WO2001043307A2

WO2001043307A2 - Method for maintaining a synchronised upward signal transmission in a radio communication system

Info

Publication number: WO2001043307A2
Application number: PCT/DE2000/004363
Authority: WO
Inventors: Lei Xiao; Stefan Bahrenburg; Michael Benz
Original assignee: Siemens Aktiengesellschaft
Priority date: 1999-12-08
Filing date: 2000-12-07
Publication date: 2001-06-14
Also published as: WO2001043307A3; CN1299220A; AU3000101A

Abstract

The invention relates to a method for maintaining a synchronised upward signal transmission in a radio communication system. Said radio communication system supports a TD/CDMAT user separation method, for the signal transmission on a radio interface, between a base station and at least one user station, whereby said radio interface is organised according to a TDD method. A relative delay in the reception of a downward signal at the user station is determined, after an effective synchronisation and said delay is used to control the timing of the subsequent transmission of signals in the upwards direction.

Description

description

Method for maintaining a synchronized upward signal transmission in a radio communication system

The invention relates to a method for maintaining a synchronized signal transmission in the upward direction from at least one subscriber station to a base station of a radio communication system. The invention is particularly suitable for use in a mobile radio system.

In radio communication systems, for example the second generation European mobile radio system GSM (Global System for Mobile Communications), information (for example voice, image information or other data) is transmitted with the aid of electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and subscriber stations, wherein the subscriber stations can be, for example, mobile stations or fixed radio stations. The electromagnetic waves are emitted at carrier frequencies that lie in a frequency band provided for the respective system. For future radio communication systems, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd generation systems, frequencies in the frequency band of approx. 2000 MHz are planned. Two modes are provided for the third mobile radio generation, one mode denoting FDD operation (frequency division duplex) and the other mode denoting TDD operation (time division duplex). These modes are used in different frequency bands. Both modes support a so-called CDMA subscriber separation procedure (Code Division Multiple Access). A proposal for a third generation mobile radio system according to “TD-SCDMA Radio Transmission Technology for IMT-2000 *, Draft VO4, the CATT from September 1998 is based on the described TDD mode with the support of a CDMA subscriber separation method. By using the CDMA subscriber separation method, transmission blocks transmitted by several subscriber stations in a time slot, which generally consist of a data part and a known training sequence, can be processed in parallel by a base station. To do this, however, it must be ensured that the transmission blocks and in particular the respective training sequences arrive at the base station location within a certain time window in order to ensure reliable detection and separation of the different signals. This problem of synchronization of the signal transmission in the upward direction occurs in the same way in known CDMA-based radio communication systems.

The respective time base of the subscriber stations is synchronized according to the prior art, as is already known, for example, from the GSM mobile radio system, and therefore generally in a connection establishment procedure in which the base station receives the time reference after receiving a signal from a subscriber station sets the respective subscriber station by signaling a correction value. Since the time reference can change continuously, for example due to a movement of the subscriber station, the time reference of the subscriber station must be periodically corrected in order to maintain temporal synchronicity. For this purpose, it is proposed for the third generation mobile radio systems to send special synchronization information with a certain periodicity ,

In this method, for example, according to a so-called closed loop, the Base station determines an arrival time of the signals sent by the subscriber station in the upward direction and signals corresponding synchronization control information in a subsequent corresponding traffic channel in the downward direction to the subscriber station. The subscriber station evaluates the synchronization information and accordingly regulates the point in time for the subsequent transmission of the signals in the upward direction to the base station.

However, this method disadvantageously leads to a large signaling load due to the periodic signaling of the control information. It must be taken into account that a slow regulation of the synchronization is sufficient for the compensation of the changes due to a movement of the subscriber station, while a fast regulation of the synchronization must take place for the compensation of the channel variation. This necessary rapid regulation can, for example, result in control information having to be inserted into each transmission frame and, as a result, less useful information being able to be transmitted in the traffic channels.

The so-called multipath transmission also has a major influence on the synchronization of the transmission in the upward direction. The multipath transmission is due to a multipath propagation of the emitted signals, these reaching the receiving subscriber station on different paths and with different transit times due to reflections on objects. As a rule, a synchronized reception of the signals of the subscriber stations can only be implemented in the base station for the strongest transmission path in each case (for example the path with the greatest reception strength), during which the respective further transmission paths are received unsynchronized. This However, multipath profile is also time-variable, which means that the strongest transmission path can vary.

This is another problem with the method described. The time interval between two transmission paths can possibly be longer than a chip duration, but a control step should be shorter than the chip duration in order to be able to regulate the synchronization with sufficient accuracy. If the base station notices that the strongest transmission path of a subscriber station has changed, a signaling, possibly over several transmission frames, is required to regulate the time of transmission of the subscriber station. In this case, for example, the case may arise that the multipath profile changes further during the signaling, the readjustment no longer takes place sufficiently quickly and, if appropriate, the synchronization is completely lost.

