Abstract:
A frequency reuse plan for a cellular radio communications system includes a plurality of antenna sites which implement twelve groups of frequencies among four antenna sites. The same frequencies are reused by other groups of four antenna sites. Each antenna site serves three hexagon shaped cells arranged in a clover-leaf shaped pattern by three groups of antennas, each of which group is separated by a pointing angle of 120 degrees.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cellular communications systems and, more particularly, to a frequency reuse plan therefor. 
     2. Description of the Related Art 
     Frequency reuse is the use of radio channels on the same carrier frequency for the coverage of geographically different areas and is necessary in order to construct practical, high-capacity cellular systems in traffic-dense areas, such as big cities. Needless to say, these geographically different areas which include the same radio carrier frequencies must be far enough apart to ensure that co-channel interference either does not arise or does not arise to an objectional level. 
     An important measurable characteristic of frequency reuse schemes is carrier-to-interference ratio (&#34;C/I ratio&#34;). The C/I ratio is defined to be the ratio of the level of the received desired signal to the level of the received undesired signal. Because of irregular terrain and the various shapes, types and numbers of local scatterers, the C/I ratio is dependent upon the instantaneous position of a mobile moving through a cell. Other factors such as antenna type, directivity and height, site elevations and positions, and the number of local interferers also affect the C/I ratio at various locations within a system. 
     The desired distribution of the C/I ratio in a system determines the number of frequency groups, F, which may be used. If the total allocation of N channels is partitioned into F groups, then each group will contain N/F channels. Since the total number of N channels is fixed, for example, there are 312 voice channels in the F.C.C. Standard A Band, a smaller number of F frequency groups would result in more channels per set and per cell site. Therefore, a reduction in the number of frequency groups would allow each site to carry more traffic, reducing the total number of sites needed for a given traffic load. However, decreasing the number of frequency groups and reducing the co-channel reuse distance results in a lower C/I distribution in the system. 
     FIGS. 1-3 are schematic illustrations of various prior art frequency reuse plans. In viewing these depictions, as well as all other illustrated frequency reuse plans, it should be appreciated that the illustrated cells are shown to have certain shapes. While in theory, cells may be envisioned as having any regular polygon shape, the important point is that the array of cells covers a plane without gaps or overlaps. Similarly, cell boundaries may be conceptually defined as lines at which the respective signal strengths of neighboring cells are equal. In reality, of course, because of such factors as random propagation effects, real cells only roughly approximate ideal cell shapes with ideal boundaries therebetween. 
     In considering the frequency reuse plans shown herein, it should be appreciated that equivalent site coverage areas, the same site locations, and 312 available voice channels are assumed in each plan. This reflects reality and allows fair comparisons to be made among the various plans. 
     Referring now to FIG. 1, there is shown a diagram of a 7/21 Cloverleaf Cell Plan as has heretofore been implemented by Ericsson, the assignee of the present invention. If may be seen that this plan employs a number of sites 2, each site serving three cells (or &#34;sectors&#34;) 4. Each cell 4 contains a dedicated antenna system, a control channel, a signal strength receiver, and voice channels. In FIG. 1, certain groups of co-channel cells, i.e., cells employing the same frequencies, are shown cross-hatched, e.g., cells 4A and 4B. Using the same terminology, sites 2A and 2B should be appreciated to be co-channel sites. 
     Further with respect to FIG. 1, it may be seen that site 2A is centrally located within the illustrated system and that the outlying sites are all shifted two (2) units in a first (&#34;i t  h&#34;) direction and one (1) unit in a second (&#34;j th  &#34;) direction from the central site. Defining i and j as shift parameters, the illustrated plan may be considered to have shift parameters of two (2) and one (1) respectively. Shift parameters are important characteristics of frequency reuse plans and will therefore be discussed with respect to each plan described herein. 
     Recalling that there are 312 available voice channels in the F.C.C. Standard A Band, the 7/21 Cloverleaf Cell Plan shown in FIG. 1 uses some 21 frequency groups in its seven repeating sites with approximately (although averaging somewhat less) 15 channels per group. 
     Supporting the three cell configurations shown in FIG. 1, each cell has antenna pointing azimuths separated by some 120°. More specifically, in practice, each cell uses 60° transmit antennas and two (2) 60° diversity receive antennas with the same pointing azimuths. 
     Referring now to FIG. 2, shown therein is a Three Rhomb Cell Plan as has heretofore been implemented by numerous operators, including the companies of the Bell System. Sites, cells and shift paraments are similarly marked in FIG. 2 as they were in FIG. 1. Examining FIG. 2, it should be appreciated that the Bell 7/21 Three Rhomb Cell Plan uses 21 Frequency groups in a seven site reuse pattern with approximately 15 channels per group. As with the Ericsson 7/21 Cloverleaf Cell Plan, the shift parameters, i and j, are two (2) and one (1) respectively. Site geometry in the Bell plan involves three cells 4 or sectors at each site 2. The antenna pointing azimuths of each cell are separated by 120°. Each cell uses 120° transmit antennas and two (2) 120° diversity receive antennas with the same pointing azimuths. Additionally, each cell is approximated by the shape of a rhomboid. 
