WO2018065039A1

WO2018065039A1 - Spectrally efficient multi-stream communication device and method

Info

Publication number: WO2018065039A1
Application number: PCT/EP2016/073738
Authority: WO
Inventors: Nikola VUCIC; Jian Luo; Mario Castaneda; Iwanow MARCIN
Original assignee: Huawei Technologies Duesseldorf Gmbh
Priority date: 2016-10-05
Filing date: 2016-10-05
Publication date: 2018-04-12
Also published as: CN109845127A

Abstract

A communication device (41) for generating a multi- stream transmission signal based upon an input signal is provided. The communication device (41) comprises a beam delay aligner (433), which is configured to determine transmission delays of a plurality of transmission beam channels (43) of the multi-stream transmission signal to a further communication device (42) and delay transmission beam signals corresponding to the transmission beam channels of the multi-stream transmission signal, compensating for the determined transmission delays. The transmission beam signals are derived from the input signal.

Description

Spectrally efficient multi-stream communication device and method

TECHNICAL FIELD The invention relates to a communication device and method, especially in a mmWave MIMO setting.

BACKGROUND In order to satisfy the ever increasing data rate demands in wireless communications, one can exploit mm Wave frequencies, as there are large portions of underutilized spectrum in these bands. However, there exist several issues related to communication in the mmWave spectrum. As mmWave channels exhibit high path loss, the communication has to resort to beamforming with large antenna arrays in order to overcome this effect. Further, the transceiver architectures have to take into account the power consumption of electronic components such as analogue- to-digital converters (ADCs) and digital-to-analogue converters (DACs), which rise with the increased sampling frequency. Finally, mmWave channels exhibit specific features such as a certain degree of sparsity and fast time-varying nature (Doppler effect). In order to reduce computational and hardware complexity, two stage transmitter (Tx) and receiver (Rx) architectures are considered for mmWave communications. One stage is a frequency independent block, performing Tx or Rx beamforming, which can be implemented in analogue or digital domains. This stage can be consided at optional at the Tx or at the Rx. The other stage is a fully digital precoder or equalizer at the Tx or the Rx, respectively. An illustration of this architecture is given in Fig. 1.

A communications system 10 comprises a transmitter 11 and a receiver 12. The

communication occurs through a channel 13. The transmitter 11 comprises a first precoding stage 111 connected to a second beamforming stage 112, which is connected to a transmitter antenna array 113. The receiver 12 comprises a receiver antenna array 121, which is connected to a first beamforming stage 122, which in turn is connected to a second equalization stage 123. Both stages can operate with multiple data streams, i.e., support MIMO transmission. The number of data streams is usually significantly smaller than the number of transmit or receive antennas. In a wireless access scenario, the Tx and Rx can be asymmetric having antenna arrays of different sizes.

Finally, similarly to other commercially used systems, mmWave communication systems can employ block based transmission, with inserted redundant blocks ensuring that no interblock interference appears due to the multipath wireless channel. Channel estimation in mmWave systems is an intricate problem due to the very large antenna arrays involved, and due to the rapidly changing mmWave channels. For this reason, designing mmWave transmitters which do not require full transmit channel state information is of particular practical interest. In a semi open loop type of transmission mode, the adaptive transmitter relies typically only on slowly changing channel characteristics, which are easier to estimate, while the receiver can have more accurate channel knowledge.

Adding redundancy in block transmission schemes can remove interblock interference and lead to very effective methods for mitigating the problem of intersymbol interference (ISI) in one block. The amount of redundancy to be added is related directly to the mmWave channel delay spread. The effective channel delay spread can be reduced by employing Tx or Rx beamforming (BF). However, in mmWave MIMO transmission, different data streams can travel over very different physical paths. Therefore, the resulting MIMO channel impulse response might still be quite long if standard beamforming techniques are applied, which leads to large redundancy blocks, or guard periods (GPs). The spectrally efficient mmWave MIMO transmission considered in this invention aims at reducing this GP overhead.

SUMMARY

Accordingly, an object of the present invention is to provide an apparatus and method, which allow for a spectrally efficient communication at very high frequencies over quickly changing channels without requiring a great deal of computational complexity or complicated hardware. The object is solved by the features of claim 1 for the apparatus and claim 15 for the method. The dependent claims contain further developments.

This invention proposes a new, semi open loop multistream (MIMO) transceiver scheme for mmWave communications with block transmission. Some of the features of the scheme can be summarized as follows:

The adaptive Tx utilizes only times of arrivals of different paths and their spatial characteristics for designing the transmit BF.

