EP1419605A1

EP1419605A1 - Method of improving transmit diversity reliability by including interleaving the transmit data in a single time slot

Info

Publication number: EP1419605A1
Application number: EP02796256A
Authority: EP
Inventors: Stephen William Wales
Original assignee: Siemens AG; Nokia Siemens Networks GmbH and Co KG
Current assignee: Nokia Solutions and Networks GmbH and Co KG
Priority date: 2001-08-23
Filing date: 2002-08-20
Publication date: 2004-05-19
Also published as: CN1309198C; CN1545780A; WO2003019849A1

Abstract

In a telecommunication system a method of transmitting digitised data via at least two transmission paths (channels) and including the step of applying interleaving in the space domain i.e. interleaving in a single time slot. This preferably includes the steps of: -coding the data by spreading the data using a number of spr eadi ng codes - applying interleaving in the space domain frame segmenting the data and - transmitting said data via at least two antennas. Interleaving may be applied across the time domain also.

Description

METHOD OF IMPROVING TRANSMIT DIVERSITY RELIABILITY BY INCLUDING INTERLEAVING THE TRANSMIT DATA IN A SINGLE TIME SLOT

BACKGROUND

5 The present invention relates to a method of improving transient diversity reliability.

In mobile communications systems, the term "transmit diversity" is a known technique which is used to improve down-link reception performance, by providing different transmission paths to a mobile 0 receiver.

For example, when a base station (transceiver) receives data from a mobile communication unit (telephone) it can be determined which path i.e. which base station, at that time is able to provide the best reception to 5 signals from the mobile communication unit. Consequently it can be assumed that subsequent reply transmissions, to the mobile communication unit, are also made from the base station that has the best reception. This is because the link can be assumed to be a reciprocal channel i.e. the channel impulse response is the same in both directions. 0

Feedback information from a receiver at the mobile communication unit, dictates from which base station antenna it is best to transmit. This technique is known as "switched antenna diversity". If characteristic signal fading, applied to each transmission path, is 5 sufficiently decorrelated then the likelihood that all transmission paths suffer equally from signal fading is reduced. PRIOR ART

Published International Patent Application no. WO-A-01/65760 (Nokia Networks OY) describes a system which utilises space-time coding combined with turbo coding. Space-time encoded signals are transmitted via a multi-channel communication system in order to improve transmission spectral efficiency.

The system appears to utilise a parallel or serial concatenated convolutional code. Polynomials output by the generator are mapped to different antennas. Interleavers are part of a normal concatenated code where interleaving is performed between coding steps. Instead of time multiplexing the outputs of different generator polynomials into a single stream, the different generator polynomials are multiplexed to different antennas. Interleaving is performed after error correction coding and not as part of the error correction coding process.

A number of transmit diversity schemes have been developed including switched antenna diversity as described above. Delay diversity is also known where the same transmission is made from each antenna but a known delay is deliberately introduced betweentransmissions.

A number of problems with switched antenna diversity exist. Firstly, there is the requirement that a reciprocal channel displays precisely the same characteristics to a signal. This may be difficult to achieve in practice. For example, if the mobile telephone is moving, then any decision made by the base station, regarding on which path to transmit a 'reply' signal, not provide the best reciprocal path. This is because by the time transmission actually occurs, a different base station may be better suited for transmission via an alternative link or path.

In systems that employ Code Division Multiple Access, different spreading codes, or different sets of spreading codes are assigned to different users. If a fixed spreading factor is used, then different transmission rates are accommodated by data being spread by a number of spreading codes by forming a multi-code group. The codes of the multi-code group are transmitted in parallel.

It is an object of the invention to overcome the aforementioned problems.

SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided in a telecommunication system, a method of transmitting digitised data via at least two transmission paths (channels) and includes the step of applying interleaving of data in the space domain.

Preferably the method is carried out using the steps of: a) coding data by spreading the data using a number of spreading codes; b) applying interleaving to the coded data in the space domain; c) frame segmenting the coded data ; and d) transmitting said frame segmented data via at least two antennae: The inventor has determined a method of overcoming the problems, when using multi-code transmissions where different codes are mapped to different antennas.

In a preferred embodiment interleaving is also applied in the time domain.

In a situation where multiple codes are assigned to a user, it is possible to employ a transmit diversity scheme where data to be sent is first interleaved and divided into M data streams. Each of the M data streams are then modulated and spread using a spreading code. Modulated and spread data is then transmitted via one of N transmit antennas. The benefit of employing this form of diversity transmission occurs when channel coding is applied and in particular when the fading duration is longer than the interleaving period applied to each block of code.

When fading is highly correlated, over the duration of a code block, the performance of any error correcting decoder will be poor. By transmitting different data streams, via different antennas, and applying interleaving across the different data streams, de-correlation of the fading, across the code block, occurs and a diversity gain is realised through improved performance of error correction code.

