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1. WO2008006930 - PROCÉDÉ, SYSTÈME RADIO, STATION DE BASE ET TERMINAL UTILISATEUR

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

Method, radio system, base station and user terminal

Field
The invention relates to a method in a radio system supporting opportunistic beamforming, to a radio system, to a base station, to a user termi-nal, and to a computer program product.

Background
Multiantenna processing and physical layer scheduling will play an important role in coming broadband wireless access (BWA), 3G LTE (Long Term Evolution) and 4G systems. A celebrated opportunistic beamforming (OBF) provides an attractive tool by which scheduling and multiantenna processing can be combined in packet switched networks. In the opportunistic beamforming technique, multiple antennas are used in a base station (BS) in order to transmit the same signal with pseudorandomly changing beamforming weights. Such variation of weights introduces an artificial fading statistics that can be efficiently coupled with channel-aware packet scheduling (PS). An example of the opportunistic beamforming technique is described in Viswanath P., Tse, D. N. C, Laroia, R., Opportunistic beamforming using dumb antennas", IEEE Transactions on Information Theory, Vol. 48, no. 6, June 2002. In the Viswanath publication, it is shown that the opportunistic beamforming is espe-daily suitable when underlying fading statistics is changing very slowly. This is typical in the case of nomadic mobility and makes opportunistic beamforming attractive especially from broadband wireless access point of view.
Conventional beamforming methods are briefly discussed next. Signal-to-noise ratio (SNR) gain that is comparable to a sectorization gain is pro-vided, a channel feedback is not needed and a beam selection is based on an uplink DOA (direction of arrival) estimate in an approach with fixed spatial beams and highly correlated transmit antennas. This is a primary beamforming technique in UTRA FDD (universal terrestrial radio access frequency division duplex). In an approach with user specific beams, highly correlated transmit antenna, in addition to a signal-to-noise ratio gain also interference nulling can be utilized. The performance greatly depends on the accuracy of the estimated DOA. This is a secondary beamforming technique in UTRA FDD. In a closed-loop (CL) transmit diversity (TD) approach, transmit antennas with a low mutual correlation are applied, transmit weights are selected based on feedback from mobiles. In UTRA FDD, two modes for two transmit antennas are sped- fied of which mode 2 will be optional. Mode 1 applies signal phasing with 2-bit accuracy while mode 2 applies 8-PSK phasing and 1-bit gain adjustment.
In contrary to the described conventional methods, opportunistic beamforming relies on multiuser diversity from the beginning. Figure 3 shows a system model of a basic opportunistic beamforming concept according to prior art. The upper part of Figure 3 marked with dashed lines 360 illustrates the estimation and feedback of signal-to-interference ratio that is executed by all active user terminals during each scheduling time interval. The base station transmitter part for pilots 302 comprises a block for generating M identical pilot signals 306 and multiple antennas 310, 312, 314 that are configured to transmit the same signal from each antenna to a receiver 330 of the user terminal. The transmitter 302 comprises a weight control unit 308 that controls varying the transmit weights in different antennas 310, 312, 314 independently in time but controlled in a pseudorandom fashion.
The user terminal comprises a feedback generation unit 330 including a channel estimation unit 336 and a signal-to-noise ratio calculation unit 338 where an overall signal-to-noise ratio is monitored. Feedback 346 about the signal-to-noise ratio is then transmitted back to the base station transmitter for data 304. A scheduling/data buffer unit 316 controls scheduling decisions on the basis of the received feedback 346. If a transmit decision is made, the data streams are transmitted via an encoder/modulator unit 318 to a unit 320 that forms signal replicas for transmission. Further, a weight control unit 322 controls the transmit weights in different antennas 324, 326, 328. The user terminal receiver 332 receives the transmitted data streams and the data is processed in a channel estimation unit 342 and in a demodulation/decoding unit 344.
The receiver monitors the overall signal-to-noise ratio and sends feedback to the base station to form a basis for scheduling. The channel monitoring is based on a single pilot signal that is repeated at different transmit an-tennas 310, 312, 314. A large system with several independent fading user terminals is likely to comprise a user terminal whose instantaneous channel gains are close to matching the current powers and phases allocated at the transmit antennas. Thus, the transmit weights are randomized and transmission is scheduled to the user terminal which is close being in the optimal beamforming configuration.

