METHOD AND APPARATUS OF MULTIPLE ANTENNAS TRANSMISSION

FIELD OF THE INVENTION

The invention relates to wireless communication systems, more particularly, relates to a method and apparatus of multiple antennas transmission for use in a wireless communication system.

BACKGROUND OF THE INVENTION

Generally, MIMO (Multiple Input Multiple Output) can be seen as a wireless communication system equipped with multiple antennas at transmitter and/or receiver. MIMO is one of the key features in future wireless communication system to bring high capacity and performance. In MIMO system, separate parallel symbol streams, which are independent or correlated, will be transmitted to wireless channel. At receiver side, the signals are captured by possible multiple antennas and recovered to be the original message in the corresponding processing part.

Among all MIMO techniques, multipaths diversity (MPD) is a well known 3GPP proposal introduced in 3GPP TR 25.876, "Multiple-Input Multiple output in UTRA "

V1.3. and 3GPP TSG RAN WGl (03) 0760: Multi-paths Diversity for MIMO (MPD), Nortel Networks. The number of parallel data streams is equal to the number of active transmitting antennas. For the case of more than two antennas are used, rate control MPD (RC MPD) is proposed, where every pair of data streams that shares the same two antennas has the same data rate and modulation. The data rate for every pair is fixed by the Node-B according to the mobile measurements. The Node-B determines the most appropriate data rate and modulation to transmit for every stream.

Fig. 1 is a block schematic diagram of a MPD transmitter and Fig. 2 is a block schematic diagram of a MPD receiver. In order to simplify explaining the basic concept of multipaths diversity, only two antennas are adopted in the transmitter.

According to Fig.l, the initial data bits are processed in a data process unit 130 to obtain two signals to be transmitted by an associated pair of antennas 140. In the MPD transmitter, space-time transmit diversity (STTD) combined with delay diversity are employed for transmission.

More specifically, the initial input data bits are converted into two parallel independent data streams with the same rate in a serial-to-parallel converter 102 and the two data streams are coded, interleaved and mapped to symbols in modulator 104. Then one copy of the symbols are spread and scrambled in a first spreader 106 to form a pair of non-STTD-encoded symbols, while another copy of the symbols are encoded with space- time transmit diversity code in a STTD encoder 108, then spread, scrambled in a second spreader 116, and delayed with 1 chip time duration in a delay circuit (118) to form a pair of STTD-encoded symbols. The pair of non-STTD-encoded symbols and the pair of STTD-encoded symbols are combined at adder 112 and 122 within single symbol duration and transmitted by two antennas 140 respectively.

Assumed that the transmitting symbols of the original pair of data streams are d _γ , d ₂ (to be transmitted at the same time), one pair of symbols are derived from the original pair of data streams after spreading and scrambling, another pair of symbols are derived from the original pair of data streams after encoding with the space-time transmit diversity (STTD) code:

where the superscript * denotes the complex conjugate. The final signals to be transmitted are combination of the two pairs of symbols, i.e., non-STTD encoded and STTD encoded symbols with one chip delay. Using the STTD code allows the data streams to be transmitted on one more antenna and thus takes advantage of the transmission diversity.

From Fig.2, at the MPD receiver the receiving data streams is de-spread and descrambled in a despreader (202) and then passed through an interference canceller named IC (204) such as Minimum Mean Squared Error (MMSE) or Multipath Parallel Interference Canceller (MPIC) to remove the interference created by multi-paths. After that, a simple STTD combiner (206) is used to distinguish the two original data streams. Next, the two data steams are de-mapped, de-interleaved and decoded in demodulators (208, 218), and the final data bits are recovered after a parallel- serial converter (220).

With combining delay diversity and space-time transmit diversity together in spread spectrum system, MPD can achieve higher data rate while maintaining the same

diversity gain as the conventional transmit diversity techniques, and thus improving system performance as a whole.

However, multipath diversity transmission encounters serious problem when overlapping of the artificial delay propagation multipath signals and the natural propagation multipath signals occur in receiving side. According to above MPD proposal, one chip delay is adopted as a fixed delay between two pair of symbols to be combined and transmitted on each single given antenna, the addressed overlapping may happen very often and thus deteriorate system performance. The cause of the existing problem and possible system degradation could be illustrated clearly with help of theoretical analysis as below.

