MXPA99005870A - Telecommunications systems - Google Patents

Abstract

A base station monitors signals received from cochannel interferers, and detects the air frame positions of the interfering transmissions. The base station then adjusts the air frame position of its own transmissions such that it minimises the possibility of interference with the cochannel interferers.

Description

TELECOMMUNICATION SYSTEMS TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for controlling the synchronization of a radio transmitter compared with other receivers in a network, in order to minimize the interference effect of other transmitters. Particularly, the invention relates to the synchronization of a network of base stations in a mobile communication system. DESCRIPTION OF THE RELATED ART In a digital cellular radio system operating in accordance with the TDMA principle such as, for example, GSM, D-AMPS or PDC, radio messages are transmitted in frames from base stations, each box includes a given number of time segments. The transmissions in the different segments are generally intended to be received by different mobile radio receivers, and therefore it is necessary to ensure that the receiver is synchronized with the transmitter. The mobile transceivers are synchronized with their respective base stations by means of a signal from the base station.

For example, in D-AMPS of total speed, 3 mobile receivers share the same frequency channel, and therefore each channel is divided into three time segments, each with a duration of 6.7 ms, three time segments forming one picture. The frames are repeated 50 times per second. Each time segment in the table is assigned a particular mobile receiver, until the call is released or until the mobile is transferred to another channel, for example in another cell. In each time segment, 324 bits are transmitted, most of which are data bits, but 28 of these bits form a synchronization word. The standard published by the Electronics Industries Association as publication IS136 of EIA / TIA, which specifies the D-AMPS system, defines 6 different synchronization words, but only 3 of these are used for a channel that operates at full capacity. Accordingly, a different synchronization word is assigned to each time segment in a frame, and the base station transmits the relevant synchronization word once during each time segment. The mobile receiver can recognize predicted transmissions by identifying the synchronization word, and similarly includes the same synchronization word in its own transmissions to the base station. The synchronization words in IS136 are chosen in such a way that there is a minimal correlation between them. Accordingly, there is only a very small probability that a receiver mistakenly identifies a synchronization word transmitted with a different synchronization word. However, a danger is that a receiver receives the expected synchronization word from an interfering transmitter that operates on the same frequency and misinterprets it as its expected synchronization word. In the prior art attempts were made to overcome these problems. A known possibility is simply to allow each base station transceiver to select its own timing, independently of other base stations, which means that there is no synchronization between the base stations. In this situation, it is possible that an interfering transmitter transmits the same synchronization word with a sufficient signal level to cause interference, and at a time point sufficiently close to the expected time for the possibility of wrong synchronization to arise. A known alternative is to synchronize the entire network in such a way that each base station is transmitting the same synchronization word at the same time. In fact, this increases the possibility that the base station mistakenly receives the synchronization word from an interfering transmitter and interprets said word as if it were its own expected synchronization word.

The probability of a wrong identification in this way depends on the carrier / interference ratio (C / I) that is related to the relative signal levels of the transmissions from a desired transmitter, and from an interfering transmitter operating in the same frequency. Since there is only a limited number of frequencies available for use in a system, it is necessary to reuse the frequencies. Frequency planning can optimize the frequency rejection distance and therefore optimize the C / I ratio, but in general it can not be guaranteed that the C / I ratio is sufficiently high to avoid any possibility of interference due to a wrong detection of the synchronization word coming from an interfering transmitter. COMPENDIUM OF THE INVENTION The present invention relates to a method of operating a base station transceiver, wherein the transceiver detects signals from interfering transmitters, and selects its own air box position, i.e. the time in which it is detected. transmits a synchronization word, in such a way that the possibility of mistakenly detecting a synchronization word coming from a co-channel interfering device is minimized. The invention also relates to the transceiver itself, and to a base station incorporating a transceiver.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a part of a telecommunications network according to the invention. Figure 2 shows an area covered by the network of the figure 1. Figure 3 represents a base station according to the invention. Figure 4 is a flow diagram showing a process used by the base station controller to obtain data used in a method according to the invention.

