Abstract:
An interference analyzing system and method analyze interference in a dual mode system having a digital network overlaid on an analog network. The digital network includes portable units operating within digital cell sites; whereas, the analog network includes mobile units operating within analog cell sites. The interference analyzing system measures the amount of interference caused by and to the portable units, and the amount of interference caused by and to the digital cell sites. The system individually compares these interference amounts to a predetermined threshold. If either of the interference amounts exceeds the predetermined threshold, the system selects one of the analog cell sites for spectrum clearing and determines the amount of spectrum to be cleared from the selected analog cell site.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications systems and more particularly to a method for minimizing interference in a dual mode Code Division Multiple Access (CDMA) system by requiring spectrum clearing of a substantially reduced number of Advanced Mobile Phone System (AMPS) sites. 
     BACKGROUND OF THE INVENTION 
     AMPS is an analog system that permits communication by mobile units operating within an analog cell site. CDMA systems, on the other hand, are digital systems that permit communication by portable units operating within a CDMA footprint; i.e., a geographical area offering digital CDMA service. In a dual mode system, the CDMA system is overlaid on an analog AMPS. 
     Since both systems operate contemporaneously in the dual mode system, some interference inherently occurs. Generally, there are four possible interference mechanisms between the AMPS and CDMA systems both operating in the 800-900 MHz frequency band (i.e., base station transmit frequency in the range of 869-894 MHz and receive frequency in the range of 824-849 MHz): (1) interference from AMPS sites to CDMA portable units; (2) interference from AMPS mobile units to CDMA sites; (3) interference from CDMA portable units to AMPS sites; and (4) interference from CDMA sites to AMPS mobile units. 
     FIG. 1 is a diagram of interference caused by an AMPS site to CDMA portable units operating within a CDMA footprint. As shown in FIG. 1, an interfering AMPS site located far from the CDMA footprint might still cause interference to CDMA portable units due to the AMPS site&#39;s excessive power transmissions, antenna height, or other operating conditions. In addition, if the interfering AMPS site is located close to a body of water, its transmission path across the water has a lower path loss, thereby permitting interference from an AMPS site located at a farther distance away from the CDMA footprint. Any AMPS site that interferes with the CDMA system requires clearing of analog channels from the frequency band being used by the CDMA system. Clearing is an expression commonly used to refer to the action of decommissioning that part of the spectrum from a cell site. 
     Even if the AMPS site does not cause excessive interference due to a very high path loss to any CDMA portable unit, for example, the path loss from a mobile unit within the AMPS site&#39;s serving area may still cause interference to the CDMA system if the path loss between the AMPS mobile unit and the CDMA base station is very low. 
     FIG. 2 is a diagram of interference caused by an AMPS mobile unit, operating within an AMPS site&#39;s serving area, to a CDMA system. In FIG. 2, the interfering AMPS mobile unit crosses a bridge, for example, within the AMPS site&#39;s serving area. Because the AMPS mobile unit is located over water, the transmission path across the water has a lower path loss, thereby facilitating interference with the CDMA system. Similar results occur when an AMPS mobile unit transmits from elevated highways, mountain roads, etc. 
     Conventional systems typically ignore such interference mechanisms. However, all of these interference mechanisms must be considered in a proper analysis of guard zones and guard bands. A guard zone is a geographical area in which cell sites must have a portion of their spectrum cleared to reduce interference between the analog AMPS and the digital CDMA systems. A guard band refers to the amount of spectrum, or frequency, that must be cleared at an AMPS site lying in the guard zone, when the digital and analog systems are overlaid. The size of the frequency band that must be cleared depends on issues such as transmit signal shape and mask and receiver filter structure and mask. 
     Conventional guard zone and guard band estimation methods are based on tessellated hexagonal grid cellular networks. These systems have found only limited direct application to real cellular network planning. In an analog system, cell sites are arranged in a hexagonal grouping and the &#34;ring&#34; of cell sites around the cell sites under consideration are referred to as tiers of cell sites. The typical result is that one or two tiers of analog cell sites must be cleared of the spectrum in order for the systems to operate over the same frequency band. 