Still taking two antennas system as an example for simplification. Assuming that the signals B _{1 5} B ₂ on the two given transmit antennas within one symbol time duration have following matrix forms:

K (1)

B ₁ = ∑ SfcCrff - S^C dJ k=ι ^κ (2)

B ₂ = ∑s _k cd ₂ + s' _k e d* ^* k=l

Wherein, s , c are the spreading code and scrambling code respectively; the superscript ' is denoted that the pair of symbols (- s _k ^' c ^' d* ^* ,s _k ^' c ^' d* ^* ) is one chip time delay relative to the pair of symbols ( s _k cd[ , s _k cd ₂ ^k ); K is the number of spreading code.

If the wireless channel has L propagation paths, the channel response is denotes as:

(3)

= ψ _{χ i} h _{2 l} \ , 1=1,2,... L,

wherein λ. _l is the channel response on t path from i transmit antenna; superscript T means transpose. The received signal can be written as the following matrix forms:

L (4) r = ∑[B _! B ₂ ]Ii;

I=I = AHd + η

wherein η is the AWGN noise; the spreading matrix A in equation (4) has the form as follows:

(5)

(NQ+W+ l)x 2KL

wherein a _k = s^c , W is the maximum time delay of all propagation paths in chips; Q is the spreading factor and NQ is the length of a data block in chips corresponding to N symbols; matrix A has (NQ+W+1) rows and 2KL columns; the channel matrix H has the form as follows:

H = [Ii ₁ O i ₂ ^ h ₂ m _2K h _L ® i _2K ] ^T _2KLx4K ^

wherein ® is Kronecker tensor product; 1 ₂ κ i ^{s an} identity matrix with 2K dimension; H has 2KL rows and 4K columns; the data vector d has the form as follows:

wherein, d _γ - d ₂ d ₂ d _γ are the transmitting symbols of k ^th spreading code.

Referring to Eq.(l) and Eq.(2), the first two symbols, i.e., d* - dζ are transmitted on the first single antenna with 1 chip time duration delay between the two symbols and the second two symbols are transmitted by the second single antenna also with 1 chip time duration delay between the two symbols.

In the receiver, various schemes may be used to detect the signals. For example, one scheme is to de- spread and descramble the signals first, and then cancel interferences according to 3GPP TSG RAN WGl (03) 0760: Multi-paths Diversity for MIMO (MPD), Nortel Networks, wherein the matrix A is cancelled as below: y = (A ⁵ A)- ¹ AT ( ⁸ )

The received signal y = [ji j ^2,1 y\,l y2,l y\,L yi,U ^can be written as (noise term is ignored here):

' v1*,1 - ^~ h ⁿ l,l d ^a l ^k + ⁺ h ⁿ 2,l d ^a 2 ^k W

Then conventional method of STTD combining can be carried out to distinguish the two data streams according to Sλlamouti, "A simple transmit diversity technique for wireless communications, " IEEE J. Select. Areas Commun., vol.16, pp.1451- 1458, Oct., 1998.

Another scheme is to use an advanced transceiver structure, according to 3GPP TSG RAN WGl (03)1102: Further results on Multi-Paths Diversity for MIMO (MPD). Nortel Networks, it cancels matrix A and H together:

MMSE receiver: y = (Z 'Z + Q ² Ff ¹ ZT ( ⁿ >

wherein Z = AH , σ is the variant of the noise in MMSE receiver.

It is known from Eq.(8) and Eq.(ll) that both of the two detection schemes have the precondition that matrix A or Z should be full rank in column. That means, if the first scheme is used, the column rank of A should be 2KL; if the second scheme is used, the column rank of Z should be AK. However, if the natural propagation paths has just one chip time delay relative to the main propagation path, i.e. W=I, the precondition can not always be satisfied. This case occurs frequently according to 3GPP TR 25.996,

"Spatial channel model for Multiple Input Multiple Output (MlMO) simulations" V6.0.0. For example, if L=I, matrix A or Z only has column rank 3K, that is, one of artificial delay propagation path signal overlaps with one of natural propagation multipath signals and thus cannot satisfy the requirement of full column rank.

In above case, the receiver cannot distinguish the different propagation paths because of the overlapping between natural propagation multipath signals and artificial delay propagation multipath signals caused by the fixed delay diversity. The overlapping of propagation paths will result in strong multipath interference, which leads to system performance degradation, most seriously, it even prevents the receiver from normal operation.

