WO2009061260A1

WO2009061260A1 - Method and arrangement in a wireless communications system

Info

Publication number: WO2009061260A1
Application number: PCT/SE2008/051114
Authority: WO
Inventors: George JÖNGREN
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2007-11-06
Filing date: 2008-10-02
Publication date: 2009-05-14

Abstract

An object of the present invention is to provide a mechanism for improving resource utilization in a wireless communications system. The object is achieved by a method in a first node (1 10) for transmitting reference symbols to a second node (120). The first node (110) and the second node ( 120) are both comprised in a wireless communications system (100). The first node (110) comprises at least three antenna ports (140, 150, 160). In the method, first and subsequent groups of reference symbols are transmitted, in two separate time periods, from each one of the antenna ports (140, 150, 160) to the second node (120). The first and subsequent groups comprise an equal number of reference symbols for each one of the antenna ports (140, 150, 160). Furthermore, each of the antenna ports (140, 150, 160), transmits its reference symbols on different frequency positions in the first and subsequent time periods.

Description

METHOD AND ARRANGEMENT IN A WIRELESS COMMUNICATIONS SYSTEM

TECHNICAL FIELD

The present invention relates generally to a method and an arrangement in a first node in a wireless communications system and to a method and an arrangement in a second node in a wireless communications system In particular it relates to transmitting reference symbols from a first node to a second node

BACKGROUND Wireless communication systems generally comprise a number of nodes, e g mobile terminals and base stations, which nodes communicate with each other using modulated signals transmitted over some type of wireless link, e g radio channels In the mobile terminals, various settings such as transmit power and modulation rate are continuously updated to account for variations in radio conditions due to fading, attenuation etc In order to determine the appropriate settings, the mobile terminals will typically perform measurements of the current channel quality and report the results back to the base station

To enable coherent demodulation and measurement, reference symbols/signals are usually required In Orthogonal Frequency Division Multiplexing (OFDM) based wireless systems like the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as Long Term Evolution (LTE), the available resources are divided into a time and frequency grid The time is divided into subframes, each 1 ms and comprising a number of OFDM symbols For normal cyclic prefix length the number of OFDM symbols is 14, which implies that time is quantized into 14 symbols during a subframe Frequency corresponds to subcamers in the OFDM symbols The nu mber of subcarriers varies depending on the system bandwidth used

In LTE, a resource block spans 12 subcarriers and 0 5 ms The smallest scheduling unit is however a resource block pair, spanning 12 subcarπers and 1 ms The two resource blocks in a resource block pair do not necessarily occupy the same frequencies A resource block is the smallest isolated group of resource elements over which a scheduling may take place

In the following disclosure, the term "resource group" will be used to denote a schedulable unit of resource elements, e g an LTE resource block pair Each square in the time-frequency grid is a resource element There are several different types of resource elements These include resource elements to be used for transmitting reference symbols as well as resource elements to be used for transmitting data symbols, i e coded information-carrying symbols In case of multi-antenna transmission, which is used e g in E-UTRAN, the mobile terminal must be able to estimate the channel corresponding to each antenna Therefore, there is at least one reference symbol transmitted for each antenna port in the transmitting node This is illustrated in Figure 1a, 1 b and 1c showing the grid for one resource block pair, or resource group, for different numbers of antenna ports, i e numbers of eNodeB Tx antennas A blank square indicates that the corresponding resource element is used for transmitting data symbols, whereas a square designated "Rx" or "X", X being an integer, indicates that the corresponding resource element is used by antenna port X for transmitting a reference symbol Figure 1 a shows the case of one antenna port, Figure 1 b shows the case of two antenna ports, and Figure 1 c shows the case of four antenna ports As seen in Figures 1 a and 1 b, the reference symbols for the different antenna ports are present in OFDM symbols 0, 4, 7, 1 1 , i e in columns 0, 4, 7, and 1 1 , for one and two ports Figure 1c shows that in the case of four antenna ports, additional reference symbols transmitted by antenna ports 2 and 3 are included in OFDM symbols 1 and 8 For the multi-port cases we also note that when a reference symbol is transmitted on a certain antenna port, the other antenna ports are silent, meaning that no power is transmitted on them The reason for this is to avoid interference between reference symbols from different transmit antennas within the same cell, thereby simplifying the channel estimation calculations in the receiving unit In Figures 1 b and 1 c, a silent antenna port is indicated by a crosshatched pattern in the corresponding resource element It should also be noted that according to the E-UTRAN standard, the same transmit power must be used on all resource elements of the same type within an OFDM symbol The type of a resource element is determined by the presence or absence of RS in the corresponding OFDM symbol This requirement applies across all antenna ports in the transmitting node and also within a single resource block, i e all antenna ports must use the same transmit power for all resource elements of the same type within a resource block

