As a higher quality (lower rank number) frequency is used as a replacement, that frequency is removed from the pool of available replacement frequencies which may be used in that same frequency dwell by other terminal stations. Removing frequencies from the pool in this manner ensures no two terminal stations attempt to modulate the same frequency with voice and/or data information during simultaneously occurring frequency dwells. 
     In yet another embodiment of the present invention, an analysis method referred to herein as the “threshold based frequency dwell ranking method” is employed. In this embodiment, the rank of each frequency within each frequency dwell is compared to a predetermined threshold value. Frequencies having rank numbers below the threshold value remain in use during their current frequency dwell and are removed from the pool of available replacement system frequencies. Among the frequencies having rank numbers above the threshold value, the lowest quality frequencies are systematically replaced with the highest quality frequencies. As a higher quality frequency is used as a replacement, that frequency is removed from the pool of available replacement system frequencies which may be used in that same frequency dwell by other terminal stations. Removing frequencies from the pool in this manner ensures no two terminal stations attempt to modulate the same frequency with voice and/or data information during simultaneously occurring frequency dwells. 
     In yet another embodiment of the present invention, regardless of the frequency hop analysis method employed, the number of frequencies which may be replaced within any one frequency hop pattern is limited by a predetermined number. 
     There are some tradeoffs and advantages associated with choosing from among the above four analysis methods. The mobile ranking method may be implemented with the least complex algorithm. The threshold based mobile ranking method requires transmitting the fewest number of messages which alert other components of impending frequency changes. The frequency dwell ranking method results in the lowest interference levels within the system. 
     As mentioned previously, each of the three base stations  102  in FIG. 1 are assigned to a time frame within a superframe. In one embodiment of the present invention, each base station may only replace frequencies within frequency hop patterns during the frame to which that base station is assigned. This limitation helps reduce the probability that system interference levels will increase due to multiple base stations simultaneously switching to the same high quality system frequencies within simultaneously occurring frequency dwells. 
     FIG. 8 is a flow chart illustrating an example of the steps for reducing interference within a frequency hopping wireless communication system according to an embodiment of the present invention. The flow chart of FIG. 8 may be implemented, for example, as a computer program or as computer hardware using well-known signal processing techniques. If implemented in software, the computer program instructions may be stored in computer readable memory, such as Read-Only Memory (ROM), Random Access Memory (RAM), magnetic disk (e.g., 3.5″ diskette or hard drive), and optical disk (e.g., CD-ROM). The stored programs may be executed, for example, by a general purpose computer which includes a processor. More particularly, the steps illustrated in FIG. 8 may be included within quality measurement module  424  and frequency hop pattern adaptation module  426  illustrated in FIG.  4 . 
     In step  802 , a base station simultaneously determines a quality value for each frequency available to the wireless communication system and ranks each system frequency as described above. These quality values may be determined using measurements obtained using OFDM methods implemented by both a base station and one or more terminal stations geographically located within the base station&#39;s cell (as described above) or may be obtained using OFDM methods implemented by only a base station. In step  804 , the base station identifies each frequency hop pattern in use by each terminal station currently communicating with this particular base station. In step  806 , the base station analyzes each frequency within each identified frequency hop pattern to ascertain those frequencies which should be replaced with system frequencies having a lower interference value. This step may be executed, for example, in accordance with one of the four above described analysis methods. In step  808 , during the appropriate frame of a superframe, this particular base station replaces the ascertained frequencies. The system executes steps  802 - 808  at each base station within the wireless communications system. This procedure is executed independently for uplink and downlink. 
     Although several embodiments are specifically illustrated herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, although the method of the present invention is described in the context of using OFDM processing techniques, other techniques may also be used to simultaneously obtain interference level measurements for each system frequency.