METHOD AND SYSTEM FOR MAINTAINING UPLINK SYNCHRONIZATION IN CDMA BASED MOBILE COMMUNICATIONS SYSTEMS

Abstract: A method and system for maintaining uplink synchronization in CDMA based mobile communication system with high accuracy and fast update frequency, said mobile communication system comprising a base station, at least one mobile station, said method comprising the steps of: receiving signals transmitted by one of said mobile stations; measuring timing of the transmitted signals from the mobile station and obtaining USActual value currently used in the mobile station; calculating a USTarget value based on the measurement; comparing USActual and USTarget to generate a SS command for increasing or decreasing said USActual; signaling said synchronization shift (SS) command in downlink to said mobile station to increase or decrease said USActual.

Description: Description METHOD AND SYSTEM FOR MAINTAINING UPLINK SYNCHRONIZATION IN CDMA BASED MOBILE COMMUNICATIONS SYSTEMS Field of the invention The present invention relates to CDMA technology, in particular, a method and system for maintenance of UL synchronization in CDMA based mobile communication systems. Technical background For most CDMA systems, the uplink channels are asynchronous. That means the signals of the users arrive at the serving Node B ( the Base Station) at different time. So the asyn- chronous spreading codes used by different users (for example PN (pseudo noise) sequence) will interfere each other. That interference is known as Multi-Access-Interference. Since CDMA is an interference-limited system, any additional interference will result in capacity reduction. Uplink synchrom- zation or so-called synchronous CDMA system keeps all user signals reaching the base station at the same time. It will greatly reduce Multi-Access-Interference. Thus uplink synchronization (US) will lead to higher capacity and simplify the demodulator in Node B at the base station side. Synchronization is more difficult to realize in mobile communications system because of the multi-path and shadow fading characteristic of mobile channel. Since the channel characteristic varies in time frequently and the accuracy for uplink (UL) synchronization must be high, the update frequency of the mobile's transmission TX timing must be fast. The problem is how to find an effective way to realize uplink synchronization and keep the system spending acceptable. The realization of UL synchronization can be divided into two parts: UL synchronization establishment and UL synchronization maintenance. In the invention a simple and effective method to maintain the uplink synchronization is described. From the layer 1 point of view, the UL synchronization is equivalent to a very high precise timing advance. In current system, there exist a scheme for timing advance. Fig. 1 illustrates a schematic diagram of the timing advance scheme. As shown in Fig. 1, there is a propagation delay when the BS receives the UL transmission signal. Therefore, a timing advance is provided to the burst during UL transmission at the mobile station (MS) site, so that the burst received at the BS can be j st in time. However, in the existing timing advance scheme, the timing advance information is transmitted from layer 3. Since the accuracy for UL synchronization is higher and the update frequency is faster than timing advance, the timing advance scheme cannot meet the requirement of UL synchronization. In current mobile communications system, there is no solution for UL synchronization. Hence before the invention, the problem is open. Summary of the invention One ob ect of the invention is to provide a method and system for maintenance of UL synchronization m CDMA based mobile communication systems with high accuracy and fast update frequency, thereby reducing multi-access-mterference effec- tively, achieving higher capacity of the CDMA system. A further object of the invention is to provide a communication system utilizing the above method, and the base station and mobile station thereof. The present invention provides a method for maintaining uplink synchronization in mobile communication system, said mobile communication system comprising a base station, at least one mobile station, said method comprising the steps of : receiving signals transmitted by one of said mobile sta- tion; measuring timing of the transmitted signals from the mobile station and obtaining an actual uplink synchronization (USA- J , value currently used in the mobile station; calculating a target uplink synchronization (US Tar ger) value based on the measurement; comparing US Ac uai and US Ta rget to generate a synchronization shift (SS) command for increasing or decreasing said US A cruai/" signaling said synchronization shift (SS) command in downlink to said mobile station to increase or decrease said US Act uai • The present invention further provides a system for maintaining uplink synchronization m mobile communication system, said mobile communication system comprising a base station, at least one mobile station, said system comprising: receiving means for receiving signals transmitted by one of said mobile station; measuring means for measuring timing of the transmitted signals from the mobile station and obtaining US A - fJα ι value currently used in the mobile station; calculating means for calculating a US Tar get value based on the measurement; comparing means for comparing US A rt U ai and US T a rge r to generate a SS command for increasing or decreasing said US, . ; signaling means for signaling said synchronization shift (SS) command in downlink to said mobile station to increase or decrease said US A j„ . The present invention further provides a mobile communication system, comprising: a base station; at least one mobile station; wherein said base station comprises: receiving means for