For the case of four antenna ports, there is a substantial problem with the existing design of the reference symbol patterns The fundamental problem is that the reference symbol patterns do not utilize all antenna ports for a particular OFDM symbol, in combination with the fact that the power output from the Power Amplifiers (PA) cannot be shared in time This problem will now be illustrated with reference to Figure 1c In OFDM symbol 0, only antenna ports 0 and 1 are used for reference symbols As stated above, antenna ports 2 and 3 must be silent while antenna ports 0 and 1 are transmitting reference symbols Hence, antenna ports 2 and 3 are active on fewer resource elements than antenna ports 0 and 1 in OFDM symbol 0, as indicated by the four crosshatched squares in OFDM symbol 0, i e column 0, for antenna ports 2 and 3 However, because of the constant power requirement per data resource element mentioned above, antenna ports 2 and 3 are not allowed to use all their available power for transmitting data symbols, as this would mean that the data symbols from antenna ports 2 and 3 would be transmitted with higher power than the data symbols from antenna ports 0 and 1 Thus, antenna ports 2 and 3 will use less than full power on this OFDM symbol The situation is similar for OFDM symbol 1 , except that now antenna ports 0 and 1 use less power Thus, for the OFDM symbols containing reference symbols, two of the PA s always run at less than full power, which means that power resources are wasted The amount of wasted power increases even further if the power on the reference symbols is boosted, i e the reference symbols are transmitted using higher power than the data symbols Reference symbol power boosting is a technique that may be used to increase coverage, e g in rural areas In the worst case, e g in the example in Figure 1c, when the reference symbols on antenna ports 0 and 1 are maximally boosted, antenna ports 0 and 1 will have no power left for transmitting data symbols in OFDM symbol 0 Consequently, no power at all will be transmitted on antenna ports 2 and 3 for OFDM symbol 0 This is because antenna ports 2 and 3 are required to use the same transmit power per data resource element as antenna ports 0 and 1 , i e zero power Hence, a ntenna ports 2 and 3 are completely useless for the OFDM symbol under study Another interesting property of the current four antenna port design is that the density in time of the reference symbols on antenna ports 0 and 1 is higher than the density on antenna ports 2 and 3 This is clearly illustrated in Figure 1c, which shows that antenna ports 0 and 1 transmit reference symbols in OFDM symbols 0, 4, 7, and 11 , whereas antenna ports 2 and 3 only transmit reference symbols in OFDM symbols 1 and 8 One reason for this is that the reference symbol pattern for the first two antenna ports is then the same as for the two antenna ports case A benefit of this design is that when a user equipment performs measurements, it can do so using the same resource elements without knowing if there are two or four antenna ports configured On the other hand, since the reference symbol density is different on different antenna ports, the channel estimation performance on antenna ports 2 and 3 will be worse than the channel estimation performance on antenna ports 0 and 1 at higher speeds However, having better estimates on the first two antenna ports is quite wasteful, as the worst channel estimates typically have a dominating impact on the performance - in essence it does not help much having a higher density for two of the antenna ports In addition, a higher reference symbol density incurs significant additional overhead, which directly reduces the data rates for the system

SUMMARY

It is therefore an object of the present invention to provide a mechanism for improving resource utilization in a wireless communications system

According to a first aspect of the present invention, the object is achieved by a method in a first node for transmitting reference symbols to a second node The first node and the second node are both comprised in a wireless communications system The first node comprises at least three antenna ports Furthermore, the first node uses a number of predetermined frequency positions for transmitting the reference symbols The number of predetermined frequency positions is at least equal to the number of antenna ports

In the method , a first group of reference symbols on a fi rst set of frequency positions among the predetermined frequency positions is transmitted from each one of the antenna ports to the second node This transmission is performed in a first time period The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports

Then, in a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions is transmitted from each one of the antenna ports to the second node The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

According to a second aspect of the present invention, the object is achieved by an arrangement in a first node for transmitting reference symbols to a second node The first node and the second node are both comprised in a wireless communications system The first node arrangement comprises at least three antenna ports The first node arrangement also comprises a transmitting unit, which is configured to use a number of predetermined frequency positions for transmitting the reference symbols The number of predetermined frequency positions is at least equal to the number of antenna ports Furthermore, the transmitting unit is configured to transmit a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions from each one of the antenna ports to the second node This transmission is performed in a first time period The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports The transmitting unit is also configured to transmit a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions from each one of the antenna ports to the second node This transmission is performed in a subsequent time period The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

According to a third aspect of the present invention, the object is achieved by a method in a second node for receiving reference symbols from a first node The first node and the second node are both comprised in a wireless communications system The first node comprises at least three antenna ports The second node uses a number of predetermined frequency positions, at least equal to the number of antenna ports in the first node, for receiving the reference symbols from the first node In the method , a first group of reference symbols is received on a first set of frequency positions among the predetermined frequency positions from each one of the antenna ports in the first node This first group is received in a first time period The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports in the first node Then, in a subsequent time period, a subsequent group of reference symbols is received on a subsequent set of frequency positions among the predetermined frequency positions from each one of the antenna ports in the first node The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports in the first node Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

According to a fourth aspect of the present invention, the object is achieved by an arrangement in a second node for receiving reference symbols from a first node The first node and the second node are both comprised in a wireless communications system The first node comprises at least three antenna ports

The second node arrangement comprises at least one antenna port The second node arrangement also comprises a receiving unit, which is configured to use a number of predetermined frequency positions for receiving the reference symbols from the first node using the at least one antenna port The number of predetermined frequency positions is at least equal to the number of antenna ports in the first node

Furthermore, the receiving unit is configured to receive a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions using the at least one antenna port The first group is received in a first time period The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports in the first node

The receiving unit is also configured to receive a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions using the at least one antenna port The subsequent group is received in a subsequent time period The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports in the first node Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

By utilizing a reference symbol pattern such that the first and second groups of reference symbols comprise an equal number of reference symbols for each one of the antenna ports, it is ensured that a single time period will contain reference symbols intended for all antenna ports Thereby, a balanced reference symbol pattern is created which enables the antenna ports to use full transmission power, bearing in mind the constant transmit power requirement per data resource element Consequently, the resource utilization in the system is improved Thus, an advantage of the present invention is that Power Amplifier balance is achieved, thereby making additional power available for data transmission.