Figures 5a-5d represent signals transmitted by the base stations that are part of the network illustrated in Figure 1. Figure 1 shows a network where a mobile services switching center MSSC 2 is connected to 4 base stations 4,6,8,10 known here also as BSl, BS2, BS3 and BS4, and controls the operation of said base stations. It will be noted that a practical network will probably involve much more base stations, but 4 base stations are sufficient to allow the description of the invention. Figure 2 represents an area to be covered by a network.

The area is shown as divided into generally hexagonal cells, each of which contains a respective base station. Each base station transmits at a particular operating frequency, but due to limited availability of adequate frequencies, each frequency must be reused in more than one cell. In the network illustrated in Figure 2, the Cl, C2, C3 and C4 cells contain the base stations BS1, BS2, BS3 and BS4, respectively, all operating on the same frequency. As shown, these cells should be as widely spaced as possible in order to minimize the possibility that stations of a base station are detected by a mobile with one of the cells associated with the other base stations. These other base stations, and the mobiles connected to them are known in this context as inter-channel co-channel devices and the proportion between the strength of the signal coming from the base station with which the mobile is in communication and the The strength of the signal from the other base stations is known here as the C / I ratio, or carrier / interference. Figure 3 is a schematic representation of one of the base stations BS1-BS4, and the general form of the base station is well known to those skilled in the art. The base station includes the transceiver 22, in communication with a controller 24, and which sends signals and receives signals from an antenna. The controller 24 analyzes the signals received by the transceiver 22 and controls its operation. In a typical system, each transceiver divides its transmissions into time segments, and communicates with a different mobile during each time segment. For example, a table that lasts 20 ms can be divided into three time segments. To allow the mobiles to determine which signals coming from the transceiver are intended for them, the transceiver transmits during each time segment a synchronization word, or synchronization word. When communications are divided into three time segments, there are three synchronization words, which are selected in such a way that they have minimal correlation, and therefore there is an extremely small probability that a mobile will detect an incorrect synchronization word and misinterpret it. as the synchronization word you are waiting for. The mobile is more likely to detect the expected synchronization word transmitted from one of the other base stations operating on the particular frequency, and act on the data transmitted with this erroneous synchronization word. In some systems of the prior art, the transceivers 22 in the base stations are controlled from MSSC 2, such that their air box positions are synchronized. That is, the transmissions of all the transceivers in the network are synchronized in such a way that each transceiver starts a new time segment at the same time as each of the other base station transceivers. As a result, each base station transceiver is transmitting a synchronization word at the same time as each of the other base station transceivers. That means that, when a mobile is waiting for the reception of a synchronization word from the base station with which it is in communication, other base stations operating on the same frequency will also be transmitting synchronization words. If these other base stations are transmitting different synchronization words, then the possibility of mistakenly interpreting one of these synchronization words is very low due to the minimum correlation between the synchronization words, as previously mentioned. However, if one of the co-channel interfering devices is transmitting the synchronization word that the