Patent Publication Number: US-2011077034-A1

Title: Method and System for Reducing Channel Interference in a Frame-Synchronized Wireless Communication System

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/617,557, filed Dec. 28, 2006, titled METHOD AND SYSTEM FOR REDUCING CHANNEL INTERFERENCE IN A FRAME SYNCHRONIZED WIRELESS COMMUNICATION SYSTEM, which is a continuation of U.S. patent application Ser. No. 09/947,650, filed Sep. 5, 2001, titled “METHOD AND SYSTEM FOR REDUCING CHANNEL INTERFERENCE IN A FRAME SYNCHRONIZED WIRELESS COMMUNICATION SYSTEM”, which issued as U.S. Pat. No. 7,177,598 on Feb. 13, 2007 which claims priority to provisional application Ser. Nos. 60/248,867, filed Nov. 15, 2000, titled “METHOD AND APPARATUS FOR REDUCING CO-CHANNEL INTERFERENCE IN A FRAME-SYNCHRONIZED WIRELESS COMMUNICATION SYSTEM” and 60/249,769, filed Nov. 16, 2000, titled “METHOD AND APPARATUS FOR REDUCING CO-CHANNEL INTERFERENCE IN A FRAME-SYNCHRONIZED WIRELESS COMMUNICATION SYSTEM”, all of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to communication systems and to a method and system for reducing channel interference in a wireless communication system. 
     2. Description of Related Art 
     A wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (either fixed or portable) and a fixed network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and the base station in order to connect the subscriber unit user with the fixed network infrastructure (usually a wired-line system). In the wireless systems using multiple access schemes, frames of time are the basic transmission unit. Each frame is divided into a plurality of slots of time. Some time slots are used for control purposes and some time slots are used for information transfer. Information is typically transmitted during time slots in the frame where the time slots are assigned to a specific subscriber unit. Subscriber units typically communicate with the base station using a “duplexing” scheme that allows for the exchange of information in both directions of connection. 
     Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art. Exemplary wireless communication systems using these schemes are described in more detail in the related U.S. Pat. No. 6,038,455, by Gardner et al., issued Mar. 14, 2000, entitled “Reverse Channel Reuse Scheme in a Time Shared Cellular Communication System”, which is herein incorporated by reference. 
     Some communication systems do not use time frames in communicating between the base station and their respective and associated subscriber units (or “terminal stations” in Broadband Wireless Access (BWA) communication systems). For example, BWA systems based on cable modem technologies do not use time frames when communicating on either the uplink or the downlink. Therefore, these systems do not allow for frame synchronization between base stations and disadvantageously do not permit coordination between the base stations for purposes of reducing co-channel interference. Similarly, un-synchronized TDD systems allow different communication cells within the system to be “free running”, in that different cells and sectors within the system operate on frames that are not synchronized in time. 
     Wireless communication systems rely upon frequency re-use because frequency allocation or bandwidth is typically limited. For example, in cellular communication systems and broadband wireless systems, geographic areas or regions are typically divided into cells that are nominally hexagonally or square shaped. As described in U.S. Pat. No. 6,038,455, each cell or sector is allocated one or more radio frequency channels. For example, in a cellular communication system utilizing frequency division multiple access (FDMA), adjacent or nearby cells are assigned separate frequencies. After all available frequencies have been allocated, it is necessary to begin reusing the frequencies. For example, if four frequencies are available, it is necessary to begin using the first frequency again starting in the fifth cell. Due to the nature of the systems described in the incorporated U.S. Pat. No. 6,038,455, and in PCS, cellular and paging systems of the prior art, frequency re-use cannot be used as aggressively as it can be used in BWA systems. For example, in PCS/cellular/paging systems, typically only a fraction of the frequency spectrum is used per cell. In contrast, in BWA, frequency re-use can be much more aggressive (for example, a frequency can be re-used at least once per cell, with multiple sectors). 
     Since base stations often operate on the same radio frequency (RF) channel or adjacent RF channels and transmit in the same general direction, terminal stations within adjacent sectors can receive unwanted RF energy from base stations. Similarly, the base station can receive unwanted RF energy from terminal stations. Receiving unwanted RF energy causes channel interference in the form of co-channel interference or adjacent channel interference. Therefore, a need exists for a method and system for reducing channel interference in a wireless communication system. By implementing such a method, the wireless communication system&#39;s capacity should increase. Furthermore, this channel interference reducing system and method can utilize uplink and downlink bandwidth in an allocation-efficient manner. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to systems and methods for reducing channel interference in a wireless communication system. 
     Channel interference in wireless communication systems that are coordinated and/or synchronized, such as a frame-synchronized broadband wireless communication system, can be reduced at selected base stations by transmitting uplink data from the selected base stations at selected times or frames while transmitting uplink data from other base stations at different selected times or frames. The embodiments described herein can be used with systems that have a framing structure on the downlink and uplink, or alternatively only on the uplink, as do time division duplex (“TDD”) or half-duplex frequency division duplexing (“FDD”) systems. A “frame synchronized” system is one in which base stations use frame based communications, and wherein all of the frames are synchronized to one another in the time domain. The embodiments described herein can be used with communication systems to reduce channel interference due to co-channel interference and adjacent channel interference. 
     One aspect includes co-channel interference reducing systems and methods that utilize sub-channels of the uplink channels to reduce co-channel interference. Another aspect includes a method and system for reducing co-channel interference in a frame-based and frame-synchronized broadband wireless communication system by using even and odd frames. Such a system can reduce co-channel interference from selected terminal stations by transmitting uplink data from the selected terminal stations at pre-selected time frames while transmitting uplink data from other terminal stations at different pre-selected time frames. The systems and methods described herein improve performance and capacity in both FDD and TDD applications. 
     Another aspect includes a method and system for reducing channel interference in a communication system that includes base stations and terminal stations with each terminal station associated with a base station, and each base station being associated with a cell. The method comprises determining a first set of terminal stations in a first cell that may potentially interfere with a second set of terminal stations in a second cell, determining a first subset of the first set of terminal stations that lie along a geographic diagonal line passing through a base station in the second cell, and determining a second subset of the second set of terminal stations that uplink using a less robust modulation technique. The method further includes selecting a more robust modulation technique for the second subset of the second set of terminal stations and coordinating uplink transmissions by the first subset of the first set of terminal stations with uplink transmissions by the second subset of the second set of terminal stations, the second subset of the second set of terminal stations utilizing the more robust modulation technique to reduce channel interference at the base station in the second cell. Additional embodiments coordinate the uplink transmissions from the first and second terminal stations to minimize channel interference. Further embodiments vary forward error correction (FEC) techniques to minimize channel interference. 
     Another aspect includes a method and system for reducing adjacent channel interference in a communication system that includes a base station and terminal stations with each base station having antenna arrays, and each terminal station associated with one antenna array for transmitting an uplink to the base station. The method comprises determining a first set of terminal stations that may potentially interfere with a second set of terminal stations, the second set of terminal stations being associated with a first antenna array of a base station, determining a second subset of the second set of terminal stations that uplink to the first antenna array using a less robust modulation technique, and selecting a more robust modulation technique for the second subset of the second set of terminal stations. The method further includes coordinating uplink transmissions by the first set of terminal stations with uplink transmissions by the second subset of the second set of terminal stations, the second subset of the second set of terminal stations utilizing the more robust modulation technique to reduce adjacent channel interference at the first antenna array. Additional embodiments coordinate the uplink transmissions from the first and second terminal stations to minimize channel interference. Further embodiments vary forward error correction (FEC) techniques to minimize channel interference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a diagram of a broadband wireless configuration incorporating frequency re-use where a plurality of base stations  1 ( a )-( d ) communicate with fixed terminal stations (i.e., “subscriber units”). 
         FIG. 1   b  shows a wireless communication system using a frequency re-use configuration that illustrates the potential problems associated with co-channel interference. 
