Method for allocating secondary synchronisation codes to a base station of a mobile telecommunication system

Abstract: The present invention relates to a method for allocating secondary synchronization codes (SSC) to a base station of a mobile telecommunication system, each base station of said system continuously transmitting a primary synchronization code (PSC) and a set of K secondary synchronization codes (SSC) respectively allocated to the cell covered by said base station so that any mobile station, when getting switched on, can, on basis of the primary synchronization code (PSC) and the set of secondary synchronization codes (SSC) received from said base station, synchronize with at least one base station in order to read broadcast information, said system using only a predetermined and fixed number M of secondary synchronization codes (SSC) amongst all the N possible and available secondary synchronization codes (SSC). According to the invention, said method comprises the steps of calculating and evaluating the auto-correlation functions and/or cross-correlation functions of N possible secondary synchronization codes (SSC) and the primary synchronization code (PSC) and choosing the M secondary synchronization codes (SSC) amongst said N possible secondary synchronization codes (SSC) so that said the M chosen secondary synchronization codes (SSC) have at least one of the statistical properties of their auto-correlation function and cross-correlation function that is best in term of detection, and allocating a sub-set of said M secondary synchronization codes (SSC) comprising K secondary synchronization codes (SSC) to said base station.

Description: The present invention relates to a method for allocating secondary synchronization codes to a base station of a mobile telecommunication system. The present invention relates to a mobile telecommunication system comprising a number of base stations which can communicate with mobile stations. A communication from a mobile station to a base station is done by means of an up-link UL and the communication from a base station to a mobile station is done by means of a down-link DL. The present invention relates also to a telecommunication systems wherein different user signals are separated both in time domain and in code domain. An example of such a system is the so called UMTS TDD system (Universal Mobile Telecommunication Systems - Time Division Duplex) or W-CDMA TDD system (Wideband Code Division Multiple Access - Time Division Duplex) in which the time domain is represented by the TDD-system component and the code domain by W-CDMA-system component. More particularly, in time-domain, transmission is for example organized based on radio frames constituted of a number (for example 15) of timeslots. The same frequency is used for both the up-link (Mobile Station to Base Station) and the down-link (Base Station to Mobile Station). Furthermore, a time-separation is used to differentiate the down-link and the up-link such that a subset of all the available timeslots per frame is exclusively allocated for down-link transmission and the remaining ones for up-link transmission. In a frame, at least one timeslot is always allocated for each down-link and up-link. In such a system, different user's signals can be transmitted in separate timeslots, e.g. N different down-link timeslots are allocated to N different down-link user signals. This is the time-division aspect of the system. Furthermore, several users signals can also be transmitted within one timeslot by using different spreading codes. This is the code-division aspect of the system. Note that each user is allocated a different spreading code and that each user bit is spread to the chip-rate as function of the employed spreading factor. In such a system, the network allocates to each cell covered by the base stations different cell parameters which enable any mobile station attempting to establish connection to said base station to read cell broadcast information needed to communicate therewith. These cell parameters indicate for example a midamble number and a scrambling code. The midamble is a complex- or real-valued chip sequence and is used by a receiver (a mobile station in the downlink) for channel estimation which is needed for the retrieval of the user's signals. The scrambling code is used by the transmitter (a base station in the downlink) to scramble the user's signals in order to average the interference caused to user's who are sending or receiving in neighboring cells. When a mobile station gets switched on, it must first find out chip, slot and frame timing of at least one cell covering the area in which it is and then find out which midamble and scrambling code are used before it can demodulate and read the cell broadcast information. Afterwards, a tracking mechanism ensures that especially chip timing is not lost, once the mobile station is "synchronized" to the cell. Each base station, for each cell, transmits the cell broadcast information on a channel which is generally the so-called Primary Common Control Physical Channel (P-CCPCH). It can also be the Secondary Common Physical Channel (S-CCPCH) when it