By also exploiting the time dimension for distributing the reference symbols over a first and subsequent time period, it is ensured that no single time period will be crowded with reference symbols. Consequently, an additional advantage of the invention is that it will not limit the possibility to measure inter-cell interference on reference symbol resource elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 a is a schematic diagram depicting a time-frequency grid according to the prior art;

Figure 1 b is a schematic diagram depicting time-frequency grids according to the prior art;

Figure 1 c is a schematic diagram depicting time-frequency grids according to the prior art;

Figure 2 is a schematic diagram illustrating embodiments of a wireless communications system;

Figure 3a is a schematic diagram depicting embodiments of time-frequency grids;

Figure 3b is a schematic diagram depicting embodiments of time-frequency grids;

Figure 4 is a flowchart illustrating embodiments of method steps in a first node;

Figure 5 is a schematic diagram depicting embodiments of time-frequency grids;

Figure 6 is a block diagram showing embodiments of a first node arrangement;

Figure 7 is a flowchart illustrating embodiments of method steps in a second node;

Figure 8 is a block diagram showing embodiments of a second node arrangement. DETAILED DESCRIPTION

In order to improve power resource utilization, the present solution uses a reference symbol pattern design where the reference symbols for the different antenna ports are distributed more uniformly over several OFDM symbols This ensures power amplifier balance and in addition does not crowd a particular OFDM symbol with only reference symbols

The invention is defined as a method and an arrangement which may be put into practice in the embodiments described below

Figure 2 shows a wireless communications system 100 comprising a first node 110 and a second node 120, which first node 110 and second node 120 are communicating with each other over a wireless link 130, i e a radio channel The wireless communications system 100 may be an OFDM-based system or any other wireless communications system being capable of addressing particular resources in the time and frequency domain

The first node 1 10 comprises at least three antenna ports 140, 150, 160, enabling multi-antenna transmission to and reception from the first node 1 10 and possibly other nodes that may be comprised in the wireless communications system 100

The second node 120 comprises at least one antenna port 170, but may also comprise two or more antenna ports for multi-antenna transmission to and reception from the first node 1 10 and possibly other nodes that may be comprised in the wireless communications system 100

Each one of the antenna ports 140, 150, 160 in the first node 1 10 is arranged to transmit a signal to the second node 120 comprising reference symbols, also known as reference signals, according to a predetermined pattern, which will be further described below The second node 120 is arranged to receive the reference symbols and may use them to measure channel quality, to ensure coherent demodulation or for other purposes

In the example in Figure 2, the first node 1 10 is represented by a base station, such as a NodeB in a Universal Mobile Telecommunications System (UMTS), an eNodeB in an evolved Universal Terrestrial Radio Access Network (E-UTRAN), or any other system using multi-antenna transmission The second node 120 is represented by a user equipment, such as a mobile terminal, Personal Digital Assistant (PDA), portable computer or any other equipment capable of receiving multi-antenna transmissions Furthermore, it should be noted that the roles of the first and second nodes may be reversed, i e the first node 110 may be a user equipment or similar, and the second node 120 may be a base station or similar In other words, the present solution is applicable to the uplink as well as the down link direction

When the first node 110 and second node 120 are communicating in accordance with the example shown in Figure 2, the second node 120 is required to demodulate the signal received from the first node 1 10 and also to perform measurements on the signal, as described above For this purpose, reference symbols are included in the signal transmitted from the first node 110 to the second node 120

In order to achieve a balanced use of the power amplifiers on the different antenna ports in the first node 1 10, all antenna ports are required to be used for transmitting reference symbols in a single OFDM symbol In addition, all antenna ports shall be active on the same number of data resource elements, and therefore all antenna ports will be able to use equal and hence full power To avoid filling a single OFDM symbol with almost only reference symbols, two closely spaced OFDM symbols may be used, such that the reference symbol energy on a certain antenna port is alternated not only in frequency, but also in time An advantage of not crowding an OFDM symbol with reference symbols is to facilitate the measurement of interference from other cells in the network, by taking advantage of the fact that other cells may transmit in the "gaps" where the current cell is not transmitting any reference symbols.

