WO2009050636A2

WO2009050636A2 - Robust cyclic delay diversity scheme

Info

Publication number: WO2009050636A2
Application number: PCT/IB2008/054200
Authority: WO
Inventors: Semih Serbetli
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-10-19
Filing date: 2008-10-13
Publication date: 2009-04-23
Anticipated expiration: 2010-04-19
Also published as: WO2009050636A3; TW200935792A

Abstract

The invention relates to a transmitter, a transmission system, a transmission method and a computer program product enabling a robust Cyclic Delay Diversity (CDD) scheme for transmissions such as - but not restricted to - wireless transmissions. The robust CDD scheme can be achieved by inserting an artificial multipath channel in a transmitter chain before an actual transmission.

Description

Robust cyclic delay diversity scheme

FIELD OF THE INVENTION

The present invention generally relates to a transmitter, a transmission system, a transmission method and a computer program product enabling a robust Cyclic Delay Diversity (CDD) scheme for transmissions such as - but not restricted to - wireless transmissions.

BACKGROUND OF THE INVENTION

Wireless transmission systems are almost omnipresent today. For example, Wireless Local Area Networks (WLANs) as defined e.g. in IEEE 802.11 based standards, cellular networks and broadcasting systems are widely used nowadays. The increase of throughput of the available channel is one major issue for such wireless transmission systems. By employing Orthogonal Frequency Division Multiplexing (OFDM), a higher throughput can be achieved. In OFDM, the given system bandwidth is split into many orthogonal subchannels, also referred to as subcarriers. Instead of transmitting data symbols sequentially through one (very broad) channel, multiple data symbols are transmitted in parallel. This leads to much longer symbol durations, such that the impact of Inter-Symbol Interference (ISI) can be reduced significantly, so that no additional measures like costly equalization are necessary.

A potential to further increase the performance of wireless transmission systems in terms of throughput and reliability is seen in multiple antenna transmission techniques. Such techniques are used e.g. in Multiple Input - Multiple Output (MIMO) antenna systems. These systems employ multiple antennas at each of a transmitter and a receiver. A plurality of channels between the transmitter and the receiver may be simultaneously established. As a result, the throughput can be increased and the Bit Error Rate (BER) can be reduced. It is intended to include MIMO techniques e.g. in the future

WLAN standard IEEE 802.1 In, in future Worldwide Interoperability for Microwave Access (WiMAX) standards based on IEEE 802.16 and in future cellular network standards.

CDD has been proposed for multiple antenna transmission systems to increase the robustness of a transmission, i.e. to improve the reliability of the transmission. CDD provides a simple and elegant method to provide transmit diversity while being backward compatible. The idea behind CDD is to increase the frequency selectivity and therefore the frequency diversity, by transmitting cyclically delayed versions of a time domain signal simultaneously from different antennas. Basically, it converts the spatial selectivity (diversity) of the antennas to frequency selectivity (diversity). The delays are done in a cyclic manner so as not to lengthen the channel and exceed a Guard Interval (GI) used to ensure that distinct transmissions do not interfere with one another. CDD provides an increase in the frequency selectivity that can be exploited by using channel coding. It is already contained in the current draft of IEEE 802.1 In and has the potential to be included in multiple antenna wireless systems such as defined e.g. in next generation WLAN standards, WiMAX standards, cellular network standards and broadcasting standards.

G. Bauch, J. S. Malik, "Cyclic Delay Diversity with Bit-Interleaved Coded Modulation in Orthogonal Frequency Division Multiple Access", IEEE Transactions On Wireless Communications, volume 5, pages 2092-2100, August 2006, discloses CDD in an OFDM system comprising an OFDM transmitter and an OFDM receiver. Such OFDM transmitter and receiver will be described in the following.

