JP2009141957A - Pre-coding transmission method of mimo system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pre-coding transmission method of a MIMO (multiple-input multiple-output) system. <P>SOLUTION: Assuming that a mobile station uses one unitary matrix defined in a code book as transmission pre-coding at a base station, and let one vector in the unitary matrix be the transmission beam vector of the user and the other vector be the transmission vector of an interference user, an equivalent channel is calculated, by using an MMSE reception weight matrix; a channel directional index and channel quality information are obtained and then sent back (feedback) to a base-station. At the base station, a ZFP transmission matrix or an adjusted ZFP transmission matrix, corresponding to a possible combination of users, is obtained, based on the feedback information; and a combination of users who simultaneously transmit is specified by a multi-user scheduling, and pre-coded by using a corresponding ZFP transmission matrix or an adjusted ZFP transmission matrix, and then transmitted. A scheduled user performs MMSE reception. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a precoding technique in a MIMO system, and more particularly to a precoding transmission method for a MIMO system.

  Future wireless communication systems will require higher information transmission rates and communication quality. In order to achieve this goal with limited frequency resources, MIMO (Multi Input Multi Output) technology has become one of the indispensable means for future wireless communication.

  In the MIMO system, the transmission side transmits signals with a plurality of antennas, and the reception side receives signals with a plurality of antennas. According to the research results, compared with the traditional single antenna transmission method, when the MIMO technology is used, the capacity of the channel can be obviously improved and the information transmission rate can be improved.

  When a pre-coding transmission method is used in a MIMO system, the performance of the MIMO system can be effectively improved. The basic idea of precoding is to perform preliminary processing such as linear processing and non-linear processing on data to be transmitted before transmission based on current channel information.

  Hereinafter, a data processing process employing precoding will be described.

である。 Assume that in a cell, the number of base stations is 1, the number of mobile stations is K, the base station has M transmit antennas, and each mobile station has N _k receive antennas. At a certain time and frequency, the base station transmits a data stream to scheduled S ≦ K users. That is, L _k data streams are transmitted to the k (k = 1, 2,..., S) th user. here,

It is.

The L _k × 1-dimensional transmission vector s _k (data symbol) of the k-th user is transmitted from M antennas via the M × L _k- dimensional precoding transmission matrix T _k . The channel characteristic matrix of user k is N _k × M-dimensional matrix H _k .

は、下記式

で示される。 The N _k × 1D signal vector y _k received by the k th user is expressed as L _k × 1D soft output vector via the L _k × N _k reception decoding matrix R _k.

Is the following formula

Indicated by

In the above equation, the second part on the right side is multi-user interference, and n _k is a noise signal.

個のベクトルが含まれる。

の場合、各ユーザは、一つの等価チャネル

のみの量子化情報を基地局にフィードバックする。ｋ番目のユーザの受信信号は、下記式

で示される。 Each user can accurately estimate their own channel characteristic matrix H _k , but only limited information can be fed back to the base station. The base station and all mobile stations have a common codebook, eg DFT (Discrete Florier Transform) matrix, the user quantizes the feedback channel into a vector in the codebook, and B Codebooks that can support bit feedback are at most

Contains vectors.

Each user has one equivalent channel

Only the quantization information is fed back to the base station. The received signal of the kth user is given by

Indicated by

は総プレコーディング送信行列であり、

は総送信信号であり、n_kはノイズ信号であり、平方誤差は

である。 In the above equation, r _k is the receiver composite vector,

Is the total precoding transmission matrix,

Is the total transmitted signal, n _k is the noise signal, and the square error is

It is.

  In the prior art, ZFP (Zero-Forcing Precoding) technology is used. This will be described in detail below.

と記する。つまり、

そして、各仮定した送信コードブックベクトルについて、対応するＳＩＮＲ（Signal to Interference plus Noise Ratio，信号対干渉とノイズの比）を求めることを試してみる。 The ZFP method directly performs SVD (Singular-Value Decomposition, singular value decomposition) on the user channel H _k , and sets the feature vector U _k | ₁ corresponding to the maximum feature value as a receiver combined vector,

It writes. That means

Then, it is tried to obtain a corresponding SINR (Signal to Interference plus Noise Ratio) for each assumed transmission codebook vector.

となる。 If the scheduled transmission vectors of each user are basically orthogonal and the error between the actual transmission beam and the assumed transmission codebook vector is very small, the approximation of the lower limit of SINR is

It becomes.

はコードブック中のコードベクトルであり、||.||はベクトルのモジュラーである。 here,

Is a code vector in the codebook, and ||. || is a modular vector.