The adaptive Tx employs delaying of different transmit streams so that they are approximately aligned at the Rx. In this way the needed GP for interblock interference removal is minimized.

The adaptive Rx effective channel (including Tx and Rx BF) estimation and a MIMO equalization scheme.

The scheme results in a computationally efficient multistream mmWave communication which can rely on practical and simple channel estimation for rapidly changing channels, and where the GP is significantly reduced.

According to a first aspect of the invention, a communication device for generating a multi- stream transmission signal based upon an input signal is provided. The communication device comprises a beam delay aligner, which is configured to determine transmission delays of a plurality of transmission beam channels of the multi-stream transmission signal to a further communication device and delay transmission beam signals corresponding to the transmission beam channels of the multi-stream transmission signal, compensating for the determined transmission delays. The transmission beam signals are derived from the input signal. Thereby, it is possible to align the arrival times of the different transmission beam signals at the receiver, thereby reducing the necessary guard period duration significantly. This allows for an efficient use of the spectrum while not requiring a great computational complexity or complicated hardware. According to a first implementation form of the first aspect, the beam delay aligner is configured to delay the transmission beam signals, so that the individual signals of the multi- stream transmission signal arrive at the further communication device simultaneously.

Thereby, a further decrease in necessary guard period length can be achieved. According to a second implementation form of the first aspect or of the first implementation form of the first aspect, the communication device comprises a beam former, which is configured to perform a beam forming of the delayed transmission beam signals, resulting in the multi-stream transmission signal to be transmitted. It is thereby possible to achieve an efficient use of the channel.

According to a third implementation form of the first aspect or the previous implementation forms, the communication device moreover comprises an antenna array comprising a plurality of antennas. Each of the antennas is supplied with a specific one of a plurality of transmission signals of the multi-stream transmission signal. The antenna array is configured to transmit the multi-stream transmission signal. It is important to note that not a single multistream transmission symbol is provided to every antenna of the antenna array, but the symbols from multiple streams can be perturbed and multiplied by the precoding coefficients in the preceding precoding stage. Additionally they are multiplied by the beamforming coefficients before reaching the antennas in the beamforming stage. Each antenna is supplied with a signal derived from a plurality of transmission signals of the multi-stream transmission signal, by the beamforming stage. This allows for an efficient beam forming using the available channel.

According to a fourth implementation form of the first aspect or the previous implementation forms, the beam delay aligner is configured to determine the transmission delay of the transmission beam signals based upon a transmission delay of a received pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels or based upon a transmission delay feedback signal received from the further communication device. It is thereby possible to easily determine the required delay of each transmission beam channel.

According to a fifth implementation form of the first aspect or the previous implementation forms, the communication device furthermore comprises a waveform processor, which is configured for generating the transmission beam signals from the pre-coded signal. This allows for an efficient generating of the transmission beam signals. According to an implementation form of the previous implementation form, the waveform processor is configured to generate the transmission beam signals by performing an inverse fast Fourier transform. In this case, the transmission beam signals are OFDM signals. This allows for an especially efficient use of the spectrum.

According to a further implementation form according to any of the previous two

implementation forms, the waveform processor comprises a guard period inserter, which is configured to insert a guard period after each block of the transmission beam signals. This allows for preventing inter-block interference.

According to an implementation form of the previous implementation form, the guard period inserter is configured to determine a length of the guard period dynamically based upon determined statistics of transmission channel conditions. This allows for an especially efficient spectrum use.

According to a further implementation form of the first aspect or the previous implementation forms, the communication device comprises a pre-coder, which is configured for performing a pre-coding of a modulated signal derived from the input signal, resulting in a pre-coded signal. The pre-coding is based on channel statistics. This further increases the efficiency of the transmission.

According to a further implementation form of the first aspect or the previous implementation forms, the communication device is a millimeter- wave communication device. Especially at higher frequencies, the previously shown features significantly increase the spectral efficiency.

According to a second aspect of the invention, a communications system is provided. The communications system comprises a communication device according to the first aspect of the invention and a further communication device. The communication device is configured to transmit the multi-stream transmission signal to the further communication device. The further communication device is configured to receive the multi-stream transmission signal. This allows for a very spectrally efficient communication between the communication device and the further communication device. According to an implementation form of the second aspect of the invention, the communication device is a communication device according to the fourth implementation form of the first aspect.

