RU2574854C2 - Method of operating secondary station - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to a method of operating a secondary station which communicates with at least one primary station via MIMO transmissions. A method of operating a secondary station which communicates with at least one primary station via MIMO transmissions is characterised by that the secondary station signals a single precoding indicator representative of at least one set of recommended precoding coefficients to the primary station, wherein the single precoding indicator is common for a plurality of available MIMO transmission modes.

EFFECT: high data rate, reduced amount of resources needed to signal recommended precoding coefficients.

7 cl, 3 dwg

Description

FIELD OF THE INVENTION

The present invention relates to methods for operating radio stations in mobile communication networks, where a primary station communicates with at least one secondary station.

This invention is, for example, suitable for IEEE 802.11 networks or also for long-term development networks of UMTS (LTE and LTE-Advanced), where the communication between the primary station and the secondary station can use different modes, for example, MIMO transmission for multiple / single user, different MIMO gear grades, or even SISO / MIMO mode.

State of the art

In a cellular telecommunication system, a plurality of user terminals within a cell communicate with a primary station serving a cell. With each subsequent generation of cellular systems, an increase in the achievable data rate is supported. In advanced systems such as UMTS and LTE, multi-antenna transmission / reception techniques, variously described as MIMO, precoding, or beamforming, are supported for transmissions from a single primary station to a mobile terminal. Due to the spatial selectivity of the beamforming mode, such transmission modes provide the possibility of an important increase in the achievable data rate and communication range, while maintaining an average level of interference.

In order to achieve beamforming, a conventional transmitting station having an antenna array applies a set of complex coefficients (forming a precoding matrix or precoding vector) to the signal transmitted from its respective antennas, so that the transmission stream is spatially directed towards the receiving station. In a wireless communication system such as LTE, both the base station and the terminal are typically equipped with multiple antennas. This provides the possibility of different MIMO operating modes along with traditional transmission modes, such as SISO transmission modes. In order to support the base station in determining channel conditions due to the selection of an appropriate transmission mode, mobile terminals typically measure downlink channels for each pair of antennas and display a channel status message to send to the base station.

The base station can then use this information for planning decisions, such as:

- to which terminals to transmit;

- what frequency / time / code resources are used;

- MIMO transmission mode parameters (for example, the number of spatial streams and SU-MIMO or MU-MIMO).

In embodiments of such systems, precoding codebooks are defined. These precoding codebooks can be considered as a way of describing a precoding matrix (or precoding vector) of channel precoding coefficients or precoding weights in a compact way, thereby reducing the amount of signaling required to indicate precoding. These codebooks also enable the user terminal (defined in LTE as user equipment or UE) to report to the network a preferred downstream encoder, in the form of an index for writing a codebook.

In this case, the preferred precoder is a set of complex coefficients to be applied to the transmitting antennas of the base station (defined in LTE as eNodeB). Similarly, precoding codebooks can also be used by a base station to signal precoding used for transmission to a user terminal. This allows the user terminal to derive the appropriate phase / amplitude reference from the common pilot symbols to demodulate each downlink transmission.

An effective way of capturing channel status information is to select an entry from the codebook of the preliminary encoders, which, when used in the transmitter, will result in the highest data rate. This information may be signaled as PMI (precoding matrix indicator). The number of estimated spatial streams, or rank indicator (RI), is usually part of such a message.

As an example of a codebook structure, the LTE Release 8 codebook for 4 transmission antennas has a nested structure. This means that for a given codebook index, the precoding vectors for a given transmission rank are a subset of the vectors for the next higher rank. This property can be useful in using the same codebook for both SU-MIMO (with multiple layers transmitted to the same terminal) and MU-MIMO (with a limited number of layers transferred to more than one terminal )

There is a requirement for LTE-A that dynamic switching between SU-MIMO and MU-MIMO transmission modes will be supported. In this case, it is desirable that the feedback of the UE be suitable for both modes. This can be achieved by sending PMI / RI and CQI for both modes. The main differences between PMI / CQI calculations for the two modes can be:

- the estimated maximum transmission rank (for example, limited only by the number of antennas for SU-MIMO and up to rank = 2 for MU-MIMO)

- available power (for example, total power for SU-MIMO and power divided between terminals for MU-MIMO)

- There may be different assumptions about interference.