Fig. 5 shows a schematic block diagram of an OFDM transmitter using CDD in coded OFDM. The transmitter comprises a Forward Error Correction (FEC) encoder 50, an interleaver 51 (block "FI"), a Quadrature Amplitude Modulation (QAM) / Phase Shift Keying (PSK) mapper 52 (block "QAM/PSK"), an Inverse Fast Fourier Transformation

(IFFT) transformer 53 (block "IFFT"), nj transmit branches 541 to 54nτ, nτ-1 cyclic shifters 552 to 55n_τ (blocks "Δ₂" to "Δ_nT"), n_τ GI adders 561 to 56n_τ (blocks "GI") and n_τ transmit antennas 571 to 57nχ. A receive antenna 58 of a receiver is depicted in Fig. 5. Further, x₀ ,

X₁ , x₂ and x₃ denote OFDM symbol samples, h/¹-* denotes a channel from the i-th transmit antenna 57i to the receive antenna 58 and A₁ denotes a cyclic shift applied at the i-th transmit antenna 57i.

The FEC encoder 50, the interleaver 51, the QAM/PSK mapper 52 and the IFFT transformer 53 are connected in series. Each of the nj transmit branches 541 to 54nτ is connected to the IFFT transformer 53 and includes a GI adder. The second to nχ-th transmit branches 542 to 54nτ respectively comprise a cyclic shifter placed before the GI adder. The first transmit branch 541 does not contain a cyclic shifter since a cyclic shift A₁ applied at the first transmit antenna 571 can be set to zero without a loss of generality. Each of the nj transmit antennas 571 to 57nτ is connected to a corresponding transmit branch of the nj transmit branches 541 to 54nχ.

An input data stream is encoded in the FEC encoder 50, interleaved by the interleaver 51, mapped by the QAM/PSK mapper 52 and transformed from the frequency domain into the time domain by the IFFT transformer 53. A resulting OFDM signal is conveyed on each of the nj transmit branches 541 to 54nχ. In each of the second to nχ-th transmit branches 542 to 54nτ the respective cyclic shifter cyclically shifts OFDM symbols of the respective OFDM signal conveyed on the transmit branch. The original OFDM signal is conveyed on the first transmit branch 541. Each of the GI adders 561 to 56nτ adds a GI (Cyclic Prefix (CP)) to the OFDM signal conveyed on the transmit branch including the respective GI adder. More specifically, the GI adders 561 to 56nτ insert a GI between OFDM symbols. Each OFDM signal resulting from the GI addition is transmitted to the receive antenna 58 via a channel between the respective transmit antenna and the receive antenna 58.

With the above described transmitter as illustrated in Fig. 5, each transmit antenna transmits a cyclically shifted version of the same OFDM symbol. In this way, a previously proposed CDD scheme as included in the current draft of IEEE 802. Hn is implemented.

Fig. 6 shows a schematic block diagram of an OFDM receiver for receiving signals from a transmitter such as illustrated in Fig. 5. The receiver comprises a receive antenna 60, a GI remover 61 (crossed out block "GI"), a Fast Fourier Transformation (FFT) transformer 62 (block "FFT"), a QAM/PSK demapper 63, a deinterleaver 64 (block "IT¹") and a FEC decoder 65. These elements are connected in series.

OFDM signals transmitted by a transmitter such as illustrated in Fig. 5 via channels from transmit antennas to the receive antenna 60 are received by the receiver. The GI remover 61 removes GIs from the received OFDM signals. Resulting signals are transformed from the time domain into the frequency domain by the FFT transformer 62, demapped by the QAM/PSK demapper 63, deinterleaved by the deinterleaver 64 and decoded by the FEC decoder 65.

Previously proposed CDD schemes such as e.g. the CDD scheme implemented with the transmitter illustrated in Fig. 5 and included in the current draft of IEEE 802.1 In are based on spatial diversity (selectivity) to increase the frequency selectivity. For OFDM multiple antenna transmission systems, CDD can be an effective way to increase the frequency selectivity of the channel and to increase the robustness of the system. The channel becomes more frequency selective due to the artificially increased channel length. Therefore, error correcting codes can pick the more reliable symbols and the robustness increases.

