CN106712817B - Low-complexity pilot frequency distribution method based on user exchange - Google Patents

Low-complexity pilot frequency distribution method based on user exchange Download PDF

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CN106712817B
CN106712817B CN201611219261.8A CN201611219261A CN106712817B CN 106712817 B CN106712817 B CN 106712817B CN 201611219261 A CN201611219261 A CN 201611219261A CN 106712817 B CN106712817 B CN 106712817B
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pilot
user
users
exchange
cell
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CN106712817A (en
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张海霞
刘见花
袁东风
周晓天
郭帅帅
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Shandong University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver

Abstract

The invention relates to a low-complexity pilot frequency distribution method based on user exchange, which can realize system performance basically consistent with an exhaustive traversal method with lower complexity, effectively weaken the influence of pilot frequency pollution, maximize the system and speed performance and simultaneously consider fairness among users with lower complexity.

Description

Low-complexity pilot frequency distribution method based on user exchange
Technical Field
The invention relates to a low-complexity pilot frequency distribution method based on user exchange, belonging to the technical field of communication systems.
Background
The pilot pollution problem is a widely studied problem in Massive MIMO systems, and is an interference problem caused by user-shared pilots, and generally occurs in TDD mode in Massive MIMO systems. In the TDD mode, the uplink and downlink channels have reciprocity, and therefore, the channel estimated in the uplink can be used as the channel for downlink transmission. Because both pilot frequency transmission and information transmission are carried out within the coherence time of the channel, the length of the pilot frequency is limited, which limits the number of available pilot frequencies in the system, so that the pilot frequency is inevitably multiplexed between adjacent cells, and the pilot frequency pollution becomes an irreparable problem in Massive MIMO.
The pilot frequency allocation taking the maximization of the user rate product as the optimization target can effectively weaken the influence of pilot frequency pollution, and can simultaneously maximize the system and the rate and take fairness among users into consideration. However, the conventional method for solving the optimization problem is an exhaustive traversal method, which refers to traversing all possibilities of pilot allocation once, and selecting a pilot allocation scheme with the largest user rate product. This scheme can achieve an optimal solution to the problem, but is very complex to implement.
Another pilot frequency allocation method based on greedy can solve the optimization problem with lower complexity, and the basic idea is that each iteration traverses the possibility of selecting one user in each cell, selects a combination which enables the maximum user rate product in the user combination as the optimal solution of the iteration, then removes the selected user from the system, and continues the iteration until all users are allocated with pilot frequencies. But such greedy-based allocation strategies can only achieve locally optimal performance.
Disclosure of Invention
The invention provides a pilot frequency allocation method based on user exchange, aiming at the problem of pilot frequency allocation with the optimization target of maximizing the product of the user rate of a system.
The method can realize the system performance basically consistent with the exhaustive traversal method, but the implementation complexity is greatly reduced, and the method is more beneficial to practical application.
The technical scheme of the invention is as follows:
a low-complexity pilot frequency distribution method based on user exchange is suitable for a Massive MIMO system, the Massive MIMO system comprises L equal hexagonal cells, each cell comprises 1 central base station and K users, the 1 central base station simultaneously serves the K users, the users in the same cell are mutually orthogonal pilot frequencies, the users in the cells multiplex the pilot frequencies, the number of the pilot frequencies which can be distributed in the Massive MIMO system is K, and the specific steps comprise:
(1) according to the distance from the user to the central base station, obtaining the large-scale fading information from each user to each central base station in each cell in the Massive MIMO system
Figure BDA0001192452030000021
i，j∈[1，2，…，L]，k∈[1，2，…，K]，
Figure BDA0001192452030000022
The large-scale fading factors of users k in the cell i to the central base station in the cell j;
(2) an objective function is constructed for the pilot frequency allocation problem, the objective function max U (P) is the product of the speeds of all users in the Massive MIMO system, and the formula (I) is shown as follows:
Figure BDA0001192452030000023
in the formula (I), the compound is shown in the specification, for the rate of users using the kth pilot in L cell, L ∈ [1,2, …, L]，
The large-scale fading factor from a user k in a cell to a central base station in the cell is referred to;
Figure BDA0001192452030000027
the large-scale fading factors of users k in the cell l to the central base station in the cell j;
(3) selecting a pilot frequency group with the maximum user rate product by using a greedy strategy each time, and iterating K times to complete pilot frequency allocation as an initial pilot frequency group;
the set of users using the same pilot is a pilot group and the users in each pilot group are from different cells, PkIndicates a pilot group using pilot K; there are K pilot groups, P, in Massive MIMO systems1，P2，…，PKEach pilot frequency group comprises L users;
(4) and (4) starting from the initial pilot frequency group obtained in the step (3), carrying out user exchange between every two pilot frequency groups: if the user rate product is larger than the user rate product before the exchange after the user exchange between the two pilot frequency groups, the exchange is carried out, otherwise, the exchange is not carried out until all the pilot frequency groups finish the user exchange.
According to the invention, the step (3) preferably comprises the following specific steps:
A. randomly selecting one user from each cell, and solving the rate product of the selected L users;
B. the L users corresponding to the maximum value of the rate product obtained in the step A are used as a pilot frequency group P1；
C. The pilot groups P are obtained in turn from the remaining users according to the method of step A, B2，…，Pk，…，PK；
D. Derived pilot group P1，P2，…，PKAs an initial pilot set state.
According to the invention, the step (4) preferably comprises the following specific steps:
E. pilot group P obtained in step (3)1，P2，…，PKAs an initial state of a pilot allocation method based on user switching;
F. setting an iteration counter l, and initializing l to 1;