A balanced reference symbol pattern according to an embodiment of the invention will now be described with reference to Figure 3a In this particular embodiment, four antenna ports 0, 1 , 2 and 3 are used Figure 3a illustrates the time-frequency grids for each antenna port Each grid, comprising 14 OFDM symbols and 12 subcamers, corresponds to one E-UTRAN resource block pair Each square corresponds to one resource element A square denoted 0, 1 , 2 or 3 in the grid indicates that the corresponding antenna port 0, 1 , 2 or 3 is transmitting a reference symbol in that resource element A crosshatched square indicates that the antenna port is silent for that resource element, because another antenna port is transmitting a reference symbol Looking first at OFDM symbol 0, i e column 0, it can be seen that all four antenna ports are transmitting one reference symbol, as indicated by the number 0, 1 , 2, or 3 in column 0 of each grid Consequently, all four antenna ports are silent for three resource elements while the other antenna ports are transmitting reference symbols, as indicated by the three crosshatched squares in column 0 of each grid Thus, all four antenna ports are active on the same number of data resource elements for OFDM symbol 0, i e eight data resource elements, as indicated by the eight blank squares in column 0 of each grid Therefore, all four antenna ports will be able to use full power, and PA balance is achieved for OFDM symbol 0 Analogously, OFDM symbols 1 , 7, and 8 also exhibit a balanced reference symbol pattern

Furthermore, when looking at OFDM symbols 0 and 1 together, i e columns 0 and 1 , it can be seen that the reference symbols for a certain antenna port are staggered in the frequency domain, i e the reference symbols are transmitted on different subcarriers in OFDM symbols 0 and 1 For example, for antenna port 0, the reference symbol in OFDM symbol 0 is transmitted on the sixth subcarrier, i e row, while the reference symbol in OFDM symbol 1 is transmitted on the twelfth subcarrier A benefit of this staggered, or zigzag, hopping pattern (which will be described in more detail in connection with Figure 5) is that the susceptibility to channel variations in the frequency domain is decreased To clarify, the reference symbols will reappear more often in the frequency domain when a staggered pattern is used, which means that the channel conditions are less likely to change significantly between two occurrences of a reference symbol Again using antenna port 0 and OFDM symbols 0 and 1 as an example, note that there is a distance of five subcarriers, i e rows, between the two occurrences of reference symbol 0 in OFDM symbols 0 and 1 If the reference symbol would instead be transmitted on the same subcarrier in OFDM symbols 0 and 1 , the distance until the next occurrence of reference symbol 0 would be 11 subcarriers This is assuming that the same reference symbol pattern is repeated over at least some part of the bandwidth, i e the next occurrence of reference symbol 0 would be in the sixth subcarrier in OFDM symbol 0 of the next resource group

The exact hopping pattern may vary and is not limited to use of the particular subcarriers depicted in Figure 3a In particular, the hopping pattern may use a longer period , e g there may be a larger number of subcarriers between each occurrence of a certain reference symbol or it takes more subcarriers until the hopping pattern repeats itself because it is irregular According to some embodiments, the two OFDM symbols that are used for creating the balanced reference symbol pattern are also closely spaced in time For example, in Figure 3a the two consecutive OFDM symbols 0 and 1 are balanced An advantage of this approach is that the channel will not fade significantly across the OFDM symbols, i e over time, which would call for a more complicated channel estimation procedure for interpolating channel e stimates Note that the two OFDM symbols are not required to be adjacent, as long as they are spaced closely enough in time that the channel variations are not significant

Figure 3b illustrates the reference symbol pattern of Figure 3a from a different viewpoint In Figure 3b, reference symbols from all four antenna ports are simultaneously shown in a single time-frequency grid, i e all four antenna ports are shown overlaid Furthermore, two adjacent resource groups are shown, to illustrate the fact that the balanced reference symbol pattern may be repeated across at least a part of the available bandwidth The border between the two resource groups is indicated by a thick line 305 in Figure 3b A square denoted 0, 1 , 2 or 3 indicates that the corresponding antenna port 0, 1 , 2 or 3 is transmitting a reference symbol in that resource element Again, OFDM symbols 0, 1 , 7, and 8 exhibit balanced reference symbol patterns, as can be verified by the presence of exactly one reference symbol from each antenna port in each of the columns 0, 1 , 7, and 8 within each resource group Note that OFDM symbols 0 and 1 , as well as symbols 7 and 8, constitute pairs of OFDM symbols that are using the zigzag hopping pattern described above In Figure 3b, the zigzag pattern is indicated by the dashed arrows 310 Although, for reasons of clarity, the zigzag pattern is only indicated by arrows for antenna port 1 , the reference symbols from the other antenna ports exhibit corresponding zigzag patterns, as can be seen in Figure 3b even if not pointed out with arrows

Note that although Figure 3b only depicts two resource groups, the reference symbol pattern may repeat for the other resource groups over part of the bandwidth, or the whole bandwidth It is also possible for the reference symbol pattern to vary across the system bandwidth For example, a certain pattern may be used for half the bandwidth and another pattern for the remaining bandwidth

The reference symbol pattern shown in Figures 3a and 3b may also be applied for the two antenna port case as well as for the one antenna port case In the two antenna port case, only the reference symbols for antenna ports 0 and 1 would be used, and for the one antenna port case, only the reference symbols for antenna port 0 would be used Such an approach will facilitate measurements, as the user equipment 120 will be able to measure on the same resource elements regardless of the number of antenna ports Moreover, it should be noted that the inventive idea can easily be generalized to other numbers of antenna ports