An U.S. Pat. No. 6,917,597 , entitled "System and method of communication using transmit antenna diversity based upon uplink measurement for the TDD mode of WCDMA " by T.M.Schmidl, issued by JuI. 12, 2005 etc., disclosed a method of multiple antennas diversity transmission. The signals for each channel are delayed at baseband, so different delays can be used for each channel. The delay between each antenna is adaptively chosen so that the strongest paths do not overlap in order to implement full transmit antenna diversity.

As the granted patent relates to a TDD system and the transmitter obtains necessary information regarding the channel delay profile by measurement of the received uplink signals with the assumption of channel reciprocity, the method cannot be generalized to non-TDD systems. Besides this, the patent only addresses the delay between signals transmitted from different antennas, and the method did not give any indication for processing the delay between signals transmitted from a given single antenna with similar propagation paths.

Therefore, it is necessary to provide a new method of multiple antenna transmission to overcome the mentioned problem when data streams with delay diversity are transmitted from a single antenna.

SUMMARY OF THE INVENTION

Amongst others it is an object of the invention to provide a method of multiple antennas transmission for use in wireless communication systems to cancel the multipath interference caused by the overlapping of natural propagation multipath signals and artificial delay propagation multipath signals. To this end, the invention provides a transmission method for use in a first transceiver with multiple antennas in a wireless communication system, the method comprising steps: receiving a feedback message from a second transceiver; determining an

artificial delay for signals to be transmitted via each of the antennas based on the feedback message so as to avoid overlapping between corresponding natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver; generating a first pair of signals based on input data and the artificial delay, wherein the first pair of signals is obtained by combining a second pair of signals and a third pair of signals with the artificial delay relative to the second pair of signals, the second and third pairs of signals being derived from a pair of original data streams based on the input data; and transmitting the first pair of signals in parallel via an associated pair of antennas. In a preferred embodiment of the method according to the invention, the second pair of signals is a pair of non-STTD encoded symbols, while the third pair of signals is a pair of STTD-encoded symbols.

The invention further provides a transmission method for use in a second transceiver in a wireless communication system, the method comprising steps: receiving multiple signals from a first transceiver with multiple transmission antennas; estimating path-delay channel profiles based on the received signals for each propagation channel; and sending a feedback message based on the estimation results to the first transceiver, the feedback message comprising information for the first transceiver to determine artificial delay for signals to be transmitted so as to avoid overlapping between corresponding natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver.

It is a further object of the invention to provide a communication system with two transceivers for multiple antennas transmission to cancel the multipath interference caused by the overlapping of natural propagation multipath signals and artificial delay propagation multipath signals.

To this end, the invention provides a first transceiver with multiple antennas, the first transceiver comprising: a receiving unit for receiving a feedback message from a second transceiver; a control unit for determining an artificial delay for signals to be transmitted via each antenna based on the feedback message to avoid overlapping between natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver; a data process unit for generating a first pairs of signals based on input data and the artificial delay, wherein the first pair of signals is obtained by

combining a second pair of signals and a third pair of signals with the artificial delay relative to the second pair of signals, the second and third pairs of signals being derived from a pair of original data streams based on the input data; and a transmitting unit for transmitting the first pair of signals in parallel via an associated pair of antennas. In a preferred embodiment of the method according to the invention, the second pair of signals is a pair of non-STTD encoded symbols, while the third pair of signals is a pair of STTD-encoded symbols.

The invention further provides a second transceiver comprising: a receiving unit for receiving multiple signals from a first transceiver with multiple transmission antennas; an estimating unit for estimating path-delay channel profiles based on the received signals for each propagation channel; and a transmitting unit for sending a feedback message based on the estimation results to the first transceiver, the feedback message comprising information for the first transceiver to determine a artificial delay for signals to be transmitted so as to avoid overlapping between corresponding natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver.

The invention further provides a communication system comprising a first transceiver and a second transceiver, the first and second transceiver being the transceivers according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which: Fig. 1 is a block schematic diagram of a multipaths diversity transmitter;

Fig. 2 is a block schematic diagram of a multipaths diversity receiver; Fig. 3 is a flow chart illustrating an embodiment of a method of multiple antennas transmission for use in a wireless communication in according with the invention; Fig. 4 is a block schematic diagram of an embodiment of a first transceiver for multiple antennas transmission in accordance with the invention;

Fig. 5 is a block schematic diagram of an embodiment of a first transceiver for multipaths diversity transmission in accordance with the invention;

Fig. 6 is a block schematic diagram of an embodiment of a second transceiver for multiple antennas transmission in accordance with the invention; Fig. 7 is a block schematic diagram of a communication system comprising a first transceiver and a second transceiver for multiple antennas transmission in accordance with the invention.