However, this requires more resources. One objective of the invention is to reduce the number of bits required to signal PMI.

Huawei: "Feedback framework consideration for single cell MIMO" 3GPP project R1-094713, November 9, 2009, reveals implicit feedback as the basis for a single SU / MU-MIMO cell in LTE-Advanced, and it offers the same PMI size for SU-MIMO and MU-MIMO. By using the same PMI feedback size (e.g. 4 bits), a feedback structure with small overhead information for SU and MU can be achieved.

Additionally, Ericsson et al .: "PMI-based Multi-Granular Feedback for SU / MU-MIMO Operation", 3GPP project R1-100852, reveals a similar concept of implicit feedback for SU- and MU-MIMO, and the same implicit feedback type can be used for SU- as well as for MU-MIMO. More specifically, two matrix indicators are transmitted by feedback, which together form the complete recommended preliminary encoder, the first matrix being responsible for the channel correlation properties, and the second matrix trying to compare the instantaneous properties of the effective channel. Thereby, the spatial resolution of precoding can be increased, while feedback overhead is limited.

Additionally, Texas Instruments: "Downlink MU-MIMO and Related Feedback Support", 3GPP project R1-095019, reveals implicit PMI / CQI feedback, while the UE does not make any specific hypothesis about whether it will be planned in SU / MU mode , and only reports SU CSI. Additionally, instead of feedbacking the single best PMI, the UE reports two recommended SU-MIMO PMIs to the eNB, i.e. first best and second best SU PMI.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method that mitigates the aforementioned problems.

Another object of the invention is to propose a method that enables a primary station to make a decision regarding the selection of a suitable transmission mode without requiring a large amount of a signaled resource.

Another object of the invention is to propose a method that reduces the amount of computation required in a secondary station to provide a recommended set of precoding coefficients.

In accordance with a first aspect of the invention, there is provided a method of operating a secondary station, the secondary station communicating with at least one primary station via MIMO transmissions, the method comprising the step of signaling the secondary station a single precoding indicator representing a set recommended precoding factors to the primary station, with a single precoding indicator common to the many available modes MIMO transmission s.

In accordance with a second aspect of the invention, a secondary station is provided, said secondary station comprising means for communicating with at least one primary station via MIMO transmissions, and means for signaling a single precoding indicator representing a set of recommended precoding coefficients, to the primary station, and the same single precoding indicator is common to many available modes before ACHI MIMO.

In accordance with a third aspect of the invention, a primary station is provided, wherein said primary station comprises means for communicating with at least one secondary station via MIMO transmissions, the method comprising the step of: receiving a single indicator from the secondary station pre-coding, representing a set of recommended precoding coefficients, while a single precoding indicator is common to many and the available transmission modes, and wherein the primary station comprises control means for selecting a transmission mode from a plurality of available MIMO transmission modes based at least in part on the recommended precoding coefficients.

Accordingly, the secondary station only signals one precoding indicator, which is used for a plurality of transmission modes. In fact, instead of providing a precoding indicator for each transmission mode, each of which represents a recommended set of coefficients for each respective transmission mode, only one single precoding indicator is required, which can represent a different set of recommended set of precoding coefficients for each of the many modes transmission. Thus, the amount of resource required to signal the recommended precoding coefficients is reduced. One embodiment of the invention is based on the finding that, in a combined manner, an optimized codebook index for two different transmission modes (e.g., SU-MIMO and MU-MIMO, or others) will not lead to significantly lower data rates in practice. Additionally, in a specific example of SU-MIMO / MU-MIMO, it is likely that the choice of a suboptimal codebook index for SU-MIMO has less impact on the transmission rate than for MU-MIMO, in which case the index selection may be biased towards High bit rate for MU-MIMO.

Moreover, signaling one single precoding indicator may require less computation and operations from the secondary station, thus leading to reduced computing power demand and reduced energy costs for the secondary station.

These and other aspects of the invention will be apparent from and will be explained with reference to the embodiments described below.

Brief Description of the Drawings

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

- Figure 1 is a block diagram of a system in which a first embodiment of the invention is implemented.

- Figa, 2B represent a signaling message used to report precoding coefficients in accordance with the traditional method and with the first embodiment, respectively.