         FIG. 2  is a graph of uplink C/I ratios for transmissions by terminal stations in sectors T 1 -T 4  towards base station  1 ( g ) of  FIG. 1   b.    
         FIG. 3   a  is a representation of uplink and downlink channels when the uplink channel is subdivided into sub-channels. 
         FIG. 3   b  is a simplified graphical representation of an exemplary broadband wireless frequency re-use configuration showing the allocation of sub-channels of a single uplink channel. 
         FIG. 3   c  is a representation of the system of  FIG. 1   b  utilizing an even-odd frame frequency re-use scheme. 
         FIG. 4  is a time frame map illustrating the even-odd frequency re-use scheme described with reference to  FIG. 3   c.    
         FIG. 5   a  is a simplified graphical description of a frequency re-use scheme having a frequency re-use factor of 2, 1 frequency set and 4 sectors per cell. 
         FIG. 5   b  is an exemplary time frame map representing the uplink and downlink allocation of sectors X and Y. 
         FIG. 6  is a simplified graphical description of a frequency re-use scheme having a frequency re-use factor of 2, 3 frequency sets and 6 sectors per cell. 
         FIG. 7  is a simplified graphical depiction of a frequency re-use scheme showing a frequency re-use factor of 3/2, with 4 available frequency sets and 6 sectors per cell. 
         FIG. 8  is a graphical representation of the uplink C/I ratio resulting from the use of various re-use schemes versus the probability of line of sight (LoS) activity of potentially interfering terminal stations. 
         FIG. 9   a  is an exemplary time frame map used in the frame-synchronized systems of  FIGS. 3   c ,  6  and  7 , showing the time frame map in the uplink of an FDD system. 
         FIG. 9   b  is an exemplary time frame map used in the frame-synchronized systems of  FIGS. 3   c ,  6  and  7 , showing the time frame map in the uplink and downlink of a TDD system. 
         FIG. 10  is a diagram of a wireless communication system that further incorporates the ability to coordinate uplink transmissions from individual terminal stations of different cells to minimize channel interference. 
         FIG. 11  is a diagram of a cell of a wireless communication system that incorporates the ability to coordinate uplink transmissions from individual terminal stations in the same cell to minimize adjacent channel interference. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than as limitations. 
     An exemplary broadband wireless communication system for use with the embodiments described herein is described in the related U.S. Pat. No. 6,016,311, by Gilbert et al., issued Jan. 18, 2000, entitled “Adaptive Time Division Duplexing Method and System for Dynamic Bandwidth Allocation within a Wireless Communication System”, which is hereby incorporated by reference. The communication system described in U.S. Pat. No. 6,016,311 is an adaptive time division duplexing (ATDD) method and system for duplexing transmissions in wireless communication systems. The ATDD system adapts to the uplink and downlink bandwidth requirements of the channels. 
       FIG. 1   a  is a diagram of a wireless communication system configuration incorporating frequency re-use where a plurality of base stations  1 ( a )-( d ) communicate with fixed terminal stations (i.e., “subscriber units”). Clusters of four sectors  4 ( a )-( d ) surround each base station  1 ( a )-( d ) to form cells  2 ( a )-( d ). The cells are shown separated by bold lines  30  and  32 . Each cell  2 ( a )-( d ) includes four sectors  4 ( a )-( d ) to form a coverage area that is square. In one embodiment, each cell  2  includes six sectors to form a hexagonal coverage area. 
     Each cell  2 ( a )-( d ) includes a base station  1 ( a )-( d ). For example, cell  2 ( a ) includes base station  1 ( a ). Cell  2 ( b ) includes base station  1 ( b ). Each base station typically includes an array of sectored antennas (not shown) for communicating with the terminal stations within its cell. A sectored antenna is often 60 or 90 degrees in beamwidth for communicating with terminal stations within an entire sector. Thus, in a four-sector case, a base station  1 ( a ) comprises at least four sectored antennas, with at least one antenna per sector  4 ( a ),  4 ( b ),  4 ( c ),  4 ( d ). In a six-sector case, the base station comprises six sectored antennas. 
     Each sector contains a plurality of terminal stations that communicate with the base station  1 ( a ) on a radio frequency (RF) channel. Each terminal station utilizes a highly directional antenna for communicating with its associated base station. The directional antenna is fixed and pointed towards the associated base station. On the downlink, a base station&#39;s sectored antenna transmits energy on an RF channel to a terminal station&#39;s highly directional antenna. On the uplink, a terminal station&#39;s highly directional antenna transmits energy on the RF channel to a base station&#39;s sectored antenna. The uplink can be less spectrum efficient than the downlink. The uplink spectrum efficiency can suffer because the base station collects RF energy using a sector antenna having a much greater beamwidth while the terminal stations collect energy using a highly directional antenna. This allows the downlink to support less robust modulation schemes (e.g., quadrature amplitude modulation-64 and quadrature amplitude modulation-16) while the uplink may require more robust modulation schemes (e.g., quadrature phase shift keying). 
     In accordance with frequency re-use methodologies and techniques, a set of RF channels is allocated for use in each cell  2 ( a )-( d ). As shown in  FIG. 1(   a ), each cell utilizes a set of four orthogonal RF channels (A, A′, B, and B′) comprising two frequencies (or frequencies bands) (A and B) wherein each frequency has two different polarizations (designated by the “non-primed” and “primed” indicators). For example, the two different polarizations can be horizontal and vertical. Each sector  4 ( a )-( d ) of each cell  2 ( a )-( d ) therefore utilizes a different orthogonal RF channel for communication between terminals in its sector and an associated base station. For example, a terminal in sector  4 ( a ) uses channel A, a terminal in sector  4 ( b ) uses channel B′, a terminal in sector  4 ( c ) uses channel B, and a terminal in sector  4 ( d ) uses channel A′. This set of four orthogonal RF channels is then re-used as shown in  FIG. 1(   a ) in adjacent cells  2 ( b )-( d ). In each cell  2 ( a )-( d ) the pattern of channel distribution is normally a mirror image of the channel distribution in adjacent and diagonal cells. Thus, the upper left-hand sector  4 ( a ) of the upper left-hand cell  2 ( a ) uses the same channel (e.g., channel A) as the upper right-hand sector  4 ( b ) of the adjacent cell  2 ( b ). Furthermore, the same channel is also used in the lower right-hand sector  4 ( d ) of cell  2 ( d ), and the lower left-hand sector  4 ( c ) of cell  2 ( c ). 
     Because channels are re-used, two cells or sectors operating on the same usable channel, though separated geographically, may interfere with each other. This is known as “co-channel interference”. The effect of co-channel interference varies with terrain and distance. In cases where path loss conditions favor the desired signal, the co-channel interference may not be strong enough to have a significant impact on receiver performance. In other cases, path loss conditions may cause the difference between the desired carrier power and the interference (known as the “C/I” ratio) to be insufficient for good receiver performance. Co-channel interference is inversely proportional to a wireless communication system&#39;s capacity (i.e., ability to communicate with multiple terminal stations). Thus, as co-channel interference increases system capacity decreases. Well-known modulation schemes such as quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) are utilized in broadband wireless communications for efficiently transmitting data between terminal stations and base stations. When the C/I ratio is low (i.e., relatively high co-channel interference) a robust modulation scheme must be used. For example, QPSK is a robust modulation scheme that can operate at lower C/I ratios than QAM-16. 
       FIG. 1   b  shows a wireless communication system using a frequency re-use configuration that illustrates the potential problems associated with co-channel interference. Terminal stations located within sectors T 1 -T 4  operate on the same RF channel, A. Similarly, base stations  1 ( a ),  1 ( e ),  1 ( f ),  1 ( g ) operate on the same RF channel, A, in sectors T 1 -T 4 . Since base stations  1 ( a ),  1 ( e ),  1 ( f ),  1 ( g ) operate on the same RF channel and transmit in the same general direction, the terminal stations within sector T 1  can receive unwanted RF energy from base stations  1 ( e ),  1 ( f ),  1 ( g ). Similarly, base station  1 ( g ) can receive unwanted RF energy from terminal stations located within sectors T 1 -T 3 . Receiving unwanted RF energy causes co-channel interference. 
