CN104113496A

CN104113496A - Method of carrying out channel estimation by using sequential pilot sequence

Info

Publication number: CN104113496A
Application number: CN201410275875.2A
Authority: CN
Inventors: 屈代明; 孔德莹; 江涛
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2014-06-19
Filing date: 2014-06-19
Publication date: 2014-10-22

Abstract

The present invention provides a method for channel estimation using sequential pilot sequences. In this way, users adopt a sequential pilot sending method, that is, different users send pilots in different time slots in a staggered manner. In a certain time slot, the base station only needs to estimate the channel of one user, and then the base station forms a beam for the user. In the next time slot, the base station estimates another user's channel. The invention can effectively solve the problem of complex orthogonal design of the pilot sequence in the traditional channel estimation method, can significantly reduce the length of the required pilot sequence, and can improve the throughput of the system.

Description

Method for channel estimation by using sequential pilot frequency sequence

Technical Field

The invention belongs to the technical field of large-scale multi-antenna wireless communication, and particularly relates to a method for performing channel estimation by using a sequential pilot frequency sequence.

Background

In the transmission scheme, a large number of low-power antennas are arranged at a base station end, and the number of the antennas far exceeds the number of single-antenna users scheduled at the same time. Massive MIMO technology can enable wireless communication systems to achieve very high throughput. In the conventional channel estimation method, users transmit pilot sequences simultaneously, and a base station performs channel estimation on all users at the same time. The conventional method requires the design of complex orthogonal pilot sequences, and the design of the pilot sequences depends on the number of users.

Disclosure of Invention

In view of the above-mentioned deficiencies or needs in the art, the present invention provides a method for channel estimation and data communication with multiple users transmitting pilots sequentially. The channel of the user can be estimated without complex pilot orthogonal sequences and the system performance is hardly lost.

In order to achieve the above object, the present invention provides a method for channel estimation using sequential pilot sequences, comprising:

step 1, K users in a cell send pilot frequencies to a base station in a sequential manner: k users in a cell simultaneously send signals to a base station on the same channel, and the base station simultaneously provides data transmission service for the K users by adopting a large-scale MIMO technology; the K users sequentially send pilot frequency to the base station, only one user sends the pilot frequency to the base station at one time point, and other K-1 users simultaneously send uplink data to the base station;

step 2, the base station carries out channel estimation in a sequential pilot frequency sending mode: a base station provides service for K users on one channel simultaneously; at a certain time point, only user K sends pilot frequency to the base station, the rest K-1 users all send uplink data to the base station, and the channels of the rest K-1 users are obtained in the previous pilot frequency sending and channel estimation stages; after receiving the signals sent by the K users, the base station estimates the channel of the K users.

In an embodiment of the present invention, in step 2, after receiving the signals sent by the K users, the base station estimates a channel of the user K, specifically, the base station estimates the channel by using a projection method:

computingThen Y' is used to channel g of user k_kCarrying out direct estimation;

wherein: y' is Y atIs a space perpendicular to the space spanned by the column vectors ofThe component of (a) to (b),is Y atThe column vector of (a),is composed ofA projection matrix of the space spanned by the column vectors of (a),a channel matrix consisting of channels of other K-1 users which have been estimated by the base station, g_iIs an N x 1 dimensional channel vector from user i to the base station,k, Y is a vector obtained after a base station receives a signal matrix Y sent by K users and performs a vectorization operator, and N is the number of antennas of the base station.

In an embodiment of the present invention, in step 2, after receiving the signals sent by the K users, the base station estimates a channel of the user K, specifically, performs channel estimation by using a linear minimum mean square error method, where the estimated user channel is:

wherein: s_kPilot sequences of length L, p, sent for a user_dPower at which data is transmitted for a user, σ²Is the power of white gaussian noise and,a channel matrix formed by the channels of other K-1 users which are estimated by the base station,is channel g_kSecond moment of (I)_N×NIs an N × N dimensional identity matrix, I_L×LThe unit matrix is L multiplied by L dimension, Y is a vector obtained after the base station receives a signal matrix Y sent by K users and carries out a vectorization operator, and N is the number of antennas of the base station.

In an embodiment of the present invention, the sequential pilot sending mode is a packet sequential pilot sending mode, and specifically includes: grouping users, different groups of users in different time slotsSending pilot frequency, wherein K is the number of groups of users, the length of a time slot used for sending the pilot frequency in each group is L symbol periods, and the correlation time of a channel is T; the users in the same group use simple orthogonal pilot to transmit pilot in the same time slot, i.e. the pilot sequences transmitted by users i and k in the same group

{s_{j}^{i}, j = 0,1, . . ., L - 1}

And

{s_{k}^{i}, j = 0,1, . . ., L - 1}

satisfy the orthogonal relation

Where L is the length of the pilot sequence.