     Referring now to FIG. 3, shown therein is a 4/24 Six Triangle Cell Plan as has heretofore been implemented by Motorola. This plan uses 24 frequency groups in a four site reuse pattern with 13 channels per group. The corresponding shift parameters i and j are two (2) and zero (0), respectively. The site geometry involves six cells 4 at each site 2 with antenna pointing azimuths separated by 60°. Each cell uses one (1) 60° antenna with the transmit and receive functions duplexed. Additionally, each cell is approximated by the shape of an equilateral triangle. 
     Further details regarding each of the above plans will be set forth below in various comparisons with the plan according to the present invention. In general however, it may be noted and should be appreciated by those skilled in the art that each of the prior art systems possesses shortcomings in that a number of important system characteristics; e.g., C/I performance, capacity, utilization, and site position tolerance, could be improved. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a frequency reuse plan having improved C/I, capacity, utilization, and site performance tolerance than prior art plans. 
     To fulfill the above and other objects, and to overcome the shortcomings and deficiencies of the prior art, the present invention provides a cellular radio communications system including a plurality of sites wherein groups of four sites reuse frequencies. The system includes means for implementing twelve groups of frequencies among the plurality of sites with each of the twelve groups of frequencies including twenty-six channels. 
     Embodiments of the frequency reuse plan according to the teachings of the present invention has shift parameters, i and j, of two (2) and zero (0), respectively. 
     Embodiments of the plan of the present invention may further include groups of three cells which surround each site. These cells may be hexagon-shaped and, further, may be arranged in a cloverleaf fashion. 
     Still further, in other embodiments of the present invention each cell may have antenna pointing azimuths, each of which may be separated by approximately 120°. Yet still further, each cell may include 60° transmit antennas and two diversity receive antennas with the same pointing azimuths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and for further objects and advantages thereof, reference may now be had to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 depicts a prior art Ericsson 7/21 Cloverleaf Cell Plan, previously discussed; 
     FIG. 2 depicts a prior art Bell 7/21 &#34;Three Rhomb&#34; Cell Plan, previously discussed; 
     FIG. 3 depicts a prior art Motorola 4/24 &#34;Six Triangle&#34; Cell Plan, previously discussed; 
     FIG. 4 depicts a frequency reuse plan according to the teachings of the present invention; 
     FIG. 5 is a graph comparing channel utilization of the four frequency reuse plans discussed herein; 
     FIG. 6 is a graph comparing C/I performance of the four frequency reuse plans discussed herein; 
     FIG. 7 illustrates C/I predictions for the Ericsson 7/21 Cloverleaf Cell Plan, which predictions were used to generate the graph shown in FIG. 6; 
     FIG. 8 illustrates C/I predictions for the Ericsson 4/12 Cloverleaf Cell Plan according to the teachings of the present invention, which predictions were used to generate the graph shown in FIG. 6; 
     FIG. 9 illustrates C/I predictions for the Bell 7/21 &#34;Three Rhomb&#34; Cell Plan, which predictions were used to generate the graph shown in FIG. 6; 
     FIG. 10 illustrates C/I predictions for the Motorola 4/24 &#34;Six Triangle&#34; Cell Plan, which predictions were used to generate the graph shown in FIG. 6; 
     FIG. 11 depicts data relating to cell-site position tolerance of the Bell 7/21 &#34;Three Rhomb&#34; Cell Plan; 
     FIG. 12 depicts data relating to cell-site position tolerance of the Ericsson 4/12 Cloverleaf Cell Plan, an embodiment of the present invention; 
     FIG. 13 depicts comparison data relating to cell-site position tolerance of the Bell 7/21 &#34;Three Rhomb&#34; Cell Plan and the Ericsson 4/12 Cloverleaf Cell Plan; 
     FIG. 14 shows the cell geometry and relative receive antenna gain for incident angles of 0°, 30°, and 60° for the Ericsson system; 
     FIG. 15 shows the cell geometry and relative receive antenna gain for incident angles of 0°, 30°, and 60° for the Bell System; 
     FIG. 16 shows the cell geometry and relative receive antenna gain for incident angles of 0°, 30°, and 60° for the Motorola system; 
     FIG. 17 is a top view of the antenna mounting arrangement used by the Ericsson and Bell systems; 
     FIG. 18 is a top view of the antenna mounting arrangement used by the Motorola system; 
     FIG. 19 illustrates transposition of frequency groups to avoid circumstances in which the received level of an adjacent channel may greatly exceed that of the desired channel in embodiments of the present invention; 
     FIG. 20 illustrates a pattern assignment for the allocation of frequency groups in a 4/12 cell plan utilizing a 1:3 cell splitting scheme; 
     FIG. 21 illustrates channel group subdivisions and overlay cell coverage restrictions for a 4/12 underlay-4/12 overlay cell pattern with multiple cell sizes; 
     FIG. 22 illustrates a 7/21 underlay-4/12 overlay cell pattern utilizing reuse partitioning; 
     FIG. 23 illustrates a control channel allocation plan which has no adjacent control channel frequencies in neighbor cells; 
     FIG. 24 illustrates a pattern assignment for the allocation of control channels in a 4/12 cell plan utilizing a 1:3 cell splitting scheme; 
     FIGS. 25-27 show the simulated evolution of a system which initially uses only the standard FCC A Band; and 
     FIGS. 28-30 illustrate various cell splitting relationships. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 4, a frequency reuse plan according to the teachings of the present invention is shown therein. This plan uses twelve (12) frequency groups in a four (4) site reuse pattern with twenty-six (26) channels per group. The corresponding shift parameters may be seen to be two (2) and zero (0), respectively. Three (3) cells are at each site. The antenna pointing azimuths of each cell are separated by 120° and the cells are arranged in a cloverleaf fashion. Each cell uses 60° transmit antennas and two (2) 60° diversity receive antennas with the same pointing azimuths. Each cell is approximated by the shape of a hexagon. 