Furthermore, it should be noted that while the present example uses a normal cyclic prefix length, other prefix lengths can be similarly handled A different cyclic prefix would correspond to a different number of OFDM symbols within a subframe, i e a different number of columns in the time-frequency grid Thus, different OFDM symbols (columns) would be used for transmitting reference symbols when using a different cyclic prefix length, but the same reference symbol patterns as in the present example may still be used

According to some further embodiments, the reference symbols primarily intended for measurements are transmitted over a limited bandwidth This may save considerable overhead and also significantly reduce the power wastage on the OFDM symbols Turning again to Figure 3b, note that OFDM symbols 4 and 1 1 comprise reference symbols from antenna ports 0 and 1 only, and thus the reference symbol pattern is not balanced for these OFDM symbols This is because OFDM symbols 4 and 1 1 do not have any neighbouring OFDM symbols containing reference symbols, and therefore the zigzag hopping pattern described above cannot be created Because OFDM symbols 4 and 1 1 are not balanced, a certain amount of power wastage will occur The reference symbols in OFDM symbols 4 and 1 1 are primarily useful from measurement perspective However, depending on the accuracy requirements on the measurements, the measurement bandwidth does not necessarily need to equal the whole system bandwidth Hence, considerable overhead may be saved by not transmitting reference symbols over the whole bandwidth for OFDM symbols 4 and 1 1 For example, the reference symbols may be limited to a 1 25 MHz bandwidth (which is the smallest possible in E-UTRAN) or perhaps 2 5 MHz in order to include some margin This may also save power better used for data transmission

The present method steps in the first node 110 for transmitting reference symbols to the second node 120 will now be described with reference to a flow chart depicted in Figure 4. As mentioned above, the first node 110 and the second node 120 are comprised in a wireless communications system 100, which according to some embodiments may be an OFDM-based system, e.g. an E-UTRAN. The first node 110, which may be an eNodeB, comprises at least three antenna ports 140, 150, 160. In some embodiments, the first node 1 10 comprises four antenna ports. Furthermore, the first node 1 10 uses a number of predetermined frequency positions, at least equal to the number of antenna ports, for transmitting the reference symbols. Throughout this disclosure, the frequency positions may correspond to subcarriers, and the second node 120 may for instance be a user equipment (UE). The method comprises the following steps:

Step 401

In a first time period, a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions is transmitted to the second node 120 from each one of the antenna ports 140, 150, 160. Throughout this disclosure, the first time period may correspond to an OFDM symbol, e.g. OFDM symbol 0, and the first group of frequency positions may correspond to the predetermined subcarriers used by each antenna port for transmitting reference symbols. The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160.

Step 402

In a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions are transmitted to the second node 120 from each one of the antenna ports 140, 150, 160. Throughout this disclosure, the subsequent time period may correspond to another OFDM symbol, e.g. OFDM symbol 1 , and the subsequent group of frequency positions may again correspond to the predetermined subcarriers used by each antenna port for transmitting reference symbols. The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160. Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions transmitted in step 401 . In other words, each antenna port will transmit its reference symbols on frequency positions, e.g. subcarriers, that are all different from the ones used in step 401 . Figure 5 illustrates how the terms used above may be interpreted according to an embodiment of the present solution In Figure 5, two columns are shown, corresponding to OFDM symbols 0 and 1 in a time-frequency grid The remainder of the grid is not shown OFDM symbol 0 correspond s to the first time period, and OFDM symbol 1 corresponds to the subsequent time period The rows in Figure 5 correspond to OFDM subcamers, or frequency positions 24 subcarriers, i e two resource groups, are shown The border between the two resource groups is indicated by the thick line As shown in Figure 5, the first group of reference symbols comprises, in this example, the two zeros in OFDM symbol 0 (ι e column 0), and the subsequent group of reference symbols comprises the two zeros in OFDM symbol 1 (ι e column 1 ) The first and subsequent sets of frequency positions are, in this example, the subcamers used to transmit the reference symbols in the first and subsequent groups

Step 402, continued Referring again to Figure 4 and step 402, according to some embodiments, the subsequent group of reference symbols is shifted a number of frequency positions relative to the preceding group of reference symbols transmitted in step 401 An example of this is shown in Figure 5 , where the subsequent group of reference symbols, corresponding to the zeros in column 1 , or OFDM symbol 1 , are all shifted by five frequency positions, or subcarriers, relative to the first group of reference symbols comprised in column 0 or OFDM symbol 0

In some embodiments, the number of frequency positions between each pair of two consecutive reference symbols is constant in both the first and subsequent groups of reference symbols for each antenna port In other words, each antenna port may transmit its reference symbols using a constant number of frequency positions, e g subcarriers, between each occurrence of a reference symbol corresponding to that antenna port For example, in Figure 5 it can be seen that in each column, or time period, there is a fixed distance of eleven subcarriers between each occurrence of reference symbol 0, corresponding to antenna port 0

Some further embodiments may comprise a combination of the features of shifting the subsequent group, and using a constant number of frequency positions between pairs of reference symbols Thereby, the reference symbols for a certain antenna port are more evenly spread out in the frequency domain, which will reduce the system's sensitivity to channel variations in frequency An example of such an embodiment is shown in Figure 3b

