Radio communication system, radio base station apparatus, user terminal and radio communication method

1. A radio communication system comprising a radio base station apparatus to form a cell and user terminals to establish radio connection with the radio base station apparatus, wherein:

the radio base station apparatus comprises:

an array antenna having a plurality of antenna elements that are arranged in one direction;

a processor configured to select precoding vectors from a codebook formed with precoding vectors that are set in advance to be uneven with respect to the direction of the arrangement of the antenna elements, such that the angle of transmission beams corresponding to the precoding vectors with respect to the direction of the arrangement of the antenna elements assumes angular intervals taking into account the unevenness of the distribution of the user terminals instead of assuming regular angular intervals, based on communication quality information that is fed back from the user terminals in the cell;

wherein the processor is configured to multiply signals to supply to each antenna element by the selected precoding vectors; and

a transmitter configured to transmit the signals multiplied by the precoding vectors to the user terminals; and

the user terminals comprise:

a receiver configured to receive a signal from the radio base station apparatus;

a processor configured to extract a reference signal from the received signal and measure channel quality; and

a transmitter configured to feed back a communication quality feedback signal, including the measured channel quality, to the radio base station apparatus, via an uplink.

2. The radio communication system according to claim 1 , wherein the codebook is formed with precoding vectors that make a reciprocal of a density of the user terminals uniform.

3. The radio communication system according to claim 1 , wherein, when ρ(L) is a distribution function of the user terminals with respect to a distance from the radio base station apparatus and L[m] is the distance from the radio base station apparatus, the codebook is formed with precoding vectors that are determined from the following equation:

CODEBOOK V [0,1, . . . M− 1]=exp(2π d·j cos(α+π/2)·[0,1,2, . . . , N− 1])

α= a tan( h /(ρ( L·[ 0,1, . . . M− 1])))

where h is a height of the radio base station apparatus, d is an interval of branches, which are groups of a plurality of antenna elements, a is a tilt angle of transmission beams directed to the user terminals UE with respect to a horizontal direction, and N is the number of branches.

4. The radio communication system according to claim 3 , wherein, when the user terminals are distributed evenly with respect to the distance from the radio base station apparatus, the codebook is formed with precoding vectors that are determined by the following equation:

CODEBOOK V [0,1, . . . M− 1]=exp(2π d·j cos(α+π/2)·[0,1,2, . . . , N− 1])

A=a tan( h /( L max/ M )·[0,1, . . . M− 1]))

5. The radio communication system according to claim 3 , wherein, when the user terminals are distributed squarefold with respect to the distance from the radio base station apparatus, the codebook is formed with precoding vectors that are determined from the following equation:

CODEBOOK V [0,1, . . . M− 1]=exp(2π d·j cos(α+π/2)·[0,1,2, . . . , N− 1])

α= a tan( h /( L max/sqrt(2)^[ M− 1, . . . 1,0]))

6. The radio communication system according to claim 2 , wherein, when ρ(L) is a distribution function of the user terminals with respect to a distance from the radio base station apparatus and L[m] is the distance from the radio base station apparatus, the codebook is formed with precoding vectors that are determined from the following equation:

CODEBOOK V [0,1, . . . M− 1]=exp(2π d·j cos(α+π/2)·[0,1,2, . . . , N− 1])

α= a tan( h /(ρ( L·[ 0,1, . . . M− 1])))

where h is a height of the radio base station apparatus, d is an interval of branches, which are groups of a plurality of antenna elements, a is a tilt angle of transmission beams directed to the user terminals UE with respect to a horizontal direction, and N is the number of branches.

7. A radio base station apparatus in a radio communication system comprising the radio base station apparatus to form a cell and user terminals to establish radio connection with the radio base station apparatus, the radio base station apparatus comprising:

an array antenna having a plurality of antenna elements that are arranged in one direction;

a processor configured to select precoding vectors from a codebook formed with precoding vectors that are set in advance to be uneven with respect to the direction of the arrangement of the antenna elements, such that the angle of transmission beams corresponding to the precoding vectors with respect to the direction of the arrangement of the antenna elements assumes angular intervals taking into account the unevenness of the distribution of the user terminals instead of assuming regular angular intervals, based on communication quality information of the user terminals in the cell;

wherein the processor is configured to multiply signals to supply to each antenna element by the selected precoding vectors; and

a transmitter configured to transmit the signals multiplied by the precoding vectors to the user terminals.

8. A user terminal in a radio communication system comprising a radio base station apparatus to form a cell and user terminals to establish radio connection with the radio base station apparatus, the user terminal comprising:

a receiver configured to receive a signal transmitted from the radio base station apparatus using a codebook formed with precoding vectors that are set in advance in the radio base station apparatus to be uneven with respect to a direction of arrangement of antenna elements, such that the angle of transmission beams corresponding to the precoding vectors with respect to the direction of the arrangement of the antenna elements assumes angular intervals taking into account the unevenness of the distribution of the user terminals instead of assuming regular angular intervals, based on communication quality information of the user terminals in the cell;

a processor configured to extract a reference signal from the received signal and measure channel quality; and

a transmitter configured to feed back a communication quality feedback signal, including the measured channel quality, to the radio base station apparatus, via an uplink.

9. A radio communication method in a radio communication system comprising a radio base station apparatus to form a cell and user terminals to establish radio connection with the radio base station apparatus, the radio communication method comprising the steps of:

at the radio base station apparatus:

selecting precoding vectors from a codebook formed with precoding vectors that are set in advance to be uneven with respect to the direction of arrangement of antenna elements, such that the angle of transmission beams corresponding to the precoding vectors with respect to the direction of the arrangement of the antenna elements assumes angular intervals taking into account the unevenness of the distribution of the user terminals instead of assuming regular angular intervals, based on communication quality information of the user terminals in the cell;

multiplying signals to supply to each antenna element by the selected precoding vectors; and

transmitting the signals multiplied by the precoding vectors to the user terminals,

at the user terminal:

receiving a signal from the radio base station apparatus;

extracting a reference signal from the received signal and measuring channel quality; and

feeding back a communication quality feedback signal, including the measured channel quality, to the radio base station apparatus, via an uplink.