METHOD FOR EQUALIZING THE PROPAGATION DELAYS AND OPTIMIZING THE POWER LEVEL IN A RADIO COMMUNICATION SYSTEM

Abstract: Method for equalizing the propagation delays and optimizing the power level in a mobile station accessing network services on a common channel, in the third generation of mobile cellular systems based on a code multiplexing (Code Division multiple Access or CDMA) and TDD-TDMA type time division (Time Division Duplex-Time Division Multiple Access) access technique and comprising at least one base station (BS) and at least one mobile station (MS). The method is characterized in that it includes various temporally distinct steps for the optimization of the "frame synchronization" and "power level" parameters during the procedure for access to the network services by a mobile station (MS).

Description: "METHOD FOR EQUALIZING THE PROPAGATION DELAYS AND OPTIMIZING THE POWER LEVEL IN A RADIO COMMUNICATION SYSTEM FIELD OF THE INVENTION This invention relates to the radio mobile telephony field, and more particularly to a method for the optimization of the «frame synchronization)) and «power level» parameters during the procedure for access to the network services by a mobile station PRIOR ART In the course of the last ten years, the radio mobile telephony systems have undergone a constant technological evolution that has brought about a gradual abandonment of the first generation of systems, characterized by analog modulations of the transmitted carriers, replaced by second generation systems, characterized instead by digital modulations, and by extensive digital processing (DSP) of the base band signal converted to digital The time is now ripe for radio mobile systems of an even more advanced design to come into operation, the so-called third generation systems which differ from their predecessors mainly in the different way the users of the service access the physical channels In particular, the different access mode is based on a code multiplexing technique, i e the Code Division Multiple Access (CDMA) technique The design of these systems made use of applications maturative in the military field following studies on the feasibility of transmissions suitable for preserving secrecy of the information transmitted and guaranteeing a certain immunity to noise created for the purposes of sabotage (jamming) These objectives were attained thanks to an artificial widening of the modulation spectrum of the transmission carrier with respect to the base band spectrum This modulation technique is therefore called spread spectrum and consists in multiplexing each symbol with a low symbol rate of the signal to be transmitted with a pseudo-noise type code sequence (chip) with a higher rate (the chip rate), the purpose being to spread the information transmitted over a wide spectrum of frequencies, accordingly making it accessible only to those duly enabled for reception To this effect, the spread spectrum receiver demodulates the signal received and reconstructs the original data by effecting a temporal correlation between the demodulated signal and a local copy of the code sequence used in the modulator From the mathematical correlation between the symbols of the demodulated signal and the exact code sequence, the original signal is obtained at the receiver output at its peak level, accordingly distinguished from the noise and from interference. In the civilian context, and more properly in the radio mobile telephony sector, a somewhat different use of the spread spectrum modulation from the former military objectives is envisaged. The particular use is that of allowing the simultaneous sharing of a single physical channel between various users distinguished by different spreading codes. The relative technique, known by the abbreviation CDMA (Channel Division Multiple Access), uses reciprocally orthogonal spreading code sequences, namely where the intercorrelation is negligible. This is what permits the discrimination between the various users entering the transmission band in that, on a channel characterized by its own code sequence, the signals of the other channels, as a result of correlation, will appear as noise. With respect to the traditional narrow band systems, the spread spectrum technique offers the additional advantage of a greater insensitivity to Rayleigh selective fading, the latter being a phenomenon caused by multiple reflections along the air path of the transmitted signal, due to the fact that the spectral fraction concerned by the strong attenuations is only a very small part of the total spectrum occupied by the effective signal. OBJECTS OF THE INVENTION In the above-mentioned third generation cellular systems, a fundamental parameter is the power level at which the Mobile Stations (MS) access the Base Station (BS). The optimization of this parameter, which consists in seeing to it that the mobile stations reach the network with the minimum power required for the desired quality, contributes to optimizing the system's capacity and also to saving power in the mobile station. For those modes which are also based on access techniques such as Time Division Duplex (TDD) - Time Division Multiple Access (TDMA), such as the TDD UTRAN mode (UMTS Terrestrial Radio Access Network) specified within the framework of 3GPP (3 rd Generation Partnership Project) or the mode TD-SCDMA proposed by CWTS (China Wireless Telecommunication Standard), another fundamental parameter is the frame synchronization between MS and BS. The optimization of this parameter, which consists in recovering the propagation delay due to the distance between MS and BS, contributes to reducing the mutual interferences between users attributed the same Time Slot (TS), and between users of adjacent TS. In the network access procedure and for the first transmission on a dedicated channel, optimization of the "power level" and "frame synchronization" parameters is particularly critical due to the fact that here there are no closed ring control mechanisms as is the case on the other