The object of the invention is to provide a method and a subscriber station which enable simple maintenance of a synchronized signal transmission in the upward direction. This object is achieved by the method having the features of claim 1 and by the subscriber station of claim 5. Advantageous further developments of the invention can be found in the dependent patent claims.

According to the invention, the subscriber station uses the existing reciprocity between the properties of the transmission channels in the upward and downward direction, since the channels are transmitted in a common frequency band at different times. Properties determined by the subscriber station can be, for example, characteristic values such as the bit error rate or, according to the invention, a time variation. The subscriber station thus uses the base station when determining a temporal change in the received signal. Station this information to control the timing of the transmission of signals in the upward direction.

The method according to the invention advantageously makes it possible to react more quickly to changes in time, since no signaling from the base station is required to control the time of transmission. Furthermore, the control is more precise, can react more flexibly to changes and can be implemented very inexpensively due to the use of functions that are largely already available in the subscriber station.

A further advantage lies in the case where a closed control loop is used in accordance with a development of the invention, in a significantly lower signaling load, since when using a closed control loop, synchronization control information is only signaled in the event of incorrect synchronization he follows. As long as the “open control loop *” of the subscriber station leads to sufficient synchronization according to the method according to the invention, additional signaling is only necessary at comparatively large time intervals or not at all.

According to further developments of the invention, the

Subscriber station based on a known channel estimate a respective strongest transmission path. Knowing this strongest transmission path, the subscriber station controls the time of transmission of the signals in the upward direction in such a way that its own strongest transmission path can be received synchronously at the location of the base station.

Embodiments of the invention are explained in more detail with reference to the accompanying drawings. Show

1 shows a block diagram of a radio communication system.

2 shows a schematic representation of the frame structure of a radio interface with a TD / CDMA subscriber separation method.

3 shows a flow chart of the control of the synchronous transmission in the upward direction.

1 shows part of a mobile radio system as an example of the structure of a radio communication system. A mobile radio system consists in each case of a multiplicity of mobile switching centers MSC which belong to a switching network (SSS - Switching Subsystem) and which are networked with one another or which provide access to a fixed network, and one or more base station systems BSS (BSS) connected to these mobile switching centers MSC - Base station subsystem). A base station system BSS in turn has at least one device RNC (RNC - Radio Network Controller) for assigning radio resources and at least one base station NB (NB - Node B) connected to it.

A base station NB can establish and maintain connections to subscriber stations UE (UE - User Equipment) via a radio interface. At least one radio cell Z is formed by each base station NB. The size of the radio cell Z is generally determined by the range of a general signaling channel (BCH - broadcast channel), which is transmitted by the base stations NB with a maximum and constant transmission power in each case. In the case of sectorization or hierarchical cell structures, several radio cells Z can also be supplied per base station NB. The functionality of this structure is based on other radio communications tion systems transferable, m where the invention can be used.

The example in FIG. 1 shows a subscriber station UE which is located in the radio cell Z of a base station NB. The subscriber station UE has set up a communication link to the base station NB, on which a selected service is transmitted in the upward UL and downward direction DL. The communication connection is separated by one or more spreading codes assigned to the subscriber station UE from communication connections set up in parallel in the radio cell Z, the subscriber station UE in each case all spreading codes currently assigned to the radio cell Z for receiving the signals of its own communication connection in accordance with the known method. Detection method uses.

The frame structure of the radio transmission of the TD-SCDMA mobile radio system can be seen from FIG. The radio interface is a broadband radio interface with a frequency band B = 1.6 MHz (thus three frequency bands per 5

MHz), with a time frame duration of 5 ms (thus two time frames for each UTRA time frame), with 7 time slots ts of a respective length of 675 us for traffic channels, and with CDMA subscriber separation using 16 different spreading codes cO to cl5.

In the TDD transmission method shown, the frequency band B for the upward direction UL corresponds to the frequency band B for the downward direction DL. The same is repeated for other carrier frequencies. Due to the variable allocation of the time slots ts for the upward or downward direction UL, DL, a variety of asymmetrical resource allocations can be made. Part of the time slots tdO ... tdn is used accordingly for the signal transmission in the downward direction DL (Downlmk) and the remaining time slots tu0 ... tum for the gnalu transmission in upward direction UL (Uplmk) used. The parameters n, m and thus the switchover point SP (Switchmg Point) can be individually adapted to a current requirement, the relationship n + m + 2 = 7 in each case. In connection with the first time slot tdO for the downward direction DL, there follows a protection time for separating the transmission directions DL and UL, which represents the switchover point SP.