- Figure 3 represents a frame used to report precoding coefficients in accordance with a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a mobile communication system, such as 802.11, for example, 802.11n, or UMTS, for example, a UMTS LTE or LTE-Advanced system, as illustrated in FIG. A primary station, for example, in an embodiment of LTE, an eNodeB, 100 communicates with a plurality of secondary stations, for example, in an embodiment of LTE, by user equipment (UE) 110.

The telecommunication system of FIG. 1 comprises a primary station 100 operating in a cell 101, where a plurality of secondary stations 110 are located. For clarity, FIG. 1 shows only two secondary stations 110. The primary station 100 comprises a plurality of transmit antennas 104 controlled by a precoder 105, which can adjust the gain and phase of signals applied to transmitting antennas to transmit in beamforming mode on one or more spatial channels. A single data beam 111 resulting from the data signals 111a and 111b is represented in FIG. 1 from the primary station 100 to the secondary station 110. For clarity, FIG. 1 shows only one data beam, in accordance with a single-user MIMO transmission mode (SU- MIMO). In the multi-user MIMO (MU-MIMO) mode of operation, additional beams could be directed to other secondary stations 110. This data beam 111 forms a spatial channel on which data can be transmitted over a data channel, such as PDSCH (Physical Downlink Shared Channel) ), eg. The secondary station may be informed by signaling the physical layer (eg, PDCCH or the physical downlink control channel) of the spatial channel (eg, antenna port or virtual antenna) used to transmit data transmitted over the PDSCH.

In order to feed back the recommended set of precoding coefficients to the primary station 100, the secondary station 110 uses the uplink control channel 115.

According to a first embodiment of the invention, it is proposed that precoding coefficients are signaled with a single message common to a plurality of transmission modes, for example, not limited to SU-MIMO and MU-MIMO. For example, the same can be done for MIMO transmission modes that differ in transmission rank. The transmission rank determines the number of data streams used and the number of antennas intended for transmission. It should be noted that in the embodiments below, the codebooks for SU-MIMO and MU-MIMO can be independent, or a combination codebook, for example, using the inherent properties found in the Release 8 codebook of the Release 8 for 4 transmit antennas.

In the case of SU-MIMO / MU-MIMO, as in FIG. 1, this embodiment is based on the assumption that in a combined manner, the optimized codebook index for SU-MIMO and MU-MIMO will not lead to significantly lower data rates at practice. In a combined way, an optimized codebook can lead to a slightly worse data rate than a dedicated optimized codebook index. However, from tests it was found that this deterioration is very small.

It should be noted that a single precoding indicator may show a plurality of sets of precoding coefficients. In fact, the extraction of the coefficients in the primary station depends on the received codebook index, and on other parameters, for example, whether the mode is MU-MIMO or SU-MIMO, or the transmission rank. In fact, the particular set of coefficients shown by a given codebook index will usually depend on the transmission mode in question (e.g., transmission rank) and the codebook structure. Typically, for transmission grades larger than one, a different part of the set of coefficients will be associated with each spatial stream (or layer).

Additionally, it was found that in some scenarios the choice of a suboptimal codebook index for SU-MIMO has less impact on the transmission rate than for MU-MIMO, in which case the index selection may be biased towards achieving a high transmission rate for MU-MIMO. This can be done in the selection phase in the secondary station, by calculating the weighted combined speed, as will be seen in the options for this implementation in the future.

для режима передачи k. Этот выбор i и/или

делается так, что объединенная скорость:

максимизируется. В этом уравнении, веса могут быть такими, что

, и

является оцененной скоростью передачи данных для режима передачи MIMO k. В конкретном случае,

.In fact, in accordance with this first embodiment, the secondary station selects a set of recommended precoding coefficients, denoted by index i, for example, from measurements performed on the received signals. In embodiments, the secondary station may also select a rank from a set of possible ranks

for transmission mode k. This choice i and / or

so that the combined speed:

maximized. In this equation, weights can be such that

, and

is the estimated data rate for the MIMO k transmission mode. In a particular case,

.