hand with the point-to-point connections. Also, during the access, the use of shared radio resources poses the problem of collisions, i.e. those events where various users simultaneously access the same radio resource. The optimization of this parameter, consisting in minimizing the probability of occurrence, also contributes to optimizing the capacity and quality offered by the system. SUMMARY OF THE INVENTION The invention described here proposes a network access procedure, based on the physical channels and control packets (bursts) already defined in the TD-SCDMA mode, which permits values to be obtained for the "frame synchronization" and "power level" parameters to be used in the access procedure and for the first transmission on the dedicated channel, at the same time minimizing the probability of collision on shared radio resources. To attain this stated object, the invention uses a method suitable for dividing the network access procedure into various temporally distinct steps. The frame timing and the power level are obtained in the following way: • In the first step, the MS uses signature bursts, called Uplink Pilot Synchronisation (UpPTS) in the TD-SCDMA, in order to obtain the correct frame timing and the correct power level, with which to access the common channel, or Random Access Channel (RACH), so as to send the network its access request; • In the second step, the MS verifies and settles the frame timing and power level parameters, again sending a signature burst, before transmitting on the dedicated resource that it has been assigned by the network. The minimization of collisions is obtained as follows. • In the first step, the time slot is defined within which the mobile station can expect to receive an answer from the network to its signature burst; this measure contributes to optimizing the collisions in the sense that the signature bursts are shared by the mobile stations lying in the area of electromagnetic coverage of the same base station. In addition, if the answer message also contains the indication of the RACH physical channel on which the MS will have to send the network its access request, the collision probability on this also shared radio resource is greatly reduced. • It may happen that under particularly strong interference conditions the MS cannot decode the answer message from the network and therefore will not know that a specific RACH channel has been assigned it. In this case, the network would be reserving a shared, i.e. precious, resource for a user who in actual fact is not in a condition to access it. This state may be resolved by fixing the instant in time, or a maximum time, within which the network waits for the booked resource to be occupied, and beyond which this resource becomes available again for the users of the cell. As a further extension of the idea, if it is established that for certain services (such as, for example, emergency calls and/or Handover requests) it is possible to send the signature burst in certain frames only of the multiframe (for example, the even frames), whereas for all the other services, the supplementary frames are allowed, then the probability of collisions in use of the signature bursts can be further reduced and the quality offered to the services accordingly improved (for example, by improving the probability of success for handovers). A further advantage of the idea described herein is obtained if: • During the first step of the procedure for updating the "Frame synchronization" and "Power level" parameters, the network in its answer to the signature burst signals the level of interference recorded on the RACH access channel (or alternatively the power level that it is appropriate to transmit) about to be used by the mobile station. The MS will use this value to define the power level with which to transmit on the RACH channel; • During the second step of the "Frame synchronization" and "Power level" parameter updating procedure, the network in its answer to the access request signals the level of interference recorded on the dedicated channel (or alternatively the power level that it is appropriate to transmit) assigned to the mobile station. The MS will use this parameter to define the power level with which to access the dedicated channel the first time. The invention is particularly, though not exclusively, suitable for the TD-SCDMA mode. The innovative characteristics of this invention may be deduced from the attached Claim 1 whereas particular embodiments may be deduced from the appended Claims 2 to 16. BRIEF DESCRIPTION OF THE FIGURE The figure 1 shows a cellular system comprising at least one base station BS and at least one mobile station MS. DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention envisages two steps for optimization of the "Frame synchronization" and "Power level" parameters during the procedure for accessing network services by a mobile station. The first step chiefly permits the recovery of a good frame synchronism between BS and MS and the indication by the BS of the physical, common access channel on which the MS can send its access request with a low probability of colliding or interfering with the other users of the cell. The second step permits the refreshing of the above parameters, before the MS starts transmitting on the dedicated resource assigned by the network with the updated timing and power values. It should be remembered that the signature burst is comparable to a correlation word; it does not carry any information or high-level message but only a signal that allows the network to calculate the timing and power level of the signal received and to correct it accordingly. In the CDMA systems, and in particular in the TD-SCDMA mode, numerous signature bursts with excellent auto and cross correlation properties can be sent in parallel by numerous mobile stations and correctly decoded by the network. Secondly, the network can answer numerous requests simultaneously, either by using numerous physical channels, code division type for