receiving signals transmitted by one of said mobile station; measuring means for measuring timing of the transmitted signals from the mobile station and obtaining US Ac r ua ι value currently used in the mobile station; calculating means for calculating a US Ta rget value based on the measurement; compar- mg means for comparing US ActU ai and US T arget to generate a SS command for increasing or decreasing said US A r υa ι; signaling means for signaling said synchronization shift (SS) command m downlink to said mobile station to increase or decrease said US Ac τjai / and wherein said mobile station comprises: adjusting means for increase or decrease said US Ac1 - U ai according to said synchronization shift command, as well as the base station and mobile station used therein. Brief description of the drawings Fig. 1 illustrates a schematic diagram of the timing advance scheme at L3 level. Fig. 2 illustrates a schematic diagram of the CDMA system according to one embodiment of the present invention. Fig. 3 illustrates a schematic diagram for examples for calculating USiarαef according to the invention. Fig. 4 shows an example of the position of the SS (synchronization shift) command in a traffic burst. Fig. 5 shows another example of the position of the SS (synchronization shift) command m a traffic burst when TFCI is transmitted. Detailed description of the preferred embodiments of the invention Fig. 2 illustrates a schematic diagram of the CDMA system ac- cording to one embodiment of the present invention. As snown m Fig.2, the base station receives the signals transmitted in uplink from the mobile station. The base station includes a measurement unit MU for measuring the timing of the received signals, and obtaining a US Ac r U ai / which is a US value (equivalent to a timing advance) currently used by the mobile station; a uplink synchronization control unit UC for calculating a US T arget which is defined as exactly that value that would be optimal for UL synchronization; a compare unit CU for comparing the US Ac r U ai with the US Ta rget ; and a SS command generating unit SSU for generating a SS command according to the comparison result. The SS command is downlink signaled in Layer 1 LI to the mobile station MS to adjust, increase or decrease the US Ac ι- u =α value used in the MS. Needless to say, the base station also includes a receiving means (not shown) for receiving signals transmitted from the MS; and a signaling means (not shown) for transmitting signals having the SS commands in downlink to the MS . They can be a known receiver or transmitter, thereby the details thereof are not described in this application. The transmission unit in the MS trans- mits signals with the US A r JαI timing, and the adjustment unit AU m the MS adjusts the US A: :r Ja value according to the SS commands . According to the embodiment of the present invention, a new LI (layer 1) signaling SS (synchronization shift) is used to maintain UL synchronization, The SS commands are transmitted in the downlink of the physical layer. As shown in Fig. 2, the measurement unit in the serving node B or base station continuously measures the timing of a transmission from the user equipment, i.e., the mobile station MS. Then, based on these measurements, the UL synchrom- zation Control UC unit calculates the US Tar e ^ • The US T arαe- is defined as exactly that value that would be optimal for UL synchronization. The US value that is currently used by the system is defined as US Act uai • The SS command is generated by the relationship between US Ac - tuai and US Targ et- and is used to increase or decrease US Act uai • Hence the SS is up- or down-command and does not signal the US Ta rge itself. The US Ac tuai is changed m steps according to SS commands and one (single stepsize) or more (multiple step- sizes) predefined stepsize. The smallest stepsize is equivalent to the necessary accuracy of UL synchronization. The process of the UL synchronization control according to an embodiment of the present invention may includes the follow- ing steps: Synchronization Control unit UC calculates a US Target; SS command is generated by comparing of US Ac rj.. and US T arαet / wherein SS is an up- or down-command; SS command is used by the MS to increase or decrease US Ac uai in predefined step(s), which are known in advance and/or are encoded in the SS command; SS command is signaled m layer 1 LI (SS field in Burst) to achieve the high update frequency of US. One embodiment of the UL synchronization m the UTRA-TDD system or m the TD-SCDMA system according to the present lnven- tion will be described below. The required accuracy for UL synchronization is, for example, ±l/8chιps. The serving node B at the BS side continuously measures the timing of a transmission from the MS, obtains the US Ac r . uai/ and calculates a US Targ et • Based on the comparison of the US Ac tuai and the US Targ et , SS commands are generated and signaled in DL. The US Actua ι used m MS is changed in steps ac- cording to SS commands, and changed one (single stepsize) or more (multiple stepsizes) predefined stepsizes. Fig. 3 illustrates a schematic diagram for explaining the calculation of the US Tar get in the UL synchronization control unit UC according to the invention. The UC has the purpose of calculating the USrarαe- • Because of the multipath effect and the time variance of the channel, a perfect UL synchronization cannot be established. Therefore the USi-arαer has the purpose to find that timing for UL transmission, where the performance of US is optimal. Two basic methods are possible, one is slow UC, the other is fast UC . As shown in Fig. 3, from the bottom to the top, there are three bursts (for exemplary purpose) in time sequence transmitted from the MS are received at the BS . For each of the burst, the BS receives corresponding taps