However, if the environment of a multiple antenna transmission system is not rich scattering and/or there is Line Of Sight (LOS) between a transmitter and a receiver, i.e. a strong LOS component, the channels from different transmit antennas to the receive antenna(s) become highly correlated. That is, the channels that each antenna will experience will be highly correlated, and the spatial diversity (selectivity) will be very limited. In such scenarios, applying previously proposed CDD schemes nulls a subset of subcarriers, and the performance gets even worse than with single antenna transmission systems. In the extreme case of a single LOS component and the channels for each antenna being the same, the previously proposed CDD schemes will lead to nulling odd subcarriers while doubling the channel gain of even subcarriers. In this case, half of the transmitted data symbols are lost, and the channel coding may not be able to recover these lost data symbols.

Fig. 7 shows a graph illustrating a combined channel frequency response for an OFDM transmitter such as shown in Fig. 5, an OFDM receiver such as shown in Fig. 6 and a LOS case with two transmit antennas. The horizontal axis represents a number of a subcarrier of an OFDM transmission, wherein 0-th to (N₈-I )-th subcarriers exist. The vertical axis represents a value |H(f)| of a channel transfer function. Highly correlated channels from each transmit antenna to the receive antenna(s) are assumed. An example for a LOS scenario with a poor scattering environment, a number of transmit antennas N_t=2 and a shift of is illustrated. The combined channel frequency response for both transmit antennas when using a previously proposed CDD scheme is depicted. As can be gathered from the graph, |H(f)|=0 for odd subcarriers. That is, a subset of the subcarriers is completely nulled. This results in a performance degradation due to the nulling effect of the CDD scheme.

It is desirable to prevent the above described kind of performance degradation in LOS or highly correlated antenna scenarios due to the high correlation among the antennas while maintaining the benefits of CDD in rich scattering environments. That is, it is desirable to provide a robust CDD scheme avoiding the performance loss in LOS and/or poor scattering scenarios while maintaining the increased frequency selectivity in rich scattering environments. SUMMARY OF THE INVENTION

It is an object of the invention to provide a robust CDD scheme capable of alleviating at least some of the above described disadvantages of previously proposed CDD schemes. This object is achieved by a transmitter according to claim 1 and a transmission method according to claim 19.

In a first aspect of the invention a transmitter comprises a plurality of transmit branches respectively configured to convey a signal, at least one processor configured to process, by means of a respective multipath channel, at least one signal conveyed on at least one transmit branch of the plurality of transmit branches, and a plurality of outputs respectively configured to output a signal supplied by a corresponding transmit branch.

In a second aspect of the invention a transmission system comprises at least one transmitter according to the first aspect and at least one receiver.

In a third aspect of the invention a transmission method comprises conveying signals on a plurality of transmit branches, processing, by means of a respective multipath channel, at least one signal conveyed on at least one transmit branch of the plurality of transmit branches, and outputting a signal supplied by a corresponding transmit branch.

In a fourth aspect of the invention a computer program product for a computer comprises software code portions for performing the steps of a method according to the third aspect when the product is run on the computer.

The first to fourth aspects enable to increase the spatial diversity (spatial selectivity). As a result, the applied CDD scheme will result in a frequency selective channel even in LOS and poor scattering environments. Thus, transmissions will be robust to any environment and will not experience problems in LOS and poor scattering environments due to CDD. Hence, a robust CDD scheme can be provided.

Further advantageous modifications are defined in the dependent claims.

These and other aspects of the invention will be apparent from and elucidated by embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described on the basis of embodiments with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic block diagram of an exemplary transmitter using a CDD scheme with multipath channels according to a first embodiment; Fig. 2 shows a schematic block diagram illustrating the construction of an exemplary multipath channel for an i-th transmit branch of a transmitter according to the first embodiment;

Fig. 3 shows a schematic block diagram of an exemplary transmitter using a CDD scheme with multipath channels according to a second embodiment;

Fig. 4 shows a flow chart illustrating basic steps of an exemplary transmission method according to the first and second embodiments;

Fig. 5 shows a schematic block diagram of an OFDM transmitter using CDD in coded OFDM; Fig. 6 shows a schematic block diagram of an OFDM receiver; and

Fig. 7 shows a graph illustrating a combined channel frequency response in a LOS case with two transmit antennas.