     Because of the use of twelve (12) frequency groups in a four (4) site reuse pattern and because of the cloverleaf arrangement of the cells, the plan according to the present invention and shown in FIG. 4 will, and for clarity and convenience, hereafter be identified as a 4/12 Cloverleaf Cell Plan. 
     To fully understand the capabilities of the present invention and the advantages it offers over the prior art, it is useful to consider traffic capacity, carrier-to-interference performance, and uplink system gain, each of which will be discussed below. 
     The three major blocking formulas, Poisson, Erlang B and Erlang C, differ in the basic assumptions made with regard to the behavior of calls failing to find an idle channel. With specific reference to the Poisson blocking formula, calls are willing to wait no longer than their intended or average holding times. If a channel becomes idle before the holding time expires, the call will seize it and use it for the remaining part or duration of its holding time. With respect to the Erlang B blocking formula, blocked calls which fail to find an idle channel immediately are not willing to wait and abandon the call immediately. Finally, with respect to the Erlang C blocking formula, blocked calls are willing to wait an indefinite period to obtain an idle channel. 
     It is common practice in the cellular industry to assume a loss system, that is, all blocked calls are cleared, when dimensioning the number of channels required per cell in a system. Therefore, the Erlang B blocking formula with a designed channel blocking probability of 2% is used as the standard. 
     With the above understandings, Table 1 below illustrates the traffic capacity per site for the frequency reuse plans discussed in the description of related art section as well as the reuse plan according to the present invention. The comparison illustrated in Table 1 assumes use of the Erlang B blocking formula with a blocking probability of 2%, 312 available voice channels, equivalent site coverage areas of 31.2 square kilometers, ideal site locations, and ideal cell boundaries. 
     
                                           TABLE 1__________________________________________________________________________FREQUENCY  FREQUENCY              CHANNELS                      CAPACITY/SITE                                CAPACITY/SITEPLAN       GROUPS  PER SITE                      (ERLANGS) (ERLANGS/SQ KM)__________________________________________________________________________ERICSSON   7/21      21      44.6(3 × 14.9)                      26.7      0.86ERICSSON   4/12      12      78.0(3 × 26)                      55.2      1.77BELL    7/21      21      44.6(3 × 14.9)                      26.7      0.86MOTOROLA   4/24      24      78.0(6 × 13)                      44.4      1.42__________________________________________________________________________ 
    
     From Table 1 above it may be noted that the Ericsson 4/12 Cloverleaf Cell Plan provides an increase in traffic capacity per site of 106% over both the Bell and Ericsson 7/21 cell plans and 24.4% over the Motorola 4/24 cell plan. In reading the chart it should be noted, however, that a cell must have an integer number of voice channels. Because it is not possible to divide 312 voice channels into an integer number between 21 cells, the 7/21 cell plans in the comparison uses an average number of voice channels per call. 
     Based on the foregoing, it should be appreciated that the reuse plan according to the present invention offers tremendous advantages over prior art systems with respect to traffic capacity and therefore the number of sites required to serve a system. 
     Considering channel utilization of the respective systems, the relationship of channel group size vs. channel utilization for the different frequency reuse plans is shown in FIG. 5. The channel utilization shown in FIG. 5 has been calculated using the formula: &#34;channel utilization (%)&#34; equals &#34;traffic capacity per cell&#34; divided by &#34;number of channels per call&#34;. 
     Examining FIG. 5, it may be noted that the channel utilization of the Ericsson 4/12 cell plan exceeds that of the Bell and Motorola cell plans by 10% and 13%, respectively. This increase is accomplished by utilizing a more efficient channel allocation scheme, that is, a larger number of voice channels per frequency group. Thus, with respect to channel utilization, the frequency reuse plan according to the teachings of the present invention constitutes a marked improvement in the art. 
     Carrier-to-Interference performance will now be considered. Co-channel interference and a multi-path fading environment has been evaluated heretofore in at least one subjective testing program. The results of this program showed that a majority of the listeners consider the transmission quality of the voice channel to be good or excellent at a C/I of 17 dB. At 17 dB C/I, an optimized cellular system will provide voice quality equal to that of a toll call off the public telephone network. 
     A goal, therefore, of cellular network system designers is to provide acceptable voice quality uniformly throughout the service area. Due to the direct relationship between C/I performance, system capacity and system cost, a system operator usually specifies a required transmission quality for X percent of the subscribers rating the call quality good or better for Y percent of the coverage area. 
     It may be noted that a C/I ratio of greater than 12 dB is considered to be required for both quality voice conversation as well as bit error rate (&#34;BER&#34;) performance within data transmission, synchronization and recognition for frequency modulated systems which use companders, limiters, and base station receive diversity. 