In the figures, the same reference number represents the same, similar or corresponding feature or function.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 3 is a flow chart illustrating an embodiment of a method of multiple antennas transmission for use in a wireless communication system in accordance with the invention. The wireless communication system comprises two transceivers and in a preferred embodiment, the first transceiver is a network base station, e.g. a Node B in the third generation communication system UMTS and the second transceiver is a mobile terminal with the function provided by this invention.

According to embodiments of the present invention, the Node B obtains feedback signalling about propagation path-delay channel profile from the mobile terminal, and then based on the feedback information, determines an optimized artificial delay for delay diversity transmission in multiple antennas system. The artificial delay is chosen in such a way that the overlapping between the natural propagation multipath signals and the artificial delay propagation multipath signal can be avoided. The signalling process of this method can be detailed in following steps shown in Fig. 3. In step SlO of the signalling process, the mobile terminal receives data blocks transmitted from the Node B when data communication is set up between them.

In step S12, the mobile terminal estimates path-delay channel profile from the received data blocks by taking advantages of conventional channel estimating method. The path-delay channel profile may comprise information of current path-delay information such as power and delay of all paths as well as the prediction information related to path-delay channel profile such as the rate and direction of path changing.

Furthermore, the mobile terminal compiles the path-delay profile into a feedback message. The feedback message comprises necessary information for Node B to

determine an optimal artificial delay for multiple antennas transmission, such as for multipaths diversity transmission, based on the feedback message.

As an embodiment, the feedback message comprise at least one of following information elements based on path-delay channel profile: at least one value of preferred artificial delay; at least one preferred range from which the artificial delay is to be selected; at least one value of artificial delay to be excluded; and at least one range of artificial delay to be excluded.

As a second embodiment, the feedback message further comprises a guard period for each value or range of preferred artificial or artificial delay to be excluded. This may allow for some drifting of the propagation paths prior to the next update. Additionally, the size of the guard period may be determined as a function of the rate with which the path delays are changing, and the said function may be predetermined, determined by the mobile terminal, or determined by the Node B and signalled to the mobile terminal. Consequently, the guard period allowed on one side of a path delay may be greater than the guard period on the other side of the same path delay. In some cases, a guard period may be used only on the side of a path delay in which the path is moving.

As a third embodiment, the mobile terminal takes into account paths not present in the current path-delay profile. For example, the mobile terminal derives the path-delay prediction information based on current path-delay profile and typical propagation model in specific communication environment. The details of a typical propagation model can be obtained from references, for example, 3GPP TR 25.996, "Spatial channel model for Multiple Input Multiple Output (MIMO) simulations" V6.0.0. In this case, the feedback message may comprise path-delay prediction information, such as a rate and direction of the change of path-delay.

As a fourth embodiment, the frequency of sending the feedback message from the mobile terminal to the Node B may be varied depending on the rate of change of path delays. The frequency may be predetermined, determined by the mobile terminal, or determined by the Node B and signalled to the mobile terminal.

In step S 14, the mobile terminal sends the compiled feedback message to the Node B. Once the Node B receives the message, it can use at least one information element in the feedback message for determining an optimal artificial delay.

In step S 18, the Node B determines an artificial delay for multipath diversity transmission according to the feedback message. The main consideration of determining the artificial delay for multipath diversity transmission is to exclude the possible overlapping of the natural propagation multipath signals and the artificial delay propagation multipath signals. How to determine or optimize the artificial delay will be explained in detail later. In step S20, the Node B generates a first pair of signals based on input data bits and the artificial delay obtained in step S 14. The first pair of signals is obtained by combining a second pair of signals and a third pair of signals with artificial delay relative to the second pair of signals, the two pairs of signals being derived from an pair of original data streams based on the input data. In a preferred embodiment, the second pair of signals is a pair of non-STTD encoded symbols, while the third pair of signals is a pair of STTD-encoded symbols. The data processing in step S20 can then be detailed as:

First, the input data bits are converted into multiple parallel original pairs of data streams by serial-to-parallel converting, coding and modulating. Then a pair of non-STTD-encoded symbols (the second pair of signals) are derived from each pair of original data streams by spreading and mapping data streams to symbols, while a pair of STTD-encoded symbols (the third pairs of signals) are derived from each pair of original data streams by STTD-encoding, spreading and delaying with the determined artificial delay; and After that each pair of non-STTD-encoded symbols and the associated pair of STTD-encoded symbols are added to generate the corresponding first pair of signals to be transmitted.