DETAILED DESCRIPTION OF EMBODIMENTS A first embodiment of the invention will be described in the following. Fig. 1 shows a schematic block diagram of an exemplary transmitter using a CDD scheme with multipath channels according to the first embodiment. The transmitter may be an OFDM transmitter using coded OFDM. It can comprise an encoder such as e.g. a FEC encoder 10, an interleaver 11 (block "FI"), a mapper 12 such as e.g. a QAM/PSK mapper (block "QAM/PSK"), a transformer such as e.g. an IFFT transformer 13 (block "IFFT"), n_τ transmit branches 141 to 14n_τ, n_τ-l cyclic shifters 152 to 15n_τ (blocks "Δ₂" to "Δ_nT"), n_τ GI adders 161 to 16nτ (blocks "GI"), nj (artificial) multipath channels (tapped delay line filters or tapped delay lines) 171 to 17nτ (blocks ¹¹W₁(X)" to "w_nτ(t)") acting as processors (processing means) and nj transmit antennas 181 to 18nτ acting as outputs. An exemplary receive antenna 19 of a receiver is depicted in Fig. 1. Further, x₀ , X₁ , x₂ and x₃ denote data symbol samples, h/¹-* denotes a channel from the i-th transmit antenna 18i to the receive antenna 19 and A₁ denotes a cyclic shift applied at the i-th transmit antenna 18i.

The FEC encoder 10 can encode an input data stream and output a result. The interleaver 11 may interleave an input supplied by the FEC encoder 10 and output a result. The QAM/PSK mapper 12 can map an input supplied by the interleaver 11 and output a result. The IFFT transformer 13 may transform an input supplied by the QAM/PSK mapper 12 from the frequency domain into the time domain and output a result. A signal supplied by the IFFT transformer 13 can be conveyed on each of the nj transmit branches 141 to 14nχ. In each of the second to nτ-th transmit branches 142 to 14nτ the respective cyclic shifter may cyclically shift data symbols of the respective signal conveyed on the transmit branch. The first transmit branch 141 does not contain a cyclic shifter since a cyclic shift A₁ applied at the first output 181 can be set to zero without a loss of generality . Thus, the original signal can be conveyed on the first transmit branch 141 without performing a cyclic shift.

The i-th GI adder 16i may add a GI (CP) to a signal conveyed on the i-th transmit branch 14i and supplied by the i-th cyclic shifter 15i or supplied by the IFFT transformer 13 without a cyclic shift as in case of the first transmit branch 141. More specifically, each of the GI adders 161 to 16nτ can insert a GI between data symbols of the respective signal. A result can be output. Each of the processors 171 to 17nτ may process a time domain signal supplied by the corresponding GI adder. More specifically, the respective time domain signal can be linearly convolved with the respective artificial multipath channel (tapped delay line). Thus, each of the elements 171 to 17nτ may be considered as a linear convolver with a respective (artificial) multipath channel (tapped delay line). In other words, data symbols of a signal conveyed on an i-th transmit branch 14i may be cyclically shifted in a first step, as with previously proposed CDD schemes. In a second step just before the transmission the signal resulting from the cyclic shifting can be linearly convolved with an i-th (short) artificial multipath channel (tapped delay line) 17i with random or pseudorandom weights, i.e. w^t), and a result can be supplied to the i-th output 18i of the transmitter. In case the i-th output 18i is a transmit antenna, the supplied signal may be transmitted to the receive antenna 19 via a channel between the i-th transmit antenna 18i and the receive antenna 19.