Interleaving is a technique used to prevent severe data loss and acts to spread errors when used with an error correcting code. When applied to a block of data, in the time domain, an interleaved block is obtained. The interleaved block is then segmented and transmitted in different time slots, if a time slotted transmission format is employed. As a result of the interleaving, bits of data are shuffled before insertion into the time slots. At the receiver, shuffled data is reordered. If the data in one time slot gets lost, corrupted or destroyed, for example due to fading, then there is a chance that data arriving in a subsequent time slot, remains intact. Errors are thus spread and subsequent application of error correction methods allows lost data to be recovered more effectively.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described by way of example only and with reference to the following Figures in which:

Figure 1 shows a schematic representation of a simple embodiment of the invention showing downlink transmission via multiple antennas using interleaving in the coding (space) domain;

Figure 2 shows a further embodiment employing interleaving in both the time and coding (space) domain; and

Figure 3 compares shows a further embodiment of the invention where time domain interleaving is also incorporated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Example 1

In the system, shown in Figure 1 , data transmitted is first encoded using a convolutional encoder 1. Other encoders, such as a Turbo encoder may alternatively be used. At the next stage 2, the coded data is interleaved in the space domain before segmentation. This is done by interleaving the data to be transmitted in a single time slot before segmentation 3 into parallel data blocks. Data blocks are transmitted by each code of the multi-code group. This interleaving is hereinafter referred to as interleaving in the space domain and any reference to such should be inteφreted as above. After this stage the data to be transmitted on each code is modulated and spread 4 according to a known spreading function. The data is then transmitted by one of the antennas 5 at the base station.

Example 2

This example is similar to that described in Example 1 above, but includes an extra stage of interleaving in the time domain shown as 7 in Figure 2. Again data is encoded. The resulting data is then interleaved across multiple frames and segmented into data to be transmitted in each frame. Data for one frame is mapped to multiple spreading codes in a single time slot. The above frame interleaving process is essentially time interleaving.

Space domain interleaving is then applied. This is done by the following process, as before by applying interleaving to data transmitted within one time slot; i.e. time slot interleaving is a way of applying code domain interleaving Next data is divided into multiple segments. Each segment is transmitted using a different spreading code. The spread data is transmitted via different antennas. If M spreading codes form a multi- code group and there are N transmit antennas, then a simple mapping of spreading codes to antennas is obtained from n = mod_w m , where n is the transmit antenna index and m the spreading code index.

Transmissions are received by a mobile telephone. The signal is de-spread, and is then subjected to a detection process, comprising a channel impulse response estimator and an appropriate detection algorithm. The transmitted signal must support the possibility for the receiver base station to estimate each of the transmission paths to the mobile telephone and to associate respective channel impulse responses with one or more of the spreading codes of the multi-code group. This can be achieved in a number of ways through either code specific known data sequences, for example as used in the UTRA TDD system or through transmit antenna specific known data sequences and knowledge of the antenna code mapping. After detection the data is de-interleaved before being decoded.

As there are N transmission paths, each characterised by its own channel impulse response, a means to estimate the channel impulse response is required. In the UTRA (UMTS Terrestrial Radio Access) TDD (Time Division Duplex) system, data is transmitted in bursts, each burst contains two data fields either side of a known midamble sequence. This unique midamble sequence can be associated with a spreading code according to a default mapping consequently the mobile telephone can obtain a channel estimate associated with each spreading code and this is sufficient for detection puφoses. The mobile telephone does not need information concerning the mapping of spreading codes to antennas.

The technique described in Example 2 provide equal performance to that of switched antenna diversity in a TDD system where the reciprocal nature of the channel is used to select from which antenna to transmit. One advantage of space-time interleaving transmit diversity, is that it is not reliant upon the presence of any feedback and can be used for common control channels.

An example of the performance achieved using space time interleaving is shown in Figure 3 for a typical multipath channel model. In this example a 64kb/s data channel is encoded using a 1/3 rate Turbo code and transmitted using 6 spreading codes mapped to two antennas. The performance of space-time interleaving (S-TI) is shown to be comparable to switched antenna diversity (SA), which relies upon transmissions from the mobile telephone to determine from which antenna to transmit.

The invention has been described by way of exemplary embodiments only, and it will be appreciated that variations may be made to the embodiments, without departing from scope of the invention.

Claims

1. A method of transmitting digitised data via at least two transmission paths (channels) and including the step of applying interleaving in the space domain.

2. A method as claimed in claim 1 including the steps of a) coding the data by spreading the data using a number of spreading codes; b) applying interleaving to the coded data in the space domain c) frame segmenting the data from step b): d) transmitting said data from step c) via at least two antennas.

3. The method of claim 1 wherein interleaving is also applied across the time domain.

4. A method as claimed in claim 3 performed by the steps of time slot interleaving and further segmentation

5. A method as claimed in any of claims 2 to 4 including the additional steps of applying modulation spreading and code antenna mapping.

6. A method of receiving data according to any preceding claim comprising the steps of: a) receiving data from a plurality of channels corresponding to a plurality of antennae; b) applying a despreading code to said data; c) estimating a channel impulse response for each channel; d) associating the channel impulse responses with one or more spreading codes of a multi-code group; e) de-interleaving said data; and, f) applying decoding.

7. A system for transmitting digitised data via at least two transmission paths (channels) having means to apply interleaving of said data in the space domain.

8. A system as claimed in claim 7 where said means to apply onterleaving comprises: a) means to code the data by spreading the data using a number of spreading codes ; b) means to apply interleaving in the space domain to said spread data; c) means to frame segment the data from step b): d) means to transmit segmented data via at least two antennas.

9. The system of claim 7 or 8 additionally having means to interleave across the time domain.

10. A system as claimed in claim 9 wherein said additional means includes means to perform time slot interleaving.

11. A system for receiving data comprising: a) means to receive data from a plurality of channels corresponding to a plurality of antennae; b) means to apply despreading to said data; c) means to estimate a channel impulse response for each channel; d) means to associate the channel impulse responses with one or more spreading codes of a multi-code group; e) means to perform de-interleaving of said data; and , f) means for applying decoding.