WO2000074255A1

WO2000074255A1 - Improvements in or relating to mobile telecommunications systems

Info

Publication number: WO2000074255A1
Application number: PCT/GB2000/002006
Authority: WO
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Limited
Priority date: 1999-05-29
Filing date: 2000-05-25
Publication date: 2000-12-07
Also published as: CN1353887A; AU4937600A; CN1157858C

Abstract

Described herein is a hybrid TDMA/TDD mobile telecommunication system in which scrambling codes and spreading codes are used to reduce the cross-correlations between users simultaneously occupying a time slot. A frame comprising a plurality of data signals (50, 60, 70) is divided into a plurality of time slots (52, 54, 56, 62, 64, 66, 72, 74, 76), each time slot for one data signal comprising the same spreading codes. Each spreading code is selected from a set of spreading codes. Each data signal in each time slot also has a scrambling code associated therewith, each scrambling code being selected from a set of scrambling codes. The scrambling code for each data signal (50, 60, 70) is periodically varied to extend the set of scrambling codes which can effectively be used for satisfactory cross-correlation.

Description

IMPROVEMENTS IN OR RELATING TO MOBILE TELECOMMUNICATIONS SYSTEMS

The present invention relates to improvements in or relating to mobile telecommunications systems, and is more particularly concerned with such systems operating on code-time division multiple access.

The UMTS terrestrial radio access (UTRA) - time division duplex (TDD) system is based on a combination of code division multiple access (CDMA) and hybrid time division multiple access (TDMA) and TDD. (UMTS is an acronym for universal mobile telecommunication system as understood by persons skilled in the art.)

In time division multiple access (TDMA), the time axis is divided into a plurality of slots and users are permitted to transmit in certain ones of those slots. Code division multiple access (CDMA), on the other hand, utilises direct sequence spread spectrum (DSSS) where a data symbol is multiplied by a pseudo-noise code of length N bits prior to transmission. In a UTRA-TDD system, orthogonal codes are used to provide a unique user code, that is, a unique code for each mobile terminal operating in the system. In this specific case, each user unique code corresponds to a user unique path within the cell. As is well understood, such a system comprises a plurality of telecommunication cells each having at least one base station and at least one mobile terminal. However, a high cross-correlation can result when codes are not aligned in time.

One method for providing reducing cross-correlation is to multiply each orthogonal user code with another unique code, the other unique code corresponding to a cell unique path. In this case, the resulting code retains its orthogonality. However, when the system is extended from a single cell to a multi- cell system, the same codes may be used in different cells. This may lead to inter-cell interference due to cross-correlation between codes.

It is therefore an object of the present invention to provide a hybrid system which utilises both TDMA and CDMA to permit users to share a radio frequency (RF) carrier by a combination of code division and time division which provides improved cross-correlation.

In accordance with one aspect of the present invention, there is provided a method of providing reduced cross-correlation in a code division multiple access mobile telecommunications system comprising a plurality of telecommunications cells, each cell having at least one base station and at least one mobile terminal which communicate by means of a plurality of frames of data, the method comprising the steps of:- dividing each frame into a plurality of time slots; utilising a spreading code for each data signal in each frame, each spreading code being unique to its associated data signal and selected from a set of spreading codes; utilising a scrambling code for each data signal in each frame for reducing inter-cell and intra-cell interference, each scrambling code being selected from a set of scrambling codes; characterised in that the method further comprises periodically varying the scrambling code for each time slot of each data signal to effectively extend the set of scrambling codes available.

Advantageously, the method further comprises setting the length of the scrambling code in accordance with the spreading code.

Preferably, a different set of scrambling codes is assigned to each telecommunications cell. The set of scrambling codes for each telecommunications cell may comprise a permutation of scrambling codes selected from a single set of scrambling codes. Alternatively, the set of scrambling codes for each telecommunications cell may comprise a discrete set of scrambling codes.

The code division multiple access mobile telecommunications system may comprise a hybrid time division duplex/time division multiple access system.

For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:- Figure 1 illustrates the operation of code-time division multiple access in accordance with the present invention; and

Figure 2 illustrates the assignment of spreading and scrambling codes on a carrier with cyclic code hopping.

Figure 1 illustrates a portion of a frame of a transmission comprising three codes 10, 20, 30. Code 10 comprises a code k and has three time slots 12, 14, 16 corresponding to time slots (m-1), m and (m+1) respectively. Similarly, code 20 comprises a code (k+1) and has three time slots 22, 24, 26 corresponding to time slots (m-1), m and (m+1) as before, and code 30 comprises a code (k+2) and has three time slots 32, 34, 36 also corresponding to time slots (m- 1 ), m and (m+ 1 ).

In a given time slot the modulated waveform may be described in the following manner:

The z^'-th data symbol of duration T_s is denoted by d,, and may take a complex value from a finite set, depending upon the type of data modulation used. For example if QPSK (quaternary phase shift keying) modulation is used then d takes values from the set [1,7, -1, -j .

A spreading code, s[^k) (where n e [0, N-l]), is defined comprising N chips of duration TJN, and associated with the k-th user in the time slot. The chips of the spreading code can in general take complex values from a finite set, depending upon the type of spreading modulation used. In addition, a so- called scrambling code which is associated with the base station to which the mobile terminal is communicating is also defined. The scrambling code, c[^p , comprises N chips, and may in general take complex values from a finite set. The significance of the index P will be explained later.

The transmitted waveform resulting from a modulated and spread data block of M symbols is given by:

where g(t) is a pulse shaping function.