The first and subsequent time periods may be closely spaced in time, e g they can be adjacent In some embodiments this corresponds to using two adjacent OFDM 5 symbols, e g OFDM symbols 0 and 1 , for transmitting the first and subsequent groups of reference symbols As mentioned above, an advantage of using closely spaced time periods is that the channel conditions are not likely to change significantly between the first and second time periods

10 Step 402a

Optionally, the step of transmitting a subsequent group of reference symbols may be repeated at least once

Step 403

15 According to some embodiments, the method comprises an additional step being performed in a further time period In this further step, an additional group of reference symbols is transmitted from at least one of the antenna ports 140, 150, 160 to the second node 120 The additional group of reference symbols is transmitted on an additional set of frequency positions among the predetermined frequency positions, but the additional

20 group of reference symbols is transmitted over only a part of the available bandwidth, in order to save transmission overhead The further time period may be an OFDM symbol, e g OFDM symbol 4 or 1 1 In some embodiments, two additional groups of reference symbols are transmitted from two antenna ports, e g one additional group from antenna port 0 and one additional group from antenna port 1 in an eNodeB

25

In some embodiments, the frequency positions in the first and the subsequent sets of frequency positions are all comprised within a single resource group This will ensure that a balanced reference symbol pattern is achieved within a resource group, which may be advantageous in a system like E-UTRAN, where a requirement of the same transmit

30 power on all antenna ports applies also to a small unit as a resource block It should be noted that the reference symbol pattern may then be repeated over one or more additional resource groups across the bandwidth

To perform the method steps for transmitting reference symbols to the second node 35 120, the first node 1 10 comprises an arrangement 600 depicted in Figure 6 The first node 110 and the second node 120 are comprised in a wireless communications system 100, which may be an OFDM-based system, e g an E-UTRAN In some embodiments, the first node 1 10 is an eNodeB and the second node 120 is a user equipment (UE)

The first node arrangement 600 is to be construed as an apparatus

The first node arrangement 600 comprises at least three antenna ports 140, 150, 160 enabling multi-antenna transmission and reception of signals According to some embodiments, the first node arrangement 600 comprises four antenna ports

The first node arrangement 600 further comprises a transmitting unit 610 configured to use a number of predetermined frequency positions, at least equal to the number of antenna ports, for transmitting the reference symbols

The transmitting unit 610 is further configured to transmit a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions from each one of the antenna ports 140, 150, 160 to the second node 120 This transmission is performed in a first time period The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 The transmitting unit 610 is also configured to transmit a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions from each one of the antenna ports 140, 150, 160 to the second node 120 This transmission is performed in a subsequent time period The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

According to some embodiments, the transmitting unit 610 is configured to perform the transmission so that the number of frequency positions between each pair of two consecutive reference symbols is constant in the first group of reference symbols for each antenna port, and the number of frequency positions between each pair of two consecutive reference symbols is also constant in the subsequent group of reference symbols for each antenna port

The transmitting unit 610 may also be configured to repeat the step of transmitting a subsequent group of reference symbols at least once In some embodiments, the transmitting unit 610 is configured to transmit the subsequent group of reference symbols so that it is shifted a number of frequency positions relative to the preceding group of reference symbols

The first and subsequent time periods may be closely spaced in time, e g adjacent According to some embodiments, the transmitting unit 610 is further configured to transmit an additional group of reference symbols on an additional set of frequency positions among the predetermined frequency positions from at least one of the antenna ports 140, 150, 160 to the second node 120 This additional transmission is performed in a further time period, e g OFDM symbol 4 or 11 The additional group of reference symbols are transmitted over only a part of the available bandwidth, thereby saving transmission power which may instead be used for data transmission In some embodiments two additional groups of reference symbols is transmitted from two antenna ports, e g one group from antenna port 0 and one group from antenna port 1 in an eNodeB The frequency positions in the first and the subsequent sets of frequency positions may all be comprised within a single resource group The transmitting unit 610 may be further adapted to transmit additional groups of reference symbols in additional resource groups

The present method steps in the second node 120 for receiving reference symbols from a first node 11 0 will now be described with reference to a flow chart depicted in Figure 7. As mentioned above, the first node 1 10 and the second node 120 are comprised in a wireless communications system 100, e g an E-UTRAN The first node 110, which may be an eNodeB, comprises at least three antenna ports 140, 150, 160 In some embodiments, the first node 1 10 comprises four antenna ports Furthermore, the second node 120 uses a number of predetermined frequency positions, at least equal to the number of antenna ports in the first node 1 10, for receiving the reference symbols The second node 120 may for instance be a user equipment (UE) The method comprises the following steps

Step 701

In a first time period, e g OFDM symbol 0, a first group of reference symbols from each one of the antenna ports 140, 150, 160 in the first node 110 is received on a first set of frequency positions among the predetermined frequency positions The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 in the first node 1 10

Step 702

5 In a subsequent time period, e g OFDM symbol 1 , a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions is received from each one of the antenna ports 140, 150, 160 in the first node 1 10 The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 in the first node 110 10 Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

According to some embodiments, the subsequent group of reference symbols is shifted a number of frequency positions relative to the preceding group of reference 15 symbols received in step 701

Step 702a

Optionally, the step of receiving a subsequent group of reference symbols may be repeated at least once 20