If the beam delay aligner is configured to determine the transmission delay of the

transmission beam signals based upon a transmission delay of a received pilot sequence on each of the transmission beam channels, the further communication device comprises a pilot sequence generator, which is configured for generating a pilot sequence, the further communication device is configured for transmitting the pilot sequence to the communication device and the beam delay aligner of the communication device is configured to determine the transmission delay of the transmission beam signals based upon a transmission delay of the pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels.

If on the other hand, the beam delay aligner is configured to determine the transmission delay of the transmission beam signals based upon the transmission delay feedback signal received from the further communication device, the communication device comprises a pilot sequence generator, configured for generating a pilot sequence, the communication device is configured for transmitting the pilot sequence to the further communication device, the further communication device is configured for determining the transmission delays of the plurality of transmission beam channels of the multi-stream transmission signal and transmitting the delay feedback signal based thereupon to the communication device. The beam delay aligner of the communication device is furthermore configured to determine the transmission delay of the transmission beam signals based upon the transmission delay feedback signal. This allows for two alternative methods of easily determining the transmission delay of the transmission beam signals.

According to a second implementation form of the second aspect or the first implementation form of the second aspect, the communication device is configured for transmitting a further pilot sequence to the further communication device. The further communication device is configured for receiving the further pilot sequence. The further communication device moreover comprises a channel estimator, which is configured for performing a channel estimation of the communication beam channels between the communication device and the further communication device, based upon the received further pilot sequence. The further communication device moreover comprises an equalizer, which is configured for performing an equalization of the received signal based upon the channel estimation. This allows for an increase of the achieved bandwidth without requiring further spectrum. According to a third aspect of the invention, a communication method for communicating from a communication device to a further communication device is provided. The communication occurs by generating a multi-stream transmission signal based upon an input signal. The method comprises determining transmission delays of a plurality of transmission beam channels of the multi-stream transmission signal to the further communication device and delaying transmission beam signals corresponding to the transmission beam channels of the multi- stream transmission signal, compensating for the determined transmission delays. The transmission beam signals are derived from the respective input signal. Thereby, it is possible to align the arrival times of the different transmission beam signals at the receiver, thereby reducing the necessary guard period duration significantly. This allows for an efficient use of the spectrum while not requiring a great computational complexity or complicated hardware.

According to a first implementation form of the third aspect, the transmission beam signals are delayed so that the individual signals of the multi-stream transmission signal arrive at the further communication device simultaneously. Thereby, a further decrease in necessary guard period length can be achieved.

According to a second implementation form of the third aspect or the previous

implementation form, a beamforming of the delayed transmission beam signals is performed, resulting in the multi-stream transmission signal to be transmitted. It is thereby possible to achieve an efficient use of the channel.

According to a third implementation form of the third aspect or the previous implementation forms, a specific one of a plurality of transmission signals of the multi-stream transmission signal is provided to an antenna of an antenna array comprising a plurality of antennas. The multi-stream transmission signal is transmitted by the antenna array. It is important to note that not a single multistream transmission symbol is provided to every antenna of the antenna array, but the symbols from multiple streams can be perturbed and multiplied by the precoding coefficients in the preceding precoding stage. Additionally they are multiplied by the beamforming coefficients before reaching the antennas in the beamforming stage. Each antenna is supplied with a signal derived from a plurality of transmission signals of the multi- stream transmission signal, by the beamforming stage. This allows for an efficient beam forming using the available channel. This allows for an efficient beam forming using the available channel.

According to a fourth implementation form of the third aspect or the previous implementation forms, the transmission delay of the transmission beam signals is determined based upon a transmission delay of a received pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels or based upon a transmission delay feedback signal, which is generated by the further communication device and transmitted to the communication device. It is thereby possible to easily determine the required delay of each transmission beam channel. According to a fifth implementation form of the third aspect or any of the previous implementation forms, a waveform processing is performed for generating the transmission beam signals from the pre-coded signal. This allows for an efficient generating of the transmission beam signals. According to an implementation form of the previous implementation form, the transmission beam signals are generated by performing an inverse fast Fourier transform and the transmission beam signals are OFDM signals. This allows for an especially efficient use of the spectrum. According to a further implementation form of any of the previous two implementation forms of the third aspect, a guard period is inserted after each block of the transmission beam signals. This allows for preventing inter-block interference.

According to a further implementation form of the previous implementation form, a length of the guard period is determined dynamically based upon determined statistics of transmission channel conditions. This allows for an especially efficient spectrum use.