       FIG. 2  is a graph of uplink C/I ratios for transmissions by terminal stations in sectors T 1 -T 4  towards base station  1 ( g ) of  FIG. 1   b . The uplink C/I ratio is graphed versus the probability that incoming RF energy along the base station&#39;s  1 ( g ) LoS interferes with the uplink transmissions from terminal stations in sector T 4 . Only sectors in nearby cells are considered in this analysis because the LoS conditions and propagation characteristics change dramatically for distant cells. A vertical axis  20  represents the ca ratio measured in dB. A horizontal axis  22  represents the probability (measured as a percentage) of a terminal station located outside of sector T 4  having a LoS position relative to the receiving base station  1 ( g ). A plot line labeled “T 1 +T 2 +T 3 ”  24  represents potentially interfering terminal stations from sectors T 1 -T 3  interfering with the uplink transmissions by terminal stations in sector T 4 . A plot line labeled “T 1  only”  26  represents potentially interfering terminal stations from only sector T 1 . Co-channel interference is greatest (i.e., worst case scenario occurs) when the probability of LoS is 100 percent. Thus, the C/I ratio reaches its smallest value (i.e., co-channel interference is at its greatest) when the probability of LoS is 100%. 
     Referring to the T 1 +T 2 +T 3  plot line  24 , when the probability of LoS of all potential interfering terminal stations located within sectors T 1 , T 2  and T 3  is greater than 50%, the C/I falls below 11 dB. Thus, the communication system&#39;s capacity is greatly reduced because not even a robust modulation scheme (i.e., QPSK) can operate when the CA falls below 11 dB. Referring to the “T 1  only” plot line  26 , when the probability of LoS of all potential interfering terminal stations located within sector T 1  is 100%, the C/I remains above 14 dB. Thus, the communication system&#39;s capacity is not as greatly affected as in the previous scenario because the most robust modulation scheme (i.e., QPSK) can operate when the C/I remains above 11 dB. However, at 100% LoS conditions, the more robust scheme is required. This increases the costs associated with deployment of the system and reduces the system&#39;s efficiency. Therefore, reducing co-channel interference from remote sectors that fall along a base station&#39;s LoS may increase the system&#39;s capacity. 
     To increase the number of available channels for implementing a frequency re-use plan, a channel can be separated into multiple bands of frequencies. Thus, an uplink channel, a downlink channel or both can be divided into sub-channels in order implement a frequency reuse pattern based in the frequency domain. Though the following example focuses upon dividing the uplink channel into two sub-channels, the method can also be applied to a downlink channel. Furthermore, the link can be divided into more than two sub-channels, for example, 3, 4, 5, 6, or more sub-channels. 
       FIG. 3   a  represents four orthogonal RF channels A, A′, B, and B′ comprising two frequencies, or frequency bands (A and B), wherein each frequency has two different polarizations (designated by the “non-primed” and “primed” indicators). In one example, each channel spans 28 MHz. In  FIG. 3   a  the channels are used in the uplink  35  and downlink  34  of a TDD system. The uplink channel A′ is further shown to be sub-divided into two 14 MHz sub-channels designated an upper sub-channel (“U”)  36  and a lower sub-channel (“L”)  37 . Depending on the frequency re-use plan, each of the uplink channels  35  can be so sub-divided as can the downlink channels  34 . In addition, different channel bands and different subdivisions of those bands can be used including non-symmetric subdivisions. 
     For example, if each of the uplink channels is sub-divided into two sub-channels, there are eight uplink sub-channels, an upper and a lower uplink sub-channel for each of A, A′, B, and B′. For systems that use four sectors per cell, each sector of the cell can utilize one uplink sub-channel. 
       FIG. 3   b  shows a plurality of base stations  38 ( a )-( n ) and their associated sectors. For ease of description, we now focus on an example of the assignment of uplink sub-channels that may potentially interfere, limiting our discussion to the sub-channels of uplink channel A′  36 ,  37 . Sectors which are assigned to the lower sub-channel A′ are marked with “+” and sectors which are assigned to the upper sub-channel A′ are marked with “X”. The same principles apply to each of the sub-channels of each of channel. 
     A base station sector which is assigned to receive the lower sub-Channel A′ (“+”) may pick-up unwanted radio frequency energy (co-channel interference) from other sectors that are assigned to the same sub-channel and are transmitting generally in its direction along a LoS. However, the base station would not pick up interference from sectors along a LoS, which are using the upper sub-channel (“X”). For example, base station  38 ( a ) utilizes the lower sub-channel in a sector  39 ( a ). The nearest sector transmitting generally in its direction and using the same sub-channel on diagonal  30  is sector  39 ( b ). 
     When the use of sub-channels is compared to a case where no sub-channels are used, for example,  FIG. 1   b , more or stronger potential co-channel interference exists in the scheme of  FIG. 1   b . In the non sub-channel system of FIG.  1   b , first level sources of co-channel interference are located at a distance of 5 times the radius of a cell (T 1  to T 4 ) and also at a distance of 5R/sqrt(2) (T 2  and T 3  to T 4 ), both from T 4 . In the sub-channel system depicted in  FIG. 3   b , first level sources of co-channel interference are located at a distance 9R/sqrt(2) (sectors  39 ( c ) and  39 ( d ) to sector  39 ( a )) and at 5R (sector  39 ( b ) to sector  39 ( a )). When all of these sources of co-channel interference are active, the use of sub-channels can improve the expected C/I ratio as compared to the non sub-channel system. 
       FIG. 3   c  is a representation of a system like that of  FIG. 1   b  but utilizing an uplink time-frame allocation scheme to minimize co-channel interference. In determining the potential for co-channel interference occurring in the system, various system level factors can be evaluated. For example, base stations that operate on the same frequency, have the same polarization, and have sectored antennas facing the same direction, for example, relative to absolute north, have the potential for co-channel interference. The system minimizes co-channel interference between parallel subsets of base stations. For ease of description,  FIG. 3   c  shows these subsets along a diagonal to the page. However, the invention is not so limited and can be implemented along any LoS between two base stations. When communicating with terminals located in sectors  4   a , base stations  1 ( a ),  1 ( e ),  1 ( f ) and  1 ( g ) operate on the same frequency (frequency A), have the same polarization and have sectored antennas facing the same direction (northwest). The base station  1 ( a ) is geographically located on the same diagonal ray  32  as the base station  1 ( g ). Base station  1 ( e ) is located on a diagonal ray  33 , and base station  1 ( f ) is located on a diagonal ray  31 . Thus, the sectors  4   a  associated with the base stations  1 ( a ),  1 ( e ) and  1 ( f ) have terminal stations that can potentially interfere with the base station  1 ( g ) when communicating on the uplink. 
     In this example, to reduce co-channel interference, the base stations of interest are synchronized and base stations  1 ( a ) and  1 ( g ) operate on a first set of time frames in the uplink. For example, base stations  1 ( a ) and  1 ( g ) only operate on even time frames in the uplink. Base stations  1 ( e ) and  1 ( f ) are not geographically located on the same diagonal ray  32  relative to the base station  1 ( g ). Base stations  1 ( e ) and  1 ( f ) can operate on a second set of time frames in the uplink. For example, base stations  1 ( e ) and  1 ( f ) operate on odd time frames in the uplink. Therefore, co-channel interference relative to the base station  1 ( g ) is reduced from terminal stations in sectors T 2  and T 3  (the sectors  4   a  associated with the base stations  1 ( f ) and  1 ( e ), respectively). 