     These conventional methods are idealistic and based on distance and a single theoretical path loss model. In addition, these methods do not consider factors such as user traffic, land use and land cover constraints, forward link margin (i.e., the value of a CDMA portable unit&#39;s forward link margin indicates the additional interference that the system can sustain at the specific location and still meet the link requirement), noise floor elevation (i.e., elevation in the effective noise at the base station due to the other portable units using the same frequency) due to multi-user traffic, or transmitter and receiver filter characteristics typically referred to as masks. Because the conventional methods do not consider these factors, they do not result in very accurate results suitable for real-life CDMA cellular deployments. 
     Therefore, a need exists in networks to efficiently determine the number of AMPS sites which need clearing of the CDMA frequency band while minimizing interference between the AMPS sites and the overlaid CDMA system. 
     SUMMARY OF THE INVENTION 
     Systems and methods consistent with the principles of the present invention address this need by efficiently determining in a dual mode system the AMPS sites lying in the guard zone where the CDMA frequency band needs to be cleared, and the amount of spectrum to be cleared at each AMPS site in the network. 
     In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the principles of the present invention analyzes interference in a dual mode system having a digital network overlaid on an analog network. The digital network includes portable units operating within digital cell sites; whereas, the analog network includes mobile units operating within analog cell sites. 
     The system measures the amount of interference caused by and to the portable units, and the amount of interference caused by and to the digital cell sites. The system individually compares these interference amounts to a predetermined threshold. If either of the interference amounts exceeds the predetermined threshold, the system selects one of the analog cell sites for spectrum clearing and determines the amount of spectrum to be cleared from the selected analog cell site. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with the description, explain the objects, advantages, and principles of the invention. In the drawings, 
     FIG. 1 is a diagram of a dual mode system in which an AMPS site causes interference to CDMA portable units operating within a CDMA footprint that covers only a small portion of the analog system; 
     FIG. 2 is a diagram of a dual mode system in which an AMPS mobile unit operating within an AMPS site&#39;s serving area causes interference to a CDMA system, and the AMPS site itself may not cause interference to the CDMA system; 
     FIG. 3 is a flow chart of an interference analysis method consistent with the principles of the present invention; 
     FIGS. 4A and 4B are flow charts of the AMPS site to CDMA portable unit interference module of FIG. 3; 
     FIG. 5 is a graph depicting a CDMA portable unit&#39;s receiver filter characteristics in an implementation consistent with the principles of the present invention; 
     FIGS. 6A and 6B are flow charts of the CDMA portable unit to AMPS site interference module of FIG.3; 
     FIG. 7 is a graph depicting a CDMA portable unit&#39;s transmitter mask requirement in an implementation consistent with the principles of the present invention; 
     FIGS. 8A and 8B are flow charts of the AMPS mobile unit to CDMA site interference module of FIG. 3; 
     FIG. 9 is a graph depicting a CDMA site&#39;s receiver filter characteristics in an implementation consistent with the principles of the present invention; 
     FIGS. 10A and 10B are flow charts of the CDMA site to AMPS mobile unit interference module of FIG. 3; and 
     FIG. 11 is a graph depicting a CDMA site&#39;s transmitter mask requirement in an implementation consistent with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description of the invention refers to the accompanying drawings The description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     The following detailed description refers to mobile units as operating within AMPS systems and portable units as operating within CDMA systems. Mobile units differ from portable units by power class. The following detailed description equally applies to portable units operating within AMPS systems and mobile units operating within CDMA systems. 
     Systems and methods consistent with the principles of the present invention consider interference between each and every possible CDMA portable unit and AMPS mobile unit location and every AMPS and CDMA site in the network. The systems and methods utilize the performance of the CDMA system to make allowances for those geographical areas where the CDMA portable unit is in handoff status, thus being able to better handle interference. The interference analysis consistent with the principles of the present invention is based on the EIA/TIA/IS-95A, IS-97, and IS-98 CDMA standards and the EIA/TIA/IS-19 and IS-20 AMPS standards, the relevant details of which are hereby incorporated by reference. 