According to a further implementation form of the third aspect or any of the previous implementation forms, a precoding of a modulated signal derived from the input signal, resulting in a pre-coded signal is performed. The pre-coding is based on channel statistics. This further increases the efficiency of the transmission.

According to a further implementation form of the third aspect or any of the previously shown implementation forms, the communication device and the further communication device are millimeter- wave communication devices. Especially in this band, the previously shown features significantly increase the spectral efficiency.

Generally, it has to be noted that all arrangements, devices, elements, units and means and so forth described in the present application could be implemented by software or hardware elements or any kind of combination thereof. Furthermore, the devices may be processors or may comprise processors, wherein the functions of the elements, units and means described in the present applications may be implemented in one or more processors. All steps which are performed by the various entities described in the present application as well as the functionality described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and

functionalities. Even if in the following description or specific embodiments, a specific functionality or step to be performed by a general entity is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be

implemented in respect of software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The present invention is in the following explained in detail in relation to embodiments of the invention in reference to the enclosed drawings, in which

FIG. 1 shows an exemplary communications system;

FIG. 2 shows a two-dimensional representation of exemplary spatial lobes of a

mm Wave channel;

FIG 3 shows exemplary power azimuth delay profiles of a mm Wave channel ;

FIG 4 shows an embodiment of the first aspect of the invention;

FIG 5 shows a first embodiment of the third aspect of the invention; FIG. 6 shows a second embodiment of the third aspect of the invention;

FIG. 7 shows achievable results by use of the invention;

FIG. 8 shows different channel delays of different communication beams in a state- of-the-art MIMO communications system;

FIG. 9 shows significantly reduced delay spread of communication beams in an

inventive communications system, and

FIG. 10 shows necessary guard period length of an inventive communications system in comparison to a common MIMO communications system.

DESCRIPTION OF EMBODIMENTS

First, along FIG. 1, the construction of a regular 2-stage MIMO communications system was described. Along FIG. 2 and FIG. 3, the special characteristics of MIMO millimeter- wave communications channels are shown. Along FIG. 4, the construction and function of an embodiment of the first aspect of the invention and the second aspect of the invention are shown. Finally, along FIG. 5 and FIG. 6, the function of different embodiments of the method according to the third aspect of the invention are shown. Along FIG. 7 - FIG. 10, achievable results by use of the invention are visualized. Similar entities and reference numbers in different figures have been partially omitted.

This invention consists of a new transceiver architecture for mmWave MIMO communications and the corresponding channel estimation and data transmission procedure based on this new transceiver architecture.

The motivation for the proposed transceiver design originates in specific properties of the mmWave channel which can be summarized as follows. The mmWave channel exhibits sparsity, in the sense that it typically has only a few spatial lobes which carry a major part of signal energy. This is a significant difference compared with sub 6 GHz wireless channels where more spatial richness can be observed. A few time clusters might appear in one lobe, e.g. wall reflections, but often only one is seen in mmWave channels. An illustration of the concept of spatial lobes is shown in Fig. 2.

There it can clearly be seen that only three spatial lobes of relatively narrow width are present. A time cluster in mm Wave channels contains typically a small number of resolvable paths.

Such a time clustering is shown in FIG. 3. It can clearly be seen that within one spatial lobe there is usually only a low delay spread. This originates therefrom, that a narrow spatial lobe is uniformly reflected by an obstacle and most parts of the spatial lobe arrive at the receiver at a similar time.

One can conclude that the channel delay spreads inside one spatial lobe and especially inside one time cluster are significantly smaller compared with the total channel delay spread. Based on these observations, a more spectrally efficient mmWave MIMO transmission scheme is proposed in the following.

The illustration of the proposed architecture, which has some similarities to the 2-stage framework of Fig. 1, is given in Fig. 4. The focus of this invention is on the downlink (DL) scenario with the Tx being a mmWave base station (BS), and the Rx representing a mmWave mobile station (MS). This is not to be understood as limiting the scope though. Also, a special case of the invention is the Rx being another BS, in a backhaul type of link.

In FIG. 4, a communications system 40 comprising a transmitter 41, for example a base station and a receiver 42, for example a mobile station, is shown. The transmitter 41 communicates with the receiver 42 through a channel 43.

The transmitter 41 comprises a pre-coder 431, which is connected to a waveform processor 432, which again is connected to a beam delay aligner 433, which is moreover connected to a beam former 434, which is furthermore connected to an antenna array 435 consisting of a plurality of transmitter antennas.