Fig. 2 shows a schematic block diagram illustrating the construction of an exemplary artificial multipath channel (tapped delay line) for an i-th transmit branch of a transmitter according to the first embodiment. As illustrated in Fig. 2, a block ^MWi(t)" as shown in Fig. 1 (wherein i=l, 2, ..., nj) can equal a tapped delay line comprising L-I delayers 202 to 2OL, L multipliers 211 to 2 IL and an adder 22. An input signal may be transmitted via all delayers 202 to 2OL. After each delayer a signal delayed by this delayer and all preceding delayers can be tapped and supplied to a respective multiplier. That is, after the first delayer 202 a signal which has been delayed by the first delayer 202 may be tapped and supplied to the second multiplier 212, after the second delayer 203 a signal which has been delayed by the first delayer 202 and the second delayer 203 can be tapped and supplied to the third multiplier 213 and so on. After the (L-l)-th delayer 2OL a signal which has been delayed by all L-I delayers may be supplied to the L-th multiplier 2 IL. In addition, before the first delayer 202 a signal can be tapped and supplied to the first multiplier 211, wherein this signal is not delayed because there are no preceding delayers in this case. Each multiplier 2 Ij (wherein j=l, 2, ..., L) may multiply the signal supplied to the same by a random or pseudorandom weight w_y and output a result. The results from all multipliers 211 to 21 L can be supplied to the adder 22. The adder 22 may add inputs supplied by the multipliers 211 to 2 IL and output a resulting signal. This signal is a sum of multiple delayed versions of the input signal with multiple gains.

According to the first embodiment, an artificial multipath channel (tapped delay line) may be inserted into each transmit branch together with CDD in the transmitter chain before an actual transmission. The tapped delay line applied to each output can be different. Thus, a different artificial channel may be provided for each output of the transmitter. Hence, each output can experience a different short channel, which results in less correlated channel frequency responses. Since the channel frequency responses are less correlated, the CDD scheme applied in the transmitter will not nullify subcarriers as frequently as previously proposed CDD schemes, while a higher frequency selectivity may still be maintained. With this scheme, CDD applied to the outputs will not have a nulling effect in LOS scenarios, while the benefits of CDD in rich scattering environments are maintained

With the first embodiment, the artificial multipath channel (tapped delay line) can be applied after the GI (CP) insertion. Thus, it will lengthen the channel. Hence, the length of the tapped delay line may be chosen short so as not to exceed the GI length.

A second embodiment of the invention will be described in the following. Fig. 3 shows a schematic block diagram of an exemplary transmitter using a CDD scheme with multipath channels according to a second embodiment. The transmitter may be an OFDM transmitter using coded OFDM. It can comprise a FEC encoder 30, an interleaver 31 (block "IT), a mapper 32 such as e.g. a QAM/PSK mapper (block "QAM/PSK"), an IFFT transformer 33 (block "IFFT"), nj transmit branches 341 to 34nτ, nτ-1 cyclic shifters 352 to 35nτ (blocks "A₂" to "Δ_nτ"), nj cyclic (circular) convolvers with (artificial) multipath channels (tapped delay line filters or tapped delay lines) 371 to 37nτ (blocks ¹¹W₁(X)" to "w_nτ(t)") acting as processors (processing means), nj GI adders 361 to 36nτ (blocks "GI") and nj transmit antennas 381 to 38nτ acting as outputs. An exemplary receive antenna 39 of a receiver is depicted in Fig. 3. Further, x₀ , X₁ , x₂ and x₃ denote data symbol samples, h/¹-* denotes a channel from the i-th transmit antenna 38i to the receive antenna 39 and A₁ denotes a cyclic shift applied at the i-th transmit antenna 38i. The FEC encoder 30 can encode an input data stream and output a result. The interleaver 31 may interleave an input supplied by the FEC encoder 30 and output a result. The QAM/PSK mapper 32 can map an input supplied by the interleaver 31 and output a result. The IFFT transformer 33 may transform an input supplied by the QAM/PSK mapper 32 from the frequency domain into the time domain and output a result. A signal supplied by the IFFT transformer 33 can be conveyed on each of the nj transmit branches 341 to 34nχ. In each of the second to nτ-th transmit branches 342 to 34nτ the respective cyclic shifter may cyclically shift data symbols of the respective signal conveyed on the transmit branch. The first transmit branch 341 does not contain a cyclic shifter since a cyclic shift A₁ applied at the first output 381 can be set to zero without a loss of generality . Thus, the original signal can be conveyed on the first transmit branch 341 without performing a cyclic shift.