In step S22, the Node B transmits each first pair of signals generated in step S20 by an associated pair of antennas. Once the mobile terminal receives those signals from the Node B, it can use conventional schemes to distinguish the two data streams and

also use the received signals to estimate path-delay channel profile for the next round multipath diversity transmission.

In order to explain the formation of the feedback message in step S 12 or determination of artificial delay in step Sl 8, an example in detail is give as below. In 3GPP HSDPA (High Speed Downlink Packet Access), spreading factor is often considered as 16. In order to reduce the interferences, different spreading codes should be used on different transmitter antennas if not wasting spread codes resource referred to section 5.2.2.2.2 in 3GPP TR 25.876, "Multiple-Input Multiple output in UTRA" Vl.3. However, if the number of spreading code channel is more than 8, same spreading codes have to be used on different transmitter antennas, that is the first and second antenna in multipaths diversity transmitter shown in Fig.1. Moreover, in general, one scrambling code is adopted for one Node B. That is to say, for multipaths diversity transmission, same spreading code and scrambling code are used when the number of spreading code channel is more than 8. In this example, number K=IO is assumed. Scrambling code is selected according to 3GPP TS 25.213, "Spreading and modulation

(FDD) (Release 6)" V6.2.0 and any of 10 columns is chosen in the following OVSF codes for spreading.

[1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1

1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1

1 1 1 1 -1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1

1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1

1 1 -1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1 1 1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1

1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1

1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1

1 1 -1 -1 1 1 -1 -1 -1 -1 1 1 -1 -1 1 1

1 -1 -1 1 1 -1 -1 1 -1 1 1 -1 -1 1 1 -1

1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1

1 -1 1 -1 -1 1 -1 1 -1 1 -1 1 1 -1 1 -1

1 1 -1 -1 -1 -1 1 1 -1 -1 1 1 1 1 -1 -1

1 -1 -1 1 -1 1 1 -1 -1 1 1 -1 1 -1 -1 1]

Propagation channel is selected according to 3GPP SCM Case 2 channel as Table 1 according to 3GPP TR 25.996, "Spatial channel model for Multiple Input Multiple Output (MIMO) simulations" V6.0.0. Comparing with the main propagation channel, the time delay of multipath are nearly 1, 3, 4, 7 and 10 chips in 3GPP FDD HSDPA (260ns/chip) system.

Table 1 3GPP SCM Case 2 channel

If the power threshold in the mobile terminal for selectable multipaths is set as 20% of power of the main propagation path, then the path with 1 delay chip is considered because of its relative high path power, i.e. W=I, L=I. In this case, if the artificial time delay is still fixed as one chip as in 3GPP TR 25.876, the matrix A and Z will not be full rank in column. That means the artificial 1 chip delay propagation path overlaps with the natural propagation path with 1 chip delay. The overlapping lead to strong interference that is difficult to be mitigated, and then the receiver cannot distinguish the two original symbols in data streams.

Using the method provided by this invention, the mobile terminal may compile above propagation path-delay information into to a feedback message and send it to the Node B. For example, the feedback message may include 2 chips delay or 2 and 3 chips delay as the value(s) of preferred artificial delay to be selected. When the Node B receives this feedback message, it may select 2 chips delay to form artificial multipath in the next transmission. In other embodiment, it is also possible the feedback message includes 1 chip delay as the value of artificial delay to be excluded. When the Node B receives this feedback message, it may exclude 1 chip delay as artificial delay and select other value, for example, 2 chip delay to form artificial multipath in the next transmission. The selection of time delay is shown in Table 2. The number in bold indicates possible overlapping between the natural propagation path with 1 chip delay (natural propagation path 2) and the artificial 1 chip delay propagation main path (artificial

delay propagation path 1). From this table, it is seen that overlapping can be removed when 2 chips artificial delay is employed for artificial multipath diversity transmission.

Table 2 Selection of time delay for two paths (number in bold means overlapping)

In further embodiment, if the power threshold in the mobile terminal is set to 10% of power of the main propagation path, the path with 1 and 3 delay chips are considered because of their relative high path power, i.e. W=3, L=3. Similar to the embodiment above, if the artificial time delay is still fixed as one chip, the matrix A will not be full rank in column, and matrix Z is probably not full rank in column, because the 1 chip artificial delay propagation path overlaps with the natural propagation path with 1 chip delay.

