Beam synthesis method for downlink beamforming in FDD wireless communication system

• US 7,359,733 B2
• Filed: 08/03/2001
• Issued: 04/15/2008
• Est. Priority Date: 09/21/2000
• Status: Expired due to Fees

• Current Assignee: National University of Singapore
• Original Assignee: National University of Singapore
• Inventors: Chin, Francois, Liang, Ying-Chang
• Primary Examiner(s): Nguyen; Duc M.
• Assistant Examiner(s): GENACK, MATTHEW W
• Application Number:
  US09/922,442
• Publication Number:
  US 20020061768A1
• Time in Patent Office: 2,447 Days
• Field of Search: 455/63.1, 455/63.4, 455/428, 455/562.1
• US Class Current: 455/562.1
• CPC Class Codes: H04B 7/0617 for beam forming
• CPC Class Codes: H04B 7/086 using weights depending on external parameters, e.g. direction of arrival [DOA], predetermined weights or beamforming

Abstract

A method for downlink beamforming in a frequency-division-duplex wireless communications system comprising a base station with an antenna array and terminals that are physically remote from said base station, the method comprising the steps of: receiving at said base station antenna array combinations of arriving uplink signals from said plurality of remote terminals, estimating an uplink beamforming weight vector for each of said terminals from said combinations of arriving uplink signal; identifying uplink nulls and an uplink main beam position from said uplink beamforming weight vector; transforming each of said uplink nulls to form a corresponding downlink null; generating a downlink beamforming weight vector from all downlink nulls; and transmitting a set of information signals from said base station antenna array according to said downlink. beamforming weights.

Claims

1. A method for downlink beamforming in a frequency-division-duplex wireless communications system comprising a base station with an antenna array defining a communication cell and terminals that are physically remote from said base station, the method comprising the steps of:

dividing the communication cell into a plurality of sectors;

identifying uplink nulls which would yield pseudo nulls in a sector;

constraining use of the system to those terminals in a sector in which no pseudo nulls will be generated;

receiving at said base station antenna array combinations of arriving signals from said plurality of remote terminals, each signal having a beam pattern incorporating a main beam and one or more nulls;

identifying an uplink beamforming weight for a signal;

generating a downlink beamforming weight based on the signal'"'"'s uplink beamforming weight using the null constraint method, wherein the null constraint method comprises

determining an uplink beam pattern'"'"'s nulls;

z_u,k(i), i=1, . . . , M−

1,

using a polynomial formed from the uplink beamforming weight;

$\underset{i=1}{\stackrel{M}{\sum <br><br>}}<br><br>w_{u,k}<br><br>\left(i\right)<br><br>z^{-i+1}=w_{u,k}<br><br>\left(1\right)<br><br>\left(1-z_{u,k}<br><br>\left(1\right)<br><br>z^{-1}\right)<br><br><br><br>\mathrm{\dots <br><br>}<br><br><br><br>\left(1-z_{u,k}<br><br>\left(M-1\right)<br><br>z^{-1}\right);<br><br>$

transforming phase components of the uplink beam pattern'"'"'s nulls and obtaining phase components of a downlink beam pattern'"'"'s nulls as;

${\mathrm{\phi <br><br>}}_{d,k}<br><br>\left(i\right)=\frac{f_d}{f_u}<br><br>{\mathrm{\phi <br><br>}}_{u,k}<br><br>\left(i\right),\text{where}<br><br><br><br>z_{u,k}<br><br>\left(i\right)=A_{u,k}<br><br>\left(i\right)<br><br>{\mathrm{e<br><br>}}^{{\mathrm{j\phi <br><br>}}_{u,k}<br><br>\left(i\right)},$

for i=1, . . . , M−

1; and

constructing the downlink beam pattern'"'"'s nulls;

z_d,k(i)=A_d,k(i)e<sup>jψ

d,k(i)</sup>, for i=1, . . . , M−

1;

and constructing the downlink beamforming weight;

$\underset{i=1}{\stackrel{M}{\sum <br><br>}}<br><br>w_{d,k}<br><br>\left(i\right)<br><br>z^{-i+1}=w_{d,k}<br><br>\left(1\right)<br><br>\left(1-z_{d,k}<br><br>\left(1\right)<br><br>z^{-1}\right)<br><br><br><br>\mathrm{\dots <br><br>}<br><br><br><br>\left(1-z_{d,k}<br><br>\left(M-1\right)<br><br>z^{-1}\right);<br><br>$

and

transmitting downlink signals using downlink beamforming weights.

2. A base station for a wireless communications system, the base station comprising:

an uplink receiver antenna array for receiving arriving signals from a plurality of remote terminals on respective uplink channels;

a downlink weight generator operable to generate downlink weights based on a signal'"'"'s uplink beamforming weight using the null constraint method;

a downlink transmit antenna array to transmit signals to the remote terminals in accordance with the generated downlink weights, the transmission cell of the antenna array being divided into a plurality of sectors;

means to identify uplink nulls which would yield pseudo nulls in a sector; and

means to constrain use of the system to those terminals in a sector in which no pseudo nulls will be generated, wherein the null constraint method comprises

determining an uplink beam pattern'"'"'s nulls;

z_u,k(i), i=1, . . . , M−

1,

using a polynomial formed from the uplink beamforming weight;

$\underset{i=1}{\stackrel{M}{\sum <br><br>}}<br><br>w_{u,k}<br><br>\left(i\right)<br><br>z^{-i+1}=w_{u,k}<br><br>\left(1\right)<br><br>\left(1-z_{u,k}<br><br>\left(1\right)<br><br>z^{-1}\right)<br><br><br><br>\mathrm{\dots <br><br>}<br><br><br><br>\left(1-z_{u,k}<br><br>\left(M-1\right)<br><br>z^{-1}\right);<br><br>$

transforming phase components of the uplink beam pattern'"'"'s nulls and obtaining phase components of a downlink beam pattern'"'"'s nulls as;

${\mathrm{\phi <br><br>}}_{d,k}<br><br>\left(i\right)=\frac{f_d}{f_u}<br><br>{\mathrm{\phi <br><br>}}_{u,k}<br><br>\left(i\right),\text{where}<br><br><br><br>z_{u,k}<br><br>\left(i\right)=A_{u,k}<br><br>\left(i\right)<br><br>{\mathrm{e<br><br>}}^{{\mathrm{j\phi <br><br>}}_{u,k}<br><br>\left(i\right)},$

for i=1, . . . , M−

1; and

constructing the downlink beam pattern'"'"'s nulls;

z_d,k(i)=A_d,k(i)e<sup>jψ

d,k(i)</sup>, for i=1, . . . , M=1;

and constructing the downlink beamforming weight;

$\underset{i=1}{\stackrel{M}{\sum <br><br>}}<br><br>w_{d,k}<br><br>\left(i\right)<br><br>z^{-i+1}=w_{d,k}<br><br>\left(1\right)<br><br>\left(1-z_{d,k}<br><br>\left(1\right)<br><br>z^{-1}\right)<br><br><br><br>\mathrm{\dots <br><br>}<br><br><br><br>\left(1-z_{d,k}<br><br>\left(M-1\right)<br><br>z^{-1}\right).$

3. A communication system incorporating a base station according to claim 2 and a plurality of remote terminals.