AU5730599A

AU5730599A - Method and device for a full-duplex radio transmission system with code division multiple access

Info

Publication number: AU5730599A
Application number: AU57305/99A
Authority: AU
Inventors: Andreas Hachenberger; Klaus Jackel; Mathias Reibe; Reinhard Schiffel; Joachim Seidel
Original assignee: Cell Q GmbH
Current assignee: Q-CELL GmbH
Priority date: 1998-08-14
Filing date: 1999-08-03
Publication date: 2000-03-06
Anticipated expiration: 2019-08-03
Also published as: EP1104595A1; AU749693B2; CA2340644A1; ZA200101603B; DE19836888A1; CN1311925A; WO2000010258A1; AR025262A1

Description

WO 00/10258 - 1 - PCT/EP99/05619 Method and device for a full-duplex-capable radio trans mission system with CDMA access The invention relates to a method and a device 5 for a full-duplex-capable radio transmission system with CDMA access, having a central radio base station and a plurality of subscriber stations which are independent of one another. In the field of radio-supported information 10 systems which operate with a central radio base station and a plurality of external stations or subscriber stations which are independent of one another and which permit information to be transmitted in full duplex form in both directions, the information in the downlink 15 which is intended for the individual users is frequently multiplexed into a telecommunications channel and transmitted organized as an access system in the uplink. Examples of such systems are mobile radio systems, public trunked mobile radio systems, point-to-multipoint 20 microwave radio systems and wireless local loop systems. Orthogonal signal domains which differ from one another are used in each case for the multiplexing or multiple access, these signal domains being, for example, - frequency division multiplex or access systems FDMA 25 (frequency division multiple access) - time division multiplex or access systems TDMA (time division multiple access) - code division multiplex or access systems CDMA (code division multiple access) or SSMA (spread spectrum 30 multiple access) - space-division multiplex or access systems. The systems differ in that the transmission of information from and to the individual users takes place in separate frequency, time, code or spatial segment 35 positions. Interleaved, coupled or respectively differ ent multiplex and access technologies within one system, so-called hybrid methods, have become known. Depending on the use and implementation, different transmission 0 -2 parameters and transmission quality criteria can be obtained with these methods. In CDMA systems, the user signal is coded by gating it with a spread function using logic operations, 5 a separate spread function which is orthogonal to the other spread functions being selected for each subscriber station. The logic operation is carried out here in each case by means of an X-OR gate, for example. At the receiver end, the coded signal can be demodulated 10 through knowledge of the associated spread function, the coded user data for other subscriber stations becoming zero during the demodulation process owing to the orthogonality. It is particularly advantageous with CDMA systems that all the users can operate in the same 15 frequency band and a relatively high degree of interference power in the band can be tolerated. Furthermore, under certain conditions it is possible that adjacent radio cells can operate on the same frequency band. It is generally a disadvantage that the 20 multi-user interference, which arises in practice as a result of implementation problems such as band limitation, level differences between the individual transmissions, multi-path propagation etc. and which leads to a loss of orthogonality. In the radio systems 25 under consideration, it is to be noted basically that because of the different signal transmit times owing to different distances between the external stations and the central station an asynchronous reception situation is normally produced in the base station receiver, which 30 situation considerably aggravates this interference to such an extent that under ideal conditions code orthogo nality is then no longer produced in the uplink. In this case, the maximum number of simultaneous transmissions M within a frequency band in the uplink of, by 35 approximation, a DS-CDMA system can be estimated as follows: M = PG/ (Eb/No)