mobile is waiting to receive, there is a danger that it misinterprets that it is the synchronization word coming from the base station with which the mobile is in communication. As mentioned above, there are only three synchronization words available for use, and therefore there is a significant probability that one of the co-channel interfering devices is transmitting the expected synchronization word to the expected time. According to the present invention, each base station can adjust its own air box position in order to avoid the possibility that mobiles connected to the base station mistakenly detect interfering synchronization words coming from inter-channel co-channel devices. Figure 4 is a flow diagram showing the process performed by the base station controller. In the first step, step 40, the controller selects one of the time segments of the transceiver and, in step 42, determines whether a mobile is connected to this time segment. If a mobile is connected to this time segment, the process proceeds to step 44, and the controller waits for the next time segment. If no mobile is connected to the time segment under consideration, the process proceeds to step 46, where it detects signals transmitted by one of the co-channel interference devices. Specifically, it detects signals received from mobiles connected to another base station transceiver during the period of a time segment. The purpose is to determine the air box position of another transceiver, that is, the time point in which the synchronization word 1 transmits, which can be considered as the beginning of a frame. The controller will know that, during a segment of time, the other transceiver will transmit a synchronization word, but will not know which of the three synchronization words to wait and, due to the lack of synchronization between the transceivers, will not know when during its own segment of time wait for the synchronization word coming from the other transceiver. During a segment of time, a base station transmits 324 bits, most of which are data, but 28 of these bits constitute a synchronization word. Accordingly, in step 46, the controller attempts to correlate the received data with the three available synchronization words. Then, in step 48, the sample having the highest correlation with one of the three synchronization words is selected. In step 50, it is determ whether this degree of correlation, between the sample that correlates more closely with one of the synchronization words, and this synchronization word, is higher than a threshold value. If this is not the case, it is decided that a reliable estimate of the air box position can not be made for this interfering co-channel device, and the process returns to step 44. If no reliable estimate can be made, this should be this may be because the signal strength for the co-channel interfering device is very low, which means that the other transceiver is a very weak interfering device, which is unlikely to cause problems. For example, there may be no dominant interfering device. If the degree of correlation measured in step 50 is greater than a certain threshold, it is considered in step 52 that the detected signal, ie the 28-bit sample that correlates more closely with one of the synchronization words, it is a synchronization word, but it is helpful to verify that it is in fact a signal for a mobile. This verification is achieved by the use of the fact that the signals transmitted by mobiles also include, during each time segment, in a conducted position in relation to the synchronization word, a digital coded voice channel (CDVCC) color code, which is unique to the particular base station. The CDVCC is a coded version of the digital voice channel (DVCC) color code that is made up of 8 bits, the CDVCC, containing 4 CRC bits. As mentioned above, the time position of the CDVCC in the transmission is known in relation to the time position of the synchronization word. Thus, in step 54 of the process, the bits that appear in this known position assuming that the detected sample is a synchronization word, are considered as CDVCC.

