MXPA99002737A - Wireless telecommunication system and method to design my - Google Patents

Abstract

A method for designing a wireless telecommunications system having a plurality of cells is described. In one embodiment of such a method, a demand for calls per cells is estimated and a demand for channel based on music is determined on a cell-in-cell basis is used to provide a reasonably strong upper limit as to the number of channels in the cell. communication required to meet the demand for calls from the entire system. It is implicit in the procedure for the estimation of the upper limit that no mutually interfering base stations use the same channel (that is, frequency). "groups" of base stations are defined to mutually intreferentes cells. A channel demand is determined for each group by adding the channel demand for each cell in the group. The higher channel demand of all the groups determines a "maximum group demand". The upper limit in the number of channels required to satisfy the call demand of the whole system is given by the expression when the mutually interfering cells are adjacent cells and is given by the expression when the mutually interfering cells are adjacent cells, where it is a minimum channel demand of all the cells in the system. By having a reasonably good estimate of the upper limit on the channel requirement of the entire system, a wireless service provider can then seek to obtain or allocate a commensurate amount of frequency spectrum to support its system.

Description

- WIRELESS TELECOMMUNICATIONS SYSTEM AND METHOD FOR DESIGNING THE SAME Field of the invention The present invention is generally concerned with telecommunications. More particularly, the present invention is concerned with a method for designing a wireless telecommunications system wherein the requirements of the frequency spectrum are estimated.

BACKGROUND OF THE INVENTION Figure 1 illustrates a schematic diagram of a portion of a wireless telecommunications system typical of the prior art. Such a system provides wireless telecommunications service to a variety of wireless terminals (e.g., wireless terminals 101-1 to 103-1) that are located within a geographic region. The heart of a typical wireless telecommunications system is the Wireless Switching Center ("WSC") 120, which may also be known as a Mobile Switching Center ("MSC") or a Mobile Telephone Switching Center ("MTSO"). ). Typically, WSC 120 is connected to a plurality of base stations (eg, base stations 103-1 to 103-5) that are dispersed throughout the geographic area to which it is given ref. 29709 service by the system. Additionally, the SC 120 is connected to local and interurban exchanges (for example, the urban center 130, the urban center 138 and the interurban plant 140). The WSC 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wired terminal, which is connected to the system via local and / or long distance networks. The geographical area served by the wireless telecommunications system is divided into a diversity of spatially distinct areas, called "cells or cells". As illustrated in Figure 1, each cell or cell is schematically represented by a hexagon; in practice, however, each cell or cell usually has an irregular shape that depends on the topography of the land. Typically, each cell contains a base station, comprising radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with the WSC 120. As an example of wireless telecommunications, when the wireless terminal 101-1 desires to communicate with the wireless terminal 101-2, the wireless terminal 101-1 transmits the desired information to the base station 103-1, which relays the information to the WSC 120. Upon receiving the information and with the knowledge that it is intended for the wireless terminal 101-2, the WSC 120 then returns the information to the base station 103-1, which relays the information, via radio, to the wireless terminal 101-2. The wireless telecommunications described above are presented in a plurality of communication channels. Such channels are characterized by a carrier frequency and a bandwidth (for example, 30 KHz) in which the carrier frequency is modulated to carry information content. Wireless service providers license, at a very substantial cost, a spectrum band of sufficient frequency to provide an adequate number of communication channels to support communications in a given wireless system. The amount of spectrum a provider must obtain to support or support such communications is predominantly a function of: (1) the amount of spectrum a channel consumes, (2) the extent to which the channels used in any of the cells can be used again in other cells, (3) call traffic ("call demand", or "traffic demand") in the system and (4) the acceptable percentage of blocked call attempts. With respect to (2), channel reuse is limited by channel interference. Such interference, which can be presented between the cells ("co-channel interference") and between numerically consecutive or almost consecutive carrier frequencies ("adjacent channel interference") must be kept within acceptable limits. Since spectrum is very expensive, it is disadvantageous for a provider to license substantially more spectrum than is required to support communications in its wireless telecommunications system. As such, it would be advantageous to have a good estimate value of that spectrum requirement. Unfortunately, little guidance is available to estimate the spectrum requirements for a wireless telecommunications system. One reason for the scarcity of information on the subject may be the difficulty of estimating such requirements, which could be abstracted as a generalization of the notoriously difficult problem of "coloring" graphics. In effect, the estimation of the frequency spectrum adds an additional degree of complexity to the already complicated standard graphics coloring problem.