     A comparison of the C/I performance of the prior art reuse plans and the reuse plan according to the present invention are shown in FIG. 6 which illustrates accumulative probability distribution curves. The evaluation of the C/I performance for the different frequency reuse plans assumes antennas of 120° for Bell and 60° for the two Ericsson and one Motorola systems. Antenna heights in all cases are assumed to be 50 meters. The propagation model employed in the Okamura/Hada, flat earth with no effect from the radio horizon. Site positions are assumed to be ideal as are the cell boundaries. The number of interferers was in all cases six (6). Finally, the effective radiated power was assumed to be equal for both the target and interfering cells. 
     The distribution curves illustrated in FIG. 6 were generated from the data illustrated in FIGS. 7-10 which show the position and level distribution of the predicted C/I values in the respective cells. 
     In examining FIG. 6, as well as FIGS. 7-10, it should be noted that the probability distribution curves and C/I predictions are predicted averages and do not include the effect of log-normal fading on the radio signal. 
     With respect to cell-site position tolerance, the level and distribution of the C/I ratio desired in a cellular system impacts on the position tolerance for locating cell sites. The Ericsson 7/21 cell plan design, for example, permits the cell site to be positioned up to one quarter of the nominal cell radius (15% of the site-to-site distance) from the ideal site location. 
     The assignee of the present invention has made a limited analysis of the cell-site position tolerance for the cell plan according to the present invention. The results of this analysis are shown in FIGS. 11-13. This analysis focused on the position tolerance which would provide an equivalent C/I distribution to the Bell 7/21 cell plan with an object site located 15% of the site-to-site distance from the nominal site position. The cell-site position tolerance was found to be more than 25% of the site-to-site distance in the 4/12 cell plan for equivalent C/I performance to the reference Bell 7/21 cell plan. However, the C/I distribution for a cell site in the 4/12 cell pattern located 25% of the site-to-site distance off grid is marginal. Therefore, a more conserative site position tolerance of 15% of the site-to-site distance is recommended for inclusion in preferred embodiments of the present invention. Adherence to this position tolerance will provide for a more favorable C/I distribution in the system. In considering the data shown in FIGS. 11-13, it may be noted that the site-to-site distance was used in the analysis due to the differing cell radii for the Bell 7/21 and Ericsson 4/12 cell plans. 
     Based on the foregoing, it should be appreciated that the cell plan according to the teachings of the present invention offers significant advantages relating to traffic capacity and C/I performance. It also offers significant advantages with respect to uplink system gain, as is discussed immediately below. 
     The uplink system gain is defined as the sum of the base station receive antenna gain, the diversity gain, and the relative distance gain in the mobile-to-base path. The diversity gain is a function of the incidence angle and the correlation coefficient of the signal received at the base station. That is: (Uplink Gain)=(Antenna Gain)+(Diversity Gain)+(Distance Gain). 
     The comparison of the four frequency reuse plans discussed herein has been undertaken. Before discussing the results of this comparison, it is important to note a number of assumptions that were made. A first assumption was that path loss is proportional to 39 log r, where r is the distance from the base station to the mobile. A second assumption was that the mobile involved in the comparison is located at the relevant cell boundary at an incidence angle to the base station of 0° to 60°. Third, with respect to the Ericsson and Motorola cell plans, the receive antenna gain for the 60° antennas is 17 dB, and further, the insertion loss (1 dB) associated with the use of duplex filters in the Motorola cell plan is subtracted from the receive antenna gain. It was also assumed that the receive antenna gain for the 120° antennas used in the Bell cell plan is 14 dB. Additionally, theoretical diversity gain for the Ericsson and Bell cell plans was assumed. Theoretical diversity gain for the Motorola cell plan minus the effect of unequal gain branches due to the 60° offset of receive antenna pointing azimuths, and a higher correlation of received signals due to a reduced antenna separation (see FIGS. 17 and 18) was assumed. Finally, a 30 meter tower with a triangular mounting platform which has standard 5 meter faces was also assumed. The antenna mounting arrangements for each cell plan shown and discussed above are shown in FIGS. 17 and 18. It should be noted that the distance gain for each cell plan is a relative value. The value is referenced to the Ericsson cell plan for a mobile located at the cell boundary with an incidence angle of 0°. 
     FIGS. 14 through 16 show the cell geometry and relative receive antenna gain for incidence angles of 0°, 30°, and 60° for the Ericsson, Bell and Motorola cell plans, respectively. 
     Based on the foregoing, an uplink system gain comparison was made and the results of that comparison are shown in Table 2 below. 
     
                                           TABLE 2__________________________________________________________________________CELL    INCIDENCE           GAIN(Db)                  GAIN(dB)                         GAIN(dB)                                GAIN(dB)PLAN    ANGLE   ANTENNA                  DIVERSITY                         DISTANCE                                TOTAL__________________________________________________________________________ERICSSON    0°           17     10     0      27   30°           14     10     2      26   60°            8     9      11     28BELL     0°           14     8      2      24   30°           13     8      4      25   60°           11     7      2      20MOTOROLA    0°           16     7      4      27   30°           13     10     2      25   60°           16     7      4      24__________________________________________________________________________ 
    
     It should now be appreciated that the frequency reuse plan according to the present invention offers excellent traffic capacity, carrier-to-interference performance, and uplink system gain characteristics. It is now appropriate to discuss implementation of an Ericsson 4/12 Cell Plan. Such implementation is discussed immediately below with special reference to frequency planning aspects, voice channel assignment aspects, control channel assignment aspects, retune scheme aspects, and cell splitting aspects, each which topic is appropriately headed. 