     FIGS. 1 and 2 are diagrams of a dual mode system. The dual mode system includes a digital CDMA cellular network overlaid on an analog AMPS network. Each network operates concurrently. An interference module analyzes interference between the two networks. The interference analyzing module is a computer, such as a personal computer or a larger main frame computer, that contains all information on both the CDMA and AMPS networks, such as coverage, traffic, etc. 
     The interference analysis module consistent with the principles of the present invention uses a defined CDMA coverage area in conjunction with the CDMA traffic pattern to create a CDMA footprint for interference analysis. Based on the interference mechanism being analyzed, the interference power is computed at the mobile/portable unit&#39;s or site&#39;s location and then compared to a threshold which includes the transmitter and receiver filter protection. The threshold is a value or level consistent with a desired service quality objective. If the interference power exceeds this threshold, then the AMPS site is determined to be lying in the guard zone and the amount of spectrum which needs to be cleared at this site is calculated. 
     The guard zone thresholds for each CDMA portable unit and CDMA site are automatically calculated in the analysis. The threshold for each CDMA portable unit is calculated based on the portable unit&#39;s forward link margin. The CDMA sites, on the other hand, use the effective noise floor, including the noise floor elevation due to multi-user traffic, for their guard zone thresholds. 
     The guard zone thresholds may alternatively be user-supplied. A user may specify fixed guard zone thresholds for the CDMA portable units and sites. These thresholds may range, for example, from -130 dBm to -50 dBm, and are used globally at each CDMA portable unit and CDMA site. 
     The guard zone analysis studies the four kinds of mutual interference possible between the AMPS and the CDMA systems, including the interference from AMPS sites to CDMA portable units, the interference from AMPS mobile units to CDMA sites, the interference from CDMA portable units to AMPS sites, and the interference from CDMA sites to AMPS mobile units. 
     The guard band analysis takes into account the transmitter mask requirements and the receiver filter characteristics of the AMPS mobile units and cell sites and the CDMA portable units and cell sites to determine the amount of guard band required at either side of the CDMA frequency band. 
     FIG. 3 is a flow chart of an interference analysis method executed by the interference analyzing module in an implementation consistent with the principles of the present invention. Prior to executing the method, the interference analyzing module obtains all information regarding the CDMA and AMPS networks, including coverage areas, traffic, etc. The method consists of four interference modules: AMPS site to CDMA portable unit interference module 3100, CDMA portable unit to AMPS site interference module 3200, AMPS mobile unit to CDMA site interference module 3300, and CDMA site to AMPS mobile unit interference module 3400. 
     To increase processing speed, the method analyzes one of the most dominant interferences, that from an AMPS site to CDMA portable units, in AMPS site to CDMA portable unit interference module 3100 first. AMPS site to CDMA portable unit interference module 3100 determines typically 80 to 95% of the AMPS sites which need clearing. The AMPS mobile units being served by these cleared sites do not need to be processed when analyzing other interference modules, thereby further improving the method&#39;s processing speed. In order to even further increase processing speed, the method concurrently analyzes the reverse link interference in CDMA portable unit to AMPS site interference module 3200 while analyzing the forward link AMPS site to CDMA portable unit interference module 3100. 
     Once the method analyzes forward and reverse link interference in interference modules 3100 and 3200, the method concurrently analyzes reverse and forward interference in AMPS mobile unit to CDMA site interference module 3300 and CDMA site to AMPS mobile unit interference module 3400, respectively. This concurrent analysis further improves the method&#39;s processing speed. 
     FIGS. 4A and 4B are flow charts of AMPS site to CDMA portable unit interference module 3100. The processing performed by AMPS site to CDMA portable unit interference module 3100 begins by creating a CDMA footprint and assigning traffic to the AMPS sites  step 4110!. Module 3100 creates the CDMA footprint by combining the CDMA coverage layer and the CDMA traffic layer. This serves to remove unwanted portable unit locations which have not been assigned traffic based on the Land Use Land Cover (LULC) categories present over the coverage area. In other words, only those CDMA portable unit locations that have a non-zero amount of Erlang traffic are considered in the analysis. This ensures that no AMPS sites that are unlikely to cause interference are cleared. 
     In addition to creating the CDMA footprint, module 3100 assigns traffic (Erlangs/sector) to the AMPS sites. The AMPS traffic assignment is used to determine whether a mobile unit exists at a certain location, and might be arbitrarily set by module 3100 or user-defined. 