最後に、対応するチャネル方向インデックスＣＤＩとチャネル品質情報ＣＱＩを取得して基地局にフィードバックする。ＣＤＩとＣＱＩはそれぞれ下記式で示される。

基地局は、全てのユーザからフィードバックされたＣＱＩとＣＤＩに基づいて、可能性のある全てのユーザ組み合わせ（Ｋ個のユーザのうちＳ個のユーザを選択する）に対応する送信行列Ｔ_ZFPを求める。送信行列Ｔ_ZFPは、下記式

で示される。ここで、

は、選択されたＳ個のユーザの量子化フィードバックチャネルの組み合わせ、肩文字Ｈは共役転置を示し、肩文字−1は逆を示す。上記ベクトル

は、各ユーザの送信パワーの割り当てである。ユーザ間のエネルギーの分布が同一の場合、

であり、Ｐは総送信パワーである。しかしながら、例えばWater fallingアルゴリズムなどの他の方法を利用して、各ユーザ間のパワー割り当てを行なってもよい。 Finally, the user equivalent channel is quantized into a code vector that maximizes SINR. It is shown by the following formula.

Finally, the corresponding channel direction index CDI and channel quality information CQI are acquired and fed back to the base station. CDI and CQI are represented by the following formulas, respectively.

Based on the CQI and CDI fed back from all users, the base station obtains a transmission matrix T _ZFP corresponding to all possible user combinations (selecting S users out of K users). . The transmission matrix T _ZFP is given by

Indicated by here,

Is the combination of the selected S user quantized feedback channels, the superscript H indicates the conjugate transpose, and the superscript -1 indicates the reverse. Vector above

Is the allocation of the transmission power of each user. If the energy distribution between users is the same,

And P is the total transmission power. However, power allocation between users may be performed using another method such as a water falling algorithm.

を用いて各ユーザのＳＩＮＲを補正する。その補正は、下記式で示される。

さらに、補正されたＳＩＮＲを用いて各ユーザ組み合わせの容量を推定する。即ち、下記式で示される。

最後に、最大容量のユーザ組み合わせを選択して送信する。その送信行列は該ユーザ組み合わせに対応するＴ_ZFPである。 And asked

Is used to correct the SINR of each user. The correction is shown by the following formula.

Further, the capacity of each user combination is estimated using the corrected SINR. That is, it is represented by the following formula.

Finally, the user combination with the maximum capacity is selected and transmitted. The transmission matrix is _TZFP corresponding to the user combination.

実のＳＩＮＲは、

であり、
実の容量は、

である。 In the finite feedback system, due to the presence of quantization error, the interference of each user is not completely eliminated, and the selected user performs MMSE reception to suppress multi-user interference. That is, it is represented by the following formula.

The actual SINR is

And
The actual capacity is

It is.

  In the above method, the actual receiver is an MMSE combiner, but the combiner used at the time of feedback is a feature vector obtained by SVD decomposition of each user's own channel, and only the user's own channel is considered. Because the interference of other users was not taken into consideration, the combined channel obtained by the above method and the actual receiving channel have a large deviation, the feeder back is inaccurate, the base station scheduling and the precoding matrix calculation are also performed. Due to inaccuracies, the frequency utilization of the system is not being utilized at maximum.

  In order to solve the problem that the frequency utilization rate is relatively low due to a large deviation between the feedback composite channel used in the conventional ZFP method and the actual reception channel, the present invention is more effective than improving the frequency utilization rate of the system. An object is to provide an effective MIMO system precoding transmission method.

  In order to achieve the above object, the present invention provides a precoding transmission method for a MIMO system. The method includes the following steps.

  In Step A, it is assumed that the mobile station uses one unitary matrix defined in the codebook as transmission precoding in the base station, and one vector of the unitary matrix is used as the transmission beam vector of the user, M-1 vectors are used as transmission beam vectors of all interfering users, an equivalent channel is calculated using the MMSE reception weight matrix, and the unitary matrix index and channel quality information are acquired and fed back to the base station. .

  In step B, the base station determines a ZFP transmission matrix or an adjusted ZFP transmission matrix corresponding to some or all possible user combinations based on the channel direction index and channel quality information fed back from the mobile station. Acquire and specify the combination of users to be transmitted simultaneously by multi-user scheduling.

  In Step C, the base station performs precoding using the ZFP transmission matrix or the adjusted ZFP transmission matrix corresponding to the user combination, and transmits the result.

  In the above method, the mobile station further includes a step D of receiving a signal using an MMSE receiver.

