Patent Application: US-50999604-A

Abstract:
a method and a system are disclosed for avoiding inter - layer inter - symbol interference . diagonally layered multi - antenna transmissions are utilised by the proposed method and system . known symbols are inserted at the borders between different layers to avoid inter - layer inter - symbol interference . the system relies on an improved method to transmit multiple data - streams . the invention describes a method for transmitting data - streams over multiple antennas in an effective and smart way when having frequency selective channels . by utilising a system using the present method problems with inter - layer inter - symbol interference between different data - streams can be avoided when changing transmit antenna for the data - streams . the invention utilises that a diagonal layered multi - antenna transmission will not create inter - symbol interference , isi , between different layers .

Description:
a diagonally layered multi - antenna transmission scheme like d - blast has a greater capacity potential than a v - blast like scheme due to transmission from several transmit antennas for each layer , as opposed to transmission from only one antenna for each layer . this invention then allows diagonally layered multi - antenna transmission schemes to be employed also for communication networks suffering from isi , without causing inter - layer isi . the inter - layer isi can be avoided by inserting known symbols into the sequence transmitted from each antenna at every border between different layers ( see fig4 ). if the number of known symbols is at least as large as the channel memory ( the number of channel taps minus one ) there will be no isi between different layers . this means that the amount of user data that can be transmitted will be reduced , since the number of data symbols is reduced . however , the known symbols could be used also for other purposes such as e . g . channel estimation and synchronisation . for the special case of two transmit antennas the burst structure employed in e . g . gsm and utra / tdd can be exploited . in those networks a burst consists of two fields of data separated by a training sequence of known symbols used for e . g . channel estimation ( see fig5 ). for this case , from the first transmit antenna the first layer is transmitted in the data field to the left of the training sequence , and the second layer is transmitted in the data field to the right of the training sequence . from the other transmit antenna the second layer is transmitted in the left data field and the first layer in the right field . as the training sequence is longer than the channel impulse responses there will be no inter - layer isi . by exploiting an existing burst structure inter - layer isi can be avoided for a diagonally layered multi - antenna transmission scheme without having to reduce the number of data symbols that are transmitted . in a document by lindskog and paulraj [ 7 ] they present a space - time block code for two transmit antennas that uses an approach similar to the one considered in this invention . as code block for the space - time block - code , they define a radio burst , where the left data field is one symbol in the space - time block - code and the right data field the second space - time block - coding symbol . these two space - time block - coding symbols are transmitted from both transmit antennas and are separated in time by the training sequence . however , compared to the present invention where multiple layers of independent data provides increased bit rates , the method of lindskog and paulraj transmits the same data on both transmit antennas , and therefore does not offer increased bit rates , only increased diversity . further , by using the training sequence to separate the space - time block coding symbols , orthogonality within the space - time code block is ensured , which enables a low complexity decoding of the space - time code , whereas this invention allows separate detection of each of the multiple layers . for diagonally layered multi - antenna transmission in case of isi with more than two transmit antennas , layering over all antennas may require transmission of quite a lot of known symbols . an alternative approach would be to divide both the transmit antennas and the layers into sub - sets , each sub - set containing a number of transmit antennas and the corresponding sub - set of layers containing a number of layers equal to the number of transmit antennas . within each sub - set the layers are layered diagonally across the transmit antennas . this is shown for the special case of two antennas within a sub - set in fig4 , where the two layers within a subset are separated by known symbols . however , there is no layering across antennas belonging to different sub - sets . this is shown in fig6 for a 4 - antenna transmission scheme . in a further application it would additionally be possible to use more than two antennas for a sub - set , even if we for simplicity here consider embodiments having just two antennas for a sub - set . it would also be possible to consider having different number of antennas , and thus also different number of layers , in different sub - sets . the division of antennas and layers into sub - sets could be changed dynamically as often as every radio burst , and different sub - set division could be allowed for different users . using the diagonally layered multi - antenna transmission scheme proposed in this invention inter - symbol interference ( isi ) between different layers is avoided . this allows the receiver to demodulate and decode the layers sequentially . a complete layer is demodulated ( including equalisation ) and decoded , before it is cancelled from the received signal . thereafter the next layer is demodulated and decoded , and so forth . using diagonal layering without consideration of isi between layers the receiver will not be able to demodulate and decode a complete layer before cancelling it . in this case the complete sequence of symbols transmitted from one antenna would have to be demodulated and cancelled . this means that only parts of each layer is demodulated before cancellation , which is performed before decoding . being able to perform decoding before cancellation reduces the amount of errors , and thus the effect of error propagation , which may occur when a cancelled layer contains estimation errors . the alternative is to use a v - blast like multi - antenna transmission scheme where there is no risk of inter - layer isi . however , transmit diversity , which improves performance , in particular for layers onto which low rate channel codes have been applied , is not provided with such a scheme . further , for good performance in a v - blast like multi - antenna transmission scheme it is important to determine which layer has experienced the best transmission quality , and to begin with this layer . this is not necessary for the diagonally layered multi - antenna transmission scheme , since all layers are transmitted equally over all transmit antennas . this means that additional gains could be achieved if different code rates are used for the different layers . the layer being demodulated and decoded first could be given a lower code rate , since this layer gains the least from diversity . by improving this layer , performance of subsequent layers may also improve . however , it should be pointed out , that the method will not be limited to any particular receiver algorithm . fig7 illustrates in a simplified base - band block diagram an exemplary illustrative embodiment of a diagonally layered multi - antenna transmission scheme , where a data stream is de - multiplexed into two individual layers , each of which is independently coded , interleaved and mapped onto symbols . thereafter the layers are mapped onto the two transmit antennas according to the diagonal layering method . fig8 illustrates in a diagram the main steps for forming a diagonally layered multi - antenna transmission according to the present method disclosed . it will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departing from the spirit and the scope thereof , which is defined by the appended claims . 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