is pointed to by the Primary Common Control Physical Channel P-CCPCH. Note that a Primary Common Control Physical Channel P-CCPCH in the W-CDMA TDD system generally uses a fixed and pre-allocated spreading code with fixed spreading factor, e.g. its spreading code is the same in all cells of the W-CDMA TDD system and therefore always known before-hand by the mobile station. A physical synchronization channel (PSCH) is also transmitted simultaneously in those timeslots in the downlink where a Primary Common Physical Channel P-CCPCH is transmitted for the purpose of synchronization to the Primary Common Control Physical Channel (P-CCPCH). The Physical Synchronization Channel essentially consists of two special signals : the primary synchronization code PSC and a set of K secondary synchronization codes SSC. The number K of secondary synchronization codes SSC is generally 3. A Primary Common Physical Channel P-CCPCH is never allocated to a particular timeslot when there is no Primary Synchronization Channel PSCH simultaneously present. If the mobile station finds out in which timeslots the Primary Synchronization Channel PSCH is sent, it knows that the Primary Common Physical Channel P-CCPCH is also in this timeslot. Each of the K in parallel transmitted secondary synchronization code SSC spreads a symbol having a number n of states, i.e. a modulated Quadrature Phase Shift Keying (QPSK) symbol, which gives a total of n K QPSK codewords. On one hand, the combination of the code sets, e.g. different triplets to spread the QPSK-symbols and, on the other hand, the modulation of these QPSK-symbols are used for indicating: A Code Group for which are defined univocally one or several cell parameters, e.g. one or several scrambling codes together with one or several basic short or long midamble codes, The position of the Primary Synchronization Channel PSCH within a double-frame period, and The position of the current Primary Synchronization Channel PSCH timeslot within one frame. Such a spread-spectrum telecommunication system that uses a primary code and secondary codes for synchronization purposes is described for example in the WO-A-99/12173 document. In this document, a primary code and a secondary code are allocated to each slot of a frame, the secondary code being further modulated by one of a number of modulation sequences. Correlations are made in order to recover the frame timing information. Finally, at power-on, a mobile station first searches by performing a correlating process for the presence of the Primary Synchronization Code PSC transmitted on the Primary Synchronization Channel PSCH by the base station of the cell under the coverage of which it is and uses the found time positions for correlating with all possible secondary synchronization codes SSC, generally 16. By performing a coherent detection, e.g. using the Primary Synchronization Code PSC as a phase reference for the secondary synchronization codes SSC, it can also detect the QPSK-symbols spread by the K detected secondary synchronization codes SSC. From this information it can derive the time position of the Primary Synchronization Channel PSCH slot within the frames period as well as the Code Group to which the base station belongs. In a last step, the mobile station can demodulate a burst on the Primary Common Control Physical Channel P-CCPCH by trying all still possible of Scrambling Codes and Basic Midamble Codes which are contained in the found Code Group. Each secondary synchronization code SSC is a different binary-valued chip sequence which is referenced by a particular index. For example, when 16 secondary synchronization codes SSC are possible in the mobile communication system, each secondary synchronization code SSC is indicated by one of the following values : SSC 0 , SSC 1 , SSC 2 , ..., SSC 15 . For example, each of the secondary synchronization code SSC is formed according to the rules defined in the Technical Specifications 3GPP TSG RAN TS25.213 v320 "Spreading and Modulation (FDD)", section 5.2.3.1 page 21ff. and 3GPP TSG RAN TS25.223 v320 "Spreading and Modulation (TDD)", section 7.1 page 10ff. Not all of the possible and available secondary synchronization codes SSC are used simultaneously in a cell for the synchronization purposes above described. Currently, for making the choice of each set of K secondary synchronization codes SSC allocated to a cell, the network takes out of the N possible ones, the first K secondary synchronization codes SSC, then the second K secondary synchronization codes SSC, etc. M out ofN are chosen, the last N-M being unused. For the W-CDMA TDD system, wherein K is 3 and N is 16, for simplicity reasons, the following 4 code groups (e.g. triplets of SSC) are chosen by the network and allocated to the specific cells: Code group 1: SSC 0 , SSC 1 , SSC 2 Code group 2: SSC 3 , SSC 4 , SSC 5 . Code group 3: SSC 6 , SSC 7 , SSC 8 . Code group 4: SSC 9 , SSC 10 , SSC 11 . Thus, only M=K*L secondary synchronization codes SSC out of N possible and available ones are used for synchronization purposes. The performance of the synchronization process above described is very sensitive to the errors than can occur during