Step 703

According to some embodiments, the method comprises an additional step being performed in a further time period In this further step, an additional group of reference symbols is received from at least one of the antenna ports 140, 150, 160 in the first node 25 1 10 The additional group of reference symbols is received on an additional set of frequency positions among the predetermined frequency positions, but the additional group of reference symbols is received over only a part of the available bandwidth In some embodiments two additional groups of reference symbols are received from two antenna ports, e g one additional group is received from antenna port 0 and one 30 additional group is received from antenna port 1 in an eNodeB

The frequency positions in the first and the subsequent sets of frequency positions may all be comprised within a single resource group It should be noted that the received reference symbol pattern may then be repeated over one or more additional resource 5 groups across the bandwidth In some embodiments, the number of frequency positions between each pair of two consecutive reference symbols is constant in both the first and subsequent groups of reference symbols for each antenna port in the first node 1 10. The first and subsequent 5 time periods may be closely spaced in time, e.g. they can be adjacent.

To perform the method steps for receiving reference symbols from the first node 1 10, the second node 120 comprises an arrangement 800 depicted in Figure 8. The first node 110 and the second node 120 are comprised in a wireless communications system 10 100, e.g. an E-UTRAN. In some embodiments, the first node 1 10 is an eNodeB and the second node 120 is a user equipment (UE).

The second node arrangement 800 is to be construed as an apparatus.

15 The second node arrangement 800 comprises at least one antenna port 170 for reception and transmission of signals.

The second node arrangement 800 further comprises a receiving unit 810, which is configured to use a number of predetermined frequency positions, at least equal to the 0 number of antenna ports in the first node 1 10, for receiving the reference symbols from the first node 110 using the at least one antenna port 170.

The receiving unit 810 is further configured to receive a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions, using the at least one antenna port 170. This reception is performed in a first 5 time period, e.g. OFDM symbol 0. The first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 in the first node 1 10.

The receiving unit 810 is further configured to receive a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined0 frequency positions using the at least one antenna port 170. This reception is performed in a subsequent time period, e.g. OFDM symbol 1. The second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports 140, 150, 160 in the first node 110. Furthermore, the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency5 positions that are not comprised in the first set of frequency positions. According to some embodiments, the number of frequency positions between each pair of two consecutive received reference symbols is constant in the first group of reference symbols received from each antenna port, and the number of frequency positions between each pai r of two consecutive received reference symbols is also constant in the subsequent group of reference symbols received from each antenna port

The receiving unit 810 may also be configured to repeat the step of receiving a subsequent group of reference symbols at least once

In some embodiments, the receiving unit 810 is configured to receive a subsequent group of reference symbols which is shifted a number of frequency positions relative to the preceding group of received reference symbols

The first and subsequent time periods may be closely spaced in time, e g adjacent

According to some embodiments, the receiving unit 810 is further configured to receive an additional group of reference symbols on an additional set of frequency positions among the predetermined frequency positions from at least one of the antenna ports 140, 150, 160 in the first node 110 This additional transmission is performed in a further time period, e g OFDM symbol 4 or 1 1 The additional group of reference symbols are received over only a part of the available bandwidth In some embodiments two additional groups of reference symbols is received from two antenna ports, e g one group is received from antenna port 0 and one group is received from antenna port 1 in an eNodeB The frequency positions in the first and the subsequent sets of frequency positions may all be comprised within a single resource group Thus, the receiving unit 810 may be configured to receive the first and second groups of reference symbols on frequency positions, e g subcamers, all comprised in one resource group The receiving unit 810 may be further adapted to receive additional groups of reference symbols in additional resource groups

The present mechanism for receiving reference symbols may be implemented through one or more processors, such as the processor 620 in the first node arrangement 600 depicted in Figure 6, or the processor 820 in the second node arrangement 800 depicted in Figure 8, together with computer program code for performing the functions of the present solution. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the present solution when being loaded into the first node 110 or the second node 120. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code can furthermore be provided as pure program code on a server and downloaded to the first node 110 or the second node 120 remotely.

It should be noted that, throughout this disclosure, the term "resource group" is used to signify a unit of resource elements that may be scheduled. Thus, the term "resource group" shall be interpreted as having the meaning "schedulable unit of resource elements" and is thus not limited to encompassing only an E-UTRAN resource block pair.

When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of.

The present invention is not limited to the above-describe preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention , which is defined by the appending clai ms.

In particular, it should be noted that although terminology from 3GPP LTE has been used in this disclosure to exemplify the invention , this should not be construed as limiting the scope of the invention to only the aforementioned system. Other wireless systems, including Wireless Code Division Multiple Access (WCDMA) , Worldwide interoperability for Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile communications (GSM), may also benefit from exploiting the ideas covered within this disclosure.

Claims

1. A method in a first node (110) for transmitting reference symbols to a second node (120), the first node (1 10) and the second node (120) being comprised in a wireless communications system (100), the first node (1 10) comprising at least three antenna ports (140, 150, 160), the first node (110) using a number of predetermined frequency positions, at least equal to the number of antenna ports, for transmitting the reference symbols, the method comprising the steps of from each one of the antenna ports (140, 150, 160), transmitting, in a first time period, a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions to the second node (120), wherein the first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160), from each one of the antenna ports (140, 150, 160), transmitting, in a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions to the second node (120), wherein the second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160), and wherein the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions.