     Next, module 3100 determines whether all of the AMPS sites in the network have been analyzed for interference  step 4120!. If some of the AMPS sites have not been analyzed, module 3100 performs analysis for each AMPS site in the network  step 4130!. Module 3100 begins the AMPS site analysis by computing the interference power from a selected AMPS site received at every CDMA portable unit location within the CDMA footprint  step 4140!. For a portable unit location under consideration, module 3100 calculates the guard zone threshold based on the forward margin available, the handoff status, and the thermal noise floor  step 4210! (FIG. 4B). Module 3100 then determines the appropriate filter rejection based on the AMPS site&#39;s transmitter mask and the CDMA portable unit&#39;s receiver filter characteristics  step 4220!. 
     Module 3100 determines whether the AMPS site causes interference to a CDMA portable unit by determining whether the interference power from the AMPS site is greater than or equal to the combination of the guard zone threshold and the filter rejection  step 4230!. This determination is expressed as follows: 
     
         Interference power from AMPS site≧(m.sub.f -1)N.sub.p -F.sub.CDMA.sbsb.--.sub.P (f)-T.sub.AMPS.sbsb.--.sub.S (f)-X dB,(1) 
    
     where (m f  -1)N p  -X dB represents the guard zone threshold for the CDMA portable unit, m f  represents the forward link margin and is defined as the amount of increase that can be applied to the thermal noise floor at a specific location of the network so that the observed Eb/No still meets the minimum Eb/No link requirement (Eb/No represents a ratio of the bit energy of the signal to the noise density in 1 Hz of bandwidth, and a minimum Eb/No is associated with any link for the system to operate satisfactorily), N p  is the CDMA portable unit&#39;s thermal noise floor, FCDMA.sbsb.--.sub. p (f) is the CDMA portable unit&#39;s receiver filter rejection (FIG. 5), and T AMPS .sbsb.-- S  (f) is the AMPS site&#39;s transmitter filter rejection which is assumed ideal and constant across the 30 kHz bandwidth. The X dB decrease in expression (1) ensures that the resulting elevation in the portable unit&#39;s noise floor due to the AMPS interference is only a trivial fraction of a dB. X can be set to levels of 6-10 dB, for example, to ensure that the interference is at a tolerable level. 
     FIG. 5 is a graph of the CDMA portable unit&#39;s receiver filter characteristics in an implementation consistent with the principles of the present invention. The graph shows that the receiver filter characteristics are constant across the CDMA bandwidth of 1.7688 MHz, which includes a 270 kHz guard band on either side of the CDMA frequency band of 1.2288 MHz, used in the guard zone analysis. 
     If the interference power from the AMPS site is greater than or equal to the combination of the guard zone threshold and the filter rejection, module 3100 determines the guard band required based on the CDMA portable unit&#39;s receiver filter characteristics in FIG. 5, and reports the AMPS site for spectrum clearing  step 4240!. Based on the information from FIG. 5, module 3100 determines that the entire Band 1 needs to be cleared. Band 1 is a 1.7688 MHz frequency band defined as 884.4 kHz on either side of the CDMA carrier frequency, and approximately equivalent to 59 AMPS channels. The AMPS site&#39;s out-of-band spurious emissions falling into the CDMA carrier frequency are negligible compared to the AMPS carrier power received by the CDMA portable unit. After reporting the AMPS site for spectrum clearing, module 3100 returns to step 4120 (FIG. 4A) where it determines whether all AMPS sites in the network have been analyzed for interference. 
     If the interference power from the AMPS site is less than the combination of the guard zone threshold and the filter rejection, however, module 3100 calls CDMA portable unit to AMPS site interference module 3200 (FIG. 6A). CDMA portable unit to AMPS site interference module 3200 performs analysis for each possible CDMA portable unit location in the network  step 6110!. Module 3200 begins the analysis by calculating for the selected CDMA portable unit location, the interference power from the CDMA portable unit at every AMPS site  step 6120!. For an AMPS site under consideration, module 3200 calculates the guard zone threshold based on the AMPS site&#39;s noise floor  step 6130!. Module 3200 then determines the appropriate transmitter and receiver filter rejection based on the AMPS site&#39;s receiver filter and the CDMA portable unit&#39;s transmitter mask characteristics  step 6210! (FIG. 6B). 