       FIG. 4  shows an exemplary time frame map  40  of the frequency re-use scheme described with reference to  FIG. 3   c . Terminal stations in sector T 1  and T 4  communicate with base stations  1 ( a ),  1 ( g ), respectively, using odd time frames  42  and even time frames  43  on channel A. Terminal stations in sector T 2  and T 3  communicate with base stations  1 ( f ),  1 ( e ), respectively, using the odd time frames  42  and the even time frames  43  also on channel A. For sector T 1  and T 4 , channel A includes a downlink portion  45  and an uplink portion  46 . For sector T 2  and T 3 , channel A includes a downlink portion  47  and an uplink portion  48 . Terminal stations located within sectors T 1  and T 4  operate on the uplink  46  (i.e., are active) only during even time frames  43 . Terminal stations located within sectors T 2  and T 3  do not operate on the uplink  48  (i.e., are inactive) during even time frames  43 . Therefore, the terminal stations in the sectors T 2  and T 3  do not contribute to C/I on the uplink  46  during the even time frames  43 . Referring to  FIG. 3   c , during even time frames, only terminal stations in sector T 1  can contribute to the C/I realized by base station  1 ( g ) in the uplink direction. 
     Referring back to  FIG. 4 , the terminal stations located within sectors T 2  and T 3  operate on the uplink  48  (i.e., are active) only during odd time frames  42 . Terminal stations located within sectors T 1  and T 4  do not operate on the uplink  46  (i.e., are inactive) during the odd time frames  42 . Therefore, the terminal stations in the sectors T 1  and T 4  do not contribute to C/I on the uplink  48  during the odd time frames  42 . Different time frame allocation schemes can be used with the embodiments herein. For example, terminal stations within sectors T 1  &amp; T 4  can operate on the uplink every third frame. In this scheme, the terminal stations within the sectors T 2  and T 3  would be inactive on the uplink during the time frames that T 1  and T 4  were active on the uplink. Additionally, the ratio between the uplink and the downlink can be varied. 
     Systems typically have different co-channel interference and thus different spectrum efficiencies on the uplink as compared with the downlink due to differences between the sectored antenna used by the base stations and the narrow beamwidth antenna used by the terminal stations. In one embodiment, a variable ratio for uplink/downlink communication time frames is used when co-channel interference dictates different spectrum efficiencies for the uplink and the downlink. Spectrum efficiency is defined herein as the amount of information that can be transmitted in a communication system as measured in bits per second per Hertz (bps/Hz). Typically, spectrum efficiency is proportional to system capacity and inversely proportional to interference. Thus, as a system&#39;s interference decreases, the system&#39;s spectrum efficiency and capacity increases. Spectrum efficiency is impacted when different re-use schemes are used for the uplink and downlink. 
     One embodiment uses ATDD techniques (such as those described in U.S. Pat. No. 6,016,311) to compensate for differences between the uplink and downlink spectrum efficiencies. This embodiment adapts the portions of the uplink and downlink to compensate for the differing inefficiencies. For example, assume it is desired to have symmetric uplink and downlink net bandwidth (i.e., 50% of the bandwidth allocation is reserved for the uplink and 50% for the downlink). However, in minimizing co-channel interference, the downlink average spectrum efficiency is approximately 3 bits per second (bps) per Hz while the uplink average spectrum efficiency is approximately 1.5 bps/Hz. To compensate for the differences in spectrum efficiencies, the allocated bandwidth is changed. For example, each frame can be divided into uplink and downlink portions having a ratio of 3 to 1.5, or 2 to 1, meaning that approximately 67% of the frame is allocated for uplink transmissions and approximately 33% is allocated for downlink transmissions. Due to the spectrum inefficiencies given, the desired 50/50 net uplink/downlink bandwidth ratio results. 
     In a second example, a downlink/uplink bandwidth allocation ratio of 80% (4 to 1) is desired (80% of the net bandwidth allocated for the downlink and only 20% of the net bandwidth allocated for the uplink). Assuming the same downlink/uplink spectrum efficiencies described above (i.e., 3 bps/Hz on the downlink but only 1.5 bps/Hz on the uplink), an ATDD approach is used to change the uplink/downlink bandwidth allocation ratio to compensate for the spectrum efficiencies differences. The desired net bandwidth allocation of 80/20 is achieved by allocating 67% of the frame for downlink communications, 33% of the frame for uplink communications. Or, described in another manner, an 80/20 desired net bandwidth allocation (downlink/uplink) is a 4/1 ratio. By allocating 2/3 for the downlink, and 1/3 for the uplink, or (2/3*3 (bps/Hz))/(1/3*3/2(bps/Hz))=2/(1/2)=4/1, the desired net 4 to 1 bandwidth allocation ratio is achieved despite the differences between the uplink and the downlink efficiencies. 
     Now we turn to the more general case. When a specific net downlink/uplink ratio is desired in a time frame communication system, the fractional part of the time frame (or bandwidth) dedicated for the downlink (Rd) is calculated and the system adjusts the bandwidth allocation accordingly. Equation 1 is utilized to calculate Rd. 
         Rd= 1/[( Sd/Su )/( Td/Tu )+1];  Equation (1)
 
     where, Sd=spectrum efficiency for the downlink;
         Su=spectrum efficiency for the uplink;   Td=time slots allocated by the system on the downlink;   Tu=time slots allocated by the system on the uplink.       

     The Td/Tu ratio can be monitored and adjusted by the communication system, for example by the base station. Thus, the time frame (the amount of bandwidth dedicated to the uplink and the downlink) can be adapted in the time domain accordingly. For example, an ATDD time frame method can allocate the downlink portion of a time frame according to the Rd value calculated utilizing Equation 1. The fractional part of the time frame dedicated to the uplink (Ru) is simply calculated from the Rd using the following formula: Ru=1−Rd. 
     When the uplink time frame allocation scheme described above (with reference to  FIGS. 3   c  and  4 ) is utilized (e.g., odd/even time frame allocation), Equation 2 is used for calculating Rd: 
         Rd= 1 /[Sd/Su )/( Td/Tu/ 2)+1]  Equation (2)
 
     Note that the time slots allocated by the system on the uplink, or Tu, is divided by a factor of “2” in Equation 2. As described above, the uplink typically experiences greater co-channel interference than does the downlink. Consequently, there is less re-use of frequencies on the uplink than there is on the downlink. The uplink is penalized by the RF planning scheme due to the increased interference in the uplink. Equation 2 compensates in time for this difference between the uplink and downlink frequency re-use efficiency by allowing for more uplink time. Thus, the methods and systems described can utilize a combination of a downlink/uplink ratio and an uplink time frame allocation scheme to reduce co-channel interference. 
     The co-channel interference reducing methods and systems have been described above in the context of a communication re-use scheme having a frequency re-use factor of 2 with 2 frequency sets (A and B) and 4 sectors per cell. The methods and systems disclosed herein are not so limited and can be used with different communication re-use schemes. Examples of such alternative schemes are now described with reference to  FIGS. 5   a - 8 . 
     Re-Use 2, 1 Frequency Set, 4 Sectors 
       FIG. 5   a  is a graphical depiction of one embodiment of a frequency re-use having a frequency re-use factor of 2 using only one frequency (one frequency set having two polarizations). A single frequency A is divided on a time frame allocation basis (even and odd frames) and polarized to derive four separate RF channels, Ae (A even), Ae′ (A prime even), Ao (A odd), and Ao′ (A prime odd). Since only RF channel A is re-used (i.e. as A′) within the cell, a re-use factor of 2 is achieved. The RF channels are distributed among 4 sectors  51 ( a ),  51 ( b ),  51 ( c ),  51 ( d ) around a base station  52  in each cell  53 . Ae and Ae′ are transmitted during even time frames  54 . Ao and Ao′ are transmitted during odd time frames  55 . This embodiment permits frequency re-use while minimizing co-channel interference between adjacent sectors of the cell. 