     IMPLEMENTATION OF AN ERICSSON 4/12 CELL PLAN 
     The Ericsson 4/12 cell plan may be implemented in an existing system and frequency plan or during the initial system planning and design of a new system. 
     Frequency Planning 
     Optimal frequency planning requires that channel assignment and channel deployment in cells be based upon required cell capacity and C/I considerations. The degree of foresight with which the channel sets are defined and used can affect the system&#39;s transmission quality, cost, and ease of adaptation of growth. 
     Voice Channel Assignment 
     The Ericsson 4/12 cell plan uses four (4) repeated cell designators, with each designator divided into three (3) frequency groups. These three (3) frequency groups, each assigned to an appropriate sector at a site, may contain 1/12 of the total number of a system&#39;s allocated voice channel frequencies. The 4/12 frequency groups are illustrated in Table 3 for the standard FCC a Band. The frequency group allocation for the FCC Extended Band and TACS may be derived in a similar manner as outlined in Table 3 below: 
     
                                           TABLE 3__________________________________________________________________________A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3__________________________________________________________________________312   311 310    309       308          307             306                305                   304                      303                         302                            301300   299 298    297       296          295             294                293                   292                      291                         290                            289288   287 286    285       284          283             282                281                   280                      279                         278                            277276   275 274    273       272          271             270                269                   268                      267                         266                            265264   263 262    261       260          259             258                257                   256                      255                         254                            253252   251 250    249       248          247             246                245                   244                      243                         242                            241240   239 238    237       236          235             234                233                   232                      231                         230                            229228   227 226    225       224          223             222                221                   220                      219                         218                            217216   215 214    213       212          211             210                209                   208                      207                         206                            205204   203 202    201       200          199             198                197                   196                      195                         194                            193192   191 190    189       188          187             186                185                   184                      183                         182                            181180   179 178    177       176          175             174                173                   172                      171                         170                            169168   167 166    165       164          163             162                161                   160                      159                         158                            157156   155 154    153       152          151             150                149                   148                      147                         146                            145144   143 142    141       140          139             138                137                   136                      135                         134                            133132   131 130    129       128          127             126                125                   124                      123                         122                            121120   119 118    117       116          115             114                113                   112                      111                         110                            109108   107 106    105       104          103             102                101                   100                       99                          98                             97 96    95  94     93        92           91              90                 89                    88                       87                          86                             85 84    83  82     81        80           79              78                 77                    76                       75                          74                             73 72    71  70     69        68           67              66                 65                    64                       63                          62                             61 60    59  58     57        56           55              54                 53                    52                       51                          50                             49 48    47  46     45        44           43              42                 41                    40                       39                           38                             37 36    35  34     33        32           31              30                 29                    28                       27                          26                             25 24    23  22     21        20           19              18                 17                    16                       15                          14                             13 12    11  10     9  8  7  6  5  4  3  2  1__________________________________________________________________________ 
    
     Through design of a mobile telephone system must include measures to limit not only co-channel interference, but adjacent channel interference as well. Although receive filters at both the cell site and mobile unit significantly attenuate adjacent channel signals, it is advisable to avoid circumstances in which the received level of an adjacent channel may greatly exceed that of the desired channel. This design goal is accomplished in the Ericsson 4/12 cell plan by eliminating the use of adjacent frequencies in neighbor cells by transposing the D2 and D3 frequency groups in the voice channel allocation plan as shown in FIG. 19. 
     Voice Channel Assignments For Split Cell Sites 
     The pattern assignment for the allocation of frequency groups in a 4/12 cell plan utilizing a 1:3 cell splitting scheme discussed further below is shown in FIG. 20. 
     Underlay-Overlay Cell Concept--Multiple Cell Sizes 
     The underlay-overlay cell concept states that in a region where cells of multiple cell sizes are present, the cellular pattern may be viewed as the superimposition of a small-cell (overlay) pattern on top of a large-cell (underlay) pattern. The channel group assigned to any cell must be subdivided into a large-cell (underlay) group and a small-cell (overlay) group to meet the required C/I objectives in the system. The subdivision of a channel group into large and small-cell groups is governed by the channel requirements of each cell to meet traffic requirements. 
     The coverage area of the small-cells resident at the sites also servicing a large-cell coverage area must be restricted to ensure an appropriate small-cell reuse distance. This small-cell coverage restriction will maintain the D/R reuse distance in the system, thus allowing the use and deployment of additional voice channels in the large-cell pattern to increase traffic capacity. 
     FIG. 21 illustrates channel group subdivisions and overlay cell coverage restrictions for a 4/12 underlay-4/12 overlay cell pattern with multiple cell sizes. 