One of the problems related to the opportunistic beamforming is related to pilot/channel estimation. The receiver estimates the sum channel at a scheduling time interval (STI) that may contain one or more transmit time intervals (TTIs), without knowing the corresponding transmit weight vector. When one scheduling time interval ends and a new scheduling time interval begins, the weight vector is changed in a pseudorandom manner and a short time period is needed for channel and SNR estimation before correct feedback can be sent to the base station. During this time period, scheduling cannot be based on SNR comparisons and thus, the system is not working efficiently. Further, beneficial filtering of consecutive channel gains is difficult due to rapid channel changes.
Another problem in the opportunistic beamforming is related to feedback channel. For example, in UTRA FDD, feedback information is sent via a dedicated control channel where accurate power control is applied. In future networks, such control channels are not attractive due to the packet switched nature of the systems.
In the conventional opportunistic beamforming, a delay can also become a problem since a receiver may need to wait for a suitable transmit weight and the corresponding coherent channel summation for a long time. This is not acceptable from the services' point of view.

Brief description of the invention
An object of the invention is to provide an improved method, an improved radio system, base station, user terminal, and an improved computer program product.
According to an aspect of the invention, there is provided a method in a radio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval. The method comprises: providing information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; calculating, at the one or more user terminals, the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; sending, by the one or more user terminals, the maximum signal-to-noise ratio of the calcu-lated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station; and controlling scheduling, by the base station, on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals.
According to another aspect of the invention, there is provided a ra-dio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval, the radio system comprising a base station and one or more user terminals. The base station includes a communication unit for providing information relating to at least two transmit weight vector sequences to one or more user termi-nals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals, the one or more user terminals each include a calculation unit for calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector, and a communication unit for sending the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station, and the base station further includes a scheduling unit for controlling scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals.
According to another aspect of the invention, there is provided a base station of a radio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time intervals, the base station communicating with one or more user ter-minals. The base station includes: a communication unit for providing information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; a receiving unit for receiving the maximum signal-to-noise ratio of calculated expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector from the one or more user terminals; a receiving unit for receiving information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio from the one or more user terminals; and a scheduling unit for controlling scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals.

According to another aspect of the invention, there is provided a user terminal of a radio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval, the user terminal communicating with at least one base sta-tion. The user terminal includes: a communication unit for receiving information from a base station relating to at least two transmit weight vector sequences, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; a calculation unit for calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; a communication unit for sending the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station for enabling the base station to control scheduling on the basis of the sent maximum signal-to-noise ratio and information relating to the transmit weight vector.
According to another aspect of the invention, there is provided a computer program product encoding a computer program of instructions for executing a computer process for data transmission method in a radio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval, the process comprising: providing information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; receiving the maximum signal-to-noise ratio of calculated expected signal-to-noise ratios of the one or more user terminals and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio of the one or more user terminals; and controlling scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals.
According to another aspect of the invention, there is provided a computer program product encoding a computer program of instructions for executing a computer process for data transmission method in a radio system supporting opportunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval, the process comprising: receiving information relating to at least two transmit weight vector sequences, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; and sending the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station for enabling scheduling control, by the base station, on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector.
According to another aspect of the invention, there is provided an integrated circuit that is configured to: provide information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; receive maximum signal-to-noise ratio of calculated expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector from the one or more user terminals; receive information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio from the one or more user terminals; and control scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals.
According to another aspect of the invention, there is provided an integrated circuit that is configured to: receive information from a base station relating to at least two transmit weight vector sequences, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; calculate the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; send the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maxi-mum signal-to-noise ratio to the base station for enabling the base station to control scheduling on the basis of the sent maximum signal-to-noise ratio and information relating to the transmit weight vector.
The invention provides several advantages. Multiuser transmission can be taken into account in scheduling and radio system structure where a set of orthogonal weight vector sequences is applied. Transmission resources are saved, and more capacity can be obtained in the radio system. Packet de- lay can be shortened. Embodiments can be easily applied to any antenna configurations.