       FIG. 5   b  shows a time frame map representing the uplink and downlink allocation for sectors X and Y of cell  53 . Sector X transmits its uplink on channel Ae′. Sector Y transmits its uplink on channel Ao′. Since terminal stations in sectors X and Y transmit on the same frequency, A′, and are located within the line of sight of site of the same base station, the potential exists for an uplink from sector X to interfere with an uplink from sector Y and vice versa. Furthermore, since sectors X and Y both share the same frequency there is also the potential for interference to occur on the downlink from the base station. To minimize co-channel interference, terminal stations in sector X transmit and receive during the even frames  54  and terminal stations in sector Y transmit and receive during the odd time frames  55 . Each even frame  54  includes a downlink portion  56  and an uplink portion  57 . Each odd frame  55  includes a downlink portion  58  and an uplink portion  59 . Sector X is active on the uplink  57  during the even time frames  54  while sector Y is active on the uplink  59  during the odd time frames  55 . The communication scheme of  FIGS. 5   a  and  5   b  can be readily adapted for use with a downlink/uplink ratio and an uplink time frame allocation scheme to reduce co-channel interference. 
     Re-Use 2, 3 Frequency Sets, 6 Sectors 
       FIG. 6  is a graphical depiction of another embodiment of a frequency re-use having a frequency re-use factor of 2. This embodiment includes 3 frequency sets (3 different frequencies having 2 polarizations) and 6 sectors per cell. Three frequency sets are polarized to derive 6 distinct RF channels (denoted in  FIG. 6  by various different shadings). Sectors having the same shading use the identical frequency and the identical polarization (i.e., sectors having the same shading use identical RF channels). Cells are nominally hexagonally shaped having 6 RF channels per cell. 
     Base stations  11 ( a ),  11 ( b ),  11 ( c ),  11 ( d ),  11 ( e ) and  11 ( f ) all use identical frequency re-use patterns. Identical re-use patterns are defined herein as cells that use identical RF channels for each similarly geographically located sector within the cell. For example, the cell associated with base station  11 ( a ) uses the same frequency re-use pattern as base station  11 ( b ). Each sector associated with base station  11 ( a ) and  11 ( b ), and facing the same direction with respect to their respective base stations  11 ( a ) and  11 ( b ), use the same RF channel. For example, sector T 15  (extending from base station  11 ( a ) in a northeast direction) uses the same RF channel as does sector T 12  (which extends from its associated base station  11 ( b ) in the same direction, northeast). Every other sector in the cells served by base station  11 ( a ) use the same RF channels as do their geographical counterpart sectors served by base station  11 ( b ). 
     In order to reduce co-channel interference in the uplink, the terminal stations that may potentially cause co-channel interference in the uplink are identified. This includes base stations that use identical frequency re-use patterns and that are sufficiently near the base station of interest as to contribute to co-channel interference. The co-channel interference reducing method and system work on a sector-by-sector basis. That is, only sectors that are sufficiently proximate to a sector of interest, using the same RF channel as the sector of interest, and facing the same direction with respect to their associated base stations as does the sector of interest, are considered. The method is then extended and repeated for every sector in a cell and along the line of sight of each sector of the cell. 
     For example, the potential uplink interferers with base station  11 ( a ) and its terminal stations located in sector T 15  are examined. Sector T 15  faces in a northeast direction along directional ray  60 . All similarly geographically located sectors that are within sufficiently close proximity to sector T 15  to potentially cause co-channel interference on the uplink are considered using the present method. More specifically, sectors are identified as potentially interfering because: (a) they use the same RF channel as does sector T 15 , (b) they face the same direction with respect to their associated base stations as does T 15  (in this case, the direction of directional ray  60 , e.g., northeast), and they are sufficiently proximate to sector T 15  as to contribute to co-channel interference on the uplink of base station  11 ( a ). Sectors T 11 , T 12 , T 13  and T 14  all meet these criteria. The terminal stations located in these sectors are therefore potential interferers on the uplink with the terminal stations located in sector T 15 . 
     Timing groups of potentially interfering sectors are identified depending upon their proximity from the sector of interest, in this case, sector T 15 . For example, sector T 11  is the closest to T 15  and is associated with base station  11 ( c ). An equidistant sector (equidistant from sector T 15 ) that also uses the identical RF channel as T 15 , is T 10 . T 10  is associated with base station  11 ( f ). However, T 10  is not in the line of sight of sector T 15  along directional ray  60 , and therefore would not significantly interfere. Nonetheless, T 10  can be logically grouped in the same timing grouping as T 11 . All other sectors having the same RF channel and along the same diagonal directional ray as indicated by diagonal directional ray  64  are logically grouped together in this first timing grouping. 
     Owing to the configurations of the sectors with respect to their associated base stations, the diagonal directional rays that define the sector timing groupings are perpendicular to the line of sight of the sector of interest. For example, as shown in  FIG. 6 , the diagonal directional ray  64  that defines the first sector timing grouping is perpendicular to the line of sight of sector T 15  which is illustrated by the directional ray  60 . 
     Base stations  11 ( b ),  11 ( d ) and  11 ( e ) use identical frequency re-use plans as base station  11 ( a ) and can be grouped along a second diagonal directional ray  66 . Similar to the first diagonal ray  64 , the diagonal ray  66  defines a second sector timing grouping comprising base stations  11 ( b ),  11 ( d ), and  11 ( e ), and sectors T 12 , T 13  and T 14 , respectively. Note that the diagonal  66  is perpendicular to the ray  60 , and necessarily parallel to the diagonal  64 . These two diagonals,  64  and  66 , are used to control the frame timing of their respective sectors. 
     In order to reduce the uplink interference that may be caused by the sector T 11  (the closest sector to sector T 15 ), terminal stations in the sector T 11  are allowed to transmit on the uplink only during frames when those in sector T 15  (of base station  11 ( a )) are inactive. Alternatively, the terminal stations in sector T 11  are inactive when T 15  is active on the uplink. Assuming a frame based communication system having “odd” and “even” frames, the first set of frames may be referred to as odd frames, and the second set of frames may be referred to as even frames. Therefore, T 11  is active in the uplink only during odd frames, and inactive during even frames. T 15  is inactive in the uplink during odd frames, and active during even frames.  FIG. 6  reflects this timing by referring to diagonal  64  as an “odd” diagonal, and diagonal  62  as an “even” diagonal. Diagonal  66  is also referred to as an “even” diagonal, because it uses the same frame timing as does sector T 15 . More specifically, sectors T 12 , T 13  and T 14  (defined by the even diagonal  66 ) are all active during even frames, and inactive during odd frames. 
     In the above example, interference on the uplink from terminal stations in the geographically closest potentially interfering sector T 11  is effectively removed during the time frames that the terminal stations in sector T 15  are active. The co-channel interference from the other potentially interfering sectors (i.e., T 12 -T 14 ) is still present, but it is at acceptable levels due to the distance that these sectors are from the sector of interest, T 15 . 
     Note that the reuse technique described above can be extended to include every sector in the cell of interest. Using the same approach as described above, a second sector of interest is selected. For example, the sector directly adjacent to T 15  and to the west of T 15  is selected as the second sector of interest. This sector now faces a second direction with respect to base station  11 ( a ). In this case diagonal ray  60  faces a new direction, i.e., the northwest direction. Consequently, the sector timing group diagonals  62 ,  64  and  66  are all rotated in a counter-clockwise fashion to point in a southwest to northeast direction. That is, the diagonals  62 ,  64  and  66  are perpendicular to the now northwest pointing directional ray  60 . 
