TW201720073A - Method for downlink channel estimation - Google Patents

Abstract

The present invention relates to a method for downlink channel estimation in a base station of a Multiple-Input Multiple-Output wireless communication system, wherein a base station of the Multiple-Input Multiple-Output wireless communication system has NT antennas and NFFT sub-carriers, the method comprising: precoding a pilot signal originated from each radio frequency link by means of a first analog precoding matrix associated with the each radio frequency link to obtain a first pilot signal; and mapping the precoded first pilot signal to all antennas connected to the each radio frequency link, and transmitting the precoded first pilot signal by the antennas.

Description

Method for downlink channel estimation

The present invention relates to wireless network technologies, and more particularly to a method for downlink channel estimation in a base station of a multiple input multiple output wireless communication system, and a downlink channel in a multiple input multiple output wireless communication system An estimated base station, a method for downlink channel estimation in a user equipment of a multiple input multiple output wireless communication system, and a user equipment for downlink channel estimation in a multiple input multiple output wireless communication system.

Hybrid analog/digital precoding provides a low-cost and efficient solution for MIMO technology due to its superior ability to reduce the number of RF links and achieve full beamforming/diversity gain. . Compared to traditional pure digital precoding (where each antenna element must be equipped with a separate RF link), hybrid precoding can significantly reduce the number of RF links and achieve similar performance at the same time. Another important advantage of mixed precoding is that it has very low training and feedback overhead, which is very useful in crossover multiplex FDD compared to full digital precoding. Due to the multi-input and multi-output MIMO system based on pure digital precoding, the required training and feedback overhead with the day The number of lines increases linearly, which makes the multi-input and multi-output MIMO system based on digital precoding difficult to implement under FDD. This problem is solved by means of hybrid precoding. In hybrid precoding, spatial signal processing operations are partitioned into high dimensional analog precoding and low dimensional digital precoding. Analog precoding is achieved using a low cost RF phase shifter network that can be implemented in different configurations. Based on the hybrid precoding algorithm, the analog precoding is designed to match the channel spatial correlation matrix, and the digital precoding is designed to match the effective channel after the analog precoding. This algorithm makes hybrid precoding suitable for frequency division multiplexing FDD systems, because the calculation of analog precoding matrix only requires slow varying broadband channel correlation matrix information, and the required training and feedback size is only linear with the number of RF links. Growth, not linear growth with the number of antennas. Since the number of RF links is much smaller than the number of antennas, the required training and feedback overhead is greatly reduced.

In order to implement hybrid precoding under the FDD system, downlink physical channel estimation is needed for the user to estimate and feed back the spatial correlation matrix. Traditional physical channel estimation techniques allow different antennas to transmit pilots over different time-frequency resources, so there is no interference between pilots of different antennas. However, under the hybrid precoding architecture, this traditional pilot transmission technique is not suitable due to the use of the phase shifter network. Antennas connected to the same RF link must use the same time-frequency resources to transmit pilots, causing interference between pilots of different antennas.

Based on the above understanding of the prior art and the technical problems that exist, the downlink (DL) channel estimation problem for mixed analog/digital precoding systems will be discussed in the present invention. Due to the specific hardware structure of the hybrid precoding, the pilots of the mutually orthogonal antennas used by the conventional downlink channel estimation techniques are not applicable here. Since downlink physical channel estimation and user feedback are important for base station to obtain channel state information for precoding, especially under FDD mode, it is also one of the key technologies for hybrid precoding. This problem will be solved in the present invention and an advanced pilot transmission and channel estimation technique is proposed to solve the problem of downlink physical channel estimation in hybrid precoding with any radio frequency phase shifting network structure.

A first aspect of the present invention provides a method for downlink channel estimation in a base station of a multiple input multiple output wireless communication system, wherein the base station of the multiple input multiple output wireless communication system has N _T Antennas and N _FFT subcarriers, the method comprising: - precoding a pilot signal originating from each radio frequency link by means of a first analog precoding matrix associated with each radio frequency link to obtain a first pilot signal; and - mapping the precoded first pilot signal to and from all antennas connected to each of the radio frequency links.