BRIEF DESCRIPTION OF THE INVENTION In accordance with an illustrative embodiment of the present invention, a method for designing a wireless telecommunications system is provided. In a modality of such a method, a cell arrangement is developed based on the field data, then the demand for calls per cell is estimated and a demand for the channel, based on the demand for calls, is determined on a basis of cell in cell. It has been found that when using cell-to-cell channel demand data, a reasonably strong upper limit as to the number of communication channels required to satisfy the demand for calls throughout the system can be estimated. In the present context, the phrase "upper limit" means that the required number of communication channels will not be greater than the estimated value obtained in accordance with the present teachings. It is implicit in the procedure to estimate the upper limit that no mutually "interfering" base stations use the same channel (ie, frequency). In a first mode, only those base stations (cells) that are adjacent or "closest" to each other are considered to be interfering. In a second mode, the nearest base stations and "closest to the nearest" are considered interfering. In accordance with the present teachings, groupings or "groups" of base stations or cells are defined. A wireless telecommunications system usually comprises many base stations; as such, a plurality of groups can be defined for any given system. A channel demand is determined for each group by adding the channel demand for each cell in the group. The highest channel demand of all groups determines a "maximum group demand"? D. The upper limit in the number of channels required to satisfy the call demand of the whole system, when the interfering cells are adjacent cells, is given by the expression:? D < 17/12"? D The upper limit in the number of channels required to satisfy the call demand of the whole system when the mutually interfering cells are adjacent cells and cells next to adjacent ones is given by the expression:? D < 2 '? D - dmin, where dmin is the minimum channel demand per cell, in the whole system, having a reasonably good estimate of the upper limit in terms of the channel requirement of the entire system, a service provider Wireless can then seek to obtain or assign a commensurate amount of the frequency spectrum to support your system.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a diagram of a wireless telecommunications system of the prior art.

Figure 2a illustrates a flow diagram of a method according to an illustrative embodiment of the present invention for designing a wireless telecommunications system that includes the estimation of spectrum requirements. Figure 2b illustrates a flow chart of a method for estimating channel demand. Figure 3 illustrates a portion of the cells in a wireless telecommunications system, some of which cells are organized in groups.