More specifically, in the case where the PMI is common to two transmission modes, for example, SU-MIMO / MU-MIMO, the secondary station calculates the combined rate for each codebook index as follows

Where:

- i is a combined codebook index used to show a codebook entry in both SU and MU-MIMO codebooks;

и

являются рангами передачи для SU-MIMO и MU-MIMO соответственно;-

and

are the transmission grades for SU-MIMO and MU-MIMO, respectively;

и

являются достижимыми скоростями передачи для SU-MIMO и MU-MIMO соответственно;-

and

are achievable transmission rates for SU-MIMO and MU-MIMO, respectively;

.- α and β are weights. As a special case, described further below

.

, который максимизирует полную (объединенную) скорость.Usually a codebook search gives a set

That maximizes the overall (combined) speed.

The selection of α (and / or β) can be used to bias the selection of the codebook index towards SU or MU-MIMO. It can also be considered as referring to the average probability that a SU or MU-MIMO transmission will be selected by the base station. In this case, the value of α can be set so as to maximize the average speed achieved during a certain time interval. This weight may be predefined. However, in one embodiment, this weight is adjusted by the primary station and signaled to the secondary station, depending on the situation and which transmission modes should be preferred. Thus, in this embodiment of the first embodiment, the base station signals a weighting factor to adjust the codebook index selection (PMI) bias towards either SU or MU-MIMO. In a further embodiment, the weighting factor may be selected by the secondary station and possibly signaled to the primary station.

2A and 2B make it possible to compare the resource conservation obtained through these embodiments. 2A and 2B show typical bit numbers for secondary station feedback for a codebook for 8 antennas, and it is assumed that 8 receive antennas should be as follows:

2A, for a conventional system:

- The SU-MIMO field includes:

- PMI (6 bits)

- RI (rank indicator) - rank 1-8 (3 bits)

- CQI (channel quality indicator) for up to two codewords (6 bits)

- MU-MIMO

- PMI (6 bits)

- RI - rank 1 or 2 (1 bit)

- CQI for up to two codewords (6 bits)

For a complete requirement of 28 bits.

FIG. 2B, for a system in accordance with the invention:

- Combined alarm

- PMI (6 bits)

- SU-MIMO

- RI - rank 1-8 (3 bits)

- CQI for up to two codewords (6 bits)

- MU-MIMO

- RI - rank 1 or 2 (1 bit)

- CQI for up to two codewords (6 bits)

For a complete requirement of 22 bits.

Then, the base station can schedule transmissions to one or more UEs based on this information, choosing, for example, a transmission scheme and a packet size. Available transmission schemes include SU and MU-MIMO, where one or more spatial layers are transmitted. In this example, with up to two codewords being transmitted, the estimated total achievable bit rate will be the sum of the speeds achievable for each codeword.

In a further embodiment, the transmission rank for SU-MIMO is jointly encoded with the transmission rank for MU-MIMO, since under normal assumptions the rank for MU-MIMO should always be equal to or lower.

In a further embodiment illustrated in FIG. 3, the transmission grades considered for SU-MIMOs do not include the transmission grades considered for MU-MIMOs. This would be appropriate if the assumptions for calculating feedback for SU-MIMO and MU-MIMO differ only in assumptions about the rank of the transmission. Thus, this again provides the opportunity to reduce the amount of data to be sent. In a further embodiment, one or more of the CQI values is also encoded in a difference manner since the values are likely to be similar.

- Combined alarm

- PMI (6 bits)

- RI - rank

- CQI for SU-MIMO (6 bits)

- MU-MIMO

- Difference CQI (3 bits). The CQI for the MU-MIMO codeword is obtained by adding the CQI for one of the SU-MIMO codewords and the difference CQI.

In similar embodiments, each set of parameters includes, for each respective transmission mode, at least one of: a preferred transmission rank, an achievable data rate, an estimate of an achievable data rate, additional information for at least one of the available transmission modes indicating a set of precoding coefficients different from that indicated by a single precoding indicator.

In a further example, the transmission rank for MU-MIMO is limited to rank 1.

In a further embodiment, feedback is transmitted for more than one part of the available transmission spectrum.

It should be noted that in this embodiment, for simplicity and brevity, only two different transmission modes have been illustrated. However, this embodiment is not limited to two transmission modes and can easily be expanded to take into account more than two transmission modes.