It is then necessary to test whether this detected code in fact represents a CDVCC or if it is simply noise, which could indicate that the detected sample does not represent a synchronization word, and therefore that it has not been possible in a 'CDVCC, and consequently that the previously detected signal represented a synchronization word. The process then proceeds to step 62. If, in step 60, the bit difference number between the recoded CDVCC and the originally detected CDVCC exceeds a threshold, it is considered that this is because the originally detected CDVCC did not in fact represented a CDVCC but was simply data received erroneously from the interfering co-channel device. In this case, it is again decided that the air box position can not be determined and the process proceeds to step 44. If the process proceeds to step 62, that is, if it is determined that a word of synchronization, followed in the appropriate interval by a CDVCC, the air box position (AFP) of the co-channel interfering device is calculated, using the position of the synchronization word detected in relation to the beginning of the time segment under consideration, and using the fact that synchronization words occur in a known sequence. Then, in step 64, the calculated AFP, the representative DVCC of the inter-channel co-channel base station, and the strength of the signal received from this base station are stored. Finally, the process continues until step 44 and can be repeated in any other inactive time segment. It will be noted that steps 54-60 can be omitted and that any sample well correlated with one of the synchronization words can be considered as a synchronization word. While the complexity of the system is reduced, this increases the probability of storage of erroneous data. When the process presented in Figure 4 has been carried out for all the detected co-channel interfering devices, and when it has been stored in a database, the controller 24 of the base station then tries to select an AFP for its own transmission, in order to minimize the risk that mobile stations that receive their transmission will receive synchronization words from interfering mobile stations. For example, with reference to the network illustrated in Figure 2, it is considered that the controller 24 of the base station BS4 in a cell C4 is carrying out the process set forth in Figure 4. Figures 5a, 5b and 5c show the calculated air box positions of the transmissions from the base stations BSl, BS2 and BS3 in the Cl, C2 and C3 cells, respectively. Thus, Figures 5a, 5b and 5c each have a table of transmissions from the respective base stations, with the time segments identified by the synchronization words SYl, SY2, or SY3 that are transmitted during these segments of time. weather. Furthermore, it will be observed from FIG. 2, that the C4 cell is considerably closer to the Cl and C2 cells than to the C3 cell. This means that it is likely that the strength of the signal received from cell C3 is significantly less than the strength of the signals received from cells Cl and C2, and it is considered for the purpose of this discussion that it is the case. Thus, when selecting the airframe position for transmissions from the base station BS4, the controller 24 can determine that it would be possible to minimize the risk of interference if the synchronization word SY1 is transmitted approximately at the same time as the station. BS1 is transmitting the synchronization word SY2, and the base station BS2 is transmitting the synchronization word SY3, as shown in FIG. 5d. In this illustrated example, the air box position of the base station BS4 is also notably of the air box position of the base station BS3, and there is only a small probability that transmissions of these two base stations have interference. The acceptability of this risk will depend on the strength of the signal from the base station BS3 detected in the base station BS4. Once an appropriate air box position has been determined for the BS4 transmissions by the controller 24, it may be necessary to adjust the air box position. This can be achieved in one of two ways. One possibility is that, if all the time segments are inactive, ie no mobile is connected to the base station, it is possible to simply turn off the transmitter of the base station and then determine the optimum air box position with reference to the information database in relation to the inter-channel co-channel devices, when the transmitter must be reactivated. Alternatively, the air box position can be adjusted while at least some time segments are occupied, by slowly changing the lengths of the time segments, i.e. by slowly altering the temporal position of the synchronization word in the desired direction. Provided that the change is made slowly enough, this will be detected simply by the mobile as a change in the path delay. As a result, the air box positions of the base stations can be changed, to ensure that interference between the base stations is minimized. The method has been described here in relation to a base station. The same method can be carried out partially in a mobile device. For example, measurements can be carried out on a mobile device and reported to a base station, which then calculates the AFP of the co-channel interfering devices and determines the optimum AFP for its own transmissions. Alternatively, the mobile device can calculate the AFP of the co-channel interfering devices, and report the results to the base station.