In one embodiment of the invention, pilot signals originating from different radio frequency links are transmitted by means of different OFDM symbols.

In one embodiment of the invention, the precoded first pilot signal is simultaneously carried out by all antennas connected to each of the radio frequency links transmission.

In one embodiment of the invention, the number S of radio frequency links used by the base station is less than the number NT of antennas.

In one embodiment of the invention, the base station selects the number s from the number S of radio frequency links it uses. S radio frequency links are used for transmission of pilot signals.

In an embodiment of the present invention, the method further includes: - partitioning a time-frequency resource including T OFDM symbols and N _FFT subcarriers to obtain a time-frequency resource block including the same number of time-frequency resource units; - precoding the pilot signal in each time-frequency resource block with a second analog precoding matrix associated with the time-frequency resource block to obtain a second pilot signal; and - the pre-coded first The two pilot signals are mapped to and transmitted by all of the antennas connected to each of the radio frequency links.

In an embodiment of the invention, each row of the corresponding different antenna in the second analog precoding matrix is orthogonal to each other.

A second aspect of the present invention provides a base station for downlink channel estimation in a multiple input multiple output wireless communication system, wherein the base station is characterized by implementing the first aspect of the present invention Methods.

The third party aspect of the present invention proposes a method for downlink channel estimation in a user equipment of a multiple input multiple output wireless communication system, the method comprising: - receiving a first base station connected to the user equipment a pilot letter No. wherein the first pilot signal is derived from each radio frequency link, precoded by means of a first analog precoding matrix associated with each radio frequency link, mapped to each of said each All antennas connected by the radio frequency link are transmitted by the antenna; and - the user equipment performs time delay estimation based on the first pilot signal.

In an embodiment of the invention, the method further comprises: - receiving a second pilot signal from a base station connected to the user equipment, wherein the second pilot signal is a pair comprising T OFDM symbols And time-frequency resources of the N _FFT sub-carriers are partitioned to obtain time-frequency resource blocks including the same number of time-frequency resource units and the pilot signals in each time-frequency resource block are associated with the time-frequency resource blocks. a second type of precoding matrix is precoded, mapped to all antennas connected to each of the radio frequency links and transmitted by the antenna to obtain a second pilot signal; and - the user equipment is The second pilot signal is used for channel estimation.

In an embodiment of the invention, the user equipment further assists channel estimation based on the delay estimate.

A fourth aspect of the present invention provides a user equipment for downlink channel estimation in a multiple input multiple output wireless communication system, wherein the base station is characterized by implementing a vendor described in accordance with the present invention. method.

As mentioned above, existing channel estimation techniques are designed for full digital pre-programming The code system, which does not apply to hybrid precoding systems with phase shifter networks, is the introduction of phase shifters. The technique according to the present invention can successfully solve the above problems and achieve the same performance as existing channel estimation using the same pilot overhead.

100‧‧‧ Schematic diagram of the pilot transmission solution in the delay estimation phase

200‧‧‧ Schematic diagram of the pilot transmission solution during the channel estimation phase

Other features, objects, and advantages of the present invention will become apparent from the Detailed Description of the Description.

Figure 1 shows a schematic diagram 100 of a pilot transmission solution during a delay estimation phase; Figure 2 shows a schematic diagram 200 of a pilot transmission solution during the channel estimation phase.

Throughout the drawings, the same or similar drawing symbols indicate the same or similar devices (module) or steps.

In order to better illustrate the method and apparatus of the present invention and the user equipment and base station for use in a wireless communication network using the method according to the invention or the apparatus according to the invention, the following two important aspects of the invention are first introduced. The prior art is a modulation broadband converter technology and a loop characteristic detection technique, respectively. The following is a brief introduction with reference to the drawings and related formulas.