example, or using a single resource in which to house a multiple answer message. The mains steps envisaged by the method proposed here are: A: The MS sends the network a signature burst (UpPTS, according to the symbols adopted by CWTS for the TD-SCDMA mode) with a power level calculated according to open ring procedures well known in the CDMA systems; the frame timing is that taken from the cell synchronization process. The MS waits to receive an acknowledge message from the network within N frames of the sending of the signature burst. The following three conditions may arise: B.1 The network answers within N frames after receiving the signature burst with a confirmation message containing the same signature as used by the mobile, the correction of the frame timing, the power level to be used for the next transmission to the network, and/or the level of interference on the physical channel about to be used, and the configuration of the RACH physical channel (in terms of time slot/frequency/coding) on which to send the network services access request message, in the case that numerous physical channels have been configured in the cell for this purpose. The RACH configuration parameters can be transmitted explicitly through signalling or taken implicitly from the mobile, for example because the association is known beforehand between the RACH channel to be used and the channel via which the mobile has received from the network the confirmation message to the signature burst B 2 The network answers within N frames after receiving the signature burst with a refusal message containing the same signature as used by the mobile and, in some cases, an indication of the degree to which the common access channels configured in the cell are occupied, from which the MS can obtain the delay time before attempting a new access B 3 In the expected interval of time, there is no answer from the network with reference to the signature used by the mobile C If we are in case B 1 , the MS sends as soon as possible, or exactly M frames after receiving the answer message from the network (the strategy selected depends on the rules established in the system, which must be known beforehand to the mobile stations), its access request on the RACH physical channel indicated to it, with the synchronization and power values obtained from the parameters configured by the network It then waits to receive from the network (up to a preconfigured maximum time) the assignment of the dedicated channel At this point, two conditions are possible C 1 The network answers affirmatively to the access request, by assigning the dedicated physical channel C 2 The MS does not receive an answer to its request, within the preconfigured maximum delay time D 1 In case C 1 , the MS can immediately set itself in reception mode on the dedicated channel and as soon as possible, that is to say K frames after the assignment of the dedicated channel, again transmit a signature burst to update the radio parameters before transmitting on the dedicated resource The MS waits for an answer from the network within X frames, and after this time interval immediately sets itself also in transmission mode on the dedicated channel the transmission will take place at any rate, even in the absence of an answer from the network The parameters "K" and "X" are configured by the network so as to optimize the collision probability In order to make this "Frame synchronization" and "Power level" parameter updating procedure more effective, the values of parameters K and X must be low D 2 In case C 2, the MS starts again from point A E If we are in case B 2, the MS starts again from point A, using where applicable the occupation value of the access channels as signalled by the network to obtain the delay time before the next access attempt F. If we are in case B.3, the MS starts again from point A. Further objects and advantages of the invention will become apparent from the claims that follow.

Claims: CLAIMS 1 Method for equalizing the propagation delays and optimizing the power level in a mobile station accessing network services on a common channel, in the third generation of mobile cellular systems based on a Code Division multiple Access, or CDMA, and Time Division Duplex-Time Division Multiple Access, or TDD-TDMA technique and comprising at least one base station (BS) and at least one mobile station (MS), and wherein provision is made for the transmission of signals organized in frames and in multiframes, also containing a correlation word called «sιgnature burst» which enables the network to calculate the timing and power level of the signal received, characterized in that it comprises a plurality of temporally distinct steps for the optimization of the «frame synchronization)) and «power level» parameters during the procedure for accessing network services by a mobile station (MS) 2 Method according to Claim 1 , characterized in that it comprises two temporally distincts steps and in particular • In the first step said at least one mobile station (MS) uses signature bursts to obtain the correct frame timing and correct power level, with which to access the common channel to send the network its access request • In the second step, said at least one mobile station (MS) verifies and settles the frame timing and power level parameters, again sending a signature burst, before transmitting on the dedicated resource which it has been assigned by the network 3 Method according to the previous claims, characterized in that in said first step the time interval is defined in which the mobile station must wait for the answer from the network to its signature burst 4 Method according to Claim 3, characterized in that the answer message from the network also contains the indication of the physical channel whereon the mobile station must send the network its access request 5 Method according to Claim 3, characterized in that the instant of time is fixed, or the maximum time, within which the network waits for the physical channel booked to