The protection time consists of a downward pilot time slot

DPTS (Downlmk Pilot Time Slot) with a length of 75 us for sending synchronization sequences distinguished by a set of so-called gold codes, from a guard time GP (Guard Period) with a length of 75 us for the switching process between sending and receiving m the base station NB, and from an upward pilot time slot UPTS (upmk pilot time slot) with a length of 125 us for sending a synchronization sequence in the event of an attempt to establish a connection by a subscriber station UE with subsequent signaling on the channel for random access RÄCH. A set of gold codes is again used to differentiate between several subscriber stations UE in this access procedure.

Information of several connections is transmitted in radio blocks within the time slots ts. The data d are spread individually with a fine structure, a spreading code c, so that, for example, n connections can be separated at the receiving end by this CDMA component. The spreading of individual symbols of the data d has the effect that sym Q chips of the duration Tc are transmitted within the symbol duration. The Q chips form the connection-specific spreading code c. A channel measurement sequence tseq for a channel estimation at the receiving end is also embedded in the radio blocks. A radio block is completed with a protection time gp. The parameters of the radio interface used for the described TD-SCDMA system are advantageously:

Chip rate: 1.28 Mchip / s

Frame duration: 5 ms

Number of time slots: 7 (traffic channels;

Duration of a time slot 675 μs

Spread factor: 1 to 16

Bandwidth: 1.6 MHz

These parameters enable the best possible harmonization with the UTRA TDD and FDD mode (FDD frequency division duplex) as well as the well-known GSM mobile radio system.

The method according to the invention will be explained by way of example with reference to FIGS. 1 and 3. The signals transmitted by the base station NB reach the receiving subscriber station UE on different transmission paths, for example on the transmission paths p1 and p2. While the signals the subscriber station UE on the

If the transmission path p1 reaches directly, the signals on the transmission path p2 arrive at the subscriber station UE due to a reflection, for example on a mountain, with a certain time delay. It is assumed that in an initial state the transmission path pl also the strongest transmission path, i.e. is the transmission path that can be received with the greatest reception power, and the signals sent in the upward direction UL can be received synchronously by the base station NB. When the subscriber station UE moves, the case now arises that the direct

Transmission path p1, for example due to shadowing, is no longer determined by the subscriber station UE as the strongest transmission path, but now the transmission path p2 is the strongest transmission path. The subscriber station UE therefore seeks synchronization assuming that its transmission path corresponding to path p2 is received synchronously by base station NB.

If, in this case, the subscriber station UE determines, for example, a time offset dt of 2.25 chips between the arrival of the signals on the transmission path p1 in the initial state and the later arrival of the signals on the transmission path p2 in a subsequent time frame, the subscriber station UE knows that it must send out its signals in the upward direction UL by 2.25 chips earlier, so that the signals on the strongest transmission path in turn arrive synchronously at the location of the base station NB.

In contrast to the closed control loop described, this regulation is independent of the determined one

Size of the relative time offset. For example, the maximum resolution can also correspond to a fraction of a chip. This independence advantageously enables very fast regulation, which can also take place within a time frame, for example.

The changes in the transmission properties occurring within a time frame or between two reciprocal transmission channels are generally not so great that the reciprocity assumed by the subscriber station UE no longer applies. Nevertheless, in practice, depending on the speed of the subscriber station UE, synchronization control information must be periodically signaled by the base station NB to compensate for control errors.

Claims

claims

1. A method for maintaining a synchronized upward signal transmission in a radio communication system, the radio communication system being a TD / CDMA subscriber separation method for signal transmission on a radio interface between a base station (NB) and at least one subscriber station (UE) supported, and the radio interface of the radio communication system is organized according to a TDD method, in which the subscriber station (UE) after a successful synchronization of the transmission of signals in the upward direction (UL) to the base station (NB) a relative time offset of the reception of signals sent in the downward direction (DL) is determined, and the determined time offset is taken into account for controlling a point in time of a subsequent transmission of the signals in the upward direction (UL).

2. The method according to claim 1, in which a closed control loop is used by the base station (NB) for the synchronization of the transmission of the signals in the upward direction (UL), the time for the transmission being controlled by means of control information signaled to the subscriber station (UE) becomes.

3. The method according to claim 1 or 2, in which a strongest transmission path (pl, p2) is determined by the subscriber station (UE) on the basis of a channel estimate of the signals sent by the base station (NB).

4. The method according to claim 3, wherein the time of transmission of the signals in the upward direction (UL) for the determined strongest transmission path is controlled by the subscriber station (UE).

5. subscriber station (UE) of a radio communication system, for performing the method according to claim 1, with means for determining a relative time offset of received signals of a base station (NB), and means for controlling the transmission of the signals in the upward direction (UL) taking into account the determined relative time offset.