The pre-coder 431 is provided with an input signal, for example an arbitrarily encoded and modulated signal comprising multiple data streams. The modulation can for example be a phase shift keying or a quadrature amplitude modulation. The pre-coder 431 performs a statistical pre-coding based upon channel statistics. Especially, this block performs pre- coding of the TX streams based on slowly changing channel parameters other than the spatial information about the dominant path. For example, it can apply beam power control. The resulting pre-coded signal is handed to the waveform processor 432, which is configured to generate a plurality of transmission beam signals. For example, this is done by performing an inverse fast Fourier transform resulting in a number of OFDM transmission beam signals. Moreover, the waveform processor 432 comprises a guard period inserter, which is configured to insert a guard period after each block of the transmission beam signals. This guard period insertion can optionally be of a dynamical nature, meaning that the length of the guard period is adapted to determined transmission channel statistics. Also, the waveform processor 432 performs a serializing and pulse shaping.

The resulting transmission beam signals are handed to the beam delay aligner 433, which determines transmission delays of a plurality of transmission beam channels of the multi- stream transmission signal to the receiver 42. Based on the determined delays, the beam delay aligner 433 delays the respective transmission beam signals compensating for the determined transmission delays. This is possible, since different TX data streams are sent over different dominant paths within the channel 43. This allows for a significant reduction in the necessary guard period duration, since the guard period now needs to only match one spatial lobe or time cluster delay spread and not the complete MIMO channel delay spread. The resulting delay transmission beam signals are handed on to the beam former 434. The beam former 434 performs a beam forming of the delay transmission beam signals resulting in a multi-stream transmission signal composed of a plurality of transmission signals to be transmitted. It should be noticed that the number of ADCs used in practice is likely to match approximately the number of dominant clusters/paths. The number of beams in the proposed scheme is equal to the number of Tx data streams, i.e. each Tx-Rx beam pair carries one data stream, which is appropriately delayed by the beam delay aligner 433. The beam former 434 can be realized in the analogue or digital domain. In the analogue beam forming case, the transmission radio frequency chains containing the ADCs can be placed either before the beam delay aligner 433, or before the beam former 434, looking at the DL signal transmission path.

Analogue beam forming can be performed in the baseband (BB), at an intermediate frequency (IF), or at the radio frequency (RF), which determines the position of other necessary radio electronics components such as mixers, upconverters, etc. In case of digital beam forming, ADCs and the remaining parts of the RF chain are placed before the transmission side antenna array 435. The resulting transmission signals are handed on to the individual antennas of the antenna array 435 and are transmitted into the channel 43 thereby.

There are different possible realizations of suitable antenna arrays for the communication scenario at hand. A fully connected antenna array has all RF chains connected to all antenna elements of the array. A subarray architecture assumes that different RF chains are connected to disjoint subsets of antenna elements in the array. In a typical DL wireless access application scenario, the Tx is a BS equipped with a larger array compared with the Rx (MS). For this reason, the Tx is able to create sharper beams and the interference coming from propagating into undesired transmit directions, depicted by dotted lines in Fig. 4 can be considered as small.

The receiver 42 again comprises an antenna array 421 , which is connected to a beam former 422. The beam former 422 is moreover connected to a waveform processor 423, which is moreover connected to an equalizer 424 and to a channel estimator 425, which is also connected to the equalizer 424.

The received transmission signals, which have passed the channel 43 are handed by the antenna array 421 to the beam former 422. This module performs the Rx beamforming. If the Rx is an MS, the Rx beams are typically broader than the Tx beams, due to a smaller number of antenna elements in the antenna array 421 than the in the antenna array 435. Therefore, the Rx sees more paths coming from the scatterers, as depicted in Fig. 4.

Analogously to the Tx beam former 434, different realizations of Rx beam former 422, analogue or digital, are possible. The RF chains containing the digital to analogue converters (DACs) can be placed after the Rx beam former 422 if this block is realized in the analogue domain. In the special case when the Rx is another BS, sharper Rx beams can be normally realized employing larger antenna arrays typically assumed for the BS unit. The case where Rx performs no BF is not precluded. After reintegrating the received transmission signals to a number of reception beam signals, these are handed on to the waveform processor 423. There, an inverse waveform processing with regard to the waveform processor 432 is performed. In the example of OFDM, the waveform processor 423 performs a guard period removal and a fast Fourier transformation, but other waveform variants are possible as well. Based on the results of the waveform processing, a channel estimation is performed by the channel estimator 425. Especially, this block performs an estimation of the instantaneous effective MIMO channel. Notice that the effective MIMO channel matrix, defined by the number of streams, is of significantly reduced dimensions compared with the radio (mm Wave) channel matrix, defined by the number of Tx/Rx antennas.