be occupied and beyond which this physical channel becomes available again for the other users of the cell 6 Method according to the previous claims, characterized in that, for predetermined services and for predetermined frames of the multiframe, it is possible to send the signature burst whereas for all the other services only the supplementary frames are allowed 7 Method according to the previous claims, characterized in that • During the first step of the "Frame synchronization" and "Power level" parameter updating procedure, the network signals in its answer to the access request the level of interference recorded on the access channel assigned to the mobile station, or alternatively the power level that it is appropriate to transmit, which will use this value for defining the power level with which to transmit on the access channel, • Duπng the second step of the "Frame synchronization" and "Power level" parameter updating procedure, the network signals in its answer to the signature burst the level of interference recorded on the dedicated channel assigned to the mobile station which will use this parameter for defining the power level with which to access the first time on the dedicated channel 8 Method according to the previous claims, characterized in that it comprises the execution in sequence of the following operative steps • The mobile station (MS) sends the network a signature burst with a power level calculated according to open ring procedures, the frame timing is that obtained from the cell synchronization process • The mobile station (MS) waits to receive an acknowledge message from the network within N frames after sending the signature burst 9 Method according to Claim 8, characterized in that if the network answers within N frames after receiving the signature burst with a confirmation message containing the same signature as used by the mobile, the mobile station (MS) sends as soon as possible, or exactly M frames after receiving the answer message from the network, the strategy selected depending on the rules established in the system which must be known beforehand to the mobile stations, its access request on the physical channel indicated to it, with the synchronization and power values obtained from the parameters configured by the network, and finally it waits to receive from the network, up to a preconfigured maximum time, the assignment of the dedicated channel 10 Method according to Claim 9, characterized in that if the network answers in the affirmative to the access request by assigning the dedicated physical channel, the mobile station (MS) can immediately set itself in reception mode on the dedicated channel and as soon as possible, that is to say K frames after the assignment of the dedicated channel, again transmits a signature burst to update the radio parameters before transmitting on the dedicated resource 1 1 Method according to Claim 10, characterized in that the mobile station (MS) waits for the answer from the network within X frames, and after this time interval immediately sets itself also in transmission mode on the dedicated channel 12 Method according to Claims 9 to 1 1 , characterized in that in order to make this "Frame synchronization" and "Power level" parameter updating procedure more effective, the values of parameters K and X are adopted in such a way as to be low 13 Method according to Claim 8, characterized in that if the mobile station (MS) does not receive an answer to its request, within the maximum preconfigured delay time, the mobile station (MS) resumes its operating program according to what was specified earlier, that is to say • The mobile station (MS) sends the network a signature burst with a power level calculated according to open ring procedures, the frame timing is that obtained from the cell synchronization process • The mobile station (MS) waits to receive an acknowledge message from the network within N frames after sending the signature burst 14 Method according to Claim 8, characterized in that if the network answers within N frames after receiving the signature burst with a refusal message containing the same signature as used by the mobile and, in some cases, an indication of the degree to which the common access channels configured in the cell are occupied, from which the MS can obtain the delay time before attempting a new access, the mobile station (MS) resumes its operating program according to what was specified earlier, that is to say • The mobile station (MS) sends the network a signature burst with a power level calculated according to open ring procedures, the frame timing is that obtained from the cell synchronization process • The mobile station (MS) waits to receive an acknowledge message from the network within N frames after sending the signature burst 15 Method according to Claim 14, characterized in that the mobile station MS resumes its operating program according to what was specified earlier using the access channel occupation value as signalled by the network for obtaining the delay time before the next access attempt 16 Method according to Claim 8, characterized in that if in the expected interval of time, there is no answer from the network with reference to the signature used by the mobile, the mobile station (MS) resumes its operating program according to what was specified earlier, that is to say • The mobile station (MS) sends the network a signature burst with a power level calculated according to open ring procedures; the frame timing is that obtained from the cell synchronization process. The mobile station (MS) waits to receive an acknowledge message from the network within N frames after sending the signature burst.

Inventors: DE BENEDITTIS ROSSELLA, DE BENEDITTIS, ROSSELLA
Applicants: BENEDITTIS ROSSELLA DE, SIEMENS INF & COMM NETWORKS, DE BENEDITTIS, ROSSELLA, SIEMENS INFORMATION AND COMMUNICATION NETWORKS S.P.A.
Application year: 2000
Main classification: H04J3
IPC codes: H04B3, H04B7, H04J3, H04J13, H04L12, H04W12, H04W28, H04W52, H04W56, H04W74