In the detailed description of the preferred embodiments that follow, reference is made to the accompanying drawings that form a part of the invention. The attached drawings are shown by way of example Particular embodiments of the invention are possible. The exemplary embodiments are not intended to be exhaustive of all embodiments in accordance with the invention. It is to be understood that other embodiments may be utilized and structural or logical modifications may be made without departing from the scope of the invention. Therefore, the following detailed description is not to be construed as limiting the scope of the invention.

The following will be considered a multi-input multiple-output MIMO-OFDM system with N _T antennas of base stations, and a single-antenna user N _FFT subcarriers. The base station uses S (S<<N _T ) radio frequency links and uses precoding with hybrid precoding technology. Hybrid precoding can employ a phase shifter network of any configuration. The proposed downlink physical channel estimation technique includes two phases, a delay estimation phase and a channel estimation phase. These two phases will be introduced separately.

First, the delay estimation phase is introduced, in which all radio links or a part of the radio link in the radio link are used to transmit pilots for the user to estimate the delay. Pilots originating from different radio frequency links are transmitted among different OFDM symbols. Only pilots from one radio frequency link are transmitted among each OFDM symbol. For each RF link, the pilots are inserted non-continuously over the entire bandwidth. The pilot originating from one radio frequency link is first precoded by an analog precoding matrix corresponding to the radio frequency link, then mapped onto all antennas connected to the radio frequency link and transmitted. Different RF links use different analog precoding matrices. An example of a pilot transmission at this stage is shown in Figure 1, where x represents the pilot signal and c _s (s = 1~S) represents the analogy of _NT *1 for the sth radio link. Precoding matrix. The precoded pilots originating from one radio link are simultaneously transmitted from all antennas connected to the radio link.

After such precoding processing on the base station side, the user will receive pilots in the time domain from the radio link, which measures the power above each delay and then averages the leads from all RF links. The power of the frequency signal and the time delay with power exceeding a predefined threshold is found as the estimated time delay. At this stage, the user does not need to know the analog precoding matrix used on the base station side.

The channel estimation phase will be described below. It is assumed that the channel is substantially flat on T _coh OFDM symbols and W _coh subcarriers. Select W and T corresponding resources to make W Min(W _coh ,S) and N _T /W T T _coh . W of the S radio links are used to transmit pilots among T consecutive OFDM symbols. The time-frequency resources on the T OFDM symbols are shared by the W radio frequency links in an orthogonal manner. The pilot from one radio frequency link is first precoded by an analog precoding matrix corresponding to the radio frequency link, mapped to all antennas connected to the radio frequency link, and then transmitted. Different RF links use different analog precoding matrices among different OFDM symbols. A pilot transmission scheme at the channel estimation stage is shown in FIG. 2, where x represents a pilot signal and c _s,t (s=1~S) represents the sth radio link in the tth OFDM The analog precoding matrix used in the symbols. In this figure, different radio frequency links transmit pilots on different subcarriers.

As mentioned above, antennas connected to the same RF link will use the same time-frequency resources for transmitting pilots, which will cause interference between the antennas at the receiving end. Therefore, the analog precoding matrix used in the channel estimation phase must be carefully designed to facilitate the elimination of inter-antenna interference. In view of this purpose, the present invention divides time-frequency resources on T OFDM symbols into N _FFT /W time-frequency resource blocks, and each time-frequency resource block includes W consecutive sub-carriers and T consecutive OFDM symbols. Due to W W _coh and T T _coh , so the channel on each time-frequency resource block can be considered flat. The analog precoding matrix is designed such that different antennas connected to the same radio frequency link are orthogonal to each other between analog precoding vectors experienced on each time-frequency resource block. For example, assume that all antennas are divided into L layers, antennas within the same layer are connected to the same RF link, and antennas in different layers are connected to different RF links. It is assumed that S(1) represents the index set of the radio frequency link connected to the antenna among the first layers and assumes W=S. Then the analog precoding matrix used in Figure 2 must be designed such that {c _s,t | s S(1), t=1~T} is a matrix composed of columns having mutually orthogonal rows.