Especially, based on the instantaneous effective channel estimation, this module performs an arbitrary MIMO equalization technique. MIMO equalization is required if the Tx-Rx beam pairs, carrying different data streams, are not perfectly isolated from each other. The approximate over the air isolation is possible in the case of both Tx and Rx having very large antenna arrays, e.g. in a backhaul BS to BS transmission. In the latter case, the scheme simplifies to multiple parallel effective single-input single-output (SISO) channels with reduced delay spread on each of these channels. The equalizer 424 then outputs equalized data streams for further processing, e.g.

demodulation, decoding, etc. It should be noted that iterative Rx design, where equalization and demodulation/decoding stages are in one block, are not precluded.

The procedure for channel estimation and data transmission for the proposed transceiver architecture in Fig. 4 is summarized in Fig. 5. In a DL scenario of interest the BS is the Tx, while the MS is the Rx, and these abbreviations will be used interchangeably. Note, however, that in some of the intermediate uplink steps, the MS is transmitting while the BS is receiving signals. In steps 50, 51, the Rx and Tx exchange training packets in order to detect dominant channel paths and perform beam alignment. One possible realization of this training stage is to use hierarchical codebooks to align directions of departure and arrival at the Tx and Rx, respectively, but other beam finding/alignment algorithms are possible. No fast fading channel estimation is needed in this stage, as only spatial and, optionally, other slowly changing channel information is needed for the subsequent steps. In step 52, the Tx calculates Tx beamformers and the statistical precoding matrix based on the available, partial CSI. One implementation option is to use conventional beamformers defined by steering vectors in the look directions, but other BF methods can be used as well. In step 53, the Rx calculates its beamformers in an analogous way as described in Step 52.

In step 54, the MS sends the pilot packets for the BS to estimate the delays of the dominant paths. Note that Step 54 can be integrated with the Steps 50 and 51. Another implementation option is that the BS sends training packets for delay estimation, possibly integrated with Step 51. In this case, Step 54 reduces to sending feedback about the estimated delays, where the estimation is done at the MS based on the DL transmission. In step 55, the BS estimates the dominant path delays. One implementation possibility is to use the algorithms similar as for the timing advance (TA) estimation based on training sequences in the LTE uplink, but other implementations are not precluded.

In an optional step 56, the MS sends training packets for estimation of other channel impulse response (CIR) parameters, such as the number of time clusters, etc. Note that this step can be integrated with Step 54 and/or steps 50, 51. In a further optional step 57, the Tx selects the preferred GP based on the available transmit CSI. In step 58, the Tx sends DL pilots for effective channel estimation. The effective channel can be seen as a convolution of the Rx BF , the radio channel, and the Tx BF.

In step 59, the Rx performs effective channel estimation using an arbitrary MIMO channel estimation method. As the effective channel matrix has a significantly reduced dimensions compared to the radio channel matrix, a number of MIMO channel estimation schemes can be applied. In step 60, based on the estimated fast fading effective CSI, the Rx performs an arbitrary MIMO equalization algorithm after the waveform processing and GP removal. In a special case of large antenna array also at the Rx, the MIMO equalization can be omitted as the streams are approximately interference free. In step 61, the Tx performs the beam delaying operation. In step 62, the Tx sends different data steams over different, delayed beams which are being received and processed by the Rx.

It should be remarked that the proposed transmission scheme shown in Fig. 5, is applicable in both frequency division duplex (FDD) and time division (TDD) communication modes. While the TDD mode is suitable due to channel reciprocity, possibly after applying calibration, the path delay and spatial channel information required at the Tx for the proposed scheme are not expected to change much in the two separate communication channels of FDD. Moreover, in FIG. 6, a basic implementation of the method according to the third aspect of the invention is shown. In a first step, transmission delays of a plurality of transmission beam channels of the multi-stream transmission signal from a communication device to a further communication device are determined. In a second step 101, transmission beam signals are delayed corresponding to the transmission beam channel delays, determined in step 100. This deliberate delaying compensates for the determined transmission delays.

Since the proposed device according to the first aspect, system according to the second aspect and method according to the third aspect very closely relate, the above elaborations regarding the first aspect and second aspect, should also be considered regarding the method according to the third aspect.