     Underlay-Overlay Cell Concept--Reuse Partitioning 
     The frequency planning technique of reuse partitioning is defined to be the coexistence of two reuse patterns, operational on a per cell basis, in a system. 
     Given that the overlay cell has a reduced cell radius, its assigned frequency group may have a reduced co-channel reuse distance and still maintain an equal D/R ratio to that of the underlay cell pattern. The voice channels allocated to the overlay cells may then be rearranged into channel groups which conform to the overlay cell reuse pattern. Thus, a two tier reuse scheme is produced and the C/I performance of the underlay cell pattern is still maintained throughout the system. 
     FIG. 22 illustrates a 7/21 underlay-4/12 overlay cell pattern utilizing reuse partitioning. 
     Supervisory Audio Tone 
     AMPS and TACS system employ a continuous out-of-band modulated audio tone known as the supervisory audio tone (SAT) for call supervision purposes. 
     The optimal allocation of the three (3) SATs at 5970, 6000, and 6030 Hz, will multiply the reuse distance ratio for supervision by the square root of 3. 
     This allocation scheme provides for an increased supervision reliability by reducing the probability of misinterpreted interference (co-channel or adjacent channel) both in the land-to-mobile and mobile-to-land path. 
     The pattern assignment for the supervisory audio tone in the Ericsson 4/12 cell plan is shown in FIG. 19. The co-channel reuse distance ratio multiplier of square root of 3 times D for supervision is also shown. 
     Control Channel Assignment 
     Control channels in the Ericsson 4/12 cell plan are assigned in a standard 7/21 cell pattern. This pattern assignment ensures system reliability during system access. FIG. 23 illustrates a control channel allocation plan which has no adjacent control channel frequencies in neighbor cells. It should be noted that the channel spacing between voice and control channels should be maintained at a frequency separation compatible with the combining equipment being used. This separation should be verified on a per cell basis because of the use of a 7/21 cell pattern for control channels and a 4/12 cell pattern for voice channels. 
     Control Channel Assignments for Split Cell Sites 
     The pattern assignment for the allocation of control channels in a 4/12 cell plan utilizing a 1:3 cell splitting scheme discussed further below is shown in FIG. 24. 
     Digital Color Code 
     Cellular systems use a digitally coded forward control channel message format which contains a digital color code (DCC). This DCC is contained in the overhead system parameter words which are transmitted on the forward control channel. The DCC must be verified from the reverse control channel message transmitted by the mobile. 
     The four (4) DCCs multiply the reuse distance for system access supervision by two (2). 
     The pattern assignment for the digital color code in the Ericsson 4/12 cell pattern is shown in FIG. 23. The reuse distance ratio multiplier of 2D for DCC assignments is also shown. 
     Retune Scheme 
     A retune scheme involves an orderly and sequential process which minimizes intra-system interference during the transition stages of converting from a 7/21 cell plan to a 4/12 cell plan. The retune scheme specifies the order in which the radios at each affected site will be retuned beginning with the core area. Temporary voice channel frequencies will be assigned or appropriate channels will be blocked in the transition region until all sites may be tuned to their final 4/12 frequency assignments. This interim procedure is necessary to ensure an acceptable level of system performance during the retune. 
     Frequency Plan Evolution 
     The evolution of existing cellular systems from a 7/21 cell plan to a 4/12 cell plan may be implemented in an orderly fashion by utilizing the underlay-overlay cell concept with the frequency planning technique of reuse partitioning. The coverage radius of the overlay cells will be controlled by software parameters and ERP restrictions to 75% of the underlay cells&#39; radii. The use of the underlay-overlay concept will thus allow the operator to increase capacity globally in the system while still maintaining the C/I performance and reuse distance of a 7/21 cell plan. 
     The simulated evolution of a system which initially uses only the standard FCC A Band is shown in FIGS. 25-27. The required rearrangement and assignment of voice channels into 7/21 and 4/12 frequency groups which use the FCC extended A Band is shown in Tables 4 to 7 immediately below: 
     
                                           TABLE 4__________________________________________________________________________7/21 FREQUENCY ASSIGNMENTSFrequency GroupA.sub.1   B.sub.1 C.sub.1    D.sub.1       E.sub.1          F.sub.1             G.sub.1                A.sub.2                   B.sub.2                      C.sub.2                         D.sub.2                            E.sub.2                               F.sub.2                                  G.sub.2                                     A.sub.3                                        B.sub.3                                           C.sub.3                                              D.sub.3                                                 E.sub.3                                                    F.sub.3                                                       G.sub.3__________________________________________________________________________352   311 310    309       308          307             306                305                   304                      303                         302                            301                               300                                  299                                     298                                        297                                           296                                              295                                                 294                                                    293                                                       292291   290 289    288       287          286             285                284                   283                      282                         281                            280                               279                                  278                                     277                                        276                                           275                                              274                                                 273    272                                                 271270   269 268    267       266          265             264                263                   262                      261                         260                            259                               258                                  257                                     256                                        255                                           254                                              253                                                 252    251                                                 250249   248 247    246       245          244             243                242                   241                      240                         239                            238                               237                                  236                                     235                                        234                                           233                                              232                                                 231    230                                                 229228   227 226    225       224          223             222                221                   220                      219                         218                            217                               216                                  215                                     214                                        213                                           212                                              211                                                 210    209                                                 208207   206 205    204       203          202             201                200                   199                      198                         197                            196                               195                                  194                                     193                                        192                                           191                                              190                                                 189    188                                                 187186   185 184    183       182          181             180                179                   178                      177                         176                            175                               174                                  173                                     172                                        171                                           170                                              169                                                 168    167                                                 166165   164 163    162       161          160             159                158                   157                      156                         155                            154                               153                                  