G. user exchange in the cell is performed between any two pilot groups: if the user rate product is larger than the user rate product before the exchange after the user exchange, carrying out the exchange, otherwise, not carrying out the exchange;
H. and judging whether L is equal to L, if so, ending, otherwise, adding 1 to L, and returning to the step G.
The invention has the beneficial effects that:
1. the invention realizes a pilot frequency allocation scheme which gives consideration to both system effectiveness and fairness by maximizing the product of the user rates of the system. Compared with the existing pilot frequency distribution mode, the pilot frequency distribution scheme considered by the invention not only maximizes the overall performance of the system, but also considers the fairness among users.
2. The invention can realize the system performance basically consistent with the exhaustive traversal method through a pilot frequency distribution method based on user exchange. Compared with an exhaustive traversal method, the pilot frequency allocation method based on user exchange has lower complexity. The L cells used in the invention, among the K user system models in each cell, need to be done when using the exhaustive method (K!)L-1Secondary traversal is performed, but the pilot frequency distribution method based on user exchange only needs to be performedA secondary traversal andand the secondary user exchange, wherein n is the number of rounds for exchange, greatly reduces the complexity of the system.
Drawings
FIG. 1 is a block diagram of the steps of the pilot allocation method based on user switching according to the present invention;
fig. 2 is a simulation effect diagram of the system user rate product when n is 1 in the embodiment;
fig. 3 is a simulation effect diagram of the system user rate product when n is 2 in the embodiment.
Detailed Description
The invention is further defined in the following, but not limited to, the figures and examples in the description.
Examples
A low-complexity pilot frequency distribution method based on user exchange is suitable for a Massive MIMO system, the Massive MIMO system comprises 3 equal hexagonal cells as shown in figure 1, each cell comprises 1 central base station and 5 users, 1 central base station simultaneously serves 5 users, and the configuration conditions of the cell users and the central base stations in the system are simply described in a model diagram of the system. It can be seen from the model diagram that since the same pilot frequency is shared, the base station receives the pilot frequency information of mutual interference at the same time in the uplink pilot frequency estimation, and the mutual interference causes pilot frequency pollution; the method comprises the following specific steps:
(1) according to the distance from the user to the central base station, obtaining the large-scale fading information from each user to each central base station in each cell in the Massive MIMO system
Figure BDA0001192452030000041
i，j∈[1，2，…，L]，k∈[1，2，…，K]，Refer to users k to in i cellLarge scale fading factors of the central base station in the j cell;
(2) an objective function is constructed for the pilot frequency allocation problem, the objective function max U (P) is the product of the speeds of all users in the Massive MIMO system, and the formula (I) is shown as follows:
Figure BDA0001192452030000043
in the formula (I), the compound is shown in the specification,
Figure BDA0001192452030000044
Figure BDA0001192452030000045
for the rate of users using the kth pilot in L cell, L ∈ [1,2, …, L]，
Figure BDA0001192452030000046
The large-scale fading factor from a user k in a cell to a central base station in the cell is referred to;
Figure BDA0001192452030000047
the large-scale fading factors of users k in the cell l to the central base station in the cell j;
(3) selecting a pilot frequency group with the maximum user rate product by using a greedy strategy each time, and iterating K times to complete pilot frequency allocation as an initial pilot frequency group; the method comprises the following specific steps:
A. randomly selecting one user from each cell, and solving the rate product of the selected L users;
B. the L users corresponding to the maximum value of the rate product obtained in the step A are used as a pilot frequency group P1；
C. The pilot groups P are obtained in turn from the remaining users according to the method of step A, B2，…，Pk，…，PK；
D. Derived pilot group P1，P2，…，PKAs an initial pilot set state.
The set of users using the same pilot is a pilot group and the users in each pilot group are from different cells, PkIndicates a pilot group using pilot K; there are K pilot groups, P, in Massive MIMO systems1，P2，…，PKEach pilot frequency group comprises L users;
(4) and (4) starting from the initial pilot frequency group obtained in the step (3), carrying out user exchange between every two pilot frequency groups: if the user rate product is larger than the user rate product before the exchange after the user exchange between the two pilot frequency groups, the exchange is carried out, otherwise, the exchange is not carried out until all the pilot frequency groups finish the user exchange. The method comprises the following specific steps:
E. pilot group P obtained in step (3)1，P2，…，PKAs an initial state of a pilot allocation method based on user switching;
F. setting an iteration counter l, and initializing l to 1;
G. user exchange in the cell is performed between any two pilot groups: if the user rate product is larger than the user rate product before the exchange after the user exchange, carrying out the exchange, otherwise, not carrying out the exchange;
H. and judging whether L is equal to L, if so, ending, otherwise, adding 1 to L, and returning to the step G.
The effect of the pilot allocation method based on user exchange according to this embodiment on maximizing the system product is shown in fig. 2 and fig. 3. As shown in fig. 2 and fig. 3, the pilot allocation method based on user exchange according to the present invention can achieve an effect comparable to the exhaustive traversal method when maximizing the system product, and particularly, in fig. 3, when n is 2, the entire system undergoes two rounds of user exchange, and the pilot allocation method based on user exchange achieves almost the same performance as the exhaustive traversal method. n is the number of times for exchanging, when n is 2, taking 3 cells and 5 users as an example, the exhaustive traversal method needs 14400 times of traversal to obtain the optimal performance, while the user exchange-based method only needs 225 times of traversal by a greedy method to obtain a pilot group in an initial state, and then the pilot group can achieve the system performance almost the same as the exhaustive traversal method by 60 times of user exchange, thereby greatly reducing the complexity. Although the method based on user exchange additionally increases 60 times of user exchange on the basis of the greedy method, the system performance is greatly improved, so that the pilot frequency allocation method based on user exchange can achieve the performance almost the same as that of the exhaustive traversal method with lower complexity.