DETAILED DESCRIPTION In accordance with an illustrative embodiment of the present invention, a method is provided for designing a wireless telecommunications system, which includes the estimation of an upper limit in terms of the requirements of the frequency spectrum. The estimated value of the upper limit is based on a channel allocation method described in the US patent application Serial No., filed on the same date herewith, entitled "Method for Operating to Wireless Telecommunications System," case of the attorney-in-fact. : Khanna 3-6. The estimated value of the frequency spectrum is based on a telecommunications system having a plurality of cells arranged in the usual hexagonal grid topology illustrated in Figure 1. According to the present teachings, the estimated value of the upper limit of the frequency spectrum is a function of the call demand in each cell and the intercell interference. For purposes of the present specification it is considered that two cells (more appropriately, base stations) are "interfering" if they are close enough to interfere with each other when they use the same carrier frequency (ie, channel) for wireless communications. In the estimation of spectrum requirements, any allocation of channels between cells in a system should not assign the same frequencies to a pair of interfering base stations. As such, the frequency spectrum requirement is the minimum number of channels required to support the demand for calls throughout the system, such that no interfering base stations share a common frequency. A method 200 for designing a wireless telecommunications system that includes the estimation of an upper edge limit to the frequency spectrum requirements, according to present teachings, is illustrated by the flow diagram of Figure 2a. In service 202, a channel demand di is determined, by known methods (e.g., standby row theory) for each cell i in the wireless system. Figure 2b illustrates the steps in a typical method for estimating the channel demand of the cell. As indicated in step 202a (FIG. 2b), a physical arrangement of the cells for the wireless system is developed by using known methods. Such methods include the collection of field data to generate an RF (radio frequency) map of the region. The data for such a map representation can be obtained by using transmitters and test receivers. For example, a transmitter is placed on a truck and is parked at a first site or location within the geographic region served by the planned wireless system. A multiplicity of other trucks each having a receiver and a location device, such as a GPS unit, are conducted in the vicinity of the first location and record the intensity of the received signal and the location. Then, the transmitter truck is moved or moved to a second location, additional readings are obtained and the procedure is repeated again. In this way, the geographical region comprising the planned wireless telecommunications system is represented on the RF (radiofrequency) map. A grid of hexagonal cells is placed on the RF map. The base stations are positioned within each cell to provide, to the extent possible, appropriate communications coverage for that cell. Such positioning considers, among other factors, the radiofrequency propagation characteristics within the cell, as they are affected by the topology of the terrain, the structures made by man and the like. In operation 202b, the call traffic within each cell is estimated by known methods, which may consider historical usage factors, projected sales of wireless terminals, etc. In operation 202c, the standby row theory is applied to determine the demand of channel di for each cell i in the system. For example, based on the estimated traffic demand and an estimated duration of the call, those skilled in the art can use Erlang tables to determine the demand of the channel. In operation 204 of method 200, is the "maximum group demand" determined? D. As used herein, the term "group" refers to a group of mutually interfering base stations. In modes where only the co-channel interference (adjacent non-channel interference) between the closest cells (that is, adjacent) is considered, the group size is 3. This is evident from Figure 3 which illustrates a grid of hexagonal cells representative of the cells of a wireless telecommunications system. Base stations (or cells) 303-1, 303-2 and 303-3 form a first CLl group. Other base stations, such as 303-4, 303-5 and 303-6 do not belong to CLl because such base stations are not "closest cells" to at least some of the base stations in the first CLl group. For example, while the base station 303-5 is adjacent to the base station 303-1 of the first group CL1, it is not adjacent to the other base stations of the first group (i.e., base stations 303-2 and 303-3 ). The base station 303-5 and the base station 303-1, however, belong to another group with 303-4; second group CL2. It is clear that a typical wireless telecommunications system, there will be many such groups. In modalities where the co-channel interference near and closest to the nearest one is considered, the group size is 7. The third group CL3 illustrated in figure 3, represents such a group of seven elements and includes the base stations ( or cells) 303-7, 303-8, 303-9, 303-10, 303-11, 303-12 and 303-13. The parameter "? D", the "maximum group demand", is defined as follows. As previously indicated, a demand for channel d is determined for each cell in the wireless system. In the first CLl group, cell 303-1 has the channel demand of f cell 303-2 has a channel demand d2 and cell 303-3 has demand d3. In group CL2, cell 303-5 has the demand of channel d6, cell 303-4 has the demand of channel d4 and as already