Systems and methods for adaptive beamforming in indoor wireless networks
View Patent ↗Systems and methods are disclosed for transmitting signals between a transmitter and a receiver by differentially estimating channel coefficients for each of the antenna elements relative to a reference antenna element; determining beam weights to remove oscillator induced phase or offset; and applying a correction (beamforming solution) at the transmitter to make the phase of the received signal from different antenna elements to be coherently combined at the receiver.
1. A method for transmitting signals between a transmitter and a receiver, comprising:
differentially estimating channel coefficients for each one of antenna elements relative to a reference antenna element;
determining beamforming weights to remove oscillator induced phase or offset; and
applying a corrective beamforming solution at the transmitter to make a phase of a received signal from different ones of the antenna elements to be coherently combined at the receiver;
wherein said beamforming is for one of single users and multiple users.
2. The method of claim 1 , comprising performing a coarse estimating of the channel coefficients.
3. The method of claim 2 , comprising:
a. measuring each pair of the antenna elements, y 1 , y 2 , y 3 using excitation (1,0) (0,1) and (1,1);
b. averaging signal magnitudes across all symbols in a packet of signals between the transmitter and receiver and the determining the median as y 1 , y 2 and y 12 ; and
c. determining Ω i =Cos −1 ((y 12 2 −y 1 2 −y 2 2 )/(2*y 1 *y 2 ));
where yi is an average received signal amplitude at the receiver when element i alone is used at the transmitter, yjk is the received amplitude when both elements j and k are excited simultaneously at the transmitter at the same phase and Ω i is a measured angle for ith antenna element with I being greater than 0.
4. The method of claim 3 , comprising:
a. exciting elements in the pairs of the antenna elements identified before using the determined Ω i and its signed compliment (Ω i , −Ω i , π−Ω i , −(π−Ω i ); and
b. determining an angle φ i yielding the maximum signal strength at the receiver with φ i being a correct phase applied to an ith antenna element.
5. The method of claim 4 , comprising selecting an angle which has a higher packet delivery for angles with similar signal strength.
6. The method of claim 4 , wherein the step of determining beam weights comprises:
setting amplitudes A i =(y i 2 /Σy i 2 ); and
applying a negative of −φ i to each element i.
7. The method of claim 1 , comprising resolving ambiguities in the channel coefficient estimation.
8. The method of claim 1 , comprising:
a. performing differential channel estimation for a single user;
b. removing data modulation induced phase;
c. identifying a frequency offset by differentiating the phase of received radio signal samples; and
d. determining offset angles between receivers.
9. The method of claim 8 , comprising determining beamforming weights for multiple users.
10. The method of claim 8 , comprising obtaining weight vectors by solving an optimization problem.
11. The method of claim 8 , wherein the step of determining beamforming weights Hw for multiple users comprises solving
Hw
=
[
h
1
T
⋮
h
M
T
]
w
=
λ
[
P
1
ⅇ
jϕ
1
⋮
P
M
ⅇ
jϕ
M
]
=
λ
p
where h 1 T . . . h K T is a column vector that denotes a channel, K denotes users, P 1 :P 2 : . . . :P K , represents a given distribution, j denotes receivers, and w denotes a weight vector.
12. The method of claim 8 , wherein the step of determining beamforming weights for multiple users comprises solving an optimal weight vector according to the relationship
max
∑
k
=
1
K
η
k
h
k
T
w
2
w
2
=
1
for a given set of η k where
∑
k
=
1
K
η
k
=
1
,
where h 1 T . . . h K T is a column vector that denotes a channel, K denotes users, and w denotes a weight vector.
13. The method of claim 12 , wherein a solution to said optimal weight vector comprises determining an eigenvector corresponding to the largest eigenvalue of the matrix
∑
k
=
1
K
η
k
h
k
h
k
H