152                                     151                                        150                                           149                                              148                                                 147    146                                                 145144   143 142    141       140          139             138                137                   136                      135                         134                            133                               132                                  131                                     130                                        129                                           128                                              127                                                 126    125                                                 124123   122 121    120       119          118             117                116                   115                      114                         113                            112                               111                                  110                                     109                                        108                                           107                                              106                                                 105    104                                                 103102   101 100     99        98           97              96                 95                    94                       93                          92                             91                                90                                   89                                      88                                         87                                            86                                               85                                                  84     82 81    80  79     78        77           76              75                 74                    73                       72                          71                             70                                69                                   68                                      67                                         66                                            65                                               64                                                  63     61 60    59  58     57        56           55              54                  53                    52                       51                          50                             49                                48                                   47                                      46                                         45                                            44                                               43                                                  42     40 39    38  37     36        35           34              33                 32                    31                       30                          29                             28                                27                                   26                                      25                                         24                                            23                                               22                                                  21     19 18    17  16     15        14           13              12                 11                    10                       9  8  7  6  5  4  3  2  1 .      .                                                 .__________________________________________________________________________ 
    
     
                                           TABLE 5__________________________________________________________________________7/21 UNDERLAY CELL FREQUENCY ASSIGNMENTS WITH REUSE PARTITIONINGFrequency Group7A.sub.1   7B.sub.1 7C.sub.1    7D.sub.1       7E.sub.1          7F.sub.1             7G.sub.1                7A.sub.2                   7B.sub.2                      7C.sub.2                         7D.sub.2                            7E.sub.2                               7F.sub.2                                  7G.sub.2                                     7A.sub.3                                        7B.sub.3                                           7C.sub.3                                              7D.sub.3                                                 7E.sub.3                                                    7F.sub.3                                                       7G.sub.3__________________________________________________________________________312   311 310    309       308          307             306                305                   304                      303                         302                            301                               300                                  299                                     298                                        297                                           296                                              295                                                 294                                                    293                                                       292291   290 289    288       287          286             285                284                   283                      282                         281                            280                               279                                  278                                     277                                        276                                           275                                              274                                                 273    272                                                 271270   269 268    267       266          265             264                263                   262                      261                         260                            259                               258                                  257                                     256                                        255                                           254                                              253                                                 252    251                                                 250249   248 247    246       245          244             243                242                   241                      240                         239                            238                               237                                  236                                     235                                        234                                           233                                              232                                                 231    230                                                 229228   227 226    225       224          223             222                221                   220                      219                         218                            217                               216                                  215                                     214                                        213                                           212                                              211                                                 210    209                                                 208207   206 205    204       203          202             201                200                   199                      198                         197                            196                               195                                  194                                     193                                        192                                           191                                              190                                                 189    188                                                 187186   185 184    183       182          181             180                179                   178                      177                         176                            175                               174                                  173                                     172                                        171                                           170                                              169                                                 168    167                                                 166165   164 163    162       161          160             159                158                   157                      156                         155                            154                               153                                  152                                     151                                        150                                           149                                              148                                                 147    146                                                 145144   143 142    141       140          139             138                137                   136                      135                         134                            133                               132                                  131                                     130                                        129                                           128                                              127                                                 126    125                                                 124123   122 121    120       119          118             117                116                   115                      114                         