,

-3 PG being the process gain or spread factor and Eb/No being the ratio of bit energy to interference power, necessary for the aimed-at bit error rate, at the demodulator. The spread factor is the ratio of tbit to 5 tchip and is typically between 101 and 104. Assuming the ratio Eb/No is, for example, 3, which corresponds to approximately 5 dB, only approximately 1/3 of the transmission capacity, based on the same bandwidth being seized, is available in the 10 uplink in comparison with the downlink or in comparison with TDMA or FDMA systems if orthogonal signals are assumed for the latter. Various methods are known which reduce the previously described disadvantage of the asynchronous 15 CDMA methods, for example the synchronization of the external station in such a way that its transmission can be processed in synchronism with the chip in the receiver of the base station. In addition, it has been proposed to implement interference concelers which, by 20 means of mathematical algorithms, subsequently eliminate the interference component of the parallel transmissions on the basis of different a priori or a posteriori know ledge. Furthermore, it has also been proposed to use multi-user detectors. A disadvantage of all these known 25 methods is that they are very costly to implement. The invention is therefore based on the technical problem of providing a method and a device for synchronization in a radio transmission system with CDMA access, by means of which method and device the multi 30 user interference in the radio base station in the uplink mode can be reduced with low cost in terms of implementation. The technical problem is solved by means of the features of patent claims 1 and 9. It is necessary 35 for the radio transmission system to be operating in time division duplex mode in which transmission and reception are separated from one another in terms of time within one telecommunications channel, which significantly simplifies the sequence control. In order VOf -4 to synchronize all the subscriber stations, the radio base station transmits a maximum sequence or gold sequence, specific to the radio transmission system, in the form of a preamble for all the subscriber stations 5 before the actual data transmission. Since the information on the direct subscriber-specific system control, such as, for example, call setup and the like is transmitted in a central service channel, a common preamble can be used for all the subscriber stations. 10 This preamble can be detected without restricting other system parameters with a significantly better signal/noise ratio, since multi-user interference is not present and the subscriber-specific signal powers can be transmitted in an additive, coherent fashion, which 15 brings about a high level of detection reliability in the subscriber stations. The preamble which is received there is fed to a matched or correlation filter whose output signal serves as a trigger criterion when a defined amplitude threshold value is exceeded. Further 20 advantageous refinements of the invention emerge from the subclaims. The averaging over time of the synchronous information which is determined, and the evaluation of the knowledge of the precise value between two 25 successive preambles, makes it possible to achieve substantially greater precision, given sufficient clock stability in the subscriber stations, since an uncertainty in terms of timing of up to 0.5 x chip duration tchip can occur with simple detection using a 30 matched filter. The transmission of the synchronous information in the uplink parallel to the transmission of user data is made more difficult by the fact that the synchronous information is a priori not known, or not 35 known sufficiently precisely, as a result of which its acquisition would lead to an asynchronous interference situation with respect to the actual user data transmission. In order to avoid this, in each case only one item of synchronous information for all the Ov -5 subscriber stations which are active in parallel is transmitted simultaneously in the delay time between the transmission cycle and reception cycle, as a result of which the time information can be detected more reliably 5 because the transmission is subject to significantly less interference. For this purpose, if appropriate the delay time must be extended somewhat, but this is acceptable in order to achieve improved detection. As a result of the transmission-end shifting of the 10 symbols by one sample value in each case, but [lacuna] symbol-based matched filtering, with fixed timing, in the receiver of the base station, the time resolution or precision of the synchronization information within only one burst is improved up to a sample value tsampie, which, 15 in the case of conventional detection, can be up to 0.5 x chip duration tchip In order to avoid data collisions, the radio base station transmits to the subscriber station via the central service channel a status signal specifying which 20 subscriber stations are to transmit their synchronization sequence consecutively. After the evaluation of the signal transmit time by the radio base station, said station transmits via the service channel the subscriber-specific starting times for the uplink 25 transmission. In a further preferred refinement of the method, orthogonal gold sequences of the length of one symbol in each case are used for the code spreading of the data both in the uplink and in the downlink, said 30 sequences being relatively easy to generate. Furthermore, the orthogonal gold sequences have defined cross-correlation properties, the result of which is the subscriber stations, in which the synchronization mechanism fails, do not cause any significant faults in 35 the other parallel transmissions. Furthermore, in comparison with Walsh sequences and similar sequences, these have the advantage of a uniform spectral power distribution, which is significant in particular in the case of short sequences.