In step A of the above method,
Step 111, in which the mobile station calculates a corresponding equivalent channel for each vector of the unitary matrix;
The mobile station obtains a corresponding SINR for each vector of the unitary matrix based on the equivalent channel;
Finding a maximum value of SINR and obtaining an equivalent channel quantized in the mobile station based on the maximum value; 113;
Obtaining channel quality information based on the maximum SINR value 114;
Feeding back 115 the channel direction index and channel hinges upward to the base station.

を選択されたＳ個のユーザの量子化チャネルの組み合わせとし、

を各ユーザの送信パワーの割り当てとすると、上記ユーザ組み合わせに対応するZFP送信行列が、下記式

で示される。 In the above method,

Is a combination of the quantized channels of the selected S users,

Is the allocation of the transmission power of each user, the ZFP transmission matrix corresponding to the above user combination is

Indicated by

を選択されたＳ個のユーザの量子化チャネルの組み合わせとし、

を各ユーザの送信パワーの割り当てとすると、上記ユーザ組み合わせに対応する調整済みＺＦＰ送信行列が、下記式

で示される。 In the above method,

Is a combination of the quantized channels of the selected S users,

Is the allocation of the transmission power of each user, the adjusted ZFP transmission matrix corresponding to the user combination is given by

Indicated by

  In the above method, in step B, a multi-user scheduling operation is performed by selecting a corresponding scheduling algorithm based on QoS.

  According to the present invention, since the CQI and CDI are calculated using the MMSE combiner in the feedback stage, the error of the channel feedback is reduced and the frequency utilization factor of the system is effectively improved.

  In the MIMO system precoding transmission method of the present invention, in a feedback stage, a mobile station calculates a channel direction index (CDI) and channel quality information (CQI) of a quantized equivalent channel using an MMSE combiner, and performs scheduling. In the stage and the precoding stage, corresponding processing is performed using the feedback information to improve the frequency utilization rate.

  As shown in FIG. 1, the MIMO system precoding transmission method of the present invention includes the following steps.

と等価チャネルh_kを得ることができないが、実際のマルチユーザシステムにおいて、基地局のスケジューラは、フィードバックチャネルが互い直交するユーザを選択する傾向がある。これは、このようにするとユーザ間の干渉を小さくし、容量を比較的に大きくすることができるからである。選択されたユーザのチャネルが互いに直交すれば、算出した送信行列Ｔも直交行列となる。後述のシミュレーション図からもわかるように、ユーザ数が多くなるほど、互いに直交するユーザグループを選択する確率も大きくなり、送信行列もますます直交行列に近似する。そこで、送信行列Ｔをユニタリー行列と仮定し（即ち、一つのユニタリー行列を基地局における送信プレコーディング行列として使用し）、式

でＭＭＳＥ合成ベクトル

を計算して、等価チャネル

を取得し、上記等価チャネルを利用してチャネル方向インデックスとチャネル品質情報を取得する。 In step 11, at the time of feedback, since each user does not know the precoding transmission matrix T, the actual receiver combined vector

Although it is impossible to obtain the equivalent channel h _k, in an actual multi-user system, a scheduler of the base station tend to feedback channel to select a user to each other perpendicular. This is because the interference between users can be reduced and the capacity can be relatively increased. If the channels of the selected users are orthogonal to each other, the calculated transmission matrix T is also an orthogonal matrix. As can be seen from the simulation diagram described later, as the number of users increases, the probability of selecting user groups that are orthogonal to each other increases, and the transmission matrix becomes more and more approximate to the orthogonal matrix. Therefore, the transmission matrix T is assumed to be a unitary matrix (that is, one unitary matrix is used as a transmission precoding matrix in the base station), and the equation

MMSE composite vector

Calculate the equivalent channel

And the channel direction index and channel quality information are acquired using the equivalent channel.

  In step 12, the mobile station feeds back CDI and CQI to the base station.

In step 13, the base station determines that some or all possible user combinations (S <= M out of K users) based on the CDI and CQI fed back from all mobile stations. The ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP corresponding to the _above is acquired, and scheduling is performed for the mobile station.

Here, the ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP corresponding to some user combinations is obtained, or the ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP corresponding to all user combinations is _acquired. Whether or not to acquire is related to the scheduling algorithm, which will be described later.

で示される。 The ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP is given by

Indicated by

は、選択されたＳ個のユーザの量子化等価チャネルの組み合わせであり、ベクトル

は、各ユーザの送信パワーの割り当てである。 here,

Is a combination of the quantized equivalent channels of the selected S users and is a vector

Is the allocation of the transmission power of each user.

  The base station can perform scheduling for the mobile station by performing multi-user scheduling operation by selecting a corresponding scheduling algorithm based on QoS.