     Module 3200 determines whether the CDMA portable unit causes interference to an AMPS site by determining whether the interference power from the CDMA portable unit is greater than or equal to the combination of the guard zone threshold and the filter rejection  step 6220!. This determination is expressed as follows: 
     
         Interference power from CDMA portable unit≧N.sub.AMPS.sbsb.--.sub.site -F.sub.AMPS.sbsb.--.sub.S.sbsb.--.sub.RF (f)-T.sub.CDMA.sbsb.--.sub.P.sbsb.--.sub.RF (f)-X dB,     (2) 
    
     where N AMPS .sbsb.-- site  -X dB represents the guard zone threshold for the AMPS site, N AMPS .sbsb.-- site  is the AMPS site&#39;s noise floor, T CDMA .sbsb.-- P .sbsb.-- RF  (f) is the CDMA portable unit&#39;s transmitter mask requirement (FIG. 7), and F AMPS .sbsb.-- S .sbsb.-- RF  (f) is the AMPS site&#39;s receiver filter rejection which is assumed to be ideal and constant across the 30 kHz band, and X, as in expression (1), ensures that the resulting interference is at a tolerable level. 
     FIG. 7 is a graph of a CDMA portable unit&#39;s transmitter mask requirement in an implementation consistent with the principles of the present invention. The graph shows that the CDMA portable unit&#39;s transmitter mask is constant across the CDMA bandwidth of 1.7688 MHz, which includes a 270 kHz guard band on either side of the CDMA frequency band of 1.2288 MHz used in the guard zone analysis. 
     If the interference power from the CDMA portable unit is greater than or equal to the combination of the guard zone threshold and the filter rejection, module 3200 determines the guard band required based on the CDMA portable unit&#39;s transmitter mask requirement in FIG. 7, and reports the AMPS site for spectrum clearing  step 6230!. Based on the information from FIG. 7, module 3200 determines that the entire Band 1 needs to be cleared. Module 3200 then calls module 3100 at step 4120 (FIG. 4A), where module 3100 determines whether all AMPS sites in the network have been analyzed for interference. 
     If the interference power from the CDMA portable unit is less than the combination of the guard zone threshold and the filter rejection, however, module 3200 determines whether all CDMA portable units within the CDMA footprint have been analyzed  step 6240!. If some of the CDMA portable units have not been analyzed, module 3200 calls module 3100 at step 4140 (FIG. 4A), where module 3100 determines the power received at the next CDMA portable unit location within the CDMA footprint. 
     If all of the CDMA portable units have been analyzed, however, module 3200 calls module 3100 at step 4120 (FIG. 4A), where module 3100 determines whether all of the AMPS sites in the network have been analyzed. If all of the AMPS sites have been analyzed for interference, module 3100 calls AMPS mobile unit to CDMA site interference module 3300 (FIG. 8A). AMPS mobile unit to CDMA site interference module 3300 begins by determining whether all of the AMPS mobile units in the network have been analyzed for interference  step 8110!. 
     If some of the AMPS mobile units have not been analyzed for interference, module 3300 performs analysis for each AMPS mobile unit in the network  step 8120!. Module 3300 begins the analysis by calculating for a selected AMPS mobile unit, the interference power from the AMPS mobile unit at every CDMA site  step 8130!. For a CDMA site under consideration, module 3300 calculates the guard zone threshold based on the CDMA site&#39;s thermal noise floor and the rise above the thermal noise floor due to multi-user traffic  step 8210! (FIG. 8B). Module 3300 then calculates the appropriate transmitter and receiver filter rejection based on the AMPS mobile unit&#39;s transmitter mask and the CDMA site&#39;s receiver filter characteristics  step 8220!. 