In cellular radio systems, it is desirable to avoid the need for code planning. This can be achieved by providing a large code set, so that only very distant base stations would re-use the same code. It is common for the spreading codes to be the same in each cell. In fact, it is desirable to use a set of orthogonal spreading codes (of which there only a number equal to the spreading factor). Consequently, the cell distinction is left to the use of the scrambling codes. Scrambling codes are derived by multiplying a user (mobile terminal) unique path code by a cell unique path code. When orthogonal spreading codes are used, the scrambling codes serve a number of purposes. First, scrambling codes can avoid high cross- correlation between transmissions from neighbouring cells, that is, their use reduces inter-cell interference, and, secondly, scrambling codes can reduce the high cross-correlations between the spreading codes for non-zero time shifts of these codes, that is, to reduce intra-cell interference. The latter is important when time dispersion is present in the communications channel. Unless -Vis very large, it is not possible to choose a large set of scrambling codes that will result in low cross-correlation between all pairs of the product of scrambling codes and spreading codes. For this to be the case, then the size of the set of scrambling codes P must be very much smaller than the length of the spreading sequence N. It is to be noted that the scrambling code is fixed.

In order to avoid code planning, the converse is required, that is, the set of scrambling codes must be very much greater than the length of the spreading sequence. Consequently, it is inevitable that very high cross- correlation can be found between pairs of the product of scrambling codes and spreading codes. This is a situation that is undesirable from the user's viewpoint because, depending on the spreading codes assigned to that user, very good performance may be obtained because the cross-correlation properties with other codes in use both within the user's cell and neighbouring cells are good, conversely very poor performance can result because of the code choice.

In accordance with the present invention, it is possible to overcome the problems associated with the use of a fixed scrambling code by making the scrambling code vary from time slot to time slot on a periodic basis. In this situation, a user may experience poor performance in a given time slot, because of the codes employed, but in other time slots good performance. If the period over which the scrambling code is changed is long then each user should experience the same performance on average. Furthermore, the length of the scrambling code is now effectively increased, consequently it is possible to easily obtain a large scrambling code set.

Defining a set of scrambling codes containing P codes: {c⁽⁰⁾,....,C^(/,~I) } where C^ip) = [c^ ,...,c_N ^(p) _}] , then in they^'-th time slot the scrambling code C ^p) is used, where p = mod( , P). Each cell (or base station) is assigned a different set of scrambling codes which may simply be a permutation of a single set of P scrambling codes, or contain entirely different scrambling codes from those in other sets. It may also be possible to change the scrambling codes on each data symbol, however this adds complexity to the receiver, particularly if the detection processes operate on blocks of data comprising M symbols. The method by which the scrambling code varies on a time slot by time slot basis is sufficient to provide all the benefits offered by varying the scrambling code without impacting upon the complexity of the receiver. If the spreading factor is user-dependent, then the length of the scrambling code should be set to the maximum spreading factor used. With multiple spreading factors in use, the complexities of choosing a set of fixed scrambling codes are increased many times. The method in accordance with the present invention periodically varies the scrambling code thereby avoiding the difficulties in obtaining a set of scrambling codes that provide equal performance independent of the choice of spreading code and spreading factor.

Figure 2 illustrates the assignment of spreading and scrambling codes on a carrier with cyclic code hopping. A portion of a frame is shown which represents three spreading codes 50, 60, 70. In a similar way to Figure 1, each code 50, 60, 70 has three time slots corresponding to time slot times of (m-1), m and (m+1). Code 50 has the same code, k, in each of its time slots 52, 54, 56. Code 60 has the same code (k+1) in each of its time slots 62, 64, 66. Code 70 has the same code (k+2) in each of its time slots 72, 74, 76. In addition to having the spreading codes 50, 60, 70, each time slot also includes a scrambling code. For example, time slot 64 has a time slot m, a spreading code, k+1, and a scrambling code, p. Although the present invention has been described with reference to a hybrid time division duplex/time division multiple access mobile telecommunications system, it will readily be appreciated that the present invention is equally applicable to other code division multiple access systems.

Claims

CLAIMS:

1. A method of reducing cross-correlation between codes in a code division multiple access mobile telecommunications system comprising a plurality of telecommunications cells, each cell having at least one base station and at least one mobile terminal which communicate by means of a plurality of frames of data, the method comprising the steps of:- dividing each frame into a plurality of time slots; utilising a spreading code for each data signal in each frame, each spreading code being unique to its associated data signal and selected from a set of spreading codes; utilising a scrambling code for each data signal in each frame for reducing inter-cell and intra-cell interference, each scrambling code being selected from a set of scrambling codes; characterised in that the method further comprises periodically varying the scrambling code for each time slot of each data signal to effectively extend the set of scrambling codes available.

2. A method according to claim 1, further comprising setting the length of the scrambling code in accordance with the spreading code.

3. A method according to claim 1 or 2, further comprising assigning a different set of scrambling codes to each telecommunications cell.

4. A method according to claim 3, wherein the set of scrambling codes for each telecommunications cell comprises a permutation of scrambling codes selected from a single set of scrambling codes.

5. A method according to claim 3, wherein the set of scrambling codes for each telecommunications cell comprises a discrete set of scrambling codes.

6. A method according to any one of the preceding claims, wherein the code division multiple access telecommunications system comprises a hybrid time division duplex/time division multiple access system.

7. A method substantially as hereinbefore described with reference to the accompanying drawings.