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.

Yet another problem related to conventional opportunistic beam- forming is that data transmission to only a single user terminal is supported. In case of more than one orthogonal resource channels, there can be as many different shared channels (separated by code, time or frequency) as there are such orthogonal resources available (in HSDPA there can be, e.g., 4 different code channels in a simultaneous use). Nevertheless, the idea of opportunistic beamforming itself has not been applied in simultaneous communication to multiple user terminals.

Brief description of the invention

An object of the invention is to provide an improved method, an improved radio system, an improved base station, an improved 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 two 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; providing at least two data rate requests of the two or more user terminals, the data rate

requests being determined on the basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences; determining, by the base station, at least one first decision variable on the basis of the at least two data rate requests of a user terminal and a filtered throughput of the same user terminal of the two or more user terminals; determining, by the base station, at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals; and controlling scheduling, by the base station, on the basis of the determined at least one first decision variable and the at least one second decision variable.

According to another aspect of the invention, there is provided a radio 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 two or more user terminals. The base station includes a communication unit for providing information relating to at least two transmit weight vector sequences to the two 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; and the base station further includes: a calculation unit for de- termining at least one first decision variable on the basis of at least two data rate requests of a user terminal of the two or more user terminals and a filtered throughput of the same user terminal, the data rate requests being determined on the basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences, a calculation unit for determining at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals, and a scheduling unit for controlling scheduling on the basis of the determined at least one first decision variable and the at least one second decision variable. 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 two or more user terminals. The base station includes: a communication unit for providing informa- tion relating to at least two transmit weight vector sequences to the two 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 calculation unit for determining at least one first decision variable on the basis of at least two data rate requests of a user terminal of the two or more user terminals and a filtered throughput of the same user terminal, the data rate requests being determined on the basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences, a calculation unit for determining at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals, and a scheduling unit for controlling scheduling on the basis of the determined at least one first decision variable and the at least one second decision variable.

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 schedul- ing time interval, the user terminal communicating with at least one base station. 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 calcula- tion unit for calculating the expected signal-to-noise ratios corresponding to each possible orthogonal transmit weight vector; a communication unit for providing feedback to the base station, the feedback comprising information for providing at least two data rate requests of the user terminal, the data rate requests being determined on the basis of the calculated signal-to-noise ratios corresponding to the at least two transmit weight vectors for enabling the base station to control scheduling.

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 a method in a radio system supporting op- portunistic beamforming, wherein more than one transmit weight vector sequence is used at the same scheduling time interval. The process comprises: providing information relating to at least two transmit weight vector sequences to two 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; providing at least two data rate requests of the two or more user terminals, the data rate requests being determined on the

basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences; determining, by the base station, at least one first decision variable on the basis of the at least two data rate requests of a user terminal and a filtered throughput of the same user terminal of the two or more user terminals; determining, by the base station, at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals; and controlling scheduling, by the base station, on the basis of the determined at least one first decision variable and the at least one second decision vari- able.

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 two or more user terminals of a radio system, the transmit weight vectors of the at least two transmit weight vector se- quences being orthogonal at the same scheduling time intervals; determine at least one first decision variable on the basis of at least two data rate requests of a user terminal of the two or more user terminals and a filtered throughput of the same user terminal, the data rate requests being determined on the basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences, determine at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals, and control scheduling on the basis of the determined at least one first decision variable and the at least one second decision variable. 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; provide feedback to the base station, the feedback comprising information for providing at least two data rate requests of the user terminal, the data rate requests being determined on the basis of the calculated signal-to-noise ratios corresponding to the at least two transmit weight vectors for enabling the base station to control scheduling.

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. Embodi- ments can be easily applied to any antenna configurations. Further, shorter feedback latency is achieved.

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 supporting 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 and 4G systems. The user terminal 170 can be, for example, user equipment, a portable communication device, a mobile computer, a mobile phone, or basi- cally 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 further described here.

On a general level, the radio system can also be defined to com- prise 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 channels. 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 ser- vices switching center. The tasks of the base transceiver station 162, 164 in-

elude: 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 simultaneously. 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 controller 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 two 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 sequences to the two 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 two or more user terminals 170, 180 each include a communication unit 176, 186 configured to send at least two data rate requests to the base station 200. The data rate requests are determined on the basis of calcu- lated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences. The signal-to-noise ratios can be calculated in calculation units 178, 188 of the user terminals 170, 180.

The base station 200 further comprises a calculation unit 204 configured to determine at least one first decision variable on the basis of the re- ceived at least two data rate request of a user terminal of the two or more user terminals 170, 180 and a filtered throughput of the same user terminal. The calculation unit 204 is also configured to determine at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals 170, 180. The base station 200 further comprises a scheduling unit 206 that is configured to control scheduling on the basis of the determined at least one first decision variable and the at least one second decision variable.