2. A method according to claim 1 , wherein the number of frequency positions between each pair of two consecutive reference symbols is constant in said first group of reference symbols for each antenna port, and wherein the number of frequency positions between each pair of two consecutive reference symbols is constant in said subsequent group of reference symbols for each antenna port.

3. A method according to any one of claims 1 -2, wherein the step of transmitting a subsequent group of reference symbols is repeated at least once.

4. A method according to any one of claims 1 -3, wherein said subsequent group of reference symbols is shifted a number of frequency positions relative to the preceding group of reference symbols. A method according to any one of claims 1-4, wherein the first and subsequent time periods are closely spaced in time, e g adjacent

A method according to any one of claims 1 -5, comprising the further step of from at least one of the antenna ports (140, 150, 160), transmitting, in a further time period, an additional group of reference symbols on an additional set of frequency positions among the predetermined frequency positions to the second node ( 120), wherein the additional group of reference symbols are transmitted over only a part of the available bandwidth

A method according to any one of claims 1 -6, wherein the frequency positions in the first and the subsequent sets of frequency positions are all comprised within a single resource group

A method according to any one of claims 1 -7, wherein the wireless communications system (100) is an OFDM-based system and the frequency positions correspond to subcamers

A method in a second node (120) for receiving reference symbols from a first node (110), the first node (1 10) and the second node (120) being comprised in a wireless communications system (100), the first node (1 10) comprising at least three antenna ports (140, 150, 160), the second node (120) using a number of predetermined frequency positions, at least equal to the number of antenna ports in the first node (1 10), for receiving the reference symbols from the first node (110), the method comprising the steps of from each one of the antenna ports (140, 150, 160) in the first node (1 10), receiving, in a first time period, a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions, wherein the first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160) in the first node (110), from each one of the antenna ports (140, 150, 160) in the first node (1 10), receiving, in a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions, wherein the second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160) in the first node (110), and wherein the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

10 A method according to claim 9, wherein the number of frequency positions between each pair of two consecutive reference symbols is constant in said first group of reference symbols for each antenna port in the first node (1 10), and wherein the number of frequency positions between each pair of two consecutive reference symbols is constant in said subsequent group of reference symbols for each antenna port in the first node (1 10)

1 1 A method according to any one of claims 9-10, wherein the step of receiving a subsequent group of reference symbols is repeated at least once

12 A method according to any one of claims 9-11 , wherein said subsequent group of reference symbols is shifted a number of frequency positions relative to the preceding group of reference symbols

13 A method according to any one of claims 9-12, wherein the first and subsequent time periods are closely spaced in time, e g adjacent

14 A method according to any one of claims 9-13, comprising the further step of from at least one of the antenna ports (140, 150, 160) in the first node (110), receiving, in a further time period, an additional group of reference symbols on an additional set of frequency positions among the predetermined frequency positions, wherein the additional group of reference symbols are received over only a part of the available bandwidth

5 A method according to any one of claims 9-14, wherein the frequency positions in the first and the subsequent sets of frequency positions are all comprised within a single resource group 6 A method according to any one of claims 9-15, wherein the wireless communications system (100) is an OFDM-based system and the frequency positions correspond to subcamers An arrangement (600) in a first node (1 10) for transmitting reference symbols to a second node (120), the first node (1 10) and the second node (120) being comprised in a wireless communications system (100), the first node arrangement (600) comprising at least three antenna ports (140, 150, 160), a transmitting unit (610) configured to use a number of predetermined frequency positions, at least equal to the number of antenna ports, for transmitting the reference symbols, the transmitting unit (610) being further configured to transmit, from each one of the antenna ports (140, 150, 160), in a first time period, a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions to the second node (120), wherein the first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160), and the transmitting unit (610) being further configured to transmit, from each one of the antenna ports (140, 150, 160), in a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions to the second node (120), wherein the second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160), and wherein the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions

An arrangement (800) in a second node (120) for receiving reference symbols from a first node (1 10) , the first node (1 10) and the second node (120) being comprised in a wireless communications system (100), the first node (1 10) comprising at least three antenna ports (140, 150, 160), the second node arrangement (800) comprising at least one antenna port (170), a receiving unit (810), configured to use a number of predetermined frequency positions, at least equal to the number of antenna ports in the first node (1 10), for receiving, using the at least one antenna port (170), the reference symbols from the first node (1 10), the receiving unit (810) being further configured to receive, using the at least one antenna port (170), in a first time period, a first group of reference symbols on a first set of frequency positions among the predetermined frequency positions, wherein the first group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160) in the first node (1 10), the receiving unit being further configured to receive, using the at least one antenna port (170), in a subsequent time period, a subsequent group of reference symbols on a subsequent set of frequency positions among the predetermined frequency positions, wherein the second group of reference symbols comprises an equal number of reference symbols for each one of the antenna ports (140, 150, 160) in the first node (110), and wherein the subsequent set of frequency positions comprises only frequency positions among the predetermined frequency positions that are not comprised in the first set of frequency positions.