Using the method provided by this invention, the mobile terminal may compile above propagation path-delay information into to a feedback message and send it to the Node B. The Node B will select suitable delay to form artificial multipaths with consideration that the selected artificial delay should exclude the overlapping between corresponding natural propagation delay multipath signals and artificial delay propagation multipath signals. The reasoning of selecting process of artificial delay in Node B or compiling feedback message in mobile terminal can be shown in Table 3.

Table 3 Selection of time delay for three paths (number in bold means overlapping)

From table 3, it can be seen that when adopting 1, 2 or 3 chip as artificial delay, overlapping between natural propagation delay path and artificial delay multipath occurs in all situations at receiver side. That means the natural propagation path 2, 3, 1 will overlap artificial delay propagation delay path 1, 2 and 1 respectively. So, the feedback message may compile [1, 2, 3] chip delay as artificial delay to be excluded or [4, 5] chip delay as preferred artificial delay. The Node B will select a suitable value to from artificial multipaths.

From above description of embodiments, it is seen that with obtaining feedback information related path-delay channel profile and determining an optimal artificial delay for multiple antenna transmission provided by this invention, the overlapping between natural propagation multipath signals and artificial delay propagation multipath signals from a single given antenna is avoided, and thus system performance is improved as a whole.

The above method for multiple antenna transmission as provided in the present invention can be implemented in software or hardware, or in combination of both.

Fig. 4 is a block schematic diagram of a first transceiver 100 for use in multiple antennas transmission in accordance with an embodiment of the present invention. Fig. 5 is a block schematic diagram of a preferred embodiment of a first transceiver 100 for multipaths diversity in accordance with the invention. Fig.6 is a block schematic diagram of an embodiment of a second transceiver 200 for multiple antenna transmission in accordance with the invention.

In Fig. 4, the first transceiver 100 comprises a receiving unit 110 for receiving a feedback message from a second transceiver 200; a control unit 120 for determining an artificial delay for signals to be transmitted via each antenna based on the feedback message to avoid overlapping between natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver 200; a data process unit 130' for generating a first pairs of signals based on input data and the

artificial delay, wherein the first pair of signals is obtained by combining a second pair of signals and a third pair of signals with the artificial delay relative to the second pair of signals, the second and third pairs of signals being derived from a pair of original data streams based on the input data; and a transmitting unit 140 for transmitting the first pair of signals in parallel via an associated pair of antennas.

From Fig. 5, it is seen that the function of the data processor 130' is similar to the data processor 130 in Fig.l and the only difference is the delay value in delay circuit 118' is variable and determined by the control unit 120. As the feedback message comprises necessary information for determining the artificial delay of multipath diversity, the overlapping between natural propagation multipath signals and artificial delay propagation multipath signals received at the second transceiver 200 is avoided, which excluding the interference between the natural propagation multipath signals and artificial delay propagation multipath signals and thus improve system performance as a whole. In Fig.6, the second transceiver 200 comprises: a receiving unit 230 for receiving multiple signals from a first transceiver 100 with multiple transmission antennas; an estimating unit 240 for estimating path-delay channel profile based on the received signals for each propagation channel; and a transmitting unit 250 for sending a feedback message based on the estimation results to the first transceiver 100, the feedback message comprising information for the first transceiver 100 to determine an optimal artificial delay for signals to be transmitted so as to avoid overlapping between corresponding natural propagation multipath signals and artificial delayed propagation multipath signals received at the second transceiver 200.

Fig.7 is a block schematic diagram of a communication system 10 comprising a first transceiver 100 and a second transceiver 200 provided by the present invention. In a preferred embodiment, the first transceiver 100 is a Node B and the second transceiver 200 is a mobile terminal in accordance with the invention.

Based on above description, it can be concluded that the first transceiver provided by this invention can obtain necessary information related to path-delay channel profile from the second transceiver and then determine an optimal artificial delay for multipaths diversity transmission on each single given antenna. The artificial delay is chosen so that the overlapping between the natural propagation delay

multipath signals and the artificial delay propagation multipath signals will be avoided and thus system performance is improved as a whole.

Although the present invention has been described primarily with reference to data transmissions on a downlink it is to be understood that the teachings of the present invention can be applied to uplink transmissions and to systems other than UMTS.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" or "comprises" does not exclude the presence of other elements or steps than those listed.

The embodiments of the present invention described herein are intended to be taken in an illustrative and not a limiting sense. Various modifications may be made to these embodiments by those skilled in the art without departing from the scope of the present invention as defined in the appended claims.