The i-th processor 37i may process a time domain signal conveyed on the i-th transmit branch 34i and supplied by the i-th cyclic shifter 35i or supplied by the IFFT transformer 33 without a cyclic shift as in case of the first transmit branch 341. More specifically, the respective time domain signal can be cyclically (circularly) convolved with the respective artificial multipath channel (tapped delay line). A result can be output. In other words, data symbols of a signal conveyed on an i-th transmit branch 34i may be cyclically shifted in a first step, as with previously proposed CDD schemes. In a second step just before the insertion of the GI and the transmission the signal resulting from the cyclic shifting can be cyclically (circularly) convolved with an i-th artificial multipath channel (tapped delay line) with random or pseudorandom weights, i.e. W₁ (wherein W₁=[W₁I w_l2 W₁₃ ... W₁L]). An output of a Cyclic (Circular) convolution (CC) of an N-block symbol vector [ x₀ , X₁ , X₂ , ..., x_N-ι ] with the artificial multipath channel (tapped delay line) W₁=[W₁,! Wi,₂ W₁^ ... Wi₁J, will be

JV-I

*„_,, = ∑X_mW₁χ_n-_{m)mOd N}, Q ^≤ n ^≤ N ^{~ ~} 1 m=0

The i-th GI adder 36i may add a GI (CP) to a signal supplied by the i-th processor 37i. More specifically, each of the GI adders 36i to 36nτ can insert a GI between data symbols of the respective signal. A result can be supplied to the i-th output 38i of the transmitter. In case the i-th output 38i is a transmit antenna, the supplied signal may be transmitted to the receive antenna 39 via a channel between the i-th transmit antenna 38i and the receive antenna 39.

According to the second embodiment, an artificial multipath channel (tapped delay line) may be inserted into each transmit branch together with CDD in the transmitter chain before an actual transmission. The tapped delay line applied to each output can be different. Thus, a different artificial channel may be provided for each output of the transmitter. Hence, each output can experience a different short channel, which results in less correlated channel frequency responses. Since the channel frequency responses are less correlated, the CDD scheme applied in the transmitter will not nullify subcarriers as frequently as previously proposed CDD schemes, while a higher frequency selectivity may still be maintained. With this scheme, CDD applied to the outputs will not have a nulling effect in LOS scenarios, while the benefits of CDD in rich scattering environments are maintained

With the second embodiment, the artificial multipath channel (tapped delay line) can be applied before the GI (CP) insertion. Therefore, the cyclic (circular) convolution rather than the linear one may be implemented. This scheme will have a similar effect as CDD. Further, it does not affect ISI issues at all, so that the length of the tapped delay line can be chosen regardless of the GI length. That is, the length of the tapped delay line may be chosen longer as compared with the first embodiment, while still avoiding ISI effects. The arrangements described above and shown in Fig. 1 and Fig. 3 are merely examples of transmitters according to the first and second embodiments. The arrangements are to be considered illustrative or exemplary and not restrictive. For example, even if Fig. 1 and Fig. 3 show that each transmit branch includes a processor, fewer processors or even only one processor may be sufficient for providing an advantageous effect in terms of robustness. Further, processors configured to process multiple signals on multiple transmit branches may be used. Moreover, a fewer number of guard interval adders and/or cyclic shifters than that illustrated in Fig. 1 and Fig. 3 can be possible. Furthermore, even if only one receive antenna is shown in each of Fig. 1 and Fig. 3, there can be multiple receive antennas. That is, a transmission system comprising a transmitter according to the first or second embodiment may be e.g. a Multiple Input - Single Output (MISO) system or a MIMO system. Moreover, other mappers than a QAM/PSK mapper can be used. For example, an Amplitude Shift Keying (ASK) mapper may be employed. Other modifications such as e.g. dispensing with the interleaver (block "FI") are conceivable. The above described CDD schemes according to the first and second embodiments can be used in OFDM or single carrier with CP systems. They may also be employed in other kinds of transmission systems enabling multiple input transmissions such as e.g. systems implementing MIMO or MISO technologies. A transmitter according to the first or second embodiment can be part of a transmission system comprising at least one transmitter and at least one receiver. A receiver may e.g. be configured such as the receiver illustrated in Fig. 6, wherein the receiver may comprise a single input or receive antenna as shown in Fig. 6 or multiple inputs or receive antennas. Each of the transmitter and receiver can e.g. be a stationary or portable terminal device. For example, each of the transmitter and the receiver may be an access point of a WLAN or user equipment such as e.g. a modem, a data card etc. for enabling access to a WLAN as well as a stationary computer or notebook provided with such modem, data card etc. or a built-in capability of accessing a WLAN. A base station of a cellular network or a broadcasting station and user equipment such as e.g. a mobile phone or a stationary or portable TV set can be other examples. The transmission system may e.g. be a WLAN, a cellular network or a broadcasting system.

