GB2379136A

GB2379136A - Method and system for transmitting data with improved diversity reliability

Info

Publication number: GB2379136A
Application number: GB0127207A
Authority: GB
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-08-23
Filing date: 2001-11-10
Publication date: 2003-02-26
Anticipated expiration: 2021-11-10
Also published as: GB2379136B; GB0120535D0; GB0127207D0

Abstract

In a telecommunication system a method of and system for transmitting digitised data via at least two transmission paths (channels) and including the step of applying interleaving in the space domain. This preferably includes the steps of: coding the data by spreading the data using a number of spreading codes; applying interleaving in the space domain; frame segmenting the data; and transmitting said data via at least two antennas. Interleaving is may be applied across the time domain also, preferably by time slot interleaving and further segmentation. Also disclosed is a receiver having a number of antenna to receive data from the same number of channels, despreading apparatus; channel impulse response estimating means for each channel and associating that response to one or more spreading codes; deinterleaving the data, and decoding.

Description

METHOD OF IMPROVING TRANSMIT DIVERSITY RELIABILITY In mobile communications systems, the term"transmit diversity"is a known technique which is used to improve the down-link reception performance by providing different transmission paths to a mobile receiver For example, when a base station (transceivers) receives data from a mobile phone it can be determined which path i. e. to which a base station, at that time gives the best reception and the subsequent reply transmissions to the mobile phone can be made using the best path, i. e. 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. Feedback information from the receiver dictates which antenna is best to transmit upon. This technique is known as"switched antenna diversity". If the characteristic signal fading applied to each transmission path is sufficiently decorrelated then the likelihood that all transmission paths equally suffer from signal fading is reduced. A number of transmit diversity schemes have been developed including the switched antenna diversity as described above. Delay diversity is also known where the same transmission is made from each antenna but a delay is introduced between the transmissions.

There are a number of problems with switched antenna diversity.

Firstly there is the requirement of a reciprocal channel. This may be difficult to achieve. For example, if the mobile telephone is moving, then

the decision regarding which path to transmit may be wrong as the new position may be better suited for transmission from an alternative 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 the data being spread by a number of spreading codes, 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 these problems. The inventor has determined a method of overcoming these problems when using multi-code transmissions where different codes are mapped to different antennas.

The invention comprises 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.

Preferably this is carried out using 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.

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

In the situation where multiple codes are assigned to a user it is possible to employ a transmit diversity scheme where the 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 one of the spreading codes and are then transmitted by 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 code block.

When the 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 then de-correlation of the fading across the code block will occur and a diversity gain is realised through the improved performance of the error correction code.

Interleaving is a known 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 the interleaved block is then segmented and transmitted in different time slots if a time slotted transmission format is employed. In the interleaving, the bits get reordered before insertion into the time slots. At the other end they are ordered. If the data in one time slot gets destroyed due to fading for example, then there is a chance that data in the next time slot remains intact. The errors are thus spread and subsequent application of error correction methods allows the lost data to be recovered more effectively.

Thus the term"interleaving"should be interpreted as above.

The invention will now be described by way of example and with reference to the following figures of 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.

Figure 3 compares shows a further embodiment of the invention

where time domain interleaving is also incorporated.

Example 1 In this system shown in figure 1, the 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 the parallel data blocks to be transmitted by each code of the multi-code group. This such interleaving is referred to herein as interleaving in the space domain and any reference to such should be interpreted as above. After this stage the data to be transmitted on each code is modulated and spread, 4, and transmitted by one of the antennas 5 at the base station.

Example 2 This example is similar to that above, but includes the extra stage of interleaving in the time domain at 7 of figure 2. Again the data is

encoded. The resulting data is then interleaved across multiple frames and segmented into the 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 the data is divided into multiple segments, each segment is transmitted via a different spreading code and the spread data is transmitted via different antennas. If M spreading codes form a multicode group and there are N transmit antennas, then a simple mapping of spreading codes to antennas is obtained from n = mod, m where n is the transmit antenna index and m the spreading code index.

The transmissions are received by the mobile phone at the other end. The signal is de-spread, and then passes a detection process, comprising a channel impulse response estimator and an appropriate detection algorithm. The transmitted signal) must support the possibility for the receiver to estimate each of the transmission paths to the mobile and to associate this channel impulse response 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 is the data is then 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 TDD 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 receiver can obtain a channel estimate associated with each spreading code and this is sufficient for detection purposes. The mobile receiver does not need to know the mapping of spreading codes to antennas.

The technique provides equal performance to that of switched antenna diversity in a TDD system where the reciprocal nature of the channel is used to select which antenna to transmit upon. 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 to determine which antenna to transmit upon.

Claims

CLAIMS 1. 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.
2. A method as claimed in claim 1 or 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 applied across the time domain also.
4. A method as claimed in claim 3 performed by the additional steps of time slot interleaving and further segmentation
5. A method as claimed in claim 2 to 4 including the additional steps in step d) of applying modulation spreading and code antenna mapping.
6. A method of receiving data according to any of preceding claim comprising the steps of : a) receiving data from a plurality channels corresponding to a plurality of antenna b) applying despreading to said data; c) estimating the channel impulse response for each channel;

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d) associating the channel impulse responses with one or more spreading codes of the multi-code group; e) de-interleaving said data; and, f) applying decoding.
7. A system a method of transmitting digitised data via at least two transmission paths (channels) and having means to apply interleaving of said data in the space domain.
8. A system as claimed in claim 7 where said means 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 c) means to frame segment the data from step b): d) means to transmitting segmented data via at least two antennas.
9. The system of claim 7 or 8 additionally having means to interleaving across the time domain also.
10. A system as claimed in claim 9 wherein said additional means includes further means to time slot interleave and further segment.
11. A system of receiving data comprising: a) means to receive data from a plurality channels corresponding to a plurality of antenna; b) means to apply despreading to said data; c) means to estimate the channel impulse response for each channel; d) means to associate the channel impulse responses with one or more spreading codes of the multi-code group;

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e) means to de-interleaving said data; and, f) means to applying decoding.