In step 14, precoding is performed on the data of the scheduled mobile station (that is, users who need to transmit at the same time) using the corresponding ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP. To send.

  Hereinafter, the method of the present invention will be described in more detail.

を該ユーザの送信ベクトルとし、ほかのＭ-１個のベクトルを干渉ユーザの送信ベクトルとする。本発明を理解するために、ステップ１１を詳しく説明する前に、該ユニタリー行列C_gとベクトルc_iについて説明する。 In step 11, the transmission matrix T is set as one unitary matrix C _g in the codebook, and one vector in the matrix

Is the transmission vector of the user, and the other M-1 vectors are the transmission vectors of the interfering user. In order to understand the present invention, before describing step 11 in detail, the unitary matrix C _g and vector c _i will be described.

を該ユーザの送信ベクトルと仮定しているだけなので、送信ベクトルが１より大きい場合、該ＭＭＳＥ合成ベクトル

はＭＭＳＥ受信ウェイト行列になる。 In step 11, one of those vectors

Is assumed to be the user's transmission vector, so if the transmission vector is greater than 1, the MMSE composite vector

Becomes the MMSE reception weight matrix.

  Although various types of codebooks can be used in the embodiment of the present invention, only a precoding codebook based on DFT will be described here as an example.

m=0，…，M−1；
i =0，…，2^B−1で示される。 The codebook is a DFT matrix composed of 2 ^B vectors. Each vector contains M elements, and the m-th element of the i-th vector is

m = 0, ..., M-1;
i = 0, ..., 2 ^B −1.

g =0，…，G−1で示される。 The 2 ^B vectors constitute G = 2 ^B / M unitary matrices. The g-th unitary matrix is

g = 0, ..., G-1.

  As shown in FIG. 2, step 11 includes the following steps in detail.

で示すように、移動局は、各c_iに対応する等価チャネル

を算出する。ここで、

である。 In step 111, the following formula

As shown by the mobile station, the equivalent channel corresponding to each c _i

Is calculated. here,

It is.

で示すように、移動局は、該等価チャネル

に基づいて、各c_iに対応する

を取得する。 In step 112,

As shown by the mobile station, the equivalent channel

Corresponding to each c _i based on

To get.

で示すように、

のうちの最大値を見つけ出し、該最大値に基づいて量子化された等価チャネル

を取得する。 In step 113, the following formula

As shown in

Finds the maximum of the equivalent channels quantized based on the maximum

To get.

のうちの最大値及び対応する等価チャネル

に基づいて、ＣＤＩ及びＣＱＩを取得する。 In step 114,

Maximum value and corresponding equivalent channel

CDI and CQI are obtained based on the above.

  Of course, in step 114, the transmission beam vector having the maximum SINR may be set as the channel direction, and an index of the transmission beam vector and channel quality information may be obtained and fed back to the base station. The results obtained by both parties match.

  In step 13, the base station performs scheduling for the mobile station, and a scheduling algorithm to be used is a Max C / I scheduling algorithm, a PF scheduling algorithm, a round robin scheduling algorithm, or the like. In a specific embodiment of the present invention, a Max C / I scheduling algorithm will be described in detail as an example.

  As shown in FIG. 3, the scheduling for the mobile station by the base station in step 13 has the following steps in detail.

となる。下記式

又は

を用いて、そのＺＦＰ送信行列Ｔ_ZFP又は調整済みＺＦＰ送信行列Ｔ_RZFPを計算する。そして、t_k(k=1,…S)を用いて該組み合わせ中の各ユーザのSINR_kを補正して、補正後のSINR_k,estを取得する。補正後のSINR_k,estは、式

で示される。 In step 131, for a possible user combination S, the quantization channel combination is

It becomes. Following formula

Or

_{Is used} to calculate the ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP . Then, SINR _k of each user in the combination is corrected using t _k (k = 1,... S), and corrected SINR _{k, est} is obtained. SINR _{k, est} after correction is

Indicated by

で示すように、補正後のＳＩＮＲを利用して各ユーザ組み合わせの容量を推定する。 In step 132, the expression

As shown by, the capacity of each user combination is estimated using the corrected SINR.

  In step 133, the user combination having the maximum capacity is set as the scheduled user.

Of course, the scheduling algorithm described above is a preferred algorithm, and for every possible user combination, the corresponding ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _RZFP is calculated to estimate the capacity, Further, selection is performed based on the result of capacity estimation. However, when the number of users is relatively large, an algorithm such as a greedy algorithm may be applied to the present invention for scheduling in order to reduce the calculation amount of the scheduling algorithm. In this case, only the ZFP transmission matrix T _ZFP or the adjusted ZFP transmission matrix T _{RZFP of} some possible user combinations may be calculated to estimate the capacity, and selection may be performed based on the result of the capacity estimation.