After the user receives pilots from all radio links on T OFDM symbols, the user will implement MMSE channel estimation. The delay estimate generated in the first phase will be used to estimate the frequency domain correlation matrix of the channel, and the estimated frequency domain correlation matrix will be used to enhance the MMSE channel estimate. Inter-antenna interference can be successfully eliminated in the MMSE channel estimation by means of the orthogonal design of the analog precoding matrix.

In the channel estimation phase, the analog precoding matrix used to transmit the pilots needs to be known. Such information is not needed in previous scenarios because different base stations can use different analog precoding matrices. Analogy pre The structure of the coding matrix depends on the structure of the phase-shifted network in the hybrid precoding used. Therefore, the base station must inform the user of the analog precoding matrix used by the downlink signaling. The signaling overhead involved is very small, as only the structure of the precoded phase shifting network used by the base station for the hybrid needs to be indicated, and it does not vary with time and frequency.

The concept of the invention will be further elucidated below by means of a formula.

The following formula is based on the premise that OFDM MIMO system with N _FFT subcarriers, the base station is equipped with N _T antennas and is equipped S <N _T S is a radio frequency link. All antennas are divided into L layers, each layer containing N _T /L antennas. Antennas of the same layer are connected to the same RF link, and antennas at different layers are connected to different RF links.

In the delay estimation phase, using the pilot transmission techniques mentioned above, M radio links in the S radio links are used to transmit pilots among M different OFDM symbols. Without loss of generality, it is assumed that the first M radio links are used to transmit pilots, assuming J denotes the index set of subcarriers for pilot transmission for each radio link and makes J=|J|, here Assume that all RF links use the same J. The pilot received by the user from the sth radio link (in the sth OFDM symbol) is represented in the frequency domain as:

Where y _s represents the received pilot signal from the sth radio link, which is a column vector of size J*1, and H represents the frequency domain channel matrix of the N _T *N _FFT dimension, H(:, J) denotes a H submatrix comprising a column with an index of J, x denotes a pilot signal and c _s (s=1~S) denotes an analog precoding matrix for the _NT *1 dimension of the sth radio link . The user then performs a discrete Fourier transform (DFT) on y _s to convert it from the frequency domain to the time domain:

among them, For the time domain representation of y _s , F is the N _FFT - order Fourier matrix, The N _T × N _FFT dimension time domain impulse response matrix for the channel and . It is well known that wireless channels with centralized antennas typically have sparse and identical delays over all antennas, so There are only very few non-zero columns and thus h _s is also a sparse vector. The position of the non-zero element in h _s represents the delay of the channel and can be estimated by:

among them, For the estimation of delay, for The dth element and Thred is a predefined threshold. It should be noted here that the estimated delay must be less than the pilot length J. This is caused by the use of non-continuously spaced pilots when F ^* (:, ) F ^T (:, Caused by the repeatability of the elements in ). Therefore, the pilot length must be chosen such that J > dmax, where dmax is the maximum delay spread.

Next, the channel estimation is introduced, where Rs={(t, w)} represents the index set of the OFDM symbol and the subcarrier pair used by the sth radio link to transmit the pilot, that is, only when (t, w) In Rs, the sth radio frequency link transmits pilots on the wth subcarrier of the tth OFDM symbol. Then the pilot received above the wth subcarrier of the tth OFDM symbol is represented as:

Where h _w is the wth column of H, Ct=[c _1,t ...c _s,t ...c _S,t ], b _t,w is a vector of S×1 dimension, its s The elements b _t,w (s) indicate whether the pilot of the sth radio link is transmitted over the wth subcarrier of the tth OFDM symbol, ie: Considering all T OFDM symbols, equation (4) can be rewritten as (assuming T(T) T _coh ) The channel above the OFDM symbol is flat)

Where A _w =[ C ₁ b _{1, w} ... C _T b _{T , w} ] (7)

Add the pilot stack received on all subcarriers to a large matrix, resulting in:

among them,

as well as

received After that, the user uses the MMSE channel estimation technique to estimate the channel matrix H, which defines:

as well as

Where σ ² is the noise variance. Among the formulas (11) and (12), R _h is defined as:

Where D=diag(d), d is the power delay property (PDP) of the channel obtained by using the delay estimation result. Then, use MMSE to get for:

followed by Valuation The estimate of the channel matrix H is obtained by transforming from vector form to matrix form.