transmission or during the correlation processes performed for the retrievals of the primary synchronization code PSC and of the secondary synchronization codes SSC. It is an object of the invention to provide a method for allocating secondary synchronization codes SSC to a base station of a mobile telecommunication system in order to improve the performance ofthe synchronization process. Generally speaking, in a system concerned with the invention, each base station continuously transmits a primary synchronization code PSC and a set of K secondary synchronization codes SSC respectively allocated to the cell covered by said base station so that any mobile station, when getting switched on, can, on basis of the primary synchronization code PSC and the set of secondary synchronization codes SSC received from said base station, synchronize with at least one base station in order to read cell parameters. Furthermore, only a predetermined and fixed number M of secondary synchronization codes SSC amongst all the N possible and available secondary synchronization codes SSC are used. The invention is defined by a method for allocating secondary synchronization codes as defined in appended claim 1. Advantageous embodiments of the invention are given in the dependent claims. The features of the invention appear clearly when reading the following description of some embodiments of the invention made in relation with : Figs. 1 and 2 respectively show the auto-correlation functions of the secondary synchronization code SSC 3 and the secondary synchronization code SSC 7 and their respective cross-correlation functions with the primary synchronization code PSC, Fig. 3 shows the difference between the auto-correlation functions of the secondary synchronization code SSC 3 and the secondary synchronization code SSC 7 that can also be observed for the statistical properties of their pair-wise mutual cross-correlation functions with all other secondary synchronization codes SSC (displayed sequential order in Fig. 3), and Fig. 4 shows tables wherein are given statistical properties of the auto-correlation functions and the cross-correlation functions of state-of-the-art synchronization codes available in a W-CDMA TDD system. Figs. 1 and 2 respectively show the auto-correlation functions of the secondary synchronization code SSC 3 and the secondary synchronization code SSC 7 and their respective cross-correlation functions with the primary synchronization code PSC. The synchronization codes used for Figs. 1 and 2 are current state-of-the-art synchronization codes available in a W-CDMA TDD system. From these examples, it becomes clear that the auto-correlation functions of the secondary synchronization codes SSC 3 and SSC 7 are quite different, the auto-correlation function of the secondary synchronization code SSC 3 being better in term of detection. Furthermore, the cross-correlation function of the primary synchronization code PSC and the secondary synchronization code SSC 3 is worse in term of detection than the cross-correlation of the primary synchronization code PSC and the secondary synchronization code SSC 7 . Fig. 3 shows the difference between the secondary synchronization codes SSC 3 and SSC 7 that can also be observed for the statistical properties of their pair-wise mutual cross-correlation functions with all other secondary synchronization codes SSC (displayed sequential order in Fig. 3). Statistically, the auto-correlation function can be characterized by its maximum auto-correlation side-lobe value (MAS-value). It can also be characterized by more than one values of its maximum side-lobes. It can still be characterized by the root of the mean of the total energy contained in all side-lobe peaks (RMS) value. Similar to the auto-correlation function, the cross-correlation function can be characterized by its maximum cross-correlation peak (MCP) value, by more than one values of its maximum peaks or by the root of the mean of the energy contained by all the cross-correlation peaks (RMS) value. Generally speaking, the detection performance of a particular synchronization code will improve, when the maximum auto-correlation side-lobe value (MAS) and the root of the mean of the energy (RMS) value of its auto-correlation function and all maximum cross-correlation peak values (MCP) and the root of the mean of the energy peaks (RMS) values of its cross-correlation with all other possible synchronization codes decrease. Choosing synchronization codes with good auto-correlation and good cross-correlation properties improves the overall Cell Search performance and as such increases the performance of the synchronization procedure and reduces mobile station processing load and its battery life. For the purpose of the invention, one looks for the best L groups each composed of K secondary synchronization codes SSC, such that M=K*L, M being the number of secondary synchronization codes to be selected out of the N possible secondary synchronization codes. Note that in any case, a selection and therefore optimization of correlation properties for the sub-set of employed synchronization codes is always possible, as long as M < N. According to the purpose