     The frame timing of the sectors having the same RF channel as the new sector of interest, and defined by the diagonals  62 ,  64  and  66  for the sectors is the same as described above for the case when T 15  was the sector of interest. Consequently, the timing of the potentially interfering sectors are defined by the even diagonals  62 ,  66  and the odd diagonal  64  in the same manner as described above. Those of ordinary skill in the art shall recognize that this same process can be repeated for every sector for a base station of interest in order to define the frame timing for every sector in the communication system. 
     Re-Use 3/2, 4 Frequency Sets, 6 Sectors 
       FIG. 7  is a graphical depiction of an embodiment of a frequency re-use scheme showing a frequency re-use factor of 3/2. This embodiment includes 4 frequency sets and 6 sectors per cell. Three of the 4 available frequency sets are polarized to derive 6 separate RF channels (denoted by various shadings) per cell because that is all that are needed. Thus, this embodiment includes a total of eight available channels for frequency re-use. Sectors with identical shadings use identical RF channels. Thus, cells corresponding to base stations  71 ( a ),  71 ( b ),  71 ( c ) and  71 ( d ) all have identical frequency re-use plans. The frequency re-use pattern is identical for every sector that extends in the same direction away from their respective base stations. For example, for the sectors extending in a Northeast direction away from the base stations  71 ( a ),  71 ( b ),  71 ( c ) and  71 ( d ), the same frequencies, having the same polarization are used. Thus, as described above, the base stations  71 ( b ),  71 ( c ), and  71 ( d ) communicate with terminal stations located in sectors T 71 , T 72 , and T 73 , respectively, that can potentially interfere on the uplink with terminal stations in sector T 74  communicating with the base station  71 ( a ). Similar to the diagonal rays described above with reference to  FIG. 6 , the base station  71 ( a ) is geographically located on the same diagonal directional ray (indicated by directional ray  60 ′) as is the base station  71 ( c ). Thus, the closest potentially interfering sector to sector T 74  is the sector T 72 . However, as shown in  FIG. 7 , there are two other potentially interfering sectors, sectors T 71  and T 73 . These sectors face the same direction (with respect to their associated and respective base stations  71 ( b ) and  71 ( d )) as sector T 74  faces with respect to its associated base station  71 ( a ). Furthermore, sectors T 71  and T 73  are closer in proximity to sector T 74  than is sector T 72 . 
     Similar to the technique described with reference to  FIG. 6 , sectors T 71  and T 73  are assigned to a first sector timing grouping designated by a first diagonal ray  76  labeled as “even.” Sector T 74  is assigned to a second timing grouping designated by a second diagonal ray  75  labeled as “odd.” Sector T 72  is also assigned to an “odd” diagonal ray  77 , and uses the same timing as used by sector T 74 . Thus, sectors T 74  and T 72  are active on the uplink only during odd time frames, and inactive during even time frames. In contrast, sectors T 71  and T 73  are active on the uplink only during even time frames and inactive during odd time frames. Therefore, any potential co-channel interference that may have been introduced by the terminal stations in sectors T 71  and T 73  (relative to uplinks from sector T 74  to base station  71 ( a )) is minimized. Potential interference does remain on the uplink from terminal stations in sector T 72  during the odd time frames. However, this interference is acceptable because sector T 72  is at a significant distance from sector T 74 . Consequently, co-channel interference on the uplink is reduced, capacity is increased, and less robust modulation schemes may be used. This re-use technique can be extended to include every sector in the cell of interest similarly to extending the re-use scheme described with reference to  FIG. 6 . 
       FIG. 8  is a graph of the uplink C/I ratio resulting from the use of various re-use schemes versus the probability of line of sight (LoS) activity of potentially interfering terminal stations. The vertical axis  80  represents the C/I ratio measured in dB. The horizontal axis  81  represents the probability (measured as a percentage) of a terminal station having a LoS position relative to the desired base station. Co-channel interference is greatest (i.e., worst case scenario occurs) when the probability of LoS is 100 percent. Thus, the C/I ratio reaches its smallest value (i.e., co-channel interference is at its greatest). 
     Four plot curves are shown that represent the observed C/I when the four re-use schemes described above are used. First, the curve labeled “4 sector case”  82  shows the C/I associated with the 4 sector re-use scheme described with reference to  FIG. 1 , wherein co-channel interference reduction is not employed. Second, the curve labeled “6 sector case”  83  shows the C/I improvements observed when using the six sector re-use scheme described with reference to  FIGS. 6 and 7  without implementing a co-channel interference reduction method. The C/I improvement using the six sector re-use scheme of  FIG. 6  with 6 channels, and implementing the co-channel interference reduction scheme, is shown in the curve labeled “Nearby interferer eliminated”  84 . Finally, the curve entitled “single interferer (relevant to FR=3/2)”  85  plot line represents the six sector re-use scheme described with reference to  FIG. 7  with eight channels and implementing co-channel interference reduction. 
     Horizontal dashed lines  86 ,  87 ,  88  represent examples of minimum C/I levels below which QPSK, QAM-16, and QAM-64 modulation schemes, respectively, do not function properly. For example, a QAM-64 modulation scheme operates properly at a C/I level of 25 dB or greater. A QAM-16 modulation scheme functions properly at a C/I level of 18 dB or greater. And a QPSK modulation scheme functions properly at a C/I level of 11 dB or greater. 
     The “4 sector case”  82  falls below the QPSK line  86  at a LoS greater than approximately 50%. Similarly, the “6 sector case”  83  falls below the QPSK line  86  at a LoS greater than approximately 55%. Thus, the system capacity of the 4 sector and the 6 sector re-use schemes is greatly reduced since the most robust modulation scheme, QPSK, does not function properly when the C/I falls below 11 dB. However, when the co-channel interference reducing method is utilized with the six sector re-use scheme described with reference to  FIG. 6  (represented by the “nearby interferer eliminated” curve  84 ), the worst case C/I scenario (i.e., 100% LoS) is 11 dB. Thus, the communication system&#39;s capacity is not greatly affected because the most robust modulation scheme (i.e., QPSK) can function when the C/I remains 11 dB or above. Furthermore, when the co-channel interference reducing method and system is utilized with the six sector re-use scheme described above with reference to  FIG. 7  (represented by the “single interferer” curve  85 ), the C/I falls below 18 dB at approximately 75% LoS. Thus, a wireless communication system can operate using a QAM-16 modulation scheme whenever the LoS is less than 75%. 
     The inventive co-channel interference reducing methods and systems have been described above in the context of various communication re-use schemes having various frequency re-use factors, frequency sets and cell sectors (e.g., communication schemes described with reference to  FIGS. 3   c ,  6  and  7 ). These methods and systems can be used during the deployment or expansion of a communication system in a region. Those of ordinary skill in the art shall recognize that any communication system that operates in a frame based and frame synchronized manner in the uplink, having frame-synchronized base stations, can be used to practice the uplink frequency allocation re-use scheme. Two exemplary time-frame maps (half-duplex FDD and TDD) that can be used in the frame-synchronized systems of  FIGS. 3   c ,  6  and  7  are now described with reference to  FIGS. 9   a  and  9   b.    
       FIG. 9   a  shows a time frame map  90  for use with the frame-synchronized systems of  FIGS. 3   c ,  6 , and  7  on the uplink of an FDD system. In  FIG. 9   a , base stations located on “even” diagonals  91  (e.g., diagonal ray  32  of  FIG. 3   c , diagonal rays  62  and  66  of  FIG. 6  and diagonal ray  76  of  FIG. 7 ) communicate with associated terminal stations on the uplink (i.e., are active) only during even time frames  92 . Base stations located on “odd” diagonals  93  (e.g., diagonal rays  31  and  33  of  FIG. 3   c , diagonal ray  64  of  FIG. 6  and diagonal rays  75  and  77  of  FIG. 7 ) communicate with associated terminal stations on the uplink (i.e., are active) only during odd time frames  94 . Therefore, the terminal stations associated with the base stations located on the odd diagonals  93  do not contribute to C/I on the uplink during the even time frames  92 . Similarly, the terminal stations associated with the base stations located on the even diagonals  91  do not contribute to C/I on the uplink during the odd time frames  94 . Therefore, when the time frame map  90  in the uplink of an FDD system is utilized, interference that may be produced by the terminal stations in nearby sectors does not degrade system performance on the uplink. 