113                            112                               111                                  110                                     109                                        108                                           107                                              106                                                 105    104                                                 103102   101 100     99        98           97              96                 95                    94                       93                          92                             91                                90                                   89                                      88                                         87                                            86                                               85                                                  84     82 81    80  79     78        77           76              75                 74                    73                       72                          71                             70                                69                                   68                                      67                                         66                                            65                                               64                                                  63     61716   715 714    713       712          711             710                709                   708                      707                         706                            705                               704                                  703                                     702                                        701                                           700                                              699                                                 698    697                                                 696695   694 693    692       691          690             689                688                   687                      686                         685                            684                               683                                  682                                     681                                        680                                           679                                              678                                                 677    676                                                 675674   673 672    671       670          669             668                667                   .  .  .  .  .  .  .  .  .  .  .      .                                                 .__________________________________________________________________________ 
    
     
                                           TABLE 6__________________________________________________________________________4/12 OVERLAY CELL FREQUENCY ASSIGNMENTS WITH REUSE PARTITIONINGFrequency Group4A.sub.1    4B.sub.1   4C.sub.1       4D.sub.1           4A.sub.2               4B.sub.2                   4C.sub.2                       4D.sub.2                           4A.sub.3                               4B.sub.3                                   4C.sub.3                                       4D.sub.3__________________________________________________________________________ 60  59  58  57  56  55  54  53  52  51  50  49 48  47  46  45  44  43  42  41  40  39  38  37 36  35  34  33  32  31  30  29  28  27  26  25 24  23  22  21  20  19  18  17  16  15  14  13 12  11  10  9   8   7   6   5   4   3   2   11023    1022   1021       1020           1019               1018                   1017                       1016                           1015                               1014                                   1013                                       10121011    1010   1009       1008           1007               1006                   1005                       1004                           1003                               1002                                   1001                                       1000999 998 997 996 995 994 993 992 991 .   .   .__________________________________________________________________________ 
    
     
                                           TABLE 7__________________________________________________________________________4/12 FREQUENCY ASSIGNMENTSFrequency GroupA1  B1  C1  D1  A2  B2  C2  D2  A3  B3  C3  D3__________________________________________________________________________312 311 310 309 308 307 306 305 304 303 302 301300 299 298 297 296 295 294 293 292 291 290 289288 287 286 285 284 283 282 281 280 279 278 277276 275 274 273 272 271 270 269 268 267 266 265264 263 262 261 260 259 258 257 256 255 254 553252 251 250 249 248 247 246 245 244 243 242 241240 239 238 237 236 235 234 233 232 231 230 229228 227 226 225 224 223 222 221 220 219 218 217216 215 214 213 212 211 210 209 208 207 206 205204 203 202 201 200 199 198 197 196 195 194 193192 191 190 189 188 187 186 185 184 183 182 181180 179 178 177 176 175 174 173 172 171 170 169168 167 166 165 164 163 162 161 160 159 158 157156 155 154 153 152 151 150 149 148 147 146 145144 143 142 141 140 139 138 137 136 135 134 133132 131 130 129 128 127 126 125 124 123 122 121120 119 118 117 116 115 114 113 112 111 110 109108 107 106 105 104 103 102 101 100  99  98  97 96  95  94  93  92  91  90  89  88  87  86  85 84  83  82  81  80  79  78  77  76  75  74  73 72  71  70  69  68  67  66  65  64  63  62  61 60  59  58  57  56  55  54  53  52  51  50  49 48  47  46  45  44  43   42                        41  40  39  38  37 36  35  34  33  32  31  30  29  28  27  26  25 24  23  22  21  20  19  18  17  16  15  14  13 12  11  10  9   8   7   6   5   4   3   2   11023    1022   1021       1020           1019               1018                   1017                       1016                           1015                               1014                                   1013                                       10121011    1010   1009       1008           1007               1006                   1005                       1004                           1003                               1002                                   1001                                       1000999 998 997 996 995 994 993 992 991 716 715 714713 712 711 710 709 708 707 706 705 704 703 702701 700 699 698 697 696 695 694 693 692 691 690689 688 687 686 685 684 683 682 681 680 679 678677 676 675 674 673 672 671 670 669 668 667 .__________________________________________________________________________ 
    
     The retune necessary to implement the simulated 7/21 underlay-4/12 overlay cell pattern with reuse partitioning from the initial system configuration will involve an initial retune of a maximum of three (3) voice channels in the 7/21 underlay cell pattern. This retune will be essential at all cell sites which use the overlay cell frequencies and are within one (1) reuse distance of the overlay cell. The control channel assignments will not be effected by the retune. 
     Once the capacity of the 7/21 underlay-4/12 overlay cell plan is exhausted, the system operator may then choose to introduce a cell pattern utilizing a 4/12 reuse scheme. 
     Cell Splitting 
     Eventually the traffic demand in some cell of a system will reach the cell&#39;s traffic-carrying capacity. The process called cell splitting implies the introduction and insertion of new cell sites between existing sites in the cell pattern. The cell splitting process revises cell boundaries so that the area formerly regarded as a single cell can now contain several cells. By reducing the area of each cell, cell splitting allows the system to adjust to a growing traffic demand density without any increase in spectrum allocation. 
     There are two (2) cell splitting plans currently used in the cellular industry, a 1:3 and a 1:4 cell split. Both of these cell splitting plans may be used in an Ericsson 4/12 cell plan. 
     1:3 Cell Split 
     In a 1:3 cell splitting plan, the new sites are located midway between three (3) existing sites. The cell site density is increased by a factor of three (3) by reducing to one-third the nominal area previously covered by each of the existing sites. Each stage of cell splitting requires the channel assignments and antennas in the cell split cluster to be rotated by 30°. This rotation is necessary in order to provide coverage without any gaps or overlaps and consistent C/I performance in the cell split cluster. These relationships are shown in FIGS. 28-30. Utilizing a 1:3 cell splitting scheme will not alter channel assignments at the existing cell sites in the Ericsson 4/12 cell plan. 
     1:4 Cell Split 
     In a 1:4 cell splitting plan, the new sites are located midway between two (2) existing sites. The cell site density is increased by a factor of four (4) by reducing to one-fourth the nominal area previously covered by each of the existing sites. Each stage of cell splitting will not require a rotation of antennas. However, each cell split will alter the channel assignments at the existing cell sites. 
     Based upon all of the foregoing, it should now be clear that the frequency reuse cell plan according to the teachings of the present invention offers tremendous advantages over prior art plans. It should also be clear that a plan according to the present invention may be relatively easy to implement. 
     It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description. While the embodiments shown and described have been characterized as preferred, it will be obvious that various changes and modifications therein without departing from the spirit and scope of the invention.