In another embodiment, transmission modes that are considered collectively to select a codebook index are MIMO transmission modes with different transmission rank ranges. In fact, in one example of this embodiment, the first transmission rank range may be r = 1, i.e. a MIMO transmission mode of rank 1, and the second transmission range may be r = {2; four; 8}, i.e. at least 2 MIMO transmission mode (multiple streams directed to the secondary station). In one example, the different transmission modes considered for joint codebook index optimization are MU-MIMO (rank 1) and MU-MIMO (rank 2). In another example, for joint codebook index optimization, more than two transmission modes are considered, for example, SU-MIMO, MU-MIMO (rank 1), MU-MIMO (rank 2).

According to a second embodiment of the invention, the system is different from the system of the first embodiment in selecting a combined codebook index. In a first embodiment, it was proposed to calculate the combined rate for each codebook index by maximizing

Where

- i is the combined codebook index used to show the codebook entry in both SU, and MU-MIMO codebooks

и

являются рангами передачи для SU-MIMO и MU-MIMO соответственно-

and

are the transmission grades for SU-MIMO and MU-MIMO, respectively

и

являются достижимыми скоростями передачи для SU-MIMO и MU-MIMO соответственно-

and

are achievable transmission rates for SU-MIMO and MU-MIMO, respectively

.- α and β are weights. As a special occasion,

.

, который максимизирует полную (объединенную) скорость.Usually a codebook search gives a set

That maximizes the overall (combined) speed.

, чтобы максимизировать:The second embodiment is not limited to SU-MIMO and MU-MIMO. It can be applied in any case where the eNB may want to use a different transmission rank than the rank shown by the UE. For example, an alternative criterion for searching for a codebook may be to select the optimal codebook index value

to maximize:

(2)

(2)

). Таким образом, вторичная станция выбирает набор рекомендованных коэффициентов предварительного кодирования i, так что объединенная скорость максимизируется. Эта объединенная скорость основывается на взвешенном среднем скоростей для ранга 1 и ранга, который дает наивысшую скорость.Here, the combined velocity is calculated as the weighted average of the velocities for rank 1 and the rank that gives the highest speed (i.e.

) Thus, the secondary station selects a set of recommended precoding coefficients i, so that the combined rate is maximized. This combined speed is based on a weighted average of the speeds for rank 1 and the rank that gives the highest speed.

In a more general case, the reported codebook index may be an optimization taking into account two or more transmission rates derived according to different assumptions (e.g., different rank, different transmission mode, other interference assumptions)

и

соответственно. Однако является желательным минимизировать служебную информацию для сообщения CQI (например, чтобы сообщать только одно значение CQI). Отметим, что разностное кодирование между двумя значениями является известным (например, сообщение одного значения CQI и разности между этим и вторым значением).The portion of the channel status message from the UE to LTE is the CQI value, which is effectively the transmission rate that the channel can support. In this case, two CQI values corresponding to

and

respectively. However, it is desirable to minimize overhead for a CQI message (for example, to report only one CQI value). Note that the difference coding between two values is known (for example, reporting one CQI value and the difference between this and the second value).

In the above embodiment, the UE may report a CQI value corresponding to the maximized combined rate from the codebook search (for example, as deduced from a criterion in equation 2)

However, in practice, this may not be the best CQI value for the message. We note that in general:

отметим, что

, что устанавливает верхнюю и нижнюю границы на максимальную скорость, которая будет сообщаться, т.е.

. Это предполагает, что сообщение

будет обеспечивать возможность eNB знать скорость для ранга 1, и оценивать скорость для оптимального ранга (например, как

. Соответственно, является предпочтительным, чтобы вторичная станция сообщала в первичную станцию значение оценки скорости на основе произведения выбранного ранга передачи и скорости передачи данных, полученной для набора рекомендованных коэффициентов предварительного кодирования с предварительно определенным рангом.Additionally, based on the Shannon-Hartley equation for channel capacity

note that

, which sets the upper and lower boundaries to the maximum speed that will be reported, i.e.

. This assumes the message

will enable the eNB to know the speed for rank 1, and evaluate the speed for the optimal rank (for example, how

. Accordingly, it is preferable for the secondary station to report the rate estimate value to the primary station based on the product of the selected transmission rank and the data rate obtained for the set of recommended precoding coefficients with a predetermined rank.