indicated, cell 303-1 has the demand of channel j. For illustrative purposes, it is assumed that d = 2 (that is, 2 channels are sufficient to satisfy the call demand in cell 303-1), d2 = 3, d3 = 3, d4 = 2 and d5 = 2. Given the previous channel demands, the CLl group has a total demand of 2 + 3 + 3 = 8 and the CL2 group has a total demand of 2 + 2 + 2 = 6. As between the CLl and CL2 groups, the CLl group has the maximum demand of the group. For the previous example,? D = 8. Thus, the maximum group demand? D is thus the demand of the group that has the highest channel demand of all the groups in the system. Having determined the maximum group demand, an upper limit on the number of channels required to satisfy the demand for system-wide calls in the wireless telecommunications system is estimated, as indicated in step 206 of method 200. It has been found that for the modalities where only the co-channel interference between the closest cells is considered, the upper limit as to the number of channels Δd required to satisfy the demand is given by: [1]? d < 17/12? D Furthermore, it has been found that for the modalities in which the co-channel interference between the cells closest to and closest to the nearest one is considered, the upper limit in terms of the number of channels required to satisfy the Demand for calls is given by: [2]? d < 2? D - c where: dm? N is the minimum channel demand per cell, throughout the system. In a previous example, it was determined that the group CLl had a demand of 8. If it is determined that the CLl group defines the maximum group demand for a system, then, considering only the co-channel interference of the closest cell. [3]? D < 12/17 (8) < 12 Thus, given a maximum group demand of 8 and considering only the co-channel interference of the nearest cell, no more than 12 frequencies are required to satisfy the call demand in the wireless system. In the previous example, which deals with the interference of the closest and closest to the closest cell, the third illustrative group CL3 (figure 3) was defined to include the cells or base stations 303-7, 303-8, 303-9 , 303-10, 303-11, 303-12 and 303-13. For illustrative purposes, it is assumed that the channel demands for those cells are d7 = 2, de = 3, d5 = 2, d10 = 2, dn = 2, di? = 3 and di3 = 2, respectively. Assuming that the CL3 group defines a maximum group demand for a system, which in the present example is 2 + 3 + 2 + 2 + 2 + 3 + 2 = 16 and given that in such channel demands, dmin is 2, then: [4]? d < 2 (16) -2 < 30. Thus, given a maximum group demand of 16 and considering the co-channel interference of the closest and closest to the closest cell, no more than 30 frequencies are required to satisfy the call demands in the wireless system . Expressions [1] - [4] do not consider the effects of adjacent channel interference on spectrum requirements. The interference of the adjacent channel, which is usually weaker than the co-channel interference, arises from the fact that the frequencies in the spectrum have a natural order that determines the use of consecutive or close frequencies. The effect of such interference from the adjacent channel is now addressed. Only the interference of the adjacent channel that occurs in a given cell is considered in the present. While there may be interference from the adjacent channel between adjacent cells, such interference is substantially weaker than the other forms of interference mentioned herein and can reasonably be ignored. It has been found that for the modalities in which the co-channel interference of the nearest cell and the adjacent channel interference are considered, the upper limit in terms of the channel requirement of the whole system? 1 is given by: [ 5]? 1 < dm? n + 2 (? d - 2dm? n) where: d "n is the minimum channel demand per cell, of the entire system. For example, assuming that *,. ^ 2, and? D = 8, then: [6] < 3 (2) + 2 [8 + 2 (2)] < 14. Thus, for the illustrative example, the upper limit on the channel requirement of the entire system is increased from 12 to 14 when the interference of the adjacent channel is considered. (See expression [3]). It has surprisingly been found that for the modalities in which the adjacent channel interference is considered, in conjunction with the co-channel interference of the closest and closest to the nearest cell, the upper limit in terms of the frequency spectrum ? 1 is given by: [7]? 1 < 2? D. The details concerning the derivation of the expressions are not necessary for the understanding or use of the present invention and as such, are not presented herein. Such details are provided in a document of the inventors entitled "On Wireless Spectrum Estimation and Generalized Graph Coloring", presented at IEEE INFOCOM * 98, March 29 - April 2, 1998 in San Francisco, CA. (17th Annual Joint Conf. Of the IEEE Computer and Communications Society), incorporated herein by reference. Finally, as indicated in operation 208 of method 200, after having estimated the upper limit of the channel requirement, a wireless spectrum provider obtains a spectrum band consistent therewith (assuming some channel bandwidth). If the provider already has the spectrum, then, it is assigned in a manner consistent with the determined upper limit. It will be understood that the embodiments described herein are illustrative only of the present invention. Other embodiments may be devised in application of the teachings present by those of ordinary skill in the art without deviating from the spirit and scope of the invention. Accordingly, it is proposed that such other embodiments be included in the scope of the following claims and their equivalents. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (7)