Claims (1)

1. CLAIMS A transceiver to receive and transmit signals at an operating frequency, the transceiver includes a controller that: controls the transceiver in order to detect interfering transmissions at the operating frequency; calculates the air box positions for said interfering transmissions; and controls the air box position of transmissions coming from the transceiver in order to reduce the possibility of interference between the transmissions from mobiles connected to the transceiver and the detected interfering transmissions. A transceiver according to claim 1, wherein the controller calculates the air box positions for said interfering transmissions by: the correlation of a received signal with all available synchronization words, and by determining the highest degree of correlation between any part of the received signal and any of the available synchronization words. A transceiver according to claim 2, wherein the controller calculates the air box positions for said interfering transmissions by, in addition: comparing the determined higher degree of correlation with a first threshold value. A transceiver according to claim 3, wherein the controller calculates the air box positions for said interfering transmissions by, in addition: the search for a color code at a predetermined location in relation to the part of the received signal that causes the highest determined degree of correlation. A transceiver according to claim 4, wherein the controller calculates the air box position for said interfering transmission, by, in addition: decoding and recoding a detected color code, and comparing the detected and recoded color codes and, only if the difference between the detected and recoded color codes is less than a second threshold value, the determination that the data is a color code, and therefore the determination that the part of the received signal that causes the most High determined correlation between the part of the received signals and the available symbolizing word, is a symbolizing word, and consequently the calculation of the air box position of the interfering transmission on this basis. A transceiver according to any of the preceding claims, wherein the controller controls the transceiver to detect interfering transmissions in each inactive time segment. A transceiver according to any one of the preceding claims, wherein the air box positions of all received interfering transmissions are calculated. A transceiver according to claim 1, wherein the air box position is set to a desired position when the transceiver begins transmissions. A transceiver according to claim 1, wherein the air box position is adjusted from the transceiver. A base station comprising one or more transceivers in accordance with that claimed in any of the preceding claims. A telecommunications network comprising a plurality of base stations in accordance with that claimed in claim 10. A method for controlling a transceiver for receiving and transmitting signals at an operating frequency, the method comprising: controlling the transceiver to detect transmissions interfering in the frequency of operation; the calculation of the air box positions for said interfering transmissions; and control of an air box position of transmissions from the transceiver to reduce the possibility of interference between the transmissions coming from the transceiver and the detected interfering transmissions. A method according to claim 12, which comprises the calculation of air box positions for said interfering transmissions by: the correlation of a received signal with all available symbolizing words, and the determination of the highest degree of correlation between any part of the received signal and any part of the received signal; the symbolizing words available. A method according to claim 13, comprising calculating air box positions for said interfering transmissions by, in addition: comparing the highest determined degree of correlation with a first threshold value. A method according to claim 14, comprising the calculation of the air box positions for said interfering transmissions by, in addition: the search for a color code at a predetermined location in relation to the part of the received signal that causes the highest determined degree of correlation. A method according to claim 15, comprising calculating the air box position for said interfering transmission by, in addition: decoding and recoding a detected color code and comparing the color codes detected and recoded and , only if the difference between the color codes detected and recoded is less than a second threshold value, the determination that the data is a color code, and therefore the determination that the part of the received signal causes the highest degree determined of correlation between the part of the received signals and one of the available synchronization words is a synchronization word and consequently the calculation of the air box position of the interfering transmission on this basis. A method according to any of claims 12 to 16, which comprises controlling the transceiver to detect interfering transmissions in each inactive time segment. A method according to any of claims 12 to 17, which comprises the calculation of the air box positions of all received interfering transmissions. A method according to claim 12, comprising setting an air box position to a desired position when signals are being transmitted. A method according to claim 12, comprising adjusting an air box position during transmissions. A base station operating on a first frequency for use in a telecommunications network having a plurality of base stations, at least some of them also operate on the first frequency, the base station is programmed to monitor transmissions from of mobile devices that are in communication with other base stations operating on the first frequency in order to determine the air box positions of such transmissions, and programmed to select an air box position for their own transmissions on the basis of to the air box positions determined in this way. A base station according to claim 21, wherein the base station is programmed to select an air box position for its own transmissions in order to minimize the possibility of mis-detection of signals from other base stations that operate on the same frequency. A method for the operation of a base station that is part of a cellular telecommunications network, the base station operates on a first frequency, and the network also comprises a plurality of other base stations operating on the first frequency, the The method comprises the monitoring of the transmissions coming from the mobile devices in communication with the other base stations that operate on a first frequency, in order to determine the air box positions of said transmissions and the selection of a frame position. air for transmissions from said base station in order to minimize the possibility of a wrong detection of a synchronization word from one of said other base stations. A base station, for use in a cellular communication network, comprising a plurality of base stations of this type, the base station comprises a transceiver operating at a predetermined frequency, and is one of several transceivers in the network operating in said network. predetermined frequency, wherein the transceiver measures a signal strength received from other transceivers operating at said predetermined frequency and determines the air box position thereof, and where the transceiver adjusts the air box position of its own transmissions based on the forces of the measured signals and air box positions. A method of operating a base station in a cellular communication network comprising a plurality of base stations of this type, the base station comprises a transceiver operating at a predetermined frequency, and is one of several transceivers in the network operating at said predetermined frequency, the method comprises measuring a signal strength received from other transceivers operating at said predetermined frequency, and determining its air box position, and adjusting the air box position of Transceiver transmissions based on measured signal strengths and air box positions.