List of drawings
In the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which
Figure 1 illustrates a radio system to which embodiments of the invention can be applied;
Figure 2 illustrates another example of a radio system to which embodiments of the invention can be applied;
Figure 3 illustrates a system model for opportunistic beamforming according to prior art;
Figure 4 illustrates an example of a system model for opportunistic multiuser beamforming according to an embodiment of the invention; and
Figure 5 illustrates an example of a method in a radio system sup-porting opportunistic beamforming according to an embodiment.

Description of embodiments
With reference to Figure 1 , examine an example of a radio system in which embodiments of the invention can be applied. A radio system in Figure 1 represents the third-generation radio systems. The embodiments are, however, not restricted to these systems described by way of example, but a person skilled in the art can apply the instructions to other radio systems containing corresponding characteristics. The embodiments of the invention can be applied, for example, to future Broadband Wireless Access (BWA), 3.9G LTE (Long Term Evolution) and 4G systems. The user terminal 170 can be, for example, user equipment, a portable communication device, a mobile computer, a mobile phone, or basically any device with a receiver/transmitter in the radio system.
The main parts of a radio system are a core network (CN) 100, a radio access network 130 and user equipment (UE) 170. The term UTRAN is short for UMTS Terrestrial Radio Access Network, i.e. the radio access network 130 belongs to the third generation and is implemented by wideband code division multiple access (WCDMA) technology. Figure 1 also shows a base station system 160 which belongs to the 2/2.5 generation and is implemented by time division multiple access (TDMA) technology, but it is not fur-ther described here.

On a general level, the radio system can also be defined to comprise user equipment, which is also known as a subscriber terminal and mobile phone, for instance, and a network part, which comprises the fixed infrastructure of the radio system, i.e. the core network, radio access network and base station system.
The structure of the core network 100 corresponds to a combined structure of the GSM and GPRS systems. The GSM network elements are responsible for establishing circuit-switched connections, and the GPRS network elements are responsible for establishing packet-switched connections; some of the network elements are, however, in both systems.
The base station system 160 comprises a base station controller (BSC) 166 and base transceiver stations (BTS) 162, 164. The base station controller 166 controls the base transceiver station 162, 164. In principle, the aim is that the devices implementing the radio path and their functions reside in the base transceiver station 162, 164, and control devices reside in the base station controller 166.
The base station controller 166 takes care of the following tasks, for instance: radio resource management of the base transceiver station 162, 164, intercell handovers, frequency control, i.e. frequency allocation to the base transceiver stations 162, 164, management of frequency hopping sequences, time delay measurement on the uplink, implementation of the operation and maintenance interface, and power control.
The base transceiver station 162, 164 contains at least one transceiver, which provides one carrier, i.e. eight time slots, i.e. eight physical chan-nels. Typically, one base transceiver station 162, 164 serves one cell, but it is also possible to have a solution in which one base transceiver station 162, 164 serves several sectored cells. The diameter of a cell can vary from a few meters to tens of kilometers. The base transceiver station 162, 164 also comprises a transcoder, which converts the speech-coding format used in the radio system to that used in the public switched telephone network and vice versa. In practice, the transcoder is, however, physically located in the mobile services switching center. The tasks of the base transceiver station 162, 164 include: calculation of timing advance (TA), uplink measurements, channel coding, encryption, decryption, and frequency hopping.
The radio access network 130 is made up of radio network subsystems 140, 150. Each radio network subsystem 140, 150 is made up of radio network controllers 146, 156 and B nodes 142, 144, 152, 154. A B node is a rather abstract concept, and often the term base transceiver station is used instead.
Operationally, the radio network controller 140, 150 corresponds approximately to the base station controller 166 of the GSM system, and the B node 142, 144, 152, 154 corresponds approximately to the base transceiver station 162, 164 of the GSM system. Solutions also exist in which the same device is both the base transceiver station and the B node, i.e. said device is capable of implementing both the TDMA and the WCDMA radio interface si-multaneously.
The user equipment 170 may comprise mobile equipment (ME) 172 and a UMTS subscriber identity module (USIM) 174. USIM 174 contains information related to the user and information related to information security in particular, for instance, an encryption algorithm.
In UMTS networks, the user equipment 170 can be simultaneously connected with a plurality of base transceiver stations (Node B) in occurrence of soft handover.
In UMTS, the most important interfaces between network elements are the Iu interface between the core network and the radio access network, which is divided into the interface IuCS on the circuit-switched side and the interface IuPS on the packet-switched side, and the Uu interface between the radio access network and the user equipment. In GSM, the most important interfaces are the A interface between the base station controller and the mobile services switching center, the Gb interface between the base station con-trailer and the serving GPRS support node, and the Um interface between the base transceiver station and the user equipment. The interface defines what kind of messages different network elements can use in communicating with each other. The aim is to provide a radio system in which the network elements of different manufacturers interwork well so as to provide an effective radio system. In practice, some of the interfaces are, however, vendor-dependent.
With reference to Figure 2, examine an example of a radio system in which embodiments of the invention can be applied. The radio system supports opportunistic beamforming wherein more than one transmit weight vector sequence is used at the same scheduling time interval. The radio system of Figure 2 comprises a base station 200 and one or more user terminals 170, 180.
In an embodiment, the base station includes a communication unit