of impulse response IR. Since the MS is moving and is in a time variant environment, the IR at the BS side is varied in time and in power, as shown in Fig. 3. For slow UC, which means the statistics of the IR is used to calculate an optimal value, the USr αr e - the US- arj _- is equivalent to the propagation delay plus the delay of that tap in the IR, that has the highest power m average. Tnis method additionally includes the average characteristics of the mobile station's IR. Therefore the UC must know the statistics of the IR. For fast UC, the US tar g et is equivalent to the propagation plus the delay of that tap in the IR, that has actually the hig ¬ hest power. This method should provide the best performance of US, however the US tar ge t will rapidly change, which causes more frequent signaling of SS commands in the DL. With either slow UC or fast UC, the value of UStaret can be obtained. The smallest stepsize is equivalent to the necessary accuracy, for example, 1/8 Chips. The stepsize can be a fixed value pre-set m the MS. It is also possible for the BS to broadcast the stepsize to the respective mobile stations, and each of the MSs use the received stepsize to increase or decrease its US Ac tuai value. The SS commands are transmitted in the physical layer, i.e., Ll-signaling. For example, the algorithm for a single stepsize is If US- -,^! > US T arαe , then the SS=0; If US Ac r Ua ι < US T arget , then the SS=1; In one embodiment of the invention, the SS information is one bit. More bits can be used for multiple stepsizes or error protection. Since SS commands are signaled in LI, they need a special field in the burst. The SS field is only added in DL timeslots because the transmission of SS is only needed in DL. Fig. 4 and Fig.5 shows two examples of the position of the synchronization shift command SS m a traffic burst, according to the present invention. As shown in Fig. 4, in this example, one symbol (two bits) for SS field is reserved. The SS field is placed around the midamble in order to achieve effective protection in time- variant environments. The 'reason for placing the SS command near the midamble is that the above measurement is made at the midamble, so that the adjustment of the US value at the MS can be more based on the real US status and more efficiently. The SS information is transmitted every frame in DL physical channel. Fig.4 shows the position of SS in traffic DL burst for UTRA (UMTS (Universal Mobile Telecommunication System) terrestrial radio access) -TDD (Time Division Duplex) system. If TFCI (Transport Format Combination Indicator) is transmit- ted, The SS is transmitted directly adjacent to the TFCI. Fig. 5 shows the position of SS in traffic DL burst when TFCI is transmitted. The adjustment of the current timing advance is changed ac- cording to the following rule: US Ar r ai , new = US A ct ' ja l , e l d + Διj_- I f SS= 1 US A=tua ι, = ι d - Δus If SS=0 Where, Δus is single stepsize for the modification of US. In this implementation, Δus is equal to 1/8 chips and it is predefined in the system. From the above description to the preferred embodiments of the present invention, it is apparent that the present invention provide an effective method to maintain the uplink synchronization in CDMA based mobile communication systems with high accuracy and fast update frequency. And consequently, it will greatly reduce multi-access-interference, and lead to higher capacity of the CDMA system as well as simplify the demodulator at the base station side. The forgoing has described the preferred embodiments of the present invention. It is contemplated that changes and modifications can be made by one of ordinary skill in the art, based on the disclosure of the present invention, without departing from the scope of the invention.

Claims: What is claimed is: 1. A method for maintaining uplink synchronization in a mobile communication system, said mobile communication system comprising a base station, and at least one mobile station, said method comprising the steps of : receiving signals transmitted by one of said mobile station; measuring timing of the transmitted signals from the mobile station and obtaining an actual uplink synchronization (US Ac tu a ι) value currently used in the mobile station; calculating a target uplink synchronization (US T arget) value based on said measurement; comparing said actual uplink synchronization value (US ActU ai) and said target uplink synchronization value (US Ta rget) to gen- erate a synchronization shift (SS) command for increasing or decreasing said actual uplink synchronization value (US ActU ai); signaling said synchronization shift (SS) command in downlink to said mobile station to increase or decrease said actual uplink synchronization (US Ac t. ua ι) value. 2. The method according to claim 1, wherein said synchronization shift (SS) command is signaled in a predetermined field in burst in layer 1. 3. The method according to claim 2, wherein said predetermined field is placed around the midamble in the burst. 4. The method according to claim 3, wherein said measurement is made at the midamble. 5. The method according to claim 1, wherein said synchronization shift (SS) command is transmitted every frame in physical channel. 6. The method according to claim 1, wherein the channel sta ¬ tistics is used to calculate said target uplink synchronization (US T arget) value in said calculating step. 7. The method according to claim 1, wherein a tap in impulse response having actually the highest power is used to determine said target uplink synchronization (US Tar αer) value in said calculating step. 8. The method according to claim 1, wherein said mobile station increases or decreases the actual uplink synchronization (US Ac t Ja ι) value according to said synchronization shift (SS) command m a predetermined stepsize or stepsizes. 