Next, we introduce the design of an analog precoding matrix for channel estimation:

In this section we will discuss how to design an analog precoding matrix {C _t , t = 1~T} to facilitate inter-antenna interference cancellation on the user side. For the sake of simplicity, assume that N _FFT /W is an integer. The N _FFT subcarriers in the T OFDM symbols are divided into N _FFT / W resource blocks, and each resource block includes W subcarriers and T OFDM symbols. The channel on each W by T resource block can be considered flat.

Assume that the analog phase shifter network is divided into L layers, each layer containing a subnetwork of (S/L) × (N _T /L) dimensions. Antennas in the same layer are connected to the same radio frequency link, and antennas of different layers are connected to different radio frequency links. Among them, the analog precoding matrix C _t must follow the following block diagonal structure:

Where C _t,1 (l=1~L) is an analog precoding matrix corresponding to the (N _T /L)×(S/L) dimension of the first layer. Process b _t,w in the same way to get:

Where b _t,w,1 is a mapping sequence used by the first layer above the wth subcarrier of the tth OFDM symbol, which is an S/L×1 dimensional vector. The mapping sequence {b _t,w,1 } is designed according to the following rules:

Each of the W radio frequency links equally shares each resource block of size W by T in an orthogonal manner.

This same mapping rule is repeated in all resource blocks of size W by T.

Follow the above two rules to design the mapping sequence. Use R _{s,j to} denote the sth (s The RF link of W) is used to transmit the index set of the pilot OFDM symbol and the subcarrier pair among the jth resource blocks, then {R _s,j } is designed in the following form, namely:

as well as

Based on equations (15) and (16), the pilot received above the wth subcarrier among the tth OFDM symbols of equation (4) can be expressed by the following formula, namely:

Where h _w,1 is the sub-vector of h _W corresponding to (N _T /L)×1 of the antenna at layer 1, ie:

Since antennas among different layers are connected to different radio frequency links and different radio frequency links transmit pilots over different time-frequency resources, there is no interference between antennas of different layers. Then, equation (20) can be simplified for a specific layer:

In formula (22) An index set representing OFDM symbols and subcarrier pairs used by the radio link of layer 1 to transmit pilots, where S(1) is the index set of the radio frequency links among the first layers. Considering all of the subcarriers among the T OFDM symbols, the pilot received from the layer 1 can be expressed as:

Where A _{w , l} =[ C _{1, l} b _{1, w , l} ... C _{t , l} b _{t , w , l} ... C _{T , l} b _{T , w , l} ] (24)

In order to facilitate the elimination of interference between antennas in different layers, {C _t,1 , t} is designed such that pilots of antennas originating from the same layer use analogy precoding with mutually orthogonal analog precoding vectors on each resource block of size W by T, so that matching can be used in the receiving segment Filters to easily remove inter-antenna interference. Consider the jth (j=1~N _FFT /W) resource blocks. The analog precoding matrix corresponding to layer 1 on the jth resource block is:

Using the mapping rules among the equations (17) to (19), the L layer uniformly shares the time-frequency resources among each resource block in an orthogonal manner, and therefore, There are only TW/L non-zero columns. (W must be a multiple of L), set for The sub-matrices of the (N _T /L) × (TW/L) dimension composed of non-zero columns. Through reasonable design {C _t,1 , Thus making Have orthogonal rows, ie:

(Note: Equation (26) is only at N _T /W In order to satisfy the requirement of equation (26), {C _t,1 } is designed such that U is a unit matrix of (TW/L)×(TW/L) dimension. Let {c _s,t,1 ,s=1~S} denote the column of C _t,1 and design c _s,t,1 as follows:

In the formula (27), [c _s,t,1 |(t,w) R ₁ ] is satisfied (t, w) c _{s,t,1 of} R ₁ is a matrix composed of columns. Combining the mapping sequence produced by equations (17) through (19) with the analog precoding matrix produced by equations (27) and (28) makes it easy to see that the requirements of equation (26) are satisfied.