Claims (2)

1. A low-complexity pilot frequency distribution method based on user exchange is suitable for a Massive MIMO system, the Massive MIMO system comprises L equal hexagonal cells, each cell comprises 1 central base station and K users, 1 central base station simultaneously serves K users, users in the same cell are mutually orthogonal pilot frequencies, users in the cells multiplex pilot frequencies, the number of the pilot frequencies which can be distributed in the Massive MIMO system is K, and the method is characterized by comprising the following specific steps:
(1) according to the distance from the user to the central base station, obtaining the large-scale fading information from each user to each central base station in each cell in the Massive MIMO systemi,j∈[1,2,…,L],k∈[1,2,…,K]，
Figure FDA0002268613260000012
The large-scale fading factors of users k in the cell i to the central base station in the cell j;
(2) an objective function is constructed for the pilot frequency allocation problem, the objective function max U (P) is the product of the speeds of all users in the Massive MIMO system, and the formula (I) is shown as follows:
in the formula (I), the compound is shown in the specification,
Figure FDA0002268613260000014
for the rate of users using the kth pilot in L cell, L ∈ [1,2, …, L]，
Figure FDA0002268613260000016
The large-scale fading factor is from a user k in a cell to a central base station in the cell;the large-scale fading factors refer to the large-scale fading factors of the central base station in the cells from k to j of the users in the cell l;
(3) selecting a pilot frequency group with the maximum user rate product by using a greedy strategy each time, and iterating M times to complete pilot frequency allocation as an initial pilot frequency group;
the set of users using the same pilot is a pilot group and the users in each pilot group are from different cells, PkIndicates a pilot group using pilot k; there are K pilot groups, P, in Massive MIMO systems1,P2,…,PKEach pilot frequency group comprises L users; the method comprises the following specific steps:
A. randomly selecting one user from each cell, and solving the rate product of the selected L users;
B. the L users corresponding to the maximum value of the rate product obtained in the step A are used as a pilot frequency group P1；
C. The pilot groups P are obtained in turn from the remaining users according to the method of step A, B2,…,Pk,…,PK；
D. Derived pilot group P1,P2,…,Pk,…,PKAs an initial pilot set state;
(4) and (4) starting from the initial pilot frequency group obtained in the step (3), carrying out user exchange between every two pilot frequency groups: if the user rate product is larger than the user rate product before the exchange after the user exchange between the two pilot frequency groups, the exchange is carried out, otherwise, the exchange is not carried out until all the pilot frequency groups finish the user exchange.
2. The method for allocating pilots based on user switching with low complexity as claimed in claim 1, wherein the step (4) comprises the following specific steps:
E. pilot group P obtained in step (3)1,P2,…,Pk,…,PKAs an initial state of a pilot allocation method based on user switching;
F. setting an iteration counter l, and initializing l to 1;
G. user exchange in the cell is performed between any two pilot groups: if the user rate product is larger than the user rate product before the exchange after the user exchange, carrying out the exchange, otherwise, not carrying out the exchange;
H. and judging whether L is equal to L, if so, ending, otherwise, adding 1 to L, and returning to the step G.
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