In step 14, the base station may perform precoding using a transmission matrix (ZFP transmission matrix T _ZFP or adjusted ZFP transmission matrix T _RZFP ) corresponding to the scheduled user combination and transmit.

で示すように、ＭＭＳＥ受信を行なう。 On the other hand, since interference between users cannot be completely suppressed on the mobile station side,

As shown by MMSE reception.

  4 and 5 are simulation diagrams when the method of the present invention is used.

  As a first simulation condition, the number of transmitting antennas M is 4, the number of receiving antennas N is 4, the antennas are not related, the number of users is 20, and 3-bit CDI and accurate CQI are fed back. To do.

  FIG. 4 is a frequency utilization efficiency curve according to the change in SNR under the first simulation condition described above. The upper curve is the frequency utilization efficiency curve according to the SNR change when the method of the present invention is used, and the lower curve is the frequency utilization efficiency curve according to the SNR change in the conventional ZFP method. is there. According to FIG. 4, when SNR = 12 dB, the frequency utilization efficiency is higher by about 2.5 bps / Hz when the method of the present invention is used compared to the conventional ZFP method. Also, as the SNR increases, the frequency utilization efficiency will become increasingly clear.

  As a second simulation condition, the number of transmitting antennas M is 4, the number of receiving antennas N is 4, the antennas are not related, and 3-bit CDI and accurate CQI are fed back, and the SNR is 12 dB. .

  FIG. 5 is a curve of frequency use efficiency according to the change in the number of users under the above-described second simulation condition. The upper curve is the frequency utilization efficiency curve according to the change in the number of users when the method of the present invention is used, and the lower curve is the frequency utilization efficiency according to the change in the number of users in the conventional ZFP method. It is a curve. According to the figure, when the number of users is 12, the frequency utilization efficiency is higher by about 1.8 bps / Hz when the method of the present invention is used compared to the conventional ZFP method. Also, as the number of users increases, the frequency utilization efficiency will become increasingly clear.

  The above are only preferred embodiments of the present invention. The present invention can be implemented as modifications and changes without departing from the spirit and scope of the present invention defined by the description of the scope of claims.

The flow of the method of the present invention is shown. The detailed flow of step 11 of the method of the present invention is shown. FIG. 9 shows a process flow employing a Max C / I scheduling algorithm in step 14 of the method of the present invention. FIG. The simulation result using the method of this invention is shown. The simulation result using the method of this invention is shown.

Claims (6)

A precoding transmission method for a MIMO system,
Assuming that the mobile station uses one unitary matrix defined in the codebook as transmission precoding at the base station, one vector of the unitary matrix is used as the transmission beam vector of the user, and the other vector is used as an interfering user. Calculating an equivalent channel using an MMSE reception weight matrix, obtaining an index in the channel direction, and channel quality information and feeding back to the base station;
The base station obtains a ZFP transmission matrix or an adjusted ZFP transmission matrix corresponding to some or all possible user combinations based on the channel direction index and channel quality information fed back from the mobile station, Step B for identifying a combination of users that perform user scheduling and transmit at the same time;
A precoding transmission method comprising: a step C in which a base station performs precoding using a ZFP transmission matrix or an adjusted ZFP transmission matrix corresponding to the user combination and transmits the same.   The method of claim 1, further comprising step D, wherein the mobile station receives a signal using an MMSE receiver. In step A,
Step 111, in which the mobile station calculates a corresponding equivalent channel for each vector of the unitary matrix;
The mobile station obtains a corresponding SINR for each vector of the unitary matrix based on the equivalent channel;
Finding a maximum value of SINR and obtaining an equivalent channel quantized in the mobile station based on the maximum value; 113;
3. The method according to claim 2, further comprising the step of: obtaining a transmission beam vector having a maximum SINR as a channel direction, obtaining its index and channel quality information, and feeding back to the base station.

Is a combination of the quantized channels of the selected S users,

Is the allocation of the transmission power of each user, the ZFP transmission matrix corresponding to the user combination is

The method of claim 3 wherein:

Is a combination of the quantized channels of the selected S users,

Is the transmission power allocation of each user, the adjusted ZFP transmission matrix corresponding to the user combination is given by

The method of claim 3 wherein:   The method according to any one of claims 1 to 5, wherein in step B, a multi-user scheduling operation is performed by selecting a corresponding scheduling algorithm based on QoS.

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