In the following, the performance of the proposed scheme is shown along FIG. 7 - 10.

The numerical evaluations are based on the implemented 2X2 (2 data streams) MIMO-OFDM link level simulation chain. The following system parameters and assumptions are used:

The mm Wave channel model assumes two scatterers for all investigated schemes. The assumed carrier frequency is 30 GHz and the channel bandwidth is 500MHz.

Subarray architecture with 2 subarrays both at the Tx and Rx. Each subarray is connected to one RF chain and transmits/receives one data stream. Fixed power constraint per beam is assumed.

Each of the two Tx subarrays has 64 antenna elements (128 antenna elements in total at the BS).

Each of the two Rx subarrays has 8 antenna elements (16 antenna elements in total at the MS).

■ Ideal knowledge of the central angles of the Tx/Rx lobes.

Approximated knowledge of delays of dominant paths.

Ideal effective channel state information (CSI) at the Rx.

Conventional Tx/Rx BF matching the central angles of the Tx/Rx lobes. Zero-forcing (ZF) receive MIMO equalization of the effective channel (effective 2X2 MIMO channel inversion).

OFDM transmission with 512 subcarriers and different GP (in the case of OFDM, GP means the cyclic prefix) lengths depending on the analysed method.

The methods compared in the simulation chain are:

Baseline method 1 : No beam delay alignment at the Tx and having a sufficiently long GP to cover for the maximum mm Wave channel delay spread (the longest MIMO CIR) in the considered scenario.

Baseline methods 2 and 3 : Methods utilizing no beam delay alignment at the Tx and having reduced GP lengths to 25% and 75% of the sufficiently long GP length of the baseline method 1 , respectively.

Proposed method: Reduced GP length to 25% of the longest MIMO CIR and beam delaying at the Tx.

The performances are illustrated by two numerical examples.

The first numerical example, shown in Fig. 7, is a link level Monte Carlo simulation including a large number of MIMO channel realizations. One can notice that the proposed method outperforms all baseline methods by 2-2.5 dB in the case of uncoded Quadrature Phase Shift Keying (QPSK) transmission, for a very large Es/No range. The Es/No refers to the effective MIMO channel (after beamforming) taking into account different GP overheads.

To explain in more detail how the proposed scheme works, one illustrative MIMO channel realization, shown in Fig. 8, is used for the second example. The notation A->B means the effective channel from the Tx subarray A to the Rx subarray B. It can be noticed that there are large void times in the effective channels which are eliminated by delaying of the beams. In the delaying operation, due to imperfect Tx BF, assuming BF with a reasonable number of Tx antenna elements, one neglects a small portion of the interference. However, as seen in Fig. 7, this effect is visible only at extremely high signal-to-noise-ratios of little practical interest for mm Wave communication. In Fig. 9, the effective MIMO channel impulse response is shown after delaying and shortening. It can be seen that the MIMO channel components are well aligned and that the overall channel impulse response exhibits potential for a significant reduction of the GP.

In Fig. 10, a link level simulation is performed for the effective 2X2 MIMO channel shown in Fig. 8. The SER for the uncoded QPSK transmission is plotted versus the GP length. It can be seen that the proposed method reaches the optimum SER with a very short GP length of less than 10, while the baseline methods, the cases 1, 2, and 3 can be treated together in this context, require more than 10 times larger CP lengths to reach their optimum, minimal, SER.

In the following, a list of abbreviation used in this document is given:

RF Radio Frequency

Rx Receiver

SC Single Carrier

SER Symbol Error Rate

TA Timing Advance

TDD Time Division Duplex

Tx Transmitter

ZF Zero Forcing

The invention is not limited to the examples and especially not to the specific wavelength or modulation scheme proposed above. The invention discussed above can be applied to many waveforms. The characteristics of the exemplary embodiments can be used in any

advantageous combination.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, 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. 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 usually 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 communication systems.

Claims

1. A communication device (41) for generating a multistream transmission signal based upon an input signal, comprising a beam delay aligner (433), configured to

- determine transmission delays of a plurality of transmission beam channels (43) of the multistream transmission signal to a further communication device (42), and

- delay transmission beam signals corresponding to the transmission beam channels (43) of the multistream transmission signal, compensating for the determined transmission delays, wherein the transmission beam signals are derived from the input signal.