The above described functionality of the first and second embodiments can be implemented in a transmission method. The steps of such method may be performed by software code portions of a computer program product for a computer when the product is run on the computer.

Fig. 4 shows a flow chart illustrating basic steps of an exemplary transmission method according to the first and second embodiments. In a step Sl signals are conveyed on a plurality of transmit branches. In a step S2 at least one signal conveyed on at least one transmit branch of the plurality of transmit branches is processed by means of a respective multipath channel. In a step S3 a signal supplied by a corresponding transmit branch is output.

With the CDD schemes according to the first and second embodiments, the spatial diversity (spatial selectivity) can be increased. Thus, the applied CDD schemes will result in a frequency selective channel even in LOS and poor scattering environments. Hence, transmissions will be robust to any environment and will not experience any problems in LOS and poor scattering environments due to CDD.

The proposed robust CDD schemes according to the first and second embodiments are expected to work especially in frequency flat / less frequency selective channels. They can maintain the advantage of an increased frequency selectivity as also obtained with previously proposed CDD schemes, avoid the nulling effect of such schemes in highly correlated channels and increase the channel length more.

With the CDD schemes according to the first and second embodiments, no information about the channel is required. Further, signals do not have to be processed in the spatial domain. No beamforming is needed. In addition, the applicable CDD length has no maximum limit. There may only be a maximum limit for the number of taps of the tapped delay line in the first embodiment so as to avoid ISI effects. Even such limit for the number of taps does not exist for the second embodiment, as a CC rather than a linear convolution is applied with this embodiment.

The above described first and second embodiments provide alternative architectures for CDD schemes. In contrast to previously proposed CDD schemes for multiple antenna transmission systems such as e.g. the CDD scheme contained in the current draft of IEEE 802.1 In, they are capable of avoiding a performance loss in LOS and/or poor scattering scenarios while enabling an increased frequency selectivity in rich scattering environments.

The invention is applicable to all multiple input transmission systems. It may be especially interesting for wireless transmission systems such as WLANs based on e.g. the future IEEE 802.1 In standard, WiMAX systems, cellular networks and broadcasting systems such as e.g. Digital Television (DTV), in particular next generation wireless systems and standards implementing MIMO or MISO technologies. The invention might even be applied to wired transmission systems, provided that they implement MIMO or MISO technologies in some way. It might even be contemplated to apply the invention to other kinds of transmission systems. While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope thereof.

In summary, the invention relates to a transmitter, a transmission system, a transmission method and a computer program product enabling a robust Cyclic Delay Diversity (CDD) scheme for transmissions such as - but not restricted to - wireless transmissions. The robust CDD scheme can be achieved by inserting an artificial multipath channel in a transmitter chain before an actual transmission.

