MIMO communication system with user scheduling and modified precoding based on channel vector magnitudes
View Patent ↗A multiple-input, multiple-output (MIMO) communication system is configured to perform user scheduling and associated precoding. The system includes multiple terminals and at least one base station configured to communicate with the terminals. The base station is operative to obtain channel vectors for respective ones of the terminals, to select a subset of the terminals based on magnitudes of the respective channel vectors, to compute a precoding matrix using the channel vectors of the selected subset of terminals, and to utilize the preceding matrix to control transmission to the selected subset of terminals. The system may be, for example, a time-division duplex (TDD) multi-user MIMO system in which the multiple terminals comprise autonomous single-antenna terminals.
1. A method of communicating information in a multiple-input, multiple-output communication system in which a base station communicates with multiple terminals, the method comprising the steps of:
obtaining channel vectors for respective ones of the terminals;
selecting a subset of the terminals based on magnitudes of the respective channel vectors;
computing a precoding matrix using the channel vectors of the selected subset of terminals; and
utilizing the precoding matrix to control transmission to the selected subset of terminals.
2. The method of claim 1 wherein the step of selecting the subset of the terminals further comprises selecting from among K terminals those N terminals having the largest channel vector magnitudes, where N<K.
3. The method of claim 1 wherein the obtaining, selecting, computing and utilizing steps are repeated for each of a plurality of coherence intervals with potentially different subsets of terminals being selected in each of the coherence intervals.
4. The method of claim 1 wherein the multiple-input, multiple-output communication system comprises a time-division duplex (TDD) multiple-input, multiple-output communication system.
5. The method of claim 2 wherein the base station communicates with the multiple terminals via a set of M base station antennas, and a forward channel between the base station and the K terminals is characterized by a K×M matrix H.
6. The method of claim 5 wherein the base station obtains the channel vectors by computing an estimate Ĥ of the forward channel matrix H, wherein the channel vectors comprise respective rows of the estimated forward channel matrix Ĥ and the estimated forward channel matrix Ĥ is computed based on training sequences transmitted by the respective terminals to the base station over a reverse channel.
7. The method of claim 6 wherein the step of selecting from among K terminals those N terminals having the largest channel vector magnitudes further comprises forming an N×M matrix Ĥ S N =[ĥ (1) ĥ (2) . . . ĥ (N) ] T where ĥ (1) T , ĥ (2) T , . . . , ĥ (K) T comprise rows of the estimated forward channel matrix Ĥ arranged in order of decreasing channel vector magnitudes.
8. The method of claim 7 wherein the step of computing a precoding matrix further comprises the step of computing the precoding matrix as
A
S
N
=
H
^
S
N
†
(
H
^
S
N
H
^
S
N
†
)
-
1
tr
[
(
H
^
S
N
H
^
S
N
†
)
-
1
]
where (·) † and tr(·) denote Hermitian conjugate and trace operations, respectively.
9. The method of claim 8 wherein the precoding matrix is normalized so that tr(A S N † A S N )=1.
10. The method of claim 5 wherein the step of utilizing the precoding matrix to control transmission to the selected subset of terminals further comprises forming an M×1 transmission signal vector s f by premultiplying an information symbol vector q=[q 1 q 2 . . . q N ] T by the precoding matrix.
11. A non-transitory processor-readable storage medium having embodied therein instruction code which when executed by a processor of a base station causes the base station to perform the steps of the method of claim 1 .
12. A base station of a multiple-input, multiple-output communication system, the base station being configured to communicate with multiple terminals of the system, the base station comprising:
an antenna array comprising a plurality of antennas for communicating with the multiple terminals;
transceiver circuitry coupled to the antennas of the antenna array; and
a processor coupled to the transceiver circuitry;
wherein the base station is operative to obtain channel vectors for respective ones of the terminals, to select a subset of the terminals based on magnitudes of the respective channel vectors, to compute a precoding matrix using the channel vectors of the selected subset of terminals, and to utilize the precoding matrix to control transmission to the selected subset of terminals.