       FIG. 9   b  shows a time frame map  95  for use in the frame-synchronized systems of  FIGS. 3   c ,  6 , and  7 , showing the time frame map in the uplink and downlink of a TDD system. As shown in  FIG. 9   b , base stations located on “even” diagonals  96  (e.g., diagonal ray  32  of  FIG. 3   c , diagonal rays  62  and  66  of  FIG. 6  and diagonal ray  76  of  FIG. 7 ) communicate with associated terminal stations on the uplink (i.e., are active) only during even time frames  97 . Base stations located on “odd” diagonals  98  (e.g., diagonal rays  31  and  33  of  FIG. 3   c , diagonal ray  64  of  FIG. 6  and diagonal rays  75  and  77  of  FIG. 7 ) communicate with associated terminal stations on the uplink (i.e., are active) only during odd time frames  99 . Therefore, the terminal stations associated with the base stations located on the odd diagonals  98  do not contribute to Ca on the uplink during the even time frames  97 . Similarly, the terminal stations associated with the base stations located on the even diagonals  96  do not contribute to C/I on the uplink during the odd time frames  99 . Therefore, when the time frame map  95  in the uplink of a TDD system is utilized, interference that may be produced by the terminal stations in nearby sectors does not degrade system performance on the uplink. Advantageously, in utilizing the time frame map  95 , the downlink of the TDD system is unaffected. As described above, due to system capabilities, the downlink is more robust (i.e., receives less interference) than the uplink and, thus, has a higher capacity. 
       FIG. 10  is a diagram of a wireless communication system, similar to the system described with reference to  FIG. 1A , that further incorporates the ability to coordinate uplink transmissions from individual terminal stations of different cells to minimize channel interference. In the system shown in  FIG. 10 , base station  102   a  communicates with terminal stations N 1  and N 2 , both of which are located within a sector  104   a . For convenience, two terminal stations are shown located within sector  104   a . However, additional terminal stations may be located within sector  104   a . A cluster of four sectors  104 ( a )-( d ) surrounds base station  102   a  to form a cell  108 . Terminal station N 1  is located close to base station  102   a  and transmits its uplink along a path  112  with a high signal to noise ratio (“SNR”). Besides distance, SNR can be affected by environmental factors, for example, the weather and local geography. A high SNR permits N 1  to transmit using a less robust modulation technique, for example, QAM-64. N 1  transmits its uplink using a nominal power level since its uplink achieves a high SNR. N 2  is located farther away from the base station  102   a  and the SNR for its uplink transmission is lower than the SNR for N 1 . N 2  transmits along a path  114  with a more robust modulation technique, for example, QAM-16. N 2  transmits using a higher power level than N 1  since its uplink has a lower SNR. N 2  transmits along a line of sight  106  towards the upper right corner of  FIG. 10 . 
     Base station  102   b  communicates with terminal stations N 3  and N 4 , both of which are located within a sector  110   a . For convenience, two terminal stations are shown located within the sector  110   a . However, additional terminal stations may be located within the sector  110   a . A cluster of four sectors  110 ( a )-( d ) surrounds base station  102   b  to form a cell  113 . Terminal station N 3  is located close to base station  102   b  and transmits its uplink along a path  116  with a high SNR. A high SNR permits N 3  to use a less robust modulation technique, for example, QAM-64. N 3  transmits its uplink using a nominal power level since its uplink achieves a high SNR. N 4  is located farther away from base station  102   b  and its SNR is lower than the SNR for N 3 . N 4  transmits along a path  118  with a more robust modulation technique, for example, QPSK. N 4  transmits using a higher power level than N 3  since its uplink has a lower SNR. 
     Due to frequency re-use, terminal stations within the sector  104   a  and the sector  110   a  transmit their uplinks using the same frequency and polarization. Thus, co-channel interference can occur due to either of the uplinks from terminal stations N 1  or N 2 , located in sector  104   a , reaching the base station  102   b  at the same time that the base station  102   b  is receiving an uplink from terminal stations N 3  or N 4  in sector  110   a.    
     Alternatively, the systems and methods described with reference to  FIG. 10  can be used to minimize adjacent channel interference. Adjacent channel interference can occur due to a terminal station or base station receiving unwanted RF energy on a channel that is adjacent to its selected channel. For example, adjacent channel interference could occur if sector  104   a  was assigned to a horizontal polarization of a channel and sector  110   a  was assigned to a vertical polarization of the same channel. Thus, in this example, adjacent channel interference can occur due to either of the uplinks from terminal stations N 1  or N 2 , located in sector  104   a , reaching the base station  102   b  at the same time that the base station  102   b  is receiving an uplink from terminal stations N 3  or N 4  in sector  110   a . For ease of description, the following text applies the systems and methods of  FIG. 10  in the context of minimizing co-channel interference. However, as explained above, these systems and methods can also be used to minimize adjacent channel interference. 
     Base station  102   b  is located along the line off sight  106 . For convenience, base station  102   b  is shown as the next adjacent base station along the line of sight  106  from base station  102   a . Alternatively, as illustrated by dividing line  120 , base station  102   b  can be located such that single or multiple cells are located between base station  102   a  and base station  102   b.    
     Base station  102   a  coordinates the uplink transmissions from terminal stations N 1  and N 2 . Base station  102   b  coordinates the uplink transmissions from terminal station N 3  and N 4 . Base station  102   a  and base station  102   b  together coordinate uplink transmissions from their terminal stations in sectors  104   a  and  110   a  to minimize co-channel interference from their respective terminals. Alternatively, this coordination can be performed by a network management system (not shown). For example, the uplink transmission path  112  from terminal station N 1  to base station  102   a  is not along line of sight  106 . The SNR, measured at base station  102   b , for an uplink transmission from N 1  are low such that it will have minimal interference with simultaneous transmissions from N 3  or N 4 . The terminal stations N 3  and N 4  within sector  110   a  can transmit their uplinks using a less robust modulation technique since N 1  will not cause co-channel interference. 
     The uplink transmission path  114  from terminal station N 2  to base station  102   a  is along line of sight  106  which passes through base station  102   b . The SNR, measured at base station  102   b , for an uplink transmission from N 2  is high such that it may interfere with a simultaneous uplink transmission from N 3  or N 4 . To reduce the potential for co-channel interference occurring at base station  102   b , base station  102   a  and base station  102   b  coordinate the uplink transmission of N 2  with the uplink transmissions from terminal stations N 3  and N 4  in sector  110   a . This coordination can limit the modulation techniques available for uplink transmissions from terminal stations N 3  and N 4  in sector  110   a  to more robust modulation techniques, for example, QPSK and QAM-16. For example, if N 3 &#39;s preferred modulation, absent interference from N 2 , is QAM-64, N 3  uplinks to base station  102   b  using QAM-16 when N 2  is transmitting its uplink to base station  102   a . Alternatively, N 3  could consistently use QAM-16 for simplicity. Since terminal station N 4 &#39;s preferred modulation, absent interference from N 2 , is a more robust technique, i.e. QPSK, N 4  does not change its modulation technique to a more robust technique when N 2  is transmitting its uplink. Thus, while terminal station N 2  is transmitting, the terminal stations N 3  and N 4  in sector  110   a  modulate their uplink transmissions using a more robust modulation technique to minimize the effect of co-channel interference at base station  102   b.    