202 for providing information relating to at least two transmit weight vector se-quences to the one or more user terminals 170, 180 of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals.
The one or more user terminals 170, 180 each include a calculation unit 178, 188 for calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector. The signal-to-noise ratios are calculated on the basis of the information relating to the transmit weight vector sequences received from the base station 200.
The one or more user terminals 170, 180 each further include a communication unit 176, 186 for sending the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station 200.
The base station 200 further comprises a scheduling unit 206 that is configured to control scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals 170, 180.
In an embodiment, the transmit weight vector sequences are retrieved from a memory of the user terminal 170, 180 when information relating to at least two transmit weight vector sequences is received from the base sta-tion 200. In an embodiment, a primary transmit weight vector sequence is stored in a memory of the user terminal 170, 180, and the transmit weight vector sequences can be generated on the basis of the primary transmit weight vector sequence when information relating to at least two transmit weight vector sequences is received from the base station.
In an embodiment, the calculated expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector are defined as:



where SNRn is a signal-to-noise ratio of n's transmit weight vector sequence (n is 1 , 2, ..., K), ytnm {t) is m's transmit weight vector of M transmit weight vectors of n's transmit weight vector sequence at scheduling time interval t, and hm(t) is m's channel response vector at scheduling time interval t.
In a two-antenna system, for example, each user terminal 170, 180 may then calculate signal-to-noise ratios relating to two transmit weight vector sequences:

and
2
SNR0 = W1Ii1 + w2h2
Then the user terminal 170, 180 may determine the maximum signal-to-noise ratio of the calculated signal-to-noise ratios, and send feedback to the base station 200 accordingly. For example, if 53VR1 > 53VR2 , then the feedback to the base station 200 can be: (1 , SNRi). Correspondingly, if 53VR1 is not greater than 53VR2 , then the feedback to the base station 200 can be: (2,