2. The communication device (41) of claim 1,

wherein the beam delay aligner (433) is configured to delay the transmission beam signals, so that the individual signals of the multistream transmission signal arrive at the further communication device (42) simultaneously.

3. The communication device (41) of claim 1 or 2,

wherein the communication device (41) comprises a beam former (434), configured to perform a beam forming of the delayed transmission beam signals, resulting in the multistream transmission signal to be transmitted.

4. The communication device (41) of any of the claims 1 to 3,

wherein the communication device (41) comprises an antenna array (435) comprising a plurality of antennas,

wherein each of the antennas is supplied with a specific one of a plurality of transmission signals of the multistream transmission signal, and

wherein the antenna array is configured to transmit the multistream transmission signal.

5. The communication device (41) of any of the claims 1 to 4,

wherein the beam delay aligner (433) is configured to determine the transmission delay of the transmission beam signals

- based upon a transmission delay of a received pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels (43), or

- based upon a transmission delay feedback signal received from the further communication device (42).

6. The communication device (41) of any of the claims 1 to 5,

wherein the communication device (41) comprises a waveform processor (432), configured for generating the transmission beam signals from the precoded signal.

7. The communication device (41) of claim 6,

wherein the waveform processor (432) is configured to generate the transmission beam signals by performing an inverse fast Fourier transform, and

wherein the transmission beam signals are OFDM signals.

8. The communication device (41) of claim 6 or 7,

wherein the waveform processor (432) comprises a guard period inserter, configured to insert a guard period after each block of the transmission beam signals.

9. The communication device (41) of claim 8,

wherein the guard period inserter is configured to determine a length of the guard period dynamically based upon determined statistics of transmission channel conditions.

10. The communication device (41) of any of the claims 1 to 9,

wherein the communication device (41) comprises a precoder (431), configured for performing a precoding of a modulated signal derived from the input signal, resulting in a precoded signal, wherein the precoding is based on channel statistics.

11. The communication device (41) of any of the claims 1 to 10,

wherein the communication device (41) is a millimeter- wave communication device.

12. Communication system (40), comprising a communication device (41) according to any of the claims 1 to 11, and a further communication device (42),

wherein the communication device (41) is configured to transmit the multistream

transmission signal to the further communication device (42), and

wherein the further communication device (42) is configured to receive the multistream transmission signal.

13. The communication system (40) of claim 12, wherein the communication device (41) is a communication device (41) according to claim 5, wherein, if the beam delay aligner (433) is configured to determine the transmission delay of the transmission beam signals based upon a transmission delay of a received pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels (43),

- the further communication device (42) comprises a pilot sequence generator, configured for generating a pilot sequence,

- the further communication device (42) is configured for transmitting the pilot sequence to the communication device (41), and

- the beam delay aligner (433) of the communication device (41) is configured to determine the transmission delay of the transmission beam signals based upon a transmission delay of the pilot sequence on each of the transmission beam channels of the plurality of transmission beam channels, and

if the beam delay aligner (433) is configured to determine the transmission delay of the transmission beam signals based upon the transmission delay feedback signal received from the further communication device (42),

- the communication device (41) comprises a pilot sequence generator, configured for generating a pilot sequence,

- the communication device (41) is configured for transmitting the pilot sequence to the further communication device (42),

- the further communication device (42) is configured for determining the transmission delays of the plurality of transmission beam channels of the multistream transmission signal and transmitting the delay feedback signal based thereupon to the communication device (41), and.

- the beam delay aligner (433) of the communication device (41) is configured to determine the transmission delay of the transmission beam signals based upon the transmission delay feedback signal.

14. The communication system of claim 12 or 13,

wherein the communication device (41) is configured for transmitting a further pilot sequence to the further communication device (42),

wherein the further communication device (42) is configured for receiving the further pilot sequence, wherein the further communication device (42) comprises a channel estimator (425), configured for performing a channel estimation of the transmission beam channels between the communication device (41) and the further communication device (42), based upon the received further pilot sequence, and

wherein the further communication device (42) comprises an equalizer (424), configured for performing an equalization of a received signal based upon the channel estimation.

15. A communication method for communicating from a communication device (41) to a further communication device (42), by generating a multistream transmission signal based upon an input signal, comprising:

- determining (100) transmission delays of a plurality of transmission beam channels of the multistream transmission signal to the further communication device (42), and

- delaying (101) transmission beam signals corresponding to the transmission beam channels of the multistream transmission signal, compensating for the determined transmission delays, wherein the transmission beam signals are derived from the input signal.