of the invention, the selection step of the M secondary synchronization codes SSC comprises the steps of discarding the N-M secondary synchronization codes SSC that have at least one of the statistical properties of their auto-correlation function and cross-correlation function which are worst in term of detection and of keeping the M remaining secondary synchronization codes SSC. Table 1, table 2 and table 3 of Fig. 4 summarize some of the statistical properties for the auto-correlation function and all cross-correlation functions of the current state-of-the-art synchronization codes which are available in the W-CDMA FDD and TDD system and that are described in the Technical Specifications 3GPP TSG RAN TS25.213 v320 "Spreading and Modulation (FDD)", section 5.2.3.1 page 21ff. and 3GPP TSG RAN TS25.223 v320 "Spreading and Modulation (TDD)", section 7.1 page 10ff. Referring to Table 1, the selection of the M = 12 secondary synchronization codes SSC gives the following result when discarding the N-M = 4 secondary synchronization codes SSC having the worst value of the off-peak maximum auto-correlation side-lobe values (MAS) in their auto-correlation function and keeping the remaining : SSC 0 , SSC 1 , SSC 2 , SSC 3 , SSC 6 , SSC 7 , SSC 8 , SSC 9 , SSC 12 , SSC 13 , SSC 14 , SSC 15 . When discarding the N-M = 4 secondary synchronization codes SSC having the worst value of the root of the mean of the energy peaks (RMS) value in their auto-correlation function and keeping the remaining, the result is : SSC 0 , SSC 1 , SSC 2 , SSC 3 , SSC 6 , SSC 7 , SSC 8 , SSC 9 , SSC 12 , SSC 13 , SSC 14 , SSC 15 . When discarding the N-M = 4 secondary synchronization codes SSC having the worst maximum cross-correlation peak value (MCP) in their cross-correlation function with the primary synchronization code PSC and keeping the remaining, the result is : SSC 0 , SSC 1 , SSC 3 , SSC 4 , SSC 5 , SSC 6 , SSC 8 , SSC 10 , SSC 12 , SSC 13 , SSC 14 , SSC 15 . When discarding the N-M = 4 secondary synchronization codes SSC having the worst value of the root of the mean of the energy peaks (RMS) in their cross-correlation function with the primary synchronization code PSC and keeping the remaining, the result is : SSC 0 , SSC 1 , SSC 4 , SSC 5 , SSC 6 , SSC 8 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 , SSC 15 . When discarding the N-M = 4 secondary synchronization codes SSC having the worst maximum cross-correlation peak value (MCP) in their cross-correlation functions with the all other secondary synchronization codes SSC and keeping the remaining, the result is : SSC 0 , SSC 1 , SSC 2 , SSC 4 , SSC 8 , SSC 9 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 , SSC 15 . When discarding the N-M = 4 secondary synchronization codes SSC having the worst value of the root of the mean of the energy peaks (RMS) in their cross-correlation function with all other secondary synchronization codes SSC and keeping the remaining, the result is : SSC 0 , SSC 2 , SSC 4 , SSC 5 , SSC 6 , SSC 7 , SSC 8 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 .

Claims: Method for allocating secondary synchronization codes (SSC) to a base station of a UMTS telecommunication system operating in Time Division Duplex mode, said system using a predetermined set of 12 secondary synchronization codes (SSC) amongst the 16 possible secondary synchronization codes (SSC 0 , ..., SSC 15 ), constructed from the position wise multiplication of a Hadamrd sequence and a sequence z, defined as : z = <b, b, b, -b, b, b, -b, -b, b, -b, b, -b, -b, -b, -b, -b>, where b = <1, 1, 1, 1, 1, 1, - 1, -1, -1, 1, -1, 1, -1, 1, 1, -1> said base station transmitting over a synchronization channel a primary synchronization code (PSC) as well as a sub-set of K secondary synchronization codes belonging to said predetermined set, said sub-set being allocated to said base station, characterized in that said predetermined set is one of the following sets: {SSC 0 , SSC 1 , SSC 2 , SSC 3 , SSC 6 , SSC 7 , SSC 8 , SSC 9 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }, {SSC 0 , SSC 1 , SSC 2 , SSC 3 , SSC 6 , SSC 7 , SSC 8 , SSC 9 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }, {SSC 0 , SSC 1 , SSC 3 , SSC 4 , SSC 5 , SSC 6 , SSC 8 , SSC 10 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }, {SSC 0 , SSC 1 , SSC 4 , SSC 5 , SSC 6 , SSC 8 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }, {SSC 0 , SSC 1 , SSC 2 , SSC 4 , SSC 8 , SSC 9 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }, {SSC 0 , SSC 2 , SSC 4 , SSC 5 , SSC 6 , SSC 7 , SSC 8 , SSC 10 , SSC 11 , SSC 12 , SSC 13 , SSC 14 }. Method for allocating secondary synchronization codes according to claim 1, characterized in that said predetermined set is {SSC 0 , SSC 1 , SSC 3 , SSC 4 , SSC 5 , SSC 6 , SSC 8 , SSC 10 , SSC 12 , SSC 13 , SSC 14 , SSC 15 }. Method for allocating secondary synchronization codes according to claim 1, characterized in that K=3.

Inventors: RUDOLF MARIAN, RUDOLF, MARIAN
Applicants: MITSUBISHI ELECTRIC INF TECH, MITSUBISHI ELECTRIC INFORMATION TECHNOLOGY CENTRE EUROPE B.V.
Application year: 2000
Main classification: H04Q7
IPC codes: H04J3, H04B7, H04J13, H04B1, H04W72, H04W56, H04W16