Claims

CLAIMS:

1. A transmitter comprising:

- a plurality of transmit branches (141, 142, ..., 14n_τ; 341, 342, ..., 34n_τ) respectively configured to convey a signal;

- at least one processor (171, 172, ... , 17n_τ; 371 , 372, ... , 37n_τ) configured to process, by means of a respective multipath channel, at least one signal conveyed on at least one transmit branch of said plurality of transmit branches; and

- a plurality of outputs (181, 182, ..., 18n_τ; 381, 382, ..., 38n_τ) respectively configured to output a signal supplied by a corresponding transmit branch.

2. A transmitter according to claim 1, wherein said respective multipath channel is an artificial multipath channel.

3. A transmitter according to claim 1, wherein said respective multipath channel is a tapped delay line.

4. A transmitter according to claim 3, wherein said tapped delay line comprises:

- a plurality of delayers (202, 203, ..., 20L) in said delay line respectively configured to delay a signal; - a plurality of multipliers (211, 212, ..., 21L) respectively configured to multiply a signal tapped from the delay line before a first delayer of said plurality of delayers or after one of the first to (L-l)-th delayer of said plurality of delayers with a weight (w,i, w_l2, ..., w_lL); and

- an adder (22) configured to add signals supplied by said plurality of multipliers.

5. A transmitter according to claim 4, wherein said weight is a random or pseudorandom weight.

6. A transmitter according to claim 1, wherein said at least one processor is placed in said at least one transmit branch.

7. A transmitter according to claim 1, wherein said transmitter comprises as many processors as transmit branches and each processor is placed in a corresponding transmit branch.

8. A transmitter according to claim 1, further comprising:

- at least one cyclic shifter (152, ..., 15nT; 352, ..., 35nT) configured to cyclically shift data symbols of said at least one signal, wherein said at least one processor is configured to process said data symbols cyclically shifted by said at least one cyclic shifter.

9. A transmitter according to claim 8, wherein said transmitter comprises a cyclic shifter for each transmit branch of said plurality of transmit branches except for a first transmit branch and each cyclic shifter is placed in a corresponding transmit branch.

10. A transmitter according to claim 1, further comprising:

- at least one guard interval adder (161, 162, ..., 16nT) placed before said at least one processor and configured to add guard intervals to said at least one signal.

11. A transmitter according to claim 10, wherein said at least one processor is a linear convolver configured to linearly convolve a signal supplied by said at least one guard interval adder with said respective multipath channel.

12. A transmitter according to claim 1, wherein said at least one processor is a cyclic convolver configured to cyclically convolve said at least one signal with said respective multipath channel.

13. A transmitter according to claim 12, further comprising:

- at least one guard interval adder (361, 362, ..., 36nT) placed after said at least one processor and configured to add guard intervals to a signal supplied by said at least one processor.

14. A transmitter according to claim 10 or claim 13, wherein said transmitter comprises as many guard interval adders as transmit branches and each guard interval adder is placed in a corresponding transmit branch.

15. A transmitter according to claim 1, further comprising:

- an encoder (10; 30) configured to encode an input data stream;

- an interleaver (11; 31) configured to interleave a signal supplied by said encoder;

- a mapper (12; 32) configured to map a signal supplied by said interleaver; and

- a transformer (13; 33) configured to transform a signal supplied by said mapper from the frequency domain into the time domain and supply a resulting signal to each transmit branch of said plurality of transmit branches.

16. A transmission system comprising :

- at least one transmitter according to claim 1 ; and

- at least one receiver.

17. A system according to claim 16, wherein said at least one transmitter and said at least one receiver are at least a part of a wireless network.

18. A system according to claim 16, wherein said system is configured to perform multiple input - multiple output transmissions.

19. A transmission method comprising:

- conveying (Sl) signals on a plurality of transmit branches (141, 142, ..., 14n_τ; 341, 342, ..., 34n_τ);

- processing (S2), by means of a respective multipath channel, at least one signal conveyed on at least one transmit branch of said plurality of transmit branches; and - outputting (S3) a signal supplied by a corresponding transmit branch.

20. A computer program product for a computer, comprising software code portions for performing the steps of a method according to claim 19 when said product is run on said computer.