6 It is advantageous for the design of cellular structures if all the radio base stations which lie in the range of mutual radio influence are synchronized in terms of the transmission/reception cycle. In particular 5 radio base stations or subscriber stations which are in line-of-sight with respect to one another owing to an exposed geographical position could otherwise give rise to considerable interference at the receivers of the respective other radio cells. The synchronization may be 10 carried out, for example, by means of GPS or beacon signals which are passed on within the radio network. The method can be particularly advantageously implemented in wireless local loop systems, since in these systems the stationary nature of the subscriber 15 stations with relatively small changes of the properties of the radio channel over time can be exploited. The invention is explained in more detail below with reference to a preferred exemplary embodiment. 20 Fig. 1 shows a signal profile of a transmission in the downlink for a subscriber station, Fig. 2 shows a signal profile of a transmission in the downlink for n subscriber stations, 25 Fig. 3 shows a schematic signal profile at the output of a matched filter in a sub scriber station, Fig. 4 shows an illustration of the polling method for the synchronization in the 30 uplink, Fig. 5 shows a structure of an uplink synchron ization sequence, Fig. 6 shows a detailed illustration of the structure according to Fig. 5, and 35 Fig. 7 shows a schematic signal profile at the output of a matched filter of a radio base station. Fig. 1 illustrates a schematic signal profile of a transmission of a radio base station to a subscriber O1 -7 station over time t. The signal comprises a preamble 1 and a data item 2 which are transmitted with an amplitude PT. The preamble 1 is in this case a radio system-specific maximum sequence or gold sequence which 5 is generated by the radio base station. The data item 2 constitutes the actual user data for the subscriber station. Since the information is transmitted for the purpose of direct subscriber-specific system control in a central service channel, a common preamble 1 can be 10 used for all the subscriber stations. Fig. 2 illustrates the signal profile of the transmission in the downlink for all n subscriber stations. Since the radio base station transmits simultaneously to all n subscriber stations, a 15 corresponding superimposition of the signal profiles occurs. Owing to the transmission of a common preamble 1 for all the subscriber stations into a service channel, a coherent addition occurs and the amplitude is Pi ~ n 2 PT. The superimposition of the user data takes 20 place in accordance with the code modulation which is used, and varies correspondingly in amplitude, on average approximately the following is true P 2 ~ nPT. In order to determine a first item of synchronization information, the preamble 1 which is 25 received by each subscriber station is fed a matched filter [sic] by means of which the reception quality can be determined. A typical signal profile at the output of the matched filter of a subscriber station is illustrated in Fig. 3. In order to determine the 30 reception time of the transmission from the radio base station to the respective subscriber station, the output signal at the matched filter is evaluated by means of an amplitude threshold value switch. If the output signal exceeds a predefinable threshold Trl, the amplitude 35 threshold value switch produces a trigger signal, that [sic] represents the starting time for the reception of the preamble. Fig. 4 illustrates the signal profiles for the synchronization in the uplink. In order to avoid inter- -8 ference, the transmission of synchronization sequences 3 by the individual subscriber stations takes place here in the form of a polling method, i.e. in the first burst only the first subscriber station transmits its 5 synchronization sequence 3 to the radio base station. Subsequently, all n subscriber stations then transmit their user data 4 simultaneously to the radio base station. In the second burst, only the second subscriber station then transmits its synchronization sequence 3, 10 until finally in the n-th burst the n-th subscriber station transmits its synchronization sequence 3. A more precise structure of the synchronization sequence 3 is illustrated in Fig. 5. The synchronization sequence 3 comprises, for example, four identical 15 symbols 5 which are transmitted successively, the distance between the symbols 5 being increased successively by one clock pulse tsample of the system clock, and the first symbol 5 serving as preamble. An exemplary profile of a symbol 5 is 20 illustrated in Fig. 6, and it corresponds to the second symbol 5 with the transition to the third symbol 5 according to Fig. 5. Figure 7 illustrates an exemplary signal profile at the output of a matched filter in the radio base 25 station when a synchronization sequence 3 according to Fig. 5 is received. Here, each of the four symbols 5 produces an output signal with an amplitude P which is larger than a predefined threshold value Tr2 of a down stream amplitude threshold value switch. The first 30 symbol 5 produces an output signal with the amplitude Pb. The second symbol 5 which is transmitted directly after the first symbol 5 also produces an amplitude Pb. The third symbol 5 which is delayed by a system clock pulse tsampie produces an amplitude Pa, and the symbol 5 35 which is correspondingly delayed by 2 x tsampie produces an amplitude Pc. The optimum reception is therefore that fRA4', of the third symbol 5 so that the signal transit time has to be corrected correspondingly by one system clock pulse tsampie. In this way, the transit time between a roV/ Q -9 subscriber station and the radio base station can be determined with corresponding precision so that the synchronization can also be performed in the order of magnitude of tsapie. RAZ,1 - 10 List of reference numerals 1) Preamble 2) Data item 3) Synchronization sequence 4) User data 5) Symbol kro