In an embodiment of the present invention, the sequential pilot sending method is a user grouping sequential pilot sending method in multiple cells, and specifically includes: grouping users of adjacent cells, wherein each group comprises a plurality of users from different cells, the users of different groups send pilot frequencies in different time slots, the users of the same group send pilot frequencies in the same time slot, and the pilot frequency sequences of the users of the same group are mutually orthogonal.

In general, compared with the prior art, the technical solution of the present invention can effectively solve the problem that the pilot sequence in the conventional channel estimation method needs to be designed in a complex orthogonal manner, and can significantly reduce the length of the required pilot sequence and improve the throughput of the system. The method is characterized in that:

1. solving complex orthogonal design problems in pilot design

In the prior art, users simultaneously transmit pilots to a base station, and in order to perform good estimation on the channels of the users, the pilot sequences transmitted by the users need to be kept orthogonal, so that complex orthogonal design is needed when designing the pilots. The technical scheme of the invention does not need to carry out orthogonal design on the pilot frequency of each user.

2. Reducing pilot length per user

In the prior art, the length of the pilot frequency required for ensuring the orthogonality of the pilot frequency sequence of each user is not less than the number of the users, so the pilot frequency length is increased along with the increase of the number of the users. The pilot frequency length in the technical scheme of the invention can be very short and cannot be increased along with the increase of the number of users.

3. Improving the throughput of a system

Although the pilot signal transmitted by the user and the data signal transmitted by other users do not have orthogonality, the base station can still obtain a good estimated channel of the user in a massive MIMO system. The technical scheme of the invention reduces the length of the pilot frequency, increases the time frequency resources for sending the data signals, thereby improving the utilization rate of the time frequency resources, and can support more users and improve the throughput of the system to a certain extent.

Drawings

Fig. 1 is a schematic diagram of a sequential pilot sequence transmission scheme in the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in fig. 1, in the massive MIMO communication system proposed by the present invention, the method for performing channel estimation and data communication by sequentially transmitting pilots by multiple users is as follows:

multiple users in a cell simultaneously transmit signals to a base station on the same channel (frequency), and the base station simultaneously provides data transmission services for the multiple users by adopting a large-scale MIMO technology. A plurality of users send pilot frequency to the base station sequentially, only one user sends the pilot frequency to the base station at one time point, and other users can send uplink data to the base station at the same time.

Suppose a base station has N antennas and serves K users simultaneously on one channel. At a certain time point, only user K sends pilot frequency to the base station, the rest K-1 users all send uplink data to the base station, and the channels of the rest K-1 users are obtained in the previous pilot frequency sending and channel estimation stages. After receiving the signals sent by the K users, the base station may use a projection method, an LMMSE (linear minimum mean square error) method, to suppress interference caused by data signals from K-1 users when estimating channels of user K.

Let the channel matrix be G ═ G₁ g₂...g_K]Wherein g is_iIs an N x 1 dimensional channel vector from user i to the base station. Assume that the pilot length is L. The signal matrix sent by the user is X ═ X₁ x₂...x_K]^TWherein x is_i＝[x_i,1 x_i,2...x_i,L]^TSignal sequence, x, transmitted in L symbol periods for the ith user_k＝s_k＝[s_i,1 s_i,2...s_i,L]^TThe signal matrix received by the base station is Y ═ GX + W, and W is zero-mean gaussian white noise in the channel. Before estimating the channel of user K, the channels of other K-1 users are estimated, and the channel matrix formed by the channels isThe matrix formed by the real channels of other K-1 users isThe K-1 users transmit data matrix ofThe implementation of the projection method and LMMSE method is described in detail below.

Projection method: when a certain user is estimated, a method of projecting a signal received by a base station to a vertical space of a known user channel space for estimation is adopted.The projection matrix of the space spanned by the column vectors ofThus Y is inProjected onto the spanned space isThus y is inIs formed as a space perpendicular to the spaceComponent ofThe channel of user k is then directly estimated using Y'.

LMMSE method: the known observation vector y ═ Vec (Y) is used to determine the unknown parameter g by using LMMSE_kThe formula for the estimation isWhere Vec (·) is the vectorization operator. Suppose a known channel g_kSecond moment of

C_{g_{k} g_{k}} = E {(g_{k} - E {g_{k}}) {(g_{k} - E {g_{k}})}^{H}} = E {g_{k} {g_{k}}^{H}} = R_{g_{k} g_{k}}

From E { g_kSubstituting 0 for E { y } into 0

{\hat{g}}_{k} = E {g_{k}} + C_{g_{k} y} C_{yy}^{- 1} (y - E {y}) = C_{g_{k} y} C_{yy}^{- 1} y .

By calculating:

let w ═ vec (w),

wherein,is a Kronecker product operation, p_dPower at which data is transmitted for a user, σ²Is the power of gaussian white noise.