13. The base station of claim 12 :
wherein said processor is configured to obtain the channel vectors, to select the subset of the terminals, to compute the precoding matrix, and to utilize the precoding matrix to control transmission.
14. The base station of claim 12 wherein the base station selects the subset of terminals by selecting from among K terminals those N terminals having the largest channel vector magnitudes, where N<K.
15. A multiple-input, multiple-output communication system comprising:
a plurality of terminals; and
at least one base station configured to communicate with the terminals;
wherein the base station is operative to obtain channel vectors for respective ones of the terminals, to select a subset of the terminals based on magnitudes of the respective channel vectors, to compute a precoding matrix using the channel vectors of the selected subset of terminals, and to utilize the precoding matrix to control transmission to the selected subset of terminals.
16. The system of claim 15 wherein the base station selects the subset of terminals by selecting from among K terminals those N terminals having the largest channel vector magnitudes, where N<K.
17. A method of communicating information in a multiple-input, multiple-output communication system in which a base station communicates with multiple terminals, the method comprising the steps of:
obtaining channel vectors for respective ones of the terminals;
computing a precoding matrix using the channel vectors; and
utilizing the precoding matrix to control transmission to the terminals;
wherein the precoding matrix is computed as a function of a product of a diagonal matrix and an estimated forward channel matrix.
18. The method of claim 17 wherein the base station obtains the channel vectors by computing an estimated forward channel matrix Ĥ based on training sequences transmitted by the respective terminals to the base station over a reverse channel.
19. The method of claim 18 wherein the step of computing a precoding matrix further comprises the step of computing the precoding matrix as
A
D
=
H
^
D
†
(
H
^
D
H
^
D
†
)
-
1
tr
[
(
H
^
D
H
^
D
†
)
-
1
]
where
H
^
D
=
D
H
^
and
D
=
diag
{
[
p
1
-
1
2
p
2
-
1
2
…
p
K
-
1
2
]
}
,
K denotes the number of terminals, and p=[p 1 p 2 . . . p K ] T is a vector of non-negative real numbers, and where (·) † and tr(·) denote Hermitian conjugate and trace operations, respectively.
20. The method of claim 19 wherein p=[p 1 p 2 . . . p K ] T is selected to approximately maximize a lower bound on forward channel weighted sum capacity.
21. The method of claim 19 further including the step of selecting from among the K terminals a subset of N terminals to receive transmissions in a given coherence interval by ordering the K terminals such that p * (1) ∥z (1) T ∥ 2 ≧ p * (2) ∥z (2) T ∥ 2 ≧ . . . ≧ p * (K) ∥z (K) T ∥ 2 and selecting the first N of the ordered terminals, where p * is an optimized version of the p vector and z (1) T , z (2) T , . . . , z (K) T denote rows of a diagonal matrix formed as a function of the estimated forward channel matrix Ĥ.
22. The method of claim 17 wherein the step of utilizing the precoding matrix to control transmission to the terminals further comprises forming an M×1 transmission signal vector s f by premultiplying an information symbol vector q=[q 1 q 2 . . . q K ] T by the precoding matrix.
23. A non-transitory processor-readable storage medium having embodied therein instruction code which when executed by a processor of a base station causes the base station to perform the steps of the method of claim 17 .
24. A base station of a multiple-input, multiple-output communication system, the base station being configured to communicate with multiple terminals of the system, the base station comprising:
an antenna array comprising a plurality of antennas for communicating with the multiple terminals;
transceiver circuitry coupled to the antennas of the antenna array; and
a processor coupled to the transceiver circuitry;
wherein the base station is operative to obtain channel vectors for respective ones of the terminals, to compute a precoding matrix using the channel vectors, and to utilize the precoding matrix to control transmission to the terminals, wherein the precoding matrix is computed as a function of a product of a diagonal matrix and an estimated forward channel matrix.