Claims

1. A method for synchronization in a full-duplex capable radio transmission system with CDMA access with TDD mode, having a central radio base station and a plurality of subscriber stations which are independent 10 of one another, in each case a matched filter with a downstream amplitude threshold value switch being assigned in each case to the individual subscriber stations at the receiver end, comprising the following method steps: 15 a) a maximum sequence or gold sequence which is speci fic for each radio transmission system is generated by the radio base station, b) the generated maximum sequence or gold sequence is transmitted, as a preamble (1) before the actual 20 user data transmission (2), to all subscriber stations, c) the preamble (1) is received by the subscriber stations, d) the received preamble (1) is fed to the input of 25 the respective matched filter of a subscriber station, e) the output signal of the matched filter is for warded to the amplitude threshold value switch, and f) a trigger signal is generated by the amplitude 30 threshold value switch when a predefinable thresh old Tri is exceeded.

2. The method as claimed in claim 1, characterized in that the subscriber stations carry out averaging over time of the synchronization information which is 35 determined, by means of a priori knowledge of the burst structure and duration.

3. The method as claimed in claim 1 or 2, characterized in that the radio base station is designed - 12 with a matched filter with a downstream amplitude threshold value switch, and in each case a subscriber station transmits a specific synchronization sequence (3) to the radio base station within the delay time of 5 the actual user data transmission (4), the radio base station receives the synchronization sequence (3) and determines the concrete signal transit time between the radio base station and the corresponding subscriber station by matched filtering with the upper 10 transgression of an amplitude threshold value being evaluated as a trigger criterion at the filter output.

4. The method as claimed in claim 3, characterized in that the synchronization sequence (3) comprises a preamble and a plurality of identical symbols (5) which 15 are spread with subscriber-specific or radio-system specific maximum sequences or gold sequences, the individual symbols (5) being transmitted shifted successively by one system clock tsample with respect to one another in each case, and the radio base station 20 uses all the amplitude values at the output of the matched filter at the precise times of the symbol change for evaluation purposes, the time when an amplitude threshold value is overwritten [sic] when a preamble is received being used as a reference time value. 25

5. The method as claimed in claim 3 or 4, characterized in that the radio base station transmits to the subscriber station via the central service channel a status signal specifying which subscriber station is to transmit its synchronization sequence (3), 30 and after the evaluation of the signal transit time determined in the radio base station an item of information is transmitted to the respective subscriber station via the service channel, said item of information specifying at which subscriber-specific 35 starting times the transmission of user data or control information in the uplink should start so that the parallel transmissions of all the subscriber stations are received in synchronism with the chip in the receiver of the radio base station. - 13

6. The method as claimed in one of the preceding claims, characterized in that orthogonal gold sequences of the length of one symbol in each case are used for the code spreading of the data both in the uplink and in 5 the downlink.

7. The method as claimed in one of the preceding claims, characterized in that adjacent radio transmission systems operate in different frequency positions and/or use different spread sequences in each 10 case and/or use spread sequences from different code families.

8. The method as claimed in claim 7, characterized in that the respective radio base stations of the adjacent radio transmission systems operate 15 synchronously with one another in the uplink cycle and in the downlink cycle.

9. A device for synchronization within a full duplex-capable radio transmission system with CDMA access with TDD mode, comprising a central radio base 20 station and a plurality of subscriber stations which are independent of one another, characterized in that a matched filter with amplitude threshold value switch is assigned to each subscriber station at the reception end. 25

10. The device as claimed in claim 9, characterized in that at least one matched filter with amplitude threshold value switch is assigned to the radio base station at the reception end.

11. The device as claimed in claim 9 or 10, 30 characterized in that the radio transmission system is designed as a wireless local loop system.