     Next, module 3300 determines whether the AMPS mobile unit causes interference to a CDMA site by determining whether the interference power from the AMPS mobile unit is greater than or equal to the combination of the guard zone threshold and the filter rejection  step 8230!. This determination is expressed as follows: 
     
         Interference power from AMPS mobile unit≧N.sub.s +N.sub.elev -F.sub.CDMA.sbsb.--.sub.S.sbsb.--.sub.RF (f)-T.sub.AMPS.sbsb.--.sub.M.sbsb.--.sub.RF (f)-X dB,     (3) 
    
     where N s  +N elev  -X dB represents the guard zone threshold for the CDMA site, N s  is the CDMA site&#39;s thermal noise floor, N elev  is the elevation in the CDMA site&#39;s noise floor due to multi-user traffic, F CDMA .sbsb.-- S .sbsb.-- RF  (f) is the CDMA site&#39;s receiver filter rejection (FIG. 9), and T AMPS .sbsb.-- M .sbsb.-- RF  (f) is the AMPS mobile unit&#39;s transmitter mask which is assumed ideal and constant across the 30 kHz bandwidth. As in expression (1), the X dB decrease ensures that the resulting elevation in the noise floor of the CDMA site due to the AMPS mobile unit interference is set to a tolerable level. 
     FIG. 9 is a graph of a CDMA site&#39;s receiver filter characteristics in an implementation consistent with the principles of the present invention. The graph shows that the CDMA site&#39;s receiver filter response has two breakpoints at approximately 750 kHz and 900 kHz. 
     If the interference power from the AMPS mobile unit is greater than or equal to the combination of the guard zone threshold and the filter rejection, module 3300 determines the guard band required based on the CDMA site&#39;s receiver filter characteristics in FIG. 9, and reports the serving AMPS site for spectrum clearing  step 8240!. Based on the information from FIG. 9, module 3300 determines that the spectrum which needs to be cleared due to this interference could be either Band 1 or Band 2 depending upon the CDMA site&#39;s receiver filter&#39;s bandpass response. Band 2 is a 1.5 MHz frequency band defined as approximately 750 kHz on either side of the CDMA carrier frequency, and equivalent to approximately 50 AMPS channels. The AMPS mobile unit&#39;s out-of-band spurious emissions falling into the CDMA carrier frequency are negligible compared to the AMPS mobile unit&#39;s carrier power received by the CDMA site. After reporting the AMPS site for spectrum clearing, module 3300 returns to step 8110 where it determines whether all the AMPS mobile units in the network have been analyzed for interference. 
     If the interference power from the AMPS mobile unit is less than the combination of the guard zone threshold and the filter rejection, however, module 3300 calls CDMA site to AMPS mobile unit interference module 3400 (FIG. 10A). CDMA site to AMPS mobile unit interference module 3400 performs analysis for each CDMA site in the network  step 10110!. Module 3400 begins the analysis by calculating for the selected CDMA site, the interference power from the CDMA site at every possible AMPS mobile unit location  step 10120!. For the AMPS mobile unit under consideration, module 3400 calculates the guard zone threshold based on the power received by the AMPS mobile unit from its best serving AMPS site  step 10130!. Module 3400 then calculates the appropriate transmitter and receiver filter rejection based on the AMPS mobile unit&#39;s receiver filter and the CDMA site&#39;s transmitter mask characteristics  step 10140!. 
     Next, module 3400 determines whether the CDMA site causes interference to an AMPS mobile unit by determining whether the interference power from the CDMA site is greater than or equal to the combination of the guard zone threshold and the filter rejection  step 10210!. This determination is expressed as follows: 
     
         Interference power from CDMA site≧P.sub.received -F.sub.AMPS.sbsb.--.sub.M.sbsb.--.sub.RF (f)-T.sub.CDMA.sbsb.--.sub.S.sbsb.--.sub.RF (f)-X dB,     (4) 
    
     where P received  -X dB represents the guard zone threshold for the AMPS mobile unit, P received  is the power received by the AMPS mobile unit from its best serving AMPS site, T CDMA .sbsb.-- S .sbsb.-- RF  (f) is the CDMA site&#39;s transmitter mask requirement (FIG. 11), F AMPS .sbsb.-- M .sbsb.-- RF  (f) is the AMPS mobile unit&#39;s receiver filter rejection which is assumed to be ideal and constant across the 30 kHz band, and X, as in expression (1), ensures that the interference is at a tolerable level. 