SNR2). Thus, the better signal-to-noise ratio is signaled to the base station 200 with the information relating to the transmit weight vector corresponding to that signal-to-noise ratio.
The scheduling unit 206 may then decide, for example, to select the user terminal 170, 180 with the best signal-to-noise ratio for transmission. Thus, for example the transmit weight vector corresponding to the highest signal-to-noise ratio can be used in transmission. The base station 200 then exe-cutes weight control by using the transmit weight vector corresponding to the maximum signal-to-noise ratio of the selected user terminal 170, 180.
In an embodiment, the information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio sent to the base station 200 is an index number associated with the transmit weight vector.
Figure 4 illustrates an example of a system model for opportunistic beamforming according to an embodiment. The upper part of Figure 4 marked with dashed lines 460 illustrates the estimation of signal-to-noise ratio that is executed by all active user terminals during each scheduling time interval. The exemplary system structure can be applied to both uncorrelated and correlated transmit antennas.
The base station transmitter part for pilots 302 comprises multiple antennas 310, 312, 314 that are configured to transmit the same signal from each antenna to a receiver 330 of the user terminal. The base station further comprises a unit for generating orthogonal pilots 410 and common pilot units 412, 414, 416. A common/dedicated pilot structure 412, 414, 416 similar to UTRA FDD (UMTS terrestrial radio access, frequency division duplex) is intra- duced, where a common pilot is transmitted as a cell-wise and dedicated pilot is transmitted as an antenna-wise. In an embodiment, the basic difference between the common pilots and the dedicated pilots is that the common pilots are used without transmit weights while the transmit weights are used in the dedi-cated pilot channels. The use of dedicated pilots is, however, optional.
In the presented solution, the transmit weights are not applied on the common pilot channels but the transmit weights are used on data channels. The antenna weights are changed with a pseudo-random manner using weight sequences. Both the transmitter and the receiver are equipped with the transmit weight information. Thus, both ends know the sequence of the transmit weights. The information on the transmit weight sequences can be provided to the user terminals, for example, in the following manner: a user terminal requests a certain service from a base station when a packet connection is being initialized, the numbers or other indicators of the applied transmit weight sequences are send to the user terminal if the service is granted, and the user terminal receiver retrieves the transmit weight vectors corresponding to the sequence numbers from a user terminal memory (alternative weight sequences can be stored in the user terminal memory beforehand). A weight vector sequence controller 420 can control the functions relating to retrieving the transmit weight vectors or calculating them, on the basis of the number or the other indicator of the applied transmit weight sequence.
Since the transmit weight vector sequences can be long, the user terminal may also know the number of the transmit weight in the sequence for a certain scheduling time interval. This information can be made available on some downlink broadcast control channel. Such a number can also be given when initializing the connection.
At this point, the user terminal is able to begin data detection. According to the example of Figure 4, orthogonal common pilots are applied on M antennas 310, 312, 314 for enabling the estimation of channels between the user terminal and the M transmit antennas 310, 312, 314. After the channel estimation from a common pilot channel in a channel estimation unit 336, the user terminal can compute the expected signal-to-noise ratios corresponding to any future scheduling time interval by applying the transmit weight sequences. The signal-to noise ratios can be calculated in an SNR calculation unit 338.

Thus, with the help of common pilots and known transmit weight vector patterns, the receiver can in advance: estimate signal-to-noise ratios corresponding to future transmit time intervals, order or process in some other ways the resulting signal-to-noise ratios and decide - based on service data rate and delay requirements - suitable transmit time interval/signal-to-noise ratio pairs.
Since the transmit weights are known in the receiver, the channel estimation can be performed on the basis of the common pilots or jointly on the basis of both common and dedicated pilots. Since the mobile receiver knows the transmit weights of the next scheduling time interval in advance, the receiver can estimate the signal-to-noise ratio corresponding to the next scheduling time interval efficiently by using the latest channel information (estimated from common pilots). The base station then has the relevant SNR information at the beginning of each scheduling time interval and the performance of the scheduling procedure remains robust, i.e. the base station can transmit data to those mobile receivers that are in good receiving conditions.
In the case of low mobility and delay tolerant services, the receiver does not have to send SNR feedback during each scheduling time interval if the detected SNR is low. Occasional feedback can be convoyed in uplink packet channels, such as random access channel.
In extreme cases of stationary channel or highly correlated antennas, the receiver knows the most suitable transmit weights long before they are applied in the base station. The receiver can then be switched off during the waiting times. Further, depending on the signal-to-noise ratio estimations and service needs, the user terminal can suspend the feedback 346 transmission when needed. When a dedicated data transmission arrives, the user terminal can utilize both common and dedicated pilots in joint channel estimation. This enables robust data detection.
The signal-to-noise ratios corresponding to the next scheduling time interval can now be reliably estimated. This improves the scheduling performance at the beginning of each scheduling time interval. Channel estimation is more robust since filtering techniques can be utilized better (channel fluctuations due to the changes in transmit weights can be taken into account better). Feedback capacity need is smaller since feedback transmission can be sus-pended from time to time. It is possible to shut off the user terminal receiver from time to time if the channel is stationary or the transmit antennas admit high mutual correlation.
A pseudorandom transmit weight sequence may be of the form:


where w(t) is the transmit weight vector at a scheduling time interval t. This vector contains M components if there are M transmit antennas. T is the length of a sequence time period. The scheduling time interval (STI) can contain one or more transmit time intervals (TTI) and the transmit weight vector is changed after each scheduling time interval. The transmit weight sequence W is known in both the transmitter (here the base station) and the receiver (here the user terminal), and channel estimation is performed from common pilots.
Consider now an M-transmit antenna system at a scheduling time interval t. The user terminal knows w (t ) and estimates M channels hγ,...,hM from common pilot channels. In prior art solutions, the user terminal effectively computes a single signal-to-noise ratio as:

M

SNR =
m=\

where SNR is a signal-to-noise ratio, wm (f) is a transmit weight vector at a scheduling time interval t, and hm is a channel response vector of the user terminal. The user terminal then sends the SNR or a corresponding data rate request to the base station scheduler 316. However, it may happen that a suit-able w(t) that would give a good SNR value to a certain user terminal will exist far away in the future. Then the packet delay can become intolerable and the scheduler 316 may decide to execute the transmission in bad channel conditions. As a result, the system efficiency is poor and interference in the network increases.
In an embodiment, M orthogonal beams can be formed in an M-antenna system. Corresponding to these orthogonal beams, there are M orthogonal transmit weight vectors that span the space of all transmit weight vectors. On the other hand, for any given transmit weight vector, say w1 , there exists M-1 weight vectors, say w 2 ,..., w M such that these M vectors to-gether are mutually orthogonal.

Thus, instead of a single transmit weight vector sequence W, M transmit weight vector sequences, say Wλ,...,WM , can be formed for which the actual transmit weight vector at each scheduling time interval are orthogonal. Assume that both the base station and the user terminal know K transmit weight vector sequences of which W1 is the primary sequence and W2,...,WK are complementary sequences formed from W1 such that the weight vectors of these sequences at the same scheduling time interval are orthogonal. The user terminal can now calculate:

M

SNR =
m=l
where n=1 ,2,..., K, and the user terminal can now decide the best weight vector on the basis of the SNRs calculated on the basis of the M transmit weight vector sequences. Let the index of this best transmit weight vector be n0. A feedback to the base station then contains the information relating to this index n0 and the corresponding SNR. The base station scheduler 316 then uses this