Thereby to obtain

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Since in practice the base station cannot know the channel matrixThus using the estimated valueInstead. Thus, it is possible to prevent the occurrence of,

in an embodiment of the present invention, the sequential pilot sending mode is a packet sequential pilot sending mode, and specifically includes: the users are grouped, and the users of different groups send the pilot frequency in different time slots. If the users are divided into K groups, the length of the time slot for transmitting the pilot frequency in each group is L symbol periods, and the correlation time of the channel is T, it is required to satisfy K × L ≤ T. The users in the same group use simple orthogonal pilot to transmit pilot in the same time slot, i.e. the pilot sequences transmitted by users i and k in the same group

{s_{j}^{i}, j = 0,1, . . ., L - 1}

And

{s_{k}^{i}, j = 0,1, . . ., L - 1}

satisfy the orthogonal relation

Where L is the length of the pilot sequence.

In another embodiment of the present invention, the sequential pilot sending method is a user grouping sequential pilot sending method in multiple cells, and specifically includes: grouping users of adjacent cells, wherein each group comprises a plurality of users from different cells, the users of different groups send pilot frequencies in different time slots, the users of the same group send pilot frequencies in the same time slot, and the pilot frequency sequences of the users of the same group are mutually orthogonal.

The following describes the method of the present invention by taking the sequential pilot transmission of multiple users in the same cell as an example, which is specifically as follows.

Example (b):

in this specific embodiment, 10 users in a cell simultaneously transmit signals to a base station with 128 antennas on the same channel (frequency), and the base station simultaneously provides data transmission services for these multiple users by using massive MIMO technology, assuming that the pilot length of each user is L ═ 1.

At a certain point in time, only user k (k ═ 1,2, …,10) transmits a pilot to the base station, the remaining 9 users all transmit uplink data to the base station, and the channels of the remaining 9 users have been obtained in the previous pilot transmission and channel estimation phase. After receiving the signals sent by the 10 users, the base station may suppress interference caused by data signals from other 9 users when estimating the channel of user k by using a projection method or an LMMSE (linear minimum mean square error) method, and estimate the channel of user k. Suppose that the signal vector received by the base station is Y, and the channel matrix formed by the known channel vectors of the users is Y

Projection method: when estimating the channel of user k, it uses the method that projects the signal received by base station to the vertical space of the known user channel space to estimate. Y is atIs formed as a space perpendicular to the spaceComponent ofAnd then Y' is used for carrying out channel estimation on the user k.

LMMSE method:

wherein sigma²Is the power of the channel Gaussian white noise, s_kPilot sequences of length L, p, sent for a user_dThe power at which the data is transmitted for the user,is channel g_kSecond order moment of (a).

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A method for channel estimation using sequential pilot sequences, the method comprising:

2. The method as claimed in claim 1, wherein in step 2, the base station estimates the channels of K users after receiving the signals transmitted by K users, specifically, the base station estimates the channels by using a projection method:

wherein: y' is Y atIs a space perpendicular to the space spanned by the column vectors ofThe component of (a) to (b),is Y atThe column vector of (a),is composed ofProjection of the space spanned by the column vectors ofThe shadow matrix is a matrix of the image,a channel matrix consisting of channels of other K-1 users which have been estimated by the base station, g_iIs an N x 1 dimensional channel vector from user i to the base station,k, Y is a vector obtained after a base station receives a signal matrix Y sent by K users and performs a vectorization operator, and N is the number of antennas of the base station.

3. The method as claimed in claim 1, wherein the base station estimates the channels of K users after receiving the signals transmitted by K users, specifically, the channel estimation is performed by using a linear minimum mean square error method:

the estimated user channel is

Wherein: s_kPilot sequences of length L, p, sent for a user_dPower at which data is transmitted for a user, σ²Is the power of white gaussian noise and,a channel matrix formed by the channels of other K-1 users which are estimated by the base station,is channel g_kSecond moment of (I)_N×NIs an N × N dimensional identity matrix, I_L×LIs an L multiplied by L dimensional unit matrix, Y is a vector obtained after a base station receives a signal matrix Y sent by K users and carries out a vectorization operator, N is the number of antennas of the base station,representing the Kronecker product operation.

4. The method according to any of claims 1 to 3, wherein the sequential pilot transmission mode is a packet sequential pilot transmission mode, and specifically comprises: grouping users, wherein users in different groups send pilot frequency in different time slots, and K is equal to or less than T, wherein K is the number of groups of users, the length of the time slot for sending the pilot frequency in each group is L symbol periods, and the relevant time of a channel is T; the users in the same group use simple orthogonal pilot to transmit pilot in the same time slot, i.e. the pilot sequences transmitted by users i and k in the same groupAndsatisfy the orthogonal relationWhere L is the length of the pilot sequence.

5. The method according to any of claims 1 to 3, wherein the sequential pilot transmission mode is a user packet sequential pilot transmission mode in a multi-cell, and specifically comprises: grouping users of adjacent cells, wherein each group comprises a plurality of users from different cells, the users of different groups send pilot frequencies in different time slots, the users of the same group send pilot frequencies in the same time slot, and the pilot frequency sequences of the users of the same group are mutually orthogonal.