1. Claims Having described the invention as above, property is claimed as contained in the following: 1. A method for designing a wireless telecommunications system having a plurality of cells, characterized in that it comprises the steps of: estimating a channel demand for each cell based on call demands within each cell; determining a maximum channel demand? d between all groups of mutually interfering cells, based on the estimated channel demand for each channel, where the interference between mutually interfering cells is co-channel interference; calculating a number of communication channels? d required to satisfy the call demands in all the cells of the system according to the expression:? d < 17/12"? D, when the mutually interfering cells with adjacent cells and in accordance with the expression? D <2 '? D -dm? N, when the mutually interfering cells are adjacent cells and cells next to adjacent ones, in where dm? n is the minimum channel demand of all cells in the system, allocating a band of the frequency spectrum for use in the system sufficient to provide the calculated number of communication channels based on a desired channel bandwidth
2. 2. The method according to the claim 1, characterized in that the step of estimating the channel demand also includes the step of estimating the demand for calls in each cell.
3. 3. The method of compliance with the claim 2, characterized in that the step of estimating the channel demand also comprises the step of developing a physical layout of the cells based on field data.
4. 4. The method of compliance with the claim 3, characterized in that it also comprises obtaining the field data when using mobile transmitters, receivers and location or location devices.
5. 5. The method of compliance with the claim 2, characterized in that the step of estimating the channel demand also comprises the step of using Erlang tables to express the demand for calls as channel demand.
6. 6. The method of compliance with the claim 1, characterized in that the desired channel bandwidth is in the range of about 20 to 50 KHz.
7. 7. A wireless telecommunications system, characterized in that it comprises: a first plurality of cells; and a base station located within each cell, each base station is operable to communicate with the wireless terminals within the same cell and other base stations in other cells; wherein the wireless telecommunications system uses a band of the frequency spectrum in which an intracell communication is presented, which band is divided into a diversity of channels sufficient to satisfy the call demand of the whole system, wherein the number of channels and hence the band of the frequency spectrum is estimated by: estimating a channel demand for each cell based on the demand for calls within each cell; determining a maximum channel demand? d between all groups of mutually interfering cells based on the estimated channel demand for each cell, wherein the interference between mutually interfering cells is co-channel interference; and calculating a number of communication channels? d required to satisfy the call demands in all the cells of the system according to the expression:? d < 17/12"cod when the mutually interfering cells with adjacent cells and according to the expression? D <2"? D -dmin when the mutually interfering cells are adjacent cells and cells next to adjacent ones, where dmln is the demand minimum channel of all cells in the system. SUMMARY OF THE INVENTION A method for designing a wireless telecommunications system having a plurality of cells is described. In a modality of such method, a call demand per cell is estimated and a channel demand based on it is determined on a cell-in-cell basis. The demand for cell-to-cell channel is used to provide a reasonably strong upper limit in the number of communication channels required to satisfy the demand for calls from the entire system. It is implicit in the procedure for the estimation of the upper limit that no mutually "interferent" base stations use the same channel (that is, frequency). "Groups" of base stations or mutually interfering cells are defined. A channel demand is determined for each group by adding the channel demand for each cell in the group. The highest channel demand of all groups determines a "maximum group demand"? D. The upper limit in the number of channels required to satisfy the call demand of the whole system is given by the expression:? D <; 17/12 '? D when the mutually interfering cells are adjacent cells and is given by the expression:? D < 2"? D - dm n when the mutually interfering cells are adjacent cells and cells next to adjacent cells, where dmin is a minimum channel demand of all the cells in the system, having a reasonably good estimate of the upper limit in Regarding the channel requirement of the entire system, a wireless service provider can then seek to obtain or assign a commensurate amount of frequency spectrum to support its system.