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 station 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 vec- tor sequences is received from the base station.

In an embodiment, the first decision variable is determined by calculating a quotient of a maximum data rate request of a user terminal 170 and the filtered throughput of the same user terminal 170. In an embodiment, the second decision variable is determined on the basis of maximum data rate re- quests, minimum data rate requests and the filtered throughputs of two user terminals 170, 180. In an embodiment, effects of disturbance are reduced from the maximum data rate requests of the two user terminals 170, 180, quotients of the maximum data rate requests where the effect of disturbance is reduced and the filtered throughputs of the two user terminals are calculated, and the second decision variable is determined by summing the calculated quotients.

In an embodiment, the determined first and second decision variables are compared, and scheduling is controlled on the basis of the comparison. In an embodiment, scheduling is controlled on the basis of a maximum decision variable of the first and second decision variables. 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 introduced, 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 dedicated 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 trans-

mit 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 number or other indicator of the applied transmit weight sequence is send to the user terminal if the service is granted, and the user terminal receiver retrieves the transmit weight vector corresponding to the sequence number 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 trans- mit 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 ba-sis 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 schedul-

ing 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 transmis- sion 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 ratio corresponding to the next scheduling time interval can now be reliably estimated. This improves the scheduling perform- ance 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 suspended 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.

Let us next consider a four-transmit antenna system with two user terminals. Assume that at first a transmit weight (1 , 1 , 1 , 1 ) is applied and then a transmit weight (1 , -1 , 1 , -1 ) is applied. Consider that with the first transmit weight vector the signal of the first user terminal admits coherent summation while the signal of the second user is attenuated. Further, when the transmission is executed with a transmit weight of (1 , -1 , 1 , -1 ), the parts are changed: the received signal of the first user terminal is attenuated but the signal of the second user terminal admits coherent summation.

Assume now that transmissions to K user terminals are performed simultaneously in an M-antenna opportunistic beamforming scenario. The received signals of separate users can then be defined as:

where h* = {h ^k ,...,h _M ^k ) consists of M channel impulse responses of channels between M transmit antennas and the /rth user terminal, w* is the transmit weight vector for the kth user and n _κ is a noise sample. This is actually a MIMO (multiple input multiple output) system but since the user terminals are not cooperating, the conventional MIMO processing in the receiver is not possible. However, with a proper selection of transmit weights the effect of interference can be reduced. Assume now that the impulse responses are independent and identically distributed complex Gaussian variables. Then it can be seen that:

E{(w*' -h ^k ) ^H (w ^k> .h*)}= (w^ fw** (2)

Hence, the mean interference between the adjusted channels depends on the selection of transmit weight vectors. In an M-antenna system, each transmit weight vector has M components which implies that it is possible to find M orthogonal weight vectors. If the weight vectors in equation (2) are orthogonal, there holds:

^ ^{•h^} π ^w ^ - ^h* )} = 0 (3)

Thus, the mean interference between the user terminals vanishes if different user terminals apply orthogonal transmit weight vectors. In the embodiment, this fact is applied to opportunistic beamforming such that both the base sta- tion transmitter and the user terminal receiver are equipped with information on transmit weight pseudorandom sequences:

W ^k = = l,2,..,K (4)

where w ^k (t) is the transmit weight vector at a scheduling time interval t, and T is the length of the sequence period. The scheduling time interval may contain one or more transmit time intervals (TTI) and the transmit weight vector is changed after each scheduling time interval in a pseudorandom manner. Next it is assumed that all or some orthogonal weight sequences W ^k are known in both the transmitter and the receiver. This information can be stored in the memories of the transmitter and the receiver beforehand, for example. Since the channel estimation is done from common pilots and K weight vector sequences are known, the nth user terminal is able to compute K signal-to-noise ratios (e.g. K different signal-to-noise ratios can be computed):

Y,' (t)-h ⁿ (t

SNR? = σ ' w

SNRl =- ^: W- ^h "W (5) σ ²

where SNR^ is a signal-to-noise ratio of the Kth transmit weight vector sequence, w ^κ (t) is a transmit weight vector of the Kth transmit weight vector sequence at a scheduling time interval t, and h"(t) is a channel response vector of the user terminal at the scheduling time interval t, and σ ² is a noise variance. The obtained information can then be used when forming the feedback to the base station. In a system according to Figure 3, the feedback contains only the first signal-to-noise ratio that is computed without knowing the transmit weights. However, according to the embodiments of the present solution, the feedback can be designed to support simultaneous transmission to multiple user terminals. A simple example where the maximum number of simultaneous served user terminals is two (there are two weight vector sequences in use) is presented. Let the feedback from the nth user terminal be of the form:

where R" , R" refer to data rate requests that are selected using the calculated signal-to-noise ratio estimates SNR" , SNR" . For this feedback, an embodiment of multiuser proportional fair scheduling is introduced. In the proposed sched-

uler, the values for the first and the second decision variables are next calculated in the base station:

T"(t) '

where R _m ⁿ Jt) = m _a χ{R? {t),R ₂ ⁿ {t)}, R _m ⁿ _m {t) = are the data rate requests of the nth user terminal corresponding to the first and the second transmit weight vectors. N is the number of active user terminals and T ⁿ (t) 0 refers to the filtered throughput of the nth user terminal. Finally, β is an efficiency factor that describes the interference rejection ability of the user terminal. Scheduling is based on a decision:

p"° = max{p;, p ₂ ¹ '' : n,l = 1,2,...,N} (8) 5 where p" is the first decision variable and p" ^J is the second decision variable. Thus, the scheduler may decide to either transmit one data stream (to a single user terminal) or two data streams (one to both user terminals) depending on the data rate requests and previously executed transmissions. The role of the 0 transmit weights can be further clarified: if the first transmit weight (from W-i) gives a good coherent channel to the nth user terminal and simultaneously the second transmit weight (from W ₂ ) strongly attenuates the channel, then:

K(t) = KJt) » KJt) = R ₂ ⁿ (t) 5

If, at the same time, there exists a user terminal (lets say the fth user) for which:

Rl(t) = KJt) « KJt) = R ₂ ^l (t) 0 then the second decision variable p"' ¹ ' ⁿ equation (7) is large since channels of user terminals n and / are orthogonal or nearly orthogonal. If such user terminals are not found, the scheduler may execute a single antenna transmission.

According to an embodiment, the filtering of previous data rates can be done according to following rules:

{l - l/t _c )T ⁿ {t) + l/t _c - {R _m ⁿ Jt)- V - RlJt)) τ"{t+ i) = {\ - \it _c )r ⁿ {t)+ \it _c - R _m ⁿ Jt) (9) {\ - \it _c )r ⁿ {t),

where transmissions to user terminals n and / were executed in the case of

(l - 1 / t _c )r ⁿ {ή + 1 / t _c ^■ (R^ {ή - β • R _m ^l _m {ή) , transmissions to a user terminal n only were executed in the case of and no transmission to user terminal n in the case of (l - 1 / t _c )τ ⁿ {ή . The embodiments of the proposed scheduler can be easily generalized to K-user cases. Also other feedback formats than described in the above examples are possible, and then also the scheduler design may be different. In certain embodiments, some of the pseudorandom sequences, e.g. W-i, can be used for video streaming and other sequences can be reserved for other packet data services. The calculation of the signal-to-noise ratio values of equation (5) can be modified as:

SIR" (10)

which assumes that interference of video streaming user terminals dominate over inter cell interference and thermal noise. The ratios between the expected data rates can be calculated accordingly.

When initializing a packet connection, the user terminal receives the number or other indication relating to the applied transmit weight from the base station. The user terminal may then retrieve the transmit weight vector sequences W _λ ,...,W _κ from its memory. 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 Also future signal-to-noise values can be calculated since the weights w ¹ (t + s),...,w ^κ (t + s) are known for any time shift s.

The data rate request can be identified as corresponding to each signal-to-noise ratio in the receiver or in the transmitter. The resulting informa-

tion can be sent to a base station transmitter for data 304 to a scheduler 316 via the feedback channel 346. The feedback can be sent in a small data packet together with other control information also. Finally, the proportional fair scheduler may decide the user terminal(s) according to the scheduling rules (7)-(9). The weight control unit 322 is then informed about the decided transmit weights. If two or more user terminals are scheduled, then the weight control unit 322 may be advised to use also complementary weight(s) 348. 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 transmis- sion. 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 channels estimation unit 342 and in a demodulation/decoding unit 344. A weight-tracking unit 406 in the receiver 322 knows the applied transmit weight and the channel estimation can therefore be done from common pilots or jointly from both common and dedicated pilots.

In an embodiment, the scheduler 316 can estimate signal-to-noise plus interference (SNIR) values in the transmitter beforehand for scheduling and link adaptation purposes. Consider an embodiment of the invention where at each transmission time interval for each possible transmit weight W ₁ there is (in two-antenna, two-user terminal system) an orthogonal transmit weight w ₂ .