     FIG. 11 is a graph of a CDMA site&#39;s transmitter mask requirement in an implementation consistent with the principles of the present invention. The graph shows that the CDMA site&#39;s transmitter mask has two breakpoints at approximately 750 kHz and 1980 kHz. 
     If the interference power from the CDMA site is greater than or equal to the combination of the guard zone threshold and the filter rejection, module 3400 determines the guard band required based on the CDMA site&#39;s transmitter mask requirement in FIG. 11, and reports the serving AMPS site for spectrum clearing  step 10220!. Based on the information from FIG. 11, module 3400 determines that the spectrum which needs to be cleared due to this interference could be either Band 1 or Band 2, depending upon the CDMA site&#39;s transmitter mask. Module 3400 then calls module 3300 at step 8110 (FIG. 8A), where module 3300 determines whether all of the AMPS mobile units in the network have been analyzed for interference. 
     If the interference power from the CDMA site is less than the combination of the guard zone threshold and the filter rejection, however, module 3400 determines whether all of the CDMA sites in the network have been analyzed  step 10230!. If some of the CDMA sites have not been analyzed, then module 3400 calls module 3300 at step 8130 (FIG. 8A), where module 3300 determines the power received from the next AMPS mobile unit at the CDMA site. 
     If all of the CDMA sites have been analyzed, however, module 3400 calls module 3300 at step 8110 (FIG. 8A), where module 3300 determines whether all of the AMPS mobile units in the network have been analyzed for interference. If all of the AMPS mobile units have been analyzed, then the interference analysis method generates a guard zone report listing all of the AMPS sites where the frequency spectrum needs to be cleared. This report might include the AMPS sites&#39;names with their corresponding sector numbers, their type (AMPS only or dual mode), and the amount of spectrum in AMPS channel numbers which needs to be cleared. 
     The following advantages are realized by the systems and methods consistent with the principles of the present invention: 
     (1) The order in which analysis is performed reduces processing speed. In the interference analysis method consistent with the principles of the present invention, the order of the analysis was chosen to quickly reduce the number of AMPS sites that have to be considered in the analysis. This reduces the remaining number of AMPS sites and mobile locations that must be investigated for each succeeding evaluation of likely interference. 
     (2) The interference analysis method consistent with the principles of the present invention addresses all of the mutual interference mechanisms. The method includes four main interference modules that account for even the case where a rogue mobile unit might cause excessive interference or the case where a high elevation base station might cause interference to CDMA portable units. Conventional methods only perform a fraction of this analysis. 
     (3) The interference analysis method consistent with the principles of the present invention considers CDMA performance in the analysis. Guard zone calculation depends upon the underlying AMPS and CDMA perform. The method considers CDMA network performance and potential areas where the CDMA network is capable of combating interference from analog signals, such as where the portable units have a high forward margin. As a result, the method does not unnecessarily expand the guard zone in cases where the CDMA network can operate with some interference. 
     (4) By taking into account the underlying CDMA network, the interference analysis method consistent with the principles of the present invention makes allowances for those geographical areas where the CDMA portable unit is in a handoff state, and thus better able to handle interference. 
     (5) The interference analysis method consistent with the principles of the present invention accounts for traffic distribution of mobile units. The method considers a user-defined traffic map such that if a potential geographical area is defined as having no traffic then the method assumes that there is a low probability that an interfering mobile unit exists at this location. As a result, the number of AMPS sites that need to have some of their spectrum cleared is reduced. 
     (6) The interference analysis method consistent with the principles of the present invention contains the ability to use automatic thresholds or manual settings. The method allows a user to override the guard zone thresholds calculated within the CDMA network, if desired, and use fixed guard zone thresholds for the CDMA sites and portable units. This may be appropriate for &#34;what-if&#34; scenarios, and in cases where the effect of different levels of interference is investigated. 
     The systems and methods consistent with the principles of the present invention efficiently determine the AMPS sites lying in the guard zone, and the amount of spectrum which needs to be cleared to reduce interference in a dual mode system. 
     The foregoing description of preferred embodiments of the present invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The scope of the invention is defined by the claims and their equivalents.