9. The method according to claim 8, wherein said stepsize is a fixed value preset in said mobile station. 10. The method according to claim 8, wherein said mobile sta- tion receives the stepsize broadcasted from said base station. 11. A system for maintaining uplink synchronization in a mobile communication system, said mobile communication system comprising a base station, and at least one mobile station, said system comprising: receiving means for receiving signals transmitted by one of said mobile station; measuring means for measuring timing of the transmitted sig- nals from the mobile station and obtaining an actual uplink synchronization (US.~r J l ) value currently used in the mobile station; calculating means for calculating a target uplink synchroni ¬ zation (USτ a rget-) value based on the measurement; comparing means for comparing said actual uplink synchronization (US Ac tuai) value and said target uplink synchronization (US Tar g e t) value to generate a synchronization shift (SS) com ¬ mand for increasing or decreasing said actual uplink synchro ¬ nization (US Ac tuai) value; signaling means for signaling said synchronization shift (SS) command in downlink to said mobile station to increase or de- crease said actual uplink synchronization (US ActU ai) value 12. The system according to claim 11, wherein said synchronization shift (SS) command is signaled m a predetermined field in burst in layer 1. 13. The system according to claim 12, wherein said predetermined field is placed around the midamble in the burst. 14. The system according to claim 13, wherein said measure- ment is made at the midamble. 15. The system according to claim 11, wherein said synchronization shift (SS) command is transmitted every frame in physical channel. 16. The system according to claim 11, wherein the channel statistics is used to calculate said target uplink synchronization (US T aret) value in said calculating step. 17. The system according to claim 11, wherein a tap in impulse response having actually the highest power is used to determine said target uplink synchronization (US Tar g e t) value m said calculating step. 18. The system according to claim 11, wherein said mobile station includes an adjusting means for increasing or decreasing the actual uplink synchronization (US A tua ι) value ac- cording to said synchronization shift (SS) command in a predetermined stepsize or stepsizes. 19. The system according to claim 18, wherein said stepsize is a fixed value preset in said mobile station. 20. The system according to claim 19, wherein said mobile station receives the stepsize broadcasted from said base station. 21. A mobile communication system, comprising: a base station; at least one mobile station; wherein said base station comprises: receiving means for receiving signals transmitted by one of said mobile station; measuring means for measuring timing of the transmitted signals from the mobile station and obtaining an actual uplink synchronization (US Act uai) currently used in the mobile station; calculating means for calculating a target uplink synchronization (US Ta rget) value based on the measurement; comparing means for comparing said actual uplink synchronization (US.a.ctuai) value and said target uplink synchronization (US T arget) to generate a synchronization shift (SS) command for increasing or decreasing said actual uplink synchronization (US A r uai ) value; signaling means for signaling said synchronization shift (SS) command in downlink to said mobile station to increase or decrease said actual uplink synchronization (US ActU ai) value, and wherein said mobile station comprises: adjusting means for increase or decrease said actual uplink synchronization (US Ac r .ua ι) value according to said synchronization shift command (SS) . 22. The mobile communication system according to claim 21, wherein said synchronization shift (SS) command is signaled in a predetermined field in burst in layer 1. 23. The mobile communication system according to claim 22, wherein said predetermined field is placed around the midam- ble in the burst. 24. The mobile communication system according to claim 23, wherein said measurement is made at the midamble. 25. The based mobile communication system according to claim 21, wherein said synchronization shift (SS) command is transmitted every frame in physical channel. 26. The mobile communication system according to claim 21, wherein the channel statistics is used to calculate said target uplink synchronization (US Tar g et ) value in said calculating step. 27. The mobile communication system according to claim 21, wherein a tap in impulse response having actually the highest power is used to determine said target uplink synchronization (US Tar get) value in said calculating step. 28. The mobile communication system according to claim 21, wherein said mobile station includes an adjusting means for increasing or decreasing said actual uplink synchronization (US Ac tuai) value according to said synchronization shift (SS) command in a predetermined stepsize or stepsizes. 29. The mobile communication system according to claim 28, wherein said stepsize is a fixed value preset in said mobile station. 30. The mobile communication system according to claim 29, wherein said mobile station receives the stepsize broadcasted from said base station. 31. The base station system used in the mobile communication system according to claims 21-30. 32. The mobile station system used in the in the mobile communication system according to claims 21-30.

Inventors: BENZ MICHAEL, SONG XIAOQIN, BENZ, MICHAEL, SONG, XIAOQIN
Applicants: BENZ MICHAEL, SIEMENS AG, SIEMENS LTD CHINA, SONG XIAOQIN, BENZ, MICHAEL, SIEMENS AKTIENGESELLSCHAFT, SIEMENS LTD. CHINA, SONG, XIAOQIN
Application year: 2000
Main classification: H04B7
IPC codes: H04B1, H04B7, H04J3, H04J13, H04L7