An additional option would be to report a rate that minimizes the combined error in rate estimation for rank 1 and optimal rank in eNB. This can be achieved by reporting a combined weighted speed, such as

or, more generally

является оптимальным рангом,

является набором оптимальных коэффициентов предварительного кодирования, и

являются скоростью передачи данных для режима передачи с рангом r, и набором коэффициентов предварительного кодирования i.where δ and ε are weights,

is an optimal rank

is a set of optimal precoding coefficients, and

are the data rate for the transmission mode with rank r, and a set of precoding coefficients i.

Based on knowledge of the reported rank and speed statistics for different ranks, the eNB can then estimate the speeds observed by the UE for the optimal rank and rank 1, which will be provided by the eNB and the UE using the same values for δ and ε. These values can be fixed in the specification, configured by signaling a higher layer, or calculated from some other parameters. They may also depend on channel conditions (e.g., SNR, optimal transmission rank).

In a more general case, the reported CQI may be a combination of two or more CQI / speed values derived according to different assumptions (e.g., different rank, different transmission mode, different interference assumptions).

In exemplary embodiments, recommended precoding coefficients are determined in the user equipment due to downlink transmission. However, in embodiments of the invention, the base station or Node B may determine the recommended precoding coefficients in accordance with the invention and signal them in the same manner as in the above detailed embodiments. Moreover, in LTE, for example, the same codebook can be used on the one hand to signal to the user terminal a precoding vector or matrix that is actually used in the downlink by the base station, and on the other hand, and as explained in the above embodiments to feedback a preferred precoding matrix by a user terminal to enable the reference phase / amplitude to be output.

The invention has a specific, but not exclusive, application to wireless communication systems that use multiple transmission modes between a primary and secondary station, especially MIMO and MU-MIMO modes. Examples include cellular systems such as UMTS, UMTS LTE, and UMTS LTE-Advanced, as well as wireless LANs (IEEE 802.11n) and broadband wireless networks (IEEE 802.16).

In the present description and the claims, an indication of an element in a singular does not exclude the presence of a plurality of such elements. Additionally, the sign “contain” does not exclude the presence of other elements or steps than those listed.

The inclusion of reference numbers in parentheses in the claims is intended to facilitate understanding, and is not intended to be limiting.

From reading the present disclosure, other modifications should be apparent to those skilled in the art. Such modifications may include other features that are already known in the art of radio technology.

Claims (7)

1. The method of operation of the secondary station, wherein the secondary station communicates with at least one primary station via MIMO transmissions, the method comprising the step of signaling the secondary station with a single precoding indicator representing at least one set recommended precoding factors to the primary station, with a single precoding indicator common to the many available MIMO transmission modes. 2. The method of claim 1, wherein the plurality of available MIMO transmission modes includes at least one single-user MIMO transmission mode and one multi-user MIMO transmission mode. 3. The method of claim 1, wherein the plurality of available MIMO transmission modes includes at least one MIMO transmission mode with a first transmission rank range and one MIMO transmission mode with a second transmission rank range, and wherein a single precoding indicator is an optimization that takes into account two or more transmission rates derived according to different assumptions. 4. The method according to any one of the preceding paragraphs, further comprising the step of the secondary station determining a set of recommended precoding coefficients for at least one of the available MIMO transmission modes to be signaled to the primary station using a single precoding indicator. 5. The method according to claim 4, in which a set of recommended precoding coefficients is selected from a precoding codebook including a set of precoding matrices, and the precoding indicator is an indicator of a precoding matrix representing an index for recording in a precoding codebook coding. 6. A secondary station comprising means for communicating with at least one primary station via MIMO transmissions, and means for signaling a single precoding indicator representing at least one set of recommended precoding coefficients to the primary station, when of this, a single precoding indicator is common to the plurality of available MIMO transmission modes. 7. A primary station comprising means for communicating with at least one secondary station via MIMO transmissions, the method comprising the step of: receiving a single precoding indicator from the secondary station representing at least one set recommended precoding ratios, with a single precoding indicator common to the many available MIMO transmission modes, and the primary station (100) contains provides control means for selecting a transmission mode from among the plurality of available MIMO transmission modes, based at least in part on a single precoding indicator.

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