Assume that two user terminals are scheduled to the same transmission time interval such that W ₁ is the best transmit weight for the first user terminal and w ₂ is the best transmit weight for the second user terminal. Thus, the SNIRs in the user terminals are:

w, h, SNIR, = ϊ — ! and

N ₀ + w ₂ Ji ₁ w, h.

SNIR 2, = 2

N ' 0 _n + w, 1 h "2

where N ₀ is the effect of noise. From the transmit weight information it can be seen that W ₁ is suitable for the first user terminal and w ₂ is suitable for the second user terminal. Assume that the feedback from the user terminals consists of (b, SNR), where b is 1 -bit information; 1 refers to present OBF transmit weight W ₁ and 0 refers to an orthogonal transmit weight w ₂ , and SNR is the

quantized SNR information (if b=1 then it corresponds to a case where W ₁ is applied and if b=0 then it corresponds to a case where w ₂ is applied). The scheduler 316 now knows Jw ₁ hj ² due to feedback and it can estimate w ₂ hj ² since there is a certain order statistics between these variables (the first one is always larger than the second one). This order statistics depends on the quantization and on the number of antennas. Similarly, the scheduler 316 knows |w ₂ h ₂ | ² and it can estimate |w _j h ₂ | ² . In this way, the scheduler

316 may obtain estimates for SNIRi and SNIR ₂ . These SNIR values can be used further while selecting rates for individual user terminals. By using fixed orthogonal transmit weights (and fixed quantization), the signal statistics can also be fixed and SNIR distributions can be investigated beforehand. Thus, the SNIR estimation can be done in the scheduler 316 with minimal feedback information.

According to the above example, the two or more user terminals each include a communication unit for sending feedback to the base station, the feedback comprising information on a signal-to-noise ratio and information on a corresponding transmit weight vector. Further, the base station comprises an estimating unit for estimating at least two signal-to-noise plus interference values corresponding to the transmit weight vector sequences for the two or more user terminals on the basis of the received feedback from the two or more user terminals, and an estimating unit for estimating the present or the future data rate requests of the two or more user terminals corresponding to the at least two transmit weight vector sequences on the basis of the estimated signal-to-noise plus interference values. 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 two 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, signal-to-noise ratios corresponding to the at least two transmit weight vector sequences are calculated by the two or more user terminals on the basis of the transmit weight vector sequences. In 506, at least two data rate request are sent to the base station by the two or more user ter- minals. In 508, at least one first decision variable is determined by the base station on the basis of the received at least two data rate requests of a user

terminal and a filtered throughput of the same user terminal of the two or more user terminals.

In 510, at least one second decision variable is determined by the base station on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals. Finally, in 512, scheduling is controlled, by the base station, on the basis of the determined at least one first decision variable and the at least one second decision variable. The method ends in 514.

An embodiment of the invention allows transmission to multiple us- ers simultaneously using the same transmission resources. This can be done in a controlled manner: multiuser transmission is taken into account in scheduling and system structure where a set of orthogonal weight vector sequences is applied. As a result, up to MxM MIMO capacity can be obtained at the system level if the transmit antennas are uncorrelated and even if there were only a single receiver antenna in the user terminals. The presented embodiments are also suitable for any antenna configurations.

Instead of a two-stage scheduling where a first beam indices and then the signal-to-noise ratios are communicated on a feedback channel, an embodiment of the invention feeds back signal-to-noise ratio values or related data rate requests in one stage resulting in a shorter feedback latency.

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 two 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; sending, by the two or more user terminals, at least two data rate requests to the base station, the data rate requests being determined on the basis of calculated signal-to-noise ratios corresponding to the at least two transmit weight vector sequences; determining, by the base station, at least one first decision variable on the basis of received at least two data

rate requests of a user terminal and a filtered throughput of the same user terminal of the two or more user terminals; determining, by the base station, at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals; and controlling scheduling, by the base station, on the basis of the determined at least one first decision variable and the at least one second decision variable.

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 two or more user terminals of a radio system, the transmit weight vectors of the at least two transmit weight vector sequences being orthogonal at the same scheduling time intervals; determine at least one first decision variable on the basis of at least two data rate requests of a user terminal of the two or more user terminals and a filtered throughput of the same user terminal, the data rate requests being determined on the basis of calculated signal-to- noise ratios corresponding to the at least two transmit weight vector sequences, determine at least one second decision variable on the basis of maximum data rate requests, minimum data rate requests and the filtered throughputs of the two or more user terminals, and control scheduling on the basis of the determined at least one first decision variable and the at least one second decision variable. 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; provide feedback to the base station, the feedback comprising information for providing at least two data rate requests of the user terminal, the data rate requests being determined on the basis of the calculated signal-to-noise ratios corresponding to the at least two transmit weight vectors for enabling the base station to control scheduling.

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.