SNR as an input. If the user terminal is selected for transmission, then the transmit weight vector is selected according to the index n0. Thus, the base station does not actually transmit the weight sequences Wλ,...,WM that are used to determined the signal-to-noise values. Instead, the sequences are generated in the user terminals according to a common rule, and the base station only applies the optimum transmit weight vector to the selected user terminal.
The system structure of an embodiment where both the base station and the user terminal know the applied transmit weight vectors in advance can be beneficially used especially in broadband wireless access (BWA) systems with nomadic mobility. Thus, according to an embodiment, the user terminal may predict the achievable data rates (or corresponding SNRs) in advance and send this information to the base station scheduler 316. Multiple transmit weight sequences that are known in both ends guarantee a certain minimum performance level and data rate predictions (that are optional) can help the base station to schedule user terminals efficiently from the radio system point of view.
The orthogonal common pilots are applied on M antennas for enabling the estimation of channels between the user terminal and M transmit an-tennas. After the channel estimation from the common pilot channel, the user terminal can compute the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector (w1(t),...,wk(ή). Also future signal-to-noise ratios can be computed since the transmit weight vectors (w1(t + 5r),...,wJf (t + 5r)) are known for any time shift s.
The resulting signal-to-noise ratios and the index n0 can be sent to a base station transmitter for data 304 to the scheduler 316 via a feedback channel 346. Finally, if the scheduler 316 has selected the user terminal for transmission, the index n0 348 is provided to the weight control unit 322. If a transmit decision is made, the data streams are transmitted via an en-coder/modulator unit 318 to a unit 320 that forms signal replicas for transmission. The use of dedicated pilots 400, 402, 404 is optional. Further, a weight control unit 322 controls the transmit weights in different antennas 324, 326, 328. The user terminal receiver 332 receives the transmitted data streams and the data is processed in a channel estimation unit 342 and in a demodula-tion/decoding unit 344. A weight-tracking unit 406 in the receiver 322 knows the applied transmit weight and the channel estimation can therefore be performed from common pilots or jointly from both common and dedicated pilots.
Figure 5 illustrates an example of a method in a radio system supporting opportunistic beamforming according to an embodiment. The method starts in 500. In 502, information relating to at least two transmit weight vector sequences is provided to one or more user terminals. The transmit weight vectors of the at least two transmit weight vector sequences are orthogonal at the same scheduling time intervals.
In 504, expected signal-to-noise ratios corresponding to each pos-sible orthogonal transmit weight vector sequence of the at least two transmit weight vector sequences are calculated by the two or more user terminals. In 506, the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio is sent to the base station.
In 508, scheduling is controlled on the basis of the maximum signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio received from the one or more user terminals. The method ends in 510.
According to an embodiment, due to the selection between or-thogonal transmit weights, the scheme of opportunistic closed-loop transmit diversity always gives the same performance as the selection over M orthogo- nal weight vectors. In addition to the performance gain, the method according to an embodiment enables avoiding situations where a signal-to-noise ratio of a certain user terminal remains at a low level for a long time. This will also shorten the packet delay. Further, when using the orthogonal transmit weight vectors, there is no waiting time either. Instead, opportunism is used in a tradeoff between a feedback requirement and performance. When using M transmission antennas and M orthogonal transmit weight vectors, the lowest performance corresponds to that of an antenna selection between M antennas. Hence, bad transmission times can be avoided and increased performance can be achieved.
The embodiments of the invention may be realized in an electronic device, comprising a controller. The controller may be configured to perform at least some of the steps described in connection with the flowchart of Figure 5 and in connection with Figures 2 and 4. The embodiments may be imple-mented as a computer program comprising instructions for executing a computer process for a method in a radio system supporting opportunistic beam-forming, wherein more than one transmit weight vector sequences at the same scheduling time interval is used. The computer process comprises: providing information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; receiving the maximum signal-to-noise ratio of calculated expected signal-to-noise ratios of the one or more user terminals and information relating to the transmit weight vector corresponding to the maxi-mum signal-to-noise ratio of the one or more user terminals; and controlling scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals. In another embodiment, the computer process comprises: receiving information relating to at least two transmit weight vector sequences, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; and sending the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight veo tor corresponding to the maximum signal-to-noise ratio to the base station for enabling scheduling control, by the base station, on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector.
The computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer pro-gram medium may be, for example but not limited to, an electric, magnetic, optical, infrared or semiconductor system, device or transmission medium. The computer program medium may include at least one of the following media: a computer readable medium, a program storage medium, a record medium, a computer readable memory, a random access memory, an erasable program-mable read-only memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunications signal, computer readable printed matter, and a computer readable compressed software package.
The embodiments of the invention may be realized in an integrated circuit that can be included in a base station or in a user terminal. It is also possible that the integrated circuit is included in a separate module outside the base station/user terminal. In an embodiment, the integrated circuit is configured to: provide information relating to at least two transmit weight vector sequences to one or more user terminals of the radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; receive maximum signal-to-noise ratio of calculated expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector from the one or more user terminals; receive information relating to the transmit weight vector corresponding to the maxi-mum signal-to-noise ratio from the one or more user terminals; and control scheduling on the basis of the maximum signal-to-noise ratio and information relating to the transmit weight vector received from the one or more user terminals. In another embodiment, the integrated circuit is configured to: receive information from a base station relating to at least two transmit weight vector sequences, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; calculate the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; send the maximum signal-to-noise ratio of the calculated signal-to-noise ratios and information relating to the transmit weight vector corresponding to the maximum signal-to-noise ratio to the base station for enabling the base station to control scheduling on the basis of the sent maximum signal-to-noise ratio and information relating to the transmit weight vector.
Even though the invention has been described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in several ways within the scope of the appended claims.