For the sake of simplicity, the design rules for the analog precoding matrix and the mapping sequence are summarized as follows: Select W and T to make W Min(W _coh ,S) and N _T /W T T _coh and W should also be an integer multiple of L; use the formulas (17) to (19) for the L layer to generate the mapping sequence {b _{t, w, 1} }; and use equations (27) and (28) to L The layer calculates the analog precoding matrix {C _t,1 }.

It is obvious to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments are to be considered as illustrative and not limiting in any way. In addition, it is obvious that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude the plural. A plurality of elements recited in the scope of the device application patent may also be implemented by one element. The first, second, etc. words are used to denote names and do not denote any particular order.

100‧‧‧ Schematic diagram of the pilot transmission solution in the delay estimation phase

Claims (12)

A method in MIMO wireless communication system base station for downlink channel estimation, wherein, in the MIMO wireless communication system base station having N _T antennas and N _FFT subcarriers, the method The method includes: - precoding a pilot signal originating from each radio frequency link by means of a first analog precoding matrix associated with each radio frequency link to obtain a first pilot signal; and - will be precoded The first pilot signal is mapped to and transmitted by all antennas connected to each of the radio frequency links. The method of claim 1, wherein the pilot signals originating from different radio frequency links are transmitted by means of different OFDM symbols. The method of claim 1, wherein the precoded first pilot signal is simultaneously transmitted by all of the antennas connected to each of the radio frequency links. The method of claim 1, wherein the number S of radio frequency links used by the base station is less than the number N _{T of} antennas. The method of claim 4, wherein the number of the radio frequency links used by the base station is selected by the base station as s S radio frequency links are used for transmission of pilot signals. The method of claim 1, wherein the method further comprises: - partitioning time-frequency resources including T OFDM symbols and N _FFT subcarriers to obtain a time-frequency resource block including the same number of time-frequency resource units; - precoding the pilot signal in each time-frequency resource block with a second analog precoding matrix associated with the time-frequency resource block to obtain a second pilot signal; and - a second pre-coded signal The pilot signals are mapped to and transmitted by all of the antennas connected to each of the radio frequency links. The method of claim 6, wherein each row of the corresponding different antenna in the second analog precoding matrix is orthogonal to each other. A base station for downlink channel estimation in a multiple input multiple output wireless communication system, wherein the base station is characterized by implementing the method according to any one of claims 1 to 7. A method for downlink channel estimation in a user equipment of a multiple input multiple output wireless communication system, the method comprising: - receiving a first pilot signal from a base station connected to the user equipment, wherein the first pilot a frequency signal originating from each radio frequency link, precoded by means of a first analog precoding matrix associated with each radio frequency link, mapped to all antennas connected to each radio frequency link and by the antenna Transmitting; and - the user equipment performs a time delay estimation based on the first pilot signal. The method of claim 9, wherein the method further comprises: - receiving a second pilot signal from a base station connected to the user equipment, wherein the second pilot signal is a pair comprising T OFDM symbols and N The time-frequency resources of the _FFT subcarriers are partitioned to obtain a time-frequency resource block including the same number of time-frequency resource units, and the pilot signal among each time-frequency resource block is associated with the time-frequency resource block. The second class is precoded with a precoding matrix, mapped to all antennas connected to each of the radio frequency links, and transmitted by the antenna to obtain a second pilot signal; and - the user equipment performs according to the second pilot signal Channel estimation. The method of claim 10, wherein the user equipment further assists channel estimation based on the delay estimate. A user equipment for downlink channel estimation in a MIMO communication system, wherein the base station is characterized by implementing the method according to any one of claims 9 to 11.