     Alternatively, uplink transmissions from terminal stations in sector  110   a  which may be susceptible to co-channel interference from N 2 &#39;s uplink can be fixed to not occur while N 2  is transmitting. For example, N 3 , which has a less robust preferred modulation technique, would not uplink to base station  102   a  while N 2  is uplinking to base station  102   b . Alternatively, N 2 &#39;s uplink transmission can be halted when terminal stations in sector  110   a  are transmitting their uplinks. For example, if N 3  and N 4  were using a less robust modulation technique for their uplinks, N 2 , which uplinks with high power along line of sight  106 , would not uplink to base station  102   a  while terminal stations N 3  or N 4  are uplinking to base station  102   h . Alternatively, forward error correction techniques can be substituted for or used in conjunction with the process described above. For example, forward error correction data can be transmitted with the data in the uplink from terminal station N 3  in sector  110   a  to increase the robustness of its preferred modulation technique, QAM-64. 
     Selecting the specific modulation technique for uplink data from terminal stations N 1  and N 2  in sector  110   a  can be based upon a measured SNR for N 2 &#39;s received signal at base station  102   b . Alternatively, the geometric locations of the terminal stations N 1  and N 2  within sector  104   a  can be used to determine the likelihood of co-channel interference occurring at base station  102   b . For example, the location and uplink path of a terminal station is determined with respect to a base station where co-channel interference may occur. A GPS can be used to determine a terminal station&#39;s location and uplink path. Once determined, potentially interfering terminal stations are identified for uplink coordination by its associated base station and the base station where the interference may occur. 
       FIG. 11  is a diagram of a cell of a wireless communication system that incorporates the ability to coordinate uplink transmissions from individual terminal stations in the same cell to minimize adjacent channel interference. A cluster of four sectors  130 ( a )-( d ) surrounds base station  132  to form cell  134 . Base station  132  communicates with terminal stations N 5 , N 6 , N 7  located within cell  134 . For ease of description, three terminal stations are shown within cell  134 . Additional terminal sectors within cell  134  can also be used. Moreover, the locations of additional terminal sectors within sectors  130 ( a )-( d ) can be varied. 
     Base station  132  includes an array of sectored antennas  136 ( a )-( d ) for communicating with the terminal stations within cell  134 . A sectored antenna is often 60 or 90 degrees in beamwidth for communicating with the terminal stations within an entire sector. Thus, in a four-sector case, base station  132  comprises at least four sectored antennas, with at least one antenna per sector  130   a ,  130   b ,  130   c ,  130   d . Sector  130   c  and sector  130   d  are separated by line  144 . Terminal stations located on opposite sides of line  144  are associated with different base station antenna. Sectored antenna  136   a  is used to transmit and receive with terminal stations located within sector  130   a . Sectored antenna  136   b  is used to transmit and receive with terminal stations located within sector  130   b . Sectored antenna  136   c  is used to transmit and receive with terminal stations located within sector  130   c . Sectored antenna  136   d  is used to transmit and receive with terminal stations located within sector  130   d.    
     Terminal station N 5  is located within sector  130   c  and uplinks along path  138  to antenna  136   c . Terminal station N 5  is located close to base station  132  and transmits its uplink with a high SNR measured at antenna  136   c . A high SNR permits N 5  to transmit using a less robust modulation technique, for example, QAM-64. N 5  transmits its uplink using a nominal power level since its uplink achieves a high SNR. N 6  is located in sector  130   c . N 6  is located farther away from base station  132  than N 5  and is also near line  144 . The SNR for its uplink transmission measured at antenna  136   c  is lower than the SNR for N 5 . N 6  transmits along path  140  with a more robust modulation technique, for example, QAM-16. N 6  transmits using a higher power level than N 5  since its uplink has a lower SNR. 
     Base station  132  communicates with terminal station N 7  within sector  130   d . Terminal station N 7  is located close to base station  132  and near line  144 . N 7  transmits its uplink along path  142  with a high SNR measured at antenna  136   d . N 7  transmits its uplink using a nominal power level since its uplink achieves a high SNR. The terminal stations within sector  130   c  use a first channel to uplink to base station  132 . The terminal stations within sector  130   d  use a second channel, which is adjacent to the first channel, to transmit their uplinks to base station  132 . Since N 7  transmits with a high SNR along line  144 , its uplink can cause adjacent channel interference at antenna  136   c  with the uplinks from terminal stations N 5  and N 6  in sector  130   c . Thus, adjacent channel interference can occur due to the uplink from terminal station N 7 , located in sector  130   d , reaching antenna  136   c  at the same time that base station  132  is receiving uplink transmissions from terminal stations N 5  and N 6  in sector  130   c.    
     Base station  132  coordinates the uplink transmissions from terminal stations N 5 , N 6 , and N 7  to minimize adjacent channel interference. For example, the uplink transmission path  142  from terminal station N 7  to base station  132  is adjacent to line  144 . Even though the uplink from terminal station N 7  will potentially cause interference with the uplink from N 5 , adjacent channel interference may not occur. The uplink from terminal station N 5  is received at antenna  136   c  with a high SNR. The SNR, measured at antenna  136   c , of an uplink transmission from N 7  is low. Thus, the base station  132  is able to differentiate between the potentially interfering signal from N 7  and the high SNR signal from N 5  if they are received simultaneously by antenna  136   c.    
     In contrast, the uplink transmission from N 6  has a lower SNR than the uplink from N 5 . Thus, the SNR of the uplink from N 6  is closer to the SNR of the uplink from N 7 . Adjacent channel interference is more likely to occur at antenna  136   c  due to the uplink from N 7  interfering with the uplink from N 6 . In this case, to reduce the potential for adjacent channel interference occurring at antenna  136   c , base station  132  coordinates the uplink transmission of N 6  in sector  130   c  with the uplink transmission from terminal station N 7  in sector  130   d . This coordination can limit the modulation techniques available for uplink transmissions from terminal station N 6  to more robust modulation techniques, for example, QPSK and QAM-16. For example, if N 6 &#39;s preferred modulation absent interference from N 7  is QAM-16, N 6  uplinks to base station  132  using QPSK when N 7  is transmitting its uplink to base station  132 . Alternatively, N 6  could consistently use QPSK for simplicity. Thus, while terminal station N 7  is transmitting, terminal station N 6  modulates its uplink transmissions using a more robust modulation technique to minimize the effect of adjacent channel interference occurring at antenna  136   c  of base station  132 . 
     Alternatively, uplinks from terminal station N 7  in sector  130   d  can be fixed to not occur while N 6  is transmitting its uplink to the base station  132 . For example, N 7 , which transmits its uplink with high power adjacent to line  144  would not uplink to the base station  132  while N 6  is uplinking to the antenna  136   a  of the base station  132 . Alternatively, N 6 &#39;s uplink transmission can be halted while terminal station N 7  is transmitting its uplink. For example, if N 6  was using a less robust modulation technique for its uplink, N 7 , which uplinks with high power adjacent to line  144 , would not uplink to the base station while terminal station N 6  is uplinking to the base station  132 . Alternatively, forward error correction techniques can be substituted for or used in conjunction with the process described above. For example, forward error correction data can be transmitted with the data in the uplink from terminal station N 6  in sector  130   c  to increase the robustness of its preferred modulation technique, QAM-16. 
     Selecting the specific modulation technique for uplink data from terminal station N 6  in sector  130   c  can be based upon a measured SNR of N 7 &#39;s received signal at antenna  136   c  of base station  132 . Alternatively, the geometric location of terminal station N 7  within sector  130   d  is used to determine the likelihood of adjacent channel interference occurring at antenna  136   c  of base station  132 . Once determined, potentially interfering terminal stations in an adjacent sector of a cell are identified for uplink coordination by the cell&#39;s base station. 
     Various aspects and a number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, different numbers of sectors of cells can be used. In addition, multi-point to point links can be used. This is equivalent to using a plurality of narrow beam sectored antennas at the base station. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment, but only by the scope of the appended claims.